New Mexico Geological Society Annual Spring Meeting — Abstracts

The buildup of in situ-produced radiocarbon (¹⁴C_is) is potentially useful for dating geomorphic surfaces exposed over the last 2000 to 20,000 years. We are developing techniques for extracting ¹⁴C_is from the McCartys, Bandera.and Twin Crater flows of the Zuni-Bandera volcanic field. Our samples are from pahoehoe flows with clear indication of original surfaces. Surface samples consist of the upper 2 to 4 cm of unshielded, horizontal pahoehoe slabs with less than 1 cm erosion and no evidence for shielding by soils, aeolian sediments, or nearby objects. All of the flows are dated by one or more previously published conventional radiocarbon dates. Shielded samples greater than 2 m below flow surfaces were also taken from lava tubes and road cuts; The shielded samples test our ability to remove contaminant ¹⁴C and to detect subsurface ¹⁴C production mechanisms. The principle challenge is removing contaminant ¹⁴C from rock surfaces covered with rock varnish and lichens. Our most promising method involves adapting conventional carbon extraction techniques used for σ¹³C measurements to sample sizes of 15 to 40 grams. Samples of this size are required in order to extract statistically significant amounts of ¹⁴C_is Whole rock samples are coarsely crushed, leached in solutions of concentrated HCI and HNO₃ to remove carbonates and rock varnish, thoroughly rinsed in distilled water, and heated at 600°C for 1 hour. The sample is then fused at 1200°C for 2 hours on a vacuum line in a circulating atmosphere of O₂ at 50 torr. CO₂ extracted from the sample is trapped in liquid nitrogen, purified, then reduced to graphite. The activity of ¹⁴C in the graphite is determined with accelerator mass spectrometry. Preliminary results indicate that shielded samples contain less than 7 x 10⁴ atoms of ¹⁴C gm^-1, indicating removaI of most contaminant ¹⁴C. A surface sample from the McCartys flow contains 2.16 x 10⁵ atoms ¹⁴C gm^-1. Conventional ¹⁴C dates on charcoal from rootlets at the base of the McCartys flows give calibrated ages of 3332-3292 and 3271-3074 cal yr bp (1 σ). The apparent ¹⁴C_is production rate from the McCartys surface is 37 ± 10 ¹⁴C atoms gm^-1 SiO₂ yr^-1 (normalized to rock surface, sea level, and latitude > 60°; rock density 2.5 ± 0.2 g cm^-3, absorption mean free path 160 ± 15 g c,^-2, geographic latitude 35.08°). This production rate is significantly greater than rates of 19 ± 3 ¹⁴C atoms gm^-1 1SiO₂ yr^-1 determined by other workers using a 17,800 cal yr bp surface. Further work is needed to determine if this variation in production rate is real.