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The formation and segregation of oceanic and continental crust from the mantle, and its return to the mantle via subduction and/or delamination, leads to the development of distinct geochemical reservoirs in the terrestrial mantle. Fundamental questions remain regarding the location, nature, and residence time of these reservoirs, as well as the respective roles of oceanic and continental crust in the development of the mantle's geochemical endmembers. The Lu-Hf and Sm-Nd isotope systems behave similarly in magmatic systems and together form the terrestrial mantle Hf-Nd isotopic array. Here we combine a geodynamic model of mantle convection with isotope and trace element (TE) geochemistry to investigate the evolution of the Hf-Nd mantle array. This study examines the sensitivity to: TE partition coefficients used in the formation of oceanic crust; density contrasts between subducting oceanic crust and the mantle; and the formation and recycling of continental crust. We show that the fractionation between the parent (Lu and Sm) and daughter (Hf and Nd) species needs to be higher than is indicated by partition coefficients determined from the present-day melting environment. This is consistent with the suggestion of deeper mantle melting earlier in Earth history and an increased role for residual garnet. Subduction and accumulation of dense oceanic crust produces a large mass of incompatible TE enriched material in the deep mantle. This deep mantle enrichment appears to play a more significant role than the extraction and recycling of continental crust in developing the Hf and Nd isotope and TE compositions of the mid-ocean ridge mantle source. The corollary of this result is that the formation of the continental crust plays a secondary role, contrary to the currently accepted paradigm. Nevertheless, the inclusion of continental crust formation and recycling produces a broader model mantle array, which better reproduces the spread in the natural data set. This model a

The formation and segregation of oceanic and continental crust from the mantle, and its return to the mantle via subduction and/or delamination, leads to the development of distinct geochemical reservoirs in the terrestrial mantle. Fundamental questions remain regarding the location, nature, and residence time of these reservoirs, as well as the respective roles of oceanic and continental crust in the development of the mantle's geochemical endmembers. The Lu-Hf and Sm-Nd isotope systems behave similarly in magmatic systems and together form the terrestrial mantle Hf-Nd isotopic array. Here we combine a geodynamic model of mantle convection with isotope and trace element (TE) geochemistry to investigate the evolution of the Hf-Nd mantle array. This study examines the sensitivity to: TE partition coefficients used in the formation of oceanic crust; density contrasts between subducting oceanic crust and the mantle; and the formation and recycling of continental crust. We show that the fractionation between the parent (Lu and Sm) and daughter (Hf and Nd) species needs to be higher than is indicated by partition coefficients determined from the present-day melting environment. This is consistent with the suggestion of deeper mantle melting earlier in Earth history and an increased role for residual garnet. Subduction and accumulation of dense oceanic crust produces a large mass of incompatible TE enriched material in the deep mantle. This deep mantle enrichment appears to play a more significant role than the extraction and recycling of continental crust in developing the Hf and Nd isotope and TE compositions of the mid-ocean ridge mantle source. The corollary of this result is that the formation of the continental crust plays a secondary role, contrary to the currently accepted paradigm. Nevertheless, the inclusion of continental crust formation and recycling produces a broader model mantle array, which better reproduces the spread in the natural data set. This model a

CO2 sources, sinks and migration mechanisms in natural CO2 gas fields provide critical analogues for developing the safe application of anthropogenic CO2 sequestration technologies. Here we use noble gas and carbon isotopes, together with other gases, to identify and quantify the origin, transport and trapping mechanisms of CO2 in the Late Cretaceous Jackson Dome CO2 gas deposit (98.75% to 99.38% CO2). Located in central Mississippi, USA, and producing from >5000 m, it is one of the deepest commercial CO2 gas fields in the world. 10 gas samples from producing wells were determined for their noble gas, chemical and stable carbon isotope composition. He-3/He-4 ratios range between 4.27R(a) and 5.01R(a) (where R-a is the atmospheric value of 1.4 x 10(-6)), indicating a strong mantle signature. Similar to CO2 deposits worldwide, CO2/He-3 decreases with increasing groundwater-derived Ne-20 (and He-4). We model several different processes that could account for the Jackson Dome data, and conclude that, similar to other CO2 dominated deposits, a Groundwater Gas Stripping and Re-dissolution (GGS-R) process best accounts for observed Ne-20/Ar-36, Kr-84/Ar-36, CO2/He-3, delta C-13(CO2), He-4, Ne-20 and Ar-36. In this context, crustal and magmatic CO2 components contribute 57% and 43%, respectively. Changes in CO2/He-3 across the field show that groundwater contact is responsible for up to 75% loss of original emplaced CO2. delta C-13(CO2) variance limits the degree of precipitation to be less than 27%, with the remaining CO2 loss being accounted for by dissolution only. A higher degree of dissolution gas loss and evidence for water contact at the reservoir crest compared to the reservoir flanks is used to argue that CO2 in this system has not undergone subsequent loss to either dissolution or precipitation since shortly after reservoir filling at over 60 Ma. (C) 2012 Elsevier Ltd. All rights reserved.

