Lena Trough Research Articles

High-resolution bathymetry coupled with 3D models of hydrothermal vents from opportunistically-acquired imagery: Aurora vent field, Arctic Ocean

ABSTRACT Active venting at the Aurora hydrothermal field was first located in 2014. In July 2023, the AUV/ROV Nereid Under Ice (NUI) expanded the known size of the Aurora hydrothermal field, discovering 7 ‘black smokers’ together with associated lower-temperature flow. In this study, we present a new high-resolution bathymetric map acquired from NUI which has allowed us to identify morphological features previously undetectable from ship multibeam. All known active vents are aligned along a single 230 m-long ridge, parallel to the Gakkel Ridge strike direction and intersected by a scarp following the general trend of the Lena Trough. The new vents were measured at up to 17 m height from 3D models generated using structure-from-motion techniques applied to opportunistically acquired imaging data collected while in exploration mode. The extent of extinct sulfides present, together with the towering height of vents are consistent with a period of sustained high-temperature venting at Aurora.

The oil and gas potential of the north of the Siberian platform and adjacent shelf

The objective of this article is an estimation of the oil and gas potential of the north of the Siberian platform and adjacent shelf. The Siberian platform is characterized by a large concentration of oil and gas resources in the southern and central parts of the platform. The main oil and gas province of the Siberian Platform is the Leno-Tungus oil and gas province. The oil- bearing strata in the Lena-Tungus oil and gas province are confined to the Riphean, Vendian, Lower-Middle Cambrian deposits, in which large oil and gas fields are discovered. In modern contours the Siberian platform in the north and north-west is fringed by the Yenisei-Khatanga regional trough. In the east, the Yenisei-Khatanga regional trough is connected with the Anabar- Lena trough, framing the north-eastern part of the Siberian platform. Analysis of the available geological and geophysical materials shows a fairly high potential of the Anabar-Lena and Yenisei-Khatanga troughs for the discovery of large hydrocarbon fields. The basic understanding of the geologic-tectonic structure and petroleum potential of the northern part of the Siberian Platform and the adjacent shelf of the Arctic zone are currently based on seismic data by the method of the common depth point (CDP), tied to the existing deep search and parametric wells. Representations of the deep structure and oil and gas potential of specific areas, the allocation of targeted promising oil and gas horizons can change dramatically as the depth of seismic acquisition of the method of the common depth point increases and the software for processing and interpreting seismic data is improved. The localized oil and gas resources of the north of the Siberian platform and adjacent shelf are 41017,3, subsequently, 27582,3 - onshore and offshore - 13435 MMT (million tons) of oil equivalent.

Spatiotemporal Relationship between Arctic Mid-Ocean Ridge System and Intraplate Seismicity of the European Arctic

AbstractIn this article, we investigate the influence of the Arctic mid-ocean ridge system (AMORS), including the Gakkel and Mohns ridges, and the Knipovich ridge–Lena trough (KL) segment, on seismicity of the Novaya Zemlya archipelago area (NZ) and the northernmost margin of the East-European Platform (EEP) for 1980–2019. For each individual area, the annual seismic energy was obtained by adding the energies of all earthquakes. To do this, we have converted various types of magnitude by different seismic networks into moment magnitude Mw. We compiled the updated catalog for the NZ, the northern EEP, and the northern part of the Ural fold belt (UFB). As a result, we constructed time distributions of annual seismic energy releases for each composing ridges of AMORS, NZ, and EEP combined with UFB. A model based on the Elsasser’s one describing the transfer of lithospheric stress disturbances in the horizontal direction was built, and quantitative calculations of the disturbance propagations from AMORS were performed. Results are in good agreement with the annual seismic energy time lags between rifts and NZ and EEP together with the UFB. We calculated correlation coefficients between the seismic energy releases over the time for the structures, enabling identification of the characteristic excitation cycles and estimation of the interval of disturbance transfer from AMORS. As a result, disturbances from the Gakkel ridge appear 3 yr later in NZ, from the KL segment in 4 yr, and from the Mona ridge in 8 yr. For the EEP + UBF combined area, we found the following disturbances spreading lags as 7 yr for the Mona ridge, 4 yr for the KL segment, and 5 yr for the Gakkel ridge. The obtained damping amplitudes of the disturbance spreading from the arctic ridges are sufficient to affect the intraplate seismic activity.

