Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.506f&link_type=abstract
Meteoritics, vol. 30, no. 5, page 506
Other
1
Chondrites, H, Geothermometry, Iron-Magnesium Ordering, Meteorites, Acfer 314, Estacado, Frontier Mountain 90049, Frontier Mountain 90076, Frontier Mountain 90156, La Villa, Miami, Raguli, Orthopyroxene
Scientific paper
The study of the thermal metamorphism of ordinary chondrites through geothermometers can provide significant constraints on the parent body thermal models which remain controversial. We report here results from Fe-Mg ordering closure temperatures (Tc) of orthopyroxenes from eight unshocked equilibrated H-chondrites obtained by means of single crystal X-ray diffraction. The method is based on the fact that cation partitioning in orthopyroxene is sensitive to temperature [1], and makes use of the experimental calibration by Molin et al. [2]. The goal of the investigation is to check how petrographic types relate to cation ordering thermal records. Results: The orthopyroxenes show a very similar degree of Fe-Mg ordering (see Table 1.). The Tc's cluster within the 384+/-48 to 480+/-28 degrees C interval, and show no correlation with petrographic type. The lack of a correlation does not mean that the distribution is random, rather, it appears to be controlled in individual samples by the degree of equilibration. In fact, the higher the petrographic type, the more coherent the results of the grains from individual chondrites. The spread of Tc's in the least equilibrated chondrites could be either a memory of heterogeneous pre-metamorphic records related to individual chondrule histories, or an artefact due to crystal defects. Therefore (1) the thermal records, inferred from the Fe-Mg ordering, are nearly the same for all the equilibrated H-chondrites; (2) the most equilibrated chondrites record distinct Tc values within the larger common Tc range; (3) the spread of Tc in H4's maybe indicative of disequilibrium and merits further study. The closure temperature conveys information on the cooling rate close to its value, regardless of the temperature regimes when the ordering process started. Extrapolation to high temperatures can be made only if the cooling path is constrained. Since we have no data to establish the temperature when ordering began and to decide whether the cooling was linear, we have not attempted quantitative estimates of cooling rates. However we can say that as equilibrated H-chondrites carry similar Tc records, they cooled at similar rates through 380-480 degrees C. However, the k(sub)D's (i.e., the intracrystalline Fe-Mg distribution coefficient) range 0.029 to 0.054, suggesting that H-chondrites cooled in petrogenetic environments intermediate between those of volcanic [3] and granulitic rocks [3,4]. Conclusion: Our results provide the following constraints: (1) Equilibrated H-chondrites, regardless the petrographic type, were located in petrogenetic environments characterized by similar temperature-time conditions when cooling through 380-480 degrees C, and, in terms of terrestrial environments, intermediate between those of volcanic and granulitic facies. (2) Different petrographic types reflect distinct metamorphic thermal regimes. Thus, to account for constraint (1), petrographic type was determined at temperatures above the 380-480 degrees C interval. This implies that equilibrated H-chondrites were placed in different geological environments characterized by distinct temperature-time regimes when still above this temperature range. In the case of the least equilibrated chondrites, for which the two events could coincide. Acknowledgments: We thank EUROMET for providing us with the Frontier Mt. samples, and PNRA for supporting this study. References: [1] Saxena S. K. and Dal Negro A. (1983) Bull. Min ral., 106, 443-449. [2] Molin G. M. et al. (1991) EPSL, 105, 260-265. [3] Bertolo S. and Nimis P. (1993) EJM, 5, 707-719. [4] Tribaudino M. and Talarico F. (1992) EJM, 4, 453-463.
Folco Luigi
Mellini Marcello
Pillinger Colin T.
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