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At least 217 records · Page 12Linked to original sources
USGS1984

Stable isotope geochemistry of acid mine drainage: Experimental oxidation of pyrite

Sulfate and water from experiments in which pyrite was oxidized at a pH of 2.0 were analyzed for sulfur and oxygen stable isotopes. Experiments were conducted under both aerobic and anaerobic sterile conditions, as well as under aerobic conditions in the presence of Thiobacillus ferrooxidans, to elucidate the pathways of oxidation. Oxygen isotope fractionation between SO2-4 and H2O varied from +4.0 %. (anaerobic, sterile) to + 18.0 %. (aerobic, with T. ferrooxidans.). The oxygen isotope composition of dissolved oxygen utilized in both chemical and microbially-mediated oxidation was also determined (+11.4 %., by T. ferrooxidans; +18.4 %., chemical). Contributions of water-derived oxygen and dissolved oxygen to the sulfate produced in the oxidation of pyrite could thus be estimated. Water-derived oxygen constituted from 23 to ~ 100 percent of the oxygen in the sulfate produced in the experiments, and this closely approximates the range of contribution in natural acid mine drainage. Oxidation of sulfides in anaerobic, water-saturated environments occurs primarily by chemical oxidation pathways, whereas oxidation of sulfides in well-aerated, unsaturated zone environments occurs dominantly by microbially mediated pathways.

Geochimica et Cosmochimica Acta↗
USGS1984

Intrinsic oxygen fugacity measurements on seven chondrites, a pallasite, and a tektite and the redox state of meteorite parent bodies

Intrinsic oxygen-fugacity ( f O 2 ) measurements were made on five ordinary chondrites, a carbonaceous chondrite, an enstatite chondrite, a pallasite, and a tektite. Results are of the form of linear log f O 2 − 1 T "> fO 2 −1T plots. Except for the enstatite chondrite, measured results agree well with calculated estimates by others. The tektite produced f O 2 values well below the range measured for terrestrial and lunar rocks. The lowpressure atmospheric regime that is reported to follow large terrestrial explosions, coupled with a very high temperature, could produce glass with f O 2 in the range measured. The meteorite Salta (pallasite) has low f O 2 and lies close to Hvittis (E6). Unlike the other samples, results for Salta do not parallel the iron-wüstite buffer, but are close to the fayalite-quartz-iron buffer in slope. Minor reduction by graphite appears to have taken place during metamorphism of ordinary chondrites. f O 2 values of unequilibrated chondrites show large scatter during early heating suggesting that the constituent phases were exposed to a range of f O 2 conditions. The samples equilibrated with respect to f O 2 in relatively short time on heating. Equilibration with respect to f O 2 in ordinary chondrites takes place between grades 3 and 4 of metamorphism. Application of P − T − f O 2 relations in the system C-CO-CO 2 indicates that the ordinary chondrites were metamorphosed at pressures of 3–20 bars, as it appears that they lay on the graphite surface. A steep positive thermal gradient in a meteorite parent body lying at the graphite surface will produce thin reduced exterior, an oxidized near-surface layer, and an interior that is increasingly reduced with depth; a shallow thermal gradient will produce the reverse. A body heated by accretion on the outside will have a reduced exterior and oxidized interior. Meteorites from the same parent body clearly are not required to have similar redox states.

Geochimica et Cosmochimica Acta↗
USGS1984

Gas composition of the January 1983 eruption of Kilauea Volcano, Hawaii

Gas collections were made from a ∼900°C vent both by conventional evacuated-bottle/wet-chemical techniques and by manual pumping of flowthrough bottles. The complete analyses suggest an equilibrium assemblage quenched at 1,010°C, about midway between fountain and vent temperatures. I suggest that the very low C S "> CS ratio is due to degassing of CO 2 during storage of the magma in a shallow reservoir before eruption. The two sampling techniques yielded analytical data in mutual agreement.

