只剩骄傲All of the above observations [(1)–(6)]are consistent with this interpretation,as are the observed zoning patterns and rim compositions of plagioclase phenocrysts (Fig.3).For example,the phenocryst assemblage and proportions of the Colima andesite (Fig.3a)can be reproduced closely
at 950–960 C (consistent with mineral thermometry on the natural lava)and p H 2O from 70to 150MPa (Moore &Carmichael,1998).The very calcic cores of some plagioclase phenocrysts (An
85)were ascribed by Moore
&Carmichael (1998)to the onset of crystallization at
even higher p H 2O but at essentially the same 0025710d561252d381eb6e31ing analyses of phenocryst-hosted melt inclusions,Blundy &Cashman (2005)advanced a similar argument
for the silicic andesites of Mount St.Helens.They pro-posed that the observed phenocryst assemblage of the white pumice of 18May 1980crystallized in response to decompression from 233to 140MPa at a near-constant temperature of $900 C,whereas the sub-sequent microlite-bearing dome lavas continued to crystallize down to pressures as low as 9MPa with negli-
gible cooling.Another example is the Soufrie `re Hills andesite,Montserrat,where An 50–60plagioclase inclu-sions in the cores of amphibole phenocrysts (Higgins &Roberge,2003),combined with experimental data (Couch et al .,2003;Rutherford &Devine,2003),indicate protracted polybaric crystallization at temperatures suffi-ciently low to stabilize amphibole (840–880 C;Murphy et al .,2000;Devine et al .,2003;Rutherford &Devine,2003).Major element chemistry of whole-rocks,pheno-
crysts and groundmass glass (Murphy et al .,2000;Harford et al .,2002)is consistent with crystallization of predominantly amphibole and plagioclase from a liquid whose initial andesite composition evolved to rhyolite as crystallization proceeded.
All of the above examples suggest that decompression crystallization can play a major role in determining the crystallization sequence,assemblage and proportions.That is not to say that cooling is not important in some circumstances,nor that reheating caused by magma mix-ing does not occur:there is compelling evidence for both processes in many andesite magmas.
An attractive attribute of polybaric,decompression-driven crystallization is that it can be very rapid in com-parison with the slow rates of crystallization expected for cooling-driven crystallization caused by heat loss from shallow magma chambers.For example,consider the case of H 2O-saturated Colima andesite.To generate the observed phenocryst proportions by isobaric cooling alone would require a temperature drop of some 125 C at p H 2O ?70MPa (Moore &Carmichael,1998).To attain the same crystallinity by isothermal decompression
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(at 960 C)would require a pressure drop of 60MPa,equivalent to an ascent of $2km.A pressure drop can be achieved much more rapidly than a temperature drop,as follows.Cooling of shallow magma chambers is controlled by conduction through the wall-rocks and convection within the magma body and the superjacent hydrothermal system (Carrigan,1988).The cooling timescale is con-trolled by the magma chamber size and the vigour of hydrothermal convection.The world’s most active geo-thermal systems associated with large silicic magma chambers have convective thermal fluxes of several W/m 2(Carrigan,1988).Assuming that the magma chamber convects internally,then the heat loss from the
chamber
0025710d561252d381eb6e31parison of experimental and natural whole-rock phase proportions (weight percent)for selected andesite compositions.(a)Volca ′n Colima,Mexico (Moore &Carmichael,1998);(b)Mont Pele ′e,Martinique (Martel et al .,1999);(c)Mount Pinatubo,Philippines (Scaillet &Evans,1999;B.Scaillet,personal communication,2004);(d)Valle de Bravo,Mexico (Blatter &Carmichael,2001).All experiments are H 2O-saturated at the pressure and temperature shown.Only experiments in which the temperature is close to that inferred from mineral thermometry of the whole-rock are shown.Also shown is the molar anorthite (An )content of plagioclase.gl,glass;plag,plagioclase;amph,amphibole;opx,orthopyroxene;cpx,clinopyroxene;ox,oxide.
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can be converted into the time required to cool the chamber to a given temperature by a heat balance calculation.For example,for a cylindrical chamber with 1km radius and 1km depth (a volume of $3km 3),the time to cool the magma internally by $100 C is calculated at 8400years for a heat flow of 2á2W/m 2and a heat loss of 220kJ/kg assuming 20%crystalliza-tion,a latent heat of 419kJ/kg and heat capacity of 1361kJ/kg per 0025710d561252d381eb6e31rger chambers or lower hydro-
thermal heat fluxes would increase crystallization times
significantly.
