Altered Basalts Research Articles

Alteration enrichment of nitrogen in the gabbroic oceanic crust: Implications for global subducting nitrogen budget and subduction-zone nitrogen recycling

Uptake of nitrogen (N) during hydrothermal alteration of oceanic crust makes the altered oceanic crust (AOC) a significant N reservoir in parallel to seafloor sediments. While N enrichment in the upper (basaltic) oceanic crust during low-temperature alteration has been widely observed, the N characteristics of moderate- to high-temperature altered gabbroic oceanic crust, which accounts for ∼70 vol% of the crustal material in subducting slabs, have been rarely studied. The lack of these data resulted in large uncertainties in calculating the N input flux of global subducting slabs and modeling the N quantities released through the arc and subducted into the deep mantle. To fill this gap, we examined the N concentrations and isotope compositions of 38 altered gabbroic rocks recovered by ODP/IODP drillings from three oceans, i.e., Hole 735B at the Atlantis Bank in the Southwest Indian Ridge, Hole 1309D at the Atlantis massif in the Mid-Atlantic Ridge, and Hole 1415P at the Hess Deep in the East Pacific Rise. All the gabbroic samples show significant N enrichment with mostly positive δ15N values (in average: 7.4±3.8 ppm and +0.6±2.0‰ for Hole 735B, 5.3±2.1 ppm and +1.4±1.3‰ for Hole 1309D and 7.9±1.9 ppm and +2.0±1.8‰ for Hole 1415P), which are comparable to those of altered basalts from global oceanic crust. The N concentrations and δ15N values of these gabbroic rocks can be readily explained by mixing between minor inherited mantle N and mainly secondary N derived from seawater. The secondary N-hosting minerals in altered gabbroic rocks are likely plagioclase, amphibole and chlorite formed during moderate- to high-temperature alteration stages. Using these new data, we obtained a global nitrogen input flux of 20.6-1.3+0.9 × 109 mol·yr−1 to 29.9±2.2 × 109 mol·yr−1 for subducting AOC, in which 50% – 64% is contributed by the altered gabbroic oceanic crust. Integrating with previous estimate of N input flux from seafloor sediments, a total N input flux of 74.9-1.3+0.9 – 84.2±2.2 mol·yr−1 was estimated for global subducting slabs. Mass balance between this input N flux and various estimates of N output flux in global arcs gave a large range (from 47% – 53% to 10% – 20%) for the fraction of slab N to be subducted beyond the sub-arc depth to the deeper mantle. The upper end (47% – 53%) of this range is more consistent with the results from other constraints, such as thermal structural control and comparison between altered gabbros and meta-gabbros.

Silicon isotopic compositions of altered Oceanic crust samples from IODP U1365 and U1368: Effect of low-temperature seawater alteration

To investigate the effects of seawater alteration on the Si isotopes of altered oceanic crust, we analyzed the Si isotopes of altered oceanic crust samples from Sites U1365 and U1368, which have different alteration degrees. Samples from Site U1365 experienced higher degrees of seawater alteration than samples from Site U1368. To further evaluate the effect of seawater alteration on oceanic crust, the samples from Sites U1365 and U1368 were separated into two groups of fresh and altered samples. The results indicated that both fresh and altered basalts from Site U1368 have δ30Si in the range of fresh MORB (−0.27 ± 0.06‰, Savage et al., 2010), except for one pair with slightly lower δ30Si (−0.45 ± 0.04‰ of fresh sample, and −0.39 ± 0.03‰ of altered sample). The average δ30Si of all oceanic crust samples from Site U1365 is similar to the value of fresh MORB, and the fresh samples from Site U1365 also have δ30Si values in the range of fresh MORB. However, the altered samples from this site have varied δ30Si (−0.58‰ to −0.27‰). Although there is no strong correlation between δ30Si and LOI, K2O/TiO2, Rb/TiO2, Cs/TiO2 and Ba/TiO2 ratios, some samples from Site U1365 with high alteration degrees have low δ30Si, which indicates that seawater alteration could affect the Si isotopes of a portion of the oceanic crust.

