CRUSTAL PROCESSES - MAJOR CONTROLS ON REYKJANES PENINSULA LAVA CHEMISTRY, SW ICELAND

Three hundred stratigraphically constrained samples om the Reykjanes P eninsula, SW Iceland, provide the basis for this study. This area is a n elevated section of mid-ocean ridge influenced by the Iceland Plume. Selected chemical, Sr, Nd and laser-assisted fluorination oxygen isot ope data are presented. The dataset is subdivided into groups bared on criteria which are independent of degree of fractionation:and petrogr aphy. Two of these groups, Depleted and Stapafell, include high-MgO ap hyric samples with delta(18)O(olivine) values in equilibrium with norm al peridotite mantle. Deleted group samples have high Nd-143/Nd-144, l ow Nb/Zr and low incompatible element abundances compared with the dat aset as a whole, the reverse of the Stapafell group. The majority of t he remaining samples have radiogenic isotope ratios, and incompatible element concentrations and ratios intermediate between the Depleted an d Stapafell groups. Some samples, however define a range in Sr-87/Sr-8 6 and delta(18)O(olivine) at constant Nd-143/Nd-144, and others posses s 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 assimilati on of xenocrysts, cumulates and hydrothermally modified crust. We conc lude that although these processes are important, the major crustal pr ocess acting to modify characteristics indicative of mantle heterogene ity is magma mixing. Chemical variation previously thought to be a con sequence of dynamic melting is more readily explained by magma mixing.