Journal of Petrology Advance Access originally published online on October 2, 2006
Journal of Petrology 2006 47(12):2303-2334; doi:10.1093/petrology/egl045
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Magma Evolution and Open-System Processes at Shiveluch Volcano: Insights from Phenocryst Zoning
DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, WILLS MEMORIAL BUILDING QUEEN'S ROAD, BRISTOL BS8 1RJ, UK
RECEIVED DECEMBER 6, 2005; ACCEPTED AUGUST 17, 2006
| ABSTRACT |
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Phenocryst zoning patterns are used to identify open-system magmatic processes in the products of the 2001 eruption of Shiveluch Volcano, Kamchatka. The lavas and pumices studied are hornblendeplagioclase andesites with average pre-eruptive temperatures of
840°C and fO2 of 1·52·1 log units above nickelnickel oxide (NNO). Plagioclase zoning includes oscillatory and patchy zonation and sieve textures. Hornblendes are commonly unzoned, but some show simple, multiple or patchy zoning. Apatite microphenocrysts display normal and reverse zoning of sulphur. The textural similarity of patchy hornblende and plagioclase, together with BaSr systematics in patchy plagioclase, indicate that the cores of these crystals were derived from cumulate material. Plagioclaseliquid equilibria suggest that the patchy texture develops by resorption during H2O-undersaturated decompression. When H2O-saturated crystallization recommences at lower pressure, reduced pH2O results in lower XAn in plagioclase, causing more Al-rich hornblende to crystallize. Plagioclase cores with diffuse oscillatory zoning, and unzoned hornblende crystals, probably represent a population of crystals resident in the magma chamber for long periods of time. In contrast, oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization. Increasing Fe/Al in oscillatory zoned rims suggests oxidation as a result of degassing of H2O during decompression. A general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization. We present evidence that the host andesite has mixed with both more felsic and more mafic magmas. Olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt. Sieve-textured plagioclase resulted from mixing of a more felsic magma with the host andesite. The mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma, which implies efficient thermal quenching, and thus small volumes of intruding magma. Mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents. We suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber, producing incremental variations in whole-rock compositions. KEY WORDS: patchy zoning; magma mixing; Shiveluch
| INTRODUCTION |
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Open-system magmatic processes are ubiquitous phenomena at island arc volcanoes. Processes may include segregation and ascent of magmas from depth, accumulation and mixing of magma batches in a shallow chamber, and eruptive processes. The intricacy of the magmatic history is reflected to varying extents in the typically complex petrography of erupted andesites. Eruptive products thus contain clues to the entire magma history, from differentiation in the deep crust to eruption at the surface.
Magma mixing phenomena are particularly widespread (e.g. Anderson, 1976
; Eichelberger, 1980
; Clynne, 1999
). Mixing is envisaged to occur in various scenarios: for example, thermal or chemical convection in a magma chamber (e.g. Huppert et al., 1982
; Couch et al., 2001
), within the conduit during eruption (Snyder & Tait, 1996
; Koyaguchi, 1985
) or during sidewall crystallization in mafic magma chambers (McBirney, 1980
). Mixing can also occur as a result of recharge of the magma chamber by the arrival of a new magma batch, which may be a trigger for eruptions (e.g. Sparks et al., 1977
; Huppert et al., 1982
; Murphy et al., 1998
). Magma recharge commonly occurs as cool, silicic melt is invaded by hotter, more mafic magma (e.g. Anderson, 1976
; Clynne, 1999
; Murphy et al., 2000
). This process is typically inferred from features such as the presence of mafic enclaves, accompanied by heavily corroded xenocrysts and significant geochemical zonation in phenocrysts (Eichelberger, 1978
, 1980
). The reverse process (i.e. injection of more silicic magma into basaltic systems) has also been inferred on the basis of field observations (Wiebe, 1987
) and geochemistry (Calanchi et al., 1993
). Less attention has been paid to the possibility of cryptic magma mixing, which we define here as the repeated mixing of small pulses of magma that have similar anhydrous bulk compositions, but may have different fO2 or volatile contents. Magma recharge would be difficult to detect if the mixing magmas were compositionally similar, because of limited disequilibrium between crystals and melt (D'Lemos, 1996
).
Recently, field evidence has shown that plutons can grow incrementally via the coalescence of many much smaller magma bodies (Bateman, 1992
; John & Blundy, 1993
; Wiebe & Collins, 1998
; Atherton, 1999
; Glazner et al., 2004
). This implies that some subvolcanic magma chambers are built up gradually and over prolonged periods of time from many small batches of magma. The magma batches may have differentiated at depth prior to ascent (Annen et al., 2006
) and been injected into a shallow-level magma chamber. The efficiency of magma mixing resulting from such recharge depends on the volume ratio of intruding magma to resident magma, and on the viscosity contrast between the two magmas (e.g. Sparks & Marshall, 1986
; Jellinek et al., 1999
). This in turn depends on the relative temperatures, dissolved volatile contents and compositions of the magmas (Sparks & Marshall, 1986
). If the intruding magma forms enclaves, enclave mineralogy will depend on the rate of quenching as well as the bulk composition of the intruding magma (Blundy & Sparks, 1992
). Further fragmentation or disaggregation of enclaves can help to hybridize the mixed magma (Clynne, 1999
, and references therein). The resulting mineral textures will depend on how close the enclave mineralogy is to equilibrium with the host magma, and thus on the composition and relative volume of the intruding magma.
As well as recharge and mixing, other processes that contribute to the textural complexity of andesites include the resorption and remobilization of earlier-formed crystals or cumulate materials, as magma ascends from its deep source to a shallow magma storage area (Pyle et al., 1988
; Turner et al., 2003
; Dungan & Davidson, 2004
). In the storage area, the magma may slowly cool and degas, promoting crystal fractionation. Convection in a zoned magma can cause compositional zoning in existing phenocrysts (Singer et al., 1995
). Late-stage crystal growth may occur during magma ascent in the conduit, in response to depressurization and degassing of H2O (e.g. Geschwind & Rutherford, 1995
).
In this paper we investigate phenocryst zoning in andesitic dome rocks and pumices from Shiveluch Volcano, Kamchatka, with the aim of identifying different magmatic processes through their effects on the phenocrysts. We identify different populations of phenocrysts based on their textural and geochemical characteristics, and attempt to produce an integrated picture of magmatic processes in their spatial and temporal context. We show that the host andesite has mixed with basalt, producing xenocrysts showing significant disequilibrium textures. The host andesite has also mixed with a mafic andesite, although efficient quenching resulted in cryptic mixing, with identical mineralogy and mineral compositions in both the enclave and the host andesite. Geochemical zonation is produced in crystals from both magmas as a consequence of variations in volatile contents between the magmas. We also discuss the remobilization of cumulates, and the effects of decompression and eruption dynamics on the evolution of the magma at both deep and shallow levels, thus revealing a complex record of open-system processes at this active andesite volcano.
| GEOLOGICAL BACKGROUND |
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Shiveluch Volcano is located in the northern Central Depression of the Kamchatka Peninsula (Fig. 1), and has been one of the most active andesitic volcanoes in the KamchatkaKurile arc during the Holocene, with at least 60 tephra fall deposits identified, marking explosive eruptions with volumes in excess of 0·1 km3 (Dirksen et al., 2006
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The most recent phase of volcanic activity started in 1964 with a strong Plinian eruption triggered by edifice failure (Belousov, 1995
Summary of andesite petrology
We studied samples of dome rock, and pumiceous material taken from pyroclastic flows, from early in the 20012004 eruption of Shiveluch Volcano (Table 1). The petrology of the andesite has been described by Dirksen et al. (2006)
, including whole-rock and modal data, and is summarized and extended here. The main phenocrysts (>300 µm) and microphenocrysts (100300 µm) are plagioclase (5670% of phenocrysts), hornblende (2735%), FeTi oxides (1·74·8%) and orthopyroxene (0·35·2%). Clinopyroxene, apatite and anhydrite are present in minor amounts. Quartz and other silica phases are absent. Xenocrysts of olivine and orthopyroxene are present. The groundmass comprises a rhyolitic glass with microlites of plagioclase, clinopyroxene, orthopyroxene and FeTi oxides. Total phenocryst contents vary from
26% (pumices) to
52% (dome rocks), calculated vesicle-free.
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Plagioclase compositions range from An29 to An74; microlite compositions range from An44 to An51. Plagioclase phenocrysts commonly contain inclusions of FeTi oxides, hornblende, apatite and glass. Orthopyroxene and clinopyroxene inclusions occur less commonly; phlogopite, bornite and anhydrite inclusions occur rarely. Hornblende compositions range from Mg-number [100 x Mg/(Mg + Fetot)] of 58 to 81, classifying as magnesio-hornblendes (Leake, 1978
Accessory minerals in the andesites studied include apatite and rare anhydrite as primary microphenocrysts, and anhedral CuFe sulphides, cassiterite (SnO2) and CuO in the groundmass and as inclusions in plagioclase and hornblende. The copper-bearing sulphide is a solid solution of bornite and digenite, ranging from Cu8FeS3 to Cu47FeS14. Apatite contains up to 5860 ppm sulphur, and is ubiquitous in the rocks studied.
