Work

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

Year: 2025

Type: article

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 TiO 2 with uniform rare-earth element (REE) patterns, enriched light rare-earth elements (LREEs) and unradiogenic 87 Sr/ 86 Sr 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 Cr 2 O 3 (0.55–2.0 wt%), and varied REE concentrations that includes HREE-depleted types. The samples have more radiogenic 87 Sr/ 86 Sr 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 TiO 2 but much lower Al 2 O 3 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 87 Sr/ 86 Sr 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.

Source: Lithos

DOI: 10.1016/j.lithos.2025.108039

Authors: Z.J. Sudholz, O. Alard, A.L. Jaques, G.M. Yaxley, Y. Gréau, C. Frigo