The Valence and Coordination of Ti in Olivine and Pyroxene in Ordinary and Enstatite Chondrites as a Function of Metamorphic Grade

Content Provider	Semantic Scholar
Author	Simon, Scott Sutton, Steve R. Grossman, Lawrence
Copyright Year	2015
Abstract	Introduction: Knowledge of the conditions to which chondrites were subjected and their responses to them is necessary for an understanding of the evolution of their parent bodies and early planetary processes in general. One way to better understand processes related to chondrite metamorphism is to evaluate changes in chondrite features, such as oxidation state, as a function of metamorphic grade. It has been suggested that ordinary chondrites underwent reduction during metamorphism from grade 3 to 4 [1] and oxidation during metamorphism from grade 4 through 6 [2, 3, 4], although [5] found no systematic variation of recorded oxygen fugacities (fO2’s) with metamorphic grade. In [1], Mossbauer spectroscopy was used, and the other studies used petrographic and mineral chemical data. We have undertaken a study of chondrites of grades 3-6 to see how Ti valence and coordination in olivine and pyroxene vary as a function of metamorphic grade and to see if the variations can be used to constrain conditions and effects of chondrite metamorphism. Results for L, LL and E chondrites have been reported [6, 7]; herein new data for H chondrites are presented and compared with all previous results. Methods: Suites of chondrules were selected for study based on examination of thin sections with the scanning electron microscope. Chondrules were classified by petrographic type based on backscattered electron imaging and quantitative energy dispersive analysis. The suite of H chondrites studied, from lowest to highest grade, consists of Asuka-881026 (3.0); Sharps (3.4); Dhajala (3.8); Weston (4); Lost City (5); and Guarena (6), all falls except for Asuka. Valence of Ti in olivine, pyroxene and mesostasis, reported as values between 3 and 4, was determined in situ by X-ray absorption near edge structure (XANES) spectroscopy using methods previously described [8], except now the beam diameter is just ~1 μm, for improved spatial resolution. Spots analyzed by XANES were found by electron probe to contain ≤0.2 wt% TiO2 (olivine) or ≤0.5 wt% (pyroxene). Orientation effects can cause spurious Ti valence determinations, especially in samples with significant tetrahedral Ti components. Analysis of multiple grains of a phase within a sample and use of merged spectra can mitigate this effect if the grains are randomly oriented, which is assumed to be the case for the porphyritic chondrules. A total of 37 chondrules (4 8 per H chondrite) were analyzed, with an average of 5.3 XANES analyses/chondrule. Results: Most of the chondrules analyzed are porphyritic, as barred and radial chondrules tend to be too fine-grained for XANES analysis. Valence measurements (sample averages and standard errors based on merged spectra for each chondrule) as a function of metamorphic grade are shown in Fig. 1 (olivine) and Fig. 2 (pyroxene). Grades are offset by +0.05 (LL) and -0.05 (L) for clarity. With one exception (Sharps pyx), the average Ti valence in H chondrites is ~3.8, with no redox trend or reduction of ranges of chondrule averages with increasing grade, and no correlation between Ti valence and Fa or Fs contents.
Starting Page	2141
Ending Page	2141
Page Count	1
File Format	PDF HTM / HTML
Alternate Webpage(s)	https://www.hou.usra.edu/meetings/lpsc2015/pdf/2141.pdf
Alternate Webpage(s)	http://geosci.uchicago.edu/~grossman/SSG15.pdf
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article