Melt inclusions are common in both hornblende and plagioclase (Humphreys, 2006
). Bubbles are common; partial devitrification or precipitation of daughter phases is common in hornblende-hosted inclusions but rare in plagioclase-hosted inclusions. Analysis of melt inclusions containing daughter minerals was avoided where possible. Because there is little compositional difference between inclusions in different host phenocrysts, or between inclusions with and without bubbles (Humphreys, 2006
), we assume that post-entrapment crystallization is limited, and that the inclusions accurately reflect the composition of the melt at the time of trapping. Detailed melt inclusion data have been given by Humphreys (2006)
and the results are summarized here. Melt inclusions are consistently rhyolitic in composition, containing 7080 wt % SiO2 (normalized to anhydrous) and 0·15·1 wt % H2O as analysed by secondary ionization mass spectrometry (SIMS). Matrix glasses are among the most silica-rich, typically containing 77·579·5 wt % SiO2, and low H2O (0·100·14 wt %). H2O correlates negatively with SiO2 content of the inclusions, and the inclusions follow a decompression trend on the QzOrAb ternary.
The andesites contain dark green, medium- to coarse-grained xenoliths. These are typically several centimetres in diameter with a well-defined, smooth, usually hornblende-rich rim. The modal mineralogy of the xenoliths is highly variable; they contain hornblende, plagioclase, phlogopite and orthopyroxene, with minor clinopyroxene, apatite, FeTi oxides and rounded chromite. No olivine is observed. Small pockets of interstitial glass are present. The xenolith texture varies from poikilitic, with plagioclase oikocrysts in optical continuity, to granoblastic, indicating variable amounts of deformation and recrystallization. Small veins containing calcic plagioclase (e.g. An93) and pyroxenes cut across some xenoliths. The xenoliths are interpreted as cumulates.
One of the samples (shiv01/#3) also contains a small (23 cm), elliptical, quench-textured andesitic enclave (Fig. 2a). There is no change in grain size towards the rim of the enclave, which is defined by the edges of crystals protruding into the host lava. The enclave has a high proportion of crystals (
85% calculated vesicle free) including randomly oriented, elongate, skeletal hornblende phenocrysts (Fig. 2a and b), phenocrysts of plagioclase, tabular FeTi oxides, acicular apatite and, unusually, abundant anhydrite crystals up to 300 µm in length. Apatite is both more acicular, more abundant, and more S-rich than in the host andesite. Dendritic protrusions extend from the rims of some crystals (Fig. 2c, d). Mineral compositions are indistinguishable from compositions of phenocrysts in the host andesite (see Tables 3 and 4). The matrix comprises a vesicle-rich, microlite-free, rhyolitic glass, with similar compositions to melt inclusions and matrix glasses in the host andesite. The enclave has slightly higher modal amphibole (Dirksen et al., 2006
, table 2) in comparison with the phenocryst assemblage in the host andesite, indicating a slightly more mafic bulk composition. Vesicles commonly occur between grain junctions, suggesting that vesiculation was induced by crystallization.
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Whole-rock geochemistry
All samples studied here are pumices and dome rocks of medium-K andesite (Gill, 1981
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| ANALYTICAL METHODS |
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Wavelength-dispersive EPMA of minerals and glasses was performed at the University of Bristol using a CAMECA SX-100 five-spectrometer instrument. Minerals were analysed using a 20 kV accelerating voltage, 10 nA beam current and a 5 µm beam diameter. Groundmass glasses and melt inclusions were analysed using a 15 kV accelerating voltage, 24 nA beam current and a 15 µm beam diameter, with Na and Si analysed first to reduce the effects of alkali migration, as described in Humphreys et al. (2006)
X-ray fluorescence (XRF) analyses were carried out using a Philips PW1400 X-ray fluoresence spectrometer at Leicester University. Analyses of major elements were performed using fused glass discs made from powders mixed with 80:20 Li metaborate:Li tetraborate. Trace elements were determined on 32 mm diameter, pressed powder briquettes.
Trace element concentrations in some hornblende and plagioclase phenocrysts were determined by inductively coupled plasma mass spectrometry (ICP-MS) using a VG Elemental PlasmaQuad 3 system coupled to a VG LaserProbe II (266 nm frequency-quadrupled NdYAG laser) at the University of Bristol. He gas was the ablation medium and a HeAr mixture carried the sample to the plasma. The laser beam was set to 20 µm diameter, 5 Hz repetition rate and about 2530 J/cm2 energy density. For each analysis, counts were collected on the background for 50 s, before firing the laser and collecting counts for 50 s. Data were collected for 45Sc, 51V, 88Sr, 90Zr and 130Ba. The glass NIST 610 (Pearce et al., 1997
) was used as primary standard, with NIST 612 as a secondary standard. Data reduction, using the method of Longerich et al. (1996)
and Si as the internal standard, was performed using PlasmaLab software.
| ESTIMATION OF INTENSIVE PARAMETERS |
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Temperature
Magma temperatures for the Shiveluch andesite were estimated by Dirksen et al. (2006)
Hblplag temperatures range from 775°C to 906°C ± 35°C, with an average of 841°C ± 35°C (n = 85). A temperature of 808836°C ± 35°C was obtained for the quenched enclave (n = 3). In comparison, FeTi oxide temperatures range from 834°C to 978°C (Table 2), with an average of 878°C (n = 32). Microlites in contact with the matrix glass return slightly higher temperatures (860978°C ) than inclusions in phenocrysts (834904°C). An oxideoxide temperature of
870°C was estimated for the andesite enclave (n = 2). Plagioclaseliquid temperatures range from 819°C to 975°C, with an average of 867°C (n = 63). These data cover a wide range of temperatures; oxide and plagioclaseliquid temperatures are consistently higher than hblplag temperatures. These differences are interpreted to reflect temperature variations throughout the magma's evolution. Hblplag data reflect temperatures early in the magmatic history, while hornblende is still crystallizing. Oxide data, which re-equilibrate rapidly (Hammond & Taylor, 1982
; Venezky & Rutherford, 1999
), are interpreted to reflect higher magma temperatures caused by release of latent heat during late-stage decompression-driven crystallization (Blundy et al., 2006
). Plagioclaseliquid temperatures reflect real variation of T and pH2O during the magma's history. From these data, we estimate that magma temperatures prior to ascent were
840°C.
Pressure
The maximum magma pressure, estimated from the maximum H2O contents of melt inclusions, is
160 MPa (Humphreys, 2006
). The depth of the magma chamber roof at Shiveluch Volcano is thus assumed to be
56 km below the summit, a result consistent with seismic data (Dirksen et al., 2006
).
Oxygen fugacity
Oxygen fugacity is estimated from FeTi oxide equilibria using the method of Andersen et al. (1993)
. Values of log(fO2) obtained are highly oxidizing, and range from 9·0 to 11·2, with an average of 10·3 (Fig. 4). The values obtained correspond to 1·52·1 log units above the nickelnickel oxide (NNO) buffer [quartzfayalitemagnetite (QFM) + 2·1 to + 2·7], with an average of NNO + 1·9. The andesite enclave gave NNO + 1·8. Uncertainties are approximately ±0·6 log units (Andersen et al., 1993
). These oxidizing conditions are consistent with the co-precipitation of sulphides (S2 oxidation state) with anhydrite (S6+) and S-bearing apatite (S6+) (Carroll & Rutherford, 1985
, 1988
).
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| PHENOCRYST ZONING PATTERNS |
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Plagioclase
Three plagioclase phenocryst textures are observed: oscillatory zoning, patchy zoning and sieve texture. Table 3 gives representative compositions of the phenocryst types, and Fig. 5 summarizes the chemical variations between the textures.
