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Preferential eruption of andesitic magmas: Implications for volcanic
magma fluxes at convergent margins
ADAM J. R. KENT
College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis,
OR, 97331, USA (e-mail: [email protected])
Abstract: Andesitic magmas (herein defined as having c. 55– 65 wt% SiO2) are abundant in convergent margin volcanoes. Although andesites and other intermediate magmas can plausibly be
produced by a number of mechanisms, a lack of evidence for andesitic liquids relative to the abundance of andesite volcanic rocks, together with widespread evidence for hybridization and
recharge, suggests that many erupted andesites form by mixing between relatively mafic
magmas that ultimately derive from the mantle wedge, and more felsic magmas or mush zones
derived from differentiation and crustal melting.
A model of recharge filtering accounts for the high abundance of andesitic volcanic rocks and
reflects the simple idea that the processes that create hybridized andesitic magmas in the shallow
crust can also initiate volcanic activity, resulting in preferential eruption of andesitic magmas relative to the parental magmas that mix to produce them. This occurs via mafic recharge – intrusion of
a mafic magma into a silicic magma or mush within the shallow crust. The overall abundance and
variability from volcano to volcano of andesitic volcanism at convergent margins also suggests
important roles for hydrous mafic magma in promoting mixing and eruption during mafic recharge,
and the crust for modulating the compositions of volcanic outputs.
Andesitic volcanic rocks represent a significant
proportion of the global eruptive output of convergent margins (Fig. 1; e.g. Ewart 1976; Gill 1981),
and are an important and characteristic component
of subduction zone magmatism – particularly in
continental subduction systems (e.g. Kuno 1968;
Anderson 1976; Ewart 1976; Eichelberger 1978;
Gill 1981; Annen et al. 2006; Hildreth 2007; Reubi
& Blundy 2009). It has also been long recognized
that the bulk composition of the continental crust
is andesitic, suggesting a link between magmatism
in orogenic settings and the growth and evolution
of the continental crust. (e.g. Rudnick 1995). In
addition, andesitic magmas are associated with
important mineral deposits and andesite-dominated
volcanoes pose significant hazards to human populations and infrastructure.
The recognition of the importance of understanding andesite petrogenesis is also matched by
many decades of research and debate regarding the
origin of andesitic magmas – particularly in the
convergent margin settings in which they are so
abundant. Despite this there remains considerable uncertainty regarding the processes by which
andesitic magmas form, and which of these are the
most important in convergent margin settings. Gill
(1981) in his landmark book, together with many
other studies (e.g. Kuno 1968; Anderson 1976;
Eichelberger 1978; Hildreth & Moorbath 1988; Carmichael 2002; Annen et al. 2006; Reubi & Blundy
2009; Kent et al. 2010; Straub et al. 2011), has
emphasized the range of mechanisms by which
andesites and other intermediate composition volcanics may form. These include: derivation of andesitic liquids directly from the mantle via hydrous
partial melting of the mantle wedge or subducted
ocean crust; partial melting of underplated basalts
within the lower crust or uppermost mantle; differentiation of basaltic precursors at the base of
the crust or within the lower crust; assimilation of
sialic crustal material by basaltic magma; and
mixing between mafic magmas derived from the
mantle with silicic magmas produced by differentiation or crustal melting. Hildreth & Moorbath
(1988) and Annen et al. (2006) also argued for an
important role for a combination of differentiation,
assimilation and mixing processes within lower
crustal zones to produce a range of intermediate
liquid compositions, including andesites. Although
examples of andesitic magmas exist that plausibly
formed by all these the mechanisms, resolution of
which process or processes are dominant, and why
andesites are so prevalent in convergent margin settings, remains one of the primary questions in the
field of igneous petrology.
This contribution focuses on the record of
subduction zone magmatic processes preserved
within volcanic sequences. While a detailed evaluation of all the mechanisms of andesite genesis
listed above is beyond the scope of this paper,
some insight can be gained from the abundances
of the different magma compositions that occur in
convergent margins (e.g. Ewart 1976; Gill 1981;
Hildreth 2007; Reubi & Blundy 2009). One clear
From: Gómez-Tuena, A., Straub, S. M. & Zellmer, G. F. (eds) 2014. Orogenic Andesites and Crustal Growth.
Geological Society, London, Special Publications, 385, 257–280.
First published online September 12, 2013, http://dx.doi.org/10.1144/SP385.10
# The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
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258
A. J. R. KENT
Andesite genesis: the role of mixing
(a) Global Arc Compilation (GEOROC)
Percent of Total
Melt Inclusions (n ~ 6500)
Glasses (n ~ 3400)
60
Lavas (n ~ 65,000)
40
Percent of Total
20
60
(b)
Iceland
40
20
60 (c)
Hawaii
40
20
40
45
50
55
60
65
70
75
80
SiO2 (wt%)
Fig. 1. (a) Global compilations of SiO2 contents of
whole rock, melt inclusion and glass (including pumice
and matrix glass) from arc settings. Data are from
GEOROC (georoc.mpch-mainz.gwdg.de/georoc/). (b),
(c) The same comparison for Iceland and Hawaii. Data
are from GEOROC and whole rock, glasses and melt
inclusions consists of c. 3100, 700 and 450 and c. 6100,
3000 and 700 analyses for Iceland and Hawaii,
respectively.
prediction of models that propose andesite genesis via differentiation or melting is that liquids of
andesitic composition should constitute the precursors of the andesitic magmas that are eventually
erupted. Examples of crystal-poor andesitic magmas that are interpreted to represent liquids do
occur (e.g. Wallace & Carmichael 1994; Ownby
et al. 2008; Schmidt & Grunder 2009); however, a
significant majority of andesites from convergent
margin settings are crystal rich (e.g. Ewart 1976;
Gill 1981; Hildreth 2007), and Ewart (1976) estimated that c. 70% of island arc and continental arc
lavas have between 20 vol% and 60 vol% phenocrysts. In such samples it is difficult to demonstrate
a priori that the erupted magmas ever existed as
a true igneous liquid. Note that, for simplicity
herein, the term andesitic is used to denote intermediate volcanics with compositions of c. 55–
65 wt% SiO2 (a compositional range similar to
that used by Gill 1981), although by currently
accepted definition true andesites have SiO2 contents that range from 57–63 wt% and basaltic andesites have SiO2 between 52 wt% and 57 wt%).
Any requirement for andesitic liquids to be the precursors for the andesitic volcanic rocks that erupt at
subduction zones is also challenged by two broad
sets of observations. The first is the presence of textures and other evidence that suggest that shallow
mixing (at ,c.10 km crustal depth) between magmas of diverse origins plays an important role in
the constitution of many erupted andesitic magmas.
In particular, the presence of disequilibrium phase
assemblages, multiple melt compositions, mineral
zoning and multiple populations of the same crystal phase that are not in equilibrium (e.g. Anderson
1976; Eichelberger 1978; Gill 1981; Gourgaud
et al. 1989; Pallister et al. 1992a, 1996; Clynne
1999; Miller et al. 1999; Sato et al. 1999; Tepley
et al. 1999; Patia 2004; Browne et al. 2006; Klemetti & Grunder 2008; Martel et al. 2006; Nakai
et al. 2008; Salisbury et al. 2008; Hirotani et al.
2009; Humphreys et al. 2009, 2013; Kent et al.
2010; Dogan et al. 2011; Koleszar 2011; Coombs
et al. 2013; Koleszar et al. 2012, Ruprecht et al.
2012; Ruprecht & Cooper 2012; Viccaro et al.
2012; Walker et al. 2012; Arce et al. 2013; Costa
et al. 2013) reveal the importance of hybridization
of crystal-bearing magmas (or crystal-rich mushes)
of different bulk compositions. Observations of
active volcanoes also suggest that mixing processes
occur within the shallow crust (,c.10 km; e.g. Pallister et al. 1992a, 1996; Aspinall et al. 1998; Miller
et al. 1999; Nishi et al. 1999; Elsworth et al. 2008;
Foroozan et al. 2010). Although the role of mixing
has been recognized for some time (e.g. Anderson
1976), increasing application of micro-analytical
techniques is promoting a burgeoning awareness
of the importance and extent of hybridization in
many intermediate magmas (e.g. Tepley et al.
1999; Sato et al. 1999; Browne et al. 2006; Martel
et al. 2006; Salisbury et al. 2008; Hirotani et al.
2009; Humphreys et al. 2009, 2013; Smith et al.
2009; Kent et al. 2010; Koleszar 2011; Koleszar
et al. 2012; Ruprecht et al. 2012; Ruprecht &
Cooper 2012; Viccaro et al. 2012; Costa et al.
2013).
Mineral chemistry, melt inclusions, mafic
enclaves and other observations also suggest that
in many cases erupted andesitic magmas are the
result of mixing between parental magmas that
have broadly mafic (basaltic-basaltic andesite) and
felsic (dacitic-rhyolitic) compositions (e.g. Walker
& Skelhorn 1966; Anderson 1976; Eichelberger
1978; Sakuyama 1978; Bacon & Metz 1984; Pallister et al. 1992a, 1996; Clynne 1999; Tepley et al.
1999; Patia 2004; Browne et al. 2006; Salisbury
et al. 2008; Hirotani et al. 2009; Humphreys et al.
