| The geochemistry of 203 fresh, glassy, zero-age basalts dredged from along the 500 km long Juan de Fuca Ridge (JDFR) illustrates the unique and complex behavior of this active spreading center located on the northern end of the East Pacific Rise. The Cobb-Eickelberg Seamount chain, the trace of the Cobb hot spot, intersects the JDFR at 46(DEGREES)N, where a young, volcanically active seamount, astride the ridge, gives clear evidence for continuing hot spot activity. In contrast with other well-documented mantle plume - ridge intersections (eg. Iceland, Azores, Galapagos), there are no systematic geochemical gradients away from the center of maximum volcanism.; Petrographic and major element evidence indicates that most JDFR basalts are normal 'depleted' mid-ocean ridge basalts (MORB). However, unlike the vast majority of MORB, the trace element ratios along the JDFR, although highly variable on the scale of single dredge hauls, are predominantly transitional (Zr/Nb = 22) between depleted MORB (Zr/Nb greater than 40) and enriched oceanic island (or plume) basalts (Zr/Nb = 10). North of the Cobb Fracture Zone at 48(DEGREES)N, the Endeavour segment is chemically distinct from the rest of the JDFR with uniformly enriched trace and rare earth element ratios (Zr/Nb 7-15, La/Sm(, ch ) greater than 1).; Geochemical variability observed along the JDFR requires a heterogeneous mantle source for JDFR basalts. Transitional trace element ratios can be explained by regionally extensive mixing of enriched (Endeavour-like) and more normal MORB magmas, however, this mixing is not simple, two component mixing. Highly evolved basalts, (Fe(,2)O(,3)* greater than 12.0 wt.%, TiO(,2) greater than 2.0 wt.%) resulting from extensive crystal fractionation requires this mixing be sporadic. |
