Thermal and baric evolution of garnet granulites from Sri Lanka

Abstract Garnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–750°C and pressures in the vicinity of 6–8 kbar. Associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening, with the prograde P–T conditions probably reflecting a combination of the conductive and advective transport of heat at the mid-levels of tectonically thickened crust. The occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions, or fluid absent conditions, during peak metamorphism and precludes pervasive advection of a CO₂-rich fluid. The advective heat component is therefore likely to have been provided by the transport of silicate melt. Intricate symplectitic textures record partial re-equilibration of the garnet granulites to lower pressures (˜ 4–6 kbar) at high temperatures (600–750°C), and testify either to the erosional denudation of the overthick crust prior to significant cooling (i.e. quasi-isothermal decompression) or to a subsequent static heating possibly of early Palaeozoic age (Pan-African). The metamorphic history of the Sri Lankan granulites is compared with high grade terrains in the neighbouring fragments of Gondwana, with the emphasis on similarities with Proterozoic granulites of the East Antarctic craton.     

Re-equilibration of CO2 fluid inclusions at controlled hydrogen fugacities

Abstract Natural, pure CO₂ inclusions in quartz and olivine (c. Fo₉₀) were exposed to controlled f_H2 conditions at T= 718–728°C and P_total= 2 kbar; their compositions were monitored (before and after exposures) by microsampling Raman spectroscopy (MRS) and microthermometry. In both minerals exposed at the graphite–methane buffer (f_H2= 73 bar), fluid speciations record the diffusion of hydrogen into the inclusions. In quartz, room-temperature products in euhedral isolated (EI type) inclusions are carbonic phases with molar compositions of c. CO₂(60) + CH₄(40) plus graphite (Gr) and H₂O, whereas anhedral inclusions along secondary fractures (AS type) are Gr-free and contain H₂O plus carbonic phases with compositions in the range c. CO₂(60) + CH₄(40) to CO₂(10) + CH₄(90). EI type inclusions in olivine evolved to c. CO₂(90–95) + CH₄(5–10) without Gr, whereas AS type inclusions have a range of compositions from CO₂(90) + CH₄(10) ± Gr to CH₄(50) + H₂(50) ± Gr; neither H₂O nor any hydrous species was detected by optical microscopy or MRS in the olivine-hosted products. Differences in composition between and among the texturally distinct populations of inclusions in both minerals probably arise from variations in initial fluid densities, as all inclusions apparently equilibrated with the ambient f_H2. These relations suggest that compositional variability among inclusions in a given natural sample does not require the entrapment of multiple generations of fluids. In addition, the absence of H₂O in the olivine-hosted inclusions would require the extraction of oxygen from the fluids, in which case re-equilibration mechanisms may be dependent on the composition and structure of the host mineral. Many of the same samples were re-exposed to identical P–T conditions using Ar as the pressure medium, yielding ambient f_H2= 0.06 bar. In most inclusions, the carbonic fluids returned to pure CO₂ and graphite persisted in the products. Reversal of the mechanisms from the prior exposure at f_H2= 73 bar did not occur in any inclusions but the AS types in olivine, in which minor CO₂ was produced at the expense of CH₄ and/or graphite. The observed non-reversibility of previous mechanisms may be attributed to: (1) slower fluid–solid reactions compared to reactions in the homogeneous fluid phase; (2) depressed activities of graphite due to poor ordering; and/or (3) low ambient f_O2 at the conditions of the second run.     

Syndeformational fluid trapping in quartz: determining the pressure-temperature conditions of deformation from fluid inclusions and the formation of pure CO2 fluid inclusions during grain-boundary migration

Abstract Observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain-boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids. Our results imply that pure CO₂ fluid inclusions in metamorphic rocks are likely to be the residue of deformation-recrystallization process rather than representing a primary metamorphic fluid.
Where the temperature of deformation can be deduced by other means, the densities of fluid inclusions trapped during recrystallization, which we call recrystallization-primary fluid inclusions, can be used to constrain the ambient pressure during deformation. Using these constraints, the data imply that the post-metamorphic Hercynian exhumation in Sardinia brought rocks at 300° C to within 3km of the surface. This conclusion is similar to that described for the rapidly uplifted Southern Alps in New Zealand.     