2009, 2010; Reubi & Blundy 2009; Christopher
et al. 2010; Kent et al. 2010; Koleszar 2011;
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PREFERENTIAL ERUPTION OF ANDESITIC MAGMA
Koleszar et al. 2012; Almeev et al. in press) rather
than the result of mixing of multiple generations
of more intermediate composition magmas –
although this undoubtedly also occurs (e.g. Smith
et al. 2009; Coombs et al. 2013). This equates to
mixing between magmas that are only moderately
evolved from basaltic compositions derived from
the mantle wedge and silicic magmas derived from
more extensive crustal differentiation and melting (e.g. Hildreth & Moorbath 1988; Annen et al.
2006; Reubi & Blundy 2009). Furthermore, geobarometry and geophysical observations (when
mixing events are associated with eruptions) suggest that mixing typically occurs within the shallow crust (,10 km; e.g. Pallister et al. 1992a,
1996; Nakada et al. 1995; Mori et al. 1996; Aspinall
et al. 1998; Barclay et al. 1998; Devine et al. 1998;
Miller et al. 1999; Elsworth et al. 2008; Foroozan
et al. 2010; Koleszar 2011; Coombs et al. 2013)
and often occurs immediately prior to eruption
(Pallister et al. 1992a, 1996; Nakamura 1995;
Devine et al. 2003; Humphreys et al. 2009; Smith
et al. 2009; Kent et al. 2010).
Secondly, amongst melt inclusion and matrix
glasses’ compositions from subduction zone volcanic rocks – which represent unambiguous samples
of liquid compositions – there is relatively little
direct evidence for andesitic liquids in proportions that appear to match the relatively high abundances of andesitic magmas in convergent margins.
Reubi & Blundy (2009) illustrated this observation
using a compilation of melt inclusion and magma
compositions from selected convergent margins,
although the dataset used was relatively small
(c. 2500 melt inclusions) and thus potentially prone
to being distorted by melt inclusion derived from
a smaller number of individual lava samples. In
light of this the author has re-examined this issue
for a number of selected subduction zones using
updated GEOROC (georoc.mpch-mainz.gwdg.de/
georoc/) melt inclusion, glass and bulk rock compilations. This is shown in Figure 1 and includes more
than 6000 melt inclusion analyses (and represents
over 250 separate localities), more than 3400 glass
analyses (primarily pumice, groundmass glass,
matrix glass and pillow rim glasses) and almost
65 000 whole-rock analyses. The new compilation
is consistent with the original findings of Reubi &
Blundy (2009) and shows that andesitic liquids are
relatively uncommon in subduction zone magmas
relative to melts of more felsic and mafic composition. The lower apparent abundances of andesitic liquids preserved as glasses or melt inclusions
also contrast with the relatively high apparent abundances of andesitic magmas (as reflected
by the number of whole-rock analyses). This difference is dramatic: overall melt inclusions with andesitic compositions appear about one third less
259
common than those ,55 wt% SiO2 (Fig. 2a), but
in terms of bulk magma compositions andesitic
and more mafic compositions are broadly sub-equal
(each represents c. 40% of all magmas sampled).
Other magmatic environments, such as the examples from Hawaii and Iceland shown in Figure 1b
and c, show much closer correspondence between
melt inclusion and erupted lava compositions.
Further, although melt inclusion compositions
can potentially be modified by a range of postentrapment processes (e.g. Kent 2008), the observation that melt inclusion patterns are independent
of host mineral (e.g. Reubi & Blundy 2009), and
have compositions that broadly parallel those evident in other glasses (although melt inclusions
have a higher proportion of mafic compositions;
Fig. 2), suggests inclusion modification has had
little effect.
Although these compilations are based on the
number of published analyses and thus do not specifically reflect magma volumes, the large amount
and geographical spread of the data suggest that
they are a reasonably robust measure of the overall output of convergent margin systems. In addition, published estimates of eruptive volumes for
arcs or arc segments, such as the Cascades and
parts of the Mexican Arc (Fig. 2b; Hildreth 2007;
Ownby et al. 2011), also suggest that erupted
volumes of andesites are high relative to their apparent low abundance in melt inclusions or glasses,
particularly where much of the volume of erupted
material occurs in large volcanic edifices (Gill
1981; Hildreth 2007). Individual arc segments or
smaller volcanic fields can show greater variation
and can be dominated by more dispersed mafic volcanism (e.g. Priest 1990; Ownby et al. 2008). In
addition a comparison between abundances based
on the number of published estimates and volume
estimates derived from Hildreth (2007) for the
major Cascades volcanic centres shown in Figure
2b shows a high degree of consistency between
these two datasets. Volume estimates that also
include dispersed volcanism in the Cascades show
higher proportions of mafic compositions relative
to the equivalent number of published analyses
(Fig. 2b), suggesting an overall bias against this dispersed mafic style of volcanism in available data
compilations, but still show higher abundances of
andesites relative to more felsic compositions.
Collectively the above evidence infers that, as
noted by Reubi & Blundy (2009), there is a dearth
of andesitic liquids in subduction zone settings,
relative to their abundance as magmas. This in
turn suggests that magma mixing and hybridization plays an important role in forming many andesitic magmas – as, unlike the other mechanisms for
andesite genesis listed above, these do not require an
andesite liquid that is the precursor to an andesite
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260
A. J. R. KENT
magma. This issue has been has been discussed in
a number of previous studies (Anderson 1976;
Eichelberger 1978; Eichelberger et al. 2006; Reubi
& Blundy 2009; Kent et al. 2010) and, given the
widespread evidence for magma mixing in andesitic systems, a case can be made for mixing being
an important, and perhaps dominant, process in the
formation of andesites in convergent margins (but
see other papers in Gómez-Tuena et al. (2014) for
alternative viewpoints). However, if this is the case
there remains an important outstanding issue to
resolve. Namely: why should andesitic volcanic
rocks be so common in convergent margin environments relative to their apparent low abundances as
liquids? Mafic and felsic magmas occur widely in
other tectono-magmatic settings (as do andesitic or
equivalent magmas, but in subordinate proportions,
Gill 1981; e.g. Fig. 1b, c), but abundant andesitic
volcanics occur only in convergent margins. In
effect this requires that andesitic magmas erupt in
significantly greater quantities than they are produced and preserved as liquids, and that andesitic
magmas must be preferentially erupted with
respect to the more mafic and felsic compositions
that appear more frequently as liquid compositions. This contribution addresses these issues and
explores the processes that control the formation
and eruption of andesitic magmas, building on earlier studies (e.g. Eichelberger 1978; Marsh 1981;
Eichelberger et al. 2006) and in particular expanding the treatment presented by Kent et al. (2010).
Magma mobility within the crust
Percent of Total Analysis Number
(a)
Arc Summary
Glasses
60
40
20
60
Melt Inclusions
40
20
60
40
Whole Rock
20
mafic
40
andesitic
50
60
70
80
SiO2 (wt%)
(b)
Percent of Total Erupted
Volume (or Total Analyses)
felsic
Quaternary Cascades
40
20
Volume including
dispersed volcanism
Number of Cascade Lava Analyses ( GEOROC n= 2617)
40
20
Volume for major volcanic centres
40
50
60
70
80
SiO2 (wt%)
Percent of Total
Erupted Volume
(c)
Tancítaro–Nueva Italia
Mexico
60
Mascota Volcanic Field
20
40
50
60
SiO2 (wt%)
70
It has long been recognized that volcanic rocks
represent a specialized subset of magmatic rocks
– namely those that are able to erupt (Stolper &
Walker 1980; Sparks et al. 1980; Marsh 1981).
The physical properties of magmas that control
mobility within the crust vary substantially and,
across the wide spectrum of known igneous compositions, magmas exhibit considerable differences in
their ability to be mobilized and to eventually erupt.
For silicate liquids with SiO2 contents between
basalt and rhyolite, density varies by more than
25% and viscosity varies by at least eight orders of
magnitude (e.g. Giordano et al. 2008; Takeuchi
2011). Larger variations in density and viscosity
80
Fig. 2. (a) SiO2 contents of whole rock, glasses and melt
inclusions from analyses of arc samples binned into
mafic (,55 wt%), andesitic (55–65 wt%) and felsic
(.65 wt%) compositions. (b) Estimates of erupted
volumes for major Cascade volcanic centres based on the
work of Hildreth (2007). Lines are shown representing
both the volcanic centres alone (boxes include the
maximum and minimum estimates) and volcanic centres
together with the dispersed and predominantly mafic
volcanism (the single line reflects the estimated value).
Compositional ranges used follow those of Hildreth
(2007) and differ slightly from those used in the rest of
this paper). The estimates of Hildreth (2007, table 6)
were converted to volumes for each volcanic centre
using the following rubric: Hildreth’s estimates of A
(Andesite) . B (Basalt) was taken to be equivalent to
67% andesite and 33% basalt; A ≫ B was taken as 90%
andesite; the composition of dispersed magmatism was
estimated to be 80% basalt and 20% andesite. Also
shown is the total number of GEOROC analyses in each
compositional range. (c) Erupted volume estimates for
the Mascota volcanic field and the Tancı́taro– Nueva
Italia region in the Mexican Arc from Ownby et al.
(2008, 2011). Compositional ranges used follow those in
the cited sources.
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PREFERENTIAL ERUPTION OF ANDESITIC MAGMA
20
Percent of Total
will also occur for magmas that are crystal or bubble
bearing. As a result, any process of igneous differentiation will produce derivative magmas whose
ability to be mobilized may be quite different from
that of their parents. Given that mobility is required
for magmas to reach the surface and erupt, there is
likely to be a strong compositional bias in volcanic
rocks towards those compositions that are able to
erupt. In addition, magmas that are formed by processes that favour magma mobility will also be
preferentially represented in the volcanic record.
