ALBERT

Description: Major ± trace element and Sr–Nd–Pb–Hf–He isotope data are presented for more than 300 geochemically diverse basalt samples collected by submersible from the Inflated Central Endeavour Segment of the Juan de Fuca Ridge. Seven chemically distinct basalt types are present, from depleted (D-) to enriched mid-ocean ridge basalt (E-MORB). By combining the geochemical data with high-resolution bathymetry and age determinations, the detailed spatial and temporal scale of on-axis mantle-derived basalt heterogeneity is determined. The basalts define binary mixing arrays in all isotope plots that are usual in their correlations, but unusual in the limited range of Sr–Nd–Hf isotope compositions for D- to E-MORB, and greater range in Pb isotopes. The basalts also define two different styles of enrichment of moderately incompatible elements. Geochemical enrichment began when the currently inflated axial ridge formed 〈10 5 years ago. One enrichment style (the Inflated Ridge Trend) characterizes basalts erupted across the ~5 km wide ridge from 〉10^000 to ~4000 years ago, whereas the other enrichment style (the Graben Trend) characterizes most basalt types erupted within the axial graben after it formed ~2300 years ago. We attribute the Inflated Ridge Trend to a relatively high proportion of pyroxenite (or melt derived therefrom) to enriched peridotite in the mantle during a phase of ridge inflation that lasted at least 6000 years. The Graben Trend reflects the reduced effect of pyroxenite after the axial graben formed. Because at least 14 different samplings of mantle components occurred within 〈1 km of ridge length and width during a time when 〈1 km of upwelling occurred, we infer that the scale of mantle heterogeneity far from a plume is 〈 1 km. The enriched mantle component at Endeavour is young with 206 Pb/ 204 Pb ~19·0; Hf and He isotope ratios trend toward HIMU characteristics. These traits are regionally widespread and are shared with the next two ridge segments to the north (West Valley and Explorer).