In contrast,magma ascent into the shallow crust is envisaged to occur in dykes (Petford et al .,1993)at speeds of cm/s to dm/s.The time taken for an H 2O-saturated andesite melt to ascend 2km would be a matter of hours (Lister &Kerr,1991;Petford et al .,1993),thereby gener-ating a significant undercooling caused by gas exsolution,leading to rapid nucleation and growth of crystals.Rapid crystallization of phenocrysts in arc magmas is consistent with U-series data (Reagan et al .,2005)and diffusion dating studies of phenocrysts (Zellmer et al .,1999,2003b ;Costa et al .,2004),which suggest crystallization on time-scales that are far more rapid than would be expected for crystallization driven by cooling alone.Rapid crystalliza-tion also provides an effective means of generating the near-closed system crystallization inferred from experi-mental studies,because the timescales are too short to permit significant crystal–melt segregation,for example by crystal settling.The physical consequences of decom-pression crystallization are discussed further in a later section.Whatever the cause of crystallization,the experimental data present a compelling argument that the chemical composition of andesites is determined at depth,prior to magma emplacement in the shallow crust.Of course,this concept does not exclude subsequent processing of andesite magmas in shallow chambers,including magma mixing and more advanced fractional crystallization.For example,at Santorini,Greece,dacites and rhyolites can be demonstrably related to andesite by low-pressure frac-tional crystallization of orthopyroxene–clinopyroxene–plagioclase–oxide assemblages (Nicholls,1971;Druitt et al .,1999),whereas at Crater Lake,USA,rhyolite magma accumulated prior to the climactic eruption of Mount Mazama by repeated injection of andesite mag-mas into a shallow chamber,and extraction of residual rhyolitic melts by filter pressing (Sisson &Bacon,1999).Partially solidified andesitic bodies,or ‘proto-plutons’,with >50%crystals can also be remobilized by sub-sequent pulses of hot magma from below,as envisaged at Soufrie `re Hills (Couch et al .,2003)and Fish Canyon Tuff,USA (Bachmann &Dungan,2002).There are also examples of zoned plutons,such as Boggy Plain,Australia (Wyborn et al .,2001)where in situ fractionation from andesite to more evolved magmas has occurred.Additionally,such magma bodies are likely to develop incrementally over long periods of time so that mixing occurs between rising batches of andesite from depth (Fig.1).The key concept is that the starting point for shallow chamber processes (e.g.further fractionation,wall-rock assimilation,magma mixing,magma recharge,repeated remobilization,etc.)is andesite,itself generated at greater depths.
EVIDENCE FOR HIGH H 2O
CONTENTS IN ARC MAGMAS
Observations (Anderson,1979;Murphy et al .,2000;Cervantes &Wallace,2003)and experimental studies (e.g.Sisson &Grove,1993;Pichavant et al .,2002a ;Barclay &Carmichael,2004)indicate that many arc basalts have H 2O contents in the range 2–6wt %.Evolved residual melt obtained by crystallization of such basalts will be even more H 2O-rich provided that the pressure is high enough for H 2O to remain in solution.For example,60%crystallization of basalts with 2–6wt %H 2O can generate intermediate to silicic melts with H 2O contents of 5–15wt %.(The figure is only slightly less if amphibole or mica are crystallizing phases.)Estim-ates of H 2O contents in calc-alkaline intermediate and silicic magmas commonly yield values of 4–6wt %(Anderson,1979;Green,1982;Barclay et al .,1998;Devine et al .,1998;Carmichael,2002,2004;Blundy &Cashman,2005),although andesite melt inclusions with up to 10%H 2O have been reported (Anderson,1979;Grove et al .,2003).These estimates are principally based on comparison of natural phenocryst assemblages with experimental products and/or melt inclusion studies.Both approaches provide good estimates of pre-eruption H 2O contents during the later stages of magma crystal-lization,but do not necessarily constrain H 2O contents at earlier stages of magma genesis.For example,Carmichael (2002,2004)inferred from experimental phase equilibria and thermodynamic calculations that andesites erupted in west–central Mexico crystallized by decompression from a melt with an original H 2O content of at least 6wt %,and possibly as much as 16wt %,almost all of which was lost during magma ascent and eruption.
Additional experimental evidence for elevated H 2O contents in arc magmas comes from the presence of aluminous amphibole phenocrysts in andesites.At Mount Shasta,USA,Grove et al .(2003)showed that pargasitic amphibole (9–12wt %Al 2O 3)overgrowth rims on magnesian olivine and pyroxenes are con-sistent with amphiboles produced experimentally from H 2O-saturated magnesian basalt at 800MPa.At this pressure the dissolved H 2O content of the melts is estim-ated at $14wt %.At Mount Pinatubo,Philippines,
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Prouteau &Scaillet (2003)observed aluminous cores (>11wt %Al 2O 3)to some amphibole phenocrysts in the 1991dacite.Amphiboles of similar composition were produced in H 2O-undersaturated experiments on the same dacite at pressures of 960MPa,under which conditions melt H 2O contents exceed 10wt %.Prouteau &Scaillet (2003)attributed the aluminous amphibole cores to generation of the 1991dacite by crystallization of a basaltic parent melt near the base of the arc crust.The lower Al 2O 3amphibole rims correspond to later crystallization at $200MPa in the sub-volcanic magma chamber.
