Using U-series disequilibria in basalts from the 2005-06 9° 50’ N EPR eruption to constrain the magnitude and timing of magmatic processes in the EPR ISS

C.J. Russo¹*, K. Rubin¹, & M. Perfit²

Corresponding author: crusso@soest.hawaii.edu
¹University of Hawaii, Department of Geology and Geophysics, Honolulu, HI, 96822
²University of Florida, Department of Geological Sciences, Gainesville, FL, 32611

Abstract:
U-series disequilibrium in volcanic rocks can be used as a unique geochemical tracer of petrologic processes because these isotopes are sensitive to the timescales over which magmatic processes occur. Previous work has shown that young lavas in the EPR ISS bullseye display a fairly limited range in ²³⁸U-²³⁰Th-²²⁶Ra radioactive disequilibria, but fall within a broader field of data from the 9-10º N segment interpreted to reflect variable rates of melt-sampling and melt-transport in the sub-ridge mantle. We have been focusing on the variations of these isotopes in lavas from the 2005-06 eruption in order to study melt dynamics within a single eruptive sequence that may have been supplied by multiple magma types. Our data span a small (²²⁶Ra/²³⁰Th) range (2.26 to 2.50), have essentially no variation in (²³⁰Th//²³⁸U) (1.110 to 1.131) and do not correlate with indices of magma differentiation such as MgO. This contrasts with prior works suggesting a substantial range among 1991-92 and older lavas within the bullseye region of the ISS (e.g., Sims et al., 2002). Despite the limited range observed in (²²⁶Ra/²³⁰Th) and (²³⁰Th/²³⁸U), we have measured considerably more variability in Th, U, Pb, Ba and Ra concentrations in the 2005-06 lavas which, correlate moderately well with MgO. The 2005-06 lavas are somewhat more differentiated than 1991-92 lavas collected at the same general lat.-long. locations, and 2005-06 lavas extend to even higher incompatible element abundances at lower MgO content at the northern and southern extremes of the eruption (locations that were not sampled or known to have experienced an eruption in 1991-92). Collectively these observations are consistent with this sequence of eruptions being supplied from a chemically zoned magma body that resided and evolved in the crust over short (perhaps even decadal) timescales (see also abstracts by Goss et al. and Perfit et al. for this workshop). The spatial and temporal distribution of this chemical zonation suggests variations in compositional attributes for the magmas feeding the 2005-06 eruption that likely reflect a segmented and heterogeneous magmatic system fed by more than one mantle source.

Keywords:
U-series, magmatic timescales, basalt geochemistry, heat budgets, 2005-06 eruption, EPR-ISS

Contributions to Integration and Synthesis:
We are currently integrating our results with those of other workers reflecting hydrothermal variability, eruption timing, shorter-lived U-series nuclides, radiogenic isotopes, eruption volume and seismicity records in an attempt to place ever tightening quantitative constraints on the timing of melt supply, magma recharge and magma residence to this mid-ocean ridge integrated-studies site. Such integrated data sets will be crucial for the development of models describing the thermal evolution of the upper mantle and crust, and therefore collectively provide a necessary link in our understanding of these systems from mantle to microbe.