Generation of Felsic Crust: A Fluid Transport Perspective

Continental crust consists mainly of felsic rocks rich in silicon and aluminum, and is formed in subduction zones via processes linked to plate tectonics. Most recent models of the felsic crust formation rely on differentiation of basaltic magma generated via partial melting of mantle wedge peridotite under influence of a fluid phase liberated from subducting hydrated oceanic crust. Here we present an alternative model that invokes metasomatic alteration of mantle peridotite due to interaction with a water-rich fluid phase. The model is based on calculations of solid phase assemblages in equilibrium with a fluid saturated in major elements at varying pressure-temperature conditions. Calculations employed THERMOLAB software [1] along with thermodynamic properties of solids (both the standard state and mixing) according to [2,3] and aqueous solution models [4]. The calculations reveal that the SiO2 content in the fluid exerts major control on the solid phase assemblage. On decompression path from 2.5 to 0.2 GPa at 700oC in the model system NCMASH it changes from a six-mineral assemblage olivine (Ol) + orthopyroxene (Opx) + pargasite(Parg) + diopside + biotite + clinochlore to a three-phase Ol+Opx+Parg. The system Si/O ratio along the path increases from 0.26 (close to that of Ol, Si/O=0.25) to 0.28, thus pre-conditioning mantle protolith for subsequent melting that would generate diorite-granodiorite-granite melts.