JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 10
PAGES 2033^2065
2013
doi:10.1093/petrology/egt040
Magma Dynamics and Petrological Evolution
Leading to the VEI 5 2000 BP Eruption of El
Misti Volcano, Southern Peru
FRANK J. TEPLEY III1*, SHANAKA DE SILVA1 AND GUIDO SALAS2
1
COLLEGE OF EARTH, OCEAN, AND ATMOSPHERIC SCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR,
97331-5506, USA
2
DEPARTMENTO DE GEOLOGIA, UNIVERSIDAD NACIONAL DE SAN AGUSTIN, AREQUIPA, PERU
RECEIVED JULY 7, 2011; ACCEPTED JUNE 17, 2013
ADVANCE ACCESS PUBLICATION JULY 17, 2013
surface in 5 days at ascent rates of at least 0·023 m s1. Further
decompression-driven crystallization is recorded in plagioclase rims
and microlite growth that may have contributed to a rapid increase
in viscosity leading to explosive eruption. This VEI 5 plinian eruption shares characteristics with other explosive events at El Misti
on a time scale of 2000^4000 years, suggesting periodic rechargedriven explosive activity.
Magma dynamics and time scales during the VEI 5, 2000 BP eruption of El Misti volcano, southern Peru (EM2000BP) are investigated to address cyclic explosive activity at this hazardous volcano.
The 1·4 km3 of pumice falls and flows have abundant mingled
pumice of high-K, calc-alkaline rhyolite and andesite composition.
Phenocryst zoning and compositions reveal mutual exchange of
plagioclase between the two magmas; amphibole in the rhyolite was
derived from the andesite. Amphiboles in the andesite are predominantly unrimmed crystals whereas those in the rhyolite mostly exhibit
reaction rims. Phase equilibria indicate that the andesite formed at
900^9508C and 2^3 kbar pressure and was water-saturated with
5·1^6·0 wt % H2O, broadly similar to El Misti magmas overall.
Amphibole, plagioclase, Ti-magnetite, and two pyroxenes were the
crystallizing phases. A separate rhyolite magma existed higher in
the crust at a temperature of 816 308C and 5% H2O in which
only plagioclase and Fe^Ti oxides were stable. The lack of cognate
amphibole in the rhyolite despite H2O saturation requires that it
staged above the stability limit of amphibole (5100 MPa).
Exchange reactions in amphibole (dominantly pargasitic) and
trace element partitioning in plagioclase indicate that both andesite
and rhyolite magmas were broadly constant in temperature and
H2O content. These constraints suggest that the initially separate
rhyolite and deeper andesite magmas interacted by an initial andesite
recharge event that resulted in mingling and crystal exchange. A
period of 50^60 days is required for amphibole introduced into the
rhyolite to develop reaction rims owing to decompression.These rims
are dominated by plagioclase, a consequence of the Al-rich nature of
the amphibole.The lack of reaction rims on amphibole in the andesite
implicates a second, more-forceful and voluminous eruption-triggering recharge event during which andesite rose rapidly from source to
Major composite cones, among the most hazardous volcanoes on the planet, are the integrated product of a prolonged history of effusive cone building activity
punctuated by explosive eruptions and edifice collapses
(Davidson & de Silva, 2000). Although the eruptive style
and attendant hazard is dominated by effusion, the rare
explosive eruptions are often the most voluminous and
hazardous. Understanding the controls on this transition
in activity is central to our efforts to fully address magmatic and volcanic evolution and hazard mitigation. Two
important clues to this effort are that explosive activity is
cyclic or quasi-cyclic (Matthews et al., 1997; Davidson &
de Silva, 2000; Ruprecht & Wo«rner, 2007) and involves
recharge, suggesting that the rhythm of open magmatic
systems is a dominant driver.
*Corresponding author. Telephone: 541 737 8199; Fax: 541 737 2064;
E-mail: [email protected].
The Author 2013. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
El Misti; explosive eruption; amphibole reaction rims;
trace element partitioning in plagioclase; magmatic time scales; recharge
KEY WORDS:
I N T RO D U C T I O N
JOURNAL OF PETROLOGY
VOLUME 54
It has long been recognized that magmatic recharge can
trigger explosive eruptions of a perched magma through
thermal, mass, and volatile exchange that results in pressurization of the system, and viscosity changes that result
in rheological and mechanical eruptive thresholds being
exceeded (e.g. Sparks et al., 1977; Blake, 1984). However, detailed studies of magmatic systems and single eruptions
reveal that these first-order results can be achieved in a
myriad of ways: mafic^mafic, mafic^silicic, and silicic^silicic interactions (Sparks et al., 1977; Eichelberger, 1978;
Feeley & Dungan, 1996; Eichelberger et al., 2000; de Silva
et al., 2008). Resident and recharge magma may or may
not achieve thermal and chemical equilibrium (e.g.
Pichavant et al., 2007). Exchange of crystals and redistribution is common (e.g. Davidson & Tepley, 1997; Ruprecht
et al., 2008). The scale of mixing and its controls on thermal
exchange and rates of equilibration, and changes in viscosity are particularly important (Huppert et al., 1982; Sparks
& Marshall, 1986; Snyder & Tait, 1995; Ruprecht &
Bachmann, 2010). All these processes are recorded in the
juvenile materials and revealed through detailed multiscale petrological studies (e.g. Tepley et al., 1999). When
based on a strong stratigraphic and volcanological foundation, such petrological studies form a crucial part of the
overall hazard assessment.
El Misti volcano (herein referred to as El Misti) in
southern Peru is one of the most hazardous volcanoes in
South America (de Silva & Francis, 1991a, 1991b; Thouret
et al., 2001; Harpel et al., 2011). Here, a population of
4800 000 live in Peru’s second largest city, Arequipa,
within 15 km of El Misti’s summit vents. During its 112
kyr eruptive history, at least three major and several smaller explosive eruptions have punctuated the effusive background activity. Reconnaissance of these eruptions has
revealed macroscopic evidence for magma mingling
(Legros, 1998; Thouret et al., 2001) and petrological studies
of plagioclase from various eruptions have revealed that
these eruptions are preceded by multiple magma recharge
events that eventually precipitated the respective eruptions
(e.g. Ruprecht & Wo«rner, 2007). If these observations hold
up to detailed scrutiny, the processes that drive explosive
volcanism at El Misti can be placed in the broader context
of the magmatic evolution of the system. To date no fully
contextual detailed study of an explosive eruption at El
Misti has been conducted.
The most recent explosive eruption at El Misti is the
VEI 5, 2000 BP eruption (Thouret et al., 2001; Harpel
et al., 2011), the products of which are exposed in multiple
drainage canyons on the south and west flanks of the volcano. This was a plinian fall and flow mixed rhyolite^
andesite tephra eruption, hypothesized to have involved a
recharge event based on abundant macroscopic and microscopic evidence from mixed pumices. As such the eruption
serves as a potential model for the other explosive
NUMBER 10
OCTOBER 2013
eruptions at El Misti. Herein we report the results of a detailed petrological study of the 2000 BP eruption at El
Mistiçthe first of its kind. We establish the magmatic conditions of the andesite and rhyolite reservoirs, and the
physical, chemical, and mineralogical signals of their
interaction, and provide constraints on the timing of the
event that led to the eruption. This work provides valuable
petrological context to a case study of the stratigraphy
and volcanology and hazard assessment of the eruption
(Harpel et al., 2011).
GEOLOGIC A L S ET T I NG
El Misti (16·2948S, 71·4098W; 5822 m above sea level) is a
major volcanic edifice of the Central Volcanic Zone of the
Andes (Bullard,1962; de Silva & Francis,1991a) in southern
Peru lying less than 15 km from the city of Arequipa
(Fig. 1). It is located within the Andean arc, and its history
is one of constructive dome growth, lava flows and explosive volcanism, endangering the growing population
center of Arequipa nearby (de Silva & Francis, 1991a;
Thouret et al., 2001; Harpel et al., 2011).
The geological history of El Misti is one typical of
Andean arc volcanoes. Based on extensive field mapping,
40
Ar/39Ar and 14C dating of rocks and organic material,
Thouret et al. (2001, and references therein), Paquereau
Lebti et al. (2006) and Ruprecht & Wo«rner (2007) have
pieced together a comprehensive volcanic history for El
Misti. The earliest remnant (c. 112 ka) of El Misti is an
eroded stratovolcano (Misti 1) that unconformably overlies
lavas and volcaniclastic deposits of Chachani Volcano
(Paquereau Lebti et al., 2006; Ruprecht & Wo«rner, 2007).
Upon this edifice lie successive edifices, termed Misti 2,
Misti 3, and Misti 4, and lava flows and pyroclastic debris
erupted since 112 ka. Historically, the volcano-building
events of El Misti are associated with alternating growth
and destruction of andesitic and dacitic domes and lava
flows with dome collapses and associated pyroclastic
flows, intermixed with explosive episodes, and avalanche
deposits (Thouret et al., 2001, and references therein;
Ruprecht & Wo«rner, 2007). Thouret et al. (2001) suggested
that, on average, ash falls occur every 500^1500 years,
with pumice fallout-producing eruptions every 2000^4000
years. The 2000 BP eruption is a plinian eruption producing
pumice falls and flows with varying proportions of
banded pumice of rhyolite and andesite compositions
amounting to 1·4 km3 of material (0·5 km3 dense rock
equivalent; Thouret et al., 2001; Harpel et al., 2011). Extensive lahars were generated by interaction of pyroclastic
flows with snow on the volcano, attesting to the potential
hazard of explosive eruptions at El Misti (Harpel et al.,
2011).
Over the course of its history, El Misti has produced
relatively homogeneous andesites and dacites with only
a few rhyolites. Thouret et al. (2001) noted that the
2034
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
70°
(a)
Caribbean Plate
(b)
10°
NVZ
0°
10°
zca
Na
El Misti
ile Trench
Peru-Ch
20°
CVZ
ge
Rid
30°
Nazca Plate
SVZ
40°
50°
Ch
ile
Ris
e
Antarctic Plate
70°
(c)
Fig. 1. (a) Map showing the location of El Misti in South America and its location in the Central Volcanic Zone. (b) Image of El Misti and the
surrounding region (from Harpel et al., 2011). Irregular white regions in bajadas are pyroclastic-flow deposits from the EM2000BP eruption.
Outlined by a white line is the city boundary of Arequipa. (c) Photograph of El Misti taken from downtown Arequipa, illustrating the proximity of a large population center to a potentially explosive volcano.
heterogeneous textures of the banded andesites and rhyolites of the 2000 BP eruption are unique to El Misti compared with other volcanoes in the region, in both texture
and the presence of a distinct mineral suite. For the purposes of our study, 50 samples were collected from throughout the eruption stratigraphy and studied to establish the
range of textures and mineralogy (compositions). Of these
four were chosen to represent the end-member textures
and compositions: two were dominantly of the white rhyolite component, and two others were composed primarily
of the black to brown andesite component. Each representative sample contains some mingled white and dark
2035
JOURNAL OF PETROLOGY
VOLUME 54
component. The results reported in this study are from thin
sections of these four samples.
A N A LY T I C A L M E T H O D S
A total of seven samples from the two lithological endmembers were analyzed for their major- and trace-element
compositions at the GeoAnalytical Lab at Washington
State University, by X-ray fluorescence spectroscopy
(XRF) and inductively coupled plasma mass spectrometry
(ICP-MS) techniques. Details of the techniques and their
associated analytical errors have been given by Johnson
et al. (1999) and Knaack et al. (1994), respectively.
Petrographic descriptions of the four representative samples provide records of the constituent phases, their abundance, and their textural relationship to the other phases.
Detailed analyses of minerals and glasses were performed
at Oregon State University using a CAMECA SX-100
electron microprobe (EMP) equipped with five wavelength-dispersive spectrometers (WDS) and high-intensity
dispersive crystals for high-sensitivity trace element analysis. Minerals and groundmass glasses were analyzed
using 15 keV accelerating voltage, 30 nA sample current,
and 1 mm beam diameter for mineral phases and 5 mm for
groundmass glasses. Counting times ranged from 10 to
60 s depending on the element and desired detection limit.
In all cases, zero-time intercept functions were applied to
reduce the effects of alkali migration. Data reduction was
performed online using a stoichiometric PAP correction
model (Pouchou & Pichoir, 1984). Back-scattered electron
(BSE) images were obtained using the same instrument
using the CAMECA Peak Site software. Precision measurements for the most significant elements in the glass,
feldspar, amphibole, pyroxene, and Fe^Ti oxides routines
are listed in Tables 5, 8, 6, 3 and 4, respectively.
Because some amphiboles in the selected samples exhibit
reaction rims whereas others do not, several amphiboles
from each lithology were selected for in situ trace element
analysis to determine population identity. Following EMP
analysis, analysis spots were chosen where EMP data
existed and in selected cores, mid-sections and rims of the
amphiboles. The analyses were carried out by laser ablation (LA)-ICP-MS in the Keck Collaboratory for Plasma
Spectrometry, Oregon State University, using a NewWave
DUV 193 nm ArF Excimer laser at 5 hz frequency, 15 ns
pulse duration and 50 mm beam size attached to a VG PQ
ExCell Quadrupole ICP-MS system and following the
techniques outlined by Kent et al. (2004). Concentrations
of single trace elements were calculated employing 43Ca
as an internal standard relative to the USGS glass standard BCR-2G. External errors are dependent on elemental
concentrations in the samples; however, calculated errors
are typically 5% for Sc, Cr, Rb, Y, Zr, Nb, La, Ce, Pr,
Nd, Sm, Eu, Gd, Dy, Er, Yb, and Pb, and 10% for V, Sr,
and Ba (1s).
NUMBER 10
OCTOBER 2013
R E S U LT S
Lithology and whole-rock textures
Both pyroclastic flow and fall deposits contain juvenile
clasts that display abundant evidence for magma mingling.
