Spatial geochemical structure in Icelandic basalts

Pb isotope binary mixing arrays

Binary mixing arrays in Pb isotope space shift systematically across Iceland, revealing a length scale on which either mixing of melts in the crust or mantle operates. The observations are similar to the recent work indicating the presence of ‘double volcanic chains’ in ocean islands such as Hawaii and the Galapagos. Figure modified from Shorttle et al. (2013).

The mantle is compositionally heterogeneous on a fine scale, this can be observed in exhumed mantle sections (e.g. Allegre & Turcotte, 1986) and in melt inclusion suites from single eruptions (e.g. Maclennan, 2008). However, this compositional variability may also show long rage structure, with basalt compositions sampling the mantle exhibiting systematic changes as a function of their eruption location. This has been demonstrated most strikingly with basalts from Hawaii (Abouchami et al. 2005), which depending on their origin north or south on the island chain exhibit distinct Pb isotopic compositions. Here we show that similar spatial patterns to those found on Hawaii are also present on Iceland, with Icelandic basalts showing systematic shifts in composition that are only recorded by Pb isotopes (see figure above).

Basalts are a probe of mantle compositional structure, and seeing such systematic spatial patterns in their compositions it is tempting to infer that there are stepped changes in the chemistry of the underlying mantle (e.g. Weis et al. 2011). However, on Iceland we observe that the composition of erupted basalts changes systematically north to south across the island. We can make this observation because in contrast to many ocean islands, such as Hawaii, volcanism on Iceland is distributed across en-echelon fissure systems affording greater spatial resolution of isotopic shifts.

Our observations from Iceland raise the question of how the geochemical asymmetry seen in double-chain volcanism truly represents underlying mantle chemical structure, if, when we have greater spatial resolution, we see more gradational shifts. To really project observations made at the surface back down into the mantle we need a lot more information on melt transport out of the mantle, to know how spatial patterns in mantle heterogeneity are being mapped into basalt chemistry.

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ReferenceShorttle, Oliver, John Maclennan, and Alexander M. Piotrowski. Geochemical provincialism in the Iceland plume. Geochimica et Cosmochimica Acta 122 (2013): 363-397