CU-ISOTOPE HETEROGENEITY IN THE LITHOSPHERIC MANTLE: EVIDENCE FROM TYPE I AND TYPE II PERIDOTITE XENOLITHS FROM THE GERONIMO VOLCANIC FIELD, SE ARIZONA

Mantle-derived magmas (e.g. MORB & OIB) exhibit relatively homogeneous δ⁶⁵Cu of ~0 ± 0.2‰, suggesting that high-temperature mantle processes do not fractionate ⁶⁵Cu/⁶³Cu. Analyzed mantle rocks (orogenic peridotites and lithospheric peridotite xenoliths), however, exhibit a wider range of values of -0.68 to +1.82‰, which has been attributed to mantle metasomatism. These data, however, are limited to Type I, olivine-rich peridotites (Cr- and Mg-rich lherzolites, harzburgites and dunites) where evidence of metasomatism is ‘cryptic’, e.g. LREE/HREE enrichment. We report results for both Type I and Type II (Fe- and Al-rich clinopyroxenites, wehrlites and amphibole clinopyroxenites) mantle xenoliths from the Geronimo Volcanic Field (GVF) of SE Arizona. Type II xenoliths are interpreted as cumulates from crystallization of basaltic melts in the lithospheric mantle; as such, they potentially retain a record of the percolating melts and metasomatic processes inferred from the cryptically metasomatized (i.e. LREE enriched) orogenic peridotites. New geothermometric analyses, based on Al partitioning between olivine and spinel, yield temperatures of 983 – 1187˚C and overlap previous temperature estimates based on two-pyroxene geothermometry of associated spinel lherzolites (1069 to 1096˚C), suggesting the xenoliths were derived from somewhere near the top of the lithospheric mantle. The Cu-isotope values for Type I xenoliths from GVF show a limited range of +0.27 to +0.38‰, within the range of unmetasomatized orogenic lherzolites worldwide. In contrast, Type II xenoliths range from +0.14 to +1.44‰. Host basalts are isotopically light (-0.23 to -1.30‰), compared with the entrained xenoliths, i.e. interaction with the host basalt is not responsible for the compositional variation in the xenoliths. Sulfide mineral petrography and microprobe analysis is ongoing to better understand the mineral host(s) for the Cu and the role these minerals play in fractionating Cu-isotopes in the mantle.