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1. Understanding diamond-forming fluids and parental lithologies using Fe, Mg, and K isotopes

   Ni, Peng; Elazar, Oded; Shirey, Steven B; Weiss, Yaakov (July 2024, Journal of International Kimberlite Conference Abstracts (JIKCA))

   With the past decades of diamond inclusion research, it is now well established that the mantle-derived diamonds are originated either from the lithospheric mantle or sublithospheric mantle. The lithospheric diamonds can be further divided into mainly the peridotitic and eclogitic suites, which can be distinguished based on their inclusion chemistry, carbon, and nitrogen isotopic compositions (1, 2). The parental lithology of sublithospheric diamonds is less well established, partly due to their much lower occurrence relative to the lithospheric diamonds. But there has been growing isotopic evidence for the involvement of subducted materials in the source region of sublithospheric diamonds, such as carbon, boron, oxygen, and iron (3–6). Precipitation of diamonds in the Earth’s mantle has been thought to require the presence of a fluid phase. Being C-O-H, saline, carbonatitic, silicic, or metallic in composition, these fluids were released upon dehydration or partial melting of the parental lithology and migrate through the mantle until they reach diamond saturation point due to either the change in pressure-temperature, or redox conditions. Understanding the parental lithology and fluid composition of different diamonds has primarily relied on their carbon and nitrogen isotope compositions and major/trace element compositions of mineral/fluid inclusions. These tools have been shown to be powerful in many cases but each could have their own disadvantages. Nitrogen isotopes, for example, are less applicable to sublithospheric diamonds due to their low N concentration. Trace element compositions, on the other hand, can be easily manipulated by small mass fractions of low degree-melt that are enriched in incompatible elements. Understanding the diamond-forming fluids and their parental lithology require new tools that can provide a different perspective than the ones discussed above. In this presentation, we show recent developments in adapting Fe, Mg, and K isotope systems to diamond inclusion studies for a better understanding of their formation. These so-called “non-traditional” stable isotope systems were typically developed for large rocks that are not limited by sample amount. In order to adapt them to mineral inclusions tens to hundreds of micrometers in size, we’ve developed dedicated procedures to: 1) clean the diamond surface to remove contamination before extracting individual inclusions; 2) scale down the columns used for chemical purification to minimize blanks; and 3) improving sensitivity on the mass spectrometer to analyze small samples. With a Nu Plasma II at the Carnegie Institution for Science, we have shown to be able to analyze inclusion samples containing as little as 200 ng of Fe (6). With an upgraded Nu Plasma Sapphire at UCLA that is equipped with a collision cell, we are now able to analyze samples with >25 ng Fe. The same strategy has now been expanded to Mg and K isotope systems, for which a low sample limit of 25 ng and 300 ng has been achieved. With examples of Fe and Mg isotopic compositions of ferropericlase in sublithospheric diamond and K isotopic composition of fluid inclusions in fibrous diamonds, we show how isotopic compositions of major elements of mineral/fluid inclusions in diamond bring us new perspectives on their origin. Our tests show promising results to extend existing Mg and Fe protocols to silicate minerals and potentially applying similar strategies to silicon, calcium, and barium isotopes in the future. 

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2. Carbonate facies‐specific stable isotope data record climate, hydrology, and microbial communities in Great Salt Lake, UT

   https://doi.org/10.1111/gbi.12386  

   Ingalls, Miquela; Frantz, Carie M.; Snell, Kathryn E.; Trower, Elizabeth J. (March 2020, Geobiology)

