Carbon dioxide, bicarbonate and carbonate ions in aqueous solutions under deep Earth conditions.

Fluid processes in the early Earth and the growth of continents

Investigating the fate of dissolved carbon dioxide under extreme conditions is critical to understanding the deep carbon cycle in Earth, a process that ultimately influences global climate change. We used first-principles molecular dynamics simulations to study carbonates and carbon dioxide dissolved in water at pressures (P) and temperatures (T) approximating the conditions of Earth's upper mantle. Contrary to popular geochemical models assuming that molecular CO2(aq) is the major carbon species present in water under deep Earth conditions, we found that at 11 GPa and 1000 K, carbon exists almost entirely in the forms of solvated carbonate ([Formula: see text]) and bicarbonate ([Formula: see text]) ions and that even carbonic acid [H2CO3(aq)] is more abundant than CO2(aq). Furthermore, our simulations revealed that ion pairing between Na+ and [Formula: see text]/[Formula: see text] is greatly affected by P-T conditions, decreasing with increasing pressure at 800 to 1000 K. Our results suggest that in Earth's upper mantle, water-rich geofluids transport a majority of carbon in the form of rapidly interconverting [Formula: see text] and [Formula: see text] ions, not solvated CO2(aq) molecules.

Representatives from four universities will start a project soon to obtain several profiles of the earth's deep crust and upper mantle to provide a better understanding of that region. Data obtained from the project could help petroleum geologists better understand sedimentary basins where much of the earth's oil deposits have been found.The work will be done under a one‐year grant of $378,100 by the National Science Foundation to Cornell University, which will coordinate the effort.

<p>The deep continental crust's chemical makeup is central to the debate of crustal formation, evolution, strength, and bulk composition. The impenetrable depths and pressures of the deep (roughly > 10 km) crust force geoscientists to rely on indirect sampling methods, studying medium- to high-grade metamorphic terrains and xenoliths to ascertain the composition of the middle and lower continental crust. Analyzing the deep crust in situ requires geophysical data, such as seismic velocities: Vp, Vs, and the Vp/Vs ratio. Each method provides a different perspective on deep crustal composition, but alone, neither is definitive. </p><p>To address the nonuniqueness in crust composition modeling, we use thermodynamic modeling software (i.e. Perple_X) to relate observed seismic velocities to bulk compositions and mineralogies. We present a multidisciplinary model for the composition of Earth's deep crust, using geochemical and geophysical data. Through a Monte Carlo modeling approach, we determine the best-fit geochemical model for bulk middle and lower crustal compositions. For 12 different tectonic regimes, we quantify uncertainties in crustal composition, temperature, and seismic velocity while recognizing our own scientific biases. We present a global model of deep crustal composition conclude that regional scale geological variations benefit from a higher resolution model. Overall, our model forecasts 77% of the deepest continental crust has 45 to 55 wt.% SiO<sub>2</sub>; 15% 55 to 65 wt.% SiO<sub>2</sub>; 8% may have > 65 wt.% SiO<sub>2</sub>. Of perhaps equal or greater importance, however, we present a scalable, modular program that can be altered to incorporate additional petrological and geophysical constraints, allowing geoscientists to more easily compare different scenarios for the deep crust.</p>

The type of collision between the European and the Adriatic plates in the easternmost Alps is one of the most interesting questions regarding the Alpine evolution. Tectonic processes such as compression, escape and uplift are interconnected and shape this area. We can understand these ongoing processes better, if we look for signs of the deformation within the Earth's deep crust of the region. By collecting records from permanent and temporary seismic networks, we assemble a receiver function dataset, and analyze it with the aim of giving new insights on the structure of the lower crust and of the shallow portion of the upper mantle, which are inaccessible to direct observation. Imaging is accomplished by performing common conversion depth stacks along three profiles that crosscut the Eastern Alpine orogen, and allow isolating features consistently persistent in the area. The study shows a moderately flat Moho underlying a seismically anisotropic middle-lower crust from the Southern Alps to the Austroalpine nappes. The spatial progression of anisotropic axes reflects the orientation of the relative motion and of the stress field detected at the surface. These observations suggest that distributed deformation is due to the effect of the Alpine indentation. In the shallow upper mantle right below the Moho interface, a further anisotropic layer is recognized, extended from the Bohemian Massif to the Northern Calcareous Alps.

