Geochemical study of near-ridge seamounts: do dunite conduits effect how the mantle melts?

C.C. Lundstrom¹*, M.R. Perfit², & N. Baxter²

Corresponding author: lundstro@uiuc.edu
¹Dept. of Geology, University of Illinois, Urbana, IL 61801
²Dept. of Geological Sciences, University of Florida, Gainesville, FL, 32611

Abstract:
We present results of a geochemical study of the relationship between near axis seamounts and adjacent well-studied portions of Pacific ridges. The work revolves around the idea that seamount chains aligned along absolute plate motion directions reflect the formation of dunite melt conduit systems in the upwelling mantle focusing melt to near ridge locations (e.g. Kelemen et al., 1995 Nature). If a dunite melt conduit system is established slightly off axis, it feeds melt to the seamount chain for time scales of a few million years. Ascending melt causes flux melting of the mantle surrounding the conduit by diffusion and may produce small temperature gradients between the magma conduit and the surrounding mantle (particularly the nascent lithosphere). Trace element, major element, U-series and non-traditional stable isotope analyses of samples from the Lamont Seamounts (located off the northern East Pacific Rise ~10°N) and the Vance Seamounts (located off the southern Juan de Fuca ridge) are being completed to relate melting systematics beneath the seamounts to that beneath the adjacent well-studied ridges (including the EPR ISS). Our current observations support such a model: average major element compositions of Vance seamount samples show distinct differences with position in the seamount chain and small but significant differences appear to occur in Fe isotopes. We are currently measuring U-Th-Ra-Pa disequilibria in very fresh appearing seamount lava samples from these seamounts and a few other near- JdF seamounts (e.g. Rouge, Hacksaw, Jinja) as well as high precision trace elements by ICPMS and will present these at the meeting.

Keywords:
Geochemistry, seamounts, MORB,

Contributions to Integration and Synthesis:
This work contributes to better understanding of the production and generation of MORB and ocean crust by providing an indication of the diversity of magma compositions and sources present beneath the ridge but obscured by all the shallow crustal processes (e.g mixing, reactions in crystal mush) that homogenize erupted axial lavas. Near-ridge seamount lavas such as those from Lamont provide an indication of the diversity of compositions generated in the mantle beneath the ISS which will allow us to better understand melting processes. Comparison between Lamont and Vance chains provides the potential to identify melting processes common to seamounts, providing insight into generation of all MOR magmas. It has been proposed that ascending melts beneath ridges coalesce into conduits by producing dunites through melt-rock reactions (Kelemen et al., Nature, 1995). While one repercussion of this is that signatures of deeper melting may be preserved in MORB, it could have another effect: as ascending melts in conduits reach shallow depths, they could cause more extensive melting of the shallow mantle through the process of diffusive fluxing (Lundstrom, Nature, 2000). This could have important ramifications for production of the ocean crust and possibly for how major element fractionation trends vary with ridge setting. For instance, a major component added at shallow depths in a polybaric melting column is alumina (plagioclase), leading increasing amounts of plagioclase fractionation and possibly affecting major element trends of depth and extent of melting.