An. Krot et Jt. Wasson, IGNEOUS RIMS ON LOW-FEO AND HIGH-FEO CHONDRULES IN ORDINARY CHONDRITES, Geochimica et cosmochimica acta, 59(23), 1995, pp. 4951-4966
Many ordinary chondrite (OC) chondrules are surrounded by rims that sh
ow evidence of appreciable melting and can be called igneous; these ri
ms formed after the melting and solidification of the host chondrule.
We studied thirteen igneous rims on low-FeO chondrules (Fa or Fs < 10
mol%) and nine rims on high-FeO chondrules (Fa or Fs > 10 mol%) in ord
inary chondrites of petrographic type less than or equal to 3.5 by ele
ctron-microprobe analysis and scanning-electron microscopy. In both se
ts, olivine and pyroxene compositions of the rims are similar to or mo
derately higher in FeO than those in the host. Igneous rims around low
-FeO chondrules show evidence of large degree (> 80%) melting includin
g: (1) resorbed chondrule surfaces, (2) rounded metal/troilite nodules
and feldspathic mesostasis, (3) euhedral and subhedral morphology of
olivine and pyroxene and limited ranges in grain sizes, and (4) a rari
ty of relict grains. Most low-FeO chondrules have olivine as their maj
or phase, but pyroxene is commonly the predominant phase in their rims
. Rims on high-FeO chondrules generally have smaller grain sizes and e
xhibit lower degrees of melting than rims around low-FeO chondrules; M
E-rich relict grains are observed in high-FeO rims. Most low-FeO chond
rules have matrix-like rims around the igneous rims, but matrix-like r
ims are rare around rims on high-FeO chondrules. There is a continuum
between igneous rims and enveloping secondaries of compound chondrules
; the existence of both sets of objects supports the view that, in the
OC region, most mm-size grain assemblages experienced more than one m
elting event. The precursors melted to produce the igneous rims on low
-FeO chondrules differ from those that comprise the rims on high-FeO c
hondrules. A sizeable fraction of the precursors of low-FeO rims are f
rom the host chondrule. Most high-FeO rims appear to consist largely o
f melted matrix-like materials. If the rim precursors are mainly indep
endent, the similar Fa and Fs contents in mafic minerals of rims and h
ost chondrules indicate that (I) OC nebular subregions (bounded in spa
ce and/or in time) were dominated by well-mixed solids, (2) many chond
rules experienced multiple heating events during a short period, and w
ere then withdrawn from the chondrule-forming region, and (3) chondrul
es formed at different times were mixed before agglomeration to form c
hondrites. If the host chondrules were the main source of the low-FeO
rim materials, the fact that pyroxene is generally more abundant than
in the host suggests that SiO2-normative matrix was also an important
ingredient and/or that kinetics favored pyroxene crystallization in th
e rim. Rims around high-FeO chondrules were heated to lower temperatur
es than rims around low-FeO chondrules. The rarity of matrix-like rims
around high-FeO igneous rims may indicate that the dust-chondrule rat
io was lower where they formed.