VOLATILES IN OLIVINE HOSTED MELT INCLUSIONS FROM THE CARRIZOZO LAVA FLOW, SOUTHERN RIO GRANDE RIFT, NEW MEXICO

The Carrizozo lava flow erupted ~5,200 years ago along the eastern margin of the southern Rio Grande Rift (RGR), New Mexico. This eruption is interesting in that it produced a 75 km long flow -good for investigating factors important to the emplacement of long lava flows. Also, it is one of the few young lavas in the southern RGR that has previously been attributed to lithospheric, rather than asthenospheric, melting. This may be related to the presence of thicker mantle lithosphere along the eastern margin of the rift, consistent with Sr-Nd-Pb isotope systematics. We measured major-, trace-, and volatile (H₂O, S, Cl, F) elements from olivine hosted melt inclusions. Volatile concentrations are of particular interest because they may impact the way the lava is erupted and how it flows once reaching the surface. In addition, we can compare geochemical evidence for volatile element enrichment to other lavas of the region to determine possible magmatic sources. Undegassed melt inclusions contain sulfur concentrations ranging from 0.2 wt% to 0.07 wt% (0.10 wt% avg.), Cl from 0.12 wt% to 0.08 wt% (0.10 wt% avg.), F from 0.04 wt% to 0.01wt % (0.03 wt% avg.). No detectable water was measured in the melt inclusions. The low volatile (water) concentrations from melt inclusions are consistent with the lack of pyroclastics near the vent.

Compared to young asthenospheric-derived southern RGR basalts, the Carrizozo lava is particularly enriched in Cl, relative to other incompatible elements. Cl/Nb ratios are variable, ranging from 17-30, consistent with other RGR basalts derived from subduction modified lithospheric mantle. However, other fluid mobile elements (e.g. Ba) show no relative enrichment and Ba/Nb ratios (8.7 avg.) are similar to basalts thought to be derived from the asthenospheric mantle. Lower Cl/Nb and Cl/K ratios appear to result from degassing however we see no evidence that crustal contamination has impacted the volatile concentrations. It is unclear at this time why we see high Cl/Nb and high Cl/K ratios and will require further investigation. The study of volatile concentrations in melt inclusions therefore provides a potential means to look at a variety of magmatic and tectonic processes, from lithospheric thinning and magma sources to how volatiles affect lava flow emplacement.