Shallow subduction of the Farallon plate during the Laramide orogeny (ca. 80–40 Ma) may have resulted in metasomatism of the western North American lithospheric mantle. Olivine- and orthopyroxene-hosted melt inclusions from the central Rio Grande Rift are variably enriched in chlorine relative to fluid-immobile elements. Subparallel trends in Cl/K versus Cl/Nb for alkali basalts and tholeiites can be explained by Cl/K fractionation during low degree partial melting, with DCl ≈ DNb < DK. The observed trace element enrichment does not correlate with host Mg# or melt SiO2 wt% as expected for crustal contamination via an assimilation–fractional crystallization (AFC) process. In addition, examples from other volcanic systems suggest that Cl/K decreases with increasing contamination, contrary to observed positive correlations between Cl/K and Ba/Nb and Sr/Nd. The positive correlation of Cl/K and Cl/Nb with typical indices of subduction enrichment (e.g., Ba/Nb and Sr/Nd) supports a model of mantle metasomatism during subduction.