Two end-member lithologies, a plagioclase^amphibole
rhyolite and a plagioclase^amphibole andesite, are found
intimately mingled at different scales. Both are moderately
porphyritic. No pure end-member clasts were found, and
all the clasts show some mingling. The rhyolite forms a distinct pervasively micro-vesicular pumiceous lithology,
whereas the andesite occurs as a more obviously vesicular
scoriaceous lithology. A wide range of mingling relationships can be seen, from rhyolite-dominated to andesitedominated (Fig. 2). Evidence of mingling is abundant in
hand specimen as millimeter-scale wisps and selvages.
Andesite within dominant rhyolite tends to be in linear
wisps, selvages, and bands, but more complex relationships
are displayed as the lithologies become more andesitedominated. Complex sheath folding relationships can be
seen and thicker (centimeter-scale) bands of rhyolite show
clear evidence of ductile deformation with recumbent
folds (Fig. 2). Complex crenulation develops on rhyolite
selvages included in andesite. In several instances we
found that some of the rhyolitic wisps were rooted in
dense rhyolitic clasts (centimeter scale), which were being
disaggregated at their margins and being incorporated
into the andesite. Some grey selvages may represent a
hybrid lithology. We did not find any systematic stratigraphic variations in the distribution of the lithologies in
either the fall or flow deposits.
Diverse textural features characterize both the rhyolitic
pumice and andesitic scoria in thin section. Some clasts
show uniform distribution of a range of vesicle sizes
throughout the slide, but more commonly, particularly in
the rhyolite, heterogeneous clasts show distinct regions
where small bubbles (diameters 5^25 mm) predominate
and are surrounded by a matrix with intermediate-size to
coarse vesicles (75^100 mm and 175 mm diameters, respectively). Independent of the degree of heterogeneity, a
marked predominance of intermediate-size to coarse vesicles is conspicuous within some slides. The andesitic
scoria is characterized by largely equant to sub-spherical
vesicles with limited evidence for bubble deformation.
However, bubble deformation is ubiquitous in the rhyolite
pumice, which typically exhibits bands of elongated vesicles crossing larger regions with more equant bubbles.
The bands tend to range in width from 50 to 500 mm,
suggesting the presence of localized shear zones on a
range of scales.
Whole-rock geochemistry
Of the 50 collected samples of the eruption, seven samples
were chosen for major- and trace-element analysis. These
were chosen to check and supplement existing data from this
2036
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
eruption. All samples are typical medium- to high-K calc-alkaline CentralVolcanic Zone (CVZ) andesites and rhyolites
(Fig. 3; Table 1). Our samples from the EM2000BP eruption
fall in a similar range in aplotof K2O v. SiO2 as other samples
from the 112 kyr history of the volcano and from the CVZ
in general (Legros, 1998; Legrende, 1999; Ruprecht &
Wo«rner, 2007; Mamani et al., 2010; Fig. 3). Similarly, for other
major or trace elements, our samples fall within the data envelope of other CVZ volcanoes.Theyare characterizedby selective enrichment in large ion lithophile and alkaline earth
elements, attesting to the probable involvement of subduction-zone fluids, and lower abundances of rare earth elements
(REE) and high field strength elements, compared with typical mid-ocean ridge basalt, confirming their arc affinity.
(a)
Summary of rhyolite and andesite
petrology
(b)
(c)
Fig. 2. Hand samples illustrating the various textures of the tephra:
(a) sample containing thick globs of rhyolite in andesite matrix; (b)
thin wisps of rhyolite in a gradational rhyolite^andesite matrix; (c)
sample containing examples of both.
The rhyolite-dominant samples are 50% vesicles, 40%
groundmass, including glass and microlites, and 10%
phenocrysts (4500 mm) and microphenocrysts (100^
500 mm) (Fig. 4). The dominant phenocryst and microphenocryst phases include sub-equal amounts of plagioclase
(6%) and amphibole (2%) with lesser amounts of pyroxene (1%), Fe^Ti oxides (1%), and high-SiO2 rhyolitic
glass (72^78 wt % SiO2). Plagioclase phenocryst and
microphenocryst compositions range from An30 to An85
and display simple to complex normal and oscillatory
zoning. Plagioclase microlite (100 mm) compositions
range from An28 to An63 encompassing two populations
of normally zoned microlites: one in the range An43 to
An63 and another in the range An28 to An44.
The andesite-dominant samples contain 40% vesicles,
50% groundmass of equal proportions of glass and microlites, and 10% phenocrysts and microphenocrysts (Fig. 4).
The groundmass comprises plagioclase, pyroxene, Ti-magnetite, and andesitic to rhyolitic glass (60^72 wt % SiO2).
Plagioclase phenocryst and microphenocryst compositions
range from An30 to An88, showing a similar but slightly
greater compositional range than the rhyolite samples;
they exhibit similar complex textural features. The plagioclase microlites range from An63 to An43 (Table 2).
Amphiboles occur in both lithologies as strongly pleochroic crystals 51·4 mm in length and are pargasitic
in composition (Mg# 0·75). They commonly contain
Fe^Ti oxide inclusions. Amphiboles in the andesite are euhedral and slightly zoned, whereas those in the rhyolite
are rimmed by plagioclase, pyroxene and Fe^Ti oxide reaction products of variable thickness (50^600 mm). The reaction rim occurs where the amphibole is in contact with
melt rather than with other crystalline phases.
Pyroxene phenocrysts and microphenocrysts are present
in both lithologies, accounting for 51% of the phenocrysts
in the rocks. Orthopyroxenes in the rhyolite are 51mm in
size, are euhedral to subhedral, and sparsely distributed.
They range in composition from En78 to En82 with an
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JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 10
OCTOBER 2013
6
CVZ
WR rho
WR and
EMP rhyo glass
EMP and glass
K2O wt%
5
4
HIGH-K
3
MEDIUM-K
2
1
LOW-K
0
50
55
60
65
70
75
80
SiO2 wt%
Fig. 3. K2O vs SiO2 diagram for whole-rocks (filled circles and squares distinguished as rhyolite and andesite) and glasses (determined by
EMPA; open circles and squares) from EM2000BP, and their positions relative to a complete sampling of the Central Volcanic Zone (Mamani
et al., 2010). Most rocks are High-K calc-alkaline.
average composition of En80 (Table 3). In the pumiceous
andesite rocks, microphenocrysts of both clinopyroxene
and orthopyroxene are present as 51% of the crystals in
the rocks. Orthopyroxene occurs as euhedral or subhedral
microphenocrysts, and ranges in composition from En79
to En82, with an average composition of En80. The clinopyroxene occurs as microphenocrysts, and like the orthopyroxene, is euhedral to subhedral and 51mm in length.
Its compositions range between Wo42 and Wo47, with an
average composition of Wo45 (Table 3). There are no
Fe^Ti oxide pairs in the andesite, therefore we used coexisting pyroxene pairs to determine magma temperatures.
These coexisting pyroxenes yield temperatures of
940 408C for the andesite based on the thermometer of
Putirka (2008).
In the rhyolite samples, Fe^Ti oxides (both ilmenite and
magnetite) occur as discrete microlites, as inclusions in
amphibole and pyroxene, and as symplectites in the reaction rims of amphibole. Crystals are typically small (2^
20 mm), accounting for 1% of the mode. For temperature
calculations in the rhyolite, groundmass ilmenite and magnetite were used, and yielded temperatures of 816 308C
in the rhyolite based on the oxide thermometer of Ghiorso
& Evans (2008) (Table 4). The Fe^Ti oxides are in equilibrium based on the method of Bacon & Hirschmann (1988).
We determined glass compositions in the four targeted
thin sections by EMP analysis. The rhyolite glass compositions have a compositional range varying between
72·5 and 76·4 wt % SiO2 whereas the glass in the ‘andesite’ ranges from 62 wt % SiO2 to 72 wt % SiO2
(Table 5). The full dataset of whole-rock XRF analyses
and EMP phase chemistries is provided as Supplementary
Data.
P H E N O C RY S T T E X T U R E S A N D
C O M P O S I T I O N A L PAT T E R N S
Amphibole
Amphibole occurs as ubiquitous crystals throughout both
rhyolite and andesite lithologies with a modal abundance
of about 2% in each lithology. The most obvious difference
between the two is that most (490%) of the amphiboles
that reside in the rhyolite have reaction rims of plagioclase,
pyroxene, and Fe^Ti oxides, or occur as ragged clusters,
whereas most (490%) of those in the andesite do not
display a reaction rim. Different rim widths may reflect
differential sectioning of crystals rather than any processrelated phenomenon. Most amphiboles in both lithologies
show some evidence of minor compositional zoning based
on EMP analyses and BSE images.
Amphiboles in both lithologies of the EM2000BP eruption show a relatively small range of major element variation, forming relatively tight trends over 3 wt %
absolute spread in SiO2 (Fig. 5; Table 6). Compositions of
the amphiboles from both lithologies are similar, although
amphibole from the andesite defines the full range for
almost all major oxides.
Cation abundances, used to constrain the amphibole
classification and decipher petrogenetic processes, were
calculated assuming a formula cation sum of 15 excluding
2038
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
Table 1: Representative whole-rock major and trace element
compositions from the 2000 BP eruption of El Misti Volcano
SiO2
TiO2
EM
EM
EM
EM
EM
EM
EM
EM
007
008
009
085
094
098
099
0401
59·97
0·890
60·76
0·799
60·56
0·822
61·21
0·774
60·64
0·804
60·93
0·807
69·63
0·359
59·99
0·776
Al2O3
17·71
17·75
17·70
17·67
17·86
17·79
15·64
17·57
FeO*
5·74
5·41
5·52
5·29
5·40
5·31
2·64
5·59
MnO
0·09
0·09
0·09
0·09
0·09
0·09
0·07
0·10
MgO
2·95
2·59
2·67
2·46
2·56
2·52
1·05
3·31
CaO
5·89
5·63
5·71
5·53
5·70
5·63
2·92
5·98
Na2O
4·31
4·43
4·40
4·38
4·41
4·34
3·86
4·25
2·18
K2O
2·15
2·25
2·22
2·31
2·23
2·28
3·70
P2O5
0·30
0·30
0·30
0·29
0·30
0·30
0·14
0·25
Total
100·00
100·00
100·00
100·00
100·00
100·00
100·00
100·00
La
24·75
25·46
25·20
25·68
25·27
25·51
31·72
25·99
Ce
50·01
51·03
50·72
51·04
50·90
51·21
57·38
51·28
Pr
6·14
6·18
6·13
6·17
6·18
6·19
6·13
6·11
Nd
24·17
24·22
23·85
23·82
24·11
24·13
21·18
23·44
Sm
4·54
4·45
4·45
4·39
4·48
4·43
3·53
4·27
Eu
1·26
1·23
1·26
1·25
1·26
1·25
0·90
1·22
Gd
3·57
3·39
3·40
3·30
3·38
3·39
2·64
3·30
Tb
0·48
0·45
0·46
0·45
0·46
0·45
0·37
0·46
Dy
2·42
2·35
2·41
2·36
2·36
2·35
2·17
2·46
Ho
0·44
0·43
0·44
0·42
0·44
0·42
0·42
0·46
Er
1·08
1·07
1·09
1·07
1·04
1·08
1·16
1·18
Tm
0·15
0·15
0·15
0·15
0·15
0·15
0·18
0·16
Yb
0·91
0·90
0·91
0·86
0·90
0·90
1·22
1·02
Lu
0·14
0·14
0·14
0·14
0·14
0·14
0·20
Ba
907
928
917
925
921
927
1092
0·16
941
Th
2·32
2·55
2·48
2·67
2·39
2·57
7·63
3·30
Nb
5·85
5·99
6·06
6·11
6·06
6·13
7·08
5·22
Y
11·28
11·00
11·08
10·91
10·98
11·07
11·53
11·93
Hf
4·01
4·06
4·09
4·01
4·03
4·03
4·04
3·91
Ta
0·36
0·37
0·36
0·37
0·37
0·37
0·61
0·30
U
0·41
0·43
0·42
0·46
0·41
0·43
1·22
0·44
Pb
13·14
13·70
13·50
13·98
13·41
13·81
23·95
12·99
Rb
37·5
40·3
39·6
42·4
39·4
41·1
94·8
44·2
Cs
Sr
Sc
Zr
0·86
836
9·3
151
0·91
834
8·1
153
0·90
835
8·6
153
0·97
829
7·7
154
0·86
850
8·1
154
0·92
840
7·8
155
2·48
513
5·1
145
trends. Variations of AlVI and (Na þ K)A with AlIV are
small and form nearly horizontal trends (Fig. 6). Mg#
[Mg/(Mg þ Fe2þ)] varies between 0·7 and 0·8 with no
distinction between amphiboles in rhyolite or andesite,
and decreases with increasing AlIV, as seen in other studies
(e.g. Rutherford & Devine, 2003). Core and rim data
from both lithologies show no preference for higher or
lower AlIV in variation with AlVI, (Na þ K)A and Mg#.
Compositional zoning within any single amphibole
phenocryst represented by either core-to-rim transects or
single EMP spots is illustrated in Fig. 7. The chemical variations within single crystals are shown in relation to BSE
images, and representative amphibole samples in both
rhyolite and andesite are illustrated. Representative compositions are given in Table 6.
Selected amphibole crystals from both rhyolite and andesite were chosen for detailed in situ LA-ICP-MS trace
element analysis primarily to determine whether a correlation exists between amphiboles in the different lithologies.
Further, analysis locations were chosen coincident with
electron microprobe locations to utilize the major-element
microprobe data for trace element calibration (see Kent
et al., 2004) and to evaluate the chemical differences between different zones in the phenocrysts. Chondrite-normalized REE patterns of in situ LA-ICP-MS data for
amphiboles from both rhyolites and andesites show a classic convex form; the patterns and normalized concentrations are nearly identical for 495% of the samples,
regardless of the host-rock lithology (Fig. 8; Table 7). Light
REE (LREE) and middle REE (MREE) abundances
(La/SmN vs LaN; not shown) and LREE and heavy REE
(HREE) abundances (La/YbN vs LaN; not shown) also
demonstrate that although variations in normalized concentrations are present, they are minor. Lastly, there are
minor variations between compatible Yand slightly incompatible Sr when the grouped data are considered. In all
cases, there is no distinction between the chemical signatures of amphiboles hosted in the rhyolite or andesite.
