Metasomatic controls on the chemical and isotopic composition of zoned clinopyroxene xenocrysts from the Mount Hope kimberlite, Gawler Craton, Australia

Abstract

Chemical and isotopic data are presented for mantle-derived clinopyroxene xenocrysts from the Mount Hope kimberlite (191 Ma), Gawler Craton (South Australia). The data are used in conjunction with single-grain pressure-temperature (PT) estimates to study the metasomatic controls on clinopyroxene composition. Our datasets reveal a complex mantle setting that consists of three suites of clinopyroxene. Clinopyroxenes from the deep cratonic lithospheric mantle (CLM) (>150km) (herein suite C) are enriched in FeO and TiO₂ with uniform rare-earth element (REE) patterns, enriched light rare-earth elements (LREEs) and unradiogenic ⁸⁷Sr/⁸⁶Sr isotopic ratios. They have chemical and isotopic compositions that overlap with clinopyroxenes from phlogopite-ilmenite-diopside (PIC) xenolith suites and low-Cr clinopyroxene megacrysts found in kimberlites elsewhere. The chemistry of the clinopyroxenes indicates an origin involving metasomatic enrichment from asthenosphere-derived melts. Clinopyroxenes from the shallow CLM (<110 km) have compositionally distinct cores (suite A) and rims (suite B). Cores have high Mg# (0.93–0.96 where Mg# = Mg/(Mg + Fe)) and Cr₂O₃ (0.55–2.0 wt%), and varied REE concentrations that includes HREE-depleted types. The samples have more radiogenic ⁸⁷Sr/⁸⁶Sr isotopic ratios in comparison to suite C. The clinopyroxenes equilibrated with garnet and orthopyroxene in an initially depleted peridotite source that had been enriched in LREEs prior to kimberlite magmatism. Clinopyroxene rims (suite B) are aegirine-rich and have rounded rim textures that contain inclusions of barite, apatite, perovskite, calcite, phlogopite, and richterite amphibole. They have similar Mg# and TiO₂ but much lower Al₂O₃ and CaO contents compared to the cores. These compositions did not equilibrate with garnet or orthopyroxene. Their trace element patterns record enrichments in LREE's and high-field strength elements (HFSE), with slight depletions for Er, Tm, Yb and Lu. Both cores and rims have similar age-corrected ⁸⁷Sr/⁸⁶Sr isotopic ratios, suggesting that they are chronologically related. We suggest that the rims formed shortly before, or during kimberlite magmatism, by melt-rock interactions with the host magma. This process may have involved dissolution and reprecipitation of portions of the suite A cores. Taken together, our results provide new insights into the metasomatic compositions of clinopyroxenes from the Gawler Craton and the important roles that low-volume melts from the asthenosphere play in the chemical and isotopic enrichment of the cratonic lithosphere, and transportation of volatiles and incompatible elements.