Light oxygen isotopes in mantle-derived magmas reflect assimilation of sub-continental lithospheric mantle material

Joan Carles Melgarejo,William L. Griffin,Kai Lu,Qiu-Li Li,Andrea Giuliani,Jing-Yao Xu

doi:10.1038/s41467-021-26668-z

Abstract

Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. Here we provide an alternative view for the origin of light oxygen-isotope signatures in mantle-derived magmas using kimberlites, carbonate-rich magmas that assimilate mantle debris during ascent. Olivine grains in kimberlites are commonly zoned between a mantle-derived core and a magmatic rim, thus constraining the compositions of both mantle wall-rocks and melt phase. Secondary ion mass spectrometry (SIMS) analyses of olivine in worldwide kimberlites show a remarkable correlation between mean oxygen-isotope compositions of cores and rims from mantle-like 18O/16O to lower ‘crustal’ values. This observation indicates that kimberlites entraining low-18O/16O olivine xenocrysts are modified by assimilation of low-18O/16O sub-continental lithospheric mantle material. Interaction with geochemically-enriched domains of the sub-continental lithospheric mantle can therefore be an important source of apparently ‘crustal’ signatures in mantle-derived magmas.

Highlights

Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities
Back-scattered electron (BSE) images show that in the studied samples olivine macrocrysts and microcrysts are commonly zoned between core and rim, with sharp variations in Mg# (Mg# = 100 × Mg/(Mg + Fe))
Internal zones are observed between core and rim in BSE images, a feature common to olivine in other kimberlites[50,58,60]

Summary

Introduction

Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. We performed 314 in situ oxygen-isotope analyses of olivine by SIMS (see “Methods” section), including 84 high-Mg cores, 69 low-Mg cores, 134 rims, 14 internal zones, and 13 olivine in the PIC-like xenoliths.

Results

Conclusion