Ultraslow-spreading Ridges Research Articles

Magma source region size controls along axis variations in crustal thickness, axial depth and relief at plate spreading centers

The amplitude of variations in axial depth and across-axis relief that define spreading center segmentation vary systematically with spreading rate. Seismic and gravity data show that crustal thickness also varies along segments. We derive a highly idealized model to relate axial lithospheric thickness, crustal thickness, and effective magma source region size to segment-scale variation in axial depth and across-axis relief. All magma to build the crust is assumed to pool in a segment-centered magma source region and is distributed along the spreading axis by flow in dikes. The magma source and dike are regarded as a closed system and the magma pressure is taken to decrease linearly with the volume of magma going into dikes. The pressure-volume relation driving lateral magma flow is taken to be proportional to the along-axis gradient of depth. Axial depth variations depend on: (1) local isostatic compensation from crustal thickness variations; and (2) across-axis relief or axial valley depth. The axial valley depth is related to crustal and lithospheric thickness using the recently published model of Liu and Buck (2018). Excellent fits to bathymetric data for 12 segments from intermediate to ultra-slow spreading ridges are found by varying three model parameters (HC0, HL and ΔP/ΔV). The dependence of pressure drop on volume withdrawn is parameterized in terms of an effective radius for a spherical source region in an elastic half-space. A key result is that the effective magma source radius increases with spreading rate.

Seafloor Hydrothermal Activity around a Large Non-Transform Discontinuity along Ultraslow-Spreading Southwest Indian Ridge (48.1–48.7° E)

Non-transform discontinuity (NTD) is one category of tectonic units along slow- and ultraslow-spreading ridges. Some NTD-related hydrothermal fields that may reflect different driving mechanisms have been documented along slow-spreading ridges, but the discrete survey strategy makes it hard to evaluate the incidence of hydrothermal activity. On ultraslow-spreading ridges, fewer NTD-related hydrothermal activities were reported. Factors contributing to the occurrence of hydrothermal activities at NTDs and whether they could be potential targets for hydrothermal exploration are poorly known. Combining turbidity and oxidation reduction potential (ORP) sensors with a near-bottom camera, Chinese Dayang cruises from 2014 to 2018 have conducted systematic towed surveys for hydrothermal activity around a large NTD along the ultraslow-spreading Southwest Indian Ridge (SWIR, 48.1–48.7° E). Five new potential hydrothermal anomaly sites (2 inferred and 3 suspected) of high or low temperature and the previously inferred Sudi hydrothermal field occurred in diverse morphotectonic settings along a 78 km long ridge axis. The calculated vent frequency (Fs, sites/100 km) was ~7.7 over the entire study area, higher than the modified value (Fs ≈ 6.5) between 48 and 52° E of SWIR. Even only for the 54 km long large NTD, three hydrothermal anomaly sites yielded an Fs of ~5.6, which is higher than that of most ridge sections and is comparable to some fast-spreading ridges with high-resolution surveys. This indicates that NTDs along ultraslow-spreading ridges could be promising areas in fertilizing hydrothermal activities. Moreover, the deeply penetrating faults on the rift valley walls and strain-focused areas may contribute to the formation of NTD-related hydrothermal circulations. We suggest that NTDs along ultraslow-spreading ridges may be potential targets for further exploration of hydrothermal activities and seafloor sulfide deposits.

