Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..484a&link_type=abstract
Meteoritics, vol. 30, no. 5, page 484
Other
Scientific paper
We previously studied Ar, Kr and Xe during stepwise heating of a sample of the eucrite Pomozdino [1], with temperature steps 400, 600, 800, 900, 1100, 1300 and 1600 C. Most (96%) of the cosmogenic and fissiogenic Xe was released in the 1100 - 1600 C fractions, with maximum release (70%) in the 1300 C fraction. The relative release of these components in the fractions was nearly identical. The fissiongenic Xe release curve is shown in Fig. 1 lower part (construction technique is discussed in part 2). The release curves of all isotopes formed in situ have a peak release in the 1300 C fraction too. The existence of this common peak for different isotopes suggests either the same release process or that the different isotopes were present in the same mineral phase. The rock-forming minerals of eucrites are plagioclase and pyroxenes. Investigation of the Sm-Nd system in the eucrite Pomozdino [2] demonstrated the nearness of the concentrations of Nd in both plagioclase and pyroxene fractions (3.34 and 2.89 nmol/g respectively). Thus the concentrations of both cosmogenic and fissiogenic Xe should be approximately equal in these minerals, and the presence of a single release peak might indicate the destruction of the crystal structure of both minerals, possibly during melting of eucrite under laboratory conditions. In order to evaluate the interval of the Pomozdino eucrite's melting temperatures we used a thermodynamic model of equilibrium crystallization of lunar magmas embodied in the LUNAMAG computer program [3] (and references in this work). The similar compositions of eucrites and lunar rocks, and also the vacuum conditions of the experiment (low fO2) make the model applicable. Fig. 1 (upper part) shows a LUNAMAG graph of equilibrium crystallization using the Pomozdino eucrite [4] chemical composition. The melting temperature of minerals during stepwise heating may be higher than the calculated values due to incomplete solid-melt equilibrium during the temperature steps, which each lasted ~1.3 hours. The real correspondence of the gas release temperatures within the 1300 C fraction to the temperature interval of equilibrium melting of Pomozdino suggests that the major mechanism of Xe release is likely to be melting. The release curves of other Ar and Kr isotopes formed in situ, aside from the common peak, have peaks at lower temperatures. The ratio between amount of gas released within the limits of these peaks is various; and 40Ar has the highest release at low temperatures. The presence of several peaks suggests that various gas migration mechanisms may exist. The application of thermodynamic models of crystallization appears to be appropriate for estimating melting temperatures of other achondrites that formed from melt during crystallization, for example in diogenites. Acknowledgments: We are very grateful to Dr. A. A. Ariskin for the LUNAMAG program. References: [1] Assonov S. S. et al. (1992) Ann. Geophys., 11, Suppl. III, Part III, 475. [2] Karpenko S. F. et al. (1991) 16th NIPR Symp. Antarc. Meteorites, 178-179. [3] Ariskin A. A. et al. (1994) LPS XXV, 37-38. [4] Warren P. H. et al. (1990) Proc. LPS, Vol. 20, 281-297.
Assonov Sergey S.
Ivanova Marina A.
Shukolyukov Yu. A.
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