PETROGRAPHY OF IMPACT SPHERULES FROM A BIF-HOSTED EJECTA LAYER IN THE HAMERSLEY BASIN, EARLY PALEOPROTEROZOIC OF WESTERN AUSTRALIA

The fourth shale band (S4) of the Dales Gorge Member of the Brockman Iron Formation is ~2.49 b.y. old and contains a single layer rich in coarse to very coarse sand-size spherules. Despite its early discovery and unusual nature (LaBerge, 1966), the spherules have not been studied in detail previously. Geochemical characteristics such as 18 ppb iridium (McDonald and Simonson, 2002) indicate they are impact ejecta. The layer primarily consists of well-sorted spherules with fibrous stilpnomelane cements filling the intergranular pore space. Splash-form shapes such as tear drops, peanuts and agglutinated grains and certain internal textures of the spherules described below indicate they were originally molten. They now consist of fibrous stilpnomelane and potassium feldspar plus smaller amounts of opaque phases. We interpret the stilpnomelane in the spherules as impact melt replaced in an iron-rich diagenetic environment. In contrast, the potassium feldspar typically takes the form of radial-fibrous aggregates centered on spherule boundaries or lath-shaped crystals, many of them hollow and some with swallow tail terminations. These hollow shapes and swallow tail terminations are typical of plagioclase crystallites, and where they are randomly oriented, the spherules have textures identical to quenched basaltic melts. We therefore believe the impact melt droplets were originally basaltic in composition, and that plagioclase crystallites in the droplets were faithfully replaced by potassium feldspar during diagenesis. A minority of the grains in the layer are angular rather than rounded, yet they show the same textures as the spherules; these grains probably represent particles of solid ejecta and/or broken fragments of larger spherules. The absence of radial-fibrous potassium feldspar along edges where spherules were broken before burial supports the interpretation of the replaced plagioclase textures as primary rather than products of later replacement.