An origin of ultraslow spreading ridges for the Yarlung-Tsangpo ophiolites

As relics of ancient ocean lithosphere, ophiolites are the most important petrological evidence for marking the sutures and also play a key role in reconstructing plate configuration. They also provide valuable windows for studying crustal accretion and mantle processes occurring at modern ocean ridges. Abundant ophiolites are distributed along the Yarlung-Tsangpo suture and represent the relics of ocean lithosphere of the Neo-Tethys. They are characterized by an incomplete litho-stratigraphy, of which the mantle section is much thicker than the crustal section. Ocean crustal rocks outcropped in the Yarlung-Tsangpo ophiolites are much thinner than normal ocean crusts (~ 7 km) or even absent. Tectonic settings from which the Yarlung-Tsangpo ophiolites originated remain highly controversial, although an origin of the supra-subduction zone is prevailing. Moreover, their incomplete litho-stratigraphy has been commonly attributed to tectonic dismemberment during the late-stage emplacement after their formation. Nevertheless, such an incompleteness resembles the ocean lithosphere generated at modern ultraslow spreading ridges, such as the Southwest Indian Ridge (SWIR). In this paper, we present several lines of evidence that support the formation of the Yarlung-Tsangpo ophiolites at ultraslow spreading ridges, during which detachment faults were developed. This suggests that the Yarlung-Tsangpo ophiolites might represent the ocean core complexes (OCC) in the Neo-Tethys Ocean. The OCC with high topography in the seafloor were clogged in the trench and preserved as ophiolites through Indo-Eurasia collision. The clogging resulted in the demise of an old subduction and a new subduction was re-initiated beneath the clogged OCC.

Anomalously hot mantle source beneath the Dragon Flag Supersegment of the Southwest Indian Ridge: New evidence from crystallisation temperatures of mid-ocean ridge basalts

Discontinuities in the magmatic activity along a spreading ridge are usually observed in ultraslow-spreading ridges with a thick crust. However, the reasons for such melting anomaly are unknown. The Dragon Flag Supersegment between the Indomed Fault Zone (FZ) and the Gallieni FZ on the Southwest Indian Ridge (SWIR) is characterised by ultra-thick oceanic crust (up to ~10 km), and it provides a unique opportunity for studying magmatic crustal accretion at an ultraslow-spreading ridge. We determined the compositions of olivine–spinel pairs in mid-ocean ridge basalts (MORBs) collected from Segments 25 and 28 of the SWIR to estimate the temperatures of crystallisation and investigate the source lithologies. The olivine-spinel pairs were carefully screened and only those having crystallized in equilibrium were used for the temperature estimates. The Al-in-olivine thermometer provided crystallisation temperatures that ranged from 1197 ± 37 to 1393 ± 21 °C. The 95th percentile calculated temperature (1345 °C) for the primitive olivine is ~75 °C hotter than for olivines in MORBs from normal ridges but comparable to the data for plume-related oceanic island basalts. The olivines have Fe/Mn ratios and Ni, Ca, and Mn contents that are similar to those of olivines in MORBs from normal ridges, which suggests their derivation from a peridotite-dominant source with little contribution from fertile mantle components. Our results suggest a hot mantle reservoir is present beneath the Dragon Flag Supersegment, and that it is probably related to a mantle plume.

Resource estimation of the sulfide-rich deposits of the Yuhuang-1 hydrothermal field on the ultraslow-spreading Southwest Indian Ridge

Abstract Seafloor massive sulfide (SMS) deposits are important deep-sea mineral resources expected to occur predominantly on slow- and ultraslow-spreading mid-ocean ridges. Resource estimates are already available for some of the largest SMS deposits on slow-spreading ridges but not on ultraslow-spreading ridges. Based on geological mapping and sampling, this study investigates the distribution and content of sulfide-rich deposits in the Yuhuang-1 hydrothermal field (YHF), located on the ultraslow-spreading Southwest Indian Ridge. The sulfide-rich deposits in the YHF are composed of two areas ~500 m apart: the southwest sulfide area (SWS) and the northeast sulfide area (NES). We calculated the volume of sulfide-rich moundsin the YHF and arrived at a total accumulation of ~10.6 × 106 tons, including at least ~7.5 × 105 tons of copper and zinc and ~18 tons of gold. Furthermore, considering the coverage of layered hydrothermal sediment mixed with sulfide-rich breccias, which may have underlying massive sulfide deposits, the maximum total mass was estimated at ~45.1 × 106 tons. This suggests that the YHF is one of the largest SMS deposits worldwide and confirm that ultraslow-spreading ridges have the greatest potential to form large-scale SMS deposits.

