干旱区生态环境知识资源中心

Our initial results indicate that three cosmogenic nuclides: Be-10, Ne-21 and 36Cl can be analyzed in sanidine. To uncover complex exposure histories or marked changes in denudation rates over time several nuclides with different half-lives (or stable) must be measured. Because of its shorter half-life, the combination of Cl-36 and a long-lived nuclide Be-10 or stable nuclide Ne-21 will provide more information than the pairs Be-10 and Al-26 or Be-10 and Ne-21 (in quartz). Sanidine (alkali feldspar) is a common high temperature mineral and often dominates the phenocryst assemblage in silicic to intermediate volcanic rocks. Bedrock surfaces studied come from the Oxaya (erupted 19-23 Ma) and Lauca (erupted 2.7 Ma) ignimbrites of northern Chile. Quartz and sanidine phenocrysts coexist; therefore, we can check the viability of sanidine through direct comparison with nuclide concentrations in quartz. In addition, as quartz has no target for Cl-36 in significant abundance we show that the unique power of sanidine is that Cl-36 can be measured. We have obtained very good agreement between Be-10 and Ne-21 concentrations measured in sanidine and coexisting quartz. No meteoric Be-10 was apparent in these sanidines. Concentrations of all three nuclides in mineral separates from rock sample CN309 from the Lauca ignimbrite in the Western Cordillera agree well and correspond to minimum exposure ages of 30-50 ka. Be-10 and Ne-21 measured in both sanidine and quartz from three rock samples from the Oxaya ignimbrite (CN19, CN23, CNI04a) in the Western Escarpment record low average landscape modification rates (< 0.70 m/Ma) over the last several million years. In contrast, Cl-36 data from sanidine in CN23 seem to indicate shorter minimum exposures and more rapid maximum erosion rates. (c) 2007 Elsevier B.V. All rights reserved.