The behaviour of copper isotopes during igneous processes

Abstract:

Application of Cu isotopes to high temperature systems has recently gained momentum and has the potential for probing sulphide fractionation during planetary differentiation [1]. This requires robust estimates for planetary reservoirs, and a fundamental understanding of how igneous processes affect Cu isotopes; this study aims to tackle the latter.

Cogenetic suites affected by both fractionation crystallisation and cumulate formation were analysed to study such effects on Cu isotopes. In S-undersatured systems, Cu behaves incompatibly during melt evolution and the Cu isotope composition of such melt is invariant over the differentiation sequence. In contrast, S-saturated systems show resolvable Cu isotope variations relative to primitive melt. Such variations are minor but imply a slightly heavy Cu isotope composition for continental crust compared to BSE, consistent with granite data [2]. Although olivine accumulation does not affect Cu isotopes, spinel-hosted Cu is isotopically light relative to the bulk.

Analysis of variably melt-depleted cratonic peridotites shows that partial melting can affect Cu isotope composition in restite, with the depleted samples isotopically light compared to BSE. This could be due to residual spinel and/or incongruent melting of sulphides – individual sulphides picked from a single xenolith reveal a range of Cu isotope compositions, dependent on composition.

Although partial melting may fractionate Cu isotopes, models suggest most mantle-derived melt will have δ⁶⁵Cu ≈ BSE, as most source Cu will be transferred to the melt. Small degree melts such as ocean island basalts are predicted to be isotopically heavier than MORB, if derived from a primitive mantle source. OIBs have a range of Cu isotope compositions: some are heavier than MORB as predicted; however, some have much lighter compositions. Since Cu isotopes can be significantly fractionated in the surface environment [e.g. 3] OIB Cu isotopic variations may be linked to recycling of sediments in the mantle; therefore Cu isotopes may be used as a tracer for mantle heterogeneities, and potentially, for sediment and/or fluid transfer through volcanic arcs.

[1] Savage et al. 2015, Geochem. Perspect. Let. 1; [2] Li et al. 2009 Chem. Geol. 258; [3] Markl et al. 2006 GCA 70.