Constraints on the application of carbon isotope thermometry in high- to ultrahigh-temperature metamorphic terranes

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 δ¹³C and δ¹⁸O values in calcite and heterogeneous single crystal δ¹³C 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 δ¹³C and δ¹⁸O values in marbles by infiltration of low δ¹³C–δ¹⁸O‐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 CO₂‐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 δ¹³C 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.     

Contrasting Carbon and Oxygen Isotopic Evolution in Metacarbonates from the Kerala Khondalite Belt,Southern India

The Kerala Khondalite belt is a Proterozoic metasupracrustal granulite facies terrain in southern India comprising garnet-biotite gneiss, garnet-sillimanite gneiss and orthopyroxene granulites as major rock types. Calc-silicate rocks and marbles, occurring as minor lithologies in the Kerala Khondalite Belt, show different mineral assemblages and reaction histories of which indicate a metamorphic P-T-fluid history dominated by internal fluid buffering during the peak metamorphism, followed by external fluid influx during decompression. The carbon and oxygen isotopic compositions of calcite from three representative metacarbonate localities show contrasting evolutionary trends. The Ambasamudram marbles exhibit carbon and oxygen isotope ratios (δ¹³C ∼ 0‰ and δ¹⁸O ∼ 20‰) typical of middle to late Proterozoic marine carbonate sediments with minor variation ascribed to the isotopic exchange due to the devolatilization reactions. The δ¹³C and δ¹⁸O values of ∼ −9‰ and 11‰, respectively, for calcite from calc-silicate rocks at Nuliyam are considerably low and heterogeneous. The wollastonite formation here, possibly corresponds to an earlier event of fluid infiltration during prograde to peak metamorphism, which resulted in decarbonation and isotope resetting. Further, petrologic evidence supports a model of late carbonic fluid infiltration that has partially affected the calc-silicate rocks, with subsequent isotope resetting, more towards the contact between calc-silicate rock and charnockite. At Korani, only oxygen isotopes have been significantly lowered (δ¹⁸O ∼ 13‰) and the process involved might be a combination of metamorphic devolatilization accompanied by an aqueous fluid influx, supported by petrologic evidence. The stable isotope signatures obtained from the individual localities, thus indicate heterogeneous patterns of fluid evolution history within the same crustal segment.     

13C12C exchange between calcite and graphite: A possible thermometer in Grenville marbles

The fractionation of ¹³C between calcite and graphite, Δ(Cc-Gr). is consistently small (2.6–4.8 permil) in 34 assemblages from upper amphibolite- and granulite-facies marbles of the Grenville Province. In 25 samples from the Adirondack Mountains, New York, it decreases regularly with increasing metamorphic temperature. The fractionations are independent of absolute δ¹³C values of calcite (?2.9 to +5.0). For T = 600–800°C, the Adirondack data are described by Δ(Cc-Gr) = ?0.00748T (°C) + 8.68. This good correlation between Δ and T suggests that carbon isotope equilibrium was attained in these high-grade marbles and that the theoretical calculations of this fractionation by Bottinga are approximately 2 permil too large in this temperature range. Because of the relatively high temperature sensitivity suggested by these results and by Bottinga's calculations, and the pressure independence of isotope fractionation, Δ(Cc-Gr) may provide a very good thermometer for high-grade marbles.Comparison of this field calibration for Δ(Cc-Gr) vs temperature with results from other terranes supports the utility of Δ(Cc-Gr) for geothermometry and suggests that graphite is much more sluggish to exchange than calcite, that exchange between calcite and graphite occurs at temperatures as low as 300°C, and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500–600°C.Because ¹³C exchange is an unavoidable metamorphic process at temperatures above 300°C, high values of δ¹³C(Gr) in moderate- to high-grade carbonate-bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite.     

Carbon,hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos,Greece

At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C.The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ¹⁸O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ¹⁸O_H2O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite.The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δD_H2O values for the metamorphic fluid decrease from ?5‰ in the glaucophane zone to an average of about ?70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend.Theδ¹⁸O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism.Detailed data on 20 marble units show systematic variations of δ¹⁸O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ¹⁸O gradients at the margins and large δ¹⁸O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ¹⁸O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates.Almost all the δ¹³C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ¹³C values of most marble units show a general correlation with δ¹⁸O values.The $\text{CO}{}_2\text{H}{}_2\text{O}$ mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ¹³C values of the CO₂ range from ?8.0 to 3.6‰ and indicate that at some localities CO₂ in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.     

