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

PI表明，从许多灰流凝灰岩（熔结凝灰岩）中精心挑选的样品应该能对古代场强做出可靠的估计。毕晓普凝灰岩的初步数据已经产生了高质量的古强度结果，成功率很高（75%）。此外，在凝灰岩的微晶玻璃中，有大量文献记载的细粒立方铁氧化物的存在范围从超顺磁性（SP）到单畴（SD），应该非常适合于古强度的测定。玻璃质基质中的细粒磁性矿物也可能在古油气田测定所需的多次实验室加热过程中得到有效的铠装。最后，灰流凝灰岩是典型的非常适合高精度的40 Ar/39 Ar定年，可以提供一个准确的年龄框架的任何paleointensity results.This项目是一个试点研究更新世主教凝灰岩的目的是评估灰流凝灰岩的绝对paleointensity研究的适用性。由于只有特定熔结凝灰岩的一部分最终被证明适合于古强度工作，因此研究的一个目标是制定一种策略，以确定最有可能获得可靠古强度的材料。磁性矿物学和灰流冷却历史的可变性也将允许测试古强度测定的几个其他方面，包括冷却速率的影响，磁性矿物学和化学剩磁对古强度结果的影响。这些是所有记录介质中古强度的重要控制因素，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。本建议的主要目标是评估是否可以从熔结凝灰岩中确定可靠的古强度。如果可以从火山灰流中确定绝对古强度，就有可能编制一个更全面的地磁场过去场强波动数据库。在本项目期间产生的所有数据将存入古地磁数据库（http：//www.earthref.org/MAGIC/index.html），其结果将在国际会议上和出版的论文中介绍。

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.