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都是不同的。这些变化记录在各种地质材料中，通过解释这些记录，我们了解了地球液体核心深处的过程（磁场产生的地方）；地球和大气是如何在地质时期演变的；我们可以利用野外变化的知识对地质构造和一些考古文物进行年龄限制。虽然恢复地球磁场的方向相对容易，但恢复磁场强度（也称为“古强度”）要困难得多。通常，用于恢复古强度的实验室程序只适用于非常小（小于几十纳米）的磁性晶体，但这些小晶体在自然界中是罕见的。火山玻璃（如黑曜石）是一种相对丰富的材料，以前的工作已经证明，在某些情况下，地质上年轻的火山玻璃准确地记录了古强度。然而，玻璃不是一种稳定的材料，玻璃的结构会随着时间的推移和水的吸收而变化。如果磁性矿物在玻璃初始形成后也发生变化，它们很可能无法为我们提供准确的古强度信息。这项工作将提供磁性矿物学如何随天然玻璃的年龄和含水量变化的理解，并将使我们能够评估天然火山玻璃作为地质时期古强度记录仪的适用性。除了对地球科学学科的影响外，天然玻璃长期以来一直被认为是核废料玻璃和其他危险废料玻璃的类似物。这项工作可能有助于我们通过磁性来监测废玻璃的早期变化。技术描述：虽然了解地球磁场随时间变化的强度（“古强度”）对我们理解磁场的产生和演化至关重要，但在整个地球历史上，古强度数据仍然很少。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（单畴）矿物学，并且在自然界和实验室再加热过程中也能抵抗蚀变。天然火山玻璃，因为它们是快速冷却的，通常含有单畴钛磁铁矿，玻璃至少部分保护钛磁铁矿不受蚀变的影响。已经证明，年轻（50-100 ka）的海底玄武岩玻璃可以可靠地记录磁场，但尚不清楚随着亚稳玻璃结构松弛和/或玻璃水化，磁性矿物学和磁化强度如何随年龄变化。对更多硅玻璃（如黑曜石）的有限研究表明，它也可以可靠地记录现场，但脱硝和/或水化作用将对古强度估计产生负面影响。虽然关于火山玻璃在地质时期的适用性已经提出了重要的问题，但还没有尝试系统地解决围绕玻璃松弛、反玻璃化和水化的问题。这项工作的目的是评估天然火山玻璃在整个地质时期恢复地球磁场强度的可靠性。