Volatile, Major and Trace Element Chemistry of Olivine-Hosted Melt Inclusions and Host Glasses in Cleft and Coaxial Segments of the Juan de Fuca Ridge

Abstract:

To assess the influence of hotspot anomalies on crustal accretion along the Juan de Fuca Ridge, we examine lavas and olivine-hosted melt inclusions (MIs) erupted at segments adjacent to (Coaxial) and isolated from (Cleft) the Cobb Hotspot, which currently intersects the ridge at Axial Seamount. Coaxial samples (host basalt N = 7; MIs N=113) were collected from the center of an axial rise, ~60 km north of Axial Seamount. Cleft samples were collected within the axial graben, ~100 km south of Axial Seamount (host basalts N=3; MIs N=38). The MIs and host glasses were analyzed for major, trace and volatile element concentrations. Vapor-saturation pressures of each MI were determined using CO₂-H₂O concentrations. Entrapment depths for Coaxial MIs range from 0-16 km below seafloor (bsf) with a broad frequency peak centered about 1.5 km bsf. By contrast, the Cleft segment MIs have a narrower range of entrapment depths (0 to 12 km bsf), with a narrow and deeper frequency distribution centered around 3 km bsf. The average rare-earth element (REE) concentrations for the MIs closely resemble those of the host-basalt glasses. Coaxial MIs display variably depleted light and heavy REE patterns and indicate variable degrees of fractional crystallization. The Cleft MIs are uniformly depleted in light REEs only, and span a narrower compositional range, indicating similar crystallization histories. This suggests a model of accretion at Cleft, where relatively homogeneous mantle melts crystallize from ~10 km bsf to the seafloor, with significant storage and crystallization in a shallow (3 km depth) melt lens. At Coaxial, crystallization begins at greater depths (~15 km bsf) with a broader, shallower peak of MI entrapment depths and more variable trace element patterns. The peaks in crystallization depths are broadly consistent with the depths for seismically imaged melt lenses (Carbotte et al., 2006) at both segments. The broader peak of MI entrapment depths observed at Coaxial may reflect the transient nature of the melt lens, as a result of variability in mantle temperatures, composition and crustal structures induced by the nearby Cobb Hotspot.