REVIEWS Diffusion chronometry and the timescales of magmatic processes F. Costa 1,2 ✉ , T. Shea3 and T. Ubide4 Abstract | Volcanic eruptions can represent major societal hazards. Placing tighter bounds on the timescales of magmatic processes that precede eruptions is, therefore, important for volcano monitoring and forecasting. Diffusion chronometry , where volcanic crystals that contain chemical gradients are treated as time capsules, allows the timescale of various magmatic processes to be constrained. In this Review , we discuss the basics of diffusion chronometry and describe how re- equilibration via chemical diffusion provides insights into the timescales of magma storage, ascent and eruption. Crystals from mafic volcanoes record timescales of days to years between magma intrusion and eruption, which broadly match those recorded by monitoring data (such as increased seismicity). The timescales recorded in crystals from large silicic calderas, however, are typically longer than those from mafic volcanoes, spanning decades to millennia, but almost two orders of magnitude shorter than the timescales obtained by U- Th isotope disequilibria in zircon. The cause of this discrepancy is debated but likely reflects the protracted magma accumulation and complex thermal history that many crystals experience before eruption. Diffusion chronometry adds the fourth dimension to volcano science (that is, time), and advances in analytical and experimental approaches (such as NanoSIMS) open up new opportunities for understanding magmatic systems. 18–20 Diffusion coefficient Magmatic processes have traditionally been studied and lava lakes , as well as the development of an ana- 1,2 The constant of proportionality through the ‘chemical- equilibria paradigm’ , where lytical solution to the diffusion equation to constrain the between the mass flux of phase- equilibria experiments3, and thermodynamic cooling histories of metamorphic rocks21. elements through a given modelling of multicomponent melts and minerals4, Owing to the increasingly precise experimental and surface and the gradient diffusion coefficient of concentration due to represent the primary methods by which magma improved theoretical constraints on the elemental diffusion. pre- eruptive storage conditions and processes are con- of key elements in volcanic minerals (such as Fe-Mg in strained. Combining experiments and/or thermody- olivine) and melts, the application of diffusion chrono- namic models with petrological and chemical analysis metry to magmatic and volcanic processes is now 22 1Earth Observatory of of erupted minerals and melt inclusions forms the basis possible . Concurrently, continued development of Singapore, Nanyang for unravelling the structure of the magmatic plumbing multiple analytical techniques has enabled precise and Technological University, system below a given volcano5,6 (Fig. 1). However, the high- resolution analysis (typically between 100 µm Singapore, Singapore. textural and petrological record preserved in volcanic and <1 µm) of the elemental and isotopic composition 2Asian School of the crystals and rocks demonstrates that equilibrium in of volcanic minerals and glass to become routine23–25. Environment, Nanyang natural systems is rarely achieved, and, therefore, other Temporal and rate information derived from diffusion Technological University, Singapore, Singapore. experimental and modelling approaches are required to chronometry (and other methods of constraining mag- 3,7–12 3Department of Earth fully understand sub-volcanic systems . matic time) enable quantification of storage and trans- Sciences, University of The presence of disequilibria in magmatic systems port timescales in the crust, offer new insights into the Hawai’i at Mānoa, Honolulu, opens the way to the field of kinetics, through which relationship between magmatic processes and geophys- HI, USA. the rate that a system departs from and attains new ical unrest signals, and provide critical time information 4School of Earth and equilibrium can be studied. The relaxation of chemical for hazard mitigation at dormant volcanoes (Fig. 1). Environmental Sciences, gradients in crystals and glass through the diffusion of In this Review, we discuss the techniques that are The University of Queensland, St Lucia, Queensland, molecules or elements permits time information to be commonly used to constrain the timescales of mag- Australia. extracted from a range of magmatic processes through matic processes, particularly U- Th- Ra dating of young 13–17 26–28 ✉e- mail: [email protected] a technique termed diffusion chronometry . Early zircons and some silicates , and summarize the basics https://doi.org/10.1038/ applications of diffusion chronometry include the of diffusion