Geochemistry

Under the Ridge

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Science  25 Mar 2011:
Vol. 331, Issue 6024, pp. 1495
DOI: 10.1126/science.331.6024.1495-b
CREDIT: MICHAEL PERFIT

Oceanic crust forms along the spine of mid-ocean ridges—massive mountain chains along the seafloor—which churn out lava and push fresh crustal material outward in both directions. Discerning how the hot material delivered from the subsurface to the ridge eventually turns into crust depends on quantifying the cooling rate of fresh lavas. In search of new geochemical clues to constrain heat transfer processes, Schmitt et al. sampled lavas from a segment of the Juan de Fuca ridge off the coast of western North America. From the rock samples collected (such as the dacite shown below), they separated tiny crystallized zircon grains, which serve as an excellent thermometer for tracking the thermal history of the melts. Dates obtained using a variety of radiometric dating techniques translate to rapid cooling rates: over an order of magnitude faster than crust formed along other segments of the same ridge. These rates are too fast to support models by which the crust cools via conduction, instead implicating cooling by permeation of convective hydrothermal fluids through the new crust.

Earth Planet. Sci. Lett. 302, 349 (2011).

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