First Recovery of Submarine Basalts from the Chukchi Borderland and Alpha/Mendeleev Ridge, Arctic Ocean

In addition to multibeam bathymetric mapping of the Amerasia Basin in the high Arctic Ocean, the August- September 2008 cruise of USCGC Icebreaker HEALY (HLY0805) conducted a total of seven dredging profiles along the southern sectors of the Alpha/Mendeleev Ridge and in the northernmost region of Northwind Ridge of Chukchi Borderland. Five of the seven dredges were recovered on relatively gentle slopes (30-40°) and yielded mostly mud with a small number of fragments of sedimentary rocks and ice rafted debris (IRD), which indicates either rapid sedimentation rates on the bathymetrically high features sampled or lack of recently active volcanism on these features. Two dredges taken from steep escarpments with slopes (\u3e 55°) at \u3e3.5 km depth recovered some of the first known submarine basaltic samples from the Arctic Ocean floor away from the Gakkel Ridge. Ragged, freshly exposed edges indicate that these samples were broken from outcrop rather than being IRD. In some cases (e.g., a rise on the ocean floor between the Alpha/Mendeleev Ridge and Northwind Ridge) the samples have well-preserved pillow�basalt structures with fresh glassy rims up to 4 cm thick. Inward from the rims, the rocks are dark-grey lavas, some with visible plagioclase laths and rare phenocrysts up to 0.5 mm in length, some with visible signs of alteration such as local occurrence of chlorite. Surfaces that were exposed to water can be covered with a thin black film of Mn oxides. Occurrence of this volcanism away from any obvious spreading centers compels us to hypothesize that forthcoming geochemical analyses are likely to identify these rocks as the first Arctic Ocean floor samples to exhibit ocean island basalt compositions. The dredge taken from the northern slope of Northwind Ridge, along slopes as steep as \u3e 45°, recovered a variety of rock types including sedimentary and basaltic rocks. Some of the basalts have columnar jointing (the size of the columns is only up to 5-6 cm across, suggesting fairly thin lava flows). These samples have a strongly altered glassy matrix with abundant plagioclase phenocrysts (up to 3 mm in length). Thin chilled basaltic crust on the lava surfaces displays pahoehoe structures, suggesting subaerial eruptions. Presence of subaerial basalts in this area supports the notion that the Chukchi Borderland has a continental origin. It is possible that further mapping using multibeam bathymetric methods and geochemical studies will show the high Arctic to possess a large igneous province built on both continental and oceanic crust