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Fig.7.Hot zone temperature (left)and melt fraction (right)evolution with time for a selection of basaltic sills injected into the lower crust.Sills 50m thick are injected every 10kyr,i.e.at an average emplacement rate of 5mm/yr.The initial H 2O content of the basalt is 2á5wt %and its injection temperature is 1285 C.The basalt is injected at a fixed depth of:(a)30km (Moho discontinuity);(b)20km (Conrad Discontinuity).The upper crust (above 20km depth)is a pelite;the lower crust is amphibolite.The basalt cools very rapidly after injection to equilibrate thermally with the surrounding crust.Successive intrusions elevate the hot zone temperature.In the first tens of thousand years after injection,sill temperatures oscillate in response to the intrusion of subsequent sills.Eventually their temperature stabilizes when they are displaced sufficiently far below the injection level.
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diminished because of lower ambient temperatures (Fig.10c and d).Temporal and spatial controls on melt compositions In the classical concept of evolved melt generation by basalt differentiation,a body of basalt slowly cools and crystallizes.The residual melt generated in this way becomes more evolved with time.In contrast,in our hot zone model the reverse is true:the system is heated with time as long as basalt continues to be injected.The formation of a residual melt from each rapidly cooling sill is instantaneous,but as the whole hot zone is heated,the generated melt becomes,on average,less evolved with time (Figs 7and 8).The first residual melts to be produced in the hot zone correspond to the most silicic compositions (rhyolite)and lowest temperatures,evolving with time through dacite to andesite;a sequence opposite to that predicted by models of fractional crystallization in magma chambers.Only when basalts cease to be injected into the hot zone does the system cool and the sequence of melt evolution reverse.
Because sill temperature and melt fraction depend on
their position on the geotherm,the compositional vari-
ation is also a function of depth.At any given time following the incubation period,a variety of residual
and crustal melt compositions coexist across a range of
depths in the hot zone (Figs 12–15).The shape of the geotherm and the consequent melt compositional pers-ity depend on the basalt emplacement rate.For example,in Figs 7and 8basalt is intruded with emplacement rates of 5and 2mm/year,respectively.After a total emplace-ment of 16km of basalt at 30km depth and an intrusion duration of 8Myr,the melt fraction for an emplacement rate of 2mm/year is 0á25–0á26,corresponding to dacitic melt compositions (Fig.8).With an emplacement rate of 5mm/year and after 3á2Myr,which also corresponds to 16km of intruded basalt,the melt fraction varies with depth from 0á25to 0á54,corresponding to melt com-position from dacite to basaltic andesite (Fig.7).In the case of randomly emplaced intrusions,screens of amphibolite are sandwiched between basalt sills (Fig.
15).
Fig.8.Hot zone temperature (left)and melt fraction (right)evolution with time for a selection of basaltic sills injected into the lower crust.Basalt sills 50m thick are injected every 25kyr (average emplacement rate of 2mm/yr)at (a)30km and (b)20km.The initial H 2O content,injection temperature and crustal compositions are as in Fig.7.In this case the temperature in the hot zone grows more slowly than in Fig.7because of the lower emplacement rate;melt fraction (and composition)are more homogeneous.
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At high temperature the amphibolite has a higher melt fraction than the basalt,whereas close to the solidus the melt fraction is higher in the basalt.Similarly,the ages of inpidual melts are spread out across the hot zone.In Figs12–15,young melt that has differentiated from the last injected sill coexists with melts that were generated more than3Myr earlier.This has important implications for the apparent timescales of magmatic differentiation.In the likely case that melts from different depths within the hot zone are mixed together during ascent(e.g.Fig.1),unravelling the timescales of differ-entiation from a single rock sample may be especially complicated.
Melt H2O content
Basalt crystallization at high pressure concentrates H2O in residual melts(Figs12–14)even in the case where hydrous minerals such as amphibole are crystallizing. For relatively low melt fractions and high initial H2O contents of the intruding basalt,the residual melt can be extremely H2O-rich.For example,65%crystallization of a basalt with an initial H2O content of2á5wt%will lead to an andesitic residual melt with$7wt%H2O.Elevated H2O contents result in low melt viscosity and density (Figs12–14).Thus,although the residual melt from basalt is rich in silica it is both buoyant and mobile.In contrast, the H2O concentration of the crustal melt is limited by the H2O content of the hydrated minerals in the proto-lith.As a consequence,crustal partial melts are much more viscous than the residual melts in the basalt (Figs13d and14d).This has implications for the relative extractability of crustal vs residual melts.
