Ultraslow-spreading Gakkel Ridge Research Articles

High-quality draft genome of Gammaproteobacterial SUP05 cluster from non-buoyant hydrothermal plumes of ultraslow spreading Gakkel Ridge (Central Arctic Ocean).

Hydrothermal plumes are an important yet understudied component of deep-sea microbial ecosystems. We report metagenome-assembled genomes (MAGs) of three Bacteria belonging to the Gammaproteobacterial SUP05 cluster (family Thioglobaceae), assembled from the metagenomes of two non-buoyant hydrothermal plumes in the ultraslow spreading Gakkel Ridge.

Hot Vents Beneath an Icy Ocean: The Aurora Vent Field, Gakkel Ridge, Revealed

Evidence of hydrothermal venting on the ultra-slow spreading Gakkel Ridge in the Central Arctic Ocean has been available since 2001, with first visual evidence of black smokers on the Aurora Vent Field obtained in 2014. But it was not until 2021 that the first ever remotely operated vehicle (ROV) dives to hydrothermal vents under permanent ice cover in the Arctic were conducted, enabling the collection of vent fluids, rocks, microbes, and fauna. In this paper, we present the methods employed for deep-sea ROV operations under drifting ice. We also provide the first description of the Aurora Vent Field, which includes three actively venting black smokers and diffuse flow on the Aurora mound at ~3,888 m depth on the southern part of the Gakkel Ridge (82.5°N). The biological communities are dominated by a new species of cocculinid limpet, two small gastropods, and a melitid amphipod. The ongoing analyses of Aurora Vent Field samples will contribute to positioning the Gakkel Ridge hydrothermal vents in the global biogeographic puzzle of hydrothermal vents.

Integrative taxonomy of a new cocculinid limpet dominating the Aurora Vent Field in the central Arctic ocean.

Deep-sea hydrothermal vents host lush chemosynthetic communities, dominated by endemic fauna that cannot live in other ecosystems. Despite over 500 active vents found worldwide, the Arctic has remained a little-studied piece of vent biogeography. Though located as early as 2001, the faunal communities of the Aurora Vent Field on the ultra-slow spreading Gakkel Ridge remained unsampled until recently, owing to difficulties with sampling on complex topography below permanent ice. Here, we report an unusual cocculinid limpet abundant on inactive chimneys in Aurora (3883–3884 m depth), describing it as Cocculina aurora n. sp. using an integrative approach combining traditional dissection, electron microscopy, molecular phylogeny, and three-dimensional anatomical reconstruction. Gross anatomy of the new species was typical for Cocculina, but it has a unique radula with broad, multi-cuspid rachidian where the outermost lateral is reduced compared to typical cocculinids. A phylogenetic reconstruction using the mitochondrial COI gene also confirmed its placement in Cocculina. Only the second cocculinid found at vents following the description of the Antarctic Cocculina enigmadonta, this is currently the sole cocculinid restricted to vents. Our discovery adds to the evidence that Arctic vents host animal communities closely associated with wood falls and distinct from other parts of the world.

Deformation beneath Gakkel Ridge, Arctic Ocean: From mantle flow to mantle shear in a sparsely magmatic spreading zone

