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Phosphoran pyroxene and olivine in silicate inclusions in natural iron-carbon alloy, Disko Island, Greenland

Authors
Journal
Geochimica et Cosmochimica Acta
0016-7037
Publisher
Elsevier
Publication Date
Volume
48
Issue
5
Identifiers
DOI: 10.1016/0016-7037(84)90202-3

Abstract

Abstract Dendritic pyroxene and olivine within intragranular silicate inclusions in natural iron-carbon alloy (~2% C) contain up to 2.7 wt.% P 2O 5. The pyroxene and olivine crystallized rapidly in supersaturated silicate liquids, enriched in FeO (13–55 wt.%), TiO 2 (2–5 wt%), and P 2O 5 (2–4 wt.%), which were trapped as immiscible liquids within iron-carbon liquid. The structural formulae of the pyroxene and olivine indicate that the P substitutes for Si in these minerals, as predicted from theoretical considerations and consistent with rare, previously reported occurrences of P-rich silicates. In the olivine P +5 appears to be charge-balanced by octahedral site vacancies. In the pyroxenes P +5 is probably charge-balanced by coupled substitution with Al +3 or Ti +3 or by octahedral site vacancies. Three factors could possibly have led to the incorporation of these unusually large amounts of P into the structures of these minerals: 1. 1) the high P 2O 5 contents, coupled with the low SiO 2 contents of the liquids in which they crystallized 2. 2) rapid crystallization from supersaturated liquids 3. 3) low ƒO 2. Further studies, particularly experimental, are needed to determine quantitatively how these factors affect P silicate crystal/liquid distribution factors. Phosphorus in the upper mantle (except in local regions, metasomatically enriched in P 2O 5) might occur principally in the structures of pyroxene and olivine, rather than in apatite.

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