CU-ISOTOPE HETEROGENEITY IN THE LITHOSPHERIC MANTLE: A ROLE FOR SUBDUCTION-DERIVED FLUIDS?

The Cu-isotope composition of the mantle is thought to be homogeneous (0.06 ± 0.20‰), and high temperature mantle processes are not believed to significantly fractionate Cu isotopes. However, metasomatized peridotites exhibit a significantly larger range of δ⁶⁵Cu values than mantle-derived basalts and unmetasomatized mantle lithologies (‒0.6 to +1.8‰). To better understand the origin of this heterogeneity, we are investigating a suite of ultramafic xenoliths hosted in Plio-Pleistocene-age alkali basalts from the Geronimo Volcanic Field, SE Arizona. The suite includes both Type I (i.e. Cr- and Mg-rich spinel lherzolite, harzburgite, and websterite) and Type II (Fe-, Al- and Ti-rich clinopyroxenite, wehrlite, and amphibole clinopyroxenite) lithologies. Evidence for both cryptic and modal metasomatism exists within the xenolith suite. The Cu isotope compositions of 18 mantle peridotites and 5 alkali basalt have been determined using both an aqua regia (AR) leaching technique and full wholerock (WR) dissolution. The WR δ⁶⁵Cu value for one unmetasomatized Type I lherzolite (+0.2‰) is within the published range for bulk silicate earth, whereas metasomatized Type I rocks range to heavier values (+0.01 to +1.12‰); AR leaching results for the same samples are much lighter (‒0.8 to ‒0.4‰). Type II xenoliths exhibit a much wider range in compositions than Type I (WR = ‒0.05 to +1.44‰; AR = ‒0.29 to +3.88‰). These compositional variations are also observed at the hand specimen scale. AR leaching results for one composite xenolith (a Type I lherzolite crosscut by a Type II clinopyroxenite vein) yield heavier δ⁶⁵Cu in the vein (+2.2‰) than in the lherzolite wall rock (‒0.8‰). Host basalts have lighter δ⁶⁵Cu than most xenoliths (‒1.3 to ‒0.23‰) and so cannot be responsible for the spread to heavy δ⁶⁵Cu seen, particularly in Type II peridotites. These data suggest that the lithospheric mantle is heterogeneous with respect to δ⁶⁵Cu, particularly among Fe-Al-Ti-rich Type II rocks. The heavy isotope ratios observed suggest a role for recycling of Cu via subduction of hydrothermally altered oceanic lithosphere, which has been reported to have δ⁶⁵Cu values up to +5‰. Future work will focus on identifying the mineral host(s) for the Cu in Type II xenoliths and the origin of the sample-scale heterogeneity observed.