CO2 Exsolution Research Articles

Abiotic processes dominate CO2 fluxes in Antarctic soils

Ecosystems within the McMurdo Dry Valleys of Antarctica are highly sensitive to environmental change. Increases in soil temperature and/or moisture content may dramatically change rates of soil respiration and soil carbon (C) turnover. Present estimates of soil respiration rates and C turnover times are based on surface carbon dioxide (CO2) fluxes and soil organic C content. However, the assumption that surface CO2 fluxes are purely biological in origin has not been rigorously tested. We use concentration and, for the first time, the stable C isotopic composition of surface soil CO2 fluxes and subsurface CO2 profiles to: 1) examine mechanisms of soil CO2 uptake and release, 2) identify the location of potential CO2 sources and sinks within the soil profile, and 3) discriminate between biotic and abiotic contributions to CO2 fluxes in soils of Taylor Valley. Surface CO2 fluxes and subsurface CO2 profiles confirm that these soils take up and release CO2 on a daily basis (during the austral summer), associated with small changes in soil temperature. Shifts in the C isotopic composition of soil CO2 are inconsistent with biological mechanisms of CO2 production and consumption. Instead, the isotopic shifts can be accounted for by Henry's Law dissolution and exsolution of CO2 into a solution of high pH, driven by changes in soil temperature. Our results constrain the biological component of soil CO2 fluxes in Taylor Valley to less than 25% (and likely to be significantly less). This finding implies that previous measurements of surface soil CO2 fluxes are overestimates of soil respiration, thus C turnover times calculated from them are underestimates. Discriminating between biotic and abiotic contributions to CO2 fluxes in Antarctic dry valley soils is essential if the effects of climate change on these sensitive ecosystems are to be accurately identified.

Geochemical Evolution of Groundwater in Meng Xu Area, Gui Ping City, Southern China

Geochemical data on dissolved major constituents in groundwater samples from Meng Xu area reveal the main processes responsible for their geochemical evolution. Inverse models were developed using PHREEQC to elucidate the chemical reactions controlling water chemistry. The inverse modeling demonstrated that relatively few phases are required to derive water chemistry in the area. In a broad sense, the reactions responsible for the hydro chemical evolution in the area fall into four categories: dissolution of CO2; calcite dissolution; incongruent dolomite dissolution, dedolomization, ex-solution of CO2, and ion exchange. Apart from this process, there are others which influence the geochemical zoning of the system. The interaction of the CO2 with the carbonate matrix of the aquifer produces an increase in the alkalinity of the water.

An Experimental Study of CO2 Exsolution and Relative Permeability Measurements During CO2 Saturated Water Depressurization

Dissolution of CO2 into brine is an important and favorable trapping mechanism for geologic storage of CO2. There are scenarios, however, where dissolved CO2 may migrate out of the storage reservoir. Under these conditions, CO2 will exsolve from solution during depressurization of the brine, leading to the formation of separate phase CO2. For example, a CO2 sequestration system with a brine-permeable caprock may be favored to allow for pressure relief in the sequestration reservoir. In this case, CO2-rich brine may be transported upwards along a pressure gradient caused by CO2 injection. Here we conduct an experimental study of CO2 exsolution to observe the behavior of exsolved gas under a wide range of depressurization. Exsolution experiments in highly permeable Berea sandstones and low permeability Mount Simon sandstones are presented. Using X-ray CT scanning, the evolution of gas phase CO2 and its spatial distribution is observed. In addition, we measure relative permeability for exsolved CO2 and water in sandstone rocks based on mass balances and continuous observation of the pressure drop across the core from 12.41 to 2.76 MPa. The results show that the minimum CO2 saturation at which the exsolved CO2 phase mobilization occurs is from 11.7 to 15.5%. Exsolved CO2 is distributed uniformly in homogeneous rock samples with no statistical correlation between porosity and CO2 saturation observed. No gravitational redistribution of exsolved CO2 was observed after depressurization, even in the high permeability core. Significant differences exist between the exsolved CO2 and water relative permeabilities, compared to relative permeabilities derived from steady-state drainage relative permeability measurements in the same cores. Specifically, very low CO2 and water relative permeabilities are measured in the exsolution experiments, even when the CO2 saturation is as high as 40%. The large relative permeability reduction in both the water and CO2 phases is hypothesized to result from the presence of disconnected gas bubbles in this two-phase flow system. This feature is also thought to be favorable for storage security after CO2 injection.

