Chrome spinel in normal MORB ‐type greenstones from the P aleozoic– M esozoic M ino terrane, E ast T akayama area, central J apan: Crystallization course with a U ‐turn

• Published in: The island arc Vol. 23; no. 1; pp. 62 - 73
• Main Authors: Agata, Takashi, Adachi, Mamoru
• Format: Journal Article
• Language: English
• Published: 01.03.2014
• ISSN: 1038-4871; 1440-1738
• DOI: 10.1111/iar.12060

Abstract Phenocrystic chrome spinel crystallized in normal MORB ‐type greenstones in the E ast T akayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg /( Mg + Fe 2+ ) and 0.21 to 0.53 in Cr /( Cr + Al ); the Fe 3+ content varies from 0.07 to 0.22 p.f.u. ( O  = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr /( Cr + Al ), and is depleted in Fe 2+ and Fe 3+ . Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe 2+ ‐ and Fe 3+ ‐rich at relatively high Cr /( Cr + Al ) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe 2+ ‐ and Fe 3+ ‐rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr ‐ and Fe 2+ ‐rich to Al ‐ and Mg ‐rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg /( Mg + Fe 2+ ) and in Cr /( Cr + Al ) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the M g/( M g+ Fe 2+ )– Cr /( Cr + Al ) diagram, exhibits a U ‐turn, and appears to be set on a double‐lane route. The U ‐turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.