Boron isotopic variations in NW USA rhyolites: Yellowstone, Snake River Plain, Eastern Oregon

Ivan P. Savov, William P. Leeman, Cin-Ty A. Lee, Steven B. Shirey
Journal of Volcanology and Geothermal Research, 2009 188:162-172

Abstract:
The geochemistry of NW USA rhyolites correlates strongly with geography and the nature of the underlying basement terranes. Rhyolites from the Snake River Plain-Yellowstone (SRPY) province have higher ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁶Pb, and lower ¹⁴³Nd/¹⁴⁴Nd than those from the Oregon High Lava Plains (HLP) province, reflecting a dominant influence of Precambrian cratonic crust east of the western Idaho suture zone versus accreted oceanic terranes of Phanerozoic age to the west. Rhyolites from the cratonic domain show significant enrichments of Th, U, and LREE/HREE, whereas B concentration and especially B/Nb and B/Rb are systematically higher west of the tectonic boundary. This decoupling of B from the other incompatible elements is best explained in terms of distinctive magmatic sources east and west of the suture zone. B isotopic composition [σ¹¹B] was measured for natural and synthetic glasses via multiple multiplier laser ablation-ICP-MS. σ¹¹B values are systematically lighter in SRPY rhyolites (-5.6 to -8.9 ‰) compared to those from the HLP (-0.8 to -3.1 ‰). These data are consistent with strongly fluid-depleted and/or metamorphosed sources for SRPY rhyolites, whereas HLP sources resemble those of typical oceanic basalts, and could reflect melting of juvenile basalt-derived protoliths in the crust. B isotope ratios of low-d18O rhyolites are indistinct from those with normal σ⁸O, suggesting that σ¹¹B values are not strongly affected by hydrothermal processes that alter source materials with meteoric water. Considering all data, it is likely that B compositions of the rhyolites are inherited from their sources in the crust. Although low σ¹¹B (<0 ‰) is also observed in many mantle-derived basalts (i.e., OIB, MORB), in SRPY rhyolites it is associated with enrichments of elements (e.g., U, Th, Rb, LREE) typically concentrated in continental crust, and thus old highgrade metamorphic continental crust is inferred to be¹¹B-depelted. If rhyolite protoliths were originally metasediments, it is likely that bulk B and σ¹¹B were selectively removed by metamorphic dehydration reactions and transported to the surface via ¹¹B-enriched fluids.

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