Re–Os constraints on harzburgite and lherzolite formation in the lithospheric mantle: a study of northern Canadian Cordillera xenoliths

Abstract

Osmium isotope data from lherzolite and harzburgite xenoliths of the northern Canadian Cordillera provide constraints on the genesis and age of the lithospheric mantle in a typical off-cratonic continental setting. The 187Os/ 188Os ratios of the lherzolites show a positive correlation with Al 2O 3 and heavy rare earth elements (HREE), which probably reflects Re/Os fractionation during various degrees of mantle melting, followed by a long period of radiogenic ingrowth. These observations are consistent with melting during the Proterozoic. Harzburgite Os isotopic ratios, however, plot above the regional correlation of the lherzolites. A positive correlation between their Os isotopic ratios and 1/Os concentrations suggests that they are the end result of the introduction of metasomatic agents with low Os contents, but high 187Os/ 188Os ratios, into the lithospheric mantle. These fluids or melts may have originated from a region of anomalously slow mantle detected seismically (Frederiksen et al., 1998) below harzburgite-rich xenolith localities (Shi et al., 1998). Alternatively, the radiogenic Os-bearing metasomatic agents may have been related to subduction processes along the western margin of the Canadian Cordillera, as has been previously suggested to explain the high Os isotopic ratios of xenoliths from the northern US Cordillera (Brandon et al., 1996a).