Stable chromium isotopic variations in peridotite mantle xenoliths: Metasomatism versus partial melting. (15th January 2022)

• Record Type: Journal Article
• Authors: Jerram, M.; Bonnand, P.; Harvey, J.; Ionov, D.; Halliday, A.N.
• Is Part Of: Geochimica et cosmochimica acta. Volume 317(2022)
• Page Start: 138
• Page End: 154
• Publication Date: 2022-01-15
• Subjects: Cr isotopes -- Mantle -- Partial melting -- Metasomatism; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2021.10.022
• Languages: English
• ISSNs: 0016-7037

Abstract

Highlights: The δ 53 Cr of the mantle, as measured in xenoliths, displays a significant range. Modelling demonstrates this is not achieved by partial melting. Chemical indices of melt depletion in peridotites show no correlation with δ 53 Cr. Non-equilibrium interactions may produce large δ 53 Cr variations. Abstract: Chromium isotope compositions have been determined for 42 mantle-derived xenoliths from Cameroon, France, Mongolia, Russia and the USA. The samples are mainly spinel and garnet lherzolites, or harzburgites, and yield δ 53 CrNIST979 from −0.33 to 0.43‰, within the range of previously reported compositions, −0.51 to 0.74‰ (Xia et al., 2017). Partial melting has been proposed as a process that alters the δ 53 Cr of the mantle residue. New non-modal melting models show that the compatible nature of Cr is inconsistent with large changes in the residue's isotopic composition. The fractionation during melting is constrained by calculated Δ 53 Crmantle-melt -values <0.09 and the measured Δ 53 / 52 Cr mantle-melt ≈ 0.07‰, which is too small to generate the observed range in δ 53 Cr at low to medium degrees of melting. Fertile to melt-depleted, unmetasomatised peridotite xenoliths show no correlations between δ 53 Cr and indicators of partial melting. Almost all LREE-enriched samples studied here define a correlation between δ 53 Cr and (La/Yb)N . This is consistent with alteration of Cr isotopic compositions by some process that is related to interactions with low degree partial melts. The low Cr content of small degree partial melts renders metasomatism via such liquids a difficult explanation for the isotopic effects found. A possible explanation is some form of disequilibrium loss of isotopically light Cr into such liquids during their percolation through the mantle. This is similar to a previously proposed mechanism of kinetic isotopic fractionation of Cr associated with interactions between silicate melts and peridotitic mantle (Xia et al., 2017).