Metamorphic CO2 Research Articles

Geochemical evolution of groundwater beneath Mexico City

The geochemical evolution of groundwaters along a 24km flow beneath Central Mexico City path from the Sierra de las Cruces towards Lake Texcoco has been investigated using stable isotopes, radiocarbon and major and trace elements to determine the natural baseline conditions, the extent of any contamination and the effectiveness of the overlying aquitard seal. Modern groundwaters of low salinity (<200mgl−1) are found up to 11km from the outcrop area and groundwater ages of up to 6000yrbp occur in the middle part of the section. Groundwater stable isotope ratios δ18O and δ2H lie close to the global meteoric water line, indicating that the groundwater originates from local rainfall. The groundwater chemistry may be interpreted as the result of inputs from the source area with progressive water–rock interaction down the horizontal flow gradient. A redox boundary is found at 9km along the line of section, coincident with the start of the confined section. Relatively low nitrate concentrations (below 9mgl−1 NO3–N) are found in the aerobic waters; low concentrations of NO3 in the aerobic waters and low Cl reflect inputs prior to the modern development. Some elements (Cr, U, As, Se, Sb) increase their concentration with distance (time) as far as the redox boundary, but low concentrations occur in the reducing aquifer section. The chemistry of several major ions (Mg, Na/Cl, K) as well as trace elements such as Li, Rb, Ba reflect the weathering of the basaltic mineral assemblage (feldspars and mafic minerals) and their increases are generally proportional to residence time; phosphate, F and I concentrations indicate a probable source from apatite in the basaltic or rhyolitic rocks. A borehole in the east of the city (some 17km downgradient) intercepted thermal water (Si geothermometry indicates 163°C at depth). This water gives a distinctive composition indicating possible addition of metamorphic CO2 which has then reacted with the igneous rocks. Increases in B and Cl are derived from volatiles trapped in the glass or vesicular basalt. A thermal anomaly found in the middle section of the heavily pumped aquifer is interpreted as the up-coming of warmer water from medium to greater depth mainly from basalts, rhyolites and possibly limestones.The geochemistry indicates that groundwater beneath Mexico City is of good quality and there is no obvious evidence of leakage of inorganic compounds from surface sources of contamination through the aquitard. The younger groundwater drawn from the western outcrop area is generally of good inorganic quality. Increased drawdowns in the confined aquifer have induced flow of warmer water with higher Cl from depth. The resources in the aquifer represent an important reserve of good quality water which need to be properly managed as a high quality resource as part of integrated plans for the City's future supplies.

Present and past nonanthropogenic CO2 degassing from the solid earth

Global carbon cycle models suggest that CO2 degassing from the solid Earth has been a primary control of paleoatmospheric CO2 contents and through the greenhouse effect, of global paleotemperatures. Because such models utilize simplified and indirect assumptions about CO2 degassing, improved quantification is warranted. Present‐day CO2 degassing provides a baseline for modeling the global carbon cycle and provides insight into the geologic regimes of paleodegassing. Mid‐ocean ridges (MORs) discharge 1–3 × 1012 mol/yr of CO2 and consume ∼3.5 × 1012 mol/yr of CO2 by carbonate formation in MOR hydrothermal systems. Excluding MORs as a net source of CO2 to the atmosphere, the total CO2 discharge from subaerial volcanism is estimated at ∼2.0–2.5 × 1012 mol/yr. Because this flux is lower than estimates of the global consumption of atmospheric CO2 by subaerial silicate weathering, other CO2 sources are required to balance the global carbon cycle. Nonvolcanic CO2 degassing (i.e., emission not from the craters or flanks of volcanos), which is prevalent in high heat flow regimes that are primarily located at plate boundaries, could contribute the additional CO2 that is apparently necessary to balance the global carbon cycle. Oxidation of methane emitted from serpentinization of ultramafics and from thermocatalysis of organic matter provides an additional, albeit unquantified, source of CO2 to the atmosphere. Magmatic CO2 degassing was probably a major contributor to global warming during the Cretaceous. Metamorphic CO2 degassing from regimes of shallow, pluton‐related low‐pressure regional metamorphism may have significantly contributed to global warming during the Cretaceous and Paleocene/Eocene. CO2 degassing associated with continental rifting of Pangaea may have contributed to the global warming that was initiated in the Jurassic. During the Cretaceous, global warming initiated by CO2 degassing of flood basalts, and consequent rapid release of large quantities of CH4 by decomposition of gas hydrates (clathrates), could have caused widespread extinctions of organisms.

