Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.460w&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 460
Mathematics
Logic
H Chondrites, Metamorphism, Petrologic Type, Shock, Trace Elements, Volatile Elements
Scientific paper
The perception among meteoriticists is that contents of the volatile trace elements systematically decrease with shock and particularly petrologic type. This perception affects views that investigators have of the early history and structure of the H chondrite parent body. Measurement of a variety of volatile trace elements in a statistically significant number of samples accompanied by chemometric data analysis techniques developed for interpretation of trace- element data [1] should maximize the amount of genetic information available from the volatile trace elements and offer clues to the early thermal history of the H chondrite parent body. Volatile trace-element data exist for 58 H chondrite falls: the complete dataset includes Co, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In (listed in increasing order of volatility) [2,3]. This dataset includes 13 H4, 32 H5, and 13 H6 chondrites, which cover the full range of shock facies from a through f. To examine the effect that shock has on volatile trace-element concentrations in H4-6 chondrites, we have compared data for the least-shocked samples (shock facies a-b) with the most shocked samples (shock facies c-f) using both univariate (Student's t-test) and multivariate techniques (linear discriminant analysis). The results demonstrate no reason to doubt the null hypothesis of no difference in volatile trace-element composition between shocked and unshocked H4-6 chondrites at any reasonable significance level. This situation contrasts sharply with the strong difference found between shocked and unshocked L chondrites [4]. The role of shock in establishing volatile trace- element contents in H and L chondrites clearly differs. Univariate comparisons between H4, H5, and H6 chondrites demonstrate that only Cs varies significantly with petrologic type (prob. > F 0.0006) with concentration decreasing monotonically with increasing petrographic type. Box- and-whisker plots of volatile trace-element contents reveal a general inverse correlation between element concentration and petrologic type for most of these elements, but the large variability of concentrations of these elements precludes use of any single element as a tool for discrimination between petrologic types. This blurring between petrologic types is significantly decreased when all 11 elements are treated by linear discriminant analysis. Linear discriminant analysis reveals three statistically significant, slightly overlapping populations with the H4 population overlapping the H5, and the H5 population overlapping the H4 and H6 populations. The H4 and H6 populations are readily distinguishable (Fig. 1). Only 21 samples are misclassified as to petrologic type on the basis of 11 volatile trace elements . All the misclassifications are restricted to the adjacent petrologic types. Reducing the number of elements demonstrate that this pattern is established primarily by the five least-volatile elements, Co, Rb, Ag, Se, and Cs. Use of the six most-volatile elements--Te, Zn, Cd, Bi, Tl, and In--reveals a similar trend, but only with greatly reduced significance levels and much more overlap between all three populations. These results support the idea of an onion- shell model for the H chondrite parent body. The results also suggest that the thermal process(es) responsible for compositional variation with petrologic type occurred at higher temperatures than those associated with fractionation of the most volatile elements. Contrary to earlier perceptions, the least- volatile elements more clearly mark the difference between petrologic types than do the most volatile elements. It might even be that condensation of H chondrite parent materials occurred at temperatures high enough to prevent incorporation of the most volatile elements, while the least-volatile ones were lost during postaccretionary metamorphism. References: [1] Wolf S. F. and Lipschutz (1993) Advances in Analytical Geochemistry (M. Hyman and M. W. Rowe, eds.) in press. [2] Lingner D. W. et al. (1987) GCA, 51, 727-739. [3] Wolf S. F. and Lipschutz (1993) JGR, submitted). [4] Huston, T. J. and Lipschutz M. E. (1984) GCA, 48, 1319-1329. Figure 1, which appears in the hard copy, is a histogram of petrologic type established by multivariate linear discriminant analysis on the basis of 11 volatile trace elements.
Lipschutz Michael E.
Wolf Steven F.
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