Plagioclase
0·66
840
12·5
150
*Total Fe given as FeO.
Na and K (15eNK). Amphiboles in both the rhyolites and
andesites are pargasitic [nomenclature of Leake et al.
(1997)]. They show a moderate but significant range in
AlIV from 1·65 to 2 atoms per formula unit (p.f.u.), although, as in the major oxides case, the amphiboles from
the andesites tend to anchor the high and low end of the
Based on EMP analyses and backscattered electron images,
plagioclase phenocrysts, microphenocrysts and microlites
in both rock types define a large compositional range.
Phenocryst sizes range from 0·1 to 1·5 mm; we define
the boundary between phenocryst and microphenocryst
at 0·5 mm and microlites as 0·1mm. We group the
crystals based on composition into two broad groups: a
Low-An group, which ranges from An60 to An30, and a
High-An group, which ranges from An88 to An65. This
classification is based on An content frequency analyses of
the total plagioclase dataset (Fig. 9), supported by a MgO
wt % frequency histogram. Mirroring the compositional
variations are two broad classes of textural varieties based
on crystal morphology and texture: clear crystals, and
crystals with alternating sieved or dusty and clear portions.
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(d)
andesite
rhyolite
rhyolite
(e)
(b)
rhyolite
andesite
(f)
(c)
andesite
andesite
Fig. 4. Photomicrographs of amphibole with and without reaction rims and Low-An and High-An Group plagioclase in both rhyolite and andesite. Field of view in all images is 2 mm, and all images are in crossed polars. (a) Boundary (dashed line) between rhyolite and andesite
with reacted and unreacted amphibole, respectively. (b) A rhyolite-hosted amphibole with reaction rim. (c) A clear elongate amphibole residing
in andesitic melt. (d) A complexly zoned plagioclase crystal in rhyolite host from the Low-An group. (e) A plagioclase crystal with complex
dusty core and clear outer rim from the High-An group, in andesitic host. (f) A predominantly clear plagioclase crystal with minor zoning, a
member of the Low-An group, in andesitic host.
Low-An plagioclase crystals are morphologically clear and
simple, and tend to represent the rhyolite, whereas HighAn plagioclase crystals are complexly zoned and textured,
and tend to reside in the andesite. Occasionally these general host^plagioclase relationships are reversed, attesting
to crystal exchange between the two hosts. Figure 10
illustrates some of the various plagioclase types and sizes,
and Table 8 gives the representative compositions.
Low-An plagioclase group (An60^30)
The Low-An plagioclase phenocryst group have maximum
core An contents of An60, and minimum rim An contents
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TEPLEY et al.
EL MISTI 2000 BP ERUPTION
Table 2: Summary of rhyolite and andesite petrography and compositions
Rock type, proportions
Phases
Characteristics
Rhyolite
50% vesicles
small and intermediate to coarse
5–25 mm, 75–100 mm
40% groundmass
glass
72–78 wt % SiO2
microlites
An28–44; An43–63 bimodal
10% phenocrysts,
plagioclase (6%)
An30–85 complexly zoned
microphenocrysts
amphibole (2%)
pargasite with reaction rims
Fe–Ti oxides (1%)
8168C 308C
pyroxene (1%)
En78–82, av. En80
Andesite
40% vesicles
equant to sub-spherical
50% groundmass
glass
62–72 wt % SiO2
microlites
An43–63
10% phenocrysts,
plagioclase (8%)
An30–88 complexly zoned
microphenocrysts
amphibole (2%)
pargasite euhedral
pyroxene (51%)
En79–82, av. En80 Wo42–47, av. Wo45
2-pyx temperature
9408C 408C
of An30. This population of phenocrysts is generally normally zoned, although most of those either imaged (BSE)
or analyzed show one zone of increased An content outboard of the core before decreasing to values An40^30
near the rims (Fig. 10a). Backscattered electron images
show that the low-An cores have rounded interior borders
that changed immediately to higher An values, although
these higher values are a few mol % An higher.
Texturally, these phenocrysts are generally simple, euhedral, clear crystals, although there are sparse crystals with
mottled cores.
The trace elements Mg, Ti and Fe were measured simultaneously with major elements during analysis transects.
Generally, their concentrations are low, given the incompatibility of these elements in plagioclase. Transects of
FeO show two patterns: one pattern shows little variation
regardless of changing An content (2B and 11E), whereas
the other pattern is antithetic to An content (e.g. samples
5I and 10G). MgO concentrations are more variable than
FeO, but they display the same patterns relative to An
content.
High-An plagioclase group (An88^65)
The second broad group of plagioclase phenocrysts includes those with compositions between An88 and An65,
with average core An contents of An80. Rim compositions
depend on whether the crystals are hosted in rhyolite or
andesite. This crystal population is also normally zoned,
but most crystals have complex zoning patterns extending
to the rim. This complex compositional zoning is reflected
in their complex textural features, characterized by obvious dusty or sieved portions alternating with clear portions. In most examples, the cores of these crystals are
sieved or dusty and alternate with clear portions outwards
towards the rim; in a few cases, the cores of these crystals
are clear. We see no compositional differences between the
crystals with clear cores versus dusty cores.
Trace element concentrations are generally higher and
their distribution patterns are different from those of the
Low-An group. In contrast to the Low-An group, FeO
variations in all cases are regular and unchanging regardless of the variations in An content (Fig. 10b). However,
MgO appears to be sensitive to changes in An content,
producing an anti-correlation in MgO.
Microlites
Microlites in the rhyolite and andesite also show some
compositional heterogeneity. Frequency histograms show
that there are two populations of microlite compositions,
one in the rhyolite, and one in the andesite. Both populations are normally zoned. The microlite population in the
andesite has cores of An54^63 and rims of An43^63, whereas
the microlite population in the rhyolite has cores of
An41^44 and rims of An28^38 (Fig. 9). These populations of
microlites are distinct in composition from the phenocrysts
in their respective host lavas: they are slightly more
evolved. We attribute these compositional characteristics
to lower pressure final equilibration and lower magma
pH2O values.
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Table 3: Representative compositions of clinopyroxene and orthopyroxene phenocrysts
clinopyroxene
orthopyroxene
Sample:
EM 10 L
EM 10 L
EM 10 N
EM 10 N
EM 11 M
EM 11 M
EM 2 J
EM 2 J
SiO2
50·32
52·46
49·61
51·73
52·09
Al2O3
3·20
1·15
3·71
1·55
1·21
FeO
8·77
8·12
10·04
9·80
MgO
14·67
15·69
13·69
CaO
20·86
21·04
20·50
Na2O
0·45
0·43
TiO2
0·88
MnO
0·38
Total
99·53
EM 10 M
52·34
53·68
52·94
52·89
1·14
0·98
1·12
1·14
9·29
9·12
19·45
19·82
15·84
14·45
14·79
24·81
18·96
21·10
21·13
1·00
0·50
0·36
0·46
0·39
0·43
1·05
0·59
0·19
0·37
0·29
0·32
0·44
99·68
99·40
99·14
99·24
EM 10 M
EM 11 L
EM 11 L
53·01
53·61
53·22
1·00
0·85
0·98
19·98
20·04
18·31
18·46
24·29
24·45
24·40
25·24
24·94
1·00
1·09
1·09
1·04
1·04
0·03
0·04
0·02
0·01
0·03
0·03
0·14
0·12
0·12
0·30
0·26
0·19
0·21
0·35
0·67
0·70
0·66
0·60
0·58
0·61
99·41
100·75
100·03
100·52
100·41
99·86
99·50
Typical 1SD: SiO2 0·12; Al2O3 0·04; FeO* 0·01; MgO 0·04; CaO 0·05; Na2O 0·04; TiO2 0·01; MnO 0·03.
Table 4: Representative compositions of magnetite and ilmenite phenocrysts
magnetite
Sample:
EM 2 O
ilmenite
EM 2 O
EM 2 O
EM 2 O
EM 2 N
EM 2 N
EM 2 O
EM 2 O
EM 2 O
EM 2 N
EM 2 N
EM 2 N
SiO2
0·05
0·05
0·07
0·05
0·04
0·04
0·00
0·01
0·01
0·03
0·03
0·04
TiO2
6·53
6·28
6·20
6·25
6·69
6·22
38·43
37·95
37·08
37·21
36·91
37·25
Al2O3
1·92
1·51
1·53
1·50
1·50
1·47
0·14
0·15
0·16
0·20
0·17
0·25
V2O3
0·44
0·48
0·45
0·50
0·44
0·48
0·27
0·32
0·31
0·34
0·30
0·35
Cr2O3
0·10
0·10
0·11
0·07
0·08
0·05
0·00
0·01
0·02
0·00
0·00
0·00
FeO*
81·82
82·47
83·64
83·45
82·94
82·23
54·79
54·56
55·64
54·29
52·95
53·57
MnO
0·39
0·51
0·52
0·53
0·49
0·58
0·57
0·60
0·52
0·57
0·51
0·56
MgO
1·84
1·73
1·76
1·67
1·67
1·58
2·37
2·38
2·41
2·62
2·53
2·76
CaO
0·03
0·01
0·02
0·01
0·04
0·01
0·02
0·01
0·01
0·01
0·03
0·03
ZnO
0·18
0·07
0·11
0·06
0·09
0·17
0·10
0·03
0·02
0·04
0·07
0·00
Total
93·30
93·21
94·42
94·11
93·99
92·84
96·70
96·04
96·18
95·32
93·95
94·83
*Total Fe given as FeO.
Typical 1SD: (magnetite) SiO2 0·01; TiO2 0·01; Al2O3 0·005; V2O3 0·02; Cr2O3 0·025; FeO* 0·3; MnO 0·02;
MgO 0·02; CaO 0·004; ZnO 0·01; (ilmenite) SiO2 0·01; TiO2 0·4; Al2O3 0·02; V2O3 0·03; Cr2O3 0·01;
FeO* 0·65; MnO 0·1; MgO 0·01; CaO 0·01; ZnO 0·01.
DISCUSSION
The details of magma mingling
The macroscopic and microscopic lithological, petrographic, and petrological observations presented above are
all consistent with extensive mingling of a relatively hot
(940 408C) andesite and a cooler (816 308C) rhyolite
magma prior to the 2000 BP eruption of El Misti. These
magmas are typical of the calc-alkaline high-K suite of
magmas that have erupted in the Central Volcanic Zone
during the Pleistocene. The pumice of the 2000 BP eruption
of El Misti is extensively ‘banded’ and heterogeneous at
macroscopic and microscopic scales, and vesicle textures in
the respective lithologies record differences in rheology and
the results of the interaction (shearing, vesicle trains, etc.).
Petrographic evidence for crystal exchange is supported
by phenocryst compositions that indicate two populations
of plagioclase phenocrysts and microlites based on composition and texture. Low-An plagioclase, morphologically
clear and simple crystals formed in the rhyolite, and
2042
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
Table 5: Representative glass compositions
Sample:
2
2
5
5
10
SiO2
TiO2
10
74·66
75·69
74·32
74·67
66·81
73·58
66·07
0·33
0·33
0·34
0·33
0·79
0·77
0·56
0·75
0·76
Al2O3
13·34
13·47
14·02
14·29
15·21
15·95
14·42
12·68
14·11
FeO*
1·49
1·47
1·48
1·41
4·91
4·18
2·26
5·88
3·76
MnO
0·05
0·07
0·00
0·08
0·08
0·09
0·03
0·13
0·10
MgO
0·30
0·29
0·18
0·18
1·96
0·96
0·41
4·20
2·17
CaO
1·15
1·03
0·73
0·82
2·88
3·04
1·55
2·84
2·55
Na2O
2·00
2·36
3·07
2·93
2·93
3·51
2·84
2·72
2·81
68·5
11 clear
11 dark
11 dark
69·46
K2O
4·64
4·66
4·81
4·71
3·38
3·36
4·26
3·56
3·83
P2O5
0·04
0·04
0·03
0·02
0·37
0·33
0·26
0·32
0·33
Cl
0·15
0·13
0·19
0·18
0·11
0·14
0·14
0·13
0·13
98·18
99·56
99·17
99·63
99·46
100·84
100·35
99·29
100·02
Total
*Total Fe given as FeO.
Typical 1SD: SiO2 0·07; TiO2 0·01; Al2O3 0·03; FeO* 0·12; MnO 0·02; MgO 0·01;
CaO 0·03; Na2O 0·04; K2O 0·03; P2O5 0·01.
high-An plagioclase, complexly zoned and textured crystals
formed in the andesite, now occur in both andesite and
rhyolite, attesting to crystal exchange. Based on the mineral
composition data, we infer that amphibole grew in the andesite magma at depth. However, amphibole can now be
found in both the rhyolite and andesite, with the amphibole
in the rhyolite exhibiting reaction rims.
Having established these baseline characteristics, below
we explore the deeper issues of the complex trace element
systematics of the plagioclase and reaction rim development on amphibole in the rhyolite, and how these relate
to the timing and development of the system as a whole.
Plagioclase trace element systematics
Plagioclase compositions in the 2000 BP eruption display a
wide range of variability; we have distinguished two
groups, the High-An Group and the Low-An Group,
based on their predominant compositions and textures.
However, overlap in An content and, in some cases, textural features limits our ability to definitively discriminate
between the two populations of plagioclase phenocrysts.
In this case, we have turned to trace element concentrations in the plagioclase as an efficient discriminator.
Plagioclase compositions are controlled by the melt
composition and its H2O content, and the intensive parameters, temperature and pressure, of the crystallizing
system (Bowen, 1928; Tsuchiyama, 1985; Housh & Luhr,
1991). Changes in these variables can lead to variations in
plagioclase composition (crystal zoning), and possibly in
the rate of crystal growth (i.e. crystal growth kinetics).
