JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 8
PAGES 1641^1659
2013
doi:10.1093/petrology/egt026
The Behavior of Metals (Pb, Zn, As, Mo, Cu)
During Crystallization and Degassing of
Rhyolites from the OkatainaVolcanic Center,
Taupo Volcanic Zone, New Zealand
EMILY R. JOHNSON*, VADIM S. KAMENETSKY AND
JOCELYN McPHIE
CODES, ARC CENTRE OF EXCELLENCE IN ORE DEPOSITS, UNIVERSITY OF TASMANIA, HOBART, TAS 7001, AUSTRALIA
RECEIVED OCTOBER 4, 2011; ACCEPTED APRIL 2, 2013
ADVANCE ACCESS PUBLICATION MAY 16, 2013
TheTaupo Volcanic Zone (TVZ), New Zealand is a region of voluminous and frequent rhyolitic volcanism and widespread geothermal
activity. Additionally, the hydrothermal systems of the TVZ contain
relatively high concentrations of base and precious metals. Here we
present an extensive dataset of major element, volatile, and trace
element (including Pb, Zn, As, Mo, Cu) abundances in melt inclusions, pumice glasses and minerals from eight eruptions within
the Okataina Volcanic Center (OVC) of the TVZ to investigate the
behavior of metals during melt evolution. The high-SiO2 rhyolites
of the OVC contain high concentrations of volatiles (6 wt %
H2O, 0·25 wt % Cl) and underwent significant degassing prior
to and during eruption.The OVC melts contain moderate concentrations of metals (11^24 ppm Pb, 20^50 ppm Zn, 2^7 ppm As,
52·5 ppm Mo, 55 ppm Cu). Ferromagnesian minerals (amphibole, biotite and orthopyroxene) in the OVC pumice have high concentrations of Zn (1500 ppm), and plagioclase and biotite contain
moderate amounts of Pb (11ppm).The melt inclusion and pumice
glass trace element data reveal complex histories of magma mixing
and mingling prior to eruption; however, discrete melt batches are
easily identified based on trace element geochemistry. Variations in
incompatible trace elements within these melt batches suggest that
the OVC rhyolites underwent at least 20^25% fractional crystallization during quartz crystallization and melt inclusion entrapment
(at pressures of 100^200 MPa) and little to no crystallization
(5%) during ascent and eruption. Comparison of melt inclusion
metal and incompatible element (e.g. U) concentrations reveals
that melt Pb, Mo and As increase, whereas melt Zn decreases,
during fractional crystallization at depth (100^200 MPa). These
observations can be explained by minor partitioning of the metals
Pb, Mo and As into the fractionating minerals and stronger
partitioning of Zn into the ferromagnesian phases, supported by
calculated metal D values and analyzed metal concentrations in
OVC minerals. Interestingly, throughout both deep, vapor-saturated
crystallization and during extensive degassing during magma ascent
and eruption (as recorded by pumice glasses), the metals analyzed
here do not appear to partition appreciably into the vapor.We propose
that the lack of volatility of the metals analyzed in this study can be
attributed to a combination of several factors. First, vapor^melt partitioning requires the presence of ligandsçcommonly Cl, S and
OHçwith which the metals may complex. Given the low Cl/H2O
ratios in the OVC melts and the extensive degassing of H2O compared with Cl, it seems likely that the rhyolites would have exsolved
H2O-rich vapor with insufficient Cl to transport metals (in particular Pb and Zn) into the vapor phase, either at depth or during
magma ascent. Second, the overall small volumes of vapor present
during crystallization at pressures of 100^200 MPa would
have impeded significant vapor^melt partitioning of the metals.
Finally, the estimated very rapid ascent of the OVC melts
from depths of 4^8 km suggests that there was insufficient time at
low pressure for diffusion of metals out of the melt. These results
imply that there may be an indirect connection between the rhyolites
and the metals of the hydrothermal systems of the TVZ. As the
metals, and other species such as Cl, remain in the rhyolitic magmas
upon eruption, they are available in the large volumes of rhyolite
emplaced in the upper crust of the TVZ for leaching by heated
meteoric waters.
*Corresponding author. Department of Geological Sciences,
University of Oregon, Eugene, OR 97403, USA. E-mail: erj@
uoregon.edu
ß The Author 2013. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
JOURNAL OF PETROLOGY
VOLUME 54
KEY WORDS: melt inclusion; rhyolite; metals, Taupo Volcanic Zone;
crystallization; degassing
I N T RO D U C T I O N
Volcanic arc magmas can transport metals from the
mantle and from subducted oceanic crust and sediments
to the Earth’s upper crust and eventually to the surface.
During ascent and degassing of these magmas, some
metals (e.g. Au, Ag, As, Pb, Zn, Cu, Hg, Sb, Sn, W, Mo)
partition from the melt into a magmatic vapor phase
(herein referring to a low-density aqueous solution with
525 wt % Cl; equivalent to a supercritical fluid under the
appropriate conditions) and/or a hydrosaline fluid
(425 wt % Cl; Webster, 2004). This behavior is evident
from experimental studies (Simon et al., 2007; Botcharnikov et al., 2010; Zajacz et al., 2010; Johnson & Canil, 2011),
from analyses of coexisting melt and fluid inclusions (e.g.
Harris et al., 2003; Halter et al., 2005; Aude¤tat et al., 2008;
Zajacz et al., 2008; Aude¤tat, 2010) and from chemical analyses of volcanic fumaroles and hot springs (e.g.
Giggenbach & Matsuo, 1991; Hedenquist et al., 1994;
Wardell et al., 2004, 2008).
The Taupo Volcanic Zone (TVZ) provides the perfect
natural laboratory to investigate the behavior of metals
during the crystallization and degassing of rhyolitic melts.
The TVZ is related to the subduction of the Pacific plate
beneath the Indo-Australian plate. The northern and
southern segments of the TVZ are dominantly andesitic,
whereas the central portion is dominated by voluminous
rhyolitic volcanism. Although there have been basaltic
eruptions in the central TVZ, rhyolites form 90% of the
total erupted volume of magma (Wilson et al., 2009). The
TVZ has been active for the past 1·6 Myr and two calderas within the TVZ are considered currently active: the
Taupo Volcanic Center in the south and the Okataina
Volcanic Center (OVC) in the north. The OVC has been
active since 550 ka, and 4600 km3 of rhyolite has been
erupted in that time (Cole et al., 2010). The most recent
eruption (Tarawera, in 1886) was basaltic. The OVC rhyolitic melts are volatile-rich and contain 6 wt % H2O
and 0·25 wt % Cl (Shane et al., 2007, 2008a; Smith et al.,
2010; Johnson et al., 2011). Furthermore, the OVC rhyolites
underwent extensive degassing of H2O, and minor, latestage (shallow) degassing of Cl during their ascent and
eruption (Johnson et al., 2011).
The TVZ also contains geothermal fields related to the
combination of high heat flux (700 mW m^2, Bibby et al.,
1995) and active extension (12 mm a^1 in the OVC,
Wallace et al., 2004). The hydrothermal fluids of the TVZ
geothermal fields are dominantly meteoric in origin, as
demonstrated by the chemical and isotopic compositions
of analyzed deep waters (Giggenbach, 1995). Hot magma
intruded into the lower crust is thought to be the heat
source that drives the deep circulation of meteoric water
NUMBER 8
AUGUST 2013
(Bibby et al., 1995; Giggenbach, 1995; Simmons & Brown,
2007). In some cases, the hydrothermal fluids contain relatively high concentrations of base and precious metals.
Simmons & Brown (2007) measured the concentrations of
metals (Au, Ag, As, Hg, Sb) in the deep hydrothermal
fluids of the TVZ, and reported high fluxes of Au and Ag
in two geothermal systems. The fluids in several hot
springs, such as the Champagne Pool at Waiotapu, have
also been shown to precipitate metals (Au, Ag, As, Sb, Tl,
Hg) at the surface (Hedenquist & Henley, 1985). In spite
of the measurable metal contents of these fluids, the TVZ
hydrothermal fluids are relatively dilute; the hydrothermal
solutions studied by Simmons & Brown (2007) were
all undersaturated in Au and many were undersaturated
in Ag.
The relationship between magmatism and the TVZ
hydrothermal systems is complex. Based on both the composition of the hydrothermal fluids and the high heat
flow, various workers have suggested that the deep
magmas driving the hydrothermal activity are likely to be
andesitic and/or basaltic (Giggenbach, 1995; Christenson
et al., 2002; Simmons & Brown, 2007). Although the hydrothermal fluids of the central TVZ are dominated by meteoric water, magmatic vapors contribute 6^14% of the
water in the hydrothermal fluids (Giggenbach, 1995).
Furthermore, magmas may play an indirect role in the
metal concentrations of the TVZ hydrothermal fluids. Volcanic rocks in the TVZ probably contribute species such
as Cl, B and metals to the hydrothermal fluids through
leaching by heated meteoric waters (e.g. Giggenbach,
1995; Simmons & Brown, 2007). Additionally, andesitic
magmas have been demonstrated to be the likely source of
metals at White Island volcano in the north (Hedenquist
et al., 1993; Wardell et al., 2004). Although it seems likely
that the mafic magmas of the TVZ contribute heat,
metals, and ligands such as Cl to the hydrothermal systems, the concentrations and behaviors of metals in the voluminous rhyolites of the TVZ have not yet been
investigated.
In this study we use analyses of melt inclusions, pumice
glasses and magmatic minerals from the airfall pyroclastic
deposits of eight OVC rhyolitic eruptions to investigate
the behavior of metals during crystallization and degassing
of the melts. Our results suggest that, throughout the
processes of fractional crystallization and extensive degassing, the metals Pb, Zn, As and Mo do not appear to partition into an exsolved vapor, but instead partition into
crystallizing minerals or remain dissolved in the melt.
These results also imply that the relatively high concentrations of these metals, and other elements such as Cl,
which remain in the melt at eruption, make the large volumes of rhyolite emplaced in the upper crust of the TVZ a
potential source of these elements for leaching by hydrothermal solutions.
1642
JOHNSON et al.
