ALBERT

Description: General Subject of research: Detailed study of the shallow water hydrothermal system around Kolbeinsey and Grimsey islands

Description: 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.

Description: Volcanic pebbles in gravels from Sites 977 and 978 in the Alboran Sea (western Mediterranean) were dated (using the 40Ar/ 39Ar single-crystal laser technique) and analyzed for their major- and trace-element compositions (determined by X-ray fluorescence and inductively coupled plasma-mass spectrometry). The samples range from basalts to rhyolites, and belong to the tholeiitic, calc-alkaline, and shoshonitic series. Single-crystal and step-heating laser 40Ar/39Ar analyses of plagioclase, sanidine, biotite, and amphibole phenocrysts from basaltic to rhyolitic samples indicate that eruptions occurred between 6.1 and 12.1 Ma. The age data conform to the stratigraphy and agree with microfossil ages, when available. The major-element and compatible trace-element data of samples with H2O 〈 4 wt% show systematic variations, consistent with fractionation of the observed phenocryst phases (plagioclase, olivine, clinopyroxene, magnetite, hornblende, quartz, and biotite). The incompatible-element patterns formed by normalizing to primitive mantle for all samples show spiked patterns with peaks generally at mobile elements and troughs at immobile elements, in particular Nb and Ta. The calc-alkaline affinities and the incompatible-element systematics are characteristic of subduction zone volcanism, which indicates that subduction occurred beneath the eastern Alboran from 6 to at least 12 Ma. We propose that the change in chemistry from calc-alkaline and potassic to sodic compositions between 5– 6 Ma reflects detachment of the subducting slab. Uplift of the Strait of Gibraltar, associated with this detachment, could have caused the Messinian Salinity Crises.

Description: The Easter microplate-Crough Seamount region located between 25° S–116° W and 25° S–122° W consists of a chain of seamounts forming isolated volcanoes and elongated (100–200 km in length) en echelon volcanic ridges oriented obliquely NE (N 065°), to the present day general spreading direction (N 100°) of the Pacific-Nazca plates. The extension of this seamount chain into the southwestern edge of the Easter microplate near 26°30′ S–115° W was surveyed and sampled. The southern boundary including the Orongo fracture zone and other shallow ridges (〈 2000 m high) bounding the Southwest Rift of the microplate consists of fault scarps where pillow lava, dolerite, and metabasalts are exposed. The degree of rock alternation inferred from palagonitization of glassy margins suggests that the volcanic ridges are as old as the shallow ridges bounding the Southwest Rift of the microplate. The volcanics found on the various structures west of the microplate consist of depleted (K/Ti 〈 0.1), transitional (K/Ti = 0.11−0.25) and enriched (K/Ti 〉 0.25) MORBs which are similar in composition to other more recent basalts from the Southwest and East Rifts spreading axes of the Easter microplate. Incompatible element ratios normalized to chondrite values [(Ce/Yb)N = 1−2.5}, {(La/Sm)N = 0.4−1.2} and {(Zr/Y)N = 0.7−2.5} of the basalts are also similar to present day volcanism found in the Easter microplate. The volcanics from the Easter microplate-Crough region are unrelated to other known South Pacific intraplate magmatism (i.e. Society, Pitcairn, and Salas y Gomez Islands). Instead their range in incompatible element ratios is comparable to the submarine basalts from the recently investigated Ahu and Umu volcanic field (Easter hotspot) (Scientific Party SO80, 1993) and centered at about 80 km west of Easter Island. The oblique ridges and their associated seamounts are likely to represent ancient leaky transform faults created during the initial stage of the Easter microplate formation (≈ 5 Ma). It appears that volcanic activity on seamounts overlying the oblique volcanic ridges has continued during their westward drift from the microplate as shown by the presence of relatively fresh lava observed on one of these structures, namely the first Oblique Volcanic Ridge near 25° S–118° W at about 160 km west of the Easter microplate West Rift. Based on a reconstruction of the Easter microplate, it is suggested that the Crough seamount (〈 800 m depth) was formed by earlier (7–10 Ma) hotspot magmatic activity which also created Easter Island.