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1.
The noble gases (He, Ne, Ar, Kr and Xe) are powerful geochemical tracers because they have distinctive isotopic compositions in the atmosphere, crust and mantle. This study illustrates how noble gases can be used to trace fluid origins in high-temperature metamorphic and mineralising environments; and at the same time provides new information on the composition of noble gases in deeper parts of the crust than have been sampled previously.We report data for H2O and CO2 fluid inclusions trapped at greenschist to amphibolite facies metamorphic conditions associated with three different styles of mineralisation and alteration in the Proterozoic Mt Isa Inlier of Australia. Sulphide fluid inclusions are dominated by crustal 4He. However, co-variations in fluid inclusion 20Ne/22Ne, 21Ne/22Ne, 40Ar/36Ar and 136Xe/130Xe indicate noble gases were derived from three or more reservoirs. In most cases, the fluid inclusions elemental noble gas ratios (e.g. Ne/Xe) are close to the ranges expected in sedimentary and crystalline rocks. However, the elemental ratios have been modified in some of the samples providing evidence for independent pulses of CO2, and interaction of CO2 with high-salinity aqueous fluids.Compositional variation is attributed to mixing of: (i) magmatic fluids (or deeply sourced metamorphic fluids) characterised by basement-derived noble gases with 20Ne/22Ne ∼ 8.4, 21Ne/22Ne ∼ 0.4, 40Ar/36Ar ∼ 40,000 and 136Xe/130Xe ∼ 8; (ii) basinal-metamorphic fluids with a narrow range of compositions including near-atmospheric values and (iii) noble gases derived from the meta-sedimentary host-rocks with 20Ne/22Ne ∼ 8-9.8, 21Ne/22Ne < 0.1, 40Ar/36Ar < 2500 and 136Xe/130Xe ∼ 2.2.These data provide the strongest geochemical evidence available for the involvement of fluids from two distinct geochemical reservoirs in Mt Isa’s largest ore deposits. In addition the data show how noble gases in fluid inclusions can provide information on fluid origins, the composition of the crust’s major lithologies, fluid-rock interactions and fluid-fluid mixing or immiscibility processes.  相似文献   

2.
Deep-sea exploration is rapidly improving our understanding of volatiles geochemistry in mid-ocean-ridge igneous products. It is also placing greater constraints on degassing processes of the Earth’s mantle, with the result that degassing models based on vapour-melt equilibrium are no longer able to explain the increasing number of data. In fact, such models force to postulate an upper mantle strongly heterogeneous at any scale, and cannot account for the widespread carbon supersaturation of the recovered igneous products. Here we review the global He-Ar-CO2 dataset of fluid inclusions in mid-ocean-ridge glasses using the framework of advanced modelling of multicomponent bubble growth in magmas. We display that non-equilibrium fractionations among He, Ar and CO2, driven by their different diffusivities in silicate melts, are common in most of the natural conditions of magma decompression and their signature strongly depends on pressure of degassing. Due to the comparable Ar and CO2 diffusivity, magma degassing at low pressure fractionates both the He/Ar and He/CO2 ratio by a similar extent, while the slower CO2 diffusion at high pressure causes early kinetic effects on Ar/CO2 ratio and dramatically changes the degassing path. On this ground, the very different geochemical signatures among suites of data coming from different ridge segments mainly depend on the depth of the magma chamber where the melt was stored. Besides, the variations inside a single suite highlight variable ascent speed and cooling rate of the emplaced lava. The large variations in both the He/CO2 and Ar/CO2 ratios at almost constant He/Ar, displayed in glasses coming from the Mid-Atlantic Ridge 24-30°N segment and the Rodriguez Triple Junction, are therefore interpreted as a high-pressure signature. In contrast, the simultaneous increase in both He/CO2 and He/Ar of the East Pacific Rise, Pito Seamount and South-East Indian Ridge data sets suggests the dominance of low-pressure fractionation, implying that the shallow magma chambers are at a lower depth than those of the Mid-Atlantic Ridge 24-30°N and Rodriguez Triple Junction. Our conclusions support the presence of a relationship between spreading rate and depth of high-temperature zones below ridges, and are consistent with the depth of magma chambers as suggested from seismic studies. Non-equilibrium degassing explains the volatile systematics of mid-ocean-ridge basalts by starting from a single mantle-derived magma, dispensing with the supposed need for heterogeneities in abundance ratios of volatiles in the mantle below oceanic ridges.  相似文献   

