BARBERTON CT3 SPHERULE LAYERS: CLUES TO THEIR FORMATION FROM PGE CONTENTS AND OS ISOTOPES
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 . 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 . 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.
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