Mantle deformation processes leading to seafloor spreading are often difficult to infer due to the highly serpentinized and weathered state of most abyssal peridotites. We investigated the development of high-temperature crystal-plastic deformation and lower temperature mylonitization processes in relatively fresh (<50% modal serpentine) and ultra-fresh (<1% serpentine) mantle peridotites derived from the heterogeneous mantle in the sparsely magmatic zone of ultraslow-spreading Gakkel Ridge system by analyzing 12 peridotites from two dredge sites (<1 km apart). Microstructurally, these 12 peridotites consist of seven high-T deformed samples and five mylonites. Modally, the 12 samples include harzburgites, lherzolites, an olivine websterite, and a plagioclase-bearing lherzolite. Based on their mineral major and trace element compositions, the lherzolites, harzburgites, and olivine websterite are residual peridotites. The lherzolites containing clinopyroxenes with flat REE patterns likely underwent refertilization with a high influx of melt. The plagioclase-bearing lherzolites probably formed by subsolidus reaction after the partial melting process. Microstructural observations support that high-T crystal-plastic deformation (most likely at temperatures exceeding 1000 °C) was active in the peridotites of the high-T deformation group, accommodating mantle flow beneath the Gakkel Ridge. The identified melt refertilization process may have contributed to the formation of [010]-fiber olivine fabrics in these peridotites. Mylonitic microstructures, decreasing fabric strength and grain-size reduction of olivine suggest that mylonitization occurred under relatively low-temperature mantle conditions (~800 °C) and probably accommodated strain localization. Water did not greatly affect the peridotites during the development of the shear zones, although amphibole with “dusty” zones developed in one mylonitic peridotite after mylonitization, indicating that late-stage metasomatism occurred locally within the shear zone. This low-T mylonitization is likely to have affected mantle peridotites of this region independently of petrogenetic processes. The development of these deformation processes in Gakkel Ridge suggests a shift from flow in the uppermost mantle to shear zone formation in the rift valley walls.

High temperature hydrothermal alteration and amphibole formation in Gakkel Ridge abyssal peridotites

Alteration mineral assemblages in abyssal peridotites offer insights into the temperature and pressure conditions during hydrothermal fluid circulation in the oceanic lithosphere. Abundant hydrothermal venting has been documented along the ultraslow spreading Gakkel Ridge and peridotites have been extensively sampled from the ridge, yet these peridotites have not been systematically evaluated for the nature and extent of their alteration. Here, we present an analysis of the alteration of 40 Gakkel peridotites from 19 dredges spanning ~600 km of the ridge within the Sparsely Magmatic and Eastern Volcanic Zones. We use a petrographic-based alteration scoring system to assign a rank to each sample of 1 (unaltered) to 5 (altered) in 0.5 step intervals, based on the alteration scale of Birner et al. (2016). Gakkel peridotites cover the full range of alteration, from essentially unaltered to completely serpentinized, but are generally moderately to highly altered (average score of 3.5). The extent of alteration is independent of peridotite lithology and is not systematically different between the two zones. Serpentine is present in all samples, magnetite occurs in all samples with alteration scores >2.5, and 27% of samples contain carbonate veins that cross-cut all other minerals. In 20% of samples, all with alteration scores >2.5, tremolite + talc occurs as rims around pyroxene, while chlorite occurs as haloes around spinel. A gabbro-veined lherzolite also contains pargasite replacing pyroxene, with later overprinting by tremolite-talc intergrowths.The abundance of tremolite, chlorite, and talc in Gakkel samples is much lower than reported for talc-tremolite-chlorite schists from oceanic core complexes, where fluid flow through the lower crust leads to open-system behavior. At Gakkel, our petrological observations suggest that alteration was nearer to closed-system conditions, which can be explained by the absence of crust along some sections of the ridge. Pseudosection modeling with Perple_X, which assumes closed-system behavior, indicates that pyroxene reacts to form tremolite + chlorite + talc at temperatures >500 °C, corresponding to fluid flow to depths >20 km for the Gakkel Ridge. The sample with pargasitic amphibole requires temperatures >750 °C to form. Overall, our results suggest that hydrothermal alteration of Gakkel peridotites, and more generally of the lithospheric mantle in regions that lack significant crust, results in different alteration mineral abundances and relationships than in regions where the mafic crust is involved in alteration.

