Carbon isotopic study on coexisting calcite and graphite in the Ryoke metamorphic rocks,northern Kiso district,central Japan |
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| Authors: | Toshiro Morikiyo |
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| Institution: | (1) Department of Geology, Faculty of Science, Shinshu University, 390 Matsumoto, Japan |
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| Abstract: | Carbon isotope fractionation between coexisting calcite and grpahite (
13Ccc-gr) has been determined in metamorphosed limestones and calc-silicate rocks from the Ryoke metamorphic belt in the northern Kiso district. In this district, the Ryoke metamorphic rocks, ranging from the lower greenschist facies to the upper amphibolite facies, are widely distributed. The fractionation of 13C/12C between calcite and graphite decreases regularly with increasing metamorphic grade and is independent of absolute 
13C values of calcite. This evidence suggests that carbon isotopic exchange equilibrium has been attained during metamorphism even in the greenschist facies and isotopic modification, possibly caused by retrogressive metamorphism, is not distinguished. For T=270–650° C, the fractionation is expressed by the following equation:
13Ccc-gr=8.9×106T–2–7.1 (T in °K).This equation has a slope steeper than the current results on the
13Ccc-gr versus 106T–2 diagram. It can be used as a potential geothermometer for almost the entire temperature range of metamorphism.
13C values of carbonaceous matter in unmetamorphosed limestones in this district are approximately –22 , due to its biogenic origin. Graphite from metamorphosed limestones is also considered to be of biogenic origin but shows enrichment of 13C due to isotopic exchange with calcite.
13C values of graphite as well as
13Ccc-gr confirm that zone II represents the lowest grade zone of Ryoke metamorphism. The maximum equilibrium fractionation of
13C between calcite and graphite is considered to be approximately 23%, which corresponds to 270° C. Below this temperature, it seems that carbon isotopic exchange between the minerals does not occur.Calcite in marble from the higher grade zones has relatively lower
13C and
18O values. The depletion of heavy isotopes is considered to be caused by the loss of 13C and 18O enriched carbon dioxide during decarbonation reactions. For oxygen, it is considered that isotopic exchange with metamorphic fluids plays an important role in lowering the
18O value of calcite in some higher grade marbles. |
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