Igneous cumulate forming processes

Cumulates are a key concept in igneous petrology and a key component of any magmatic series. A cumulate is the result of crystal segregation from its equilibrium melt. A cumulate is therefore refractory, enriched in compatible elements (those that like minerals) and depleted in incompatible elements (those that prefer melts) relative to segregated melts. The main cumulate formation processes recognised for several decades are crystal settling and crystal-rich magma (mush) compaction. Recently, in light of the mush-dominated character of igneous reservoirs and of the importance of melt-mush reactive percolation processes, which appear to be widespread in several geodynamic contexts, an additional cumulate process has been proposed: the melt flush process (Boulanger & France, 2023). The melt flush process conceptualises a continuous reactive porous flow: in continuously (or oscillatingly) replenished mushy reservoirs, freshly recharged melt percolates through the mush, flushing out previous interstitial (and relatively more evolved) melt and reacting with the mush-forming minerals. This thermodynamically feasible process results in crystal-melt segregation as less evolved melts replace fractionated ones; the resulting assemblage is more refractory and corresponds to a cumulate.

Crystal settling, mush compaction or melt flush are three cumulate forming processes that are buoyancy assisted. Here I discuss the potential role of an alternative process, not based on melt buoyancy or the density difference between melt and crystals, in cumulate formation. I show that heterogeneous nucleation has a strong potential to locally segregate evolved melt pockets from crystal clusters when crystallization is rather static and the magma becomes a mush (>50% crystal). Crystal nucleation requires a certain nucleation energy to be overcome before crystallization can proceeds, so nucleation is often delayed during early crystallization of magmas. In heterogeneous nucleation, the presence of pre-existing nuclei or impurities can allow crystal growth without the need to overcome the nucleation energy, resulting in a heterogeneous crystal distribution within the melt that is directly related to the presence of nuclei and can ultimately form crystal clusters. The maturation of such crystal clusters can then segregate the melt from the various clusters, eventually forming evolved melt pockets, and cumulative domains. Other modes of crystal clustering (e.g. synneusis) might also be involved in the acquisition of the cumulate signature. In the natural record, such a cumulate formation process can either be locally preserved or might act as a catalyser for crystal-melt segregation by deformation or buoyancy-assisted processes. During this presentation I will show the potential chemical implications of such a process and possible natural examples.

 

Boulanger, M. & France, L. (2023). Cumulate formation and melt extraction from mush-dominated magma reservoirs: the Melt Flush process exemplified at Mid-Ocean ridges. Journal of Petrology 64, egad005. https://doi.org/10.1093/petrology/egad005