Low-Magnitude Earthquakes at the Eastern Ultraslow-Spreading Gakkel Ridge, Arctic Ocean

AbstractThanks to the new permanent seismic stations installed in the Franz Joseph Land and Severnaya Zemlya arctic archipelagoes, it has become possible at present to record earthquakes occurring in the eastern Gakkel ridge with a much lower detection threshold than that provided by the global network. At present, the lowest recorded magnitude is ML 2.4 and the magnitude of completeness is 3.4. We examined the results of seismic monitoring conducted from December 2016 through January 2020 to show that the earthquake epicenters are not uniformly distributed both in space and over time within the eastern part of the ridge. There were periods of quiescence and seismic activity. Most of the epicenters are confined to the area between 86° and 95.0° E. Relative location techniques were used to locate the single major swarm of earthquakes recorded so far. Most earthquakes were recorded by two or three stations only, so that relative location techniques have been able to yield reliable data for an analysis of the swarm. We showed that there have been actually two swarms that contained different numbers of events. The earthquakes in the larger swarm were occurring nonuniformly over time and clustered at certain depths. The ML scale was calibrated for the Eurasian Arctic based on records of the seismic stations installed in the Svalbard Archipelago, Franz Joseph Land, and on Severnaya Zemlya: −logA0(R)=1.5×logR100+1.0×10−4(R−100)+3.0. The results will help expand our knowledge of the tectonic and magmatic processes occurring within the ultraslow Gakkel ridge, which are reflected in the local seismicity.

Competing effects of spreading rate, crystal fractionation and source variability on Fe isotope systematics in mid-ocean ridge lavas

Two-thirds of the Earth is covered by mid-ocean ridge basalts, which form along a network of divergent plate margins. Basalts along these margins display a chemical diversity, which is consequent to a complex interplay of partial mantle melting in the upper mantle and magmatic differentiation processes in lower crustal levels. Igneous differentiation (crystal fractionation, partial melting) and source heterogeneity, in general, are key drivers creating variable chemistry in mid-ocean ridge basalts. This variability is reflected in iron isotope systematics (expressed as δ57Fe), showing a total range of 0.2 ‰ from δ57Fe = + 0.05 to + 0.25 ‰. Respective contributions of source heterogeneity and magma differentiation leading to this diversity, however, remain elusive. This study investigates the iron isotope systematics in basalts from the ultraslow spreading Gakkel Ridge in the Arctic Ocean and compares them to existing data from the fast spreading East Pacific Rise ridge. Results indicate that Gakkel lavas are driven to heavier iron isotope compositions through partial melting processes, whereas effects of igneous differentiation are minor. This is in stark contrast to fast spreading ridges showing reversed effects of near negligible partial melting effects followed by large isotope fractionation along the liquid line of descent. Gakkel lavas further reveal mantle heterogeneity that is superimposed on the igneous differentiation effects, showing that upper mantle Fe isotope heterogeneity can be transmitted into erupting basalts in the absence of homogenisation processes in sub-oceanic magma chambers.

New data about small-magnitude earthquakes of the ultraslow-spreading Gakkel Ridge, Arctic Ocean

At the present time there is available detailed bathymetry, gravimetric, magnetometer, petrological, and seismic (mb>4) data for the Gakkel Ridge. However, so far not enough information has been obtained on the distribution of small-magnitude earthquakes (or microearthquakes) within the ridge area due to the absence of a suitable observation system. With the ZFI seismic station (80.8°N, 47.7°E), operating since 2011 at the Frantz Josef Land Archipelago, we can now register small-magnitude earthquakes down to 1.5ML within the Gakkel Ridge area. This article elaborates on the results and analysis of the ZFI station seismic monitoring obtained for the period from December 2011 to January 2015. In order to improve the accuracy of the earthquakes epicenter locations, velocity models and regional seismic phase travel-times for spreading ridges in areas within the Euro-Arctic Region have been calculated. The Gakkel Ridge is seismically active, regardless of having the lowest spreading velocity among global mid-ocean ridges. Quiet periods alternate with periods of higher seismic activity. Earthquakes epicenters are unevenly spread across the area. Most of the epicenters are assigned to the Sparsely Magmatic Zone, more specifically, to the area between 1.5°E and 19.0°E. We hypothesize that assignment of most earthquakes to the SMZ segment can be explained by the amagmatic character of the spreading of this segment. The structuring of this part of the ridge is characterized by the prevalence of tectonic processes, not magmatic or metamorphic ones.