   Abstract Organic and inorganic stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial ecosystems and environments. Microbial activity and local hydrological inputs can alter porewater chemistry (e.g., pH, alkalinity) and isotopic composition (e.g., δ¹⁸O_water, δ¹³C_DIC), which in turn has the potential to impact the stable isotopic compositions recorded and preserved in lithified carbonate. The fingerprint these syngenetic processes have on lacustrine carbonate facies is yet unknown, however, and thus, reconstructions based on stable isotopes may misinterpret diagenetic records as broader climate signals. Here, we characterize geochemical and stable isotopic variability of carbonate minerals, organic matter, and water within one modern lake that has known microbial influences (e.g., microbial mats and microbialite carbonate) and combine these data with the context provided by 16S rRNA amplicon sequencing community profiles. Specifically, we measure oxygen, carbon, and clumped isotopic compositions of carbonate sediments (δ¹⁸O_carb, δ¹³C_carb, ∆₄₇), as well as carbon isotopic compositions of bulk organic matter (δ¹³C_org) and dissolved inorganic carbon (DIC; δ¹³C_DIC) of lake and porewater in Great Salt Lake, Utah from five sites and three seasons. We find that facies equivalent to ooid grainstones provide time‐averaged records of lake chemistry that reflect minimal alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud show greater alteration by local microbial influence and hydrology. Further, we find at least one occurrence of ∆₄₇isotopic disequilibrium likely driven by local microbial metabolism during authigenic carbonate precipitation. The remainder of the carbonate materials (primarily ooids, grain coatings, mud, and intraclasts) yield clumped isotope temperatures (T(∆₄₇)), δ¹⁸O_carb, and calculated δ¹⁸O_waterin isotopic equilibrium with ambient water and temperature at the time and site of carbonate precipitation. Our findings suggest that it is possible and necessary to leverage diverse carbonate facies across one sedimentary horizon to reconstruct regional hydroclimate and evaporation–precipitation balance, as well as identify microbially mediated carbonate formation. 

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3. Evaluating the Efficacy of Collagen Isolation Using Stable Isotope Analysis and Infrared Spectroscopy

   Trayler, R. B. (January 2023, Journal of archaeological science)

   Stable isotope analysis of bone and dental collagen is one of the most common methods to investigate the ecology of modern and extinct human and animal populations. However, since bone and dentine are composite materials with both organic and mineral components, the mineral component must be removed prior to analysis. In this study we investigated the timing and efficacy of mineral removal from bone and dentine. We performed a series of time-step experiments that show that mineral removal can be quantified over short periods of time using Fourier Transform Infrared Spectroscopy (FTIR), and collagen alteration can be tracked using a combination of stable isotope analysis and elemental analysis. We tested our methods on three modern materials: mammalian bone, mammalian dentine, and shark dentine. Our results show: 1) mineral removal is a necessary step, as structural carbonate has a strong influence on stable isotope compositions; 2) demineralization using weak acid (0.1M HCL) does not appear to alter the elemental and isotopic compositions of collagen. Our methods can be used as a framework to evaluate the need-for and efficacy of other demineralization methods in use today including EDTA-demineralization and lipid removal. 

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4. Lead isotopes and rare earth elements geochemistry of global phosphate rocks: Insights into depositional conditions and environmental tracing

   https://doi.org/10.1016/j.chemgeo.2023.121715  

   Wang, Zhen; Hill, Robert; Williams, Gordon; Dwyer, Gary S; Hu, Jun; Schnug, Ewald; Bol, Roland; Sun, Yajie; Coleman, Drew S; Liu, Xiao-Ming; et al (November 2023, Chemical Geology)

   Phosphate rock bears both geologically and environmentally significant information. Rare earth elements and yttrium (i.e., REY) characteristics have been commonly used for reconstructing the redox conditions of depositional environments of and the effects of post-depositional diagenetic alteration on phosphate rock. In addition, phosphate rock is typically enriched in a range of trace elements such as uranium (U) and cadium (Cd) that can be dispersed as contaminants into the environment with phosphate mining and phosphate fertilizer application. Here we report the lead (Pb) isotope compositions combined with Pb and REY concentrations of both global sedimentary and igneous phosphate rocks, aiming to evalute the geological origin of phosphate rocks over time and the potential of using them for environmental tracing. Phopshate rocks samples analyzed in this study were sourced from major economic phosphate deposits in the world, including China, Southern Tethys (e.g., Morocco, Tunisia, Israel), the U.S., India, South Africa and Russia. Our results show a wide range of 208Pb/204Pb (35.70 to 60.58), 207Pb/204Pb (15.20 to 18.25), and 206Pb/204Pb (16.369 to 71.806) ratios in phosphate rocks, with sedimentary phosphate rocks being significantly more radiogenic than igneous rocks. The majority of the sedimentray phosphate rocks show a notable isotopic overprinting by non-radiogenic terrestrial Pb, except for those from Israel and Morocco that have the most radiogenic Pb isotope compositions. Correspondingly, phosphate rocks with more radiogenic Pb isotope ratios show relatively pristine seawater REY features, likely suggesting their preservation of the original oxic seawater conditions and/or minimal diagenetic alteration. In contrast, phosphate rocks with less radiogenic Pb isotope compositions show REY indications for more anoxic seawater redox conditions and/or greater diagenetic alteration. We further evaluate the potential utility of Pb isotopes for tracing the associated contamination with phosphate rock mining and fertilizer application in the environment. In most cases, the radiogenic Pb isotope composition of phosphate rocks and corresponding P-fertilizers is distinctive from both natural crustal Pb and major anthropogenic Pb sources (e.g., Pb ore deposits and pesticides), which provides a great advantage for applying Pb isotopes as environmental tracers for metal(loid) contamination from phosphate sources. The combination of Pb isotope ratios and REY proxies could further constrain the Pb source discrimination. Overall, this study provides new Pb isotopic and REY geochemical data on global phosphate rocks and fertilizers, which lays the groundwork for future regional and local studies on both their geological and environmental implications. 

