Dating Early Archean partial melting events: insights from Re-Os dating of micrometric Os-minerals from Kalahari Craton mantle xenoliths

Our understanding on the formation, mechanism and timing of the formation of the Earth’s earliest continents hinges on obtaining robust and accurate ages of melt depletion and lithosphere stabilisation. The Re-Os geochronometer has been used to obtain such information. However, recent advances indicate that metasomatism can impact on the signatures obtained. This thesis utilises the combination of Re-Os and highly siderophile element (HSE) systematics to accurately assess the impact secondary mineralisation has on the Re-Os ages determined. A suite of samples from the Kalahari craton (Kaapvaal and Zimbabwe cratons) are investigated in terms of their metasomatic overprint and Re-Os systematics. The sub-suite of peridotites from the Kaapvaal craton are first analysed for whole-rock major and trace elements, Re-Os and HSE systematics. The peridotites were also analysed for the 187Os/188Os compositions at the single grain base metal sulphide (BMS) and platinum-group mineral (PGM) scale. These analyses indicate that the Kaapvaal peridotites have experienced high degrees of melt depletion followed by variable and significant enrichment in the incompatible trace elements and HSE. The enrichment of HSE led to the precipitation of metasomatic BMS which impact on the Re-Os ages determined. As such, the whole-rock and single grain BMS TRD (rhenium depletion model age) indicate that the mantle was pervasively metasomatised as early as 3.2 Ga. Nano-particle PGM (Pt-alloys) with radiogenic 187Os/188Os (0.1294-0.1342) were found included within unradiogenic BMS (187Os/188Os 0.1066-0.1084). This signifies that the PGM formed in the presence of Re and evolved to high 187Os/188Os compositions over a long time scale. The Os composition of the Pt-alloys, combined with their nano-particle nature and the Os dichotomy with the host BMS signifies that the Pt-alloys formed in a HSE-Si-rich melt. This provides further evidence for the metasomatic overprinting of the Kaapvaal peridotites. Despite the high degree of metasomatism experienced by the Letlhakane peridotites (Zimbabwe craton), as evidenced by their re-enriched HSE-Se-Te systematics, single grain BMS preserve evidence of partial melting events. The 187Os/188Os analyses of the BMS provide ages >2.5 Ga older than the whole-rock. The oldest BMS TRD age of 3.7 Ga is preserved in a metasomatic BMS associated with secondary clinopyroxene and phlogopite. The attainment of an Eoarchean age from a metasomatic BMS suggests that the metasomatic fluid is able to entrain or nucleate on residual BMS. As such, the obtained ages reflect a mixing between the two different Os signatures. Whereas older ages reflect the dominance of the residual PGM on the bulk Os composition, younger ages are due to the control from the metasomatic melt. The 3.7 Ga TRD obtained age also pushes the age of initial stabilisation of the Zimbabwe lithosphere to within the age of the oldest crustal rocks in this region. The combined results of the Letlhakane and Kaapvaal peridotites indicate that whilst the Re-Os system can be affected by the metasomatic addition of BMS, the combination of HSE and Re-Os at the whole-rock and micro-scale can still resolve geologically significant ages

Crustal rejuvenation stabilised Earth’s first cratons

This work was funded by Australian Research Council grant FL160100168 and Australian Research Council grant DP180100580.The formation of stable, evolved (silica-rich) crust was essential in constructing Earth’s first cratons, the ancient nuclei of continents. Eoarchaean (4000–3600 million years ago, Ma) evolved crust occurs on most continents, yet evidence for older, Hadean evolved crust is mostly limited to rare Hadean zircons recycled into younger rocks. Resolving why the preserved volume of evolved crust increased in the Eoarchaean is key to understanding how the first cratons stabilised. Here we report new zircon uranium-lead and hafnium isotope data from the Yilgarn Craton, Australia, which provides an extensive record of Hadean–Eoarchaean evolved magmatism. These data reveal that the first stable, evolved rocks in the Yilgarn Craton formed during an influx of juvenile (recently extracted from the mantle) magmatic source material into the craton. The concurrent shift to juvenile sources and onset of crustal preservation links craton stabilisation to the accumulation of enduring rafts of buoyant, melt-depleted mantle.Publisher PDFPeer reviewe

Metal anomalies in zircon as a record of granite-hosted mineralization

Funding: This work was supported by Australian Research Council grant FL160100168 (NJG, ANW, JAM, and PAC); Leverhulme Trust RPG-2015-422 and EM-2017-047\4 (CJH). LJR acknowledges the support of the DSI-NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis (DSI-NRF CIMERA), at the University of Johannesburg, towards this research.Granite-hosted magmatic-hydrothermal mineral deposits are major sources of Cu, Mo, Sn, Li, and W, originating via mineralizing fluids exsolved from volatile-saturated magmas. We show how trace elements in zircon sampled from the granite-hosted Zaaiplaats tin deposit, Bushveld Complex, preserve a record of both the enrichment of incompatible metals during magma fractionation and those arising from magmatic-hydrothermal mineralization processes. The Zaaiplaats granites are subdivided into three groups; mineralized, altered, and unmineralized. Zircon trace element contents define two trends in a plot of Sn against Gd: Sn/Gd ratios in zircons from the unmineralized samples, as well as the majority of altered samples, define a magma fractionation trend with increasing Y at constant Sn/Gd, whereas those from the mineralized samples are displaced to high Sn/Gd ratios at similar Y. Elevated Sn in the Zaaiplaats zircons is attributed to the introduction of a Sn-rich mineralizing fluid during zircon growth, which occurred at an advanced stage of crystallization (>85%) of the host magma. This model is consistent with the preservation of whole-rock Sn zonation in the Zaaiplaats granites modelled by closed-system magma differentiation and the ensuing exsolution of an acidic, saline Sn-rich magmatic-hydrothermal fluid (Groves and McCarthy, 1978). A metal anomaly, Sn/Sn*, is defined which describes the deviation of Sn over that expected through magma fractionation alone (Sn*), and arises from Sn mobilization due to magmatic-hydrothermal mineralization processes. Identification of metal anomalies such as Sn/Sn* and Cu/Cu* in mineral archives or at the whole-rock level, provides an empirical link to the onset of mineralization processes in magmatic-hydrothermal systems, and can be coupled with geochemical proxies to yield a better understanding of the conditions leading up to, and subsequent to, volatile saturation.PostprintPeer reviewe