On the evolution of large ultramafic magma chambers and timescales for flood basalt eruptions

[1] Large igneous provinces are characterized by magmatic activity on two distinct timescales. While these provinces have total active lifetimes of order 10–30 Ma, most of the erupted volume is emplaced within 1 Ma in many cases. The longer timescale is consistent with plume or tectonic models for mantle melting responsible for flood volcanism, but the shorter “main stage” timescale is enigmatic. We hypothesize that cessation of main stage eruptions reflects shutoff of dike propagation from the deep crust due to the onset of thermally activated creep on a ∼1 Ma timescale, with intrusive processes and minor eruptions continuing over 10–30 Ma. To test this hypothesis we model deep magma differentiation and the stability of Moho level magma reservoirs. Assuming mantle volatile contents, fractionation results in concentration and deep exsolution of CO2, with the geothermal gradient and melt influx setting the timescale for buoyancy production and thus timing between individual eruptions. Chemical evolution generally occurs rapidly compared to heat conduction from the magma body. Although the viscous response of surrounding rocks depends on lower crustal rheology, we find that thermally induced creep can reasonably prevent dike propagation within 1 Ma of intrusion. However, if melt influx is high or heat transfer from the magma chamber is low, viscous creep may outpace differentiation. In this regime, continued melt influx spreads along the Moho until external stresses provide a destabilizing trigger. The coevolution of large-scale melting and lower crustal rheology may thus control a progression of large igneous province emplacement from largely eruptive to largely intrusive magmatism.

Influence of the March 11, 2011 M w 9.0 Tohoku-oki earthquake on regional volcanic activities

Two days after the March 11, 2011, M w 9.0 Tohoku-oki earthquake, Shinmoedake volcano, located on the Japanese island of Honshu, erupted. Was this eruption triggered by the Tohoku-oki earthquake? Could Mount Fuji and Changbaishan volcanoes also be triggered to erupt? By calculating changes in the regional stress-strain field that resulted from the earthquake, we find that Mount Fuji, Shinmoedake and Changbaishan volcanoes are all located in regions of volumetric expansion. The volumetric expansions at a depth of 10 km are up to ∼220 nano-strain, ∼8 nano-strain, and ∼14 nano-strain, respectively, for the three volcanoes. The strain changes inferred from GPS co-seismic displacements also suggest that these three volcanoes are located in regions with surface areal expansion. Considering that the expansional stress may cause the opening of magma channels, exsolution of CO2 gases stored in magma, and a series of positive feedback effects, the Tohoku-oki earthquake may result in an increase in the activity of these volcanoes. Attention should be paid to potential triggering of volcanic eruptions by stress changes induced by the Tohoku-oki earthquake.

Laboratory experiments on core-scale behavior of CO2 exolved from CO2 -saturated brine

Abstract During the process of carbon dioxide storage in deep saline aquifers a large quantity of brine will become saturated with dissolved CO2. Although saturated brine is denser than pure brine, there are several plausible mechanisms by which CO2-saturated brine could be transported to a shallower depth, where the CO2 would come out of solution (exsolve), forming a mobile gas phase. In this paper we describe preliminary results of laboratory experiments performed in order to observe the formation of exsolved CO2 as the pressure experienced by the core sample was gradually decreased from reservoir conditions (∼124 bars or 1800 psi). X-ray CT scanning was used to measure CO2 saturation and observe its spatial distribution within the core sample as a function of time and pressure.