3.
We report new stepped heating He, Ar, CO2 and water data on a petrogenetically diverse suite of lavas from the Manus back-arc basin, where a plume component has previously been identified. The aim of this study is to evaluate the superimposed effects of degassing and contamination in order to identify mantle source characteristics. CO2 abundances and carbon isotopes in both the vesicle ([CO2] up to 180ppm; δ13C as low as -33.6 ‰) and glass ([CO2] up to 270ppm; δ13C as low as -34.3 ‰) phases reveal that samples have been modified by varying degrees of degassing. High water concentration samples (back-arc basin basalts (BABB) and arc type samples) show the highest degrees of degassing (i.e. lower δ13C values and lower CO2 contents). The results are modelled for both the glass and vesicle phases using batch and fractional degassing models. Parental melt compositions can be constrained to show the following CO2 concentration trend: arc-type > BABB s.r. (southern rift) > MORB-2, E-MORB, X-BABB (extreme BABB), BABB > MORB-1 and MORB-smt. 4He/40Ar∗ ratios of samples (14.6-1100) are consistent with residual volatiles from a degassed source. Variations in CO2/3He values are likely due to degassing, followed by contamination from a crustal source (either the subducting Solomon Sea Plate or the pre-existing crust through which the lavas erupt), as evidenced by high K2O/TiO2 ratios and low δ13C. The CO2/3He of the Manus plume is best estimated by the MORB-smt and MORB-1 samples at 3.1 ± 0.6 x 109. This value is similar to previous estimates of plume CO2/3He values, which are either equal to or slightly greater than the upper mantle average of 2 x 109.  相似文献   

4.
In the Czech-German border region of the Vogtland and NW Bohemia (western Eger rift, Central Europe), chemical and isotopic compositions (C, N, He, Ar) of free gas from a thermal water escape (fluorite mine, Schönbrunn), two mineral springs (“Eisenquelle,” Bad Brambach; “Sprudel III,” Bad Elster) and a mofette (Bublak) located along an ∼40-km long traverse are reported. The gases of Bublak and Bad Brambach are CO2-rich (>99 vol.%) and have δ13C values of −1.95 and −4.29‰, respectively. With distance from the center of CO2 degassing (Bublak) the δ13C values decrease, most likely due to physico-chemical fractionation of CO2 between gaseous and aqueous phases rather than to admixture of organic/biogenic CO2. The δ15N values range between −3.2 and −0.6‰, compared to an upper mantle value of −4.0 ± 1.0‰. The four locations are characterized by 3He/4He ratios decreasing from 5.9 Ra in the center (Bublak) to 0.8 Ra in the periphery (Schönbrunn) and give evidence for mixing of He from a deep-seated magmatic source with a crustal source. The location with the highest 3He/4He ratio (5.9 Ra) is accompanied by the highest 40Ar/36Ar (550). We argue that the nitrogen of the Bublak mofette gas is a mixture of predominantly atmospheric and mantle-derived components, whereas at the other three locations crustal nitrogen may also be present. The Bublak δ15N value of ≈−4.5 ± 1.0‰ represents the first free gas δ15N reference from the European subcontinental mantle (ESCM) and indicates that, in contrast to the 3He/4He ratios, the δ15N values are equal for ESCM and MORB, respectively.  相似文献   

5.
《Applied Geochemistry》1998,13(4):441-449
Noble gas elemental and isotopic compositions have been measured as well as the abundance of C and its isotopic ratios in 11 glasses from submarine pillow basalts collected from the Mariana Trough. The 3He/4He ratios of 8.22 and 8.51 Ratm of samples dredged from the central Mariana Trough (∼18°N) agree well with that of the Mid-Ocean Ridge Basalt (MORB) glasses (8.4±0.3 Ratm), whereas a mean ratio of 8.06±0.35 Ratm in samples from the northern Mariana Trough (∼20°N) is slightly lower than those of MORB. One sample shows apparent excess of 20Ne and 21Ne relative to atmospheric Ne, suggesting incorporation of solar-type Ne in the magma source. There is a positive correlation between 3He/4He and 40Ar/36Ar ratios, which may be explained by mixing between MORB-type and atmospheric noble gases. Excess 129Xe is observed in the sample which also shows 20Ne and 21Ne excesses. Observed δ13C values of ∼20°N samples vary from −3.76‰ to −2.80‰, and appear higher than those of MORB, and the corresponding CO2/3He ratios are higher than those of MARA samples at ∼18°N, suggesting C contribution from the subducted slab.  相似文献   

