Systematics of metals, metalloids, and volatiles in MORB melts: Effects of partial melting, crystal fractionation and degassing (a case study of Macquarie Island glasses)

Abstract

Magnesian basaltic glasses from the Miocene Macquarie Island ophiolite (SW Pacific) are used for understanding the effects of progressive partial melting of the mantle peridotite, and subsequent magma crystallisation and degassing on the composition of mid-ocean ridge basalts. These glasses are represented by two suites, near-primitive (Group I) and fractionated (Group II), which show clear parent–daughter relationships. Their exceptional compositional diversity in major, trace lithophile and volatile elements is shared by a set of metallic elements and metalloids, analysed in this study by laser ablation ICPMS. The Group-I glasses provide concentrations of compatible metals before they are severely modified by crystal fractionation (e.g., V, Sc, Co, Ni, Cr, Zn) or melt degassing (Cu). The constant or nearly constant ratios of the elements in the Macquarie Island primitive and fractionated melts are used for identification of similar bulk distribution coefficients during melting and crystal fractionation, respectively. The estimated relative degree of incompatibility during mantle melting provides constraints on the siderophile, chalcophile and volatile element abundances in the model “primitive” and “depleted” mantle sources. The chemical systematics observed in the studied glasses can be further used to explore mantle source compositions, including mineral phases involved in magma generation, and processes controlling fractionation of chemical elements in both mantle source and mantle-derived melts.