Ar-40/Ar-39 thermochronology using alkali feldspars: real thermal history or mathematical mirage of microtexture?

The Multiple Diffusion Domain (MDD) theory of Ar loss, based on the Ar-40/A r-39 method, has been used to obtain thermal histories, over tens of millio ns of years, in the range 500 degrees C to near-surface temperatures, of al kali feldspars from slowly cooled geological environments and recovered in situ from sedimentary basins and geothermal systems. It assumes that Ar dif fuses in Nature, over geological time, from the same domains, and by the sa me processes, that control its diffusion during laboratory step-heating exp eriments. Mathematically the model has many stringent requirements: the dom ains must form at T above those being modelled and must remain unchanged du ring geological time and during step heating, must not interact, must occur in discrete size ranges sharing one simple geometrical shape, contain no c ompositional gradients in K-40, and release Ar into an infinite reservoir. We describe the nature of possible diffusion domains, and the fast pathways for diffusion between them, known to exist in alkali feldspars from direct electron microscopic observations, and explore their time-temperature evol ution, based on established phase behaviour, both over geological time and during step heating. Strain-controlled microtextures, whose morphology is c ontrolled by the minimization of coherency strain energy, do not provide fa st pathways for Ar loss, and behave as crystallographically complex but sin gle domains for Ar. Sub-regular dislocations may form over a range of Tin s ome strain-controlled intergrowths, but in Nature the majority form closed loops and do not provide pathways to the crystal surface. Deuteric microtex tures cut across strain-controlled microtextures, and are composed of micro porous subgrain and grain mosaics with many pathways and a wide spectrum of possible domain sizes and shapes. The deuterically altered zones provide p athways for Ar diffusion out of the crystal, although some Ar is retained i n fluid inclusions. All microtextures likely to define diffusion domains fo rm below 500 degrees C, over a range of T and tin slowly cooled rocks) of t imes. The most important involve fluid-feldspar reactions, which can reset Ar, and continue to surface T. Feldspars in geothermal systems and young se dimentary basins are subject to ongoing chemical and microtextural changes. Because the microtextures are metastable during step-heating experiments i n the T range over which most Ar is released, domains and pathways for rapi d diffusion of Ar between domains will be significantly modified during ste p heating, particularly above 750 degrees C, when departures from a simple Arrhenius diffusion law are most marked. Pre-melting is likely to be an imp ortant cause of complexity in Ar loss behaviour. Few, if any, of the requir ements of the MDD model are met, and we conclude that thermal histories can not be obtained from alkali feldspars using this method.