Thermal metamorphism of primitive meteorites—II. Ten trace elements in Abee enstatite chondrite heated at 400–1000°C |
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| Authors: | M Ikramuddin CM Binz ME Lipschutz |
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| Institution: | Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, U.S.A.;Departments of Chemistry and Geosciences, Purdue University, West Lafayette, Indiana 47907, U.S.A. |
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| Abstract: | We used neutron activation analysis to determine ten trace elements retained in Abee (E4) samples heated at 400–1000°C for 1 week in a low-pressure (initially ~ 10?5atm H2) environment. Eight elements generally are lost progressively with increasing temperature although gas(es) evolved from the samples apparently affect retention of some elements. In the extreme, ‘open-system’ losses are: Se—23%, Cs—40%; Te—87%; Ag, Bi, In, Tl, Zn— ≥93%. Under these conditions Co is not lost; Ga is lost only at 1000°C. At 900°C elements are lost from Abee chips in the same relative order as from Abee powder but the loss is somewhat less facile. Three of the most mobile elements—Bi, In, Tl—are lost more readily from Abee than from Allende (C3), the only other primitive chondrite studied to date. Assuming that elemental loss is a kinetic process involving mobilization from spherical grains, Bi, In, Se, Tl and Zn have different activation energies at high and low temperatures either because each element was originally present in two different sites or each has more than one loss mechanism (diffusion or desorption) in different temperature ranges.Comparison of elemental abundance patterns, patterns of statistically-significant correlations, factor analysis results and two-element correlation diagrams indicate strong similarities between heated Abee and ‘as-received’ enstatite chondrites for mobile elements. These results are consistent with a two-stage evolutionary model for enstatite chondrites involving condensation of cosmochemically fractionated primitive nebular material and subsequent loss of mobile elements from parent material by metamorphism. |
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| Keywords: | Author to whom correspondence should be addressed |
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