Distinct metamorphic evolution of alternating silica-saturated and silica-deficient microdomains within garnet in ultrahigh-temperature granulites: An example from Sri Lanka

Here we report the occurrence of garnet porphyroblasts that have overgrown alternating silica-saturated and silica deficient microdomains via different mineral reactions. The samples were collected from ultrahigh-temperature (UHT) metapelites in the Highland Complex, Sri Lanka. In some of the metapelites, garnet crystals have cores formed via a dehydration reaction, which had taken place at silica-saturated microdomains and mantle to rim areas formed via a dehydration reaction at silica-deficient microdomains. In contrast, some other garnets in the same rock cores had formed via a dehydration reaction which occurred at silica-deficient microdomains while mantle to rim areas formed via a dehydration reaction at silica-saturated microdomains. Based on the textural observations, we conclude that the studied garnets have grown across different effective bulk compositional microdomains during the prograde evolution. These microdomains could represent heterogeneous compositional layers (paleobedding/laminations) in the precursor sediments or differentiated crenulation cleavages that existed during prograde metamorphism. UHT metamorphism associated with strong ductile deformation, metamorphic differentiation and crystallization of locally produced melt may have obliterated the evidence for such microdomains in the matrix. The lack of significant compositional zoning in garnet probably due to self-diffusion during UHT metamorphism had left mineral inclusions as the sole evidence for earlier microdomains with contrasting chemistry.     

Origin of CO2-rich fluid inclusions in leucosomes from the Skagit migmatites, North Cascades, Washington, USA

CO₂–CH₄ fluid inclusions are present in anatectic layer-parallel leucosomes from graphite-bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water-rich fluid during high-temperature metamorphism and migmatization. CO₂-rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta-igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO₂-rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post-entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO₂-rich fluid is attributed to water-saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al₂SiO₅+ water-rich fluid = garnet + melt + CO₂–CH₄. The presence of CO₂-rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis. Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high-temperature metamorphism. The infiltrating fluid was a water-rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.     

On the origin of CO2-rich fluid inclusions in migmatites

Abstract Nearly pure CO₂ fluid inclusions are abundant in migmatites although H₂O-rich fluids are predicted from the phase equilibria. Processes which may play a role in this observation include (1) the effects of decompression on melt, (2) generation of a CO₂-bearing volatile phase by the reaction graphite + quartz + biotite + plagioclase = melt + orthopyroxene + CO₂-rich vapour, (3) selective leakage of H₂O from CO₂+ H₂O inclusions when the pressure in the inclusion exceeds the confining pressure during decompression, and (4) enrichment of grain-boundary vapour in CO₂ by subsolidus retrograde hydration reactions.     

Charnockitic alteration: evidence for CO2 infiltration in granulite facies metamorphism