Carbon isotope thermometry in marbles of the Adirondack Mountains, New York

Abstract Carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the Adirondack Mountains, New York. Eighty-nine calcite-graphite pairs from the amphibolite grade NW Adirondacks change systematically in temperature north-westwards from 680 to 640 to 670° C over a 30-km distance, reflecting transitions from amphibolite facies towards granulite facies to the north-west and to the south-east. Temperature contours based on calcite-graphite thermometry in the NW Adirondacks parallel mineral isograds, with the orthopyroxene isograd falling above 675° C, and indicate that regional metamorphic temperatures were up to 75° C higher than temperatures inferred from isotherms based on cation and solvus thermometry (Bohlen et al. 1985). Fifty-five calcite-graphite pairs from granulite grade marbles of the Central Adirondacks give regional metamorphic temperatures of 670–780° C, in general agreement with cation and solvus thermometry. Data for amphibolite and granulite grade marbles show a 12%oo range in δ¹³C_cal and δ¹³C_gr. A strong correlation between carbon isotopic composition and the abundance of graphite (C_gr/C_rock) indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism. The trends seen in δ¹³C _vs. C_gr/C_rock and δ¹³C_cal vs. δ¹³C_gr could not have been preserved if significant amounts of CO₂-rich fluid had pervasively infiltrated the Adirondacks at any time. The close fit between natural data and calculated trends of δ¹³C vs. C_gr/C_rock indicates a biogenic origin for Adirondack graphites, even though low δ¹³C values are not preserved in marble. Delamination of 17 graphite flakes perpendicular to the c-axis reveals isotopic zonation, with higher δ¹³C cores. These isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature, and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite. Samples located >2km from anorthosite show a decrease of 0.5-0.8%oo in the outer 100 μ of the grains, while samples at distances over 8 km show smaller core-to-rim decreases of c.0.2%oo. Correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism. Core graphite compositions indicate contact metamorphic temperatures were 860–890° C within 1 km of the Marcy anorthosite massif. If samples with a significant contact metamorphic effect (Δ(_cal-gr) <3.2%oo) are not included, then the remaining 38 granulite facies samples define the relation Δ¹³C(_cal-gr) = 3.56 ± 10⁶T^-2 (K).     

Assessment of grain-scale homogeneity and equilibration of carbon and oxygen isotope compositions of minerals in carbonate-bearing metamorphic rocks by ion microprobe

Nineteen samples of metamorphosed carbonate-bearing rocks were analyzed for carbon and oxygen isotope ratios by ion microprobe with a ∼5-15 μm spot, three from a regional terrain and 16 from five different contact aureoles. Contact metamorphic rocks further represent four groups: calc-silicate marble and hornfels (6), brucite marble (2), samples that contain a reaction front (4), and samples with a pervasive distribution of reactants and products of a decarbonation reaction (4). The average spot-to-spot reproducibility of standard calcite analyses is ±0.37‰ (2 standard deviations, SD) for δ¹⁸O and ±0.71‰ for δ¹³C. Ten or more measurements of a mineral in a sample that has uniform isotope composition within error of measurement can routinely return a weighted mean with a 95% confidence interval of 0.09-0.16‰ for δ¹⁸O and 0.10-0.29‰ for δ¹³C. Using a difference of >6SD as the criterion, only four of 19 analyzed samples exhibit significant intracrystalline and/or intercrystalline inhomogeneity in δ¹³C at the 100-500 μm scale, with differences within individual grains up to 3.7‰. Measurements are consistent with carbon isotope exchange equilibrium between calcite and dolomite in five of six analyzed samples at the same scale. Because of relatively slow carbon isotope diffusion in calcite and dolomite, differences in δ¹³C can survive intracrystalline homogenization by diffusion during cooling after peak metamorphism and likely represent the effects of prograde decarbonation and infiltration. All but 2 of 11 analyzed samples exhibit intracrystalline differences in δ¹⁸O (up to 9.4‰), intercrystalline inhomogeneity in δ¹⁸O (up to 12.5‰), and/or disequilibrium oxygen isotope fractionations among calcite-dolomite, calcite-quartz, and calcite-forsterite pairs at the 100-500 μm scale. Inhomogeneities in δ¹⁸O and δ¹³C are poorly correlated with only a single mineral (dolomite) in a single sample exhibiting both. Because of relatively rapid oxygen isotope diffusion in calcite, intracrystalline inhomogeneities in δ¹⁸O likely represent partial equilibration between calcite and fluid during retrograde metamorphism. Calcite is in oxygen isotope exchange equilibrium with forsterite in one of four analyzed samples, in equilibrium with dolomite in none of six analyzed samples, and in equilibrium with quartz in neither of two analyzed samples. There are no samples of contact metamorphic rock with analyzed reactants and products of an arrested metamorphic reaction that are in oxygen isotope equilibrium with each other. The degree of departure from equilibrium in analyzed samples is variable and is often related, at least in part, to alteration of δ¹⁸O of calcite during retrograde fluid-rock reaction. In situ sub-grain-scale carbon and oxygen isotope analyses of minerals are advisable in the common applications of stable isotope geochemistry to metamorphic petrology. Correlation of sub-mm scale stable isotope data with imaging will lead to improved understanding of reaction kinetics, reactive fluid flow, and thermal histories during metamorphism.     

