Browsing by Author "Weldeghebriel, MF"

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    Ca-SO4 crossover points in Phanerozoic seawater: evidence from fluid inclusions in halite
    (American Geophysical Union (AGU), 2021-12-15) Weldeghebriel, MF; Lowenstein, TK; García-Veigas, J; Cendón, DI
    The major ion (Mg2+, Ca2+, Na+, K+, SO42-, Cl-, HCO3-) chemistry of seawater has varied in the Phanerozoic and Neoproterozoic. Transitions between SO4-rich and Ca-rich seas, here called “Ca2+- SO42- crossover points” are of great biogeochemical interest because of their coincidence with shifts in seawater Mg2+/Ca2+ ratio and the mineralogies of potash evaporites. Crossover periods occurred four times over the past 550 Myr, when seawater chemistry shifted from Ca-rich to SO4-rich or vice versa. Here, chemical analyses of 2,466 fluid inclusions in marine halite define paleoseawater [Ca2+] and [SO42-] over the past 550 Myr, with emphasis on Ca2+- SO42- crossover periods. Three types of brine chemistries are recognized from [SO42-] and [Ca2+] concentrations: (1) SO4-rich (SO4>>Ca): inclusions with measurable SO42- (7-993 mmol/kg H2O, here termed mmolal) and low Ca2+ (<91 mmolal) (Late Neoproterozoic, Permian, Triassic, and Late Cenozoic); (2) Ca2+-rich (Ca >>SO4): inclusions with measurable Ca2+ (150-2000 mmolal) and very low SO42-, below detection (Cambrian, Silurian, Devonian, Mississippian, Pennsylvanian, Jurassic, and Cretaceous); (3) crossover periods (SO4≈Ca): inclusions have measurable but low concentrations of both Ca2+ (10-120 mmolal) and SO42- (10-210 mmolal) (Late Triassic to Early Jurassic, Eocene to Oligocene). The Late Pennsylvanian Paradox Formation represents a second type of crossover period. In that case, fluid inclusions vary from Ca2+-rich (3-670 mmolal) to SO4-rich (6-590 mmolal).The fourth transition from SO4-rich to Ca-rich seawater occurred during the Terminal Proterozoic to the Early Cambrian (ca. 544-515 Ma) (Brennan et al., 2004). Salts predicted to precipitate during evaporation of crossover periods, for example Eocene and Oligocene seawater are calcite, gypsum, halite, polyhalite, sylvite, carnallite, kieserite, and bischofite. The four crossover points parallel major transitions in: (1) Mg2+/ Ca2+ ratio of seawater; (2) aragonite and calcite seas; (3) MgSO4 and KCl potash evaporites; and (4) icehouse and greenhouse climates, suggesting a link between the composition of seawater, marine potash evaporites, carbonate mineralogy, and climate.
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    Combined 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, I
    Quantitative 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.
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    Seafloor 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, E
    Long-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)