Binary Mixing Processes at a Ridge Segment as Shown by Historic Reykjanes Peninsula Lavas, Iceland

Binary mixing between compositionally distinct melts has been recognised as an important process in generating the compositional diversity of oceanic magmatism at several length scales, and it has been argued that local variations in ²³⁸U-²³⁰Th of MORB lavas are controlled by such a process. Additional information about the melting behaviour of the mixing end-members that might potentially be provided by ²²⁶Ra is limited by the lack of precise age control for most MORB samples. The Reykjanes Peninsula (SW Iceland) is essentially an onshore, 80 km long, plume-influenced 'mid-ocean' ridge segment, but with a well-dated record of historic basaltic lavas (MgO 6.5-9.2 wt%) erupted between c. 940 AD and 1340 AD. Sr-Nd isotope data indicate minimal shallow-level crustal assimilation. Coherent linear trends shown by high-precision (double-spike) Pb isotope data and correlations with incompatible element ratios (e.g. La/Yb) provide evidence for binary mixing between a 'depleted' end-member with ²⁰⁶Pb/²⁰⁴Pb < 18.7 and La/Yb_N < 1.3 and an 'enriched' end-member with ²⁰⁶Pb/²⁰⁴Pb > 18.9 and La/Yb_N > 2.7. The historic lavas are dominated by the 'enriched' end-member (~ 50-90 %), and the 'depleted' end-member is only found in a relatively undiluted form in picrites erupted during the last deglaciation. Work is in progress to obtain U-Th-Ra disequilibria data on these historic lavas to see if they preserve systematic correlations with Pb isotope and trace element data and thus place critical constraints on the melt generation processes.