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SEM/EDS analyses of late Archaean — early Proterozoic Hamersley Basin impact spherules

A.Y. Glikson, B.M. Simonson1 and S. Hassler2 1 Oberlin College, Ohio 2 State University of California, Hayward

Since 1992 several impact fallout units have been identified in the Hamersley Basin and Oakover syncline, including (with increasing age): Dales Gorge Member, Brockman Iron Formation, 2470+/-4 Ma; Bee Gorge Member, Wittenoom Formation, 2561+/-8 Ma; top Jeerinah Formation below Marra Mamba Iron Formation, pre-2630 Ma post-2687 Ma; lower Carawine Dolomite; top Lewin Shale (Simonson, 1992, Geol. Soc. Am. Bull., 104, 829-839); Simonson and Hassler, 1997, Aust. J. Earth. Sci., 44, 37-48). A field trip was undertaken by AYG during July-September, 2001, visiting impact fallout localities in the Hamersley Basin and Rippon Hills, followed by field work in the central and western Hamersley Basin aimed at extending the search for these units in other parts of the Basin. The thickness of impact spherule units generally increases eastward, where impact spherule-bearing debris flows reach a thickness of near 25 meters. Impacts on the scale indicated by the Hamersley fallout deposits trigger high energy seismicity and earthquakes, perhaps up to a magnitude of 9 on the Richter scale. The occurrence of boulder-scale rip-up clasts at the bottom of impact fallout units represents the effects of earthquakes and/or powerful tsunami waves effecting the sea floor. Impact spherules in the Hamersley Basin typically contain inward-radiating crystals of K-feldspar, surrounding off-center central cavities filled with carbonate, quartz and iron oxides. Other spherules are dominated by hydrous Mg-rich silicates – chlorite and stilpnomelane. The lack of shocked quartz grains in these deposits suggests that the spherules originated from impacts impinging on basaltic oceanic crust and not on quartz-bearing continental crust. PGE studies of Hamersley spherules indicate Iridium anomalies of up to about 1.7 ppb, with mean values of about 0.5—0.6 ppb – about an order of magnitude higher than background sediments (Simonson et al., 1999, Geol. Soc. Am. sp. Pap., 339, 249-261). Hydrous alteration resulted in obliteration of other PGE and trace metal signatures. SEM/EDS studies of the Dales Gorge spherules identify cores of stilpnomelane enveloped by shells of K-feldspar which contain euhedral ilmenite blades and micro-scale Ni, NiO, NiS and Co-bearing NiAs particles (Figure 1a, 1b). Due to their micron-scale size no pure probe analyses were obtained of the Ni particles and ilmenites. SEM/EDS analyses of zoned spherules from the top Jeerinah spherule layer indicate abundance of Ni-rich (up to 0.9% NiO) hydrous iron oxides, whichoccur both as veins through the spherules and in the inter-spherule matrix.

Figure 1: (1) SEM Backscattered Electron (BSE) image of Ilmenite (Il) and Ni particles (Ni) in feldspar shell (fl) and surrounding stilpnomelane (s), Dales Gorge Member spherules; (2) detailed BSE image of Ilmenite blades and Ni-particle.