Calcium isotope constraints on OIB and MORB petrogenesis: The importance of melt mixing

Abstract

Listen

Translate article

Lithological and isotopic heterogeneity in the mantle archive the mantle's melting history and fingerprint crustal recycling processes coupling the deep Earth with the terrestrial biosphere. Oceanic basalts, consisting of ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs), are invaluable tools for studying the chemical evolution of the mantle because they sample mantle heterogeneity at a variety of depths and length scales. In this study, we probe mantle heterogeneity by exploring the systematics of stable Ca isotopes in OIBs and MORBs. We find that OIBs from Iceland have δ44/40Ca values (∼0.85‰) that are indistinguishable from MORBs, while OIBs from Mangaia, the Azores, the Canary Islands and Hawaii have an average δ44/40Ca value ∼0.08‰ lower than the MORB-Iceland group. Moreover, MORB and Iceland δ44/40Ca values are relatively invariable, while OIBs range from 0.70‰ to 0.85‰ and correlate negatively with La/Lu and Nb/Y. The Ca isotope signature of the MORB-Iceland group is consistent with equilibrium fractionation during high-degree melting of average spinel/garnet peridotite with bulk silicate Earth (BSE) δ44/40Ca. Calcium-isotope variability in OIBs requires two-component mixing of melts with distinct δ44/40Ca values, one of which is a high-degree melt consistent with derivation from average peridotite with BSE δ44/40Ca. The melt-mixing endmember with isotopically light Ca (≤0.70‰) must be a deep, low-degree melt. This low-degree melt cannot be derived from a peridotite source with BSE δ44/40Ca, but instead requires either a source with lower-than-BSE δ44/40Ca or a different source mineralogy. The isotopically light, low-degree melt is consistent with derivation from a pyroxene- and garnet-rich source, but the effects of mineralogical heterogeneity in the source regions of OIBs cannot be easily distinguished from the possible influence of Ca isotope heterogeneity in the mantle. Regardless, mixing of melts from a shallow peridotite source and a deep source appears to be a ubiquitous part of OIB petrogenesis.