Mathematics – Logic
Scientific paper
Mar 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999m%26ps...34..285o&link_type=abstract
Meteoritics & Planetary Science, vol. 34, no. 2, pp. 285-300 (1999).
Mathematics
Logic
10
Scientific paper
Thirteen phosphate minerals are found in IIIAB iron meteorites. Four of these, sarcopside-graftonite, johnsomervilleite, galileiite comprise the majority of occurrences. IIIB irons are confined to occurrences of only these four phosphates. IIIA irons may contain one or more of these four phases, and may also contain other rarer phosphates, and, in two instances, silica, and in one instance, a silicate rock. Thus, the IIIA lithophile chemistry is more varied than that of the IIIBs. Based on petrographic relations, sarcopside appears to be the first phosphate to form. Graftonite is probably formed by recrystallization of sarcopside. Johnsomervilleite and galileiite exsolved as enclaves in sarcopside or graftonite at lower temperatures, although some of these also nucleate as separate crystals. IIIAB phosphates are carriers of a group of incompatible lithophile elements: Fe, Mn, Na, Ca and K, and (rarely) Mg, as well as Pb. These elements, and oxygen, were concentrated in a residual, sulfur-rich liquid during igneous fractional crystallization of the IIIAB core mass. The phosphates formed by oxidation of phosphorus as the core solidified and excluded oxygen, which increased its partial pressure in the residual liquid. The trace siderophile trends in bulk IIIAB metal are paralleled by a mineralogical trend of the phosphate minerals that formed. For IIIABs with low Ir contents in the metal, the phosphates are mainly Fe-Mn phases; at intermediate Ir values more Na-bearing phosphates appear; at the highest Ir values the rarer Na, K, Mg, Cr, Pb bearing phosphates appear. The absence of significant amounts of Mg, Si, Al and Ti suggest depletion of these elements in the core by the overlying mantle.
Bunch Ted E.
Davis Andrew M.
Hutcheon Ian D.
Kracher Alfred
Olsen Edward J.
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