Investigation of Cyclic Solvent Injection Process for Heavy Oil Recovery

Summary This paper summarizes numerical and experimental simulation results of a cyclic solvent injection process study, which was part of a continuing investigation into the use of solvents as a follow-up process in Cold Lake and Lloydminster reservoirs that have been pressure-depleted by cold heavy oil production with sand (CHOPS). Typically only 5% - 10% of the original oil in place (OOIP) is recovered during cold production; therefore, an effective follow-up process is required. The cyclic solvent injection (CSI) experiment consisted of primary production followed by six solvent (28% C3H8 - 72% CO2) injection cycles. Oil recovery after primary production and six solvent cycles was 50%, which indicates the potential viability of the CSI process. Concurrently with the laboratory physical simulation, a numerical simulation model was developed to represent the physical behaviour of the experimental results. A history match of the primary production portion of the experiment was obtained using an Alberta Innovates - Technology Futures (AITF) foamy oil model. This resulted in the characterization (fluid saturations and pressures) of the oil sandpack at the start of the solvent injection process. The history match of the subsequent six solvent injection cycles was used to validate the numerical model of the CSI process developed at AITF. This model includes nonequilibrium rate equations that simulated the delay in solvent reaching its equilibrium concentration as it dissolves or exsolves in the oil in response to changes in the pressure and/or gas-phase composition. Dissolution of CH4, C3H8 and CO2 in oil and CO2 in water were considered, as was exsolution of CH4, C3H8 and CO2 from oil and CO2 from water. Reduced gas-phase permeabilities resulting from gas exsolution were also included. The history match simulations indicated that: The important mechanisms were represented in the simulations. Significant oil swelling by solvent dissolution occurs during solvent injection periods. This can reduce solvent injectivity and penetration into a heavy oil reservoir during solvent injection periods. Low oil and gas-phase relative permeabilities are required during production periods to match the experimental oil and gas production during solvent cycles. A parametric simulation study showed that the quantity of gas injected in an injection period was relatively insensitive to the oil-phase diffusion coefficients, but was sensitive to solvent solubility in oil, dissolution rates, gas-phase diffusion coefficients, molar densities in the oil phase, gas-phase relative permeability and capillary pressure. It was shown that oil production is highly dependent on how quickly solvent can dissolve in the oil during injection and exsolve from the oil during production.

Geochemical evolution of groundwater in the carbonate aquifers of Sierra de Segura (Betic Cordillera, southern Spain)

Sierra de Segura (Betic Cordillera), with a total area of over 3000 km2, is the source of the two principal rivers in southern Spain, the Guadalquivir and the Segura. Due to the orographic effect of these mountains, precipitations are considerably more abundant than in nearby lowland areas, where the climate is semi-arid. Sierra de Segura is constituted of Mesozoic and Cenozoic sedimentary rocks, among which there are thick limestone–dolomitic formations which have given rise to extensive outcrops of permeable materials. In geomorphological terms, there is a large plateau intensively karstified that constitutes the main recharge area. Discharge takes place via a large number of springs, of which the 50 most important add up to a mean spring flow of about 13,500 l/s. The active geochemical processes in aquifers of Sierra de Segura, with their corresponding time sequence, are: dissolution of CO2, dissolution of calcite, incongruent dissolution of dolomite, dedolomitization, exsolution of CO2, and precipitation of calcite. More evolved water has higher temperature, magnesium content and Mg/Ca ratio; therefore, these parameters can be utilised as indicators of the degree of hydrochemical evolution. In addition, a good correlation between water temperature and magnesium concentration (or Mg/Ca ratio) indicates that an increase in temperature accelerates the kinetics of the dissolution of dolomite. Finally, the distribution of the temperatures in the vadose zone, determined by atmospheric thermal gradient, implies an apparent stratification of the predominant hydrochemical processes and of the groundwater physical and chemical characteristics.

