Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p51e1246b&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P51E-1246
Physics
1028 Composition Of Meteorites (3662, 6240), 3662 Meteorite Mineralogy And Petrology (1028, 6240), 6240 Meteorites And Tektites (1028, 3662)
Scientific paper
Metachondrites: Metachondrites are newly recognized groups of stony meteorites that lack chondrules, but which have elemental and oxygen isotopic compositions and textures suggesting that they have been transformed by metamorphism or partial melting from precursor ordinary and carbonaceous chondrites on relatively large parent bodies [1]. The best known examples have affinities to CR (e.g., LEW 88763), CV (e.g., NWA 3133), H, L and LL chondrites; conversely there is evidence that winonaites and acapulcoites also are metachondrites derived from chondritic precursors (represented by rare chondrites such as NWA 1463 and Monument Draw). With increased sampling of new meteorites from both hot and cold desert regions, there is an emerging realization that the early solar system was populated with many relatively large differentiated planetary bodies complete with metallic cores, silicate mantles and chondritic regoliths of various types. The affinity of a particular metachondrite to a specific chondrite class relies mainly on oxygen isotopic analysis combined with distinctive elemental ratios in bulk rocks and constituent minerals (notably Fe/Mn and Ca/Na ratios, which are quite different for ordinary vs. various carbonaceous chondrite classes). Northwest Africa 2788: This specimen exhibits a metamorphic texture with triple grain junctions (grain size is mostly <0.5 mm, a few grains reach nearly 1 mm), and is composed of orthopyroxene (63 vol.%, Fs18.0Wo1.3, FeO/MnO = 30), olivine (27 vol.%, Fa21.4, FeO/MnO = 57-61), clinopyroxene (5 vol.%, Fs7.4Wo49.8, TiO2 = 0.74 wt.%, Cr2O3 = 0.63 wt.%, FeO/MnO = 19), plagioclase (5 vol.%, An53.9Or3), and accessory merrillite, troilite and metal. Replicate oxygen isotopic analyses of acid-washed bulk samples by laser fluorination gave δ18O = 6.004, 6.082; δ17O = 3.082, 3.102; Δ17O = -0.076, -0.097 per mil (for TFL slope of 0.526); these values plot close to but below the TFL. The elevated Fe/Mn ratios in the mafic silicates coupled with the relatively calcic plagioclase are hallmarks of carbonaceous chondrite bulk compositions [2], but the oxygen isotopic compositions are quite unlike those of any known chondrite class. Bulk rock abundances by INAA relative to Allende analyzed simultaneously are 0.89 times for Fe and ~1.2 times for Cr, REE and Hf. We conclude that NWA 2788 is a unique metachondrite sampled from a new, perhaps now-disaggregated parent body accreted from a distinctive oxygen isotopic reservoir. It is possible that this body also may harbor or have harbored chondrule-bearing carbonaceous regolith materials ('CT chondrites') that are yet to be found or recognized. [1] Irving A. J. et al. (2005) 68th Met. Soc. Mtg., #5218; Bunch T. E. et al. (2005) LPS XXXVI, #2308 [2] Wasson J. T. and Kallemyn G. W. (1988) Phil. Trans. Roy. Soc. Lond. A325, 535-544.
Bunch Ted E.
Irving Anthony J.
Korotev Randy L.
Rumble Doug
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