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Carbon isotope exchange during polymetamorphism in the Panamint Mountains, California, USA
Author(s) -
BERGFELD D.,
NABELEK P. I.,
LABOTKA T. C.
Publication year - 1996
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1046/j.1525-1314.1996.05848.x
Subject(s) - schist , calcite , geology , metamorphic rock , geochemistry , isotopes of carbon , carbonate minerals , metamorphism , mineralogy , carbon fibers , mineral , carbonate , total organic carbon , chemistry , materials science , metallurgy , composite number , environmental chemistry , composite material
Carbon isotope fractionations between calcite and graphite in the Panamint Mountains, California, USA, demonstrate the importance of mass balance on carbon isotope values in metamorphosed carbon‐bearing minerals while recording the thermal conditions during peak regional metamorphism. Interbedded graphitic marbles and graphitic calcareous schists in the Kingston Peak Formation define distinct populations on a δ 13 C (gr) –δ 13 C (cc) diagram. The δ 13 C values of both graphite and calcite in the marbles are higher than the values of the respective minerals in the schists. δ 13 C values in both rock types were controlled by the relative proportions of the carbon‐bearing minerals: calcite, the dominant carbon reservoir in the marble, largely controlled the δ 13 C values in this lithology, whereas the δ 13 C values in the schists were largely controlled by the dominant graphite. This is in contrast to graphite‐poor calcsilicate systems where carbon isotope shifts in carbonate minerals are controlled by decarbonation reactions. The marbles record a peak temperature of 531±30 °C of a Jurassic low‐pressure regional metamorphic event above the tremolite isograd. In the schists there is a much wider range of recorded temperatures. However, there is a mode of temperatures at c. 435 °C, which approximately corresponds to the temperatures of the principal decarbonation metamorphic reactions in the schists, suggesting that the carbon exchange was set by loss of calcite and armouring of graphite by newly formed silicate minerals. The armouring may explain the relatively large spread of apparent temperatures. Although the modal temperature also corresponds to the approximate temperature of the Cretaceous retrograde event, retrograde exchange is thought less likely due to very slow exchange rates involving well‐crystallized graphite, armouring of graphite by silicates during the earlier event, and because of other barriers to retrograde carbon exchange. Thus, only the calcite–graphite carbon isotope fractionations recorded by the marbles demonstrate the high‐temperature conditions of the low‐pressure Jurassic metamorphic event that was associated with the emplacement of granitic plutons to the west of the Panamint Mountains.