Thermochemical Stability of Low-Iron, Manganese-Enriched Olivine in Astrophysical Environments

By Keith Cowing
Status Report
July 5, 2023
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Thermochemical Stability of Low-Iron, Manganese-Enriched Olivine in Astrophysical Environments
Compositions of olivine in primitive solar system materials. Panel “a” shows detail in the dashed rectangle of panel “b”. Reported MnO contents of meteoritic and IDP olivines are plotted as a function of wt% FeO, where mole fractions tephroite (XTep) and fayalite (XFa) are nearly equal to wt% MnO/100 and wt% FeO/100, respectively, at the low XTep and XFa of “a”. Long dashed lines indicate constant wt% FeO/MnO ratios. Data are from Klöck et al. (1989, their Table 1 and Fig. 1), Weisberg et al. (1995, their Fig. 6; 2004), Zolensky et al. (2006, their Fig. S2b), Rubin (1984, his Table 2, px: pyroxene, ol: olivine), Steele (1990, his Tables 1,3), Sugiura et al. (2009, their Table 1), and Nakamura et al. (2008, their Table S1). Matrix olivines in “a” are from CM2 chondrites (E: EET83226, and M: Murchison; inverted triangles) and Orgueil (CI; open rectangles). Two populations of Semarkona (LL3.0) matrix olivines (open circles) are FeO-rich, in “a”, and MnO-rich, in “b” (Klöck et al. 1989). Core and MnO-rich rim analyses of olivine from two AOAs (Sugiura et al. 2009) are indicated. The dotted line in “a” indicates the condensation trend for a vapor of solar composition at Ptot = 10-4 bar, with equipartition of Mn between pyroxenes and olivine (see Fig. 3). — astro-ph.EP

Low-iron, manganese-enriched (LIME) olivine grains are found in cometary samples returned by the Stardust mission to comet 81P/Wild 2. Similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix.

LIME olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar, but enrichment of solar composition vapor in a dust of chondritic composition causes the FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in primitive materials indicate oxygen fugacities close to that of a very reducing vapor of solar composition.

The compositional zoning of LIME olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine, without reequilibration at lower temperatures. A similar history is likely for LIME olivines found in comet samples and in interplanetary dust particles. LIME olivine is not likely to persist in nebular conditions in which silicate liquids are stable.

Denton S. Ebel, Michael K. Weisberg, John R. Beckett

Comments: 15 pages, 5 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Materials Science (cond-mat.mtrl-sci)
Cite as: arXiv:2307.00652 [astro-ph.EP] (or arXiv:2307.00652v1 [astro-ph.EP] for this version)
Journal reference: Meteoritics and Planetary Sciences 47, 585-593 (2012)
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From: Denton Ebel
[v1] Sun, 2 Jul 2023 20:03:04 UTC (396 KB)

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