2019 | Partial Support for a Workshop: The Significance of Zircon Age Spectra for Understanding Crustal Magma Reservoirs

Zircon is a nearly ideal timekeeper for crystallization and crystal residence in magma reservoirs because of its chemical and mechanical stability as well as the extremely slow diffusivities for geochronologically and geochemically relevant components in zircon. In consequence, zircon is increasingly used to date magmatic processes down to millennial timescales (e.g., Schmitt, 2011). In addition, absolute ages of zircon crystal- lization can be compared and contrasted to relative ages of crystal storage derived from diffusion modelling of major phases and, increasingly, also from zircon itself using fast-diffusing species such as lithium (e.g., Rubin et al., 2017). One intriguing observation from such studies is that zircon — even in an individual volcanic or plutonic hand-sample — often shows evidence for protracted crystallization, where ages define spectra that extend over timescales of 105-106 years. These patterns of heterogeneous zircon age distribu- tions that are clearly outside analytical uncertainties have emerged independently of the analytical method applied: high-precision thermal ionization mass spectrometry (TIMS) dating of single crystals and spatially selective analysis of individual zircon growth domains by secondary ionization mass spectrometry (SIMS) or laser ablation inductively coupled mass spectrometry (LA-ICP-MS) techniques both show similar behav- ior, although the some method-related differences exist. Zircon age spectra thus form an important observ- able which can be parameterized as a time-temperature history for evolved magma reservoirs in which zircon crystallized between the temperature limits of zircon saturation (~850 °C) and the solidus (~700 °C). Existing models aim to match observed age spectra to model thermal histories for magma as constrained by numerical modelling of open-system magma reservoirs. These thermochemical models permit permu- tations of magma injection into crustal reservoirs to constrain magma recharge rates and overall volumes, and predictions of where and when zircon crystallizes based on experimentally constrained zircon satura- tion (Caricchi et al., 2014; 2016; Tierney et al., 2016). This approach, however, has limitations in that dating uncertainties can “blurr” zircon age spectra. Moreover, numerical models not implementing magma dynam- ics such as multi-phase flow are limited in constraining the pathways of zircon crystals formed at different times and in separate compartments of a magma reservoir (e.g., Kent and Cooper, 2017). Thus, in order to fully utilize the potential of zircon as a reliable probe into the thermal histories of real-world magma reservoirs, it is important to assess analytical (e.g., resulting from mixing of different age domains) as well as conceptual limitations related to magma reservoir dynamics. It is the aim of this proposal to further boost existing efforts for initiating a constructive dialog between experts in zircon petrochemistry, geochronology, and numerical modeling to define the state-of-the-art for zircon age spectra modeling and to outline potential future improvements. As the core activity to achieve this aim, we propose a 3-day Work- shop “ZASSy” (Zircon Age Spectra Systematics) to be held in fall 2019 that will bring together approximately 10-12 junior and senior scientists with complementing expertise in this field of research.

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