BORON ISOTOPIC VARIATIONS IN NW USA RHYOLITES: YELLOWSTONE, SNAKE RIVER PLAIN, EASTERN OREGON

Boron isotopic composition was determined in rhyolites from the Snake River Plain- Yellowstone (SRPY) hotspot track and the High Lava Plains (HLP) of Eastern Oregon to explore its utility in understanding magmatic sources and processes. Because B is strongly fluid-mobile element absent in the upper mantle and enriched in sediments and crust, it is an excellent tracer for interactions between hotspot-related melts, crustal fluids and metamorphic basement. Compositions of NW USA rhyolites correlate strongly with geography and the nature of the underlying basement terranes. Those from the SRPY province have higher ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁶Pb, and lower ¹⁴³Nd/¹⁴⁴Nd than those from the HLP, reflecting a dominant influence of Precambrian cratonic crust east of the western Idaho suture zone versus accreted oceanic terranes to the west. Rhyolites from the cratonic domain also show significant enrichments of Th, U, and LREE/HREE. However, B contents and especially B/Nb and B/Rb are systematically higher west of the tectonic boundary. Decoupling between B and the other incompatible elements is significant, because it requires processes other than normal magmatic differentiation.

B isotopic measurements were made on natural and synthetic glasses via Multiple Multiplier Multi Collector Laser Ablation-ICP-MS at DTM. B isotopic compositions are lighter in SRPY rhyolites (δ¹¹B range: -5.6 to -8.9 ‰) compared to HLP (δ¹¹B range: -0.8 to -3.1‰); these data are consistent with strongly fluid-depleted and/or metamorphosed sources for SRPY, whereas HLP sources resemble those of MORB. B isotope ratios of low-δ¹⁸O rhyolites are indistinct from those with normal δ¹⁸O, suggesting that δ¹¹B values are not strongly affected by hydrothermal processes, but rather inherited from the melt source regions. Although low δ¹¹B is also observed in mantle-derived basalts (i.e. OIBs), in the SRPY rhyolites it is associated with enrichments of U, Th, Rb, LREE typical for continental crust, and thus light δ¹¹B must be characteristic of the old metamorphosed continental crust as well. Accordingly, SRPY protoliths are inferred to be high-grade metamorphic rocks. If they represent metasediments, it is likely that bulk B and ¹¹B were selectively removed by metamorphic dehydration reactions and transported to the surface via ¹¹B-enriched fluids.