Major and trace element data for a suite of lavas from fifty-six dredges and ALVIN dives on the ridge axis and adjacent abyssal hills have been used to investigate the geometry and evolution of magmatic systems beneath the Endeavour Segment, Juan de Fuca Ridge. The morphology of the Endeavour Segment between the northward propagating Cobb Offset and the recently formed (<0.2 m.y.) Endeavour Offset is dominated by a shallow, rifted, elongate crestal volcano (Endeavour Ridge) that deepens along-strike into a broad, deep basin at each offset. A set of ridges, interpreted to be previous crestal volcanoes rifted apart by spreading, flank the Endeavour Ridge and chronicle the ''dueling'' propagator history of the Cobb Offset. The tectonic evidence strongly suggests that a large portion of the Endeavour Segment may be a failing rift segment at this time. Lavas from the current axis of the Endeavour Segment are moderately fractionated (MgO: 6--8.5 wt %) and have generally higher SiO2, Al2O3, Na2O, and K2O, and lower FeO* than lavas from south of the Cobb Offset (SOCO lavas). Incompatible trace element abundances and ratios indicate the Endeavour lavas are primarily enriched E-MORBs and T-MORBs (e.g., Zr/Nb: 7--24; Zr/Y: 2.5--5.9, and Ba/TiO2; 6--64), in contrast with the SOCO lavas, which are more depleted in character. Thus, the 30-km wide Cobb Offset appears to mark a major geochemical boundary beneath the Juan de Fuca Ridge.

In contrast with the Endeavour Segment axial lavas, samples from adjacent abyssal hills are more similar to the SOCO lavas in their major and trace element characteristics. These observations suggest that the parental magmas of the Endeavour Segment exhibit temporal variability, with more enriched material arriving only recently beneath the ridge axis. Pronounced compositional variability is observed at small spatial scales within the Endeavour Segment axial lavas, which does not correlate with axial morphology. This variability is interpreted to reflect ubiquitous small scale mantle heterogeneity and poor mixing of multiple parental magmas during migration from the melt region or within axial magma chambers.

Highly enriched samples (Zr/Nb<10) are localized near the summit region of Endeavour Ridge, whereas slightly less enriched samples occur along the length of the axis and on the older flanking ridges. Recent enrichment may result from diminishing extents of partial melting of a heterogeneous source in response to tectonic reconfiguration, causing more fusible enriched domains to dominate the chemical signature of melts produced. Small scale heterogeneity along-strike seems incompatible with models of centralized upwelling of melts beneath the summit region of the ridge axis, with shallow lateral injection of melts to distal ends of the segment, unless these spatial variations actually reflect temporal variations in the source composition and collapse of the shallow magmatic systems toward the summit region as rift failure has progressed. ¿ American Geophysical Union 1990