6.
During the Devonian magmatism (370 Ma ago) ∼20 ultrabasic-alkaline-carbonatite complexes (UACC) were formed in the Kola Peninsula (north-east of the Baltic Shield). In order to understand mantle and crust sources and processes having set these complexes, rare gases were studied in ∼300 rocks and mineral separates from 9 UACC, and concentrations of parent Li, K, U, and Th were measured in ∼70 samples. 4He/3He ratios in He released by fusion vary from pure radiogenic values ∼108 down to 6 × 104. The cosmogenic and extraterrestrial sources as well as the radiogenic production are unable to account for the extremely high abundances of 3He, up to 4 × 10−9 cc/g, indicating a mantle-derived fluid in the Kola rocks. In some samples helium extracted by crushing shows quite low 4He/3He = 3 × 104, well below the mean ratio in mid ocean ridge basalts (MORB), (8.9 ± 1.0) × 104, indicating the contribution of 3He-rich plume component. Magnetites are principal carriers of this component. Trapped 3He is extracted from these minerals at high temperatures 1100°C to 1600°C which may correspond to decrepitation or annealing primary fluid inclusions, whereas radiogenic 4He is manly released at a temperature range of 500°C to 1200°C, probably corresponding to activation of 4He sites degraded by U, Th decay.Similar 4He/3He ratios were observed in Oligocene flood basalts from the Ethiopian plume. According to a paleo-plate-tectonic reconstruction, 450 Ma ago the Baltica (including the Kola Peninsula) continent drifted not far from the present-day site of that plume. It appears that both magmatic provinces could relate to one and the same deep-seated mantle source.The neon isotopic compositions confirm the occurrence of a plume component since, within a conventional 20Ne/22Ne versus 21Ne/22Ne diagram, the regression line for Kola samples is indistinguishable from those typical of plumes, such as Loihi (Hawaii). 20Ne/22Ne ratios (up to 12.1) correlate well with 40Ar/36Ar ones, allowing to infer a source 40Ar/36Ar ratio of about 4000 for the mantle end-member, which is 10 times lower than that of the MORB source end-member. In (3He/22Ne)PRIM versus (4He/21Ne)RAD plot the Kola samples are within array established for plume and MORB samples; almost constant production ratio of (4He/21Ne)RAD ≅ 2 × 107 is translated via this array into (3He/22Ne)PRIM ∼ 10. The latter value approaches the solar ratio implying the non-fractionated solar-like rare gas pattern in a plume source.The Kola UACC show systematic variations in the respective contributions of in situ-produced radiogenic isotopes and mantle-derived isotopes. Since these complexes were essentially plutonic, we propose that the depth of emplacement exerted a primary control on the retention of both trapped and radiogenic species, which is consistent with geological observations. The available data allow to infer the following sequence of processes for the emplacement and evolution of Kola Devonian UACC: 1) Ascent of the plume from the lower mantle to the subcontinental lithosphere; the plume triggered mantle metasomatism not later than ∼700 to 400 Ma ago. 2) Metasomatism of the lithosphere (beneath the central part of the Kola Peninsula), including enrichment in volatile (e.g., He, Ne) and in incompatible (e.g., U, Th) elements. 3) Multistage intrusions of parental melts, their degassing, and crystallisation differentiation ∼370 Ma ago. 4) Postcrystallisation migration of fluids, including loss of radiogenic and of trapped helium. Based on model compositions of the principle terrestrial reservoirs we estimate the contributions (by mass) of the plume material, the upper mantle material, and the atmosphere (air-saturated groundwater), into the source of parent melt at ∼2%, 97.95%, and ∼0.05%, respectively.  相似文献   

7.
We present new data on mineralogical, major and trace element compositions of lavas from the northernmost segment of the Kolbeinsey Ridge (North Kolbeinsey Ridge, NKR). The incompatible element enriched North Kolbeinsey basalts lie on a crystal fractionation trend which differs from that of the other Kolbeinsey segments, most likely due to higher water contents (~0.2%) in the NKR basalts. The most evolved NKR magmas erupt close to the Jan Mayen Fracture Zone, implying increased cooling and fractionation of the ascending magmas. Mainly incompatible element-enriched basalts, as well as some slightly depleted lavas, erupt on the NKR. They show evidence for mixing between different mantle sources and magma mixing. North Kolbeinsey Ridge magmas probably formed by similar degrees of melting to other Kolbeinsey basalts, implying that no lateral variation in mantle potential temperature occurs on the spreading axis north of the Iceland plume and that the Jan Mayen Fracture Zone does not have a cooling effect on the mantle. Residual garnet from deep melting in garnet peridotite or from enriched garnet pyroxenite veins does not play a role. The incompatible element-enriched source has high Ba/La and Nb/Zr, but must be depleted in iron. The iron-depleted mantle is less dense than surrounding mantle and leads to the formation of the North Kolbeinsey segment and its shallow bathymetry. The enriched NKR source formed from a relatively refractory mantle, enriched by a small degree melt rather than by recycling of enriched basaltic crust. The depleted mantle source resembles the mantle of the Middle Kolbeinsey segment with a depletion in incompatible elements, but a fertile major element composition.  相似文献   

