ARGON DIFFUSION DOMAINS IN K-FELDSPAR .1. MICROSTRUCTURES IN MH-10

Samples of MH-10, a K-feldspar thought to contain discrete diffusion domains for argon, have been examined by light and TEM microscopy to search for domain boundaries. We examined both unheated K-feldspar separates and samples heated in vacuum between 750 and 1150-degrees-C in order to characterize the initial microstructure as well as any changes resulting from laboratory heating. The MH-10 rock shows almost no evidence of plastic deformation, and the K-feldspar does not possess networks of dislocations which might define a subgrain structure derived from external stresses. Three classes of substructure are present in the K-feldspar, but only the third appears to have been affected by laboratory heating: (1) Cross-hatched extinction in light microscope is common but variably developed. TEM reveals almost no albite/pericline twinning but tweed microstructure is ubiquitous. (2) Turbid zones occur with about 5 volume% abundance. Micropores, < 1 to 2 mum in diameter, characterize these regions, which often also contain blebs of albite, up to 40 mum in diameter. TEM shows these turbid zones to be very complex with intricate twin and tweed structures at the sub-micron scale and numerous dislocation and strain features. (3) Albitic exsolution lamellae ( < 1 mum thick, 20 mum long and separated by 1 mum) arc rarely seen in the light microscope but TEM shows that 0.01 mum thick by 0.2-1 mum long lamellae, separated by 0.1 to 0.5 mum, occur in about 20% of the sample. These lamellae are disk-shaped, have a semi-coherent or coherent relationship to the host K-feldspar, and seem to disappear upon heating to 750-degrees-C. Extensive diffusion studies have been conducted on MH-10, and the results analyzed in terms of distribution of sizes of diffusion domains. The preferred result of the analyses was a distribution consisting of three distinct sizes. The largest domain size identified in this way is approximately 50 to 100 mum, and this corresponds to blocks of K-feldspar defined by the network of fractured/turbid zones. The smallest domain size inferred from diffusion analysis is approximately 0.1 mum which corresponds to the spacing between albite exsolution lamellae that may represent fast diffusion pathways. Alternatively, the modulated argon distribution accompanying the lamellar variation in feldspar composition may result in an overestimation of the initial diffusion coefficients. Even though a promising agreement exists between domain analyses based upon diffusional and microstructural analyses, at least two questions remain unanswered. Firstly, no clear candidate has been identified for a domain of intermediate size. Secondly, the diffusion analysis has been formulated with independent gas release from all domains, whereas the observed structures are likely to be nested (i.e., small domains exist inside bigger domains) with the network of semi-coherent lamellar boundaries having connectivity and permeability which can provide only a modest increase in argon diffusivity over bulk values.