OKATAINA VOLCANIC CENTER RHYOLITES
epoxy and made into thick sections for analysis. The
pumice clasts are typically glassy, with very few to no
microphenocrysts in the groundmass. Mineral phases identified within the pumice clasts were also analyzed (Fig. 2).
M ET HODS
Sample preparation
Pumice lapilli were sampled from fall deposits from eight
recent (550 ka) rhyolitic eruptions in the OVC (Fig. 1).
Pumice clasts were sampled from multiple layers throughout the stratigraphic sections of the fall deposits to capture
any variations in the magma composition during the
eruptions. The pumice clasts were lightly crushed and
quartz grains were picked from the smaller size fractions
(0·5^2 mm). Quartz grains hosting suitable melt inclusions (naturally glassy, rarely containing small vapor bubbles) were mounted in epoxy, singly intersected, and
polished. A subset of quartz crystals was removed from
the epoxy mounts and their melt inclusions were doubly
intersected in preparation for Fourier transform infrared
(FTIR) analysis. Pumice clasts from one to three stratigraphic layers from each eruption were impregnated with
174°E
Analytical methods
Melt inclusions, pumice glasses and minerals (feldspar,
orthopyroxene, amphibole and biotite) were analyzed for
their major and minor element compositions (including
Cl, S, F) using the Cameca SX-100 electron microprobe
at the University of Tasmania. Glass analyses utilized a
high-SiO2 glass standard (VG-568) that contains 76·71wt
% SiO2 (Jarosewich et al., 1980; Streck & Wacaster, 2006).
The glasses and minerals were analyzed using a beam current of 10 nA, an operating voltage of 15 kV and a 10 mm
electron beam spot size to minimize alkali loss. Analyses
of S and F in the melt inclusions and pumice glasses
178°E
North Island
New Zealand
36°S
38°S
TVZ
Rotoiti
40°S
100 km
38°S
Mamaku
Rotoma
Whakatane
HLVZ
Rotorua
Okareka
TLVZ
10 km
Rerewhakaaitu
Kaharoa
176°30’E
Fig. 1. Sample location map. HLVZ, Haroharo Linear Vent Zone; TLVZ, Tarawera Linear Vent Zone. Inset shows the North Island of New
Zealand with the approximate outline of the Taupo Volcanic Zone (TVZ) and a box depicting the area of the Okataina Volcanic Center
shown by the sample location map.
1643
JOURNAL OF PETROLOGY
VOLUME 54
Bt
Cgt
Pl
Cgt
Cgt
500 μm
Fig. 2. Reflected light photomicrograph of a Rerewhakaaitu pumice
clast with plagioclase feldspar (Pl), cummingtonite (Cgt) and biotite
(Bt).
NUMBER 8
AUGUST 2013
indicate that concentrations of these elements are below
the detection limit (0·07 wt % for S; 0·09 wt % for F).
However, concentrations of F above the detection limit
were measured in the hydrous minerals (amphibole, biotite). Average melt inclusion and pumice glass major element compositions, including Cl, are reported in Table 1,
with standard deviations based on replicate analyses (reported here and in the tables and figures on a volatile-free
basis). The complete suite of both uncorrected and corrected glass composition data is reported in Electronic
Appendix 1 (available for downloading at http://www.pet
rology.oxfordjournals.org). Average mineral major element
composition and standard deviations for mineral analyses
are reported in Table 2; the complete dataset is in
Electronic Appendix 2.
Water (CO2) contents were analyzed in a subset of
melt inclusions by FTIR spectroscopy. Of the 575 melt
Table 1: Average melt inclusion and pumice glass major, minor, and trace element compositions for all eruptions
Eruption:
Rotoiti
Okareka
Rerewhakaaitu
Rotorua
Rotoma
Mamaku
Whakatane
Kaharoa
Lat. (S):
37·85375
38·17687
38·20388
38·17256
38·08997
38·08411
38·11780
38·27295
Long. (E):
176·57932
176·60196
176·58251
176·32896
176·47308
176·45739
176·47873
176·51353
MI
MI
MI
MI
MI
MI
MI
MI
pum
pum
pum
pum
pum
pum
pum
pum
SiO2
77·76
76·97
77·66
77·68
77·43
77·66
77·41
77·08
77·56
76·45
77·51
76·92
77·71
76·80
77·17
TiO2
0·14
0·14
0·09
0·11
0·09
0·10
0·08
0·16
0·13
0·11
0·13
0·11
0·12
0·13
0·10
0·07
Al2O3
12·57
12·72
12·52
12·53
12·66
12·56
12·59
12·72
12·66
12·51
12·62
12·56
12·58
12·60
12·85
12·66
0·79
77·23
FeO
0·89
0·91
0·86
0·91
0·87
0·95
0·93
1·06
0·91
0·89
0·89
0·88
0·88
0·81
0·85
MnO
0·06
0·06
0·05
0·05
0·06
0·05
0·05
0·06
0·06
0·06
0·05
0·05
0·05
0·04
0·05
0·06
MgO
0·14
0·13
0·07
0·11
0·07
0·09
0·07
0·07
0·13
0·11
0·12
0·09
0·11
0·02
0·08
0·03
CaO
0·92
0·83
0·69
0·74
0·70
0·82
0·65
1·00
0·83
0·69
0·77
0·71
0·74
0·64
0·66
0·49
Na2O
3·97
4·60
3·73
3·86
3·67
3·82
3·82
3·99
3·96
5·41
3·90
4·78
3·85
4·74
3·84
4·23
K 2O
3·34
3·43
4·13
3·83
4·23
3·78
4·18
3·65
3·57
3·60
3·82
3·74
3·75
4·01
4·20
4·27
Cl
0·19
0·19
0·20
0·16
0·21
0·16
0·21
0·17
0·19
0·17
0·19
0·15
0·18
0·18
0·19
0·16
H2O
4·47
1·30
4·54
1·30
4·64
1·30
4·99
1·30
5·26
1·30
4·82
1·30
4·48
1·30
5·07
CO2 (ppm)
n.a.
n.a.
n.a.
b.d.
n.a.
b.d.
n.a.
n.a.
n.a.
Pressure (MPa)
Li
Cu
Zn
As
118
43·9
1·54
30·3
2·82
12
37·4
3·12
28·0
3·41
27
126
60·0
1·37
28·4
4·56
135
12
39·0
1·85
31·3
3·19
121
124
70·5
1·32
27·1
4·71
147
12
43·3
2·55
32·3
4·72
121
144
12
61·5
40·4
1·37
3·66
32·9
29·3
4·02
134
4·42
114
156
54·9
2·14
35·5
3·46
107
12
37·8
5·29
34·9
4·44
112
59
137
54·1
1·31
31·9
3·57
111
n.a.
12
38·5
0·89
34·1
3·73
121
42
124
51·1
0·91
31·3
3·77
114
n.a.
12
34·7
8·43
36·2
4·40
114
47
150
64·2
1·12
31·6
4·11
126
1·30
n.a.
12
42·4
3·45
30·3
4·28
Rb
90
95
Sr
76·1
61·6
47·8
63·1
43·9
62·5
42·3
72·2
71·3
57·6
65·0
60·4
63·5
40·8
49·2
25·5
Y
18·9
21·7
22·9
23·2
21·6
20·9
28·0
22·8
23·9
21·7
24·3
23·9
24·2
21·1
25·6
23·7
Zr
88·6
87·3
82·9
93·5
77·8
83·2
97·9
116·6
92·2
78·1
92·0
84·1
89·9
68·4
83·4
55·4
Nb
7·18
7·71
7·72
7·29
7·82
7·33
8·86
7·71
7·69
6·95
7·58
7·85
7·70
7·26
7·72
Mo
1·12
1·22
1·40
1·22
1·64
1·24
1·47
1·11
1·17
1·35
1·19
1·31
1·22
1·22
1·39
Ba
936
1014
781
866
752
798
773
757
860
861
873
927
900
819
901
121
7·39
1·27
810
La
21·8
24·0
25·3
23·1
26·3
23·1
27·0
22·9
23·2
21·8
23·3
24·0
23·9
21·2
23·9
16·4
Ce
44·6
49·4
51·7
49·7
52·8
49·3
56·5
46·9
49·6
48·5
50·0
52·3
50·6
47·2
50·3
38·3
(continued)
1644
JOHNSON et al.
