Journal of Conference Abstracts

Texture, Structure and Pore Visualisation in Microdioritic Enclaves, Ross of Mull, Scotland

Stefano Pugliese (s.pugliese@kingston.ac.uk) & Nick Petford (n.pet@kingston.ac.uk)

School of Geological Sciences, Kingston University, Kingston-upon-Thames, Surrey KT1 2EE, U.K.

A complex suite of microdioritic enclaves exposed in tidal sections along the western margin of the Ross of Mull granite (ROM) Scotland preserve unusual evidence of mingling, incomplete mixing and infiltration processes between contemporaneous acidic and dioritic magmas. Using field, textural and mineralogical criteria, the ROM microdioritic enclaves have been classified into two types: 1) megacrystic and 2) megacrystic free. Megacrystic enclaves are further subdivided on megacrystic assemblage into: a) plagioclase and b) amphibole + K-spar + plagioclase types. Corona and mantling textures are a common feature in all enclave megacrysts.

In exceptional circumstances, a pervasive veining of the enclaves by felsic strands composed predominantly of plagioclase (An_1-25), have formed complex channel-like morphologies that preserve a relict porosity. These channel networks provide the basis of a simple methodology for estimating key petrophysical characteristics of partially molten (igneous) porous media. By serial sectioning through individual enclaves, we show that the preserved channel (porosity) network is interconnected in three dimensions. Using image analysis techniques, we have estimated the porosity (phi) of individual enclave sections to obtain the variation in porosity with depth. Simple representation of the pore-channel network on a branch and node chart allows useful petrophysical characteristics including the connectivity, genus and tortuosity of the network to be estimated. Computer enhanced reconstructions of the pore network in three dimensions are shown that provide a powerful way of visualising the complex geometries that can arise in porous igneous media.

Computer Simulation of Trace Element Partitioning

J. A. Purton¹ (J.A.Purton@bris.ac.uk), J. D. Blundy¹ (Jon.Blundy@bris.ac.uk) & N. L. Allan² (N.L.Allan@bris.ac.uk)

¹ CETSEI, Dept of Geology, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, U.K.

² School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, U.K.

We present a new approach for the rationalisation of trace element partitioning between silicate minerals and melts, which is not based on the empirical, parameterised continuum models in common use. We calculate the energetics of ion substitution using atomistic simulation techniques, which include explicit evaluation of the relaxation energy (strain energy) contribution to this process. Solution energies are estimated for isovalent and aliovalent impurities in diopside, enstatite and forsterite. These show a parabolic dependence on ionic radius, similar to the variation of mineral-melt partition coefficients with ionic radius. The solution energies are used to assess the relative importance of size mismatch between host and dopant cations. For aliovalent substitution the mode of solution has also been investigated.