Ion microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua supracrustal belt, West Greenland: Relationship between metamorphism and carbon isotopic composition

In-situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland. The δ¹³C values of micron-scale graphite globules in the metasediments and the carbonate rocks vary from -18 to +2‰ and from -7 to -3‰, respectively. The maximum δ¹³C value of graphite globules in the metasediment rises from -14 to -5‰, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the δ¹³C values of graphite inclusions in garnet are ∼7‰ lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower δ¹³C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively ¹³C-depleted graphite indicates that the regional metamorphism increased the δ¹³C values of the Isua graphite. This is consistent with the regional trend of ¹³C-enrichment accompanied by the increase of metamorphic grade. The minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 °C. These observations indicate that isotopic exchange with isotopically heavy carbonate caused ¹³C-enrichment of Isua graphite. The δ¹³C values of graphite reported here (δ¹³C > -18‰) were produced either as a metamorphic modification of organic carbon with initially much lower δ¹³C values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the ¹³C-enrichment of Isua graphite, previously reported large ¹³C-depletion of graphite, especially armored by apatite (Mojzsis et al., 1996) was probably premetamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga).     

Graphite content and isotopic fractionation between calcite-graphite pairs in metasediments from the Mgama Hills,Southern Kenya

Amphibolite-grade metasediments from the Mgama Hills region, Kenya, contain conspicuous quantities of graphite, most probably derived from organic progenitor materials. The highest graphite contents (5.1–20.4%) are found in schists whereas calcite marbles intercalated in the sequence contain relatively low amounts (0.1–2.0%). The graphitic constituents are consistently enriched in ¹³C relative to common sedimentary organic material, with the highest isotopic ratios in graphite from the marbles (δ¹³C = ?7.3 ± 5.0%.; n = 10). Carbon isotope fractionations between calcite and graphite mostly vary between 3.3 and 7.1‰, which comes close to both empirically recorded and thermodynamically calculated fractionations in the temperature range of the upper amphibolite faciès (550–650°C). However, larger values occasionally encountered in the marbles suggest that complete isotopic equilibrium is not always attained in amphibolite-facies metamorphism.     

Calcite–graphite isotope thermometry: a test for polymetamorphism in marble, Tudor gabbro aureole, Ontario, Canada

