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

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• 作者：L.J. Elkins ; K.W.W. Sims ; J. Prytulak ; J. Blichert-Toft ; T. Elliott ; J. Blusztajn ; S. Fretzdorff ; M. Reagan ; K. Haase ; S. Humphris ; J.-G. Schilling
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：15 February, 2014
• 年：2014
• 卷：127
• 期：Complete
• 页码：140-170
• 全文大小：2184 K

文摘

We present new ²³⁸U-²³⁰Th-²²⁶Ra-²¹⁰Pb, ²³⁵U-²³¹Pa, 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 (²³⁰Th/²³⁸U) activity ratios ranging from 1.165 to 1.30 and 1.101 to 1.225, respectively. The high ²³⁰Th 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掳E on the Gakkel Ridge, on the other hand, overlie little to no crust and range from small (鈭?%) ²³⁰Th excesses to small ²³⁸U excesses (鈭?%). The strong negative correlation between (²³⁰Th/²³⁸U) values vs. axial ridge depth among Arctic ridge basalts is controlled not only by solidus depth influence on ²³⁸U-²³⁰Th 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 ²³¹Pa 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 (²²⁶Ra/²³⁰Th) ratios ranging from 3.07 to 3.65 卤 3%, which lie along and extend the global negative correlation between ²²⁶Ra and ²³⁰Th excesses observed in MORB. The new ²³⁰Th-²²⁶Ra 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.