High CO 2 emissions through porous media: transport mechanisms and implications for flux measurement and fractionation

Diffuse emissions of CO 2 are known to be large around some volcanoes and hydrothermal areas. Accumulation-chamber measurements of CO 2 flux are increasingly used to estimate the total magmatic or metamorphic CO 2 released from such areas. To assess the performance of accumulation chamber systems at fluxes one to three orders of magnitude higher than normally encountered in soil respiration studies, a test system was constructed in the laboratory where known fluxes could be maintained through dry sand. Steady-state gas concentration profiles and fractionation effects observed in the 30-cm sand column nearly match those predicted by the Stefan-Maxwell equations, indicating that the test system was functioning successfully as a uniform porous medium. Eight groups of investigators tested their accumulation chamber equipment, all configured with continuous infrared gas analyzers (IRGA), in this system. Over a flux range of ∼200–12,000 g m −2 day −1, 90% of their 203 flux measurements were 0–25% lower than the imposed flux with a mean difference of −12.5%. Although this difference would seem to be within the range of acceptability for many geologic investigations, some potential sources for larger errors were discovered. A steady-state pressure gradient of −20 Pa/m was measured in the sand column at a flux of 11,200 g m −2 day −1. The derived permeability (50 darcies) was used in the dusty-gas model (DGM) of transport to quantify various diffusive and viscous flux components. These calculations were used to demonstrate that accumulation chambers, in addition to reducing the underlying diffusive gradient, severely disrupt the steady-state pressure gradient. The resultant diversion of the net gas flow is probably responsible for the systematically low flux measurements. It was also shown that the fractionating effects of a viscous CO 2 efflux against a diffusive influx of air will have a major impact on some important geochemical indicators, such as N 2/Ar, δ 15N–N 2, and 4He/ 22Ne.

Nature and role of CO 2 in some hot and cold HCO 3 /Na/CO 2 -rich Portuguese mineral waters: a review and reinterpretation

At the northern part of the Portuguese mainland, the upflow zone of several hot and cold HCO3/Na/CO2-rich mineral waters is mainly associated with important NNE–SSW faults. Several geochemical studies have been carried out on thermal and non-thermal hydromineral manifestations that occur along or near these long tectonic alignments. The slight chemical differences that exist between these meteoric hot and cold HCO3/Na/CO2-rich mineral waters seem to be mainly caused by CO2. δ13C(TIDC) values observed in these groundwaters range between –6.00 and –1.00‰ versus V-PDB (V denotes Vienna, the site of the International Atomic Energy Agency; PDB originates from the CaCO3 of the rostrum of a Cretaceous belemnite, Belemnitella americana, collected in the Peedee formation of South Carolina, USA) indicating a deep-seated (mantle) origin for most of the CO2. Nevertheless, in the case of the heavier δ13C(TIDC) values, the contribution of metamorphic CO2 or the dissolution of carbonate rock levels at depth cannot be excluded. Concerning the hot waters, the lack of a positive 18O-shift should be attributed to water-rock interaction in a low temperature environment, rather than to the isotopic influence of CO2 on the δ18O-value of the waters.

Geochemical features of the geothermal CO2-watercarbonate rock system and analysis on its CO2 sources

Taking Huanglong Ravine and Kangding, Sichuan, and Xiage, Zhongdian, Yunnan, as examples, the authors summarize the hydrogeochemical and carbon stable isotopic features of the geothermal CO 2 -water-carbonate rock system and analyze the CO 2 sources of the system. It was found that the hydrogeochemical and carbon stable isotopic features of such a system are different from those of shallow CO 2 -water-carbonate rock system, which is strongly influenced by biosphere. The former has higher CO 2 partial pressure, and is rich in heavy carbon stable isotope. In addition, such a geothermal system is also different from that developed in igneous rock. The water in the latter system lacks Ca 2+ , and thus, there are few tufa deposits on ground surface, but it is rich in light carbon stable isotope. Further analysis shows that CO 2 of the geothermal CO 2 -water-carbonate rock system is a mixture of metamorphic CO 2 and magmatic CO 2 .

