Oceanic basalts are geochemically and isotopically heterogeneous owing to variations in melting processes and mantle source compositions. Although enriched heterogeneities associated with recycled and primordial material are well known to contribute to oceanic basalt genesis in many settings, important roles for highly depleted heterogeneities are also being increasingly recognised. However, variability in oceanic basalts is not restricted to their isotopic and trace-element contents, but also manifests in their major element contents. This is important because variations in major element contents result in variable phase equilibria relations that in turn have the potential to affect magma transport and storage processes. Here we will use Icelandic basalts as a case study to discuss how mantle-derived geochemical variability imposes fundamental controls on the evolution of oceanic basalts and their crystal cargoes. Firstly, we will discuss how measuring incompatible elements in olivine reinforces inferences made about mantle heterogeneity from melt inclusion archives. We will then discuss the results of phase equilibria experiments performed on synthetic analogues of Icelandic lavas that reveal earlier and more extensive crystallisation in magmas from depleted sources, which may bias erupted records of mantle geochemistry towards magmas from enriched sources. We will then consider how the crystallisation of widespread but cryptic depleted melts at depth may produce the disequilibrium cargoes of anorthitic plagioclase observed in many oceanic lavas, including in Iceland. Finally, we will discuss how isothermal mixing between geochemically variable basalts affects plagioclase stability and may trigger mush disaggregation and crystal entrainment in the run up to Icelandic eruptions.