OKATAINA VOLCANIC CENTER RHYOLITES
Table 1: Continued
Eruption:
Rotoiti
Okareka
Rerewhakaaitu
Rotorua
Rotoma
Mamaku
Whakatane
Kaharoa
Lat. (S):
37·85375
38·17687
38·20388
38·17256
38·08997
38·08411
38·11780
38·27295
Long. (E):
176·57932
176·60196
176·58251
176·32896
176·47308
176·45739
176·47873
176·51353
MI
MI
MI
MI
MI
MI
MI
MI
Nd
16·7
pum
19·4
19·2
pum
19·6
18·5
pum
18·2
pum
22·5
18·2
20·2
pum
18·0
20·3
pum
20·1
20·0
pum
17·8
20·1
pum
16·1
Sm
3·05
3·65
3·60
3·75
3·37
3·33
4·46
3·88
3·98
3·51
3·92
4·26
3·89
3·75
4·01
3·36
Eu
0·52
0·49
0·43
0·51
0·35
0·50
0·50
0·59
0·65
0·56
0·66
0·64
0·63
0·47
0·54
0·38
Gd
2·69
3·46
3·24
3·79
2·96
2·98
4·11
3·24
3·54
3·40
3·61
3·39
3·64
3·12
3·76
3·20
Dy
2·85
3·41
3·53
3·72
3·28
3·28
4·46
3·37
3·83
3·54
3·88
3·87
3·83
3·41
4·04
3·70
Er
1·89
2·25
2·37
2·80
2·17
2·17
2·95
2·29
2·45
2·19
2·48
2·55
2·55
2·36
2·68
2·40
Yb
2·17
2·74
2·64
2·49
2·58
2·51
3·21
2·61
2·65
2·73
2·72
2·66
2·70
2·50
2·87
2·71
Hf
2·73
2·82
2·92
2·91
2·85
2·92
3·44
3·27
3·03
2·91
3·08
3·00
3·06
2·60
2·97
2·32
Ta
Pb
0·59
0·64
0·79
0·82
0·86
0·69
0·86
0·73
0·65
0·68
0·65
0·74
0·67
0·65
0·73
0·68
12·0
12·6
17·7
18·4
18·7
18·9
19·1
22·8
16·1
18·8
16·2
17·4
16·4
17·2
17·1
16·9
Th
8·5
9·4
13·9
11·1
15·4
11·5
13·8
11·0
10·1
10·0
10·5
10·9
11·1
10·2
11·7
9·6
U
2·06
2·41
3·23
2·92
3·61
2·96
3·27
2·78
2·39
2·67
2·53
2·87
2·71
2·79
2·82
2·84
Major elements and Cl analyzed by electron microprobe and reported in weight per cent; trace elements analyzed by LAICP-MS and reported in parts per million. H2O (wt %) and CO2 measured by FTIR (detection limit for CO2 15 ppm; b.d.,
below detection; n.a., not analyzed). Errors on H2O are 10%. Pumice H2O and pressure estimates (italics) from Johnson
et al. (2011), based on FTIR measurements, microprobe totals and estimates of fragmentation depths. Pressure calculated
using the solubility model of Liu et al. (2005). Average standard deviations for electron microprobe analyses (based on 3–4
spots): SiO2 0·24, Al2O3 0·07, Cl 0·01, K2O 0·11, CaO 0·02, FeO 0·03, MnO 0·01, TiO2 0·02, Na2O 0·17,
MgO 0·01. Microprobe totals were typically 93–96% for melt inclusions and 97–99% for pumice glasses. Average
analytical precision (1s) for LA-ICP-MS analyses: Li (6%), Cu (33%), Zn (8%), As (17%), Rb (6%), Sr (5%), Y (5%),
Zr (5%), Nb (6%), Mo (15%), Ba (5%), La (5%), Ce (5%), Nd (6%), Sm (9%), Eu (11%), Gd (9%), Dy (7%), Er (8%),
Yb (8%), Hf (7%), Ta (8%), Pb (6%), Th (5%), U (6%).
inclusions analyzed by electron microprobe, 115 were analyzed for H2O using reflectance FTIR (Johnson et al.,
2011) and 34 were analyzed by transmission FTIR for
H2O and CO2 (averages reported in Table 1; full dataset
reported in Electronic Appendix 1). There is good agreement between the H2O contents measured by the two techniques and errors in the H2O analyses are estimated to be
10% (Johnson et al., 2011).
All samples were analyzed by laser ablation inductively
coupled plasma mass spectrometry (LA-ICP-MS) at the
University of Tasmania. This study reports the trace element compositions of 440 melt inclusions (440 mm in diameter), 26 pumice glasses, and 74 minerals. Spot sizes for
melt inclusion analyses ranged from 35 to 95 mm (depending on the melt inclusion size); spot sizes were typically
35 mm for pumice glasses (to avoid vesicles) and 44 mm for
mineral analyses. A suite of 25 trace elements was analyzed, including the metals Pb, Zn, Cu, Mo, and As.
Silver was also measured by LA-ICP-MS; however, concentrations typically ranged from below detection to
0·2 ppm and thus analyses of Ag are not reported.
Analyses of Pt and Au were also attempted, but their concentrations were consistently below the detection limit
and therefore no further analyses were attempted. Major
elements Ca and Al were analyzed, and Al (from electron
microprobe analyses) was used as the internal standard.
The NIST-612 glass was used as the primary standard for
glass analyses; NIST-610 was used for mineral analyses
and BCR-2 G was used as the secondary standard in all
runs. The preferred standard values for these glasses were
obtained from GeoRem (http://georem.mpch-mainz.
gwdg.de/). The average analytical precision (per cent) for
each element is reported with the average geochemical
data for melt inclusions and pumice glass from each eruption (Table 1) and average mineral analyses (Table 2). The
complete LA-ICP-MS data, including detection limits and
details of analyses, can be found in Electronic Appendix 1.
Deviations of analyzed BCR-2 G from the preferred values
are reported in Table 3.
R E S U LT S
Melt inclusion and glass compositions
Major elements and volatiles
The OVC glasses analyzed here are high-SiO2 rhyolites.
Melt inclusion and pumice glasses exhibit a restricted range
in SiO2 (76^79 wt %; Fig. 3a). The concentrations of FeO
and Al2O3 (typically compatible) decrease with increasing
1645
JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 8
AUGUST 2013
Table 2: Average major and trace element compositions of OVC minerals and average analytical errors
Mineral:
Plagioclase
Orthopyroxene
Hornblende
Cummingtonite
Biotite
Analytical errors*
n ¼ 33
n ¼ 11
n¼6
n¼8
n ¼ 16
Average standard deviations
average
SiO2
60·23
TiO2
b.d.
Al2O3
25·10
SD
average
SD
average
SD
average
SD
average
SD
pl
opx
hbl
cgt
bt
51·45
0·53
47·01
0·53
53·03
0·54
35·49
0·37
0·87
0·32
0·8
0·22
0·09
0·02
1·35
0·26
0·28
0·05
4·35
0·05
0·01
0·04
0·14
0·05
0·07
1·17
0·46
0·14
6·75
0·49
1·60
0·21
13·61
0·22
0·56
0·16
0·54
0·28
0·11
0·05
1·81
—
0·28
FeO
0·20
25·66
1·14
16·88
2·55
20·66
1·51
22·09
0·58
0·03
0·59
0·81
0·41
0·29
MnO
b.d.
—
2·02
0·23
0·70
0·09
1·60
0·08
0·35
0·04
0·03
0·12
0·05
0·09
0·04
MgO
b.d.
—
19·10
1·07
12·88
1·39
18·06
1·05
9·82
0·30
0·01
0·38
0·47
0·3
0·16
CaO
6·45
1·38
0·79
0·15
10·36
0·33
1·83
0·36
0·04
0·10
0·63
0·08
0·19
0·26
0·02
Na2O
7·44
0·70
0·03
0·01
1·38
0·12
0·40
0·06
0·47
0·04
0·33
0·03
0·07
0·07
0·04
0·16
K 2O
0·47
b.d.
—
0·34
0·07
0·02
0·01
8·38
0·40
0·06
0·02
0·03
0·02
0·19
Cl
b.d.
—
b.d.
—
0·12
0·03
0·04
0·01
0·30
0·03
0·01
0·01
0·02
0·01
0·02
F
b.d.
—
b.d.
—
0·22
0·05
0·16
0·04
0·33
0·05
0·02
0·03
0·04
0·03
0·03
0·56
Total
99·93
Li
13·55
Cu
0·42
Zn
5·06
As
b.d.
Rb
Sr
0·66
629
0·31
99·65
0·57
97·97
0·25
97·69
0·34
95·28
5·63
7·20
2·76
6·60
1·35
8·27
1·30
51·5
0·78
0·31
0·52
0·14
0·68
0·29
—
1·26
—
0·43
93
783
—
b.d.
—
0·06
0·06
12·36
4·57
Y
0·27
Nb
b.d.
—
b.d.
Mo
b.d.
—
0·35
210
b.d.
Ba
532
0·12
326
b.d.
—
0·12
—
205
b.d.
18
—
399
—
—
0·78
0·14
b.d.
27·36
14·34
1·84
252
47
88
b.d.
52·6
9·19
332
b.d.
852
Average analytical precisions (%, 1s)
88·6
8
12·82
b.d.
52
—
894
12
b.d.
14
9
9
9
8
28
30
42
17
8
8
8
8
b.d.
b.d.
b.d.
b.d.
b.d.
8
b.d.
10
1·00
11·59
11·91
8
37
8
9
8
9·5
26·2
43·9
15
8
7
8
10
15·83
5·53
1·89
0·46
43·20
5·64
b.d.
0·19
0·03
0·19
0·03
0·48
0·14
b.d.
47·9
9·1
2·34
3·51
La
8·10
1·54
0·05
0·03
22·5
6·9
2·62
4·57
5629
28·1
43·9
Ce
10·82
1·59
0·15
0·06
101·4
27·9
14·6
24·3
Eu
2·15
0·47
0·05
0·02
Dy
b.d.
—
1·62
0·58
Yb
0·09
—
3·20
1·23
Pb
6·76
Th
b.d.
U
b.d.
926
8
8
b.d.
26
b.d.
28
8
9
8
29
32
22
8
17
8
8
11
11
8·47
11·84
8
15
8
9
11
1·95
0·38
0·12
1·01
1·58
9
35
8
12
17
48·4
8·4
8·32
1·58
4·76
6·42
44
11
8
8
13
23·7
4·6
7·67
1·33
1·97
2·97
b.d.
9
8
8
20
0·13
b.d.
—
3·29
1·96
8
b.d.
10
59
8
4·68
b.d.
—
0·61
—
b.d.
—
b.d.
—
b.d.
—
b.d.
—
b.d.
b.d.
12
25
23
—
b.d.
—
b.d.
—
b.d.
—
b.d.
—
b.d.
b.d.
23
31
32
1·68
Major elements, Cl and F analyzed by electron microprobe and reported in wt %; trace elements measured by LA-ICP-MS
and reported in ppm. pl, plagioclase; opx, orthopyroxene; hbl, hornblende; cgt, cummingtonite; bt, biotite; b.d., below
detection limit.
*Analytical errors are reported as the average standard deviations (based on analysis of 2–4 points) for microprobe
analyses, and the average analytical precision for LA-ICP-MS analyses.
SiO2, and CaO and FeO are positively correlated (Fig. 3).
Melt inclusions from some fall deposits reveal wide ranges
in melt compositions (e.g. Kaharoa, Rerewhakaaitu,
Okareka) and in others, distinct but very diverse melt
batches are present, as exhibited by the high-FeO and lowFeO melt batches from the Rotorua samples (Fig. 3b). In
contrast, some fall deposits show a more restricted range in
melt compositions (e.g. Rotoiti, Whakatane, Rotoma,
Mamaku). Furthermore, the pumice glasses from some deposits appear to be compositionally distinct from some melt
inclusion populations (e.g. Rotorua).
The rhyolitic melts erupted fromthe OVC are volatile rich.
