Calcite–graphite isotope thermometry in amphibolite facies marble, Bancroft, Ontario

Abstract

This study presents calcite-graphite carbon isotope fractionations for 32 samples from marble in the northern Elzevir terrane of the Central Metasedimentary Belt, Grenville Province, southern Ontario, Canada. These results are compared with temperatures calculated by calcite-dolomite thermometry (15 samples), garnet-biotite thermometry (four samples) and garnet-hornblende thermometry (three samples). D cal-gr values vary regularly across the area from >6.5& in the south to 4.0& in the north, which corresponds to temperatures of 525°C in the south to 650°C in the north. Previous empirical calibration of the calcite-graphite thermometer agrees very well with calcite-dolomite, garnet-biotite and garnet-hornblende thermometry, whereas, theoretical calibrations compare less well with the independent thermometry. Isograds in marble based on the reactions rutile + calcite + quartz ¼ titanite and tremolite + calcite + quartz ¼ diopside, span temperatures of 525-600°C and are consistent with calculated temperature-X(CO 2 ) relations. Results of this study compare favourably with large-scale regional isotherms, however, local variation is greater than that revealed by large-scale sampling strategies. It remains unclear whether the temperature-D cal-gr relationship observed in natural materials below 650°C represents equilibrium fractionations or not, but the regularity and consistency apparent in this study demonstrate its utility for thermometry in amphibolite facies marble.

Relationship between structure, morphology, and carbon isotopic composition of graphite in marbles: Implications for calcite-graphite carbon isotope thermometry

“…Here, the carbon isotopic composition of graphite is, in principle, controlled by temperature-dependent isotope fractionation with the matrix carbonate minerals (Bottinga 1969;Valley and O'Neil 1981;Wada and Suzuki 1983). Several earlier studies have considered the carbon isotopic exchange between graphite and calcite, wherein they have attempted to formulate and generalize regression equations that are consistent with the temperature conditions of metamorphism derived from other petrological geothermometers, or from experimental and theoretical constraints (see Dunn 2005 and references therein for a review on calibrations other than the recent work of Deines and Eggler 2009). The results from these studies suggest that carbon isotope thermometry is an effective tool to retrieve peak metamorphic temperature conditions in high-grade metamorphic rocks (Dunn and Valley 1992;Kitchen and Valley 1995;Satish-Kumar et al 2002;Dunn 2005), even up to ultrahigh-temperature metamorphism (Satish-Kumar 2000).…”

“…Carbon isotope exchange between calcite and graphite has been widely and efficiently applied for estimating the peak metamorphic temperature conditions in marbles, because (1) only two carbon-bearing phases exist in marbles, (2) diffusion of carbon in graphite is extremely slow and restricts retrograde isotope exchange, (3) graphite is low in modal abundance, (4) the temperature-dependent isotope fractionation is large and measurable, and (5) retrograde isotope alterations can be easily detected in calcite (Eiler et al 1993;Valley 2001;Satish-Kumar et al 2002;Dunn 2005). However, in some instances, the behavior of carbon isotope exchange between calcite and graphite during metamorphism seems to give variable fractionations and erroneous temperature estimates (Kreulen and van Beek 1983;Arneth et al 1985).…”

Origin of graphite, and temperature of metamorphism in Precambrian Eastern Ghats Mobile Belt, Orissa, India: A carbon isotope approach

“…The d 13 C value of both the phases will be modified depending on the relative amount of the phases along with the magnitude of the isotopic fractionation and temperature of metamorphism (Valley and O'Neil, 1981;Dunn and Valley, 1992;Kitchen and Valley, 1995;Dunn, 2005). Optical microscopic study of calcite-bearing graphite samples from the EGMB show that specks, scales and flakes of graphite are intergrown with calcite (Fig.…”

“…8). Temperatures calculated using the Kitchen and Valley (1995) equation, best suited for temperatures above 650°C (Dunn, 2005), correspond to a temperature range of 620-950°C in one group of samples while a range of 90-290°C is obtained for another group (Fig. 8).…”

Experimental determination of carbon isotope fractionation between CaCO3 and graphite

“…13 C fractionation between calcite and graphite in metamorphic rocks. Observed fractionations (d): Valley and O'Neil (1981), Wada and Suzuki (1983), Kreulen and van Beek (1983), Morikiyo (1984), Kitchen and Valley (1995), Rathmell et al (1999), andDunn (2005). The temperature estimates for the metamorphic rocks were taken from the original publications and were based on element distributions and mineral stability data.…”