The influence of igneous processes on the chromium isotopic compositions of Ocean Island basalts

• Published in: Earth and planetary science letters 2020-02, Vol.532, p.116028, Article 116028
• Main Authors: Bonnand, P., Doucelance, R., Boyet, M., Bachèlery, P., Bosq, C., Auclair, D., Schiano, P.
• Format: Article
• Language: English
• Subjects: Cr isotopes; Earth Sciences; Fangataufa Archipelago; Geochemistry; magmatic differentiation; Ocean Island basalts; partial melting; Sciences of the Universe; Volcanology
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2019.116028

•Compatible element (Cr and Ni) are mainly controlled by fractional crystallisation.•Cr isotopic fractionation during magmatic differentiation on Earth is small.•Parental melts have Cr isotopic composition lighter than mantle xenoliths.•Low-degree partial melts have lighter Cr isotopic compositions. We present the first stable chromium isotopic data for a suite of ocean island basalts (OIB) in order to investigate the Cr isotope fractionation during major igneous processes such as partial melting and fractional crystallisation. Twenty-one basaltic samples from Fangataufa Island (Tuamotu Archipelago, Pacific Ocean) have been analysed for major- and trace-element concentrations, and Sr, Nd and Cr isotopic compositions. They define two distinct series: medium to high-K calc-alcaline and low to medium-K calc-alcaline. The variations in incompatible elements such as La and Yb mostly result from varying degrees of partial melting of a mixed mantle source composed of two lithologies: garnet bearing peridotite and a recycled “fertile” component. The recycled component is also identified with the Sr and Nd isotopic composition of Fangataufa basalts. In contrast, the variations in compatible element contents such as Cr and Ni are governed by fractional crystallisation of a mixture of olivine, clinopyroxene and spinel. The samples analysed in this study are also characterised by small Cr isotope variations from −0.24 to −0.17‰. The Cr-poor samples have on average lighter Cr isotopic compositions compared to Cr-rich ones. The observed variations in the low-K series can be modelled by a Rayleigh fractionation model with a fractionation factor (Δ53Crcrystals-melt) of −0.010 ± 0.005‰. The fractionation is more limited than that observed in lunar basalts and two hypotheses may explain this observation: a change in crystallising phases (cpx + spinel on Earth and spinel on the Moon) and/or the difference in temperature and oxygen fugacity between the crystallisation of lunar and terrestrial basalts. The more primitive basalts from Fangataufa have an average Cr isotopic composition of −0.18 ± 0.01‰, lighter than the Cr isotopic composition of pristine mantle xenoliths. Chromium isotopes are therefore slightly fractionated during partial melting with the melts depleted in heavy Cr isotopes. The difference between silicate melts and mantle xenoliths indicates that partial melting could produce small but resolvable shift in mantle xenoliths Cr isotopic composition of up to 0.05