Graphitization and coarsening of organic material in carbonate-bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory, pressure-independent geothermometer. Comparison of observed isotopic fractionations between calcite and graphite (δ¹³C_Cal–Gr) with independent petrological thermometers provides the following empirical calibration over the range 400–800°C: δ¹³C_Cal–Gr= 5.81 times 10⁶×T^–2(K) - 2.61. This system has its greatest potential in marbles where calcite + graphite is a common assemblage and other geothermometers are often unavailable. The temperature dependency of this empirical calibration differs from theoretical calibrations; reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications. Both calcite-graphite isotopic thermometry and calcite-dolomite solvus thermometry are applied to marble adjacent to the Tudor gabbro in the Grenville Province of Ontario, Canada. The marble has undergone two metamorphic episodes, early contact metamorphism and later regional metamorphism. Values of δ¹³C_Cal–Gr decrease regularly from c. 8‰ in samples over 2 km from the pluton to values of 3–4‰ within 200 m of the contact. These samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer, and indicate temperatures of 450–500° C away from the intrusion and 700–750°C near the gabbro. This thermal profile around the gabbro is consistent with conductive heat flow models. In contrast, the distribution of Mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c. 500°C, even at the contact. Graphite textures are important for interpreting the results of the calcite–graphite thermometer. Coarsening of graphite approaching the Tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion. In contrast to isotopic exchange during prograde metamorphism, which is facilitated by graphitization, retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally. Resistance of the calcite–graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems.     

Contrasting fluid evolution of granulite-facies marbles: implications for a high-T intermediate-P terrain in the Famatinian Range, San-Juan, Argentina

High-temperature, intermediate-pressure calc-silicate marbles occur in the granulite-facies terrain of the La Huerta Range in the Province of San Juan, NW-Argentina, in three bulk-compositional varieties: Type (1) dolomite-absent scapolite-wollastonite-grandite-clinopyroxene-quartz—calcite marbles; Type (2) diopside-forsterite-spinel-corundum—calcite marbles with dolomite exolution lamellae in calcite; Type (3) serpentinized forsterite-spinel-dolomite marbles. An isobaric cooling path from peak-metamorphic conditions of 860°C to 750°C at 6.5 kbar is inferred from scapolite-wollastonite-grandite reaction textures in Type (1) and is consistent with cooling after an advective heat input from related gabbroic and tonalitic intrusive bodies. Stable carbon and oxygen isotope geochemistry was used to decipher the fluid/rock evolution of the three marble types. An interpreted four-stage temperature-time-fluid flow path comprises: (1) infiltration of pre-peak-metamorphic fluids, depleted in δ¹⁸O, that caused a shift of primary sedimentary δ¹⁸O ratios to lower values (19.6–20.0); (2) syn-metamorphic fluid liberation from Type (1) marbles with evidence for processes close to batch devolatilization that caused a weak coupled ¹³C and ¹⁸O depletion during prograde metamorphism. A different devolatilization behaviour, close to Rayleigh fractionation, texturally associated with fold settings indicates that granulite-facies fluid flow was focused rather than pervasive; (3) H₂O-absent conditions were dominant when coronal grandite formed during incipient high-temperature isobaric cooling at the expense of scapolite and wollastonite in the Type (1) marbles; (4) intense post-peak- hydration of Type (2) and Type (3) marbles is the last recognizable metasomatic event. In combination, the three marble types record fluid infiltration both before and after the metamorphic peak.     

Fluid flow and mass transport at the Valentine wollastonite deposit, Adirondack Mountains, New York State

The Valentine wollastonite skarn in the north-west Adirondack Mountains, New York, is a seven million ton deposit which resulted from channellized infiltration of H₂O-rich, silica-bearing fluids. The wollastonite formed by reaction of these fluids with non-siliceous calcite marble. The skarn formed at the contact of the syenitic Diana Complex and was subsequently overprinted by Grenville-age granulite facies metamorphism and retrograde hydrothermal alteration during uplift. Calcite marbles adjacent to the deposit have generally high δ¹⁸O values (c. 21‰), typical of Grenville marbles which have not exchanged extensively with externally derived fluids. Carbon isotopic fractiona-tions between coexisting calcite and graphite in the marbles indicate equilibration at 675d? C, consistent with the conditions of regional metamorphism. Oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14‰ variation (-1‰ to 13‰). Some isotopic heterogeneity is preserved from skarn formation, and some represents localized exchange with low-δ¹⁸O retrograde fluids. Detailed millimetre- to centimetre-scale isotopic profiles taken across skarn/marble contacts reveal steep δ¹⁸O gradients in the skarn, with values increasing towards the marble. The gradients reflect isotopic evolution of the fluid as it reacted with high δ¹⁸O calcite to form wollastonite. Calcite in the marble preserves high δ¹⁸O values to within <5 mm of the skarn contact. The preservation of high δ¹⁸O values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid. Thus, the marbles were relatively impermeable during both the skarn formation and retrograde alteration. Skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts. Although these steep isotope gradients resemble fluid infiltration fronts, they actually represent the sides of the major flow system. Because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock, such models are not applicable to this hydrothermal system and, by extension, to many other metamorphic systems where low-permeability rocks restrict fluid migration pathways. Minimum time-integrated fluid fluxes have been calculated at the Valentine deposit using oxygen isotopic mass balance, reaction progress of fluid buffering reactions, and silica mass balance. All three approaches show that large volumes of fluid were necessary to produce the skarn, but silica mass balance calculations yield the largest minimum flux and are hence the most realistic.     

