Source heterogeneity, melt mixing and the thermal state of the mantle for the North Atlantic Large Igneous Province

The east Greenland flood basalts provide a window into source composition and melting dynamics in the ancestral Iceland plume during continental breakup forming the northeast Atlantic. Approximately 6000 m of lavas preserved along the Blosseville Kyst include >350 flow units erupted in less than one myr, probably within a few hundred kyrs, with no significant hiatus in volcanism. Previous work has shown marked variations in rare earth element chemistry through the stratigraphy attributed to secular cooling of the mantle and variable efficiencies in melt pooling with time (1). Previously reported radiogenic isotope compositions of subsets of lavas taken from relatively restricted portions of the stratigraphy exhibit uniformity, but differ from each other, suggesting a temporal evolution in the mantle source (2). To distinguish the relative importance of source and process in the generation of the east Greenland flood basalts, we have measured major and trace elements, and Sr-Nd-Hf isotope compositions on a nearly flow-by-flow basis through the entire stratigraphy. When combined with published data, this high-resolution geochemical record of magmatism shows that the vast majority of the lavas are evolved (MgO ~7 wt. %) with a range in incompatible trace element ratios (i.e., Nb/Zr) and isotopic compositions similar to, but larger than observed for present-day Iceland. We propose that the variation in lava compositions comprising this temporal record is governed to a first order by the delivery of enriched and depleted melts from heterogeneous mantle that are variably mixed en route to the surface, as is also proposed for Iceland (3). This effect dampens the signals of secular changes in mantle temperature and composition. Recognizing the role of such melt mixing is essential to place robust constraints on the thermal and compositional state of the mantle during flood basalt volcanism.

(1) Tegner et al., 1998, Nature, 395; (2) Barker et al., 2006, Geology, 34; (3) Shorttle et al., 2014, EPSL, 395.