8.
We present new He-Ne data for geothermal fluids and He-Ne-Ar data for basalts from throughout the Icelandic neovolcanic zones and older parts of the Icelandic crust. Geothermal fluids, subglacial glasses, and mafic phenocrysts are characterized by a wide range in helium isotope ratios (3He/4He) encompassing typical MORB-like ratios through values as high as 36.8 RA (where RA = air 3He/4He). Although neon in geothermal fluids is dominated by an atmospheric component, samples from the northwest peninsula show a small excess of nucleogenic 21Ne, likely produced in-situ and released to circulating fluids. In contrast, geothermal fluids from the neovolcanic zones show evidence of a contribution of mantle-derived neon, as indicated by 20Ne enrichments up to 3% compared to air. The neon isotope composition of subglacial glasses reveals that mantle neon is derived from both depleted MORB-mantle and a primordial, ‘solar’ mantle component. However, binary mixing between these two endmembers can account for the He-Ne isotope characteristics of the basalts only if the 3He/22Ne ratio of the primordial mantle endmember is lower than in the MORB component. Indeed, the helium to neon elemental ratios (4He/21Ne∗ and 3He/22Nes where 21Ne∗ = nucleogenic 21Ne and 22Nes = ‘solar’-derived 22Ne) of the majority of Icelandic subglacial glasses are lower than theoretical values for Earth’s mantle, as observed previously for other OIB samples. Helium may be depleted relative to neon in high-3He/4He ratio parental melts due to either more compatible behavior during low-degree partial melting or more extensive diffusive loss relative to the heavier noble gases. However, Icelandic glasses show higher 4He/40Ar∗ (40Ar∗ = radiogenic Ar) values for a given 4He/21Ne∗ value compared to the majority of other OIB samples: this observation is consistent with extensive open-system equilibrium degassing, likely promoted by lower confining pressures during subglacial eruptions of Icelandic lavas. Taken together, the He-Ne-Ar systematics of Icelandic subglacial glasses are imprinted with the overlapping effects of helium depletion in the high-3He/4He ratio parental melt, binary mixing of two distinct mantle components, degassing fractionation and interaction with atmospheric noble gases. However, it is still possible to discern differences in the noble gas characteristics of the Icelandic mantle source beneath the neovolcanic zones, with MORB-like He-Ne isotope features prevalent in the Northern Rift Zone and a sharp transition to more primitive ‘solar-like’ characteristics in central and southern Iceland.  相似文献   

9.
The Hugoton-Panhandle giant gas field, located across SW Kansas and the Texas and Oklahoma panhandles in the USA, is the case type example of high nitrogen concentrations in a natural gas being linked with high helium concentrations. We collected 31 samples from producing wells in a north-south traverse of the 350-km-long field. The samples reflect the previously observed north-south change in 4He/N2, with values changing from 0.020 to 0.049 respectively. 3He/4He, 21Ne/22Ne, and 40Ar/36Ar vary between 0.14-0.25 Ra, 0.0373-0.0508, and 818-1156 respectively, and are caused by quantifiable contributions from mantle, crustal, and atmosphere-derived sources. The atmosphere-derived 20Ne/36Ar ratios are indistinguishable from groundwater values. The crustal 4He/21Ne* and 4He/40Ar* ratios show a 60% excess of 4He compared to predicted production ratios in the crust and are typical of noble gases released from the shallow crust. The mantle 3He/N2 and groundwater-recharge 36Ar/N2 ratios enable us to rule out significant magmatic or atmosphere contributions to the gas field N2, which is dominantly crustal in origin.Correlated 20Ne/N2 and 4He/N2 shows mixing between two distinct crustal N2 components. One N2 component (N2*) is associated with the crustal 4He and groundwater-derived 20Ne, and the other with no resolvable noble gas contribution. Measured δ15NN2 values vary from +2.7‰ to +9.4‰. The N2* and non-He-associated N2 endmembers are inferred to have δ15NN2 = −3‰ and +13‰ and contribute from between 25-60% and 75-40% of the nitrogen respectively. The non-He-associated nitrogen is probably derived from relatively mature organic matter in the sedimentary column. The δ15NN2* value is not compatible with a crystalline or high-grade metamorphic source and, similar to the 4He, is inferred to be from a shallow or low metamorphic-grade source rock. 4He mass balance requires a regional crustal source, its association with significant magmatic 3He pointing to a tectonically active source to the west of the Hugoton system. The volume of groundwater required to source the 20Ne in the gas field demonstrates the viability of the groundwater system in providing the collection, transport, and focusing mechanism for the 4He and N2*. The N2*/20Ne ratio shows that the N2* transport must be in the aqueous phase, and that the degassing mechanism is probably contact between the regional groundwater system and the preexisting reservoir hydrocarbon gas phase.  相似文献   

