Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) An erratum has been published Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (8) Citing Articles via Google Scholar Google Scholar Articles by Tsujimori, T. Articles by Liou, J. G. Search for Related Content GeoRef GeoRef Citation

Coexisting chromian omphacite and diopside in tremolite schist from the Chugoku Mountains, SW Japan: The effect of Cr on the omphacite-diopside immiscibility gap

¹ Research Institute of Natural Sciences, Okayama University of Science, Okayama 700-0005, Japan
² Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.

Chromian clinopyroxenes with exsolution textures were found in high-pressure (HP) tremolite schist from the Osayama serpentinite melange in the Chugoku Mountains, Southwestern Japan. Chromian omphacite [jadeite (Jd)_23.3–41.4 diopside (Di)_46.7–60.7 kosmochlor (Ko)_4.5–19.6 aegirine (Ae)_<7.0 ] with up to 6.6 wt% Cr₂O₃ occurs as neoblastic crystals in a foliated tremolite-rich matrix, or as pseudomorphs after relict chromian spinel, and contains irregular-shaped thin lamellae of chromian diopside (Jd_3.8–18.2Di_72.5–93.6Ko_1.1–13.9Ae_<3.4). Chromian omphacite contains roughly constant jadeite plus kosmochlor components at 38.2–51.6 mol%; this is equivalent to the jadeite component of ordered P2/n omphacite. Systematic analyses of coexisting chromian pyroxenes yield a clear immiscibility gap between "omphacite" and "diopside." The compositional gap becomes much narrower with increasing Ko component; addition of only 10 mol% Ko component narrows the omphacite-diopside gap by an order of magnitude. Such an effect is similar to, but more effective than, the introduction of Fe ³⁺ on the omphacite-diopside immiscibility gap.

Chromian pyroxenes replacing relict chromian spinel are associated with other chromian silicates including phengite, chlorite, and pumpellyite. The wide compositional gap of chromian pyroxenes and the presence of chromian pumpellyite and Si-rich chromian phengite indicate T <300–400 °C and P >0.8 GPa. This P-T estimate is consistent with parageneses of minerals in the host serpentinite. The variation of Cr content in chromian silicates reflects the extent of Cr Al substitution, and may be related to a chemical heterogeneity of the Cr-bearing fluid.

This article has been cited by other articles:

				G. H. Shi, B. Stockhert, and W. Y. Cui Kosmochlor and chromian jadeite aggregates from the Myanmar jadeitite area Mineralogical Magazine, December 1, 2005; 69(6): 1059 - 1075. [Abstract] [Full Text] [PDF]

				T. Tsujimori and J. G. Liou Low-pressure and low-temperature K-bearing kosmochloric diopside from the Osayama serpentinite melange, SW Japan American Mineralogist, October 1, 2005; 90(10): 1629 - 1635. [Abstract] [Full Text] [PDF]

				T. Tsujimori, J. G. Liou, and R. G. Coleman Coexisting retrograde jadeite and omphacite in a jadeite-bearing lawsonite eclogite from the Motagua Fault Zone, Guatemala American Mineralogist, May 1, 2005; 90(5-6): 836 - 842. [Abstract] [Full Text] [PDF]

				K. Ikehata, K. Ikehata, and S. Arai Metasomatic formation of kosmochlor-bearing diopside in peridotite xenoliths from North Island, New Zealand American Mineralogist, October 1, 2004; 89(10): 1396 - 1404. [Abstract] [Full Text] [PDF]