Geochimica et Cosmochimica Acta↗
USGS1984

Time and the crystallization of apatite in seawater

Carbonate fluorapatite has been synthesized in seawater in an experiment of nearly 10-years duration. The addition of phosphate to seawater whose fluoride concentration had been increased to 7.6 mg/l brought about an initial amorphous phosphate precipitate. After 20 months, a crystalline magnesium phosphate phase developed within the amorphous phosphate. Crystallization of apatite, which occurred during the last 3 years of the experiment, was accompanied by dissolution of the crystalline magnesium phosphate phase. The MgO content of the apatite (1.9 percent) is high in comparison to Tertiary and older apatite but similar to some young apatite; the CO 2 content (3.6 percent) is medium, and the fluorine content (2.2 percent) is low but again similar to some young apatite. The hydroxyl ion (OH − ) likely fills the need for additional fluorine-position atoms. The mole ratio of Ca plus substituent elements to P plus substituent elements (1.50) is low in comparison to the expected ratio of 1.67. The substitution of the hydronium ion (H 3 O + ) for Ca may account for this difference. The synthesis of apatite in seawater demonstrates that the factor of time overcomes the well known inhibiting effect of magnesium upon the crystallization of apatite. It also implies that given an adequate supply of phosphate, apatite can form in most ocean environments and likely plays a major pan in the control of the phosphate content of seawater.

Geochimica et Cosmochimica Acta↗
USGS1984

The solubility of strontianite (SrCO 3 ) in CO 2 -H 2 O solutions between 2 and 91°C, the association constants of SrHCO + 3 (aq) and SrCO 0 3 (aq) between 5 and 80°C, and an evaluation of the thermodynamic properties of Sr 2+ (aq) and SrCO 3 (cr) at 25°C and 1 atm total pressure

Seventy new measurements (Sr T -P co2 of the solubility of strontianite were used to evaluate the equilibrium constant for the reaction SrCO 3 ( cr ) = Sr 2+ ( aq ) + CO 2− 3 ( aq ) between 2 and 91 °C. The temperature dependence of the equilibrium constant is given by the expression Log K = 155.0305 − 7239.594/ T − 56.58638 log T where T is in degrees Kelvin. The log K of strontianite, the Gibbs energy, enthalpy and entropy of the reaction at 25°C are −9.271 ± 0.020, 52.919 ± 0.08 kJ · mol −1 , −1.67 ± 1.30 kJ · mol −1 , and −183.1 ± 4.0 J · mol −1 · K −1 , respectively. The equilibrium constants are consistent with an aqueous model that includes the ion pairs SrHCO + 3 (aq) and SrCO 0 3 (aq) which were evaluated by potentiometric methods between 5 and 80°C. The equilibrium constant for the association reaction Sr 2+ ( aq ) + HCO − 3 ( aq ) = SrHCO + 3 aq ) is given by the expression Log K SrHCO + 3 = −3.248 + 0.014867 T . The log of the association constant, the Gibbs energy, enthalpy and entropy of the reaction at 25°C are 1.18, −6.76 kJ · mol −1 , 25.30 kJ · mol −1 , and 107.5 J · mol −1 · K −1 , respectively. The equilibrium constant for the association reaction Sr 2+ ( aq ) + CO 2− 3 ( aq ) = SrCO 0 3 aq ) is given by the expression Log K SrCO 0 3 = −1.019 + 0.012826 T . The log of the association constant, the Gibbs energy, enthalpy, and entropy of the reaction at 25°C are 2.81, −16.01 kJ · mol −1 ,21.83 kJ · mol −1 , and 126.9 J · mol −1 · K −1 , respectively. These results lead to reliable calculation of the aqueous speciation and solubility of strontianite in the system SrCO 3 -CO 2 -H 2 O from 0 to more than 90°C. Literature data on the solubility of strontianite have been evaluated and compared with these results. Our new data for strontianite have been used in an evaluation of the thermodynamic properties of Sr 2+ (aq), SrCO 3 (cr) and related compounds. The following values are recommended for the standard enthalpy (kJ · mol −1 ), Gibbs energy (kJ · mol −1 ), and entropy (J · mol −1 · K −1 ), respectively, of Sr 2+ aq): −550.90 ± 0.50, −563.83 ± 0.8 and −31.50 ± 2.0, and for SrCO 3 (cr): −1225.77 ± 1.1, −1144.73 ± 1.0 and 97.2.