Melt inclusions typically contain 4^6 wt % H2O and
0·15^0·25 wt % Cl (Fig. 4; see alsoJohnson et al., 2011). Only
10 melt inclusions out of the 35 analyzed by transmission
FTIRcontain detectable CO2, and measured concentrations
of CO2 are low (15^150 ppm; Table 1 and Electronic
Appendix 1). Melt H2O and Cl contents are similar for all
fall deposits, except for some Rotorua melt inclusions that
have higher Cl contents (Fig. 4). Pumice glasses contain
slightly lower concentrations of Cl (0·15^0·2 wt %) than
the melt inclusions (Johnson et al., 2011).
1646
JOHNSON et al.
OKATAINA VOLCANIC CENTER RHYOLITES
Kaharoa
Whakatane
Table 3: Average LA-ICP-MS analyses of secondary glass
standard (BCR-2 G)
BCR-2 G (average values, ppm)
Ref.
Mamaku
Rotoma
Rotorua
Rerewhakaaitu
14
Okareka
Rotoiti
1σ
error
Per cent difference
Li
45
9·8
66
8·7
9·4
95
35
9·2
9·0
8·9
45
–3·2
66
4·2
95
2·4
Cu
16
16
15
15
21
–24
–25
–30
–30
Zn
136
134
131
122
125
9
7
5
–2
As
0·98
1·09
0·84
0·70
1·10 –11·1
–0·8 –24·0 –36·5
Rb
43
43
42
41
47
–9
–8
–11
–12
Sr
314
317
312
313
342
–8
–7
–9
–9
Y
32
32
31
31
35
–9
–9
–11
–12
Zr
169
169
167
164
184
–11
Nb
11·6
11·6
11·3
11·4
12·5
Mo
221
218
208
206
230
Ba
629
637
625
628
683
La
23·4
23·6
23·1
22·9
24·7
–8
–8
–9
–7·5
–7·5
–9·2
–4
–5
–10
(a)
11
76
–8·7
1.2
–10
–8
–7
–9
–8
–5·3
–4·5
–6·6
–7·4
Ce
48·3
48·9
47·9
48·1
53·3
–9·3
–8·2 –10·1
–9·8
Nd
26·5
26·8
26·6
26·5
28·9
–8·2
–7·4
–7·8
–8·5
Sm
6·2
6·4
6·1
6·2
6·6
–6·0
–3·3
–7·0
–6·3
Eu
1·77
1·82
1·77
1·78
1·97 –10·1
–7·7 –10·1
–9·5
Gd
6·09
6·07
6·04
5·91
6·71
–9·6 –10·0 –11·9
Dy
6·06
6·17
5·98
5·83
6·44
–5·9
–4·2
–7·1
Er
3·48
3·47
3·39
3·33
3·70
–6·0
–6·1
–8·4 –10·0
Yb
3·23
3·31
3·16
3·14
3·39
–4·9
–2·4
–6·9
–7·3
Hf
4·76
4·89
4·79
4·64
4·84
–1·6
1·1
–1·1
–4·2
Ta
0·68
0·68
0·65
0·64
Pb
9·6
9·5
9·2
8·8
Th
5·68
5·75
5·60
5·47
5·90
–3·7
–2·6
U
1·55
1·59
1·52
1·50
1·70
–9·0
–6·6 –10·8 –11·8
–9·2
13
12
FeO wt%
Spot size (mm): 35
Al2O3 wt%
value*
77
SiO2 wt%
78
79
1σ
error
1.0
0.8
–9·4
(b)
0.6
0.2
0.4
0.6
0.8
CaO wt%
1.0
1.2
0·78 –12·8 –12·9 –16·5 –17·5
11·0
–12·9 –13·4 –16·5 –20·3
–5·1
–7·3
*Reference value for BCR-2 G from GeoRem preferred
values (http://georem.mpch-mainz.gwdg.de/).
Trace elements and metals
The OVC melt inclusions and pumice glasses display large
variations in trace element concentrations. The concentrations of elements that are typically compatible (e.g. Sr) decrease with increases in typically incompatible elements
(e.g. Rb, Th, U; Fig. 5). Relatively large variations in the
concentrations of Sr (20^100 ppm) and U (1·5^4·5 ppm;
Fig. 5) suggest that these eight eruptions sampled a range
of more evolved (low-Sr, high-U) and less evolved (highSr, low-U) rhyolitic melts. In some cases, all melt inclusions (or a population of inclusions) are compositionally
similar to, or less evolved than, their associated pumice
glass compositions (e.g. Rotoiti, Whakatane, Mamaku,
Rotoma). However, for other samples the melt inclusion
compositions are more evolved than, or compositionally
distinct from, their associated pumice glasses (e.g.
Rerewhakaaitu, Okareka, Kaharoa, Rotorua). This
Fig. 3. Melt inclusion (filled symbols) and pumice glass (white symbols) wt % Al2O3 vs SiO2 (a) and FeO vs CaO (b).
relationship suggests that, in some cases, at least two separate melt batches mingled prior to eruption. These observations are consistent with the demonstrable involvement of
multiple magma batches and/or crystal mushes in many of
the OVC eruptions (e.g. Nairn et al., 2004; Smith et al.,
2004, 2010; Shane et al., 2007, 2008a, 2008b; Saunders et al.,
2010).
The concentrations of metals in the OVC rhyolitic
melts are moderate (11^24 ppm Pb, 20^50 ppm Zn,
2^7 ppm As, 55 ppm Cu, 52·5 ppm Mo; Fig. 6). These
values are generally comparable with other analyses of
metals in melt inclusions from explosive rhyolitic eruptions, such as Sumisu, in the Izu^Bonin arc (4^5 ppm Pb,
30^60 ppm Zn, 52 ppm Mo; Allen et al., 2010), the Kos
Plateau Tuff, Greece (10^14 ppm Pb, 10^23 ppm Zn,
51·3 ppm Cu; Bachmann et al., 2010) and the Toba Tuff,
Indonesia (38^66 ppm Pb; Chesner & Luhr, 2010). There
are positive correlations between As and Pb (Fig. 6b) and
As and Mo in the OVC data, suggesting broadly similar
behavior of these elements in all rhyolitic melts. In general,
pumice glasses have similar to slightly higher
1647
JOURNAL OF PETROLOGY
Kaharoa
Whakatane
Mamaku
Rotoma
VOLUME 54
Okareka
Rotoiti
Rotorua
Rerewhakaaitu
Exsolve
vapor
H2O wt%
.05
1σ
error
5
C
O
l/H 2
=0
4
Exsolve
hydrosaline
liquid
3
2
1
0.1
0.2
Cl wt%
AUGUST 2013
The behavior of Cu is more erratic than that of the other
metals, and the higher Cu concentrations in pumice glasses
could be the result of Cu diffusion out of the melt inclusions and into the surrounding melt. Copper has been
demonstrated to diffuse rapidly in and out of rhyolitic
melt inclusions (Kamenetsky & Danyushevsky, 2005;
Zajacz et al., 2009). Given the low and variable concentrations of Cu in melt inclusions, the highly variable Cu in
pumice glasses, and the demonstrated rapid diffusion of
Cu, it is not possible to make definitive conclusions about
the behavior of Cu in this study, and Cu is not included in
subsequent discussions of metal behavior.
7
6
NUMBER 8
Mineral compositions
0.3
Fig. 4. Melt inclusion (filled symbols) and pumice glass (open symbols) H2O vs Cl (wt %). Pumice glass H2O is estimated to be 1·3 wt
% from FTIR and microprobe analyses and based on expected pressures of magma fragmentation (Johnson et al., 2011). Also shown is a
line corresponding to a Cl/H2O ratio of 0·05. Melts of granitic composition at 200 MPa with Cl/H2O ratios less than 0·05 should exsolve
a vapor, whereas melts with Cl/H2O40·05 should exsolve a hydrosaline liquid (Webster, 2004). The Cl/H2O ratio required for hydrosaline
liquid exsolution increases with decreasing pressure (Webster et al.,
2003; Webster, 2004), so it is likely that the OVC melts exsolved only
a vapor during degassing.
The focus of the mineral analyses in this study is on the
trace element compositions, to assess whether or not
metals partitioned into the crystallizing phases. The minerals analyzed in this study are typically free, euhedral
crystals, although intergrowths of some minerals (e.g. biotite) are observed (Fig. 2). The analyzed crystals typically
range in size from 0·5 to 2 mm. Reflected light images
of the analyzed minerals do not reveal evidence of resorption, and when multiple analyses per crystal were possible,
the mineral compositions are typically homogeneous.
Plagioclase
Kaharoa
Whakatane
Mamaku
Rotoma
Rotorua
Rerewhakaaitu
Okareka
Rotoiti
Plagioclase is common in all the OVC deposits and its
composition is variable. Plagioclase crystals contain 3^
10 wt % CaO, and K2O contents are low (51wt %; Fig. 7),
corresponding to An18^50. The concentrations of trace
elements also display broad ranges; Ba contents (200^
1000 ppm) increase with decreasing Sr concentrations
(450^800 ppm; Table 2). In plagioclase the only trace
metals with abundances above the detection limit are Pb
(4^11ppm) and Zn (3^10 ppm; Figs 7 and 8, Table 2).
Concentrations of Pb in plagioclase increase with decreasing CaO (Fig. 7), suggesting that in the more evolved rhyolites, Pb partitions to a greater extent into plagioclase.
120
1σ error
Sr ppm
100
80
60
40
20
1
2
3
U ppm
4
Amphibole
5
Fig. 5. Melt inclusion (filled symbols) and pumice glass (white symbols) Sr vs U (ppm). Decreases in Sr with increasing U indicate fractionation of plagioclase feldspar during and after melt inclusion
entrapment. It should be noted that in some deposits (e.g.
Rerewhakaaitu, Rotorua) the pumice glasses are compositionally less
evolved than the melt inclusions.
concentrations of Pb, As and Mo compared with their
associated melt inclusions, whereas the concentrations of
Zn are typically lower (Fig. 6). Concentrations of Cu in
melt inclusions and pumice glasses show significant scatter
(values range from below detection to 5 ppm; rare analyses contain 10 ppm Cu), but in many cases pumice
glasses have higher Cu contents (Fig. 6d).
