Browsing by Author "Sendula, E"
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- ItemCombined LA-ICP-MS and cryo-SEM-EDS: an improved technique for quantitative analysis of major, minor, and trace elements in fluid inclusions in halite(Elsevier B. V., 2020-09-30) Weldeghebriel, MF; Lowenstein, TK; García-Veigas, J; Collins, D; Sendula, E; Bodnar, RJ; Graney, JR; Cendón, DI; Lensky, NG; Mor, Z; Sirota, IQuantitative multi-element analyses of single fluid inclusions in halite and other sedimentary minerals can provide information on the origin and chemical evolution of ancient surface waters on Earth. Integrated laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and cryogenic-scanning electron microscopy-energy dispersive spectroscopy (cryo-SEM-EDS) were used here for the quantitative analysis of fluid inclusions in halite. Single phase fluid inclusions in modern and ancient halite were analyzed using a 193 nm ArF excimer laser ablation system coupled with a quadrupole mass spectrometer to test a new calibration technique using magnesium (Mg) as an internal standard. Mg concentrations obtained by cryo-SEM-EDS analyses of fluid inclusions were used to convert LA-ICP-MS concentration ratios into absolute elemental concentrations. Mg concentrations of ancient fluid inclusions from cryo-SEM-EDS analyses were reproducible to better than 5% relative standard deviation (RSD). Comparison between the chemical composition of modern Dead Sea brine measured using ICP-OES (optical emission spectroscopy) and the composition of fluid inclusions in Dead Sea halite formed from those brines, shows that fluid inclusions in halite faithfully record the chemistry of the brines from which they precipitated. Overall LA-ICP-MS analytical precision for major ions K, Ca, S in SO4 (above 50 mmol/kg H2O) is better than 10% RSD and accuracies range from 2% to 21%. Mean concentrations of Li, B, Sr, Rb and Ba agree within 7% of their expected values and are reproducible within 15%, whereas Cs concentrations above detection limit are typically reproducible to within 15 to 25% RSD. For trace elements in seawater, such as U and Mo, quantitative analyses in fluid inclusions are achieved at concentrations above 20 nmol/kg H2O. The results from this study confirm that the precision and accuracy of major and minor elemental analysis is improved with Mg as an internal standard instead of Na and Cl used in previous studies. Controlled, optimized ablation of >30 μm fluid inclusions in halite improved the accuracy and precision and reduced the overall limit of detection (LOD) by one order of magnitude compared to previous studies. Wide ranges of LODs, between 0.7 nmol/kg H2O and 10 mmol/kg H2O, reflect variations in inclusion volume and elemental concentrations. Analytical accuracies obtained for major elements demonstrate that cryo-SEM-EDS and LA-ICP-MS are complementary microbeam techniques for chemical analysis of individual fluid inclusions in halite. © 2021 Elsevier B.V.
- ItemSeafloor hydrothermal systems control seawater chemistry: evidence from fluid inclusion in halite(The Geological Society of America, 2019-09-24) Weldeghebriel, MF; Lowenstein, TK; Demicco, RV; Graney, JR; García-Veigas, J; Cendón, DI; Bodnar, RJ; Sendula, ELong-term changes in the major ion and isotopic composition of seawater coincide with icehouse-greenhouse climate fluctuations, calcite-aragonite seas, and sea level changes. However, there is disagreement over what processes controlled the changes in ocean chemistry. This study uses a new record of Li concentration in paleoseawater to explore how temporal variations in the flux of MOR hydrothermal brines, the largest source of Li to seawater, and reverse weathering of seafloor basalts (important sink) control the oceanic Li cycle on multimillion-year time scales. Here we present a 350-million-year record of seawater lithium concentrations [Li+]sw from direct measurement of primary fluid inclusions in marine halite using combined LA-ICP-MS and cryo SEM-EDS. We also present a 150 Myr forward model of [Li+]sw. From 350-0 Ma, the lithium concentration of seawater oscillated systematically, parallel to secular variations of sea level, greenhouse-icehouse climates, and major ion chemistry such as the Mg2+/Ca2+ ratio. Highest seawater Li occurred during the Cretaceous, up to one order of magnitude higher than modern [Li+]sw, which coincides with low seawater Mg2+/Ca2+ ratios, high atmospheric CO2, and Mesozoic-Early Cenozoic Greenhouse climates. Such high Li concentrations require high MOR hydrothermal activity. Conversely, Permian and Cenozoic (35-0 Ma) seawater had relatively low Li, consistent with high Mg2+/Ca2+ ratios, low atmospheric CO2, and late Paleozoic and Cenozoic icehouse periods. The forward model involves 10 Kyr time steps and variable cycling of hydrothermal fluids through the axial portion of the MOR system and variable rates of low-temperature weathering of seafloor basalts. The model agrees well with paleoseawater fluid inclusion data for Li. The same model parameters, with variable Li isotope fractionation of off-axis oceanic crust, are used to successfully model the 9‰ increase of δ7Li in seawater from 60-0 Ma. Our data and modeling suggest that seafloor hydrothermal systems exerted the dominant control on the [Li+] and δ7Li composition of Phanerozoic seawater. These data will be further used to test the long-term relationships between seafloor MOR activity, the carbon cycle, and climate. © Copyright 2019 The Geological Society of America (GSA)