Quantifying the Seawater Sulfate Concentration in the Cambrian Ocean

Although the earliest animals might have evolved in certain “sweet spots” in the last 10 million years of Ediacaran (550–541 Ma), the Cambrian explosion requires sufficiently high levels of oxygen (O2) in the atmosphere and diverse habitable niches in the substantively oxygenated seafloor. However, previous studies indicate that the marine redox landscape was temporally oscillatory and spatially heterogeneous, suggesting the decoupling of atmospheric oxygenation and oceanic oxidation. The seawater sulfate concentration is controlled by both the atmospheric O2 level and the marine redox condition, with sulfide oxidation in continents as the major source, and sulfate reduction and pyrite burial as the major sink of seawater sulfate. It is thus important to quantify the sulfate concentration on the eve of the Cambrian explosion. In this study, we measured the pyrite contents and pyrite sulfur isotopes of black shale samples from the Yurtus Formation (Cambrian Series 2) in the Tarim Block, northwestern China. A numerical model is developed to calculate the seawater sulfate concentration using the pyrite content and pyrite sulfur isotope data. We first calibrate some key parameters based on observations from modern marine sediments. Then, the Monte Carlo simulation is applied to reduce the uncertainty raised by loosely confined parameters. Based on the geochemical data from both Tarim and Yangtze blocks, the modeling results indicate the seawater sulfate concentration of 8.9–14 mM, suggesting the seawater sulfate concentration was already 30–50% of the present level (28 mM). High seawater sulfate concentration might be attributed to the enhanced terrestrial sulfate input and widespread ocean oxygenation on the eve of the Cambrian explosion.

Sulfur isotope evidence of geochemical zonation of the Samoan mantle plume

<p>The radiogenic Pb isotope compositions of basalts from the Samoan hotspot suggest various mantle endmembers contribute compositionally distinct material to lavas erupted at different islands [1]. Basalts from the Samoan islands sample contributions from all of the classical mantle endmembers, including extreme EM II and high <sup>3</sup>He/<sup>4</sup>He components, as well as dilute contributions from the HIMU, EM I, and DM components. Here, we present multiple sulfur isotope data on sulfide extracted from subaerial and submarine whole rocks associated with several Samoan volcanoes—Malumalu, Malutut, Upolu, Savaii, and Tutuila—that sample the full range of geochemical heterogeneity at Samoa and allow for an assessment of the S-isotope compositions associated with the different mantle components sampled by the Samoan hotspot. We observe variable S concentrations (10-1000 ppm) and δ<sup>34</sup>S values (-0.29‰ to +4.84‰ ± 0.3, 2σ). The variable S concentrations likely reflect weathering, sulfide segregation and degassing processes. The range in δ<sup>34</sup>S reflects mixing between the primitive mantle and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal Δ<sup>33</sup>S within uncertainty of Δ<sup>33</sup>S=0 ‰ ± 0.008, suggesting Δ<sup>33</sup>S is relatively well mixed within the Samoan mantle plume. Important exceptions to this observation include: (1) a negative Δ<sup>33</sup>S (-0.018‰ ±0.008, 2σ) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (2) a previously documented small (but resolvable) Δ<sup>33</sup>S values (up to +0.027±0.016) associated with the Vai Trend (associated with a diluted HIMU component) [2]. The variability we observed in Δ<sup>33</sup>S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. Δ<sup>36</sup>S vs. Δ<sup>33</sup>S relationships suggest all recycled S is of post-Archean origin. The heterogeneous S isotope values and distinct isotopic compositions associated with the various compositional trends confirms a prior hypothesis; unique crustal materials are heterogeneously delivered to the Samoan mantle plume and compositionally influence the individual groups of islands.</p><p>[1] Jackson et al. (2014), <em>Nature; </em>[2] Dottin et al. (2020), <em>EPSL</em></p>

Hera: Evidence for Multiple Mineralization Events and Remobilization in a Sediment-Hosted Au-Pb-Zn-Ag Deposit, Central New South Wales, Australia

The Hera Au-Ag-Pb-Zn deposit of central New South Wales, Australia with a total undepleted resource of 3.6 Mt @ 3.3 g/t Au, 25 g/t Ag, 2.6% Pb and 3.8% Zn occurs on the SE margin of the Cobar Basin. It is hosted by the shallow marine Mouramba Group and overlying turbiditic Amphitheatre Group. The siltstones comprise various mixtures of quartz, plagioclase, muscovite-phengite, biotite and clinochlore, along with accessory titanite and ilmenite. The deposit comprises a number of discrete lodes which are steeply west-dipping and strike NNW. Each lode has different abundances of the main ore minerals sphalerite, galena, chalcopyrite, pyrrhotite and electrum-gold. The North Pod and Far West lenses have the most diverse mineralogy in additionally containing arsenopyrite, native silver, gudmundite, Ag-tetrahedrite, acanthite, dyscrasite, native antimony, nisbite and breithauptite. Electrum (continuous spectrum from Ag-rich to Au-rich) is associated with sulfides in the main ore lenses while native gold occurs in the host rocks along cleavages/lineations and away from the main ore. The sulfur isotope data from across the deposit indicates a magmatic source. Most of the deposit has experienced greenschist facies metamorphism with pervasive green chlorite alteration, though the North Pod differs in being distinctly Ag- and Sb-rich and has reached at least amphibolite facies metamorphism with a garnet-wollastonite-vesuvianite-tremolite assemblage. Tremolite is relatively abundant throughout most of the deposit suggesting widespread low-T skarn alteration. Cross-cutting pegmatites comprise quartz, plagioclase (labradorite-andesine) and microcline. Hydrothermal remobilization is relatively extensive and best explains the unusual Ag-Sb-As assemblages of the North Pod and Far West lodes.