10.
Chemical and isotopic compositions have been measured for N2-He-rich bubbling gases discharging from hot springs in the Hainan Island, Southern China. Observed 3He/4He ratios (0.1–1.3 RA) indicate the occurrence of a mantle component throughout the island, which has been highly diluted by a crustal radiogenic 4He component. The occurrence of mantle-derived helium is high in the northern island (12%–16% of total He) and gradually decreases towards southern coast (1%–3% of total He). Such a distribution pattern is most likely controlled by the Pleocene-Quaternary volcanic activities in the northern island and groundwater circulation along the deep major faults. The 40Ar/36Ar and N2/Ar ratios suggest that N2 and Ar of the hot spring gases are mostly meteoric. Although δ13C values of CO2 (–20‰ to –27‰) with low concentrations are consistent with the biogenic origin, the combination of 3He/4He and d13CCO2 suggests a two end-member mixing of mantle and crustal components with CO2/3He ratios of 2×109 and 8×1011, respectively. However, the low CO2/3He ratios (1–22×106) can not be ascribed in terms of the simple mixing but has to be explained by the addition of radiogenic 4He and loss of CO2 by calcite precipitation in the hydrothermal system, which is most likely controlled by the degree of gas-water-rock interaction.  相似文献   

11.
Oxygen isotope ratios have been determined using laser fluorination techniques on olivine and plagioclase phenocrysts and bulk glasses from the Reykjanes Ridge and Iceland. δ18O in Reykjanes Ridge olivines shows hyperbolic correlations with Sr-Nd-Pb isotope ratios that terminate at δ18O = +4.5‰ at compositions almost identical to those of moderately enriched lavas on the Reykjanes Peninsula, Iceland. Samples with low δ18O show no indication of contamination by oceanic crust such as elevated Cl/K, and are too deep to have been influenced by meteoric water hydrothermal systems. They cannot represent Icelandic melts contaminated in the crust and transferred laterally along the ridge since fissure systems are strongly oblique to the ridge axis. It follows that Icelandic mantle advected along the ridge has low δ18O. The hyperbolic 143Nd/144Nd-δ18O correlation appears to be more strongly curved than magma mixing trajectories and suggests that melt fractions are ∼4.5× greater and source Nd contents ∼9× greater in the mantle at 63°N compared with that at 60°N. Primitive lavas from the Reykjanes Peninsula show linear correlations between olivine δ18O and 143Nd/144Nd or 206Pb/204Pb, extending to δ18O of +4.3‰ at 143Nd/144Nd close to the lowest ratios observed in Icelandic magmas. These correlations cannot be produced by melt mixing or crustal contamination because these would yield strongly hyperbolic trajectories. Lower δ18O seen in more evolved samples from the Eastern Rift Zone may reflect crustal contamination, though there is some evidence of a mantle source with lower δ18O in eastern Iceland. It is very difficult to explain the low δ18O of enriched Icelandic mantle sources on current understanding of mantle and crustal oxygen isotopes. There is no obvious reason why such low-δ18O sources should not contribute to other ocean islands. No oceanic crustal lithologies exist that could produce the low-δ18O enriched sources by recycling into the mantle, and there is no evidence for changes in δ18O of ophiolite suites with time, nor of changes during high-P metamorphism. Low δ18O appears to be associated with high 3He/4He, and we speculate that this signature may be characteristic of the host mantle into which ocean crust was recycled.  相似文献   

12.
Here we present the first data on He, Ne, Ar isotopic and elemental composition in fluid phases of tholeiitic chilled glasses from the Bouvet Triple Junction (BTJ). The chilled glasses from several dredging stations situated at different segments of BTJ have been investigated: Spiess Ridge, Mid Atlantic Ridge (MAR) and in a valley of the Southwest Indian Ridge (SWIR). The data allow to distinguish within BTJ three segments characterized by different geochemical behavior of He, Ne and Ar. MAR and Spiess samples contain MORB-like helium and neon while SWIR is characterized by addition of plume type He and Ne. The strong atmospheric contamination is typical of all segments, but for MAR it is less pronounced. The Ne-Ar isotope systematics suggests that the atmospheric component was most probably introduced into the mantle source of the fluids with fragments of oceanic crust/sediments.  相似文献   

