Slow-spreading Mid-ocean Ridges Research Articles

Depth-varying seismogenesis on an oceanic detachment fault at 13°20′N on the Mid-Atlantic Ridge

Extension at slow- and intermediate-spreading mid-ocean ridges is commonly accommodated through slip on long-lived faults called oceanic detachments. These curved, convex-upward faults consist of a steeply-dipping section thought to be rooted in the lower crust or upper mantle which rotates to progressively shallower dip-angles at shallower depths. The commonly-observed result is a domed, sub-horizontal oceanic core complex at the seabed. Although it is accepted that detachment faults can accumulate kilometre-scale offsets over millions of years, the mechanism of slip, and their capacity to sustain the shear stresses necessary to produce large earthquakes, remains subject to debate. Here we present a comprehensive seismological study of an active oceanic detachment fault system on the Mid-Atlantic Ridge near 13°20′N, combining the results from a local ocean-bottom seismograph deployment with waveform inversion of a series of larger teleseismically-observed earthquakes. The unique coincidence of these two datasets provides a comprehensive definition of rupture on the fault, from the uppermost mantle to the seabed. Our results demonstrate that although slip on the deep, steeply-dipping portion of detachment faults is accommodated by failure in numerous microearthquakes, the shallow, gently-dipping section of the fault within the upper few kilometres is relatively strong, and is capable of producing large-magnitude earthquakes. This result brings into question the current paradigm that the shallow sections of oceanic detachment faults are dominated by low-friction mineralogies and therefore slip aseismically, but is consistent with observations from continental detachment faults. Slip on the shallow portion of active detachment faults at relatively low angles may therefore account for many more large-magnitude earthquakes at mid-ocean ridges than previously thought, and suggests that the lithospheric strength at slow-spreading mid-ocean ridges may be concentrated at shallow depths.

Ultrafast magmatic buildup and diversification to produce continental crust during subduction

Abstract The processes and fluxes that produce the distinct compositional structure of Earth’s continental crust by subduction remain controversial. The rates of oceanic crust production, in contrast, are well quantified and are generally believed to be faster than those responsible for building magmatic systems in subduction settings. Here we show that a recently recognized crustal section, the 30-km-thick Ordovician Sierra Valle Fértil–Sierra Famatina complex in Argentina, was built magmatically within only ∼4 m.y. More than half of the crustal section represents additions from the mantle, and is preserved as mafic igneous rocks and mafic-ultramafic cumulates; the remainder is tonalite to granodiorite with evidence for widespread assimilation from highly melted metasedimentary units. U-Pb zircon geochronology reveals that the construction of the arc was not a simple bottom-up construction process. This continuous exposure of the arc crust allows the quantification of field constrained magmatic addition rates of 300–400 km3 km–1 m.y.–1. These rates are similar to those determined for modern slow-spreading mid-ocean ridges and are of the same magnitude as magmatic addition rates required to build certain large segments of the continental masses such as the Arabian-Nubian shield, among others. The implication is that significant convective removal of arc roots is required over time in order to build the modern continental crust via subduction-related magmatism.

Experimental study of carbonate formation in oceanic peridotite

Interactions of CO2-rich aqueous fluids with mantle peridotite have major implications for geochemical budgets and microbial life in the shallow oceanic lithosphere through the formation of carbonate minerals and reduced carbon species. However, the underlying mechanisms controlling the transformation of CO2 to carbonates in ultramafic-hosted hydrothermal systems remain incompletely understood. A long-term laboratory experiment was conducted at 300°C and 35MPa to investigate serpentinization and carbonate formation pathways during hydrothermal alteration of peridotite. Powdered harzburgite was initially reacted with a Ca-rich aqueous fluid for 14,592h (608days) and changes in fluid composition were monitored with time. Once the system reached a steady state, a CO2(aq)-rich fluid was injected and allowed to react with the system for 5907h (246days). Fluid speciation and mineral analyses suggest that serpentinization of harzburgite in the CO2-poor system led to the precipitation of serpentine, brucite, magnetite, and minor calcite, in addition to other minor phases including chlorite and sulfur-poor Ni sulfides. The addition of the CO2(aq)-rich fluid caused dolomite, Ca-rich dolomite, and high-Mg calcite to form at the expense of olivine, calcite, and brucite, while serpentine remained unreactive. Replacement textures and mineral assemblages mimic those documented in carbonate-altered seafloor serpentinites, particularly those from the Mid-Atlantic Ridge and the Iberia Margin. In contrast to thermodynamic predictions, magnesite did not form in the experiment because the dissolution of clinopyroxene, in combination with the lack of serpentine reactivity, maintained low Mg/Ca ratios in solution. Clinopyroxene dissolution and unreactive serpentine may similarly maintain low Mg/Ca ratios in submarine serpentinization systems and limit magnesite formation in subseafloor environments. Results of this study suggest that the formation of Ca–Mg carbonates by mineral carbonation is favorable in subseafloor serpentinization systems and likely represents a significant, but poorly quantified, carbon sink in hydrothermally altered oceanic lithosphere at slow-spreading mid-ocean ridges.

