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

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Margaret Hartley, Oliver Shorttle , Cees-Jan de Hoog


Trace element and volatile heterogeneity in the Earth's mantle is inuenced 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
inuence from crustal contamination, identifying high [B] and low X18O as ngerprints of
assimilation processes. Only the most primitive melt inclusions with MgO8 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 Ÿ5-20% 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.



Earth Sciences, Geochemistry, Geology, Physical Sciences and Mathematics, Volcanology


mantle, Iceland, boron isotopes, volatiles, melt inclusion, crustal assimilation


Published: 2020-11-13 17:01


CC BY Attribution 4.0 International

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