Detection of subsurface basaltic sheets and associated structures utilising forward modelling and inversion of 2D electrical resistivity data: A case study from Jebel-Qatrani, Fayoum, Egypt

In the north of Fayoum Governate, Egypt, numerous extrusive flows of basalt rocks exposed on the surface and affected by weathering and erosion, forming altered basalt sheets which is the case of the Jebel-Qatrani. The study objective comes in two main folds: the first, to choose the optimum parameters required for the filed survey by generating forward models using several arrays with 3% Gaussian random noise, simulating the geology of the study area deploying a finite element modelling approach; the second, to acquire the real field data and generate the electrical resistivity tomograms to ascertain the existence, extension, and characteristics of the basaltic sheets in the subsurface. Hence, four electrical resistivity profiles with a Wenner-alpha array with 48 electrodes, 5 metre electrode spacing, and a total length of 235 metres were acquired in suitable locations based on previous aeromagnetic results in the vicinity of the study area. These profiles were inverted to derive the true resistivity distribution of the subsurface. Another objective is to detect the near-surface, Oligocene normal faulting structures associated with rifting of the Red Sea. Results from two inverted resistivity tomograms show a possible normal fault cutting through the early Oligocene Qatrani Formation at the same time as red sea rifting, confirming previous geological studies in the Fayoum region based on satellite imagery, aeromagnetic data, and geological outcrops. It is concluded that near these faults, a sheet-like body of relatively high resistivity values, representing basaltic extrusions, was detected, confirming that the basaltic presence is associated with these structural zones.

Alkaline picritic volcanism on northern Ellesmere Island associated with initial rifting of the Sverdrup Basin, Canadian Arctic

The Caledonian and Ellesmerian orogenies were followed by extension and development of intracontinental rift basins since the late Paleozoic, as represented by the Sverdrup Basin along the Canadian Arctic Islands. The initial rifting was accompanied by pulses of volcanic activity during the Carboniferous and Permian. A new occurrence of mafic volcanic rocks, the “Taconite volcanics” (informal name), was discovered on northern Ellesmere Island between the head of Ayles Fiord and M’Clintock Inlet. The mainly alkali-picritic lavas are exposed within the central part of the Pearya Terrane as an outcrop in faulted contact with upper Carboniferous red beds of the Canyon Fiord Formation. The contact to the Ordovician island-arc volcanic rocks of the Pearya Terrane is unclear. The outcrop is characterized by a small circular magnetic anomaly. Ar–Ar whole-rock geochronology on the volcanic rocks yielded an age of 290 ± 19 Ma suggesting emplacement of the lavas during the early Permian. Whole rock geochemical analyses for eight samples revealed a geochemical affinity to ocean island basalt (OIB) and indicate variable mixing of low-degree melts in the fields of garnet and spinel peridotite (∼80–90 km depth). The involvement of a metasomatized subcontinental lithospheric mantle is indicated by high Pb, Nb, and Ta concentrations. Geochemical differences (as enrichments in Ti, Nb, Zr, and the light rare earth elements (REE)) to the known Carboniferous and Permian spilitic altered basalt occurrences of northwestern Ellesmere and northern Axel Heiberg islands are probably based on differences in the mantle source. The Sr isotope ratios of the Taconite volcanics are primitive ((87Sr/86Sr)t: 0.7037–0.7042) and its Nd isotope ratios are moderately depleted (εNd(t): +2.20 to +2.85) in contrast to the enriched εNd(t) values of the Permian Esayoo formation.

Multiple parameters enable deconvolution of water-rock reaction paths in low-temperature vent fluids of the Kamaʻehuakanaloa (Lōʻihi) seamount

The contribution of venting fluids at mid-ocean ridges to global ocean biogeochemical cycles is well recognized. Less is known about the role of magmatically-active intra-plate volcanoes. In this study, new compositional fluid data were acquired from 20 to 50 °C vent fluids at Kamaʻehuakanaloa (previously known as Lōʻihi) seamount (Hawai’ian archipelago) and used to model the wide diversity of reaction conditions capable of producing the Fe-, Si- and CO2-rich vent fluids observed. Our conceptual model includes a first step where seawater reacts with increasing proportions of basalt and gas as the temperature increases, and a second step where the resulting hydrothermal fluid mixes with unaltered seawater while continuing to react with basalt until the fluid mixture reaches 20 °C. A series of reaction paths were chosen to vary: the maximum temperature during Step 1 (50 to 400 °C) and the proportions of basalt and gas reacting; the degree, F, of low-temperature basalt alteration during Step 2, which corresponds to the extent to which the hot fluid generated during Step 1 continues to react with more basalt as it ascends to the seafloor. Our model shows that the 20–50 °C vent fluids are greatly dependent on the degree of low-temperature basalt alteration during fluid upwelling. Indeed, the compositions of Kamaʻehuakanaloa vent fluids cannot be reconciled with a general model of subsurface mechanical mixing of high-temperature end-member vent fluid and seawater alone. Instead, they require both subsurface equilibrium mixing between a ≥350 °C hydrothermal fluid end-member and seawater and further basalt alteration that must occur as the fluid mixture rises to the seafloor. Although it involves only ∼4% of the amount of basalt having reacted during Step 1, this low-temperature basalt alteration during Step 2 leads to the characteristic enrichments in Fe observed in the Kamaʻehuakanaloa vent fluids and a concomitant depletion in H2S. We hypothesize that low-temperature basalt alteration during an extended path of fluid upwelling through the subseafloor might arise as a direct consequence of the height and steep-sloped topography of Kamaʻehuakanaloa seamount. If correct, this suggests a more general case - that input from magmatically-active intraplate volcanoes, which have been relatively overlooked throughout the history of submarine vent investigations to date, could differ significantly from global mid-ocean ridge fluxes and contribute more substantially than previously recognized to the global ocean Fe cycle.

