Elemental and isotopic geochemistry of crystal-melt systems: Elucidating the construction and evolution of silicic magmas in the shallow crust, using examples from southeast Iceland and southwest USA

Abstract

Silicic magmas (>65 wt.% SiO2) play an integral role in creating permanent continental crust. Understanding how silicic magmatic systems evolve can help us better understand the processes that control whether silicic magmas remain trapped within Earth’s crust or ultimately reach Earth’s surface, culminating in volcanic eruptions. In this study, I use elemental and isotopic compositions of major and accessory minerals from diverse rock types produced by silicic magmatism in southeast Iceland and southwest USA to investigate geochemical relationships between crystals and their associated melt(s). I present an extensive partition coefficient dataset for 8 mineral phases from a high-silica rhyolite, and demonstrate that accessory minerals (zircon, titanite, chevkinite, and apatite) exert a strong control on the distribution of rare earth and high field strength elements in a magma, while major minerals (amphibole, biotite, plagioclase, sanidine) dominantly control large ion lithophile elements. In addition, I offer a mathematical expression to estimate the relative abundances of Eu and Ce in their multivalent states (2+, 3+, and 4+), which may yield insight into the oxidation state of magmas based on the occurrence Ce and Eu anomalies in coexisting mineral phases. I conducted a detailed geochemical study of zircons from Icelandic intrusive rocks, and show that their compositions form a coherent array consistent with the signature of zircons from Icelandic silicic volcanic rocks. I demonstrate that oxygen isotopes and hafnium isotopes (εHf) provide strong evidence for the existence of isotopically diverse magmatic sources in the Icelandic crust, including the influence of meteoric-hydrothermal processes and recycling of highly altered crust as a major contributor to silicic magmas in many Icelandic silicic magmatic systems. The results presented here offer an additional dimension in helping us better understand the accumulation of silicic magmas and subsequent evolution of silicic magmatic systems within the Earth’s crust.