Effects of axially variable diking rates on faulting at slow spreading mid-ocean ridges

Magma supply for dike injection can be highly variable within a segment of a slow-spreading mid-ocean ridge but the tectonic impact of this variability is not fully understood. Here, we use three-dimensional numerical models to quantify the effects of variable diking rates on the faulting mode at a 20 km-long slow spreading ridge segment. In addition to end-member faulting modes in which long-lived detachment faults or short-lived normal faults form along the whole segment, we newly identify a transitional mode in which a detachment and a short-lived normal fault form simultaneously but in respective domains separated by a transfer fault. Different faulting modes can be better correlated with the average dike intrusion rate, rather than the highest or lowest rate along the segment. Along-axis stress coupling tends to homogenize fault offset along the segment, inhibiting the domination of a particular faulting mode associated with an extreme local diking rate. This homogenizing effect explains why detachment faults can sometimes form even in the regions previously considered as unfavorable. Our results further suggest that a long (>15 km) and continuous detachment, partially overlain by younger faults, can create an oceanic core complex when faults weaken fast and diking rate is low. When faults weaken slow and diking rate is moderate, however, faulting occurs in the transitional mode, producing a detachment over only a part of the segment length.

Sulfur and lead isotopic compositions of massive sulfides from deep-sea hydrothermal systems: Implications for ore genesis and fluid circulation

Studies of sulfur and lead isotopic compositions in hydrothermal deposits are an important tool to determine the source and processes of both sulfur and lead, and to understand the origin of hydrothermal ore deposits. Here, the sulfur and lead isotopic compositions of sulfide minerals have been studied for different hydrothermal fields in the East Pacific Rise (EPR), Mid-Atlantic Ridge (MAR), Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), and North Fiji Basin (NFB). The sulfur isotopic compositions of the studied sulfide samples are variable (δ34S 0.0 to 9.6‰, avg. δ34S 4.7‰; n=60), being close to the associated igneous rocks (~0‰ for, e.g., basalt, serpentinized peridotite), which may reflect the S in the sulfide samples is derived mainly from the associated igneous rocks, and a relatively small proportion (<36%) of seawater sulfur incorporated into these sulfides during mixing between seawater (δ34S 21‰) and hydrothermal fluid. In contrast for a mixed origin for the source of S, the majority of the lead isotopic compositions (206Pb/204Pb 17.541±0.004 to 19.268±0.001, 207Pb/204Pb 15.451±0.001 to 15.684±0.001, 208Pb/204Pb 37.557±0.008 to 38.988±0.002, n=21) of the sulfides possess a basaltic Pb isotopic composition, suggesting that the lead in the massive sulfide is mainly leached from local basaltic rocks that host the sub-seafloor hydrothermal systems in sediment-free mid-ocean ridges and mature back-arc basins. Furthermore, sulfide minerals in the super-fast and fast spreading mid-ocean ridges (MORs) exhibit less spread in their the δ34S values compared to sulfides from super-slow, and slow spreading MORs, which is most easily explained as a lesser degree of fluid-rock interaction and hydrothermal fluid-seawater mixing during hydrothermal ore-forming process. Additionally, the S and Pb isotope compositions of sulfides are controlled by the fluid processes for forming seafloor massive sulfide deposits. We demonstrate that the variable sulfur and lead isotopic compositions exhibit a relationship with the sulfur and lead sources, fluid–rock interaction, and fluid–seawater mixing.

