Untitled Document

SEM/EDS and laser ICPMS study of c. 3.24—3.225 Ga Barberton impact spherules

A.Y. Glikson, G. Byerly1, C. Allen, W. Taylor and I.H. Campbell 1 University of Louisiana

Least-altered samples of Barberton impact spherules (microkrystites) were studied by optical microscopy, scanning electron microscopy, electron probe EDS spectrometry, and laser-ICPMS mass spectrometry. The spherules are dominated by chlorite and are set in heterogeneous siliceous matrix. Some spherules contain partly resorbed octahedral nickel-rich chrome spinel concentrated in annular zones. EDS analyses of chromites indicate high levels of Ni, Co, V and Zn identified earlier by Byerly and Lowe (1994, Geochim. Cosmochim. Acta, 58, 3469-3486). Laser-ICPMS Analyses of micron-scale PGE nuggets indicate about an order of magnitude enrichment in Pt and Rh relative to the other PGEs (RSES-2000, p. 124—125). Geochemical evidence for a meteoritic component in the spherules includes: (1) the spherules display PGE patterns depleted in the volatile Pd and Au (Kyte et al., 1992, Geochim. Cosmochim. Acta, 56, 1365-1372) consistent with their suggested condensation from impact-released vapor; (2) the spherules contain high Ni, Co and Cr levels (Ni and Cr up to about 3000 ppm), Ni/Co ratios (~40—80) higher than Barberton komatiites (<10) and C1 chondrites (21), and Ni/Cr ratios (~0.5—1.0) higher than Barberton komatiites (0.1—0.6) and lower than C1 chondrites (~4.0) (Figure 1). These relations allow mass balance mixing estimates of the proportion of meteoritic component (MC) admixed in the impact released vapor, showing that the MC varies between spherules, which range from compositions akin to komatiites and high-Mg basalts to compositions including a high meteoritic component of about 30 percent. Rare Earth element patterns in the spherules display marked relative depletion in the light REE (La/Sm)N ~ 0.4—0.8 as compared to Barberton komatiites with ratios of about unity or higher, potentially related to loss attendant with vapor condensation.

Figure 1: Ni-Co-Cr relations in Barberton spherules, indicating meteoritic signatures in impact condensate spherule SA306-L5, with distinct high Ni/Co (40—80) and high Ni/Cr (0.5—1.0) as compared to Barberton komatiites and high-Mg basalts.