CU-ISOTOPE HETEROGENEITY IN MANTLE PERIDOTITE XENOLITHS FROM THE GERONIMO VOLCANIC FIELD, SE ARIZONA: IS METASOMATISM A POSSIBLE MECHANISM FOR FRACTIONATING STABLE CU ISOTOPES IN THE LITHOSPHERIC MANTLE?

There are few published data on the stable Cu-isotope composition of the mantle. Studies to date have inferred that (i) δ⁶⁵Cu in the mantle is relatively homogeneous, ranging from 0 to 0.1‰, and (ii) high temperature mantle processes do not significantly fractionate Cu isotopes. However, new data (Liu et al., 2015) show that metasomatized peridotites exhibit a significantly larger range of δ⁶⁵Cu values than unmetasomatized mantle lithologies. This suggests that mantle processes, in particular mantle metasomatism, may be able to fractionate Cu isotopes under certain conditions. To test this hypothesis, we are investigating a suite of ultramafic xenoliths hosted in Pleistocene- to Pliocene-age alkali basalt flows from the Geronimo Volcanic Field, SE Arizona. This suite includes a wide range of mantle rock types, including Type I (i.e. Cr- and Mg-rich) spinel lherzolites, harzburgites, and websterites as well as Type II (Fe-, Al- and Ti-rich) clinopyroxenites, wehrlites, and amphibole clinopyroxenites. Evidence for both cryptic and modal metasomatism exists within the xenolith suite. The Cu isotope compositions of 7 mantle peridotites and 1 alkali basalt have been determined using the aqua regia leaching technique of Mathur et al. (2009). Mantle samples were chosen to represent the range of lithologies present, including metasomatized and unmetasomatized Type I peridotites as well as a range of Type II lithologies. Preliminary δ⁶⁵Cu values for the xenolith suite range from -0.4 to 3.9‰, with Type I rocks exhibiting a much smaller range than Type II lithologies (-0.4 to -0.8‰ vs 0.3 to 3.9‰). These compositional variations are also observed at the hand specimen scale. One composite xenolith, consisting of a Type I spinel lherzolite crosscut by a Type II clinopyroxenite vein, has a significantly higher δ⁶⁵Cu in the vein (+2.2‰) than in the lherzolite wall rock (-0.8‰). The alkali basalt, host for several of the analyzed xenoliths, has a δ⁶⁵Cu of 1.6‰. These data suggest that a significant range in δ⁶⁵Cu may exist in the lithospheric mantle, particularly among Fe-Al-Ti-rich Type II rocks. To our knowledge, this study represents the first analysis of Type II mantle peridotites. Future work will focus on identifying the mineral host(s) for the Cu in Type II xenoliths and the role they play in fractionating Cu-isotopes in the mantle.