Sulfur formation associated with coexisting sulfide minerals in the Kemp Caldera hydrothermal system, Scotia Sea

Abstract

The Kemp Caldera is a submarine caldera located in the southernmost part of the intra-oceanic South Sandwich arc, Scotia Sea. The caldera comprises a unique hydrothermal system in which elemental sulfur (S⁰) coexists with sulfide minerals at unusually high pH-values (pH_{25 °C} > 5). During the 2019 R/V Polarstern PS119 expedition, samples from the white smoker vent fields in the center of the caldera were recovered by a remotely operated vehicle. The sampling of elemental sulfur took place at the vent sites “Great Wall” and “Toxic Castle”, which are located < 80 m apart. At Great Wall, sulfur is fine-crystalline, while at Toxic Castle S⁰ occurs in liquid form, forming amorphous and globular structures. Here, tabular plates of covellite are found as inclusions in the quenched sulfur. The fluids from both sites have pH-values of about 5.4 to 5.7 (at 25 °C) and show a relatively wide temperature range from 63 to > 200 °C. The isotope values of sulfide and elemental sulfur range from 4.6 to 5.8 ‰, while sulfate concentrations at Great Wall are lower than in seawater.

Elemental sulfur in submarine arc/back-arc systems is commonly believed to form by disproportionation of SO₂. It generally shows negative δ³⁴S values and precipitates from acid-sulfate type hydrothermal fluids with pH-values as low as < 1. This formation mechanism, however, cannot explain the high pH and low sulfate concentration of the Great Wall fluid alongside high δ³⁴S values of elemental sulfur and massive sulfides found at Kemp. Instead, we suggest that S⁰ formation at Kemp Caldera can be attributed to synproportionation of SO₂ and H₂S. This formation mechanism is thermodynamically feasible but has not been demonstrated to actually take place in hydrothermal systems. The association of elemental sulfur with covellite is also uncommon, but not unexpected in magmatic-hydrothermal systems. The uncommonly high pH-value is likely responsible for the precipitation of the metal sulfides, which are unusual for known acid-sulfate systems studied to date. Our study shows that the geochemical behavior of sulfur in arc/back-arc hydrothermal systems is more diverse than previously recognized.

Introduction

Since the discovery of the Galápagos Spreading Center in the late 1970’s, the deep sea has been explored extensively with more and more hydrothermal systems having been discovered. Deep-sea hydrothermal systems are strongly linked to geodynamic processes and volcanic activity. Hence, they are mainly located at mid-ocean ridges (MORs), e.g., at the Mid-Atlantic Ridge, volcanic arcs (e.g., Kermadec arc) and back-arc basins such as the Manus Basin and Lau Basin (Leat and Larter, 2003; Reid et al., 2013; de Ronde and Stucker, 2015; Diehl and Bach, 2020). Compared to MORs, fluids of subduction-related arc and back-arc hydrothermal systems are significantly enriched in metals and magmatic volatiles (e.g., de Ronde et al., 2001; Butterfield et al., 2011; de Ronde et al., 2003; Pereira et al., 2022; Seewald et al., 2015). Their environments are relatively shallow so that temperatures are lower and phase separation results in relatively large variation of Cl concentrations compared to MOR hosted hydrothermal systems (Hein and Mizell, 2013; Kleint et al., 2019; Diehl et al., 2020).