Carbon Dioxide Discharged through the Las Cañadas Aquifer, Tenerife, Canary Islands

Carbon dioxide is one of the first gases to escape the magmatic environment due to its low solubility in basaltic magmas at low pressures. The exsolved CO2 gas migrates towards the surface through rock fractures and high permeability paths. If an aquifer is located between the magmatic environment and the surface, a fraction of the CO2 emitted is dissolved in the aquifer. In this paper, an estimation of the water mass balance and the CO2 budget in Las Canadas aquifer, Tenerife, Canary Islands, is presented. Magmatic CO2 is transported by groundwater and discharged through man-made sub-horizontal drains or galleries that exist in this island, and by the flow of groundwater discharged laterally towards other aquifers or to the ocean. In addition, the pCO2 at the gallery mouth (or entrance) and at the gallery bottom (internal and deepest discharge point where the gallery starts) are calculated and mapped. The total CO2 advectively transported by groundwater is estimated to range from 143 to 211 t CO2 d−1. Considering that the diffuse soil emission of CO2 for the same area is 437 t d−1, the diffuse/dissolved CO2 flux ratio varies between 2 and 3. The high dissolved inorganic carbon content of groundwater explains the ability of this low temperature hydrothermal water to dissolve and transfer magmatic CO2 at volcanoes, even during quiescence periods.

Study on the origin of the Hongshan brecciated copper deposit in Huichang County, Jiangxi Province, China

The Hongshan copper deposit is a typical cryptoexplosive breccia-type deposit, which occurs in a metamorphic rock series of the Mesoproterozoic Taoxiyuan Formation. Orebodies are distributed inside and outside porphyry-cryptoexplosive breccia pipes. The isotope geochemistry of the deposit is consistent with the origin of porphyry breccia: the δ18\(O_{H_2 O} \) values ranging from 1.2‰ to 6.1‰ and the δ34S values varying from 0 to 2.5‰. 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of pyrite, which coexists with ore minerals, indicate it was derived from the orogenic belt. Thermodynamic analysis indicates that the main metals were deposited largely as a result of the decreasing of proton concentrations associated with H2S and CO2 exsolution during explosion and temperature dropping. Based on K-Ar dating of quartz coexisting with ore minerals, the age of mineralization was estimated to be 97.1–98.8 Ma, which suggests that mineralization occurred between the Early and Late Cretaceous. According to the relevant information obtained, a diagenetic and metallogenic pattern in the area has been presented in this paper.

Constraints on the preeruptive volatile concentrations in the Columbia River flood basalts

Many of the Columbia River flood basalts that erupted in the states of Oregon, Washington, and Idaho during the Miocene are remarkably iron rich (12–17 wt% total FeO) and under volatile-free conditions would have been too dense to erupt through continental crust. Their eruption through >20 km of sialic crust sets limits on their minimum, preeruptive total volatile (H2O + CO2) concentrations. These limits are obtained by quantitatively comparing the average density of the overlying crustal column with the calculated effect of dissolved H2O or exsolved CO2 on the density of various Columbia River tholeiitic basalts as a function of depth. Minimum total volatile (H2O + CO2) concentrations range to >4 wt% and are therefore considerably higher than those found in tholeiitic basalts at oceanic islands, like Hawaii. These conclusions are consistent with the strong enrichments in other incompatible elements (e.g., K, P, Ba) observed in the Columbia River flood basalts. Such high concentrations of H2O + CO2 would have had a marked effect on the solidus temperature of the source region, which may explain the remarkably large individual flow volumes (∼103 km3) that characterize this flood-basalt province.

