Diversity of Iceland’s Basalt Geochemistry: Implications for Petrogenesis and Crust-Mantle Dynamics

Building on recent analysis of Iceland’s volcanic zones, its basalt compositions are further evaluated to provide insights into the crust-mantle dynamics and petrogenesis. Magnesium number, trace element ratios and ternary relationships reveal distinctive trends within and between specific zones. Cr and Ni concentrations versus high field strength elements and Th indicate that crystal fractionation of minor phases is involved in generating globally unusual Ta and Nb/Ta relationships in some rift zone basalts. The nature of the compositional variations cannot be explained solely in terms of different degrees of partial melting and/or fractional crystallisation in magma chambers. Mantle heterogeneities at local and regional scales are required. A conceptual model combining sequential, dynamic partial melting of an ancient variably veined, upwelling mantle with complex interconnections between lower crustal magma chambers can account for the observed compositions. Average trace-element and isotopic compositions effectively distinguish each zone. The rift zones display MORB-like compositions, distinct from the off-rift zones. These differences require distinct interactions between crustal and mantle processes. The ternary relationship Ce/Yb-Hf/Th-²⁰⁷Pb/²⁰⁶Pb reveals an expansive trend extending from more depleted compositions than N-MORB in some of the Iceland volcanic zones, towards other Atlantic Ocean islands and the world average intraplate-basalt compositions. A new multi-stage incremental hybrid decompression (MIHD) partial melting model is developed and applied to model trace-element changes in melts generated during polybaric partial melting as a veined mantle plume rises through the upper mantle.