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Constraints on the application of carbon isotope thermometry in high‐ to ultrahigh‐temperature metamorphic terranes
Author(s) -
SatishKumar M.,
Wada H.,
Santosh M.
Publication year - 2002
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.1525-1314.2002.00371.x
Subject(s) - granulite , geology , calcite , graphite , metamorphic rock , metamorphism , geochemistry , terrane , isotopes of carbon , isotopes of oxygen , mineralogy , facies , isotope , materials science , composite material , geomorphology , paleontology , physics , structural basin , quantum mechanics , tectonics
Nine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of δ 13 C and δ 18 O values in calcite and heterogeneous single crystal δ 13 C values (− 1 to − 10‰) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of δ 13 C and δ 18 O values in marbles by infiltration of low δ 13 C–δ 18 O‐bearing fluids, the extent of alteration being a direct function of the fluid‐rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO 2 ‐rich fluid, and that the infiltration had occurred under high temperature and low f O2 conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid+calcite carbon reservoir, following a Rayleigh‐type distillation process within fluid‐rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite–graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800–1000 °C), with fractionations reaching <2‰. Zonation studies on graphite show a nominal rimward lowering δ 13 C on the order of 1 to 2‰. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are > 1000 °C and would be consistent with UHT metamorphism.