Re-Os Isotopes Distinguish Crust vs. Slab Inputs to Northern Cascade Arc Basalts

Abstract:

Delineating the relative contributions of mantle, slab, and crust to arc magmas is particularly challenging in the Cascades where the continental crust is juvenile and contrasts little with magmas in traditional radiogenic isotope systems (Sr-Nd-Hf-Pb). The Re-Os isotope system offers a sensitive technique for evaluating these contributions because even young crust has significantly higher Os ratios than the mantle. We analyzed Re-Os isotope ratios in 33 primitive basalts from 9 volcanic centers of the northern Cascade Arc (Garibaldi Volcanic Belt, GVB). Although GVB basalts have mantle-like Sr-Pb-Nd-Hf ratios (Mullen & Weis, 2015, EPSL), the range in ¹⁸⁷Os/¹⁸⁸Os is very large (0.13-0.99) with [Os] of <1 to 17 ppt. Ten samples of subducting Cascadia Basin sediment and altered Juan de Fuca MORB are associated with high ¹⁸⁷Os/¹⁸⁸Os (0.68-0.92) and [Os] = 25-132 ppt. The GVB basalts define two groups: Group 1 (Cinder Cone/Mt. Garibaldi, Salal Glacier, Mt. Meager, Indian Pass/Glacier Peak) has low Os isotopic ratios (0.13–0.19), only slightly elevated relative to global mantle wedge peridotites (≤0.16), indicating minimal crustal contamination. Group 1 samples lie on Os-Sr isotope mixing curves indicating variable sediment input to the mantle. Os ratios of Group 2 basalts (Silverthrone, Bridge River, Elaho, Cheakamus, Mt. Baker, Glacier Peak) extend to extremely high values, up to 0.99, and lie on different Os-Sr mixing curves indicating addition of a crustal contaminant. Sr-Nd-Hf-Pb isotopic ratios cannot be used to identify this crustal input because Groups 1 and 2 are identical in these isotope systems. Interaction with a mafic underplate from older Cenozoic or accreted Mesozoic arcs is likely, and Re-Os analyses of basement samples (in progress) will provide a test of this hypothesis. This study reveals that most primitive magmas in the Cascades have suffered variable crustal contamination, but only the Re-Os isotope system has the potential to delineate the extent of this involvement.