Spinel group minerals in carbonaceous and ordinary chondrites

Abstract: Meteorites are easily altered in the oxygen-rich environment on the Earth surface. Fossil meteorites are thus uncommon and in the ones, which are found, almost all minerals are secondary. However, the spinel group mineral chromite is resistant to alteration and has successfully been used as a proxy for ordinary chondrites in sediment. The frequency and composition range of spinel group minerals in carbonaceous chondrites are not known well enough to be used as proxies for carbonaceous chondrites. Two carbonaceous chondrites NWA 4428 (CM2) and Allende (CV3) have been systematically investigated in order to determine the frequencies of the spinel group minerals in each chondrite, where in the chondrites the spinel group minerals are located and size and composition of the minerals. The long term aim of this project is to establish a method for detecting spinel group minerals from carbonaceous chondrites in sediment, thus being able to constrain the flux of carbonaceous chondrites further. For references and to compare with previous studies two ordinary chondrites, Jilin (H5) and Mbale (L5/6) were studied in the same way. Spinel is the most common spinel group mineral in the two carbonaceous chondrites investigated here, whereas chromite is the most common spinel group mineral in the two ordinary chondrites. Spinel is not present in the ordinary chondrites. Chromite is present in the carbonaceous chondrites but is considerably smaller than chromite in the ordinary chondrites (~25 μm compared to ~270 μm). Carbonaceous chondritic chromite also has a larger variety in composition than ordinary chondritic chromite, which is due to the higher amount of thermal metamorphism on the ordinary chondrite parent bodies than the carbonaceous chondrite parent bodies. Spinel in NWA 4428 is essentially pure MgAl2O4 whereas spinel in Allende varies from pure MgAl2O4 to FeO contents as high as 24.0 wt%. Magnetite is present in Allende and in Mbale; in Allende, both as spherule and as poorly defined grains with a secondary nature. In Mbale, magnetite only occurs as grains with a secondary nature. In Jilin (H5) Cr-spinel occurs in one aggregate. To detect extraterrestrial spinel group minerals in sediment, these have to differ from their terrestrial equivalents. To distinguish the CM2 and CV3 spinel analysed here from terrestrial spinel on compositional bases is probably not possible. The carbonaceous chondritic chromite displays a larger variety of compositions than chromite in ordinary chondrites and is therefore probably less suitable as a proxy than chromite in ordinary chondrites. Magnetite spherules in Allende (CV3) appear to have a composition distinct from terrestrial magnetite. Magnetite might however be unsuitable as a proxy due to oxidation on the Earth surface. Further studies are needed in order to get a clear picture of the compositions of spinel group minerals in carbonaceous chondrites.