Cumulate formation and melt extraction from mushy reservoirs: the "melt flush" model

Abstract:

Volcanism is the surface expression of extensive magmatic systems, with their intrusive counterpart representing ~80% of the total magma budget. Our knowledge of igneous processes therefore largely relies on our understanding of deep plutonic processes. In continental or oceanic environments, most of the intrusive igneous rocks bear geochemical cumulate signatures (e.g., depletion in incompatible elements, enrichment in compatible ones) that are commonly explained by minerals-melt segregation during differentiation. Nevertheless, the processes aiding melt segregation still need to be further constrained.

In oceanic environments, deformation-assisted compaction aided by melt buoyancy he main process involved in melt extraction. However, buoyancy alone is not sufficient and a number of cumulative rocks are lacking any compaction evidence, opening the potential for the involvement of other processes. Here, relying on the mushy nature of magmatic reservoirs, and on their likely cyclic replenishment, we propose the involvement of a new igneous process. In this model, continuously injected fresh melt hybridizes within the injected mush triggering mineral dissolution and crystallization, and partial extraction of the former interstitial melt: the "melt flush model".

This model is consistent with the widespread occurrence of reactive porous flow (RPF) in oceanic igneous systems, and matches the petrographical (e.g., olivine & plagioclase dissolution) and geochemical constraints (trace element signatures) brought by natural oceanic samples. More specifically, it has been shown that RPF proceeds following melt consuming reactions that ultimately result in a progressive closure of the mush porosity. The porosity closure, associated with the extraction of the evolved interstitial melt, both triggered by a cyclic melt injection are here proposed to account for some of the cumulative signature observed in igneous rocks. The proposed "melt flush model" eventually adds to the other processes involved in cumulates formation from various settings like magma compaction or crystal settling.