Charnockitic alteration (arrested orthopyroxene formation in biotite- and amphibole-bearing rocks) occurs in high-grade terranes of all ages. Three criteria are used to show that this alteration was produced in many locations by a migrating fluid phase: (i) diffuseness of the alteration—the alteration zones are often quite unlike discrete migmatitic veins; (ii) relation to deformation—most occurrences show alteration closely associated with warping of foliation or dilation cracks; (iii) open-system alteration—whilst some occurrences represent nearly isochemical alteration, slight changes in bulk composition, often loss of mafic constituents and gain of Na and Si, are evident in detailed mass-balance analysis. Y and sometimes Rb are characteristically depleted. Partial melting sometimes accompanied volatile infiltration, as evidenced by more discrete veins and euhedral orthopyroxene. It is quite unlikely, however, that open-system alteration was produced by escape of viscous quartzo-feldspathic melts. Pervasive migration of low-T lamprophyric (mafic–alkaline, CO₂-charged) interstitial liquids is a possibility by virtue of their extreme fluidity, but CO₂ infiltration was needed to generate these liquids. Vapour-deficient dehydration melting is another feasible mechanism of orthopyroxene formation which may have operated in conjunction with CO₂ infiltration. Characteristic development of charnockitic alteration in some prograde amphibolite to granulite facies transitions, as in the Dharwar Craton of South India, suggests that the alteration is a fundamental feature of the granulite facies metamorphism, implying active and causal participation of migrating fluids. In other high-grade terranes like the Adirondack Mountains of New York, this kind of alteration is rare, and fluid action does not seem to have been important in the metamorphism. A vapour phase participating in charnockitic metamorphism was necessarily one of relatively low H₂O, therefore presumably rich in CO₂. Consideration of possible large CO₂ sources leads to the conclusion that emanations from volatile-rich basalts emplaced in the lower crust are the most probable source of charnockitizing fluids. The ultimate source would therefore be enriched subcontinental lithosphere or asthenosphere. The Rb-depleted pyroxene gneiss (charnockitic) terranes may be characteristic of zones of large-scale transcurrent or oblique-motion faults which tap such great depths.     

Ultrahigh-temperature Metamorphism (1150{degrees}C, 12 kbar) and Multistage Evolution of Mg-, Al-rich Granulites from the Central Highland Complex, Sri Lanka

Mg- and Al-rich granulites of the central Highland Complex,Sri Lanka preserve a range of reaction textures indicative ofa multistage P–T history following an ultrahigh-temperaturemetamorphic peak. The granulites contain a near-peak assemblageof sapphirine–garnet–orthopyroxene–sillimanite–quartz–K-feldspar,which was later overprinted by intergrowth, symplectite andcorona textures involving orthopyroxene, sapphirine, cordieriteand spinel. Biotite-rims, kornerupine and orthopyroxene-rimson biotite are considered to be late assemblages. Thermobarometriccalculations yield an estimated P–T of at least 1100°Cand 12 kbar for the near-peak metamorphism. Isopleths of Al₂O₃in orthopyroxene are consistent with a peak temperature above1150°C. The P–T path consists of four segments. Initialisobaric cooling after peak metamorphism (Segment A), whichproduced the garnet–sapphirine–quartz assemblage,was followed by near-isothermal decompression at ultrahigh temperature(Segment B), which produced the multiphase symplectites. Furtherisobaric cooling (Segment C) resulted in the formation of biotiteand kornerupine, and late isothermal decompression (SegmentD) formed orthopyroxene rims on biotite. This evolution canbe correlated with similar P–T paths elsewhere, but thereare not yet sufficient geochronological and structural dataavailable from the Highland Complex to allow the tectonic implicationsto be fully assessed. KEY WORDS: central Highland Complex; granulites; multistage evolution; Sri Lanka; UHT metamorphism     

Role of fluids in migmatites: CO2-H2O fluid inclusions in leucosomes from the Deep Freeze Range migmatites (Terra Nova Bay, Antarctica)

ABSTRACT The metasedimentary sequence of the Deep Freeze Range (northern Victoria Land, Antarctica) experienced high-T/low-F metamorphism during the Cambro-Ordovician Ross orogeny. The reaction Bt + Sil + Qtz = Grt + Crd + Kfs + melt was responsible for the formation of migmatites. Peak conditions were c. 700–750° C, c. 3.5–5 kbar and x_H2Oc. 0.5). Distribution of fluid inclusions is controlled by host rock type: (1) CO₂-H₂O fluid inclusions occur only in graphite-free leucosomes; (2) CO₂–CH₄± H₂O fluid inclusions are the most common type in leucosomes, and in graphite-bearing mesosomes and gneiss; and (3) CO₂–N₂–CH₄ fluid inclusions are observed only in the gneiss, and subordinately in mesosomes. CO₂–H₂O mixtures (41% CO₂, 58% H₂O, 1% Nad mol.%) are interpreted as remnants of a synmig-matization fluid; their composition and density are compatible P–T–a_H2O conditions of migmatization (c. 750° C, c. 4 kbar, x_H2Oc. 0.5). CO₂-H₂O fluid in graphite-free leucosomes cannot originate via partial melting of graphite-bearing mesosomes in a closed system; this would have produced a mixed CO₂–CH₄ fluid in the leucosomes by a reaction such as Bt + Sil + Qtz + C ± H₂O = Grt + Crd + Kfs + L + CO₂+ CH₄. We conclude that an externally derived oxidizing CO₂-H₂O fluid was present in the middle crust and initiated anatexis. High-density CO₂-rich fluid with traces of CH₄ characterizes the retrograde evolution of these rocks at high temperatures and support isobaric cooling (P–T anticlockwise path). In unmigmatized gneiss, mixed CO₂–N₂–CH₄ fluid yields isochores compatible with peak metamorphic conditions (c. 700–750° C, c. 4–4.5 kbar); they may represent a peak metamorphic fluid that pre-dated the migmatization.     