Collectively these effects are described as the eruption filter, and by definition the volcanic record
consists only of magmas that have successfully passed through this filter. In earlier studies Stolper &
Walker (1980) and Sparks et al. (1980) discussed the
role of density filtering in producing the spectrum of
erupted mid-ocean ridge basalt and other basaltic
rocks. Marsh (1981) focused on the role of crystallinity and its effect on viscosity in terms of controlling the eruption of more evolved magmas,
including andesites. More recently the concept of
eruption filtering has received less attention.
This issue becomes all the more germane when
it is considered that only a relatively small fraction
of magmas present within subvolcanic magma
storage zones are ever erupted. Although it is a difficult parameter to constrain precisely, current
estimates place the ratio of extruded to intruded
magmas in various crustal settings at c. 3–30%
(Hildreth & Moorbath 1988; White et al. 2006).
Given this, and the likelihood that the composition
and mode of formation of magmas is likely to
strongly influence their mobility within the crust,
it is highly likely that the compositions of volcanic
rocks alone provide an incomplete record of the
range of magma compositions produced and present in a given crustal-scale magmatic system. Magmas that are more mobile, or that are formed by
processes that also initiate eruptions, are likely to
be over-represented, and those magmas that do
erupt are probably strongly biased in favour of eruptible compositions.
A corollary of this conclusion is that conditions
within the crust itself – such as the thermal and
stress state, composition, density and strength –
will influence magma mobility, and thus can exert
a significant control on the compositions of rocks
erupted in a given region or from a given magmatic system. As crustal conditions and lithology
can also vary over short lateral length scales it is
likely that the influence of the crust varies considerably – within individual subduction zone segments
or even between neighbouring volcanoes. This
effect is evident when looking at the variations in
magma compositions of rocks erupted in volcanoes
along a given arc, where considerable variations in
the range and proportions of erupted compositions
261
Mount St. Helens (n=240)
20
Mount Hood (n=107)
12
Mount Jefferson (n=48)
44
48
52
56
60
64
68
72
76
SiO2 (wt%)
Fig. 3. Histograms showing the number of analysed
compositions in the GEOROC database for Mount
St Helens, Mount Hood, and Mount Jefferson. Data for
Mount Hood also include data from Woods (2004),
Kent et al. (2010) and Koleszar (2011).
are apparent – even in adjacent volcanoes (e.g.
Fig. 3; Hildreth & Moorbath 1988; Hildreth 2007;
Klemetti & Grunder 2008; Kent et al. 2010).
Within individual arc segments it is unlikely that
the overall forcing factors that control melting during subduction (convergence rate, slab age, slab
and wedge thermal structure) vary substantially,
and are unlikely to drive major compositional differences between nearby volcanoes. Local crustal
conditions, and the interplay between these and
variations in magma flux, should thus play an important role in dictating the compositions of volcanic
rocks erupted.
Eruption filtering and the origin
of andesitic magmas
Kent et al. (2010) argued that eruption filtering
plays an important role in andesitic volcanism in
convergent margins, and identified a critical role
for mafic recharge, magma mixing and hybridization in andesitic volcanism. This idea originated
from a study of Mount Hood, Oregon, and this andesitic volcano provides a compelling case for eruption filtering of this style.
Mount Hood – a case study of a
compositionally restricted andesitic
volcano controlled by mafic recharge
Mount Hood, Oregon, forms part of the Cascadia
subduction zone, and represents a volcanic system
whose behaviour appears fundamentally controlled
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262
A. J. R. KENT
by recharge and magma mixing. As such, it is useful
here to briefly summarize the characteristics of the
volcano and the magmas it erupts. The following
section and Table 1 summarize a number of previous studies (e.g. Cribb & Barton 1997; Scott
et al. 1997; Woods 2004; Darr 2006; Kent et al.
2010; Koleszar 2011; Eppich et al. 2012; Koleszar
et al. 2012).
The current edifice of Mount Hood has been
constructed over the last c. 500 ka, with the most
recent growth phases occurring between 15 ka
and12 ka (Polallie), c. 1500 years (Timberline)
and c. 220 years (Old Maid) before present (Scott
et al. 1997). In comparison with many other subduction volcanoes, Mount Hood exhibits a marked
degree of homogeneity in the compositions of
erupted magmas, and contrasts with other nearby
Cascade volcanoes (Fig. 3). Some 95% of the lavas
erupted over the life of the current edifice have
SiO2 contents that range between 58 wt% and 66
wt%, (Figs 3 & 4), and lie primarily within the andesite field (although some compositions extend to
low SiO2 dacite). Erupted lavas are crystal-rich
and mineralogically similar, being dominated by
plagioclase with lesser amounts of amphibole, pyroxene, oxides and olivine.
Mount Hood lavas exhibit a range of textural,
mineralogical and composition features that suggest that magma mixing and hybridization play a
dominant role in petrogenesis.
† The presence of distinct populations of plagioclase and amphibole – the dominant modal
phases (Kent et al. 2010; Koleszar 2011; Koleszar et al. 2012). Kent et al. (2010) defined two
populations of plagioclase on the basis of
kinked crystal size distributions (CSD), textural
form and chemical compositions (Figs 5 & 6).
Population 1 plagioclase are smaller, elongate
or acicular in form, show little chemical zoning
and have higher Fe, Ti, Sr contents and lower
Sr/Ba and Sr/light rare earth elements (LREE)
ratios consistent with derivation from a more
mafic melt. Population 2 plagioclase are larger
and more tabular, often exhibit complex internal zoning and have lower Fe, Mg, Ti, Sr/Ba
and Sr/LREE, consistent with formation from
more felsic melts. Zoning profiles, specifically
thin An-rich rims (with higher Fe, Mg, Ti, Sr
and Sr/Ba) on Population 2 plagioclase (Fig.
6), show that the two plagioclase populations
were juxtaposed at a late stage of magmatic
development, immediately prior to eruption.
230
Th– 226Ra disequilibria measurements also
suggest that each plagioclase population has distinctly different ages (Eppich et al. 2012), with
the larger Population 2 grains, having average
ages of .4.5– 5.5 ka, with cores that extend to
older than 10 ka (the oldest resolvable age for
the 230Th – 226Ra system). The ages of smaller
Population 1 plagioclase are more difficult to
estimate but are ,3 ka and likely close to the
eruption ages. Two populations of amphibole
are also apparent, with magnesiohornblende
crystallized from mafic melts formed at 10 –
16 km depth and at temperatures of c. 900–
1000 8C, and tschermakitic pargasites from
more felsic melt at 3.5–7 km and c. 800–870 8C
(data from Koleszar (2011) recalculated using
the calibration of Ridolfi & Renzulli 2012).
† Quenched mafic enclaves are widespread and
occur in almost all lavas. These typically have
quenched groundmass textures, lower SiO2 contents than their host lava (Fig. 4) and higher
proportions of phenocrysts derived from mafic
melt, although there is also evidence for substantial crystal exchange. Enclaves are viewed
as partially hybridized samples of unmixed
mafic melt left following recharge and mixing
(e.g. Bacon 1986).
† Whole-rock compositions of Mount Hood lavas
define linear trends on bivariate plots between
major element oxides (Fig. 4), suggesting mixing between mafic and felsic magmas. The compositions of the endmember compositions that
define these arrays have remained broadly constant through time (although there is greater
diversity for the mafic magma in terms of trace
element compositions; Woods 2004; Koleszar
2011). Plots of highly incompatible v. highly
compatible trace elements (e.g. Rb v. Cr; Fig. 7)
also show linear or fan-shaped trends rather than
the strongly curved trends predicted by crystal
fractionation.
In order to investigate the dynamics of magmatic
systems dominated by recharge and mixing it is
important to establish the compositions of the parental magmas involved (e.g. Sparks & Marshall
1986). However, this can also be difficult as, even
with the simple binary mixing such as that observed
at Mount Hood, it is easy to define a line along
which parental compositions must lie – which is
simply the extension of the mixing array outside
the compositional space defined by mixed lava
compositions – but more difficult to establish
exactly where along these arrays the parental compositions lie. Previous studies on other systems
have used mass balance approaches based on mineral chemistry, whole-rock analyses, glass inclusions and quenched mafic enclaves (e.g. Nakamura
1995; Holtz et al. 2005; Humphreys et al. 2013),
although these approaches also have their limitations. At Mount Hood major element oxides are
highly correlated with the proportion of Population
1 plagioclase (that derived from a mafic source)
Mount Hood
At least 2.5 ka and possibly
up to 14 ka
c. 175 ka
54– 66 wt% SiO2
(Timberline and Old Maid
periods, c. 1500 and c. 220
years BP c. 62 –64 wt%
SiO2)
50– 60 wt% SiO2
58 –67 wt% SiO2
(1992 –1995: 59 –65 wt%
SiO2)
57 –66 wt% SiO2
(1902 –1930: 58 –66 wt%
SiO2)
58 – 68 wt% SiO2
(1995 –2007: 59– 65 wt%
SiO2)
51 –59 wt% SiO2
51 –58 wt% SiO2
51 – 55 wt% SiO2
Plagioclase, amphibole,
orthopyroxene,
clinopyroxene (Fe– Ti
oxides, olivine, apatite)
Plagioclase, orthopyroxene;
clinopyroxene; amphibole,
(Fe –Ti oxides, olivine)
Plagioclase, amphibole,
orthopyroxene, Fe –Ti
oxides
c. 25 –60 vol%
68– 76 wt% SiO2
Plagioclase, amphibole,
biotite, clinopyroxene,
orthopyroxene, quartz,
magnetite (Fe– Ti oxides,
olivine)
17 –48 vol%
72 –77 wt% SiO2
30 –50 vol%
67 –72 wt% SiO2
c. 40 vol%
65 – 70 wt% SiO2
Magnesiohornblende
100– 200 MPa (3.5– 7 km)
790 –900 8C
110 –160 MPa (3.9 –5.6 km)
790– 850 8C
100 – 180 MPa (3.5 – 6.3 km)
c. 850 8C
100 – 160 MPa (3.5 – 5.6 km)
720 –850 8C
Tschermakitic
pargasite
300– 500 MPa
(c. 10.5– 17.5 km)
910 –9008C
1.8– 8.8 km
300 –400 MPa (10.5 – 14 km)
900– 950 8C
340 – 500 MPa (12 –17.5 km)
910 –1050 8C
350 – 530 MPa
(12.3 –18.6 km)
910 –950 8C
5 –7 km
Shallow
Deep
Shallow
5 –8 km
c. 11 km
Deep
150 – 250 MPa (5.2 – 8.8 km)
12 – 17 km
50 – 100 MPa (1.8 –3.5)
PREFERENTIAL ERUPTION OF ANDESITIC MAGMA
Phase equilibria constraints
on magma storage
Soufriére Hills
c. 500 ka*
Range of mafic inclusion
compositions
Dominant modal phases
Geophysical estimates of
magma residence depth
Mont Pelée
c. 500 ka*
Duration of
andesitic-dominated
eruptions
Range of erupted magma
compositions†
Crystallinity
Estimated resident magma
composition (see text
for method)
Estimated temperature and
pressure from amphibole
compositions‡
Mount Unzen
c. 400 MPa (c. 14 km)
See text for references unless noted.