13.
Argon analyses by both high-resolution stepheating and stepcrushing of MORB and Loihi basalt glasses were performed to separate pristine mantle-derived Ar and contaminating atmospheric Ar. In high-vesicularity glasses (> 0.8% vesicles), most of the mantle argon resides in vesicles, from which it is released by crushing or stepheating between 600 and 900 °C. By contrast, in low vesicularity glasses (< permil vesicularity), most mantle argon is dissolved in the glass matrix, as inferred from the correlation with neutron-induced, glass-dissolved argon isotopes (39Ar, 37Ar, 38Ar from K, Ca, Cl). The distribution of mantle Ar between vesicles and glass matrix is well explained by melt-gas equilibrium partitioning at eruption according to Henry’s law, which is compatible with previously determined Henry constants of ∼(5-10) × 10−5ccSTP 40Ar mantle/g bar. Atmospheric Ar is heterogeneously distributed in all samples. Only a very minor part is dissolved in the glass matrix; a significant part correlates with vesicularity and is released by crushing, most probably from a rather small fraction of vesicles or microcracks that equilibrated with unfractionated air. Other carriers of atmospheric argon are pyroxene microlites and minor phases decomposing at intermediate temperatures that were probably contaminated upon eruption by fractionated atmospheric rare gases. Our high-resolution stepheating and stepcrushing analyses of low vesicularity samples with extraordinary high solar-like 20Ne/22Ne indicate successful discrimination of unfractionated air as a contamination source and suggest an upper mantle 40Ar/36Ar of 32,000 ± 4000 and a Hawaiian mantle plume source 40Ar/36Ar ratio close to 8000.  相似文献   

14.
We describe and compare the two transform zones that connect the Icelandic rift segments and the mid-Atlantic Ridge close to the Icelandic hot spot, in terms of geometry of faulting and stress fields. The E–W trending South Iceland Seismic Zone is a diffuse shear zone with a Riedel fault pattern including N0°–N20°E trending right-lateral and N60°–N70°E trending left-lateral faults. The dominant stress field in this zone is characterised by NW–SE extension, in general agreement with left-lateral transform motion. The Tjörnes Fracture Zone includes three major lineaments at different stages of development. The most developed, the Húsavík–Flatey Fault, presents a relatively simple geometry with a major fault that trends ESE–WNW. The stress pattern is however complex, with two dominant directions of extension, E–W and NE–SW on average. Both these extensions are compatible with the right-lateral transform motion and reveal different behaviours in terms of coupling. Transform motion has unambiguous fault expression along a mature zone, a situation close to that of the Tjörnes Fracture Zone. In contrast, transform motion along the immature South Iceland Seismic Zone is expressed through a more complicate structural pattern. At the early stage of the transform process, relatively simple stress patterns prevail, with a single dominant stress field, whereas, when the transform zone is mature, moderate and low coupling situations may alternate, as a function of volcanic–tectonic crises and induce changes in stress orientation.  相似文献   

15.
Chemical and isotopic compositions have been measured for CO2-rich bubbling gases discharging from cold springs in Wudalianchi intra-plate volcanic area, NE China. Observed 3He/4He ratios (2–3 RA) and δ13C values of CO2 (−5‰ to −3‰) indicate the occurrence of a mantle component released and transferred to the surface by the Cenozoic extension-related magmatic activities. The CO2/3He ratios are in wide range of (0.4–97 × 109). Based on the apparent mixing trend in a 3He/4He and δ13C of CO2 diagram from all published data, the extracted magmatic end-member in the Wudalianchi Volcano has 3He/4He, δ13C and CO2/3He value of ∼3.2 RA, ∼−4.6‰ and ∼6 × 1010, respectively. These values suggest that the volatiles originate from the sub-continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events, as revealed by geothermal He and CO2 from the adjacent Changbaishan volcanic area.  相似文献   

16.
Identification of the source of CO2 in natural reservoirs and development of physical models to account for the migration and interaction of this CO2 with the groundwater is essential for developing a quantitative understanding of the long term storage potential of CO2 in the subsurface. We present the results of 57 noble gas determinations in CO2 rich fields (>82%) from three natural reservoirs to the east of the Colorado Plateau uplift province, USA (Bravo Dome, NM., Sheep Mountain, CO. and McCallum Dome, CO.), and from two reservoirs from within the uplift area (St. John’s Dome, AZ., and McElmo Dome, CO.). We demonstrate that all fields have CO2/3He ratios consistent with a dominantly magmatic source. The most recent volcanics in the province date from 8 to 10 ka and are associated with the Bravo Dome field. The oldest magmatic activity dates from 42 to 70 Ma and is associated with the McElmo Dome field, located in the tectonically stable centre of the Colorado Plateau: CO2 can be stored within the subsurface on a millennia timescale.The manner and extent of contact of the CO2 phase with the groundwater system is a critical parameter in using these systems as natural analogues for geological storage of anthropogenic CO2. We show that coherent fractionation of groundwater 20Ne/36Ar with crustal radiogenic noble gases (4He, 21Ne, 40Ar) is explained by a two stage re-dissolution model: Stage 1: Magmatic CO2 injection into the groundwater system strips dissolved air-derived noble gases (ASW) and accumulated crustal/radiogenic noble gas by CO2/water phase partitioning. The CO2 containing the groundwater stripped gases provides the first reservoir fluid charge. Subsequent charges of CO2 provide no more ASW or crustal noble gases, and serve only to dilute the original ASW and crustal noble gas rich CO2. Reservoir scale preservation of concentration gradients in ASW-derived noble gases thus provide CO2 filling direction. This is seen in the Bravo Dome and St. John’s Dome fields. Stage 2: The noble gases re-dissolve into any available gas stripped groundwater. This is modeled as a Rayleigh distillation process and enables us to quantify for each sample: (1) the volume of groundwater originally ‘stripped’ on reservoir filling; and (2) the volume of groundwater involved in subsequent interaction. The original water volume that is gas stripped varies from as low as 0.0005 cm3 groundwater/cm3 gas (STP) in one Bravo Dome sample, to 2.56 cm3 groundwater/cm3 gas (STP) in a St. John’s Dome sample. Subsequent gas/groundwater equilibration varies within all fields, each showing a similar range, from zero to ∼100 cm3 water/cm3 gas (at reservoir pressure and temperature).  相似文献   

