Experimental evidence for the hydrothermal formation of native sulfur by synproportionation

Elemental sulfur (S0) is known to form in submarine acid-sulfate vents by disproportionation of magmatic SO2. S0 formed upon disproportionation shows δ34SS values considerably lower than the influxing magmatic SO2, which results in δ34SS values typically <0‰. The peculiar occurrence of isotopically heavy sulfur in the Kemp Caldera hydrothermal system (δ34SS > 5‰) and Niua North (δ34SS = 3.1‰) led to the suggestion that disproportionation is not the only sulfur forming process in submarine hydrothermal systems. We conducted hydrothermal experiments to investigate if synproportionation of SO2 and H2S can explain the occurrence and isotopic composition of S0 observed in some vent fields. Provided that SO2 and H2S are both abundant, this formation mechanism is thermodynamically conceivable, but it has not yet been demonstrated experimentally that this process actually takes place in submarine hydrothermal systems. We conducted the experiments in collapsible Ti-cells under pT-conditions (20–30 MPa, 220°C) that are relevant to S0 formation in submarine hydrothermal systems. We used starting concentrations of 10 mM sulfite and 20 mM sulfide of known isotopic composition. Under acidic conditions (pH25 °C = 1.2), S0 was the most abundant reaction product, but small amounts of sulfate were also produced. A Rayleigh fractionation model was applied to determine the isotopic composition of SO42–, SO2, H2S and S0 expected to form by SO2 disproportionation, H2S oxidation, and SO2–H2S synproportionation. The sulfur isotopic signatures of the sulfur produced in the experiments can only be explained by synproportionation of sulfite and sulfide. These results provide strong evidence that synproportionation is likely responsible for exceptionally high δ34SS values observed in S0 from some arc/back-arc hydrothermal environments, like the Kemp Caldera in the South Sandwich arc. Coeval degassing of H2S and SO2 is likely required to have this particular reaction dominate in the H–S–O reaction network and produce noticeable accumulations of isotopically heavy native sulfur at the seafloor

Seafloor investigations of the Kemp Caldera, the southernmost arc caldera volcano from the South Sandwich Island Arc

Kemp Caldera, situated in the south of the intra-oceanic South Sandwich arc, is one of the least explored submarine calderas that hosts hydrothermally active vent sites. The caldera was discovered in 2009. Since then, the focus has been primarily on biological studies. During the R/V Polarstern cruise PS119 in 2019, we gained new insights into the morphology, petrology and the formation of the Kemp Caldera. The ship's multibeam data provide an overview of the caldera bathymetry and backscatter characteristics. The new data revealed that the caldera is nested with two or possibly three concentric calderas. TV-sled and remotely operated vehicle (ROV) observations provide detailed visual data for the hydrothermally active sites of the vent field at the central resurgent cone and flare site at the NNW caldera rim. The central vent field is dominated by white smokers, where clams, sponges and other fauna thrive, while at the flare site inactive as well as actively venting chimneys have been found. The latter are characterized by metal-enriched fluids of temperatures ≥200°C. During ROV dives, rock samples were collected from the cone, providing the first information about the Kemp Caldera rock composition. The caldera rocks are dacitic, in contrast to the basalts and andesites of the neighboring Kemp Seamount. This suggests that the dacitic cone was formed by one or more later eruptions of differentiated magma, probably stored in shallow intrusions which are driving hydrothermal activity

Fluid-rock interaction at the backstop to the Mediterranean Ridge Accretionary Complex South of Crete : R/V SONNE Cruise Report SO278 : Emden (Germany), 12.10.2020 - Emden (Germany), 01.12.2020 : FRINGE

The research cruise to the Eastern Mediterranean (GPF-18-2-40) originally planned on RV METEOR was relocated to RV SONNE (Fig. 1.2) due to the reduced number of scientists as part of the corona pandemic. The main objective of the Bremen Ocean Cluster expedition (DFG, EXC2077) was to investigate the interactions between the seabed and ocean water in Greek waters, whereby the plate tectonic constellation of a broad collision zone represents a special tectonic drive. A secondary goal was the sampling of the Sartori mud volcano, which is being processed in Italian waters as part of a separate DFG project and for which the GPF granted an additional permit for ship time (GPF 20-1_054). The expedition began on 12 October in Emden/Germany and ended on 01 December 2020, in Emden. Investigations on mud volcanoes were carried out divided into 3 working areas (Fig. 1.1, the Sartori mud volcano in the Calabrian arc, the so-called Cobblestone Area, the Olimpi mud volcano field including the United Nation Ridge). With the MARUM AUV SEAL (Fig. 1.3) 11 dives were successfully carried out to create high-resolution detailed maps of certain seafloor structures. A total of 38 gravity cores (Fig. 1.4), 30 multicorers (Fig. 1.5) and 4 minicorers were used for sampling sediments and 6 CTD stations for sampling methane in the water column. Furthermore, 10 profiles were carried out with the heat flow lance and 5 observation profiles with the on-board OFOS. In four different provinces, 16 mud volcanoes were examined, 10 of which are characterized by pore waters that show a distinct freshening, while three mud volcanoes, Napoli, Heraklion and Gelendzhik, are characterized by very high salt concentrations. The salt accumulations in these structures are derived from the Messinian salt deposits in the subbed, from which salty brines arise through subrosion, which interact in various ways with the mud volcanoes. The study areas were selected based on preliminary surveys and morphological structures and increased backscatter patterns from multibeam mapping carried out over 3580 nautical miles in Italian and Greek waters.32

