2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 290-9
Presentation Time: 10:15 AM


OZDEMIR, Seda1, SCHULZ, Toni1, KOEBERL, Christian2, REIMOLD, Wolf Uwe3 and MOHR-WESTHEIDE, Tanja3, (1)Department of Lithospheric Research, University of Vienna, Althanstrasse 14, Vienna, 1090, Austria, (2)Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, also of the Natural History Museum, Burgring 7, A-1010 Vienna, Austria, (3)Mineralogy, Museum für Naturkunde - Leibniz-Institute at Humboldt University Berlin, Invalidenstrasse 43, Berlin, 10115, Germany

Spherule-rich layers in the Barberton Greenstone Belt (South Africa) occur in the 3.4 Ga contact between the Fig-Tree and Onverwacht Groups [1,2]. Although subjected to intense research over the last two decades, their exact origin remains ambiguous. Whereas Cr isotope data seem to favor a meteoritic source, noble metal enrichments, locally exceeding the chondritic abundance by a factor of up to five, require further consideration. Our study focuses on new spherule layers that have recently been recognized in the drill core CT3 in the north-eastern part of the Barberton Greenstone Belt (Kaapval Craton at the grid location 25°30`50.76``S, 31°33`10.08``E). We aim to determine their diagenetic and metamorphic history and to assess their possible impact origin by petrographic and geochemical as well as Os isotope analyses.

Spherule bearing layers are intercalated by shale, chert, carbonates and sulfides. Spherules typically exhibit variable size distributions and vary in composition, texture and shapes. Vesicles as well as teardrops or dumbbell shaped spherules support an impact origin [3]. Our preliminary Os isotope data reveal a clear dichotomy between spherule bearing samples with a typically unradiogenic Os isotope signature (187Os/188Os between 0.11 and 0.16 and Os concentrations up to ~1 ppm) and non-spherule bearing samples (typically exhibiting radiogenic 187Os/188Os values of up to 1.1 and low Os concentrations in the low ppb range). Our isotope data thus confirm an earlier study performed on spherule layers S1 to S4, exhibiting similar correlations [4]. However, any meteoritic Os isotope signals in spherule bearing samples from the Barberton Greenstone belt may be obscured by the near chondritic Os isotope evolution paths of their mafic precursors. A possible confirmation of such a signal has to await Re data, which, together with concentration data for Ir, Pt, Ru, and Pd, will be presented at the conference.

[1] Lowe D.R. and Byerly G. R. 1986. Geology 14(1):83-86. [2] Lowe D.R. et al. 2003. Astrobiology 3:7-48. [3] Glass B. P. and Simonson B. M. 2012. Elements 8:15-60. [4] Morel et al. (2002), Goldschmidt Conference Abstracts, A523