17.
We determined total CO2 solubilities in andesite melts with a range of compositions. Melts were equilibrated with excess C-O(-H) fluid at 1 GPa and 1300°C then quenched to glasses. Samples were analyzed using an electron microprobe for major elements, ion microprobe for C-O-H volatiles, and Fourier transform infrared spectroscopy for molecular H2O, OH−, molecular CO2, and CO32−. CO2 solubility was determined in hydrous andesite glasses and we found that H2O content has a strong influence on C-O speciation and total CO2 solubility. In anhydrous andesite melts with ∼60 wt.% SiO2, total CO2 solubility is ∼0.3 wt.% at 1300°C and 1 GPa and total CO2 solubility increases by about 0.06 wt.% per wt.% of total H2O. As total H2O increases from ∼0 to ∼3.4 wt.%, molecular CO2 decreases (from 0.07 ± 0.01 wt.% to ∼0.01 wt.%) and CO32− increases (from 0.24 ± 0.04 wt.% to 0.57 ± 0.09 wt.%). Molecular CO2 increases as the calculated mole fraction of CO2 in the fluid increases, showing Henrian behavior. In contrast, CO32− decreases as the calculated mole fraction of CO2 in the fluid increases, indicating that CO32− solubility is strongly dependent on the availability of reactive oxygens in the melt. These findings have implications for CO2 degassing. If substantial H2O is present, total CO2 solubility is higher and CO2 will degas at relatively shallow levels compared to a drier melt. Total CO2 solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H2O contents (<1 wt.%). We found that total CO2 solubility is negatively correlated with (Si + Al) cation mole fraction and positively correlated with cations with large Gibbs free energy of decarbonation or high charge-to-radius ratios (e.g., Ca). Combining our andesite data with data from the literature, we find that molecular CO2 is more abundant in highly polymerized melts with high ionic porosities (>∼48.3%), and low nonbridging oxygen/tetrahedral oxygen (<∼0.3). Carbonate dominates most silicate melts and is most abundant in depolymerized melts with low ionic porosities, high nonbridging oxygen/tetrahedral oxygen (>∼0.3), and abundant cations with large Gibbs free energy of decarbonation or high charge-to-radius ratio. In natural silicate melt, the oxygens in the carbonate are likely associated with tetrahedral and network-modifying cations (including Ca, H, or H-bonds) or a combinations of those cations.  相似文献   

