The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite from the southern Sesia Zone, Western Alps, Italy
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Accession number;05A0449257
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| Title;The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite from the southern Sesia Zone, Western Alps, Italy |
| Author;
MATSUMOTO KEISAKU
(Kyoto Univ., Kyoto, Jpn)
HIRAJIMA TAKAO
(Kyoto Univ., Kyoto, Jpn)
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Journal Title;J Mineral Petrol Sci
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Journal Code:G0150B
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ISSN:1345-6296
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VOL.100;NO.2;PAGE.70-84(2005)
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| Figure&Table&Reference;FIG.11, TBL.3, REF.55 |
| Pub. Country;Japan |
| Language;English |
| Abstract;An association of jadeite and omphacite has been newly found in an eclogite-facies metaquartz diorite collected from the Orco Valley area, southern part of the Eclogitic Micaschist Complex of the Sesia Zone, Western Alps, Italy. Both the jadeite and omphacite occur as idiomorphic to subidiomorphic grains in the matrix, and some of them are in contact with each other with a sharp grain boundary. Most of the jadeite shows faint and irregular zoning, with a chemical variation of Xjd(Al'VI'jd/(Na+Ca))=0.75-0.90. Half of the omphacite grains is homogeneous, and the remainder shows various zoning patterns. Some omphacite grains exhibit prograde zoning with an increase in Xjd from 0.31 to 0.55 and a decrease in Xaeg(=Fe'3+'/(Na+Ca)) from 0.14 to 0.04 from the core to the rim. The rim composition of the zoned omphacite is similar to that of homogeneous omphacite (Xjd=0.40-0.56). The average rim composition of the jadeite-omphacite pairs in direct contact shows an apparent miscibility gap between Xjd=0.50.+-.0.06, Xaeg=0.09.+-.0.03, and Xaug(=Ca/(Na+Ca))=0.41.+-.0.05 in omphacite, and Xjd=0.79.+-.0.04, Xaeg=0.08.+-.0.03, and Xaug=0.13.+-.0.03 in jadeite. Application of Powell (1985) garnet-clinopyroxene geothermometer gives T=470.+-.30.DEG.C. at P=12kbar and application of Waters and Martin (1993) garnet-omphacite-phengite geobarometer gives P=12.4kbar at T=440.DEG.C. and P=12.0kbar at T=500.DEG.C. as peak metamorphic conditions.... (author abst.) |
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