Wainwright, Ashlea N.: Dating Early Archean partial melting events: insights from Re-Os dating of micrometric Os-minerals from Kalahari Craton mantle xenoliths. - Bonn, 2015. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5n-39288
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5n-39288,
author = {{Ashlea N. Wainwright}},
title = {Dating Early Archean partial melting events: insights from Re-Os dating of micrometric Os-minerals from Kalahari Craton mantle xenoliths},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2015,
month = mar,

note = {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.},

url = {http://hdl.handle.net/20.500.11811/6427}

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