Ar-Ar Dating

Core Idea

The 40Ar/39Ar method is a refined version of K-Ar dating where the sample is irradiated with neutrons to convert 39K to 39Ar, enabling measurement of both parent and daughter isotopes on the same aliquot by mass spectrometry. Step-heating (incrementally increasing temperature) releases argon from different crystallographic sites, producing an age spectrum that reveals the sample's thermal history. A flat (plateau) age spectrum indicates a single undisturbed cooling age. Disturbed spectra with young edges and old cores record partial argon loss during thermal events. The method dates cooling through the closure temperature of the mineral analyzed (K-feldspar ~150-350 C, biotite ~300 C, hornblende ~500 C, muscovite ~350 C), making it a thermochronometer that records when a rock cooled through specific temperatures.

Explainer

The Ar-Ar method improved upon K-Ar dating by measuring the parent (K, via proxy 39Ar) and daughter (40Ar) on the same aliquot, eliminating the need for separate K concentration analysis and the assumption of sample homogeneity. The neutron irradiation that converts a known fraction of 39K to 39Ar is the key innovation -- it turns a two-aliquot measurement into a single-aliquot isotope ratio measurement.

The step-heating technique extracts argon at progressively higher temperatures, exploiting the fact that argon in different crystallographic sites has different retentivity. Loosely held argon (from grain boundaries, fluid inclusions, or partially disturbed zones) is released at low temperatures; argon from the intact crystal lattice is released at high temperatures. Plotting apparent age versus cumulative 39Ar fraction produces an age spectrum. A plateau -- a series of contiguous steps with statistically indistinguishable ages comprising >50% of the total 39Ar -- provides a robust age estimate.

The concept of closure temperature (Tc) is central. As a mineral cools, it transitions from a state where argon diffuses freely out of the crystal (open system, no age accumulation) to a state where argon is quantitatively retained (closed system, age accumulates). The closure temperature depends on the mineral's crystal structure, grain size, and cooling rate. Hornblende (Tc ~500 C) closes at higher temperature than muscovite (~350 C) or biotite (~300 C) or K-feldspar (~150-350 C). Dating multiple minerals from the same sample provides a cooling history -- a series of time-temperature points that define the cooling path.

Applications span tectonic studies (timing of metamorphism, exhumation, faulting), volcanology (eruption ages), extraterrestrial chronology (ages of meteorites and lunar rocks), and paleomagnetic calibration (dating lava flows for the geomagnetic polarity time scale). The method works from ~100 ka (young volcanic rocks with very little radiogenic 40Ar) to the age of the solar system (~4.56 Ga), making it one of the most versatile geochronological tools available.