Changes in the temperature of the system will change the
equilibrium composition of the plagioclase in that system,
as will increases or decreases in the pH2O of the system
(e.g. Housh & Luhr, 1991; Lange et al., 2009). Closedsystem processes that effect compositional changes, such as
crystal entrainment in convective currents within a
magma chamber (Singer et al., 1995) or density currents
induced from overburdened sidewall or roof crystallization
(Marsh, 1989), may occur without the interaction of different magmas. Open-system processes, such as magma recharge, change not only the composition of the system
and its temperature but also the equilibrium plagioclase
composition.
Discriminating between competing intensive and extensive variables requires evaluation of the minor and trace
element compositions of the plagioclase, which are less susceptible to changing intensive parameters. In closed systems, equilibrium crystallization of plagioclase and the
associated partitioning of trace elements into plagioclase
will be governed by crystal chemical controls on elemental
partitioning (e.g. Blundy & Wood, 1994) and melt compositional controls (Nielsen & Drake, 1979; Nielsen &
Dungan, 1983). An exception to these rules is the non-equilibrium effects of variable diffusion of trace elements to
and from the crystal^melt interface during rapid crystal
growth that may lead to significant departures from ‘equilibrium’ element partitioning (Albare'de & Bottinga, 1972;
Shimizu, 1983; Singer et al., 1995). Recharge events bring
about changes in temperature, pressure, pH2O and melt
composition, which may change plagioclase compositions
and the trace element composition of the melt, and therefore the equilibrium partitioning of that trace element
into plagioclase. Recharge may also mix two populations
of plagioclase crystals with different compositions.
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1.0
pargasite
Mg/(Mg+Fe2+)
edenite
magnesiosadanagaite
0.8
0.6
0.4
Amphiboles in
rhyolite melt
0.2
ferropargasite
ferro-edenite
0.0
7.5
7.0
Amphiboles in
andesite melt
6.5
6.0
5.5
5.0
4.5
Si pfu
14.0
13.0
Amphiboles in rhyolite melt
12.5
Amphiboles in andesite melt
13.0
CaO
Al2O3
13.5
12.5
12.0
11.5
12.0
11.0
11.5
10.5
11.0
41.0
42.0
43.0
10.0
41.0
44.0
42.0
SiO2
18.0
14.0
17.0
MgO
13.0
FeO*
44.0
43.0
44.0
SiO2
15.0
12.0
11.0
16.0
15.0
14.0
10.0
13.0
9.0
8.0
41.0
43.0
42.0
43.0
12.0
41.0
44.0
SiO2
42.0
SiO2
Fig. 5. Upper diagram shows EM2000BP amphibole phenocryst compositions from both rhyolite- and andesite-dominated samples plotted in
the classification scheme of Leake et al. (1997) using the Mg# [Mg/(Mg þ Fe2þ)] vs Si p.f.u. (per formula unit) diagram. All are pargasitic
amphibole. Lower diagrams show amphibole variations in Al2O3, FeO* (total iron), CaO and MgO vs SiO2.
Plotted in Fig. 11 are equilibrium partitioning concentration curves of MgO, TiO2 and FeO based on the plagioclase^melt trace element partitioning experiments of
Bindeman et al. (1998) and Tepley et al. (2010). In modeling
the plagioclase concentrations, we use starting melt compositions obtained from the rhyolite and andesite whole-rock
compositions (see Table 1), and temperatures calculated
from oxide pairs in the rhyolite and pyroxene pairs in the
2044
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
Table 6: Representative compositions and structural formulae of rhyolite- and andesite-hosted hornblende types
rhyolite-hosted amphiboles
Sample:
EM 2
EM 2
EM 2
EM 2
EM 2
EM 2
EM 2
EM 5
EM 5
EM 5
EM 5 A
EM 5
EM 5
EM 5
EM 5
I
I
I(2)
I(2)
I(2)
I(3)
I(3)
cl1
cl1
cl2A
cl2A
cl2A
cl2B
cl2B
cl2B
42·36
SiO2
42·28
42·30
42·47
42·43
42·38
42·50
42·75
42·84
42·97
43·03
42·32
42·72
42·50
42·53
TiO2
2·48
2·53
2·44
2·46
2·54
2·43
2·37
2·43
2·40
2·30
2·41
2·40
2·35
2·55
2·50
Al2O3
12·92
12·98
12·60
12·56
12·54
12·36
12·19
12·59
12·36
12·22
12·52
12·28
12·26
12·34
12·51
Cr2O3
0·02
0·02
0·02
0·00
0·01
0·00
0·00
0·00
0·00
0·17
0·02
0·00
0·07
0·02
0·06
FeO*
12·03
12·22
11·39
12·00
11·91
12·07
11·75
11·81
11·99
10·64
11·71
11·67
11·33
11·92
11·93
MnO
0·10
0·13
0·10
0·15
0·07
0·08
0·11
0·11
0·15
0·11
0·10
0·11
0·09
0·15
0·15
MgO
14·58
14·24
14·80
14·58
14·48
14·58
14·69
14·68
14·88
15·48
14·76
14·88
14·97
14·63
14·50
CaO
11·81
11·73
11·58
11·56
11·56
11·62
11·55
11·71
11·73
11·41
11·57
11·45
11·37
11·47
11·37
Na2O
2·28
2·30
2·24
2·32
2·31
2·23
2·25
2·31
2·29
2·25
2·26
2·28
2·30
2·27
2·27
K2O
0·53
0·63
0·60
0·60
0·56
0·52
0·51
0·55
0·55
0·59
0·54
0·53
0·58
0·53
0·58
Cl
Total
0·02
0·02
0·02
0·02
0·03
0·02
0·02
0·03
0·03
0·02
0·02
0·02
0·01
0·02
0·02
99·09
99·18
98·34
98·74
98·43
98·46
98·25
99·11
99·40
98·26
98·25
98·36
97·89
98·49
98·31
Si
6·103
6·122
6·170
6·157
6·166
6·175
6·221
6·184
6·184
6·230
6·153
6·203
6·199
6·181
6·172
AlIV
1·897
1·878
1·830
1·843
1·834
1·825
1·779
1·816
1·816
1·770
1·847
1·797
1·801
1·819
1·828
SUM T
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
AlVI
0·301
0·336
0·328
0·307
0·318
0·293
0·312
0·328
0·281
0·316
0·299
0·305
0·307
0·295
0·321
Ti
0·265
0·272
0·263
0·265
0·274
0·262
0·255
0·260
0·256
0·247
0·260
0·258
0·254
0·275
0·270
Fe3þ
0·320
0·228
0·224
0·236
0·203
0·279
0·218
0·213
0·281
0·192
0·282
0·227
0·212
0·225
0·202
Cr
0·002
0·002
0·002
0·000
0·002
0·000
0·000
0·000
0·000
0·019
0·002
0·000
0·008
0·002
0·007
Mg
3·137
3·072
3·204
3·153
3·142
3·157
3·186
3·158
3·192
3·341
3·197
3·221
3·256
3·170
3·150
Fe2þ
0·974
1·090
0·979
1·039
1·062
1·011
1·029
1·042
0·994
0·885
0·960
0·989
0·964
1·033
1·050
Mn
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
SUM C
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
Fe2þ
0·158
0·161
0·181
0·180
0·185
0·177
0·183
0·172
0·169
0·212
0·182
0·201
0·207
0·191
0·201
Mn
0·012
0·016
0·013
0·018
0·009
0·010
0·013
0·013
0·018
0·014
0·012
0·013
0·012
0·018
0·019
Ca
1·827
1·819
1·803
1·797
1·802
1·809
1·800
1·811
1·809
1·770
1·802
1·781
1·777
1·786
1·775
Na
0·004
0·004
0·004
0·004
0·004
0·004
0·004
0·004
0·004
0·004
0·004
0·004
0·004
0·005
0·005
SUM B
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
Na
0·635
0·641
0·628
0·648
0·647
0·624
0·632
0·642
0·634
0·627
0·634
0·638
0·645
0·635
0·636
K
0·097
0·117
0·112
0·110
0·104
0·096
0·096
0·102
0·100
0·109
0·099
0·097
0·108
0·099
0·108
SUM A
0·733
0·757
0·739
0·758
0·750
0·720
0·727
0·744
0·735
0·736
0·733
0·736
0·753
0·734
0·745
Mg/(Mg þ Fe2þ)
0·735
0·711
0·734
0·721
0·716
0·727
0·724
0·722
0·733
0·753
0·737
0·730
0·736
0·721
0·716
andesite-hosted amphiboles
Sample:
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 11
EM 11
EM 11
EM 11
H
H
H
I
I
I
K
K
H
H
H
I
I
K(2)
K(2)
42·63
SiO2
41·99
41·95
41·62
43·88
42·74
43·80
43·03
42·27
42·72
42·58
42·45
43·21
42·69
42·77
TiO2
2·58
2·53
2·64
2·30
2·43
2·29
2·39
2·47
2·51
2·54
2·54
2·22
2·35
2·44
2·33
Al2O3
12·86
12·84
13·13
11·87
12·59
11·98
12·62
12·54
12·37
12·55
12·74
12·67
11·95
12·53
12·64
Cr2O3
0·03
0·01
0·00
0·12
0·11
0·50
0·07
0·04
0·05
0·02
0·02
0·14
0·00
0·17
0·02
FeO*
11·41
11·76
11·92
10·11
11·62
10·18
11·47
11·89
11·20
11·26
12·04
10·51
11·65
11·52
12·35
(continued)
2045
JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 10
OCTOBER 2013
Table 6: Continued
andesite-hosted amphiboles
Sample:
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 10
EM 11
EM 11
EM 11
EM 11
H
H
H
I
I
I
K
K
H
H
H
I
I
K(2)
K(2)
MnO
0·12
0·14
0·08
0·07
0·16
0·07
0·09
0·11
0·09
0·11
0·11
0·10
0·11
0·10
0·12
MgO
14·86
14·67
14·41
16·15
14·87
16·03
15·03
14·42
15·32
14·87
14·42
15·86
14·68
15·02
14·51
CaO
11·21
11·53
11·34
11·41
11·59
11·61
11·75
11·75
11·60
11·66
11·75
11·23
11·55
11·45
11·39
Na2O
2·32
2·39
2·37
2·28
2·31
2·29
2·35
2·21
2·36
2·28
2·28
2·39
2·20
2·30
2·23
K2O
0·54
0·57
0·58
0·52
0·60
0·61
0·53
0·54
0·60
0·56
0·58
0·59
0·59
0·59
0·54
Cl
0·02
0·01
0·02
0·02
0·02
0·03
0·02
0·02
0·02
0·02
0·02
0·02
0·02
0·01
0·02
97·99
98·46
98·16
98·75
99·06
99·42
99·42
98·37
98·89
98·47
99·01
98·96
97·83
98·97
98·87
Total
Si
6·119
6·098
6·077
6·298
6·168
6·260
6·181
6·152
6·165
6·173
6·144
6·204
6·241
6·175
6·174
AlIV
1·881
1·902
1·923
1·702
1·832
1·740
1·819
1·848
1·835
1·827
1·856
1·796
1·759
1·825
1·826
SUM T
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
8·000
AlVI
0·328
0·299
0·337
0·307
0·310
0·278
0·318
0·303
0·269
0·318
0·318
0·348
0·300
0·308
0·332
Ti
0·278
0·273
0·285
0·244
0·260
0·242
0·255
0·267
0·268
0·273
0·272
0·234
0·254
0·260
0·250
Fe3þ
0·227
0·270
0·229
0·156
0·223
0·167
0·225
0·275
0·245
0·208
0·236
0·178
0·214
0·215
0·258
Cr
0·004
0·002
0·000
0·013
0·013
0·056
0·008
0·005
0·005
0·002
0·002
0·016
0·000
0·019
0·003
Mg
3·228
3·180
3·136
3·456
3·199
3·416
3·219
3·128
3·297
3·214
3·112
3·394
3·198
3·233
3·133
Fe2þ
0·935
0·977
1·014
0·824
0·995
0·841
0·976
1·023
0·916
0·985
1·060
0·829
1·037
0·964
1·024
Mn
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
0·000
SUM C
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
5·000
Fe2þ
0·228
0·182
0·212
0·233
0·184
0·209
0·178
0·150
0·191
0·172
0·160
0·255
0·174
0·211
0·214
Mn
0·015
0·018
0·010
0·008
0·019
0·008
0·011
0·014
0·011
0·013
0·013
0·012
0·013
0·012
0·014
Ca
1·751
1·796
1·773
1·754
1·793
1·778
1·808
1·833
1·794
1·811
1·822
1·728
1·809
1·772
1·767
Na
0·005
0·004
0·005
0·005
0·004
0·004
0·004
0·003
0·004
0·004
0·004
0·005
0·004
0·005
0·004
SUM B
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
2·000
Na
0·650
0·668
0·666
0·629
0·643
0·631
0·651
0·622
0·656
0·638
0·636
0·662
0·619
0·640
0·622
K
0·100
0·105
0·107
0·096
0·111
0·112
0·097
0·101
0·110
0·104
0·108
0·109
0·109
0·109
0·099
SUM A
0·750
0·773
0·773
0·724
0·754
0·743
0·748
0·722
0·766
0·742
0·744
0·770
0·728
0·748
0·721
Mg/(Mg þ Fe2þ)
0·735
0·733
0·719
0·766
0·731
0·765
0·736
0·727
0·749
0·735
0·718
0·758
0·725
0·733
0·717
*Total Fe given as FeO.
