Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.605z&link_type=abstract
Meteoritics, vol. 30, no. 5, page 605
Other
Condensation, Inclusions, Refractory, Metasomatism, Meteorites, Acfer 182, Maralinga, Mighei, Murchison, Trace Elements, Troctolite
Scientific paper
An unusual inclusion of triangular cross section (5 mm x 2.5 mm in size) from the Maralinga CK chondrite has a zonal structure, consisting of core, mantle, and crust. The core (2.5 x 1 mm) consists mainly of plagioclase-olivine intergrowth (troctolite) with ophitic texture, but also contains minor clinopyroxene and Cl-apatite, as well as some calcite, which partially fills pore space. The mantle varies in thickness (0.1-1.5 mm) and consists of a dense intergrowth of green spinel and plagioclase with abundant dispersed magnetite grains of widely varying sizes (1-100 micrometer) and shapes. The spinel-plagioclase intergrowth has, in places, symplectitic texture and variable plag/sp ratios and grain-sizes. The mantle is frequently cut by plagioclase-rich veins connecting the core with the crust. Minor phases in the mantle are ilmenite (exsolution lamellae in magnetite) and calcite (in rare pore space). The thin (^about10 micrometer) discontinuous crust consists mainly of plagioclase with some olivine and magnetite and is commonly intimately intergrown with the chondrite matrix. An indentation contains an olivine-plagioclase intergrowth with subophitic texture in places. A super-crust of calcite almost continuously covers the inclusion. Phase compositions, as determined by EMP, are: Olivine - Fa = 33.1, NiO = 0.62 wt%, plagioclase - An 55-74 with high-An compositions in the mantle, clinopyroxene - Fs 10, Wo 46.7, spinel - Fe/Fe+Mg = 0.55, NiO = 1.53 wt%, and magnetite - TiO2 = 0.50 wt%, NiO = 0.57 wt%. Abundances of up to 37 trace elements were determined by secondary ion mass spectrometry[1]. Most phases are rich in trace elements and have group II REE patterns[2] with depletions of the refractory HREEs, a strong positive Tm anomaly and, commonly, a negative Eu anomaly. The exceptions are olivine, which has LREE depletions relative to the HREEs, and calcite, which does not show any significant REE fractionation at the 1xCI abundance level. Thus, trace element abundances in most phases are determined by volatility[3] and not by interphase distribution coefficients, and only olivine appears to have lost the LREEs. This suggests formation of the Maralinga inclusion by condensation from a gas depleted in the superrefractory REEs. Similar patterns have been reported from spinel-hibonite inclusions in Murchison[4], spinel-rich inclusions in Mighei[5], grossite-hibonite inclusions in Acfer 182[6], and spinel inclusions in micrometeorites[7]. The essentially unfractionated REE abundances in calcite must represent an independent reservoir. A search for ^26Al in plagioclase was mostly negative, with only one measurement giving a small ^26Mg excess. The Ti isotope ratios in ilmenite are normal, as expected. The formation history of the inclusion appears to be the following: plagioclase and olivine condensed simultaneously from a gas depleted in superrefractory elements, forming the igneous-looking troctolite core. Plagioclase continued to condense and was joined by spinel and magnetite (or a phase that was subsequently replaced by magnetite). Finally, plagioclase was again joined by olivine, forming the crust. Subsequently, metasomatic exchange reactions under oxidizing conditions in a volatile-rich gas added Fe^2+ and Na (among others) to the phases and led to the formation of phosphate (from phosphide ?) and magnetite (from a reduced precursor ?). Continuing oxidizing conditions caused mobilization of Ca from inside (and probably also outside) the inclusion that precipitated calcite into available pore spaces and at the inclusion's surface. References: [1] Zinner E. and Crozaz G. (1986) Int. J. Mass Spectr. Ion Processes, 69, 17-38. [2] Mason B. and Martin P. M. (1974) Nature, 249, 333-334. [3] Boynton W. V. (1975) GCA, 39, 569-584. [4] Ireland T. R. et al. (1988) GCA, 52, 2841-2854. [5] MacPherson G. J. and Davis A. M. (1994) GCA, 58, 5599-5625. [6] Weber D. et al. (1995) GCA, 59, 803-823. [7] Kurat G. et al. (1994) LPS XXV, 763-764.
Brandstatter Frans
Kurat Gero
Zinner Emst
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