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Oscillatory zoned phenocrysts (
6875% of plagioclase) typically have rather diffuse, symmetrical zones in their cores, in contrast to the sharper, more asymmetrical boundaries of outer zones (Fig. 6a and b). This may reflect partial re-equilibration of the cores during long residence times in the magma chamber. Compositional variation in oscillatory zoning is typically less than 30 mol % Anorthite. Inner zones show an approximately inverse relationship between Fe/Al and XAn. Outer zones commonly show a sharp boundary to higher XAn, followed by gradual decay to more sodic compositions (Fig. 6a and b). At some outer zone boundaries resorption has taken place, as shown by irregular surfaces (Fig. 7a and b) and rounded corners (Fig. 7c). Melt inclusions are occasionally trapped at these resorption boundaries. Rim compositions are usually consistent across each sample. The rims of oscillatory zoned crystals typically show a gradual increase in the ratio Fe/Al over several zones (Fig. 6), unaccompanied by changes in Mg. Groundmass plagioclase also has higher Fe contents and Fe/Al values (Fig. 5b), as observed in other orogenic andesites (e.g. Nakada & Motomura, 1999
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Patchy zoned crystals (
2025% of plagioclase) have an irregular or chequerboard, central corroded core with abundant melt and mineral inclusions, suggesting partial resorption of an older core. In back-scattered scanning electron microscope (SEM) images, melt inclusions are closely associated with more albitic (darker) plagioclase of An2954 (Fig. 8a). This suggests that the more An-rich patches (An4274, Fig. 8a) formed the original core, and that the more albitic plagioclase crystallized after partial resorption, causing entrapment of melt inclusions. The change in XAn between early, anorthite-rich (brighter) plagioclase and later, albite-rich (darker) plagioclase is typically not accompanied by changes in minor elements (e.g. Fe, Mg). Trace element contents do vary, however: An-rich patches typically contain 120220 ppm Ba whereas Ab-rich patches contain 210360 ppm Ba. Sr contents are similar (9201520 ppm Sr in An-rich patches, 10001400 ppm in Ab-rich patches). The rims of patchy phenocrysts (An3757) are typically clear and zoned; many show oscillatory zoning (Figs 6c and 7d). In the oscillatory zoned rims, the same range of XAn and pattern of increasing Fe/Al is observed as in oscillatory phenocrysts (Fig. 6c). Outermost individual zones can be correlated between the two phenocryst types (Fig. 6), implying that they had a common magmatic history during the late stages of crystallization.
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Sieve-textured phenocrysts (
39% of plagioclase) have a sodic core (An3437, Fig. 5a), surrounded by a narrow rim of more An-rich plagioclase (Fig. 5b). The core is commonly anhedral, and in some crystals oscillatory zonation is observed in the core (Fig. 9a). The boundary between the core and rim comprises a network of micrometre-scale melt channels. The channelized zone is usually a narrow ring (Fig. 9b), but sometimes occupies the whole plagioclase interior (Fig. 9c); these variations could be an artefact of sectioning, or related to the extent of disequilibrium encountered by the crystal (Clynne, 1999
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Hornblende
Many hornblende phenocrysts (6580%) are compositionally rather uniform, unzoned magnesio-hornblendes. However, the compositions of unzoned hornblendes vary in terms of Aliv, Alvi, Mg-number and (Na,K)A (Fig. 10). Three other textural types are observed: patchy zoning (<10% of phenocrysts), simple zoning (420%) and multiple zoning (03%) (Fig. 11; Table 4). Overall, hornblendes show a very weak AlivAlvi (Tschermak) correlation (R2 = 0·30; n = 420) (Fig. 12a). No correlation is observed within any single zoned phenocryst (Fig. 12b), but multiple analyses from the same grain plot in a cluster around a given AlivAlvi. Rims of patchy zoned crystals have lower AlivAlvi than simple-zoned phenocrysts (Figs 10 and 12b). Qualitatively, this relationship indicates rim crystallization at lower pressure, though quantitative estimates cannot be made because of the lack of K-fsp and quartz in the crystallizing assemblage (Johnson & Rutherford, 1989
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Patchy zoning in hornblende phenocryst cores (Fig. 11a) comprises irregularly shaped dark and light patches in back-scattered SEM images (Fig. 8b). The crystals often contain abundant mineral and melt inclusions. The texture is strongly reminiscent of patchy zoning in plagioclase (see Fig. 8a). Melt inclusions are commonly associated with brighter patches (Fig. 8b), and mineral inclusions are commonly edged with a sliver of melt, suggesting that they grew from a pre-existing melt pool. Bright regions have higher Aliv and (Na,K)A, and lower Mg-number than dark regions (Fig. 10; Table 4). Bright and dark regions are indistinguishable in terms of trace elements, except for Ba, which is slightly more abundant in the Al-rich hornblende (Fig. 13a). This probably results from the higher A-site occupancy of the Al-rich hornblende (Table 4), with Ba substituting for K (Tiepolo et al., 2003
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Simple-zoned hornblende phenocrysts are characterized by a fairly uniform core, and a sharp change in composition to the rim (Figs 11b and 14df), with only occasional evidence of embayment or rounding. Aliv and (Na,K)A are well correlated (Fig. 14f). Rims have higher Mg-number (Fig. 14e), Na and Si, higher Fe3+/Fe2+ (calculated using Holland & Blundy, 1994
Zonation in multiple-zoned hornblendes (Fig. 14gi) is texturally similar to that in simple-zoned hornblendes, but the zones are less clearly defined (Fig. 11c). Zoning is characterized by broad, diffuse zones that lack correlation of Aliv with Mg-number (Fig. 14h) or (Na,K)A (Fig. 14i), and are marked by large jumps in Mg-number (Fig. 14g); for example, from
77 to
58 and back. Dark zones have high Mg-number, Na and Si, and low Ti, Mn, K and Fe compared with bright zones (Fig. 10).
Apatite
Both normal and reverse zoning patterns are observed in apatite. Normally zoned crystals have a rimward increase in S, whereas reversely zoned crystals have a rimward decrease. S correlates with Na2O (Fig. 15), suggesting a coupled substitution mechanism such as S6+ + Na+ = P5+ + Ca2+ (Liu & Comodi, 1993
) or 2S6+ + 4REE3+ + Si4+ + 2Na+ = 4P5+ + 5Ca2+ (Parat et al., 2002
). In general, microphenocrysts and microlites from the quenched enclave are more sulphur-rich (5505860 ppm) than those from the host andesite (2353300 ppm, Fig. 15).
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Olivine
Olivine occurs as large, isolated crystals of composition Fo8087, and occasionally contains inclusions of En-rich orthopyroxene and chromite. Strong evidence of disequilibrium is seen, varying from a reaction corona of titanomagnetite, orthopyroxene and amphibole, to complete replacement by sub-grains of orthopyroxene, titanomagnetite, amphibole and sometimes plagioclase. The remnant olivine is commonly strongly zoned, with Mg decreasing rimward. These reaction rims are attributed to reaction of olivine with a silica-rich, oxidizing melt (e.g. van Lamoen, 1979
| DISCUSSION |
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Crystal zoning and dissolution textures are produced by changes in some combination of pressure (P), temperature (T), pH2O, fO2 or melt composition. Changes in these parameters reflect physical processes acting at different stages of the magmatic history. For example, prolonged periods of cooling and fractional crystallization in the deep crust result in the segregation of evolved, H2O-rich residual melts (Annen et al., 2006
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Origin of patchy zoning
Patchy zoning in plagioclase
Patchy plagioclase is characterized by an irregular, partially resorbed, An-rich, Ba-poor core, filled in and overgrown by Ab-rich, Ba-rich plagioclase that trapped abundant melt inclusions (Fig. 16a). Similar textures have previously been attributed to skeletal growth (e.g. Kawamoto, 1992
Under equilibrium conditions, the concentrations of minor and trace elements in plagioclase are controlled by melt concentrations and partition coefficients (Ginibre et al., 2002
). We calculated Ba and Sr concentrations of melts coexisting with patchy plagioclase using the method of Blundy & Wood (1991)
for T = 840°C. In general, the calculations indicate that early, An-rich plagioclase was in equilibrium with melts considerably enriched in Ba and Sr compared with the Ab-rich plagioclase (Fig. 17a). The concentrations of Ba and Sr in the melt in equilibrium with early, An-rich plagioclase decrease markedly from approximately 1000 ppm Ba and 700 ppm Sr to approximately 400 ppm Ba and 200 ppm Sr (Fig. 17a). Ba and Sr concentrations in the melt then remain constant at these values while the Ab-rich plagioclase crystallizes. The early, decreasing trend is consistent with evolution of the melt during fractional crystallization of plagioclase (An62) + hornblende + phlogopite ± pyroxenes (Fig. 17a). This assemblage is consistent with the mineralogy of cumulate xenoliths, and with the whole-rock data, which require involvement of a K-rich mineral (see Fig. 3). These early formed plagioclase crystals are then partially resorbed, forming the irregular cores. The residual melt following the early fractional crystallization stage has similar trace element chemistry to the later melt that crystallized the Ab-rich plagioclase infill. This lack of variation in melt chemistry implies that the patchy texture develops in response to an external factor such as P, T or pH2O.
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In the absence of varying melt composition, variations in XAn can be produced by varying any of temperature, pressure and pH2O (e.g. Housh & Luhr, 1991
T < 30°C) is required to move from early, An-rich to late, albitic plagioclase for a typical plagioclaseliquid pair (Fig. 18). Conversely, at constant T, large changes in pH2O (e.g.
P
40 MPa) are required (Fig. 18), equating to significant changes in depth. A positive
T is also possible, but
pH2O must be even larger (Fig. 18). These calculations show that patchy textures in plagioclase develop during conditions of decreasing temperature or decreasing pH2O.
