CO2 Fluid Inclusions Research Articles

Fluid-inclusion studies of high-grade metamorphic rocks of the Ashuanipi complex, eastern Superior Province: constraints on the retrograde P–T path and implications for gold metallogeny

The high-grade metamorphic rocks of the Ashuanipi complex have been the subject of a microthermometric fluid-inclusion study. Four types of fluid inclusions were observed: CO2-rich fluids; low-temperature, high-salinity H2O fluids; CH4 ± N2-rich fluids; and high-temperature, low-salinity H2O fluids. The regionally distributed CO2-rich fluids are the earliest fluids, and their calculated isochores indicate a clockwise post-peak metamorphic P–T–t path for the Ashuanipi complex. The low-temperature, high-salinity aqueous fluid inclusions are also distributed regionally and can be interpreted as late brines, retrograde metamorphic fluids, or the wicked-off aqueous component of H2O–CO2 fluid inclusions. Both CH4 ± N2-rich fluids and the high-temperature, low-salinity aqueous fluid inclusions were found only locally in gold-bearing metamorphosed banded iron formations. Fluid-inclusion microthermometry, arsenopyrite thermometry, and metamorphic petrologic study at Lac Lilois, one of the principal gold showings, suggest that some gold deposition may have occurred during regional post-peak metamorphic exhumation and cooling at P–T conditions near the amphibolite–greenschist transition. However, it is possible that gold deposition began at higher near-peak metamorphic P–T conditions. Another major gold showing, Arsène, is characterized by CH4 ± N2-rich fluid inclusions, tentatively inferred to be either directly related to gold deposition or responsible for secondary gold enrichment. The association of CH4 ± N2-rich fluids with gold occurrences in the Ashuanipi complex is comparable to gold deposits of the Abitibi greenstone belt and of Wales, Finland, and Brazil.

The origin and significance of non-aqueous CO2 fluid inclusions in the auriferous veins of Bin Yauri, northwestern Nigeria

Non-aqueous CO2 and CO2-rich fluid inclusions are found in the vein quartz hosting mesothermal gold-sulphide mineralization at Bin Yauri, northwestern Nigeria. Although mineralizing fluids responsible for gold mineralization are thought to be CO2-rich, the occurrence of predominantly pure to nearly pure CO2 inclusions is nevertheless unusual for a hydrothermal fluid system. Many studies of similar CO2-rich fluid inclusions, mainly in metamorphic rocks, proposed preferential loss (leakage) of H2O from H2O-CO2 inclusions after entrapment. In this study however, it is proposed that phase separation (fluid immiscibility) of low salinity CO2-rich hydrothermal fluids during deposition of the gold mineralization led to the loss of the H2O phase and selective entrapment of the CO2. The loss of H2O to the wallrocks resulted in increasing oxidizing effects. There is evidence to suggest that the original CO2-rich fluid was intrinsically oxidized, or perhaps in equilibrium with oxidizing conditions in the source rocks. The source of the implicated fluid is thought to be subducted metasediments, subjected to dehydration and devolatilization reactions along a transcurrent Anka fault/shear system, which has been described as a Pan-African (450–750 Ma) crustal suture.

Origin of xenoliths in the trachyte at Puu Waawaa, Hualalai Volcano, Hawaii

Rare dunite and 2-pyroxene gabbro xenoliths occur in banded trachyte at Puu Waawaa on Hualalai Volcano, Hawaii. Mineral compositions suggest that these xenoliths formed as cumulates of tholeiitic basalt at shallow depth in a subcaldera magma reservoir. Subsequently, the minerals in the xenoliths underwent subsolidus reequilibration that particularly affected chromite compositions by decreasing their Mg numbers. In addition, olivine lost CaO and plagioclase lost MgO and Fe2O3 during subsolidus reequilibration. The xenoliths also reacted with the host trachyte to form secondary mica, amphibole, and orthopyroxene, and to further modify the compositions of some olivine, clinopyroxene, and spinel grains. The reaction products indicate that the host trachyte melt was hydrous. Clinopyroxene in one dunite sample and olivine in most dunite samples have undergone partial melting, apparently in response to addition of water to the xenolith. These xenoliths do not contain CO2 fluid inclusions, so common in xenoliths from other localities on Hualalai, which suggests that CO2 was introduced from alkalic basalt magma between the time CO2-inclusion-free xenoliths erupted at 106±6 ka and the time CO2-inclusion-rich xenoliths erupted within the last 15 ka.