Geochimica et Cosmochimica Acta↗
USGS1985

Some mineral stability relations in the system CaO MgO SiO2 H2O HCl

Mineral-aqueous solution equilibria for the assemblages talc-quartz, tremolite-talc-quartz, diopside-tremolite-quartz, wollastonite-diopside-quartz and wollastonite-quartz have been studied at 2 kb total pressure, 500° to 700°C and chloride concentrations from 0.03 to 6.0 molal. Most work was at 1 m chloride. Both buffered and unbuffered data were obtained and a recalibration of the Ag-AgCl buffer is presented. Log equilibrium quotients at 500°, 600° and 700°C are respectively: Ta-Qz ( m MgCl 2 m HCl 2 "> mMgCl2mHCl2 ) 2.57, 1.71, 0.73; Tr-Ta-Qz and Di-Tr-Qz ( m CaCl 2 m MgCl 2 m HCl 2 "> mCaCl2mMgCl2mHCl2 ) 4.98, 3.99, 2.21 and 7.29, 5.30, 3.56; WoDi-Qz ( m CaCl 2 m MgCl 2 "> mCaCl2mMgCl2 ) 3.30, 3.00, 2.79: Wo-Qz ( m CaCl 2 m HCl 2 "> mCaCl2mHCl2 ) 5.15, 3.95, 2.68. Mineral stability fields plotted in terms of these concentration data more tangibly represent the compositional character of real systems and the mass transfer capabilities of their fluids than do the analogous theoretical activity diagrams. Overall dissociation constants of MgCl 2 and CaCl 2 were calculated from the experimental data using the calculated ionic activity constants for the reactions and the established dissociation constants of HCl. The negative log values are respectively: 3.88. 6.63, 9.20 for CaCl 2 and 4.60, 7.54, 10.37 for MgCl 2 at 500°, 600° and 700°C, 2 kb. The Ca values are about an order of magnitude more positive than the conductance-derived values by Frantz and Marshall (1982). The phase relations developed in this study have application to the genesis of talc, tremolite, and diopside-bearing assemblages in some regional metamorphic rocks, but more specifically to the calcsilicate skarn assemblages of many metasomatic aureoles. The equilibrium fluids are characterized by high concentrations of Ca relative to Mg and increasing Ca Mg "> CaMg ratios with decreasing temperatures. The stability fields of talc, tremolite, and quartz expand relative to those of diopside and wollastonite with decreasing temperature, hence their more common appearance as retrograde products in skarn systems.

Geochimica et Cosmochimica Acta↗
USGS1985

The heat-capacity of ilmenite and phase equilibria in the system Fe-T-O

Low temperature adiabatic calorimetry and high temperature differential scanning calorimetry have been used to measure the heat-capacity of ilmenite (FeTiO 3 ) from 5 to 1000 K. These measurements yield S 298 0 = 108.9 J /( mol · K ). Calculations from published experimental data on the reduction of ilmenite yield Δ 298 0 ( I 1) = −1153.9 kJ /( mol · K ). These new data, combined with available experimental and thermodynamic data for other phases, have been used to calculate phase equilibria in the system Fe-Ti-O. Calculations for the subsystem Ti-O show that extremely low values of ƒO 2 ">ƒ ƒO2 are necessary to stabilize TiO, the mineral hongquiite reported from the Tao district in China. This mineral may not be TiO, and it should be re-examined for substitution of other elements such as N or C. Consideration of solid-solution models for phases in the system Fe-Ti-O allows derivation of a new thermometer/oxybarometer for assemblages of ferropseudobrookite-pseudobrookite ss and hematite-ilmenite ss . Preliminary application of this new thermometer/oxybarometer to lunar and terrestrial lavas gives reasonable estimates of oxygen fugacities, but generally yields subsolidus temperatures, suggesting re-equilibration of one or more phases during cooling.

