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

智力上的功绩。如果大陆玄武岩岩浆要被用作熔体生成过程和地幔来源成分的可靠探测器，那么我们必须定量地了解它们的组成在多大程度上受到了与地壳物质的相互作用和在前往地球表面的过程中的结晶的影响。我们拟议的研究的目的是通过观察不同长度尺度(全岩样品与晶体中熔融包裹体)小体积火山田单生喷发的成分不均一性，为地壳同化过程提供新的约束。我们选择了单独的单一成因系统，而不是流动的地层序列，以使我们能够更好地控制样品年龄，并减少同化过程和岩浆补给事件的时间变化带来的复杂性。拟议的研究将使用各种技术(熔融包裹体、U系列不平衡、高精度铅同位素)，这些技术结合在一起将为理解地壳同化和分馏过程的机制和速率提供一种新的途径，这些过程影响穿过地壳的岩浆成分。我们将重点关注两个主要的样品地点：犹他州西南部的冰泉玄武岩熔岩流(约公元1290年)和墨西哥的帕里库廷火山(公元1943-1952年)。这两次喷发都是年轻的(1000年)单生喷发，保存了最容易归因于地壳同化和分离结晶的成分变化，也包含部分熔融的地壳捕虏体。首先，橄榄石和斜长石斑晶中的熔融包裹体将被用作了解地壳同化的早期阶段和岩浆管道系统中熔体聚集过程的窗口，这些过程为这两次单生喷发提供了条件。成分的多样性将制约岩浆分化史不同阶段的地壳同化程度。再均一次熔体包裹体的分析将采用电子探针(主要元素，氯和S)、激光烧蚀电感耦合等离子体质谱(痕量元素)和激光烧蚀电感耦合等离子体质谱(SIMS)或激光烧蚀MC-ICPMS(原位铅同位素)。其次，对受不同污染的全岩样品进行痕量元素和高精度铅同位素测量将限制地壳同化物的成分。第三，全岩样品中226Ra/230Th不平衡的变化将提供有关分异速率的信息。最后，对从部分熔融的地壳包体中分离出来的矿物和玻璃进行的226Ra-230Th-238U分析将首次直接观察到这些核素在地壳熔融过程中的行为和相对分馏。该项目将支持一位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。它将通过以下几种方式为教育和推广计划做出贡献：(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班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。拟议的研究结果将引起广泛的科学家的兴趣，包括那些对更好地了解火山系统的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。

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.