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Ancient oceanic mantle below the Cascades inferred from the highly siderophile elements


Mantle harzburgite xenoliths from Lorena Butte, Simcoe Mountains volcanic field, Washington, are examined to infer processes occurring in the mantle wedge below the Cascade arc. Previous studies have characterized these xenoliths with radiogenic 187Os/188Os and high oxidation states, suggesting intermixing with a slab component with highly radiogenic osmium. In this study, I report osmium isotope systematics and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances from a new suite of xenoliths collected from Lorena Butte. These data are coupled with mineral analyses and whole-rock major- and trace-element abundances.The new 187Os/188Os measurements for ten harzburgites average 0.1231 ±20 and are less radiogenic than xenoliths previously reported from the same locality, while the new estimates of oxygen fugacity within the xenoliths agree with literature data. The Lorena Butte harzburgites resemble abyssal harzburgites in 187Os/188Os, HSE abundance systematics, major element chemistry, and estimated extent of melt depletion. The host melt bears a strong intraplate-melt signature despite its proximity to the Cascade arc. These geochemical lines of evidence suggest that the harzburgites are less influenced by subduction-related processes than previously thought. I propose that they represent mantle residuum from the oceanic lithosphere which has been entrained into the mantle wedge, and that the host magma represents an intraplate melt related to the melt which formed the Columbia River flood basalts. This illustrates the complex petrogenesis of the sub-arc lithosphere below the Cascades and suggests that subduction zones can preserve unusually old and melt-depleted mantle derived from a non-subduction tectonic setting.

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