RELATIVE ABUNDANCES OF CHONDRULE PRIMARY TEXTURAL TYPES IN ORDINARY CHONDRITES AND THEIR BEARING ON CONDITIONS OF CHONDRULE FORMATION

Abstract

A petrographic survey of > 1600 chondrules in thin‐sections of 12 different mildly to highly unequilibrated H‐, L‐, and LL‐chondrites, as well as morphological and textural study of 141 whole chondrules separated from 11 of the same chondrites, was used to determine the relative abundances of definable chondrule primary textural types. Percentage abundances of various chondrule types are remarkably similar in all chondrites studied and are ∼ 47–52 porphyritic olivine‐pyroxene (POP), 15–27 porphyritic olivine (PO), 9–11 porphyritic pyroxene (PP), 3–4 barred olivine (BO), 7–9 radial pyroxene (RP), 2–5 granular olivine‐pyroxene (GOP), 3–5 cryptocrystalline (C), and ≤ 1 metallic (M). Neither chondrule size nor shape is strongly correlated with textural type. Compound and cratered chondrules, which are interpreted as products of collisions between plastic chondrules, comprise ∼ 2–28% of nonporphyritic (RP, GOP, C) but only ∼ 2–9% of porphyritic (POP, PO, PP, BO) chondrules, leading to a model‐dependent implication that nonporphyritic chondrules evolved at number densities (chondrules per unit volume of space) which were 102 to 104 times greater than those which prevailed during porphyritic chondrule formation (total range of ∼ 1 to ∼ 106 m−3). Distinctive “rims” of fine‐grained sulfides and/or silicates occur on both porphyritic and nonporphyritic types and appear to post‐date chondrule formation. Apparently, either the same process(es) contributed chondrules to all unequilibrated ordinary chondrites or, if genetically different, the various chondrule types were well mixed before incorporation into chondrites. Melting of pre‐existing materials is the mechanism favored for chondrule formation.