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These conditions are consistent with textural indications of partial resorption, if the magma is initially undersaturated with respect to H2O. This is because significant resorption can occur during ascent of H2O-rich, but undersaturated, magmas (e.g. Holtz & Johannes, 1994
70°C/GPa; Annen et al., 2006
Patchy zoning in hornblende
The textural similarity between patchy plagioclase and hornblende (see Fig. 8) suggests a common resorption origin. Bright (late crystallizing) hornblende compositions have higher Aliv and (Na,K)A, but lower Mg-number than the earlier (Mg-rich, Al-poor) hornblende (Fig. 16b). Experimental studies indicate that this is consistent with crystallization of the later, Al-rich hornblende from a hotter melt (Scaillet & Evans, 1999
; Rutherford & Devine, 2003
). However, patchy plagioclase compositions indicate that temperatures changes are small, and that resorption is driven by changing pH2O (see Fig. 18). An alternative explanation is therefore that the composition of hornblende crystallizing is modified in response to the changing plagioclase composition. For example, decreasing pH2O in the melt at constant T promotes crystallization of Ab-rich plagioclase (Fig. 18). Hornblende is thus driven to more Al-rich compositions, reflecting the edeniterichterite compositional exchange (Holland & Blundy, 1994
):
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This explanation requires Al-rich hornblende to coexist with albitic plagioclase (An2954), and Al-poor hornblende to coexist with anorthitic plagioclase (An4274), without significant temperature changes. Hornblendeplagioclase thermometry shows that pairs of patchy hornblende compositions can coexist with pairs of patchy plagioclase compositions, at temperatures that agree within error. Larger changes in XAn require a greater shift in hornblende Al2O3. Changes in plagioclase composition, as a result of changing pH2O at near-constant T, can therefore cause the observed variations in hornblende composition.
Summary
Textures and compositions of patchy plagioclase indicate resorption as a result of H2O-undersaturated, adiabatic decompression (Fig. 16a). The textural similarity of patchy hornblende (Fig. 16b) suggests that a similar mechanism causes resorption of hornblende. The crystals originate at depth in an H2O-rich, but undersaturated magma. Resorption occurs during adiabatic ascent as a result of superheating relative to the liquidus (Annen et al., 2006
). When crystallization recommences, H2O contents in the magma are lower as a result of degassing, and the temperature may also be lower because of adiabatic cooling and the heat of resorption. This results in crystallization of Ab-rich plagioclase and entrapment of melt inclusions. The composition of the accompanying hornblende therefore becomes more Al-rich. Thus crystallization under water-saturated conditions produces the observed patchy textures in plagioclase and hornblende. The origin of resorbed phenocrysts has been attributed to restite from partial melting (Raia & Spera, 1997
) or cumulate material (Dungan & Davidson 2004
; Annen et al., 2006
). At Shiveluch, falling Ba in the melt during crystallization of An-rich patchy plagioclase (see Fig. 17a) and K2O variations in whole-rocks (see Fig. 3b) suggest some earlier crystallization of phlogopite. Inclusions of phlogopite occur in both plagioclase and hornblende phenocrysts, and phlogopite is also observed in cumulate xenoliths. We infer that phlogopite was a stable phenocryst phase early in the magmatic history, possibly during prolonged fractional crystallization of basalts at high pH2O in a crustal hot zone (Annen et al., 2006
). We therefore suggest, following Annen et al. (2006)
, that the patchy zoned plagioclase and hornblende phenocrysts are derived ultimately from cumulates or the high-P crystallization products of magmas.
Changes in oxygen fugacity
Compositional zoning in both hornblende (simple and multiple zoning) and apatite can be explained by changes in the oxygen fugacity of the melt. In this section we use experimental constraints to match compositional changes in hornblende and apatite to changes in external parameters (P, T, fO2), because the lack of trace element variation suggests that the melt composition did not change significantly.
The composition of hornblende responds to P, T, fO2 and pH2O, as described by Scaillet & Evans (1999)
. In their experiments on the Mount Pinatubo dacite, a magma very similar in bulk composition, mineralogy and oxygen fugacity to the Shiveluch andesite, temperature increases caused increased Altot, Na2O and TiO2 of hornblende. The behaviour of hornblende Mg-number with changing temperature is ambiguous: Scaillet & Evans (1999)
reported that Mg-number increases slightly with temperature, but this was contradicted by Rutherford & Devine (2003)
. We therefore assume that the effect of temperature on Mg-number is small. In contrast, Mg-number is strongly increased at higher fO2 (Scaillet & Evans, 1999
), because the activity of Fe2+ in the melt is reduced (Czamanske & Wones, 1973
).
Simple zoning in hornblende is characterized by a sharp increase in Mg-number, Na and Si, and decrease in Ti from the core to the rim (Fig. 16c). The cores also show slowly decreasing Aliv, NaA and Ti, mirrored by a small increase in Mg-number (see Fig. 14). This pattern is consistent with gradual cooling during crystallization of the core (Bachmann & Dungan, 2002
) followed by an increase in melt fO2 during rim crystallization. The (rare) normally zoned hornblende phenocrysts show a decrease in rim Mg-number. They are therefore inferred to have undergone a reduction in oxygen fugacity. The magnitude and nature of the compositional variations correspond well in simple- and multiple-zoned hornblende (Fig. 16c and e), suggesting that a similar mechanism causes both zoning types.
In apatite, zoning is most clearly seen in S. Apatite, Ca5(PO4)3(OH,Cl,F), incorporates sulphur by partial replacement of
by
. High sulphur contents in apatite are generated by high oxygen fugacity, because S6+ (
) is stabilized, and by high melt sulphur contents (e.g. Peng et al., 1997
). The effect of increasing temperature on sulphur contents in apatite is unclear: recent studies cite both increasing (Parat & Holtz, 2004
) and decreasing (Peng et al., 1997
) sulphur in apatite. Normal zoning in apatite could therefore be explained either by the melt becoming enriched in S, or by an increase in oxygen fugacity (Peng et al., 1997
; Parat & Holtz, 2004
). Apatite in the quenched enclave is more S-rich than apatite in the host andesite (see Fig. 15). However, fO2 in the quenched enclave is the same as that of the host andesite (see Fig. 4). This suggests that the high sulphur contents in apatite result from high melt sulphur, not high fO2.
Magma recharge
The xenocrysts of olivine and orthopyroxene, characterized by their Mg-rich compositions, strong normal zoning, well-developed reaction rims (olivine) or overgrowth rims (orthopyroxene), and inclusions of chromite (olivine), are clear indications of recharge of the andesite by a more mafic magma (e.g. Sakuyama, 1984
; Clynne & Borg, 1997
). Using the widths of reaction rims surrounding olivine, Dirksen et al. (2006)
constrained residence times of olivines in the andesite magma to between 2 months and 4 years, suggesting that mafic recharge occurred both shortly before and during the 20012004 eruption.
Mixing with felsic magma
As well as the presence of xenocrysts, zoning in phenocrysts can be used to detect the influx of a new magma. Sieve plagioclase phenocrysts (Fig. 16d) are observed at many arc volcanoes, and the texture is commonly attributed to reaction between the plagioclase and a melt in which more An-rich plagioclase is in equilibrium (e.g. Tsuchiyama, 1985
; Nakamura & Shimakita, 1998). This melt could be more mafic, or hotter, or both. A change in minor and trace elements between the sieve plagioclase core and rim would indicate a change in melt composition (Singer et al., 1995
); heating alone would not affect minor and trace element concentrations except through changes to the partition coefficients. We calculated Ba and Sr contents of melts coexisting with the sieve plagioclase (Fig. 17b), using the same method as for the patchy plagioclase. The calculations indicate that the melt in equilibrium with the rims was richer in Ba and Sr than that in equilibrium with the cores (Fig. 17b). This indicates that sieve plagioclase rim crystallization occurred in a hotter, more mafic melt than core crystallization. This interpretation is supported by higher values of Mg, Fe and XAn in sieve plagioclase rims (see Table 3 and Fig. 5), and higher temperatures as calculated by geothermometry (see Fig. 9d). The variable width of the sieved region is probably related to the degree of disequilibrium experienced by an individual crystal (Clynne, 1999
). However, the sieved cores probably come from a more felsic magma that mixed with the host andesite, because the cores are albitic and Fe-poor compared with most plagioclase phenocrysts (Fig. 5).
Mixing with mafic magma
The quench texture of the andesite enclave gives a clue to the zoning observed in both hornblende and apatite. To summarize, quenching is indicated by the large amounts (
15%) of microlite-free, vesicular glass (e.g. Eichelberger, 1980
), by the acicular habit of apatite (Wyllie et al., 1962
) and hornblende, and by dendritic extensions on some phenocryst and microphenocryst rims (see Fig. 2c and d). The composition of the interstitial glass is identical to that of the more evolved melt inclusions and matrix glasses in the host rock. Modal analysis suggests that the enclave bulk composition is slightly more mafic than the host silicic andesite. Estimated crystallization temperatures of the quenched enclave are consistent with those of the host magma (see Table 2). We propose the following sequence of events to explain these observations.