The Kemp Caldera hydrothermal system, Scotia Sea – Morphological, mineralogical and geochemical characteristics

Submarine calderas are a unique form of hydrothermal systems, which are not yet well understood in terms of how they form or how they develop over time. One of the least explored submarine calderas is the Kemp Caldera, which is located in the Scotia Sea in the rear-arc region of the southernmost part of the South Sandwich island arc. Since its discovery in 2009, the caldera has been of great interest primarily to bioscience researchers, but within the last few years, the Kemp Caldera has also been increasingly studied from a geoscientific perspective. One of the objectives of the R/V Polarstern PS119 expedition in 2019 was the investigation of the Kemp Caldera and its hydrothermal system in more detail. New bathymetric data together with visual seafloor observations and rock samples now show that the caldera was formed by two collapse events, resulting in a prominent morphology. The shape of the resurgent cone, which occurs in the central part of the caldera, and the results of rock analyses indicate a dacitic post-caldera eruption that was responsible for the formation and development of several vent fields. Two of these hydrothermal fields, Great Wall and Toxic Castle, located on the eastern slope of the central resurgent cone, are of particular interest. Here, contrary to other hydrothermal systems in this area, elemental sulfur occurs not only in fine-crystalline, but also in liquid form. Sampling and later investigation of the sulfur and other hydrothermal precipitates showed that the elemental sulfur is isotopically heavy and thus cannot be attributed to the generally accepted formation by SO2 disproportionation. Instead, the observed isotopic composition of sulfur must be the result from synproportionation of SO2 and H2S. Although this reaction has not been documented from other hydrothermal systems, the use of a thermodynamic computation software and a Rayleigh fractionation model demonstrated that synproportionation is thermodynamically possible both at Great Wall and Toxic Castle, and capable of producing the observed sulfur isotopic composition. Moreover, experiments carried out under hydrothermal conditions produced artificially generated sulfur, whose measured isotope values matched those predicted by the Rayleigh fractionation model for synproportionation. This thesis therefore proves, for the first time, that synproportionation can form sulfur in hydrothermal environments and might thus represent a previously unknown source of isotopically heavy sulfur

Elemental sulfur formation in the Kemp Caldera hydrothermal system, Scotia Sea

&amp;lt;p&amp;gt;The Kemp Caldera is a submarine arc caldera volcano and belongs to the southernmost part of the South Sandwich island arc, located in the Scotia Sea. In 2009, the caldera was discovered by the R/V &amp;lt;em&amp;gt;James Clark Ross&amp;lt;/em&amp;gt; research cruise JR224 during a geophysical survey. At this time, first hydrothermal activities were observed within the caldera. Around a resurgent cone in the center of the caldera, extinct chim&amp;amp;#173;neys and whi&amp;amp;#173;te smo&amp;amp;#173;ker vent fiel&amp;amp;#173;ds are found. A special feature of the Kemp Caldera hydrothermal system is the occurrence of elemental sulfur (S&amp;lt;sup&amp;gt;0&amp;lt;/sup&amp;gt;) at uncommonly high pH values. Sulfur samples of the white smoker vent fields &amp;amp;#8220;Great Wall&amp;amp;#8221; and &amp;amp;#8220;Toxic Castle&amp;amp;#8221; at the eastern flank of the resurgent cone were recovered with a remotely operated vehicle during the R/V &amp;lt;em&amp;gt;Polarstern&amp;lt;/em&amp;gt; PS119 expedition in 2019. These two sites are no more than 80 m apart, but the occurrence of S&amp;lt;sup&amp;gt;0&amp;lt;/sup&amp;gt; is different: at Great Wall, the sulfur is crystalline, while at Toxic Castle the sulfur is liquid and forms amorphous, pearl-like structures. Both sites are characterized by fluids with pH&amp;lt;sub&amp;gt;25 &amp;amp;#176;C &amp;lt;/sub&amp;gt;values &amp;gt; 5 and show a temperature range from 63 to &amp;gt; 200 &amp;amp;#176;C. Most interesting, however, are the &amp;amp;#948;&amp;lt;sup&amp;gt;34&amp;lt;/sup&amp;gt;S values of elemental sulfur, ranging from +5.2 to +5.8 &amp;amp;#8240;.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Disproportionation of magmatic SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; commonly explains the formation of S&amp;lt;sup&amp;gt;0&amp;lt;/sup&amp;gt; in arc/back-arc systems. Elemental sulfur precipitates from highly acidic hydrothermal fluids with pH-values &amp;amp;#8804; 1 and show negative &amp;amp;#948;&amp;lt;sup&amp;gt;34&amp;lt;/sup&amp;gt;S values due to isotope fractionation. However, this formation mechanism cannot explain the moderate pH of the fluids and the lack of significantly negative &amp;amp;#948;&amp;lt;sup&amp;gt;34&amp;lt;/sup&amp;gt;S values for sulfur that would indicate SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; disproportionation. We suggest that the formation of sulfur in the Kemp Caldera is a result of SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;S synproportionation. From a thermodynamic point of view, this formation mechanism is possible, but it has not yet been demonstrated that it actually takes place in hydrothermal systems. Our study focuses on the formation of elemental sulfur in the Kemp Caldera hydrothermal system and shows that the diversity of hydrothermal arc/back-arc systems may be greater than previously assumed.&amp;lt;/p&amp;gt;</jats:p