*This is the life of the current edifice.
†
Total range and most recent eruptions are shown. For Mount Hood this includes the two most recent prehistoric eruptive phases.
‡
Estimates are from data summarized by Koleszar (2011) and calculated using the calibrations of Ridolfi & Renzulli (2012).
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Table 1. Summary of erupted magmas and magma storage conditions for Mount Hood, Mount Unzen, Mont Pelée and Soufriére Hills volcanoes
263
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264
A. J. R. KENT
Total Alkalis (wt%)
7
Calculated felsic
endmember
magma
Lavas (K&K Data)
Enclaves (K&K Data)
Lavas (other sources)
Enclaves (other sources)
6
5
4
3
endmember
magma
CaO (wt%)
10
8
6
4
TiO2 (wt%)
2.0
1.2
0.6
MgO (wt%)
6
5
4
3
2
1
SiO2 (wt%)
Fig. 4. Total alkalis, CaO, TiO2 and MgO v. SiO2 for Mount Hood lava and mafic enclave samples. Data from this study
are reported in Kent et al. (2010) and Koleszar (2011) and are identified separately (‘K&K Data’) as all come from the
same analytical facility; data from other sources are from GEOROC. Calculated mafic and felsic parental magma
compositions and uncertainties are from Kent et al. (2010).
present in each lava (as estimated from CSD
measurements). This is precisely what would be
expected from mixing of two parental magmas of
differing crystallinity and composition. Extension
of these regressions to 0 and 100% of Population 1
plagioclase provides an estimate of the parental
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PREFERENTIAL ERUPTION OF ANDESITIC MAGMA
(a)
265
10
Population 1 (0.0448 - 0.448 mm)
Mid Interval (mm)
5
0
Population 2 (0.71 - 2.82 mm)
-5
Plagioclase Crystal Size Distribution
-10
0
1
2
3
Mid Interval (mm)
M4
An70
40
M2 M1
An70
Sr/Ba
T C
M1
M2
M3
M4
1000
1000
1000
800
12000
9000
6000
9000
2.5
2.5
2.0
2.5
F1
F2
F3
800
800
1000
4200
3000
4200
1.0
0.7
1.0
{
{
An60
M3
30
Model
Mafic
(b) 50
Population 2
WR-0501, MH-08-12
HM-0401
PD-0401, PD-0501
MH-08-08
Felsic
Population 1
MH-08-12
HM-0401
PD-0401,PD-0501
MH-08-08
CC-0401
An90
F1
Ti µg/g Sr/Ba
An90
20
Rims
F2
An80
An30
F3
An30
An30
An90
10
An30
An30
An30
0
0
200
Population 2
400
An30
600
Population 1
800
Ti (μg g–1)
Fig. 5. Textural and compositional populations in plagioclase from Mount Hood lavas. (a) Crystal size distributions
(CSD) for plagioclase from eight lavas ranging in age from c. 470 000 to 220 years (Kent et al. 2010). Populations 1 and
2 plagioclase are defined on the basis of crystal length as shown. Solid lines represent linear regressions for crystals from
0.0448–0.448 mm (Population 1) and 0.710– 2.82 mm (Population 2). (b) Sr/Ba v. Ti (mg g21) measured in
Populations 1 and 2 plagioclase crystals, modified from Kent et al. (2010). Also shown are curves representing
equilibrium partitioning between plagioclase and a range of representative mafic and felsic melt compositions
(see inset table). Anorthite contents of equilibrium plagioclase are shown starting at An30 (see Kent et al. 2010 for more
details). Each point marked along the curves represents 10 mol. percent higher An content. Open circle symbols with dot
in the middle represent rims to Population 2 plagioclase crystals. Data and further sample details are in Kent
et al. (2010).
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266
A. J. R. KENT
Fig. 6. Backscattered electron (BSE) photos of typical textures from four Mount Hood lava samples (see Kent
et al. 2010 for sample details). Anorthite (An) and total iron (FeO*) contents are marked for selected points.
Morphologies representing Populations 1 and 2 plagioclase are shown for HM-04-01. All other photos show Population
2 plagioclase.
magma compositions involved (Kent et al. 2010),
and shows that these parental magmas have SiO2
contents of 50 + 4 and 72 + 4 wt% SiO2 (the
M
Fractional Crystallization
Cr (μg g–1)
40
uncertainties reflect the uncertainties in regressions). Note that all other lavas lie along similar
trends in bivariate plots of major element
30
20
10
Lavas (K&K Data)
Lavas (other sources)
f=0.2
0.4
0
0
20
0.6
40
0.8
60
80
100
Rb (μg g–1)
Fig. 7. Rb v. Cr for Mount Hood lavas. Data sources as per legend and Figure 4. Also shown is a calculated fractional
crystallization trend of representative mafic melt (M) with and initial composition of 15 mg g21 Rb and 35 mg g21 Cr.
Calculations used bulk mineral-melt partition coefficients of DRb ¼ 0.01 and DCr ¼ 13, calculated for crystallization of
an assemblage consisting of 0.55 (number refers to modal mass fraction) plagioclase, 0.15 orthopyroxene, 0.05
clinopyroxene, 0.20 amphibole, and 0.05 titanomagnetite, using mineral-melt partition coefficient from Rollinson
1993). Crystal fraction values ( f ) are marked on the fractional crystallization trend.
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PREFERENTIAL ERUPTION OF ANDESITIC MAGMA
composition (e.g. Fig. 4), suggesting that broadly
similar parental magma compositions are common
to all magmas erupted at Mount Hood.
The estimated compositions of parental magmas from Mount Hood are shown on the major
element oxide v. silica plots in Figure 4. Importantly
there is little or no overlap between the erupted
magma compositions and those of the parental
magmas. Mafic enclaves extend to more mafic
compositions and are close to the estimated mafic
parental magma, although only cumulate texture
enclaves fall within the mafic parent magma
range. Thus, although mafic and parental magmas
demonstrably contribute to each of the erupted
lavas at Mount Hood they appear to never erupt
by themselves, and an obvious conclusion is that,
although both mafic and felsic precursors are key
participants in magma genesis, they are themselves
unable to erupt.
Another feature of Mount Hood lavas that is
important for the interpretation of the petrogenesis
are mineral zoning profiles that reveal the timing
of magma mixing events. This is most evident in
the larger Population 2 plagioclase crystals that
exhibit narrow (,c. 50 –100 mm) rims of plagioclase with higher anorthite contents (c. An50 – 70)
and higher Mg, Fe, Ti, Sr and Sr/Ba (Figs 5 & 6).
These rims have compositions that approach those
of the smaller Population 1 plagioclase that derive
from the mafic parental melt, and are the result of
plagioclase growth following mixing, whereas the
interior of Population 2 plagioclase grew within
the felsic parental magma. The compositions that
are evident as rims are not observed within the
interior of Population 2 plagioclase grains, suggesting that the only time that mixing occurs is
immediately prior to eruption – and thus strongly
implies that mixing and eruption are linked processes. Diffusion of Mg across the high An rims
on Population 1 crystals can be used to estimate
the time at which rims remained at high temperature prior to eruption and quenching, and this timescale ranges between a few days to a few weeks
(Kent et al. 2010).
Mixing proportions
One additional feature of the homogeneity of
Mount Hood magmas that must be explained is the
broad constancy of the proportions with which
mafic and felsic parental melts are combined to
produce the erupted andesitic magmas. A homogeneous series of magmas will not necessarily
result from mixing processes alone, as theoretically
any composition along the length of the mixing
array can be produced. Thus, there must be other
mechanisms that restrict the possible range of mixing proportions (Eichelberger et al. 2006). From
267
the estimated parental magma compositions at
Mount Hood it is possible to calculate the proportion
of felsic magma added to mafic ( ffelsic) to produce
each magma composition. As shown in Figure 8
these are quite restricted; over 90% of the erupted
lavas have ffelsic values that lie within 0.5 + 0.2
(calculated using SiO2 contents, although mixing
proportions estimated using other major elements
are similar). Thus, not only is it apparently difficult
for the endmember parental mafic and silicic magmas to erupt at Mount Hood, but it also appears
that mixed magmas with relatively small admixtures
(,20%) of either parental melt either do not form
or, if they do, are also unable to erupt.
