Studies of the production rate of cosmic-ray produced ¹⁴C in rock surfaces

We have developed a method of extracting ¹⁴C in rock samples which is produced by the direct spallation of oxygen in the rock. The low levels of ¹⁴C produced in rocks, approximately 10⁶ atoms/g of rock at an altitude of 2000 m, are detectable only by accelerator mass spectrometry (AMS). This radioisotope can be extracted from the rock by melting in an rf furnace in a flow of oxygen, and earlier work has discussed this in some detail. Several improvements have been made to this system, allowing for processing of up to 100 g of rock sample at one time, and reduction of the blank. Our measurements from Tabernacle Hill, Utah, an intermediate-altitude basalt with well-constrained conventional ¹⁴C ages, yield an estimated cosmogenic ¹⁴C production rate of about 50 ¹⁴C atoms/g/yr. When corrected to sea level we obtain a value of 20 ± 2 ¹⁴C/g/yr for high-latitude samples.