Diffusion of Ar in K-feldspar: Present and absent

Abstract

The mechanisms of Ar release from K-feldspar samples in laboratory experiments and during their geological history are assessed here. Modern petrology clearly established that the chemical and isotopic record of minerals is normally dominated by aqueous recrystallization. The laboratory critique is trickier, which explains why so many conflicting approaches have been able to survive long past their expiration date. Current models are evaluated for self-consistency, especially Arrhenian non-linearity which leads to paradoxes. The models' testable geological predictions suggest that temperature-based downslope extrapolations often overestimate observed geological Ar mobility substantially. An updated interpretation is based on the unrelatedness of geological behaviour to laboratory experiments. The isotopic record of K-feldspar in geological samples is not a unique function of temperature, as recrystallization promoted by aqueous fluids is the predominant mechanism controlling isotope transport. K-feldspar should therefore be viewed as a hygrochronometer. Laboratory degassing proceeds from structural rearrangements and phase transitions such as are observed in situ at high temperature in Na and Pb feldspars. These effects violate the mathematics of an inert Fick's Law matrix and preclude downslope extrapolation. The similar upward-concave non-linear shapes of Arrhenius trajectories of many silicates, hydrous and anhydrous, are likely common manifestations of structural rearrangements in silicate structures.

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

“…3). The degassing of Ar in the laboratory is not expected (Villa 2013) and not observed (this work, see below) to proceed from volume diffusion alone.…”

“…Solid line, trajectory of a fine-grained sieve fraction. If the nonlinearity is an intrinsic property of the diffusion coefficient D of the Kfs crystal structure (Villa 2013;Cassata and Renne 2013;Foland 1994), the Arrhenius trajectory of a coarser sieve fraction (dashed line) is parallel to the solid line; the vertical distance for all temperatures below melting is given by the ratio of the squares of the sieved grain sizes. e Schematic prediction of Arrhenius trajectories for different sieve fractions if the nonlinearity is the effect of a distribution of diffusion ''domain'' sizes superimposed on an intrinsically linear trajectory that results if D is viewed as the Fick's Law diffusion coefficient (Lovera et al 1993).…”

“…In the CPP-02-01 sample, the phases formed last are outgassed first; the most recent alterations precede the adularia, which is followed (with partial overlap) by magmatic Kfs, and finally by the phase with the oldest apparent age, restitic plagioclase. As the in vacuo degassing rate directly reflects the phase's interatomic bond length/strength (Villa 2006;2013), the implication is that the metasomatic replacement phases formed later, at decreasing P-T, and also have a decreasing bond strength. The degassing sequence mirrors the P-T conditions of the respective replacement reactions along the rock's exhumation path.…”

“…They obtained curved trajectories. Such curvatures can be either extrinsic, being the effect of a perfectly linear D modulated by a variable geometric factor, a (Lovera et al 1993), or intrinsic, being due to nonlinear Arrhenian behavior inherent in D (Villa 2013;Cassata and Renne 2013, Fig. 8).…”

Protolith provenance and thermotectonic history of metamorphic rocks in eastern Jamaica: Evolution of a transform plate boundary

“…These patterns are not uncommon in K-feldspar release spectra from slowly cooled terranes (McDougall and Harrison, 1999;Lovera et al, 2002) and have typically been interpreted to refl ect different diffusion domains within K-feldspar that have different closure temperatures with respect to 40 Ar diffusion. While multiple diffusion domain (MDD) studies (e.g., Lovera et al, 1989;Richter et al, 1991) have the potential to reveal complexities, these approaches have their limitations (see the recent reviews in Harrison and Lovera [2013] and Villa [2013]). In the absence of detailed MDD studies, it has become common practice (e.g., Karlstrom et al, 2010;Wintsch et al, 2010) to assign a closure temperature of ~150 °C for the lower experimental temperature apparent ages (in our case those in the 33-36 Ma age range), and a closure temperature of ~250 °C for the higher extraction temperature apparent ages (in our case, those in the 60 Ma age range).…”