Variations in melting dynamics and mantle compositions along the Eastern Volcanic Zone of the Gakkel Ridge: insights from olivine-hosted melt inclusions

We present major element, trace element, and volatile concentrations from 66 naturally glassy, olivine-hosted melt inclusions erupted along the Eastern Volcanic Zone (EVZ) of the ultraslow-spreading Gakkel Ridge. Melt inclusion compositions suggest that there are systematic variations in the mantle source composition and melting dynamics from the eastern to the western end of the EVZ. This includes increasing water contents and highly incompatible trace element concentrations (e.g., Ba and Nb) and decreasing light and middle rare earth element concentrations. Ratios of light to heavy rare earth elements in the easternmost melt inclusions are relatively homogeneous, but become more variable to the west. To determine the source of the geochemical variability observed along the EVZ, we model trace elements associated with mantle melting in one- and two-component systems. We consider four possible mantle sources and a range of melting regime shapes, from a full melting triangle to a vertical melting column centered beneath the ridge axes. The observed geochemical variations can be explained by melting of a heterogeneous mantle source composed of depleted MORB mantle plus a metasomatized mantle, where the proportion of the metasomatized component and the extent of melting increases toward the west. Lower rare earth element concentrations and trace element ratios in the westernmost sites also suggest inefficient melt focusing from the outer edges of the melting region. Our results indicate that despite variations in the size of the melting zone and the composition of the mantle source along the ridge axis, the region over which the melts are pooled back to the ridge axis is relatively constant (~10–20 km), suggesting that there is a limit to the distance melts can be transported from off-axis in ultraslow-spreading environments.

Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges

We present new 238U–230Th–226Ra–210Pb, 235U–231Pa, and Nd, Sr, Hf, and Pb isotope data for the slow- to ultraslow-spreading Mohns, Knipovich, and Gakkel Ridges. Combined with previous work, our data from the Arctic Ridges cover the full range of axial depths from the deep northernmost Gakkel Ridge shallowing upwards to the Knipovich, Mohns, and Kolbeinsey Ridges north of Iceland. Age-constrained samples from the Mohns and Knipovich Ridges have (230Th/238U) activity ratios ranging from 1.165 to 1.30 and 1.101 to 1.225, respectively. The high 230Th excesses of Kolbeinsey, Mohns, and Knipovich mid-ocean ridge basalts (MORB) are erupted from ridges producing relatively thin (Mohns, Knipovich) to thick (Kolbeinsey) oceanic crust with evidence for sources ranging from mostly peridotite (Kolbeinsey) to eclogite-rich mantle (Mohns, Knipovich). Age-constrained lavas from 85°E on the Gakkel Ridge, on the other hand, overlie little to no crust and range from small (∼5%) 230Th excesses to small 238U excesses (∼5%). The strong negative correlation between (230Th/238U) values vs. axial ridge depth among Arctic ridge basalts is controlled not only by solidus depth influence on 238U–230Th disequilibria, but also by variations in mantle source lithology and depth to the base of the lithosphere, which is expected to vary at ultra-slow spreading ridges. Small 231Pa excesses (65% excess) in age-constrained basalts support the presence of eclogite in the mantle source for this region. Conversely, the ultraslow-spreading Gakkel Ridge basalts are homogeneous, with Sr, Nd, and Hf radiogenic isotopic signatures indicative of a long time-averaged depleted mantle source. The Gakkel samples have minimum (226Ra/230Th) ratios ranging from 3.07 to 3.65±3%, which lie along and extend the global negative correlation between 226Ra and 230Th excesses observed in MORB. The new 230Th–226Ra data support a model for global MORB production in which deep melts record interaction with shallower materials. This scenario requires either mixing with shallow-derived melts, or melt-rock reaction with shallower rocks in the lithosphere or crust.

