The positions of Laurentia and other landmasses in the Precambrian supercontinent of Rodinia are controversial. Although geological and isotopic data support an East Antarctic fit with western ...Laurentia, alternative reconstructions favor the juxtaposition of Australia, Siberia, or South China. New geologic, age, and isotopic data provide a positive test of the juxtaposition with East Antarctica: Neodymium isotopes of Neoproterozoic rift-margin strata are similar; hafnium isotopes of ~1.4-billion-year-old Antarctic-margin detrital zircons match those in Laurentian granites of similar age; and a glacial clast of A-type granite has a uraniun-lead zircon age of ~1440 million years, an epsilon-hafnium initial value of +7, and an epsilon-neodymium initial value of +4. These tracers indicate the presence of granites in East Antarctica having the same age, geochemical properties, and isotopic signatures as the distinctive granites in Laurentia.
Significant variations in the isotopic composition of marine calcium have occurred over the last 80 million years. These variations reflect deviations in the balance between inputs of calcium to the ...ocean from weathering and outputs due to carbonate sedimentation, processes that are important in controlling the concentration of carbon dioxide in the atmosphere and, hence, global climate. The calcium isotopic ratio of paleo-seawater is an indicator of past changes in atmospheric carbon dioxide when coupled with determinations of paleo-pH.
The effect of continental weathering on the iron isotope compositions of natural materials is investigated. Unweathered igneous rocks, pelagic clay, and dust fall within the range
δ
56Fe=0±0.3‰. ...Rivers with large suspended loads also have
δ
56Fe values near zero. Dilute streams have
δ
56Fe values that trend towards lower
δ
56Fe (∼−1) suggesting that dissolved riverine iron is isotopically light relative to igneous rocks. Bulk soil and soil leaches display systematically different
δ
56Fe profiles, indicating that isotopically distinct Fe pools are generated during pedogenesis. Nannofossil ooze, which contains Fe scavenged from the ocean water column, has
δ
56Fe≈0, but is consistent with seawater dissolved Fe having negative
δ
56Fe. It is inferred that continental weathering under modern oxidizing Earth surface conditions preferentially releases dissolved Fe with negative
δ
56Fe, which is transported in rivers to the ocean. A preliminary analysis of the marine Fe budget suggests that riverine Fe has a substantial role in determining the
δ
56Fe of both the modern and ancient oceans, but other inputs, particularly that from diagenesis of marine sediments, may also be important. Since the chemical pathways of Fe processing during weathering are dependent on oxidation state and biological activity, Fe isotopes may prove useful for detecting changes in these parameters in the geologic past.
Isotopic fractionation of water during evaporation Cappa, Christopher D.; Hendricks, Melissa B.; DePaolo, Donald J. ...
Journal of Geophysical Research - Atmospheres,
27 August 2003, Letnik:
108, Številka:
D16
Journal Article
Recenzirano
Odprti dostop
Variations in the isotopic content (18O/16O and D/H ratios) of water in the natural environment provide a valuable tracer of the present‐day global hydrologic cycle and a record of the climate over ...at least 400,000 years that is preserved in glacial ice. The interpretation of observed isotopic ratios in water vapor, rain, snow, and ice depends on our understanding of the processes (mainly phase changes) that produce isotopic fractionation. Whereas equilibrium isotopic fractionation is well understood, kinetic effects, or diffusion‐controlled fractionation, has a limited experimental foundation. Kinetic effects are significant during evaporation into unsaturated air and during condensation to form ice from vapor. Kinetic effects are also thought to control the deuterium excess (d = δD − 8δ18O) of precipitation. We describe experiments to observe kinetic effects associated with evaporation. Analysis of our own and previous experiments shows that surface cooling of the liquid is a crucial variable affecting fractionation from evaporating water that has not been properly considered before. Including the effects of evaporative surface cooling reconciles observed D/H fractionation with kinetic theory and removes the need to invoke an unusual size for the HDO molecule. Thus the isotopic molecular diffusivity ratios are D(H218O)/D(H216O) = 0.9691 and D(HD16O)/D(H216O) = 0.9839. Implications of this work for representation of kinetic fractionation in global circulation models and cloud physics models are briefly discussed.
