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New osmium isotope evidence for intracrustal recycling of crustal domains with discrete ages

¹ Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA
² U.S. Geological Survey, MS 910, Menlo Park, California 94025, USA
³ Carnegie Institution of Washington, Washington, D.C. 20015, USA

New ¹⁸⁷Os/¹⁸⁸Os ratios of Quaternary Mount Adams volcanic rocks from the Cascade arc in southern Washington vary by >300% (¹⁸⁷Os/¹⁸⁸Os = 0.165–0.564) and fall into high (>0.319) and low (0.166 to 0.281) groups of ¹⁸⁷Os/¹⁸⁸Os ratios that are substantially more radiogenic than mantle values. These Os isotope compositions and groupings are interpreted to reflect recycling of discrete intracrustal domains with high ¹⁸⁷Os/¹⁸⁸Os ratios but differing ages, thus recording the process of crustal hybridization and homogenization. Os isotope compositions provide new constraints on amounts of intracrustal recycling in young subduction-zone environments that reflect the magmatic history of the arc. Sr, Nd, Hf, and Pb isotope variations in this young, mafic arc complex are too small to allow such constraints.

Key Words: Mount Adams • Cascade Range • crust • recycling • osmium • hybridization