Amphibole is a common ferromagnesian mineral in the
OVC eruptive rocks (e.g. Smith et al., 2005). Two types of
amphibole are present in the OVC deposits: hornblende
and cummingtonite. Hornblende is common in the OVC
samples and contains 12^15 wt % MgO and 13^19 wt %
FeO (Fig. 9) and a restricted range of Al2O3 (6^7 wt %;
Table 2). Cummingtonite is less common and contains
higher MgO (16^20 wt %) and higher FeO (19^23 wt %
FeO; Fig. 9) and much lower Al2O3 (1^2 wt %; Table 2).
The Cl content of the amphiboles ranges from 0·02 to
0·14 wt % (Table 2). The metal concentrations in the
amphiboles are variable; the most abundant metal analyzed is Zn (150^550 ppm; Fig. 8), whereas other metal concentrations are much lower (51ppm Pb, 50·2 ppm Mo, As
is below detection; Table 2).
1648
JOHNSON et al.
OKATAINA VOLCANIC CENTER RHYOLITES
Kaharoa
Whakatane
Mamaku
Rotoma
Rotorua
Rerewhakaaitu
3
60
Okareka
Rotoiti
Zn ppm
1σ error
Mo ppm
50
2
40
30
1
20
(a)
(c)
As ppm
7
Cu ppm
4
6
5
3
4
2
3
1
2
1
(d)
(b)
8
12
16
Pb ppm
20
0
24 8
12
16
Pb ppm
20
24
Fig. 6. Variation of Zn (a), As (b), Mo (c) and Cu (d) vs Pb in melt inclusions (filled symbols) and pumice glasses (white symbols). The average
analytical precisions are shown in each panel.
Orthopyroxene
Orthopyroxene is common in the OVC deposits (e.g.
Smith et al., 2005), especially in the less evolved rhyolites
(e.g. Rotoiti, Rotoma, Mamaku). Compositions are in the
range of 17^21wt % MgO and 22^27 wt % FeO (Fig. 9).
The OVC orthopyroxenes have the highest concentrations
of Zn amongst the analyzed minerals, although the concentrations are variable (400^1400 ppm; Fig. 8). Orthopyroxene also contains some Mo (0·5 ppm), and all other
metals (Pb, As) are below detection.
variable amounts of Cl (0·25^0·35 wt %) and F (0·3^
0·45 wt %; Table 2). Concentrations of trace elements such
as Ba (typically 5000^6000 ppm) and Rb (500^
2000 ppm) are high (Table 2). The abundances of metals
in biotite tend to be highly variable, and in some cases,
higher than in the OVC glasses. Biotites contain high concentrations of Zn (250^500 ppm) and moderate concentrations of Pb (2^8 ppm; Fig. 8). Biotite can contain some
Mo (0·8 ppm) whereas the concentrations of As are
below the detection limit.
Biotite
Biotite crystallization is variable within the OVC melts. It
typically crystallizes in trace amounts to 54 vol. % (e.g.
Nairn et al., 2004; Schmitz & Smith, 2004; Smith et al.,
2004, 2006; Shane et al., 2008a) with larger proportions of
biotite crystallization in the Rerewhakaaitu melts (10^
15 vol. %; Shane et al., 2007). Biotite is most common in
the more evolved rhyolites (Smith et al., 2005). Analyzed
biotite crystals have a narrow range of MgO (9^10 wt
%) and FeO (21^23 wt %; Fig. 9; Table 2) and contain
DISCUSSION
The OVC melt inclusion data provide snapshots of the melt
evolution during the crystallization of the host mineral, in
this case quartz. In the following sections, we combine the
melt inclusion, pumice glass and mineral analyses to discuss the evolution of the OVC rhyolites, via both magma
mixing and fractional crystallization, and the behavior of
metals during fractional crystallization and ascent, degassing, and eruption of these magmas.
1649
JOURNAL OF PETROLOGY
VOLUME 54
AUGUST 2013
1500
1.1
1σ
error
0.9
plagioclase
plagioclase
biotite
hornblende
cummingtonite
orthopyroxene
1σ error
1000
0.7
OVC pl
Zn ppm
K2O wt%
NUMBER 8
0.5
500
0.3
(a)
0
0
2
4
6
Pb ppm
8
10
12
Fig. 8. Variation of Zn vs Pb in OVC plagioclase, biotite, hornblende,
cummingtonite and orthopyroxene. Ferromagnesian minerals contain
large amounts of Zn (200^1500 ppm) and little Pb (58 ppm, in biotite). Plagioclase contain only small amounts of Zn (3^8 ppm) and
greater Pb (510 ppm).
Pb ppm
10
5
35
(b)
3
5
7
CaO wt%
9
11
biotite
OVC opx
hornblende
cummingtonite
orthopyroxene
30
FeO wt%
0
Fig. 7. Plagioclase K2O vs CaO (a) and Pb vs CaO (b). Also shown
for comparison in (a) are the ranges of plagioclase K2O and CaO
contents from other OVC studies (Nairn et al., 2004; Smith et al.,
2006; Shane et al., 2007). The correlation of Pb and CaO in (b) indicates that Pb partitions more strongly into less calcic plagioclase.
25
OVC bt
20
OVC cgt
OVC amph
15
Evidence for magma mixing and mingling
10
To properly assess the crystallization history of the OVC
rhyolitic melts, the melt inclusions with or without pumice
glasses need to originate from a single melt batch. The
melt inclusion and pumice glass data presented here are
consistent with the involvement of multiple magma
batches, as has been previously demonstrated for many
OVC eruptions (e.g. Nairn et al., 2004; Smith et al., 2004,
2010; Shane et al., 2007, 2008a, 2008b; Saunders et al., 2010).
In particular, plots of middle versus heavy rare earth elements illuminate distinct melt batches present within a
single sample (Fig. 10). The melt inclusions from some eruptions appear to be generally homogeneous and less evolved
than their associated pumice glasses (e.g. Rotoiti,
Whakatane, Rotoma, Mamaku), suggesting that the analyzed melt inclusions and glasses record the compositional
evolution of a single melt batch. However, melt inclusions
from the Rerewhakaaitu deposit, although they largely originate from a single magma batch (with the exclusion of a
few outliers; Fig. 10), are compositionally more evolved
than the pumice glasses (Fig. 5). In two deposits (Rotorua,
Kaharoa), the melt inclusions represent two distinct melt
batches, as illustrated by the high-Dy and low-Dy melt
1σ error
5
10
15
MgO wt%
20
25
Fig. 9. Variation of FeO vs MgO in OVC biotite, hornblende, cummingtonite and orthopyroxene. Also shown for comparison are fields
of published data for OVC biotite (bt), amphibole (amph) and orthopyroxene (opx) from Deering et al. (2010) and OVC cummingtonite
(cgt) from Smith et al. (2006) and Shane et al. (2008).
batches from Rotorua (Fig. 10). The melt inclusions from
the Okareka deposit have a very wide range of compositions that possibly reflect mixing of multiple magma
batches during the entrapment of the melt inclusions and/
or recharge by mafic melts (e.g. Shane et al., 2008a).
We have distinguished the dominant melt batches
(Electronic Appendix 2) from each deposit; this allows us
to investigate the extent of fractional crystallization, crystallization depth, and the behavior of metals during the
crystallization and degassing of these melt batches. The
inclusions from Okareka, however, proved too compositionally heterogeneous and do not appear to reflect crystallization processes, and therefore have been excluded
from further modeling. Although separate melt batches
1650
JOHNSON et al.
Kaharoa
Whakatane
OKATAINA VOLCANIC CENTER RHYOLITES
Mamaku Rotorua
Rotoma Rerewhakaaitu
Okareka
Rotoiti
Fractional crystallization of OVC melt
batches
7
1σ error
Dy ppm
6
5
4
3
2
10
15
20
25
La ppm
30
35
Fig. 10. Melt inclusion (filled symbols) and pumice glass (white symbols) Dy vs La (ppm). Dashed lines highlight the two unique groups
of Kaharoa melt inclusions, and dotted lines outline the two groups
of Rotorua melt inclusions.
1100
Ba ppm
1000
Kaharoa
Mamaku
Whakatane Rotoma
Rotorua
Rerewhakaaitu
no biotite fractionation
(DBa = 0)
Rotoiti
some biotite
fractionation
(DBa = 0.6)
900
800
700
600
70
1σ error
90
increased biotite
fractionation
(DBa =1.7)
110
130
Rb ppm
150
170
Fig. 11. Ba vs Rb in melt inclusions (filled symbols) and pumice
glasses (white symbols) from single melt batches. Variations in Ba
within single melt batches are dependent on fractionation of biotite
(OVC biotite contains 4000^7000 ppm Ba). The arrows fitted to
the data show expected fractionation trends with no biotite fractionation (calculated assuming DBa ¼ 0), which fits the Rotoiti data,
some biotite fractionation (DBa ¼ 0·6), which fits the Kaharoa,
Whakatane, Mamaku and Rotoma data, and increased biotite fractionation (DBa ¼1·7) which fits the Rotorua and Rerewhakaaitu data.
have been identified in the melt inclusion and pumice glass
data, the H2O and Cl contents of most melt batches are
the same and the pumice glass Cl concentrations appear
to be relatively homogeneous in all deposits (with the exception of the melt batches from Rotorua; Johnson et al.,
2011). This suggests that, although the bulk compositions
of the rhyolites are variable, the volatile contents and extents of degassing of these melt batches are similar.
