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.

由地球外壳（称为岩石圈）形成的构造板具有长期强度和进化，取决于地幔矿物质中所含的水量。水将自己插入矿物缺陷中，在晶格中缺少Mg或Si的地方，并且这样做会削弱矿物结构。地幔岩石圈的水含量是通过研究异种岩的主要方式，这是对近期研究的地幔的研究，但是在几分钟内，在岩浆温度（1000°C）下，有时会从矿物质中获得或流失水。如果这是真的，那么地幔Xenoliths的保真度就会受到质疑。异种石不一定会保留它们在地幔中的浓度（深度为40 km），而是将其重置为将它们传递到表面的岩浆中的浓度。此外，在熔岩流中的脱气，喷发和冷却过程中可能会流失。如果这是真的，那么必须重新诠释地幔异种石中的大多数现有水测量值，并且有关重新考虑岩石圈强度的许多注释。该项目的目的是通过对异种石的上下文进行密集研究来确定地幔异种石的水记录的保真度？他们的大小和宿主岩浆的水含量。该项目提出三种不同的方法。 1）研究异种石岩浆对的研究。大多数异种石研究是在没有有关宿主岩浆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约束。简单的测试是异种石矿物质和宿主岩浆表晶和熔体夹杂物是否反映了H2O平衡。异种石斜二烯和宿主岩浆熔体夹杂物的试验数据与H2O平衡一致。 2）研究扩散长度尺寸与晶粒尺寸，异种石内的位置和异种石大小的函数的研究。如果存在扩散的交换，则小晶粒应反映出比大型谷物更大的平衡。大型异种石经典会在爆发后冷却，并可能反映出脱水的范围更大。 3）研究异种石器时代橄榄石和辉石中特定地点的区域模式。 H与特定的红外（IR）苦难带确定的橄榄石和辉石中的几个点缺陷有关，有些人可能会缓慢地扩散以在热岩浆中运输时间，而另一些则可以记录脱气和H2O-alss。我们将通过对美国西部碱玄武岩阳离子锥（CIMA和Grand Canyon火山场）中爆发的橄榄石和辉石的IR研究来解决这些测试。项目预算的大部分是为了支持哥伦比亚博士学位。学生（Henry Towbin）已经进行了初步分析。本科生还将受到支持，以完成实地考察，并完成暑期实习和高级论文。

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