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1. 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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2. Data report: 87Sr/86Sr in pore fluids from IODP Expedition 355 Arabian Sea Monsoon

   https://doi.org/10.14379/iodp.proc.355.201.2017  

   Carter, S.C.; Griffith, E.M.; Scher, H.D. (October 2017, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   Here we report the strontium isotope ratios (87Sr/86Sr) from pore fluids collected during International Ocean Discovery Program (IODP) Expedition 355. Ratios from Sites U1456 (N = 21) and U1457 (N = 20) are reported. Pore fluid 87Sr/86Sr is a useful tool to establish fluid-rock reactions, sources of Sr, and fluid mixing. The measured 87Sr/86Sr of the pore fluid has significant variations at both sites, and three distinct zones are identified. At Site U1456, 87Sr/86Sr starts at values similar to that of modern seawater (~0.7092) from near the seafloor down to ~100 meters below seafloor (mbsf). Over this interval, Sr concentration increases, whereas Ca decreases (Zone 1). Below 100 mbsf, 87Sr/86Sr values increase to a max of ~0.7100 at ~224 mbsf, and Sr concentrations decrease (Zone 2). Isotopic values then gradually decrease to ~0.7085, with an increase in Sr concentrations (Zone 3). Site U1457 displays the same general trends in pore fluid 87Sr/86Sr composition; however, there are distinct differences. First, Zone 1 occurs over a shorter interval (~0–54 mbsf) due to a lower sedimentation rate, and it also has lower Sr concentrations compared to Site U1456. Additionally, 87Sr/86Sr reaches a higher peak value in Zone 2 than at Site U1456. Finally, in Zone 3 the maximum Sr concentration reached is significantly lower than that at Site U1456. 

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3. The Role of Crustal Contamination throughout the 1329–2005 CE Eruptive Record of Mt. Etna Volcano, Italy

   https://doi.org/10.1093/petrology/egae028  

   Takach, Marie K; Bohrson, Wendy A; Spera, Frank J; Viccaro, Marco (April 2024, Journal of Petrology)

   Abstract The nearly continuous volcanic eruption record at Mt. Etna dating back ~700 years provides an excellent opportunity to investigate the geochemical evolution of a highly active volcano. Of particular interest is elucidating the cause of selective enrichment in alkali elements (K and Rb) and 87Sr/86Sr observed in various episodes of past activity. More recently, this alkali enrichment trend started to manifest in the 17th century and accelerated after 1971, and was accompanied by an increase in the volume, frequency, and explosivity of eruptions. Explanations for this signature include recharge of alkali-enriched magmas and/or crustal contamination from the subvolcanic basement. This study quantitatively examines the role of crustal contamination in post-1971 Etnean magma compositions via hundreds of open-system phase equilibria and trace element calculations based upon whole-rock major oxides, trace elements, 87Sr/86Sr ratios, and mineral compositional data. Available pre-1971 petrochemical data are satisfactorily reproduced by fractional crystallization of a high whole-rock MgO (12–17 wt.%), Ni (135–285 ppm), and Cr (920–1330 ppm) parental magma composition that is documented in Etna's ~4-ka fall-stratified deposit. Observed post-1971 whole-rock and glass trends and phase equilibria are reproduced via modeled assimilation of a skarn and flysch mixture, lithologies that represent the uppermost 10 to 15 km of sedimentary rocks beneath Etna. Notably, models show that K2O (wt.%) and Rb (ppm) behave incompatibly during partial melting of skarn/flysch. Additionally, the observed elevation of 87Sr/86Sr in post-1971 samples is consistent with the addition of radiogenic Sr from wallrock partial melts. In best-fit models, which yield observed post-1971 K2O, Rb, and 87Sr/86Sr trends, ~17% anatectic melt is assimilated and there may be a subordinate stoped wallrock component of ≤2% (percentage is relative to the starting mass of pristine magma). Previous work has shown that metasomatized spinel lherzolite and garnet pyroxenite can be melted in different proportions to reproduce long- and short-term changes observed in Etna’s geochemical products. We propose that the alkali enrichment signature observed after 1971 can be fully explained through the combination of mantle heterogeneity and crustal contamination. In particular, up to ~20% crustal input coupled with mantle heterogeneity of primitive melts explains the geochemical signals quite well. The influence of crustal contamination on post-1971 lavas is, in part, the result of frequent recharge of magmas that thermally primed the middle to upper crust and enhanced its partial melting. 