Juvenile CO2 in enderbites of Tromøy near Arendal, southern Norway: a fluid inclusion and stable isotope study

The enderbites from Tromøy in the central, granulite facies part of the Proterozoic Bamble sector of southern Norway contain dominantly CO₂ and N₂ fluid inclusions. CO₂ from fluid inclusions in quartz segregations in enderbites was extracted by mechanical (crushing) and thermal decrepitation and the δ¹³C measured. Measurement was also made on samples washed in 10% HCl, oxidized with CuO at high temperatures, and step-wise extracted with progressive heating. Results between the different techniques are systematic. The main results show δ¹³C of -4.5±1.5% for crushing and -7±2% for thermal decrepitation. δ¹³C is about constant for CO₂ extracted at different temperatures and points to a homogeneous isotopic composition. Due to the presence of carbonate particles and/or induced contaminations for the extraction by thermal decrepitation, the results for the crushing experiments are assumed the most reliable for fluid-inclusion CO₂. Very low values of δ¹³C have not been found in enderbite samples and δ¹³C combined with δ¹⁸O of the host quartzes (8-11%) indicates juvenile values. In addition, the fluid inclusions were examined by microthermometry and Raman analysis and host quartz by acoustic emission and cathodoluminescence. CO₂ fluid inclusions have varying densities with a frequency maximum of 0.92 g cm^-3 and generally do not concur with trapping densities at granulite conditions. Textures show that CO₂ must have been trapped in fluid inclusions in one early event, but transformed to different extents during late isothermal uplift without important fractionation of isotope compositions. The present data support a model of intrusion and crystallization of a CO₂-rich enderbitic magma at granuiite conditions.     

Reactions and textures in grossular–wollastonite–scapolite calc–silicate granulites from Maligawila, Sri Lanka: evidence for high-temperature isobaric cooling in the meta-sediments of the Highland Complex

Grossular–wollastonite–scapolite calc–silicate granulites from Maligawila in the Buttala klippe, which form part of the overthrusted rocks of the Highland Complex of Sri Lanka, preserve a number of spectacular coronas and replacement textures that could be effectively used to infer their P–T–fluid history. These textures include coronas of garnet, garnet–quartz, and garnet–quartz–calcite at the grain boundaries of wollastonite, scapolite, and calcite as well as calcite–plagioclase and calcite–quartz symplectites or finer grains after scapolite and wollastonite respectively. Other textures include a double rind of coronal scapolite and coronal garnet between matrix garnet and calcite. The reactions that produced these coronas and replacement textures, except those involving clinopyroxene, are modelled in the CaO–Al₂O₃–SiO₂–CO₂ system using the reduced activities. Calculated examples of T–X_CO2 and P–X_CO2 projections indicate that the peak metamorphic temperature of about 900–875 °C at a pressure of 9 kbar and the peak metamorphic fluid composition is constrained to be low in X_CO2 (0.1<X_CO2<0.30). Interpretation of the textural features on the basis of the partial grids revealed that the calc–silicate granulites underwent high-temperature isobaric cooling, from about 900–875 °C to a temperature below 675 °C, following the peak metamorphism. The late-stage cooling was accompanied by an influx of hydrous fluids. The calc–silicate granulites provide evidence for high-temperature isobaric cooling in the meta-sediments of the Highland Complex, earlier considered by some workers to be confined exclusively to the meta-igneous rocks. The coronal scapolite may have formed under open-system metasomatism.     

