^1,2McCanta, M. C., ¹Beckett, J. R., ¹Stolper, E. M.
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²Department of Earth and Ocean Sciences, Tufts University, Medford, Massachusetts, USA

Phosphorus zoning is observed in olivines in high-FeO (type IIA) chondrules in H chondrites over the entire range of petrologic grades: H3.1–H6. Features in P concentrations such as oscillatory and sector zoning, and high P cores are present in olivines that are otherwise unzoned in the divalent cations. Aluminum concentrations are low and not significantly associated with P zoning in chondrule olivines. In highly unequilibrated H chondrites, phosphorus zoning is generally positively correlated with Cr. Atomic Cr:P in olivine is roughly 1:1 (3:1 for one zone in one olivine in RC 075), consistent with Cr3+ charge-balancing P5+ substituting for Si4+. Normal igneous zonation involving the dominant chrome species Cr2+ was observed only in the LL3.0 chondrite Semarkona. In more equilibrated chondrites (H3.5–H3.8), Cr spatially correlated with P is occasionally observed but it is diffuse relative to the P zones. In H4–H6 chondrites, P-correlated Cr is absent. One signature of higher metamorphic grades (≥H3.8) is the presence of near matrix olivines that are devoid of P oscillatory zoning. The restriction to relatively high metamorphic grade and to grains near the chondrule–matrix interface suggests that this is a response to metasomatic processes. We also observed P-enriched halos near the chondrule–matrix interface in H3.3–H3.8 chondrites, likely reflecting the loss of P and Ca from mesostasis and precipitation of Ca phosphate near the chondrule surface. These halos are absent in equilibrated chondrites due to coarsening of the phosphate and in unequilibrated chondrites due to low degrees of metasomatism. Olivines in type IA chondrules show none of the P-zoning ubiquitous in type IIA chondrules or terrestrial igneous olivines, likely reflecting sequestration of P in reduced form within metallic alloys and sulfides during melting of type IA chondrules.

Reference
McCanta MC, Beckett JR, Stolper EM (2016) Correlations and zoning patterns of phosphorus and chromium in olivine from H chondrites and the LL chondrite Semarkona. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12604]
Published by arrangement with John Wiley & Sons

¹Frans J.M. Rietmeijer
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA

In Stardust tracks C2044,0,38, C2044,0,39, and C2044,0,42 (Brennan et al. 2007) and Stardust track 10 (this work) gold is present in excess of its cosmochemical abundance. Ultra-thin sections of allocation FC6,0,10,0,26 (track 10) show a somewhat wavy, compressed silica aerogel/silica glass interface which challenges exact location identification, i.e., silica glass, compressed silica aerogel, or areas of overlap. In addition to domains of pure silica ranging from SiO2 to SiO3 glass, there is MgO-rich silica glass with a deep metastable composition, MgO = 14 ± 6 wt%, due to assimilation of Wild 2 Mg-silicate matter in silica melt. This magnesiosilica composition formed when temperatures during hypervelocity capture reached >2000 °C followed by ultrafast quenching of the magnesiosilica melt when it came into contact with compressed aerogel at ~155 °C. The compressed silica aerogel in track 10 has a continuous Au background as result of the melting point depression of gold particles

Reference
Rietmeijer FJM (2016) At the interface of silica glass and compressed silica aerogel in Stardust track 10: Comet Wild 2 is not a goldmine. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12608]
Published by arrangement with John Wiley & Sons