Subduction is a key component of Earth's long‐term sulfur cycle; however, the mechanisms that drive sulfur from subducting slabs remain elusive. Isotopes are a sensitive indicator of the speciation of sulfur in fluids, sulfide dissolution‐precipitation reactions, and inferring fluid sources. To investigate these processes, we report δ34S values determined by secondary ion mass spectroscopy in sulfides from a global suite of exhumed high‐pressure rocks. Sulfides are classified into two petrogenetic groups: (1) metamorphic, which represent closed‐system (re)crystallization from protolith‐inherited sulfur, and (2) metasomatic, which formed during open system processes, such as an influx of oxidized sulfur. The δ34S values for metamorphic sulfides tend to reflect their precursor compositions: −4.3 ‰ to +13.5 ‰ for metabasic rocks, and −32.4 ‰ to −11.0 ‰ for metasediments. Metasomatic sulfides exhibit a range of δ34S from −21.7 ‰ to +13.9 ‰. We suggest that sluggish sulfur self‐diffusion prevents isotopic fractionation during sulfide breakdown and that slab fluids inherit the isotopic composition of their source. We estimate a composition of −11 ‰ to +8 ‰ for slab fluids, a significantly smaller range than observed for metasomatic sulfides. Large fractionations during metasomatic sulfide precipitation from sulfate‐bearing fluids, and an evolving fluid composition during reactive transport may account for the entire ~36 ‰ range of metasomatic sulfide compositions. Thus, we suggest that sulfates are likely the dominant sulfur species in slab‐derived fluids.
Sulfur belongs among H2O, CO2, and Cl as one of the key volatiles in Earth’s chemical cycles. High oxygen fugacity, sulfur concentration, and δ34S values in volcanic arc rocks have been attributed to significant sulfate addition by slab fluids. However, sulfur speciation, flux, and isotope composition in slab-dehydrated fluids remain unclear. Here, we use high-pressure rocks and enclosed veins to provide direct constraints on subduction zone sulfur recycling for a typical oceanic lithosphere. Textural and thermodynamic evidence indicates the predominance of reduced sulfur species in slab fluids; those derived from metasediments, altered oceanic crust, and serpentinite have δ34S values of approximately −8‰, −1‰, and +8‰, respectively. Mass-balance calculations demonstrate that 6.4% (up to 20% maximum) of total subducted sulfur is released between 30–230 km depth, and the predominant sulfur loss takes place at 70–100 km with a net δ34S composition of −2.5 ± 3‰. We conclude that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver34S-enriched sulfur to produce the positive δ34S signature in arc settings. Most sulfur has negative δ34S and is subducted into the deep mantle, which could cause a long-term increase in the δ34S of Earth surface reservoirs.
more » « less- Award ID(s):
- 1650329
- NSF-PAR ID:
- 10154281
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We present a quantitative assessment of the input and output of CO2and N2along the Hikurangi margin based on the chemical and stable isotope composition of sediments and basalts (from IODP 375), previously accreted metasedimentary rocks, and volcanic/hydrothermal gases (together with noble gas data for the latter). We compare these results with 3‐D thermo‐petrologic models for four lithologic structures, representing different plateau inputs. The model results indicate that 59%–85% of initially subducted C and 5%–12% of N is lost from the slab during metamorphism, with both volatiles being dominantly sourced from altered oceanic crust with some contribution from subducted sediment at the forearc‐arc transition (75–90 km depth). The δ13CVPDBand CO2/3He values for the arc gases range from −8.3 to −1.4‰ and 2 × 109to 2.7 × 1011, indicating contributions from slab carbonate, organic C, and mantle C of 67%, 30%, and 3%, respectively. The δ15Nairand N2/36Ar values of arc gases are −1.0 to +2.3‰ and 1.54 × 104to 1.9 × 105, indicating slab and mantle contributions of 74% and 26%. The δ13C signature of gases requires addition of organic C by tectonic erosion and/or shallow crustal assimilation. These calculations yield whole‐margin fluxes of 5.4–7.0 Tg/yr for CO2and 0.0022–0.0057 Tg/yr for N2,corresponding to ∼2.2% and 1%–30% of the global CO2and N2flux from subaerial volcanoes worldwide (assuming no loss during transit). This unique assessment of volatile cycling could prove useful in refining regional and global estimates of volatile recycling efficiency.
