Almahata Sitta-Fragment MS-CH: Characterization of a new chondrite type

Abstract

Among the several hundred, mostly small meteorite fragments, recovered within the Almahata Sitta strewn field, one fragment (MS-CH), weighing 5.68 g, was detected that represents a new type of chondritic meteorite. The detection of short-lived cosmogenic radionuclides clearly indicates that this chondrite fragment results from a fresh meteorite fall consistent with the Almahata Sitta event in October 2008. The fundamental mineralogical characteristics of the Almahata Sitta fragment MS-CH can be summarized as follows: (1) the almost equilibrated olivine has high Fa contents of about 36 mole%. The fragment is of petrologic type 3.8 +/- 0.1; (2) the metal abundance of the rock is on the order of 2.5 vol%; (3) the mean chondrule size has been determined to be roughly 450 mu m; (4) point-counting and imaging indicate that the matrix abundance is approximately 45 vol%; (5) Cr-spinels have much lower TiO2 concentrations than typical spinels within R chondrites; (6) calcium-aluminum-rich inclusions are spinel-rich and severely altered having abundant Na- and/or Cl-rich alteration products. Spinel also contains significant concentrations of Fe and Zn; (7) magnetites and platinum-group element-rich phases (sulfides, tellurides, and arsenides) characteristic of both R and CK chondrites were not found in fragment MS-CH; and (8) the mean oxygen isotope composition of three small fragments of Almahata Sitta MS-CH is delta 17O = +4.35 parts per thousand, delta 18O = +4.94 parts per thousand, and Delta 17O = +1.76 parts per thousand. The oxygen isotopes relate MS-CH to R chondrites. No established chondrite group having all these characteristics exists.