Middle Miocene magmatic activity in the Sophia Basin, Arctic Ocean—evidence from dredged basalt at the flanks of Mosby Seamount

The area to the northwest of Svalbard was repeatedly affected by tectono-magmatic events during the opening of the Arctic Ocean including the formation of the Cretaceous High Arctic Large Igneous Province, the Late Cretaceous/early Cenozoic birth of the Eurasian Basin, and the establishment of a full seafloor-spreading regime along the Lena Trough/Fram Strait in the middle Miocene. These processes also affected the Sophia Basin located between the Yermak Plateau and the northern Svalbard Shelf. In 2013 a piece of basalt was dredged from the southern flank of the Mosby Seamount, the central landmark within the Sophia Basin. According to Ar–Ar dating on fresh plagioclase the basalt erupted at ~ 13 Ma, contemporaneous with incipient seafloor spreading in the nearby Lena Trough and volcanic activity on northern Svalbard. If the dredged basalt is temporally related to sediment-covered lava flows and sill intrusions around Mosby Seamount, which were revealed by seismic reflections, then the age of the sedimentary cover must be middle Miocene or younger. This finding will improve the regional seismo-stratigraphy.

An introduction of Markov chain Monte Carlo method to geochemical inverse problems: Reading melting parameters from REE abundances in abyssal peridotites

Markov chain Monte Carlo (MCMC) simulation is a powerful statistical method in solving inverse problems that arise from a wide range of applications. In Earth sciences applications of MCMC simulations are primarily in the field of geophysics. The purpose of this study is to introduce MCMC methods to geochemical inverse problems related to trace element fractionation during mantle melting. MCMC methods have several advantages over least squares methods in deciphering melting processes from trace element abundances in basalts and mantle rocks. Here we use an MCMC method to invert for extent of melting, fraction of melt present during melting, and extent of chemical disequilibrium between the melt and residual solid from REE abundances in clinopyroxene in abyssal peridotites from Mid-Atlantic Ridge, Central Indian Ridge, Southwest Indian Ridge, Lena Trough, and American-Antarctic Ridge. We consider two melting models: one with exact analytical solution and the other without. We solve the latter numerically in a chain of melting models according to the Metropolis–Hastings algorithm. The probability distribution of inverted melting parameters depends on assumptions of the physical model, knowledge of mantle source composition, and constraints from the REE data. Results from MCMC inversion are consistent with and provide more reliable uncertainty estimates than results based on nonlinear least squares inversion. We show that chemical disequilibrium is likely to play an important role in fractionating LREE in residual peridotites during partial melting beneath mid-ocean ridge spreading centers. MCMC simulation is well suited for more complicated but physically more realistic melting problems that do not have analytical solutions.

186Os/188Os variations in upper mantle peridotites: Constraints on the Pt/Os ratio of primitive upper mantle, and implications for late veneer accretion and mantle mixing timescales