New basaltic-andesite melt, which was hotter and/or richer in dissolved volatiles (e.g. S) than the host andesite, intrudes the shallow magma storage region. The close similarity of temperature estimates and mineral and melt compositions of the enclave compared with those of the host magma suggests that the intruding magma was efficiently quenched, to a temperature very close to that of the host (Blundy & Sparks, 1992
. This means that the intruding magma must be very small in volume compared with the volume of the host magma (Sparks & Marshall, 1986
). Unlike many quenched inclusions, the edges of the enclave are defined by crystal margins, rather than a fine-grained chilled layer. This suggests that a rigid crystalline framework was present at its incorporation into the host. We infer that the enclave represents part of a larger inclusion that broke up in a semi-brittle fashion, although it is also possible that the intruding magma may have been already partially crystallized.
S contents in the melt of the enclave increase as crystallization occurs. This results in the formation of S-rich zones in apatite microphenocrysts (Fig. 16f) and the nucleation of anhydrite (Peng et al., 1997
). Reversely zoned apatite crystals (Fig. 16f) are probably derived from the intruding magma and transferred into the host andesite. When the melt vesiculates, S partitions into the vapour phase with H2O because the solubility of S in oxidized, silicic melt is low (Carroll & Rutherford, 1985
). Anhydrite, which is abundant in the enclaves, comes into contact with the host magma as the enclaves break up, where it begins to dissolve, releasing O2 and thus oxidizing the surrounding melt:
![]() |
The nominal CaSiO3 component is probably incorporated into hornblende or pyroxene. The Mg-rich zones of simple- and multiple-zoned hornblende (Fig. 16d and e) form under these more oxidizing conditions. It is possible that the rapid exsolution and vesiculation event in the intruding magma could also cause oxidation (Candela, 1986
; Peng et al., 1997
). We speculate that the rare, multiple-zoned hornblende phenocrysts may have undergone more than one oxidation event, perhaps during repeated injection or intricate mixing with the intruding andesite.
An important aspect of magma chamber evolution is highlighted by the mineralogy of the quenched enclave. The quenched enclave recorded crystallization temperatures of 808836°C, equivalent to those of the host andesite (see Table 2). Crystallization at this temperature resulted in the assemblage plag + hbl + oxides + opx, which is different from the expected near-liquidus assemblage of perhaps ol + cpx + plag. Blundy & Sparks (1992)
showed experimentally that when a magma is rapidly undercooled, the liquidus and near-liquidus assemblages are by-passed and only the low-T assemblage forms. Such efficient and rapid thermal equilibriation implies a very small volume of intruding magma relative to the volume of the host (Sparks & Marshall, 1986
). In contrast, more prolonged crystallization at high temperatures would be needed for an assemblage including olivine and pyroxene to form. This would require a proportionally larger volume of intruding magma (Sparks & Marshall, 1986
). In the latter case, clearly recognizable xenocrysts would form on break-up of enclaves. However, break-up of the hbplag enclave described here would result in xenocrysts indistinguishable, in terms of major elements, from primary phenocrysts.
These observations suggest that some sub-volcanic magma chambers may be kept topped up by a continuous (over long time-scales) drip-feed of small parcels of magma that are derived from depth. At Shiveluch, the above discussion shows that there is evidence for intrusion of both more mafic and more felsic magmas. The process may be punctuated by the arrival of larger batches of mafic magma, injected shortly prior to or during eruption. The consequence of drip-feeding very small parcels of magma into the system is that the major element composition of the residual liquid or melt will remain relatively constant because the residual liquids of the intruding magmas are close in composition (Fig. 19). In contrast, bulk-rock compositions may be drawn out towards the compositions of intruding magmas (Fig. 19) and by assimilation of cumulate material, as suggested in Fig. 3. We anticipate that, although the major element composition of the residual liquid remains relatively constant, this process may leave a trace element signature.
|
Oscillatory zoning
Various factors causing oscillatory zoning in plagioclase have been proposed, including kinetically or diffusion-controlled growth (Bottinga et al, 1966
Eruption dynamics
Recent work on numerical modelling of conduit flow processes (Melnik & Sparks, 2005
) during decompression-driven crystallization showed that unstable magma flow regimes can cause periodic variations in magma discharge rate, and hence oscillatory pressure fluctuations within the magma chamber. In addition to the pressure oscillations, replenishment by more mafic material (as occurs at Shiveluch) can produce oscillatory temperature changes at the top of the conduit (Melnik & Sparks, 2005
). Because the Shiveluch magma is H2O-saturated (see below), variations in pressure would affect pH2O of the magma. Because of the strong dependence of plagioclase XAn on both pH2O and T, the presence of unstable flow regimes provides a mechanism for the formation of oscillatory zoning in the rims of plagioclase phenocrysts. The pressure fluctuations can also explain the typical sharp increase in XAn and slower decay of the outer zones. As the pressure increases, plagioclase growth will slow and resorption may occur. The next zone will therefore begin by crystallizing An-rich plagioclase, and then decay back to more An-poor plagioclase.
To test this further, we used the plagioclaseliquid thermometer (Putirka, 2005
) to estimate the
pH2O or
T required to produce the observed XAn variation in the plagioclase zoning. XAn zoning ranges from 5 to 19 mol %, but is typically 915 mol % An. For a typical melt inclusion composition, this requires
pH2O of approximately +16 to +26 MPa, or
T of approximately +6 to +13°C. At low pressures, typical values are
pH2O of +3 to +7 MPa and
T of +8 to +15°C. These changes in pH2O of a few megapascals are equivalent to large vertical movements (e.g. 6501000 m, or 120300 m at shallower pressures), so convection in the magma chamber is also a possible explanation. However, the temperature and pressure oscillations predicted from numerical modelling are
720 MPa and
6080°C (Melnik & Sparks, 2005
), and thus sufficient to account for the observed oscillatory zoning in plagioclase.
Decompression-driven crystallization
The model requires that magma ascent from the chamber is accompanied by decompression-driven crystallization. At Shiveluch, this is indicated by the volatile contents and compositions of melt inclusions, which record the changing composition and volatile content of the evolving melt phase during crystallization of phenocryst rims and groundmass. The melt inclusions show a negative correlation of H2O with SiO2, and track towards the QzOr join on a H2O-saturated haplogranite ternary diagram (Humphreys, 2006
). These trends indicate H2O-saturated, decompression-driven crystallization (Blundy & Cashman, 2001
; 2005
), in keeping with numerous recent studies reporting decompression-driven crystallization at andesitic and dacitic volcanoes, such as Montserrat (Sparks et al., 2000
), Mount St. Helens (Blundy & Cashman, 2005
) and Unzen (Nishimura et al., 2005
).
The increasing Fe/Al in plagioclase rims can also be explained by H2O-saturated, decompression-driven crystallization. Increasing Fe or XAn could reflect changes in melt chemistry. However, this seems unlikely because the highest Fe/Al values are observed at the outermost rim. Experimental data indicate that both DFe and Fe/Al in plagioclase increase with increasing fO2 (Wilke & Behrens, 1999
). We therefore ascribe increasing Fe and Fe/Al in plagioclase to gradually increasing oxygen fugacity during the late stages of crystallization. Candela (1986)
showed that the release of aqueous vapour results in oxidation of the melt as a result of preferential loss of H2. At Shiveluch, this is supported by observations that groundmass magnetite is concentrated near bubbles (P. Pletchov (2004), personal communication). We note that, because DFe also increases with T (Wilke & Behrens, 1999
), it is possible that increasing Fe/Al could also result from rising temperatures as a result of release of latent heat during decompression-driven crystallization (Blundy et al., 2006
). Either way, the high Fe/Al in plagioclase phenocryst rims and groundmass plagioclase is consistent with crystallization driven by degassing of H2O.
Prolonged history of crystallization
We now consider the origin of the diffuse, broad zones found in the cores of oscillatory zoned phenocrysts (Fig. 6). These are probably produced by a different mechanism compared with the narrower, sharp outer zones because they show an approximately inverse relationship between XAn and Fe/Al, which is not seen in the outermost zones (see Fig. 6). The diffuse nature of the core zoning implies long residence times to allow for slow CaAlNaSi diffusion (Grove et al., 1984
). We therefore suggest that the cores of oscillatory zoned crystals represent the dominant crystal population resident in the chamber over long periods of time. The compositional zoning could result from regular inputs of fresh magma and continued slow growth as a result of cooling or degassing, whereas the rims reflect later growth. This interpretation is consistent with studies that have shown a wide discrepancy between the ages of phenocryst cores and rims (e.g. Turner et al., 2003
). We suggest that the unzoned hornblendes also represent this resident crystal population.
