Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.414n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 414
Other
Diopside, Dunite, Emery, Enstatite, Mesosiderites, Mincy, Rare-Earth Elements
Scientific paper
A dunitic clast from the Mincy mesosiderite was studied using microprobe and INAA techniques. This clast (1.5 x 1.2 cm) consists of olivine with minor amounts of enstatite, diopside, plagioclase, merrillite, troilite, taenite, kamacite, schreibersite, chromite, pentlandite, magnetite, and rust [1]. The texture is porphyroclastic with large olivine porphyroclasts (up to 0.3 cm) embedded in a fine-grained matrix consisting mainly of olivines (up to 60 micrometers) commonly forming triple point junctions. The opaque minerals show traces of a network penetrating the silicates. Olivine has a narrow range in composition (Fo(sub)90.9-Fo(sub)92.2) with both fine-grained and porphyroclastic olivines being slightly zoned with cores more forsteritic than rims. The FeO/MnO ratio decreases with increasing Fo, which is opposite from the trend reported for olivine clasts from mesosiderites [2]. Enstatite (En(sub)90.6Fs(sub)7.9Wo(sub)1.4) is associated with diopside (En(sub)51.9Fs(sub)3.1Wo(sub)45.0), chromite, and both types of olivine. Chromite occurs as euhedral and irregular grains. Compared to the wide variation of olivine compositions within and between other mesosiderites (Fo(sub)53-Fo(sub)92), the Mincy dunite contains only highly magnesian olivine. Triple points between fine-grained equigranular olivines clearly indicate recrystallization of coarse-grained porphyroclasts. Enstatite, diopside, and olivine appear to be in chemical equilibrium. The estimated equilibration temperature for enstatite-diopside pairs varies between 940 degrees and 1000 degrees C [3,4]. This temperature is consistent with those estimated by Nehru et al. [5] for the Emery mesosiderite. Chromite- olivine pairs reflect closure temperatures of about 550 degrees C [6]. The large differences in equilibration temperatures of pyroxenes and olivine- chromite pairs appear to be incompatible with a uniform steady-rate cooling history indicating drastically different cooling regimes for the pyroxenes and olivine-chromite pairs respectively. The dunitic clast has low REE abundances that display a V-shaped pattern (Fig. 1). This pattern indicates mixing of LREE-depleted and LREE-enriched components. The siderophile elements have high abundances (~0.2-0.5x CI) (Fig. 1) and an unfractionated Ni/Ir. Normalized abundances of Co and Fe are greater than those of Ni, indicating a substantial contribution from oxidized species. According to the planetesimal model, dunitic rocks should be common in mesosiderites [5,7,8], but are in fact rare. Besides the clast described here, only one other was reported from Morristown [9]. Trace elements indicate that the Mincy dunite is not of a simple magmatic origin. Lithophile elements indicate mixing of at least two reservoirs (high-LREE and low-LREE) similar to ureilites [10] and terrestrial metasomatic peridotites [11]. The high abundance of siderophile elements also excludes a simple magmatic origin. It rather suggests mixing of a metal with unfractionated Ir/NI ratio and an oxidized silicate of low siderophile contents. References: [1] Ntaflos Th. et al. (1991) Europ. J. Miner., 3, 200. [2] Delaney J. S. et al. (1980) Proc. LPSC 11th, 1073-1087. [3] Wells P. (1977) Contrib. Mineral. Petrol., 62, 129-139. [4] Bertrand P. and Mercier C. (1985) EPSL, 76, 109-122. [5] Nehru C. E. et al. (1980) GCA, 44, 1103-1118. [6] Fabries J. (1979) Contrib. Mineral. Petrol., 69, 329-336. [7] Mittlefehldt D. W. (1980) EPSL, 51, 29-40. [8] Hewins R. H. (1981) GCA, 45, 123-126. [9] Ruzicka A. and Boynton W. V. (1991) Meteoritics, 26, 391. [10] Janssens M. J. et al. (1987) GCA, 51, 2275-2283. [11] Frey F. A. (1984) REE Geochemistry (P. Henderson, ed.), 153-203. Fig. 1 appears here in the hard copy.
Brandstatter Frans
Koeberl Christian
Kurat Gero
Ntaflos Th.
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