Seismicity and structure of the 85°E volcanic complex at the ultraslow spreading Gakkel Ridge from local earthquake tomography

Accretion mechanisms at ultraslow spreading ridges are still only poorly understood due to difficult survey conditions for seismic experiments at these ridges. Melts gets focused in distinct magmatic centres, with thin crust in between. At the 85°E/85°N volcanic complex at Gakkel ridge, Arctic Ocean, where a recent spreading episode has been observed, perennial sea ice cover challenges traditional investigations of seismic structure and microseismicity. We used an unusual survey set-up to gather seismological data during 16 days in July 2007, using seismometer arrays mounted on ice floes. Despite only 12 stations, the drift of the ice floes over the survey area resulted in a multitude of crossing rays in the rift valley. The data included 303 microearthquakes of which 248 events could be confidently located. We compiled a 1D velocity model by localizing a subset of these earthquakes with a suite of randomly created velocity models. In this model, the Moho is placed at 7 km depth below seafloor, inferring a thick, basaltic crust. Using 124 events which were recorded by at least two arrays, we inverted for P-wave velocity structure in a local earthquake tomography. Resolution tests indicate reliable results in the central rift valley, illuminating the thermal structure underneath the Asgard volcanic chain in the aftermath of its most recent spreading episode. Our results show microearthquake activity down to 16 km beneath seafloor, inferring a cold lithosphere. Most hypocentres cluster at the centre of the rift valley at the site the Asgard volcanic chain. This may mean that existing thermal models for this class of ridges have to be refined. An area of decreased seismic velocities crosses the rift valley at this location and microearthquake activity is located at its eastern fringe where the velocity gradient is highest. We therefore speculate that the reduced velocities may be caused by warm intruded material and that the observed seismicity predominatly reflects the relaxation of thermal stresses following recent intrusion.

Effusive and explosive volcanism on the ultraslow‐spreading Gakkel Ridge, 85°E

We use high‐definition seafloor digital imagery and multibeam bathymetric data acquired during the 2007 Arctic Gakkel Vents Expedition (AGAVE) to evaluate the volcanic characteristics of the 85°E segment of the ultraslow spreading Gakkel Ridge (9 mm yr−1full rate). Our seafloor imagery reveals that the axial valley is covered by numerous, small‐volume (order ∼1000 m3) lava flows displaying a range of ages and morphologies as well as unconsolidated volcaniclastic deposits with thicknesses up to 10 cm. The valley floor contains two prominent volcanic lineaments made up of axis‐parallel ridges and small, cratered volcanic cones. The lava flows appear to have erupted from a number of distinct source vents within the ∼12–15 km‐wide axial valley. Only a few of these flows are fresh enough to have potentially erupted during the 1999 seismic swarm at this site, and these are associated with the Oden and Loke volcanic cones. We model the widespread volcaniclastic deposits we observed on the seafloor as having been generated by the explosive discharge of CO2 that accumulated in (possibly deep) crustal melt reservoirs. The energy released during explosive discharge, combined with the buoyant rise of hot fluid, lofted fragmented clasts of rapidly cooling magma into the water column, and they subsequently settled onto the seafloor as fall deposits surrounding the source vent.

Performance of localization algorithms for teleseismic mid-ocean ridge earthquakes: the 1999 Gakkel Ridge earthquake swarm and its geological interpretation

