E-MORB glasses from the Gakkel Ridge (Arctic Ocean) at 87°N: evidence for the Earth's most northerly volcanic activity

During the ARCTIC '91 expedition aboard RV Polarstern (ARK VIII/3) to the Central Arctic Ocean, a box corer sample on the Gakkel Ridge at 87 degrees N and 60 degrees E yielded a layer of sand-sized, dark brown volcanic glass shards at the surface of the sediment core. These shards have been investigated by petrographic, mineralogical, geochemical and radiogenic isotope methods. The nearly vesicle-free and aphyric glass shards bear only minute microphenocrysts of magnesiochromite and olivine (Fo(88-89)). Most glasses are fresh, although some show signs of incipient low-temperature alteration. From their shapes and sizes, the glass shards most likely formed by spalling of glassy rinds of a nearby volcanic outcrop. Geochemically, the glasses are relatively unfractionated tholeiites with E-MORB trace element compositions. Thus, they are quite similar to the previously investigated ARK IV/3-11-370-5 basalts from 86 degrees N. The Nd and Sr isotopic ratios of PS 2167-2 glasses are significantly lower than for ARK IV/3-11-370-5 basalts and suggest an isotopically heterogeneous mantle source of Gakkel Ridge MORE between 86 degrees and 87 degrees N. The positive Delta-8/4 Pb value (similar to 16) and high Sr-87/Sr-86 ratio (0.70270), found for PS 2167-2 glasses are similar to that of ARK IV/3-11-370-5 basalts and show the influence of the DUPAL isotopic anomaly in the high Arctic mantle. These results argue against the presence of an 'anti-DUPAL anomaly' in the mantle below the North Pole region and simple models of whole-mantle convection

Active submarine volcanism on the Society hotspot swell (west pacific): A geochemical study

The present work deals with the petrography and geochemistry of lavas dredged from five active submarine volcanoes (named Mehetia, Moua Pihaa, Rocard, Teahitia, and Cyana) from the southeast end of the Society Islands hotspot trace. Most samples are basic and alkaline, ranging from 16 to 5 wt % MgO, with about 5% normative nepheline. Fractionation modelling based on major and minor compatible element variations suggests that olivine and minor clinopyroxene were the major fractionating phases and implies a maximum range of fractionation of 30–35%. Rocard and Cyana have yielded more evolved, trachy-phonolitic, glassy samples. These evolved samples are thought to be derived by removal of 70% cumulate from the basalts. Both basaltic and phonolitic samples are incompatible-element enriched, with La/YbN ≈ 15 in most of the basalts. The trachy-phonolite patterns show middle rare earth element (REE) depletion and negative Eu anomalies. The Moua Pihaa basalts have flatter patterns than the other basalts (La/YbN = 7.5–12.4). All samples, with the exception of a sample from Moua Pihaa which has elevated 206Pb/204Pb, fall on linear Sr-Nd-Pb isotopic arrays, suggesting two end-member mixing. The most depleted end-member is shown to be a pristine ocean island basalt magma with no detectable contribution from a depleted, mid-ocean ridge basalt (MORB) upper mantle. The flatter REE patterns and higher 206Pb/204Pb of the Moua Pihaa sample are taken to indicate a more depleted, U-enriched (high μ) component in its source. This component may be altered oceanic crust. The Sr isotopic variations in the samples excluding Moua Pihaa correlate positively with Rb/Nb, Pb/Ce, and SiO2 variations, indicating a component of mantle enriched by injection of material from a subducted oceanic slab. Correlation of 207Pb/204Pb with 87Sr/86Sr suggests that the subducted material is geochemically old. Mapping the geochemical variations shows that the contribution to the lavas from the subduction component is greater over the north of the hotspot than in the south. The absence of a MORB component in the Society magmatism, the small volumes of the Polynesian hotspot volcanoes, and the lack of more intense volcanic activity near the center of the Pacific Supers well, all lead us to conclude that the latter is unlikely to be caused by a large convective plume. The Superswell is more probably located above a region in the asthenospheric mantle which, due to its higher content of recycled continental debris, is anomalously hot

Current geoscientific knowledge on the High Arctic submarine Alpha-Mendeleev complex

