Fluid-Mobile Elements as Tracers in Intraplate Volcanic Environments: Mt. Erebus, Ross Island, Antarctica.
Gene Foster, Jeffrey G. Ryan (Department of Geology, University of South Florida)
Philip Kyle (Dept. Earth and Environmental Sciences, New Mexico Tech)
The "fluid mobile" elements B, Cs, Rb, Pb, U, and Li are powerful tracers of subduction-related inputs to mantle source regions beneath volcanic arcs. The nature of subduction inputs to the mantle sources of intraplate volcanic systems has been debated for many years. This suite of elements (in particular B and Li) may offer insights into the role of subduction in producing intraplate sources, but only if the effects of open system, magmatic differentiation processes can be resolved.
The extensively characterized suites of lavas from Mt. Erebus, Antarctica, afford a unique opportunity to examine the systematics of Li, B, and other fluid-mobile species during the progressive differentiation of intraplate parental magmas. Representative samples from the Erebus Lineage and EFS series lavas (after Kyle et al. 1991) representing compositions from picrite to phonolite, have been analyzed for light element contents. Li abundances for these samples were determined by isotope dilution mass spectrometry at Dept. Terrestrial Magnetism, while Be and B are being measured via DC Plasma emission spectrometry at USF. Alkali basalts from the Crary Mountains, a Pliocene volcanic province in Marie Byrd land (Panter et al. 2000) have also been analyzed to try and resolve changes in light element signatures in the sub-Antarctic mantle over time.
While Be, Li, and other incompatible species increase regularly with increasing SiO2 in Erebus lavas, the incompatible element ratios Li/Yb, U/Nb, and Rb/Nb show marked increases in lavas with SiO2 > 50% wt. By contrast, Be/Nd ratios show no apparent increase, as expected given the largely similar partitioning behaviors of these elements during shallow level crystallization. While changes in crystallizing mineral assemblages cannot be ruled out, it is probable that progressive assimilation of crustal materials beneath Erebus may be increasing abundances of Li, U, and Rb. Given these effects, one must examine intraplate magmatic systems with some care to determine which lavas (if any) may reflect mantle chemical signatures.