The apparent restriction of mixing proportions
probably reflects, at least in part, the requirement
that any two magmas must have similar viscosities
after thermal equilibration in order to promote
efficient mixing and hybridization (e.g. Sparks &
Marshall 1986; Koyaguchi & Blake 1989; Jellinek
et al. 1999; Jellinek & Kerr 1999). Sparks &
Marshall (1986) suggest that there is a ‘window’
of mixing proportions, controlled by the temperature and crystallinity of the mixing magmas,
where hybridization is strongly favoured. Application of the approach set out by Sparks & Marshall
(1986) to Mount Hood using the calculated parental mafic and silicic magma compositions (Fig. 8)
suggests that the mixing proportions where the
thermally equilibrated parental magmas will have
similar viscosities (and where the silicic parent is
crystal-rich) closely match the peak in the observed
mixing proportions. Thus the ability of magmas to
mix and hybridize during recharge probably exerts
an important influence on the composition of
hybrid magmas erupted at Mount Hood and other
andesitic volcanoes.
Furthermore, the observation that only hybridized magmas erupt at Mount Hood suggests that
there is also a link between magma mobility and
efficient magma mixing. One simple option, as
noted by Kent et al. (2010) is that the mixed magmas occupy a ‘goldilocks’ zone in terms of physical
properties that allow them to be sufficiently mobile
to erupt. Addition of small amounts of mafic material to felsic magmas, or vice versa, may not sufficiently modify viscosity or density to surmount the
same barriers to eruption that restrict the parental
magmas. Several studies (Ruprecht & Bachmann
2010; Takeuchi 2011; Koleszar et al. 2012) have
highlighted the role of recharge by hotter mafic
magmas in reducing the viscosity of cooler crystalrich magma stored at shallow crustal levels. For
Mount Hood the estimated thermal effects of
recharge reduce the viscosity of silicic parental
magma by a factor of four (Koleszar et al. 2012).
In this instance the effectiveness of recharge in
reducing the viscosity may also play an important
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268
A. J. R. KENT
(a) 10
Mafic parental magma
logη (Pa s)
8
ystals)
s (50% cr
l magma
ta
n
re
a
p
Silicic
)
o crystals
agma (n
rental m
a
p
c
ci
ili
S
6
4
2
(b) 12
mean = 0.50
Frequency
standard deviation = 0.11
8
median = 0.51
4
0
0.2
0.4
0.6
0.8
1
ffelsic (mixing fraction)
Fig. 8. (a) Calculated viscosity (log h) for mafic and felsic parental magma compositions v. ffelsic, the proportion of
felsic parental magma present in mixed magmas. Curves represent the viscosities after thermal equilibration but prior to
hybridization and were calculated following the method outlined in Sparks & Marshall (1986). The approach of
Giordano et al. (2008) was used to estimate the silicate liquid viscosities and the MELTS algorithm used to estimate the
crystallinity of the mafic parent magma during cooling below a liquidus temperature of 1144 8C. Calculations were
performed for 50% and zero crystals for the felsic parent composition. (b) Calculated mixing proportions of felsic and
mafic parent magmas contributing to erupted Mount Hood magma compositions, based on SiO2 contents and the
estimated parental magma compositions from Kent et al. (2010).
role in promoting effusive styles of eruption
(Ruprecht & Bachmann 2010; Koleszar et al. 2012).
Other examples of compositionally
restricted andesitic volcanoes
Several other well-characterized intermediate volcanic systems are remarkably similar to Mount
Hood, in that they produce compositionallyrestricted andesitic magmas over significant periods (several thousand years or more), but erupt
magmas with clear evidence of mixing between
mafic and felsic parental magmas during mafic
recharge. Collectively, these volcanoes are described here as compositionally restricted andesitic
volcanoes. Three volcanoes of this type – Mount
Unzen, Mont Pelée and Soufriere Hills – are particularly well known, partly due to historic or
recent eruptive activity.
The compositional range of lavas and mafic
enclaves from Mount Unzen, Soufriére Hills and
Mont Pelée are shown in in Figure 9 and summarized in Table 1. In these three volcanoes the longterm eruptive output is dominated by andesitic
magmas (e.g. Gourgaud et al. 1989; Smith &
Roobol 1990; Murphy et al. 1998, 2000; Nakada
& Motomura 1999; Venezky & Rutherford 1999;
Pichavant et al. 2002; Zellmer et al. 2003; Sugimoto
et al. 2005; Davidson & Wilson 2011), with SiO2
contents typically between 58 wt% and 66 wt%.
Most erupted magmas also contain abundant
quenched mafic enclaves with compositions that
are more mafic than their host magmas (50–59
wt%, Table 1, Fig. 9 (Gourgaud et al. 1989; Smith
& Roobol 1990; Nakada et al. 1999; Murphy et al.
2000; Browne et al. 2006)). Erupted magmas lie
on linear compositional arrays and also contain
multiple mineral populations, consistent with
magma mixing (e.g. Fig. 9; Gourgaud et al. 1989;
Smith & Roobol 1990; Nakada et al. 1999, Sato
et al. 1999; Venezky & Rutherford 1999; Browne
et al. 2006; Martel et al. 2006; Humphreys et al.
2009, 2010; Koleszar 2011). Zoning patterns in
plagioclase, oxides and other minerals also show
the late influence of mafic recharge as an eruption trigger (Devine et al. 2003, Nakamura 1995;
Murphy et al. 2000; Stewart & Fowler 2001;
Browne et al. 2006; Humphreys et al. 2009). Estimates of the proportions and compositions of the
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PREFERENTIAL ERUPTION OF ANDESITIC MAGMA
269
Total Alkalis (wt%)
(a)
6
Estimated felsic
parental magma
5
Lavas > ~2.5 ka
Lavas < ~2.5 ka
1902-1932
Mafic enclaves
4
3
Total Alkalis (wt%)
(b)
6
Estimated felsic
parental magma
South Soufriére Hills
5
4
Older lavas (< ~300 ka)
1995- present
Mafic enclaves
3
(c)
Total Alkalis (wt%)
6
77
Estimated
wt.%
felsic
parental
magma
5
4
Older lavas
1991-1995
Mafic enclaves
3
50
55
60
65
70
SiO2 (wt%)
Fig. 9. Total alkali v. SiO2 for three other compositionally restricted andesitic volcanoes. (a) Mont Pelée, Martinique
(data sources: Gourgaud et al. 1989; Smith & Roobol 1990; Pichavant et al. 2002). Lavas from the most recent
phase of volcanism (younger than c. 2.5 ka; Smith & Roobol 1990) represent a series of crystal-rich andesites. Older
lavas are more variable and more mafic (note that Gourgaud et al. (1989) and Pichavant et al. (2002) consider that
the homogeneous andesitic eruptions commenced at c. 14 ka.) (b) Soufriére Hills, Montserrat (data sources: Murphy
et al. 1998, 2000; Zellmer et al. 2003). (c) Mount Unzen, Japan (data from GEOROC). In all plots the range of
SiO2 estimated for felsic parental melts following the approach in the text is shown by a grey field. For (c) the
composition of this field extends off scale to 77 wt% SiO2.
magmas involved in mixing vary (e.g. Nakamura
1995; Murphy et al. 2000; Pichavant et al. 2002;
Devine et al. 2003; Holtz et al. 2005; Sugimoto
et al. 2005; Humphreys et al. 2013). However, recognition of the ubiquitous presence of microlites
and microphenocrysts derived from the recharging
mafic magma suggests that the proportion of mafic
magma involved may be greater than previously
considered (Browne et al. 2006; Martel et al.
2009; Humphreys et al. 2009, 2013), and that the
resident magma is also more evolved in composition. A rough estimate for the composition of
resident magmas can be made by assuming that mixing to produce the erupted magmas occurs in
broadly 50:50 proportions of recharge and resident
magma, as suggested by the dynamics of mixing
between mafic and felsic magmas (e.g. Sparks &
Marshall 1986), and as occurs at Mount Hood (see
discussion above; Fig. 8). For Mont Pelée, Soufriére
Hills and Mount Unzen resident magma compositions can be estimated by assuming ffelsic ¼ 0.5–
0.6, by using the composition of mafic enclaves
(51 –55 wt% SiO2) as the composition of the mafic
recharging magma, and the median values of 61,
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270
A. J. R. KENT
62 and 65 wt% SiO2 of erupted lavas from the most
recent eruptive episodes (Fig. 9). This approach
suggests bulk rock SiO2 contents of the resident
magmas of 67– 72, 65 –70 and 72 –77 wt% for
Mont Pelée, Soufriére Hills and Mount Unzen,
respectively (also see Table 1). These ranges
overlap with the composition of the felsic parental
magma given above for Mount Hood, and suggest
that resident magmas are relatively silicic in all
these systems (ranging overall from dacite to rhyolite) and rarely, if ever, erupt directly. Our estimates
are more SiO2-rich than previous studies that
suggested that resident magmas were broadly similar to the composition of erupted andesites (e.g.
