Role of Microtexture in Isotopic Exchange and Weathering in Alkali Feldspars
Character of fast diffusion paths for argon and oxygen isotopic exchange Feldspars are often unretentive of radiogenic 'mAr ('mAr) and give young apparent ages using the K-Ar and 39Ar/'mAr methods. Similarly, feldspars frequently exchange oxygen isotopes to low temperatures and are the last mineral to close in multiphase assemblages. Because of their abundance and reactivity, feldspars in many crustal rocks approximate to the infinite reservoir of exchangeable oxygen implicit in the formulation
... in the formulation of closure temperature, To. For both isotopic processes, Tc is often much lower than would be expected using volume diffusion coefficients determined experimentally using 'gem quality' crystals. The reasons are microtextural and are ultimately related to intracrystal phase behaviour. In nature, 4~ is lost and 180 exchanged via routes that permit rapid, non-volume diffusion or involve a non-diffusional process such as solutionredeposition. Calculations of thermal history require estimates of the effective diffusion dimension (or dimensions in 'multidomain' samples) and the effective grain shape, together with knowledge of the reaction process and the time-temperature evolution of the microtextures. Pathways for fast diffusion will be illustrated using transmission and scanning electron microscopy, and include lattice-scale dislocations, pores, and subgrain boundaries. Dislocations occur along perthite lamellae in Ab-rich ternary feldspars from syenitic rocks and granulites and in Or-rich crypto-and micro-perthites in many granites. Their development is a function of bulk composition and is a response to the need to reduce coherency strain. TEM micrographs will be presented of features in Or-rich feldspar phenocrysts from the subsolvus granite from Shap, northern England, which show that, once formed, the dislocations provide pathways into the interior of crystals which lead to dissolution and reprecipitation of feldspar far from the crystal surface. The resulting microtexture contains numerous subgrains with a network of walls and micropores between them. A similar microporous texture forms along fractures permeated by aqueous solutions; together, these features are responsible for the variable translucency of common feldspars. The textures imply abrupt decrease in effective diffusion dimension during < o cooling, by a factor I> 103, at T 350 C, leading to a reactive, easily modified, micropermeable structure. Microporous texture also develops in An-poor feldspars from hypersolvus syenites but in this compositional range dislocations do not develop as a result of coherency strain and recrystallisation is not preceeded by development of dislocations independently of fluid-crystal interactions; it may occur at T ~< 450~ Driving forces Free energy changes associated with 180--160 exchange are trivial (Giletti 1985) and a driving force for recrystallisation is required. Crystal bulk compositions usually remain constant during deuteric reactions (although not during diagenetic albitization) and there are two main driving forces for recrystallisation, collectively called 'unzipping'. Elastic strain energy stored in coherent cryptoperthites is large, N 2.5-4 kJmol-1; reduction in free energy by this amount will occur when straincontrolled intergrowths recrystallise to coarser, largely incoherent intergrowths. A second driving force is loss of strain energy of ~2-3 kJmo1-1 in the 'tweed' orthoclase domain texture on its transformation into 'tartan' twinned microcline.