Mathematics – Logic
Scientific paper
Sep 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001m%26ps...36.1189k&link_type=abstract
Meteoritics & Planetary Science, vol. 36, no. 9, pp. 1189-1216 (2001)
Mathematics
Logic
29
Scientific paper
The metal-rich chondrites Hammadah al Hamra 237 (HH 237), and QUE 94411 (QUE), paired with QUE 94627, contain relatively rare (< 1 vol%) Ca, Al-rich inclusions (CAIs) and Al-diopside-rich chondrules. Forty CAIs and CAI fragments and 7 Al-diopside-rich chondrules were identified in HH 237 and QUE. The CAIs, ~50-400 μm in apparent diameter, include: (a) 22 (56%) pyroxene-spinel melilite [+forsterite rim], (b) 11 (28%) forsterite-bearing, pyroxene-spinel melilite anorthite [+forsterite rim] (c) 2 (5%) grossite-rich [+spinel-melilite-pyroxene rim], (d) 2 (5%) hibonite-melilite [+spinel-pyroxene forsterite rim], (e) 1 (2%) hibonite-bearing, spinel-perovskite [+melilite-pyroxene rim], (f) 1 (2%) spinel-melilite-pyroxene-anorthite, and (g) 1 (2%) amoeboid olivine aggregate. Each type of CAI is known to exist in other chondrite groups, but the high abundance of pyroxene-spinel melilite CAIs with igneous textures and surrounded by a forsterite rim are unique features of HH 237 and QUE. Additionally, oxygen isotopes consistently show relatively heavy compositions with ?17O ranging from -6deg to -10deg (1? = 1.3deg) for all analyzed CAI minerals (grossite, hibonite, melilite, pyroxene, spinel). This suggests that the CAIs formed in a reservoir isotopically distinct from the reservoir(s) where "normal", 16O-rich (?17O < -20deg) CAIs in most other chondritic meteorites formed. The Al-diopside-rich chondrules, which have previously been observed in CH chondrites and the unique carbonaceous chondrite Adelaide, contain Al-diopside grains enclosing oriented inclusions of forsterite, and interstitial anorthitic mesostasis and Al-rich, Ca-poor pyroxene, occasionally enclosing spinel and forsterite. These chondrules are mineralogically similar to the Al-rich barred-olivine chondrules in HH 237 and QUE, but have lower Cr concentrations than the latter, indicating that they may have formed during the same chondrule-forming event, but at slightly different ambient nebular temperatures. Aluminum-diopside grains from two Al-diopside-rich chondrules have O-isotopic compositions (?17O ~ -7+/-1.1deg ) similar to CAI minerals, suggesting that they formed from an isotopically similar reservoir. The oxygen isotopic composition of one Ca, Al-poor cryptocrystalline chondrule in QUE was analyzed and found to have ?17O ~ -3 +/- 1.4deg. The characteristics of the CAIs in HH 237 and QUE are inconsistent with an impact origin of these metal-rich meteorites. Instead they suggest that the components in CB chondrites are pristine products of large-scale, high-temperature processes in the solar nebula and should be considered bona fide chondrites.
Fagan Timothy J.
Keil Klaus
Krot Alexander N.
Krot Tatiana V.
McKeegan Kevin D.
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