Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: Equilibration temperatures of petrologic types 3.7 to 6

Abstract

1) The H3.7 to 3.8 and the L3.7 to 3.8 chondrites analyzed show temperatures of 625 to 680 °C; 2) Equilibrated chondrites show a range of olivine/Cr-spinel temperatures between 700 and 820°C, and the same average temperatures for type 4 to 6 (number of analyzed meteorites in brackets): H4 (9) 766 °C, H5 (7) 774 °C, H6 (3) 775 °C, L4 (5) 752 °C, L5 (4) 754 °C, L6 (1) 754 °C. These temperatures are interpreted as equilibration temperatures. One indication is that the measured isotherms are straight lines down to low Cr/(Cr + Al) ratios, which have a higher Fe/Mg interdiffusion coefficient than grains with high ratios. And there is no correlation of measured temperature with grain size of Cr spinel.3) Chromites sensu stricto show temperatures about 50 to 100 °C lower than Cr spinel, and a correlation with grain size. This is a closure temperature established during cooling and in situ crystallization.These results can best be interpreted by a "rubble pile" model of parent body evolution. This model cannot explain, however, the absence of type 4 to 6 chondrites with temperatures as low as for type 3.7 to 3.8.

Petrographic classification of Middle Ordovician fossil meteorites from Sweden

“…Chromite in every petrographic type from type 4 to 6 also contained a range of average equilibration temperatures of 752-775 °C. Wlotzka (2005) suggested this could be explained by the chromite grains and their parent meteorites having formed within rubble pile environments where rocks of different types were stored and mixed together. However, the larger D max chromite sizes in type 6 chondrites measured here are consistent with a longer history of heating and slow cooling than for the lower petrographic types even though no correlation between chromite grain size and maximum equilibration temperature was identified in recent meteorites by Wlotzka.…”

“…4). In a study of ordinary chondrite chromite, Wlotzka (2005) determined equilibration temperatures of up to 820 °C for types 4, 5, and 6. Other processes, i.e., parent body shock, are not associated with formation of such relatively large chromite grains (Stöffler et al 1991).…”

“…Schmitz et al (2001) analyzed chromites in 33 chondrite falls and finds and showed by comparison that the chromites in the fossil meteorites most probably are from L, or possibly LL, chondrites. Moreover, Schmitz et al (2001) and Wlotzka (2005), following Bunch et al (1967) and Ramdohr (1967), demonstrated that chromite compositions can be used to distinguish between chondrite groups and to reveal information on the heating and cooling histories of their parent bodies. Chromite textures and chemistry are thus of particular importance in identifying the components in the flux of meteorite material preserved within the fossil record.…”

Concentration of Platinum Group Elements during the Early Earth Evolution: A Review*

“…chondrite [315]; the H chondrites contain ~0.22 wt% chromite and the L chondrites contain about ~0.27 wt% chromite [314]; chromite is the most common spinel group mineral in the ordinary chondrites [316]; chromite is more common in type 4 -6 ordinary chondrites than in type 3 ordinary chondrites [317].…”

The mineralogy of the Yaringie Hill meteorite—A new H5 chondrite from South Australia

“…However, temperatures have been determined using the olivine-spinel thermometer of Wlotzka (2005). The resulting temperatures, which are in the range 640-830 °C, depend strongly on the textural type of the spinels (Fig.…”