Sulfide geochronology along the Northern Equatorial Mid-Atlantic Ridge

Hydrothermal processes and seafloor massive sulfide (SMS) deposits have different characteristics at fast and slow spreading mid-ocean ridges. One such parameter is the age of a SMS deposit, which differs by 1–2 orders of magnitude between the fast spreading East Pacific Rise (EPR) and the slow spreading Mid-Atlantic Ridge (MAR). The large collection of SMS samples dated from the 18 hydrothermal fields of the northern equatorial part of the Mid-Atlantic Ridge (194 samples) demonstrates a relatively old average age of hydrothermal fields here (~66ka) with the oldest one estimated as ca. 223ka (Peterburgskoye field). Based on geochronological data it was confirmed that hydrothermal discharge has an episodic character: active and inactive periods of the SMS formation alternate. The distribution of events at all hydrothermal fields demonstrates that maximum activity occurred at 38–35, 30–20, and 8–2ka and increased with time. Based on statistical analyses, dating variations can be explained as a superposition of several periods of activity with the duration of ~15, 10 and 5ka.Relationship between the age and distance from the axial rift zone as well as between the age and aerial distribution is different for SMS deposits hosted by basalts and by gabbro-peridotites depending on their geological setting on the particular MAR segment. This difference can be explained by a variety of hydrothermal processes determined by “tectonic” or “magmatic” segment evolution and symmetrical or asymmetrical mode of accretion (Escartin et al., 2008).

Geomorphology of the central Red Sea Rift: Determining spreading processes

Continental rifting and ocean basin formation is occurring today in the Red Sea, providing a possible modern analogue for the creation of mid-ocean ridges. Yet many of the seafloor features observed along the axis of the Red Sea appear anomalous compared to ancient and modern examples of mid-ocean ridges in other parts of the world, making it unclear, until recently, whether the Red Sea is truly analogous. Recent work suggests that the main morphological differences between the Red Sea Rift (RSR) and other mid-ocean ridges are due to the presence and movement of giant, submarine salt flows, which blanket large portions of the rift valley and thereby the oceanic crust. Using ship-based, high-resolution multibeam bathymetry of the central RSR between 16.5°N and 23°N we focus here on the RSR volcanic terrains not covered by salt and sediments and compare their morphologies to those observed along slow and ultra-slow spreading ridges elsewhere. Regional variations in style and intensity of volcanism can be related to variations in volcanic activity and mantle heat flow. The Red Sea oceanic seafloor shows typical features of mature (ultra)slow-spreading mid-ocean ridges, such as 2nd order discontinuities (overlapping spreading centres) and magma focussing in the segment centres (forming spreading-perpendicular volcanic ridges of thick oceanic crust). The occurrence of melt-salt interaction at locations where salt glaciers blanket the neovolcanic zone, and the absence of large detachment faults are unique features of the central RSR. These features can be related to the young character of the Red Sea and may be applicable to all young oceanic rifts, associated with plumes and/or evaporites. Thus, the RSR falls in line with (ultra)slow-spreading mid-ocean ridges globally, which makes the Red Sea a unique but highly important type example for initiation of slow rifting and seafloor spreading and one of the most interesting targets for future ocean research.

Petrogenetic conditions at 18°–20° N MAR: Interaction between hydrothermal and magmatic systems

The paper presents petrological and geochemical data on mantle peridotite, basalt, and metamorphic rocks sampled in Cruise 36 of the R/V Professor Logachev at the MAR axial zone between 17° and 20° N. These data are interesting not only as providing new information on the inner structure of the oceanic crust in the still-poorly known axial MAR segment but also in the context of the fundamental problem of interaction between magmatic and hydrothermal systems in slow-spreading mid-oceanic ridges. The MAR axial zone between 17° and 20° N was determined to host both weakly and strongly depleted residual peridodites, which suggests that the degree of mantle source melting significantly varied along the ridge axis in this segment. The MAR crest zone comprises slabs of serpentinized peridotite brought to the seafloor surface at various time. The most strongly depleted mantle peridotites likely uplifted later than the mildly and weakly depleted rocks in the same areas. A mantle reservoir beneath the MAR axial zone at 20° N is not isotopically related to the mantle source of the parental MORB melts, and high-Mg metabasites exposed at 17°56- N were derived from a crustal source that was modified near the root zone of a high-temperature hydrothermal system. The studied area seems to display traces of an extinct hydrothermal field and likely an ore occurrence related to it.

Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet)

Ophiolites in the Indus–Yarlung Zangbo (IYZ) suture (southern Tibet) have been interpreted as remnants of the Neo-Tethyan lithosphere. However, the discovery of diamonds and super-reducing, ultra-high pressure (SuR-UHP) mineral assemblages (e.g., coesite after stishovite, olivine after wadsleyite, native metals, alloys, and moissanite) in some of these massifs and associated chromitites requires a re-evaluation of their origin and evolution. A new petrological and geochemical study of the Purang ophiolite in the western IYZ suture sheds new lights on these issues.The depleted harzburgites of the Purang massif have low modal contents of clinopyroxene (<2%), and high Cr# [100*Cr3+/(Cr3++Al3+)] in spinel (>40~70) and pyroxenes (>16 in orthopyroxene, and >20 in clinopyroxene), suggesting high degrees of melt extraction (>20%). These features are not consistent with formation in a (ultra-) slow-spreading mid-ocean ridge. These peridotites have high modal contents of orthopyroxene; this, and the extremely high Cr# of spinels in these peridotites, suggests modification in a subduction zone. The clinopyroxene-rich harzburgites and lherzolites contain rare spinel–pyroxene symplectites after garnet. Their clinopyroxenes have low MREE-to-HREE ratios ((Sm/Yb)N<0.1) at relatively high HREE concentrations, and are Na-rich but Nd-poor. The relatively enrichment of Na but depletion of Nd in clinopyroxene cannot be explained by refertilization with MORB melts but are consistent with an origin from Na-rich subcontinental lithospheric mantle (SCLM). All lines of evidence suggest that these peridotites underwent initial melting in the stability field of garnet–facies peridotites, followed by additional melting in the spinel–facies mantle.Whole-rock Os isotopic compositions of the Purang peridotites give ancient TRD model ages (up to 1.3Ga), indicating that the formation of these ancient depletion residues predated the opening of Neo-Tethyan Ocean. These observations, together with recent studies on other IYZ peridotites, suggest that the Purang peridotites are genetically unrelated to the associated mafic crust. Instead, they represent ancient SCLM domains, initially formed beneath a continental margin, and then modified by subduction, before they were incorporated into the Neo-Tethyan ocean basin. This model is consistent with the deep-mantle-recycling model for the presence of SuR-UHP phases in the IYZ ophiolites. The infiltration of MORB melts through these ancient depleted peridotites during their final exhumation in a (ultra-) slow-spreading center may have refertilized them to produce the clinopyroxene-rich peridotites.

Petrological and tectono-magmatic significance of ophiolitic basalts from the Elba Island within the Alpine Corsica-Northern Apennine system

Two distinct ophiolitic units, which represent remnants of the Jurassic Ligurian-Piedmont Ocean, crop out in the Elba Island. They are the Monte Strega unit in central-eastern Elba and the Punta Polveraia-Fetovaia unit in western Elba. Ophiolitic rocks from the Monte Strega unit are commonly affected by ocean floor metamorphism, whereas those from the Punta Polveraia-Fetovaia unit are affected to various extent by thermal metamorphism associated with the Late Miocene Monte Capanne monzogranitic intrusion. Both ophiolitic units include pillow lavas and dykes with compositions ranging from basalt to basaltic andesite, Fe-basalt, and Fe-basaltic andesite. Basaltic rocks from these distinct ophiolitic units show no chemical differences, apart those due to fractional crystallization processes. They display a clear tholeiitic nature with low Nb/Y ratios and relatively high TiO2, P2O5, Zr, and Y contents. They generally display flat N-MORB normalized high field strength element patterns, which are similar to those of N-MORB. Chondrite-normalized rare earth element patterns show light REE / middle REE (LREE/MREE) depletion and marked heavy (H-) REE fractionation with respect to MREE. This HREE/MREE depletion indicates a garnet signature of their mantle sources. Accordingly, they can be classified as garnet-influenced MORB (G-MORB), based on Th, Nb, Ce, Dy, and Yb systematics. We suggest that the Elba Island ophiolitic basalts were generated at a magma starved, slow-spreading mid-ocean ridge. REE, Th, and Nb partial melting modelling shows that the compositions of the relatively primitive Elba Island ophiolitic basalts are compatible with partial melting of a depleted MORB mantle (DMM) source bearing garnet-pyroxenite relics. Hygromagmatophile element ratios suggest that basalts from both ophiolitic units were originated from chemically very similar mantle sources. A comparison with basalts and metabasalts from Alpine Corsica and northern Apennine ophiolitic units shows that the composition of the inferred mantle source for the Elba Island basalts is similar to that of some Lower Schistes Lustres metabasalts of Alpine Corsica ophiolites, and some basalts from the Internal Ligurian units of northern Apennine. In contrast, it slightly differs from those of other ophiolitic units of Alpine Corsica and northern Apennine. The chemical differences observed between basalts and metabasalts from different Ligurian-Piedmont ophiolitic units were likely associated with different partial melting degrees of either DMM source or garnet-pyroxenite relics and/or different mixing proportions of melts derived from them, as well as to different compositions of garnet-pyroxenite relics.

