Assessing the Oxidative History of Miller Range Martian Meteorites

Miller Range (MIL) Martian meteorites are oxidized nakhlites. Early studies attribute their oxidation to reduction-oxidation reactions involving assimilated sulfate. I utilize the sulfur isotope and major element composition of the MIL pairs to assess their oxidative history. MIL sulfides display an average sulfur isotope composition that is different from Nakhla sulfate and sulfide. The sulfur isotope differences produce a mixing array between juvenile sulfur and mass-independent sulfur signatures, indicating assimilation of anomalous sulfur into the melt. I estimate an fO2 of QFM (+3.5 ± 0.4) and a sulfur content of 360 ppm ± 12 – 1300 ppm ± 50. With these results, I test the hypothesis of sulfate assimilation through models of charge balance, isotope mixing, and degassing of sulfur bearing compounds. I conclude that sulfate assimilation was significant in the oxidation of the MIL pairs but, additional oxidants were assimilated.

The Paragould meteorite / Sharat Kumar Roy -- and Robert Kriss Wyant --

v.10:no.23(1955)

Meteorite studies -- by Oliver Cummings Farrington, Ph.D., Curator, Dept. of Geology.

v.1:no.11(1902)

The Navajo meteorite, by Sharat Kumar Roy -- and Robert Kriss Wyant --.

v.7:no.8(1949)

Meteorite studies. by Oliver Cummings Farrington --

v.3:no.8(1910)

The Walters meteorite / Sharat Kumar Roy -- , Jewell J. Glass -- [and] Edward P. Henderson --.

v.10:no.37(1960)

The Smithonia meteorite / by Sharat Kumar Roy -- and Robert Kriss Wyant --.

v.7:no.9(1950)

The La Porte Meteorite / by Sharat Kumar Roy -- and Robert Kriss Wyant --.

v.7:no.10(1950)

The Rodeo meteorite / by Oliver Cummings Farrington --

v.3:no.1(1905)

Meteorite studies : Oliver Cummings Farrington --

v.3:no.6(1907)

The Benld meteorite / by Sharat Kumar Roy -- and Robert Kriss Wyant --.

v.7:no.11(1951)

Handbook and catalogue of the meteorite collection / by Oliver C. Farrington, Ph.D., Curator, Dept. of Geology.

v.1:no.1(1895)

The carbon isotope composition of natural SiC (moissanite) from the Earth's mantle: New discoveries from ophiolites

Moissanite (natural SiC) has been recovered from podiform chromitites of several ophiolite complexes, including the Luobusa and Donqiao ophiolites in Tibet, the Semail ophiolite in Oman and the United Arab Emirates, and the Ray-Iz ophiolite of the Polar Urals, Russia. Taking these new occurrences with the numerous earlier reports of moissanite in diamondiferous kimberlites leads to the conclusion that natural SiC is a widespread mineral in the Earth's mantle, which implies at least locally extremely low redox conditions. The ophiolite moissanite grains are mostly fragments (20 to 150 mu m) with one or more crystal faces, but some euhedral hexagonal grains have also been recovered. Twinned crystals are common in chromitites from the Luobusa ophiolite. The moissanite is rarely colorless, more commonly light bluish-gray to blue or green. Many grains contain inclusions of native Si and Fe-Si alloys (FeSi(2), Fe(3)Si(7)). Secondary ion mass spectrometric (SIMS) analysis shows that the ophiolite-hosted moissanite has a distinctive (13)C-depleted isotopic composition (delta(13)C from -18 to -35 parts per thousand, n=36), much lighter than the main carbon reservoir in the upper mantle (delta(13)C near -5 parts per thousand). The compiled data from moissanite from kimberlites and other mantle settings share the characteristic of strongly (13)C-depleted isotopic composition. This suggests that moissanite originates from a separate carbon reservoir in the mantle or that its formation involved strong isotopic fractionation. The degree of fractionation needed to produce the observed moissanite compositions from the main C-reservoir would be unrealistically large at the high temperatures required for moissanite formation. Subduction of biogenic carbonaceous material could potentially satisfy both the unusual isotopic and redox constraints on moissanite formation, but this material would need to stay chemically isolated from the upper mantle until it reached the high-T stability field of moissanite. The origin of moissanite in the mantle is still unsolved, but all evidence from the upper mantle indicates that it cannot have formed there, barring special and local redox conditions. We suggest, alternatively, that moissanite may have formed in the lower mantle, where the existence of (13)C-depleted carbon is strongly supported by studies of extraterrestrial carbon (Mars, Moon, meteorites). (C) 2009 Elsevier B. V. All rights reserved.