Thermo‐Mechanical State of Ultraslow‐Spreading Ridges With a Transient Magma Supply

AbstractThe thermal structure of mid‐ocean ridge axes is a critical control on the mechanical properties of young lithosphere and modulates the faulting styles that shape the Earth's seafloor. Models that balance a steady input of magmatic heat with a vigorous hydrothermal output are generally successful at explaining the thermal state of magmatically robust mid‐ocean ridges. However, the magma supply of ultraslow spreading ridges is often subdued and highly episodic, and the depth‐extent and vigor of hydrothermal circulation in these settings remain poorly known, requiring modifications to the standard magmatic‐hydrothermal paradigm. We develop models that couple repeated magmatic intrusions with hydrothermal convection in a permeable domain whose boundaries are controlled by the extent of grain‐scale thermal cracking. Our simulations reveal decreasing trends of venting temperatures with increasing intrusion periodicity, as well as with increasing permeability. Our model explains the seismically inferred depth (∼10 km) to the brittle‐ductile transition and the depth of MORB crystallization at the Mid‐Cayman Spreading Center by invoking repeated intrusion of sills every 20–50 kyrs, depending on their size. Doubling this periodicity predicts a thicker (∼15 km) seismogenic layer and lower‐temperature venting, as observed at the Southwest Indian Ridge (SWIR) at 64˚E. However, if fluid convection is not possible below ∼6 km, our model requires that high‐temperature venting at ultraslow ridges (e.g., Longqi at SWIR 49.7˚E) be fueled by shallow episodic magma intrusions into the long‐term convective region, which maintain high‐output circulation for at most a few tens of kyrs.

Segment‐Scale Seismicity of the Ultraslow Spreading Knipovich Ridge

AbstractUltraslow spreading ridges form the slowest divergent plate boundaries and exhibit distinct spreading processes in volcanically active magmatic sections and intervening amagmatic sections. Local seismicity studies of ultraslow spreading ridges until now cover only parts of segments and give insight into spreading processes at confined locations. Here, we present a microseismicity data set that allows to study spreading processes on the scale of entire segments. Our network of 26 ocean bottom seismometers covered around 160 km along axis of the ultraslow spreading Knipovich Ridge in the Greenland Sea and recorded earthquakes for a period of about 1 year. We find seismicity varying distinctly along‐axis. The maximum earthquake depths shallow over distances of 70 km toward the Logachev volcanic center. Here, swarm activity occurs in an otherwise aseismic zone. Melts may thus be guided along the subparallel topography of the lithosphere‐asthenosphere boundary toward major volcanic centers explaining the uneven along‐axis melt distribution typical for ultraslow ridges. Absence of shallow seismicity in the upper 8 km of the lithosphere with a band of deep seismicity underneath offsets presumably melt‐poor regions from magma richer sections. Aseismic deformation in these regions may indicate weakening of mantle rocks by alteration. We do not find obvious indications for major detachment faulting that characterizes magma‐poor spreading at some ultraslow spreading segments. The highly oblique spreading of Knipovich Ridge may be the reason for a fine‐scale segmentation of the seismic activity with zones of weak seismicity possibly indicating transform motion on short obliquely oriented faults.