Where are the undiscovered hydrothermal vents on oceanic spreading ridges?

In nearly four decades since the discovery of deep-sea vents, one-third of the length of global oceanic spreading ridges has been surveyed for hydrothermal activity. Active submarine vent fields are now known along the boundaries of 46 out of 52 recognized tectonic plates. Hydrothermal survey efforts over the most recent decade were sparked by national and commercial interests in the mineral resource potential of seafloor hydrothermal deposits, as well as by academic research. Here we incorporate recent data for back-arc spreading centers and ultraslow- and slow-spreading mid-ocean ridges (MORs) to revise a linear equation relating the frequency of vent fields along oceanic spreading ridges to spreading rate. We apply this equation globally to predict a total number of vent fields on spreading ridges, which suggests that ~900 vent fields remain to be discovered. Almost half of these undiscovered vent fields (comparable to the total of all vent fields discovered during 35 years of research) are likely to occur at MORs with full spreading rates less than 60mm/yr. We then apply the equation regionally to predict where these hydrothermal vents may be discovered with respect to plate boundaries and national jurisdiction, with the majority expected to occur outside of states' exclusive economic zones. We hope that these predictions will prove useful to the community in the future, in helping to shape continuing ridge-crest exploration.

Reply to “Comment on Manuella et al. ‘The Hyblean xenolith suite (Sicily): an unexpected legacy of the Ionian–Tethys realm’ by Beccaluva et al. (2015)”

One of the most important pieces of background information left in our pen (Manuella et al. 2015) regards the circumstance that, during the last 25 years, international marine geology expeditions brought crucial advances in understanding the composition and tectonic evolution of present and fossil oceanic lithosphere (e.g., Pearce 2002; Dick et al. 2003; Boschi et al. 2006; Snow and Edmond 2007; Ildefonse et al. 2007; Miranda and Dilek 2010; Silantyev et al. 2011). In particular, we would draw attention to some fault-bounded abyssal highs, called oceanic core complexes (OCCs), located in the crest zone of (ultra) slow-spreading mid-ocean ridges. OCCs mostly consist of serpentinized mantle peridotites and gabbroic rocks exhumed to the ocean floor along systems of detachment faults, related to serpentinite diapirism. Most elevated blocks even reach the ocean surface to form non-volcanic ocean islands, as well as the St. Peter and St. Paul Rocks located near the axial zone of MAR in the equatorial region (e.g., Campos et al. 2010; Sharkov 2012). More in general, magmatic layers of the normal oceanic crust are very thin or even absent at OCCs sites, seismic profiles being compatible with a serpentinite layer overlying almost unaltered mantle ultramafics (e.g., Blackman et al. 2004a, b). In this respect, the concept of a “crust” had to be called into question, and hence, the Moho can be regarded as a serpentinization front (e.g., Minshull et al. 1998). Oxide-rich gabbros with sheared texture are considered obliged components of the gabbroic suite of present and fossil OCCs (e.g., Sharkov 2012). Veins of plagiogranites are also relatively common in these oceanic structures, intruding both gabbros and peridotite bodies. Oxide gabbros and plagiogranites from OCCs typically bear zircon as accessory phase (e.g., Aranovich et al. 2013). OCC basalts, Introduction

Carlsberg Ridge and Mid-Atlantic Ridge: Comparison of slow spreading centre analogues

