PETROGRAPHIC AND COMPOSITIONAL CHARACTERIZATION OF METALLIC SPHERULES IN IMPACT MELT AT MONTURAQUI CRATER, CHILE

Monturaqui is a simple, ~350 m diameter impact crater in northern Chile, emplaced into Paleozoic granite + Tertiary ignimbrite. Its age is ~570-750 ka based on (U-Th)/He zircon/apatite, TL and cosmogenic nuclide methods. Impact melt occurs as scattered cm-sized chunks, and contains small spherules (< 1 µm to ~2 mm) of Fe-Ni metal±sulphides in a glass matrix. Previous work showed significant compositional variations with spherule size: smaller spherules have higher Ni and Co and lower Fe and P relative to larger spherules. We are doing a petrographic, SEM, EMP, LA-ICP-MS and SIMS compositional study of individual spherules to address models proposed for the origin of these variations (vaporization-condensation, vapor fractionation or selective volatilization of Fe, selective oxidation processes, selective shock-melting of sulphide-metal inter-growths at meteorite grain boundaries, ionic-radius-controlled mobility of certain elements). Individual Fe-Ni±S spherules (< ~200 µm) are spherical with homogeneous Fe and Ni contents and occasional interstitial troilite, while larger areas of Fe-Ni±S have irregular shapes, often with hollow centres filled with secondary barite, and can show Ni-rich margins. New EMP analyses (n=225) show a continuum of compositions from 8 wt% Ni to 58 wt% Ni, and the larger spherules have compositions similar to unaltered parts of ‘iron shale’ – weathered fragments of iron meteorite found around the crater. Preliminary in-situ trace element analysis by LA-ICP-MS shows enrichments in PGEs, Au, Mo and Cu with increasing Ni/Fe and decreasing spherule size. The spherules show no significant Re/Os fractionation with Ni/Fe ratio, which limits their potential use for Os isotope dating of crater formation. We are currently investigating the extent of inter-element fractionation within the PGE group of elements in the spherules and glass, as PGE patterns are often used to ascertain impactor type and contribution to impact melts. We have successfully used electric pulse disintegration methods (selFrag) to cleanly separate individual metal spherules from glass matrix ready for future solution ICP-MS analysis and stable isotope analysis (Fe, Ni).