Geochemical provincialism in the Iceland plume

Abstract

We present new Pb–Sr–Nd isotope, major and trace element data for a suite of basalts from central Iceland. We combine this new data with existing sample sets and interrogate it using spatial statistical methods. On the ∼100km scale of a volcanic zone we find strong correlation between the three isotope systems. However, on greater length scales we identify two types of spatial structure, both of which are most strongly observed in the Pb isotopes. Firstly, the mean Pb-isotopic composition of basalts becomes progressively less radiogenic from south to north Iceland, with our central Iceland dataset falling at intermediate compositions. Secondly, there is a shift in the pseudo-binary mixing array that samples fall along as the neovolcanic zones are stepped through south to north, both in Pb–Pb, and Pb–Sr/Nd isotope space. The Pb isotope systematics of Icelandic basalts therefore appear to be decoupled from those of Sr and Nd isotopes on length scales >140km. Only within individual neovolcanic zones are there coherent relationships between the Pb, Sr and Nd isotopic compositions of basalts. The spatial structure uniquely recorded by Pb isotopes complements observations from previous authors that Pb isotope dynamics are fundamentally distinct from other isotope systems.Iceland lies at the centre of a geochemical transition that occurs in the North Atlantic mantle. Our analysis shows that this shift occurs progressively from the southwest to north east of the island. However, geochemical spatial structure on Iceland does not map simply into adjacent ridges. We find that at length scales >200km partial melting of the plume head during outflow can explain some of the transition to unradiogenic Pb isotopic compositions north of Iceland. However, the shift in binary mixing arrays to sources with higher time integrated Th/U, requires the North Atlantic mantle to be highly provincial in its Pb isotopic composition.The spatial trends that we observe on Iceland are similar to those seen on Hawaii between the Loa and Kea volcanic chains, which also show shifts in mean Pb isotopic composition and binary mixing array. However, on Iceland we are able to see that, rather than representing dichotomous compositional domains on either side of the island, the change in mean composition and mixing array occurs continuously. Geochemical structure only shows segmentation on the scale of a neovolcanic zone. Discrete arrays in isotopic space on the scale of volcanic systems may develop in response to mixing in the melt transport and storage system and require neither the shallow nor the deep mantle to exhibit sharp (<100km) lateral contrasts in composition.