18.
《Applied Geochemistry》2001,16(4):419-436
The chemical and isotopic compositions of gases from hydrocarbon systems of the Taranaki Basin of New Zealand (both offshore and onshore) show wide variation. The most striking difference between the western and south-eastern groups of gases is the helium content and its isotopic ratio. In the west, the Maui gas is over an order of magnitude higher in helium concentration (up to 190 μmol mol−1) and its 3He/4He ratio of 3.8 RA (where RA=the air 3He/4He ratio of 1.4×10−6) is approximately half that of upper mantle helium issuing from volcanic vents of the Taupo Volcanic Zone. In the SE, the Kupe South and most Kapuni natural gases have only a minor mantle helium input of 0.03–0.32 RA and low total helium concentrations of 10–19 μmol mol−1. The 3He/C ratio (where C represents the total carbon in the gas phase) of the samples measured including those from a recent study of on-shore Taranaki natural gases are generally high at locations where the surface heat flow is high. The 3He/CO2 ratio of the Maui gases of 5 to 18×10−9 is higher than the MORB value of 0.2 to 0.5×10−9, a feature found in other continental basins such as the Pannonian and Vienna basins and in many high helium wells in the USA. Extrapolation to zero CO2/3He and CO2/C indicates δ13C(CO2) values between −7 and −5‰ close to that of MORB CO2. The remaining CO2 would appear to be mostly organically-influenced with δ13C(CO2) c.−15‰. There is some evidence of marine carbonate CO2 in the gases from the New Plymouth field. The radiogenic 4He content (Herad) varies across the Taranaki Basin with the highest Herad/C ratios occurring in the Maui field. δ13C(CH4) becomes more enriched in 13C with increasing Herad and hydrocarbon maturity. Because 3He/4He is related to the ratio of mantle to radiogenic crustal helium and 3He/C is virtually constant in the Maui field, there is a correlation between RC/RA (where RC=air-corrected 3He/4He) and δ13C(CH4) in the Maui and New Plymouth fields, with the more negative δ13C(CH4) values corresponding to high 3He/4He ratios. A correlation between 3He/4He and δ13C(CO2) was also observed in the Maui field. In the fields adjacent to Mt Taranaki (2518 m andesitic volcano), correlations of some parameters, particularly CO2/CH4, C2H6/CH4 and δ13C(CH4), are present with increasing depth of the gas reservoir and with distance from the volcanic cone.  相似文献   

19.
A Late Paleocene (∼60 Ma BP) lateritic soil from Northern Ireland (the Antrim paleosol, herein referred to as Nire) contains coexisting goethite, gibbsite, phyllosilicate, and hematite. The Fe(III) oxides exhibit pisolitic and Liesegang-type morphologies that are mutually exclusive in hand specimens. X-ray diffraction (XRD) measurements of Al substituted for Fe in goethite indicate two populations: (1) low-Al, Liesegang-type goethites (∼0 mol% Al) and (2) high-Al, pisolitic goethites (∼9 to ∼24 mol% Al). Selective dissolution and incremental vacuum dehydration-decarbonation were used to determine the concentration and δ13C values of CO2 occluded in the respective structures of the goethites and gibbsites in this complex mixture of Nire lateritic minerals. The Fe(CO3)OH component in the high-Al goethites appears to retain a proxy carbon isotopic record of vadose zone CO2 in the ancient soil. The δ13C values of CO2 occluded in coexisting goethites and gibbsites indicate that these minerals did not form in equilibrium with the same environmental CO2.The measured mole fractions (X) of Fe(CO3)OH in the high-Al goethites range from 0.0059 (±0.0005) to 0.0077 (±0.0006) and correspond to soil CO2 concentrations of ∼28,000 to ∼37,000 ppmV. The average values of X and δ13C for the four high-Al goethites are 0.0067 ± 0.0007 and −20.1 ± 0.5‰, respectively. The δ13C value of the organic matter undergoing oxidation in this midlatitude (∼55°N) Late Paleocene soil appears to have been ∼ −28.2‰. Taken together, these data indicate an atmospheric CO2 concentration of ∼2400 ppmV (± ∼1200 ppmV) at ∼60 Ma BP. The inferred high concentration of atmospheric CO2 would have been coincident with the warm global climate of the Late Paleocene and is consistent with the idea that CO2 plays an important role in climate variation.  相似文献   

20.
This study is focused on geothermal heat flow and the origin of non-hydrocarbons in natural gases in terms of the isotope geochemical characteristics of Ar, He, CO2 and N2 in natural gases from the Sanshui Basin, Guangdong Province. China.3He/4He ratios are of (1.60-6.39) × 10-6, and40Ar/36Ar ratios of 450–841. The carbon isotopic composition (δl3C PDB) of carbon dioxide ranges from -20‰ to -2‰. δl5N(air) ratios have a wider range of-57 ‰- +95 ‰. The isotope geochemical characteristics of non-hydrocarbons indicate that He, Ar and N2 in the gas reservoirs enriched in non-hydrocarbons were derived largely from the upper mantle. Non-hydrocarbons in gaseous hydrocarbon reservoirs consist mainly of crustal radiogenic He and40Ar and some mantle-derived He and Ar, as well as of13C-depleted carbon dioxide and nitrogen generated as a result of thermal decomposition of organic matter in strata. Carbon dioxide enriched in13C was derived largely from carbonate rocks and partially from the lower crust and upper mantle. Based on the relationship between geothermal heat flow (Q) and3He/4 He ratio in natural gases, the Q values for the area studied have been calculated. Similar Q values are reported from the upper mantle uplift area (77 mWm-2) in Huabei and the Tancheng-Lujiang Rift Zone (88 mWm-2). More than 60 percent of geothermal heat flow in the Sanshui Basin may have been derived from the upper mantle. The project is financially supported by the National Natural Science Foundation of China.  相似文献   

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