186Os/188Os variations in mantle peridotites provide constraints on the long-term Pt/Os evolution of the depleted mantle and the Pt/Os ratio of the primitive upper mantle (PUM). We report new 186Os/188Os data for mantle peridotites from continental (Rio Grande Rift and Colorado Plateau) and oceanic (Lena Trough and Hawaiian Islands) settings that span a wide range in fertility (Al2O3 ≈0.67-4.42 wt.%) and 187Os/188Os ratios (0.1138-0.1305). Although peridotite 186Os/188Os values span only a narrow range (from 0.1198345 to 0.1198384), 186Os/188Os broadly correlates with indices of melt depletion including bulk rock Al2O3, spinel Cr#, and clinopyroxene Cr#, consistent with Pt depletion in residual peridotites.PUM 186Os/188Os is estimated to be 0.1198378±23 (2 SD) based on extrapolation of 186Os/188Os-fertility trends, which is very slightly lower than H-chondrites [≈0.1198398±16 (2 SD); Brandon et al., 2006]. This value is consistent with a PUM Pt/Os of 1.7±0.2, similar to average Pt/Os ratios of fertile continental peridotites. The inferred PUM Pt/Os is slightly lower than but within error of Pt/Os values measured in several classes of chondrites [Carbonaceous ≈1.8±0.2, Ordinary ≈1.9±0.1, and Enstatite ≈1.9±0.1 (Brandon et al., 2006)] indicating that PUM Pt/Os is broadly chondritic. In contrast, estimates for PUM Ru/Ir and Pd/Ir (cf. Becker et al., 2006) are suprachondritic. The addition of a chondritic late veneer alone cannot create a combination of chondritic and suprachondritic HSE ratios for the PUM. Instead, minor core segregation occurring concurrently with the addition of a late veneer may explain the observed mantle HSE abundances and ratios.Combined 186Os/188Os-187Os/188Os isotopic and Pt/Os and Re/Os variability in peridotites suggest an average mantle homogenization timescale of ~1.2 Ga. In contrast, combined Hf-Nd isotopic and Lu/Hf and Sm/Nd variability in peridotites indicate much shorter homogenization timescales (<0.4 Ga), potentially reflecting enhanced homogenization by melt-rock interaction to which the Pt-Os and Re-Os systems are relatively immune. The mechanical mixing timescale inferred from Os isotopes is consistent with timescales predicted for whole mantle convection.

Magnetic signature of North-East Greenland, the Morris Jesup Rise, the Yermak Plateau, the central Fram Strait: Constraints for the rift/drift history between Greenland and Svalbard since the Eocene

A compilation of several aeromagnetic surveys across North Greenland and the Fram Strait area is presented. Across North Greenland, the data are related to the known onshore geology. The correlation between the geological structure and the large-scale magnetic anomalies is excellent. Along the North Greenland coast, the data indicate that the Late Mesozoic Kap Washington volcanics might have a larger areal extent than known from onshore geological mapping. The submarine Morris Jesup Rise is characterized by strong positive magnetic anomalies, and the anomalies across the plateau indicate the presence of distinct volcanic centers. This massive magmatism has occurred latest in conjunction with the Oligocene initial stages of plate divergence between Svalbard and North Greenland that led to the development of Fram Strait. Both plateaux, the Morris Jesup Rise and the Yermak Plateau, are interpreted to be of continental origin and, in the initial rift stage of the Eurasia Basin around 56Ma, to have been contiguous with the Lomonosov Ridge. More toward the east in the Fram Strait interpretations of seismic and bathymetric data indicate the presence of an active segment of mid-ocean ridge, the Lena Trough. This approximately 280-km-long segment terminates in the south at the NW–SE striking Spitsbergen Fracture Zone, while in the north it bends to connect with the Gakkel Ridge. A shallow water pathway along the line of the trough might have existed between those times and ~21Ma, above the propagating rift system in the north and, further south, over extending and subsiding continental crust of the Svalbard and North-East Greenland margins. Magnetic anomaly identifications indicate that the Lena Trough has remained an ultraslow spreading system, or possibly even all of the magnetic lineations may be signals of serpentinized exhumed mantle rocks rather than of basaltic crust.

Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle

The Lena Trough is a highly oblique, sparsely magmatic, ultra-slow spreading center located at the smallest distance between North America and Eurasia in the Arctic basin. Competing models suggest that it is either floored by oceanic mantle abyssal peridotites (APs) exposed by lithospheric necking, or by subcontinental mantle exposed in a still juvenile rift. To distinguish between these hypotheses, we have examined mineral major and trace element and whole rock Os-isotope variations in Lena Trough peridotites. Lena Trough peridotites are predominantly LREE-depleted, similar to other AP suites, and have 187Os/188Os ranging from ∼0.118 to 0.130 (Ave.=0.1244). This distribution is nearly identical to that of abyssal peridotites globally. Both the REE patterns and the Os-isotope distribution of the Lena Trough peridotites differ starkly from subcontinental mantle xenoliths sampled at Svalbard adjacent to Lena Trough. This suggests that Lena Trough is a site of oceanic spreading, although mid-ocean ridge volcanism as such has not yet begun.Highly refractory APs from several settings have Os- and Hf-isotope compositions indicating ancient (>1 Ga) melt depletion. Some researchers have proposed that at least some APs do not directly sample the convecting upper mantle source of MORB, but instead sample highly melt-depleted residues either entrained in the convecting mantle or present as a buoyant “slag” floating atop the less-depleted MORB-source mantle. However, ocean island peridotite xenoliths and APs reveal an essentially identical, non-Gaussian distribution of Os-isotopes and also span a similar range in Hf-isotopes. The similar mean and distribution of Os-isotopes between APs and ocean island xenoliths indicate that these two sample types derive from the same heterogeneous mantle reservoir. This similarity is inconsistent with the AP “slag hypothesis” due to the significantly greater depth of origin of ocean island xenoliths with respect to APs.Global correlation between peridotite fertility indices (e.g., whole rock Al2O3, spinel Cr#) and 187Os/188Os in both APs and ocean island xenoliths suggests ancient melting events have played a role in their petrogenesis, with an average age of melt depletion of ∼1.5 Ga. However, APs display a much steeper correlation than ocean island xenoliths, but have a similar y-intercept. The contrasting trends can be explained by recent on-ridge melt extraction superimposed on an ancient melt-depletion trend. The average Os-isotope composition of both APs and ocean island xenoliths is distinctly sub-chondritic, and requires significantly greater average melt depletion of the upper mantle (>10%) than suggested by previous inversions of MORB compositions.

Eocene to Miocene igneous activity in NE Greenland: northward younging of magmatism along the East Greenland margin

Radiometric dating by the 40 Ar– 39 Ar incremental heating method was carried out on lavas, sills, dykes, and a central intrusion from NE Greenland. Eighteen samples gave acceptable crystallization ages. Lavas of both Lower and Upper Plateau Lava Series gave ages in the range 55.5–53.5 Ma and cannot be constrained to better than 56–53 Ma. Sills and dykes from Traill Ø to Shannon, with compositions fairly similar to those of the lavas, gave ages of 55.1–51.3 Ma, contemporaneous with and slightly younger than the lavas. Alkaline lavas on inland nunataks have ages of 53–50 Ma, and the Kap Broer Ruys intrusive centre has an age of 48.7 ± 0.5 Ma. An alkaline sill on Hvalrosø is much younger at 20.3 ± 0.1 Ma. There are no pre-breakup lavas onshore NE Greenland. We surmise that the hot mantle of the Iceland plume arrived and melted extensively beneath the northern basins only at the time of breakup around 55 Ma. Post-breakup intrusive events in NE Greenland coincided with plate-tectonic events such as reorganization, uplift and opening in the north. The Hvalrosø sill represents a local small melting event that may be related to coeval opening of the Lena Trough. Supplementary materials: Details of the source data, results, and the compositions and locations of the dated samples, are available at www.geolsoc.org.uk/SUP18738 .

High-pressure Reactive Melt Stagnation Recorded in Abyssal Pyroxenites from the Ultraslow-spreading Lena Trough, Arctic Ocean

Pyroxenites are an essential component in petrological and geochemical models for melt formation at mid-ocean ridges and ocean islands. Despite their rarity, their origin has been widely discussed and various processes have been invoked for their formation. Here, we present a detailed study of the microtextures and major, minor and trace element compositions of relatively fresh pyroxenites and associated harzburgites from the ultraslow-spreading Lena Trough, Arctic Ocean. Microtextural and geochemical characteristics suggest an origin by magmatic assimilation–fractional crystallization with a high ratio of mass crystallized to mass assimilated. The major element compositions of pyroxenes suggest that this process occurred at high pressures (>0·7 GPa), although interstitial plagioclase in two of the pyroxenites indicates that melt–rock reaction continued at lower pressures. The parental melt to the pyroxenites was most probably depleted mid-ocean ridge basalt similar to basalts from the North Lena Trough and westernmost Gakkel Ridge; basalts from the Central Lena Trough cannot have functioned as parental melts. The melt was generated close to the garnet–spinel facies transition by variable degrees of partial melting and reacted with the local refractory harzburgite. Pyroxenites from this study provide further evidence, together with plagioclase-bearing and vein-bearing peridotites, for significant melt stagnation below the Lena Trough that occurred over a range of depths, either continuously or stepwise. Comparison with abyssal pyroxenites reveals common characteristics, suggesting that, consistent with results of high-pressure crystallization experiments, they mark the onset of (reactive) crystallization of melts passing through the deeper parts of the mid-ocean ridge plumbing system.

