Water in the Lithosphere: The Fidelity of Mantle Xenoliths

构造板块由地球的外部刚性外壳(称为岩石圈)形成，具有长期的强度和演化，这取决于地幔矿物中所含的水的数量。水插入到矿物缺陷中，在晶格中缺少镁或硅的地方，这样做会削弱矿物结构。了解地幔岩石圈水分含量的主要方法是研究捕捉体，即地幔碎片意外地卷入上升的岩浆中并喷发出火山。然而，最近的研究表明，在岩浆温度(1000摄氏度)下，矿物有时可以在几分钟内获得或失去水分。如果这是真的，那么地幔捕虏体的保真度就受到了质疑。捕虏体不一定会保持它们在地幔(40公里深)中的浓度，但会被重置为将它们输送到地表的岩浆中的浓度。此外，在熔岩流中的脱气、喷发和冷却过程中，水可能会丢失。如果这是真的，那么现有的对地幔包体中水的测量结果将不得不重新解释，许多关于岩石圈强度的概念将不得不重新考虑。该项目旨在通过深入研究地幔包体背景下的包体来确定地幔包体中水记录的保真度。它们的大小和它们宿主岩浆的水分含量。这个项目提出了三种不同的方法。1)捕虏体-寄主岩浆对研究。大多数包体研究是在没有关于宿主岩浆中H2O含量的数据的情况下进行的，因此对H2O交换的边界条件没有限制。简单的检验是捕虏体矿物和寄主岩浆斑晶和熔体包裹体是否反映了H2O平衡。捕虏体、单斜辉石和寄主岩浆熔体包裹体的先导数据与H2O平衡一致。2)研究作为颗粒大小、包体内位置和包体大小的函数的扩散长度关系。如果存在扩散交换，那么小颗粒应该比大颗粒反映更大的平衡。大型捕虏体碎屑在喷发后冷却较慢，可能反映更大程度的脱水。3)捕虏器时代橄榄石和辉石中特定地点的分带模式研究。H与橄榄石和辉石中的几个点缺陷有关，如特定的红外(IR)吸收带所确定的那样，其中一些扩散得足够慢，足以在高温岩浆中存活数小时，而另一些则记录了脱气和水的损失。我们将通过对来自美国西部(Cima和大峡谷火山田)的碱性玄武岩渣锥中喷发的不同大小的橄榄岩包体中的橄榄石和辉石的红外研究来解决每一项测试。该项目的大部分预算是为了支持一名哥伦比亚大学的博士生(亨利·托宾饰)，他已经在进行初步分析。还将支持一名本科生进行实地考察，并完成暑期实习和高级论文。

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.