Geochimica et Cosmochimica Acta↗
USGS1985

Pyrolysis-mass spectrometry/pattern recognition on a well-characterized suite of humic samples

A suite of well-characterized humic and fulvic acids of freshwater, soil and plant origin was subjected to pyrolysis-mass spectrometry and the resulting data were analyzed by pattern recognition and factor analysis. A factor analysis plot of the data shows that the humic acids and fulvic acids can be segregated into two distinct classes. Carbohydrate and phenolic components are more pronounced in the pyrolysis products of the fulvic acids, and saturated and unsaturated hydrocarbons contribute more to the humic acid pyrolysis products. A second factor analysis plot shows a separation which appears to be based primarily on whether the samples are of aquatic or soil origin.

Geochimica et Cosmochimica Acta↗
USGS1985

Chemistry and transport of soluble humic substances in forested watersheds of the Adirondack Park, New York

Studies were conducted in conjunction with the Integrated Lake-Watershed Acidification Study (ILWAS) to examine the chemistry and leaching patterns of soluble humic substances in forested watersheds of the Adirondack region. During the summer growing season, mean dissolved organic carbon (DOC) concentrations in the ILWAS watersheds ranged from 21–32 mg C l −1 in O/A horizon leachates, from 5–7 mg C l −1 in B horizon leachates, from 2–4 mg C l −1 in groundwater solutions, from 6–8 mg C l −1 in first order streams, from 3–8 mg C l −1 in lake inlets, and from 2–7 mg C l −1 in lake outlets. During the winter, mean DOC concentrations dropped significantly in the upper soil profile. Soil solutions from mixed and coniferous stands contained as much as twice the DOC concentration of lysimeter samples from hardwood stands. Results of DOC fractionation analysis showed that hydrophobia and hydrophilic acids dominate the organic solute composition of natural waters in these watersheds. Charge balance and titration results indicated that the general acid-base characteristics of the dissolved humic mixture in these natural waters can be accounted for by a model organic acid having an average p K a of 3.85, an average charge density of 4–5 μeq mg −1 C at ambient pH, and a total of 6–7 meq COOH per gram carbon.

New York↗
USGS1985

Degradation of carbohydrates and lignins in buried woods

Spruce, alder, and oak woods deposited in coastal sediments were characterized versus their modern counterparts by quantification of individual neutral sugars and lignin-derived phenols as well as by scanning electron microscopy, 13 C NMR, and elemental analysis. The buried spruce wood from a 2500 yr old deposit was unaltered whereas an alder wood from the same horizon and an oak wood from an open ocean sediment were profoundly degraded. Individual sugar and lignin phenol analyses indicate that at least 90 and 98 wt% of the initial total polysaccharides in the buried alder and oak woods, respectively, have been degraded along with 15–25 wt% of the lignin. At least 75% of the degraded biopolymer has been physically lost from these samples. This evidence is supported by the SEM, 13 C NMR and elemental analyses, all of which indicate selective loss of the carbohydrate moiety. The following order of stability was observed for the major biochemical constituents of both buried hardwoods: vanillyl and p- hydroxyl "> p-hydroxyl lignin structural units > syringyl and lignin structural units > pectin > α-cellulose > hemicellulose. This sequence can be explained by selective preservation of the compound middle lamella regions of the wood cell walls. The magnitude and selectivity of the indicated diagenetic reactions are sufficient to cause major changes in the chemical compositions of wood-rich sedimentary organic mixtures and to provide a potentially large in situ nutrient source.