A model study of the first ventilated regime of the Arctic Ocean during the early Miocene

The tectonic opening of Fram Strait during the Neogene was a significant geological event that transferred the Arctic Ocean from a poorly ventilated enclosed basin, with weak exchange with the North Atlantic, to a fully ventilated ‘‘ocean stage’’. Previous tectonic and physical oceanographic analyses suggest that the early Miocene Fram Strait was likely several times narrower and less than half as deep as the present-day 400 km wide and 2550 m deep strait. Here we use an ocean general circulation model with a passive age tracer included to further address the effect of the Fram Strait opening on the early Miocene Arctic Ocean circulation. The model tracer age exhibits strong spatial gradient between the two major Arctic Ocean deep basins: the Eurasian and Amerasian basins. There is a two-layer stratification and the exchange flow through Fram Strait shows a bi-layer structure with a low salinity outflow from the Arctic confined to a relatively thin upper layer and a saline inflow from the North Atlantic below. Our study suggests that although Fram Strait was significantly narrower and shallower during early Miocene, and the ventilation mechanism quite different in our model, the estimated ventilation rates are comparable to the chemical tracer estimates in the present-day Arctic Ocean. Since we achieved ventilation of the Arctic Ocean with a prescribed Fram Strait width of 100 km and sill depth of 1000 m, ventilation may have preceded the timing of a full ocean depth connection between the Arctic Ocean and North Atlantic established through seafloor spreading and the development of the Lena Trough. Keywords: Ocean modelling; Miocene Arctic Ocean; palaeoceanography; ocean ventilation; age tracer (Published: 30 July 2012) Citation: Polar Research 2012, 31 , 10859, http://dx.doi.org/10.3402/polar.v31i0.10859

Oblique nonvolcanic seafloor spreading in Lena Trough, Arctic Ocean

Passive rifting and the early non-volcanic formation of ocean basins are fundamental aspects of the plate tectonic cycle. Cenozoic plate margins where this has occurred are rare. Here we present new observations from Lena Trough in the Arctic Ocean that bear on the early phase of oceanic spreading in such rifts. Lena Trough is an oblique seafloor rift system bounding the North American and Eurasian plates, and connecting neighboring Gakkel Ridge with the rest of the global mid-ocean ridge system. Mapping and sampling show widespread mantle outcrop along two parallel basement ridges bounded by steeply dipping normal faults. Volcanism is limited to the intersection with Gakkel Ridge and to minor eruption of strongly potassic alkali basalts in a single location. Non-eruptive magmatism is shown by an increase in plagioclase- and vein-bearing lithologies over residual peridotite in the center of Lena Trough. Normal mid-ocean ridge stairstep geometry and obvious low-angle detachments as seen at other ridges are absent. Lena Trough thus is an example of a young nonvolcanic continental rift that is just now beginning the transition to oblique nonvolcanic seafloor spreading. This style of oblique rifting, without the formation of striated large-scale low-angle detachments appears to be a major mode of crust formation on ultraslow spreading ridges. The sharp transition from the continental margins on either side to nonvolcanic rifting, with mantle slab exhumation in the rift may provide a model for the early evolution of oblique continental rifts, such as the Cote d'Ivoire/NE Brazil conjugate margins.

Geochemical Composition of K-rich Lavas from the Lena Trough (Arctic Ocean)

Geochemical Composition of K-rich Lavas from the Lena Trough (Arctic Ocean)

Seismicity and active tectonic processes in the ultra-slow spreading Lena Trough, Arctic Ocean

