Water in the Lithosphere: The Fidelity of Mantle Xenoliths

构造板块由地球的外层刚性壳层（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量。水将自己插入矿物缺陷中，在晶格中缺少Mg或Si的地方，这样做会削弱矿物结构。了解地幔岩石圈含水量的主要方法是通过研究捕虏体，捕虏体是地幔的碎片，偶然被上升的岩浆携带并喷发出火山。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得或损失。如果这是真的，那么地幔捕虏体的真实性就受到了质疑。捕虏体不一定会保持它们在地幔（40公里深处）中的浓度，但会被重置为将它们带到地表的岩浆中的浓度。此外，水可能会在熔岩流的脱气、喷发和冷却过程中损失。如果这是真的，那么大多数现有的地幔捕虏体中水的测量结果将不得不重新解释，许多关于岩石圈强度的概念也将重新考虑。该项目旨在通过对地幔捕虏体的深入研究，确定捕虏体中水记录的真实性。它们的大小和其宿主岩浆的含水量。该项目提出了三种不同的方法。1)捕虏体-寄主岩浆对的研究。大多数捕虏体研究是在没有寄主岩浆H2O含量数据的情况下进行的，因此没有H2O交换边界条件的限制。简单的测试是捕虏体矿物和寄主岩浆斑晶和熔融包裹体是否反映了H2O平衡。捕虏体单斜辉石和寄主岩浆熔融包裹体的先导数据与H2O平衡一致。2)作为捕虏体中颗粒大小、位置和捕虏体大小函数的扩散长度比例关系的研究。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，可能反映出更大程度的脱水。3)捕虏体中橄榄石和辉石的分带模式研究。H与橄榄石和辉石中的几个点缺陷有关，如通过特定的红外（IR）吸收带所识别的，并且一些可能扩散得足够慢以在热岩浆中的运输中存活数小时，而另一些将记录脱气和H2O损失。我们将解决每个这些测试的橄榄石和辉石在不同大小的橄榄岩捕虏体从美国西部（西马和大峡谷火山田）的碱性玄武岩火山锥喷发的红外研究。大部分项目预算都是为了资助一个哥伦比亚的博士。学生（亨利托宾）已经进行了初步分析。本科生也将支持开展实地工作，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.