Fluid inclusions in granulites and eclogites from the Bergen Arcs, Caledonides of W. Norway

AbstractThe Grenvillian granulite-facies complex on Holsnøy island, Bergen Arcs, W. Norway, has been metamorphosed at eclogite-facies conditions during the Caledonian orogeny (ca. 425 Ma). The granulite-eclogite facies transition takes place along shear zones and fluid pathways. Mineral thermobarometry indicates PT conditions of 800–900°C and 8–10 kbar for the Proterozoic granulite facies metamorphism and 700–800°C and 16–19 kbar for the eclogite-forming event. Quartz in the granulite facies complex contains CO2 fluid inclusions with less than 2.5 mole percent N2; the molar volumes are compatible with the PT conditions of the Proterozoic granulite metamorphism. Quartz in pegmatitic quartz + omphacite and quartz + phengite/paragonite veins coeval with shear-zone eclogites contain N2 ± CO2 fluid inclusions. Combined laser Raman microanalysis and microthermometry show that the least disturbed inclusions have XCO2 = 0.1–0.3, and molar volumes less than 40 cm3/mole, which may agree with the PT conditions during Caledonian high-pressure metamorphism. Younger, low-density N2 and N2-H2O fluid inclusions are the results of decrepitation and redistribution of early inclusions during the retrograde PT evolution of the eclogites.

CO2 fluid inclusions in ultramafic xenoliths from the Iblean Plateau, Sicily, Italy

AbstractThe Iblean Plateau (Southeastern Sicily, Italy) consists of a thick Meso-Cenozoic carbonate sequence with interbedded volcanic horizons (alkaline and tholeiitic basalts). The alkaline basalts contain ultramafic (peridotites and pyroxenites) and mafic xenoliths. The peridotites are spinel-bearing lherzolites and lherzolitic harzburgites, with porphyroblastic to protogranular texture. Pyroxenites consist of Cr-diopside-bearing and Al-augite-bearing websterites. The mineral chemistry of the nodules indicates temperatures between 700 and 1050°C.Fluid inclusions containing CO2 and (sometimes) various proportions of silicate glass have been studied in olivine, orthopyroxene and clinopyroxene. The secondary inclusions occur as trails of CO2-rich inclusions, often cross-cutting deformation lamellae. The few primary inclusions, generally empty, show clear evidence of decrepitation. Of the 390 inclusions examined, 97% homogenized to the liquid phase (Th → L = −43.9 to +30.9°C); 3% homogenized to the vapour phase (Th → V = + 20.5 to +30.3°C, yelding CO2 densities in the range 0.20–1.13 g/cm3. Assuming a trapping temperature of 1100°C, the corresponding trapping pressure for a pure CO2 system lies in the range 0.6–11.0 kbar, i.e. a depth of ∼2.2 to 42 km.The majority of CO2 trapping events in the xenoliths occurred from 2.2 to 11.0 kbar, with no major trapping events at pressures less than 2.3 kbar, indicating the absence of a shallow magma reservoir below the Iblean Plateau.

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.

Carbon isotope compositions of fluid inclusions in charnockites from southern India

Models for the formation of crustal granulites, which include the metamorphism of anhydrous lithologies1,2, the extraction of hygroscopic granite melts3,4 and fluxing by CO25–7 constrain the thermal and geochemical evolution of the lower crust and its interaction with mantle-derived volatiles. Here we present abundances and carbon isotope compositions of CO2 in fluid inclusions in quartz and garnet from gneiss–granulite pairs from the incipient char-nockite quarries of southern India, determined by a stepped combustion/thermal decrepitation carbon–extraction technique, which quantifies problematic sample contamination and allows precise lithological control. The results show that the CO2 in incipient charnockite is more abundant and isotopically heavier than the CO2 in associated gneiss, providing evidence for the influx of CO2-rich fluids during their formation. The inferred carbon isotope compositions of these fluids are consistent with a mantle source, and at Ponmudi, the isotopic composition of coexisting graphite and CO2 implies fluid/rock volume ratios in excess of 0.15.

