MIXING OF MAGMAS FROM ENRICHED AND DEPLETED MANTLE SOURCES THE NORTHEAST PACIFIC - WEST-VALLEY SEGMENT, JUAN-DE-FUCA RIDGE

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORE to enriched (E-) MORE. Heck Seamount lavas have chondrite-normalized La/Sm-cn similar to 0.3, Sr-87/Sr-86 = 0.70235-0.70242, and Pb-206/Pb-204 = 18.22-18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Sm-cn similar to 1.8, Sr-87/Sr-86 = 0.70245-0.70260, and Pb-206/Pb-204 = 18.73-19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with ''plums'' of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the ''plums'', initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the ''plums'' and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the ''plums'' could evolve to their more evolved isotopic composition after a period of 1.5-2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a less-depleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as ''plums'' throughout the upper mantle, transported from depth by mantle plumes.