The OVC melt inclusion and pumice glass data reveal that
U, Th, and in some cases Rb behaved incompatibly
during crystallization, whereas Sr behaved compatibly
and partitioned strongly into plagioclase (400^800 ppm
Sr; Table 2). Negative correlations between Sr and U
(Fig. 5) demonstrate that plagioclase crystallized concurrently with quartz in all OVC melts. Decreases in melt
MgO content with increasing differentiation (e.g. increasing U, Th, Rb) in the OVC melts are indicative of crystallization of ferromagnesian phases: amphibole biotite orthopyroxene. Although the major element compositions
of the ferromagnesian minerals are broadly similar, the
trace element compositions can be used to discern fractionation trends in the OVC rhyolite data. For example,
the variable concentrations of Ba in the OVC melts
(650^1050 ppm; Fig. 11) relative to Rb reflect the variable
fractionation of biotite, which contains high concentrations
of Ba (4000^7000 ppm, Table 2). The incompatible behavior of Ba with increasing Rb in the least evolved OVC
rhyolite analyzed in this study (Rotoiti) indicates that no
biotite crystallized from these melts (Fig. 11). The more
evolved rhyolite compositions show increasing compatibility of Ba during crystallization, suggestive of increased
biotite fractionation in these melts (Fig. 11). These observations are consistent with the lack of biotite in the early
phase of the Rotoiti eruption (Schmitz & Smith, 2004;
Smith et al., 2010) and the presence of biotite in the more
evolved melts from Kaharoa, Rerewhakaaitu and Rotorua
(Nairn et al., 2004; Smith et al., 2004; Shane et al., 2007).
Although the OVC melt inclusion data clearly reflect fractionation of plagioclase and ferromagnesian minerals, estimating the quantities of the fractionating phases is beyond
the scope of this study, and such estimates would pertain
only to the differentiation of the melt during quartz crystallization. However, increases in incompatible elements (U,
Th) can be used to calculate the extents of fractional crystallization during quartz crystallization (Fig. 12). These estimates are minima, as the data here do not constrain any
crystallization that took place prior to quartz crystallization
and melt inclusion entrapment. Also, owing to the complexities of mixing and the involvement of multiple magma
batches in most OVC eruptions, these estimates pertain
only to the melt batch from which the quartz crystallized,
and do not represent the total extent of fractionation for all
magmas involved in one eruption. Estimates of the extent of
crystal fractionation were not possible for the Okareka samples in which mixing or recharge clearly affected the melt inclusion compositions. Our data suggest that the extents of
fractional crystallization in all other OVC melt batches are
very similar; melt inclusions record 20^25% crystallization for most melt batches (Fig. 12). In most samples,
pumice glass compositions overlap with the most evolved
1651
JOURNAL OF PETROLOGY
VOLUME 54
20
1σ error
30%
20%
Th ppm
15
10%
Kaharoa
Whakatane
Mamaku
Rotoma
Rotorua
Rerewhakaaitu
Rotoiti
30%
10
5
20%
10%
1
2
3
U ppm
4
5
Fig. 12. Variation of Th vs U in melt inclusions (filled symbols) and
pumice glass (white symbols). Only melt inclusions ( pumice
glasses) determined to originate from a single melt batch are shown.
Also shown are two fractionation trends assuming bulk D values of
zero for both Th and U; small white circles indicate increments of
10% fractional crystallization. These fractionation trends suggest
that bothTh and U are incompatible during fractional crystallization,
and that single batches of magma underwent 20^25% crystallization during melt inclusion entrapment. Slightly higher Th and U contents in some pumice glasses are indicative of an additional 55%
crystal fractionation between melt inclusion entrapment and eruption.
melt inclusions, suggesting that fractional crystallization
post-melt inclusion entrapment was not significant.
However, the pumice glasses of the Rotoiti deposit are
slightly more evolved than the melt inclusions, suggesting
that an additional 55% crystal fractionation occurred between quartz crystallization and eruption (Fig. 12). Overall,
it appears that single melt batches in the OVC typically
underwent at least 20^25% crystal fractionation prior to
eruption. These estimates are similar to the modeled
amount of crystal fractionation (25^30%) calculated for
the Rotoiti magma by Schmitz & Smith (2004).
Depths of crystallization
The well-established relationship between H2O and CO2
solubilities and pressure (e.g. Newman & Lowenstern,
2002; Liu et al., 2005; Papale et al., 2006) allows estimation
of the pressure of melt inclusion entrapment, and thus pressure (and depth) of quartz crystallization. Although only
a subset of melt inclusions were analyzed for CO2, very
few of those melt inclusions contain CO2 and the measured
CO2 concentrations are uniformly low (5150 ppm).
Furthermore, CO2 concentrations of the order of 100 ppm
would have very little effect on the melt inclusion entrapment pressure (15 MPa). Therefore, for those melt inclusions without CO2 measurements the pressure estimated
by H2O alone (PH2O) should be approximately equivalent
to the total pressure at melt inclusion entrapment.
However, these pressure estimates are valid only if the
melts are saturated in volatiles. Several lines of evidence
NUMBER 8
AUGUST 2013
suggest that the OVC melts were vapor saturated. First,
the very high H2O contents (56 wt %) on their own
and the observed presence of large vapor bubbles in
some inclusions (avoided during analysis), which may
have been trapped during crystallization, point to vapor
saturation. Second, estimations of entrapment pressures
using H2O and Cl (e.g. Webster, 1997; see Johnson et al.,
2011, fig. 1) and using H2O CO2 (Liu et al., 2005) give
very similar results. Finally, we have investigated melt inclusion pumice glass H2O and Cl behavior with crystallization (as indicated by increases in incompatible
elements). We find that, overall, melt H2O and Cl contents
do not increase with increasing U, Th and Rb as would
be expected if the melts were vapor-undersaturated (Fig.
13; see also Johnson et al., 2011). Instead, the volatile concentrations of most melt batches typically remain approximately constant during crystallization. There is some
ambiguity in the volatile behavior in melts from two of
the most evolved eruptions (Fig. 13); however, we believe
that this is probably due to the increased fractionation of
biotite and amphibole in these most evolved rhyolites.
Overall, our data suggest that, although the melts contain
little CO2, the OVC melt batches were probably vapor
saturated and some minor degassing of volatiles accompanied fractional crystallization at depth.
Melt inclusion entrapment pressures of 50^200 MPa
were calculated following the solubility model of Liu
et al. (2005); the majority of melt inclusions were trapped
between 100 and 200 MPa (Fig. 13a, and Johnson et al.,
2011). As these pressures represent the trapping pressures
of the melt inclusions, they also correspond to the pressures and depths of quartz crystallization. Assuming a
crustal density of 2500 kg m^3, the majority of quartz crystallization in the OVC rhyolites occurred at depths of
4^8 km.
The OVC melt inclusion data demonstrate that the
rhyolitic melts underwent a minimum of 20^25% fractional crystallization at pressures of 100^200 MPa. Most
melt inclusion compositions do not record a clear relationship between pressure and melt evolution (Fig. 13a). The
rare entrapment of melt inclusions at lower pressures
(5100 MPa) and the overlap in melt inclusion and pumice
glass compositions suggest that, for many eruptions, most
of the crystallization and melt evolution occurred in the
mid- to upper crust, and there was little crystallization of
the melts at pressures 5100 MPa.
Behavior of metals during crystallization
and degassing of the OVC melts
The range in melt inclusion and pumice glass H2O and
Cl contents demonstrates that the OVC melts efficiently
degassed large quantities of H2O (74^78% of the original melt H2O), and lesser amounts of Cl (9^34% of
the original melt Cl), during eruption (Johnson et al.,
2011). Trace element data indicate that significant
1652
JOHNSON et al.
OKATAINA VOLCANIC CENTER RHYOLITES
5
150
4
100
3
50
2
1
Metal behavior during fractional crystallization
PH2O (MPa)
H2O wt%
Kaharoa
Mamaku Rotorua
Whakatane Rotoma Rerewhakaaitu Rotoiti
7
250
200
6
1σ error
10
(a)
30% DCl = 0
Cl wt%
0.25
20%
DCl = 0.3
10%
0.20
0.15
(b)
1.5
2.0
2.5
3.0
3.5
U ppm
4.0
4.5
Fig. 13. Melt inclusion (filled symbols) and pumice glass (white symbols) H2O and entrapment pressure (a) and Cl (b) vs U. Only melt
inclusions pumice glasses determined to originate from a single
melt batch are shown. The top panel (a) illustrates that most crystallization occurred at pressures of 100^200 MPa. Pressures (MPa)
were calculated from melt inclusion H2O ( CO2) at temperatures
of 765^8008C (Johnson et al., 2011) using the solubility model of Liu
et al. (2005). Pressures of 12 MPa for pumice glasses are based on estimated H2O contents of 1·3 wt %. In (b), two possible fractionation
paths are shown for Cl and U using different D values for Cl;
the path with DCl ¼ 0 illustrates the expected trend if Cl were completely incompatible and the melts were vapor-undersaturated,
and the path DCl ¼ 0·3 was calculated assuming that maximum biotite and amphibole fractionation occurred (12 and 14 vol. %, respectively, estimated from the maximum modal abundances reported in
the literature; Smith et al., 2006; Shane et al., 2008a). Biotite and
amphibole KD values (1·4^1·7 for biotite, 0·4^0·7 for amphibole) were
calculated using the concentrations of Cl measured in these minerals
and the concentrations of Cl in the melt inclusions. The fractionation
path for DCl ¼ 0 is most reasonable for the least-evolved, biotite-free
magma batches, and the trajectories between the two paths are
probably reasonable for the other eruptions. Small white circles indicate increments of 10% fractional crystallization. This figure illustrates that the melts were most likely vapor-saturated at depth
during melt inclusion entrapment. The large scatter in Cl in the most
evolved eruptions is probably due to increased biotite fractionation.
fractional crystallization (at least 20^25%) both preceded and accompanied melt degassing. Here we assess
the behavior of metals (Pb, Zn, As, Mo) during two
stages of rhyolite melt evolution: (1) the fractional crystallization-dominated evolution of the melts at pressures of
100^200 MPa, and (2) the dominantly degassing-only
evolution of the melts during ascent and eruption
(5100 MPa, by comparison of the most evolved melt inclusions and pumice glasses).
To assess metal behavior during fractional crystallization,
the concentrations of metals and U (incompatible) in
melt inclusions from single melt batches were compared
(Fig. 14). The OVC data demonstrate that the metals As,
Pb, and Mo generally increase with increasing U, suggesting that these metals behaved relatively incompatibly
during crystallization. This is true both of the melts
within a single melt batch and in general of the melts
erupted throughout the OVC; the more evolved rhyolites
(higher U) typically have the highest As, Pb and Mo contents (Fig. 14). The concentrations of Zn, however, decrease
slightly within a single melt batch, and overall, the Zn contents of the OVC melts decrease with increasing melt evolution (Fig. 14a).