Was the Himalayan orogen a climatically significant coupled source and sink for atmospheric CO 2 during the Cenozoic?

The hypothesis that the Himalayan orogen was a climatically significant coupled source and sink for atmospheric CO 2 during the Cenozoic is evaluated in light of the timing, duration and CO 2 fluxes associated with Himalayan metamorphism and chemical weathering. We suggest that diachronous Eohimalayan metamorphism occurred over a ∼20 m.y. time span (Middle Eocene to Early Oligocene) with total metamorphic CO 2 production of ∼4–10×10 18 mol. Because this is much greater than the amount of carbon stored in the atmosphere and oceans, and because uplift and accelerated erosion began at least ∼5 m.y. after the peak of metamorphism, we conclude that it is implausible that CO 2 produced by metamorphism in the Himalayan orogen was consumed millions of years later by erosion-enhanced weathering in this orogen. Assuming a global climate/silicate-weathering feedback, we estimate that metamorphic CO 2 degassing from the Himalayan orogen would have produced a warming of <0.5°C, and enhanced weathering in this orogen would have produced a cooling of <0.2°C; thus, direct climate effects of this degassing and weathering were likely to have been minor.

Global chemical erosion over the last 250 my; variations due to changes in paleogeography, paleoclimate, and paleogeology

We utilize predictions of runoff from two series of GENESIS (version 1.02) climate model experiments to calculate chemical erosion rates for 12 time slices that span the Mesozoic and Cenozoic. A set of control experiments where geography is altered according to published paleogeographic reconstructions and atmospheric pCO2 is held fixed at the present-day value was designed to elucidate climate sensitivity to geography alone. A second series of experiments, where the (elevated) atmospheric CO2 level for each time slice was adapted from Berner (1991), was executed to determine the additional climate sensitivity to this parameter. By holding other climate forcing factors (for example, vegetation) constant throughout the sequence of experiments we evaluate the effects of systematic/coherent paleogeographic changes on runoff and temperature, and thus on global rates of chemical weathering. By using empirical relationships between runoff and bicarbonate fluxes for different rock types and maps of paleogeology we calculate global bicarbonate fluxes, taking into account spatial variations in lithology and hydrology. The climate model predicts that many regions experienced dramatic changes in runoff (for example, from wet to very arid) since the Early Triassic. In general, the supercontinent (Pangean) paleogeographic regime was driest, times of dispersed continents (Tethyan regime) wettest, and the modern geography intermediate. Total bicarbonate fluxes, as well as those from silicate mineral weathering only, closely parallel these trends. We find that spatial variations in lithology account for little variation in the total or silicate chemical erosion rates. In contrast, changes in hydrology due to differences in paleogeography are significant and are the main clear trend in our results. Here we also add an Arrhenius-type temperature dependency that modifies the runoff-determined fluxes for each rock type. High activation energies for the weathering reactions substantially increase absolute values, but we find that the ratio between the flux for a particular time slice and the Present Day remains very similar for different temperature dependencies, following the paleogeographic-controlled trend in runoff. Our calculations suggest a weaker-than-expected CO2-climate weathering feedback. The reasonable atmospheric pCO2 variations specified for the climate-model simulations do not lead to climatic effects that support large changes in the chemical erosion rate, compared to those generated by changing paleogeography. In general, however, we find that silicate weathering rates are similar to outgassing rates of volcanic and metamorphic CO2. Times of supercontinental stasis represent low outgassing but also high aridity due to extreme continentality and thus low chemical erosion fluxes. In contrast, times of continental dispersion represent high outgassing as well as high runoff (and fluxes) due to increased proximity to moisture sources. Where a mismatch occurs, particularly in the case of the early to mid-Cretaceous, we infer higher CO2 levels than those used in our GCM simulations to balance the carbon cycle.

