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Boron isotopic signatures of melt inclusions from North Iceland reveal recycled material in the Icelandic mantle source

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Hartley, Margaret 
de Hoog, Cees-Jan 


Trace element and volatile heterogeneity in the Earth's mantle is in uenced by the recycling of oceanic lithosphere through subduction. Oceanic island basalts commonly have high concentrations of volatiles compared to mid-ocean ridge basalts, but the extent to which this enrichment is linked to recycled mantle domains remains unclear. Boron is an ideal tracer of recycled subducted material, since only a small percentage of a recycled component is required to modify the bulk X11B of the source mantle. Boron isotopic compositions of primary melts thus have potential to trace the fate of recycled subducted material in the deep mantle, and to constrain the lengthscales of lithologic and compositional heterogeneities in diverse tectonic settings. We present new measurements of volatiles, light elements and boron isotopic ratios in basaltic glasses and melt inclusions that sample the mantle at two endmember spatial scales. Submarine glasses from the Reykjanes Ridge sample long-wavelength mantle heterogeneity on the broad scale of the Iceland plume. Crystal-hosted melt inclusions from the Askja and Bárðarbunga volcanic systems in North Iceland sample short-wavelength mantle heterogeneity close to the plume centre. The Reykjanes Ridge glasses record only very weak along-ridgeenrichment in B content approaching Iceland, and there is no systematic variability in X11B along the entire ridge segment. These observations constrain ambient Reykjanes Ridge mantle to have a X11B of -6.1h (2SD=1.5h, 2SE=0.3h). The North Iceland melt inclusions have widely variable X11B between -20.7 and +0.6h. We screen melt inclusions against in uence from crustal contamination, identifying high [B] and low X18O as ngerprints of assimilation processes. Only the most primitive melt inclusions with MgO 8 wt.% reliably record mantle-derived X11B. In North Iceland, incompatible trace element (ITE)-depleted primitive melt inclusions from Holuhraun record a X11B of -10.6h, a signal that has also been seen in melt inclusions from southwest Iceland (Gurenko and Chaussidon, 1997). In contrast, primitive ITE-enriched melt inclusions from nearby Askja volcano record a X11B of -5.7h, overlapping with our new constraint on the X11B of Reykjanes Ridge mantle. Coupled [B], X11B and X18O signatures of more evolved melt inclusions from North Iceland are consistent with primary melts assimilating 5-20% of hydrothermally altered basaltic hyaloclastite as they ascend through the upper crust. Our data reveal the presence of a depleted, low-X11B and an enriched, higher-X11B mantle component, both intrinsic to the Icelandic mantle source and distinct from Reykjanes Ridge mantle. Non-modal melting calculations suggest that the enriched and depleted mantle components both contain 0.085 g/g B, slightly lower than the 0.10-0.11 g/g calculated for Reykjanes Ridge mantle. These data are consistent with the Icelandic mantle containing B-depleted dehydrated recycled oceanic lithosphere, in keeping with the low B/Pr of Icelandic melt inclusions in comparison to Reykjanes Ridge glasses or MORB. Our new data provide strong support for the role of recycled subducted lithosphere in melt generation at ocean islands, and highlight the need for careful screening of melt inclusion compositions in order to study global volatile recycling in ocean island basalts.



Journal Title

Geochimica et Cosmochimica Acta

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Meteoritical Society