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

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.

Deformation-assisted compaction aided by melt buoyancy is usually referred to as the 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. In addition, our view of magmatic systems has shifted in the last decades from large melt-rich bodies to crystal-rich mushy reservoirs. This paradigm shift challenges some of the long-established first-order igneous concepts. The idea that melt differentiation at depth is solely governed by (fractional) crystallization processes is now debated.

We propose a novel igneous process, consistent with the mushy nature of oceanic igneous reservoirs, their continuous/cyclic replenishment by primitive melts, and the widespread occurrence of reactive porous flow (RPF) during magma differentiation identified in a growing number of magma systems. The “melt flush” process relies on reactions between the primitive recharge melt(s) and crystal mush at decreasing porosities and the continuous extraction of more evolved interstitial melt by buoyancy, both participating in the acquisition of the cumulate signature.

This model is consistent with the widespread occurrence of RPF in oceanic igneous systems, and matches the petrographic (e.g., olivine & plagioclase dissolution evidence) and geochemical constraints (trace element signatures) brought by natural oceanic samples. We tested different RPF scenarios on which the melt flush model relies to account for their thermodynamic feasibility with the Magma Chamber Simulator*. The first results show that one-step equilibration of primitive melt with primitive to moderately differentiated crystal mush triggers assimilation. The results of the thermodynamic models are consistent with the constraints established from the natural rock record, and strengthen the idea that RPF is a key process for magma differentiation in mushy reservoirs at different evolution stages. The proposed "melt flush" model eventually adds to other processes involved in cumulates formation like magma compaction or crystal settling, and is likely to apply to any other magma system from various settings sharing similar reservoir characteristics.

*Bohrson et al. 2014, Journal of Petrology (doi: 10.1093/petrology/egu036)