Microscopic mechanisms operating at the mineral–aqueous interface control rates of growth and dissolution, isotope fractionation and trace element partitioning during crystal growth. Despite the ...importance of characterizing surface kinetic controls on isotopic partitioning, no self-consistent microscopic theory has yet been presented which can simultaneously model both mineral growth rate and isotopic composition. Using a kinetic theory for AB or di-ionic crystal growth, we derive a model to predict precipitation rate and isotope fractionation as a function of growth solution oversaturation and solution stoichiometry and apply the theory to calcium isotope fractionation during calcite precipitation.
Our model assimilates the current understanding of surface controlled isotope fractionation with kinetic theories of ion-by-ion mineral growth to predict isotopic partitioning during the growth of ionic crystals. This approach accounts for the effect of solution composition on microscopic mineral surface structure and composition, providing numerous testable hypotheses for growth of sparingly soluble AB crystals such as calcite, namely:
(1)Both oversaturation and solution stoichiometry control growth rate and partitioning of isotopes during precipitation;(2)for growth driven primarily by step propagation, distinct expressions describe dislocation- and 2D nucleation-driven growth rates, while the expression for isotope fractionation is the same for both mechanisms;(3)mineral precipitation occurring via the formation of an amorphous precursor will generate isotope effects that are not compatible with ion-by-ion growth theory and may therefore be excluded from comparison; and,(4)the absolute kinetic limit of isotope fractionation may not be accessible at high oversaturation due to the formation of amorphous precursors.
Using calcite as a model system, we derive expressions for growth rate and isotopic fractionation as a function of oversaturation and Ca2+:CO32− in solution. Increasing oversaturation increases mineral growth rate and drives isotope partitioning towards the kinetic limit, while increasing the concentration of Ca2+ relative to CO32− at a given oversaturation tends to drive crystal growth towards isotopic equilibrium. These competing effects attenuate the magnitude of isotope fractionations observable in terrestrial environments.
In-situ recovery (ISR) of uranium (U) from sandstone-type roll-front deposits is a technology that involves the injection of solutions that consist of ground water fortified with oxygen and carbonate ...to promote the oxidative dissolution of U, which is pumped to recovery facilities located at the surface that capture the dissolved U and recycle the treated water. The ISR process alters the geochemical conditions in the subsurface creating conditions that are more favorable to the migration of uranium and other metals associated with the uranium deposit. There is a lack of clear understanding of the impact of ISR mining on the aquifer and host rocks of the post-mined site and the fate of residual U and other metals within the mined ore zone. We performed detailed petrographic, mineralogical, and geochemical analyses of several samples taken from about 7m of core of the formerly the ISR-mined Smith Ranch–Highland uranium deposit in Wyoming. We show that previously mined cores contain significant residual uranium (U) present as coatings on pyrite and carbonaceous fragments. Coffinite was identified in three samples. Core samples with higher organic (>1wt.%) and clay (>6–17wt.%) contents yielded higher 234U/238U activity ratios (1.0–1.48) than those with lower organic and clay fractions. The ISR mining was inefficient in mobilizing U from the carbonaceous materials, which retained considerable U concentrations (374–11,534ppm). This is in contrast with the deeper part of the ore zone, which was highly depleted in U and had very low 234U/238U activity ratios. This probably is due to greater contact with the lixiviant (leaching solution) during ISR mining.
EXAFS analyses performed on grains with the highest U and Fe concentrations reveal that Fe is present in a reduced form as pyrite and U occurs mostly as U(IV) complexed by organic matter or as U(IV) phases of carbonate complexes. Moreover, U–O distances of ~2.05Å were noted, indicating the potential formation of other poorly defined U(IV/VI) species. We also noted a small contribution from UO at 1.79Å, which indicates that U is partially oxidized. There is no apparent U–S or U–Fe interaction in any of the U spectra analyzed. However, SEM analysis of thin sections prepared from the same core material reveals surficial U associated with pyrite which is probably a minor fraction of the total U present as thin coatings on the surface of pyrite.
