Assessing Mechanisms and Rates of Crustal Assimilation and Fractionation From Compositional Heterogeneity in Monogenetic Eruptions

从单源喷发的成分异质性评估地壳同化和分馏的机制和速率

• 批准号: 0609652
• 负责人: David Peate
• 金额: $ 25.68万
• 依托单位: University of Iowa
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-11-01 至 2010-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0609652&HistoricalAwards=false
• 关键词: Assessing; Mechanisms; Rates; Crustal; Assimilation

Intellectual Merit. If continental basaltic magmas are to be used as a reliable probe of melt generation processes and mantle source composition, then we have to understand quantitatively to what extent their compositions are modified by interaction with crustal materials and crystallization en route to the Earth's surface. The aims of our proposed research are to provide fresh constraints on crustal assimilation processes by looking at compositional heterogeneity in monogenetic eruptions from small-volume volcanic fields at different length-scales (whole rock samples vs. melt inclusions in crystals). We have selected individual monogenetic systems rather than stratigraphic sequences of flows to give us a better control over sample ages and to reduce complexities from temporal variations in the assimilation process and magma replenishment episodes. The proposed research will use a variety of techniques (melt inclusions, U-series disequilibria, highprecision Pb isotopes) that in combination will provide a new approach to the understanding of mechanisms and rates of crustal assimilation and fractionation processes that influence magma compositions as they traverse the crust. We will focus our efforts on two main samples localities: the Ice Springs basaltic lava flow (c. 1290 AD) in SW Utah and Paricutin volcano (1943-1952 AD) in Mexico. These are both young ( 1000 years), monogenetic eruptions that preserve compositional variations that can most easily be attributed to crustal assimilation and fractional crystallisation and that also contain partially-melted crustal xenoliths.The principal objectives of the project are four-fold. Firstly, melt inclusions in olivine and plagioclase phenocrysts will be used as a window into the earlier stages of crustal assimilation and the melt aggregation processes in the magmatic plumbing systems feeding these two monogenetic eruptions. The diversity of compositions will constrain the extent of crustal assimilation at different stages in the differentiation history of the magma. Re-homogenized melt inclusions will be analysed by electron-microprobe (major elements, Cl & S), laser ablation ICP-MS (trace elements), and SIMS or laser ablation MC-ICP-MS (in-situ Pb isotopes). Secondly, trace element and high-precision Pb isotope measurements of variably-contaminated whole-rock samples will constrain the composition of the crustal assimilant. Thirdly, variations in 226Ra/230Th disequilibria in whole rock samples will provide information on rates of differentiation. Finally 226Ra-230Th-238U analyses of mineral and glass separates from partially-melted crustal xenoliths will provide the first direct observations of the behaviour and relative fractionation of these nuclides during crustal melting.Broader Impacts. This project will support the early career development of a promising young scientist in a post-doctoral research position. It will contribute to educational and outreach programs in several ways: (i) by providing support for one or more graduate students, (ii) by the involvement of undergraduate students in the research through senior research projects, and (iii) by providing summer research experiences for high-school teachers through a University of Iowa Department of Education summer school. Funding of the proposal will also support and enhance the analytical facilities at the University of Iowa. The results of the proposed research will be of interest to a broad range of scientists, including those interested in better understanding the evolution of volcanic systems and those who use basaltic magmas as probes of mantle composition.

知识价值。如果要用大陆玄武岩岩浆作为熔体生成过程和地幔源组成的可靠探测器，那么我们必须定量地了解它们的组成在多大程度上被地壳物质的相互作用和到达地球表面的结晶所改变。我们提出的研究目的是通过观察不同长度尺度（整个岩石样品与晶体中的熔融包裹体）的小体积火山场单成因喷发的成分异质性，为地壳同化过程提供新的约束。我们选择了单独的单成因系统，而不是流动的地层序列，以便我们更好地控制样品的年龄，并减少同化过程和岩浆补充事件的时间变化带来的复杂性。拟议的研究将使用多种技术（熔体包裹体，u系列不平衡，高精度铅同位素），这些技术的结合将为理解地壳同化和分异过程的机制和速率提供一种新的方法，这些过程会影响岩浆在地壳中的组成。我们将把精力集中在两个主要的采样地点：犹他州西南部的冰泉玄武岩熔岩流（约公元1290年）和墨西哥的Paricutin火山（公元1943-1952年）。这些都是年轻的（1000年），单成因喷发，保存了成分的变化，最容易归因于地壳同化和部分结晶，也包含部分熔化的地壳捕虏体。该项目的主要目标有四个方面。首先，橄榄石和斜长石斑晶中的熔体包裹体将作为一个窗口，用于研究地壳同化的早期阶段和岩浆管道系统中的熔体聚集过程，这些管道系统为这两次单成因喷发提供了物质来源。在岩浆分异史的不同阶段，成分的多样性将限制地壳同化的程度。再均匀化的熔体包裹体将通过电子探针（主要元素，Cl & S），激光烧蚀ICP-MS（微量元素）和SIMS或激光烧蚀MC-ICP-MS（原位Pb同位素）进行分析。其次，变污染全岩样品的微量元素和高精度Pb同位素测量将限制地壳同化剂的组成。第三，整个岩石样品中226Ra/230Th不平衡的变化将提供有关分化速率的信息。最后，对部分熔融的地壳捕虏体中矿物和玻璃分离物的226ra -230 - th - 238u分析将首次提供对这些核素在地壳熔融过程中的行为和相对分异的直接观察。更广泛的影响。该项目将支持一名有前途的青年科学家在博士后研究岗位上的早期职业发展。它将以几种方式为教育和推广项目做出贡献：(i)为一名或多名研究生提供支持，（ii）通过高级研究项目让本科生参与研究，以及（iii）通过爱荷华大学教育部暑期学校为高中教师提供暑期研究经验。该提案的资金还将支持和加强爱荷华大学的分析设施。提议的研究结果将引起广泛科学家的兴趣，包括那些对更好地理解火山系统演化感兴趣的科学家，以及那些使用玄武岩岩浆作为地幔成分探测器的科学家。