Siderophile elements in silicate melts - A review of the mechanically assisted equilibration technique and the nanonugget issue

Solubilities for a wide range of siderophile elements (SE: Ni, W, Re, Ir, Os, Pt, Rh) have been determined in a haplobasaltic melt composition (An₄₂Di₅₈) using the mechanically assisted equilibration (MAE) technique. Here we present a review of the data obtained and the experience gained over the past decade using this approach. In particular, the major experimental and analytical challenges encountered (and inferred to result from so-called "nanonugget" formation) are described. After a general background overview of siderophile element solubility experiments, the experimental (MAE) and analytical techniques are described. 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-ICPMS) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS)). The last technique, in particular, has demonstrated its powerful capabilities for micro-analyses, supplying information on both absolute trace element concentrations and on small scale heterogeneities in run products. All investigated SEs exhibit solubilities whose systematic variation with oxygen fugacity (fO₂) is consistent with their presence as stoichiometrically dissolved oxide species in the melt phase. There is no indication of zero-valence species dissolved at any 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 raises 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. The combination of the MAE technique with LA-ICPMS micro-analytical methods has enabled us 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.