Typical 1SD: SiO2 0·11; TiO2 0·04; Al2O3 0·05; Cr2O3 0·02; FeO* 0·17; MnO 0·02; MgO 0·07; CaO 0·06;
Na2O 0·07; K2O 0·04; Cl 0·005.
andesite, and then calculate the trace element equilibrium
concentration in the plagioclase. On each diagram, two
equilibrium-partitioning curves are plotted representing
the equilibrium conditions of plagioclase crystals growing
in the rhyolite (Low-An type) and those growing in the andesite (High-An type), labeled as low Tand high T, respectively. Plotted with these equilibrium-partitioning curves are
MgO, TiO2 and FeO compositions measured simultaneously with An content via EMP analysis. In the MgO and
TiO2 diagrams, two swaths of data are prominent, which
plot on or near the equilibrium concentration lines. The
first observation is that trace element concentrations in
plagioclase allow us to discriminate between the two
populations of crystals. The second observation is that, for
the most part, equilibrium crystallization of plagioclase
occurred, and the large variations in An content in both
clusters are consistent with closed-system evolution associated with small variations in H2O and/or temperature of
the host magma. In contrast, FeO shows large variations in
An content with small or no changes in FeO, which suggest
that other factors, such as fO2, contributed to the partitioning of Fe in plagioclase phenocrysts that did not affect Mg
or Ti partitioning. Based on these observations, we conclude
that plagioclase phenocrysts in both the rhyolite and andesite grew independently of each other in relatively consistent
environments before being mingled together and erupted.
2046
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
0.6
0.5
AlVI (pfu)
The advantage of these geochemical discriminators is
that regardless of whether a plagioclase crystal is found in
the rhyolite-dominated end-member or the andesite-dominated end-member, the trace element characteristics
coupled with the An content can reveal the original premixed environment of crystallization: the andesite reservoir or the rhyolite reservoir. With a system dominated by
mixed tephra containing mixed crystal populations, this
gives us the ability to elucidate the mixing process.
Amphibole in rhyolite melt
Amphibole in andesite melt
0.4
0.3
0.2
10K
10H
0.1
(a)
0.0
(Na+K)A (pfu)
1.0
Amphibole textures: the significance of reaction rims
0.9
0.8
0.7
0.6
(b)
0.5
Ti (pfu)
0.4
0.3
0.2
(c)
0.1
Mg/(Mg+Fe2+
1.0
0.9
0.8
0.7
0.6
(d)
0.5
1.6
1.7
1.8
AlIV
1.9
2.0
2.1
(pfu)
Fig. 6. Amphibole atomic (p.f.u., per formula unit) compositions and
evaluation of substitution mechanisms. (a) AlVI shows no change
with increasing AlIV in the pressure-sensitive Al-Tschermak substitution, reflecting no change in pressure at time of crystallization and
growth. Included in the diagram are lines labeled 10K and 10H indicating the range of AlIV for two amphibole crystals (10K and 10H)
from the andesite. Temperature-dependent exchanges, such as the edenite exchange (b) and the Ti-Tschermak exchange (c), indicate slight
temperature fluctuations. (d) A slight decrease in Mg# [Mg/
(Mg þ Fe2þ)] with increasing AlIV is indicative of growth in a fractionating liquid.
The reaction rims on amphibole in the rhyolite lavas are
composed of intergrowths of plagioclase, orthopyroxene,
clinopyroxene and Fe^Ti oxides; the rims occur only
where amphibole edges are in contact with melt, not other
crystals. The rims are of relatively uniform thickness
around selected amphiboles and generally retain the precursor euhedral shape of the amphibole. These observations suggest that the reaction rims grew inward from the
amphibole edge such that the host melt plays an integral
role in the development of the rim (e.g. Rutherford &
Hill, 1993; Browne & Gardner, 2006; Buckley et al., 2006).
These gabbro-type reaction rims on amphibole are often
interpreted as resulting from volatile exsolution as a consequence of H2O loss during magma decompression during
movement to or storage at shallow depth (e.g. Garcia &
Jacobson, 1979; Rutherford & Hill, 1993; Rutherford &
Devine, 2003). The major percentage of amphiboles in the
andesitic host magma have no reaction rims, whereas the
majority of amphiboles in the rhyolite have reaction rims.
We evaluate the re-equilibration process through a detailed
mass-balance analysis of the amphiboles, their reaction
rims, and the surrounding melt, because this information
has bearing on the mechanism, timing and evolution of
the reaction rims.
We determined the distribution and proportion of phases
in the reaction rims using high-resolution X-ray element
mapping on the OSU electron microprobe. X-ray intensity
maps of Al, Fe, Ca and Mg were produced for selected
reacted amphiboles to determine the spatial distribution
and proportion of phases, in which Al is diagnostic for
plagioclase, Fe for Fe^Ti oxides, and Mg and Ca for clinopyroxene and orthopyroxene. The X-ray images were imported and modified in Adobe PhotoshopTM. The layers
for each element were stacked, the reaction rim on the
inside and outside was outlined, and the interior and exterior pixels were cut away leaving only reaction rim pixels.
Within each element layer, pixels of a limiting threshold
were highlighted and counted, and the total pixels from
each layer were summed. To obtain the areal proportion
of a phase in the reaction rim, each layer’s pixels were
ratioed to the summed pixels of the reaction rim. Table 9
lists the relative proportion of plagioclase, orthopyroxene,
clinopyroxene and Fe^Ti oxides in five amphibole rims.
2047
JOURNAL OF PETROLOGY
(a)
VOLUME 54
NUMBER 10
OCTOBER 2013
Laser spot
Microprobe spot
Microprobe transect
500 um
1000 um
0.80
0.80
EM10 andH
EM10 andI
0.75
Mg#
Mg#
0.75
0.70
0.70
0.65
0.65
0
200
400
600
1.5
800
1.6
1.7
1.8
1.9
2.0
2.1
2.2
AlIV pfu
Distance (μm)
1000 um
200 um
0.80
0.80
EM10 andK
EM11 rhyH
0.75
Mg#
Mg#
0.75
0.70
0.70
0.65
0.65
0
200
400
600
0
800
200
Distance (μm)
400
600
800
Distance (μm)
200 um
500 um
0.80
0.80
EM11 rhyI
EM11 rhyK2
0.75
Mg#
Mg#
0.75
0.70
0.70
0.65
0.65
1.5
1.6
1.7
1.8 1.9
AlIV pfu
2.0
2.1
2.2
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
AlIV pfu
Fig. 7. BSE and compositional diagrams for representative amphibole phenocrysts from rhyolite and andesite. Illustrated are BSE images
plotted with EMP traverse or spot point locations and LA-ICP-MS spot locations. In traverses, the Mg# data are plotted versus distance
from rim, and in spot analyses, the Mg# data are plotted versus AlIV p.f.u.
(continued)
2048
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
(b)
Laser spot
Microprobe spot
Microprobe transect
500 um
200 um
0.80
0.80
EM2 rhyI3
EM2 rhyI
0.75
Mg#
Mg#
0.75
0.70
0.70
0.65
0.65
1.5
1.6
1.7
1.8
1.9
AlIV
2.0
pfu
2.1
2.2
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
AlIV pfu
200 um
200 um
0.80
0.80
EM5 rhy2B
EM5 rhy cl1
0.75
Mg#
Mg#
0.75
0.70
0.70
0.65
0.65
0
200
400
600
1.5
800
1.6
1.7
1.8
1.9
2.0
2.1
AlIV pfu
Distance (μm)
500 um
200 um
0.80
0.80
EM5 rhy2A
EM2 rhyI2
0.75
Mg#
Mg#
0.75
0.70
0.70
0.65
0.65
0
200
400
600
800
0
200
400
600
Distance (μm)
Distance (μm)
Fig. 7. (Continued)
2049
800
2.2
JOURNAL OF PETROLOGY
VOLUME 54
Concentration/
Chondrite
100
hbl þ melt ! cpx þ opx þ plag þ ilm:
10
ð1Þ
However, Buckley et al. (2006) re-evaluated the Mount St.
Helens data and determined that using amphibole compositions close to the rim instead of an averaged amphibole
composition, the reaction equation can be written
La
Ce
Pr
Nd Sm Eu
Gd
Dy
Er
Yb
hbl ! cpx þ opx þ plag þ mag þ ilm:
100
Concentration/
Chondrite
OCTOBER 2013
reduce their residuals, Rutherford & Hill (1993) needed to
include melt compositions in the equation that took the
form
EM2 Rhyolite
1
EM10 Andesite
10
1
La
Ce
Pr
Nd Sm Eu
Gd
Dy
Er
Yb
100
Concentration/
Chondrite
NUMBER 10
EM11 Rhyolite and Andesite
10
1
La
Ce
Pr
Nd Sm Eu
Gd
Dy
Er
Yb
Fig. 8. Chondrite-normalized REE element patterns illustrating the
similarity in trace element abundances between reacted (mostly rhyolite) and unreacted (mostly andesite) phenocrysts. Each frame represents an single hand sample with several LA-ICP-MS analysis points
within one or two amphibole phenocrysts from the listed sample,
regardless of the composition of the glass.
Mass-balance calculations were performed using a multiple linear regression least-squares mixing algorithm
coded in MATLAB (Dymond et al., 1973). The code uses
the chemical compositions of the reaction rim phases
(plagioclase, pyroxene, and Fe^Ti oxides) in oxide weight
per cent to calculate a modal best-fit solution to a target
composition (amphibole composition) with the lowest residuals. The algorithm in this code is similar to Petmix
(Wright & Doherty, 1970) used by both Rutherford & Hill
(1993) and Buckley et al. (2006), and reproduces solutions
to mineral proportions in amphibole reaction rims in
these studies accurately.
Rutherford & Hill (1993) noted that there was no combination of reaction rim phases in their calculation equivalent to the amphibole. To balance their equation and
ð2Þ
They applied this equation to amphiboles from Soufrie're
Hills Volcano and found that the mass balance similarly
followed equation (2), but required an open system in
which some components in the amphibole are exchanged
with the melt, and vice versa. The main difference between
equation (1) and equation (2), therefore, is that wholesale
melt interaction is required in the former, whereas selective
component interaction occurs in the latter.
Applying this method to the El Misti amphiboles, it is
noted that the mineral phase mode in the reaction rim on
El Misti amphiboles is dominated by plagioclase followed
by orthopyroxene, clinopyroxene and Fe^Ti oxides
(Table 9). This is in contrast to amphibole reaction rims
from Mount St. Helens (Rutherford & Hill, 1993) and
Soufrie're Hills (Buckley et al., 2006) in which the dominant
phase is clinopyroxene, followed by orthopyroxene, plagioclase and some oxides. Furthermore, using reaction rim
phase compositions, we were not able to reproduce the
target amphibole composition with similar observed mineral modes in the reaction rim or with low residuals. In
the El Misti case, host melt is required to balance the equation, acting as both an element supplier and element reservoir as observed in the case of the Soufrie're Hills
amphiboles (Buckley et al., 2006). The plagioclase-dominated mode in the reaction rims of the EM2000BP rhyolite
amphiboles is probably controlled by the breakdown of
the Al-rich amphibole pargasite; excess Al and Ca
from the decomposing amphibole may contribute to the
preferential growth of plagioclase and then pyroxene
respectively.
Magma evolution and dynamics
The detailed petrological evidence, in particular the disparate plagioclase populations and the complex amphibole
provenance involving transfer from andesite to rhyolite
and reaction rim growth, provides a framework within
which we now attempt to piece together the magma dynamics that led to the 2000 BP eruption of El Misti Volcano.
The andesitic magma: phase equilibria constraints
The main crystallizing phases in the EM2000BP andesite
are amphibole and plagioclase with lesser amounts of
Fe^Ti oxides and pyroxenes. Notably, amphibole and, in
some cases, plagioclase contain small inclusions of Fe^Ti
2050
EM 2
20·24
6·32
1·98
5·40
3·81
1·47
1·08
Nd
Sm
Eu
Gd
Dy
Er
Yb
9·00
2051
4·42
1·70
4·55
2·86
1·19
0·64
EM 11
Nd
Sm
Eu
Gd
Dy
Er
Yb
Sample:
0·08
0·84
Yb
0·05
0·10
0·22
0·18
0·73
1·23
3·24
4·97
1·73
5·39
1·00
1·40
3·51
4·93
1·77
5·29
19·11
3·26
16·97
3·96
H-3
EM 11
0·08
0·12
0·18
0·30
0·06
0·48
0·93
0·24
1·72
0·84
1 SE
0·15
0·11
0·16
0·68
0·16
EM 2
0·05
0·05
0·22
0·24
0·04
0·43
0·27
0·14
0·35
0·08
1 SE
0·75
1·30
2·76
4·32
1·94
5·40
18·25
3·53
18·62
3·65
I-1
EM 10
1·22
1·59
4·21
5·88
2·22
7·70
27·82
5·35
36·75
11·65
I3-3
EM 2
0·65
0·87
2·26
3·77
1·41
3·97
13·91
2·29
12·82
2·58
I-1
EM 11
0·08
0·23
0·09
0·23
0·09
0·29
0·98
0·12
0·56
0·09
1 SE
0·13
0·04
0·09
0·20
0·09
0·07
0·30
0·35
0·16
0·28
0·11
1 SE
0·75
1·36
2·69
3·74
1·83
4·32
17·71
3·14
17·32
3·46
I-2
EM 10
0·98
3·17
1·14
0·12
4·47
1·83
5·38
18·26
3·24
17·76
3·56
I2-1
0·07
0·69
0·17
0·87
2·81
0·78
6·20
2·94
1SE
Sample values and standard errors are in mg g1.
3·81
1·42
Dy
Er
1·78
5·12
5·02
Eu
0·30
0·20
18·12
Nd
Sm
Gd
0·10
2·96
0·47
Pr
0·13
3·80
16·73
3·52
19·40
1 SE
Ce
0·16
0·07
0·10
0·34
0·09
0·33
0·77
0·08
La
H-2
2·85
16·32
Pr
20·14
4·95
0·34
3·33
15·72
La
EM 10
1·11
1·69
3·58
4·12
1·82
1·43
0·26
5·13
0·25
2·41
1·04
1SE
21·73
4·02
28·22
Ce
0·06
EM 2
I3-2
H-5
1 SE
0·06
0·10
0·24
0·26
0·05
0·61
0·36
0·11
0·63
0·20
1SE
H-4
EM 10
3·84
Pr
Sample:
4·29
19·55
Ce
I3-1
La
Sample:
Table 7: REE LA-ICP-MS compositions of amphibole
0·08
0·09
0·33
0·12
0·05
0·15
0·47
0·11
0·34
0·14
1 SE
0·15
0·09
0·20
0·42
0·05
0·27
0·34
0·12
0·63
0·10
1SE
EM 2
0·55
1·11
2·91
3·70
1·47
4·48
15·12
2·70
14·07
2·88
I-2
EM 11
1·07
1·40
3·54
4·67
1·92
6·47
20·95
3·88
22·04
4·60
I-3
EM 10
1·10
1·21
3·72
5·04
1·76
6·21
20·92
3·50
20·01
4·28
I2-2
0·11
0·05
0·25
0·31
0·09
0·21
0·71
0·05
0·37
0·14
1 SE
0·08
0·09
0·33
0·12
0·05
0·15
0·47
0·11
0·34
0·14
1 SE
0·15
0·09
0·20
0·42
0·05
0·27
0·34
0·12
0·63
0·10
1SE
EM 2
0·77
1·31
3·62
5·36
1·81
5·91
20·55
3·82
20·56
4·59
I-3
EM 11
0·99
1·44
3·66
6·03
1·92
5·67
21·78
3·75
20·98
4·70
J(2)-1
EM 10
0·92
1·34
2·79
4·63
1·58
5·03
19·39
3·52
21·36
5·06
I2-3
0·14
0·10
0·31
0·42
0·07
0·37
0·36
0·17
0·38
0·07
1 SE
0·14
0·09
0·32
0·18
0·06
0·51
1·51
0·15
0·41
0·15
1 SE
0·07
0·08
0·21
0·26
0·14
0·47
0·43
0·17
0·81
0·17
1SE
EM 2
0·09
0·95
1·45
3·36
5·43
1·53
5·01
18·53
3·31
17·33
3·87
K(2)-1
EM 11
0·07
1·28
0·95
0·20
0·09
0·11
0·24
0·57
0·18
1·30
0·55
1 SE
0·14
0·08
0·16
0·19
0·02
0·34
0·35
0·14
0·70
0·16
1SE
2·87
4·60
1·83
5·47
19·50
3·54
19·77
4·80
J(2)-2
EM 10
1·38
1·33
3·41
5·64
1·81
5·89
23·24
3·74
20·60
4·56
I-1
EM 2
0·15
0·18
0·14
0·43
0·07
0·11
0·47
0·10
0·22
0·09
1 SE
0·75
0·79
2·27
3·51
1·37
4·00
14·79
2·48
14·59
2·87
K-1
EM 10
0·72
1·43
3·81
5·39
1·78
6·20
20·16
3·62
19·23
4·21
I-2
0·07
0·09
0·20
0·29
0·07
0·44
0·57
0·05
0·44
0·07
1 SE
0·13
0·07
0·23
0·33
0·07
0·29
0·56
0·19
0·69
0·17
1SE
EM 2
0·83
1·17
3·49
4·73
1·69
5·28
20·29
3·53
19·50
4·09
K-2
EM 10
1·02
1·48
3·43
4·20
1·72
5·25
18·21
3·41
21·45
6·63
I-3
0·09
0·09
0·16
0·16
0·09
0·25
0·55
0·07
0·33
0·21
1 SE
0·13
0·07
0·23
0·33
0·07
0·29
0·56
0·19
0·69
0·17
1SE
EM 10
1·15
1·68
4·17
6·64
2·30
8·22
29·12
5·51
32·87
6·76
K-3
EM 10
0·65
1·05
2·70
3·41
1·60
4·35
16·02
2·63
14·53
2·98
H-1
0·09
0·09
0·16
0·16
0·09
0·25
0·55
0·07
0·33
0·21
1 SE
0·11
0·08
0·16
0·22
0·06
0·39
0·38
0·06
0·37
0·07
1SE
EM 10
0·79
0·98
2·29
4·34
1·36
4·08
17·24
3·59
24·87
8·28
K-4
EM 10
0·94
1·19
3·15
4·66
1·73
4·71
18·45
2·93
16·98
3·52
H-2
0·18
0·07
0·16
0·32
0·15
0·37
1·45
0·23
1·15
0·47
1 SE
0·10
0·07
0·17
0·46
0·07
0·07
0·44
0·05
0·28
0·22
1SE
EM 10
0·84
1·61
2·94
5·27
1·53
5·55
17·15
2·97
15·34
3·48
H-1
EM 11
0·81
1·12
2·62
3·93
1·54
5·08
16·60
2·79
15·49
3·39
H-3
0·05
0·10
0·22
0·18
0·08
0·20
0·30
0·10
0·47
0·13
1 SE
0·10
0·07
0·17
0·46
0·07
0·07
0·44
0·05
0·28
0·22
1SE
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
20
40
60
80
Number
Number
160
140
120
100
80
60
40
20
0
NUMBER 10
100
An Content
8
6
4
2
0
40
50
60
70
80
An Content
Number
Number
Microlites
Total # analyses = 63
Total # crystals = 35
10
9
±2σ
8
7
6
5
4
3
2
1
0
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
Relative probability
12
Relative probability
14
30
160
±2σ
140
120
100
80
60
40
20
0
0.00 0.02 0.04 0.06 0.08 0.10 0.12
MgO wt%
16
20
OCTOBER 2013
Relative probability
Phenocrysts
VOLUME 54
Relative probability
Total # analyses = 1850
Total # crystals = 47
JOURNAL OF PETROLOGY
MgO wt%
Fig. 9. Cumulative abundance plots of plagioclase phenocryst and microlite An content and corresponding MgO contents. The relative
frequency of occurrence of compositions and 2s errors for MgO are plotted.
oxides revealing a paragenetic sequence. There are no relative crystallization clues between plagioclase and amphibole (i.e. no inclusions of plagioclase in amphibole or vice
versa) to indicate saturation order.
Experimental data for andesite and dacite phase equilibria reveal the effect of pressure and melt H2O content
(Eggler, 1972; Eggler & Burnham, 1973; Moore &
Carmichael, 1998; Martel et al., 1999), and fO2
(Rutherford & Devine, 1988; Martel et al., 1999) on amphibole stability. These studies show that crystallization of
plagioclase as the main liquidus phase at low pressure is
suppressed by increasing pH2O, and that the amphibole
stability field increases at high pH2O and temperature
and fO2 Ni^NiO þ1 at the expense of clinopyroxene,
orthopyroxene and plagioclase (Moore & Carmichael,
1998; Martel et al., 1999). Water-saturated conditions also
change the plagioclase phase equilibria such that plagioclase compositions increase in An content with increasing
pH2O (Housh & Luhr, 1991; Lange et al., 2009). Phase equilibria for a starting material similar to the El Misti andesite indicate that hornblende is the first crystallizing phase,
followed closely by plagioclase and Fe^Ti oxides at pressures between 2 and 2·5 kbar at H2O-saturated conditions
(5^6 wt % H2O) and temperatures between 950 and
9758C (Moore & Carmichael, 1998). This temperature
range is similar to our two-pyroxene thermometer calculation for the EM2000BP andesite. A similar relationship is
found in the experiments of Martel et al. (1999) using silicic
andesites from Mount Pele¤e, although at somewhat higher
pressures of 3·5 kbar. These experimental studies suggest
that the EM2000BP andesite magma reservoir was located
at 2^3·5 kbar pressure (7^12 km depth), 9408C
(calculated in this study) and was H2O saturated with
5^6 wt % H2O. We confirm these data using the plagioclase^liquid hygrometer of Lange et al. (2009), which
yields 5^6 wt % (H2O), and the amphibole thermobarometer of Ridolfi et al. (2010), which yields similar temperature
and water saturation values (Fig. 12). Our results for the
EM2000BP andesite are concordant with conditions of
crystallization of 900^9508C and 2^3 kbar pressure
(Legrende, 1999) under conditions of maximum water
solubility for andesite to dacite magmas of 5·1^6·0 wt %
(Ruprecht & Wo«rner, 2007) for El Misti overall.
The andesite: constraints from amphibole compositions
Variations in formula cation abundances in the amphiboles
allow us to describe the pre-mixing thermal and pressure
history of the crystals and the andesite in which they
grew. Experimental studies by Spear (1981) and Blundy &
Holland (1990) found that variations in AlIV in amphibole
are strongly temperature dependent, expressed in the edenite exchange [SiIV þ œA ¼ AlIV þ (Na þ K)A)], and the
Ti-Tschermak exchange (2SiIV þ MnVI ¼ 2AlIV þ TiVI)
(Fig. 6c), which is applicable as long as a Ti-rich phase
such as magnetite or ilmenite is present in the mineral assemblage (Spear, 1981). In EM2000BP amphiboles, the edenite exchange accounts for most of the total observable Al
variation (Fig. 6a), whereas Ti (p.f.u.) vs AlIV (p.f.u.)
2052
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
shows a slightly positive correlation, indicative of the TiTschermak exchange. Further support for temperature
control on the EM2000BP amphibole compositions is seen
in the trend of decreasing Mg# (0·5 Mg# values) and
increasing AlIV (0·4 AlIV p.f.u.) (Fig. 6a and d), consistent with the work of Rutherford & Devine (2003). Finally,
our data show no increase in AlVI with any other geochemical indicator obviating a role for the Al-Tschermak
exchange (2SiIV þ MgVI ¼ 2AlIV þAlVI) favored by
increasing pressure (Johnson & Rutherford, 1989; Thomas
& Ernst, 1990; Schmidt, 1992).
Thus, on the basis of previously published experimental
results and our observed mineral chemistry variations, we
suggest that the EM2000BP amphiboles crystallized in a
near isobaric environment with modest temperature
(a)
fluctuations during crystal growth, possibly as a result of
convective rotation in a small magma body or as a result
of repeated small recharge events of similar composition
into a small magma body.
This model is consistent with the zoning characteristics
of two amphibole crystals (Fig. 6) that show large variation
in AlIV, Mg# and other geochemical parameters.
Amphibole 10H and 10K, a non-rimmed and a rimmed
amphibole both in mixed andesite 10, respectively, together
account for the same range in (Na þ K)A, AlVI, and
Mg# vs AlIV as the complete amphibole dataset. Their
variation in AlIV also covers the full range exhibited by
the complete dataset. This type of zoning, increasing AlIV
and decreasing Mg# followed by decreasing AlIV and
increasing Mg#, has been shown in experimental studies
An Content
Low An Content Plagioclase
±1σ
±1σ
500 um
An Content
Distance (μm)
±1σ
±1σ
200 um
Distance (μm)
Fig. 10. BSE images, An transects, and corresponding MgO (open circles) and FeO (filled squares) concentration profiles for representative
Low-An (a) and High-An group (b) plagioclase phenocrysts from EM2000BP tephra. White lines on plagioclase (BSE images) represent transect locations. Dashed black line represents average limit of detection for MgO. Also plotted are 1s errors for MgO (open circle) and FeO
(filled square).
(continued)
2053
JOURNAL OF PETROLOGY
(b)
VOLUME 54
NUMBER 10
OCTOBER 2013
High An Content Plagioclase
±1σ
±1σ
200 um
Distance (μm)
±1σ
±1σ
500 um
Distance (μm)
Fig. 10. (Continued)
to represent crystallization in a hotter, more Al-rich
magma followed by crystallization in cooler, more
Al-poor magma (Scaillet & Evans, 1999; Rutherford &
Devine, 2003).
Similar zoning has been interpreted by Humphreys et al.
(2006) to be the result of changes in pH2O and its associated
effect on plagioclase composition. In effect, increasing the
pH2O of the melt at constant temperature promotes crystallization of higher An plagioclase and Al-poor amphibole.
In the study by Humphreys et al. (2006), this mechanism of
amphibole^plagioclase compositional exchange is mirrored
in the edenite exchange in (Na þ K)A vs AlIV (their fig. 12c
and d). In the EM2000BP case, (Na þ K)A shows only
small variation with AlIV (Fig. 6). Additionally, the plagioclase exchange (SiIV þ NaA ¼ AlIV þCaA) (not illustrated)
also shows no variation over the full range of AlIV variations. Both of these observations suggest that plagioclase
crystallization did not affect the amphibole compositions
and that zoning and changes in amphibole composition
were the result of crystal growth in a melt of fluctuating
temperature. The amphibole adjusted its composition as
the temperature fluctuated, utilizing whatever exchange
mechanism necessary to maintain chemical and thermal
equilibrium with the surrounding melt. Plagioclase data
from the andesite (and rhyolite) support these conclusions
as trace element partitioning between plagioclase and melt
generally falls along equilibrium partitioning curves
(Fig. 11). Although changing pH2O may play a role in the
changing An contents, it did not play a major role in amphibole crystallization and zoning.