Our data show the presence of different structurally variable uranium forms associated with the mined cores. U associated with carbonaceous materials is probably from the original U mobilization that accumulated in the organic matter-rich areas under reducing conditions during shallow burial diagenesis. U associated with pyrite represents a small fraction of the total U and was likely deposited as a result of chemical reduction by pyrite. Our data suggest that areas rich in carbonaceous materials had limited exposure to the lixiviant solution, continue to be reducing, and still hold significant U resources. Because of their limited access to fluid flow, these areas might not contribute significantly to post-mining U release or attenuation. Areas with pyrite that are accessible to fluids seem to be more reactive and could act as reductants and facilitate U reduction and accumulation, limiting its migration.
•Carbonaceous fragments and pyrite in post-mined cores contain significant U.•U predominantly occurs as U(IV)-carboxylate complexes.•U associated with organic matter resisted oxidation or leaching during ISR mining.•Pyrite acted as a reducing phase to partially attenuate U.•Organic-rich sediments with significant U unlikely to contribute to U attenuation.
Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. ...Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria (
238U-
230Th-
226Ra and
235U-
231Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of
238U-
230Th-
226Ra and
235U-
231Pa disequilibria; Hf, Nd, Sr, and Pb isotopic; and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9°28′ and 9°52′N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples,
230Th excesses and
226Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of
226Ra, this inverse correlation between
230Th and
226Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study,
226Ra and
230Th excesses also vary with lava composition:
226Ra excesses are negatively correlated with Na
8 and La/Yb and positively correlated with Mg#. Conversely,
230Th excesses are positively correlated with Na
8 and La/Yb and negatively correlated with Mg#. Th/U,
230Th/
232Th, and
230Th excesses are also variable and correlated to one another.
231Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na
8. The isotope ratios
143Nd/
144Nd,
176Hf/
177Hf,
87Sr/
86Sr, and
208Pb/
206Pb are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf, Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U,
230Th, and
226Ra excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources.
Reactive transport modeling is an essential tool for the analysis of coupled physical, chemical, and biological processes in Earth systems, and has additional potential to better integrate the ...results from focused fundamental research on Earth materials. Appropriately designed models can describe the interactions of competing processes at a range of spatial and time scales, and hence are critical for connecting the advancing capabilities for materials characterization at the atomic scale with the macroscopic behavior of complex Earth systems. Reactive transport modeling has had a significant impact on the treatment of contaminant retardation in the subsurface, the description of elemental and nutrient fluxes between major Earth reservoirs, and in the treatment of deep Earth processes such as metamorphism and magma transport. Active topics of research include the development of pore scale and hybrid, or multiple continua, models to capture the scale dependence of coupled reactive transport processes. Frontier research questions, that are only now being addressed, include the effects of chemical microenvironments, coupled thermal–mechanical–chemical processes, controls on mineral–fluid reaction rates in natural media, and scaling of reactive transport processes from the microscopic to pore to field scale.
Calcium from bone and shell is isotopically lighter than calcium of soft tissue from the same organism and isotopically lighter than source (dietary) calcium. When measured as the 44Ca/40Ca isotopic ...ratio, the total range of variation observed is Note: Equation omitted. See the image of page 13709 for this equation., and as much as Note: Equation omitted. See the image of page 13709 for this equation. variation is found in a single organism. The observed intraorganismal calcium isotopic variations and the isotopic differences between tissues and diet indicate that isotopic fractionation occurs mainly as a result of mineralization. Soft tissue calcium becomes heavier or lighter than source calcium during periods when there is net gain or loss of mineral mass, respectively. These results suggest that variations of natural calcium isotope ratios in tissues may be useful for assessing the calcium and mineral balance of organisms without introducing isotopic tracers.
Fluids associated with the San Andreas and companion faults in central and south-central California have high $^3$He/$^4$He ratios. The lack of correlation between helium isotopes and fluid chemistry ...or local geology requires that fluids enter the fault system from the mantle. Mantle fluids passing through the ductile lower crust must enter the brittle fault zone at or near lithostatic pressures; estimates of fluid flux based on helium isotopes suggest that they may thus contribute directly to fault-weakening high-fluid pressures at seismogenic depths.
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