Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.498c&link_type=abstract
Meteoritics, vol. 30, no. 5, page 498
Other
1
Chondrites, Cv, Ion Probe, Isotopes, Oxygen, Magnetites, Meteorites, Allende
Scientific paper
Magnetite is relatively abundant in the oxidized CV chondrites. This study was undertaken to examine possible origins in nebular or in parent-body processes. Magnetite in Allende has several distinct characteristics compared to that of the other CV chondrites. Unlike Mokoia and Vigarano, magnetite in Allende is almost exclusively found in chondrules, where it typically forms spherical nodules often associated with metal and/or sulfides. In addition, the Ni contents of metal in most CV chondrites seldom exceed 50 wt%, while in Allende nearly all of metal grains are Ni rich (64-71 wt% Ni [e.g., 1]), which we designate awaruite even though the Ni range is below that of awaruite. Rare kamacite grains are present in chondrules [2]. A few rare Allende nodules have euhedral to subhedral awaruite grains [e.g., 3]. Nodules of magnetite + sulfides, magnetite + metal, and metal + sulfide exhibit different textures implying that magnetite grains are not simply pseudomorphs of preexisting metal grains. Tiny magnetite grains (< 1 micrometer) are found in Ni-rich metal ; in turn, the host metal is surrounded by magnetite. The textural evidence is not sufficient to distinguish between a high temperature and a low temperature origin for the magnetite. Six magnetite nodules, which had relatively clean areas of ~ 25 micrometers diameter, from 4 Allende PO chondrules were selected for in situ oxygen isotope study using the UCLA Cameca ims 1270, a high resolution - high sensitivity ion microprobe. A ~0.3 nA primary Cs+ beam was defocused to a 20-25 micrometers spot and a field aperture inserted into the secondary ion beam to restrict the analyzed area to the central 10-12 micrometers of each nodule. Negative ions were collected at a mass resolving power of ~6500, sufficient to eliminate hydride interferences. A normal incidence electron gun was employed to compensate possible sample charging. To monitor the instrumental mass fractionation (-11.2+/-0.5 per mil/amu) analyses of terrestrial magnetite LP204a [4] were made interspersed amongst the chondrule analyses. Chondrule data corrected for mass fractionation and normalized to the SMOW scale are shown in Figure 1. Oxygen isotope ratios from inclusion and crack free regions in four magnetite nodules are tightly clustered below the terrestrial mass fractionation (TF) line and are distinct from oxygen isotope fields observed in magnetite from CI and Essebi chondrites [5] and from isolated grains in CI meteorites [6]. The data plot near the ^(16)O mixing line of refractory minerals in Allende (CCAM), but the spread in the ratios is too low relative to the precision of the measurements to define any linear trend for these samples. In 2 chondrules, magnetite nodules with cracks or Ni-Fe metal have very different oxygen compositions; they plot near the TF line at low delta^(18)O (-10 per mil). At this time, it is not clear if this difference could be due to a sputtering artifact, or if it reflects alteration or inclusion of another phase in these "magnetite" nodules. The question of the origin of CV magnetite has not yet been resolved. Data on additional samples including other CV chondrites will help clarify this point. References: [1] McSween H. Y. Jr. (1977) GCA, 41, 1777-1790. [2] Fuchs L. H. and Olsen E. (1973) EPSL, 18, 379-384. [3] Rubin A. E. (1991) Am. Mineral., 76, 1356-1362. [4] Valley J. W. and Graham G. M. (1991) Contrib. Mineral. Petrol., 109, 38-52. [5] Rowe M. W. et al. (1994) GCA, 58, 5341-5347. [6] Hyman M. et al. (1991) Meteoritics, 26, 350.
Choi Byung Goo
Coath Christopher D.
Leshin Lauri
McKeegan Kevin D.
Wang Jeffrey
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