Eighty per cent of all mid-ocean spreading centres are slow. Using a mixture of global bathymetry data and ship-board multibeam echosounder data, we explore the morphology of global mid-ocean ridges and compare two slow spreading analogues: the Carlsberg Ridge in the north-west Indian Ocean between 57°E and 60°E, and the Kane to Atlantis super-segment of the Mid-Atlantic Ridge between 21°N and 31°N. At a global scale, mid-ocean spreading centres show an inverse correlation between segment length and spreading rate with segmentation frequency. Within this context, both the Mid-Atlantic Ridge super-segment and Carlsberg Ridge are similar: spreading at 22 and 26mm/yr full rates respectively, being devoid of major transform faults, and being segmented by dextral, non-transform, second-order discontinuities. For these and other slow spreading ridges, we show that segmentation frequency varies inversely with flank height and ridge axis depth. Segments on both the Mid-Atlantic Ridge super-segment and Carlsberg Ridge range in aspect ratio (ridge flank height/axis width), depth and symmetry. Segments with high aspect ratios and deeper axial floors often have asymmetric rift flanks and are associated with indicators of lower degrees of melt flux. Segments with low aspect ratios have shallower axial floors, symmetric rift flanks, and evidence of robust melt supply. The relationship between segmentation, spreading rate, ridge depth and morphology, at both a global and local scale, is evidence that rates of melting of the underlying mantle and melt delivery to the crust play a significant role in determining the structure and morphology of slow spreading mid-ocean ridges.

Stability of rift axis magma reservoirs: Spatial and temporal evolution of magma supply in the Dabbahu rift segment (Afar, Ethiopia) over the past 30 kyr

Unravelling the volcanic history of the Dabbahu/Manda Hararo rift segment in the Afar depression (Ethiopia) using a combination of cosmogenic (36Cl and 3He) surface exposure dating of basaltic lava-flows, field observations, geological mapping and geochemistry, we show in this paper that magmatic activity in this rift segment alternates between two distinct magma chambers. Recent activity in the Dabbahu rift (notably the 2005–2010 dyking crises) has been fed by a seismically well-identified magma reservoir within the rift axis, and we show here that this magma body has been active over the last 30 kyr. However, in addition to this axial magma reservoir, we highlight in this paper the importance of a second, distinct magma reservoir, located 15 km west of the current axis, which has been the principal focus of magma accumulation from 15 ka to the subrecent. Magma supply to the axial reservoir substantially decreased between 20 ka and the present day, while the flank reservoir appears to have been regularly supplied with magma since 15 ka ago, resulting in less variably differentiated lavas. The trace element characteristics of magmas from both reservoirs were generated by variable degrees of partial melting of a single homogeneous mantle source, but their respective magmas evolved separately in distinct crustal plumbing systems.Magmatism in the Dabbahu/Manda Hararo rift segment is not focussed within the current axial depression but instead is spread out over at least 15 km on the western flank. This is consistent with magneto-telluric observations which show that two magma bodies are present below the segment, with the main accumulation of magma currently located below the western flank, precisely where the most voluminous recent (<15 ka) flank volcanism is observed at the surface.Applying these observations to slow spreading mid-ocean ridges indicates that magma bodies likely have a lifetime of a least 20 ka, and that the continuity of magmatic activity is maintained by a system of separate relaying reservoirs, which could in return control the location of spreading. This long term (>105 yr) alternation between distinct crustal reservoirs located broadly at the same location relative to the segment appears to be a key feature for organizing and maintaining active spreading centres over stable soft points in the mantle.

Formation of H2, CH4 and N-species during low-temperature experimental alteration of ultramafic rocks