Seafloor hydrothermal circulation at a rifted margin of the South China Sea: Insights from basement epidote veins in IODP Hole U1502B

To better understand the seafloor hydrothermal circulation at the passive rifted margins, we present a detailed geochemical investigation of epidote veins in eight altered basalts from IODP Hole U1502B, which is located in the continental-oceanic transition zone at the northern margin of South China Sea (SCS). The studied epidotes have XFe values [XFe = Fe3+/ (Fe3+ + Al3+)] of 0.21 to 0.37, suggesting Fe-rich epidote in composition and equilibration in lower greenschist facies. The occurrence of cataclastic and zoning structures in the epidotes suggests variations in mineral growth environment. Combined rare earth element and Sr-isotope signatures of these epidotes revealed the involvement of three types of circulating fluids, i.e., modified seawater (negative Ce anomaly, and 87Sr/86Sr = ∼0.708), hydrothermal fluid (positive Eu anomaly, and 87Sr/86Sr = ∼0.706), and magmatic fluid (negative Eu anomaly, and 87Sr/86Sr = ∼0.704). These observations suggest that basement epidote veins in Hole U1502B were likely formed under a tectonically active and open environment, with the growth process and compositions of epidotes varying during the evolution of circulating fluids. A new seafloor hydrothermal circulation model is envisaged at Hole U1502B, in which an intrusive dike instead of a magma chamber supplied the heat source, and the extensional faults formed during continental breakup to seafloor spreading provided the channel of upwelling circulating fluids. The low heat flux of intrusive dikes caused the shallow occurrence depth of basement epidote veins, which were formed in the discharge zone of the hydrothermal system. Our study provides new mineral chemistry constraints on the hydrothermal process and environment at the passive rifted margin of SCS, which is different from those suggested for a typical fast-spreading mid-ocean ridge.

Evidence of hydrothermal fluid migration pathways in pores and microfractures based on nuclear magnetic resonance and electron microscopic analyses from the Southwest Indian Ridge

The formation of massive sulfide deposits on the seafloor is closely related to sub-seafloor hydrothermal circulation. Hydrothermal circulation depends on the connected pores and fractures. Most previous studies have focused on hydrothermal circulation at the macroscopic scale; however, the transport pathways of hydrothermal fluids have rarely been studied at the microscopic scale. This study showed that multiple measurement techniques could characterize the pores of rocks and the mineral fraction surrounding these pores. Nuclear magnetic resonance (NMR) was used to measure the porosity of fresh basalt, altered basalt, massive sulfide, and sulfide chimneys obtained from the Southwest Indian Ridge. Meanwhile, the distribution of pore radius and pore throats in these samples was obtained using the NMR transverse relaxation time distribution. The scanning electron microscopy results and polarizing light microscope images showed that the pores of altered basalts are more developed than those of fresh basalts, which is consistent with the NMR measurement results that altered basalts have a large pore size distribution. In addition, metallic minerals such as pyrite were found to be precipitated in these pores, and microfractures were observed in altered basalts. This demonstrates that the pores and microfractures in these altered basalts provide pathways for fluid transport and are enriched with metal minerals. Altered basalt is closely related to the formation of massive sulfide deposits and is an important ore-prospecting indicator. Therefore, the study of rock pore structures and components can provide a better understanding of hydrothermal circulation and contribute to the exploration of mineral resources.

