Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..451u&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 451
Mathematics
Logic
Cv Chondrites
Scientific paper
A carbonaceous chondrite with a total mass of 30.6 g was found in the Mundrabilla area on the Nullarbor Plains in Southwest Australia in 1992. The meteorite contains more than 30 vol% chondrules and is heavily altered. No metal or glassy chondrule mesostasis are preserved. The chondrules vary from spheres with 1 mm diameter to ellipsoids up to 2 mm long with an aspect ratio of 3 that seems to be aligned. The chondrule textures are typically olivine-microporphyritic; one "barred olivine" chondrule contains rosy spinel. Spherical metal grains (now iron-oxide pseudomorphs) are frequently seen at chondrule margins. The olivines have highly magnesian cores (Fa 0.3-6) whereas margins and narrow zones along fractures are strongly Fe-enriched (Fa 20). The altered mesostasis retained its primary chemistry in part, but shows strong local enrichment of K. Hydrothermal alteration has turned some chondrules into "ghosts" that are just barely recognizable. The fine-grained matrix is semitransparent with a reddish-brown color and consists of sheet silicates. The matrix contains diffuse, recrystallized patches that consist of iron-rich olivine (Fa 44), diopside, and calcium carbonate. Iron-sulphide-rich domains are typically associated with magnetite. High-T(?) Inclusions: We found a single, 20-my-sized, green, transparent spinel grain with an irregular surface in the matrix. In addition, we found a 0.5-mm kidney-shaped inclusion with a submicroscopic Ca-Al-silicate-rich matrix with tiny, dispersed grains of ilmenite (hem 8.5). Low totals of the microprobe analyses suggest that the matrix contains carbonates or hydroxides. Secondary Processes: Our observations indicate that the chondrite parent body underwent a long period of pervasive hydrothermal alteration under relatively high-P/lowT conditions where hydrous silicates and calcium carbonate crystallized in the matrix. We believe that oxidation of the metal took place during this event along with the Fe enrichment of the surface and fractures in olivine--terrestrial oxidation of metal would probably have created a rusty crust and vein filllings of goethite instead. The coarse-grained matrix patches were formed by a mild metamorphic event that did not cause any notable equilibration. Because of the predominating hydrothermal imprint, we propose "petrologic type 2" for the meteorite although the metamorphic event probably was in the low part of the "type 3" range or of very brief duration. Classification and Discussion: The size and abundance of chondrules, the abundance of "metal" and the presence of a Ca-Al inclusion are characteristic of the CV chondrite group and inconsistent with the properties of known CI, CO, CM, CR, and CK carbonaceous chondrites. Aligned, elongate chondrules also occur in Leoville (CV3). Most known CV chondrites belong to petrologic type 3 or 4 and hydrothermally altered members are unusual though not unique. Thorough hydrothermal alteration also affected Read Bluff (CV3), which, however, still contains some metal [1]. Acknowledgments: This work was supported by the Carlsberg Foundation. Our description is based on a polished thin section (150 mm^2) kindly provided by David New. References: [1] Treiman A. H. and DeHart J. M. (1992) Meteoritics, 27, 299.
Grundvig S.
Rasmussen Lars K.
Ulff-Moller Finn
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