Considerable experience with integrated geological and geophysical studies has enabled definition of deep crustal structures and, within limits, composition and processes within the deep crust, and to determine their association with metallogeny in the USSR.By means of seismic experiments, stratification of the Earth's crust and the upper mantle to a depth of about 100 km has been revealed. Numerous heat flow data have been compiled. Magneto-telluric soundings made it possible to determine the position of conductive strata in the crust and upper mantle for a number of areas. Gravity surveys coupled with the results of seismic profiling enabled the finding of a number of empirical laws that are useful for investigation into the deep crust. Magnetic data analysis has enabled evaluation of the magnetic layering of the deep crust. Kimberlite and ore provinces can be considered examples of these concepts.For more detailed studies of deep crustal structure the territory of the USSR is the subject of a system of regional investigation of the deep crust and upper mantle. This system is based principally upon a network of interconnected regional profiles (geotraverses) tied to deep and superdeep boreholes. The system includes predicted geophysical observations to control investigation of the geophysical field data. The geotraverse network is the basis for detailed studies within the bounds of petroleum and ore provinces.The most accurate data obtained allows the formation of a crustal model and reveals empirical relationships with metallogeny.Based on the deep crustal structure data a regional oregenesis prediction map has been made. The endogenous mineralization prediction was based on special features of the upper layering of the crust and on data relating to deep crustal permeability zones.

Mineral-aqueous fluid partitioning of trace elements at 900°C and 2.0 GPa: Constraints on the trace element chemistry of mantle and deep crustal fluids

Electromagnetic measurements have demonstrated that the lower continental crust has remarkable electrical anomalies of high conductivity and electrical anisotropy on a global scale (probably with some local exceptions), but their origin is a long-standing and controversial problem. Typical electrical properties of the lower continental crust include: (1) the electrical conductivity is usually 10−4 to 10−1 S/m; (2) the overlying shallow crust and underlying upper mantle are in most cases less conductive; (3) the electrical conductivity is statistically much higher in Phanerozoic than in Precambrian areas; (4) horizontal anisotropy has been resolved in many areas; and (5) in some regions there appear to be correlations between high electrical conductivity and other physical properties such as seismic reflections. The explanation based on conduction by interconnected, highly conductive phases such as fluids, melts, or graphite films in grain boundary zones has various problems in accounting for geophysically resolved electrical conductivity and other chemical and physical properties of the lower crust. The lower continental crust is dominated by mafic granulites (in particular beneath stable regions), with nominally anhydrous clinopyroxene, orthopyroxene, and plagioclase as the main assemblages, and the prevailing temperatures are mostly 700–1,000°C as estimated from xenolith data, surface heat flow, and seismic imaging. Pyroxenes have significantly higher Fe content in the lower crust than in the upper mantle (peridotites), and plagioclase has higher Na content in the lower crust than in the shallow crust (granites). Minerals in the lower continental crust generally contain trace amounts of water as H-related point defects, from less than 100 to more than 1,000 ppm H2O (by weight), with concentrations usually higher than those in the upper mantle. Observations of xenolith granulites captured by volcano-related eruptions indicate that the lower continental crust is characterized by alternating pyroxene-rich and plagioclase-rich layers. Experimental studies on typical lower crustal minerals have shown that their electrical conductivity can be significantly enhanced by the higher contents of Fe (for pyroxenes), Na (for plagioclase), and water (for all minerals) at thermodynamic conditions corresponding to the lower continental crust, e.g., to levels comparable to those measured by geophysical field surveys. Preferred orientation of hydrous plagioclase, e.g., due to ductile flow in the deep crust, and alternating mineral fabrics of pyroxene-rich and plagioclase-rich layers can lead to substantial anisotropy of electrical conductivity. Electrical conductivity properties in many regions of the lower continental crust, especially beneath stable areas, can mostly be accounted for by solid-state conduction due to the major constituents; other special, additional conduction mechanisms due to grain boundary phases are not strictly necessary.

