MICROCHONDRULES IN ORDINARY CHONDRITES - IMPLICATIONS FOR CHONDRULE FORMATION

Citation
An. Krot et al., MICROCHONDRULES IN ORDINARY CHONDRITES - IMPLICATIONS FOR CHONDRULE FORMATION, Geochimica et cosmochimica acta, 61(2), 1997, pp. 463-473
Citations number
29
Categorie Soggetti
Geochemitry & Geophysics
ISSN journal
00167037
Volume
61
Issue
2
Year of publication
1997
Pages
463 - 473
Database
ISI
SICI code
0016-7037(1997)61:2<463:MIOC-I>2.0.ZU;2-C
Abstract
Previous studies of ordinary chondrites (OC) reported isolated microch ondrules within the fine-grained or elastic matrix of many type-3 samp les; larger sets of microchondrules have also been found as rare clast s in type-3 OC. We report here another major setting for microchondrul es; lame numbers (n greater than or equal to 100) of microchondrules ( less than or equal to 40 I-lm in apparent diameter) occur together wit h irregularly shaped fragments in fine-grained rims around four low-Fe O porphyritic olivine chondrules and one barred olivine chondrule. The se chondrules are from LL3.1 Bishunpur, L3.4 EET90161,L3.4 EET90261, L L3.4 Piancaldoli, and LL3.0 Semarkona. The two kinds of microchondrule s observed are (1) numerous low-FeO radial and cryptocrystalline micro chondrules consisting of low-Ca pyroxene and (2) relatively rare high- FeO olivine microchondrules. Both types of microchondrules are embedde d in high-FeO fine grained matrix materials; they are typically accomp anied by irregularly shaped pyroxene fragments that form a continuum i n composition and shape with the low-FeO microchondrules. The pyroxene -rich surfaces of the host chondrules project into the surrounding rim s as peninsulas with rounded embayments, consistent with remelting; al though on average less ferroan, the peninsulas overlap the fragments a nd low-FeO microchondrules in composition and appear to have been the main source of these objects. The occurrence of numerous low-FeO pyrox ene-rich microchondrules with similar textures and compositions within high-FeO fine-grained matrix rims around normal-size chondrules and t he apparent remelting of the surfaces of the host chondrules indicate that the microchondrules formed after solidification of the host chond rules mainly by remelting of their pyroxene-rich surfaces (or, possibl y, igneous rims). The remnants of these pyroxene-rich rims are preserv ed as peninsulas extending outward from the chondrule surface and as i rregularly shaped pyroxene fragments coexisting with the microchondrul es. Because a newly formed microchondrule ''cloud'' in the nebula woul d dissipate quickly due to random motions of the individual microchond rules, it seems inescapable that the fine-grained material, which now surrounds the microchondrules, was in their immediate vicinity when th ey formed and served as a trapping matrix. The material probably occur red as a highly porous, aerogel-like aggregate of FeO-rich nebular dus t. The rare high-FeO olivine microchondrules probably formed at the sa me time as the low-FeO pyroxene microchondrules by melting adjacent po rtions of the porous dust. Copyright (C) 1997 Elsevier Science Ltd.