Thematic Articles

The formation of continental crust via plate tectonics strongly influences the physical and chemical characteristics of Earth’s surface and may be the key to Earth’s long-term habitability. However, continental crust formation is difficult to observe directly and is even more difficult to trace through time. Nontraditional stable isotopes have yielded significant insights into this process, leading to a new view both of Earth’s earliest continental crust and of what controls modern crustal generation. The stable isotope systems of titanium (Ti), zirconium (Zr), molybdenum (Mo), and thallium (Tl) have proven invaluable. Processes such as fractional crystallization, partial melting, geodynamic setting of magma generation, and magma cooling histories are examples of processes illuminated by these isotope systems.

T he variability of iron isotopes among rocky bodies in the inner Solar System provides a window onto the diversity of materials and mechanisms from which they formed. The magnitude of isotopic variation in mantle-derived rocks within a given body is similar to that between different planetary bodies. Isotopic signatures arising from primordial events, namely, evaporation/condensation, core formation and melting/crystallization, may be progressively diluted, modified, and redistributed over time by global recycling processes such as plate tectonics. Here, we assess the relative influence of these primordial mechanisms on the iron isotope compositions of igneous rocks and their implications for the structure and accretion histories of rocky planets.

Evaporation of magma oceans exposed to space may have played a role in the chemical and isotopic compositions of rocky planets in our Solar System (e.g., Earth, Moon, Mars) and their protoplanetary antecedents. Chemical depletion of moderately volatile elements and the enrichment of these elements’ heavier isotopes in the Moon and Vesta relative to chondrites are clear examples. Evaporation is also thought to be an important process
in some exoplanetary systems. Identification of evaporation signatures among the rock-forming elements could elucidate important reactions between melts and vapors during planet formation in general, but the process is more complicated than is often assumed.

The detection of exoplanets and accretion disks around newborn stars has spawned new ideas and models of how our Solar System formed and evolved. Meteorites as probes of geologic deep time can provide ground truth to these models. In particular, stable isotope anomalies in meteorites have recently emerged as key tracers of material flow in the early Solar System, allowing cosmochemists to establish a “planetary isotopic genealogy”. Although not complete, this concept has substantially advanced our understanding of Solar System evolution, from the collapse of the Sun’s parental molecular cloud to the accretion of the planets.

Stable isotopes provide deep insights into processes across a wide range of scales, from micron- to cosmic-size systems. Here, we review how continued advances in mass-spectrometry have enabled the analysis of ever-smaller samples and brought the field of heavy stable isotope geochemistry to its next frontier: the single-crystal scale. Accessing this record can be as enlightening as it is challenging. Drawing on novel systematics at different stages of development (from well-established to nascent), we discuss how the isotopes of heavy elements, such as magnesium, iron, zirconium, or uranium, can be used at the single-crystal and subcrystal scales to reconstruct magma thermal histories, crystal growth timescales, or, possibly, magma redox conditions.

Igneous and metamorphic rocks exhibit greater isotopic heterogeneity than expected from equilibrium. Large nonequilibrium isotope effects can arise from diffusion and chemical reactions, such as crystal growth and dissolution. The effects are time-dependent and can, therefore, be used to probe timescales of igneous and metamorphic processes that are inaccessible to direct observation. New discoveries of isotopic variability in nature, informed by diffusion and reaction modeling, can provide unique insights into the formation of rocks in the interiors of planetary bodies.

The isotopic variability of the elements in our planet and Solar System is the end result of a complex mixture of processes, including variable production of isotopes in stars, ingrowth of daughter nuclides due to decay of radioactive parents, and selective incorporation of isotopes into solids, liquids, or gases as a function of their mass and/or nuclear volume. Interpreting the isotopic imprints that planetary formation and evolution have left in the rock and mineral record requires not only precise and accurate measurements but also an understanding of the drivers behind isotopic variability. Here, we introduce fundamental concepts needed to “read” the isotopic code, with particular emphasis on heavy stable isotope systems.