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null (Ed.)Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones and suprasubduction zone lithosphere. International Ocean Discovery Program (IODP) Expedition 366 was designed to address the nature of these processes in the shallow to intermediate depth of the Mariana subduction channel. Although no technology is available to permit direct sampling of the subduction channel of an intraoceanic convergent margin at depths up to 18 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access this zone. Active conduits, resulting from fractures in the forearc, are prompted by along- and across-strike extension that allows slab-derived fluids and materials to ascend to the seafloor along associated faults, resulting in the formation of serpentinite mud volcanoes. Serpentinite mud volcanoes of the Mariana forearc are the largest mud volcanoes on Earth. Their positions adjacent to or atop fault scarps on the forearc are likely related to the regional extension and vertical tectonic deformation in the forearc. Serpentinite mudflows at these volcanoes include serpentinized forearc mantle clasts, crustal and subducted Pacific plate materials, a matrix of serpentinite muds, and deep-sourced formation fluid. Mud volcanism on the Mariana forearc occurs within 100 km of the trench, representing a range of depths and temperatures to the downgoing plate and the subduction channel. These processes have likely been active for tens of millions of years at this site and for billions of years on Earth. At least 10 active serpentinite mud volcanoes have been located in the Mariana forearc. Two of these mud volcanoes are Conical and South Chamorro Seamounts, which are the furthest from the Mariana Trench at 86 and 78 km, respectively. Both seamounts were cored during Ocean Drilling Program (ODP) Legs 125 and 195, respectively. Data from these two seamounts represent deeper, warmer examples of the continuum of slab-derived materials as the Pacific plate subducts, providing a snapshot of how slab subduction affects fluid release, the composition of ascending fluids, mantle hydration, and the metamorphic paragenesis of subducted oceanic lithosphere. Data from the study of these two mud volcanoes constrain the pressure, temperature, and composition of fluids and materials within the subduction channel at depths of about 18 to 19 km. Understanding such processes is necessary for elucidating factors that control seismicity in convergent margins, tectonic and magma genesis processes in the forearc and volcanic arc, fluid and material fluxes, and the nature and variability of environmental conditions that impact subseafloor microbial communities. Expedition 366 centered on data collection from cores recovered from three serpentinite mud volcanoes that define a continuum of subduction-channel processes defined by the two previously cored serpentinite mud volcanoes and the trench. Three serpentinite mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoro) were chosen at distances 55 to 72 km from the Mariana Trench. Cores were recovered from active sites of eruption on their summit regions and on the flanks where ancient flows are overlain by more recent ones. Recovered materials show the effects of dynamic processes that are active at these sites, bringing a range of materials to the seafloor, including materials from the lithosphere of the Pacific plate and from subducted seamounts (including corals). Most of the recovered material consists of serpentinite mud containing lithic clasts, which are derived from the underlying forearc crust and mantle and the subducting Pacific plate. Cores from each of the three seamounts drilled during Expedition 366, as well as those from Legs 125 and 195, include material from the underlying Pacific plate. A thin cover of pelagic sediment was recovered at many Expedition 366 sites, and at Site U1498 we cored through serpentinite flows to the underlying pelagic sediment and volcanic ash deposits. Recovered serpentinites are largely uniform in major element composition, with serpentinized ultramafic rocks and serpentinite muds spanning a limited range in SiO2 , MgO, and Fe2 O3 compositions. However, variation in trace element composition reflects pore fluid composition, which differs as a function of the temperature and pressure of the underlying subduction channel. Dissolved gases H2 , CH4 , and C2 H6 are highest at the site furthest from the trench, which also has the most active fluid discharge of the Expedition 366 serpentinite mud volcanoes. These dissolved gases and their active discharge from depth likely support active microbial communities, which were the focus of in-depth subsampling and preservation for shore-based analytical and culturing procedures. The effects of fluid discharge were also registered in the porosity and GRA density data indicated by higher than expected values at some of the summit sites. These higher values are consistent with overpressured fluids