Zircon age-temperature-compositional spectra in plutonic rocks

Geochronology can resolve dispersed zircon dates in plutonic rocks when magma cooling time scales exceed the temporal precision of individual U-Pb analyses; such age heterogeneity may indicate protracted crystallization between the temperatures of zircon saturation (Tsat) and rock solidification (Tsolid). Diffusive growth models predict asymmetric distributions of zircon dates and crystallization temperatures in a cooling magma, with volumetrically abundant old, hot crystallization at Tsat decreasing continuously to volumetrically minor young, cold crystallization at Tsolid. We present integrated geochronological and geochemical data from Bergell Intrusion tonalites (Central Alps, Europe) that document zircon compositional change over hundreds of thousands of years at the hand-sample scale, indicating melt compositional evolution during solidification. Ti-in-zircon thermometry, crystallization simulation using MELTS software, and U-Pb dates produce zircon mass-temperature-time distributions that are in excellent agreement with zircon growth models. These findings provide the first quantitative validation of longstanding expectations from zircon saturation theory by direct geochronological investigation, underscoring zircon’s capacity to quantify supersolidus cooling rates in magmas and resolve dynamic differentiation histories in the plutonic rock record. INTRODUCTION Zircon is an inimitable chronicler of the temporal, thermal, and compositional evolution of many crustal magmatic systems. U-Pb zircon geochronology by chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS; Mattinson, 2005) is the most robust method available for constraining the tempo of pluton assembly and the longevity of magma reservoirs in deep time, having been applied to discern the incremental nature of composite intrusive suites (Coleman et al., 2004). As a result of steadily improved analytical precision, CA-ID-TIMS can resolve heterogeneous U-Pb zircon dates, or zircon spectra, in many igneous rocks at the hand-sample scale (Miller et al., 2007; Schoene et al., 2012; Broderick et al., 2015). Such studies demonstrate that zircon age distributions can reflect protracted zircon crystallization time scales, supplanting convention that zircon retains a singular emplacement age of its host rock (e.g., see discussion in Samperton et al., 2015). Therefore, zircon age information combined with complementary petrologic data from zircon and other phases can be used to constrain the time scales of magma emplacement, calculate supersolidus cooling rates, and track magma compositional evolution. An important step in using zircon to further quantify magmatic processes is to compare zircon geochronology of natural systems with theoretical and experimental models for zircon saturation and growth (Watson, 1996; Harrison et al., 2007; Ferry and Watson, 2007; Boehnke et al., 2013; Bindeman and Melnik, 2016). Ab initio diffusive saturation calculations predict noninstantaneous, nonlinear volumetric zircon growth during monotonic magma cooling, with an asymmetric distribution of zircon mass crystallized as a function of time (t) or temperature (T) (Watson, 1996). This model predicts high initial crystallization rates at the system’s zircon saturation temperature (Tsat) and continuous, near-exponentially decreasing growth to the solidus temperature (Tsolid). While the treatment of Watson (1996) has been employed to describe zircon dissolution during thermal rejuvenation (Frazer et al., 2014), no attempt has been made to quantitatively compare theoretical expectations with measured zircon age spectra. If a zircon population is the product of crystallization in a cooling, closed system, then the following criteria may be observed: (1) resolvable dispersion in U-Pb zircon dates, reflecting protracted crystallization time scales; (2) trends in zircon composition through time, reflecting evolving melt composition during fractional crystallization; (3) a systematic decrease in Ti-in-zircon crystallization temperature with time, reflecting magma undercooling (Ferry and Watson, 2007); and (4) asymmetric distributions of zircon mass crystallized as functions of time and temperature (Watson, 1996). While such features have been previously documented individually (e.g., Ickert et al., 2011; Tierney et al., 2016), no prior study has demonstrated these criteria simultaneously. As such, the geochronological, experimental, and modeling perspectives of zircon crystallization have yet to be concisely unified, and our ability to link U-Pb geochronology, petrology, and numerical modeling in magmatic systems thus remains limited. Here we address this shortcoming through geochronological, geochemical, and thermometric characterization of zircon from mid-crustal granitoids. PREVIOUS GEOCHRONOLOGY AND HYPOTHESIS We use the Bergell Intrusion, Central Alps, Europe (Fig. 1), as an excellent locale to test the four criteria presented here. Thermal insulation of *Current address: Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA; E-mail: samperton1@llnl.gov. †Current address: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA. GEOLOGY, November 2017; v. 45; no. 11; p. 983–986 | Data Repository item 2017333 | doi:10.1130/G38645.1 | Published online 23 August 2017 © 2017 eological Society of A erica. For permission to copy, contact editing@geosociety.org. Insubric line Enga dine line tona lite gran odio rite hybr id. San Martino