The secondary classification of unequilibrated chondrites

Abstract

The multiplication of decimal petrologic schemes for different or the same chondrite groups evinces a lack of unified guiding principle in the secondary classification of type 1-3 chondrites. We show that the current OC, R and CO classifications can be a posteriori unified, with only minor reclassifications, if the decimal part of the subtype is defined as the ratio $m=Fa_I/Fa_{II}$ of the mean fayalite contents of type I and type II chondrules rounded to the nearest tenth (with adaptations from Cr systematics for the lowest subtypes). This parameter is more efficiently evaluable than the oft-used relative standard deviations of fayalite contents and defines a general metamorphic scale from M0.0 to M1 (where the suffixed number is the rounded $m$). Type 3 chondrites thus span the range M0.0-M0.9 and M1 designates type 4. Corresponding applications are then proposed for other chondrite groups. Known type 1 and 2 chondrites are at M0.0 (i.e. the metamorphic grade of type 3.0 chondrites). Independently, we define an aqueous alteration scale from A0.0 to A1.0, where the suffixed number is the (rounded) phyllosilicate fraction (PSF). For CM and CR chondrites, the subtypes can be characterized in terms of the thin-section-based criteria of previous schemes which are thus incorporated in the present framework. The rounding of the PSF to the (in principle) nearest tenth makes the proposed taxonomy somewhat coarser than those schemes, but hereby more robust and more likely to be generalized in future meteorite declarations. We propose the corresponding petrologic subtype to be 3-PSF, rounded to the nearest tenth (so that type 1 would correspond to subtypes 2.0 and 2.1). At the level of precision chosen, nonzero alteration and metamorphic degrees remain mutually exclusive, so that a single petrologic subtype $\approx$ 3+$m$-PSF indeed remains a good descriptor of secondary processes.