In 1999, an unusual earthquake swarm at the 85°E/85°N volcanic centre on the ultraslow spreading Gakkel Ridge, Arctic Ocean, was detected teleseismically. The swarm lasted over 9 months and counted 252 events with mb ≥ 3.1. It represents the strongest and largest ever recorded mid-ocean ridge earthquake swarm, and it occurred at a site where spreading rates are only about 10 mm yr–1. We relocated the earthquake swarm comparing the performance of three different localization algorithms: (1) the absolute least-squares routine HYPOSAT, (2) the absolute probabilistic routine NonLinLoc and (3) the relative least-squares routine Mlocate. The epicentres as calculated by each algorithm mostly did not agree within their error ellipses. Thus, the choice of location algorithm proved more critical than, for example, the choice of a local velocity model.We compiled a set of well-localized events which closely agree in at least two routines, mostly Mlocate and NonLinLoc. We conclude that the earthquake swarm of 1999 was related to a spreading episode and shows a complex interplay of tectonic and magmatic events. Our geological interpretation revealed three phases in swarm activity: In the first phase from January 17 up to February 1 fracturing of the crust took place, either as a result of or enabling magmatic intrusion. Seismicity in the second phase from February 2 to April 6 expanded along- and across axis. It showed signs of magmatic interaction, but a clear dyke migration pattern is absent. At the beginning of the third phase, a distinct break in the event rate suggested a change in the physical process, either an adjustment of the stress field to the new regime or a transition to an effusive stage.

Cooling-induced fractionation of mantle Li isotopes from the ultraslow-spreading Gakkel Ridge

Li isotopic compositions of magmatic rocks have gained considerable attention recently as probes of mantle-scale processes. However, the concentrations and isotopic composition of Li in mantle minerals from mid-ocean ridges remain relatively unconstrained. This is largely because of the general presence of seawater alteration in abyssal peridotites. Lithium elemental and isotopic compositions for mineral separates of coexisting olivine, clinopyroxene, orthopyroxene and bulk rocks of serpentine-free Gakkel Ridge peridotites were investigated. Bulk rocks have Li contents of 1.6 to 2.7ppm and δ7Li values of 3 to 5‰, which fall within the range of reported normal pristine “MORB mantle” values. Lithium concentrations vary in the order cpx (2.1–4.7ppm)>opx (0.9–1.7ppm)≥olivine (0.4–0.9ppm), the opposite found in “equilibrated” mantle peridotite xenoliths (Seitz and Woodland, 2000). The Li isotopic compositions indicate a systematic mineral variation with δ7Liolivine (7.14‰–15.09‰)>δ7Liopx (1.81‰–3.66‰)>δ7Licpx (−2.43‰−−0.39‰). The δ7Li values of cpx are negatively correlated with their Li concentrations with the lightest value for the most enriched cpx grains. There is a first order negative linear correlation between Δolivine–cpx (δ7Liolivine−δ7Licpx) and ol/cpxD (Liolivine/Licpx).Numerical simulations indicate that the observed systematic inter-mineral variations of Li concentrations and isotopic compositions could be explained by a cooling driven diffusive redistribution between minerals in a closed system if there is a temperature dependent partitioning of Li between olivine and clinopyroxene. The studied Gakkel Ridge abyssal peridotites may alternatively have cooled under a variable cooling rate with a rapid cooling before the Li system was closed, which is less likely given the tectonic setting. Our calculations confirm that Li systematics in minerals, especially in coexisting mineral phases could potentially be used as a mantle geospeedometer, even for slowly cooled mantle rocks.

Crystal growth during dike injection of MOR basaltic melts: evidence from preservation of local Sr disequilibria in plagioclase

Profiles of a total of 23 plagioclase crystals erupted within the 1982–1991 and 1993 flows of the Coaxial segment of the Juan de Fuca ridge, the 1996 flow of the North Gorda ridge, and from the Western Volcanic Zone of the ultra-slow spreading Gakkel Ridge, have been studied for variations in major and trace element concentrations. We derive equilibration times for the relatively rapidly diffusing Sr in mid-ocean ridge basalt (MORB) plagioclase crystals of the order of months to a few years in each case. All crystals preserve diffusive disequilibria of strontium and barium. Crystal residence times at MORB magmatic temperatures are thus significantly shorter, of the order of days to a few months at most, precluding prolonged crystal storage in axial magma chambers and instead pointing to rapid crystal growth (up to ~10−8 cm s−1) and cooling (up to ~1°C h−1) shortly prior to eruption of these samples. Growth of these crystals is therefore inferred to occur almost entirely within oceanic layer 2 during dike injection. Crystals that grew at lower crustal levels or earlier in the differentiation sequence appear to have been excluded from the erupted magmas, as might occur if most of the gabbroic rocks in oceanic layer 3 formed an interlocking crystal framework, with viscosities that are too high to carry earlier formed crystals with the melt. The vertical extent of eruptible, crystal-poor melt lenses within the gabbroic zone is constrained to ~1 m or less by considering the width of local equilibrium growth zones, equilibration times, and crystal settling velocities. This lengthscale is consistent with field evidence from ophiolites. Finally, crystal aggregates within the Gakkel ridge sample studied here are the result of synneusis within the propagating dike during melt ascent.