Geochimica et Cosmochimica Acta↗
USGS1985

Kinetic and thermodynamic factors controlling the distribution of SO32- and Na+ in calcites and selected aragonites

Significant amounts of SO 4 2− , Na + , and OH − are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO 4 2− . Aragonites and most biogenic low Mg-calcites contain significant amounts of Na + , but very low concentrations of SO 4 2− . The SO 4 2− content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na + and SO 4 2− increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO 4 2− beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO 4 2− is the same as that of aragonite. Na + appears to have very little effect on the solubility product of calcites. The amounts of Na + and SO 4 2− incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient ( D "> D ) of SO 4 2− in calcite at 25.0°C and 0.50 molal NaCl is described by the equation D = k 0 + k 1 R "> D = k 0 + k 1 R where k 0 "> k 0 and k 1 "> k 1 are constants equal to 6.16 × 10 −6 "> 6.16 × 10 −6 and 3.941 × 10 −6 "> 3.941 × 10 −6 , respectively, and R "> R is the rate of crystal growth of calcite in mg·min −1 ·g −1 of seed. The data on Na + are consistent with the hypothesis that a significant amount of Na + occupies interstitial positions in the calcite structure. The distribution of Na + follows a Freundlich isotherm and not the Berthelot-Nernst distribution law. The numerical value of the Na + distribution coefficient in calcite is probably dependent on the number of defects in the calcite structure. The Na + contents of calcites are not very accurate indicators of environmental salinities.

Geochimica et Cosmochimica Acta↗
USGS1985

Geochemistry of Great Salt Lake, Utah I: Hydrochemistry since 1850

The hydrochemistry of Great Salt Lake, Utah, has been defined for the historic period, 1850 through 1982, from published data combined with new observations. The water balance depends largely on river inflow, atmospheric precipitation onto the lake surface and evaporation. Input of the major solutes can best be accounted for by mixing dilute calcium-bicarbonate type river waters with NaCl-dominated hydrothermal springs. Prior to 1930, lake concentrations fluctuated inversely with lake volume in response to small climatic variations. Since then, salt precipitation and dissolution have significantly modified lake brine compositions and have led to density stratification and the formation of brine pockets of differing composition. Brine mixing has become an important component of brine evolution. We have used calculated evaporation curves with mineral precipitation and dissolution to clarify these processes. Pore fluids represent important storage for solutes. Solute profiles can be modeled by simple one-dimensional diffusion calculations. Short-term historic variations in lake composition affect shallow pore fluids in the upper 2 metres of sediment. ?? 1985.

Utah↗
USGS1985

Geochemistry of great Salt Lake, Utah II: Pleistocene-Holocene evolution

Sedimentologic and biostratigraphic evidence is used to develop a geochemical model for Great Salt Lake, Utah, extending back some 30,000 yrs. B.P. Hydrologie conditions as defined by the water budget equation are characterized by a lake initially at a low, saline stage, rising by about 17,000 yrs. B.P. to fresh water basin-full conditions (Bonneville level) and then, after about 15,000 yrs. B.P., dropping rapidly to a saline stage again, as exemplified by the present situation. Inflow composition has changed through time in response to the hydrologie history. During fresh-water periods high discharge inflow is dominated by calcium bicarbonate-type river waters; during saline stages, low discharge, NaCl-rich hydrothermal springs are significant solute sources. This evolution in lake composition to NaCl domination is illustrated by the massive mirabilite deposition, free of halite, following the rapid drawdown until about 8,000 years ago, while historic droughts have yielded principally halite. Hydrologic history can be combined with inferred inflow composition to derive concentration curves with time for each major solute in the lake. Calcium concentrations before the drawdown were controlled by calcite solubility, and afterwards by aragonite. Significant amounts of solutes are removed from the lake by diffusion into the sediments. Na+, Cl- and SO42- are also involved in salt precipitation. By including pore fluid data, a surprisingly good fit has been obtained between solute input over the time period considered and the amounts actually found in lake brines, pore fluids, salt beds and sediments. Excess amounts are present for calcium, carbonate and silica, indicating detrital input.

