New Mexico Geological Society Annual Spring Meeting — Abstracts

We are investigating helium isotope systematics for Quaternary lavas inthe southern Rio Grande rift, with emphasis on well-preserved flow surfaces from the Potrillo volcanic field of southcentral New Mexico. The field is young; K/Ar dating indicates ages less than 1 Ma (Hoffer, 1976; Seager et al., 1984). Our research confirms this general estimate, with surface exposure dates ranging 280 ka to 20 ka.

The ³He surface exposure dates (figure) from this field are consistent with geologic field relationships. Further, duplicate surfaces from a single flow and multiple samples from a single feature for instance, the dates from Aden Crater) yield the same dates within analytical error. Duplicate analyses of ejecta from Hunt's Hole give dates of14 ± 7 and 17 ± 9 ka, which may represent a minimum exposure age due to some period of cover by surge deposits from that maar. Many individual volcanic centers have lifetimes less than the current resolution of the ³He dating technique (i.e. 5 to 10 ka). Our research is also focussed on determining the oldest flows for which we can obtain reliable ³He surface exposure dates. The oldest surface we have dated thus far is from a flow capping the surge unit erupted from Malpais maar.

Volcanic rocks in the Potrillo volcanic field are exclusively nepheline-normative, therefore this field differs from other volcanic fields within the axis of the rift. There is no relationship between the age of anyone volcanic center and its position in the geochemical trends. Some of the most evolved lavas comprise the oldest deposits. We have documented differing geochemistry for the maar-related lavas relative to the less explosively erupted flows comprising the majority of the field. The maare magmas represent small degrees of partial melt and lack a low pressure fractionation history. Overall, the volcanic centers show limited chemical variation relative to the entire field, suggesting that the various centers tap small, multiple magma chambers undergoing shallow-level differentiation.