Magmato-tectonic mechanism of Southwest Indian Ridge (49°-50°E) inferred from quantitative morphotectonic analysis based on high-resolution multibeam bathymetry

Southwest Indian Ridge (SWIR) is a typical ultra-slow spreading ridge which has been extensively studied based on geophysical survey, geochemical sampling and morphological feature extraction. In this paper, based on the high-resolution multibeam bathymetric data, we extract seven types of topography, including fault, axial rift, axial volcanic ridge (AVR), axial trough, AVR's remnant, volcanic cone and oceanic core complex (OCC). We perform quantitative morphotectonic statistic, verifying the relations between the topography and magmato-tectonic mechanism of SWIR 49–50°E. The analyzed results of flank topography show that the average seafloor depth of the southern flank is shallower than that of the northern flank. The Total Cumulative Length of the faults in the northern and southern flank is 624.6 km and 948.2 km. The number, Total density, Average Length and Maximum Length of the faults in the southern flank are higher than those in the northern flank. An OCC and detachment faults locate in the southern flank. The asymmetry in flank topography indicates the asymmetric spreading mode of the study area. The analyzed results of the axial topography show that AVRs and troughs develop in the rift. The rift is narrower and shallower in the AVR regions, and wider and deeper in the trough regions. These observations support the partition of magmatic and less-magmatic stretching in the study area, and indicate the relation between the variation of rift's depth-width and the magmato-tectonic accretion.

The Xigaze ophiolite: fossil ultraslow-spreading ocean lithosphere in the Tibetan Plateau

The crust and mantle both in ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are critical to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogues for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it with the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dykes and sills cutting both mantle and lower crust. The gabbro units are thin ( c . 350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable with the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analogue of ultraslow-spreading ocean lithosphere. Supplementary material: Single-spot analysis and average compositions for mineral major element compositions (wt.%) of the Jiding gabbro section, Xigaze ophiolite are available at https://doi.org/10.17632/dtzpmwkscp.1

GIS-based Mineral Prospectivity Mapping of Seafloor Massive Sulfide on Ultraslow-spreading Ridges: A Case Study of Southwest Indian Ridge 48.7°–50.5° E

With the depletion of mineral resources on land, seafloor massive sulfide deposits have the potential to become as important for exploration, development and mining as those on land. However, it is difficult to investigate the ocean environment where seafloor massive sulfide deposits are located. Thus, improving prospecting efficiency by reducing the exploration search space through mineral prospectivity mapping (MPM) is desirable. MPM has been used in the exploration for seafloor deposits on regional scales, e.g., the Mid-Atlantic Ridge and Arctic Ridge. However, studies of MPM on ultraslow-spreading ridges on segment scales to aid exploration for seafloor massive sulfide have not been carried out to date. Here, data of water depth, geology and hydrothermal plume anomalies were analyzed and the weights-of-evidence method was used to study the metallogenic regularity and to predict the potential area for seafloor massive sulfide exploration in 48.7°–50.5° E segments on the ultraslow spreading Southwest Indian Ridge. Based on spatial analysis, 11 predictive maps were selected to establish a mineral potential model. Weight values indicate that the location of seafloor massive sulfide deposits is correlated mainly with mode-E faults and oceanic crust thickness in the study area, which correspond with documented ore-controlling factors on other studied ultraslow-spreading ridges. In addition, the detachment fault and ridge axis, which reflect the deep hydrothermal circulation channel and magmatic activities, also play an important role. Based on the posterior probability values, 3 level A, 2 level B and 2 level C areas were identified as targets for further study. The MPM results were helpful for narrowing the search space and have implications for investigating and evaluating seafloor massive sulfide resources in the study area and on other ultraslow-spreading ridges.

Evidence for Multi-stage Melt Transport in the Lower Ocean Crust: the Atlantis Bank Gabbroic Massif (IODP Hole U1473A, SW Indian Ridge)