DataSheet1_Experimental evidence for the hydrothermal formation of native sulfur by synproportionation.docx

Elemental sulfur (S0) is known to form in submarine acid-sulfate vents by disproportionation of magmatic SO2. S0 formed upon disproportionation shows δ34SS values considerably lower than the influxing magmatic SO2, which results in δ34SS values typically 34SS > 5‰) and Niua North (δ34SS = 3.1‰) led to the suggestion that disproportionation is not the only sulfur forming process in submarine hydrothermal systems. We conducted hydrothermal experiments to investigate if synproportionation of SO2 and H2S can explain the occurrence and isotopic composition of S0 observed in some vent fields. Provided that SO2 and H2S are both abundant, this formation mechanism is thermodynamically conceivable, but it has not yet been demonstrated experimentally that this process actually takes place in submarine hydrothermal systems. We conducted the experiments in collapsible Ti-cells under pT-conditions (20–30 MPa, 220°C) that are relevant to S0 formation in submarine hydrothermal systems. We used starting concentrations of 10 mM sulfite and 20 mM sulfide of known isotopic composition. Under acidic conditions (pH25 °C = 1.2), S0 was the most abundant reaction product, but small amounts of sulfate were also produced. A Rayleigh fractionation model was applied to determine the isotopic composition of SO42–, SO2, H2S and S0 expected to form by SO2 disproportionation, H2S oxidation, and SO2–H2S synproportionation. The sulfur isotopic signatures of the sulfur produced in the experiments can only be explained by synproportionation of sulfite and sulfide. These results provide strong evidence that synproportionation is likely responsible for exceptionally high δ34SS values observed in S0 from some arc/back-arc hydrothermal environments, like the Kemp Caldera in the South Sandwich arc. Coeval degassing of H2S and SO2 is likely required to have this particular reaction dominate in the H–S–O reaction network and produce noticeable accumulations of isotopically heavy native sulfur at the seafloor.</p

Native sulfur at the seafloor: Composition and origin

Highlights • Native S precipitates in a variety of seafloor settings. • Oxidative alteration of pyrrhotite results in S0 formation at the mid-ocean ridges. • S0 forms via disproportionation of SO2 at volcanic arcs, back-arcs and hot spots. • SO2 - H2S synproportionation results in S0 formation at volcanic arcs and back-arcs. • Bacterial sulfate reduction and sulfide oxidation play a role in S0 precipitation. Mineralogy, geochemistry and sulfur (S) isotope composition of native sulfur (S0) precipitated at intraoceanic and intracontinental back-arc rift, submarine and subaerial volcanic arc, sediment and sediment-free mid-ocean ridge, hot spot, accretionary wedge, and submarine and continental cave settings were investigated with a main goal to understand the mode of formation of all the types of native S at the modern seafloor. Native S occurs as various depositional forms: chimneys, colloform flows, liquid ponds, slabs; fills in cavities and pore space of the sediment, vesicles and cracks in volcanic rocks; cements and coats the sediment; stains the conduits or fills in pores of the sulfide chimneys; forms fine-grained layers within the sediment; coats the walls, stalactites and stalagmites in the caves. Mineralogically, the native S from the seafloor is pure rhombic S0 with negligible impurities of metal sulfides, aluminosilicates, and carbonates inferred from the chemistry data. Mineral interrelations and S isotope data suggest that native S from different geologic settings has different origin. In the sulfide chimneys and mounds at the mid-ocean ridges, native S appears to be a result of oxidative alteration of primary pyrrhotite. The native S from back-arc rifts, volcanic arcs and hot spots can be a result of either disproportionation of magmatic SO2 (δ34S 0 ‰). The native S from the sediments in anoxic brine-filled deeps (accretionary wedge setting) is a result of bacterial sulfate reduction and consequent sulfide (δ34S < 0 ‰) oxidation. The native S coating the cave walls and forms also has a bacterial origin (δ34S < 0 ‰)