Cu?Isotope Evidence for Subduction Modification of Lithospheric Mantle

Ultramafic xenoliths from southeastern Arizona, USA, provide evidence for Cu-isotope heterogeneity in the lithospheric mantle. We report new data on Type I (Cr-, Mg-rich) peridotites, but also first Fe-Ti-Al-rich II pyroxenite (±amphibole) xenoliths. Whole rock ?65Cu values of pyroxenites and cryptically metasomatized lherzolites range to isotopically heavier compositions than asthenospheric mantle (i.e., up +1.44‰ +1.12‰, respectively, vs. ?0‰ ± 0.2‰). Copper leached using aqua regia, assumed be hosted interstitial sulfides, is even more variable (?65Cu ?0.78 +3.88‰), indicating considerable isotopic within individual samples. Host basalts have low (?0.23‰ ?1.30‰), so basalt—xenolith interactions are not responsible compositional variations observed. While mass-dependent fractionation may partly responsible, metasomatism by fluids derived recycled crustal materials predominant control Amphibole megacrysts amphiboles separated amphibole-bearing clinopyroxenite normal, mantle-like 18O/16O ratios H-isotope (?2HSMOW ?82‰ ?45‰) that between nominally anhydrous (?80 10‰) seawater (0‰). enriched 34S relative depleted mantle, having ?34SCDT +8‰, i.e., commonly attributed a component or hydrated oceanic crust. These suggest formation metasomes beneath Basin Range Province was associated with subduction Farallon plate alkali basalt magmatism extension.