The PIs suggest that carefully selected samples from many ash flow tuffs (ignimbrites) should yield reliable estimates of the ancient field intensity. Preliminary data from the Bishop Tuff have yielded high quality paleointensity results with a high success ratio (75%). In addition, the well documented presence of fine-grained cubic Fe-oxides spanning the superparamagnetic (SP) to single domain (SD) size range in silicic glasses from tuffs should be well suited for paleointensity determinations. These fine grained magnetic minerals in a glassy matrix may also be effectively armored during the multiple laboratory heatings necessary for a paleofield determination. Finally, ash flow tuffs are typically well suited for high precision 40Ar/39Ar dating that can provide an accurate age framework for any paleointensity results. This project is a pilot study of the Pleistocene Bishop Tuff aimed at evaluating the suitability of ash flow tuffs for absolute paleointensity studies. Because only portions of a particular ignimbrite will ultimately prove suitable for paleointensity work, a goal of the study will be to develop a strategy to identify the most likely materials for reliable paleointensities. The variability in magnetic mineralogy and cooling history in ash flows will also allow the testing of several additional aspects of paleointensity determinations, including the effects of cooling rate, magnetic mineralogy and the influence of chemical remanence on paleointensity results. These are important controls on paleointensity in all recording media and so these tests will yield valuable information for interpreting other paleointensity results. The main goal of this proposal is to evaluate whether reliable paleointensities can be determined from ignimbrites. If absolute paleointensities can be determined from ash flows, it may be possible to compile a much more comprehensive database of past field intensity fluctuations of the geomagnetic field. All data generated during the present project will be archived in the paleomagnetic data base (www.earthref.org/MAGIC/index.html) and the results will be presented at international conferences and in published papers.