The decreases in melt Zn, and deviations from perfectly
incompatible behavior of Pb, Mo and As, during crystal
fractionation could be explained by variable partitioning
of these metals into crystallizing minerals and/or an
exsolved vapor phase. Given the measured concentrations
of metals Zn, Pb and Mo in the OVC minerals (e.g.
Fig. 8, Table 2), we suggest that the variations in metal content with increasing U can be best explained by minor to
moderate partitioning of the metals into fractionating
phases. To test this, we used published mineral^melt partition coefficients and estimates of the modal mineralogy of
the OVC magmas to calculate bulk D values for each of
the metals analyzed in this study (Table 4). The published
mineral^melt KD values that we use here (Table 4) are
very similar to those calculated from our mineral and
melt inclusion analyses, thus lending further support to
this approach. The range in mineral abundances (Table 4)
reflects the diversity exhibited by the OVC magmas.
Using this range in both mineralogy and KD values, we
have calculated a range in bulk D values for each metal.
The minimum and maximum D values are reported in
Table 4 and shown in Fig. 14.
As illustrated in Fig. 14, variable partitioning of the
analyzed metals into the fractionating minerals can
explain the trends in metal concentrations with increasing
crystallization. The effect is strongest for Zn, which
partitions strongly into biotite, amphibole, and orthopyroxene, as evidenced by the elevated Zn contents measured in these minerals. Calculated DZn values of 1·7^5·2
encompass much of the melt inclusion data, with the
higher D values being more appropriate for the biotitebearing magmas. The lower D values for Pb and Mo,
which also coincide with lower abundances of these
metals measured in OVC minerals, reflect minor partitioning of these elements into ferromagnesian minerals
and feldspar (Pb). Arsenic appears to have the highest affinity for the melt, and DAs values of 0^0·3 can explain
many of the melt inclusion data. Thus, metal behavior at
depth (100^200 MPa) appears to have been controlled
1653
JOURNAL OF PETROLOGY
Melt inclusions:
Kaharoa
Whakatane
VOLUME 54
NUMBER 8
AUGUST 2013
Pumice glasses:
Mamaku
Rotoma
Rotorua
Rerewhakaaitu
Rotoiti
Whakatane
Mamaku
Rotoma
Rotoiti
3
50
Zn ppm
Mo ppm
1σ error
DMo = 0.2
40
2
30
20
DZn = 5.1
(a)
Pb ppm
DMo = 1.7
1
DZn = 1.7
(c)
DPb = 0.2
7
As ppm
DAs = 0
6
20
5
DPb = 0.7
DAs = 0.3
4
15
3
2
10
1
(b)
5
1.5
2.0
2.5
3.0
U ppm
3.5
4.0
(d)
0
4.5 1.5
2.0
2.5
3.0
U ppm
3.5
4.0
4.5
Fig. 14. Variation of Zn (a), Pb (b), Mo (c) and As (d) vs U in melt inclusions (filled symbols) and pumice glasses (white symbols) from single
melt batches. Also shown are fractionation trends using calculated minimum and maximum metal D values (U is assumed to be completely incompatible; small white circles indicate increments of 10% fractional crystallization). D values were calculated using mineral^melt partition coefficients from the literature and a range of published mineral abundances appropriate for the OVC rhyolites (see Table 4). The calculated D
values can explain the behavior of metals during fractional crystallization. The higher DZn values explain the decreases in melt Zn via partitioning of Zn into mainly ferromagnesian minerals (a). The behavior of the other metals can be explained by minor to moderate partitioning into
ferromagnesian minerals feldspar. The pumice glass metal concentrations overlap with the most-evolved melt inclusion data, suggesting negligible partitioning of metals into a vapor phase during ascent, degassing, and eruption.
by partitioning of metals into the fractionating minerals;
partitioning into an exsolved vapor seems to have been
negligible.
Metal behavior during ascent, eruption, and degassing
Comparison of the metal contents of the OVC melt inclusions and pumice glasses can reveal the behavior of metals
during the time between melt inclusion entrapment and
eruption. In the melt batches analyzed here, pumice glass
incompatible element concentrations generally overlap
with the most evolved melt inclusions, suggesting that
magma ascent was dominated by significant degassing of
the melts but that little to no additional crystallization
occurred. We also find that the metal contents of the OVC
pumice overlap with those of the most evolved melt
inclusions from the same eruption (Fig. 14, Table 1). That
the metal contents of the OVC melts remain relatively unchanged during ascent and extensive degassing suggests
that the metals remained in the melt and did not partition
into the vapor. These results are surprising, given the documented volatility of these metals in rhyolitic melts (e.g.
Simon et al., 2007; Zajacz et al., 2008; Johnson & Canil,
2011); possible explanations for this behavior are explored
below.
I M P L I C AT I O N S
Although the OVC rhyolites are evolved (high-SiO2;
20^25% crystal fractionation) and are volatile-rich,
magmatic metals appear to have remained in the melt
1654
JOHNSON et al.
OKATAINA VOLCANIC CENTER RHYOLITES
Table 4: Mineral proportions, KD values, and calculated
bulk D-values for Zn, Pb, Mo and As
Mineral
Mineral
KD and bulk D values
proportions
(vol. %)
Zn
Pb
Mo
As
KD values from literature:
Plagioclase
25–67
0·12–0·23
0·35–0·97
n.a.
0·02–0·12
Quartz
21–45
n.a.
n.a.
n.a.
n.a.
Orthopyroxene
0–3
7·5–19
0·37–0·37
n.a.
0·23
Amphibole
7–14
7·8–11·4
0·19–0·32
n.a.
n.a.
n.a.
Biotite
0–12
13–19
0·1–1·6
1·7–5·7
Magnetite
0–4
22–41
0·80
6–16
2·60
Ilmenite
0–6
0·54–1
3–11
n.a.
8·3–12·4
Calculated bulk D values:
1·7–5·1
0·2–0·7
0·2–1·8
0–0·3
Mineral proportions represent the minimum and maximum
abundances as reported for several OVC eruptions (Schmitz
& Smith, 2004; Smith et al., 2006; Shane et al., 2008a).
Mineral–melt KD values for rhyolites and high-SiO2 rhyolites
compiled from Mahood & Hildreth (1983), Nash & Crecraft
(1985) and Ewart & Griffin (1994).
and/or partitioned to some extent into crystallizing minerals during fractional crystallization at depth; similarly,
metals remained largely dissolved in the melt during
magma ascent and degassing, instead of partitioning into
the exsolved vapor. These results contradict qualitative
analysis of Cl-, Cu- and Zn-bearing fluid bubbles exsolved
in melt inclusions within OVC lavas (Davidson &
Kamenetsky, 2007) as well as previous experiments and
field measurements of volcanic gases that report partitioning of these metals into a magmatic vapor phase. Recent
experimental work on the diffusivity of metals (including
As, Pb, Mo and Cu) in rhyolites at atmospheric conditions
(0·1MPa) has demonstrated that As and Pb diffuse into
the vapor, whereas the behavior of Mo and Cu is erratic
(Johnson & Canil, 2011). Partitioning of As between rhyolitic melt and vapor was investigated at higher pressures
(120 MPa) by Simon et al. (2008), who reported As vapor^
melt partition coefficients 1. Recently, Zajacz et al. (2008)
assessed fluid^melt partitioning of metals through comparison of fluid and melt inclusions in granites, and reported D values 41 for As and Mo, and 45 for Pb and
Zn. Furthermore, analyses of fumaroles and hot springs
associated with volcanoes yield elevated concentrations of
the metals analyzed in this study (e.g. Giggenbach &
Matsuo, 1991; Hedenquist et al., 1994; Wardell et al., 2008).
The results of our work contradict these studies; possible
explanations and implications for the melt-affinity of the
OVC rhyolite metals are discussed in more detail below.
Explanations for the melt-affinity of metals
in the OVC
Metal complexing
The ability of metals to partition out of the melt depends
on the presence both of a vapor phase and of ligands, or
elements in the exsolving vapor (and/or hydrosaline fluid)
with which the metals can form complexes (Hedenquist &
Lowenstern, 1994, and references therein; Williams-Jones
& Heinrich, 2005, and references therein). The common
ligands in magmatic vapors and fluids for metals are Cl,
OH and HS (e.g. Hedenquist & Lowenstern, 1994, and references therein; Simon et al., 2007; Zajacz et al., 2008;
Borisova et al., 2010). Based on the review by Hedenquist
& Lowenstern (1994) and other recent experimental work,
the likely ligand for Pb and Zn would be Cl (Zajacz et al.,
2008), whereas Mo and As would most form hydroxyl complexes (Simon et al., 2007; Zajacz et al., 2008; Borisova
et al., 2010), although As also complexes with S (Simon
vap=melt
1 for
et al., 2007). Simon et al. (2007) calculated DAs
S-free rhyolites, demonstrating that As partitions into a
vap=melt
vapor by complexing with OH, and DAs
2·5 in Sbearing rhyolites, indicating that the presence of S causes
increased partitioning of As into the vapor.
It is possible, therefore, that the availability of ligands in
the magmatic vapors affected partitioning of metals in the
OVC rhyolites. It seems likely that the S contents of the
OVC rhyolites were low, based on concentrations below detection in the melt inclusions; this could help to explain
the lack of vapor^melt partitioning of As. However, As
should also form complexes with OH (Simon et al., 2007;
Zajacz et al., 2008), so the melt affinity of As remains puzzling. The other common ligand, Cl, is available in relatively large quantities in the OVC melts, but the Cl
concentration and the salinity of the exsolved vapors will
influence the partitioning of metals into the vapors (e.g.