Weathering processes in the Ganges–Brahmaputra basin and the riverine alkalinity budget

We present river chemistry data for a network of rivers draining the western and central Nepal Himalaya. Our sampling locations cover the system from the sources of rivers in Tibet to the Gangetic plain. Water samples were collected throughout the year, including the monsoon season, for rivers in Nepal and for the Ganges and Brahmaputra in Bangladesh. Rivers draining the North Himalaya are characterized by low discharge under a cold and arid climate. Main stream waters have δ13CDIC near 0‰ and high [SO42−]/([SO42−]+[HCO3−]) ratios (XSO4) with values around 40 Eq% and high [Cl−]. Ca is the dominant cation (Ca2+/∑cations=55 to 75 Eq%, after correction of sodium by chloride). Dissolved sulfate is produced in waters from the Tethian Sedimentary Series whereas chlorine is related to thermal waters. Dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution. δ13CDIC values of up to 3.9‰ show that metamorphic CO2 is an important weathering agent. Rivers of the North Himalayan basins have about 50% of their alkalinity derived from carbonate dissolution, 20% from biogenic activity and 30% from metamorphic CO2. On the south flank of the Himalaya, rivers are more depleted in 13C and have, on average, lower XSO4. Most rivers have δ13CDIC and XSO4 values compatible with a simple mixing between soil CO2 and sulfate derived from sulfide oxidation. During the monsoon, discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced. XSO4 and δ13C decrease while Ca/∑cations increases, implying enhanced dissolution of pedogenic calcite. As a whole, the G–B riverine flux of alkalinity derived from silicate weathering is around 2.7×1011 mol/year, modest at the global scale. Sulphuric acid controls 20–30% of the weathering reaction in the Brahmaputra and 6 to 9% in the Ganges. Na+ and K+ balance 60 to 65% of the silicate-derived alkalinity flux, and the long term CO2 consumption by Ganges and Brahmaputra is near 6.4×1010 mol/year. The flux of metamorphic CO2 converted to alkalinity via weathering reactions is ca. 1×1010 mol/year.

Geological implications of a permeability-depth curve for the continental crust

The decrease in permeability ( k ) of the continental crust with depth ( z ), as constrained by geothermal data and calculated fluid flux during metamorphism, is given by log k = −14 − 3.2 log z, where k is in meters squared and z is in kilometers. At moderate to great crustal depths (>∼5 km), this curve is defined mainly by data from prograde metamorphic systems, and is thus applicable to orogenic belts where the crust is being thickened and/or heated; lower permeabilities may occur in stable cratonic regions. This k-z relation implies that typical metamorphic fluid flux values of ∼10−11 m/s are consistent with fluid pressures significantly above hydrostatic values. The k-z curve also predicts that metamorphic CO2 flux from large orogens may be sufficient to cause significant climatic effects, if retrograde carbonation reactions are minimal, and suggests a significant capacity for diffuse degassing of Earth (1015–1016 g/yr) in tectonically active regions.