that minimize compaction of serpentinite mud deposits. In contrast, flank sites have significantly greater decreases in porosity with depth, suggesting that processes in addition to compaction are required to achieve the observed data. Thermal measurements reveal higher heat flow values on the flanks (~31 mW/m2) than on the summits (~17 mW/m2) of the seamounts. The new 2G Enterprises superconducting rock magnetometer (liquid helium free) revealed relatively high values of both magnetization and bulk magnetic susceptibility of discrete samples related to ultramafic rocks, particularly in dunite. Magnetite, a product of serpentinization, and authigenic carbonates were observed in the mudflow matrix materials. In addition to coring operations, Expedition 366 focused on the deployment and remediation of borehole casings for future observatories and set the framework for in situ experimentation. Borehole work commenced at South Chamorro Seamount, where the original-style CORK was partially removed. Work then continued at each of the three summit sites following coring operations. Cased boreholes with at least three joints of screened casing were deployed, and a plug of cement was placed at the bottom of each hole. Water samples were collected from two of the three boreholes, revealing significant inputs of formation fluids. This suggests that each of the boreholes tapped a hydrologic zone, making these boreholes suitable for experimentation with the future deployment of a CORK-lite. An active education and outreach program connected with many classrooms on shore and with the general public through social media.more » « less
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null (Ed.)Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones and suprasubduction zone lithosphere. International Ocean Discovery Program Expedition 366 was designed to address the nature of these processes in the shallow to intermediate depth of the Mariana subduction channel. Although no technology is available to permit direct sampling of the subduction channel of an intraoceanic convergent margin at depths up to 19 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access materials from this zone. Active conduits, resulting from fractures in the forearc, are prompted by along- and across-strike extension that allows slab-derived fluids and materials to ascend to the seafloor along associated faults, resulting in the formation of serpentinite mud volcanoes. Serpentinite mud volcanoes of the Mariana forearc are the largest mud volcanoes on Earth. Their positions adjacent to or atop fault scarps on the forearc are likely related to the regional extension and vertical tectonic deformation in the forearc. Serpentinite mudflows at these volcanoes include serpentinized forearc mantle clasts, crustal and subducted Pacific plate materials, a matrix of serpentinite muds, and deep-sourced formation fluid. Mud volcanism on the Mariana forearc occurs within 100 km of the trench, representing a range of depths and temperatures to the downgoing plate and the subduction channel. These processes have likely been active for tens of millions of years at the Mariana forearc and for billions of years on Earth. At least 19 active serpentinite mud volcanoes have been located in the Mariana forearc. Two of these mud volcanoes are Conical and South Chamorro Seamounts, which are the farthest from the Mariana Trench at 86 and 78 km, respectively. Both seamounts were cored during Ocean Drilling Program Legs 125 and 195, respectively. Data from these two seamounts represent deeper, warmer examples of the continuum of slab-derived materials as the Pacific plate subducts, providing a snapshot of how slab subduction affects fluid release, the composition of ascending fluids, mantle hydration, and the metamorphic paragenesis of subducted oceanic lithosphere. Data from the study of these two mud volcanoes constrain the pressure, temperature, and composition of fluids and materials within the subduction channel at depths of up to 19 km. Understanding such processes is necessary for elucidating factors that control seismicity in convergent margins, tectonic and magma genesis processes in the volcanic arc and backarc areas, fluid and material fluxes, and the nature and variability of environmental conditions that impact subseafloor microbial communities. Expedition 366 focused on data collection from cores recovered from three serpentinite mud volcanoes that define a continuum of subduction-channel processes to compare with results from drilling at the two previously cored serpentinite mud volcanoes and with previously collected gravity, piston, and remotely operated vehicle push cores across the trench-proximal forearc. Three serpentinite mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoro) were chosen at distances 55 to 72 km from the Mariana Trench. Cores were recovered from active sites of eruption on their summit regions and on the flanks where ancient flows are overlain by more