Spinel Iherzolite nodules ffom Oahu Island (Hawaii) : a fluid inclusion study

Essentially pure CO2 fluid inclusions have been found in orthopyroxene, clinopyroxene and olivine of 11 spinel lherzolite nodules in nepheline basalts from Pali (Koolau Range) and Salt Lake Crater, Oahu, Hawaii. Most of the inclusions occur along healed fractures (are thus secondary) and are often associated with different proportions of silicate glass ; some contain a dark globule, probably a sulfide. They represent the coexistence of two or three immiscible fluid phases — silicate melt, sulfide melt, and dense supercritical CO2. Only a few ones, not associated with glass, are most likely primary ; they are generally bigger in size and show clear decrepitation phenomena. 361 secondary inclusions were studied ; homogenization temperatures (Th) of vapour (V) and liquid (L) CO2 range from — 54.2 to + 31.1 °C (95 % Th : L + V → L ; 5 % Th : L + V → V), yielding CO2 densities from 1.16 to 0.08 g/cm3 with preferential distribution near 1.11, 0.96, 0.89, 0.79 and 0.74 g/cm3. Assuming a trapping temperature of 1 000 °C, the corresponding trapping pressure for a pure CO2 system would lie in the range 0.2 to 11.5 kbar, i.e. a depth of 1 to 43 km. The CO2 fluid inclusions give evidence that the spinel lherzolite nodules originate in the upper mantle and represent accidental fragments randomly sampled by the host nepheline basalt magma. The distribution of CO2 densities reflects the succession of fracturing episodes during the ascent.

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 \(X_{{\text{CO}}_{\text{2}} } \)>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 δ13C 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.

Microfracture and crack healing in experimentally deformed peridotite

Secondary CO2 fluid inclusion arrays in peridotite xenoliths from basalt are formed by the healing of CO2‐filled microcracks. We have produced healed microcracks and deformation textures comparable to those in xenoliths by deforming peridotite ifi the presence of excess CO2 fluid at a constant strain rate of 10−5 s−1, a confirming pressure of 1.0–1.5 GPa, and a temperature of 800°–1050°C. Plastic and micro‐clastic deformation textures coexist, and healed microcracks extending out of (100) and (011) olivine deformation lamellae have the crystallographic orientation of microcracks nucleated on pileups of (100)[010] and (011)[100] edge dislocations. Healed microcracks in olivine grains without deformation lamellae tend to be in these orientations or in the orientation of microcracks nucleated on pileups of (110)[001] and (010)[001] edge dislocations. The same crystallographic orientation of healed microcracks is present in peridotite xenoliths from basalt. The steady flow stress data of the experiments without added CO2 fluid fit a power law with a stress exponent n of 3 and a thermal activation coefficient of 410 ± 50 kJ mol−1. The maximum stress that can be supported by samples with added CO2 fluid is 100–300 MPa less than the flow stress of samples without added CO2 fluid. The reduction in niechanical strength correlates with an increase in the concentration of intragranular healed microcracks and an increase in the proportion of healed microcracks inclined to the maximum compressive stress direction. Strength and microcrack characteristics of experiments with CO2 performed at 1.5 GPa are similar to experiments without CO2 at 1.0 GPa. The dependence of strength and microcrack concentration on confining pressure suggests that the principal effect of the CO2 fluid is to reduce the effective confining pressure and to promote microclastic deformation mechanisms. The similarity of experimental textures to xenoliths suggests that CO2 fluid may produce semibrittle mechanical behavior in the regions of the lower lithosphere where xenoliths originate.

