Insights into the petrogenesis of the West Kimberley lamproites from trace elements in olivine

摘要

The Miocene lamproites of the West Kimberley region, Western Australia include olivine-leucite lamproites (≤10?wt% MgO) containing olivine and leucite microphenocrysts, and diamondiferous olivine lamproites (20–30?wt% MgO) containing olivine phenocrysts and larger (1–10?mm) olivine as mantle xenocrysts and dunite micro-xenoliths. Olivine phenocrysts and thin (<100?μm) magmatic rims define trends of decreasing Cr and Ni, and increasing Ca and Mn, with decreasing olivine Mg#, consistent with fractional crystallisation of olivine (and minor chromite). Many phenocrysts are zoned, and those with cores of similar Mg# and trace element abundances to the mantle xenocrysts may be xenocrysts overgrown by later olivine crystallised from the lamproite magma. Magmatic olivines Mg#~(91–92)are estimated to have been in equilibrium with olivine lamproite magma(s) containing ~22–24?wt% MgO. The xenocrystic mantle olivines Mg~(90–92.5)in the olivine lamproites are inferred from trace element abundances to be mostly derived from garnet peridotite with equilibration temperatures estimated from the Al-in-olivine thermometer (Bussweiler et al. 2017) to be ~1000–1270 °C at depths of 115–190?km. Olivines from the deeper lithosphere are less depleted (lower Mg#, higher Na, Al, P, Ti, Zr etc) than those at shallower depths, a feature suggested to reflect the combined effects of metasomatic re-enrichment of the craton roots (Ti, Fe, Zr etc) and increasing temperature with depth of origin (Na, Al, Ca). The West Kimberley lamproite olivines are not enriched in Li, as might be expected if their source regions contained continental sedimentary material as has been previously inferred from lamproite large-ion-lithophile trace elements, and Sr and Pb isotopes.