Chromites in Ordinary chondrite fusion crusts

We studied 5 fall Ordinary Chondrites of different groups (H4, H5, LL5, LL6, L3.6) and an Antarctic meteorite (H5), in order to investigate possible compositional differences between the chromites present in the bulk and the chromites formed within the fusion crust. We report here the composition of about 50 chromites measured within the bulk and 70 chromites found in the crust.

Chromites found in the bulk are usually anhedral and relatively large in size (several tens of micrometers), whereas chromites formed within the crust are consistently smaller (few micrometers in size) and can display anhedral, or subhedral to euhedral habit.

The Mg# and Al# determined for all the chromites found in the bulk show a fair agreement with data reported in the literature for chromite compositions in ordinary chondrites (Bunch et al., 1967; Ramdohr, 1967; Rubin, 2003; Wlotzka, 2005), which display a small scatter of the Al# (ca.0.13±0.025) and a large variation of the Mg# (from 0.05 to 0.30).

When compared with the ones found in the bulk, chromites found within the fusion crusts generally exhibit similar values of the Al#; however, they display a much larger scatter of the Mg# and, usually, also larger average Mg# (up to 0.65) than their conterparts in the bulk.

Chromite in the fusion crusts are often associated to magnetite dendrites made up by magnetite octahedral crystals 200-400 nanometers wide; occasionally, other spinel group minerals can be found, as magnesiochromites and magnesioferrites. In most of the samples studied, several chromite crystals are mantled by magnetite crystals, whereas no magnetite crystal has been found mantled by chromites. Textural data so far collected suggest a crystallization sequence in the fusion crust: Olivine, Chromite, Magnetite.

 

References:

Bunch T.E., Keil K. and Snetsinger K.G. (1967). Chromite composition in relation to chemistry and texture of ordinary chondrites. Geochimica et Cosmochimica Acta, 31, 1569-1582.

Ramdohr P. (1967). Chromite and chromite chondrules in meteorites-I. Geochimica et Cosmochimica Acta, 31, 1961-1967.

Rubin A.E. (2003). Chromite-Plagioclase assemblages as a new shock indicator; implications for the shock and thermal histories of ordinary chondrites. Geochimica et Cosmochimica Acta, 67, 2695–2709.

Wlotzka F. (2005) Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: Equilibration temperatures of petrologic types 3.7 to 6. Meteoritics and Planetary Science, 40, 1673-1702