Webster, 2004; Williams-Jones & Heinrich, 2005; Zajacz
et al., 2008). As shown by Johnson et al. (2011), degassing of
H2O from the OVC rhyolites is far more extensive (74^
78% of original melt H2O) than Cl (9^34% of original
melt Cl), and such degassing would produce H2O-rich
vapors. Furthermore, research on the exsolution of vapors
and hydrosaline liquids (fluids with 425 wt % Cl) from
magmas by Webster (2004) demonstrated that there is a
minimum Cl/H2O melt concentration required for melts
to exsolve a hydrosaline liquid. For melts of granitic composition at 200 MPa, the Cl/H2O ratio must be 0·05;
melts with ratios lower than this will exsolve an H2O-rich
vapor. The OVC melts dominantly have Cl/H2O ratios
50·05, which indicates that these melts would exsolve a
vapor, not a hydrosaline liquid, during degassing (Fig. 4).
Furthermore, this relationship is pressure-dependent; the
minimum Cl/H2O ratio required to exsolve a hydrosaline
liquid from a phonolite melt increases from 0·15 at
200 MPa to 0·2 at 50 MPa (Webster et al., 2003; Webster,
1655
JOURNAL OF PETROLOGY
VOLUME 54
2004). This suggests that, at pressures 5200 MPa, the
Cl/H2O ratio required for exsolution of a hydrosaline
liquid from the OVC melts would be 40·05, and thus it is
likely that the OVC melts exsolved an H2O-rich vapor
during degassing both at depth and during ascent.
Therefore, it is possible that the melt-affinity of Pb and
Zn, whose vapor^melt partitioning is strongly dependent
on the chlorinity of the vapor (Zajacz et al., 2008), was inhibited by the high proportion of H2O compared with Cl
in the vapors exsolved from the OVC melts. However, this
hypothesis does not explain the lack of Mo or As degassing,
as these metals have been shown to complex with OH
(Simon et al., 2007; Zajacz et al., 2008).
Additionally, although the melts were vapor-saturated
during crystallization at depth, the amount of exsolved
vapor in the magmas at 100^200 MPa was probably relatively low, thus further impeding metal vapor^melt partitioning. Although it is not possible to accurately estimate
the total amount of exsolved vapor in the system at these
pressures, given that some amount of vapor may have already been present prior to melt inclusion entrapment, we
can make a rough estimate of the amount of H2O degassed
during fractional crystallization as recorded by the melt
inclusions. Assuming DH2O ¼ 0 during 20^25% crystallization, melt H2O should increase by 25^33%.
Comparison of the estimated maximum H2O contents
with the measured H2O contents of the most evolved melt
inclusions suggests that 1^2 wt% H2O was lost during
crystallization. Although this is only a rough estimate,
and does not take into account degassing of other volatiles,
it demonstrates that there was most probably little vapor
present during the fractional crystallization of the OVC
rhyolites at depth. Therefore, the vapor^melt partition coefficients of the metals would need to be fairly high before
significant vapor^melt partitioning would occur, and this
could also explain the lack of metal volatility observed in
the OVC rhyolites at depth.
Pressure and ascent rates
The lack of metal partitioning into the vapor during the
ascent of the OVC rhyolites could be due to rapid ascent
rates of the magmas from depth. Studies demonstrating
metal volatility in natural systems have been based on
melt and fluid inclusions in granites (Zajacz et al., 2008) or
melt inclusions in granites and other intrusive rocks
(Aude¤tat, 2010). Therefore, it is possible that the enhanced
vapor^melt partitioning of metals reported in these studies
is in part due to the fact that the samples originate from intrusions that underwent protracted cooling and crystallization in the crust compared with the extrusive OVC
rhyolites. Several observations indicate that the ascent
rates of the OVC rhyolites would have been extremely
rapid. First, estimates based on the lack of reaction rims
on hornblende from the Kaharoa eruption imply rapid
ascent rates of 54 days (Leonard et al., 2002). Second,
NUMBER 8
AUGUST 2013
our melt inclusion data suggest that at least 20^25% crystallization occurred at pressures of 100^200 MPa and
only very minor crystallization and melt inclusion entrapment occurred at lower pressures; these estimates are also
similar to those in other melt inclusion studies from the
OVC (Shane et al., 2007, 2008a; Smith et al., 2010). This
indicates a lack of protracted residence, cooling and crystallization of the melts at low pressures. Furthermore, the
minor crystallization (55%) that occurred post-melt inclusion entrapment implies that the melts must have ascended rapidly to inhibit crystallization during
decompression, which is further supported by the lack of
microphenocrysts observed in the pumice glasses. Thus,
we suggest that the melt affinity of metals in the OVC
melts can be explained in part by the lack of magma residence at low pressures and the rapid ascent rates of the
OVC melts compared with previous observations in intrusions and in experiments.
The rapid decompression and quenching of the OVC
rhyolites studied here could also explain the discrepancy
between our results and the measurements of Cl-rich, Cuand Zn-bearing fluid bubbles from the OVC lavas
(Davidson & Kamenetsky, 2007). The post-trapping fluid
bubbles in quartz analyzed by Davidson & Kamenetsky
(2007) originate from lava dome samples from the OVC,
which would have cooled much more slowly and at much
lower pressures than the melts from the explosive rhyolitic
eruptions. Their data imply that, with sufficient residence
time at low pressure, the OVC rhyolitic melts can become
saturated in a saline, metal-bearing fluid.
Therefore, it seems likely that a combination of factors
can explain the melt-affinity of metals in the OVC rhyolites. First, the low melt Cl/H2O ratios and extensive
degassing of H2O compared with Cl suggest that the
OVC melts would have exsolved an H2O-rich vapor
(not a hydrosaline fluid), which would have impeded
partitioning of Pb and Zn, in particular, into the vapor
at depth and during magma ascent. Second, the probably small amounts of vapor present during crystallization at depth would have impeded vapor^melt
partitioning of the metals. Finally, the rapid ascent rates
and quenching of the OVC rhyolites would probably
have prevented low-pressure exsolution of Cl- and
metal-rich brines.
Rhyolites as a potential source of
metals and other species for the TVZ
hydrothermal systems
Although the hydrothermal waters of the TVZ are dominantly meteoric, magmas could contribute species such as
Cl, B, S and metals to the hydrothermal solutions, both
via leaching and through incorporation of minor amounts
of magmatic vapor (514%; Giggenbach, 1995) into the
hydrothermal fluids (Giggenbach, 1995; Simmons &
1656
JOHNSON et al.
OKATAINA VOLCANIC CENTER RHYOLITES
Brown, 2007). Given the documented minimal contribution
of magmatic vapors to the hydrothermal systems, particularly in the western TVZ (6% magmatic; Giggenbach,
1995), it has been suggested that the metals in the TVZ
hydrothermal solutions are probably derived by leaching
of country rocks, which can be either volcanic or metasedimentary (e.g. Giggenbach, 1995; Simmons & Brown,
2007). The lack of volatility of metals in the OVC rhyolite
magmas, as demonstrated here, suggests that the voluminous rhyolites emplaced in the upper crust could potentially
be such a source of metals, and other species, to hydrothermal fluids. Although andesitic magmas have been hypothesized to be the source of hydrothermal Au, As and Sb,
based on gas emissions from White Island (Hedenquist
et al., 1993; Wardell et al., 2004), our analyses demonstrate
that the concentrations of As in the OVC rhyolitic rocks
(2^8 ppm) are much higher than in the TVZ andesites
(0·6^4·0 ppm) and basalts (0·2^0·8 ppm; Simmons &
Brown, 2007). Additionally, the concentrations of Cl remaining in the rhyolites (0·15^0·20 wt %) are higher
than in the matrix glasses of the White Island andesites
(0·1wt %; Rapien et al., 2003). Thus, our data suggest
that as metals and large proportions of magmatic Cl
remain dissolved in the melt during ascent, degassing and
eruption, the OVC rhyolites could be a potential source of
Cl, metals and other magmatic species through leaching
of the rocks by heated meteoric waters.
relatively low-salinity exsolved vapors; (2) relatively small
amounts of total exsolved vapor present during crystallization and (3) the rapid ascent rates and lack of shallow residence of the OVC magmas, which probably impeded
metal vapor^melt partitioning. The results of our research
imply that, given the remaining metal and Cl concentrations in the rhyolites upon eruption, the voluminous OVC
rhyolites emplaced in the upper crust of the TVZ could be
a viable source of some metals, Cl and other species to the
hydrothermal systems via leaching of the rocks by heated,
meteoric waters.
AC K N O W L E D G E M E N T S
The authors thank Karsten Goemann for assistance with
electron microprobe analyses, Thomas Rodemann for assistance with FTIR analyses at the University of
Tasmania, and Paul Wallace for kindly analyzing H2O in
some melt inclusions by FTIR at the University of
Oregon. Leonid Danyushevsky, Sarah Gilbert and Marcel
Guillong are also thanked for their assistance with LAICP-MS analyses. The authors thank Isabelle Chambefort
for comments on an earlier version of this paper, and Jeff
Mauk and two anonymous reviewers for their insightful
reviews that greatly improved this manuscript.
FU N DI NG
This research was funded by the Australian Research
Council (ARC) Centre of Excellence in Ore Deposits.
CONC LUSIONS
The combination of melt inclusion, pumice glass and mineral geochemistry presented here allows a detailed assessment of the crystallization and degassing histories of the
OVC rhyolites, and the behavior of metals during these
differentiation processes. The OVC rhyolites underwent at
least 20^25% vapor-saturated fractional crystallization at
pressures of 100^200 MPa. During ascent from these
pressures the melts underwent extensive degassing of H2O
and minor degassing of Cl, and little to no crystallization
occurred (5%). During both of these stages of magma
evolution the metals Pb, Zn, As and Mo remained largely
dissolved in the melt and/or partitioned into fractionating
minerals. Decreases in melt Zn during magma crystallization can best be explained by partitioning of Zn into ferromagnesian minerals, whereas minor partitioning of the
Pb, Mo As into ferromagnesian minerals feldspar can
explain the general increases of these metals in the melts
during crystallization. None of the analyzed metals
appear to partition into the vapor during ascent and
degassing, as indicated by the overlap in metal concentrations in the most evolved melt inclusions and pumice
glasses. The lack of volatility of these metals can be
explained by a combination of: (1) the low Cl/H2O contents of the OVC melts and the extensive degassing of
H2O compared with Cl, which would probably result in
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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