Tracing Lithosphere formation and evolution with highly siderophile elements and Os isotopes
Final Report Abstract
The absolute and relative abundances of the PGE combined with Re-Os isotope systematics are a powerful tool to date of melt extraction from the mantle and attendant lithosphere stabilisation, and to trace its subsequent evolution that frequently involves multi-stage metasomatic enrichment with distinct signatures. The laboratory and analytical routines to obtain precise and accurate PGE-Re abundances and Os isotope ratios employing isotope dilution ICPMS techniques have now been successfully established at Frankfurt University. This is demonstrated by repeated analyses of internationally accepted reference material UB-N, which yield data that are indistinguishable from previous results, and in addition show similar reproducibilities. These bulk methods complement recently perfected analytical protocols to study sulphide minerals – the main carriers of PGE under many mantle conditions – in situ by laser ICPMS (DFG-funded project), which now allows analysis of this important group of elements in mantle and other samples on the micro- and bulk scale. The new methods were applied to a suite of peridotite xenoliths sampling the mantle beneath the nominally Proterozoic Rehoboth Terrane, which have been the subject of detailed recent lithophile trace-element and radiogenic isotope investigations, with the aims to obtain new insights into the distribution of PGE-Re and Os isotopes in mantle rocks, their behaviour during different types of metasomatism and the age and evolution of the this mantle section. Results emphasise the importance of different types of sulphides (residual-included vs magmatic/metasomatic-interstitial) in controlling the budgets of the compatible Or-Ir-Ru and the incompatible Pd-Pt and further revealed opx-rich fractions in these samples to be the carriers of the most unradiogenic and hence oldest Os component. Although the dataset is comparatively small, it would appear that modal metasomatism (addition of cpx and presumably magmatic sulphide) and silicate melt metasomatism add incompatible Pd and lead to addition of radiogenic Os and/or Re followed by ingrowth of radiogenic Os, whereas cryptic metasomatism has little effect on PGE-Re-Os isotope systematics. While Os model ages are in agreement with prior studies, combination of the new data with constraints from recently published lithophile element data allows formulation of a new model for the Os isotopic evolution of the mantle beneath the Rehoboth Terrane. In addition, stock solutions of new single PGE and Re spikes were prepared and calibrated using a reverse calibration method in order to determine their concentrations. From these stock solutions, custom-designed mixed PGE and Re-Os spikes can now be prepared that will be appropriate for the precise determination of the concentration of these elements in rock types with very different relative abundances of these elements. A set of spikes for average Archaean organic-rich sediments has already been prepared, directly benefiting from the DFG-funded project mentioned, which aims, inter alia, to characterise the PGE-Re-Os isotope systematics of Paleoarchaean organic-rich rocks from the Barberton Greenstone Belt.
