Cumulate Melting and the Evolution of Igneous Rocks

Abstract:

Magmatic evolution creates crystal cumulates. In the current paradigm for the genesis of evolved magmas, cumulate crystal mushes dominate trans-crustal magma systems that feed volcanoes and supervolcanoes erupting intermediate and felsic magmas. Both melts and cumulates may show broad compositional variation with depth, from mafic/ultramafic near the base of the crust to more evolved at shallow depths. Upward movement of melt through the system (recharge) thus brings relatively mafic magmas into contact with fusible crystal mush deposited by more evolved magma. Recharge mobilizes and melts crystal mush thus contributing to the total mass of crystal-poor eruptible magma in the system, generates compositional zoning, and may trigger eruptions. By their nature, cumulate mushes compositionally resemble melts higher on the liquid line of descent in any one system, hence the chemical imprint of cumulate melting may be hard to distinguish from that of simple parent-to-daughter fractional crystallization. Here, we explore the influence of cumulate melting on the evolutionary path of igneous suites and ways of identifying cumulate contributions to phenocryst-poor volcanic rocks. One approach is to use minor and trace elements that serve as markers for particular mineral phases, and hence respond strongly to both crystallization and melting of that phase; examples are Ba and Eu in alkali feldspar, Zr in zircon, P in apatite, and F in biotite. We attribute strong enrichments in Ba and positive Eu anomalies in intermediate to felsic rocks and glasses to melting of cumulates rich in alkali feldspar, crystallized from melts in the rhyolite-trachyte-phonolite spectrum. The dependence of accessory mineral solubilities on melt composition and temperature controls their survival in systems subject to recharge. Melting of a feldspar-dominated mush dilutes volatile abundances in the liquid, potentially destabilizing interstitial (OH,F)-phases such as micas and amphiboles. In summary, cumulate melting can produce a plethora of petrologic effects that await detailed investigation.