No abstract

Assessing water contents of subduction zone mantle peridotites can gain insight into the compositions of slab‐derived fluids/melts and the active margin water cycle. Here eight mantle xenoliths from Alligator Lake (northern Canadian Cordillera) are examined to address these issues. The harzburgites have less water, on average, but are more oxidized (ΔFMQ ~ 0.1) than the lherzolites (ΔFMQ ~ −1.0). The lherzolites have major and trace element compositions close to primitive mantle, while the harzburgite major element and heavy rare earth element compositions are indicative of higher degrees of melt depletion but with light rare earth element‐enriched profiles. Correlations between lherzolite pyroxene water contents and bulk rock Ba/Nb and Ba/Yb ratios likely result from interaction with subduction related fluids. The trace element compositions of the harzburgite clinopyroxenes are successfully modeled by melting of a fertile mantle lithosphere and interaction with a carbonatite melt. Correlations between the harzburgite water contents and clinopyroxene Ca/Al ratios and Mg# are also consistent with the influence of carbonatite metasomatism. Metasomatism likely resulted from opening of a slab window beneath the region, detected as a low‐velocity seismic anomaly, which heated and mobilized a heterogeneous mantle lithosphere veined with carbonatite. This study confirms that subduction zone mantle lithosphere is not necessarily more water‐rich or more oxidized than oceanic lithosphere or other off‐cratonic settings. Moreover, local oxidation is not necessarily related to the ingress of subduction zone fluids but can also be related to melting of a heterogeneous lithosphere following heating above a slab window.