Hydrogeochemistry of the groundwater flow system in a crystalline terrain: a study from the Kurunegala district,Sri Lanka

Water samples collected from dug wells and tube wells from the Kurunegala District of Sri Lanka have been studied for their major hydrogeochemical parameters to understand the chemical quality of water in the terrain. The region is composed of Precambrian metamorphic rocks where groundwater is only available in the regolith and along weak structural discontinuities. The study of the major chemical constituents of groundwater revealed several relationships with the aquifer lithology. Groundwater from mafic rocks have high dissolved solids, while quartzose metaclastic rocks yield water with low dissolved solids. The study area displays very low SO₄ ²⁻ contents of the groundwater. The chloride content is higher in the dry regions and in terrains underlain by pink granite and marble/calc gneiss while areas with marble, as expected, show high concentrations of Ca and Mg ions. The waters in the region can be classified into non-dominant cations to Na + K dominant and Cl⁻ and HCO₃ ⁻ dominant types. Water from charnockite-bearing areas tends to have non-dominant cations and more CO₃ ²⁻ + HCO₃ ⁻ types. Effects such as soluble salts in the regolith, fracture intensity and climatic variations play a significant role in the behavior of the hydrogeochemistry in the area.     

Ionic association in H2O–CO2 fluids at mid-crustal conditions

The equilibrium constant, K _a, of the association reaction to form ion pairs from charged solute species in supercritical solutions can be calculated from a model based on published equations. Log K _a at constant pressure is a linear function of the inverse in the dielectric constant of the fluid times temperature. The dielectric properties of H₂O and CO₂ at supercritical pressures and temperatures can also be evaluated using the Kirkwood equation. Using Looyenga mixing rules, the dielectric constant of H₂O–CO₂ mixtures can be obtained and the change in log K _a with addition of CO₂ in aqueous solutions evaluated. These changes in log K _a with addition of CO₂ are consistent with measured changes of log K _a with addition of Ar in supercritical H₂O–Ar solutions.
Log K _a of KCl and NaCl increase to an increasing extent as the mole fraction of CO₂ increases in H₂O–CO₂ solutions. For instance, at 2 kbar and constant temperature between 400 and 600° C, log K _a of KCl increases by about two orders of magnitude whilst that of NaCl increases by over four orders of magnitude as the CO₂ mole fraction increases from 0.0 to 0.35. Such changes in log K _a will have dramatic effects on the solubility of minerals in CO₂-rich environments.     

Calculated phase equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O for silica-undersaturated sapphirine-bearing mineral assemblages

Silica‐undersaturated, sapphirine‐bearing granulites occur in a large number of localities worldwide. Such rocks have historically been under‐utilized for estimating P–T evolution histories because of limited experimental work, and a consequent poor understanding of the topology and P–T location of silica‐undersaturated mineral equilibria. Here, a calculated P–T projection for sapphirine‐bearing, silica‐undersaturated metapelitic rock compositions is constructed using THERMOCALC for the FeO‐MgO‐Al₂O₃‐SiO₂ (FMAS) and KFMASH (+K₂O + H₂O) chemical systems, allowing quantitative analysis of silica‐undersaturated mineral assemblages. This study builds on that for KFMASH sapphirine + quartz equilibria [Kelsey et al. (2004) Journal of Metamorphic Geology, vol. 22, pp. 559–578]. FMAS equilibria are significantly displaced in P–T space from silicate melt‐bearing KFMASH equilibria. The large number of univariant silica‐undersaturated KFMASH equilibria result in a P–T projection that is topologically more complex than could be established on the basis of experiments and/or natural assemblages. Coexisting sapphirine and silicate melt (with or without corundum) occur down to c. 900 °C in KFMASH, some 100 °C lower than in silica‐saturated compositions, and from pressures of c.≤1 to ≥12 kbar. Mineral compositions and composition ranges for the calculated phases are consistent with natural examples. Bulk silica has a significant effect on the stability of sapphirine‐bearing assemblages at a given P–T, resulting in a wide variety of possible granulite facies assemblages in silica‐undersaturated metapelites. Calculated pseudosections are able to reproduce many naturally occurring silica‐undersaturated assemblages, either within a single assemblage field or as the product of a P–T trajectory crossing several fields. With an understanding of the importance of bulk composition on sapphirine stability and textural development, silica‐undersaturated assemblages may be utilized in a quantitative manner in the detailed metamorphic investigation of high‐grade terranes.     

