Seeing is believing: Visualization of He distribution in zircon and implications for thermal history reconstruction on single crystals

Abstract

Zircon (U-Th)/He thermochronometry is an established radiometric dating technique used to place temporal constraints on a range of thermally sensitive geological events, such as crustal exhumation, volcanism, meteorite impact, and ore genesis. Isotopic, crystallographic, and/or mineralogical heterogeneities within analyzed grains can result in dispersed or anomalous (U-Th)/He ages. Understanding the effect of these grain-scale phenomena on the distribution of He in analyzed minerals should lead to improvements in data interpretation. We combine laser ablation microsampling and noble gas and trace element mass spectrometry to provide the first twodimensional, grain-scale zircon He "maps" and quantify intragrain He distribution. These maps illustrate the complexity of intracrystalline He distribution in natural zircon and, combined with a correlated quantification of parent nuclide (U and Th) distribution, provide an opportunity to assess a number of crystal chemistry processes that can generate anomalous zircon (U-Th)/He ages. The technique provides new insights into fluid inclusions as potential traps of radiogenic He and confirms the effect of heterogeneity in parent-daughter isotope abundances and metamictization on (U-Th)/He systematics. Finally, we present a new inversion method where the He, U, and Th mapping data can be used to constrain the high-and low-temperature history of a single zircon crystal.

Thermochronological insights into the structural contact between the Tian Shan and Pamirs, Tajikistan

“…The youngest single grain ZHe age for Gm‐19 (197 ± 13 Ma) matches well with the mean ZHe age for Gm‐21 (199 ± 10 Ma, Table S3). The other single grain ages exceed the zircon U–Pb age obtained for the rock (281 ± 16 and 301 ± 17 Ma, Konopelko et al., ), due to radiation damage (Danišík et al., ; Guenthner, Reiners, Ketcham, Nasdala, & Giester, ).…”

“…Three other single grain AHe ages were largely consistent and produced a mean AHe age of 23.7 ± 1.6 Ma (Table ; Table S3). The meaning of the ~24 Ma AHe age is uncertain and could indicate the presence of undetected, He‐bearing inclusions (Table , Table S3, Ehlers & Farley, ; Danišík et al., ). However, the remarkable consistency between both AFT and AHe populations likely indicates that some apatite grains (partially) preserve different cooling steps in the thermal history of the fault (e.g.…”

Fault Slip and Exhumation History of the Willard Thrust Sheet, Sevier Fold‐Thrust Belt, Utah: Relations to Wedge Propagation, Hinterland Uplift, and Foreland Basin Sedimentation

“…Inheritance envelopes, calculated using the radiation‐damage‐accumulation‐annealing model of Guenthner et al (), are broadly consistent with the range of ZHe‐eU grain values for the Monte Cristo transect (Figure d), and the range of values in upper level samples from the Wellsville and Promontory transects (Figure a). However, detailed relations among grain size, U‐Pb age, eU, and ZHe age fail to precisely match model predictions, suggesting additional sources of dispersion, such as U‐Th zoning, presence of flaws that act as He traps (fluid inclusions) and fast pathways (microcracks), nonideal grain shapes, and limitations of the current model (Danišík et al, ; Meesters & Dunai, ). For this study, zircon grains were selected based on lack of U‐Th zonation in laser‐ablation depth profiles, but U‐Th contents may have varied in other parts of grains.…”

Differential Exhumation Across the Longriba Fault System: Implications for the Eastern Tibetan Plateau

“…It should be noted that overdispersion among single‐apatite or single‐zircon FT or (U‐Th)/He ages has been reported in several studies as resulting from different grain response either to annealing process in the case of the FT technique (e.g., Tagami & O'Sullivan, ) or to spatial variability of U‐He concentration and He diffusivity within the grains for the (U‐Th)/He method (Danišík et al, ; Farley et al, ; Hourigan et al, ).…”