AbstractThe architecture of lower oceanic crust at slow- and ultraslow-spreading ridge is diverse, yet the mechanisms that produce this diversity are not well understood. Particularly, the 660-km2 gabbroic massif at Atlantis Bank (Southwest Indian Ridge) exhibits significant compositional zonation, representing a high magma supply end member for accretion of the lower ocean crust at slow and ultraslow-spreading ridges. We present the petrographic and geochemical data of olivine gabbros from the 809-metre IODP Hole U1473A at Atlantis Bank gabbroic massif. Structurally, the upper portion of U1473A consists of a ∼600-metre shear zone; below this, the hole is relatively undeformed, with several minor shear zones. Olivine gabbros away from the shear zones have mineral trace element compositions indicative of high-temperature reaction with an oxide-undersaturated melt. By contrast, olivine gabbros within shear zones display petrographic and chemical features indicative of reaction with a relatively low-temperature, oxide-saturated melt. These features indicate an early stage of primitive to moderately evolved melt migration, followed by deformation-driven transport of highly evolved Fe–Ti-rich melts to high levels in this gabbroic massif. The close relationship between shear zones and the reaction with oxide-saturated melts suggests that syn-magmatic shear zones provide a conduit for late-stage, Fe–Ti-rich melt transport through Atlantis Bank lower crust. This process is critical to generate the compositional zonation observed. Thus, the degree of syn-magmatic deformation, which is fundamentally related to magma supply, plays a dominant role in developing the diversity of lower ocean crust observed at slow- and ultraslow-spreading ridges.

New interpretation of the spreading evolution of the Knipovich Ridge derived from aeromagnetic data

SUMMARYInsights into the spreading evolution of the Knipovich Ridge and development of the Fram Strait are revealed from a recent aeromagnetic survey. As an ultraslow spreading ridge in an oblique system located between the Svalbard–Barents Sea and the Northeast Greenland rifted margins, the dynamics of the Knipovich Ridge opening has long been debated. Its 90° bend with the Mohns Ridge, rare in plate tectonics, affects the evolution of the Fram Strait and motivates the study of crustal deformation with this distinctive configuration. We identified magnetic isochrons on either side of the present-day Knipovich Ridge. These magnetic observations considerably reduce the mapped extent of the oceanic domain and question the present understanding of the conjugate rifted margins. Our analysis reveals a failed spreading system before a major spreading reorganization of the Fram Strait gateway around magnetic chron C6 (circa 20 Ma).

Sulfur isotopic compositions of sulfides along the Southwest Indian Ridge: implications for mineralization in ultramafic rocks

The recently explored Tianzuo hydrothermal field in serpentinized ultramafic rocks of the amagmatic segment of the ultraslow-spreading Southwest Indian Ridge displays high-temperature sulfide mineralization (isocubanite, sphalerite, and minor pyrrhotite) and low-temperature (pyrite and covellite) phases. Pyrite can be subdivided into pyrite-I and -II, with the former generally having a pseudomorphic texture after pyrrhotite and the latter typically growing around isocubanite, sphalerite, and pyrite-I or occurring as individual grains in quartz veinlets. The sulfide minerals have the greatest range of δ34S values (− 23.8 to 14.1‰), found so far among modern sediment-starved ridges, with distinct δ34S values for low- and high-temperature mineral phases. The high δ34S values of isocubanite (9.6 to 12.2‰) and sphalerite (9.1 to 14.1‰) suggest that sulfate, which precipitated from seawater during an early low-temperature phase of hydrothermal circulation, was the main sulfur source for these sulfides. Pyrite-II has the lowest and most variable δ34S values (− 23.8 to − 3.6‰), suggesting microbial sulfate reduction. Pyrite-I has variable and generally positive δ34S values (− 0.1 to 12.0‰), with sulfur being inherited from pyrrhotite from the original thermochemical reduction of sulfate, mixed with volcanogenic sulfur. Intermittent magmatism represented by gabbroic intrusions, and high permeability caused by well-developed fractures associated with detachment faults, contributed to the formation of sulfides in the Tianzuo hydrothermal field. These factors possibly control sulfide mineralization in amagmatic segments of ultraslow-spreading ridges.

Hydration of a Mantle Shear Zone Beyond Serpentine Stability: A Possible Link to Microseismicity Along Ultraslow Spreading Ridges?

