Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..358g&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 358
Other
3
Enstatite Chondrites, Oldhamite, Rare-Earth Elements, Reduction
Scientific paper
LEW87223 (paired with LEW87057, 87220, and 87234) is an enstatite chondrite with unique textural and compositional properties. The Si content of metal (~0.5%), the presence of alabandite instead of niningerite, and the bulk WNi ratio in this chondrite are all consistent with an EL classification, but the high metal (23 wt%) and siderophile element abundances are closer to those in the EH group [1,2]. The moderately volatile elements, Mn, Na, K, As, Ga, and Se are all depleted relative to EH and EL chondrites [2]. The presence of olivine is consistent with an E3 classification. Oxygen isotopes in LEW87220 and 87234 are in the EH and EL chondrite field [R. Clayton, priv. comm., 1993]. Zhang et al. [2] suggested that this meteorite formed from an EL3 chondrite that experienced fractionation during impact and brecciation. Shock and thermal processing undoubtedly occurred, but our work suggests that LEW87223 was not derived from normal EL starting material. A precursor with properties intermediate between H and E chondrites seems to be indicated. LEW87223 has an opaque-mineral assemblage unlike that of other E chondrites. It has EH-like amounts of metal and troilite, schreibersite is common, but penyite, sphalerite, and djerfisherite (all found in EH3 and EL3 chondrites; [3]) were not observed. Oldhamite does occur as rare, transparent, pink crystals up to 100 micrometers in size, completely enclosed in kamacite. Metal grains are comparable in size to chondrules, and equant. Although it is a type 3, LEW87223 chondrules commonly appear to be welded together, and share long boundaries with other chondrules and metal grains. The olivines show features consistent with shock stage 2 of Stoffler et al. [4]. Alabandite is Fe-rich [2] and occurs as recrystallized aggregates along FeS grain boundaries [A. El Goresy, priv. comm., 1993]. Many chondrules appear dark or opaque in transmitted light due to abundant sub-micrometer, pure Fe metal intergrown with enstatite and silica, all of which partially replace ferroan pyroxene ^FS(sub)15-18). All of these features are consistent with a history involving strong reduction, shock, and heating. We analyzed four large, unweathered oldhamite grains in LEW87220 and 87234 by ion probe, and found trace element characteristics similar to those seen in MAC88136, the only known EL3 chondrite [3]: Mg, Sr, and Zr are lower, and Mn is higher than in EH oldhamite. One grain has a REE pattem that is flat at 60 x CI for LREE, and decreases from 46 x CI at Eu to 15 x CI at Lu, with a positive Yb anomaly (Yb/Yb*=3). The other grains have flat REE patterns near 75 x CI, with negative Eu anomalies (Eu/Eu*=0.4). The first pattern is most likely nebular in origin. The second, more common pattern, with the negative Eu anomaly is unlikely to be nebular, and may be metamorphic in origin, in which case the meteorite contains a mixture of nebular and metamorphic oldhamite. Zhang et al. [2] explained the low abundances of alkalis, Mn, Ga, and Se, and high abundance of siderophiles in LEW87223 as being due to the addition of EL metal, and removal of sulfides during shock and thermal processing of a normal EL3. This seems highly implausible to us because of the physical difficulty of completely separating minor sulfides from FeS and metal, and because perryite, which is intimately associated with metal in E chondrites, is missing. It is more likely that the chondrite represents an entirely new group of E chondrites. The high abundance (10s of %) of blackened (reduced) chondrules also indicates a population of chondrules that was initially quite oxidized; in fact, the measured compositions of surviving ferroan pyroxene are in the H-chondrite range. Also evident in the published trace element data [2] is a siderophile element abundance pattern identical to that in H chondrites but different from those in EH and EL chondrites. LEW87223 may be a link between the ordinary and enstatite chondrite groups. References: [1] Mason (1989,1992) Ant. Meteor. Newslett., 12(1) and 15(1,2). [2] Zhang et al. (1993) LPS XXIV, 1571. [3] Lin et al. (1991) LPS XXll, 811. [4] Stoffler et al. (1991) GCA, 55, 3845.
Crozaz Ghislaine
Grossman Jeffrey N.
MacPherson Glenn J.
No associations
LandOfFree
LEW 87223: A Unique E Chondrite with Possible Links to H Chondrites does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with LEW 87223: A Unique E Chondrite with Possible Links to H Chondrites, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and LEW 87223: A Unique E Chondrite with Possible Links to H Chondrites will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1072342