D6 Isotopic Geochemistry and Geochronology

Topic

Isotope geochemistry and geochronology utilize the composition of radiogenic isotopes—produced by the decay of another isotope—and radioactive isotopes—which are actively decaying—found within minerals and rocks to determine a rock's formation age, thermal history, the petrological processes it underwent, and the fluid pathways that traversed it. The most important chronometric systems are the uranium-lead system, measured in zircons, which allows for dating ages ranging from one million to 4.5 billion years; the argon-argon system, measured in feldspars and hornblendes, covering a range from just ten thousand to 4 billion years; and the uranium-thorium-helium system, measured in apatite and zircons, which is primarily used to reconstruct a rock's low-temperature thermal history.

Thermochronology specifically measures the closure age of one of these isotopic systems—that is, the temperature below which a mineral retains the radiogenic product it has generated without significant loss through diffusion. Apatite closes its helium system at approximately 70°C and its fission-track system at around 110°C, whereas zircon closes its uranium-lead system at a much higher temperature, near 900°C. By combining several of these systems—each with its own closure temperature—it is possible to reconstruct the complete temperature-versus-time path followed by a rock, a path that directly reflects the burial and exhumation history the rock experienced. Unlike radiogenic isotopes, stable isotopes of oxygen, carbon, and sulfur do not decay; however, they do fractionate differently between phases in thermodynamic equilibrium. This fractionation depends directly on temperature—the higher the temperature at which equilibrium occurs, the lower the fractionation between phases—allowing the isotopic composition measured today to serve as a paleothermometer capable of reconstructing past temperatures.

Cosmogenic isotopes—such as beryllium-10, aluminum-26, and chlorine-36—are produced directly on rock surfaces by the action of cosmic rays. Their accumulation makes it possible to date how long a geomorphic surface (such as a glacial moraine or a fault scarp) has been exposed, with a precision of 5 to 10 percent. Together with the previously described radiogenic chronometric systems and stable isotopes, they complete the toolkit used in isotope geochemistry to reconstruct the age and thermal history of a rock.

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