Water in the Lithosphere: The Fidelity of Mantle Xenoliths

构造板块由地球坚硬的外壳（称为岩石圈）形成，具有长期的强度和进化，这取决于地幔矿物中所含的水量。水将自己插入到矿物缺陷中，在晶格中缺少镁或硅的地方，这样做会削弱矿物结构。了解地幔岩石圈含水量的主要方法是通过对捕虏体的研究，捕虏体是偶然被上升的岩浆带走并喷发出火山的地幔碎片。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质中获得或失去。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，而是会被重置为将它们带到地表的岩浆中的浓度。此外，在脱气、喷发和熔岩流冷却过程中，水可能会流失。如果这是真的，那么大多数现有的地幔捕虏体中水的测量将不得不重新解释，许多关于岩石圈强度的观念也将重新考虑。本项目旨在通过对地幔捕虏体的深入研究，确定其水记录的保真度。它们的大小和宿主岩浆的含水量。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。大多数捕虏体研究是在缺乏寄主岩浆的H2O含量数据的情况下进行的，因此不受H2O交换边界条件的约束。简单的测试是捕虏体矿物和宿主岩浆斑晶和熔体包裹体是否反映H2O平衡。捕虏体斜辉石和寄主岩浆熔体的中试数据与水平衡一致。2)研究粒径、捕虏体内部位置和捕虏体尺寸与扩散长度尺度的关系。如果存在扩散交换，那么小颗粒应该比大颗粒反映出更大的平衡。大的捕虏体碎屑在喷发后冷却得更慢，可能反映出更大程度的脱水。3)新石器时代橄榄石和辉石的地点分带模式研究。H与橄榄石和辉石中的几个点缺陷有关，可以通过特定的红外吸收带识别出来，有些可能扩散得足够慢，可以在热岩浆中运输数小时，而另一些则会记录脱气和h2o损失。我们将通过对美国西部（西马和大峡谷火山场）碱性玄武岩火山渣锥中喷发的不同大小橄榄岩捕虏体中的橄榄石和辉石的红外研究来解决这些测试。大部分的项目预算是支持哥伦比亚大学的博士生（亨利·托宾）已经从事初步分析。本科生也将被支持开展实地工作，并完成暑期实习和毕业论文。

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.