E-MORB GLASSES FROM THE GAKKEL RIDGE (ARCTIC-OCEAN) AT 87-DEGREES-N -EVIDENCE FOR THE EARTH 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 shard s have been investigated by petrographic, mineralogical, geochemical a nd radiogenic isotope methods. The nearly vesicle-free and aphyric gla ss shards bear only minute microphenocrysts of magnesiochromite and ol ivine (Fo(88-89)). Most glasses are fresh, although some show signs of incipient low-temperature alteration. From their shapes and sizes, th e glass shards most likely formed by spalling of glassy rinds of a nea rby volcanic outcrop. Geochemically, the glasses are relatively unfrac tionated tholeiites with E-MORB trace element compositions. Thus, they are quite similar to the previously investigated ARK IV/3-11-370-5 ba salts from 86 degrees N [1]. The Nd and Sr isotopic ratios of PS 2167- 2 glasses are significantly lower than for ARK IV/3-11-370-5 basalts a nd suggest an isotopically heterogeneous mantle source of Gakkel Ridge MORE between 86 degrees and 87 degrees N. The positive Delta-8/4 Pb v alue (similar to 16) and high Sr-87/Sr-86 ratio (0.70270), found for P S 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 ma ntle. These results argue against the presence of an 'anti-DUPAL anoma ly' in the mantle below the North Pole region and simple models of who le-mantle convection. (C) 1997 Elsevier Science B.V.