Utah↗
USGS1985

Thermodynamic stability of CoOOH and its coprecipitation with manganese

A precipitate of cobalt oxyhydroxides formed by bubbling oxygen through a dilute solution of Co(NO 3 ) 2 held at pH 9.0 and 25°C was aged for 23 months in contact with the original solution, with access to atmospheric oxygen. Co 3 O 4 and CoOOH were identified in the precipitate by X-ray diffraction. Chemical equilibria involving these solids were evaluated by measurements of solution pH and Co 2+ activities and by redox potential measurements and gave a ΔG coOOH 0 "> ΔGcoOOH0 of −92.3 ± 0.5 kcal/mole "> −92.3 ± 0.5 kcal/mole . This value and other thermodynamic data show relative feasibility of hypothetical reaction steps and changes in reaction paths during automated coprecipitation titrations and subsequent aging of a precipitate that finally contained βMnOOH, MnO 2 (birnessite) and CoOOH.

Geochimica et Cosmochimica Acta↗
USGS1985

Geochemistry and petrogenesis of lamproites, late cretaceous age, Woodson County, Kansas, U.S.A.

Lamproite sills and their associated sedimentary and contact metamorphic rocks from Woodson County, Kansas have been analyzed for major elements, selected trace elements, and strontium isotopic composition. These lamproites, like lamproites elsewhere, are alkalic (molecular K 2 O + Na 2 O Al 2 O 3 = 1.6–2.6 "> K 2 O + Na 2 OAl 2 O 3 = 1.6–2.6 ), are ultrapotassic ( K 2 O Na 2 O = 9.6–150) "> (K 2 ONa 2 O= 9.6–150) , are enriched in incompatible elements (LREE or light rare-earth elements, Ba, Th, Hf, Ta, Sr, Rb), and have moderate to high initial strontium isotopic compositions (0.7042 and 0.7102). The silica-saturated magma (olivine-hypersthene normative) of the Silver City lamproite could have formed by about 2 percent melting of a phlogopite-garnet lherzolite under high H 2 O CO 2 "> H 2 OCO 2 ratios in which the Iherzolite was enriched before melting in the incompatible elements by metasomatism. The Rose Dome lamproite probably formed in a similar fashion although the extreme alteration due to addition of carbonate presumably from the underlying limestone makes its origin less certain. Significant fractional crystallization of phases that occur as phenocrysts (diopside, olivine, K-richterite, and phlogopite) in the Silver City magma and that concentrate Co, Cr, and Sc are precluded as the magma moved from the source toward the surface due to the high abundances of Co, Cr, and Sc in the magma similar to that predicted by direct melting of the metasomatized Iherzolite. Ba and, to a lesser extent, K and Rb and have been transported from the intrusions at shallow depth into the surrounding contact metamorphic zone. The Silver City lamproite has vertical fractionation of some elements due either to volatile transport or to variations in the abundance of phenocrysts relative to groundmass most probably due to flow differentiation although multiple injection or fractional crystallization cannot be conclusively rejected.

Geochimica et Cosmochimica Acta↗
USGS1986

10Be distribution in soils from Merced River terraces, California

The distribution and residence time of cosmogenic 10 Be in clay-rich soil horizons is fundamental to understanding and modelling the migration of 10 Be on terrestrial sediments and in groundwater solutions. We have analyzed seven profiles of clay-rich soils developed from terrace sediments of the Merced River, California. The terraces and soils of increasing age are used to compare the 10 Be inventory with a simple model of accumulation, decay and erosion. The data show that the distribution of 10 Be varies with soil horizon clay content, that the residence time of 10 Be in these horizons exceeds 10 5 years, and that to a rough approximation the inventory of 10 Be in a thoroughly sampled soil profile fits the equation: N = ( q − Em )(1 − e − λι )/ λ where q is delivery rate, E is erosion rate, m is the concentration of 10 Be in the eroding surface layer, λ is the decay constant, and t is the age of the depositional unit from which the soil has developed. The general applicability of this model is uncertain and warrants further testing in well-calibrated terrace sequences.