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4. Strontium and carbon isotopic evidence for decoupling of pCO2 from continental weathering at the apex of the late Paleozoic glaciation

   https://doi.org/10.1130/G40093.1  

   Chen, Jitao; Montañez, Isabel P.; Qi, Yuping; Shen, Shuzhong; Wang, Xiangdong (March 2018, Geology)

   Earth’s penultimate icehouse (ca. 340–285 Ma) was a time of low atmospheric pCO2 and high pO2, formation of the supercontinent Pangaea, dynamic glaciation in the Southern Hemisphere, and radiation of the oldest tropical rainforests. Although it has been long appreciated that these major tectonic, climatic, and biotic events left their signature on seawater 87Sr/86Sr through their influence on Sr fluxes to the ocean, the temporal resolution and precision of the late Paleozoic seawater 87Sr/86Sr record remain relatively low. Here we present a high-temporal-resolution and high-fidelity record of Carboniferous– early Permian seawater 87Sr/86Sr based on conodont bioapatite from an open-water carbonate slope succession in south China. The new data define a rate of long-term rise in 87Sr/86Sr (0.000035/m.y.) from ca. 334–318 Ma comparable to that of the middle to late Cenozoic. The onset of the rapid decline in 87Sr/86Sr (0.000043/m.y.), following a prolonged plateau (318–303 Ma), is constrained to ca. 303 Ma. A major decoupling of 87Sr/86Sr and pCO2 during 303–297 Ma, coincident with the Paleozoic peak in pO2, widespread low-latitude aridification, and demise of the pan-tropical wetland forests, suggests a major shift in the dominant influence on pCO2 from continental weathering and organic carbon sequestration (as coals) on land to organic carbon burial in the ocean. 

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5. Changing inputs of continental and submarine weathering sources of Sr to the oceans during OAE 2

   https://doi.org/10.46427/gold2020.1889  

   Nana Yobo, L.; Brandon, A.D.; Holmden, C.; Lau, K.V.; Eldrett, J. (June 2021, Geochimica et cosmochimica acta)

   Jacobson, A. (Ed.)

   Ocean anoxic events (OAE) are characterized by increased organic content of marine sediment on a global scale with accompanying positive excursions in sedimentary organic and inorganic d 13C values. To sustain the increased C exports and burial required to explain the C isotope excursion, increased supplies of nutrients to the oceans are often invoked during ocean anoxic events. The potential source of nutrients in these events is investigated in this study for Oceanic Anoxic Event 2, which spans the Cenomanian-Turonian boundary. Massive eruptions of one or more Large Igneous Provinces (LIPs) are the proposed trigger for OAE 2. The global warming associated with volcanogenic loading of carbon dioxide to the atmosphere has been associated with increased continental weathering rates during OAE 2, and by extension, enhanced nutrient supplies to the oceans. Seawater interactions with hot basalts at LIP eruption sites can further deliver ferrous iron and other reduced metals to seawater that can stimulate increased productivity in surface waters and increased oxygen demand in deep waters. The relative importance of continental and submarine weathering drivers of expanding ocean anoxia during OAE 2 are difficult to disentangle. In this paper, a box model of the marine Sr cycle is used to constrain the timing and relative magnitudes of changes in the continental weathering and hydrothermal Sr fluxes to the oceans during OAE 2 using a new high-resolution record of seawater 87Sr/86Sr ratios preserved in a marl-limestone succession from the Iona-1 core collected from the Eagle Ford Formation in Texas. The results show that seawater 87Sr/86Sr ratios change synchronously with Os isotope evidence for the onset of massive LIP volcanism 60 kyr before the positive C isotope excursion that traditionally marks the onset of OAE 2. The higher temporal resolution of the seawater Sr isotope record presented in this study warrants a detailed quantitative analysis of the changes in continental weathering and hydrothermal Sr inputs to the oceans during OAE 2. Using an ocean Sr box model, it is found that increasing the continental weathering Sr flux by  1.8-times captures the change in seawater 87Sr/86Sr recorded in the Iona-1 core. The increase in the continental weathering flux is smaller than the threefold increase estimated by studies of seawater Ca isotope changes during OAE 2, suggesting that hydrothermal forcing may have played a larger role in the development of ocean anoxic events than previously considered. 

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