A continuous 65 ka time series of magma compositions at the Juan de Fuca ridge from sediment-hosted volcanic glass

Abstract:

A major frustration to studies of mid-ocean ridge (MOR) magmatism is the inability to construct detailed time-series of erupted magma compositions. Lava on the rift flanks are quickly covered in sediments making direct sampling difficult and even when these rocks are recovered, for example from fault scarps, precise radiometric dating is not possible. Existing time-series of ridge magmatism over periods of 10s-100s ka therefore typically involve sparse sampling and uncertain age estimates. Recent work by Clague et al [JVGR, 2009] has shown that small pyroclastic fragments of volcanic glass are a near ubiquitous component of surficial sediments in the axial regions of MORs. These fragments are dispersed into the water column during seafloor eruptions and can have transport distance of several km. In this study we show that these glasses are preserved in the sedimentary record and can be used to provide detailed records of temporal trends in MOR magmatism over many 10s of ka. Our samples are from a piston core taken on the western side of the Cleft segment of the Juan de Fuca (JdF) ridge. This core penetrated the entire sedimentary succession, impacting the underlying ~610 ka volcanic basement. Sampling of the lowermost sediment layers directly above the basal lavas provides many hundreds of particles of pristine volcanic glass suitable for electron microprobe analysis. Combining the results of the glass analysis with oxygen isotope stratigraphy of the host sediments allows a detailed time-series to be developed, providing a continuous record of magma composition over a period of 65 ka, with a nominal temporal resolution of a few ka. Our result reveals two striking step-wise transitions in magma compositions, first in MgO and then in K₂O/TiO₂ contents, that occur rapidly (< 10 ka) and are preceded and followed by periods of relatively low compositional variability. These coincide in time with a marked increase in crustal thickness at Cleft, previously attributed to the capture of the Cobb hotspot by the JdF ridge. Our study demonstrates the potential of the sediment-hosted glass method to produce high resolution temporal records of ridge magmatism, providing a new approach to investigating the magmatic and volcanic evolution of MORs over ~10s-100s ka.