Hydrogen, methane and ammonium are important chemical species for chemolithotrophic microorganisms that sustain subsurface endolithic communities. It is well known that production of H2 through serpentinization of the primary Fe(II)-containing minerals, olivine and pyroxene, in ultramafic rocks and reduction of CO2 to CH4 are processes that occur at higher temperatures. Knowledge is, however, limited about these reactions under low-temperature conditions, and how they are affected by Fe(II)-containing secondary minerals, such as brucite and serpentine, which are commonly found in these environments. In this experimental study, we explored the formation of H2, CH4 and NH4 during low-temperature (25°C) reactions between deionised water and 1) unaltered, 2) medium altered and 3) highly altered dunites, over a period of 99days.Hydrogen was detected in all three experiments and the concentration increased over time. The O2 concentration decreased over time, but O2 was still present at significant levels when H2 started to form. This suggests that the H2 was formed locally at the surface of mineral grains rather than in the hypoxic solution. The highly altered dunite gave the highest and the unaltered dunite the lowest H2 concentrations, and while the O2 concentration decreased fast with the medium altered dunite, significant amounts of O2 still remained with the unaltered dunite at the end of the experiments. Both the medium and highly altered dunite gave O2 concentrations close to or below the detection limit during the last part of the experiment. Factors that likely affected the H2 production are Fe-content, mineralogy, dissolution rates and surface areas of the minerals as well as the presence of catalytic minerals such as chromite and magnetite.Methane was also detected and increased with time in all experiments. The results indicate that CH4 formation also occurred locally on the surface of minerals. Detection of both NH4 and NO3 in the aqueous solutions indicates that previously absorbed N-species were leached out during the experiment. Increasing NH4 and decreasing NO3 concentrations over time suggest NO3 reduction, likely catalysed by mineral surfaces.The results imply that low-temperature water–rock reactions in moderately to highly altered ultramafic rocks provide reduced chemical species that can be utilized as electron donors in energy-yielding metabolic processes by subsurface microorganisms. Such reactions could play an important role in sustaining subsurface microbial communities, particularly in ophiolites and near seafloor parts of the ultramafic oceanic lithosphere at slow-spreading mid-ocean ridges and possibly also in other rocks and sediments. The results suggest that brucite and possibly also serpentine may be more important sources of Fe(II) and thus H2 formation than olivine in low-temperature ultramafic systems. Formation of H2 in reducing microenvironments on mineral surfaces surrounded by oxic conditions increases the extent of H2 formation and the subsurface habitat for aerobic H2-oxidizing microorganisms.

Nanoparticulate pyrite and other nanoparticles are a widespread component of hydrothermal vent black smoker emissions

The presence of nanoparticulate pyrite is reported in hydrothermal emissions from Rainbow, TAG and Snakepit on the Mid-Atlantic Ridge (MAR). When coupled with previously collected data from East Pacific Rise 9°N (EPR) and Lau Basin, these data demonstrate that pyrite nanoparticles are a widespread component of black smoker emissions from hydrothermal vents and are found in significant concentrations at a fast spreading mid-ocean Ridge (EPR), a back-arc basin (Lau Basin), and a slow spreading mid-ocean Ridge (MAR). The maximum percentage of filtered iron emitted as nanoparticulate pyrite was found to be as high as 25%, 10%, and 5%, respectively. As a widespread component of hydrothermal vent emissions, these nanoparticles may be an important source of iron to the world's oceans. Metals such as Cu and Zn are detected in pyrite-containing aggregates at all sites, and chalcopyrite was a component of nanoparticle aggregates at MAR. Iron containing silicate nanoparticles are also identified, and indicate that nanoparticles other than sulfides should also be considered when determining transport implications of hydrothermal vent emissions. The varied morphologies and the presence of different minerals within these nanoparticles provide insight into their formation and stability.

Post-Mesozoic Rapid Increase of Seawater Mg/Ca due to Enhanced Mantle-Seawater Interaction

The seawater Mg/Ca ratio increased significantly from ~ 80 Ma to present, as suggested by studies of carbonate veins in oceanic basalts and of fluid inclusions in halite. We show here that reactions of mantle-derived peridotites with seawater along slow spreading mid-ocean ridges contributed to the post-Cretaceous Mg/Ca increase. These reactions can release to modern seawater up to 20% of the yearly Mg river input. However, no significant peridotite-seawater interaction and Mg-release to the ocean occur in fast spreading, East Pacific Rise-type ridges. The Mesozoic Pangean superocean implies a hot fast spreading ridge system. This prevented peridotite-seawater interaction and Mg release to the Mesozoic ocean, but favored hydrothermal Mg capture and Ca release by the basaltic crust, resulting in a low seawater Mg/Ca ratio. Continent dispersal and development of slow spreading ridges allowed Mg release to the ocean by peridotite-seawater reactions, contributing to the increase of the Mg/Ca ratio of post-Mesozoic seawater.

