Melt generation beneath Arctic Ridges:Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Elkins, L. J., Sims, K. W W, Prytulak, J., Blichert-Toft, J., Elliott, T., Blusztajn, J., Fretzdorff, S., Reagan, M., Haase, K., Humphris, S., Schilling, J. G.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: Arctic; Gakkel Ridge; Kolbeinsey; Iceland
• Online Access: https://hdl.handle.net/1983/cb4146f5-8de1-4718-80b5-ca422665973c; https://research-information.bris.ac.uk/en/publications/cb4146f5-8de1-4718-80b5-ca422665973c; https://doi.org/10.1016/j.gca.2013.11.031; http://www.scopus.com/inward/record.url?scp=84891704409&partnerID=8YFLogxK

We present new 238 U- 230 Th- 226 Ra- 210 Pb, 235 U- 231 Pa, and Nd, Sr, Hf, and Pb isotope data for the slow- to ultraslow-spreading 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 ( 230 Th/ 238 U) activity ratios ranging from 1.165 to 1.30 and 1.101 to 1.225, respectively. The high 230 Th 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°E on the Gakkel Ridge, on the other hand, overlie little to no crust and range from small (~5%) 230 Th excesses to small 238 U excesses (~5%). The strong negative correlation between ( 230 Th/ 238 U) values vs. axial ridge depth among Arctic ridge basalts is controlled not only by solidus depth influence on 238 U- 230 Th 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 231 Pa 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 ( 226 Ra/ 230 Th) ratios ranging from 3.07 to 3.65±3%, which lie along and extend the global negative correlation between 226 Ra and 230 Th excesses observed in MORB. The new 230 Th- 226 Ra data support a model for global MORB production in which deep melts record interaction with shallower materials. This scenario ...