Numerical Modeling of REE-SiO2 Systematics During Island Arc Lower Crustal Amphibolite Melting: the Role of Partial Disequilibrium Melting Conditions.

Abstract:

Brophy (2008) proposed that REE-SiO₂ systematics could provide a method for discriminating between a melting and fractional crystallization origin for silicic magmas in intra-oceanic island arcs. Specifically, for liquid SiO₂ contents in excess of 63% fractional crystallization and lower crustal amphibolite melting should lead to a positive and negative REE-SiO₂ correlation respectively. A drawback to the melting modeling was the assumption of instantaneous equilibration between solids and melts (equilibrium melting). However, estimated rates of melt production coupled with solid state diffusion rates of REE all but ensure that melting actually occurs under partial disequilibrium conditions. A numerical method has been developed that permits the modeling of liquid trace element evolution for complex source rocks undergoing either combined congruent or incongruent melting. For a given source rock and melting history the input variables include the solid state trace element diffusivity and the duration of melting. Depending upon the input parameters it can model anything from equilibrium through partial disequilibrium to complete disequilibrium melting. The nature of melting and melt extraction is a critical factor in any such modeling. Recent work on lower crustal melting suggests that, in the absence of shear deformation, melt will accumulate until around 20-25% melting at which point it leaves the source region. In the presence of shear melting and melt extraction will occur in multiple cycles of melting with extraction occurring at 7-8%. Using the original amphibolite melting model of Brophy (2008) and REE diffusion coefficients ranging from 10^-16 to 10^-23 cm²/s , which fully encompasses the likely rate of diffusion at lower crustal temperatures (10^-19 - 10^-20 cm²/s), modeling has been carried out for the two end-member cases of a single batch melting event with extraction at 20-25% and multiple cycles of batch melting with extraction occurring at 7-8%. The results indicate that for both end member cases, melting still yields a negative REE-SiO₂ correlation in the liquid, thus permitting REE-SiO₂ systematics to continue to be an important method for discriminating a melting versus fractional crystallization origin for silicic magma.

Brophy JG (2008) Contrib Mineral Petrol 156: 337-357