In general, arc- and back-arc-related submarine hydrothermal systems can be divided into two main types: magmatic-hydrothermal systems, in which influxing magmatic vapors provide heat and solutes to the fluids, and seawater-rock-dominated hydrothermal systems, where circulating seawater is heated by and reacts with hot rock in the subseafloor (Reeves et al., 2011; de Ronde et al., 2019; Kleint et al., 2019; Seewald et al., 2019). Magmatic-hydrothermal systems have a direct magmatic input and are characterized by acid-sulfate fluids, which have relatively low temperatures (< 250 °C) and are acidic (pH ≤ 1–3) (Butterfield et al., 2011; Seewald et al., 2015). They are analogs of fumaroles and subaerial high-sulfidation environments in epigenetic porphyry-related systems (Resing et al., 2007; Seewald et al., 2015). Acid-sulfate fluids are metal-poor and contain oxidized sulfur species (especially SO₂ and HSO₄⁻). Iron oxyhydroxide crusts and advanced argillic alteration assemblages are commonly found in magmatic-hydrothermal systems, while sulfide minerals are rare or even absent (e.g., de Ronde et al., 2011; Seewald et al., 2019). The term “advanced argillic” refers to mineral associations that can include quartz, pyrophyllite, and alunite formed by alteration of volcanic rocks. Elemental sulfur (S⁰) is commonly associated with these minerals. Typically, sulfur also precipitates from acid-sulfate type fluids due to the disproportionation of magmatic SO₂ upon cooling and reaction with H₂O (Kim et al., 2009; Seewald et al., 2015; Seewald et al., 2019). Besides elemental sulfur, H₂S can also form by disproportionation reactions, which invariably yield sulfuric acid. Isotopic fractionation results in isotopically lighter elemental sulfur and H₂S and isotopically heavier sulfate than the incoming SO₂, which is often around 4‰ in δ³⁴S, but can be as heavy as 10 ‰ (Alt et al., 1993). Hence, δ³⁴S values of S⁰ and H₂S are commonly < 0 ‰ (de Ronde et al., 2005; de Ronde et al., 2011; McDermott et al., 2015). By contrast, seawater-rock-dominated hydrothermal systems (commonly seen as a later stage of magmatic-hydrothermal systems; de Ronde et al., 2019) are characterized by seawater-derived hydrothermal fluids with temperatures of > 300 °C, although diffuse venting with lower temperatures (< 70 °C) can also occur (de Ronde and Stucker, 2015). The fluids have pH-values around 3 and are enriched in dissolved metals and reduced sulfur species (e.g., H₂S), but elemental sulfur is typically absent (Seewald et al., 2019). Rapid cooling of these hydrothermal fluids due to mixing with cold seawater or conductive cooling can result in precipitation of typical sulfate-sulfide minerals (including Cu- and Zn-rich phases) on, or beneath, the seafloor (e.g., de Ronde and Stucker, 2015; McDermott et al., 2018; Seewald et al., 2019; Pereira et al., 2022). Common additional minerals are quartz, opaline silica, adularia and clay minerals as well as pyrite, (Fe-rich) sphalerite, galena, tennantite, chalcopyrite and bornite as sulfide minerals (de Ronde et al., 2005). Note that both types of hydrothermal systems involve water-rock interaction processes. Compared to seawater-rock-dominated hydrothermal systems, however, magmatic-hydrothermal systems receive an additional magmatic input and fluids are hence commonly not rock-buffered.

Two large submarine caldera volcanoes have been studied in some detail with respect to their hydrothermal systems: Niuatahi in the NE Lau Basin and Brothers volcano in the Kermadec arc, both host the two types of hydrothermal systems mentioned above (Peters et al., 2021). At Niuatahi (previously known as “Volcano O” or “MTJ-1 caldera”), alunite, silica and fine-grained pyrite were noted along with molten native sulfur, suggesting the formation of acid-sulfate type fluids due to disproportionation of magmatic SO₂ (Kim et al., 2009; Seewald et al., 2019). At Brothers, both massive sulfides and advanced argillic assemblages were found at two different vent sites. The NW Caldera site is characterized by assemblages of chlorite-illite-smectite-barite-sulfides occurring with vent fluid pH-values > 3.2. By contrast, native sulfur, natroalunite and amorphous silica are dominant at the Upper Cone site where vent fluid pH-values are as low as 1.9 (de Ronde et al., 2005; de Ronde et al., 2011). At the Lower Cone site of Brothers, elemental sulfur is also present but here pH-values are more moderate (e.g., pH 4–5) (de Ronde et al., 2011; Kleint et al., 2019). However, at both Cone sites, the δ³⁴S values of elemental sulfur deposits are noticeably negative. The molten sulfur from Niuatahi has isotope values ranging from −8.2 to −7.5 ‰ (Kim et al., 2011). At Brothers, Lower Cone δ³⁴S values range from −8.3 to −3.9 ‰ for S⁰ (de Ronde et al., 2005), and from −8.0 to −4.8 ‰ for the Cone vent fluids (de Ronde et al., 2011).

These two main types of hydrothermal systems can also be found in the Kemp Caldera (formerly “McIntosh Crater/Caldera”). The prominent caldera represents an arc caldera volcano, similar in shape and size to Brothers volcano or Niuatahi; it is the only known submarine arc caldera volcano in the (southern) Atlantic Ocean. The Kemp Caldera was discovered during a geophysical survey by the R/V James Clark Ross research cruise JR224 in 2009, and hydrothermal activity was first observed within this submarine volcanic crater (Larter, 2009; Cole et al., 2014). Since then, several biological investigations have been carried out, but from a geochemical, petrological and volcanological point of view, Kemp Caldera is still largely unknown.

This paper provides the first vent fluid compositional and isotopic data as well as mineralogical results of hydrothermal precipitates collected at Kemp Caldera hydrothermal vent fields during the R/V Polarstern PS119 expedition in 2019. The focus of this study is on the formation mechanisms of elemental sulfur, which coexists with covellite and pyrite.