The rhyolite: bulk geochemistry and phase equilibria
constraints
Above we showed that whole-rock major element variations follow similar evolutionary trends to previously
published data from various eruptions in El Misti’s past
2054
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
Table 8: Representative compositions of Low- and High-An plagioclase phenocrysts and microlites
Low-An plagioclase phenocrysts
Sample:
EM 2 B
EM 2 B
EM 2 B
EM 5 I
EM 5 I
EM 5 I
EM 10 G
EM 10 G
EM 10 G
EM 11 E
EM 11 E
EM 11 E
Core
Inter
Rim
Core
Inter
Rim
Core
Inter
Rim
Core
Inter
Rim
SiO2
57·47
55·62
57·00
55·11
53·76
58·91
56·55
54·98
59·69
54·28
52·90
56·20
Al2O3
26·61
27·92
27·20
28·48
28·87
25·70
27·49
28·27
25·82
28·59
29·60
27·53
FeO*
0·36
0·33
0·40
0·28
0·29
0·56
0·29
0·31
0·35
0·40
0·44
0·43
MgO
0·02
0·02
0·01
0·01
0·01
0·05
0·01
0·01
0·01
0·03
0·02
0·02
CaO
8·71
10·11
9·07
10·44
11·31
7·95
9·36
10·41
7·12
10·85
12·11
9·70
Na2O
5·82
5·03
5·26
4·84
4·58
5·21
5·38
4·91
6·21
4·69
3·96
4·67
0·43
K2O
0·58
0·42
0·51
0·37
0·32
0·57
0·50
0·37
0·77
0·35
0·27
TiO2
0·03
0·04
0·02
0·02
0·01
0·05
0·02
0·01
0·02
0·02
0·02
0·02
Total
99·59
99·50
99·48
99·54
99·14
99·01
99·60
99·26
99·98
99·21
99·32
99·02
An content
44
51
47
53
57
44
48
53
37
55
62
52
High-An plagioclase phenocrysts
Sample:
EM 2 D
EM 2 D
EM 2 D
EM 5 H
EM 5 H
EM 5 H
EM 10 E
EM 10 E
EM 10 E
EM 11 C
EM 11 C
EM 11 C
Core
Inter
Rim
Core
Inter
Rim
Core
Inter
Rim
Core
Inter
Rim
SiO2
47·64
49·04
54·71
46·65
50·77
53·64
45·99
48·16
50·02
47·86
48·29
50·82
Al2O3
33·14
32·35
28·16
34·03
30·94
29·72
33·94
32·25
31·28
32·76
32·78
31·39
FeO
0·55
0·62
0·53
0·54
0·64
0·58
0·59
0·57
0·58
0·57
0·57
0·58
MgO
0·04
0·06
0·06
0·04
0·09
0·07
0·03
0·05
0·06
0·05
0·05
0·07
CaO
16·55
15·34
10·71
17·07
13·84
11·86
17·34
15·47
14·12
15·85
15·69
14·01
Na2O
1·98
2·51
4·83
1·60
3·25
4·33
1·48
2·41
3·06
2·20
2·40
3·14
K2O
0·06
0·10
0·31
0·05
0·15
0·20
0·04
0·08
0·11
0·07
0·08
0·11
TiO2
0·03
0·03
0·03
0·01
0·03
0·03
0·02
0·02
0·03
0·02
0·02
0·03
Total
99·99
100·05
99·33
99·99
99·71
100·43
99·42
99·02
99·27
99·37
99·87
100·15
An content
82
54
85
70
86
78
71
80
78
77
59
71
Low- and High-An plagioclase microlites
Sample:
EM 2
EM 2
EM 2
SiO2
56·86
59·68
61·00
Al2O3
26·36
24·56
24·00
FeO
0·40
0·25
MgO
0·01
CaO
8·42
EM 5
EM 5
EM 5
EM 10
EM 10
EM 10
EM 11
EM 11
EM 11
54·64
54·85
54·27
54·11
54·50
58·75
52·86
53·01
53·45
28·69
27·67
28·10
28·21
27·79
24·92
29·32
29·44
29·09
0·31
0·66
0·75
0·69
0·78
0·79
1·36
0·63
0·65
0·60
0·01
0·00
0·07
0·10
0·08
0·07
0·10
0·14
0·08
0·06
0·08
6·22
5·34
10·78
10·33
10·70
10·97
10·50
8·00
12·09
11·98
11·75
Na2O
5·81
6·62
6·94
4·96
5·07
4·81
4·71
4·93
4·93
4·30
4·28
4·47
K2O
0·63
0·94
1·32
0·28
0·32
0·30
0·30
0·36
1·01
0·19
0·20
0·26
TiO2
0·00
0·02
0·03
0·04
0·05
0·04
0·05
0·06
0·18
0·04
0·03
0·04
Total
98·59
98·32
98·96
100·14
99·15
98·99
99·23
99·07
99·35
99·52
99·67
99·74
An content
43
32
27
52
54
55
53
44
60
60
58
54
*Total Fe given as FeO.
Typical 1SD: SiO2 0·14; Al2O3 0·15; FeO* 0·03; MgO 0·01; CaO 0·1; Na2O 0·05; K2O 0·015; TiO2 0·01.
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JOURNAL OF PETROLOGY
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detection limit
100
90
80
An Content
OCTOBER 2013
and thus a case can be made that the rhyolite endmember is related to the andesite magma by crystal
fractionation of plagioclase, amphibole, pyroxene and
magnetite. However, we have no definitive evidence that
and within the Central Volcanic Zone as a whole (Fig. 3).
Although the rhyolite is related to the system as a whole,
its petrogenesis is not wholly understood. For El Misti as
a whole, all compositions fall on a liquid line of descent
(a)
NUMBER 10
Phenocrysts
Microlites
Hi T eq
Lo T eq
±2σ
70
60
50
40
30
20
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
MgO wt%
100
detection limit
(b)
90
An Content
80
Phenocrysts
Microlites
Hi T eq
Lo T eq
±2σ
70
60
50
40
30
20
0.00
0.02
0.04
0.06
0.08
0.10
TiO2 wt%
Fig. 11. Trace-element [MgO (a), TiO2 (b), FeO*(c)] variations vs An content in plagioclase from EM200BP tephra. Plotted are phenocryst
(open symbols) and microlite (shaded symbols) values. Also plotted are equilibrium partitioning curves for andesite (Hi T eq) and rhyolite
(Lo Teq) and partitioning curve uncertainties based on the partitioning behavior of these elements into plagioclase as reported by Bindeman
et al. (1998) and Tepley et al. (2010). Values for each element used to determine the equilibrium partitioning curves are whole-rock values of the
andesite (EM0401) and the rhyolite (EM099), as an estimate of the melt composition, and T is determined through Fe^Ti oxide (rhyolite;
8168C) and two-pyroxene (andesite; 9408C) geothermometry. Limit of detection for each trace element is depicted as a gray dashed line. Also
plotted are 2s errors.
(continued)
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TEPLEY et al.
EL MISTI 2000 BP ERUPTION
(c) 100
Phenocrysts
Microlites
Hi T eq
Lo T eq
90
±2σ
70
60
50
detection limit
An Content
80
40
30
20
0.0
0.2
0.4
0.6
0.8
1.0
FeO wt%
Fig. 11. (Continued)
the EM2000BP rhyolite was derived from the EM2000BP
andesite. Whatever its exact relation to the EM2000BP andesite, the rhyolite magma must have separated from an
El Misti andesite magma sometime in the past and staged
at shallower levels in the crust as required by its separate
plagioclase phenocryst population, the lack of cognate
amphibole, its lower temperatures of equilibrium, and its
evolved residual liquid composition. This magma sat in
the upper crust fractionating plagioclase and Fe^Ti
oxides, stagnated and partially solidified. It seems that an
injection of andesitic magma ‘reactivated’ the rhyolite on a
local level, mingled with it intimately, and a later recharge
induced it to erupt explosively.
The main crystallizing phases in the rhyolite are plagioclase, Fe^Ti oxides and pyroxene, which are similar to
those crystallizing in the rhyolitic component of a zoned
eruption deposit from the 1912 eruption at Novarupta,
Alaska (Hildreth, 1983). Phase equilibria experimental results from the Novarupta rhyolite (Coombs & Gardner,
2001) demonstrate that at temperatures and pressures similar to the El Misti system (T ¼ 8168C, P5100 MPa), a
similar mineral assemblage was produced. The experiments also show that amphibole is not on the liquidus
5100 MPa in pressure despite being water saturated.
Plagioclase^melt equilibria (plagioclase rims^adjacent
glass compositions) indicate that the rhyolite was water
saturated with 5 wt % H2O (Lange et al., 2009). This
probably represents the conditions of the rhyolite when it
first formed and not just before eruption, as otherwise it
would have been water-saturated with the potential to
erupt through crystallization-driven overpressure. The
EM2000BP rhyolite must then represent a degassed remnant of some prior episode in El Misti’s past, or it had passively degassed. In either case, based on phase equilibria
experiments and plagioclase^melt equilibria, the lack of
cognate amphibole requires that the magma was stored
above the stability limit of amphibole (100 MPa or
53 km at c. 816 308C). This is consistent with the correspondence of the EM2000BP rhyolitic glass to the 0·5^1
kbar granite ternary eutectic in Petrogeny’s Residua
System (Tuttle & Bowen, 1958).
Time scales of magma dynamics during EM2000BP:
chemical equilibration and amphibole rim development
Three observations can be used to provide time scales for
the magmatic interactions preceding and during the
EM2000BP eruption of El Misti volcano: (1) the lack of
pervasive chemical equilibration despite minimal diffusive
length scales; (2) the absence of reaction rims on amphibole in the andesite; (3) the presence of reaction rims on
the andesite-originated amphibole mixed into the rhyolite.
The lack of chemical equilibration of andesite and rhyolite melt despite intimate mixing of the two magmas supports very short time scales of interaction prior to
eruption. Macroscopic and microscopic textural evidence
shows that mingling on the crystal scale and significant
folding and stretching of the magmas occurred. The equilibration process is dependent on the diffusivities of major
elements, which are of the order 10^12 m2 s1 (Liang et al.,
1996). Using t x2/D, for length scales of 1mm to 1cm,
which would be necessary for pervasive equilibration,
chemical equilibration would be reached in the order of
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1000
Table 9: Amphibole reaction rim phase proportions, and
comparison with other studies
plagioclase
pyroxene*
oxidesy
EM 2 I
68
25
7
EM 2 I3
64
26
9
EM 2 I2
64
30
6
EM 5 K
61
36
3
EM 10 K
55
40
5
Soufrière Hills (B)
18·2
77·4
4·5
Soufrière Hills (B)
18·1
78·3
3·6
MSH (B)z
23·7
67·5
8·7
MSH (R&H)§
43
53
3
P (MPa)
Sample
EM rhyolite
EM andesite
800
maximum thermal
stability limit
600
400
200
0
700
800
900
1000
1100
T (°C)
1150
T (°C)
1050
*Clinopyroxene and orthopyroxene are combined as
‘pyroxene’.
yIlmenite and magnetite are combined as ‘oxides’.
zRecalculation of Rutherford & Hill (1993) amphibole reaction rim mineral modes of Buckley et al. (2006) using the
method described by those researchers.
§Original calculation of reaction rim mineral modes of
Rutherford & Hill (1993).
B, Buckley et al. (2006); R&H, Rutherford & Hill (1993);
MSH, Mount St. Helens amphibole.
10 days to 1^2 years over those length scales. Meanwhile,
thermal equilibration (10^6 m2 s1) and cooling would
have been achieved orders of magnitude fasterçin a
matter of seconds to minutes. Thus, because very limited
hybridization seems to have taken place, the process must
have been arrested rather rapidly, in the order of days.
Although not meant to be definitive, the point here is that
even though diffusive length scales were minimal, chemical equilibration is too slow to compete with freezing of
the system. The interaction between the rhyolite and
andesite could not have been prolonged and had to have
happened just prior to eruption.
The lack of reaction rims on amphibole in the
EM2000BP andesite allows us to assign an upper time
limit on the mixing process during this eruption. Multistep decompression experiments for starting materials
from the 1989 eruption of Redoubt volcano, Alaska,
USA, by Browne & Gardner (2006) suggest that as
few as 4 days and as many as 7 days elapsed before reaction rims developed on amphiboles that were moved outside their stability range, depending on the rate of
decompression. A similar time frame was demonstrated
for the 1980 eruption of Mount St. Helens volcano,
Washington, USA, in the decompression experiments
of Rutherford & Hill (1993). Therefore, for the
EM2000BP andesite, we suggest a conservative time
950
850
maximum thermal
stability limit
750
EM rhyolite
EM andesite
650
550
2
4
6
8
10
H2Omelt (wt.%)
Fig. 12. P (MPa) vsT (8C), and T (8C) vs H2Omelt (wt %) based on
the amphibole reduction of Ridolfi et al. (2010), showing the coherence
of data from amphiboles residing in both the rhyolite and andesite.
This figure also illustrates T and P of formation for the amphibole,
and the water content in the andesitic melt.
frame of 5 days for magma migration from the storage
reservoir to the surface.
Reaction rim development on andesite-originated
amphiboles in the EM2000BP rhyolite is controlled by diffusion of constituent material from the melt to the crystal
surface and diffusion of unused elements away from the
crystal surface into the melt (Liang, 2000; Coombs &
Gardner, 2004). Whereas the rate of the reaction will
depend on the temperature and pressure of the system
and the amount of dissolved water in the melt, the rates of
crystal decomposition and crystal rim growth should be
governed by the diffusive exchange of amphibole rim material and the surrounding melt. In a study by Browne &
Gardner (2006), amphibole decompression rim growth
was determined for various systems at a range of temperatures. Those researchers found that reaction rim growth
rate is probably related to melt viscosity and associated
temperature, and that it would take 50^60 days as a minimum for the reaction rims on amphiboles to develop.
Given that the EM2000BP rhyolite has a similar temperature to the 1989 Mount Redoubt dacite (8168C
and 8408C, respectively), we infer a similar time scale
2058
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
of 50^60 days for the development of rims on the amphiboles in the EM2000BP rhyolite.
Microlite crystallization
Plagioclase microlites dominate the groundmass mineral
assemblage and constitute a significant proportion of the
total crystal fraction in both lithologies. Microlite composition histograms show two populations of microlites
(Fig. 9), the compositions of which correlate with the host
magma composition and are lower in An than the phenocryst compositions in the respective host magmas. This
suggests that the microlites grew at lower pH2O during
magma ascent (e.g. Geschwind & Rutherford, 1995).
Moreover, the microlites are normally zoned and texturally display elongate, tabular and swallowtail morphologies that indicate rapid crystallization. This suggests that
the plagioclase microlites crystallized during ascent in
their respective host magmas rather than in a shallow
holding chamber such as in the 1980^1986 eruptions of
Mount St. Helens volcano (e.g. Geschwind & Rutherford,
1995).
Magma dynamics and physical model of the EM2000BP
eruption
Based on the above, a petrological model for the El Misti
2000 BP eruption can now be proposed (Fig. 13). Two separate, compositionally distinct magma reservoirs existed beneath El Misti, of andesite and rhyolite composition, each
with their own sets of phenocryst and microphenocryst
populations. The andesite reservoir was located at
7^12 km depth in the crust (200^350 MPa) and the
magma within it was at 9408C and was water saturated
(5^6 wt % H2O). Although sparse, the crystallizing
phases were amphibole, plagioclase, Fe^Ti oxides and pyroxenes. Conditions of crystallization in the reservoir were
relatively constant with minor perturbations in temperature perhaps reflecting small-scale convection currents or
small-volume recharge of similar composition magma.
