A Primitive Achondrite With Oxygen Isotopic Affinities to CV Chondrites: Implications for Differentiation and Size of the CV Parent Body

Details A Primitive Achondrite With Oxygen Isotopic Affinities to CV Chondrites: Implications for Differentiation and Size of the CV Parent Body A Primitive Achondrite With Oxygen Isotopic Affinities to CV Chondrites: Implications for Differentiation and Size of the CV Parent Body

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p31c..02i&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #P31C-02

Other

6240 Meteorites And Tektites, 3660 Metamorphic Petrology, 3662 Meteorites, 1040 Isotopic Composition/Chemistry

Scientific paper

NWA 3133 found in Northwest Africa (as several stones totaling ˜ 4 kg) has a metamorphic texture with ˜120° triple grain junctions (mean grainsize = 0.28 mm); no chondrules are present. Olivine (46 vol.%; Fa22, FeO/MnO = 64) and orthopyroxene (28 vol.%; Fs18.9Wo2.3, FeO/MnO = 42) are the most abundant phases, with less intermediate plagioclase (An53.5Or2.3), Cr-diopside (Fs7.5Wo48.3, Cr2O3 = 0.71 wt.%), Al-Ti-bearing chromite (Cr/(Cr+Al) = 0.73, TiO2 = 2.6 wt.%), Na-Mg-bearing merrillite, troilite (1-5 wt.% Ni) and Fe-Ni metal (15-20 wt.% Ni). Clinopyroxene, chromite and merrillite are inhomogeneously distributed as relatively large grains. Metal is partially altered to limonite (W1-2), and minor limonite and calcite occur along grain boundaries. Oxygen isotopic compositions (δ 17O, δ 18O) determined in two laboratories by laser fluorination on handpicked olivine (-3.67, +0.94; -3.62, +0.89; -2.91, +1.78 per mil) and on acid-washed whole rock fragments (-2.25, +2.46; -1.75, +3.06 per mil) plot on the mixing line for Allende and other CV3 chondrites. Thus, NWA 3133 could be regarded as the first known "CV7 chondrite", and may have been formed by metamorphic recrystallization (or perhaps igneous processes) in the CV parent body. Other meteorites with oxygen and/or Cr isotopic affinities to the CV3 chondrites are the three members of the Eagle Station pallasite grouplet and the silicated irons Bocaiuva and NWA 176 (Clayton and Mayeda., 1996; Liu et al, 2001; Shukolyukov and Lugmair, 2001). Our inference from these data is that the CV parent body was an at least partially differentiated (and relatively large) object consisting of a metal+silicate core region surrounded by a presumably silicate-rich mantle and a chondrule-CAI-rich regolith, the deepest portions of which were metamorphosed and/or partially melted to form primitive achondritic lithologies.

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