Deep pooling of low degree melts and volatile fluxes at the 85°E segment of the Gakkel Ridge: Evidence from olivine-hosted melt inclusions and glasses

We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85°E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO 2-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO 2 contents in the melt inclusions extend to higher values (167–1596 ppm) than in the co-existing glasses (187–227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO 2 melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (∼ 4 km) and ∼ 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25–40 km) and cold (1240°–1325 °C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous; this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9–20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO 2 is perfectly incompatible, we show that the highest CO 2 concentrations of the melt inclusions (∼ 1600 ppm) are consistent with the calculated CO 2 concentrations of primary undegassed melts. The highest measured CO 2/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO 2 content of 134 ppm and would result in a global ridge flux of 2.0 × 10 12 mol CO 2/yr.

Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)

Constraints on the highly siderophile element (HSE: including Os, Ir, Ru, Pt, Pd and Re) budget of the upper mantle upwelling beneath mid-ocean ridges have until now been based on samples that have experienced varying degrees of alteration, partial melting and secondary igneous processes. Here we present results from a set of abyssal peridotites that have been relatively unaffected by these phenomena. Eighteen abyssal peridotites from two localities along the ultra-slow spreading Gakkel Ridge (Arctic Ocean) were selected for this study. Samples from one locality (PS66-238) are extremely fresh, while those from HLY0102-D70 are typical abyssal serpentinites. Comparison of HSE data between fresh peridotites and weathering products supports the contention that HSE are stable during serpentinization, but that Pd and Re are mobile during subsequent weathering. HSE budgets of spinel lherzolites suggest that all platinum group elements (PGEs: including Os, Ir, Ru, Pt and Pd) are compatible during very low degree of partial melting (5–8%), whereas Re behaves as an incompatible element. Harzburgites from each locality were subjected to ~ 12% fractional partial melting, which is lower than that expected for total consumption of sulfides in mantle peridotites (ca. 16%). The harzburgites are depleted in PPGE (Pt, Pd) and Re relative to IPGE (Os, Ir, Ru), which reflects the extraction of sulfide melt along with the silicate melt. Low bulk-rock contents of both Na 2O and TiO 2 in the fresh PS66-238 lherzolites indicate that they were not transformed from harzburgites through melt refertilization, but represent pristine fertile mantle rocks. Their systematic suprachondritic Ru/Ir and Pd/Ir ratios, but chondritic Pt/Ir ratios, cannot be ascribed to partial melting or secondary igneous processes. This signature may reflect the primary HSE signature of the Earth's upper mantle. Non-chondritic HSE patterns discovered in PS66-238 lherzolites are consistent with the primitive upper mantle (PUM) inferred from the study of orogenic peridotites and mantle xenoliths.

Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean

The Earth's mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution.

