The Genesis of Intermediate and Silicic Magmas in Deep Crust(8)

The storage of evolved residual melts in the deep crustal hot zone depends critically on the dynamics of cooling and crystallization of each sill.Two end-member scen-arios can be envisaged,and mirror observations of differ-entiation in high-level intrusions.In one end-member the magma body retains its suspended crystals and consolid-ates as a physically undifferentiated layer of partially molten rock.Crystallization is near equilibrium and porous media processes are required to segregate the evolved residual melts.In the other end-member,crystals and melt are efficiently separated during cooling;for example,by crystal settling and floor crystallization with compositional convection (Tait et al .,1984).The sill thus becomes strongly physically differentiated and evolution may be closer to fractional crystallization.In either case the sill may,in principle,develop into a lower layer of cumulates and an upper layer of buoyant crystal-free evolved residual melt,which may detach immediately or shortly after sill consolidation.Studies of shallow sills suggest that both these end-members,and intermediate situations,can occur depending on many parameters,including sill thickness,density and viscosity of melt,and whether convection

develops.

Fig.11.Melt production rates (mm/yr)for different basalt intrusion rates (2,5,10mm/yr),for (a)residual melt,and (b)crustal melt as a function of the thickness of the intruded basalt.The production rate is defined as the thickness of melt generated per year.The basalt emplacement depth is 30km.The curves were smoothed to eliminate peaks caused by model discretization.

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Several mechanisms have been proposed for the segregation of buoyant partial melts;namely,compaction of partially molten rock,large-scale Rayleigh–Taylor instabilities of the entire hot zone,and tectonic deforma-tion.The rate of segregation by compaction is very sensitive to melt viscosity (McKenzie,1985).The high H 2O content of hot zone residual melts will result in low viscosity (Figs 12–14)and low density (<2300kg/m 3).For a typical viscosity of 103Pa s compaction-driven segrega-tion times are estimated to be in the range 104–106years for a porosity of the order of 10%(McKenzie,1985).Long residence times of evolved melts in the deep crust are consistent with data from studies of 230Th/226Ra disequilibrium in many intermediate and silicic arc vol-canic rocks (Reagan et al .,2003;Zellmer et al .,2003b ).Jackson et al .(2003)coupled heat transfer from basaltic sills,partial melting of crustal rocks and melt segregation by compaction processes.Their model focuses on the segregation of partial melts from heated crustal rocks,but the same principles can be applied to residual melts in basaltic sills.Melt segregates to produce porosity waves,which move upwards because of buoyancy and start to accumulate at depths just above the solidus.Unmelted rocks above the depth at which the solidus is reached are considered impermeable,so melt cannot ascend by compactional mechanisms.Thus the depth in the crust at which the geotherm reaches the solidus tem-perature is highly significant,because residual melts from basalt and partial crustal melts can only exist below this depth.Jackson et al .(2003)also showed that segregated melts in high-porosity zones can have the geochemical attributes of highly evolved melts.

Melts can segregate more rapidly when the partially molten rock is deformed (Petford,2003).Tectonic processes and large-scale Rayleigh–Taylor buoyancy instabilities in the entire hot zone can cause deformation and melt segregation (De Bremond d’Ars et al .,1995).Interaction between extracted melts from one layer

and

Fig.12.Variation of (a)temperature,(b)melt fraction,(c)melt H 2O content,and (d)melt viscosity as a function of depth,taken as a snapshot 3á2Myr after initiation of the hot zone shown in Fig.7a.The total added thickness of intruded basalt at this stage is 16km;the original crustal thickness was 30km.Melt viscosity is calculated using the equation of Baker (1998).Continuous line shows basalt with initial H

2O content of 2á5wt %and an injection temperature of 1285 C;dashed line shows basalt with initial H 2O content of 1á5wt %and injection temperature of 1302 C.The amphibolite lower crust has become partially melted.Its melt fraction is high but its H 2O content is low because of low initial H 2O content.Thus,the viscosity of the crustal melt is high compared with the viscosity of the H 2O-rich residual melt.

524JOURNAL OF PETROLOGY VOLUME 47NUMBER 3MARCH 2006

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melts from other layers,with lower or higher melt frac-tions,will result in chemical mixing between more and less evolved melts,leading to a wide spectrum of melt compositions extracted from the hot zone (e.g.Fig.1).These hybrid melts should define linear chemical trends in contrast to the curved trends diagnostic of fractional crystallization alone.PETROLOGICAL CONSTRAINTS ON THE ASCENT OF ANDESITE MELT We now consider the ascent of evolved residual melts segregated from a deep hot zone.This exercise requires information on the liquidus temperature (T L )of a particu-lar andesite composition as a function of H 2O content from pressures between those of the lower crust and the depth of H 2O saturation.Unfortunately,to date,no such study has been carried out experimentally,and the desired information can only be compiled for a range of experi-mental studies,and must be considered semi-quantitative.Figure 16shows the liquidus surface,contoured for H 2O content,of a typical silicic andesite.This was con-structed from available phase equilibria experiments in which the residual melt composition was within 2SD of the average 1980–1986Mount St.Helens silicic andesite

for the components SiO 2,Al 2O 3,MgO,FeO,CaO and

Na 2O tK 2O (see Fig.16caption for average values).

We assume that the minor components TiO 2,MnO and P 2O 5have negligible effect on phase relations.We have

also used some experimental data from older near-

liquidus experimental studies (e.g.Eggler,1972;Green,1972)in which the residual melt composition was not analysed,but where the starting composition is within 2SD of the Mount St.Helens average.Our approach involves a number of approximations.We assume that f O 2has only a small effect on phase relations,which is not strictly true for amphibole (Allen &Boettcher,

1983;

Fig.13.Variation of (a)temperature,(b)melt fraction,(c)melt H 2O content,and (d)melt viscosity as a function of depth,3á2Myr after initiation of the hot zone shown in Fig.7b.The total added thickness of basalt,original crustal thickness and melt viscosity are as in Fig.12.Continuous line shows basalt with initial H 2O content of 2á5wt %and an injection temperature of 1285 C;dashed line shows basalt with initial H 2O content of 1á5wt %and injection temperature of 1302 C.The pelitic upper crust has become partially melted.The low H 2O initial content of the pelitic melt accounts for the low viscosity of the crustal melt relative to the H 2O-rich residual melt.Saturation of the melt in H 2O is reached at the base of basalt column.Saturation values are taken from Zhang (1999).

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