Basalt volatile fluctuations during continental rifting: An example from the R io G rande R ift, USA

Hydration and metasomatism of the lithospheric mantle potentially influences both the magmatic and tectonic evolution of southwestern North America. Prior studies have suggested that volatile enrichments to the mantle underlying western North America resulted from shallow subduction of the Farallon Plate during the Laramide (∼74–40 Ma). This study examines temporal and spatial variations in volatile elements (H 2 O, Cl, F, and S) determined from olivine and orthopyroxene‐hosted melt inclusions along and across the Rio Grande Rift, the easternmost extent of Laramide shallow subduction. Maximum chlorine enrichments are observed in the southern rift with a Cl/Nb of ∼210 and reduce with time to MORB‐OIB levels (∼5–17). Measured water abundances are <0.8 wt % in rehomogenized inclusions; however, calculated H 2 O, based on Cl/Nb systematics, primarily varies from 0.5 to 2 wt % H 2 O. Sulfur abundances (<0.61 wt %), and calculated sulfide saturation, indicate magmas with high Cl/Nb also contain oxidized sulfur. The abundance of fluorine in melt inclusions (up to 0.2 wt %) is not correlated to other volatile elements. Temporal variations in melt inclusion volatile abundances coupled with varying isotopic (Sr‐Nd‐Pb) whole‐rock systematics suggest a transition from lithospheric to asthenospheric melt generation in the southern RGR and potential lithosphere‐asthenosphere melt mixing in the central RGR. East to west decrease in volatile enrichment likely reflects a combination of varying mantle sources and early removal of metasomatized lithospheric mantle underlying regional extension. Results indicate, from multiple causes, subduction‐related volatile enrichment to the lithospheric mantle is ephemeral, and strong enrichments in volatiles are not preserved in active magmatic‐tectonic provenances.