Chromites in ordinary chondrite fusion crusts

Chromites from ordinary chondrites of groups H4, H5, LL5, LL6, L3.6 and L6 were studied and compared to an H5 ordinary chondrite processed to form a synthetic fusion crust. Chromites found in the bulk are usually anhedral and relatively large in size (several tens of micrometres), as opposed to chromites formed within the crust, which are consistently smaller (a few micrometres in size) and can display anhedral or subhedral to euhedral habit. The Mg# and Al# values of the chromites in the bulk display typical composition reported for ordinary chondrites, with a limited scatter of Al# (ca. 0.13 ±0.025) but a large variation in Mg# (from 0.05 to 0.30). Chromites within fusion crusts generally exhibit similar Al# values with respect to those in the bulk but a much larger scatter of Mg# values and a larger average Mg# (up to 0.65). Chromites in the fusion crusts are often associated with magnetite dendrites made up of magnetite octahedral crystals that are 100–400 nm wide; occasionally, other spinel group minerals can be found, such as magnesiochromites and magnesioferrites. In most of the studied samples, several chromite crystals are mantled by magnetite crystals. Textural data collected so far suggest a crystallisation sequence in the fusion crust, starting with olivine and going over chromite to magnetite. A small but significant fraction (∼15 %) of chromites grown in the fusion crust display a detectable Ni content, in marked contrast with those found in the bulk, where only ca. 2 % of the analysed chromites display Ni contents above the detection limit. These observations are additionally supported by measurements on the El Hammami (H5) meteorite sample, which was previously processed with an induction-heated plasma wind tunnel in order to create a synthetic fusion crust. Also in this case, chromites found within the fusion crust formed under controlled conditions (temperature measured at the sample surface at 2400–2200 K for 20 s) display similar Al# and considerably higher Mg# compared to those in the bulk, which is similar to the chromites in the natural fusion crusts studied here. Our study shows that chromites of meteoritic origin retrieved from sediment can display significantly higher Mg# values than those of the parent meteoroid if they originate within the fusion crust, providing a way to recognise chromites from ablation spherules.