Supplemental Material: Phanerozoic variation in dolomite abundance linked to oceanic anoxia

Paleontology and sulfur isotope data, methods, Figures S1–S6, and Tables S1–S3.<br>

Supplemental Material: Phanerozoic variation in dolomite abundance linked to oceanic anoxia

Paleontology and sulfur isotope data, methods, Figures S1–S6, and Tables S1–S3.<br>

Sulfur cycling in carbonatite of the Kaiserstuhl volcanic complex (Germany)

&lt;p&gt;The redox sensitive element sulfur is used for reconstructing the oxygen fugacity during magmatic melt evolution applying the sulfur isotopic composition of sulfide and sulfate minerals. Especially fast ascending sulfur-rich alkaline magma from the upper mantle provides the possibility for determining the oxidation state of Earth`s mantle via a detailed investigation of the sulfur cycling. Here we present the first sulfur isotope data of sulfides, sulfates as well as carbonate associated sulfate (CAS) of carbonatite (s&amp;#246;vite) from two well-studied locations (Orberg and Badberg) of the Kaiserstuhl volcanic complex, situated in the southern part of the Upper Rhine Graben (Germany). Based on our results, s&amp;#246;vites are 25000 times more enriched in sulfate than in sulfide. Sulfides display a &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S value of 0.6&amp;#160;&amp;#8240; (V-CDT), whereas water-soluble sulfate (e.g. anhydrite) show a sulfur isotopic composition between 3.8&amp;#160;&amp;#8240; and 6.1&amp;#160;&amp;#8240;. &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CAS &lt;/sub&gt;data are at 6&amp;#160;&amp;#8240; at the Orberg and 9&amp;#160;&amp;#8240; at Badberg locality. Our sulfur isotope data are comparable to other carbonatite occurrences worldwide (e.g. Phalabora, South Africa), emplaced at similar temperatures (ca. 860&amp;#160;&amp;#176;C). However, the strongly elevated sulfate content recorded here for s&amp;#246;vites formed at this high temperature is unique and indicates an enhanced oxidation state during s&amp;#246;vite formation in the Kaiserstuhl volcanic complex.&lt;/p&gt;

The Genesis of the Salt Diapir-Related Mississippi Valley-Type Ba-Pb-(± Zn) Ore of the Slata District, Tunisia: The Role of Halokinesis, Hydrocarbon Migration, and Alpine Orogenesis

The juxtaposition of a Triassic evaporite diapir with the organic matter-rich Fahdene Formation (Albian-Vra-conian) along major faults in the Slata ore district raises the question of the roles played by halokinesis, hydrocarbons, and tectonics in mineralization. The Slata mining district, located in the Tunisian salt diapiric zone, contains Ba-Pb-(± Zn) ore hosted in the Aptian carbonates. The mineralogical paragenetic sequence consists of barite (Ba-1)–galena ± sphalerite ± calcite (Ca-1)–barite (Ba-2) and finally, late calcites (Ca-2 and Ca-3). Fluid inclusions from early barite reveal that it was precipitated from a warm (134°–157°C), H2O-NaCl-KCl-CaCl2, moderately saline (13.3–24.6 wt % NaCl equiv) basinal brine. This fluid is thought to have resulted from the mixing of a deep-seated, hot, metal-bearing fluid with a cooler, dilute SO42−-rich fluid. Early calcite and cogenetic sulfides (galena and sphalerite) precipitated from fluids of similar salinities and temperatures as the barite-forming fluids, but with the additional involvement of hydrocarbons. Sulfur isotope data suggest that thermochemical sulfate reduction of Triassic gypsum was the main source of reduced sulfur for sulfides. Late barite precipitated as a result of the mixing between a Ba-rich, hot, ascending fluid with a cooler, dilute Triassic sulfate-rich fluid in the absence of hydrocarbons. The homogeneous Pb isotope compositions of galena along with the Sr isotope compositions of barite point to a Paleozoic reservoir as the main source of metals with a contribution from the Triassic-Cretaceous rocks. The emplacement of the ore occurred during the Eocene-Miocene Alpine compressional tectonic activity that triggered the circulation of Paleozoic-derived metal-bearing fluids.