PETROLOGY AND GEOCHEMISTRY OF BASALTS FROM THE SOUTHERN JUAN-DE-FUCA RIDGE - CONTROLS ON THE SPATIAL AND TEMPORAL EVOLUTION OF MIDOCEAN RIDGE BASALT

Three morphologically distinct regions within the neovolcanic zone of the Cleft segment of the southern Juan de Fuca Ridge were investigated and sampled in detail using the DSRV Alvin. Additional along-strike a nd off-axis samples were recovered by dredge. The southernmost region, the Southern Cleft site, is characterized by a 3-km-wide axial valley floored by ponded sheet flows and bisected by a 30- to 50-m-wide clef t. Farther north at the ''Young Sheet Flow'' site, the ridge axis is c haracterized by a distinct 500-m-wide inner graben that is largely cov ered by distinctly younger looking sheet flows. The northernmost of th e three regions is defined by a linear series of discontinuous constru ctional pillow mounds that extend the trend of the Cleft segment well into the zone of overlap with the neighboring Vance segment. The pillo wed lavas at the ''Young Pillow Mound'' site represent the most recent episode of volcanism along the Cleft segment. Strong correlations exi st between degrees of fractionation, relative ages of lavas, and latit ude; lavas are progressively younger looking and more mafic to the nor th. The compositional range of mid-ocean ridge basalts from the neovol canic zone can generally be accounted for by 35-40% low-pressure fract ional crystallization of relatively primitive, but not primary, deplet ed (N-type) melts. Scatter of the geochemical data about calculated li quid lines of descent is probably the result of mixing of magmas with slightly different parental compositions, generated from small-scale m antle heterogeneities. Furthermore, the chemical variability may be th e result of mixing of very depleted and more enriched sources or melts that are present beneath the southern Juan de Fuca Ridge. The more pr imitive nature of the young pillow mound basalts and their slightly di fferent chemical characteristics indicates they cannot be simply relat ed to the older southernmost lavas by along-axis flow in a continuous axial magma chamber or conduit. Rather, the data suggest lavas were de rived from discrete magma chambers or lenses, each in a different stag e of evolution. The youngest events may be associated with a new influ x of magma into the northern part of the segment and subsequent northw ard diking and propagation to form the new pillow mounds. The oldest s tage (approximately 40% additional crystallization of the most mafic c omposition) is associated with focused hydrothermal activity and tecto nic extension, whereas the youngest events are characterized by ridge inflation, diking, and dispersed hydrothermal activity. Geochemical an d tectonomagmatic features observed along the Cleft segment are simila r to those recently documented along the East Pacific Rise from 9-degr ees-30'N-10-degrees-N suggesting the scales, processes, and stages of magmagenesis are similar along medium to fast spreading ridge segments .