Catchment-averaged denudation rates from cosmogenic 36Cl in magnetite: Evaluation and application to an andesitic landscape

10Be in quartz is widely used to constrain catchment-averaged denudation rates. However, the absence of quartz in most intermediate to mafic igneous lithologies has largely precluded the use of this method in environments dominated by those rock types and creates an interest in exploring new nuclide/target mineral pairs. Magnetite is an especially attractive target mineral candidate for determining denudation rates. It is a common accessory mineral in a wide range of rock types, including many that do not have quartz, and, like quartz, is resistant to weathering in soil environments. Furthermore, magnetite may be easily collected and purified from detrital sediment using magnetic separation techniques and dissolved without the use of the large quantities of concentrated HF required for quartz. Magnetite has appreciable production pathways for several cosmogenic nuclides including 36Cl through spallation on Fe. This nuclide is particularly well-suited for determining denudation rates from soil or sediment samples because meteoric 36Cl is readily leached from the weathering profile, allowing the in situ-produced 36Cl signal to be reliably isolated from the meteoric signal. Here we test the suitability of 36Cl in magnetite for determining catchment-averaged denudation rates by comparing denudation rates produced by the 36Cl/magnetite pair with those from 10Be/quartz pair at 12 watersheds in the Sierra Nevada region of California. We find that 36Cl in magnetite produces denudation rates that are in good agreement with those from 10Be in quartz. The magnetite to quartz denudation rate ratios range from from 0.70 to 1.32 with a mean of 0.98 +/- 0.06. We also present a series of five denudation rate measurements using 36Cl in magnetite along the reach of a single stream draining a catchment developed on basaltic andesite near Mt. Lassen, California and show that these compare favorably to the catchment-averaged denudation rate inferred from a reconstruction of the original volcanic form.