Age and Eruptive Style of Volcanic Rocks Dredged from the Alpha Ridge,
\nArctic Ocean

The Alpha–Mendeleev ridge complex is a prominent physiographic and 
\ngeological feature of the Arctic Amerasia Basin. The Alpha and Mendeleev 
\nridges are, respectively, the eastern and western components of a continuous
\nseafloor high that is approximately 2000 km long and 200–400 km wide. 
\nA surge of interest in the tectonic evolution of Arctic submarine features has 
\nled to a wealth of new geophysical data collected from the Alpha Ridge. Current
\ninterpretations of its origin vary but there is compelling evidence that the Alpha Ridge 
\nmay have formed as an oceanic plateau during the Late Cretaceous. Geological 
\nsamples are rare but most samples recovered indicate a genetic link with the High 
\nArctic Large Igneous Province (HALIP).
\nIn August 2016, Canada’s Extended Continental Margin-United Nations Convention 
\non the Law of the Sea Program dredged approximately 100 kg of volcanic rocks from 
\nthe Alpha Ridge. The large size and pristine state of the samples enabled the first 
\ncomprehensive study of a single eruptive event in the volcanic record of the Alpha 
\nRidge. The dredge sample is a lapilli tuff containing vitric and basaltic clasts. Textural 
\nevidence and the coexistence of juvenile and cognate clasts suggest a phreatomagmatic 
\neruption. The vitric fragments consist of sideromelane glass with abundant plagioclase 
\nmicrolites. Texturally, these basaltic glass lapilli display a fresh glassy core surrounded
\nby Fe- and Ti-rich zones and a palagonite rim. Major and trace element analyses of 
\nglassy cores indicate remarkably uniform, mildly alkaline basaltic compositions. The plagioclase-bearing glass yielded a 40Ar/39Ar plateau age of 90.40±0.26 Ma (2σ error) which included 89% of 39 Ar 
\nreleased. We interpret this result to represent the eruption age of the plagioclase microlites 
\nand consequently, of the host basaltic glass lapilli in the tuff. Volatile species analyses by 
\ninfrared spectroscopy on the fresh basaltic glass suggests that the melt was effectively 
\ndegassed to shallow level. Assuming equilibrium degassing, the homogeneous resulting 
\nvalues of H2O total in the range 0.1 to 0.19 wt.% (1σ error) indicate subaerial or shallow 
\neruption (surface to 80 m).
\nThe new 40Ar/39Ar age for the sample is consistent with a 40 Ar/39Ar age of 89±1 Ma 
\nobtained for a sample of tholeiitic basalt dredged from the central part of the 
\nAlpha Ridge, and with the range of ages reported for HALIP igneous rocks exposed 
\nonshore in the Canadian Arctic Archipelago (130-80 Ma). Our new data provide
\nevidence for local emergence of the Alpha Ridge in the Late Cretaceous. A comparison 
\nthe Alpha Ridge and Kerguelen Plateau–Broken Ridge Large Igneous Province (LIP) 
\nprovides new insights on the episodic nature of LIP magmatism and variations in 
\neruptive style through time.

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