CHLORINE AS AN INDICATOR OF CRUSTAL CONTAMINATION AND LITHOSPHERIC METASOMATISM IN THE RIO GRANDE RIFT AND JEMEZ LINEAMENT, NEW MEXICO

The origins of “arc-like” trace element signatures in basaltic lavas within the Rio Grande Rift and Jemez Lineament have long been debated. High ratios of fluid-mobile to fluid-immobile elements are often considered reliable tracers of metasomatism of the lithospheric mantle as a result of addition of a slab-derived fluid or melt, rich in fluid-soluble elements. However, many of these geochemical signatures are also shared by the regional granitic basement. Therefore, assimilation of minor amounts of granitic basement may also result in “arc-like” trace element signatures. In contrast, crustal estimates of Cl abundances are relatively low compared to a potentially large flux of Cl originating from a subducting slab. Therefore Cl abundances relative to other fluid-mobile elements may provide a way to distinguish crustal contamination from subduction modification of the mantle. To distinguish between these respective sources of fluid mobile elements, olivine- and orthopyroxene-hosted melt inclusions from Rio Grande Rift and Jemez Lineament basalts are utilized to track the evolution of individual magma batches.

In several cases where crustal contamination has previously been documented, Cl/K ratios in melt inclusions decrease with increasing melt evolution and assimilation. In the case of Paricutin Volcano, Mexico, Cl/K decreases with increasing SiO₂ and Ba/Nb, consistent with the hypothesis that assimilation of granitic basement results in low Cl/K ratios. In basaltic lavas of the Rio Grande Rift and Jemez Lineament Cl/K and Cl/Nb ratios are positively correlated to Ba/Nb and Sr/Nd and show little variation as a function of SiO₂ or host composition. This is inconsistent with the proposed model for crustal contamination. However, the positive correlation between Cl/K (and Cl/Nb) and Sr/Nd and Ba/Nb is consistent with the model for a mantle source enrichment in fluid mobile elements, characteristic of a subduction-derived fluid. Subduction modification of the lithospheric mantle beneath the Rio Grande Rift may have occurred both during Proterozoic subduction, and shallow subduction of the Farallon Plate (~80-40 Ma). Measuring Cl variability in basaltic melt inclusions both along and across the rift may help to distinguish between potential sources of subduction fluids.