Geochimica et Cosmochimica Acta↗
USGS1986

Sr, Nd and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador: Implications for crustal contamination and basement mapping

We report Sr, Nd and Pb isotopic compositions of mid-Proterozoic anorthosites and related rocks (1.45-1.65 Ga) and of younger olivine diabase dikes (1.4 Ga) from two complexes on either side of the Grenville Front in Labrador. Anorthositic or diabasic samples from the Mealy Mountains (Grenville Province) and Harp Lake (Nain-Churchill Provinces) complexes have very similar major, minor and trace element compositions, but distinctly different isotopic signatures. All Mealy Mountains samples have I Sr = 0.7025−0.7033, ε Nd = +0.6 to +5.6 and Pb isotopic compositions consistent with derivation from a mantle source depleted with respect to Nd/Sm and Rb/Sr. Pb isotopic compositions for the Mealy Mountains samples are slightly more radiogenic than model mantle compositions. All Harp Lake samples have I Sr = 0.7032−0.7066, ε Nd = −0.3 to −4.4 and variable, but generally unradiogenic 207 Pb/ 204 Pb and 206 Pb/ 204 Pb compared to model mantle, suggesting mixing between a mantle-derived component and a U-depleted crustal contaminant. Crustal contaminants are probably a variety of Archean high-grade quartzofeldspathic gneisses with low U/Pb ratios and include a component that must be isotopically similar to the early Archean (>3.6 Ga) Uivak gneisses of Labrador or the Amitsoq gneisses of west Greenland. This would imply that the ancient gneiss complex of coastal Labrador and Greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the Labrador Trough. If Harp Lake and Mealy Mountains samples were subjected to the same degree of contamination, as suggested by their chemical similarities, then the Mealy contaminants must be much younger, probably early or middle Proterozoic in age. The Labrador segment of the Grenville Front, therefore, appears to coincide with the southern margin of the Archean North Atlantic craton and may represent a pre mid-Proterozoic suture.

Geochimica et Cosmochimica Acta↗
USGS1986

Nd, O and Sr isotopic constraints on the origin of Precambrian rocks, southern Black Hills, South Dakota

The Nd, O and Sr isotopic characteristics of Precambrian metasedimentary, metavolcanic and granitic rocks from the Black Hills of South Dakota are examined. Two late-Archean granites (2.5-2.6 Ga) have T dm ages of 3.05 and 3.30 Ga, suggesting that at least one of the granites was derived through the melting of significantly older crust. Early-Proterozoic metasedimentary rocks have T dm ages that range from 2.32 to 2.45 Ga. These model ages, in conjunction with probable stratigraphic ages ranging from 1.9 to 2.2 Ga, indicate that mantle-derived material was added to the continental crust of this region during the early-Proterozoic. Previous studies of the Harney Peak Granite complex have reported U-Pb and Rb-Sr ages of about 1.71 Ga and most granite samples examined in this study have Sr isotopic compositions consistent with that age. Two granite samples taken from the same sill, however, give two-point Rb-Sr and Sm-Nd ages of 2.08 ±0.08 and 2.20 ±0.20Ga (∑ 2200 Nd = −15.5), respectively. In addition, whole-rock and apatite samples of the spatially associated Tin Mountain pegmatite give a Sm-Nd isochron age of 2000 ±100 Ma (∑ 2200 Nd = −5.8 ±1.8). The Sm-Nd, O and Rb-Sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post-crystallization processes, and the age of the pegmatite and parts of the Harney Peak Granite complex remain uncertain. Processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material, assimilation, mixing of REE between granite and country rock during crystallization via a fluid phase and post-crystallization mobility of Sr. The Nd isotopic compositions of the pegmatite and the Harney Peak Granite indicate that they were not derived primarily from the exposed metasedimentary rocks.

Geochimica et Cosmochimica Acta↗