New Mexico Geological Society Annual Spring Meeting — Abstracts

New ⁴⁰Ar/³⁹Ar age detenninations of basalts from the Zuni-Bandera, Red Hill-Quemado, and Potrillo volcanic fields have been obtained as part of an ongoing study exploring intercalibration of the ⁴⁰Ar/³⁹Ar dating technique with other methods (including ¹⁴C, ³He, and ³⁶CI) of dating young basaltic rocks. Procedures used in this study include resistance-furnace and laser step-heating of phenocryst-free "whole-rock" matrix concentrates and separates of xenocrystic anorthoclase.

Approximately two thirds of matrix concentrate samples yield consistent plateau or isochron ages with precisions on the order of ±15 to ±100 ka (1 sigma); these data are interpreted as relatively accurate records of eruption age. The remaining one third of matrix concentrate samples have low radiogenic yields and non-flat age spectra probably related to hydration of matrix glass and/or alteration of plagioclase. Neither isochrons nor age spectra from matrix concentrates indicate significant excess argon.

Anorthoclase xenocrysts from some localities in the Potrillo volcanic field give nearly flat, high-radiogenic yield, precise (±2 to ±20 ka) age spectra interpreted as records of eruption age of the enclosing basaltic host rock. Anorthoclase xenocrysts from other localities give strongly climbing age spectra potentially indicative of incomplete degassing of ⁴⁰Ar prior to eruption.

Results from matrix concentrates of basalts of the Potrillo field range from 59 ± 10 ka for a lava in Potrillo Ma,ar to 902 ±70 ka for a reversed polarity lava near the western part of the volcanic field. Paired samples of anorthoclase xenocrysts and matrix concentrates from two flows in the western part of the volcanic field give nearly concordant ages which illustrate the potential for anorthoclase xenoliths to yield precise, accurate eruption ages (Flow 1: anorthoclase =521 ± 17 ka, matrix = 475 ± 23 Ma; flow 2: anorthoclase =847 ± 2 ka, matrix =916 ± 67 ka). ⁴⁰Ar/³⁹Ar ages from several localities agree well with He exposure ages determined by E. Anthony, W. Williams, and J. Poths. One basalt from Malpais Maar gives widely discordant ⁴⁰Ar/³⁹Ar (405 ± 20 ka) versus He exposure (54 ka ) ages. If both data are correct, they may reflect an interval of post-eruption burial by surge deposits follewed by erosional exhumation of the flow surface.

Results from matrix concentrates of basalts from the Red Hill-Quemado volcanic field range from 0.71 ± 12 ka to 7.92 ± 0.2 Ma, and indicate regular periodic basaltic volcanism in this Iarea from 8 Ma to the present Intervals between eruptions were generally 0.5 to 1.0 million years, except for an apparent eruptive hiatus from 2.5 to 5.2 Ma.

Basalts from the Zuni-Bandera volcanic field have so far proven difficult to date by the ⁴⁰Ar/³⁹Ar method, compared with units from the Potrillo and Red Hill-Quemado fields. Many of the young (<0.5 Ma), low-potassium, tholeiitic lavas give low radiogenic yields and disturbed age spectra. A matrix concentrate from one young alkalic basalt, the Bandera flow, yields a flat spectrum with a plateau age of 41 ±7 ka, significantly older than the 10 ka ¹⁴C date obtained by W. Laughlin from charcoal beneath a cinder deposit associated with the flow. Abundant anorthoclase xenocrysts from this cinder deposit yield ages as old as 633 ± 70 ka, indicating incomplete degassing of excess or inherited ⁴⁰Ar present in the xenocrysts prior to eruption. It is possible that small amounts of xenocrystic contaminants are responsible for the anomalously old results from the matrix concentrate sample from the Bandera flow. Samples from the Cebollita Mesa basalts, older lavas exposed near the eastern edge of the Zuni-Bandera volcanic field, yield flat age spectra and plateau ages ranging from 3.5 to 4.1 Ma.