Ultrahigh-Temperature Metamorphism in Madurai Granulites, Southern India: Evidence from Carbon Isotope Thermometry

Ultrahigh-temperature (UHT) metamorphism in the Madurai Block of the southern Indian granulite terrain has been verified using the calcite-graphite isotope exchange thermometer. Carbon isotope thermometry has been applied to marbles from a locality near the reported occurrence of sapphirine granulites that have yielded temperature estimates of around 1000 degrees C. The delta(13)C and delta(18)O values of calcite are homogenous, implying equilibration of the isotopes during metamorphism. However, the delta(13)C values of single graphite crystals show variations in the order of 1 per thousand within a hand specimen. Detailed isotopic zonation studies indicate that graphite preserves either the time-integrated crystal growth history or reequilibrium fractionation during its cooling history. The graphite cores preserve higher delta(13)C values than the rims. The fractionation between calcite and graphite cores gives the highest metamorphic temperature of about 1060 degrees C, which matches the petrologically inferred temperature estimates in the high-magnesian pelites. The fractionation between graphite rims and calcite suggests a temperature of around 750 degrees C, which is interpreted to reflect retrograde cooling. This event is also observed in the sapphirine granulites. Calcite-graphite thermometry thus provides a useful tool to define UHT metamorphism in granulite terrains.     

Carbon isotopic study on coexisting calcite and graphite in the Ryoke metamorphic rocks,northern Kiso district,central Japan

Carbon isotope fractionation between coexisting calcite and grpahite ( ¹³C_cc-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: ¹³C_cc-gr=8.9×10⁶T^–2–7.1 (T in °K).This equation has a slope steeper than the current results on the ¹³C_cc-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 ¹³C_cc-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.     

Short‐duration contact metamorphism of calcareous sedimentary rocks by Neoproterozoic Franklin gabbro sills and dykes on Victoria Island,Canada

This contribution addresses contact metamorphism and fluid flow in calcareous rocks of the Neoproterozoic Shaler Supergroup on Victoria Island, Arctic Canada. These processes occurred due to intrusion of gabbroic sills and dykes at c. 720 Ma during the Franklin magmatic event, which was associated with the break‐up of Rodinia. The intrusive sheets (sills and dykes) are a few metres to ～50 m thick. Metasedimentary rocks were examined in three locations with very good exposures of vertical dykes feeding horizontal sills, the Northern Feeder Dyke (NFD) complex, the Southern Feeder Dyke (SFD) complex and the Uhuk Massif. In the NFD and SFD complexes, protoliths were limestones and dolostones with minor silicates, and at the Uhuk Massif, the protoliths were silty dolostones. At the time of magma emplacement, these locations were at depths of 1–4 km. The widths of contact aureoles are only several decametres wide, commensurate with thicknesses of the dykes and sills. Splays of tremolite mark incipient metamorphism. Highest grade rocks in the NFD and SFD complexes contain the prograde assemblage diopside + phlogopite whereas at Uhuk they contain the assemblage vesuvianite + garnet + diopside. The assemblages are successfully modelled with T–X(CO₂)^fluid pseudosections that suggest achievement of CO₂‐rich fluid compositions due to early decarbonation reactions, followed by influx of aqueous fluids after peak metamorphism. Rapid heating of host rocks and short near‐peak temperature intervals are demonstrated by the prevalent morphology of diopside as radial splays of acicular crystals that appear to pseudomorph tremolite and by incomplete recrystallization of calcite in marbles. Calcsilicates in the roof of one sill at Uhuk experienced metasomatic influx of Fe that is evidenced by nearly pure andradite rims on grossular garnet. Vesuvianite, which overgrew the grossular portions of garnet, also contains ferric iron. Vesuvianite was partially consumed during retrograde growth of serpentine and andradite. The occurrence of serpentine in high‐grade portions of aureoles is consistent with eventual levelling‐off of temperatures between 350 and 400 °C, an inference that is supported by modelled conductive heat transfer from the cooling magma sheets. Focused fluid flow near intrusion‐wall rock contacts is demonstrated by narrow zones of anomalously low δ¹³C and δ¹⁸O values of carbonate minerals. Although the up to 5‰ decrease of both δ¹³C and δ¹⁸O values from sedimentary values is much smaller than is typical for calcsilicate aureoles around large plutons, it is greater than what could have been achieved by decarbonation alone. The decrease in δ¹³C is attributed to fluid‐mediated exchange with organic low‐¹³C carbon that is dispersed through the unmetamorphosed rocks and the decrease in δ¹⁸O is attributed to fluid‐mediated isotopic exchange with the gabbroic intrusive sheets. This study shows that when gabbroic sills and dykes intrude a sedimentary basin, (i) contact aureoles are likely to be narrow, only on the scale of several decametres; (ii) short high‐temperature regimes prevent achievement of equilibrium metamorphic textures; and (iii) T–X(CO₂)^fluid paths in calcareous contact aureoles are likely to be complex, reflecting a transition from prograde decarbonation reactions to influx of aqueous fluids during cooling.     