Eclogite is potentially an important constituent in local regions in the deep crust and upper mantle. The electrical conductivity of omphacite and garnet in eclogite has been measured at 1 GPa and 350–800 °C with pre-annealed OH-bearing samples. The conductivities were determined using a piston–cylinder apparatus and a Solartron-1260 Impedance/Gain Phase Analyser in the frequency range of 106–1 Hz. The sample water contents show almost no change before and after the experimental runs. The conductivity of both omphacite and garnet increases with temperature, and the activation enthalpy is ~ 82 kJ/mol for omphacite and 90 kJ/mol for garnet, which is nearly independent of water content in each mineral. The conduction is probably dominated by protons, and for both minerals, the conductivity increases linearly with water content, with a water content exponent of ~ 1. These data are used to model the bulk conductivity of an eclogite with different water contents and modal compositions. In combination with reported data, the conductivity of the eclogite is similar to that of typical granulites above 600 °C, but is much larger than that of olivine, assuming small to moderate water contents. This would mean that the contribution of eclogites, if present, to the electrical structure of the deep continental crust cannot be easily separated from that of granulites, and that the regional enrichments of eclogites in the upper mantle may cause high electrical anomalies. The results also provide information for the electrical property of orogen-related thickened deep crust where eclogites may be locally abundant, e.g., in the Dabieshan region and the Tibet plateau. At mantle depths, eclogitized portions of subducted slabs are usually of very low conductivities as suggested by geophysical observations, implying small water contents in the constitutive omphacite and garnet and the limited ability of these minerals in recycling water into the deep mantle.

[1] The viscosity of the upper mantle has a large control on the dynamics of plate tectonic processes or the response of the Earth's crust after a period of glaciation. Temperature variations within the upper mantle, time-dependent stress changes due to glaciations, and/or variations in the microstructural characteristics of upper mantle rocks (grain size, water content) will result in orders of magnitude variations in upper mantle viscosity. In this study we have taken a microphysical approach to determine variations in viscosity under Scandinavia. We combined experimentally determined flow laws for olivine, data on olivine grain size from Scandinavian xenoliths and peridotites, and stress changes within the upper mantle over the last glaciation period (30 kyr B.P.–present) with two data sets of the temperature distribution within the upper mantle under Europe from Goes et al. (2000) derived from seismic tomography and Artemieva (2006) derived from heat flow measurements. Modeling of olivine viscosity under Scandinavia shows large lateral, radial, and temporal variations in upper mantle viscosity. Lateral temperature variations cause up to four orders of magnitude lateral viscosity variations. Glaciation-induced time-dependent changes of stress cause two orders of magnitude variations in viscosity. Mean viscosity values for a dry upper mantle under Scandinavia are expected to be larger than 1022 Pa s, whereas for a wet upper mantle lower viscosity values in the range of 1019–1022 Pa s are predicted. Estimates of the upper mantle viscosity under Scandinavia from glacial isostatic adjustment studies (1020–1021 Pa s) would indicate that a wet upper mantle below Scandinavia is most likely present. The viscosity modeling furthermore shows that the type of rheology of the upper mantle (linear versus nonlinear rheology) is very sensitive to the microstructural state of the upper mantle. Both mechanisms are active in the upper mantle under Scandinavia on timescales of glacial isostatic adjustment, and their relative contribution varies radially, laterally, and temporally.

Three techniques for analyzing rare earth elements (REE) in geological materials are described, i.e. instrumental neutron activation analysis (INAA), neutron activation analysis with pre-irradiation chemical REE separation (PCS-NAA) and radiochemical neutron activation analysis (RNAA). The knowledge of REE concentrations in eclogites, peridotites and minerals from the earth's lower crust and upper mantle is very useful in constraining their petrogenetic history.

Three-dimensional density model of the Earth's crust and the upper mantle for the area of Poland

Remote sensing observations have been interpreted to indicate that the Crisium basin‐forming event excavated deep crust and upper mantle. Samples from the highlands adjacent to the Crisium basin returned by Luna 20 (L‐20) bring a unique perspective for evaluating this concept. The magmatic lithologies returned from the noritic Hilly and Furrowed Terrain (nHFT) by L‐20 are coarse‐grained feldspar (>300 μm) with inclusions of pyroxene, and finer‐grained norites, troctolites, spinel troctolites, and gabbros (<100 μm). These two suites represent ferroan anorthosites (FANs) and the Mg‐suite, respectively. There is limited evidence for mantle or deep crustal material within the nHFT samples. Ultramafic rocks such as dunites and orthopyroxenites are absent, and Mg‐rich olivine‐ and orthopyroxene‐bearing‐assemblages are derived from magmatic rocks emplaced in the shallow crust. These lithic fragments represent pre‐Crisium episodes of magmatism (Mg‐suite) and lunar magma ocean products (FANs). The lack of deep lithologies at the L‐20 site seems contradictory to excavation models for Crisium. Mineralogical‐chemical differences suggest a higher FAN component in the rim and that this represents FANs excavated from the deep lunar crust. If it exists, the Mg‐rich olivine previously identified within the Crisium rim is most likely related to deep, complementary versions of the Mg‐suite rocks from L‐20. The material associated with the Crisium basin is not derived from the lunar mantle but represents crustal lithologies from the shallow to deep crust, a substantial mantle component may have been incorporated into the Crisium basin impact melt sheet, or that our “Earth‐analog” for the lunar upper mantle is incorrect.

