クロムの地球化学的挙動 Geochemical Behavior of Chromium

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One of the trace elements, chromium, which is incorporated most effectively into the crystallizing minerals with octahedral sites from a magma and therefore most sensitive to magmatic fractionation, best indicates the degree of fractionation from the Earth's mantle, and can be used to trace the evolution of the continental crust.<BR>The early crystallized Cr-rich spinels react with the magma to form pyroxenes. Pyroxenes like spinels remove most of the Cr from the magma. In some high-Mg andesites, however, the early formed pyroxenes that coexist with spinels are rather low in Cr, and the Cr contents in the pyroxenes increase with decreasing Mg/Fe in the pyroxenes at the expense of spinels. In primary mafic magmas Cr is reduced relative to mantle rocks by a factor of three to ten through partial melting (1000-300 ppm). However, effects on Cr content of the melting degree are small relative to those of crystal fractionation. Crystal fractionation reduces Cr in the latest felsic differentiates to less than 10 ppm.<BR>Chromium abundances of peridotite constituting the upper mantle are around 3000 ppm, which are slightly lower than those of chondrite. Spinel with Cr as a major constituent, chromite, crystallizes first from most basaltic magmas, and chromite is the only Cr-mineral of economic interest. Chromite orebodies can be divided into layered and podiform types. Chromitite layers in large ultramafic-mafic intrusions, which occupy more than 90 % of world chromite resources, originate from contamination of the mafic magma with silicic liquid, because the increase of SiO<SUB>2</SUB> decreases the Cr solubility in basaltic melt. Podiform chromitites, which provide more than half of chromite produced, also form by interaction between basaltic melt and depleted peridotite. Spinels with the highest Cr in terrestrial rocks are found in podiform chromitites, low-Ca boninites and inclusions in diamond. The chromites from low-Ca boninites are higher in Cr/Al than chromites from komatiites with high Cr/Al relative to boninites. The Cr contents of spinels in volcanic rocks tend to increase with increasing SiO<SUB>2</SUB> in primary magmas. They also increase with increasing degrees of depletion of basaltic components in the source peridotites.<BR>Chromium contents in clastic sediments are controlled dominantly by the Cr abundances in the source areas, and can be used to estimate the ratio of ultramafic and mafic to felsic rocks in provenance. The Cr abundances of the weathered crust estimated from clastic sediments are 50 to 100 ppm, which are higher than the Cr abundance, 35 ppm, of the exposed Canadian Precambrian shield. Many Archean sediments show elevated Cr concentrations relative to post-Archean sediments, and immature, mafic oceanic terranes have higher Cr abundances than mature, felsic continental crust. The Cr abundances of the Earth's surface have decreased with the evolution of the continental crust.


  • Mining Geology  

    Mining Geology 47(6), 319-330, 1997-12-01 

    The Society of Resource Geology

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    Journal Article
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    ZM49(科学技術--地球科学--地質) // ZM46(科学技術--地球科学--岩石・鉱物・鉱床)
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    CJP  CJPref  NDL  J-STAGE 
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