Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.275p&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 275
Other
1
Scientific paper
Phosphates in ordinary chondrites are frequently associated with metal and troilite (e.g., Murrell and Burnett, 1983). They also constitute a large fraction of the inclusions present inside these two phases (Perron et al., 1990), in which they seem to have evolved from tiny precipitates formed from P solid solution in Fe-Ni (Zanda et al., 1990). Here, we further characterize these inclusions, and the external phosphates as well ("external" and "internal" will refer to phosphates located outside and inside metal grains, respectively). Polished sections of Dhajala, H3.8, and Forest Vale, H4 (hereafter DH and FV) were etched to reveal tracks in phosphates. Track densities were obtained with an SEM in internal phosphates in DH and FV, and in external phosphates in DH . The area of all phosphates larger than about 4 micrometers (224 grains in DH, 106 in FV) was measured, and their location noted. REE were measured with an ion probe in internal phosphates. Xe was analysed by stepwise heating in separated metal grains from both chondrites. A ^244Pu fission Xe component (Xe(sub)f) was released at low temperature, attributed to recoils from external phosphates, and another one at high temperature, attributed to internal phosphates. The strong link between phosphates and metal is confirmed: 74% by area (and thus, approximately, by mass) of the phosphates in DH are associated with metal, whether internal, or external in contact with metal (62% in FV). This translates into a ratio of metal-associated P to metal of 0.3% by mass, comparable to the P concentration in the metal of CM2 chondrites (Grossman and Olsen, 1974), of Semarkona (Paris, 1992), and of Leoville (under the form of tiny inclusions, Zanda et al., 1990). It is thus likely that a large proportion of chondritic phosphates formed from P originally contained in Fe-Ni as solid solution, through exsolution followed by growth of the original precipitates during parent-body metamorphism. Mean track densities of internal merrillites agree with those measured here in external grains for DH, and by Lavielle et al. (1992) for FV. So, in the moderately metamorphosed chondrites studied here, merrillite inclusions in metal had already their full share of Pu, as compared to external merrillites. On the other hand, REE abundances in internal merrillites are about a factor of two lower than in typical OC merrillites (Crozaz et al., 1989), with a less pronounced Eu anomaly. The elemental pattern varies somewhat from grain to grain (especially in FV) displaying sometimes a steady decrease of the CI-normalized abundances from light to heavy REE. This, together with the presence, inside the metal, of "exotic" phases like farringtonite (Perron et al., 1990), shows that internal phosphates are not completely equilibrated with the external ones. Track densities are considerably lower in DH (-50 x 10^6 cm^-2) than in FV (-350 x 10^6 cm^-2), which points to a lower Pu concentration or a slower cooling rate for the former. On the other hand, the fission Xe released at high temperature by the metal is larger for DH than for FV (~5 and ~3 x 10^-14 cm^3 ^136Xe/g, respectively). This is also true for low temperature Xe(sub)f (a factor -2 difference between DH and FV). Resolution of this conflict must await the measurement of Xe(sub)f in DH external phosphates. References: Crozaz G., Pellas P., Bourot-Denise M., de Chazal S.M., Fieni C., Lundberg L.L. and Zinner E. (1989) Earth Planet. Sci. Lett. 93, 157-169. Grossman L. and Olsen E. (1974) Geochim. Cosmochim. Acta 38, 173-187. Lavielle B., Marti K., Pellas P. and Perron C. (1992) Meteoritics, submitted. Murrell M.T. and Burnett D.S. (1983) Geochim. Cosmochim. Acta 47, 1999-2014. Perron C., Bourot-Denise M., Pellas P. and Marti K. (1990) Meteoritics (abstract) 25, 398-399. Zanda B., Bourot-Denise M. and Perron C. (1990) Meteoritics (abstract) 25, 422-423.
Bourot-Denise Michèle
Crozaz Ghislaine
Kim Jinyoung Serena
Marti Kurt
Perron Claude
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