Throughout most of Canada the regional Bouguer anomaly is inversely proportional to the depth of the crust-mantle boundary; the constant of proportionality indicates that the density contrast at the crust-mantle boundary is about 0.2 g/cm3. Seismic data suggest that compressional wave velocities of 7.0 to 7.5 km/sec occur near the base of the crust, and interpretations of magnetic and electromagnetic measurements in Canada combined with analyses of heat flow data and upper mantle seismic velocity distributions from other areas suggest that the temperature in the vicinity of the crust-mantle boundary in a stable area is of the order of 1000°K and that the vertical gradient of temperature is about 15° to 20°K/km. The above results, considered together with laboratory measurements of compressional-wave velocities in rocks and petrological investigations of mafic and ultramafic mineral assemblages, are interpreted as follows: in cratonic areas of Canada, rocks such as amphibolite and-or intermediate to basic granulite probably comprise the deep crust; garnet-peridotite is probably an important constituent of the upper mantle; rocks of a given composition should be stable at different levels in the crust and upper mantle, and therefore, assuming that the crust-mantle boundary represents a change in chemical composition, it can be understood why large fluctuations can exist in the depth of the crust-mantle boundary. In Canada, variations of compressional wave velocity in the upper mantle may be due to regional variations of deep heat flow in some areas and, in other areas, to anisotropic propagation of seismic waves. Lateral variations of density and compressional wave velocity in the upper mantle appear to extend to a depth of at least 100 km.

[1] Detailed 3-D tomographic images of P and S wave velocity (Vp, Vs) and Poisson's ratio (σ) under the central and western Tien Shan orogenic belt are determined by using a large number of high-quality P and S wave arrival times from local earthquakes. The results show that under the Tien Shan orogenic belt high-Vp, high-Vs, and low-σ anomalies are revealed in the upper and middle crust, possibly indicating the existence of the Paleozoic crystalline basement rocks, while low-Vp, low-Vs, and high-σ anomalies appear in the lower crust and upper mantle, perhaps suggesting that the hot and wet material is upwelling under the Tien Shan orogenic belt from the mantle. Some high-Vp, high-Vs, and low-σ anomalies are tilted toward the Tien Shan along with the seismicity. These are found in the collision zones between the Tien Shan and the Tarim basin in the south and the Kazakh shield in the north and suggest the underthrusting of the Tarim and Kazakh lithosphere beneath the Tien Shan. Meanwhile, some low-Vp, low-Vs, and high-σ anomalies are imaged in other parts of these collision zones, perhaps indicating the intrusion of the hot and wet material into the crust from the upper mantle. These results indicate that both the upwelling of the hot and wet material and the underthrusting of the Tarim and Kazakh lithosphere may have played an important role in the mountain building. Under the Tarim and Fergana basins, low-Vp, low-Vs, and high-σ anomalies are revealed in the upper crust, while high-Vp, high-Vs, and low-σ anomalies are visible in the lower crust and upper mantle. These may reflect the existence of less compacted sedimentary material in the shallow crust and more highly compacted craton-like structures in the deeper crust and upper mantle under the basins. The Talas-Fergana fault shows an obvious tectonic boundary between central and western Tien Shan. The central Tien Shan displays high-Vp, high-Vs, and low-σ anomalies in the upper and middle crust, while western Tien Shan exhibits low-Vp, low-Vs, and high-σ anomalies. However, the pattern of seismic structure between central and western Tien Shan reverses in the lower crust. Such a correlation may extend down to the upper mantle, suggesting that the Talas-Fergana fault may be a lithospheric-scale boundary. Additionally, a columnar low-Vp and low-Vs anomaly is clearly observed around the turning point of the Talas-Fergana fault from the NWN to NWW trending orientations and may indicate that the fault provides a channel for the hot and wet material upwelling from the mantle to the surface.