NaHCO3 as a carrier of CO2 and its enhancement effect on mineralization during hydrothermal alteration of basalt

Reaction of carbon dioxide (CO2) with minerals to generate stable carbonates, also known as CO2 mineralization, has been regarded as one of the most promising methods for safe and permanent carbon storage. As a promising feedstock, basaltic rock has gained special interest, and elevating basalt carbonation efficiency with the reduction of negative environmental impact is the main challenge for CO2 mineralization system development. Considering multiple potential positive effects of the CO2 carrier, NaHCO3, we conducted this study to experimentally evaluate the CO2 storage efficiency during water-basalt-NaHCO3 interactions under hydrothermal conditions at 200–300°C. The inclusion of NaHCO3 was confirmed to drastically promote the alteration of basalt, especially at higher temperatures. As revealed by experiments conducted at the saturated vapor pressure of water, the carbon storage efficiency at 300°C reached 75 g/kg of basalt in 5 days, which was 12 times higher than that at 200°C. In such hydrothermal systems, basalt was carbonated to generate calcite (CaCO3), where the Ca was mainly from plagioclase; Mg and Fe were incorporated into smectite, and Na in the saline system participated in the formation of Na silicates (i.e., analcime in the case of basalt). Due to the presence of additional Na in solution, all the released elements were consumed quickly with generation of secondary minerals in turn promoted basalt dissolution to release more Ca for CO2 storage. This study illuminated the role of NaHCO3 in basalt carbonation and provided technical backup to the design of advanced CO2 mineralization systems.

Rock‐Matrix Porosity and Permeability of the Hydrothermally Altered, Upper Oceanic Crust, Oman Ophiolite

AbstractPorosity and permeability are key controls on hydrothermal circulation and alteration in magmatically heated upper oceanic crust. However, the hydraulic properties of basalts altered above 200°C are largely unknown, leaving their role in high‐temperature systems unclear. Here, we assess rock‐matrix porosities and permeabilities of pervasively altered MORB‐like basalts from outcrops in the sheeted dikes and axial lavas of the Semail ophiolite, Oman. The samples represent regional spilite alteration (chlorite–albite–quartz ± actinolite; 150–440°C) and localized epidosite alteration (epidote–quartz; 255–435°C). Porosity and permeability of spilitized rocks vary as follows: interpillow hyaloclastites (14–27 vol.%, 10−17–10−15 m2) > pillow cores (4–12 vol.%; ∼10−19–6 × 10−18 m2) > pillow rims (4–11 vol.%; ∼10−19–2 × 10−18 m2) > massive sheet flows (1–9 vol.%; ∼10−19 m2) ≥ dikes (1–5 vol.%; ∼10−19 m2). Pillow values fall within the ranges of existing data on fresh and low‐temperature altered basalts in in situ crust. However, hyaloclastite permeabilities are 1–4 orders of magnitude higher and are clearly preferred flow paths. Epidosites have elevated porosity and permeability irrespective of rock morphology (18–26 vol.%; ∼10−16–10−14 m2). Pillow stacks have upscaled (∼104 m3) matrix porosities and permeabilities of ∼9 vol.% and ∼10−17–10−16 m2 when spilitized and 16 vol.% and up to ∼10−14 m2 when epidotized. Upscaled permeabilities of spilites meet minimum requirements for observed heat and fluid discharge from high‐temperature seafloor systems even without fracture networks, and reflect strong flow through the rock‐matrix.

The dissolution rates of naturally altered basalts at pH 3 and 120 °C: Implications for the in-situ mineralization of CO2 injected into the subsurface

Due to their widespread distribution in the Earth's crust, it seems likely that altered basalts could be targeted for CO2 storage via subsurface carbon mineralization. To assess the potential efficiency of this approach, the steady state release rates of major elements from a suite of altered basalts have been measured at pH 3, 120 °C, and far from equilibrium conditions. The altered basalt samples have chemical compositions close to that of fresh basalt, but their mineralogy varies due to their alteration either at the Earth's surface or hydrothermal conditions at temperatures up to 250 °C. The studied altered basalts contain variable amounts of primary plagioclase and pyroxene, and substantial secondary phases including quartz, zeolites, epidote, chlorites and clay minerals. Despite their differing mineralogy, the steady-state element release rates of all the altered basalts are similar to each other when normalized to geometric surface area. These rates, however, are one to three orders of magnitude slower than corresponding release rates of basaltic glass and fresh crystalline basalt, depending on the element and on whether the rates are normalized to initial BET or geometric surface area. If present in small amounts in the altered basalts, calcite dissolves rapidly in the acidic reactive fluids, and does not contribute to the measured steady-state calcium release rates. Taken together, the results of this study indicate that altered basalt formations can provide sufficient divalent cations for subsurface carbon mineralization. As the element release rates of these altered basalts are lower than those of basaltic glass or fresh crystalline basalt, efforts to carbonate subsurface altered basalts may be best targeted at systems having temperatures in excess of 100 °C to compensate for the lower reactivity of these rocks.

Reservoir properties and reactivity of the Faroe Islands Basalt Group: Investigating the potential for CO2 storage in the North Atlantic Igneous Province

Offshore injection of CO2 into volcanic sequences of the North Atlantic Igneous Province may present a large-scale, permanent storage option through carbonate mineralization. To investigate this potential, onshore studies of reservoir properties and reactivity of the subaerially erupted Faroe Islands Basalt Group have been conducted. Outcrop and borehole samples reveal that the lava flow crusts commonly contain vesicles that have been filled with secondary minerals due to hydrothermal fluid circulation, however, unmineralized and highly porous layers do occur. Bulk density measurements, micro-computed tomography (µ-CT) image analysis, and microscope studies of samples from onshore boreholes give present-day porosities ranging from 0.5% to 36.2% in the volcanic sequences. The unmineralized brecciated lava flow crusts contain the largest estimated porosity and simulated absolute permeability (reaching up to 10−12 m2). µ-CT studies of the mineralized, brecciated flow crusts indicate initial porosities reaching up to 45%, before clogging. Kinetic experiments of rock dissolution show that the reactivity of the basalt and volcaniclastic sediments depends on the alteration state with more altered basalt being less reactive. However, the presence of reactive, high porosity, and high permeability flow crusts prior to clogging indicate the existence of promising and very large CO2 reservoirs in less altered offshore sequences.

Potassium isotopic fractionation during multistage alteration of oceanic crust in the southern Mariana Trench

To further constrain the migration trajectory of K isotopes during oceanic crust alteration, we present detailed mineral, elemental, and K isotopic data of oceanic crust samples from the southern Mariana Trench. These samples are divided into four groups with various colors and alteration intensities, in which secondary mineral assemblages indicate a progressive alteration of oceanic crust at low temperatures (<100 °C). Group I samples are fresh basalts with chemical compositions identical to the mid-ocean ridge basalts (MORBs). Their higher δ41K values (0.08‰ to 0.11‰) than MORBs (−0.44 ± 0.17‰) might point to a 41K-enriched magmatic source. Group II samples are slightly altered basalts at the initial stage of low-temperature alteration and have lower δ41K values (−0.31‰ to 0.05‰) than Group I, which are interpreted as a result of chemical exchange with seawater via equilibrium and kinetic isotopic fractionations. Group III samples are celadonite phase-dominated rocks at the second stage. They have higher K2O contents (3.53–8.39 wt%) and lower δ41K values (−0.71‰ to −0.45‰) than Group II, reflecting the preferential incorporation of isotopically light K. In contrast, the clay mineral phase at the late stage dominates Group IV samples with lower K2O contents (1.79–4.20 wt%) and higher δ41K values (−0.22‰ to −0.07‰) than Group III, indicating a loss of isotopically light K during the transformation of the high-K celadonite phase to the low-K clay mineral phase. Overall, systemic variations in secondary minerals, K2O contents, and δ41K values from Group I to Group IV samples suggest a multistage K isotopic fractionation and shed new light on a key role of the high-K celadonite phase in K isotopic behavior during oceanic crust alteration.

Phyllosilicate formation on early Mars via open-system acid alteration of basaltic glass

Smectites are widespread on Mars, but the water-rich, neutral-to-alkaline pH conditions favorable for smectite formation would be expected to have also produced abundant carbonates on early Mars, which are not observed. Smectite formation from basaltic glass on Mars could occur in acidic environments unfavorable for carbonate formation. Acidic smectite formation has been previously demonstrated in batch experiments (closed hydrologic systems), however, the mechanisms and octahedral composition of smectite forming in acidic flow-through (open hydrologic systems) environments are still not fully understood. We conducted hydrothermal (190°C) alteration experiments on Stapafell basaltic glass at 0.01 and 0.25 mLmin−1 flow rates corresponding to low and high water to rock ratio (W/R) flow conditions, and initial pH (pH0) values of 2, 3, 4 and 6. A batch low W/R experiment was conducted at pH0 2 for comparison to the open system experiment. Kaolinite, montmorillonite and chlorite formed at pH0 2 at low W/R; no phyllosilicates formed at pH0 2 at high W/R; and lizardite formed at pH0 ≥ 3 at both W/R ratios. Lizardite, kaolinite, and montmorillonite in these experiments formed by precipitation from solution and chlorite likely formed through alteration of montmorillonite and/or basalt. Saponite formed at pH0 2 in batch conditions by alteration of basaltic glass. Comparison of experimental data with martian observations of phyllosilicate assemblages indicated that smectite formation on Mars likely occurred under water-limited environmental conditions. Al-rich smectite could form in low W/R open system subsurface environments under a very narrow range of pH (pH < 3) while saponite could form in closed low W/R systems under acidic to alkaline conditions. The combination of open and closed hydrological regimes could be responsible for development of clay mineral stratigraphies observed on Mars. The acidic conditions required for formation of Al-rich smectite montmorillonite were unfavorable for carbonate precipitation, but carbonate precipitation could occur together with Fe/Mg-smectite saponite in closed systems at pH > 4. The lack of widespread carbonates on Mars could not therefore be explained solely by acidic conditions. Phyllosilicate formation under acidic conditions on Mars may affect biosignature stability in martian regolith as preservation capacity is lower in phyllosilicates that formed in or experienced acidic pH.

Natural hydrogen potential and basaltic alteration in the Asal–Ghoubbet rift, Republic of Djibouti

The Asal–Ghoubbet active rift in the Republic of Djibouti is a site of interest for geothermal energy and natural hydrogen, and previous studies have indicated that dihydrogen (H2) emanates from this rift. However, the well-known serpentinization reaction does not appear to be the main mechanism generating H2 at this site. Rather, the H2 is generated as follows: (1) by alteration of basaltic lava at depth via reaction with seawater flowing from Ghoubbet Bay towards Lake Asal; (2) by simple degassing of the volcanic chamber located a few kilometers below the Fiale Caldera in the rift axis; or (3) as a result of pyritization processes via the oxidation of H2S. Study of microorganisms did not indicate any production or consumption of H2, CO2, or CH4; therefore, it is unlikely that microorganisms affected H2 gas contents measured at the surface. However, air contamination at fumaroles is typically considerable and may limit interpretation of such processes. Drill cuttings from the Fiale 1 (F1) and Gale le Goma 1 (Glc1) wells (located on the inner and outer rift margins, respectively) were analyzed to determine where H2 is generated. Total rock analyses indicated distinct zones at depths of 464 m and 280 m for F1 and Glc1, respectively, representing the boundary between the Asal and Stratoïd Basalts. 57Fe Mössbauer analyses show a decrease in the percentage of Fe3+ at depth, indicating that Fe2+-rich minerals, particularly in the Stratoïd Basalts, may be a source of H2. Based on well data from the rift center and the outer rift margin, it is evident that H2 is present at the surface in the rift axis and that this area offers good remnant potential because of the presence of Fe-rich chlorite. Conversely, few H2 emissions were measured at the surface on the outer rift margins, although well data showed some H2 (∼0.25%) at depth. The presence of a cap rock in the rift axis has not yet been proven; however, the high loss on ignition and the mineralogy in well Glc1 may indicate that the rocks are sufficiently altered into clays to offer potential as a H2 seal. If so, the rift margins would offer greater exploration potential than the rift center.

Hydrothermal Friction Experiments on Simulated Basaltic Fault Gouge and Implications for Megathrust Earthquakes

AbstractNucleation of earthquake slip at the plate boundary fault (décollement) in subduction zones has been widely linked to the frictional properties of subducting sedimentary facies. However, recent seismological and geological observations suggest that the décollement develops in the subducting oceanic crust in the depth range of the seismogenic zone, at least in some cases. To understand the frictional properties of oceanic crustal material and their influence on seismogenesis, we performed hydrothermal friction experiments on simulated fault gouges of altered basalt, at temperatures of 100–550°C. The friction coefficient (μ) lies around 0.6 at most temperature conditions but a low μ down to 0.3 was observed at the highest temperature and lowest velocity condition. The velocity dependence of μ, (a−b), changes with increasing temperature from positive to negative at ∼100°C and from negative to positive at ∼450°C. Compared to gouges derived from sedimentary facies, the altered basalt gouge showed potentially unstable velocity weakening over a wider temperature range. Microstructural observations and microphysical interpretation infer that competition between dilatant granular flow and viscous compaction through pressure‐solution creep of albite contributed to the observed transition in (a−b). Alteration of oceanic crust during subduction produces fine grains of albite and chlorite through interactions with interstitial water, leading to reduction in its frictional strength and an increase in its seismogenic potential. Therefore, shear deformation possibly localizes within the altered oceanic crust leading to a larger potential for the nucleation of a megathrust earthquake in the depth range of the seismogenic zone.

Nitrogen enrichment in the altered upper oceanic crust: A new perspective on constraining the global subducting nitrogen budget and implications for subduction-zone nitrogen recycling

Studies of alteration enrichment of nitrogen (N) in seafloor basalts clearly indicate that the upper oceanic crust is an important reservoir to transfer crustal N into Earth's interior. However, since previous studies have focused on relatively few DSDP/ODP/IODP drill cores, the factors controlling N enrichment in the altered upper oceanic crust at a global scale have not been revealed. Here we report N concentrations and isotope compositions of altered basalts from four additional DSDP/ODP Holes (556, 1224F, 417A and 543A). These new data, together with the published data from Holes 801C, 1149D, 1256D and 504B, enable systematic assessment of the potential affecting factors such as alteration degree, crustal age, spreading rate and sediment type, on N enrichment in the upper oceanic crust. The new data demonstrate ubiquitous N enrichment (1.3 – 48.4 ppm) in global seafloor basalts. The results indicate that, (i) the secondary N (in the form of ammonium) in altered basalts was sourced from sediment/seawater with δ15N values of 0‰ to +8‰, with a minor source from abiotic N2 reduction with δ15N values of -12‰ to -21‰; (ii) N enrichments in the upper oceanic crust do not correlate with crustal age, suggesting that the N uptake should mainly occur over early ages (likely within ∼20 Myr of crustal formation); (iii) the magnitude of N enrichment in the upper oceanic crust is primarily controlled by the N availability in the surrounding environment, which is ultimately related to the N abundance of basal sediments and/or seawater. The highest degree of N enrichment occurs in the upper oceanic crust overlain by N-rich basal sediments (claystone), i.e., at Hole 543A (N = 24.9±8.3 ppm; 1σ). In contrast, the lowest degree of N enrichment occurs in the upper oceanic crust overlain by N-poor basal sediments (chert), i.e., at Hole 1149D (N = 2.0±0.5 ppm; 1σ). With the consideration of this primary controlling factor, a global N input flux of 3.7±0.3 ×109 mol⋅yr−1 for the 300 – 600 m upper oceanic crust is calculated. These new data also demonstrate that N concentrations are relatively low in global altered basalts (average: 10.1±7.8 pm; 1σ). In comparison to their metamorphic equivalents, blueschists have much higher N concentrations (up to ∼122 ppm) whereas eclogites have N concentrations comparable to altered basalts. This implies further N enrichment in basaltic oceanic crust during early subduction and N devolatilization during deep subduction (by eclogite facies).

Mapping the structural configuration of the western Gulf of Guinea using advanced gravity interpretation methods

The western Gulf of Guinea has important implications for seismic-hazard estimates of the adjoining sub-Sahara West Africa region because of the presence of four active transform faults, namely the St. Paul, Romanche, Chain and Charcot fault zones. A satellite gravity data analysis was performed over the western Gulf of Guinea to delineate the structural configuration of the area. Initially, the energy spectrum analysis was used to determine regional and residual gravity effects. Further, the advanced techniques such as the tilt angle of the gradient amplitude (TAHG), the improved horizontal tilt angle (ITDX), the enhanced gradient amplitude (EHGA), and the fast sigmoid-based edge detection filter (FSED) were applied to the residual gravimetric anomaly to extract lineaments of the western Gulf of Guinea. The major trend of the detected lineaments has an ENE-WSW direction and depths of the gravity sources range from 3.4 to 13 km corresponding to widespread altered basalt, breccia, serpentinized peridotite and minor gabbro. Our findings closely agree with known information but are also able to confirm the presence of various other structures. Our results thus provide a structural map that helps us to have a better understanding of the tectonic and structural framework of the western Gulf of Guinea.

Fe3O4 Nanoparticles Loaded Bentonite/Sawdust Interface for the Removal of Methylene Blue: Insights into Adsorption Performance and Mechanism via Experiments and Theoretical Calculations

Herein, magnetite nanoparticles (MNPs) synthesized from altered basalt were used to develop a composite with H2O2–activated bentonite (BE) and fibrous sawdust (SD). The as-prepared BE/SD–MNPs were characterized by FTIR, FESEM, TEM, TGA, DSC, and Zeta potential techniques and utilized as an effective multifunctional composite for removing methylene blue (MB). The adsorption isotherms of MB at 25–55 °C were analyzed via kinetics, classical, and advanced statistical physics models. Theoretically, the pseudo-second-order of kinetics and the Freundlich isotherms model fit the experimental data well without microscopically clarifying the adsorption mechanism. Studying a multilayer model’s steric and energetic parameters was a reliable approach to understanding the MB uptake mechanism at the molecular scale. Sterically, the removed MB molecules offered a combination of horizontal and vertical geometry (i.e., mixed orientation). The MB adsorption capacity at saturation (Qsat) increased from 829 to 849 mg/g with temperature, suggesting endothermic interactions. Energetically, the MB uptake by BE/SD–MNPs was controlled by physical interactions (i.e., adsorption energy &lt; 20 kJ/mol). The BE/SD–MNPs retained more than 85% of the MB uptake after five adsorption-desorption rounds. Overall, this study aimed to understand the MB adsorption mechanism using a magnetic clays/lignocellulosic interface such as the utilized BE/SD–MNPs composite as a promising strategy in wastewater remediation.

