GEOCHEMISTRY AND GEOCHRONOLOGY OF THE FIRST INTRAPLATE LAVAS RECOVERED FROM THE ARCTIC OCEAN

An August-September 2008 Arctic Ocean cruise of icebreaker USCGC HEALY recovered some of the first known submarine basaltic samples from the Amerasian Basin at 3.2-3.5 km depth. One dredge was taken on a seamount on the border between the Mendeleev Abyssal Plain and the Canada Basin (79°44’31”N; 155°06’43”W), and another one on the northernmost edge of the Northwind Ridge of the Chukchi Borderland (78°32’02”N; 156°42’00”W). Two chemically distinct suites of rocks were recovered from the seamount. The first one (SiO₂ = 43.7%; TiO₂ = 2.79%; Na₂O+K₂O = 3.4%, MgO = 9.1%; Mg# = 51) is characterized as a low-silica subalkaline basalt. The second suite (SiO₂ = 43.9-46.0%; TiO₂ = 3.98-4.37%; Na₂O+K₂O = 4.4-4.9%, MgO = 4.5 – 5.0%; Mg# = 34 – 37) is identified as alkali basalt. The Chukchi Borderland lavas (SiO₂ = 47.6 wt%; TiO₂ = 1.42 wt%; Al₂O₃ = 16.9 wt%; Na₂O+K₂O = 5.0 wt%; and MgO = 7.59 wt%, Mg# = 53.8) are transitional between alkaline and subalkaline basalt and appear to have a different origin compared to the seamount lavas. The seamount volcanic rocks are similar to late Mesozoic basaltic rocks of Ellesmere Island and Franz Josef Land, all part of the High Arctic Large Igneous Province (HALIP). Trace element composition of the seamount rocks (enrichment in most incompatible LILE relative to primitive mantle; Ce_N/Yb_N = 1.7-2.8, Gd_N/Lu_N = 1.0-1.6; pronounced trough at Sr with Sr_N/Nd_N = 0.28-0.45) are very similar to those of other continental flood basalts or CFBs. However, the elemental ratios of highly incompatible elements like Th/Ce (0.05) and the absence of negative Nb and Ta anomalies show that crustal assimilation was not a significant process during the evolution of these basalts. None of the samples display chemical characteristics typical for MORB. All seamount basalts are depleted in terms of Sr, Nd and Hf isotope ratios relative to the Bulk Silicate Earth or BSE (¹⁴³Nd/¹⁴⁴Nd = 0.512669-0.512919; ⁸⁷Sr/⁸⁶Sr = 0.703894-0.704764; ¹⁷⁶Hf/¹⁷⁷Hf = 0.283128-0.283191) with isotopic ratios lying between MORB and BSE. Occurrence of this volcanism away from any obvious spreading centers, geochemical characteristics that are different from those of lavas from the Gakkel Ridge, and lack of evidence for rifting, suggest that they originated from deep-seated plume sources, the first to be recognized in the Arctic Ocean.