recent ones. Recovered materials show the effects of dynamic processes that are active at these sites, bringing a range of materials to the seafloor, including materials from the crust of the Pacific plate, most notably subducted seamounts (even corals). Most of the recovered material consists of serpentinite mud containing lithic clasts, which are derived from the underlying forearc crust and mantle and the subducting Pacific plate. A thin cover of pelagic sediment was recovered at many Expedition 366 sites, and at Site U1498 we cored through distal serpentinite mudflows and into the underlying pelagic sediment and volcanic ash deposits. Recovered serpentinized ultramafic rocks and mudflow matrix materials are largely uniform in major element composition, spanning a limited range in SiO2, MgO, and Fe2O3 compositions. However, variation in trace element composition reflects interstitial water composition, which differs as a function of the temperature and pressure of the underlying subduction channel. Dissolved gases H2, CH4, and C2H6 are highest at the site farthest from the trench, which also has the most active fluid discharge of the Expedition 366 serpentinite mud volcanoes. These dissolved gases and their active discharge from depth likely support active microbial communities, which were the focus of in-depth subsampling and preservation for shore-based analytical and culturing procedures. The effects of fluid discharge were also registered in the porosity and gamma ray attenuation density data indicated by higher than expected values at some of the summit sites. These higher values are consistent with overpressured fluids that slow compaction of serpentinite mud deposits. In contrast, flank sites have significantly greater decreases in porosity with depth, suggesting that processes in addition to compaction are required to achieve the observed data. Thermal measurements reveal higher heat flow values on the flanks (~31 mW/m2) than on the summits (~17 mW/m2) of the seamounts. The new 2G Enterprises superconducting rock magnetometer (liquid helium free) revealed relatively high values of both magnetization and bulk magnetic susceptibility of discrete samples related to ultramafic rocks, particularly dunite. Magnetite, a product of serpentinization, and authigenic carbonates were observed in the mudflow matrix materials. In addition to coring operations, Expedition 366 focused on the deployment and remediation of borehole casings for future observatories and set the framework for in situ experimentation. Borehole work commenced at South Chamorro Seamount, where the original-style CORK was partially removed. Work then continued at each of the three summit sites following coring operations. Cased boreholes with at least three joints of screened casing were deployed, and a plug of cement was placed at the bottom of each hole. Water samples were collected from two of the three boreholes, revealing significant inputs of formation fluids. This suggests that each of the boreholes tapped a hydrologic zone, making these boreholes suitable for experimentation with the future deployment of a CORK-Lite.more » « less
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Abstract Lithium isotopes have been identified as a promising tracer of subducted materials in arc lavas due to the observable variations in related reservoirs such as subducting sediments and altered oceanic crust. The Tonga–Kermadec arc–Lau back‐arc provides an end‐member of subduction zones with the coldest thermal structure on Earth. Reported here are Li isotope data for 14 lavas from the arc front and 7 back‐arc lavas as well as 12 pelagic and volcaniclastic sediments along a profile through the sedimentary sequence at DSDP Site 204. The arc and back‐arc lavas range from basalts to dacites in composition with SiO2 = 48.3–65.3 wt% over which Li concentrations increase from 2 ppm to 16 ppm. Li/Y ratios range from 0.08 to 0.77 and from 0.24 to 0.65 in the arc and back‐arc lavas, respectively. The majority of the lavas have δ7Li that ranges from 2.5 ‰ to 5.0 ‰ with an average of (3.6 ±0.7) ‰, similar to that reported from other arcs and there is no distinction between the arc front and back‐arc lavas. The pelagic sediments have variable Li concentrations (33–133 ppm) and δ7Li that ranges from 1.2 ‰ to 10.2 ‰ while the volcaniclastic sediments have an even greater range of Li concentrations (3.6–165 ppm) and generally higher δ7Li values (8–14 ‰). However, δ7Li in the lavas does not correlate with commonly used trace element ratio or isotope signatures indicative of slab‐derived fluids or the sediments. This is probably because the range of δ7Li in the lavas and sediments overlap. Calculated sediment mass‐balance models require significantly more sediment than previous estimates based on Th–Nd–Be isotopes. This may indicate that a sizeable proportion of the total Li budget in the lavas is provided by Li‐enriched fluids from the subducting sediments and/or altered oceanic crust.