Heat transfer and melting in subglacial basaltic volcanic eruptions: implications for volcanic deposit morphology and meltwater volumes

Abstract Subglacial volcanic eruptions can generate large volumes of meltwater that is stored and transported beneath glaciers and released catastrophically in jökulhlaups. At typical basaltic dyke propagation speeds, the high strain rate at a dyke tip causes ice to behave as a brittle solid; dykes can overshoot a rock-ice interface to intrude through 20–30% of the thickness of the overlying ice. The very large surface area of the dyke sides causes rapid melting of ice and subsequent collapse of the dyke to form a basal rubble pile. Magma can also be intruded at the substrate-ice interface as a sill, spreading sideways more efficiently than a subaerial flow, and also producing efficient and widespread heat transfer. Both intrusion mechanisms may lead to the early abundance of meltwater sometimes observed in Icelandic subglacial eruptions. If meltwater is retained above a sill, continuous melting of adjacent and overlying ice by hot convecting meltwater occurs. At typical sill pressures under more than 300 m ice thickness, magmatic CO 2 gas bubbles form c. 25 vol% of the pressurized magma. If water drains and contact with the atmosphere is established, the pressure decreases dramatically unless the overlying ice subsides rapidly into the vacated space. If it does not, further CO 2 exsolution plus the onset of H 2 O exsolution has the potential to cause explosive fragmentation, i.e. a fire-fountain that forms at the dyke-sill connection, enhancing melting and creating another candidate pulse of meltwater. The now effectively subaerial magma body becomes thicker, narrower, and flows faster so that marginal meltwater drainage channels become available. If the ice overburden thickness is much less than c. 300m the entire sill injection process may involve explosive magma fragmentation. Thus, there should be major differences between subglacial eruptions under local or alpine glaciers compared with those under continental-scale glaciers.

Groundwater recharge and chemical evolution in the southern High Plains of Texas, USA

The unconfined High Plains (Ogallala) aquifer is the largest aquifer in the USA and the primary water supply for the semiarid southern High Plains of Texas and New Mexico. Analyses of water and soils northeast of Amarillo, Texas, together with data from other regional studies, indicate that processes during recharge control the composition of unconfined groundwater in the northern half of the southern High Plains. Solute and isotopic data are consistent with a sequence of episodic precipitation, concentration of solutes in upland soils by evapotranspiration, runoff, and infiltration beneath playas and ditches (modified locally by return flow of wastewater and irrigation tailwater). Plausible reactions during recharge include oxidation of organic matter, dissolution and exsolution of CO2, dissolution of CaCO3, silicate weathering, and cation exchange. Si and 14C data suggest leakage from perched aquifers to the High Plains aquifer. Plausible mass-balance models for the High Plains aquifer include scenarios of flow with leakage but not reactions, flow with reactions but not leakage, and flow with neither reactions nor leakage. Mechanisms of recharge and chemical evolution delineated in this study agree with those noted for other aquifers in the south-central and southwestern USA.

An exhumed palaeo‐hydrocarbon migration fairway in a faulted carrier system, Entrada Sandstone of SE Utah, USA

AbstractThe Moab Anticline, east‐central Utah, is an exhumed hydrocarbon palaeo‐reservoir which was supplied by hydrocarbons that migrated from the Moab Fault up‐dip towards the crest of the structure beneath the regional seal of the Tidwell mudstone. Iron oxide reduction in porous, high permeability aeolian sandstones records the secondary migration of hydrocarbons, filling of traps against small sealing faults and spill pathways through the Middle Jurassic Entrada Sandstone. Hydrocarbons entered the Entrada Sandstone carrier system from bends and other leak points on the Moab Fault producing discrete zones of reduction that extend for up to 400 m from these leak points. They then migrated in focused stringers, 2–5 m in height, to produce accumulations on the crest of the anticline. Normal faults on the anticline were transient permeability barriers to hydrocarbon migration producing a series of small compartmentalized accumulations. Exsolution of CO2 as local fault seals were breached resulted in calcite cementation on the up‐dip side of faults.Field observations on the distribution of iron oxide reduction and calcite cements within the anticline indicate that the advancing reduction fronts were affected neither by individual slip bands in damage zones around faults nor by small faults with sand: sand juxtapositions. Faults with larger throws produced either sand: mudstone juxtapositions or sand: sand contacts and fault zones with shale smears. Shale‐smeared fault zones provided seals to the reducing fluid which filled the structural traps to spill points.

