Mantle vs. crustal processes: testing the tracer potential of arc olivines

Columbia University, New York NY

Investigators

Abstract

This project helps explain why volcanoes in Earth’s subduction zones erupt explosively. Subduction zones are major fault regions where the crust and upper mantle, are pulled into the mantle below. Volcanoes at subduction zones have high amounts of silica and volatiles, which make them prone to explosive eruptions. The ‘Ring of Fire’ around the Pacific rim, includes active volcanoes in Alaska, the Cascades, and Mexico that are linked to subduction. Eruptions of subduction zone volcanoes can devastate communities, halt air travel, cause crop failures, famine, social unrest, and lead to large-scale human migration. Scientists aim to understand what causes volcanic eruptions. This knowledge helps improve predictions and reduce destructive impacts. This project investigates the origin of silicic, volatile-rich melts in young Mexican volcanoes. Do they form deep within the Earth’s mantle, or do they develop in the crust above the mantle? Researchers will use the compositions of minerals from silicic magmas to address these questions. These minerals, called ‘olivines’ and ‘Cr-spinels,’ are less than a millimeter in size. The temperature and composition of the magmas will also be determined using high-precision analytical methods. Modeling this information will clarify how the magmas form and how they contribute to explosive eruptions. This project will offer many learning opportunities for high school and undergraduate students. A high school teacher will design classroom materials to teach high school students about volcanic hazards. Students will also learn how tiny crystals can inform scientists about magmas formed deep beneath Earth’s surface. Hands-on, research projects will provide undergraduate students with valuable research experiences. An interactive exhibit based on this research will be developed for the annual Lamont Open House event. Olivines with Fo>88 are recognized for their potential to contain unique information on mantle lithology, composition, and melt processing, which cannot be obtained from bulk rock studies. This potential is especially valuable for arc magmas, where Cr-spinel-bearing forsteritic olivine phenocrysts or antecrysts are often the only remaining clues of mafic melt input. However, there is reasonable doubt whether olivine trace element patterns can withstand ‘transcrustal overprinting’ caused by fractional crystallization, melt mixing, and diffusion during melt ascent through the crustal basement. The Transmexican Volcanic Belt (TMVB), where high-Mg# and high-MgO magmas erupt through thick continental crust with minimal contamination, provides an ideal setting for testing whether olivines can preserve primary melt signatures and what this implies for mantle wedge processes. Over many years of fieldwork and collection of bulk rock and crystal-scale compositional data, we have assembled a carefully selected, well-characterized set of Cr-spinel-bearing olivines from 36 samples that represent the broader spectrum of high-Mg# magmas erupting at both the arc front and the rear-arc of the Transmexican Volcanic Belt. The He-O-Os systematics of these olivines constrain their crystallization in (near-) primary mantle melts. The main aim of this project is to select a subset of already mounted and pre-characterized olivine + Cr-spinels for new high-quality laser-ablation ICP-MS analyses, targeting elements such as Li, Na, Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, and Yb. High-Fo>88 olivines will be tested for their ability to retain the trace element spectrum of their (near-) primary equilibrium melts by correlating olivine composition with bulk rock diversity, the source-sensitive Cr# of Cr-spinel inclusions, and by comparing olivines from intraplate versus mid-ocean ridge settings. Selected TMVB olivines with Fo<88~80 will be used to evaluate whether primary olivine signatures survive transcrustal processes. Since the Trans-Mexican Volcanic Belt has an end-member character within the global spectrum of arcs, the results from this study are relevant not only for arc olivines worldwide but also for olivine research in intraplate and mid-ocean ridge environments. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.