Quite often sanidine (alkali feldspar) dominates the phenocryst assemblage in silicic to intermediate volcanic rocks. Indeed some intermediate to alkaline rocks contain no free quartz at all. We are therefore investigating the use of sanidine for surface exposure dating with 10Be, 21Ne and 36Cl. Rock surfaces studied come from the Oxaya and Lauca ignimbrites of northern Chile. Eruption of the ignimbrite sheets (welded tuffs) was around 19-23 Ma and 2.7 Ma, respectively. Since then the rocks have under gone very slow denudation but have never been covered. As quartz and sanidine phenocrsyts coexist, we can check the viability of the different cosmogenic nuclides by directly comparing nuclide concentrations measured in both quartz and sanidine separates. Results from four sanidine separates have shown that sanidine retains neon isotopes quantitatively but that 3He diffuses (Kober et al., 2005). Sanidine is a relatively robust mineral. Therefore, similar to quartz, it does not disintegrate during the chemical etching steps necessary to remove possible meteoric 10Be contamination and only minor modifications of the standard procedures are required. First results indicate 10Be concentrations in sanidine that are about 80% of the 10Be concentrations in coexisting quartz. We are presently analyzing 10Be, 21Ne and 36Cl in additional sanidine separates. Production in sanidine is composition dependent, therefore major element analyses must be carried out and production rates calculated on a sample-by-sample basis. Adding sanidine to the palette of potential minerals would allow in depth studies of rates and modes of landscape change at more sites as ignimbrite sheets are rather widespread in many regions across the globe.
F. Kober, S. Ivy-Ochs, I. Leya, H. Baur, T. Magna, R. Wieler, P.W. Kubik, EPSL (2005) in press.
See more of New Developments in Terrestrial Cosmogenic Nuclide Research
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)