A Biotite Isograd in South-Central Maine, U.S.A.: Mineral Reactions, Fluid Transfer, and Heat Transfer

The biotite isograd in pelitic schists of the Waterville Formationinvolved reaction of muscovite + ankerite + rutile + pyrite+graphite + siderite or calcite to form biotite + plagioclase+ ilmenite. There was no single reaction in all pelites; eachrock experienced a unique reaction depending on the mineralogyand proportions of minerals in the chlorite-zone equivalentfrom which it evolved. Quartz, chlorite, and pyrrhotite werereactants in some rocks and products in others. All inferredbiotite-forming reactions involved decarbonation and desulfidation;some were dehydration reactions and others were hydration reactions.P-T conditions at the biotite isograd were near 3500 bars and400 °C. C-O-H-S fluids in equilibrium with the pelitic rockswere close to binary CO₂-H₂O mixtures with X_CO2 = 0.02–0.04.During the biotite-forming reaction, pelitic rocks (a) decreasedby 2–5 percent in volume, (b) performed – (4–11)kcal/liter P-V work on their surroundings, (c) absorbed 38–85kcal/liter heat from their surroundings, and (d) were infiltratedby at least 0.9–2.2 rock volumes H₂O fluid. The biotite isograd sharply marks the limit of a decarbonationfront that passed through the terrane during regional metamorphism.Decarbonation converted meta-shales with 6–10 per centcarbonate to carbonate-free pelitic schists. One essential causeof the decarbonation event was pervasive infiltration of theterrane by at least 1–2 rock volumes H₂O fluid early inthe metamorphic event under P-T conditions of the biotite isograd.Average shale contains 4–13 per cent siderite, ankerite,and/or calcite, but average pelitic schist is devoid of carbonateminerals. If the Waterville Formation serves as a general modelfor the metamorphism of pelitic rocks, it is likely that worldwidemany pelitic schists developed by decarbonation of shale caused,in part, by pervasive infiltration of metamorphic terranes byseveral rock volumes of aqueous fluid during an early stageof the metamorphic event.     

High integrated fluid/rock ratios during metamorphism at Naxos: evidence from carbon isotopes of calcite in schists and fluid inclusions

Calcite in schists of the metamorphic complex at Naxos is depleted both in ¹³C and in ¹⁸O with respect to massive marbles. This effect is attributed to isotope exchange with circulating CO₂-rich fluids, which had an >0.5 according to fluid inclusions. The carbon isotopic composition of the calcites is close to equilibrium with fluid inclusion CO₂ at metamorphic temperatures. Mass balance calculations assuming initial ¹³C values of 0 for calcite and –5 for the fluid, give integrated fluid/rock volume ratios between 0.1 and 2.0. Such high fluid/rock ratios are supported by observations on the distribution of CO₂/H₂O ratios of fluid inclusions, carbon isotopic compositions of fluid inclusion CO₂ and oxygen isotope systematics of silicates.     

