High Ca and low Ca boninites from Chichijima, Bonin (Ogasawara) archipelago

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Abstract

Chichijima, Bonin archipelago, is a Tertiary volcanic island approximately 1000 km south-south east of Tokyo, and has been known as the type locality of boninite. Two types of boninite are found in the Bonin archipelago: One is high-Ca boninite from the Mikazukiyama Formation (Mk-HCB) and the Marubewan Formation (Mr-HCB) in Chichijima, and another is low-Ca boninite from the Marubewan Formation (Mr-LCB), Anijima (Ani-LCB), and Mukojima (Muko-LCB). HigbrCa boninite is distinguished by CaO content from dominant low-Ca boninite, and contains augite phenocrysts. The bulk MgO, Ni and Cr contents and the magnesian compositions of olivines and pyroxenes in Mk-HCB and Mr-LCB indicate their primitive nature. Bronzite phenocrysts in Mk-HCB and Mr-HCB are highly enriched in Wo molecule, implying existence of high-Ca primitive boninite magmas. Mr-HCB is a mixing product between a high-Ca boninite magma and a differentiate from Mr-LCB. Sr isotopic ratios and incompatible element ratios of Mr-HCB, Mr-LCB, Ani-LCB and Muko-LCB are nearly the same and therefore they could have been produced from the common source. In contrast, Mk-HCB has different Sr isotopic and incompatible element ratios from the others and then it may have been produced from different parental materials. Both REE model calculations and Cr-spinel compositions imply that the parental mantle of the Mr-LCB is more depleted than that of the Mk-HCB. The large enrichment of LILE and LREE in Mr-LCB compared to Mk-HCB suggests that Mr-LCB has been generated under more hydrous condition than Mk-HCB. © 2001, Japan Association of Mineralogical Sciences. All rights reserved.

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Yajima, K., & Fujimaki, H. (2001). High Ca and low Ca boninites from Chichijima, Bonin (Ogasawara) archipelago. Japanese Magazine of Mineralogical and Petrological Sciences, 30(5), 217–236. https://doi.org/10.2465/gkk.30.217

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