Crustal Processes: Major Controls on Reykjanes Peninsula Lava Chemistry, SW Iceland

Abstract

Three hundred stratigraphically constrained samples from the Reykjanes Peninsula, SW Iceland, provide the basis for this study. This area is an elevated section of mid-ocean ridge influenced by the Iceland Plume. Selected chemical, Sr, Nd and laser-assisted fluorination oxygen isotope data are presented. The dataset is subdivided into groups based on criteria which are independent of degree of fractionation and petrography. Two of these groups, Depleted and Stapafell, include high-MgO aphyric samples with Δ18Oolivine values in equilibrium with normal peridotite mantle. Depleted group samples have high 143Nd/144Nd, low Nb/Zr and low incompatible element abundances compared with the dataset as a whole, the reverse of the Stapafell group. The majority of the remaining samples have radiogenic isotope ratios, and incompatible element concentrations and ratios intermediate between the Depleted and Stapafell groups. Some samples, however, define a range in 87Sr/86Sr and δ18Oolivine at constant 143Nd/144Nd, and others possess positive Sr anomalies when normalized to primitive mantle values. We explore the possibility that these and other chemical characteristics have been produced by shallow crustal processes, including assimilation of xenocrysts, cumulates and hydrothermally modified crust. We conclude that although these processes are important, the major crustal process acting to modify characteristics indicative of mantle heterogeneity is magma mixing. Chemical variation previously thought to be a consequence of dynamic melting is more readily explained by magma mixing.