Fluid inclusions in Scourian granulites from the Lewisian complex of NW Scotland: evidence for CO2-rich fluid in Late Archaean high-grade metamorphism

In this paper the first fluid-inclusion data are presented from Late Archaean Scourian granulites of the Lewisian complex of mainland northwest Scotland. Pure CO₂ or CO₂-dominated fluid inclusions are moderately abundant in pristine granulites. These inclusions show homogenization temperatures ranging from − 54 to + 10 °C with a very prominent histogram peak at − 16 to − 32 °C. Isochores corresponding to this main histogram peak agree with P-T estimates for granulite-facies recrystallization during the Badcallian (750–800 °C, 7–8 kbar) as well as with Inverian P-T conditions (550–600 °C, 5 kbar). The maximum densities encountered could correspond to fluids trapped during an early, higher P-T phase of the Badcallian metamorphism (900–1000 °C, 11–12 kbar). Homogenization temperatures substantially higher than the main histogram peak may represent Laxfordian reworking (≤ 500 °C, < 4 kbar). In the pristine granulites, aqueous fluid inclusions are of very subordinate importance and occur only along late secondary healed fractures. In rocks which have been retrograded to amphibolite facies from Inverian and/or Laxfordian shear zones, CO₂ inclusions are conspicuously absent; only secondary aqueous inclusions are present, presumably related to post-granulite hydration processes. These data illustrate the importance of CO₂-rich fluids for the petrogenesis of Late Archaean granulites, and demonstrate that early fluid inclusions may survive subsequent metamorphic processes as long as no new fluid is introduced into the system.     

Stability of sapphirine-bearing mineral assemblages in the system FeO-MgO-Al2O3-SiO2 and metamorphic P-T parameters of aluminous granulites

Consistent thermodynamic data on the properties of pure mineral end members and the mixing properties of solid solutions in the system FeO-MgO-Al₂O₃-SiO₂ were employed to simulate phase relations of sapphirine, garnet, spinel, orthopyroxene, cordierite, quartz, Al silicates, and corundum. Compositional variations of the solid solutions with temperature notably modify the topology of the P-T diagrams, which differ from the petrogenetic grids widely used in the literature. It is worth noting that the evaluation of P-T metamorphic conditions based on reaction relations in sapphirine-bearing assemblages cannot be so far considered reliable enough. The lower stability limit of the sapphirine + quartz assemblage in the system in question is possibly located at much lower P-T parameters: at least 835°C and ∼6 kbar. The sapphirine + kyanite assemblage can be stable at temperatures below 860°C and a pressure of ∼11 kbar, and the stability field of the sapphirine + olivine assemblage is narrow and constrained to temperatures no higher than ∼800°C.     