Empirical tests of carbon isotope thermometry in granulites from southern California

Abstract ‘Peak’metamorphic carbon isotope fractionations between calcite and graphite (Δ_Cal–Gr) in marbles and calc-silicates from the Cucamonga granulite terrane (San Gabriel Mountains, California) range from 3.48 to 2.90%. The data are used to test three previously published calibrations of the calcite–graphite carbon isotope thermometer. An empirical calibration of the calcite–graphite carbon isotope thermometer gives temperatures of 700–750°C; a theoretical–experimental calibration of the system gives temperatures of 760°–870°C; an experimental calibration gives temperatures of 870–1300°C. Temperatures calculated using the empirical calibration are in agreement with those calculated from garnet-based cation exchange thermometry when uncertainty is considered. Temperatures calculated using the theoretical–experimental calibration overlap the upper range of cation exchange thermometry temperatures and range to 50°C higher. The experimental calibration yields temperatures from 50 to 480°C higher than those from cation exchange thermometry. Moreover, temperatures from the experimental calibration are also inconsistent with mineral and melt equilibria in the granulite phase assemblage. Despite the better agreement between cation exchange thermometry and the empirical calibration of the calcite–graphite system, temperatures calculated using the theoretical–experimental calibration may be real peak metamorphic temperatures. If retrograde diffusion partially reset garnet-based cation exchange thermometers by c. 50°C, then the cation exchange temperatures are consistent with those from the theoretical–empirical calibration. Thermometric evidence from biotite dehydration melting equilibria is consistent with either the empirical calibration if melting was fluid-present, or the theoretical–experimental calibration if melting was fluid-absent.     

Fractionation of carbon and oxygen isotopes and magnesium between coexisting metamorphic calcite and dolomite

Fractionations of carbon and oxygen isotopes and magnesium between coexisting dolomite and calcite have been determined for marbles and calcareous schists of a wide variety of metamorphic environments from Vermont and the Grenville Province of Ontario. Concordant equilibrium fractionations are given by 83% of the samples. Calibration of the isotopic thermometers using the Mg-calcite solvus thermometer gave in the temperature range: 650°>T°>100°C $$ \begin{gathered} 1,000\ln \alpha _{D - Ct}^{O^{18} } = 0.45 (10^6 T^{ - 2} ) - 0.40 \hfill \\ 1,000\ln \alpha _{D - Ct}^{O^{18} } = 0.18 (10^6 T^{ - 2} ) + 0.17. \hfill \\ \end{gathered} $$ These isotopic fractionation expressions differ significantly from the experimentally derived relations, including the dolomite-Mg-calcite C¹³ partial exchange experiments of this study. Temperature ranges obtained for the metamorphic zones of Vermont are: chlorite zone, 210° to 295° C; biotite zone, 255° to 400° C; staurolite-kyanite zone, 110° to 550° C. In amphibolite-facies rocks the quenched partition relations can be complex. The temperature of quench or recrystallization may be as large as 400° C below the inferred metamorphic maximum. Oxygen isotope disequilibrium in high grade rocks, particularly from the Chester dome area, Vermont, is characterized by large negative δO _D ¹⁸ –δO _Ct ¹⁸ values. The size of the equilibrium exchange system for carbon and oxygen isotopes and magnesium is small, less than a few inches across the inferred relict bedding. This is attributed to the lack of a mobile pore fluid except in systems undergoing decarbonation. C¹³/C¹² ratios in Grenville and Vermont marbles and O¹⁸/O¹⁶ ratios in Grenville and greenschist-facies Vermont carbonates span the range of ancient limestones. Staurolite-kyanite zone calcareous schists and marbles from the Chester dome area, Vermont are depleted in O¹⁸(δO¹⁸=12 to 20‰) due to equilibrium or disequilibrium decarbonation and some partial exchange. Extrapolation of the dolomite-calcite fractionation expressions to 20° C indicates that dolomite is enriched in O¹⁸ by about 4.9‰ and in C¹³ by about 2.4‰.     

