- © The Mineralogical Society Of America
Mid-ocean ridges account for more than 75% of the annual magmatic output of planet earth and are a critical piece of the plate tectonic model. Understanding the mantle melting process beneath ridges is essential for discerning basic physical and chemical processes of the earth. For instance, ridges play an intrinsic role in the geochemical flux balance for many elements on the earth and provide a tectonic framework for investigating the physical process of solid mantle flow. Ridges are also the simplest tectonic regime to study the mantle melting process because interactions with the lithosphere are thought to be minimal. Yet major questions remain about the rates and timescales of melt transport and solid mantle flow. Because uranium series (U-series) nuclides are sensitive to processes occurring at timescales similar to their half-lives (e.g., Allègre and Condomines 1976; McKenzie 1985), U-series disequilibria can provide constraint on the velocities of ascending melts and upwelling solid mantle as well as constraint on the depth of melting beneath mid-ocean ridges.
This chapter begins with a brief introduction to the most important decay series nuclides for studying melting processes. Next, I discuss analytical issues involved with measuring U-series disequilibria in mid-ocean ridge basalts (MORB), including sample preparation and instrumental techniques. I summarize the observations of U-series disequilibria in MORB to date and applications of U-series data to geochronology of MORB. Geochemical data bearing on the issue of secondary contamination are used to argue that the majority of U-series disequilibria observed in MORB reflect the melting process and do not reflect alteration processes. Given the observation of disequilibria, geochemical models for MORB genesis are described and assessed for their ability to realistically explain the disequilibria observations. Finally, I discuss the interpretations of the combined U-series models and observations and relate these to constraints on MORB genesis …