Textural overlap between phenocrysts
We have tried to explain the textural variations between phenocrysts by relating them to processing occurring during ascent and mixing of separate batches of magma (Fig. 20). These processes include crystallization, remobilization and resorption of cumulates (patchy phenocrysts), breakdown of anhydrite during mixing of sulphur-rich magmas (apatite and hornblende), and mixing with mafic magmas, whether basalt or basaltic andesite. If different crystal populations share a textural feature (e.g. shared rims), this would imply that batches of crystals had a common crystallization environment for some length of time. In contrast, if there is no overlap between phenocryst types, the different crystal populations must be spatially or temporally isolated during crystallization. In the lavas studied, there is limited evidence of different textures overlapping between phenocryst populations. Most phenocryst populations tend to record a distinct and individual magmatic history, implying that they were mixed together very late or even during eruption. However, their close spatial relationship in the erupted samples suggests that crystals with different histories may not have been far apart in absolute distance (e.g. Kouchi & Sunagawa, 1985
).
|
There are three cases where textural overlap does occur. First, oscillatory zoned cores are observed in some sieve plagioclase crystals (Fig. 9). Second, oscillatory zoned rims occur on patchy plagioclase cores (Figs 6 and 16a). Third, some patchy hornblende phenocrysts seem to have similar oscillatory zonations in their rims (see Fig. 14a). The occurrence of oscillatory zoning in patchy plagioclase rims indicates that the patchy phenocrysts experienced the same rhythmic fluctuations in temperature or pH2O as the oscillatory phenocrysts. In our interpretation, this is because the patchy cores crystallized much earlier and were derived from cumulate material, but, after mixing, shared the later stages of crystallization with the oscillatory plagioclase. The oscillatory zoning in some hornblende rims may be a response to the same PT fluctuations. In contrast, sieve plagioclase crystals only show a single zone in the rim, but sometimes have oscillatory zoning in their cores. This implies that these sieve-textured crystals are rather younger than most other plagioclase crystals (i.e. they were mixed in even later in the magmatic history). Figure 20 illustrates schematically the possible spatial relationships between phenocryst populations.
Our main temporal constraint on the time-scales of mixing comes from the lack of breakdown rims on hornblende phenocrysts. Work on the growth rate of amphibole breakdown rims suggests that this final mixing must take place over very short time-scales; that is, less than 56 days (Rutherford & Hill, 1993
; Browne & Gardner, 2002
). Even taking into account the much lower diffusivity of Si in a melt with 80 wt % SiO2, low H2O and at lower temperatures, ascent times must still be of the order of 23 weeks (Buckley, 2003
) to avoid hornblende breakdown. This implies that the last stages of crystallization occurred rapidly.
| CONCLUSIONS |
|---|
|
|
|---|
Andesites from Shiveluch Volcano contain petrological evidence for open-system magmatic processes. Patchy zoned hornblende and plagioclase phenocrysts represent high-pressure crystals or cumulates, remobilized as H2O-undersaturated melts rise into the upper crust, and resorbed during ascent. When crystallization starts again, resorbed plagioclase cores are infilled and overgrown with more albitic plagioclase, driving hornblende to crystallize more Al-rich compositions. The magma chamber accumulates slowly, by intermittent addition of small volumes of magma, forming a dominant population of unzoned hornblende and oscillatory zoned plagioclase cores. The replenishing magmas vary in composition, including cooler silicic dacite and hotter mafic andesite, which forms enclaves rather than cryptic mixing. The crystallization temperature and mineralogy of the hotter magma are identical to those of the host andesite, indicating efficient heat transfer and thus small volumes of intruding magma. Differences in dissolved volatile contents between intruding magmas can result in significant geochemical zonation in hornblende and apatite.
Mg-rich orthopyroxene and olivine xenocrysts indicate late-stage influx of hotter magma from depth, causing chamber mixing contemporaneous with eruption. Decompression in the conduit during eruption leads to crystallization of plagioclase and other groundmass phases, accompanied by oxidation resulting from exsolution of H2O. Oscillatory zonation in plagioclase may result from pressure and temperature oscillations in the subvolcanic plumbing system, related to instabilities in magma discharge rate.
| ACKNOWLEDGEMENTS |
|---|
M.C.S.H. acknowledges the support of an NERC studentship. J.D.B. is supported by an NERC Senior Research Fellowship, and R.S.J.S. acknowledges a Royal Society Wolfson Merit Award. We are very grateful to Stuart Kearns and Bruce Paterson (University of Bristol), Nick Marsh and Tim Brewer (Leicester University) and the staff of the NERC ion microprobe facility (University of Edinburgh) for assistance with the acquisition of analytical data. The manuscript was significantly improved following thorough and constructive reviews by Mike Clynne, Catherine Ginibre and Dan Morgan, and thought-provoking critical comments from an anonymous reviewer.
*Corresponding author. Present address: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. Telephone: 0122 376 5260. Fax: 0122 333 3450. E-mail: mcsh2{at}cam.ac.uk
| REFERENCES |
|---|
|
|
|---|
Allègre C. J., Provost A., Jaupart C. (1981) Oscillatory zoning: a pathological case of crystal growth. Nature 294:223228.[CrossRef]
Andersen D. J., Lindsley D. H., Davidson P. M. (1993) QUILF: a Pascal program to assess equilibria among FeMgTi oxides, pyroxenes, olivine and quartz. Computers and Geosciences 19:13331350.
Anderson A. T. (1976) Magma mixing: petrological process and volcanological tool. Journal of Volcanology and Geothermal Research 1:333.[CrossRef][Web of Science]
Annen C., Blundy J. D., Sparks R. S. J. (2006) The genesis of intermediate and silicic magmas in deep crustal hot zones. Journal of Petrology 47:3505539.
Arculus R. J. and Wills K. J. A. (1980) The petrology of plutonic blocks and inclusions from the Lesser Antilles island arc. Journal of Petrology 21:743799.
Atherton M. P. (1999) Shape and intrusion style of the Coastal batholith, Peru. 4th International Symposium on Andean Geodynamics pp. 6063 (extended abstract).
Bachmann O. and Dungan M. A. (2002) Temperature-induced Al-zoning in hornblendes of the Fish Canyon magma, Colorado. American Mineralogist 87:10621076.
Baranov B.V., Seliverstov N.I., Murav'ev A. V., Muzurov E. L. (1991) The Komandorsky Basin as a product of spreading behind a transform plate boundary. Tectonophysics 199:237269.[CrossRef][Web of Science]
Bateman P. C. (1992) Journal of Volcanology and Geothermal Research 66:357365.
Belousov A. B. (1995) The Shiveluch volcanic eruption of 12 November 1964explosive eruption provoked by failure of the edifice. Journal of Volcanology and Geothermal Research 66:357365.[CrossRef][Web of Science]
Belousov A. B., Belousova M., Voight B. (1999) Multiple edifice failures, debris avalanches and associated eruptions in the Holocene history of Shiveluch volcano, Kamchatka, Russia. Bulletin of Volcanology 61:324342.[CrossRef][Web of Science]
Blundy J. and Cashman K. (2001) Ascent-driven crystallisation of dacite magmas at Mount St. Helens, 19801986. Contributions to Mineralogy and Petrology 140:631650.[Web of Science]
Blundy J. and Cashman K. (2005) Rapid decompression-driven crystallization recorded by melt inclusions from Mount St. Helens volcano. Geology 33:793796.
Blundy J., Cashman K., Humphreys M. (2006) Magma heating by decompression-driven crystallisation beneath andesite volcanoes. Nature 443:7680.[CrossRef][Medline]
Blundy J. D. and Sparks R. S. J. (1992) Petrogenesis of mafic inclusions in granitoids of the Adamello Massif, Italy. Journal of Petrology 33:10391104.
Blundy J. D. and Wood B.J. (1991) Crystal-chemical controls on the partitioning of Sr and Ba between plagioclase feldspar, silicate melts and hydrothermal solutions. Geochimica et Cosmochimica Acta 55:193209.[CrossRef][Web of Science]
Bottinga Y., Kudo A., Weill D. (1966) Some observations on oscillatory zoning and crystallization of magmatic plagioclase. American Mineralogist 51:792806.[Web of Science]
Browne B. L. and Gardner J. E. (2002) Experimental calibration of amphibole breakdown rates in response to decompression and heating. EOS Transactions, American Geophysical Union 83:F1464.
Buckley V. J. E. (2003) Hornblende dehydration during magma decompression. Unpublished Ph.D. thesis, University of Bristol.
Calanchi N., De Rosa R., Mazzuoli R., Rossi P., Santacroce R., Ventura G. (1993) Silicic magma entering a basaltic magma chamber: eruptive dynamics and magma mixingan example from Salina (Aeolian islands, South Tyrrhenian Sea). Bulletin of Volcanology 55:504522.[CrossRef][Web of Science]
Candela P. A. (1986) The evolution of aqueous vapour from silicate melts: effect on oxygen fugacity. Geochimica et Cosmochimica Acta 50:12051211.[CrossRef][Web of Science]
Carroll M. R. and Rutherford M. J. (1985) Sulfide and sulfate saturation in hydrous silicate melts. Journal of Geophysical Research 90:C601C612.[Web of Science]
Carroll M. R. and Rutherford M. J. (1988) Sulfur speciation in hydrous experimental glasses of varying oxidation state: results from measured wavelength shifts of sulfur X-rays. American Mineralogist 73:845849.[Abstract]
Clemens J. D., Petford N., Mawer C. K. (1997) Ascent mechanisms of granitic magmas: causes and consequences. In Holness M. (Ed.). Deformation-enhanced fluid transport in the Earth's crust and mantle(Chapman & Hall, London) pp. 144171.
