Kinetics of CO_2(g)-H_2O(l) isotopic exchange, including ^(13)C^(18)O^(16)O

The analysis of mass 47 isotopologues of CO_2 (mainly ^(13)C^(18)O^(16)O) is established as a constraint on sources and sinks of environmental CO_2, complementary to δ^(13)C and δ^(18)O constraints, and forms the basis of the carbonate ‘clumped isotope’ thermometer. This measurement is commonly reported using the Δ_(47) value — a measure of the enrichment of doubly substituted CO_2 relative to a stochastic isotopic distribution. Values of Δ_(47) approach 0 (a random distribution) at high temperatures (≥ several hundred degrees C), and increase with decreasing temperature, to ~1 ‰ at 25 °C

The hydrogen isotopic composition and water content of southern Pacific MORB: A reassessment of the D/H ratio of the depleted mantle reservoir

In this paper, we re-investigate the isotopic composition of hydrogen in MORB and the possible effects of contamination on δD and water content. A suite of 40 N-MORB from the Pacific–Antarctic ridge, far from any hotspot, was analyzed for chlorine content by electron microprobe and for water content and δD with silica tubes. Cl concentrations (from 29 to 2400 ppm) indicate widespread contamination, more intense with faster spreading rates, while water contents (from 840 to 7800 ppm) are mainly controlled by igneous processes. δD values range from −76 to −48‰−48‰, with an average value of −61‰−61‰. The lack of correlation between Cl content and either H_2O/Ce or δD indicate that contamination has a negligible effect on δD for our samples, which is therefore characteristic of the mantle below the Pacific–Antarctic ridge. We suggest that the 20‰ lower δD value reported for the North Pacific and North Atlantic is highly unlikely from geodynamical arguments. We propose that the convecting mantle is characterized by a δD of −60±5‰−60±5‰, as supported by the most recent data from North Atlantic N-MORB

A reconnaissance study of 13C- 13C clumping in ethane from natural gas

Ethane is the second most abundant alkane in most natural gas reservoirs. Its bulk isotopic compositions (δ13C and δD) are used to understand conditions and progress of cracking reactions that lead to the accumulation of hydrocarbons. Bulk isotopic compositions are dominated by the concentrations of singly-substituted isotopologues (13CH3-12CH3 for δ13C and 12CDH2-12CH3 for δ D). However, multiply-substituted isotopologues can bring additional independent constraints on the origins of natural ethane. The 13C2H6 isotopologue is particularly interesting as it can potentially inform the distribution of 13C atoms in the parent biomolecules whose thermal cracking lead to the production of natural gas. This work presents methods to purify ethane from natural gas samples and quantify the abundance of the rare isotopologue 13C2H6 in ethane at natural abundances to a precision of ±0.12‰ using a high-resolution gas source mass spectrometer. To investigate the natural variability in carbon-carbon clumping, we measured twenty-five samples of thermogenic ethane from a range of geological settings, supported by two hydrous pyrolysis of shales experiments and a dry pyrolysis of ethane experiment. The natural gas samples exhibit a range of ’clumped isotope’ signatures (Δ13C2H6) at least 30 times larger than our analytical precision, and significantly larger than expected for thermodynamic equilibration of the carbon-carbon bonds during or after formation of ethane, inheritance from the distribution of isotopes in organic molecules or different extents of cracking of the source. However we show a relationship between the Δ13C2H6 and the proportion of alkanes in natural gas samples, which we believe can be associated to the extent of secondary ethane cracking. This scenario is consistent with the results of laboratory experiments, where breaking down ethane leaves the residue with a low Δ13C2H6 compared to the initial gas. Carbon-carbon clumping is therefore a new potential tracer suitable for the study of kinetic processes associated with natural gas

The MAT-253 Ultra — a novel high-resolution, multi-collector gas source mass spectrometer

We present the design, performance and representative applications of the MAT 253 Ultra – the first prototype of a new class of high-resolution gas source isotope ratio mass spectrometers

Insights into the response of coral biomineralisation to environmental change from aragonite precipitations in vitro