AbstractHydration of the oceanic mantle is a fundamental process of the global water cycle promoting chemical and volumetric changes and facilitating mantle exhumation along detachment faults. At which depth these processes occur and how fluids circulate along ductile mantle shear zones are, however, less well constrained. Here we present field, chemical, and microstructural evidence of hydration processes of peridotite mylonites within an upper mantle shear zone from an Alpine ophiolite (Lanzo massif, Italy). Mylonitic and ultramylonitic areas of the anastomosing shear zone are enriched in Cl‐bearing amphibole. Electron backscatter diffraction (EBSD) data indicate the activation of the (100)[001] amphibole slip system arguing for synkinematic growth and deformation at temperatures consistent with Mg‐hornblende stability between 800°C and 850°C. High Cl contents in amphibole (0.15–0.61 wt%) as well as oxygen isotope data (δ18Owhole‐rock between 4.4‰ and 4.7‰) indicate accumulation and focusing of seawater‐derived fluid in mylonitic and ultramylonitic domains. Such hydration processes are consistent with strain partitioning between water‐poor (less deformed) and water‐rich (intensely deformed) layers, consistent with changes in olivine and pyroxene crystallographic preferred orientations (CPOs). Our results support recent geophysical data from ultraslow spreading mid‐ocean ridge systems that fluids might penetrate beyond the stability of serpentine to depth between 6 and 15 km. Such peridotite shear zones act as fluid pathways for long‐lived detachment faults or oceanic transform faults, along which upper mantle rocks are exhumed to the seafloor. Fracturing and fluid flow along such peridotite shear zones might be recorded by deep microseismicity along ultraslow spreading ridges.

Variations in faulting style of the Southwest Indian Ridge (46°–53.5°E): Implications for crustal accretion process at ultraslow-spreading ridges

The variations in faulting style at the Southwest Indian Ridge (SWIR) 46°–53.5°E segment, correspond to different patterns of crustal accretion under different magma supply in an ultraslow-spreading ridge. We use multi-beam and filtered bathymetry to identify the fault characteristics and measure the fraction of magmatic accretion M and axial valley depth D. Four styles of faulting, namely, mode-C, mode-E, one-side detachment, and flip-flop detachment faults, are identified in the study area. The heave of these four faulting styles tends to increase gradually, and these faults can be progressive evolve in order from symmetrical to asymmetrical spreading. The reducing M significantly affects the faulting styles from mode-C to mode-E faults by largening fault heave, resulting in a deeper D at mode-E faults. However, there was no significant difference in M and D from mode-E to one-side detachment faults with the increase of heave. We consider that more magma may preferentially intrude into the asthenosphere at one-side detachment faults. Moreover, the flip-flop detachment faults are developed at the amagmatic segments, constrained by the extreme low M, talc- serpentinization, and dense transform faults.

Chemical composition and petrogenesis of plagioclases in plagioclase-phyric basalts from the Southwest Indian Ridge (51°E)

Electron microprobe analysis was conducted on plagioclase from the plagioclase ultraphyric basalts (PUBs) erupted on the Southwest Indian Ridge (SWIR) (51°E) to investigate the geochemical changes in order to better understand the magmatic processes occurring under ultraslow spreading ridges and to provide insights into the thermal and dynamic regimes of the magmatic reservoirs and conduit systems. The phenocryst cores are generally calcic (An74–82) and are depleted in FeO and MgO. Whereas the phenocryst rims (An67–71) and the plagioclase in the groundmass (An58–63) are more sodic and have higher FeO and MgO contents than the phenocryst cores. The crystallization temperatures of the phenocryst cores and the calculation of the equilibrium between the phenocrysts and the matrix suggest that the plagioclase cores are unlikely to have crystallized from the host basaltic melt, but are likely to have crystallized from a more calcic melt. The enrichment in incompatible elements (FeO and MgO), as well as the higher FeO/MgO ratios of the outermost phenocryst rims and the groundmass, are the result of plagioclase-melt disequilibrium diffusion during the short residence time in which the plagioclase crystallized. Our results indicate that an evolved melt replenishing under the SWIR (51°E) drives the eruption over a short period of time.