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5. Isotopic fractionation accompanying CO2 hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA

   https://doi.org/doi.org/10.1016/j.gca.2021.01.003  

   Christensen, J. (January 2021, Geochimica)

   Teagle, Damon A (Ed.)

   The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric CO2 and at times via mixing with surface water. These processes lead to distinct carbonate morphologies with ‘‘floes” forming at the atmosphere-water interface, ‘‘snow” of fine particles accumulating at the bottom of pools and terraced constructions of travertine. Floe material is mainly composed of aragonite needles despite CaCO3 precipitation occurring in waters with low Mg/Ca (<0.01). Precipitation of aragonite is likely promoted by the high pH (11.5–12.0) of pool waters, in agreement with published experiments illustrating the effect of pH on calcium carbonate polymorph selection. The calcium carbonates exhibit an extreme range and approximately 1:1 covariation in d13C (
   9 to
   28‰ VPDB) and d18O (0 to
   20‰ VPDB) that is characteristic of travertine formed in high pH waters. The large isotopic fractionations have previously been attributed to kinetic isotope effects accompanying CO2 hydroxylation but the controls on the d13C-d18O endmembers and slope have not been fully resolved, limiting the use of travertine as a paleoenvironmental archive. The limited areal extent of the springs (0.5 km2) and the limited range of water sources and temperatures, combined with our sampling strategy, allow us to place tight constraints on the processes involved in generating the systematic C and O isotope variations. We develop an isotopic reaction–diffusion model and an isotopic box model for a CO2-fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO3. The box model includes four sources or sinks of DIC (atmospheric CO2, high pH spring water, fresh creek water, and CaCO3 precipitation). Model parameters are informed by new floe D44Ca data (
   0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to 8  10
   7 mol/m2/s) and by previously published elemental and isotopic data of local water and DIC sources. Model results suggest two processes control the extremes of the array: (1) the isotopically light end member is controlled by the isotopic composition of atmospheric CO2 and the kinetic isotope fractionation factor (KFF (‰) = (a
   1)  1000) accompanying CO2 hydroxylation, estimated here to be
   17.1 ± 0.8‰ (vs. CO2(aq)) for carbon and
   7.1 ± 1.1‰ (vs. ‘CO2(aq)+H2O’) for oxygen at 17.4 ± 1.0
   C. Combining our results with revised CO2 hydroxylation KFF values based on previous work suggests consistent KFF values of
   17.0 ± 0.3‰ (vs. CO2(aq)) for carbon and
   6.8 ± 0.8‰ for oxygen (vs. ‘CO2(aq)+H2O’) over the 17–28
   C temperature range. (2) The isotopically heavy endmember of calcium carbonates at The Cedars reflects the composition of isotopically equilibrated DIC from creek or surface water (mostly HCO- 3, pH = 7.8–8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate d13C and d18O values of modern and ancient travertines therefore reflect the proportion of calcium carbonate formed by processes (1) and (2), with process (2) dominating the carbonate precipitation budget at The Cedars. These results show that recent advances in understanding kinetic isotope effects allow us to model complicated but common natural processes, and suggest ancient travertine may be used to retrieve past meteoric water d18O and atmospheric d13C values. There is evidence that older travertine at The Cedars recorded atmospheric d13C that predates large-scale combustion of fossil fuels. 

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