Formation of wollastonite-bearing marbles during late regional metamorphic channelled fluid flow in the Upper Calcsilicate Unit of the Reynolds Range Group,central Australia*

Abstract Granulite facies marbles from the Upper Calcsilicate Unit of the Reynolds Range, central Australia, contain metre-scale wollastonite-bearing layers formed by infiltration of water-rich (X_CO2= 0.1–0.3) fluids close to the peak of regional metamorphism at c. 700° C. Within the wollastonite marbles, zones that contain <10% wollastonite alternate on a millimetre scale with zones containing up to 66% wollastonite. Adjacent wollastonite-free marbles contain up to 11% quartz that is uniformly distributed. This suggests that, although some wollastonite formed by the reaction calcite + quartz = wollastonite + CO₂, the wollastonite-rich zones also underwent silica metasomatism. Time-integrated fluid fluxes required to cause silica metasomatism are one to two orders of magnitude higher than those required to hydrate the rocks, implying that time-integrated fluid fluxes varied markedly on a millimetre scale. Interlayered millimetre -to centimetre-thick marls within the wollastonite marbles contain calcite + quartz without wollastonite. These marls were probably not infiltrated by significant volumes of water-rich fluids, providing further evidence of local fluid channelling. Zones dominated by grandite garnet at the margins of the marl layers and marbles in the wollastonite-bearing rocks probably formed by Fe metasomatism, and may record even higher fluid fluxes. The fluid flow also reset stable isotope ratios. The wollastonite marbles have average calcite (Cc) δ¹⁸O values of 15.4 ± 1.6% that are lower than the average δ¹⁸O(Cc) value of wollastonite-free marbles (c. 17.2 ± 1.2%). δ¹³C(Cc) values for the wollastonite marbles vary from 0.4% to as low as -5.3%, and correlations between δ¹⁸O(Cc) and δ¹³C(Cc) values probably result from the combination of fluid infiltration and devolatilization. Fluids were probably derived from aluminous pegmatites, and the pattern of mineralogical and stable isotope resetting implies that fluid flow was largely parallel to strike.     

Carbon and Oxygen Isotopic Composition of Surface‐Sediment Carbonate in Bosten Lake (Xinjiang,China) and its Controlling Factors

Bosten Lake is a mid-latitude lake with water mainly supplied by melting ice and snow in the Tianshan Mountains. The depositional environment of the lake is spatially not uniform due to the proximity of the major inlet and the single outlet in the western part of the lake. The analytical results show that the carbon and oxygen isotopic composition of recent lake sediments is related to this specific lacustrine depositional environment and to the resulting carbonate mineralogy. In the southwestern lake region between the Kaidu River inlet and the Kongqi River outlet, carbon isotope composition (δ13C) values of the carbonate sediment (?1‰ to ?2‰) have no relation to the oxygen isotope composition of the carbonate (δ18O) values (?7‰ to ?8‰), with both isotopes showing a low variability. The carbonate content is low (<20%). Carbonate minerals analyzed by X-ray diffraction are mainly composed of calcite, while aragonite was not recorded. The salinity of the lake water is low in the estuary region as a result of the Kaidu River inflow. In comparison, the carbon and oxygen isotope values are higher in the middle and eastern parts of the lake, with δ13C values between approximately +0.5‰ and +3‰, and δ18O values between ?1‰ and ?5‰. There is a moderate correlation between the stable oxygen and carbon isotopes, with a coefficient of correlation r of approximately 0.63. This implies that the lake water has a relatively short residence time. Carbonate minerals constitute calcite and aragonite in the middle and eastern region of the lake. Aragonite and Mg–calcite are formed at higher lake water salinity and temperatures, and larger evaporation effects. More saline lake water in the middle and eastern region of the lake and the enhanced isotopic equilibrium between water and atmospheric CO2 cause the correlating carbon and oxygen isotope values determined for aragonite and Mg–calcite. Evaporation and biological processes are the main reasons for the salinity and carbonate mineralogy influence of the surface-sediment carbonate in Bosten Lake. The lake water residence time and the CO2 exchange between the atmosphere and the water body control the carbon and oxygen isotope composition of the carbonate sediment. In addition, organic matter pollution and decomposition result in the abnormally low carbon isotope values of the lake surface-sediment carbonate.