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 following the past literature). This parameter is more efficiently evaluable than the oft-used relative standard deviation 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 (i.e. subtypes 3.0–3.9) and M1 designates type 4. Corresponding applications are then proposed for other chondrite groups (with, e.g., CV secondary classification reduced to essentially three grades from M0.0 to M0.2, that is, subtypes 3.0–3.2). Known type 1 and 2 chondrites are at M0.0 (i.e. the metamorphic grade of type 3.0 chondrites), even so-called “CY” chondrites, since our metamorphic scale is insensitive to brief heating. 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 alteration degrees can be characterized in terms of the thin-section-based criteria of previous schemes which are thus incorporated in the present framework, if in a coarser, but hereby more robust form. We propose their 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). Since nonzero alteration and metamorphic degrees remain mutually exclusive at the level of precision chosen, a single petrologic subtype ≈3+m-PSF indeed remains a good descriptor of secondary processes for all unequilibrated chondrites, obviating the explicit mention of our separate scales unless finer subdivisions are adopted for the most primitive chondrites.