Age and Eruptive Style of Volcanic Rocks Dredged from the Alpha Ridge, Arctic Ocean.

The Alpha–Mendeleev ridge complex is a prominent physiographic and geological feature ofthe Arctic Amerasia Basin. The Alpha and Mendeleev ridges are, respectively, the eastern andwestern components of a continuous seafloor high that is approximately 2000 km long and200–400 km wide. A surge of interest in the tectonic evolution of Arctic submarine featureshas led to a wealth of new geophysical data collected from the Alpha Ridge. Currentinterpretations of its origin vary but there is compelling evidence that the Alpha Ridge mayhave formed as an oceanic plateau during the Late Cretaceous. Geological samples are rarebut most samples recovered indicate a genetic link with the High Arctic Large IgneousProvince (HALIP). In August 2016, Canada’s Extended Continental Margin-United Nations Convention onthe Law of the Sea Program dredged approximately 100 kg of volcanic rocks from the AlphaRidge. The large size and pristine state of the samples enabled the first comprehensive studyof a single eruptive event in the volcanic record of the Alpha Ridge. The dredgesample is a lapilli tuff containing vitric and basaltic clasts. Textural evidence and thecoexistence of juvenile and cognate clasts suggest a phreatomagmatic eruption. Thevitric fragments consist of sideromelane glass with abundant plagioclase microlites.Texturally, these basaltic glass lapilli display a fresh glassy core surrounded byFe- and Ti-rich zones and a palagonite rim. Major and trace element analyses ofglassy cores indicate remarkably uniform, mildly alkaline basaltic compositions. Theplagioclase-bearing glass yielded a 40Ar/39Ar plateau age of 90.40 ± 0.26 Ma (2σ error)which included 89% of 39Ar released. We interpret this result to represent the eruption age ofthe plagioclase microlites and consequently, of the host basaltic glass lapilli inthe tuff. Volatile species analyses by infrared spectroscopy on the fresh basalticglass suggests that the melt was effectively degassed to shallow level. Assumingequilibrium degassing, the homogeneous resulting values of H2Ototal in the range0.1 to 0.19 wt.% (1σ error) indicate subaerial or shallow eruption (surface to 80m). The new 40Ar/39Ar age for the sample is consistent with a 40Ar/39Ar age of 89 ± 1 Maobtained for a sample of tholeiitic basalt dredged from the central part of the Alpha Ridge,and with the range of ages reported for HALIP igneous rocks exposed onshore in theCanadian Arctic Archipelago (130-80 Ma). Our new data provide evidence for localemergence of the Alpha Ridge in the Late Cretaceous. A comparison of the Alpha Ridge andKerguelen Plateau–Broken Ridge Large Igneous Province (LIP) provides new insights on theepisodic nature of LIP magmatism and variations in eruptive style through time. [ABSTRACT FROM AUTHOR]