THE SIMPLEST MODEL FOR HYDROUS MAGMA FLOW THROUGH A MANTLE WEDGE (Invited Presentation)

Melting, crystallization, and mass transport in a mantle wedge were computed from transport theory developed for infiltration-driven metamorphism [1]. The chemical model is the simplest possible: the system rock-H₂O with 3 phase components [solid rock (rs), molten rock (rm), and H₂O]. There is one reaction, rs = rm. P-T conditions along a vertical flow path through a ~70-km-thick wedge are from [2]. Hydrous magma is stable throughout the wedge; all H₂O is dissolved in melt with mass fraction H₂O (M_H2O) specified according to [3] for peridotite. M_H2O was fit to distance (z, km) between 107 km (depth to base of wedge) and 36 km (base of crust), taking z = 3P (kbar). Two input fluids are considered at the inlet at z = 107 km: (1) hydrous magma in equilibrium with rs and (2) pure H₂O. In Case (1), melting/crystallization along the flow path (expressed as ξ, g/cm³) was computed for a given time-integrated input fluid flux (q°, g/cm²) with reaction driven by P and T gradients along the ascent path. For all q°, melting occurs between 107 and 65 km with maximum positive ξ at 76 km. Melting switches to crystallization at z = 65 km. Crystallization continues between 65 and 36 km depth with minimum negative ξ at 53 km. Mass of rm passing through z = 65 km is 4.6 times the mass of rm that enters at 107 km. 90.2% of the melted rock recrystallizes in the mantle at 36-65 km. The mass of rm that exits the outlet at z = 36 km is 1.9 times the rm mass that enters at z = 107 km. For q° = (1-2) x10⁶ g/cm², melt delivered at the outlet adds a rock layer 4.2-8.4 km thick to the crust. In Case (2), input of the same mass of H₂O as in Case (1), but as pure H₂O, produces a reaction front at z = 106-103 km between completely melted rock upstream and rock in equilibrium with hydrous melt downstream. Downstream from the reaction front melting and crystallization are identical to Case (1). Highly incompatible elements are stripped from the mantle and added to the crust; moderately incompatible elements are preferentially removed at 107-65 km and enriched at 65-36 km. Results apply to flow in grain boundaries, cracks, tubes, and porosity waves, provided there is reaction between melt and surrounding rock. Rock melting and liquid crystallization experiments do not reproduce infiltration processes in the wedge.

[1] Ferry (1994) AmMin 79, 719. [2] Till et al. (2012) CMP 162, 669. [3] Grove et al. (2006) EPSL 249, 74.