EVIDENCE FOR MAGMA MIXING AND A HETEROGENEOUS MANTLE ON THE WEST VALLEY SEGMENT OF THE JUAN-DE-FUCA RIDGE

We studied basaltic rocks of the northern end of the Juan de Fuca Ridge to compare the volcanology, geochemistry, and petrogenesis of spatially related rocks of an active spreading center (West Valley and West Ridge), the Sovanco Fracture Zone, the Middle Ridge, and the adjacent Endeavour and Northern Symmetrical ridges. Rocks were analyzed for mineral chemistry and major, trace, and rare earth elements (REE). Crystallization temperatures and equilibrium phase compositions were calculated. The complex zoning and disequilibrium compositions of plagioclase and olivine megacrysts and phenocrysts from all areas except West Ridge, Sovanco Fracture Zone (7123-7), and Endeavour Ridge indicate mixing in shallow level magma chambers. The rocks range from primitive (up to Mg # = 68) in the West Valley south and West Ridge areas to evolved ferrobasalts (Mg # as low as 52) at West Valley central, West Valley north, and Northern Symmetrical Ridge. The rocks from the West Valley axis span the range of Mg #. Rocks from West Valley are transitional to enriched (e.g., [La/Ce]n = 0.89-1.06), as are rocks from Endeavour Ridge (e.g., [La/Ce]n = 0.88-1.06). Rocks from West Valley wall, Middle Ridge, Sovanco Fracture Zone, and Northern Symmetrical Ridge are light REE depleted (e.g., [La/Ce]n = 0.76-0.88), and the most depleted rocks are from West Ridge (e.g., [La/Ce]n = 0.62). The chemically heterogeneous character of the lavas exist both within site and within a single dredge haul (West Valley north and Endeavour Ridge rocks) and between sites, indicating that the mantle source is heterogeneous on a variety of scales. The range in incompatible element ratios can be modelled as binary mixing between an enriched source and a depleted source. Further, trace element systematics require variations in the degree of partial melting. Lava chemistry and ridge morphology are consistent with southward propagation of spreading in West Valley to form an overlap with the Endeavour Segment. West Valley (West Valley north, central and south) is dominated by extension over volcanism and is thus thermally and magmatically depressed. Rocks from the West Valley Segment span a broader compositional range than ridge systems to the south, reflecting the heterogeneity of the source and the lack of robust volcanism to homogenize the chemistry. Zero age rocks from West Valley are similarly enriched in [La/Ce]n as zero age rocks from the Endeavour and Explorer ridges. Rocks from the West Valley wall, the Middle Ridge, and the older Sovanco Fracture Zone rocks are depleted similar to rocks of the Juan de Fuca Ridge south of the Cobb Offset. This difference suggests that enrichment of the mantle source north of the Cobb Offset has been a recent phenomena.