Microseismicity of the ultraslow-spreading Gakkel ridge, Arctic Ocean: a pilot study

SUMMARY The active mid-ocean ridge of the Arctic Ocean, named Gakkel ridge, is the slowest spreading ridge of the global system of mid-oceanic ridges with full spreading rates declining from about 12.5 to 6 mm yr−1 from west to east. Geological models of seafloor spreading predict a decreasing intensity of magmatic processes with decreasing spreading rate. In summer 2001, the multidisciplinary Arctic Mid-Ocean Ridge Expedition (AMORE2001) discovered robust magmatism at western Gakkel ridge, an amagmatic section further east and pronounced volcanic centres at eastern Gakkel ridge. During AMORE2001, an attempt was made at recording the microearthquake activity of the ridge which allows important insights into the character and dynamics of active crustal accretion at the ridge axis. Due to the permanent ice cover of the Arctic Ocean, the use of ocean-bottom seismometers bears the risk of instrument and data loss. In this pilot study, we used for the first time drifting ice floes as platforms for small seismological arrays. The arrays consisted of four three-component seismometers equipped with GPS devices and arranged as a triangle with a central seismometer and a side length of about 1 km. Three such arrays were deployed in different rift segments and recorded the seismic activity continuously for 5–11 days at a sampling rate of 100 Hz. The array technique allowed to distinguish clearly between icequakes and earthquakes and to localize the earthquake source to within few kilometres or less depending on epicentral distance. We intensively discuss the detection capabilities and the location accuracy of this single array on a drifting ice floe. Earthquake magnitudes could not be calculated in our pilot study but are estimated to be significantly smaller than magnitude 2 by comparison with a regional earthquake of known magnitude. Furthermore, we analyse the characteristics of the recorded seismic events ranging from long waveforms of regional events to short local events with reverberations in the water column. All of the arrays recorded numerous microearthquakes in the central rift valley and on its flanks which we interpret as tectonic earthquakes. A swarm of microearthquakes was localized with high accuracy underneath the crest of a volcanic ridge in the rift valley and is proposed to have magmatic origin. The pilot study was thus successful in detecting, localizing and interpreting microearthquakes below magnitude 2 at Gakkel ridge. However, we suggest improvements of the method for a comprehensive microearthquake survey of Gakkel ridge which should aim at an understanding of active magmatism and faulting at ultraslow-spreading ridges.

Geophysical characteristics of the ultraslow spreading Gakkel Ridge, Arctic Ocean

The northernmost spreading centre of the world, the Gakkel Ridge, is also an end‐member in terms of global spreading velocities. Models show that full spreading rates vary between 1.3 and 0.63 mm yr−1 along the almost 1800 km long ridge system in the Central Arctic Ocean. The western part of the ridge was investigated in great detail by a two‐ship expedition in summer 2001. The complete data sets and the modelling of the seismic refraction and aeromagnetic experiments gathered during this expedition are shown in this study. The magnetic signals along the dense (2 km spacing) aeromagnetic flight lines acquired at the same time show a good correlation between high amplitudes and a shallowing of the rift valley and the presence of large volcanic constructions at the rift shoulders. The magnetic anomalies rapidly fade out east and west of these centres of focused magmatism. This might indicate that the basaltic layer producing the magnetic anomaly thins away from the volcanic centres. A continuous magnetic anomaly is observed along the rift valley west of 3°30′E, consistent with increasing and more robust magmatism. The crustal thickness along the Gakkel Ridge varies greatly. Beneath some of the centres of focused magmatism, the oceanic crust thickens up to 3.5 km. In the amagmatic segments in between the crust thins to 1.4–2.9 km. This observation is also valid for the Western Volcanic Zone west of 3°30′E, where despite the stronger magnetic anomaly the crust does not significantly thicken. The strength of the magnetic anomaly along the rift valley is thus not a reliable indicator of crustal thickness beneath the Gakkel Ridge. The data show that the crustal thickness does not change dramatically across 3°30′E. Only the occurrence of a large elongate volcanic ridge significantly influences this parameter. More frequent volcanic eruptions along such ridges are most likely responsible for the basalts found in the westernmost part of the Gakkel Ridge. In the non‐transform segments some seismic stations indicate that mantle rocks are exposed at the seafloor, with no indication of the presence of a basaltic cover or normal oceanic crust. Both the seismic and magnetic data support models in which the uppermost basaltic cover is responsible for the magnetic anomaly in the rift valley.