Evaluating aging and hydration effects on geomagnetic paleointensity in natural glass

评估天然玻璃中的老化和水合对地磁古强度的影响

• 批准号: 1547483
• 负责人: Julie Bowles
• 金额: $ 14.79万
• 依托单位: University of Wisconsin-Milwaukee
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-12-15 至 2019-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1547483&HistoricalAwards=false
• 关键词: Evaluating; aging; hydration; effects; geomagnetic

The strength and direction of Earths magnetic field varies in both time and space. These variations are recorded in a variety of geological materials, and by interpreting the records we learn about processes deep in Earth's liquid core (where the field is generated); about how the Earth and our atmosphere have evolved over geologic time; and we can use knowledge of field variations to put age constraints on geologic formations and some archeological artifacts. Although it is relatively easy to recover the direction of Earth's field, it is far more difficult to recover field strength (also known as 'paleointensity'). Typically, laboratory procedures designed to recover paleointensity are only appropriate for very small (less than a few tens of nanometers) magnetic crystals, but these small crystals are rare in nature. Volcanic glass (such as obsidian) is one material in which they are relatively abundant, and previous work has demonstrated that in some cases geologically young volcanic glass accurately records paleointensity. Glass is not a stable material, however, and the glass structure changes over time and with the absorption of water. If the magnetic minerals also change after the initial formation of the glass, they most likely will not provide us with accurate paleointensity information. The proposed work will provide an understanding of how magnetic mineralogy varies with the age and water content of natural glasses and will allow us to evaluate the suitability of natural volcanic glass as a paleointensity recorder over geologic time. In addition to the impact on geoscience disciplines, natural glasses have long been considered an analogue for nuclear waste glasses and other hazardous waste glasses. The proposed work may contribute to our ability to monitor incipient alteration of waste glasses via magnetic properties.Technical description:Although knowledge of the strength of Earth's magnetic field through time ('paleointensity') is critical in our understanding of field generation and evolution, paleointensity data remain sparse throughout Earth's history. This stems partly from the fact that few natural materials have the required fine-grained (single-domain) mineralogy and are also resistant to alteration both in nature and during laboratory reheating. Natural volcanic glasses, because they are quickly cooled, typically contain single-domain titanomagnetite, and the glass serves to at least partially protect the titanomagnetite against alteration. It has been demonstrated that young (50-100 ka) submarine basaltic glass can reliably record the field, but it is unclear how the magnetic mineralogy and hence magnetization might change with age as the metastable glass structure relaxes and/or the glass becomes hydrated. Limited studies of more silicic glass (such as obsidian) suggest that it may also reliably record the field, but that devitrification and/or hydration will negatively impact paleointensity estimates. While important questions have been raised regarding the suitability of volcanic glass over geologic time, there has been no attempt to systematically address the issues surrounding glass relaxation, devitrification, and hydration. The objective of the proposed work is to assess the reliability of natural volcanic glasses to recover the intensity of Earth's magnetic field throughout geologic time.

地球磁场的强度和方向在时间和空间上各不相同。这些变化记录在多种地质材料中，并通过解释我们了解地球液体中深处的过程的记录（生成了田间）；关于地球和我们的大气在地质时期的发展；我们可以利用田间变化的知识来对地质形成和一些考古文物施加年龄限制。尽管恢复地球场的方向相对容易，但要恢复场强度（也称为“ Palesenty”）要困难得多。通常，旨在恢复古显度的实验室程序仅适用于非常小的（少于几十纳米）磁性晶体，但这些小晶体本质上很少见。火山玻璃（例如黑曜石）是一种相对丰富的材料，并且先前的工作表明，在某些情况下，地质年轻的火山玻璃可以准确记录古显着。但是，玻璃不是一种稳定的材料，并且随着时间的流逝和水吸收而变化。如果磁性矿物质在玻璃初始形成后也发生了变化，则它们很可能不会为我们提供准确的古显微信息。拟议的工作将对磁性矿物学的年龄和自然眼镜的水分含量变化，使我们能够评估天然火山玻璃在地质时期内作为古'强度录音机的适用性。除了对地球科学学科的影响外，天然眼镜长期以来被认为是核废料和其他危险垃圾眼镜的类似物。拟议的工作可能有助于我们通过磁性特性监测垃圾玻璃的初期改变的能力。技术描述：尽管通过时间（“ Palesenty”）了解地球磁场强度的知识对于我们对田间产生和田间生成的理解至关重要在整个地球历史上，数据仍然很少。这部分源于这样一个事实，即很少有天然材料具有所需的细粒度（单域）矿物学，并且在自然界和实验室重新加热过程中也具有抵抗力。天然火山眼镜，因为它们会很快冷却，通常包含单域钛磁铁矿，并且该玻璃至少可以部分保护钛磁铁矿免受改动。已经证明，年轻的（50-100 ka）海底玄武岩玻璃可以可靠地记录该田地，但尚不清楚磁性矿物学和磁化如何随着亚稳态玻璃结构的放松而随着年龄而变化，并且/或玻璃被水合变化。对更多硅胶玻璃（例如黑曜石）的有限研究表明，它也可能可靠地记录该领域，但是Depitrification和/或水合会对古显着性估计产生负面影响。尽管在地质时期的火山玻璃上提出了重要的问题，但仍未尝试系统地解决玻璃放松，脱落和水合的问题。 拟议工作的目的是评估自然火山眼镜的可靠性，以恢复整个地质时间的地球磁场强度。