MELT INCLUSIONS, BASALTIC MAGMATISM AND MANTLE METASOMATISM IN THE RIO GRANDE RIFT

The Rio Grande Rift in the southwest United States provides an excellent opportunity to investigate temporal and spatial variations in mantle sources and basaltic magmatism in an active continental rift setting. The Rio Grande Rift, extending from southern New Mexico to northwest Colorado, has undergone varying amounts of extension, with the total amount of extension decreasing northwards along the rift. Prior rheological studies have indicated that metasomatism and hydration may have a significant effect on the strength of the lithospheric mantle. In the present study we combine primitive melt inclusion geochemistry, whole rock geochemistry and tomographic imaging to examine mantle source regions and potential mantle metasomatism and its effects on basaltic magma compositions in the Rio Grande Rift, the easternmost expression of Basin and Range extension. Melt inclusions are critical to this investigation since they provide the only direct means to measure pre-eruptive magmatic volatile concentrations (S, Cl, F, and H₂O).

Major, trace, and volatile elements have been measured in melt inclusions from both tholeiitic and alkali basalts. Melt inclusions and whole rock compositions have been screened for potential crustal contamination based on negative correlations between Cl/K and Ba/Nb as described by Rowe and Lassiter (2009). For the purpose of identifying mantle metasomatism we have focused on chlorine variations in melt inclusions, specifically variations in Cl/Nb. The only significant temporal variation along the rift in Cl/Nb is in the southernmost region where the greatest Cl/Nb ratios are observed in olivine-hosted melt inclusions from a ~37Ma basalt (~150 Cl/Nb) compared to melt inclusions from a young (< 50ka) basalt (~10 Cl/Nb). For basaltic volcanism younger than 5 m.y. the highest Cl/Nb ratios are measured in the central Rio Grande Rift. By utilizing the formulations of Lee et al (2009) we have calculated temperatures and pressures of melt generation beneath the rift. Combining melt inclusion evidence for mantle metasomatism with temperature and pressure estimates of mantle melting from whole rock geochemistry and tomographic imaging we can finally begin to look at the relationship between metasomatism and regions of melt generation in the mantle beneath the rift zone.