The boron isotope systematics of Icelandic basalts and their constraints on the Icelandic mantle source

The boron isotope compositions (δ¹¹B) of ocean-island basalts (OIBs) place constraints on the cycling of volatile elements in the Earth's interior. Because most OIB localities are small islands that lie on oceanic crust, OIBs often assimilate seawater-altered oceanic crust (AOC) as they migrate to the surface, obscuring the δ¹¹B value of their mantle source (e.g. [1]). In contrast, Iceland is an OIB locality that contains no underlying seawater AOC and consequently its lavas are not affected by seawater-derived boron. Previous boron isotope studies of Icelandic basalts and melt inclusions focused on only a few localities (n = 4) within Iceland, and therefore how δ¹¹B values are controlled by mantle heterogeneity and magmatic processes in Iceland is still unknown. We present new high-precision (2SE @ 1ppm B = 1.6‰) SIMS δ¹¹B values and boron concentrations from a suite of well-characterized subglacial Icelandic basaltic glasses (n = 40) that spans the known spatial and geochemical variability of rift and off rift basalts to better assess how mantle heterogeneity and magmatic processes control boron isotope compositions.

Measured δ¹¹B values of Icelandic basalts range from -9.2‰ to -1.0‰, with a mean of -4.2‰. Remarkably, most Icelandic basalts fall in a narrow range of compositions (from -6‰ to -1‰) despite our broad sampling strategy. As a whole, our measured values have higher δ¹¹B values than those measured in previous studies of Icelandic basalts and melt inclusions.

The boron isotope compositions of Icelandic basalts are clearly distinct from DMM (-7.1‰; [2]) and do not clearly correlate with indices of fractional crystallization or assimilation (e.g. MgO, δ¹⁸O). Excluding samples with MgO < 8 wt%, Cl/K > 0.08, or low δ¹⁸O does not shift the average to a DMM-like δ¹¹B value. The ¹¹B-enriched composition of Icelandic basalts is most simply explained by an ¹¹B-enriched component in the mantle source. Hawaii and Iceland are the only OIB localities thought to have an ¹¹B-enriched component in their mantle sources, potentially highlighting a common mantle component in the two localities.

[1] Genske, F.S., et al. (2014) Chem. Geol., v. 373, p. 27-36

[2] Marschall, H.R., et al. (2017) Geochim. et Cosmochim. Acta, v. 207, p. 102-138