Oxygen Isotopes in Mantle and Crustal Magmas as Revealed by Single Crystal Analysis

Abstract

Oxygen is the most abundant element in the Earth’s crust, mantle, and fluids and therefore its isotopic composition provides robust constraints on magma genesis. Application of oxygen isotope geochemistry to volcanology and igneous petrology provides a much needed foundation for radiogenic isotope and trace element approaches. Since isotope fractionations at high temperature are small, there is a demand for high analytical precision in order to recognize and interpret small (tenths of permil) variations in isotopic composition. Recently improved analytical techniques involving lasers and ion microprobes, and reduction in sample and spot size, has painted a picture of isotope complexity on a single crystal scale that is helpful in interpreting magma genesis and evolution. In this chapter a review is provided for several classic examples of silicic and basic magmatism, including Yellowstone and Iceland, that shows isotope zoning and heterogeneity reaching several permil. Isotope heterogeneity fingerprints crystal sources and provides constraints on diffusive and recrystallizational timescales. These new lines of evidence reveal that magma genesis happens rapidly, at shallow depths, and through batch assembly processes. Oxygen isotope geochemistry spans more than 50 years of investigation and is the most developed among other traditional (e.g., C, N, H, S, Li, B) and less-traditional (e.g., Fe, Mo, Cu) stable isotope systems. While we provide basic concepts of isotope fractionation below, the reader is referred to three prior RiMG volumes on stable isotopes (Valley, Taylor, and O’Neil 1986 – volume 16; Valley and Cole 2001 – volume 43; Johnson et al. 2004 – volume 55), and the Hoefs (2005) and Sharp (2006) textbooks for greater treatment and historic perspective. Finally this Chapter does not deal with oxygen isotopic variations in meteorites and planetary igneous materials, and interested readers are referred to RiMG volume 68, “Oxygen in the Solar System” (MacPherson et al. 2008). …