With its remote location in the ice-covered Fram Strait, Lena Trough is a poorly known segment of the global mid-ocean ridge system. It is a prominent member of the ultra-slow spreading mid-ocean ridges but its spreading mechanisms are not well understood. We relocalized teleseismically recorded earthquakes from the past five decades to identify tectonic processes in Lena Trough and the adjacent Spitsbergen Fracture Zone (FZ). During two cruises with RV Polarstern in 2008 and 2009 we deployed seismic arrays on ice floes to record the local seismicity of Lena Trough. We could identify and localize microseismic events which we assume to be present in the entire rift valley. In contrast, our relocalization of teleseismically recorded earthquakes shows an asymmetric epicentre distribution along Lena Trough with earthquakes occurring predominately along the western valley flanks of Lena Trough. In 2009 February/March, several high-magnitude earthquakes peaking in an Mb 6.6 event occurred in an outside-corner setting of the Spitsbergen FZ. This is the strongest earthquake which has ever been recorded in Fram Strait and its location at the outside-corner high of the ultra-slow spreading ridge is exceptional. Comparing the seismicity with the magnetic anomalies and high-resolution multibeam bathymetry, we divide Lena Trough in a symmetrically spreading northern part and an asymmetrically spreading southern part south of the South Lena FZ. We propose that a complex interaction between the former De Geer Megashear zone, which separated Greenland from Svalbard starting at Late Mesozoic/Early Cenozoic times, and the developing rift in the southern Lena Trough resulted an increasing eastward dislocation towards the Spitsbergen FZ between older spreading axes and the recent active spreading axis which we believe to be located west of the bathymetric rift valley flanks in a wide extensional plain.

Rare gases in lavas from the ultraslow spreading Lena Trough, Arctic Ocean

Mid‐ocean ridge basalts (MORB) from the Arctic Ocean have been much less studied than those from the Indian, Atlantic, and Pacific due to the difficulty of access related to ice cover. In 2001 and 2004 the Arctic ridges (Gakkel Ridge and Lena Trough) were intensively sampled. In this study we present the first helium, neon, and argon concentrations and isotopic ratios in a suite of samples from the ultraslow spreading Lena Trough (∼0.75 cm/yr effective full rate). Central Lena Trough (CLT) lavas display 4He/3He between 89,710 and 97,530 (R/Ra between 7.4 and 8.1), similar to the mean MORB ratio of 90,000 ± 10,000 (R/Ra = 8 ± 1). In a three neon isotope diagram, the samples fall on the MORB line, without showing any excess of nucleogenic 21Ne. The 40Ar/36Ar ratios vary from 349 to 6964. CLT samples have a typical MORB He and Ne isotopic composition. Rare gases do not indicate any mantle heterogeneities or contribution of subcontinental lithospheric mantle, although this has been suggested previously on the basis of the Sr‐Nd and Pb isotopic systems. Based on noble gas systematics, a DUPAL‐like anomaly is not observed in the Arctic Ocean. We propose two possible models which reconcile the rare gases with these previous studies. The first is that the Lena Trough mantle has a marble cake structure with small‐scale heterogeneities (&lt;1 km), allowing rapid diffusion and homogenization of rare gases compared to elements such as Sr, Nd, and Pb. The second model proposes that the recycled component identified by other isotopic systems was fully degassed at a recent date. It would therefore have a negligible mass budget of rare gases compared to other isotopic systems. This would suggest that the mantle enrichment beneath Lena Trough was generated by rift‐forming processes and not by recycling.

Tectonic structure and geodynamics of the divide between the Atlantic and Arctic oceans

Tectonic analysis of the divide between the Atlantic and Arctic oceans, where Greenland and Eurasia are located close to each other, is accompanied by description of the main structural units of this region, including the Lena Trough, continental Yermak Block (Plateau), Molloy and Spitsbergen fracture zones, and Molloy interfault tectonic complex. The tectonic features of these structural units allow us to combine them into a single domain of the interoceanic tectonic divide, which separates the spreading zones of the North Atlantic and the Gakkel Ridge of the Arctic Ocean. This dividing domain is compared with other interoceanic divides of the Earth, the classification of which is proposed in my previous publications. Their common feature is the development of fault systems particularly pronounced in the Australian-Antarctic and African-Antarctic oceanic domains. The morphology, crustal structure, and magmatism of the considered region show that it is currently undergoing rifting, which probably predates oceanic spreading.