Clynne M. A. (1999) A complex magma mixing origin for rocks erupted in 1915, Lassen Peak, California. Journal of Petrology 40:105132.[CrossRef][Web of Science]
Clynne M. A. and Borg L. E. (1997) Olivine and chromian spinel in primitive calc-alkaline and tholeiitic lavas from the southernmost Cascade Range, California: a reflection of relative fertility of the source. Canadian Mineralogist 35:2453472.[Web of Science]
Couch S., Sparks R. S. J., Carroll M. R. (2001) Mineral disequilibrium in lavas explained by convective self-mixing in open magma chambers. Nature 411:10371039.[CrossRef]
Czamanske G. K. and Wones D. R. (1973) Oxidation during magmatic differentiation, Finnmarka Complex, Oslo Area, Norway: Part 2, The mafic silicates. Journal of Petrology 14:349380.
D'Lemos R. S. (1996) Mixing between granitic and dioritic crystal mushes, Guernsey, Channel Islands, UK. Lithos 38:233257.[CrossRef][Web of Science]
Dirksen O., Humphreys M. C. S., Pletchov P., Melnik O., Demyanchuk Y., Sparks R. S. J., Mahony S. (2006) The 20012004 dome-forming eruption of Shiveluch Volcano, Kamchatka: observation, petrological investigation and numerical modelling. Journal of Volcanology and Geothermal Research doi:10.1016/j.jvolgeores.2006.03.029.
Dungan M. A. and Davidson J. (2004) Partial assimilative recycling of the mafic plutonic roots of arc volcanoes: an example from the Chilean Andes. Geology 32:773776.
Eichelberger J. C. (1978) Andesitic volcanism and crustal evolution. Nature 275:2127.[CrossRef]
Eichelberger J. C. (1980) Vesiculation of mafic magma during replenishment of silicic magma reservoirs. Nature 288:446450.[CrossRef]
Firstov P. P., Gavrilov V. A., Zhadnova E., Yu E., Kiriyanov V. Yu. (1994) Onset of new extrusive eruption of Shiveluch in April 1993. Volcanology and Seismology 45:3347 (in Russian).
Geist E. L. and Scholl D. W. (1994) Large-scale deformation related to the collision of the Aleutian arc with Kamchatka. Tectonics 13:538560.[CrossRef][Web of Science]
Gerlach D. C. and Grove T. L. (1982) Petrology of Medicine Lake Highland volcanicscharacterization of end-memebers of magma mixing. Contributions to Mineralogy and Petrology 80:147159.[CrossRef][Web of Science]
Geschwind C.-H. and Rutherford M. J. (1995) Crystallization of microlites during magma ascent: the fluid mechanics of 19801986 eruption at Mount St Helens. Bulletin of Volcanology 57:356370.[Web of Science]
Gill J. B. (1981) Orogenic Andesites and Plate Tectonics(Springer, Berlin).
Ginibre C., Woerner G., Kronz A. (2002) Minor- and trace-element zoning in plagioclase: implications for magma chamber proceses at Parinacota volcano, northern Chile. Contributions to Mineralogy and Petrology 143:300315.[Web of Science]
Glazner A. F., Bartley J. M., Coleman D. S., Gray W., Taylor R. Z. (2004) Are plutons assembled over millions of years by amalgamation from small magma chambers? GSA Today 14:411.
Grove T. L., Baker M. B., Kinzler R. J. (1984) Coupled CaAlNaSi diffusion in plagioclase feldspar: experiments and applications to cooling rate speedometry. Geochimica et Cosmochimica Acta 48:21132121.[CrossRef][Web of Science]
Hammond P. A. and Taylor L.A. (1982) The ilmenite/titano-magnetite assemblage: kinetics of re-equilibration. Earth and Planetary Science Letters 61:143150.[CrossRef][Web of Science]
Hofmeister A. M. and Rossman G. R. (1984) Determination of Fe3+ and Fe2+ concentrations in feldspar by optical-absorption and electron-paramagnetic-resonance spectroscopy. Physics and Chemistry of Minerals 11:213224.[CrossRef][Web of Science]
Holland T. and Blundy J. D. (1994) Non-ideal interactions in calcic amphiboles and their bearing on amphiboleplagioclase thermometry. Contributions to Mineralogy and Petrology 116:433447.[CrossRef][Web of Science]
Holtz F. and Johannes W. (1994) Maximum and minimum water contents of granitic melts: implications for chemical and physical properties of ascending magmas. Lithos 32:149159.[CrossRef][Web of Science]
Holtz F., Johannes W., Tamic N., Behrens H. (2001) Maximum and minimum water contents of granitic melts generated in the crust: a reevaluation and implications. Lithos 56:114.[CrossRef][Web of Science]
Housh T. B. and Luhr J. F. (1991) Plagioclasemelt equilibria in hydrous systems. American Mineralogist 76:477492.[Abstract]
Humphreys M. C. S. (2006) Andesite petrogenesis and magmatic processes at Shiveluch Volcano, Kamchatka. Ph.D. thesis, University of Bristol.
Humphreys M. C. S., Kearns S., Blundy J. D. (2006) SIMS investigation of electron-beam damage to hydrous, rhyolitic glasses: implications for melt inclusion analysis. American Mineralogist 91:667679.
Huppert H. E., Sparks R. S. J., Turner J. S. (1982) Effect of volatiles on mixing in calc-alkaline magma systems. Nature 297:554557.[CrossRef]
Jellinek A. M., Kerr R. C., Griffiths R. W. (1999) Mixing and compositional stratification produced by natural convection. 1. Experiments and their application to Earth's core and mantle. Journal of Geophysical Research 104:B471837201.[CrossRef]
Johannes W. and Holtz F. (1996) Petrogenesis and Experimental Petrology of Granitic Rocks(Springer, Berlin).
John B. E. and Blundy J. D. (1993) Emplacement-related deformation of granitoid magmas, southern Adamello Massif, Italy. Geological Society of America Bulletin, 105:1215171541.
Johnson M. C. and Rutherford M. J. (1989) Experimental calibration of the aluminium-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks. Geology 17:837841.
Kawamoto T. (1992) Dusty and honeycomb plagioclaseindicators of processes in the Uchino stratified magma chamber, Izu Peninsula, Japan. Journal of Volcanology and Geothermal Research 49:191208.[CrossRef][Web of Science]
Kouchi A. and Sunagawa I. (1985) A model for mixing basaltic and dacitic magmas as deduced from experimental data. Contributions to Mineralogy and Petrology 89:1723.[CrossRef][Web of Science]
Koyaguchi T. (1985) Magma mixing in a volcanic conduit. Journal of Volcanology and Geothermal Research 25:365369.[CrossRef][Web of Science]
Leake B. E. (1978) Nomenclature of amphiboles. American Mineralogist 63:10231052.[Abstract]
Lindsley D. H. (1983) Pyroxene thermometry. American Mineralogist 68:477493.[Abstract]
Liu Y. and Comodi P. (1993) Some aspects of the crystal-chemistry of apatites. Mineralogical Magazine 57:709719.[Web of Science]
Longerich H. P., Jackson S. E., Günther D. (1996) Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. Journal of Atomic Analytical Spectrometry 11:899904.[CrossRef]
Longhi J., Walker D., Hays J. F. (1976) Fe and Mg in plagioclase. Proceedings of the 7th Lunar Science Conference. Geochimica et Cosmochimica Acta Supplement 12811300.
Matthews S. J., Jones A. P., Beard A. D. (1994) Buffering of melt oxygen fugacity by sulphur redox reactions in calc-alkaline magmas. Journal of the Geological Society, London 151:815823.
McBirney A. R. (1980) Mixing and unmixing of magmas. Journal of Volcanology and Geothermal Research 7:357371.[CrossRef][Web of Science]
Melekestsev I. V., et al. (1991) Shiveluch Volcano. In Fedotov S. A., Masurenkov S. A., Yu P. (Eds.). Active Volcanoes of Kamchatka(Nauka, Moscow) Vol. 1:.
Melnik O. and Sparks R. S. J. (2005) Controls on conduit magma flow dynamics during lava dome building eruptions. Journal of Geophysical Research 110: B02209, doi:10.1029/2004JB003183.
Menyailov A. A. (1955) Shiveluch Volcano, its Geological Structure, Composition and Eruptions. Transactions of the Volcanology Laboratory, USSR Academy of Sciences 9:264 pp.
