Sub-km HIMU-type Enriched Mantle at a Mid-ocean Ridge Far From a Plume: Endeavour, JdFR

Abstract:

The Endeavour segment of the Juan de Fuca Ridge is characterized by abundant enriched (E) MORB since the currently inflated axial ridge formed <10⁵ years ago, and by the full range of depleted (D) to E-MORB during the last 2300 years in the km-wide axial graben. Two different styles of enrichment of moderately incompatible elements are present. The first characterized basalts across the ~5 km-wide ridge from >10,000 to ~4000 years ago, whereas the second characterizes more recent basalts erupted in the axial graben. We attribute the first to a higher proportion of pyroxenite to enriched peridotite in the mantle source during ridge inflation. The more recent style reflects the reduced role of pyroxenite after the axial graben formed. The enriched component for both styles is a HIMU-type because it has low ⁸⁷Sr/⁸⁶Sr and ¹⁷⁶Hf/¹⁷⁷Hf relative to ¹⁴³Nd/¹⁴⁴Nd, lower ³He/⁴He (~8.1 R_A) than in the more depleted basalts, shallow slopes on Pb isotope diagrams, and high Nb/LREE ratios. It is regionally widespread and shared with the West Valley and Explorer segments to the north. 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, indicating that the scale of mantle heterogeneity is <1 km in this setting that is far from a plume.