Funding: This work was supported by the UK Natural Environment Research Council (NE/S001417/1) to NA, KP, RK, MC and AF. We thank Gavin Peters, University of St Andrews, for assistance with BET analyses. Electron microscopy was carried out in the Aberdeen Centre for Electron Microscopy, Analysis and Characterisation (ACEMAC).Precipitation of marine biogenic CaCO3 minerals occurs at specialist sites, typically with elevated pH and dissolved inorganic carbon, and in the presence of biomolecules which control the nucleation, growth, and morphology of the calcium carbonate structure. Here we explore aragonite precipitation in vitro under conditions inferred to occur in tropical coral calcification media under present and future atmospheric CO2 scenarios. We vary pH, ΩAr and pCO2 between experiments to explore how both HCO3- and CO32- influence precipitation rate and we identify the effects of the three most common amino acids in coral skeletons (aspartic acid, glutamic acid and glycine) on precipitation rate and aragonite morphology. We find that fluid ΩAr or [CO32-] is the main control on precipitation rate at 25°C, with no significant contribution from HCO3- or pH. All amino acids inhibit aragonite precipitation at 0.2-5 mM and the degree of inhibition is inversely correlated with ΩAr and, in the case of aspartic acid, also inversely correlated with seawater temperature. Aspartic acid inhibits precipitation the most, of the tested amino acids (and generates changes in aragonite morphology) and glycine inhibits precipitation the least. Previous work shows that ocean acidification increases the amino acid content of coral skeletons and probably reduces calcification media ΩAr, both of which can inhibit aragonite precipitation. This study and previous work shows aragonite precipitation rate is exponentially related to temperature from 10-30°C and small anthropogenic increases in seawater temperature will likely offset the inhibition in precipitation rate predicted to occur due to increased skeletal aspartic acid and reduced calcification media ΩAr under ocean acidification.Publisher PDFPeer reviewe

Insights into the response of coral biomineralisation to environmental change from aragonite precipitations in vitro

This work was supported by the UK Natural Environment Research Council (NE/S001417/1) to NA, KP, RK, MC and AF. We thank Gavin Peters, University of St Andrews, for assistance with BET analyses. Electron microscopy was carried out in the Aberdeen Centre for Electron Microscopy, Analysis and Characterisation (ACEMAC).Precipitation of marine biogenic CaCO3 minerals occurs at specialist sites, typically with elevated pH and dissolved inorganic carbon, and in the presence of biomolecules which control the nucleation, growth, and morphology of the calcium carbonate structure. Here we explore aragonite precipitation in vitro under conditions inferred to occur in tropical coral calcification media under present and future atmospheric CO2 scenarios. We vary pH, ΩAr and pCO2 between experiments to explore how both HCO3- and CO32- influence precipitation rate and we identify the effects of the three most common amino acids in coral skeletons (aspartic acid, glutamic acid and glycine) on precipitation rate and aragonite morphology. We find that fluid ΩAr or [CO32-] is the main control on precipitation rate at 25°C, with no significant contribution from HCO3- or pH. All amino acids inhibit aragonite precipitation at 0.2-5 mM and the degree of inhibition is inversely correlated with ΩAr and, in the case of aspartic acid, also inversely correlated with seawater temperature. Aspartic acid inhibits precipitation the most, of the tested amino acids (and generates changes in aragonite morphology) and glycine inhibits precipitation the least. Previous work shows that ocean acidification increases the amino acid content of coral skeletons and probably reduces calcification media ΩAr, both of which can inhibit aragonite precipitation. This study and previous work shows aragonite precipitation rate is exponentially related to temperature from 10-30°C and small anthropogenic increases in seawater temperature will likely offset the inhibition in precipitation rate predicted to occur due to increased skeletal aspartic acid and reduced calcification media ΩAr under ocean acidification.Publisher PDFPeer reviewe

Clumped isotope and Δ17O measurements of carbonates in CM carbonaceous chondrites: new insights into parent body thermal and fluid evolution

The CM carbonaceous chondrites are key archives for understanding the earliest history of the solar system. Their C-complex asteroid parent body(ies) underwent aqueous alteration, among the products of which are carbonate minerals that can faithfully record the conditions of their formation. In this study we report carbon, triple oxygen and clumped isotope compositions of carbonates in six CM chondrites which span a range in degrees of aqueous alteration (Allan Hills 83100, Cold Bokkeveld, LaPaz Icefield 031166, Lonewolf Nunataks 94101, Murchison, Scott Glacier 06043). Δ¹⁷O values range from −2.6 to −1.0 ‰ (±0.1), and where calcite and dolomite co-exist their Δ¹⁷O differ by 0.6 permil, suggesting precipitation from distinct fluids. Calculated crystallization temperatures range from 5 to 51 °C for calcite (typically ± 10 °C) and 75 to 101(±15) °C for dolomite. The δ¹⁸ᴼVSMOW of the aqueous fluids from which they formed ranges from −6.6 to 2.3 ‰, with no relationship to the δ¹³C of carbonates. As the population of carbonates in any one CM chondrite can include multiple generations of grains that formed at different conditions, these values represent the mode of the temperature of carbonate formation for each meteorite. We observe that in the more altered meteorites carbonate Δ¹⁷O values are lower and formation temperatures are higher. These correlations are consistent with aqueous alteration of the CM chondrites being a prograde reaction whereby the hotter fluids had undergone greater isotope exchange with the anhydrous matrix. Our data are broadly consistent with the closed system model for water/rock interaction, but carbonate mineral formation in the latter stages of aqueous alteration may be linked to fluid movement via fractures