Metamorphic CO 2 degassing from orogenic belts

Kerrick and Caldeira (1993, 1994a) concluded that metamorphic CO 2 degassing in collisional orogens, and especially the Himalayan orogenic belt, could have been an important factor in enhancing paleoatmospheric CO 2 levels and contributing to early Cenozoic global greenhouse warming [Kerrick, D.M., Caldeira, K., 1993. Paleoatmospheric consequences of CO 2 released during early Cenozoic regional metamorphism in the Tethyan orogen. In: Touret, J.L.R., Thompson, A.B. (Guest-Eds.), Fluid–Rock Interaction in the Deeper Continental Lithosphere. Chem. Geol. 108, 201–230.] [Kerrick, D.M., Caldeira, K., 1994a. Metamorphic CO 2 degassing and early Cenozoic paleoclimate. GSA (Geol. Soc. Am.) Today 4, 57–65.]. However, our revised CO 2 mass loss computations for regional metamorphism in the Himalaya–Karakoram belt incorporating recent geochronologic data and revised estimates of the proportion of carbonate source rocks indicate that metamorphic CO 2 degassing from this orogen cannot explain Early Eocene warmth. Widespread pluton-induced hydrothermal flow occurred during the Eocene in the Cordilleran belt of western North America. Synmetamorphic intrusions, which are common in metamorphic belts, may cause significant regional fluid flow. To obtain a representative CO 2 flux from such environments, we computed a CO 2 flux of 1.5×10 12 mol km −2 Ma −1 from petrologic and geochemical studies of the Paleozoic plutonic–metamorphic belt in New England (northeastern United States). For the 2×10 6 km 2 area of Eocene metamorphism in the North American Cordillera, the CO 2 fluxes derived from the New England metamorphic belt yield an area-integrated flux of ∼3×10 18 mol Ma −1. If a significant fraction of this CO 2 entered the atmosphere, this degassing flux would alone account for Eocene greenhouse global warming. For the Ominica belt within the Cordilleran orogen, a volumetric estimate of the mass of carbonate veins indicates that the consumption of CO 2 by precipitation of carbonate veins may not significantly decrease the amount of CO 2 in fluids that convect to near-surface crustal levels. Compared to other Eocene metamorphic belts, the widespread hydrothermal activity in the North American Cordillera may have been the largest, and most climatically significant, source of metamorphic CO 2 to the Eocene atmosphere. CO 2 degassing by active metamorphism is most significant in extensional regimes of high heat flow. Extensional tectonism and hydrothermal activity in metamorphic belts may have substantially contributed to atmospheric CO 2 content throughout the Phanerozoic. Examples include the Mesozoic circum-Pacific metamorphic belt, and Oligocene–Miocene regional metamorphism in the Himalayan orogen.

Low temperature diagenetic–metamorphic and magmatic contributions of external CO 2 gas to a shallow ground water system

A small (<15 km 2), low temperature (<20°C), CO 2 gas overpressured, gently dipping Paleozoic carbonate aquifer rests on Precambrian granite in a narrow canyon along the eastern edge of the Rocky Mountain Front Range, Colorado. The carbonate aquifer is bounded on three sides by granite and in the down dip direction by the Front Range fault. The fault, in a major continental intraplate zone of weakness along which magmatic fluids and gases could migrate to the surface and near surface from great depths. The fault has also overthrusted slices of carbonate and clastic rocks several kilometres below the granitic basement. Stable isotopic ( δ 2H and δ 18O) and discharge temperature data suggests that carbonate aquifer ground waters are of meteoric origin and have not circulated to depths greater than the base of the carbonate aquifer (≈650 m). Elevated CO 2 and the δ 13C of HCO 3 − in the carbonate aquifer suggest an external crustal source of CO 2 gas. 3He/ 4He, O 2/N 2 and Ar/N 2 gas ratios indicate gas contributions from both magmatic and atmospheric sources. Atmosphere contributions account for about 25% of the exsolving gas, whereas magmatic CO 2 accounts for 7 to 14%. Possible external CO 2 sources, which are consistent with the mean HCO 3 − δ 13C=−2.4‰ (PDB), are clay–carbonate mineral diagenesis or low temperature metamorphism of siliceous–carbonate rocks that have been overthrust by 3 to 6 km of granite. Diagenetic or metamorphic CO 2, mixed with some magmatic gas, appears to have migrated from the source rock area upward along the Ute Pass thrust fault until it encountered the shallow carbonate aquifer ground water system where it was further diluted with atmospheric gas.

Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia

Metabasalts surrounding the metasediment-hosted base metal deposit at Mount Isa have been modified by metamorphism, polyphase deformation, and several stages of hydrothermal alteration. This study aims to understand the large-scale hydrothermal processes that determined the chemical composition of syn to postmetamorphic ore brines, prior to being focused into chemicaly anomalous metasediments that acted as a chemical trap for localized copper precipitation. Field mapping of the metabasalts provided the framework to determine the relative timing of district-scale alteration events, which were then studied by mineralogic observations and bulk mass balance measurements. Microthermometry, Raman microspeetrometry and Br/Cl analyses of fluid inclusions, and H, O, C, and S stable isotope measurements on vein minerals were used to characterize the hydrothermal fluids associated with the alteration events. All fluids are moderately to highly saline, variably Ca enriched with high Br/Cl compared to modern seawater, and isotopiealy most akin to modern evolved basin or low-grade metamorphic brines. The timing and fluid-tracer observations in conjunction with thermodynamic mass transfer modeling permit the following correlation between district-scale alteration processes and the mine-scale hydrothermal events associated with copper deposition at Mount Isa.District-scale epidote-sphene alteration of metabasalts temporally overlapped with the peak of greenschist facies metamorphism and deformation and is widely distributed in flow-top breccias and as halos around synmetamorphic quartz-calcite veins. It is associated with calcic brines with 12 to 38 wt percent CaCl 2 + NaCl equiv, and thermodynamic modeling suggests that the observed mineralogy and bulk mass changes can be explained by focusing of metabasalt-equilibrated fluids and by reaction with similar rocks at somewhat higher temperature. Similar timing and fluid characteristics are associated with dolomitic alteration at the Mount Isa mine, where mixing of metabasalt-derived calcic brine with metamorphic CO 2 from the metasediments led to massive dolomite precipitation.Kilometer-scale fracture zones associated with carbonate-Fe oxide alteration (calcite + albite + magnetite + or - hematite + or - biotite + or - chlorite) cut all penetrative foliations in the metabasalts east of Mount Isa. They are locally associated with uranium mineralization and are characterized by medium-salinity NaCl-rich fluid inclusions (8-93 wt % NaCl equiv). Carbonate-Fe oxide alteration is interpreted to mark channelways of infiltration by oxidized (sulfate-bearing) brines from outside the metabasalt sequence, possibly from an overlying evaporitic cover sequence. Fluid characteristics, evidence for extreme copper mobility, and timing criteria of alteration relative to deformation suggest a temporal, and possibly genetic, correlation with the main copper and silica deposition stage at the Mount Isa mine.Mg chlorite-rutile alteration is restricted to metabasalts immediately below the copper deposit and the vicinity of the Mount Isa fault zone, where higher grade amphibolite facies metabasalts were upthrust against the greenschist facies metabasalts and metasediments now hosting the copper ore bodies. Mg chlorite-rutile alteration of metabasalts, and copper mineralization in the adjacent reduced sediments, can both be explained by fluid focusing along an actively extending contact zone between the oxidized metabasalts and the reducedmetasediments. The brines focused into this contact zone were initially oxidized as a result of infiltration through the carbonate-Fe oxide fracture zones and less altered metabasalts, and became reduced by reaction with the metasediments. The unusual geochemical and isotopic (low 13 C and high 34 S) composition of Mg chlorite-rutile alteration can be explained as a result of back flow of reduced fluids from the metasediments into the metabasalts. Although Mg chlorite-rutile alteration of metabasalts probably liberated some of the copper now present in the orebodies, this alteration may be a relatively localized effect associated with the site of ore deposition rather than an essential sourcing mechanism for the copper.The regional feature of greatest significance to ore formation at Mount Isa, and to exploration for similar high-grade copper deposits elsewhere, is not the somewhat elevated initial copper content of the metabasalts nor their locally significant copper depletions, but their moderately high oxidation state evident from the ubiquitous occurrence of Fe-rich epidote and the large-scale Fe oxide-altered fracture zones. Oxidation of the basalts (or any other silicate rocks) favored high copper solubility along extensive transport paths and provided the essential chemical contrast to the depositional environment, which was anomalously reduced and sulfide rich as a result of an earlier process of sedimentary to diagenetic Pb-Zn-Fe-S mineralization.

Surface fluid convection during Cordilleran extension and the generation of metamorphic CO2 contributions to Cenozoic atmospheres

Modeling of hydrogeological regimes associated with crustal extension indicate that flux values for deeply convected meteoric water are on the order of 10 −3 m 3 ċ m −2 ċ yr −1 . Calculated CO 2 fluxes produced by infiltration-driven, metamorphic decarbonation reactions along the circulation path are 2.4 × 10 −4 m 3 ċ m −2 ċ yr −1 . Appication of the model to Cenozoic extension in the North American Cordillera demonstrates that CO 2 generated in this manner may have been a major contributor to elevated CO 2 contents of Cenozoic atmospheres and the resulting global warming due to CO 2 greenhouse effect.