For low residual melt fractions in the basalt,the H2O concentration can reach sufficiently high levels to become saturated,resulting in exsolution.This situation occurs if the parent basalt has more than$1wt%H2O.Exsolved volatiles can flux the overlying crust and induce further melting.The transfer of H2O from the basalt into the crust is difficult to constrain in low-porosity crust and we have,therefore,not explicitly modelled flux melting of the crust.For this reason,the amounts of crustal melt generated in our models should be considered as minima. H2O exsolution may also enhance the extraction of melt by gas-driven filter pressing(Sisson&Bacon, 1999).Seismic observations are consistent with fluid ex-solution and movement in the deep arc crust(Obara, 2002;Katsumata&Kamaya,
2003).
Fig.9.Incubation time between the first intruded basalt and the beginning of melt accumulation for different average basalt emplacement rates
and emplacement depths.(a)Residual melt;(b)crustal melt.The basalt initial H2O content is2á5wt%,inpidual sill thickness is50m and the
different emplacement rates correspond to different time intervals between intrusions.
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ANNEN et al.DEEP CRUSTAL HOT ZONES
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CONTRASTS BETWEEN DIFFERENTIATION IN DEEP HOT ZONES AND SHALLOW MAGMA CHAMBERS Generation of evolved andesite and dacite melts in deep crustal hot zones can be contrasted with melt differenti-ation in shallow magma chambers.In the former case sills consolidate quickly and the evolved residual melt is gen-erated on (geologically)short timescales.The residual melt can then be stored for long periods without further differentiation,because on a large scale the thermal pro-file of the hot zone evolves very slowly,on timescales governed by thermal conduction.In contrast,a shallow magma chamber requires high rates of magma input to be maintained in a molten state,as the surroundings are cold.Unless the heat input from the new magma input balances the heat loss from the chamber walls,then temperature and melt composition will evolve continu-ously.U-series data for evolved arc rocks commonly suggest that,following U–Th fractionation,magmas
have residence times of the order of 104–105years
(Reagan et al .,2003;Zellmer et al .,2003b ).We suggest that such long residence times are hard to reconcile with a shallow magma storage system because of the require-
ment of stable thermal conditions.In a deep hot zone the
residual melt is stored at temperatures governed by the geotherm,allowing melt to remain compositionally stable for long periods.
The high H 2O content of residual melts also has important implications for their physical properties,spe-cifically reduced density and viscosity,which mean that melts can be readily concentrated by compaction,facilit-ating extraction,and rapid ascent toward the surface.Carmichael (2002)arrived at a similar conclusion regard-ing the H 2O-rich andesites of the Mexican volcanic arc.He argued that their low viscosity and density allowed magma ascent to be near-adiabatic.The adiabat for hydrous andesite melts is of the order of 25–50 C/GPa (Mastin &Ghiorso,2001;Carmichael,2002).Consequently,there may be only 50–100 C
difference
Fig.10.Productivity for (a)residual melt from basalt crystallization,and crustal melts from:(b)amphibolitic lower crust;(c)greywacke upper crust;(d)pelite upper crust.Productivity is defined as the thickness of the accumulated melt pided by the total thickness of the intruded basalt.Basalt initial H 2O content is 2á5wt %.One 50m thick sill is injected every 10kyr.Results are shown for fixed injection depths of 10,20and 30km and for random intrusion between 20and 30km.
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between andesite generation temperatures in the lower crust and low-pressure equilibration temperatures inferred from mineral thermometry or phase equilibria.A further implication of our model is the significant thickening of the crust that results from basalt emplace-ment.If melt is efficiently extracted then the residual basalts will have a cumulate character,with associated
high densities and seismic velocities,as observed in
exposed deep crustal sections through arcs (Debari &Coleman,1989).In some cases,the processes of delamination and recycling of dense mafic cumulates in the less dense underlying mantle can be invoked to limit the extent of crustal thickening (Kay &Kay,1993;Jull &Kelemen,2001).In the Sierra Nevada,USA,for example,seismic refraction and gravity data (Fliedner et al .,1996,2000)show that the granitic rocks of this young mountain belt are not supported by an isostatic root,which Fliedner et al .(1996,2000)attributed to delamination of the mafic counterpart to the granites.Such an interpretation is consistent with rapid Pliocene uplift in the region and the change in the petrology of xenoliths in basalt lavas from predominantly lower crus-tal granulites to predominantly mantle peridotites between 10and 3Ma (Ducea &Saleeby,1996,1998a ,1998b ;Ducea,2002;Farmer et al .,2002).A more recent seismic experiment across the southern Sierra Nevada,by Zandt et al .(2004),using receiver functions,has identified a welt of thickened crust and a ‘hole’in the Moho,which those workers ascribed to asymmetric flow of dense lower crust into a delaminating mantle drip beneath the Great Valley.In other arcs,for which there is less compelling evidence for delamination,the apparent lack of a deep cumulate root may simply be a consequence of the difficulty of seismically distinguishing pyroxenites and/or garnet-rich mafic rocks from mantle peridotite.For example,Fliedner &Klemperer (2000)proposed that beneath the Aleutians volcanic arc some 10km of ultramafic cumulates lie below the geophysical Moho.MELT SEGREGATION
