Abstract
Experimentally Derived Intermediate to Silica-rich Arc Magmas by Fractional and Equilibrium Crystallization at 1·0 GPa: an Evaluation of Phase Relationships, Compositions, Liquid Lines of Descent and Oxygen Fugacity
Highlights
The production of magmas at convergent plate margins is an important process forming juvenile island arc and continental crust (e.g. Taylor & White, 1965; Kushiro, 1990; Rudnick, 1995)
A number of processes have been proposed to account for their genesis, including the following: (1) fractional crystallization of mantle-derived basaltic precursors (Bowen, 1928; Cawthorn et al, 1976; Green, 1980; Sisson & Grove, 1993; Grove et al, 2003); (2) additional extensive assimilation of and hybridization with crustally derived partial melts generated by heating of mantle-derived magmas over prolonged time (MASH/FARM) (Hildreth & Moorbath, 1988; Thompson et al, 2002); (3) partial melting of deep crustal lithologies, as well as pelitic micaschists (e.g. Clemens et al, 2011); (4) direct mantle partial melts; (5) slab melting (Kay, 1978; Drummond & Defant, 1990)
We explored the crystallization-differentiation of two different starting compositions representing variable mantle source extraction conditions—a high-Mg basalt from the Adamello batholith (Italy) (Ulmer, 1988; Hurlimann et al, 2016) and a high-Mg basaltic andesite from Mt Shasta (California) (Baker et al, 1994)—to evaluate the potential influence of the ‘mantle input’
Summary
The production of magmas at convergent plate margins is an important process forming juvenile island arc and continental crust (e.g. Taylor & White, 1965; Kushiro, 1990; Rudnick, 1995). To identify and quantify the relative significance of these processes, phase equilibria constraints, melt fractions and compositions, as well as proportions and compositions of solid residues generated by the proposed endmember processes need to be known This contribution provides experimental constraints on both the equilibrium and fractional crystallization of primary, mantle-derived, hydrous, arc tholeiitic to calc-alkaline parental melts at pressure conditions corresponding to lower crustal levels (1Á0 GPa, 30–35 km). This study provides new experimental data to evaluate the geochemistry and modal proportions of silica-rich magmas generated by equilibrium and fractional crystallization in the lower crust, prior to decompression and mixing in shallow reservoirs Both geochemically based calculations as well as phase equilibria (experimental and by thermodynamic modeling) constraints imply that in excess of 50% crystallization of ultramafic rocks from initial, mantle-derived magmas must have taken place to produce andesitic liquids at lower crustal conditions The diversity of experimentally derived cumulates and their relevance for the formation of arc lower crust are discussed in Muntener & Ulmer (2018)
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