26Al: measurement and applications

Abstract 26Al(r1/2 = 0.7 Ma), like 10Be (r1/2 = 1.5 Ma), is produced by cosmic rays in the atm osphere, but its production rate is low (ca 4 x 10-3 that of 10Be) and the crustal abundance o f 27Al is high, resulting in 26A1:27Al ratios rarely exceeding 10~14. Both isotopes are also produced in surficial rocks by cosmic rays; in quartz the production rates of 26Al and 10Be are ca. 70 atoms g-1 a-1 and ca. 10 atoms g-1 a-1, respectively (at sea level). Because quartz frequently contains 27Al at less than 100 p.p.m. (by mass), the 26Al :27A1 ratio resulting from this situ production can be as high as 3 x 10-11. In extraterrestrial m atter, where the production rates are several hundred times greater, samples containing a few percent o f 27Al have 26Al:27Al ratios of 10-11 or larger. We describe an experim ental arrangem ent that was developed at the University of Pennsylvania for measuring 26A1:27Al ratios as low as 10-15. The size requirements for samples are approximately a factor of 100000 smaller than those of decay counting, and measurement times are less than an hour, compared with weeks or months. We discuss several applications of the technique to the measurement of terrestrial as well as extraterrestrial samples. Emphasis is placed on the combined measurement of 10Be and 26Al and the various uses to which the ratio o f 26Al: 10Be in a sample can be put. In particular, we discuss the very high 26Al: 10Be ratios that are found in the outer few millimetres of extraterrestrial bodies, resulting from low-energy protons emitted by solar flares, and the diagnostic value of these ratios in determining the size of objects in space. The low value of the 26A1: 10Be ratio resulting from atmospheric production is used to prove the terrestrial origin of tektites. And finally, we discuss the manner in which the variable 26Al: 10Be ratio that results from in situ production in objects on the Earth ’s surface can be used to measure rates of weathering and exposure histories.