Nakamura 1995; Murphy et al. 1998, 2000; Pichavant et al. 2002; Devine et al. 2003) – although
many of these estimates were made prior to the
realization of the extent of mafic material present
within mixed magmas. For example, recent work
by Humphreys et al. (2013) suggests that there is
c. 6% of ‘cryptic’ mafic-derived crystalline material
in Soufriére Hills andesite (not including those in
quenched mafic enclaves). Although it is difficult
to estimate the amount of mafic liquid that must
accompany these crystals, it is likely significant.
Using the typical crystallinities observed in arc
basalts of 20 –30% (Ewart 1976) suggests that the
composition of the resident magma (calculated
using the same compositions for mafic enclaves
and andesites used above) has SiO2 contents
between 65 wt% and 68 wt%.
Amphibole barometry also reveals remarkably
similarities in magmatic architecture between
Mont Pelée, Soufriére Hills, Mount Unzen and
Mount Hood. Two compositionally-distinct populations of amphibole occur in erupted magmas
from all these volcanoes (Table 1; Fig. 10; Koleszar
2011): (1) tschermakitic pargasite which crystallized from more mafic magma (molar Al/Simelt
c. 0.3–0.4) at 900 –990 8C and 300– 500 MPa and
(2) magnesiohornblende which crystallized from
770 to 870 8C and ,200 MPa from a more silicic
host magma (molar Al/Simelt c. 0.15–0.25). Note
that all amphibole classifications given herein use
the nomenclature scheme of Leake et al. (1997)
and Ridolfi et al. (2010), and calculation of pressure, temperature and molar Al/Simelt follows the
methods given in Koleszar (2011) and Ridolfi &
Renzulli (2012). Estimates of the depth of magma
storage from phase equilibria, gas and geophysical
observations (e.g. Nakada et al. 1995; Aspinall
et al. 1998; Barclay et al. 1998; Devine et al.
1998; Nishi et al. 1999; Pichavant et al. 2002; Elsworth et al. 2008; Christopher et al. 2010; Foroozan
et al. 2010) overlap with pressures calculated from
amphibole (Table 1), showing that in all these volcanoes hotter mafic magmas ascend from deeper
sources to mix with silicic resident magma within
the shallow crust. The depth of shallow magma
storage of the felsic magma also overlaps with the
c. 4 –10 km depth estimated by Annen et al.
(2006) for viscous stalling of the ascent of silicic
melts derived from within the lower and middle
crust.
It is also important to note that this basic architecture evident at Mount Hood and other compositionally restricted andesite volcanoes appears to be
a consistent global feature of subduction zone volcanic systems. A compilation of amphibole compositions of intermediate lavas from convergent
margin reveals that calculated pressures, temperature and co-existing liquid compositions (using the
method of Ridolfi & Renzulli 2012) are also
strongly bimodal (e.g. Fig. 10c). One population
of amphibole forms between c. 100 and 200 MPa,
at 770–850 8C and from relatively felsic liquids,
the second population formed at 300–425 MPa
and 900–970 8C, and from more mafic liquids.
These closely mirror the amphibole populations
observed at Mount Hood and the other andesitic volcanoes discussed above (e.g. Fig. 10c). The widespread presence of these bimodal amphibole
populations emphasizes the global importance of
mixing between deep and hot mafic magmas and
shallowly stored felsic magmas in the production
of intermediate magmas, and is further evidence
for the important role of mixing processes in the formation of many andesitic volcanic rocks.
Other examples of compositionally restricted
intermediate volcanoes that show evidence for an
important role for hybridization and mixing
include Mount Dutton, Alaska (Miller et al. 1999),
the Plat Plays complex in the Lesser Antilles
(Halama et al. 2006) and Volcán Aucanchilcha,
Chile (Klemetti & Grunder 2008; Walker et al.
2012).
The recharge filtering model for andesitic
volcanoes
Any model for magma genesis at Mount Hood and
other compositionally restricted andesitic volcanoes must explain the following observations: (1)
the broad homogeneity of magma compositions
through time; (2) the macro- and microscopic features which indicate that magma mixing between
mafic and felsic magmas plays a key role in
magma genesis, and that these parental magmas
seem to rarely erupt without mixing; (3) the observation that magma mixing often occurs immediately prior to eruption, which is initiated by
episodes of mafic recharge.
Kent et al. (2010) proposed a model to account
for the behaviour of Mount Hood via a process
they termed recharge filtering. This is a reference
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PREFERENTIAL ERUPTION OF ANDESITIC MAGMA
271
(a)
Depth (km)
10
5
15
Count
8
6
4
2
Mount Unzen
Soufriére Hills
Mont Pelée
(b)
Mount Hood
Count
8
6
4
2
100
200
300
400
500
Pressure (MPa)
6
4
Mt Hood
Tsch. Pargasite
10
8
Mt Hood
Mg hornblende
Percen of total
(c)
2
0
Mt Hood
Tsch. Pargasite
1,000
T (°C)
950
900
Mt Hood
Mg hornblende
850
GEOROC Compilation
Al/Simelt < 0.25
800
Al/Simelt > 0.25
200
400
P (MPa)
600
0 2 4 6 8 10
Percent of total
Fig. 10. Summary of crystallization pressures calculated from amphibole for (a) Mount Unzen, Mont Pelée and
Soufriére Hills, (b) Mount Hood and (c) a global summary of pressure, temperatures calculated from amphiboles from
convergent margin volcanoes from GEOROC. Samples have been grouped according to calculated Al/Simelt values,
with Al/Simelt ,0.25 corresponding to more silicic liquids. Pressure, temperature and Al/Simelt estimates have been
calculated using the procedure of Ridolfi & Renzulli (2012). Amphibole compositions from GEOROC were filtered to
include only analyses from the cores of amphibole phenocrysts from subduction-related volcanic rocks. The global
compilation includes data from 300 different amphiboles taken from 15 arcs and more than 50 different volcanic centres.
Fields showing the range of pressures calculated for Mount Hood amphiboles are also shown for reference. For (a) and
(b) all amphibole crystallized at pressures greater than 250 MPa are tschermakitic pargasite, all those at lower pressures
are magnesiohornblende, see Koleszar (2011) for more details.
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272
A. J. R. KENT
both to the general phenomenon of eruption filtering, and the recognition that at Mount Hood,
as with many other andesitic volcanoes, it is mafic
recharge that is the driving force for initiating
eruptions.
The observation that mafic and silicic parental
magmas involved in magma genesis at compositionally-restricted andesitic volcanoes appear unable
to erupt without mixing suggests that there are significant barriers to the movement and eruption of
these parental magmas within the shallow crust.
This is readily understood in terms of the physical
properties expected for these magmas, but is also
probably heavily influenced by the nature of the
crust itself. Felsic magmas are typically cooler and
difficult to mobilize due to high crystallinities
and viscosities (e.g. Marsh 1981; Rubin 1995; Takeuchi 2011; Fig. 8), and mafic magmas are restricted by their higher densities relative to many
crustal lithologies and by difficulty in propagating
through (or near) overpressured magma reservoirs
(e.g. Wiebe 1994; Pinel & Jaupart 2000; Eichelberger et al. 2006; Hildreth 2007; Karlstrom et al.
2009). Crustal limits on magma mobility also likely
reflect regional and local crustal stress fields, basement lithology, thermal state and strength of the
crust and the presence of existing magma reservoirs
and the volcanic edifice. These factors are also
likely to interact as the changes in magmatic overpressure and the size of the edifice influence local
stress conditions (e.g. Pinel & Jaupart 2000; Karlstrom et al. 2009; Kenedi et al. 2010).
In volcanic systems where crustal and other
conditions reduce magma mobility and limit the
ability of magmas to erupt, mafic recharge appears
to be a common means by which eruptions can
initiate. Mafic recharge is widely recognized as a
powerful means of mobilizing magmas and initiating eruptions, and has been documented in a considerable number of volcanic systems (e.g. Sparks
et al. 1977; Eichelberger 1980; Huppert et al. 1982;
Pallister et al. 1992a, 1996). During recharge increases in magma reservoir overpressure (required
to propagate a fracture to the surface) and magma
mobility (required to allow magma to move along
the fracture or conduit without freezing) results
from thermal and volatile exchanges between mixing magmas, changes in volatile solubility, addition
of material to a fixed-volume magma chamber
and changes in physical properties of the new
mixed magma (e.g. Sparks et al. 1977; Eichelberger
1978, 1980; Huppert et al. 1982; Snyder & Tait
1996; Ruprecht et al. 2008; Ruprecht & Bachmann
2010; Burgisser & Bergantz 2011; Takeuchi 2011;
Koleszar et al. 2012).
In addition to initiating eruptions, mafic recharge
is also often associated with rapid and intense
hybridization between the mafic recharging magma
and the magma or crystal-rich mush (typically more
felsic in composition) residing in shallow storage
zones. Petrographic, experimental and numerical
studies demonstrate that vigorous convective overturn and other dynamic processes associated with
recharge and magma migration can produce highly
hybridized magmas (Eichelberger 1980; Huppert
et al. 1982; Sparks & Marshall 1986; Snyder &
Tait 1996; Jellinek & Kerr 1999; Eichelberger
et al. 2006; Ruprecht et al. 2008; Burgisser & Bergantz 2011). Studies of mineral compositions in
many andesitic systems suggest that the timescales
of mixing and hybridization following recharge
are rapid (e.g. Nakamura 1995; Venezky & Rutherford 1999; Devine et al. 2003; Browne et al. 2006;
Martel et al. 2006; Salisbury et al. 2008; Humphreys
et al. 2009, 2010; Kent et al. 2010; Koleszar 2011;
Ruprecht & Cooper 2012). Less complete hybridization can also result in magma mingling textures and
the presence of mafic enclaves, which are also
common in andesitic magmas (e.g. Bacon 1986;
Sparks & Marshall 1986).