Paleoatmospheric consequences of CO 2 released during early Cenozoic regional metamorphism in the Tethyan orogen

The Eocene was the warmest epoch of the Cenozoic, with published estimates of Eocene atmospheric CO 2 content ranging from two to six times the current value. Calculations of CO 2 consumption by silicate weathering show that CO 2 fluxes to the atmosphere of ca. 10 18/Myr could account for inferred Paleocene/Eocene atmospheric CO 2 contents, and the resulting greenhouse effect would have contributed to Eocene warmth. Extensive portions of the Tethyan orogen underwent regional metamorphism culminating in the Eocene. Prograde metamorphism may have been contemporaneous with the Late Paleocene global warming. We calculated the amount of metamorphic CO 2 produced at depth in the Himalayan orogen with data on the timing of the India/Asia collison, duration of prograde regional metamorphism, and proportions and bulk compositions of metamorphic CO 2 source rocks. If CO 2 was generated at a constant rate over a 10 Myr period of prograde metamorphism in the Himalayan orogen, we estimate that ca. 10 18–10 19 moles/Myr of metamorphic CO 2 were produced at depth. Significant expulsion of metamorphic CO 2 to the atmosphere may have occurred by focused fluid flow along shear zones such as the extensive Main Central Thrust in the Himalayan orogen. An additional ca. 10 18/Myr could have been contributed by Eocene regional metamorphism in the Mediterranean Tethys (i.e., from the Alps to Turkey). The extensive metamorphism associated with the India/Asia collision, and the closing of Tethys, may have contributed to CO 2-greenhouse warming in the early to mid Cenozoic.

Continental-pelagic carbonate partitioning and the global carbonate-silicate cycle.

A carbonate-silicate geochemical cycle model is developed and used to explore dynamic and climatic consequences of constraints on shallow-water carbonate burial and possible carbon loss to the mantle associated with sea-floor subduction. The model partitions carbonate deposition between shallow-water and deep-water environments and includes carbon fluxes between the mantle and lithosphere. When total lithospheric carbonate mass is constant, there are two stable steady states, one in which the carbonate burial flux is mostly continental and another in which it is mostly pelagic. The continental steady state is characterized by a low metamorphic CO2 flux to the atmosphere and predominantly shallow-water carbonate burial. The pelagic steady state is characterized by a high metamorphic CO2 flux and predominantly deep-water carbonate burial. For reasonable parameter values, when total lithospheric carbonate mass is allowed to vary, the model oscillates between predominantly continental and predominantly pelagic modes. Model results suggest that carbonate deposition patterns established during the Cenozoic may be pushing the Earth system from the continental to the pelagic mode on a time scale of 10(8) yr, with a possible consequent order-of-magnitude increase in the metamorphic CO2 flux to the atmosphere.

Devolatilization equilibria in graphite-pyrite-pyrrhotite bearing pelites with application to magma-pelite interaction

The proportions of species in a C-O-H-S fluid in equilibrium with graphite, pyrite and pyrrhotite were calculated for a range of pressure, temperature and \(f_{{\text{O}}_{\text{2}} } \) conditions, using the equilibrium constants and mass balance method, for ideal and non-ideal mixing in the fluid. Under typical metamorphic conditions, H2O, CO2, CH4 and H2S are the principal fluid species with H2S favored by higher temperatures, lower pressures and lower \(f_{{\text{O}}_{\text{2}} } \) conditions. The dominance of H2S in the fluid at high temperatures leads to values of \(X_{{\text{H}}_{\text{2}} {\text{O}}} \) becoming significantly less than 1, and causes hydrous minerals to dehydrate at lower temperatures than the case when \(X_{{\text{H}}_{\text{2}} {\text{O}}} = 1\). The production of H2S-bearing fluids provides a mechanism for the selective transfer of sulfur from a graphite-pyrite-pyrrhotite bearing pelite into a pluton via a fluid phase, without requiring wholesale melting and assimilation of rocks. Such a process is feasible if a magma is intruded by stoping, which allows a significant volume of pelite country rock to be raised rapidly to temperatures approaching that of the magma. H2S-bearing fluids produced from graphite-pyrite-pyrrhotite pelites (due either to magmatic intrusion or regional metamorphism) may also mobilize ore-forming metals as sulfide complexes.