SOURCE COMPONENTS FROM SEAFLOOR LAVAS OF THE ALEUTIAN ISLAND ARC

Trace element and isotopic compositions of seafloor lavas are used to constrain the nature of the source mixture that produces volcanic rocks of the Aleutian island arc. Samples were collected by dredging of small volcanic cones on the Aleutian seafloor from locations between the much larger emergent volcanoes, and in back-arc locations up to 39 km from the volcanic front. Most of the samples are from three locations in the eastern (Umnak), central (Seguam) and western (Little Sitkin) parts of the arc. The samples include basalts and andesites, with 48-57% SiO₂ and 4-12% MgO. Some basalts are primitive, with Mg/Mg+Fe greater than 0.60. Several samples have slightly higher K₂O (>1.24 wt. %) and more fractionated rare-earth patterns (La/Yb>5.5) compared to common lavas from the emergent volcanoes. Strontium isotopes in seafloor lavas from each location have generally less radiogenic Sr than lavas from nearby emergent volcanoes. In the central Aleutian area, seafloor lavas have ⁸⁷Sr/⁸⁶Sr of 0.7029-0.7035 compared to greater than 0.7036 in lavas from Seguam Island. Available Nd and Hf isotope data appear to be inversely correlated with Sr isotopes, which are well correlated with Ba and Pb enrichments. Samples with the least radiogenic Sr (⁸⁷Sr/⁸⁶Sr<0.7028) have trace element ratios approaching those of ocean ridge basalts (Ba/La=12, Ce/Pb=19, La/Ta=16). These patterns indicate that Aleutian seafloor lavas carry a stronger source component from depleted mantle and show a reduced role for subducted sediment compared to common lavas of the arc.