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Abstract
Plate tectonics and mantle dynamics necessitate mantle recycling throughout Earth’s history, yet direct geochemical evidence for mantle reprocessing remains elusive. Here we present evidence of recycled supra-subduction zone mantle wedge peridotite dredged from the Mid-Atlantic Ridge near 16°30′N. Peridotite trace-element characteristics are inconsistent with fractional anhydrous melting typically associated with a mid-ocean ridge setting. Instead, the samples are best explained by hydrous flux melting which changed the melting reactions such that clinopyroxene was not exhausted at high degrees of melting and was retained in the residuum. Based on along-axis ridge depth variations, this buoyant refractory arc mantle is likely compensated at depth by denser, likely garnet-rich, lithologies within the mantle column. Our results suggest that highly refractory arc mantle relicts are entrained in the upper mantle and may constitute >60% of the upper mantle by volume. These highly refractory mantle domains, which contribute little to mantle melting, are under-represented in compilations of mantle composition that rely on inverted basalt compositions alone.
Highlights
Plate tectonics and mantle dynamics necessitate mantle recycling throughout Earth’s history, yet direct geochemical evidence for mantle reprocessing remains elusive
Nd and Hf isotope systematics in basalts and olivine hosted melt inclusions have provided additional evidence for ancient, ultra-depleted mantle domains beneath the Azores Rise[14,15]. These results show that our understanding of mantle composition and how it relates to basalt chemistry at slow and ultra-slow spreading mid-ocean ridges needs major modification, where ultramafic samples appear to be uncorrelated to local basalt compositions[4,9]
Samples are ultra-depleted in heavy rare earth element (HREE), middle rare earth elements, and high field strength elements (HFSE), whereas light rare earth elements (LREE) define a shallower slope than one would expect from fractional melting models based on lanthanide compatibility (Supplementary Fig. 2)
Summary
Plate tectonics and mantle dynamics necessitate mantle recycling throughout Earth’s history, yet direct geochemical evidence for mantle reprocessing remains elusive. Nd and Hf isotope systematics in basalts and olivine hosted melt inclusions have provided additional evidence for ancient, ultra-depleted mantle domains beneath the Azores Rise[14,15] These results show that our understanding of mantle composition and how it relates to basalt chemistry at slow and ultra-slow spreading mid-ocean ridges needs major modification, where ultramafic samples appear to be uncorrelated to local basalt compositions[4,9]. We use clinopyroxene (Cpx) from 16°N abyssal peridotites to show that hydrous melting is required to explain their highly depleted heavy rare earth element (HREE) and Ti concentrations, suggesting that parcels of entrained mantle are recycled from subduction zones to mid-ocean ridges and are likely a ubiquitous mantle component
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