Where recharge involves mafic magma mixing
with a more silicic composition, efficient hybridization must also produce a bulk magma with a composition that is intermediate between the two,
producing mixed magmas of intermediate composition (e.g. Walker & Skelhorn 1966; Eichelberger
et al. 2006). Thus an important link exists between
the increased probability of eruption via mafic
recharge and the production and eruption of mixed
andesitic magmas. Kent et al. (2010) termed this
effect recharge filtering as it results in filtering of
erupted magmas in favour of the mixed andesitic
compositions over the original more diverse parental magma compositions. As a result recharge filtering involves the preferential production and
eruption of andesitic magmas.
A recharge filtering explanation for
compositionally restricted
andesitic volcanoes
The recharge filtering model provides a straightforward answer to the question of how compositionally-restricted andesitic volcanoes form – in that
these are systems where crustal or other conditions
are highly restrictive for mobility and eruption of
more mafic and silicic parental melts. As a result
the only means by which eruption can occur is via
mafic recharge, which results only in eruption of
hybridized andesites, effectively filtering the volcanic record in favour of these compositions.
Eichelberger (1978) argued that compressional settings common in convergent margins
also favour andesite production by promoting
recharge and, although this may be broadly true,
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PREFERENTIAL ERUPTION OF ANDESITIC MAGMA
compositionally-restricted andesitic volcanoes appear to occur in a range of local stress regimes,
including extensional (Unzen) and transtensional
(Soufriére Hills). Mount Hood also lies within a
local extensional zone, close to the propagating tip
of the High Cascade graben, although compressional faults related to the northward movement of
the Oregon coast block also occur in the vicinity
of the edifice (Scott et al. 1997). This variability
underscores the likelihood that the ultimate combinations of crustal conditions that promote or
retard magma mobility are probably complex.
Parameters, such as integrated magma flux and
the age and maturity of a given magma plumbing
system, are also influential as they control thermal
conditions within the shallow crust (e.g. Annen
et al. 2006; Klemetti & Grunder 2008; Gregg
et al. 2012), and establishment of silicic magma
chambers in the shallow crust are likely to be particularly important for retarding mobility of mafic
magmas (e.g. Wiebe 1994; Eichelberger et al.
2006; Karlstrom et al. 2009). It is probable that
magma plumbing systems with relatively low and/
or sporadic magma fluxes are more likely to be
restricted by recharge filtering, as in these cases it
is conditions within the crust that will have the primary influence on magma mobility. Smaller magma
storage zones would also be able to react faster
following recharge, reducing the time between
recharge and eruption (e.g. Miller et al. 1999;
Ruprecht & Wörner 2007). Where magma fluxes
and magma accumulation rates are higher, heat
flow from magma to the surrounding crust can
strongly influence the temperature and viscosity of
wallrocks (e.g. Annen et al. 2006; Lipman 2007;
Gregg et al. 2012) and, for larger systems, the triggering of eruptions can occur via thermal weakening and collapse of the crust overlying the magma
reservoir (e.g. Gregg et al. 2012). In addition, in
some systems (e.g. Mount Pinatubo) it appears
that mafic recharge results in mixing to produce
andesites, but conditions within the shallow magma
reservoir are such that the resident magma may
also be mobilized and erupt in significant quantities
(e.g. Pallister et al. 1992a, 1996; Eichelberger et al.
2006).
Previous explanations for compositionally
restricted andesitic volcanism
The occurrence of volcanoes that produce consistent intermediate magma compositions through
time has been noted previously in the literature
(e.g. Pichavant et al. 2002; Annen et al. 2008; Klemetti & Grunder 2008). In the most detailed studies
conducted to date, Pichavant et al. (2002) and
Annen et al. (2008) argued that the relative
273
constancy of andesitic compositions erupted by
Mont Pelée over the last 13 000 years suggests
that the physical state of magma beneath Mont
Pelée has remained largely constant over this time,
with temperature maintained by periodic input of
hotter mafic material. Annen et al. (2008) used thermal models to estimate the magma flux required
to maintain temperatures above 875 8C (as estimated from experimental petrology; Pichavant et al.
2002) at 4–5 × 1024 km3 a21. Although frequent
intrusions of mafic magma could maintain constant
temperatures, it is unlikely that this represents a
general model to explain compositionally-restricted
andesitic volcanoes. In many andesitic systems,
including Mont Pelée, the restriction of crystalline
material derived from mafic magmas to the rims
of larger plagioclase phenocrysts and microlitesized crystals (e.g. Eichelberger 1978; Gourgaud
et al. 1989; Browne et al. 2006; Martel et al. 2006;
Humphreys et al. 2009, 2013; Kent et al. 2010), as
well as the limited diffusional equilibration (Nakamura et al. 1995; Venezky & Rutherford 1999;
Devine et al. 2003; Kent et al. 2010), show that interaction with mafic melts occurs immediately prior to
eruption, rather than semi-continuously throughout
magmatic evolution (although some systems do
show evidence for periodic influx of mafic material
with only the final interaction being sufficient to
initiate eruption; e.g. Ruprecht & Wörner 2007;
Smith et al. 2009). The presence of microlite-sized
plagioclase derived from mafic magma in many
andesites is also inconsistent with long periods of
storage after mafic intrusion, as it would be expected
that textural and chemical equilibration would
modify these small crystals.
Recharge filtering in other volcanoes
Recharge filtering is also unlikely to occur solely
in compositionally restricted andesitic systems.
The petrographic features evident at Mount Hood
and other andesite-dominated volcanoes described
above are not unique – and, in fact, features such
as the presence of disequilibrium mineral assemblages, reverse zoning patterns and mineral dissolution, and quenched mafic enclaves are often
considered defining features of intermediate magmas from convergent margins (e.g. Eichelberger
1978; Gill 1981). Petrological and other evidence
that demonstrates the important role of mafic
recharge in the formation of andesites and other
intermediate magmas in convergent margins is
also widespread (e.g. Anderson 1976; Eichelberger
1978; Gill 1981). Thus, even where volcanoes
might be hosted in crustal settings that permit eruption of more diverse magma types, the strong
link between mafic recharge, hybridization and
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274
A. J. R. KENT
initiation of eruption is still likely to result in preferential eruption of andesitic magmas. As a result
many other convergent margin volcanoes show
high relative abundances of andesites amongst
their eruptive output (e.g. Ewart 1976; Gill 1981;
Hildreth & Moorbath 1988; Hildreth 2007), even
where more diverse compositions also occur. In
these volcanoes the high overall abundance of andesitic magmas is consistent with recharge filtering
and is not well explained by either the volumes of
andesitic magma expected from progressive crystal fractionation of a basaltic parent – where andesitic magmas should be less abundant than more
mafic magmas as they require greater extents of
crystal fractionation (e.g. Marsh 1981), or from
melting of crustal lithologies, which produce more
silicic magma compositions for most crustal lithologies at reasonable degrees of melting (e.g. Sisson
et al. 2005).
One interesting example is Mount St Helens,
located c. 100 km to the NW of Mount Hood.
Over the previous 300 000 years Mount St Helens
has erupted a diverse range of magma compositions from basalts through to low silica rhyolite
(Fig. 3; c. 48–70 wt% SiO2; Clynne et al. 2008). In
particular the significant proportion of lavas with
SiO2 ,55 wt% suggests that at times mafic magmas have been able to erupt with little or no mixing with evolved melts. In addition, some magmas
such as the 1980– 1986 and 2004–2008 dacites
(c. 65 wt% SiO2; Pallister et al. 1992b, 2008)
show little petrological evidence for mafic recharge,
appearing to derive directly from melting of basaltic
protolith within the lower crust (e.g. Smith &
Leeman 1993). These recent eruptions also appear
to have been initiated by mechanisms other than
mafic recharge (e.g. Kent et al. 2007; Pallister
et al. 2008). Despite this, significant volumes of
andesites also occur within the eruptive output of
the volcano, and these are considered to result
from hybridizing between mafic and dacitic magmas (Pallister et al. 1992b, 2008; Smith & Leeman
1993; Clynne et al. 2008). Petrological evidence
indicative of mafic recharge and hybridization is
also relatively common (e.g. Clynne et al. 2008).
Thus in this system it is likely that recharge filtering
plays an important, but not dominant role.
other settings, they are generally subordinate to
other magma types (e.g. Fig. 1b, c). Gill (1981)
suggested that two conditions are required for orogenic andesites to predominate: (i) the presence
of subduction-modified asthenosphere (but not
necessarily active subduction) to produce parental
magmas, and (ii) compressional stress or thick
sialic crust to promote differentiation of mantlederived magmas. Eichelberger (1978) also emphasized the role of compressional environments for
andesite formation, but in this case argued that this
promoted a dependence on mixing to produce
andesites.
If it is accepted that mafic recharge and mixing drive production of a significant proportion of
andesitic magmas in convergent margins and also
promote their preferential eruption, then the high
abundances of water and other volatiles in the basaltic melts that form in subduction zones are also
likely to play a decisive role in andesite genesis.
