MANTLE HYDRATION AND SR-ND ISOTOPE SYSTEMATICS IN THE YELLOWSTONE REGION

Any attempt to discern the nature of Yellowstone magmatism at depth requires a thorough appreciation of the chemical and physical nature of the pre-existing lithosphere. The well-documented isotopic peculiarities of the Archaean craton provide a clear geochemical contrast between Yellowstone-Snake River Plain “asthenospheric” melts and those derived from ancient lithosphere. Such is not necessarily the case in Proterozoic and younger terranes to the west. Seismic attenuation studies suggest there is also a marked difference in mantle hydration, imaging relatively dry Yellowstone hotspot mantle adjacent to markedly hydrated Archaean mantle. This hydration may serve to explain the isotope systematics of the Archaean mantle-derived melts, where Sr ratios are confined to a narrow range compared to those of Yellowstone basalts. Examination of the systematics for several suites of regional non-Yellowstone volcanic rocks indicate that Sr and Rb/Sr isotopic ratios may have been reset sometime in the late Phanerozoic whereas those of Nd and Sm/Nd were relatively unaffected. High Sr and low Nd concentrations are characteristic of seawater, which may have been introduced into the mantle during low-angle subduction of the Farallon plate and/or Eocene Absaroka magmatism. Such fluids would also be depleted in high-field-strength elements such as Nb. The Archaean mantle-derived suites are all low-Nb; while Yellowstone-Snake River Plain basalts are relatively high-Nb, as has been previously noted by many authors (e.g. Brandon and Goles, 1988). The Newberry volcanics of western Oregon are also low-Nb, making it less likely that they were derived from the same mantle source as Yellowstone magmas.