An. Krot et al., MICROCHONDRULES IN ORDINARY CHONDRITES - IMPLICATIONS FOR CHONDRULE FORMATION, Geochimica et cosmochimica acta, 61(2), 1997, pp. 463-473
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.