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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.

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An early call for plate tectonics The composition of continental crust far back in Earth's history gives us insight into when plate tectonics ramped up and has influenced ocean chemistry. Greber et al. looked at titanium isotopes in shales, which form from eroded continental crustal sediments, to estimate the composition 3.5 billion years ago, closer to the origins of Earth. They found a silica-rich composition, which indicates that plate tectonics was happening deep in our distant past. Other changes in crustal composition might be linked to changing ocean chemistry and major events such as the oxygenation of our atmosphere. Science , this issue p. 1271