O, H, C isotope geochemistry of carbonated mineral springs in central Victoria, Australia: sources of gas and water–rock interaction during dying basaltic volcanism

Waters discharging from carbonated cold mineral springs in the central Victorian Highlands of Australia have δ2H and δ18O values of −45 to −35‰ and −8 to −5.5‰, respectively, that suggest that they were derived from local meteoric water recharged under cooler climatic conditions than present. δ13C values of entrained CO2 (−10.4 to −7.0‰) and dissolved inorganic carbon (−3.9 to −0.1‰) imply that the C was probably derived from outgassing of Recent igneous rocks within the Newer Volcanic Province. The waters lie to the left of both local and global meteoric water lines due to CO2 exsolution from waters that were never involved in high-temperature water–rock interaction.

Deep sea explosive activity on the Mid-Atlantic Ridge near 34°50′N: Magma composition, vesicularity and volatile content

Submersible observations and sampling were carried out in the rift valley of the Mid-Atlantic Ridge (MAR) near 34°40′N–35°N. The 4-km-wide rift valley consists of a Neo Volcanic Zone (NVZ) (<1km wide) bounded at the west by a Median Ridge (MR) (5km wide and 20km long) and at the east by the first scarps of the eastern wall. The MR and the eastern wall are characterized by volcanic cones about 200–300m height culminating at depths of 1500–1900m which are made up of volcaniclastic deposits (pyroclasts and hyaloclasts) suggestive of explosive volcanism. Based on their surface morphology, degree of vesicularity, and composition, the erupted deposits are classified into four groups: (1) poorly vesicular (<15% vesicles) N- and T-MORBs (K/Ti <0.25, Na2O+K2O<2.9%) consisting of sheet flows and pillows formed during fissure eruptions in the NVZ at 2000–2300m depths; (2) vesicular (15–30% vesicles) E-MORBs (K/Ti=0.25−0.45,Na2O+K2O>2.8−3.2%) and alkali basalts (K/Ti=0.45−0.70,Na2O+K2O>3.3−4) made up mainly of pillows; (3) highly vesicular (>35% vesicles) pillow lava and pyroclastic (scoria-like) alkali basalts (K/Ti>0.45−0.80,Na2O+K2O>3−4%); and (4) hyaloclastites consisting of glassy shards of alkali basalt composition. The total water and carbon contents of the deposits increase with the incompatible element concentrations. The estimated initial H2O content for the N- and T-MORBs is less than 3500ppm, whereas for the E-MORBs and alkali basalts the H2O content is near 4000 and 7000ppm, respectively. While the H2O is mainly in the melt, the carbon is in the form of CO2 filling vesicles. The vesicles are formed from magma with an initial carbon content of 1000–3000 for the N- and T-MORBs, 3000–6500ppm for the E-MORBs and higher than 1wt% for the alkali basalts.The various lava types were derived from a heterogeneous mantle source composed of enriched and depleted components during sequential eruptions of N-, T- and E-MORBs and alkali basalts (K/Ti>0.7). The amount of CO2 and H2O in equilibrium with the dissolved species present in the vesicles indicates that CO2 (XCO2=1−0.84) was the main exsolved compound responsible for bubble nucleation. The increase in the degree of vesicularity and pressure of the volatile phases is mainly due to the early exsolution of CO2 from an alkali melt. The exsolution of significant amounts of dissolved water occurred only for the alkali basalt a few hundred meters beneath the seafloor and contributed to late bubble expansion. This subsequent addition of magmatic water to the vesicles increased the gas pressure and triggered explosions. An alternative hypothesis for the explosive volcanism is based on field observations. During crater collapsed, seawater could have been trapped in fractured volcanic conduits and later sealed by hydrothermal fluid circulation and precipitation. In such an environment, this seawater will be heated and vaporized during renewed magmatic upwelling. Both scenarios give rise to fragmented debris (hyaloclasts and pyroclasts) and the explosive events create turbulent flows followed by differential gravity settling of the particles (shards versus lapilli) through the seawater.

