Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges
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We present new U-238-Th-230-Ra-226-Pb-210, U-235-Pa-231, and Nd, Sr, Hf, and Pb isotope data for the slow-to ultraslowspreading Mohns, Knipovich, and Gakkel Ridges. Combined with previous work, our data from the Arctic Ridges cover the full range of axial depths from the deep northernmost Gakkel Ridge shallowing upwards to the Knipovich, Mohns, and Kolbeinsey Ridges north of Iceland. Age-constrained samples from the Mohns and Knipovich Ridges have (Th-230/U-238) activity ratios ranging from 1.165 to 1.30 and 1.101 to 1.225, respectively. The high Th-230 excesses of Kolbeinsey, Mohns, and Knipovich mid-ocean ridge basalts (MORB) are erupted from ridges producing relatively thin (Mohns, Knipovich) to thick (Kolbeinsey) oceanic crust with evidence for sources ranging from mostly peridotite (Kolbeinsey) to eclogite-rich mantle (Mohns, Knipovich). Age-constrained lavas from 85 degrees E on the Gakkel Ridge, on the other hand, overlie little to no crust and range from small (similar to 5%) 230 Th excesses to small U-238 excesses (similar to 5%). The strong negative correlation between (Th-230/U-238) values vs. axial ridge depth among Arctic ridge basalts is controlled not only by solidus depth influence on U-238-Th-230 disequilibria, but also by variations in mantle source lithology and depth to the base of the lithosphere, which is expected to vary at ultra-slow spreading ridges. Small Pa-231 excesses (65% excess) in age-constrained basalts support the presence of eclogite in the mantle source for this region. Conversely, the ultraslow-spreading Gakkel Ridge basalts are homogeneous, with Sr, Nd, and Hf radiogenic isotopic signatures indicative of a long time-averaged depleted mantle source. The Gakkel samples have minimum (Ra-226/Th-230) ratios ranging from 3.07 to 3.65 +/- 3%, which lie along and extend the global negative correlation between Ra-226 and Th-230 excesses observed in MORB. The new Th-230-Ra-226 data support a model for global MORB production in which deep melts record interaction with shallower materials. This scenario requires either mixing with shallow-derived melts, or melt-rock reaction with shallower rocks in the lithosphere or crust. (C) 2013 Elsevier Ltd. All rights reserved.