Geochemical ‘Scouts’ for Processes in Earth’s Lithospheric Mantle
Investigation of the geochemical composition of the Earth’s subcontinental lithospheric mantle provides insights into the Earth’s evolution, including early magmatic and metasomatic processes. Samples of the lithospheric mantle are exposed on the Earth’s surface predominantly in orogenic peridotite massifs or entrained as mantle xenoliths in volcanic rocks, such as kimberlites and alkali basalts. The silicate phases olivine, orthopyroxene, clinopyroxene and garnet are the main constituents of the mantle peridotites and thus their trace element composition records petrogenetic processes through time. The Earth’s oldest subcontinental lithospheric mantle rocks are highly depleted peridotites that experienced up to 40% partial melting, thus completely consuming clinopyroxene and garnet, leaving behind a residue composed almost exclusively of olivine and orthopyroxene. Therefore, these minerals bear great potential to record depletion and re-enrichment events in the mantle and promise to be useful petrogenetic indicators for our understanding of geochemical construction of lithospheric profiles. However, trace element concentrations in olivine and orthopyroxene are very low, and often close to analytical detection limits, complicating the acquisition of low-level abundances elements.
This study focused on the optimisation of a conventional and widely used in situ LA-ICP-MS procedure to enable the acquisition of low-level trace elements in olivine and orthopyroxene. The analytical limitations were addressed by the addition of molecular hydrogen gas (5 mL min-1) to the carrier gas flow and tested on the reference material glasses NIST 612, BCR-2G as well as a natural olivine. The measured elements were extended to a palette of 72 masses, including rare earth elements and the rarely analysed volatile chalcophile and siderophile group of elements. The improved method resulted in slightly enhanced sensitivities (1.5-fold), highly improved precision (e.g., on Ga from 68 to 34 RSD%) and slightly improved limits of detection ranges (e.g., Cu from 6–10 to 4–6 ng g-1), which enable acquisition of a complete rare earth element pattern in natural olivines.
The enhanced LA-ICP-MS method was then applied on natural mantle peridotite xenoliths from Mt Gambier, southeastern Australia. Mt Gambier is a well-studied locality in Australia and represents relatively unmetasomatised continental mantle. New trace element analyses of olivine, orthopyroxene and clinopyroxene and the extended element palette to REE (especially in olivine and orthopyroxene) and the volatile chalcophile and siderophile elements (e.g., Mo, Ag, Cd, In, and Sb) allow us to estimate a more complete composition of the lithosphere, which makes it a potential keystone for composition of continental lithospheric mantle.
The improved method was also used to collect new trace and ultra-trace element data for minerals in cratonic mantle xenoliths from the Kaapvaal craton (southern Africa) and North Atlantic craton (West Greenland). The compositions are compared and contrasted with each other and with the off-craton continental mantle xenoliths from Mt Gambier. Both cratons experienced a high degree of partial melting, vii yet their mineralogical and geochemical compositions show differences, which are related to different later re-enrichment processes. The addition of Si-rich melt resulted in higher modal orthopyroxenes in the Kaapvaal peridotites (28 }10 %) compared to West Greenland (11 }12 %), whereas carbonate-rich melt addition produces olivine and clinopyroxene whilst orthopyroxene is consumed, resulting in higher olivine/orthopyroxene. Later metasomatic enrichment processes introduced secondary clinopyroxene and garnet, which are recorded in increased LREE and MREE in orthopyroxenes from West Greenland and Kaapvaal peridotite xenoliths.