New Mexico Geological Society Annual Spring Meeting — Abstracts

Preliminary ⁴⁰Ar/³⁹Ar dating of the Ocate volcanic field along with previous K-Ar dating reveals an eruptive history spanning 0.8-8.2 Ma. The Ocate volcanic field lies in north-central New Mexico along the Jemez lineament just east of the Rio Grande depression within the transition zone between the Great Plains and the Rocky Mountain physiographic provinces. The field is a series of late Miocene-Pleistocene basaltic to dacitic flows whose physiographic expressions roughly reflect their ages, where the oldest flows cap the highest mesas and younger flows cap lower mesas. Precision (at ±2σ ) on previous conventional K-Ar dates for flows from the Ocate field are typically ±3-9%, whereas the precision on individual ages using the ⁴⁰Ar/³⁹Ar dating technique is typically ±0.5-1.5%. Thus, the ⁴⁰Ar/³⁹Ar dating technique has the potential to resolve past eruptive events differing in age by 100,000 years or less.

The volcanic evolution began with discrete erupti ve pulses focused in the northern and central portions of the fleld from 8.2 to 5.5 Ma. From 5.5 to 2.2 Ma activity increased and was less episodic throughout the field. A second stage of discrete eruptive pulses then followed from 2.2 to 0.8 Ma focused in the central and southern portion of the field including the monogenetic eruption of Maxson Crater at 1.58 ± 0.08 Ma. Four flows emanating from Maxson Crater were stratigraphically sampled at the Mora/Canadian confluence and all samples agree within error of each other and within error of samples collected proximal to the vent.

The new precise ⁴⁰Ar/³⁹Ar dates agree within error with previously published K-Ar dates for the same flows. Of the fifty samples dated thus far, the vast majority display well-behaved age spectra and minimal presence of excess Ar, 40Ar* loss, and 39Ar_K recoil. Duplicate analyses performed on aliquots of a given sample and of samples collected from separate flows proximal and distal from the vent show excellent within-sample and between-samples repeatability and agreement with stratigraphy. Geochronological studies of the neighboring Raton and Taos volcanic fields also show a similar range in age of volcanic activity and an increased period of activity ~2.5 to 4.5 Ma.