The rhyolite reservoir is less well constrained but it appears that the only crystallizing phases were plagioclase
and Fe^Ti oxides. The rhyolite reservoir was located at
3 km depth in the crust (5100 MPa) and the magma
within was at 816 308C and either degassed or partially
degassed. The lack of stable hydrous phases and the temperature of the system also suggest low-pressure
(5100 MPa), shallow crustal residence (3 km depth).
We envision the development of the eruption as a twostage process (Fig. 13a). The first stage initiated as a dike
of andesitic magma intruding into the rhyolite magma.
The andesite was water-saturated and would have been
vesiculating as it rose and, on encountering the rhyolite, it
may have vigorously mixed with the latter. Exchange of
minerals occurred between the two magmas during this
intrusion event, but was limited by the large temperature
and viscosity contrasts. Given that the rhyolite resides in
the upper crust above the stability limit of amphibole, it is
at this stage that amphiboles and plagioclase from the andesite are mixed into the rhyolite along a thin interactive
zone, and the amphibole, now out of its stability field,
develops reaction rims over a period of 50^60 days.
Eruption was precluded by the relatively small volume of
the recharging magma with respect to the host rhyolite
magma.
Around 50^60 days after the initial recharge event,
spurred on either via continued recharge or by simple
buoyant rise, another larger pulse of vesiculating andesite
magma forced its way through the earlier stalled
recharged zone in the perched rhyolite and initiated the
eruption. In this second stage, further limited crystal exchange may have occurred at the margins of the andesitic
dike or eruption conduit, with amphiboles grown in the
andesite being added to the rhyolite and plagioclase from
each lithology being mutually exchanged.
The time for the EM2000BP second pulse of andesite to
reach the rhyolite at 3 km depth (above the amphibole
stability limit) is indeterminate, but the absence of reaction
rims on the amphiboles in the andesite recharge magma
requires that it travelled from its storage reservoir to the
surface within 5 days. This yields an average ascent rate
of at least 0·023 m s1.
The presence of abundant microlites in the EM2000BP
rhyolitic and andesitic groundmasses is consistent with
such ascent rates from low pressures (m s1 and
5100 MPa; e.g. Klug & Cashman, 1994; Metrich &
Rutherford, 1998; Cashman & Blundy, 2000; Martel &
Schmidt, 2003). The formation of plagioclase microlites
was most probably driven by magma undercooling owing
to exsolution of volatiles associated with decompression
(e.g. Muncill & Lasaga, 1988; Hammer, 2008). The loss of
dissolved volatiles has the effect of increasing the relative
liquidus temperature of the magma, thereby decreasing
the An content of any crystallizing plagioclase. The lower
An contents of the rims on both High-An and Low-An
plagioclase populations, when compared with their core
An contents, attest to this process. This process similarly
affects andesite and rhyolite microlite compositions, which
in El Misti’s case, show an overall reduction in An content
in comparison with their respective plagioclase phenocryst
populations (Figs 9 and 10).
Given the relatively small volume of the EM2000BP
tephra deposits, we prefer a model in which the magmatic
interactions take place along and within a dike of watersaturated andesite. Mixing between the hot (9408C) andesitic magma and a cooler rhyolitic (8168C) magma will
initially be limited, given the temperature and viscosity
contrasts between them (Huppert et al., 1982; Campbell &
Turner, 1985; Sparks & Marshall, 1986; Turner & Campbell, 1986; Snyder & Tait, 1995); however, shearing along
the edges and progressive physical mixing will allow
2059
JOURNAL OF PETROLOGY
(a)
VOLUME 54
NUMBER 10
OCTOBER 2013
Stage 2
Stage 1
Eruption
More in-conduit
magma mixing
and exchange
of crystals
Limited magma mixing
and exchange of crystals
rhyolite crystal mush reservoir
Duration of stagnation:
50-60 days
hbl stability limit (in rhy)
lnitial dike emplacement
and stagnation
hbl stability limit (in and)
Duration of event:
<5 days
Second more forceful
dike emplacement
of recharge magma
andesite reservoir
(b)
eruption
Stage 2
Stage 1
~5 days
e1d
>200 MPa
amphibole stability
(in andesite)
max
max
Stage 2 dike emplacement
ceme
amphibole stability
(in rhyolite)
ike e
mpla
100 MPa
Stag
Pressure
nt
50-60 days
T
T
min
max
min
max
Vm/Vf
Vm/Vf
min
max
μ
min
conduit
cross section
min
max
μ
min
conduit
cross section
Time
Fig. 13. Petrogenetic model illustrating andesite reservoir location and perched rhyolite magma lens. (a) Schematic model and simplified development of the 2000 BP eruption of El Misti. Diagram illustrates the initial conditions of each reservoir in relationship to the amphibole stability
limit. Stage 1 is initiated with dike emplacement into and stagnation in the existing rhyolitic mush. Limited magma mixing occurs during this
stage, resulting in mixed crystal populations and development of reaction rims on amphibole. Stage 2 occurs when a stronger recharge pulse reactivates the emplaced dike, causing more magma mixing, mixed crystal populations and eruption. (b) Right panel illustrates the detail associated with mixed magma and crystal exchange. Included are box models schematically illustrating the interaction between andesite and
rhyolite at the initial contact deep in the system (left), and interactions in the conduit during eruption (right). The box models offer acrossdike and conduit qualitative assessments associated with variations in temperature (T), volume of mafic to felsic magma (Vm/Vf) and viscosity
(m). These gradients are steep at the initial contact deep in the system and become more gentle higher in the system with continued shearing
and diffusion of material and heat. The left panel illustrates the pressure^time relationship showing the time scales of eruption based on amphibole stability. Elongate dike represents the initial intrusion of andesite into the rhyolite magma reservoir (Stage 1). This magma resides
above the amphibole stability limit for 50^60 days, before being recharged by another pulse of andesite magma, which initiates evacuation
and eruption in the time frame of 5 days (Stage 2). Diagram is not to scale.
2060
TEPLEY et al.
EL MISTI 2000 BP ERUPTION
thermal equilibration to commence rapidly (of the order of
seconds to minutes for length scales of millimeters to centimeters respectively), reducing the viscosity contrast
between the two magmas (e.g. Ruprecht et al., 2012;
Fig. 13b). With time (and decreasing depth of the rising
magma in the dike), this will promote magma mingling.
Shear along the interface between the magmas might aid
mixing of liquid and crystals. As seen in the ejecta, interaction between the two magma compositions takes many
forms, from thick toothpaste-like globs of rhyolite in a
matrix of andesite, to thin wisps of alternating andesite
and rhyolite (e.g. Fig. 2), to intermixing melt fractions and
crystal population transfers. We see this as reflecting an
evolving gradient in mingling in space and time from the
edges to the center of the dike, with more intimate mingling the further upwards it travels (Fig. 13b).
Texturally, vesicle shapes and glass distortion provide
evidence for the viscosity differences between the rhyolite
and the andesite. Bubble coalescence, merging of smaller
bubbles into larger bubbles, can be seen in most of the
rhyolite thin sections [‘donut-like’ features of Klug et al.
(2002)]. As in Mazama pumices (Klug et al., 2002), interaction between equal-sized bubbles often results in very
thin, planar melt films (1 mm), inferred to be caused by
approximately equal pressures acting on the film from
inside each bubble. These textures suggest significant
shear within the rhyolitic melt that may have been buffered
in the andesite by its lower viscosity. The combined effects
of groundmass crystallization and loss of volatiles from
the melt lead to increased magma and melt viscosity, but
combine with vesiculation to increase the potential for explosive eruption, overcoming the ‘viscous death’ described
by Annen et al. (2006).
Magma recharge and associated mixing with a preexisting magma is often cited as a triggering mechanism
for volcanic eruptions (Sparks et al., 1977; Eichelberger,
1978; Huppert et al., 1982; Pallister et al., 1992; Suzuki &
Nakada, 2007; de Silva et al., 2008; Kent et al., 2010). This
may be due to a simple hydraulic pressure increase induced
by addition of mass to a magma reservoir (e.g. Blake, 1981,
1984), by exsolution of volatiles from a resident felsic
magma induced through superheating owing to recharge
by hot mafic magma (Sparks et al., 1977), or by cooling of
the more mafic recharge magma forcing saturation and
vapor phase exsolution (e.g. Huppert et al., 1982; Tait et al.,
1989; Pallister et al., 1992; Folch & Marti, 1998). In all
these cases, over-pressurization of small magma chambers
beyond the tensile strength of the wall-rocks is thought to
be the trigger for explosive eruptions (see Gregg et al.,
2012). Alternatively, volatile exsolution and syn-eruptive
crystallization driven by depressurization during adiabatic
rise of magma to the surface can drive explosive eruptions
(Geschwind & Rutherford, 1995; Hammer et al., 1999;
Nakada & Motomura, 1999; Cashman & Blundy, 2000;
Blundy & Cashman, 2001). In the case of the 2000 BP eruption of El Misti, all these processes probably conspired to
cause the explosive eruption, but the fundamental trigger
for the eruption was andesite recharge.
The magmatic architecture at El Misti
Lastly, we consider the size and nature of the El Misti
magmatic system over the lifetime of the volcano.
Ruprecht & Wo«rner (2007) concluded that a single, large,
often-recharged magma reservoir existed below El Misti
rather than a plexus of smaller, interconnected magma reservoirs and dikes. Given that the eruption history of El
Misti is one dominated by effusive edifice-building andesite
and dacite domes and flows, this interpretation is plausible.
However, the 2000 BP eruption of El Misti was an explosive
eruption precipitated by a recharge event(s) of andesite
into rhyolite. Rhyolite at El Misti is rare and is found only
in the explosive eruptions that punctuate its effusive-dominated eruption history on a time scale of 2000^4000 years
(e.g. Thouret et al., 2001). The similarity of the EM2000BP
juvenile material in composition and magmatic conditions
and in physical appearance to those from the other (less
studied) explosive events during the history of El Misti
allows a model for the explosive events to be presented.
We modify the model of Ruprecht & Wo«rner (2007) to include periods when a small rhyolitic reservoir develops at
shallower levels in the crust. Recharge by andesite results
in an explosive eruptionça fast and transient event in the
history of El Misti. We suggest that these events do not tap
a large single reservoir but perhaps represent the interaction between a deeper long-lived andesitic reservoir and
a small transient shallow rhyolitic magma that may form
cyclically on a 2000^4000 years time scale. This time
frame may represent the period required to produce the
rhyolite from the andesite and segregate it to a high level
in the plumbing system shortly before eruption.
Interaction does not have to be chamber wide, but more
probably occurs along a dike that penetrates, interacts locally and erupts at the surface. If, as in the case of the
2000 BP eruption, the explosive eruption coincides with
periods when El Misti has significant snow cover, the inevitable ash-fall hazard would be magnified by the triggering
of extensive lahars by small pyroclastic flows (Harpel
et al., 2011).
CONC LUSIONS
The architecture, dynamics, and time scales of andesite^
rhyolite interaction during the 2000 BP, VEI 5 eruption of
El Misti in southern Peru have been revealed through
detailed petrological study. Bulk-rock chemistry, mineral
textures and compositions reveal macroscopic and microscopic evidence for magma mingling and crystal exchange
that record how an initial dike tapping a deep (7^12 km),
hot, water-saturated andesite magma reservoir intruded
2061
JOURNAL OF PETROLOGY
VOLUME 54
into a cooler, dryer, shallow (3 km) rhyolitic magma
higher in the crust and stalled. During the initial intrusion,
limited exchange of crystals from the two magmas
occurred. Amphibole crystals grown in the andesite
magma were transported into a cooler, shallower, and
chemically different environment where over a period of
at least 50^60 days they decompressed in both the rhyolitic
and andesitic melt to form plagioclase-dominated reaction
rims. A subsequent recharge via an andesitic dike remobilized the small magma storage system and resulted in extensive magma mingling and crystal exchange at a variety
of scales with mingling diminishing away from the andesite dike^rhyolite magma interface. Explosive eruption of
pervasively to minimally banded pumice reveals that although decompression crystallization of plagioclase microlites occurred, there was no wholesale equilibration of
melt and no reaction rims developed on amphiboles in the
andesite from the second recharge event. These observations require that during this latter stage, transport of andesite magma above the amphibole stability zone,
interaction with the rhyolite, and eruption all happened
within a period of 5 days at an average ascent rate of
0·02 m s1.
The 2000 BP VEI 5 plinian eruption shares characteristics with other explosive events that punctuate the background effusive activity at El Misti with a period of
2000^4000 years. It may therefore serve as a model for explosive events at this hazardous volcano. Our model for
the El Misti system includes the interaction of a deeper
and larger body of andesitic magma with a small rhyolitic
reservoir resulting in cyclic explosive eruptions. The periodicity may represent the time scales required for rhyolite
development, rapid andesite recharge and eruption.
AC K N O W L E D G E M E N T S
J. Permenter, C. Harpel, W. Scott, B. Anders, Y. Lavallee
and J. Burns, as well as Ms. C. Harpel-Avendano and
other students from UNSA, were helpful during various
fieldwork sessions at El Misti when these samples were collected. We thank S. Marcott for help with MATLAB
code, H. Diettrich for help with EMPA work, and C.
Bouvet de la Maisonneuve for an internal review of the
paper. A. Allan, P. Ruprecht, M. Rutherford, and M.
Streck provided very thorough and constructive reviews
that are appreciated. These, and G. Wo«rner’s editorial
handling of the paper, have helped clarify and strengthen
our ideas.
F U N DI NG
This work has been variously supported by the National
Science Foundation (EAR 0087181 to S.d.S.) and the
Volcano Disaster Assistance Program (VDAP) of the US
Geological Survey. This work was initiated when G.S. was
NUMBER 10
OCTOBER 2013
a visiting scientist supported by Oregon State University,
Department of Geosciences.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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