Der Vredefort-Dom in S�dafrika

A meteorite impact origin of the Vredefort structure is in conflict with the results of recent geological, petrological, and fluid-inclusion studies. They indicate that the shock event was preceded by domal uplift and static thermal metamorphism within the dome rising to temperatures above 900 °C in the central region now exposed. Here heating has also continued after the shock and caused strong annealing recrystallization and nearly complete obliteration of the shock features in the former lower crustal granulites. High-density CO2 fluid inclusions are still aligned along shock-induced planar elements in quartz despite complete post-shock recrystallization of this mineral. Fluid pressures of 2–4 kbar at the time of trapping, that is shortlyafter the shock catastrophy, are not compatible with a near-surface cause of the shock by meteorite impact but may point to a deep-seated gas explosion that occurred as one episodic event during the long-term tectonic and thermal evolution of the dome. However, difficulties still exist to explain the high intensity of the shock and its pervasive character through internal terrestrial mechanisms.

Solid and fluid phases in lherzolite and pyroxenite inclusions from the Hoggar, central Sahara.

Major and trace elements have been determined in spinel lherzolite, spinel pyroxenite and garnet websterite xenoliths from the Hoggar and in their host basanites and nephelinites. Transition metal and REE abundances do not support a genetic link between these ultramafic rocks and the lavas erupted at the surface. Mineral chemistry and textures suggest a complex evolution and subsolidus re-equilibration in the range 800–1, 000°C. High-density CO2 fluid-inclusions define minimum pressures of 10–12kb. Thus the ultramafic xenoliths came from a relatively low pressure regime within an inhomogeneous lithospheric mantle segment characterized by a disturbed geotherm.

Stable isotope and fluid inclusion studies of gem-bearing granitic pegmatite-aplite dikes, San Diego Co., California

Late Cretaceous, granitic pegmatite-aplite dikes in southern California have been known for gem-quality minerals and as a commercial source of lithium. Minerals, whole-rock samples, and inclusion fluids from nine of these dikes and from associated wall rocks have been analyzed for their oxygen, hydrogen, and carbon isotope compositions to ascertain the origins and thermal histories of the dikes. Oxygen isotope geothermometry used in combination with thermometric data from primary fluid inclusions enabled the determination of the pressure regime during crystallization. Two groups of dikes are evident from their oxygen isotope compositions (δ18Oqtz≃+10.5 in Group A, and ≃+8.5 in Group B). Prior to the end of crystallization, Group A pegmatites had already extensively exchanged oxygen with their wall rocks, while Group B dikes may represent a closer approximation to the original isotopic composition of the pegmatite melts. Oxygen isotope fractionations between minerals are similar in all dikes and indicate that the pegmatites were emplaced at temperatures of about 730 ° to 700 ° C. Supersolidus crystallization began with the basal aplite zone and ended with formation of “quench aplite” in the pocket zone, nearly to 565 ° C. Subsolidus formation of gem-bearing pockets took place over a relatively narrow temperature range of about 40 ° C (approximately 565–525 ° C). Nearly closed-system crystallization is indicated. Hornblende in gabbroic and noritic wall rocks (δDw.r. = −90 to −130) in the Mesa Grande district crystallized in the presence of, or exchanged hydrogen with, meteoric water (δD≃ −90) prior to the emplacement of the pegmatite dikes. Magmatic water was subsequently added to the wall rocks adjacent to the pegmatites. Groups A and B pegmatites cannot be distinguished on the basis of their hydrogen isotope compositions. A decrease in δD of muscovite inward from the walls of the dikes reflects a decrease in temperature. δD values of H2O from fluid inclusions are: −50 to −73 (aplite and pegmatite zones); −62 to −75 (pocket quartz: Tourmaline Queen and Stewart dikes); and −50 ± 4 (pocket quartz from many dikes). The average δ13C of juvenile CO2 in fluid inclusions in Group B pegmatites is −7.9. In Group A pegmatities, δ13C of CO2 is more negative (−10 to −15.6), due to exchange of C with wall rocks and/or loss of 13C-enriched CO2 to an exsolving vapor phase. Pressures during crystallization of the pockets were on the order of 2,100 bars, and may have increased slightly during pocket growth. A depth of formation of at least 6.8 km (sp. gr. of over burden = 3.0, and P fiuid=P load) is indicated, and a rate of uplift of 0.07 cm/yr. follows from available geochronologic data.