SGER - Ash Flow Tuffs: a New Material for Reliable Paleointensities

pi表明，从许多火山灰流凝灰岩（烟灰岩）中精心挑选的样本应该可以对古代磁场强度做出可靠的估计。Bishop凝灰岩的初步数据获得了高质量、高强度的结果，成功率高达75%。此外，在凝灰岩的硅玻璃中，有充分记录的细粒立方铁氧化物跨越超顺磁（SP）到单畴（SD）大小范围的存在，应该很适合于古强度测定。这些玻璃状基质中的细粒磁性矿物也可以在古场测定所需的多次实验室加热期间有效地铠装。最后，火山灰流凝灰岩通常非常适合用于高精度的40Ar/39Ar定年，它可以为任何古强度结果提供准确的年龄框架。本项目是对更新世主教凝灰岩的初步研究，旨在评价灰流凝灰岩对绝对古强度研究的适用性。由于一种特定的褐煤只有一部分最终被证明适合于古强度研究，因此这项研究的一个目标将是制定一种策略，以确定最可能用于可靠古强度研究的材料。磁矿物学和火山灰流冷却历史的可变性也将允许测试古强度测定的几个附加方面，包括冷却速率、磁矿物学和化学残留物对古强度结果的影响。这些是对所有记录介质中古强度的重要控制，因此这些测试将为解释其他古强度结果提供有价值的信息。本提案的主要目标是评估是否可以从火成岩中确定可靠的古强度。如果可以从火山灰流中确定绝对的古强度，就有可能编制一个关于过去地磁场强度波动的更全面的数据库。在本项目中产生的所有数据将被存档在古地磁数据库（http://www.earthref.org/MAGIC/index.html）中，结果将在国际会议上发表并发表论文。

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 qualitypaleointensity results with a high success ratio (75%). In addition, the well documented presenceof fine-grained cubic Fe-oxides spanning the superparamagnetic (SP) to single domain (SD) sizerange in silicic glasses from tuffs should be well suited for paleointensity determinations. Thesefine grained magnetic minerals in a glassy matrix may also be effectively armored during themultiple laboratory heatings necessary for a paleofield determination. Finally, ash flow tuffs aretypically well suited for high precision 40Ar/39Ar dating that can provide an accurate ageframework for any paleointensity results.This project is a pilot study of the Pleistocene Bishop Tuff aimed at evaluating the suitability of ashflow 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 ofseveral additional aspects of paleointensity determinations, including the effects of cooling rate,magnetic mineralogy and the influence of chemical remanence on paleointensity results. Theseare important controls on paleointensity in all recording media and so these tests will yieldvaluable information for interpreting other paleointensity results.The main goal of this proposal is to evaluate whether reliable paleointensitiescan be determined from ignimbrites. If absolute paleointensities can be determined from ash flows, it maybe possible to compile a much more comprehensive database of past field intensity fluctuationsof the geomagnetic field. All data generated during the present project will be archived in thepaleomagnetic data base (http://www.earthref.org/MAGIC/index.html) and the results will bepresented at international conferences and in published papers.