The Search for Subducted Components in the Mantle: A Boron and Lithium Isotope, and Fluid-Mobile Element Study of Mount Erebus

University Of South Florida, Tampa FL

Investigators

Abstract

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports research examine volcanic rocks from Mount Erebus, Antarctica, for evidence of previously subducted materials that might be mixed into the mantle source region beneath this volcano. Many workers have, over the years, suggested that the radiogenic isotopic signatures of basaltic volcanic rocks from intraplate volcanic centers reflect the development of heterogeneities in the Earth's mantle brought about largely by the chemical processes of subduction at convergent plate margins. Generally, however, data for incompatible trace elements, which should reflect to some degree the variability recorded by the isotopes, suggests little heterogeneity in intraplate mantle sources relative to one another, or to the mantle source of mid-ocean ridge basalts (MORBs). While part of the difference between the isotopic and trace element record of intraplate sources may certainly relate to the time factor involved in "growing-in" a radiogenic isotope signature, it is reasonable to expect that given the unique chemical signatures of subducted materials, and the profound changes that basaltic crust and sediments on the slab undergo during subduction metamorphism, that some recognizable "fingerprint" of subduction-related material recycling should be left on some portion of the mantle. A problem in seeing such a signature may lay in the evolutionary history of intraplate lavas, as trace element signatures in particular may be obscured by crystallization and assimilatory processes occurring as intraplate magmas evolve in crustal magma chambers. This project is a focused study that uses new and sensitive tracers of subduction effects - boron and lithium isotopes, and the abundances of "fluid-mobile" elements (B, Cs, Rb, Pb, U, As, Sb) - to examine the volcanic products of a single intraplate site - Mount Erebus, in Antarctica - as a means of discovering the influence that subduction has had on the development of intraplate mantle sources. Mount Erebus is a well characterized intraplate volcano of HIMU isotopic affinities, and its different lava series are documented as recording the effects of both closed and open system evolution of parental magmas. Erebus lavas ranging from primitive basanites to evolved phonolites and trachytes, along with a selection of associated anorthoclase phenocrysts are available for study. Boron isotope and B abundance systematics during subduction-zone processes are well understood, and intraplate lavas record both boron abundance and B isotope anomalies, reflecting a mantle source distinct from that of ocean ridge basalts or lavas from volcanic arcs. The systematics of Li during subduction are well understood also, and Li isotopes, while a relatively new system, show great potential as a means of resolving among mantle sources, especially those with subduction zone affinities. Both to, we shall conduct High precision B and Li isotope and abundance measurements, and high precision analyses of "fluid-mobile" elements will be undertaken in order to characterize the mantle source region of Erebus and to search for distinctions between the mantle source of Erebus and the mantle source of other basaltic volcanoes. These data should allow the effects of magma differentiation and assimilation in Erebus lavas to be determined so that source signatures can be identified and key questions can be addressed about whether or not subduction processes (and/or subducted materials) were involved in the development of this intraplate mantle source region. This project will contribute to the training of undergraduate students and will contribute to a greater understanding of the sources of basaltic magmas in general.