Mathematics – Logic
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.485k&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 485
Mathematics
Logic
Breccia, Core Sampling, Iowa, Meteorite Craters, Meteoritic Composition, Electron Probes, Factor Analysis, Lithology, Mineralogy, Recrystallization
Scientific paper
Eleven drill cores were recently recovered from the Manson, Iowa, impact structure. Core M-1, which was drilled at the edge of the large central uplift, contains three major impact rock types: sedimentary clast breccia, melt matrix breccia, and fragmental breccia. Breccia matrices from different depths in the M-1 core were investigated by defocused beam electron probe microanalysis. To obtain average compositions, matrix areas free of lithic and mineral clasts were covered with an orthogonal grid of analyses points. Variations in composition have two main causes: differences in source lithology and postimpact processes. With increasing depth distinct changes in composition can be observed. All compositions are highly feldspathic and surprisingly low in SiO2 for putative source lithologies of mostly granitic composition. Within the lithological units containing melt matrix breccia increasing recrystallization can be observed with depth. Compositions in this interval are marginally corundum normative and not as high in normative feldspar as other matrices. The thoroughly recrystallized matrices from the lowest intervals of the melt matrix breccia are more feldpathic, higher in Na/K ratio, and not corundum normative. Unlike the chemical variation between rock types, we tentatively ascribe the chemical changes within the melt matrix breccia to reaction of matrix with clasts. In particular, quartz clasts in breccias with highly recrystallized matrices develop fringes of K-spar, cpx, and ilmenite. The formation of these minerals is presumably accompanied by complementary changes in matrix composition. Factor analysis on data sets of individual melt matrix analyses provides further clues to the influence of source lithology and post impact processes respectively in the chemical makeup of the breccia matrices. The most significant factors are a mafic component and a plagioclase component.
Anderson Rachel
Koeberl Christian
Kracher Alfred
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