^1,2Etienne Médard, ²Max W. Schmidt, ^2,3Markus Wälle, ^4,5Nicole S. Keller, ³Detlef Günther
¹Laboratoire Magmas et Volcans, Université Blaise Pascal – CNRS – IRD, OPGC, 5 rue Kessler, 63038 Clermont Ferrand, France
²Institut für Geochemie und Petrologie, ETH Zürich, Clausiusstrasse 25, CH-8092 Zürich, Switzerland
³Laboratory of Inorganic Chemistry, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland
⁴University of Iceland, Institute of Earth Sciences, Sturlugata 7, IS-101 Reykjavik, Iceland
⁵Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

High-pressure, high-temperature experiments have been performed at ∼1.2 GPa and 1360-2100 °C to investigate the partitioning of Pt between a silicate melt and a metallic melt. Our experiments indicate that nanonuggets encountered in previous experiments are experimental artifacts, formed at high temperature by oversaturation caused by high oxygen fugacity during the initial stages of an experiment. Experiments at high-acceleration using a centrifuging piston-cylinder show that nanonuggets can be removed by gravity during the experiment. Formation of nanonuggets can also be avoided by using initially reduced starting materials. The presence of iron is also a key element in reducing the formation of nanonuggets. Our nanonugget-free data are broadly consistent with previous nanonuggets-filtered data, and suggest that Pt partitioning becomes independent of oxygen fugacity below an oxygen fugacity of at least IW+2. Pt is thus possibly dissolved as a neutral species (or even an anionic species) at low fO2, instead of the more common Pt2+ species present at higher fO2. Due to low concentration, the nature of this species cannot be determined, but atomic Pt or Pt- are possible options. Under core-formation conditions, Pt partitioning between metal and silicate is mostly independent of oxygen fugacity, silicate melt composition, and probably also pressure. Partition coefficient during core formation can be expressed by the following equation: View the MathML sourcelogDPtMmetal/silicate=1.0348+14698/T (in weight units). Calculations indicate that the Pt content (and by extension the Highly Siderophile Elements content) of the Earth’s mantle cannot be explained by equilibrium partitioning during core formation, requiring further addition of HSE to the mantle. The mass of this late veneer is approximately 0.4% of the total mass of the Earth (or 0.6% of the mass of the mantle).

Reference
Médard E, Schmidt MW, Wälle M, Kellern NS, Günther D (2015) Platinum partitioning between metal and silicate melts: Core formation, late veneer and the nanonuggets issue. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.019]

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¹N.E.B. Zellner, ²J.W. Delano
¹Department of Physics, Albion College, Albion, MI 49224 USA
²New York Center for Astrobiology, Department of Atmospheric and Environmental Sciences, University at Albany (SUNY), Albany, NY 12222 USA

Lunar impact glasses, which are quenched melts produced during cratering events on the Moon, have the potential to provide not only compositional information about both the local and regional geology of the Moon but also information about the impact flux over time. We present in this paper the results of 73 new 40Ar/39Ar analyses of well-characterized, inclusion-free lunar impact glasses and demonstrate that size, shape, chemical composition, fraction of radiogenic 40Ar retained, and cosmic ray exposure (CRE) ages are important for 40Ar/39Ar investigations of these samples. Specifically, analyses of lunar impact glasses from the Apollo 14, 16, and 17 landing sites indicate that retention of radiogenic 40Ar is a strong function of post-formation thermal history in the lunar regolith, size, and chemical composition. This is because the Ar diffusion coefficient (at a constant temperature) is estimated to decrease by ∼3-4 orders of magnitude with an increasing fraction of non-bridging oxygens, X(NBO), over the compositional range of most lunar impact glasses with compositions from feldspathic to basaltic. Based on these relationships, lunar impact glasses with compositions and sizes sufficient to have retained ∼90% of their radiogenic Ar during 750 Ma of cosmic ray exposure at time-integrated temperatures of up to 290K have been identified and are likely to have yielded reliable 40Ar/39Ar ages of formation. Additionally, ∼50% of the identified impact glass spheres have formation ages of ⩽500 Ma, while ∼75% of the identified lunar impact glass shards and spheres have ages of formation ⩽2000 Ma. Higher thermal stresses in lunar impact glasses quenched from hyperliquidus temperatures are considered the likely cause of poor survival of impact glass spheres, as well as the decreasing frequency of lunar impact glasses in general with increasing age. The observed age-frequency distribution of lunar impact glasses may reflect two processes: (i) diminished preservation due to spontaneous shattering with age; and (ii) preservation of a remnant population of impact glasses from the tail end of the terminal lunar bombardment having 40Ar/39Ar ages up to 3800 Ma. A protocol is described for selecting and analysing lunar impact glasses.

Reference
Zellner NEB, Delano JW (2015) 40Ar/39Ar ages of lunar impact glasses: Relationships among Ar diffusivity, chemical composition, shape, and size. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.013]

Copyright Elsevier