Computer Science
Scientific paper
Jun 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975gecoa..39..991i&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 39, Issue 6-7, pp.991-1008
Computer Science
48
Scientific paper
A mechanism of origin for chromite-rich layers in stratiform ultramafic-gabbroic intrusions is proposed whereby the layers are precipitated on occasions when the basic parental magma of the intrusion is suddenly extensively contaminated with granitic liquid melted from salic roof rocks. It is inferred that the increase of silica and alkalies in the basic liquid should cause it to become more polymerized with a lower frequency of octahedral sites, so that on continued crystallization, Cr 3+ is preferentially expelled (into chromite) owing to its large octahedral crystal-field stabilization energy. The feasibility of this process is demonstrated by experimental data on forsterite-picroohromite crystallization relations in the system K 2 O-MgO-Cr 2 O 3 -SiO 2 , and its apparent applicability to magmas is illustrated through a comparison of the differentiation patterns of Cr and Ni in the Muskox intrusion. The granitic melt is produced because most of the crystals formed in the intrusion accumulate on its floor, leaving the roof rocks to be continuously exposed to the high temperature of the basic magma. Between episodes of contamination, the melt tends to accumulate on top of the basic magma and to remain separate because of its low density and high viscosity. If not assimilated, it eventually resolidifies as granophyre. In the Muskox intrusion there are two chromite-rich layers, each occurring in a stratigraphic unit showing the layer sequence, peridotite-chromitite-orthopyroxenite. This sequence is explained in a model in which the basic magma is contaminated while coprecipitating olivine and minor chromite. A period follows when chromite precipitates alone, and then, because the liquid is enriched in silica, orthopyroxene crystallizes instead of olivine. Variations on the model are described that simulate the main layer sequences involving chromitite in the Stillwater, Great Dyke and Bushveld intrusions. Evidence of contamination is found in the concentrated chromite crystals in the form of small spherical, composite silicate inclusions, rich in alkalies, apparently representing trapped droplets of the contaminant granitic melt in various stages of assimilation. It is suggested that the same type of contamination mechanism may also yield concentrated deposits of magnetite and of immiscible sulphide liquid.
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