CONSTRAINING EMPLACEMENT TEMPERATURE OF VOLCANIC DEPOSITS USING THERMOVISCOUS MAGNETIZATION BEHAVIOR: INITIAL RESULTS FROM THE 8.7 KA BOULDER CREEK ASSEMBLAGE, MT. BAKER, WASHINGTON

Mt Baker is an active andesitic volcano found in the northwest corner of Washington State. The Boulder Creek Assemblage is a lahar flow from an eruption 8.700 ka (Hildreth 2003). If this lahar flow was emplaced “warm”, these rocks will show a partial thermal remanent magnetization (TRM) direction at relatively low (~100º C) temperatures. TRM studies of clasts found in other pyroclastic and volcanoclastic deposits have been used successfully to identify depositional temperature of units in many other volcanic settings (Giordano et al 2008), (Paterson et al 2010), (Zanella et al 2007), so we are confident that this technique can provide useful constraints on the emplacement temperature of the Boulder Creek Assemblage. A set of clasts, 10 cm to 60 cm in diameter were sampled from two locations within the Boulder Creek deposits relative to the source on the flanks of Mt. Baker- one site was more proximal (about 3km from the source) and the other more distal (6km from source) within the banks of Boulder Creek. Proximal and distal samples were measured with AMS, then a paleomagnetic study using thermal demagnetization to characterize the components of magnetization and their unblocking temperatures. The distal deposits had two components of magnetization, with unblocking temperatures between 0 and 130 ºC for the first-removed component, and between 130 and 560 ºC for the second removed component. Proximal deposits had between two and three components of magnetization, with unblocking temperatures for the first component at 0 to 131 ºC, 171 to 285 ºC for the second removed component, and 171 to 560 ºC for the third removed component. Using Watson’s test for randomness, calculated R values from the data are below the 95% confidence level. Data from samples of both distal and proximal deposits fail the randomness test, and were thus likely emplaced cold. It is possible that some of the low unblocking temperature components would suggest elevated (~100 ºC) emplacement temperatures- however compositional aspects of the sampled clasts could also produce similar unblocking behavior. More detailed rock-magnetic work will be used to determine if these unblocking temperatures are related to compositional variations, or to elevated (but moderate) emplacement temperature.