Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.428s&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 428
Other
Chondrites, H, Chondrules, Chromite, Ion Probe, Isotopic Fractionation
Scientific paper
Cr-rich objects (chromite-rich inclusions and chondrules) have been documented in several ordinary chondrites in recent years [1]. They are most abundant in H chondrites and the dominant phases in these objects are chromite and sodic plagioclase with ilmenite, pyroxene, and phosphate occurring as accessory phases. The genesis of these objects and their interrelationship are not clearly understood as yet. Condensation from nebular gas of nonsolar composition, gas-phase metasomatism, and oxidation of metal phases are some of the proposed mechanisms for the formation of the chromite phases found in these objects [2]. We have carried out ion microprobe studies of isotopic compositions of magnesium, chromium, and iron in a set of Cr-rich objects in the Raguli (H3.8) chondrite to further address these questions. The measurements were carried out at appropriate mass resolution to resolve hydride and other isobaric interferences. The analyzed phases include two chromite-rich chondrules, one of which contains two large euhedral grains of chromite, two chromite-rich inclusions, and isolated chromite grains in matrix. The magnesium isotopic compositions of plagioclase phases in these objects were measured to look for the possible presence of excess 26Mg. The contribution from chromite toward the magnesium signal made these measurements difficult and a relatively clean signal could be seen only for the plagioclase phase in one of the chondrules (measured 27Al+/24Mg+ = 52). No evidence for excess 26Mg was found. Magnesium (24,25,26), chromium (52,53), and iron (56,57) isotopic compositions of the chromite phases in all the objects were measured to determine isotopic mass fractionation. Terrestrial chromite (USNM 117075) was used as a standard. The measured magnesium isotopic mass fractionation for the chromite phases in the inclusions and in the chondrules are similar and are also close to the measured value (-21.0 +- 0.72 permil/amu) for the terrestrial chromite. The data for the isolated chromite grain in matrix are suggestive of a small intrinsic fractionation (a few per mil per amu) favoring the lighter isotope. The measured chromium isotope mass fractionation for all the chromite phases, including the matrix grain, are similar and again these values are also close to the measured value for the terrestrial standard (-10.3 +- 1.43 permil/amu) There is no hint in the data for an intrinsic mass fractionation favoring either the lighter or heavier isotopes of magnesium and chromium for the chromite phases in both the Cr-rich chondrules and inclusions. The above results suggest that the precursor material from which the Cr-rich objects were formed had nearly unfractionated magnesium and chromium isotopic compositions and also the process(es) leading to the formation of these objects did not result in any detectable isotopic fractionation in the chromite phases. This would argue against the suggestion that the chromite phases in these objects could be of condensation origin. On the other hand, the isotopic data are not incompatible with the suggestion that incomplete melting of chromite-rich inclusions followed by rapid crystallization led to the formation of the chromite-rich chondrules. References: [1] Krot A. N. and Ivanova M. A. (1992) LPSC XXIII, 729-730. [2] Krot A. N. et al. (1992) LPSC XXIII, 731-732.
Goswami Jitendra N.
Ivanova Marina A.
Sahijpal Sandeep
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