CO2 sources, sinks and migration mechanisms in natural CO2 gas fields provide critical analogues for developing the safe application of anthropogenic CO2 sequestration technologies. Here we use noble gas and carbon isotopes, together with other gases, to identify and quantify the origin, transport and trapping mechanisms of CO2 in the Late Cretaceous Jackson Dome CO2 gas deposit (98.75% to 99.38% CO2). Located in central Mississippi, USA, and producing from >5000 m, it is one of the deepest commercial CO2 gas fields in the world. 10 gas samples from producing wells were determined for their noble gas, chemical and stable carbon isotope composition. He-3/He-4 ratios range between 4.27R(a) and 5.01R(a) (where R-a is the atmospheric value of 1.4 x 10(-6)), indicating a strong mantle signature. Similar to CO2 deposits worldwide, CO2/He-3 decreases with increasing groundwater-derived Ne-20 (and He-4). We model several different processes that could account for the Jackson Dome data, and conclude that, similar to other CO2 dominated deposits, a Groundwater Gas Stripping and Re-dissolution (GGS-R) process best accounts for observed Ne-20/Ar-36, Kr-84/Ar-36, CO2/He-3, delta C-13(CO2), He-4, Ne-20 and Ar-36. In this context, crustal and magmatic CO2 components contribute 57% and 43%, respectively. Changes in CO2/He-3 across the field show that groundwater contact is responsible for up to 75% loss of original emplaced CO2. delta C-13(CO2) variance limits the degree of precipitation to be less than 27%, with the remaining CO2 loss being accounted for by dissolution only. A higher degree of dissolution gas loss and evidence for water contact at the reservoir crest compared to the reservoir flanks is used to argue that CO2 in this system has not undergone subsequent loss to either dissolution or precipitation since shortly after reservoir filling at over 60 Ma. (C) 2012 Elsevier Ltd. All rights reserved.

At least 20% of the world's natural gas originates from methanogens subsisting on organic-rich coals and shales; however in-situ microbial methane production rates are unknown. Methanogens in the Upper Devonian New Albany Shale in the Illinois Basin extract hydrogen from low salinity formation water to form economic quantities of natural gas. Because of this association, constraining the source and timing of groundwater recharge will enable estimation of minimum in-situ metabolic rates. Thirty-four formation water and gas samples were analyzed for stable isotopes (oxygen and hydrogen), chloride, tritium, (14)C, and noble gases. Chloride and delta(18)O spatial patterns reveal a plume of water with low salinity (0.7 to 2154 mM) and delta(18)O values (-0.14 to -7.25 parts per thousand) penetrating similar to 1 km depth into evapo-concentrated brines parallel to terminal moraines of the Laurentide Ice Sheet, suggesting glacial mediated recharge. However, isotopic mixing trends indicate that the recharge endmember (similar to-7 parts per thousand delta(18)O) is higher than the assumed bulk ice sheet value ( For the majority of samples the atmosphere derived (4)He contribution is negligible, and the (4)He is dominated by a crustal radiogenic source, with near complete transfer of dissolved noble gases to the gas phase. In addition, mantle derived helium is negligible for all samples (<1%). Helium-4 ages of formation waters associated with natural gas accumulations range from 0.082 to 1.2 Ma. Thermogenic methane is associated with older fluids (average 1.0 Ma), as compared to microbial methane (average 0.33 Ma), consistent with chloride and delta(18)O data. However, all groundwater in the study area was influenced by Pleistocene recharge. Estimated in-situ microbial methane production rates range from 10 to 1000 TCF/Ma - similar to 10(4) to 10(6) times slower than average laboratory rates from coals. Findings from this study have implications for targeting undeveloped micr

The possibility of dating minerals by the accumulation of ^4He from U and Th decay has been recognized for many years (e.g., Strutt 1905), but in the century since the idea was first conceived, the method has rarely been applied successfully. After several investigations of (U-Th)/He dating of various minerals (e.g., Damon and Kulp 1957; Fanale and Kulp 1962; Damon and Green 1963; Turekian et al. 1970; Bender 1973; Leventhal 1975; Ferreira et al. 1975) the technique was essentially abandoned as yielding unreliable and usually low ages, presumably as a result of diffusive He loss possibly associated with radiation damage. In 1987, Zeitler and coworkers rekindled interest in the method by proposing that in the case of apatite, He ages might be meaningfully interpreted as ages of cooling through very low temperatures. Laboratory diffusion data presented by these authors indicated a closure temperature of about 100ºC, a value supported by more recent studies (Lippolt et al. 1994; Wolf et al. 1996b; Warnock et al. 1997). Consistent with this interpretation Wolf et al. (1996a) found that apatite He ages increase systematically with sample elevation in a mountain range, as expected for exhumation-induced cooling through a low closure temperature. Based on the strength of these results and additional laboratory (Farley 2000) and natural (Warnock et al. 1997; House et al. 1999; Stockli et al. 2000) constraints on He diffusivity, recent attention has focused on applications of apatite He thermochronometry. There is also renewed interest in He dating of other U- and Th-bearing minerals both for dating mineral formation and for thermochronometry. For example, Lippolt and coworkers have undertaken detailed studies of He diffusion and dating of various phases, most notably hematite formed in hydrothermal systems (Lippolt and Weigel 1988; Wernicke and Lippolt 1992; Lippolt et al. 1993; Wernicke and Lippolt 1994a,b). Here I present an overview of recent techniques, calibrations, an