New Mexico Geological Society Annual Spring Meeting — Abstracts

As part of a 1993 field conference addressing the difficult problem of dating Quaternary basalts, we sampled 9 young basalt flows in the Zuni-Bandera volcanic field for surface age determinations using measurement of the cosmogenic ³⁶CI buildup in the exposed surface of a
lava flow. ³⁶CI accumulates in rocks exposed to cosmic rays at a rate proportional to the concentrations of target nuclides, mainly ³⁵CI, ³⁹K, and ⁴⁰Ca. The production rate of ³⁶CI from the parent nuclides is known, as is the decay rate of the long-lived ³⁶CI radionuclide (half-life = 301,000 yrs). The buildup of ³⁶CI is thus known as a function of time, and correcting for chemical composition of the rock as well as longitudinal, latitudinal and elevational variations in production rates, it is possible to calculate the exposure time of a rock at the earth's surface, which, for a volcanic rock, should be the eruption age.

Samples were taken of 8 Zuni-Bandera flows, including the McCarthys, Bandera, Twin Crater, Laguna, Bluewater, Ramah Navajo, Fence Lake, and North Plains. The best available flow surfaces were sampled, in areas where there was 00 shielding of cosmogenic rays by local landforms. Samples were collected of the top 5 cm of flow surface. The flow surface present on the first three flows listed above was good, the next two were fair, and the final three were poor. In the laboratory, samples were crushed, then sieved and leached to remove any clay or carbonate that might contain meteoric Cl. The CI content of the samples was determined by specific ion electrode measurement, using a diffusion cell technique to remove the Cl from the sample matrix. The mean CI contents (n=3-5) of the samples listed above are: 63.0±3, 52.0±2, 79.1±5, 61.8±2, 53.5±5, 57.2±2, 67.1±3, 75.6±4.

The CI for ³⁶Cl analysis was removed from ground rock samples by dissolving the rock in a mixture of hot concentrated hydrofluoric+nitric acid. The CI was precipitated from the dissolved sample using silver nitrate. A number of cleaning steps followed the initial Cl sample separation, including a number of steps to remove S from the sample by precipitating the S as barium sulfate. Removal of S is important due to an isobaric interference of S on ³⁶Cl. The pure CI samples were then analysed using the tandem accelerator mass spectrometer at Purdue University. The ³⁶CI exposure ages are as follows: Bandera 9.5±0.9 ka; Twin Craters 17.8±0.9 ka; Laguna 33.4±3 ka; Bluewater 35.6±3.4 ka; Ramah Navajo 184±35 ka; Fence Lake 232.8±21 ka; and North Plains 77.8±7.2 ka. The errors shown here are simply analytical, and do not take into account uncertainties due to erosion or in production rates. Unfortunately, we do not yet have a exposure age date for the Bandera flow. Although two samples were submitted for analysis, both had excessively high S despite several S removal steps during CI purification. The exposure ages of the Bandera and Twin Craters flow agree well with ¹⁴C and ³He determination. The Bluewater flow ³⁶CI age is younger than both ³He exposure age and U/Th disequilibrium ages (see Laughlin et at., 1994), although if the error based on low degrees of erosion were quantified, the ³He and ³⁶Cl determinations would be likely to overlap. The effect of erosion on ³⁶Cl exposure ages results in an apparently younger age, and this process has certainly affected the exposure age determinations of the Ramah Navaho, Fence Lake, and North Plains basalt flows. Particularly the North Plains flow, which had undergone a high degree of erosion, yielded a very anomalously young age.