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

University Of Iowa, Iowa City IA

Investigators

Abstract

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.