Seawater versus mantle sources of mercury in sulfide-rich seafloor hydrothermal systems, Southwest Indian Ridge

The Southwest Indian Ridge (SWIR) is an ultraslow-spreading ridge where large hydrothermal fields (HFs) are widely distributed. The HFs differ in geological settings, basement rock compositions and mineral associations, but are commonly associated with massive sulfides and sulfide-rich hydrothermal vents that are rich in mercury (Hg). However, the source of Hg remains not well understood. This is a first report on the concentration and isotopic composition of Hg in sulfides from two large HFs, named Duanqiao and Yuhuang, in the SWIR. Sulfides from Duanqiao and Yuhuang showed elevated Hg concentrations, ranging from 3.5 × 102 to 8.1 × 103 ng/g and 4.4 × 102 to 4.4 × 104 ng/g, respectively, which suggest that seafloor mass sulfide deposits can be an important sink of Hg to the deep marine environment. In both HFs, pyrite (4.4 × 102–4.4 × 104 ng/g) and sphalerite (8.3 × 102–6.0 × 103 ng/g) show higher Hg concentrations than chalcopyrite (3.5 × 102 ng/g), suggesting that the replacement of Fe(II) and Zn(II) by Hg(II) is a major form of Hg incorporation in sulfides. Sulfides from Yuhuang show relatively larger δ202Hg and Δ199Hg ranges of −1.23 to −0.05‰ and −0.10 to 0.20‰, respectively, compared with those from Duanqiao (δ202Hg: −0.63 to −0.12‰; Δ199Hg: 0.02–0.10‰), suggesting that the Hg sources are different in the two HFs. The differences in Δ199Hg suggest the dominance of magmatic/mantle Hg in the Duanqiao HF, but a mixture of magmatic/mantle and seawater Hg to the Yuhuang HF. We thus propose Hg isotopes as a potential source tracer of Hg in HFs and infer that magmatism may not only serve as a direct Hg source in HFs, but may also drive seawater circulation and cause the precipitation of Hg from hydrothermally circulated seawater.

Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity: COMMENT

Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity: COMMENT

Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity: REPLY

Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity: REPLY

Pulsated Global Hydrogen and Methane Flux at Mid‐Ocean Ridges Driven by Pangea Breakup

AbstractMolecular hydrogen production occurs through the serpentinization of mantle peridotite exhumed at mid‐ocean ridges. Hydrogen is considered essential to sustain microbial life in the subsurface; however, estimates of hydrogen flux through geological time are unknown. Here we present a model of the primary, abiotic production of molecular hydrogen from the serpentinization of oceanic lithosphere using full‐plate tectonic reconstructions for the last 200 Ma. We find significant variability in hydrogen fluxes (1–70 • 1016 mol/Ma or 0.2–14.1 • 105 Mt/Ma), which are a function of the sensitivity of evolving ocean basins to spreading rates and can be correlated with the opening of key ocean basins during the breakup of Pangea. We suggest that the primary driver of this hydrogen flux is the continental reconfiguration during Pangea breakup, as this produces ocean basins more conducive to exhuming and exposing mantle peridotite at slow and ultraslow spreading ridges. Consequently, present‐day flux estimates are ~7 • 1017 mol/Ma (1.4 • 106 Mt/Ma), driven primarily by the slow and ultraslow spreading ridges in the Atlantic, Indian, and Arctic oceans. As methane has also been sampled alongside hydrogen at hydrothermal vents, we estimate the methane flux using methane‐to‐hydrogen ratios from present‐day hydrothermal vent fluids. These ratios suggest that methane flux ranges between 10 and 100% of the total hydrogen flux, although as the release of methane from these systems is still poorly understood, we suggest a lower estimate, equivalent to around 7–12 • 1016 mol/Ma (1.1–1.9 • 106 Mt/Ma) of methane.