阿勒泰恰夏铜矿床的富CO2流体与矿床成因

恰夏铜矿床位于新疆阿尔泰山南缘克兰火山-沉积盆地内,赋矿地层主要为下泥盆统康布铁堡组上亚组变质岩系.脉状铜矿化主要特征为:早期顺层石英脉,呈脉状或透镜状沿变质片理分布,有星点状黄铁矿产出;晚期含铜黄铁矿-石英脉,斜切变质围岩,黄铜矿以浸染状分布于石英脉裂隙中.石英脉中流体包裹体主要为富CO2包裹体,其次为水溶液包裹体,同时含有少量的碳质流体包裹体.显微测温研究表明,早期顺层石英中原生富CO2流体包裹体,CO2三相点温度(tm,CO2)集中在-61.5～-57.5℃,CO2部分均一温度(th,CO2)集中在25～27℃,完全均一温度(th,tot)集中于223～280℃,流体密度为0.82～0.90 g/cm3;含铜黄铁矿-石英脉中原生富CO2包裹体的tm,CO2集中于-61.5～-58.7℃,th,CO2集中在23.5～28.7℃,th,tot集中在230 ～ 310℃,流体密度0.81～0.86 g/cm3.成矿流体为中高温、中低盐度、富CO2的CO2-H2O-NaCl±CH4±N2体系.恰夏铜矿脉状铜矿化的成矿流体特征与造山型金矿床的流体包裹体特征类似,结合矿床产出的地质背景、控矿构造特征,认为脉状铜矿化的成因与造山-变质热液有关,是阿尔泰山南缘晚泥盆世一二叠纪造山-变质作用的产物.SRXRF测试富CO2流体包裹体中金属微量元素,显示其富集Au,可能表明富CO2流体对金的富集起到一定作用.     

High-density early CO2 fluids in the ultrahigh-temperature granulites of Ihouhaouene (In Ouzzal, Algeria)

Fluid inclusion studies in rocks from the Lower Proterozoic granulites from western Hoggar (Algeria) provide new evidence for the hypothesis that a CO₂-rich, H₂O-poor fluid was present during the high-grade metamorphism. CO₂ inclusions represent the main fluid trapped in the Ihouhaouene ultrahigh-temperature (over 1000 °C) and high-pressure (10 to 14 kbar) granulites. The microthermometric and Raman microspectrometric measurements indicate that the carbonic fluid is mainly composed of CO₂ with minor amounts of CH₄ and N₂ detected in some inclusions (< 4 mol% CH₄). Carbonic fluid densities range from 1.18 to 0.57 g/cm³. The highest densities are recorded in superdense carbonic inclusions presenting evidence of the earliest trapping and they correspond to the fluid densities expected for the P–T conditions of the peak of metamorphism in the area previously determined from mineral geothermobarometers. Lower densities of carbonic fluids mainly result from the reequilibration of earlier trapped fluid inclusions during retrograde metamorphism and final uplift of the metamorphic terrane, but a new influx of carbonic fluids during the retrograde event remains possible. Carbonic fluids can be produced in situ from decarbonation reactions in interlayered impure marbles during the prograde event or derived from CO₂ flushing from underlying basic intrusions. The aqueous fluids present large variations of composition (0.5 to 30 wt.% NaCl equivalent) and densities (1.16 to 0.57 g/cm³). They clearly correspond to post-metamorphic fluids because they mainly occur along microfractures, they do not show any evidence of immiscibility with the carbonic fluids and mixed aquo-carbonic inclusions have not been observed. The percolation of aqueous fluids is related to the Pan-African tectonometamorphic event.     

Anthropogenic CO2 in Southern Ocean surface waters: evidence from stable organic carbon isotopes

We present an approach for tracing the fate of anthropogenic CO₂, compiling a large data set of stable organic carbon isotope ratios from surface sediments, plankton, and sinking matter in the Atlantic Ocean. The δ¹³C values of sinking matter are generally lower by 0.5–4.6‰ compared to the surface sediments. This difference increases with increasing latitude, which is explained by a stronger modern increase in surface water [CO₂ (aq)] in the Southern Ocean relative to the Tropical/Subtropical Ocean. Preindustrial dissolved CO₂ concentrations in Atlantic surface waters, estimated from the δ¹³C_org of surface sediments, are compared to recently measured surface water [CO₂ (aq)] values taken from literature. We obtain only a slight increase in [CO₂ (aq)] at lower latitudes but a significant change of about 7 ± 2 μ m in high latitudinal surface waters which we attribute to anthropogenic perturbation. Our results suggest that CO₂ released by human activities has been stored in Southern Ocean surface waters.