Opening of the Fram Strait gateway: A review of plate tectonic constraints

We have revised the regional crustal structure, oceanic age distribution, and conjugate margin segmentation in and around the Lena Trough, the oceanic part of the Fram Strait between the Norwegian–Greenland Sea and the Eurasia Basin (Arctic Ocean). The Lena Trough started to open after Eurasia–Greenland relative plate motions changed from right-lateral shear to oblique divergence at Chron 13 times (33.3 Ma; earliest Oligocene). A new Bouguer gravity map, supported by existing seismic data and aeromagnetic profiles, has been applied to interpret the continent–ocean transition and the influence of Eocene shear structures on the timing of breakup and initial seafloor spreading. Assuming that the onset of deep-water exchange depended on the formation of a narrow, oceanic corridor, the gateway formed during early Miocene times (20–15 Ma). However, if the initial Lena Trough was blocked by terrigenous sediments or was insufficiently subsided to allow for deep-water circulation, the gateway probably formed with the first well developed magnetic seafloor spreading anomaly around Chron 5 times (9.8 Ma; Late Miocene). Paleoceanographic changes at ODP Site 909 (northern Hovgård Ridge) are consistent with both hypotheses of gateway formation. We cannot rule out that a minor gateway formed across stretched continental crust prior to the onset of seafloor spreading in the Lena Trough. The gravity, seismic and magnetic observations question the prevailing hypotheses on the Yermak Plateau and the Morris Jesup Rise as Eocene oceanic plateaus and the Hovgård Ridge as a microcontinent.

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES

Deep melting and sodic metasomatism underneath the highly oblique-spreading Lena Trough (Arctic Ocean)

Abyssal peridotites collected along the highly oblique-spreading Lena Trough north of Greenland and Spitsbergen have mineral compositions that are similar to residual abyssal peridotites, except for high sodium concentrations in clinopyroxene (cpx). Most samples are lherzolites with light rare earth element (REE)-depleted cpx trace element patterns, but significantly fractionated middle to heavy REE ratios at relatively high heavy REE concentrations. Such characteristics can only be explained by initial melting of a garnet peridotite followed by low degrees of melting in the stability field of spinel peridotite. The residual garnet signature requires either a high potential temperature of the upwelling mantle, or elevated solidus-lowering water contents. The limited spinel field melting suggests a deep cessation of melt extraction, possibly because of the presence of a thick lithospheric cap. This is consistent with the extremely low effective spreading rate and the vicinity to a passive continental margin, which allow conductive cooling to reach deeper levels than commonly estimated for faster mid-ocean ridges. High sodium concentrations in cpx are neither explainable by melt refertilization, nor by a simple diffusion mechanism. The efficient fractionation of sodium from the light REE requires post-melting metasomatism, which is typically restricted to the subcontinental lithosphere. This might imply that the Lena Trough peridotites represent unroofed subcontinental mantle, from which no melt was extracted during the opening of the Lena Trough. It is more likely that sodic metasomatism occurred after partial melting underneath the Lena Trough, and that such an enrichment process is responsible for elevated sodium concentrations in abyssal peridotites elsewhere. Sodium in cpx of residual peridotites can therefore not serve as an indicator of partial melting or melt refertilization.

Hydroacoustic Earthquake Detection in the Arctic Basin with the Spinnaker Array

Eighteen Arctic Basin earthquakes were hydroacoustically detected during 64 days of monitoring with the Spinnaker hydrophone array in the Lincoln Sea (Arctic Ocean). Epicentral estimates were made for 12 of these events. Event ranges were estimated via the arrival time difference between the solid earth P -wave and the hydroacoustic T wave. Azimuths were estimated via plane-wave beamforming to the array elements in the 5–10 Hz band. The 12 events fall on the North American/Eurasian plate boundary at the extreme southern end of the Gakkel Ridge, and along the Spitsbergen Fracture Zone (Lena Trough). One of the events ( m b 5.4, 11 May 1996, Spitsbergen Fracture Zone) was detected by the Global Seismic Network (GSN) with an epicenter that differs from our hydroacoustic estimate by 51 km. Our hydroacoustic detection threshold appears to be m b ∼3–4 within a ∼1100 km radius of the Spinnaker array, confirming that T waves propagate efficiently in the Arctic Basin and that hydroacoustic techniques hold promise for seismic monitoring of the region.