Highly siderophile element systematics of the 3.3 Ga Weltevreden komatiites, South Africa: Implications for early Earth history

Details Highly siderophile element systematics of the 3.3 Ga Weltevreden komatiites, South Africa: Implications for early Earth history Highly siderophile element systematics of the 3.3 Ga Weltevreden komatiites, South Africa: Implications for early Earth history

Nov 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011e%26psl.311..253c&link_type=abstract

Earth and Planetary Science Letters, Volume 311, Issue 3, p. 253-263.

Physics

Re–Os Isotope System, Highly Siderophile Elements, Early Archean Komatiites, Late Accretion, Barberton Greenstone Belt, Mantle Plume

Scientific paper

Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundances, Re-Os isotopic systematics, and major-, minor-, and lithophile trace element abundances are determined for a suite of remarkably fresh komatiites from the Weltevreden Formation (Fm.) of the Barberton Greenstone Belt, South Africa. The komatiite lavas examined are calculated to have contained ca. 31% MgO upon emplacement, and are among the most MgO-rich lavas ever erupted onto Earth's surface. Based on the partitioning behavior of V, indicating reduced conditions of magma formation, as well as the low calculated H2O abundance of ~ 0.02% in the mantle source, anhydrous melting for the Weltevreden Fm. komatiites is inferred. The high calculated liquidus temperatures of the emplaced komatiite lavas of ca. 1600 °C require an unusually hot (~ 1800 °C) mantle source, > 100 °C hotter than the ambient contemporaneous mantle. These observations are most consistent with formation of the Weltevreden komatiites via deep (~ 530 km) melting in a mantle plume. The Re-Os isotopic data for twelve whole-rock samples and an olivine separate define a precise Re-Os isochron age of 3266 ± 8 Ma (2σmean) and an initial 187Os/188Os of 0.10440 ± 5 (γ187Os = - 0.13 ± 0.05). The Re-Os data, coupled with the relatively high calculated HSE abundances in the mantle source totaling ~ 80% of those in modern Primitive Mantle (PM) estimates, indicate that at least some portions of the deep, early Archean mantle evolved with essentially chondritic Re/Os and absolute HSE abundances that are typical of the sources of late Archean komatiites. If the HSE budget of the terrestrial mantle was established via late accretion of several large planetesimals following the cessation of major chemical interaction between the core and the mantle, our data indicate that by 3.3 Ga, these materials were well homogenized within the mantle domains sampled by the Weltevreden komatiites and, by inference, within the mantle as a whole.

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