Rapid cooling rates at an active mid-ocean ridge from zircon thermochronology

Oceanic spreading ridges are Earth's most productive crust generating environment, but mechanisms and rates of crustal accretion and heat loss are debated. Existing observations on cooling rates are ambiguous regarding the prevalence of conductive vs. convective cooling of lower oceanic crust. Here, we report the discovery and dating of zircon in mid-ocean ridge dacite lavas that constrain magmatic differentiation and cooling rates at an active spreading center. Dacitic lavas erupted on the southern Cleft segment of the Juan de Fuca ridge, an intermediate-rate spreading center, near the intersection with the Blanco transform fault. Their U-Th zircon crystallization ages (29.3(-4.6)(+4.8)ka; 1 sigma standard error se.) overlap with the (U-Th)/He zircon eruption age (32.7 +/- 1.6 ka) within uncertainty. Based on similar U-238-Th-230 disequilibria between southern Cleft dacite glass separates and young mid-ocean ridge basalt (MORB) erupted nearby, differentiation must have occurred rapidly, within similar to 10-20 ka at most. Ti-in-zircon thermometry indicates crystallization at 850-900 degrees C and pressures > 70-150 MPa are calculated from H2O solubility models. These time-temperature constraints translate into a magma cooling rate of similar to 2 x 10(-2) degrees C/a. This rate is at least one order-of-magnitude faster than those calculated for zircon-bearing plutonic rocks from slow spreading ridges. Such short intervals for differentiation and cooling can only be resolved through uranium-series (U-238-Th-230) decay in young lavas, and are best explained by dissipating heat convectively at high crustal permeability. (c) 2010 Elsevier B.V. All rights reserved.