EFFECT OF METAMORPHIC CONDITIONS ON MINERALOGY AND TRACE ELEMENT RETENTION IN THE MURCHISON C2 CHONDRITE - IMPLICATIONS FOR THE EVOLUTIONARY HISTORY OF CERTAIN CHONDRITIC METEORITES
Abstract
Neutron activation analysis was used to determine 11 trace elements (Ag, Bi, Cd, Co, Cs, Ga, In, Se, Te, Tl and Zn) in Murchison (C2) chondrite powder subjected to simulated open-system thermal metamorphism (one-week heating in a low-pressure H(,2) environment at 100(DEGREES)C increments from 400-1400(DEGREES)C). Cadmium is the most mobile element (96% lost at 500(DEGREES)C) while Co is the least (lost (GREATERTHEQ) 1200(DEGREES)C). Temperatures of incipient release vary widely and elemental loss progresses with temperature to extremes of >99%. Treating elemental mobilization as kinetically-controlled by diffusion from spherical grains of uniform size, Co, Ga, Cs and In are lost from a single site and/or process while Ag, Te, Zn, Se, Bi and Tl are lost from two sites and/or by two processes. Magnitudes of apparent activation energies for loss of Co, Ga, Cs and In and, at low temperatures, the latter six elements, are consistent with volume diffusion; all other data suggest a low energy process like desorption. Bivariate interelement relationships involve only elements of similar mobility. Factor analysis yields coherent results. Comparison, where possible, between data for heated Murchison and heated Allende (C3) indicates differences in the genetic histories of these carbonaceous chondrites, possibly including mild closed-system metamorphism of Allende parent material. The 11 aforementioned trace elements were also determined in as-received Karoonda (C4), a significantly metamorphosed chondrite. The data suggest that our experiments do not properly model Karoonda's metamorphic environment. Karoonda's depletion pattern might be indicative of open-system metamorphism in severly oxidizing conditions. Alternatively, Karoonda's abundances may be those established during nebular condensation and preserved in subsequent closed-system metamorphism, although the data are difficult to explain in terms of the two-component condensation model. Similarities between heated Murchison data and literature data for Warrenton hint that the abundance pattern of at least this C30 chondrite was established by open-system metamorphism. Available data for the remaining C30's are also consistent with this suggestion, although they can be explained equally well by the two-component condensation model. Presumably, the metamorphic temperature of most C30's was too low to produce substantial loss of mobile trace elements. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of school.) UMI
Degree
Ph.D.
Subject Area
Analytical chemistry
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