E-MORB GLASSES FROM THE GAKKEL RIDGE (ARCTIC OCEAN) AT 87ON: EVIDENCE FOR THE EARTHS MOST NORTHERLY VOLCANIC ACTIVITY

Abstract

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°N and 60°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 (Fo88-89). 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°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 MORB between 86° and 87°N. The positive Δ-8/4 Pb value (~16) and high 87Sr/86Sr 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.