Basin scale evolution of zebra textures in fault-controlled, hydrothermal dolomite bodies: insights from the Western Canadian Sedimentary Basin

Structurally controlled dolomitization typically involves the interaction of high-pressure (P), high-temperature (T) fluids with the surrounding host rock. Such reactions are often accompanied by cementation and recrystallization, with the resulting hydrothermal dolomite (HTD) bodies including several ‘diagnostic’ rock textures. Zebra textures, associated with boxwork textures and dolomite breccias, are widely considered to reflect these elevated P/T conditions. Although a range of conceptual models have been proposed to explain the genesis of these rock textures, the processes that control their spatial and temporal evolution are still poorly understood. Through the detailed petrographical and geochemical analysis of HTD bodies, hosted in the Middle Cambrian strata in the Western Canadian Sedimentary Basin, this study demonstrates that a single genetic model cannot be applied to all the characteristics of these rock textures. Instead, a wide array of sedimentological, tectonic and metasomatic processes contribute to their formation; each of which is spatially and temporally variable at the basin scale. Distal to the fluid source, dolomitization is largely stratabound, comprising replacement dolomite, bedding-parallel zebra textures and rare dolomite breccias (non-stratabound, located only proximal to faults). Dolomitization is increasingly non-stratabound with proximity to the fluid source, comprising bedding-inclined zebra textures, boxwork textures and dolomite breccias that have been affected by recrystallization. Petrographical and geochemical evidence suggests that these rock textures were initiated due to dilatational fracturing, brecciation and precipitation of saddle dolomite as a cement, but significant recrystallization occurred during the later stages of dolomitization. These rock textures are closely associated with faults and carbonate-hosted ore deposits (e.g. magnesite, rare earth element and Mississippi Valley–type mineralization), thus providing invaluable information regarding fluid flux and carbonate metasomatism under elevated P/T conditions

The chlorine isotopic composition of the Moon: Insights from melt inclusions

The Moon exhibits a heavier chlorine (Cl) isotopic composition compared to the Earth. Several hypotheses have been put forward to explain this difference, based mostly on analyses of apatite in lunar samples complemented by bulk-rock data. The earliest hypothesis argued for Cl isotope fractionation during the degassing of anhydrous basaltic magmas on the Moon. Subsequently, other hypotheses emerged linking Cl isotope fractionation on the Moon with the degassing during the crystallization of the Lunar Magma Ocean (LMO). Currently, a variant of the LMO degassing model involving mixing between two end-member components, defined by early-formed cumulates, from which mare magmas were subsequently derived, and a KREEP component, which formed towards the end of the LMO crystallization, seems to reconcile some existing Cl isotope data on lunar samples. To further ascertain the history of Cl in the Moon and to investigate any evolution of Cl during magma crystallization and emplacement events, which could help resolve the chlorine isotopic variation between the Earth and the Moon, we analysed the Cl abundance and its isotopic composition in 36 olivine- and pyroxene-hosted melt inclusions (MI) in five Apollo basalts (10020, 12004, 12040, 14072 and 15016). Olivine-hosted MI have an average of 3.3 ± 1.4 ppm Cl. Higher Cl abundances (11.9 ppm on average) are measured for pyroxene-hosted MI, consistent with their formation at later stages in the crystallization of their parental melt compared to olivines. Chlorine isotopic composition (δ37) of MI in the five Apollo basalts have weighted averages of +12 ± 2.4‰ and +10.1 ± 3.2‰ for olivine- and pyroxene-hosted MI, respectively, which are statistically indistinguishable. These isotopic compositions are also similar to those measured in apatite in these lunar basalts, with the exception of sample 14072, which is known to have a distinct petrogenetic history compared to other mare basalts. Based on our dataset, we conclude that, post-MI-entrapment, no significant Cl isotopic fractionation occurred during the crystallization and subsequent eruption of the parent magma and that Cl isotopic composition of MI and apatite primarily reflect the signature of the source region of these lunar basalts. Our findings are compatible with the hypothesis that in the majority of the cases the heavy Cl isotopic signature of the Moon was acquired during the earliest stages of LMO evolution. Interestingly, MI data from 14072 suggests that Apollo 14 lunar basalts might be an exception and may have experienced post-crystallization processes, possibly metasomatism, resulting in additional Cl isotopic fractionation recorded by apatite but not melt inclusions

Methane Clumped Isotopes: Progress and Potential for a New Isotopic Tracer

The isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric greenhouse gas concentrations, the global carbon cycle, and life in extreme environments. Recent analytical developments focusing on multiply substituted isotopologues (‘clumped isotopes’) are opening a valuable new window into methane geochemistry. When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions. We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism