East Pacific Rise Research Articles

Microseismicity Indicates Atypical Small‐Scale Plate Rotation at the Quebrada Transform Fault System, East Pacific Rise

AbstractClosely spaced, multi‐strand ridge transform faults (RTFs) accommodate relative motions along fast spreading mid‐ocean ridges. However, the relations between RTFs and plate spreading dynamics are poorly understood. The Quebrada system is one of the most unique RTF systems at the East Pacific Rise, consisting of four transform faults connected by three short intra‐transform spreading centers (ITSCs). We use seven‐months of ocean bottom seismograph data to study the Quebrada system, and find abundant earthquakes unevenly distributed among three active faults. We identify two deep, diffuse seismicity clouds at the inside corners of the ITSC‐transform fault intersections, and one seismically active fracture zone. The observations suggest a complex regional plate‐motion pattern, including possible heterogeneous rotations within the Quebrada system. Evolution of multi‐strand RTFs may have resulted from a strong three‐dimensional local thermal and fluid effects, while the RTFs may have also regulated regional tectonics, forming an intricate feedback system.

Tracking Crustal Permeability and Hydrothermal Response During Seafloor Eruptions at the East Pacific Rise, 9°50’N

AbstractPermeability controls energy and matter fluxes in deep‐sea hydrothermal systems fueling a 'deep biosphere' of microorganisms. Here, we indirectly measure changes in sub‐seafloor crustal permeability, based on the tidal response of high‐temperature hydrothermal vents at the East Pacific Rise 9°50’N preceding the last phase of volcanic eruptions during 2005–2006. Ten months before the last phase of the eruptions, permeability decreased, first rapidly, and then steadily as the stress built up, until hydrothermal flow stopped altogether ∼2 weeks prior to the January 2006 eruption phase. This trend was interrupted by abrupt permeability increases, attributable to dike injection during last phase of the eruptions, which released crustal stress, allowing hydrothermal flow to resume. These observations and models suggest that abrupt changes in crustal permeability caused by magmatic intrusion and volcanic eruption can control first‐order hydrothermal circulation processes. This methodology has the potential to aid eruption forecasting along the global mid‐ocean ridge network.

Kinematics of the East Pacific Rise Retrodicted From Pacific and Farallon/Nazca Subduction‐Related Torques: Support for Significant Deep Mantle Buoyancy Controlling EPR Spreading

AbstractSlab‐pull together with other boundary‐related torques of the Pacific and Farallon/Nazca plates are reconstructed from paleo‐agegrid datasets over the past 80 Ma in the no‐net rotation (NNR) frame of reference. The resulting reconstructions of both the retrodicted Pacific‐Farallon/Nazca age distribution and relative motion history of the East Pacific Rise (EPR) are compared with global plate kinematic reconstructions of these same aspects of Pacific basin evolution. There are mismatches between the predicted and observed age distributions and relative motion history of the EPR. Slab‐pull related torques mostly retrodict west to east motion of the EPR, whereas global kinematics result in south to north motions, orthogonal to each other. Slab‐pull torques are augmented with estimates of subduction and transform resistances together with spatially variable viscosities to assess their potential contributions to Pacific‐Farallon/Nazca relative motions and EPR motions in the NNR frame of reference. We argue that the discrepancies between these different reconstructed histories result from contributions associated with basal tractions acting on the Pacific and Farallon/Nazca plates resulting from deep mantle buoyancy. We propose that the latter has controlled the EPR evolution over the past 80 Ma and significantly contributed to driving plate motions at least in the Pacific basin.

Balanced Polymorphism at the Pgm-1 Locus of the Pompeii Worm Alvinella pompejana and Its Variant Adaptability Is Only Governed by Two QE Mutations at Linked Sites.

The polychaete Alvinella pompejana lives exclusively on the walls of deep-sea hydrothermal chimneys along the East Pacific Rise (EPR), and displays specific adaptations to withstand the high temperatures and hypoxia associated with this highly variable habitat. Previous studies have revealed the existence of a balanced polymorphism on the enzyme phosphoglucomutase associated with thermal variations, where allozymes 90 and 100 exhibit different optimal activities and thermostabilities. Exploration of the mutational landscape of phosphoglucomutase 1 revealed the maintenance of four highly divergent allelic lineages encoding the three most frequent electromorphs over the geographic range of A. pompejana. This polymorphism is only governed by two linked amino acid replacements, located in exon 3 (E155Q and E190Q). A two-niche model of selection, including ‘cold’ and ‘hot’ conditions, represents the most likely scenario for the long-term persistence of these isoforms. Using directed mutagenesis and the expression of the three recombinant variants allowed us to test the additive effect of these two mutations on the biochemical properties of this enzyme. Our results are coherent with those previously obtained from native proteins, and reveal a thermodynamic trade-off between protein thermostability and catalysis, which is likely to have maintained these functional phenotypes prior to the geographic separation of populations across the Equator about 1.2 million years ago.

Correlation of fault plane structure variations with the 2008 Mw6.0 earthquake slip behaviors along the Gofar transform fault, East Pacific Rise

Correlation of fault plane structure variations with the 2008 Mw6.0 earthquake slip behaviors along the Gofar transform fault, East Pacific Rise

Thermal History of Lithosphere Formed Beneath Fast Spreading Ridges: Constraints From the Mantle Transition Zone of the East Pacific Rise at Hess Deep and Oman Drilling Project, Wadi Zeeb, Samail Ophiolite

AbstractWe investigate the cooling histories of peridotites and gabbros from localities that expose oceanic lithosphere formed beneath two fast seafloor spreading centers: Hess Deep as recovered from IODP Expedition 345 and ODP Leg 147, and the Oman Ophiolite as sampled by the Oman Drilling Project, ICDP Expedition 5057 (OmanDP). At these locations, relict crust‐mantle transition zones are directly sampled, enabling characterization of the thermal history of the crust‐mantle transition, and by inference, the depth extent of hydrothermal circulation beneath spreading centers. We measured major and trace element abundances in crustal gabbros and mantle peridotites from Hess Deep and OmanDP, and applied major and trace element‐based thermometers. Geospeedometric interpretation of the temperatures suggests similar cooling histories at both locations; cooling rates ranged from 0.02 to 2.6 °C/y from peak temperatures up to 1,350°C. The rates are consistent on either side of the paleo‐Moho (i.e., in the crust and mantle). Models for conductive cooling of the lower oceanic crust predict rates more than two orders of magnitude slower at the crust‐mantle transition zone, while thermal models that invoke deep and efficient hydrothermal circulation throughout the entire crustal section predict rates consistent with our observations. We infer that hydrothermal cooling extended to or near the petrologic Moho beneath the East Pacific Rise and the OmanDP paleo‐spreading center, consistent with the Sheeted Sills model for crustal accretion. Comparison with previously published rates recalculated using the methods we employed suggests the oceanic lower crust is cooled hydrothermally in some places and by conduction at others.

ВНУТРИПЛИТНАЯ ВУЛКАНОТЕКТОНИЧЕСКАЯ АКТИВНОСТЬ В СЕВЕРО-ВОСТОЧНОМ И ЮЖНОМ СЕКТОРАХ ТИХООКЕАНСКОЙ ЛИТОСФЕРНОЙ ПЛИТЫ В СВЯЗИ С ИЗМЕНЕНИЕМ ЕЕ ОТНОСИТЕЛЬНОГО ДВИЖЕНИЯ

The analysis of altimetric data in combination with bathymetry and gravimetry materials in the north-eastern and southern sectors of the Pacific Ocean, as well as detailed data on the underwater relief, the structure of the sedimentary cover, the composition and absolute age of basalts obtained within the area of domestic geological exploration for ferromanganese nodules (the Clarion-Clipperton zone) is carried out. Structural trends formed by local cone-shaped local structures of presumably volcanic nature, grouped along transform faults belonging to various stages of the kinematics of the Pacific Plate, have been traced in the structure of the oceanic lithosphere at various scale levels. The first trend corresponds to the extension of the fault system corresponding to the spreading system on the crest of the East Pacific rise before the restructuring of its planned geometry in the Paleocene-Eocene, the second coincides with their extension after the change in the relative movement of the Pacific Plate. The trends are characterized by planned disagreement, and an increase in the number of seamounts is observed in the areas of their intersection. Within the area of detailed studies, obvious signs of volcanic-tectonic activity were revealed: high dissection of the underwater relief, hills of different heights with steep slopes, whose volcanic nature is confirmed by differentiated basalts raised from their slopes, the absolute age of which indicates the multistage outpourings that occurred in an intraplate environment. The angular velocity of rotation of the spreading axis and the linear velocity of its advance with changes in the kinematics of the Pacific plate are estimated and possible reasons for changes in its relative motion are considered. An improved scheme of adaptation of the spreading zone to a change in the direction of relative plate movement is proposed, acc0ording to which an essential factor of intraplate volcanic-tectonic activity is the relaxation of stresses in the plate caused by external influence on it.

Frictional Characteristics of Oceanic Transform Faults: Progressive Deformation and Alteration Controls Seismic Style

AbstractOceanic transform faults are inferred to be weak relative to surrounding oceanic crust and primarily slip aseismically. Neither their weakness nor tendency to creep are well‐explained. We test the effects of fault‐rock evolution on oceanic transform fault frictional strength and stability using direct‐shear experiments (at room temperature, 10 MPa normal stress, and fluid‐saturated conditions) on dolerite from the East Pacific Rise and natural fault rocks from the exhumed Southern Troodos Transform, Cyprus. Dolerites and cemented breccias are frictionally strong (μ = 0.52–0.85) and velocity‐weakening (strength decreases with increasing slip velocity, characteristic of earthquakes). In contrast, matrix‐rich chlorite‐bearing fault breccias and gouges are frictionally weak (μ = 0.25–0.48) and velocity‐strengthening (characteristic of stable creep). This transition implies that seismic behavior is controlled by degree of damage and alteration, such that earthquakes can nucleate within relatively intact oceanic crust, whereas fault segments of increased damage and chlorite content tend to slip aseismically.

Molybdenum isotope systematics of lavas from the East Pacific Rise: Constraints on the source of enriched mid-ocean ridge basalt

Mid-ocean ridge basalts (MORB) reveal large mantle compositional heterogeneity, whose origin remains debated. Here we present a systematic study of molybdenum isotopes on well-characterized MORB glass samples from the East Pacific Rise (EPR) and near-EPR seamounts. Our analyses show significant Mo isotope variations with δ98/95Mo (relative to NIST SRM3134) ranging from −0.23‰ to −0.06‰. We argue that these Mo isotope variations are not caused by processes of MORB melt generation and evolution but reflect mantle isotopic heterogeneity. Taking together with the literature data, we show that MORB Mo isotope compositions vary systematically with geochemical parameters indicating mantle enrichment. These observations are best explained by two-component mixing between an incompatible element depleted endmember (e.g., low La/Sm, Nb/La, Nb/Zr and Th/Yb, and high Sm/Nd and 143Nd/144Nd) with low δ98/95Mo (∼−0.21‰) and an incompatible element enriched endmember (e.g., high La/Sm, Nb/La, Nb/Zr and Th/Yb, and low Sm/Nd and 143Nd/144Nd) with high δ98/95Mo (∼−0.05‰). The association of heavier Mo isotope compositions with the geochemically more enriched MORB is inconsistent with recycled ocean crust with or without sediment being the enriched endmember. Instead, this is consistent with the enriched endmember being of magmatic origin, most likely lithologies of low-degree melt metasomatic origin dispersed in the more depleted peridotite matrix in the MORB mantle. Thus, with MORB Mo isotope systematics, we confirm that recycled oceanic mantle lithosphere metasomatized by low degree melt plays a key role in the formation of E-MORB source lithologies. Our study also highlights Mo isotopes as an effective tool for studying upper mantle processes.

Formation of the Moho transition zone in the Oman ophiolite, and comparison with sub-Moho melt lenses at fast spreading ridges

Our knowledge of melt distribution in the lower crust and upper mantle at oceanic fast spreading centers is very limited. Evidence of melt accumulation, sometimes away from the axis, has been imaged and interpreted at the Moho of the East Pacific Rise; but the detailed structures of these deep magma lenses remains much more difficult to unveil than that of the shallow axial melt lens at the top of the plutonic crust. Ophiolites offer on-land sections of oceanic lithosphere that can complement marine geological and geophysical observations. We present results of a geological survey of the Moho transition zone at a paleospreading center in the Oman ophiolite. We find that the thickness of this dunite-rich horizontal layer increases from a few meters at the axis to hundreds of meters 6 km away from the axis, and is reduced to a few meters 2–3 km further away. The base of the Moho transition zone contains dunite and very depleted harzburgite with isotropic plagioclase and clinopyroxene impregnations, and stockwork-like magmatic breccia, indicative of episodic high melt fractions. We conclude that the melt-free dunite horizontal layer may stop the progression of ascending melt; this lead to melt accumulation within the uppermost harzburgite beneath the Moho transition zone and forms the isotropic impregnations. As the melt dissolves the harzburgite orhtopyroxene, and is flushed to the top of the MTZ though the breccia, it leaves new dunite at the base of the Moho transition zone. Repetition of this process renders the Moho transition zone thicker as it moves away from the ridge axis, until it leaves the main area of mantle melt delivery. Then, tectonic thinning and intrusion of parts of the MTZ into the lower crust reduce the MTZ thickness. These processes seem coherent with several marine geophysical observations.

Heterogeneous nickel isotope compositions of the terrestrial mantle – Part 2: Mafic lithologies

We report stable Ni isotope compositions (δ60/58Ni, relative to SRM986) for mafic lavas with a range of −0.16‰ to +0.20‰ (n = 44), similar to that of peridotite samples. Ocean island basalts (OIB) have been analysed from Iceland (n = 6), the Azores (n = 3), the Galápagos Islands (n = 2), and Lōʻihi, Hawaii (n = 1). Samples from Iceland (average δ60/58Ni = +0.13 ± 0.16‰, 2s, n = 7) display the greatest range in Ni isotope compositions from a single OIB location in this work, of +0.01‰ to +0.23‰. Samples from the Azores (average δ60/58Ni = −0.10 ± 0.10‰, 2s) and Galápagos (average δ60/58Ni = −0.01 ± 0.04‰, 2s) are generally isotopically lighter. The single Lōʻihi sample has a δ60/58Ni of +0.17‰. The lightest analysed bulk rock δ60/58Ni in this work, −0.16‰, is from the Azores island, Pico. Enriched mid ocean ridge basalts (E-MORB), which have (La/Sm)N > 1, are isotopically lighter than normal type MORB (N-MORB), as shown by data from the Mid Atlantic Ridge (n = 9) and East Pacific Rise (n = 3). All E-MORB average δ60/58Ni = +0.00 ± 0.06‰ (2s, n = 7), whereas N-MORB average δ60/58Ni = +0.14 ± 0.10‰ (2s, n = 5).A suite of 15 mafic samples from the Cameroon Line, comprising lithologies ranging from nephelinites to hypersthene-normative basalts, have Ni isotope compositions that are identical within analytical uncertainty (average δ60/58Ni = +0.08 ± 0.06‰, 2s). Similarly, MORB samples display no relationship between δ60/58Ni and geochemical indicators of degree of partial melting or fractional crystallisation. Host lavas for two previously analysed ultramafic xenolith suites have δ60/58Ni identical to the average δ60/58Ni of their respective xenolith suites. This is consistent with previously published evidence from peridotites and komatiites that Ni isotopes are not greatly fractionated by melting. Therefore, mafic rocks may preserve the δ60/58Ni of their mantle source. Sampling a greater volume of mantle, their average Ni isotope composition +0.07 ± 0.17‰ (2s, n = 44) may also be a better representation of the Bulk Silicate Earth (BSE), than estimates based purely on peridotites.The δ60/58Ni of MORB co-varies with La/Sm, Rb/Sr, europium anomaly (Eu/Eu*), and K2O/(K2O + Na2O). The relationships between these parameters and δ60/58Ni are consistent with mixing between two model endmembers. One could be depleted MORB or depleted MORB mantle (DMM) with a relatively heavy Ni isotope composition; the other a more enriched endmember that has isotopically lighter δ60/58Ni. The link between lighter δ60/58Ni and enriched lithologies in the mantle is further supported by published evidence of light Ni isotope compositions associated with some pyroxenite xenoliths. However, the curvature of the apparent mixing arrays defined by basalts is hard to reconcile with admixing of geochemically enriched but isotopically fractionated oceanic crustal lithologies. High [Ni] enriched magmas such as kimberlites may be a closer match to the enriched endmember. However, this needs further study.

Dispersed Carbon in Basalts of the Altered Oceanic Crust: Isotope Composition and Mechanisms of Formation

Carbon contents and isotopic compositions were compared in the basalt groundmass of the oceanic crust of different age in the zone of the East Pacific Rise. In samples the basalt groundmass of the ancient oceanic crust (~270 Ma, ODP Site 801C) in which a carbonate phase was found, the isotopic composition of the oxidized carbon (δ13C = ±1.5‰) indicates that this carbon was formed by the precipitation of seawater dissolved inorganic carbon (DIC). In the samples in which no carbonate phase was identified, the low concentration (<0.1 wt % CO2) of oxidized dispersed carbon and its isotopic composition (δ13C < –7‰) are in the range of values typical of carbon dissolved in basalt glasses without crystallinity. This makes it possible to relate the oxidized dispersed carbon to residual carbon dissolved in the magmatic melt after CO2 degassing. The precipitation of DIC results in a positive correlation between the concentration of total carbon and its δ13C values, along with the formation of a carbonate phase. The application of this criterion to basalt groundmass samples of the young crust (~15 Ma, ODP Site 1256D) showed that oxidized dispersed carbon in the young oceanic crust groundmass was not formed by the precipitation of DIC, contradicting the generally accepted paradigm. Constant concentration and δ13C values of the reduced dispersed carbon in the basalt groundmass of the young and ancient oceanic crusts, including lithological zones where microbial activity has not been recorded, indicate that the most probable model is high-temperature abiogenic generation of reduced dispersed carbon near the ridge axis. The Fischer–Tropsch synthesis and/or Bell–Boudouard reaction provide a possible basis for the abiogenic model. The Bell–Boudouard reaction 2CO = C + CO2 leads to the formation of an adsorbed layer of elemental carbon on the fresh surfaces of minerals during background alteration of the oceanic basalt crust. The CO2–CO gas-phase equilibrium maintains the necessary depletion of the newly formed elemental carbon in the 13C isotope to δ13C < –20‰. Abiogenic models for the origin of the isotopically light reduced dispersed carbon in the basalt groundmass do not assume the presence of carbon depleted in the heavy 13C isotope in the magmatic melt.

Impact of Upward Oxygen Diffusion From the Oceanic Crust on the Magnetostratigraphy and Iron Biomineralization of East Pacific Ridge-Flank Sediments

Recent measurements of pore-water oxygen profiles in ridge flank sediments of the East Pacific Rise revealed an upward-directed diffusive oxygen flux from the hydrothermally active crust into the overlying sediment. This double-sided oxygenation from above and below results in a dual redox transition from an oxic sedimentary environment near the seabed through suboxic conditions at sediment mid-depth back to oxic conditions in the deeper basal sediment. The potential impact of this redox reversal on the paleo- and rock magnetic record was analyzed for three sediment cores from the Clarion-Clipperton-Zone (low-latitude eastern North Pacific). We found that the upward-directed crustal oxygen flux does not impede high quality reversal-based and relative paleointensity-refined magnetostratigraphic dating. Despite low and variable sedimentation rates of 0.1–0.8 cm/kyr, robust magnetostratigraphic core chronologies comprising the past 3.4 resp. 5.2 million years could be established. These age-models support previous findings of significant local sedimentation rate variations that are probably related to the bottom current interactions with the topographic roughness of the young ridge flanks. However, we observed some obvious paleomagnetic irregularities localized at the lower oxic/suboxic redox boundaries of the investigated sediments. When analyzing these apparently remagnetized sections in detail, we found no evidence of physical disturbance or chemical alteration. A sharp increase in single-domain magnetite concentration just below the present lower oxic/suboxic redox boundary suggests secondary magnetite biomineralization by microaerophilic magnetotactic bacteria living as a separate community in the lower, upward oxygenated part of the sediment column. We therefore postulate a two-phased post-depositional remanent magnetization of ridge flank sediments, first by a shallow and later by a deep-living community of magnetotactic bacteria. These findings are the first evidence of a second, deep population of probably inversely oriented magnetotactic bacteria residing in the inverse oxygen gradient zone of ridge flank sediments.

Effective global mixing of the highly siderophile elements into Earth’s mantle inferred from oceanic abyssal peridotites

Late accretion occurred through addition of massive impactors to Earth, leading to potential heterogeneity in the distribution of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) within the mantle. Abyssal peridotites sample the present-day convecting mantle, which make them useful for examining the distribution of the HSE within the mantle. Here we report new HSE abundance data and 187Os/188Os ratios, in conjunction with mineral chemistry and bulk rock major- and trace-element compositions for abyssal peridotites from the fast-spreading Pacific Antarctic Ridge (PAR) and East Pacific Rise (Hess Deep), and for slow to intermediate spreading ridges from the Southwest Indian Ridge, Central Indian Ridge and Mid-Atlantic Ridge. These analyses expand the global abyssal peridotite Os isotope and HSE database, enabling evaluation of potential variations with spreading rate, from ultraslow (<20 mm/yr, full spreading rate) to fast (135–150 mm/yr). Accounting for likely effects from seawater modification and serpentinization, the Pacific data reveals heterogeneous and sometimes significant melt depletion for PAR (3–23% melt depletion; 187Os/188Os from 0.1189 to 0.1336, average = 0.1267 ± 0.0065; 2SD) and Hess Deep abyssal peridotites (15–20% melt depletion; 0.1247 ± 0.0027). Abyssal peridotites from fast to intermediate spreading ridges reveal no systematic differences in the distribution and behavior of the HSE or Os isotopes, or in degrees of melt depletion, compared with slow to ultraslow spreading ridges. These observations arise despite significant differences in melt generation processes at mid-ocean ridges, suggesting that the effects of ancient melt depletion are more profound on HSE compositions in abyssal peridotites than modern melting beneath ridges. Using global abyssal peridotites with Al2O3 content > 2 wt.%, the average composition of the primitive mantle is 0.3 ppb Re, 4.9 ppb Pd, 7.1 ppb Pt, 7.2 ppb Ru, 3.8 ppb Ir and Os, showing no Pd/Ir, but a positive Ru/Ir anomaly, relative to chondrites. There is ∼50% variation of the HSE abundances in the oceanic mantle, with much of this variation being observed at small length scales (<1 km) and due entirely to both modern and more ancient partial melting effects. Consequently, any significant HSE heterogeneities formed during late accretion or early Earth differentiation processes are no longer recognizable in the mantle sampled within ocean basins, implying generally efficient mixing of Earth’s mantle for these elements. By contrast, relatively ancient heterogeneity in Os and other radiogenic isotopes has been effectively preserved in the convecting mantle over the last ∼2 Ga, through recycling processes and through preservation and isolation of melt-depleted refractory residues.

High‐Resolution Isotopic Variability Across EPR Segment 16°N: A Chronological Interpretation of Source Composition and Ridge‐Seamount Interaction

AbstractA high‐resolution sampling profile constituted of 52 basalt samples from across the East Pacific Rise (EPR) was investigated. These samples provide a unique opportunity to study the coeval recording of isotopic signals derived from sub‐marine eruptions at a fast spreading ridge over a time interval of ∼320 kyrs. Additionally, the study area is marked by the presence of a neighboring seamount chain that has recently caused the ridge to jump twice toward it. Combining previous geochemical studies and bathymetry, we established a first‐order chronology between analyzed samples, and have reconstructed the evolution of basalt compositions as the ridge and seamounts advance and finally merge. Our data reveal the existence of two distinct types of isotopic variability within the samples. One that has a low amplitude and frequency and is accounted for by the continuous melting of the ambient mantle, indicating a process with a ∼125 kyr periodicity. The other, of higher amplitude, is discontinuous in time, and likely reflects the seamounts source influence on the ridge. Our results support a two‐step hotspot‐ridge interaction including a first stage (≥600 ka) of regional enrichment of the depleted ridge mantle by hotspot material; and a second, more recent (at least 300 ka) even wherein ambient mantle melts mixed with proximal melts from heterogeneous seamounts sourced nearby. We also propose that the ancient gabbroic component previously identified in this region appeared very recently (&lt;hundreds ya) as erupted lavas are exclusively recorded in a cluster proximal to the axial zone of the ridge.

Fe-catalyzed sulfide oxidation in hydrothermal plumes is a source of reactive oxygen species to the ocean

Historically, the production of reactive oxygen species (ROS) in the ocean has been attributed to photochemical and biochemical reactions. However, hydrothermal vents emit globally significant inventories of reduced Fe and S species that should react rapidly with oxygen in bottom water and serve as a heretofore unmeasured source of ROS. Here, we show that the Fe-catalyzed oxidation of reduced sulfur species in hydrothermal vent plumes in the deep oceans supported the abiotic formation of ROS at concentrations 20 to 100 times higher than the average for photoproduced ROS in surface waters. ROS (measured as hydrogen peroxide) were determined in hydrothermal plumes and seeps during a series of Alvin dives at the North East Pacific Rise. Hydrogen peroxide inventories in emerging plumes were maintained at levels proportional to the oxygen introduced by mixing with bottom water. Fenton chemistry predicts the production of hydroxyl radical under plume conditions through the reaction of hydrogen peroxide with the abundant reduced Fe in hydrothermal plumes. A model of the hydroxyl radical fate under plume conditions supports the role of plume ROS in the alteration of refractory organic molecules in seawater. The ocean's volume circulates through hydrothermal plumes on timescales similar to the age of refractory dissolved organic carbon. Thus, plume-generated ROS can initiate reactions that may affect global ocean carbon inventories.

Experimental study on Fe solubility in vapor-rich hydrothermal fluids at 400–500 °C, 215–510 bar: Implication for Fe mobility in seafloor vent systems

In seafloor hydrothermal systems, vent fluids usually contain elevated dissolved iron (Fe) that is significantly enriched relative to deep ocean seawater. It is commonly thought that Fe is preferentially transported in dense Cl-rich fluids due to the formation of aqueous Fe-Cl complexes. However, Fe enrichment in vapor-rich low-density fluids with low Cl concentrations (<550 mmol/kg) underscores the efficacy of the low-Cl vapor-rich phase to transport Fe in both subaerial and submarine hydrothermal systems. Currently, transport of Fe in low-density vapor-rich fluids is poorly understood due to the lack of high temperature–pressure (T-P) solubility experiments and requisite thermodynamic data. Here, we report new data of Fe solubility from experiments conducted at 400–500 °C, 215–510 bar, targeting fluids with low-density (∼0.1–0.35 g/cm3). The experiments were performed in the KCl-H2O system with hematite-magnetite and K-feldspar-muscovite-quartz as mineral buffering assemblages. Our results show that Fe solubility positively correlates with density and fluid chlorinity, which are affected by temperature and pressure. The equilibrium constants (log Khm) for Fe-buffering reaction Fe3O4(s) + 2HCl(aq) = Fe2O3(s) + FeCl2(aq) + H2O were determined. The new data and the data calculated using Helgeson-Kirkham-Flowers (HKF) equation of state were fit into a density model to extrapolate log Khm for hematite-magnetite Fe buffering reaction over a wide T-P range. The density models for magnetite dissolution reaction and pyrite-pyrrhotite equilibrium were also fit based on HKF to allow redox constraints. We show that calculated Fe solubility are in good agreement with measured values in vapor-rich fluids formed via phase separation in mineral buffered and basalt alteration experiments at elevated T-P. The density model was further applied to model Fe transport in fluids at the Brandon hydrothermal field at East Pacific Rise (EPR) 21°S, with T-P constrained by Si-Cl geothermobarometer. The calculations suggest that the reported Fe concentrations of vent fluids at Brandon reflect phase separation occurring at depth in the seafloor, with T-P up to 450 °C, 400 bar, and redox conditions buffered by pyrite-pyrrhotite-magnetite equilibrium.

Copper recycling and redox evolution through progressive stages of oceanic subduction: Insights from the Izu-Bonin-Mariana forearc

Addition of subducted materials from the slab to the mantle wedge is often thought to elevate the oxygen fugacity of arc magmas and also to fertilize the mantle wedge in metals for subduction-related Cu-Au deposits. However, it remains controversial if slab-driven metal addition is effective and whether it occurs at all stages of subduction. The Izu-Bonin-Mariana (IBM) forearc preserves a full record of arc development from the initiation of subduction of the Pacific plate to mature arc volcanism. Here, we study the ore-forming and redox-sensitive metal Cu and its isotopes (δ65Cu) in the type-locality forearc basalts (FABs), boninites and high-Mg andesites from the IBM forearc, as well as MORBs from the East Pacific rise. Overall, the FABs display variably high Cu contents and MORB-like δ65Cu, consistent with limited Cu isotopic fractionation during partial melting of typical MORB mantle sources. Beginning with the boninites, the magma products from the IBM record strong slab signals, high water contents and high fO2 (ΔFMQ > +1). However, the oxidized, hydrous boninites and subsequent high-Mg andesites display low Cu contents, and mantle-like Cu/Sc ratios and δ65Cu, with no clear indication of the addition of Cu from the slab. The boninites show noticeably lower TiO2, Yb and Cu contents than the FABs, consistent with refractory mantle sources. Combined with available data from boninites and arc basalts worldwide, our results lead to a general conclusion that subducted oceanic slabs from initial to mature subduction contribute little Cu to the mantle wedge. This is further supported by the compiled Cu contents of arc peridotites. Adding Cu-poor, water-rich slab melts to the mantle wedge at any stage of Pacific oceanic subduction causes limited release of Cu, which remains trapped predominantly by reduced sulfides in the subducting slab. This likely reflects the overall reducing nature of slab materials added to the mantle wedge. However, we propose that the subsequent reaction of such reducing, hydrous slab melts with peridotites and flux melting produce oxidized primitive arc magmas deep in the mantle wedge. The reactive process promotes sulfide dissolution and metal release from the mantle wedge itself to oxidized arc magmas with high sulfur solubility, which explains the inheritance of mantle-like δ65Cu and the sulfide-undersaturated early-stage evolution in boninites and arc basalts. This study elucidates the role of subducted oceanic slabs in metal transfer and redox evolution and implies no significant Cu enrichment in the metasomatized mantle sources of magmatic arcs.

Wet magmatic processes during the accretion of the deep crust of the Oman Ophiolite paleoridge: Phase diagrams and petrological records

The Oman Ophiolite is regarded as an analogue to modern fast-spreading ocean ridge systems in an environment of subduction zone initiation. In contrast to recent mid-ocean ridge basalts from the East Pacific Rise, parental melts at the Oman paleoridge are assumed to be hydrous in nature. In order to constrain the role of water during magmatic accretion processes in the deep crust at the Oman paleoridge, we evaluated several experimental studies in hydrous tholeiitic systems performed at shallow pressures. We concluded that the wehrlitic phase assemblage (olivine coexisting with clinopyroxene but without plagioclase) is the most significant feature indicative of high prevailing water activities. The stability of the wehrlitic assemblage decreases with decreasing pressure (not stable in the upper plutonic crust) and depends on the chemical system (only stable in primitive MORB systems).We applied these results to plutonic rocks from cores drilled as part of the Oman Drilling Project (OmanDP). A key observation is the presence of coherent wehrlitic layers within the layered gabbro series, which are frequent in the lowermost gabbros (20%), relative sparse in the mid-crust (6%), and absent from the top of the plutonic crust at the dike/gabbro transition. Based on the combined phase relations for hydrous MORB-type systems at shallow pressures, we interpret this as a direct consequence of the presence of a significant water activity during the accretion of the plutonic crust of the Oman paleoridge, and not as a local phenomenon related to variations in temperature or bulk chemistry. These findings have implications for the mechanism of accretion of the lower crust at the Oman paleoridge, supporting a model that significant parts of the plutonic crust were produced by in-situ crystallization of primitive melt sills.

Rhyolitic and basaltic reference materials for TC/EA analysis: Investigation of water extraction and D/H ratios

Increasingly, water and D/H ratios of silicic and basaltic glasses are used to investigate magmatic degassing and secondary hydration, as well as for study these parameters in the mantle and crustal magmas. The advent of the High Temperature Conversion Element Analyzer (TC/EA) continuous flow mass spectrometry made the determination of hydrogen isotopes relatively quick and precise (±0.04 wt% H2O and 1–3‰ δ2H). Many labs around the world have such systems, thus there is a need to develop both silicic and basaltic volcanic glass reference materials (RMs) that can be used for interlaboratory comparison by bulk and microanalytical methods. Here, we report results of such investigation run against solid RMs (USGS micas) and water RMs (including VSMOW) in three different labs and describe analytical protocols. We report on the effects of glass size fraction, the mass of aliquots measured, and yield dependency for two glasses: UOR (drill cutting of IDDP-1 rhyolite, Iceland) and UOB (an E-MORB basalt from the East Pacific Rise). Proposed RM values are:UOB rhyolitic glass (n = 31):H2O=1.84±0.06wt%1s.d.,andδ2H=−115.5±2.6‰1s.d..UOB basaltic glass (n = 60):H2O=0.37±0.03wt%1s.d.,andδ2H=−82.1±5.7‰1s.d..Glass reference materials are available in 100 mg quantities from the Stable Isotope Laboratory at the University of Oregon for a nominal fee.