Claystone Constraints on Models of the Long-Term Chemical Evolution of Buffer Porewaters

Geochemical models of clay-water interaction are evaluated using data characterizing the hydrochemical, isotopic and mineralogical properties of a natural claystone that is similar to bentonite buffers that have evolved over long periods of time in a deep geologic repository for nuclear wastes. Model predictions in general compare favorably with the mineralogy and hydrochemistry of this formation if increases in pH due to partial CO2(g) exsolution from porewater samples is accounted for. This suggests the models can be used with enhanced confidence to accurately predict the chemistry of pore fluids in bentonite buffers, enabling more defensible estimates to be made of radioelement solubilities that partially define the source term in performance assessments. Modifications are needed, however, to improve the accuracy of thermodynamic data supporting the models and to account for potential effects on porewater compositions of reactions that are kinetically inhibited from attaining equilibrium over experimental time scales.

The San Venanzo maar and tuff ring, Umbria, Italy: eruptive behaviour of a carbonatite-melilitite volcano

The late Pleistocene San Venanzo maar and nearby Pian di Celle tuff ring in the San Venanzo area of Umbria, central Italy, appear to represent different aspects of an eruptive cycle accompanied by diatreme formation. Approximately 6x106 m3 of mostly lapillisized, juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted. The stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions. The pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion, roughly stratified lapilli tuff deposited by concentrated pyroclastic surge, chaotic scoriaceous pyroclastic flow and inverse graded grain-flow deposits. The key feature of the pyroclastics is the presence of concentric-shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits. The rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite, phonolite and calcitic melilitite pyroclasts. Juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin. Pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds. The San Venanzo maar-forming event is believed to have been trigered by fluid-rich carbonatite-phonolite magma. The eruptive centre the moved to the Pian di Celle tuff ring, where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area. In both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of CO2 which, flowing through the diatremes, produced the peculiar intrusive pyroclastic facies and triggered explosions.

A fluid inclusion study of vug minerals in dawsonite-altered phonolite sills, Montreal, Quebec: implications for HFSE mobility

Ordovician limestones in the Francon quarr on the island of Montreal, Quebec, are host to three sills of Cretaceous age composed of phonolite that has been extensively altered to dawsonite. An interesting feature of the sills is the presence of abundant vugs containing a wide variety of minerals, including several in which one or more high field strength elements (Zr, Hf, Nb, Ti) is a major component. The most important of these latter minerals is weloganite, a rare strontium zirconium hydrous carbonate, first identified in the Francon, quarry. Four types of inclusions have been recognized in vug minerals: aqueous, aqueous-carbonic, carbonic and solidbearing. Aqueous inclusions homogenize at temperatures mainly between 70° and 170°C and between 230° and 390°C. The homogenization temperatures of primary inclusions cluster around 350°C. Aqueous inclusions and the aqueous phase in aqueous-carbonic inclusions have salinities ranging between 10 and 24 eq.wt.% NaCl equivalent. Primary aqueous-carbonic inclusions have low XCO2 ( 0.7); carbonic inclusions are all secondary. Nahcolite, dawsonite and weloganite occur as daughter minerals or trapped solids. Nahcolite and possibly natron or mirabilite appear to form in frozen inclusions. Analyses of fluid inclusion decrepitates detected high concentrations of Na, Cl, Al, S, and C. The extraordinarily high concentration of Al in the fluid (possibly exceeding 1 wt.%) suggests a pH of approximately 10. Pressure and temperature conditions, estimated from stratigraphic reconstruction and the isochores of primary aqueous fluid inclusions, were 450 bar and 360 to 400°C, respectively. The relatively high temperatures and compositions of primary fluid inclusions suggest that vug filling was the result of mineral precipitation from an orthomagmatic fluid. A model is proposed in which a partially crystallized phonolite melt started exsolving a homogeneous low XCO2 fluid immediately prior to or after intrusion. Sodium, aluminium, chlorine, fluorine, sulphur and HFSE elements such as Zr, Hf, Nb and Ti were partitioned into the hydrous phase, in the case of Zr, possibly to a concentration of 300 ppm. The near horizontal orientation of the sills and the chilled margins, produced by quenching of the magma, created a tight seal that inhibited escape of the fluids. As a result, the phonolite “stewed in its own juices” long after crystallization, giving rise to widespread replacement of primary igneous minerals by dawsonite, and precipitation of this and other minerals in vugs. Once the sills had colled to temperatures between 200 and 300°C, the aqueous fluid exsolved a high CO2 fluid which was trapped as the secondary three-phase type II and type III inclusions. Decreasing temperature is considered to have been the principal control of mineralization, although in the case of the lower temperature minerals, decreased bicarbonate or carbonate ion activity, and a lower dielectric constant, as a result of CO2 exsolution, may have played a role in the deposition of HFSE-bearing minerals.

