A re-examination of petrogenesis and 40Ar/39Ar systematics in the Chain of Ponds K-feldspar: “diffusion domain” archetype versus polyphase hygrochronology

Abstract

K-feldspar (Kfs) from the Chain of Ponds Pluton (CPP) is the archetypal reference material, upon which thermochronological modeling of Ar diffusion in discrete ''domains'' was founded. We reexamine the CPP Kfs using cathodoluminescence and back-scattered electron imaging, transmission electron microscopy, and electron probe microanalysis. 40 Ar/ 39 Ar stepwise heating experiments on different sieve fractions, and on handpicked and unpicked aliquots, are compared. Our results reproduce the staircase-shaped age spectrum and the Arrhenius trajectory of the literature sample, confirming that samples collected from the same locality have an identical Ar isotope record. Even the most pristine-looking Kfs from the CPP contains successive generations of secondary, metasomatic/retrograde mineral replacements that postdate magmatic crystallization. These chemically and chronologically distinct phases are responsible for its staircase-shaped age spectra, which are modified by handpicking. While genuine within-grain diffusion gradients are not ruled out by these data, this study demonstrates that the most important control on staircase-shaped age spectra is the simultaneous presence of heterochemical, diachronous post-magmatic mineral growth. At least five distinct mineral species were identified in the Kfs separate, three of which can be traced to external fluids interacting with the CPP in a chemically open system. Sieve fractions have size-shifted Arrhenius trajectories, negating the existence of the smallest ''diffusion domains.'' Heterochemical phases also play an important role in producing nonlinear trajectories. In vacuo degassing rates recovered from Arrhenius plots are neither related to true Fick's Law diffusion nor to the staircase shape of the age spectra. The CPP Kfs used to define the ''diffusion domain'' model demonstrates the predominance of metasomatic alteration by hydrothermal fluids and recrystallization in establishing the natural Ar distribution among different coexisting phases that gives rise to the staircase-shaped age spectrum. Microbeam imaging of textures is as essential for 40 Ar/ 39 Ar hygrochronology as it is for U-Pb geochronology. Keywords Hygrochronology Á K-feldspar geochronology Á Cathodoluminescence Á Electron probe microanalysis Á 40 Ar/ 39 Ar age spectra Á Arrhenius trajectories Communicated by J. Hoefs.

Dating protracted fault activities: microstructures, microchemistry and geochronology of the Vaikrita Thrust, Main Central Thrust zone, Garhwal Himalaya, NW India

“…This indicates that "cooling" (Sen et al 2015) is unlikely to be the only factor controlling the biotite ages. Because age spectra only provide an incomplete information (Chafe et al 2014), it is necessary to also take into account the information provided by the (often neglected) isotopes 38 Ar and 37 Ar, which are produced from Cl and Ca in the reactor, respectively (Merrihue 1965). From the measured 38 Ar/ 39 Ar and 37 Ar/ 39 Ar ratios and the known production factors it is possible to calculate the Cl/K and Ca/K ratios, respectively (which can, but need not, be validated by EPM analyses: Villa et al 2000).…”

Apatite U-Pb Thermochronology: A Review

“…Aqueous fluid interaction in the crust is key in modifying the isotope record in nature [46], while deformation plays a role as a conduit for fluid transport on a large-scale (e.g., faults) or by creating short circuit pathways for fluid ingress within minerals (e.g., micro-fractures and other crystallographic defects) [47]. The debate on whether isotope transport in minerals is dominated by thermally-activated diffusion or if mineral dates are 'geohygrometers' that date fluid circulation episodes [46] is contentious in the literature, particularly for the interpretation of Ar isotope distributions in K-feldspar and muscovite [48][49][50][51][52]. Apatite is also highly susceptible to various fluidinduced chemical and textural changes over a wide P-T range [28], and can also exhibit phases of metamorphic growth [40,53].…”

Thermochronology of Alkali Feldspar and Muscovite at T > 150 °C Using the 40Ar/39Ar Method: A Review

“…Kinetic rate constants for fluid mediated transport of solutes are orders of magnitude larger, with significantly smaller activation energies than thermally driven diffusion (e.g., [66]). Consequently, many authors conclude that Ar redistribution and loss from K-feldspar and white micas in nature is controlled by sub-solidus transformations and/or fluid interaction (e.g., [2,3,12,14,30,[54][55][56][67][68][69][70][71]), and volume diffusion is subordinate.…”