chronometry. In addition, we highlight the s43017-020-0038- x extraction of cooling- rate information from meteorites applications of diffusion chronometry in both mafic and NATURE REVIEWS | EARTH & ENVIRONMENT REVIEWS Key points which can be converted into ascent rates if the crustal pressure gradient is known or some reasonable assump- • Diffusion modelling of the chemical gradients in crystals can be used to extract tion can be made35. Bubble nucleation occurs owing to invaluable time information from magmatic systems. the decrease in volatile solubility associated with magma • Crystals from mafic volcanoes record the timescales of magma and transfer towards decompression. Nucleation rates are largely controlled eruption that correlate with surface- monitoring data. by the magnitude and duration of decompression, • Crystals from silicic volcanoes record timescales of magma remobilization and storage as well as the rate at which volatiles can diffuse to a bub- on the order of decades to hundreds of years, much shorter than U- Th dating of zircons. ble nucleus36. Building on the classical nucleation theory, • Further studies that integrate diffusion chronometry, using a wide range of elements relationships between the number of bubbles and and minerals, with thermal models of magmatic systems are required to understand decompression rates have been calibrated for vari- the timescales of magma storage and remobilization. ous magma compositions37. Therefore, the number of vesicles per unit volume melt in erupted pyroclasts silicic systems. The applications reviewed here demon- can be used to calculate magma- decompression rates. strate that diffusion chronometry — in combination However, it is necessary to assume that bubbles nucleate with radiogenic dating via the U- Th- Ra disequilibria either homogeneously (without mineral substrates) or series — provides a detailed, though currently debated, heterogeneously (aided by a mineral substrate), and the image of magma residence in the crust prior to erup- assumption made will greatly alter the values retrieved tion. We finish by summarizing the main challenges by this method38. As a result, there are still important and opportunities that are associated with the continued limitations to the use of vesicle textures as a measure of development of diffusion chronometry. magma- ascent rates. Insights into the timescales of magmatic processes Approaches to obtain timescales can also be derived by measuring the thickness of crystal- Various techniques have been developed to enable growth rims. Rim- thickness measurements offer a the timescales of magmatic processes to be recovered. powerful method for constraining the approximate Experimental calibrations of the kinetics of crystalli- timescales of magma mobilization prior to eruption, zation and bubble nucleation, as well as the U- Th- Ra provided that continuous crystal growth during low disequilibrium- series method, have been successfully undercooling can be identified based on crystal shape applied to magmatic systems for more than 30 years29–31. and composition39,40. Similarly, the stability of hydrous Below, we summarize these approaches and note some minerals, such as amphibole, are sensitive to the amount of their advantages and disadvantages. of water in the melt and, therefore, to ambient pressure. In deep-seated magma-storage regions, water-rich mag- Mineral and bubble textures mas stabilize hydrous minerals41. However, as magmas As magma decompresses towards the surface, water loss ascend towards the surface, pressure decreases, water drives nucleation and growth of crystals and bubbles in exsolves from the melt and hydrous minerals become the melt. Textural analysis of bubbles and/or crystals unstable. Breakdown of amphibole, induced by decom- formed by decompression can be used to determine pression and H2O loss, results in the formation of reac- magma- ascent rates prior to eruption5. For example, tion rims formed from pyroxene, plagioclase, Fe- Ti microlites, which are small (typically <100 µm) crystals oxides and melt. Experimental calibrations can then be in the groundmass of volcanic products, can form by used to convert reaction- rim thicknesses into the time both cooling and decompression- induced degassing of available for magma ascent42. For example, reaction- rim magma32. Microlite textures (that is, their size, num- measurements have been used to calculate decompres- ber, shape and distribution) have been used to extract sion rates of 0.001–1 MPa s−1 for the 1980 eruption information on the timescale of ascent and/or of the of Mount St Helens42, the 1989 eruption of Mount cooling of magmas from storage to the surface. Most Redoubt43 and the 1997 eruption at Soufriere Hills vol- texture- based techniques