Phosphorus Behaviour in Pallasites: Experimental Constraints

Abstract

Pallasites are stony-iron meteorites mainly composed of olivine (Fa11-20) and iron-nickel metal in approximately equal weight proportions. They are believed to have formed at the core-mantle boundary of planetary bodies. An alternative model is that these meteorites represent pieces of metal pools or 'raisins' isolated within the silicate mantle of their parent body. The minor-element content of the pallasites metal suggest that pallasites are linked to the IIIAB iron-meteorite group (Scott, 1977). Trace and minor element variations within the IIIAB group has been successfully modelled considering that these iron meteorites are pieces of a single asteroidal core material which solidified according to a fractional crystallisation process (Haack and Scott, 1993). Following this model, the high Ni-content of the pallasites metal relative to IIIAB iron-meteorites and the presence of olivine crystals is consistent with a late crystallizing core-liquid, enriched in incompatible elements, which has intruded the asteroid olivine-mantle as already proposed by Scott (1977). A remarkable feature in pallasites is the occurrence of phosphates coexisting with schreibersite, (Fe,Ni)3P. Whitlockite, Ca3(PO4)2, farringtonite, ( M g , F e ) 3 ( P O 4 ) 2 , a n d s t a n f i e l d i t e , Ca4(Mg,Fe)5(PO4)6, are the most widespread species. Phosphoran olivine, a forsterite-rich olivine containing up to 4 -5 wt.% P205 , has also been reported in some pallasites. Phase equilibria involving both phosphates and phosphides have been used to infer oxygen fugacity in pallasites. If we assume that pallasites are composed of metal (either metal pools or core) and olivine of mantle origin and since phosphorus is a siderophile element, accordingly schreibersite phosphorus was dissolved in the metal. The origin of the phosphorus in phosphates, however, is not as obvious. Phosphates could be of mantle origin where they would coexist with olivine. This would rule out the core-mantle origin for pallasites since in terrestrial magmatic systems, phosphates are known to be late crystallizing phases unlikely to occur in magmatic cumulates. However, on the basis of R E E pattern analyses, Davis and Olsen (1991) pointed out that some calcium-rich phosphates (whitlockite and stanfieldite) in Springwater and Santa Rosalia have crystallised from a chondritic melt. The R E E pattern and content of some other phosphate crystals occurring in phosphate-pyroxenetroilite intergrowths between olivine and metal, is different and seem inherited from olivine. In that case, phosphates are supposed to have formed by subsolidus reaction between metal and olivine as suggested by the textural observations. Buseck and Holdsworth (1977) have described striking phosphate textures which rather suggest that phosphates where present as molten droplets attached onto the olivines surface. Phosphates appear to be key minerals in order to decipher the pallasites origin. To our knowledge, no experimental data on the silicate-metal-phosphate system relevant to pallasite formation. Only thermodynamic calculations have been performed in that system in order to derive oxygen fugacity in pallasites (e.g. Righter et al., 1990).