²³¹Pa systematics in postglacial volcanic rocks from Iceland

Several recent studies have highlighted the potential of combined ²³⁸U–²³⁰Th and ²³⁵U–²³¹Pa systematics to constrain upwelling rates and the role of recycled mafic lithologies in mantle plume-derived basalts. Accordingly, we present measurements of the ²³¹Pa concentrations from 26 mafic volcanic rocks from Iceland, including off-axis basalts from the Snaefellsnes Peninsula, to complement previously published ²³⁸U–²³⁰Th–²²⁶Ra data. ²³¹Pa concentrations vary from 27 to 624 fg/g and ( ²³¹Pa/²³⁵U) ratios from 1.12 to 2.11 with the exception of one anomalous sample from the Southeast Rift which has a ²³¹Pa deficit with (²³¹Pa/²³⁵U) = 0.86. An important new result is that basalts from the Southeast Rift and the Snaefellsnes Peninsula define a trend at relatively low (²³¹Pa/²³⁵U) for a given (²³⁰Th/²³⁸U) ratio. Many of the remaining samples fall in or around the global field for ocean island basalts but those from the Mid-Iceland Belt and the Southwest Rift/Reykjanes Peninsula extend to higher (²³¹Pa/²³⁵U) ratios at a given (²³⁰Th/²³⁸U), similar to mid-ocean ridge basalts. In principle, these lavas could result from melting of peridotite at lower pressures. However, there is no reason to suspect that the Mid-Iceland Belt and the Southwest Rift lavas reflect shallower melting than elsewhere in Iceland. In our preferred model, these lavas reflect melting of garnet peridotite whereas those from the Southeast Rift and the Snaefellsnes Peninsula contain a significant contribution (up to 20%) of melt from garnet pyroxenite. This is consistent with incompatible trace element and radiogenic isotope evidence for recycled oceanic crust in these lavas. There is increasing agreement that the displacement of ocean island basalts to lower (²³¹Pa/²³⁵U) ratios at a given (²³⁰Th/²³⁸U), compared to mid-ocean ridge basalts, reflects the role of recycled mafic lithologies such as garnet pyroxenite as well as higher average pressures of melting. It now seems likely that this interpretation may have more general application to plume-affected MORB that exhibit the same U-series disequilibria characteristics.