Today Alpha-Mendeleev Ridge is the largest single submarine feature in the Arctic Ocean, which geological origin is still unknown. A better understanding of the evolution of the ridge complex in relation to the opening of the Canada Basin would have profound consequences for Arctic geodynamic models. Currently models in which Alpha Ridge represents a former spreading centre or "hot spot" trail are favoured. In this contribution the state of knowledge will be reviewed.The ridge was discovered by the US ice station Alpha during its drift in the years 1957-1958, which acquired the first information on the ridges topography and sedimentary thickness. The largest single-channel seismic data set was gathered during the drift of the US ice station T-3 from February 1967 to June 1970. The width of Alpha Ridge ranges from 250 to 800 km. In bathymetric cross sections it is roughly symmetrical with greatest elevation at the centre. The existing single channel seismic reflection lines acquired from ice stations Alpha, T-3 and CESAR and ship based seismic experiments show that Alpha-Mendeleev Ridge is mainly covered by a sedimentary sequence, which can reach up to 1000 m in thickness. Along most of the profiles the sediments lie conformably on the basement. Deep seismic experiments of Canadian and Russian researchers indicate that the crust beneath the ridge has a thickness well above 30 km, and high seismic velocities above 7.0 km/s are present at lower crustal levels.The most important and complete geophysical data sets in that area are aeromagnetic and aerogravity data acquired by US and Russian researchers. The magnetic data across the Alpha Ridge indicate the presence of mostly irregular magnetic anomalies up to 2000 nT. No clear evidence of magnetic seafloor spreading anomalies has been obtained. Based on the existing geophysical data various researchers suggested that the ridge must have been formed during the Cretaceous positive polarity chron from 124 to 83 Ma, if the irregular magnetic anomalies are due to oceanic basalts. At three locations Cretaceous and Early Cenozoic sediments were recovered from western part of Alpha-Mendeleev Ridge. Two volcanic rock samples were dredged from the Alpha-Mendeleev Ridge. One of them could be dated to 83 Ma. So far, all existing geoscientific data support a formation of this complex in Cretaceous times. However, details on the responsible processes and the plate movements during this period are rather hypothetical

Chem. Geol.

The ultraslow spreading Gakk-el Ridge represents one of the most extreme spreading environments on the Earth. Full spreading rates there of 0.6-1.3 cm/year and Na-8.0 in basalts of 3.3 imply an extremely low degree of mantle partial melting. For this reason, the complementary degree of melting registered by abyssal peridotite melting residues is highly interesting. In a single sample of serpentinized peridotite from Gakkel Ridge, we found spinels which, though locally altered, have otherwise unzoned and thus primary compositions in the cores of the grains. These reflect a somewhat higher degree of melting of the uppermost oceanic mantle than indicated by basalt compositions. Cr/(Cr + Al) ratios of these grains lie between 0.23 and 0.24, which is significantly higher than spinels from peridotites collected along the faster spreading Mid-Atlantic and Southwest Indian Ridges. Crustal thickness at Gakkel Ridge can be calculated from the peridotite spinel compositions, and is thicker than the crustal thickness of less than 4 km estimated from gravity data, or predicted from global correlations between spreading rate and seismically determined crustal thickness. The reason for this unexpected result may be local heterogeneity due to enhanced melt focussing at an ultraslow spreading ridge. (C) 2002 Elsevier Science B.V. All rights reserved

Mantle melting beneath Gakkel Ridge (Arctic Ocean): abyssal peridotite spinel compositions

The ultraslow spreading Gakk-el Ridge represents one of the most extreme spreading environments on the Earth. Full spreading rates there of 0.6-1.3 cm/year and Na-8.0 in basalts of 3.3 imply an extremely low degree of mantle partial melting. For this reason, the complementary degree of melting registered by abyssal peridotite melting residues is highly interesting. In a single sample of serpentinized peridotite from Gakkel Ridge, we found spinels which, though locally altered, have otherwise unzoned and thus primary compositions in the cores of the grains. These reflect a somewhat higher degree of melting of the uppermost oceanic mantle than indicated by basalt compositions. Cr/(Cr + Al) ratios of these grains lie between 0.23 and 0.24, which is significantly higher than spinels from peridotites collected along the faster spreading Mid-Atlantic and Southwest Indian Ridges. Crustal thickness at Gakkel Ridge can be calculated from the peridotite spinel compositions, and is thicker than the crustal thickness of less than 4 km estimated from gravity data, or predicted from global correlations between spreading rate and seismically determined crustal thickness. The reason for this unexpected result may be local heterogeneity due to enhanced melt focussing at an ultraslow spreading ridge. (C) 2002 Elsevier Science B.V. All rights reserved