The Lake Nyos gas disaster: chemical and isotopic evidence in waters and dissolved gases from three Cameroonian crater lakes, Nyos, Monoun and Wum

Abstract To better understand the cause of the Nyos gas disaster of August 21, 1986, we conducted geochemical and limnological surveys in October 1986, of three lakes (Nyos, Monoun and Wum) which are located in the Cameroon volcanic zone that is characterized by a prevalence of young alkaline basalts and basanitoids. Lake Wum was studied as a non-active control: CO2 is dissolved in significant concentrations (about 1 5 of saturation) in gas-active lakes (Nyos and Monoun), but is virtually absent in Lake Wum. Stable isotopic ratios of total dissolved carbon (δ13C= −3% for Nyos and −5.5% for Monoun) and of helium (5.7 Ratm for Nyos and 3.6 Ratm for Monoun) indicate a mantle origin of these gases. However, SO42− and Cl concentrations are found to be very low. Concentrations of dissolved chemical species like Fe2+, Mg2+, Ca2+, and HCO3− are high in the two gas-active lakes, whereas they are very low in the gas-inactive lake. High salinities in the gas-active lakes are probably due to dissolution of indigenous mafic rocks and transported lateritic soil in acidic CO2-rich, warm water. The gas-active lakes are characterized by increasing temperature and salinity with increasing depth, indicating an active influx of heat and dissolved materials at the bottom. Density estimates show that the lake water is stably stratified in spite of the inverse temperature profile of the lakes, on account of dissolved chemical species. The concentrations of dissolved carbonate species (CO2(aq) and HCO3−) are positively correlated with those of ionic dissolved species, indicating their common occurrence in the bottom water. The August 1986 gas bursts from Lake Nyos were most likely caused by rapid exsolution of dissolved CO2 within the lake; an explosive process such as a phreatic eruption or a CO2 gas-jetting from beneath the bottom is unlikely because of low concentrations of Cl− and SO42−, no oxygen isotopic shift, low turbidity, and no reported perturbation of the bottom sediments. Exsolution of CO2 bubbles could occur if CO2-saturated bottom water was displaced upwards by an increased influx of high salinity water from the bottom during the rainy season. Exsolution of CO2 at the upper layers was possibly accelerated by upwelling of a two-phase fluid (CO2 bubbles and solution), a mechanism known as a pneumatic lift pump, resulting in discharge of a large amount of CO2 gas. The H2S concentration in the gas cloud must have been kept far below the lethal level because of a high Fe2+ concentration of the lake water.