Sustained volcanically-hosted venting at ultraslow ridges: Piccard Hydrothermal Field, Mid-Cayman Rise

At slow spreading mid-ocean ridges sustained submarine venting and the deposition of large seafloor massive sulfide deposits have previously been ascribed to tectonically-controlled hydrothermal circulation unrelated to young volcanic activity. Here, by contrast, we show that the Piccard Hydrothermal Field (PHF), on the ultraslow spreading Mid-Cayman Rise, represents a site of sustained fluid flow and sulfide formation hosted in a neovolcanic setting. The lateral extent and apparent longevity associated with the PHF are comparable to some of the largest tectonically-hosted vent sites known along the slow-spreading Mid-Atlantic Ridge. If such systems recur along all ultraslow ridges, which comprise ∼20% of the ∼55,000 km global ridge crest, potential implications would include (i) a higher probability of locating large, economically valuable, mineral deposits along ultraslow ridges together with (ii) larger fluxes than previously anticipated of chemicals released from high-temperature venting entering the oceans along the Atlantic–Indian Ocean sectors of the deep-ocean thermohaline conveyor.

Two-stage evolution of mantle peridotites from the Stalemate Fracture Zone, northwestern Pacific

This paper reports the results of a mineralogical study of 14 mantle peridotite samples dredged in 2009 from the eastern slope of the northwestern segment of the Stalemate Ridge in the northwestern Pacific during cruise SO201-KALMAR Leg 1b of the R/V Sonne. The sample collection included four serpentinized and silicified dunites and ten variably serpentinized lherzolites. The compositions of primary minerals (clinopyroxene, orthopyroxene, and spinel) change systematically from the lherzolites to dunites. Spinel from the lherzolites shows higher Mg# and lower Cr# values (0.65-0.68 and 0.26-0.33, respectively) compared with spinel from the dunites (Mg# = 0.56-0.64 and Cr# = 0.38-0.43). Clinopyroxene from the lherzolites is less magnesian (Mg# = 91.7-92.4) than clinopyroxene from dunite sample DR37-3 (Mg# = 93.7). Based on the obtained data, it was concluded that the lherzolites of the Stalemate Fracture Zone were derived by 10-12% near-fractional melting of a DMM-type depleted mantle reservoir beneath the Kula-Pacific spreading center. The dunites were produced by interaction of residual lherzolites with sodium- and titaniumrich melt and are probably fragments of a network of dunite channels in the shallow mantle. The moderately depleted composition of minerals clearly distinguishes the lherzolites from the strongly depleted peridotites of the East Pacific Rise and indicates the existence of slow-spreading mid-ocean ridges in the Pacific Ocean during the Cretaceous-Paleogene.

Perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon

The formation of oceanic plagiogranite has been attributed primarily to either 1) extreme fractional crystallization of a mantle melt, or 2) partial melting of hydrated mafic crust, with support for the latter from field evidence and recent melting experiments. Remelting of hydrothermally-altered ocean crust could yield rocks (and minerals) with diverse primary magmatic δ18O values forming proximal to the magmatic center where crustal growth is occurring. To constrain the magmatic δ18O of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis, we characterized the δ18O of zircons in 22 plagiogranite samples (tonalite and trondhjemite) from 8 different ophiolites and one dacite sampled along the East Pacific Rise using Secondary Ion Mass Spectrometry (SIMS). The δ18O values of 202 magmatic zircons from ophiolites range from 3.9 to 5.6‰ (n=244 spots; average 4.9±0.6‰; 2SD), extending ~1‰ below typical zircon in equilibrium with mantle and from gabbroic massifs along slow-spreading mid-ocean ridges (4.7–5.9‰). East Pacific Rise dacite zircons range from 4.6 to 5.0‰ (n=12 spots). Plagiogranite from the dike-gabbro transition zone of the northern Oman Ophiolite yield the lowest δ18O(Zrn), with rock-average values of 4.3–5.0‰. The low-δ18O values are best explained by remelting of crust altered by hydrothermal fluids with seawater-like isotopic compositions at high temperatures, possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high-temperature hydrothermal system in the overlying crust. If the partial melt was assimilated into a fractionating melt lens with MORB-like δ18O, as envisioned for km-scale plagiogranite bodies in Oman, up to 20% contamination by a protolith with δ18O=2‰ would be required. Previous oxygen isotope constraints from quartz in Oman plagiogranite suggested melting of both high and low δ18O crust had occurred; comparison of quartz–zircon pairs indicates that quartz has been modified in most samples, and we find no evidence for the involvement of high-δ18O rocks during plagiogranite formation.