Petrological evidence, together with experimental and numerical studies, highlights the important role of density instabilities driven by vapour
exsolution within an underplated mafic recharge
magma to drive convective overturn associated
with mafic recharge events (e.g. Eichelberger 1978,
1980; Huppert et al. 1982; Bacon 1986; Ruprecht
et al. 2008; Burgisser & Bergantz 2011). Hydrous
mafic magmas are key to this process as the higher
volatile abundances are more liable to promote
the density-driven overturn and subsequent mixing required to produce and erupt mixed andesitic magmas during recharge events. The volatile
content of the silicic parental magmas is probably
less important, and the melt component of silicic
magmas produced in a range of tectonic settings
is typically saturated or close to saturated with
vapour at shallow pressures, due to partial melting
of hydrous crustal lithologies or extensive differentiation involving anhydrous phases. Importantly, it
is only in subduction zone environments where
volatile-rich mafic magmas predominate (e.g. Wallace 2005) and where significant volumes of
andesitic magmas occur. The likelihood of andesite production and eruption is correspondingly
reduced in environments, such as intraplate volcanic
fields, ocean islands and mid-ocean ridges, where
mafic magmas are less hydrous.
Formation of andesites in convergent
margins
Conclusions and outstanding issues
The idea that recharge filtering results in preferential
eruption of andesitic magmas also provides an
explanation for the overall high abundance of andesitic magmas in convergent margin settings relative
to other tectono-magmatic environments. Although
andesitic magmas and their equivalents occur in
The global record of volcanism represents a tremendous resource for geologists, geochemists and
others who seek to understand the inner workings
of the Earth. Melting of the crust and mantle is
strongly correlated with plate tectonic and other
deep Earth processes. Moreover, as magma
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PREFERENTIAL ERUPTION OF ANDESITIC MAGMA
generation occurs deep within the crust and mantle,
any direct observations of the processes that create
magmas, and direct samples of the materials that
undergo melting, are difficult or impossible to
obtain. Volcanic rocks represent the endpoint of
these processes and the material is conveniently
transported to the surface of the Earth, where
sampling and observation are possible. As a result
the physical state and the mineralogical, chemical
and isotopic compositions of volcanic rocks represent one of the primary observational datasets
used to constrain the inner workings of the Earth.
However, the volcanic record should not be considered to simply represent an unbiased sampling
of magmas produced within a given setting, and it
must always be remembered that volcanic rocks represent a special set of magmas that are capable of
being erupted.
In this contribution I have promoted the idea that
the high proportions of andesitic magmas erupted at
convergent margin volcanoes need not directly
reflect production of large volumes of andesitic
melt produced by igneous differentiation or melting processes associated with subduction. Instead,
many erupted andesitic magmas result from preferential eruption of andesitic magmas formed by
mafic recharge and hybridization processes within
the shallow crust. Although this does not rule out
other modes of andesite formation, the process of
preferential eruption via recharge filtering is likely
to play an important, and possibly dominant role
in controlling the high abundances of andesitic
magma erupted from convergent margin volcanoes. The hydrous nature of mantle-derived magmas in subduction zones also favours andesite
formation relative to other tectono-magmatic settings as this promotes the vigorous convection and
overturn following mafic recharge required to initiate eruptions and to produce hybridized magmas. Andesite formation is also promoted by
crustal conditions that limit the ability of more
mafic and felsic parental magmas to erupt, and
thus an important corollary of the recharge filtering
concept is that local conditions within the crust,
probably in concert with magma fluxes, play an
important role in controlling the range and compositional proportions of magmas erupted at a
given volcanic system. Volcanoes that erupt compositionally-restricted andesitic sequences through
time probably reflect crustal settings where parental
magmas are restricted from erupting by crustal
density and viscosity filters or other factors and,
thus, where recharge is the dominant form of eruption initiation. For volcanoes in crustal settings
where a broader range of compositions erupt, the
common prevalence of andesitic magmas formed by
recharge and hybridization suggest that recharge filtering is still an important process. In addition, as
275
crustal conditions such as lithology and stress field
may change over short lateral length scales, it is
likely that the role of recharge filtering varies
considerably between neighbouring volcanoes, providing an explanation for the common juxtaposition of compositionally-restricted volcanoes with
those that erupt more variable compositions and
for the general variations observed in erupted compositions that occur within individual arc segments. It is also possible to envisage changes in
crustal conditions and regional or local stress
fields driven by far-field tectonic processes, by
development of the volcanic plumbing system or
by growth and collapse of the volcanic edifice. Thus,
the degree to which eruption filtering controls
erupted compositions at an individual volcano
may also change, sometimes abruptly.
Another implication of the recharge filtering
model is that in many convergent margins the
rocks that constitute the volcanic record may be an
unreliable guide to the magmatic outputs of the
crustal magmatic systems involved. Andesitic and
other intermediate volcanics that are formed from
mixing and hybridization will be over-represented
relative to the parental magmas that mix to form
them. This paper has not focused on the plutonic
record of convergent margin magmatism, and
overall reconciliation of the volcanic and plutonic
records of convergent margin magmatism remains
an area of active debate (e.g. Bachmann et al.
2007; Lipman 2007). Recharge filtering and other
forms of eruption filtering will result in significant differences between the volcanic and plutonic
record of magma fluxes in convergent margins,
and complicate the comparison of the plutonic and
volcanic records. In addition – although the role
of mixing and hybridization in the formation of
magmas that occur in large batholiths in exhumed
subduction terranes is often less evident than in
many volcanic rocks – macroscopic and other indications of magma mixing in plutonic rocks are
common, and mixing probably plays a prominent
role in the formation of many intermediate magmas that occur in convergent margin batholiths
(e.g. Sisson et al. 1996; Wiebe 1994; Reubi &
Blundy 2009). Thus, it is also possible that mixing
and magma mobility are also linked in the construction of batholiths in convergent margins.
Several other important avenues related to this
topic await further study. One of the most important
is the development and application of techniques
to identify the compositions and physical state of
the parental magmas prior to mafic recharge.
Modern micro-analytical techniques provide unprecedented access to the compositions of minerals
and melts (via melt inclusions and matrix glasses)
within hybridized magmas. Although these studies
are driving an increasing appreciation of the role
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276
A. J. R. KENT
of mixing and hybridization in andesite genesis,
there are still limitations. In particular, it remains
difficult to estimate the compositions and the physical conditions of the parental magmas that are
involved in hybridization. These parameters are
critical for understanding the dynamics of recharge,
mixing and eruption, but are also difficult to estimate from mineral or melt inclusion chemistry
alone. Mineral compositions can be inverted to estimate the compositions of the liquid from which
they crystallize, but it can be difficult to determine
the crystallinity of the magma involved, which is
key to understanding viscosity. Mineral thermometry can also be used to estimate temperatures of
various minerals or mineral pairs, but these reflect
the conditions at the time of crystallization of
the mineral or most recent re-equilibration. For the
case of a felsic magma or mush present within the
shallow crust, significant cooling to solidus or subsolidus conditions may occur after mineral formation and prior to recharge, and thus the state of
the magma at the time of remobilization may
differ substantially from that inferred by mineral
equilibria.
One way to address these issues is to combine
multiple lines of inquiry. For example recent
studies combine textural quantification using crystal size distributions or other approach with mineral and bulk rock geochemistry (e.g. Salisbury
et al. 2008; Kent et al. 2010). Kent et al. (2010)
combined CSD, mineral and bulk rock chemistry to estimate parental magma compositions at
Mount Hood. Martel et al. (2006), Salisbury et al.
(2008) and Humphreys et al. (2013) have also
demonstrated novel approaches for identifying
crystalline contributions from mafic magma in
hybrid lavas.
Further work is also needed to illuminate the
dynamics of magma mixing and hybridization
during mafic recharge and eruption. A basic understanding of these processes is already known from
numerical, analogue and other approaches, but
micro-analytical and related studies are increasingly revealing the high degree to which andesitic
magmas are hybridized, with mineral components
derived from parental magmas intimately juxtaposed. In particular mafic magma-derived microlitesize crystals are now known to occur throughout
many andesites, but important questions remain
as to whether mixing occurs via hybridization of
two magmas (e.g. Sparks & Marshall 1986) or disaggregation and shear of quenched mafic enclaves
(e.g. Clynne 1999; Humphreys et al. 2009).
Timing constraints also suggest that such efficient
mixing is also extremely rapid – occurring within
days to weeks prior to eruption (e.g. Nakamura
1995; Venezky & Rutherford 1999; Devine et al.
2003; Browne et al. 2006; Salisbury et al. 2008;
Kent et al. 2010; Ruprecht & Cooper 2012) and,
although models that describe such mixing are
now being developed (e.g. Huber et al. 2009; Burgisser & Bergantz 2011), understanding this rapid
timescale remains a challenge. Finally, there is
also the puzzling observation that for long-lived
eruptions mixing timescales appear to be shorter
than the duration of the eruption (Nakamura 1995;
Venezky & Rutherford 1999; Devine et al. 2003),
suggesting that, despite the compositional homogeneity of individual eruptive units, mixing is a
continual process throughout the eruptions.
The author thanks many colleagues for discussions regarding the origin of andesites, and especially A. Koleszar,
C. Darr, K. Cooper, G. Eppich, W. Scott and C. Gardner
for collaboration on studies of Mount Hood. Members of
the Oregon State University VIPER group also contributed to lively discussions on andesites and other topics.
Comments from R. Lange and an anonymous reviewer,
together with comments and editorial suggestions from
A. Gómez-Tuena, improved the manuscript significantly.
This work was supported by National Science Foundation
grant EAR 1144555.
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