40Ar–39Ar analysis of phlogopite in the Horoman Peridotite Complex, Hokkaido, Japan and implications for its origin |
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| Authors: | Ichiro Kaneoka Natsuko Takahashi and Shoji Arai |
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| Institution: | Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan (email:;)Faculty of Science, Chiba University, Inage-ku, Chiba 263-0022, Japan,;Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa 920-1191, Japan |
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| Abstract: | Abstract 40Ar–39Ar analysis of phlogopite separated from a plagioclase lherzolite of the Horoman Peridotite Complex, Hokkaido, Japan, has yielded a plateau age of 20.6 ± 0.5 Ma in an environment where the metamorphic fluid was characterized by an almost atmospheric Ar isotopic ratio. The age spectrum is slightly saddle-shaped, implying some incorporation of excess 40Ar during the formation of the phlogopite at a depth. As the phlogopite has been inferred to have formed in veins and/or interstitials during exhumation of the peridotite body, metasomatic fluids, to which ground- and sea water might have contributed, were probably involved in the formation of phlogopite in the crustal environment. A total 40Ar–39Ar age of 129 Ma of a whole rock sample of the plagioclase lherzolite, from which the phlogopite was separated and is representative of the main lithology of the Horoman Peridotite Complex, indicates the occurrence of excess 40Ar. Hence, the age has no geological meaning. |
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| Keywords: | Ar–Ar age excess Ar Horoman Peridotite Complex metasomatic fluids phlogopite |
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