Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.329b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 329
Other
Cooling Rates, Eucrites, Exsolution, Pyroxenes
Scientific paper
It has been widely recognized that the non-cumulate eucrites consist of two groups, which reflect different degrees of post- crystallization equilibration. The unequilibrated eucrites, exemplified by Pasamonte, retain clear evidence of primary igneous zoning in pyroxenes, while equilibrated eucrites such as Stannern and Juvinas have lost their primary magmatic mineral compositions. Although some petrographic observations have been made, the details of the thermal histories of unequilibrated and equilibrated eucrites remain unclear [e.g., 1,2]. We have begun a systematic study of pyroxene mineral chemistry and microstructures in Pasamonte, Stannern, and Juvinas to examine this problem in detail. In Pasamonte pigeonites show strong Mg-Fe zoning from core to rim and contain very thin exsolution lamellae. TEM observations show that lamellae of augite have exsolved primarily parallel to (001), but rare, extremely thin (100) lamellae are also present. The average width of the lamellae is 120 nm with a wavelength of 270 nm in the rims and 70 nm with a wavelength of 200 nm in the Mg-rich cores. The compositions of the existing lamellae determined by AEM show that as the bulk pigeonite composition becomes more Ca and Fe-rich (i.e. towards the rims), the exsolved augite lamellae decrease in Ca content. The relationship is consistent with experimental data for the subsolidus phase relations of augite and pigeonite, which show that equilibration of the lamellae occurred between 800-850 degrees C [3], indicating rapid cooling. The microstructures present in Stannern pigeonites are significantly different from Pasamonte. Exsolution has occurred exclusively on (001) pigeonite and the lamellae have extremely variable widths, ranging from 70 nm up to a maximum of 1 micrometer. In any one area [30 x 30 micrometers) the lamellae widths are all similar, but there is considerable variation in the mean lamella width from region to region. The compositions of coexisting pigeonite and augite lamellae show that extreme unmixing of the two phases has occurred and that equilibration of coexisting lamellae occurred at temperatures between 500 and 600 degrees C [3]. As reported by [1] we have found chromites exsolved within the Stannern pyroxenes, but no ilmenite or metal particles have been observed. Chromite has exsolved exclusively within augite lamellae and appears to have nucleated coherently at pigeonite-augite interfaces. AEM of augite lamellae in Stannern shows that they are depleted in Cr and Al relative to augite in Pasamonte consistent with exsolution of chromite. Cr contents in both pigeonite and augite in Stannern are essentially zero, showing that the Cr systematics measured by electron microprobe are the result of the presence of fine-grained exsolved chromite. For Pasamonte an initial rapid cooling stage is indicated by the small wavelength of the exsolution lamellae in pigeonite cores. Cooling in the subsolidus region slowed after exsolution in the Mg-rich cores had occurred. The thermal history of Stannern appears to be more complex. The variability in the thickness and wavelength for exsolution lamellae in Stannern strongly indicates that there were compositional gradients in the crystal while exsolution was occurring, i.e. in the subsolidus region. Therefore equilibration could not have taken place above the solvus. The magma may have been emplaced into a hot environment such as the base of a thick flow and cooled slowly producing thick exsolution lamellae. After significant exsolution had occurred cooling slowed, perhaps as a result of burial under additional flows. During this period compositional equilibration of pyroxene occurred, but the spatial distribution of exsolution lamellae was preserved. Alternatively a second separate reheating event could be invoked, perhaps as a result of contact metamorphism by other, later intrusive rocks. In the latter scenario the exsolution of opaque phases is probably associated with this reheating event. Funded by NASA grant NAGW-3347 to J. J. Papike (P.I.). References: [1] Duke M. B. and Silver L. T. (1967) GCA, 31, 1637. [2] Harlow G. E. and Klimentidis R. (1980) LPS XI, 1131. [3] Lindsley D.H. (1983) Am. Mineral., 68, 477-493.
Brearley Adrian J.
Papike James J.
Spilde Michael N.
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