Water in the Lithosphere: The Fidelity of Mantle Xenoliths

岩石圈中的水：地幔包体的真实性

• 批准号: 1731784
• 负责人: Terry Plank
• 金额: $ 23.25万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-09-01 至 2021-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1731784&HistoricalAwards=false
• 关键词: Water; Lithosphere; Fidelity; Mantle; Xenoliths

The tectonic plates, formed from the outer rigid shell of Earth (termed lithosphere), have a long term strength and evolution that depends on the amount of water contained in mantle minerals. Water inserts itself into mineral defects, in places where Mg or Si are missing from the lattice, and in so doing, weakens the mineral structure. The primary way the water content of the mantle lithosphere is known is through study of xenoliths, pieces of the mantle that get accidently entrained in ascending magma and erupted out volcanoes. Recent research, however, has shown that water can sometimes be gained or lost from minerals at magmatic temperatures (1000°C) in a matter of minutes. If this is true, then the fidelity of mantle xenoliths is questioned. Xenoliths would not necessarily preserve the concentrations they had in the mantle ( 40 km depth), but be reset to the concentration in the magma that conveyed them to the surface. Additionally, water may be lost during degassing, eruption and cooling in lava flows. If this is true, then most of the existing measurements of water in mantle xenoliths would have to be reinterpreted, and many notions about the strength of the lithosphere reconsidered. This projects aims to determine the fidelity of the water record in mantle xenoliths by intensive study of xenoliths in their context ? their size and the water content of their host magma.This project proposes three different approaches. 1) Study of xenolith-host magma pairs. Most xenoliths studies are carried out in the absence of data on the H2O content of the host magma and thus without constraints on the boundary conditions for H2O exchange. The simple test is whether xenolith minerals and host magma phenocrysts and melt inclusions reflect H2O equilibration. Pilot data on xenolith clinopyroxene and host magma melt inclusions are consistent with H2O equilibration. 2) Study of diffusive lengthscale relationships as a function of grain size, position within the xenolith, and xenolith size. If there is diffusive exchange, then small grains should reflect greater equilibration than large ones. Large xenolith clasts will cool more slowly post-eruption, and may reflect greater extents of dehydration. 3) Study of site-specific zonation patterns in xenolithic olivines and pyroxenes. H is associated with several point defects in both olivine and pyroxene, as identified by specific infrared (IR) absorption bands, and some may diffuse slowly enough to survive hours of transport in hot magma, while others will record degassing and H2O-loss. We will address each of these tests with an IR study of olivines and pyroxenes in variably-sized peridotite xenoliths erupted in alkali basalt cinder cones from the western US (Cima and Grand Canyon volcanic fields). Most of the project budget is in support of a Columbia Ph.D. student (Henry Towbin) already engaged in preliminary analyses. An undergraduate will also be supported to carry out field work, and complete a summer internship and senior thesis.

由地球刚性外壳（称为岩石圈）形成的构造板块具有长期的强度和演化，这取决于地幔矿物中所含的水量。水会插入到含有镁或硅的地方的矿物缺陷中。晶格中缺失，从而削弱了矿物结构。了解地幔岩石圈含水量的主要方法是通过研究捕虏体，即上升过程中意外夹带的地幔碎片。然而，最近的研究表明，在岩浆温度（1000°C）下，水有时可以在几分钟内从矿物中获取或流失，如果这是真的，那么地幔捕虏体的真实性就受到质疑。包体不一定会保留它们在地幔（40公里深）中的浓度，而是会重置为将它们输送到地表的岩浆中的浓度。此外，水可能会在脱气过程中损失。如果这是真的，那么大多数现有的地幔捕虏体中水的测量都必须重新解释，并且许多关于岩石圈强度的概念需要重新考虑。该项目旨在确定水的保真度。通过深入研究捕虏体的大小和宿主岩浆的含水量来记录地幔捕虏体。该项目提出了三种不同的方法 1) 捕虏体宿主的研究。大多数捕虏体研究都是在缺乏母岩浆 H2O 含量数据的情况下进行的，因此对 H2O 交换的边界条件没有限制，简单的测试是捕虏体矿物和母岩浆斑晶和熔体包裹体是否反映 H2O。捕虏体单斜辉石和宿主岩浆熔体包裹体的初步数据与 H2O 平衡一致 2) 作为函数的扩散长度关系的研究。颗粒大小、捕虏体内的位置以及捕虏体大小的影响。如果存在扩散交换，则小颗粒比较大的颗粒应该反映更大的平衡，喷发后冷却得更慢，并且可能反映更大程度的脱水。 ) 对捕虏石橄榄石和辉石中特定位点的分带模式的研究 H 与橄榄石和辉石中的几个点缺陷有关，如通过特定的鉴定所确定的。红外（IR）吸收带，有些可能扩散得足够慢，可以在热岩浆中存活数小时，而另一些则记录脱气和水的损失，我们将通过对不同情况下的橄榄石和辉石进行红外研究来解决这些测试。大小橄榄岩捕虏体在美国西部（西马和大峡谷火山田）的碱性玄武岩渣锥中喷发，大部分项目预算用于支持哥伦比亚。已经进行初步分析的博士生（Henry Towbin）也将获得支持进行实地工作，并完成暑期实习和毕业论文。

项目成果

Rates of dehydration of olivines from San Carlos and Kilauea Iki

• DOI: 10.1016/j.gca.2018.08.050
• 发表时间: 2018-12
• 期刊: Geochimica et Cosmochimica Acta
• 影响因子: 5
• 作者: E. Ferriss;T. Plank;M. Newcombe;D. Walker;E. Hauri