Melt/rock ratios and melt fluxes during reactive percolation: from matrix- to melt-controlled dynamics

Abstract

Deep melt migration processes occurring beneath spreading ridges largely occur by porous flow and involve reaction with the pre-existing crystal matrix. The control of the melt/rock ratios and melt fluxes involved in these reactive percolation processes on the structural and chemical evolution of oceanic magmatic systems is yet to be fully constrained. We here report a combined petro-geochemical study of variably evolved gabbroic layers in the Oman Moho Transition Zone, atop the Maqsad mantle diapir, ranging from dunites, troctolites and wehrlites to olivine gabbros. The layering characterizing the base of the crustal section formed during a process of reactive porous flow and hybridization of a dunitic precursor. Positive feedback between melt distribution and deformation focusing allowed for the development of two distinct percolation behaviours, between focused melt percolation and diffuse melt impregnation. This geological setting provides an ideal case study to assess the impact of the melt/rock ratios and percolation dynamics on the evolution of textures and chemical compositions during focused and diffuse percolation. Namely, the former leads to a modification of the crystallographic preferred orientation and complete chemical reequilibration of the matrix, while the latter allowed for preservation of the pre-existing structure and buffer of the melt composition by the matrix and reactive processes. We quantify the melt/rock ratios associated with the two magmatic systems using Plate Models to demonstrate that focused percolation easily resets the matrix composition from melt/rock ratios integrated over time ~ 2–3, whereas diffuse, low-flux melt impregnation would require elevated melt/rock ratios (> 20) to allow for chemical reequilibration. Furthermore, we provide a global overview of the evolution of mineral compositions and textures of a percolated olivine-rich protolith as a function of the melt migration style and the involved melt/rock ratios, both instantaneous and integrated over time.