Mixed carbonate-siliceous hydrothermal chimneys ahead of the Asal propagating rift (SE Afar Rift, Republic of Djibouti)

Numerous active and fossil hydrothermal carbonate chimneys have been reported at Lake Asal and Lake Abhe, in the southeastern part of the Afar Rift (Republic of Djibouti): a tectonically and volcanically active region for the last 30 million years. In this study, we report on a detailed mineralogical and geochemical investigation of unusual hydrothermal mixed carbonate-siliceous chimneys recently discovered in the Allols area (Harralol, Haralé Awda and Sakallol), northwest of the Asal Rift. These inactive chimneys, up to 20 m high, consist of tall spires partly stained by Fe-oxides. They are located along major faults and were formed sub-aqueously when the area was occupied by a lake. Hydrothermal carbonate structures were also found in the same area. The oxygen (δ18O) and carbon (δ13C) isotopic compositions of the chimneys range from −4.5 to 25.3‰ and from −9.44 to 8.92‰, respectively, indicating mixing between magmatic and surficial waters. Their radiogenic Sr isotopic compositions (87Sr/86Sr = 0.7049–0.7056) show values typical of altered basaltic rocks. Chimneys also exhibit negative Ce anomaly suggestive of a genetic link with basalt alteration. Overall, this study suggests that lake water chemistry in the Allols area experienced major changes in the past, reflecting various contributions of mixing fluids (lake water and hydrothermal fluids), leading alternatively to deposition of carbonate chimneys and precipitation of (and/or replacement with) silica.

Complex fluid source of the multistage pyrite-bearing Huilvshan gold deposit (west Junggar, NW China): Insight from pyrite texture, sulfur isotope and trace element compositions

Hydrothermal gold deposits commonly form during protracted multistage ore-forming processes, however, discriminating among different ore-forming fluids and tracing their origin to unique or different repositories is a challenging task. The Huilvshan gold deposit in West Junggar (Xinjiang province, NW China), was affected by three hydrothermal stages and thus provides an opportunity to examine complex ore-forming processes. The deposit consists of gold-bearing quartz-sulfide veins and disseminated sulfides, hosted within Early Carboniferous basalts and tuffs. Three stages of hydrothermal pyrite (Py1, Py2, and Py3) were identified. Gold is only present in Py2 where it occurs as native gold inclusion and in invisible form. Anhedral Py1, disseminated in the altered basalt, is characterized by higher Ni (13.7 to 635 ppm) concentrations than Py2 and Py3. Euhedral-subhedral Py2 is the richest in Au (1.46 to 25.7 ppm) among three stages of pyrite. Py3 is subdivided into grains with zonal texture (Py3a) and irregular grains (Py3b). Both sub-types have high Sb (56.8 to 1599 ppm) and Tl (0.02 to 68.7 ppm) concentrations. In situ δ34S values of Py1 (-6.0 to 2.7 ‰) are similar to those of Py2 (-5.9 to 3.9 ‰), whereas Py3 has extremely negative δ34S values (-44.3 to-18.0 ‰).Ar-Ar ages of hydrothermal muscovite coexisting with Py2 indicate that the Huilvshan gold deposit formed at ∼300 Ma. The sulfur isotopic systematics of Py1 and Py2, close spatial and temporal relationship between the Huilvshan gold deposit and adjacent felsic intrusions, and paucity of regional metamorphic rocks in the region all suggest that hydrothermal fluids in stage I (Py1) and II (Py2) were derived from magmatism. On the other hand, stage III pyrite and associated sulfides more likely precipitated from a hydrothermal fluid that circulated in tuffs, based on the extremely negative δ34S values of Py3, similar to those measured from framboidal pyrite in the host tuffs.Therefore, textural, trace elemental and sulfur isotope data from pyrite suggest different sources for the hydrothermal fluids in the Huilvshan gold deposit, and that magmatic activity may have contributed the most to the main ore-forming stage. The genesis of this Huilvshan gold deposit could provide significant insights into the origin of other hydrothermal gold deposits that show multiple sulfide generations.