Miyashiro A. (1974) Volcanic rock series in island arcs and active continental margins. American Journal of Science 274:321355.[Abstract]
Murphy M. D., Sparks R. S. J., Barclay J., Carroll M. R., Lejeune A. M., Brewer T. S., Macdonald R., Black S., Young S. (1998) The role of magma mixing in triggering the current eruption at the Soufrière Hills volcano, Montserrat, West Indies. Geophysical Research Letters 25:34333436.[CrossRef][Web of Science]
Murphy M. D., Sparks R. S. J., Barclay J., Carroll M. R., Brewer T. S. (2000) Remobilization of andesite magma by intrusion of mafic magma at the Soufrière Hills Volcano, Montserrat, West Indies. Journal of Petrology 41:2142.
Nakada S. and Motomura Y. (1999) Petrology of the 19911995 eruption at Unzen: effusion pulsation and groundmass crystallization. Journal of Volcanology and Geothermal Research 89:173196.[CrossRef][Web of Science]
Nakamara M. and Shimakita S. (1998) Dissolution origin and syn-entrapment compositional change of melt inclusion in plagioclase. Earth and Planetary Science Letters 161:119133.[CrossRef][Web of Science]
Nash W. P. and Crecraft H. R. (1985) Partition coefficients for trace elements in silicic magmas. Geochimica et Cosmochimica Acta 49:23092322.[CrossRef][Web of Science]
Nelson S. T. and Montana A. (1992) Sieve-textured plagioclase in volcanic rocks produced by rapid decompression. American Mineralogist 77:12421249.[Abstract]
Newman S. and Lowenstern J. B. (2002) VolatileCalc: a silicate meltH2OCO2 solution model written in Visual Basic for Excel. Computers and Geosciences 28:597604.[CrossRef]
Nishimura K., Kawamoto T., Kobayashi T., Sugimoto T., Yamashita S. (2005) Melt inclusion analysis of the Unzen 19911995 dacite: implications for crystallisation processes of dacite magma. Bulletin of Volcanology 67:648662.[CrossRef][Web of Science]
Parat F. and Holtz F. (2004) Sulfur partitioning between apatite and melt and effect of sulfur on apatite solubility at oxidizing conditions. Contributions to Mineralogy and Petrology 147:201212.[CrossRef][Web of Science]
Parat F., Dungan M. A., Streck M. J. (2002) Anhydrite, pyrrhotite, and sulfur-rich apatite: tracing the sulfur evolution of an Oligocene andesite (Eagle Mountain, CO, USA). Lithos 64:6375.[CrossRef][Web of Science]
Pearce T. H. and Kolisnik A. M. (1990) Observations of plagioclase zoning using interference imaging. Earth-Science Reviews 29:926.
Pearce N. J. G., Perkins W. T., Westgate J. A., Gorton M. P., Jackson S. E., Neal C. R., Chenery S. P. (1997) A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glass reference materials. Geostandards Newsletter 21:115144.[CrossRef][Web of Science]
Peng G., Luhr J. F., McGee J. J. (1997) Factors controlling sulfur concentrations in volcanic apatite. American Mineralogist 82:12101224.[Abstract]
Portnyagin M., Hoernle K., Avdeiko G., Hauff F. (2005) Transition from arc to oceanic magmatism at the KamchatkaAleutian junction. Geology 33:12528.
Pouchou J. L. and Pichoir F. (1984) PAP (
z) correction procedure for improved quantitative microanalysis. In Armstrong J. T. (Ed.). Microbeam Analysis(San Francisco Press, San Francisco, CA) pp. 104106.
Putirka K. D. (2005) Igneous thermometers and barometers based on plagioclase + liquid equilibria: tests of some existing models and new calibrations. American Mineralogist 90:336346.
Pyle D. M., Ivanovich M., Sparks R. S. J. (1988) Magmacumulate mixing identified by UTh disequilibrium dating. Nature 331:157159.[CrossRef]
Raia F. and Spera F. J. (1997) Simulation for the growth and differentiation of continental crust. Journal of Geophysical Research 102:2262922648.[CrossRef][Web of Science]
Rutherford M. J. and Devine J. D. (2003) Magmatic conditions and magma ascent as indicated by hornblende phase equilibria and reactions in the 19952002 Soufrière Hills magma. Journal of Petrology 44:14331454.
Rutherford M. J. and Hill P. M. (1993) Magma ascent rates from amphibole breakdown: an experimental study applied to the 19801986 Mount St. Helens eruptions. Journal of Geophysical Research 98:1966719685.[CrossRef]
Sakuyama M. (1984) Magma mixing and magma plumbing systems in island arcs. Bulletin of Volcanology 47:685703.[CrossRef]
Scaillet B. and Evans B. W. (1999) The 15 June 1991 eruption of Mount Pinatubo. I. Phase equilibria and pre-eruption PTfO2fH2O conditions of the dacite magma. Journal of Petrology 40:381411.[CrossRef][Web of Science]
Singer B. S., Dungan M. A., Layne G. D. (1995) Textures and Sr, Ba, Mg, Fe, K, and Ti compositional profiles in volcanic plagioclase: clues to the dynamics of calc-alkaline magma chambers. American Mineralogist 80:776798.[Abstract]
Snyder D. and Tait S. (1996) Magma mixing by convective entrainment. Nature 379:529531.[CrossRef]
Sparks R. S. J. and Marshall L. A. (1986) Thermal and mechanical constraints on mixing between mafic and silicic magmas. Journal of Volcanology and Geothermal Research 29:99124.[CrossRef][Web of Science]
Sparks R. S. J., Sigurdsson H., Wilson L. (1977) Magma mixingmechanism for triggering acid explosive eruptions. Nature 267:315318.[CrossRef]
Sparks R. S. J., Murphy M. D., Lejeune A. M., Watts R. B., Barclay J., Young S. R. (2000) Control on the emplacement of the andesite lava dome of the Soufrière Hills Volcano, Montserrat by degassing-induced crystallization. Terra Nova 12:1420.[CrossRef][Web of Science]
Stormer J. C. (1983) The effects of recalculation on estimates of temperature and oxygen fugacity from analyses of multicomponent iron titanium oxides. American Mineralogist 68:586594.[Abstract]
Sugawara T. (2000) Thermodynamic analysis of Fe and Mg partitioning between plagioclase and silicate liquid. Contributions to Mineralogy and Petrologyz 138:101113.
Tiepolo M., Zanetti A., Oberti R., Brumm R., Foley S., Vannucci R. (2003) Trace-element partitioning between synthetic potassic-richterites and silicate melts, and contrasts with the partitioning behaviour of pargasite and kaersutites. European Journal of Mineralogy 15:329340.
Tischendorf G., Förster H.-J., Gottsman B. (2001) Minor- and trace-element composition of trioctahedral micas: a review. Mineralogical Magazine 65:249276.
Tsuchiyama A. (1985) Dissolution kinetics of plagioclase in the melt of the system diopsidealbiteanorthite, and origin of dusty plagioclase in andesites. Contributions to Mineralogy and Petrology 89:116.[CrossRef][Web of Science]
Turner S., George R., Jerram D. A., Carpenter N., Hawkesworth C. (2003) Case studies of plagioclase growth and residence times in island arc lavas from Tonga and the Lesser Antilles, and a model to reconcile discordant age information. Earth and Planetary Science Letters 214:279294.[CrossRef][Web of Science]
Vance J. A. (1965) Zoning in igneous plagioclase: patchy zoning. Journal of Geology 73:636651.
van Lamoen H. (1979) Coronas in olivine gabbros and iron ores from Susimaki and Riuttamaa, Finland. Contributions to Mineralogy and Petrology 68:259268.[CrossRef][Web of Science]
Venezky D. Y. and Rutherford M. J. (1999) Petrology and FeTi oxide re-equilibration of the 1991 Mount Unzen mixed magma. Journal of Volcanology and Geothermal Research 89:213230.[CrossRef][Web of Science]
Wiebe R. A. (1987) Rupture and inflation of a basic magma chamber by silicic liquid. Nature 326:6971.[CrossRef]
Wiebe R. A. and Collins W. J. (1998) Depositional features and stratigraphic sections in granitic plutons: implications for the emplacement and crystallization of granitic magma. Journal of Structural Geology 20:12731289.[CrossRef][Web of Science]
Wilke M. and Behrens H. (1999) The dependence of the partitioning of iron and europium between plagioclase and hydrous tonalitic melt on oxygen fugacity. Contributions to Mineralogy and Petrology 137:102114.[CrossRef][Web of Science]
Wyllie P. J., Cox K. G., Biggar G. M. (1962) The habit of apatite in synthetic and igneous systems. Journal of Petrology 3:238243.
Zharinov N. A., Bogoyavlenskaya G. E., Khubunaya S. A., Demyanchuk S. A., Yu V. (1995) New eruptive cycle on Shiveluch Volcano19801993. Volcanology & Seismology 1:2028 (in Russian).
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