EARLY ARCHEAN SPHERULE BEDS IN THE BARBERTON MOUNTAIN LAND, SOUTH-AFRICA - NO EVIDENCE FOR IMPACT ORIGIN

Detailed petrographical and geochemical studies are reported for sever al well-documented occurrences of spherule-rich layers in the Barberto n Mountain Land, South Africa, which were previously inferred by Lowe and co-workers to be the result of early Archaean meteorite impacts. I n contrast, we made the following observations. The textures of most s pherules are not necessarily the result of impact, but could instead b e the product of radial or intersertal growth of crystals during secon dary mineral formation. There is no difference in the content of sider ophile elements between spherule layers and layers devoid of spherules . The siderophile element abundances are high where sulphide minerals (e.g., pyrite, gersdorffite, and chalcopyrite) and/or chromite are pre sent, independent of the presence or absence of spherules. High conten ts of, e.g., Ir (up to 2700 ppb), Ni (0.96 wt%), and Cr (1.6 wt%) were found in various samples. Abundances of these elements in chondritic meteorites are approximately 600 ppb, 1.4 wt%, and 0.35 wt%, respectiv ely, resulting in respective meteoritic components in these samples of 450%, 70%, and 460%. We do not accept these high concentrations as pr imary meteoritic signatures. Impact melt rocks have typically much les s than 1% of a meteoritic component. Furthermore, spherules of any kin d are very rare in known impact deposits (including the K-T boundary). If present at all, they are not usually associated with any significa nt Ir or PGE anomaly. The high abundances of the siderophile elements in some Barberton samples and their enrichment in secondary minerals i ndicate that these elements have been remobilized and reconcentrated. The PGE interelement ratios have changed during remobilization as well . Thus, the PGE abundance patterns and ratios are not primary and cann ot be used as an argument in favour of an impact origin either. Nickel -rich Cr-spinels, which are found in some of the Barberton spherule sa mples, have low Fe3+/Fe(total) ratios. These ratios are incompatible w ith an extraterrestrial or impact origin, because such impact-derived spinels (including those found at the K-T boundary) are highly oxidize d and also have different chemical compositions. There is also no indi cation of any evidence of shock metamorphism associated with the Barbe rton spherule layers, which is the commonly accepted definitive criter ion for recognition of an impact origin. This is unusual, because such evidence is preserved even in heavily altered samples from deeply ero ded Archaean impact structures. We conclude that, while these spherule layers are clearly unusual and deserve further attention, there is no convincing evidence for an origin by impact. We suggest that they wer e formed by volcanic processes, followed by extensive hydrothermal alt eration.