HR: 10:50h
AN: V22B-03 INVITED    [Abstracts]
TI: Mechanically assisted equilibration of Siderophile elements in silicate melts
AU: Ertel-Ingrisch, W
AF: Earth and Environment, LMU-University of Munich, Theresienstr. 41/III, Munich, 80333, Germany
AU: * Dingwell, D B
EM: Dingwell@lmu.de
AF: Earth and Environment, LMU-University of Munich, Theresienstr. 41/III, Munich, 80333, Germany
AB: Solubilities of a wide range of siderophile elements (SE: Ni, W, Re, Ir, Os, Pt, Rh) in an analog basaltic melt (corresponding to the 1 atm anorthite-diopside eutectic) have been determined using the mechanically assisted equilibration (MAE) technique of Dingwell et al. (1994). Here we present a review of the data obtained and the experience gained in this decade of work. In particular, the major experimental and analytical challenges of the nanonugget problem as well as implications for core/mantle equilibria and core formation scenarios in terrestrial planets are reviewed. To accomplish this, a comprehensive and detailed description of the MAE technique is provided. A general background overview of solubility experiments regarding siderophile elements (SE) is also supplied. In these studies, major element composition was routinely determined by electron microprobe analyses (EMP), whereas trace elements were determined using a wide variety of analytical techniques (Ni, W: INAA, EMP, ICP-AES; Re, Ir, Pt, Rh and Os: INAA, SIMS, dissolution- (diss-ICP-MS) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS)). The last technique, in particular, has demonstrated its powerful capabilities as a truly micro-analytical technique supplying information both on absolute trace element concentrations and on small scale heterogeneities in run products. All investigated SEs exhibited solubilities consistent with their presence as stoichiometrically dissolved oxide species in the melt phase. There was no indication for zero-valent species dissolved at any oxygen fugacity (fO2) condition. In the case of highly-SE (HSE: Pt, Rh, Re, Os, Ir), INAA results appear to indicate a decrease of HSE solubility with decreasing fO2 down to a fO2 limit which depends on the investigated HSE. Below this limit, bulk HSE concentrations remain either constant with large variations or increase with further fO2 decrease. Duplicate analyses of samples by LA-ICPMS reveal increasing amounts of so-called nanonuggets with decreasing fO2, which lead to high HSE concentrations in the glass samples obtained by bulk analytical methods such as INAA. The formation of HSE (and potentially some SE) nanonuggets in low fO2 samples raise the question of whether nanonuggets are formed either during the quench by precipitation from precursor species dissolved homogeneously in the melts, or are precipitated in situ at high temperature due to true thermodynamic oversaturation. Through the combination of MAE technique with LA-ICPMS micro-analytics it has been possible to extend our knowledge of the solubility behaviour of HSE to unprecedentedly low fO2 values. Clarification of the solubility mechanism for SE as well as the nanonugget issue, however, will undoubtedly require further novel experimental designs.
DE: 3612 Reactions and phase equilibria (1012, 8412)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting