No abstract

In this study we describe the mobility and fractionation of REEs in two deep (up to 30 m) tropical weathering profiles developed on two granites from the Kuala Lumpur pluton, Malaysia, sampled at Cheras and Rawang. On the basis of Na2O and K2O both are S-type granites, but Rawang has higher CaO, MgO and FeO than Cheras and lower SiO2. With respect to Al-saturation Rawang is I-type and Cheras is S-type. We compared the two profiles in terms of total REEs, magnitude and changes in Ce and Eu anomalies, REE mobility and LREE/HREE fractionation. Rawang profiles have higher REE contents, display lower mobility for most except the heaviest REEs and show higher LREE/HREE fractionation than those from Cheras. These differences can be linked to differences in primary mineralogy and degree of weathering, the latter controlling the type and volume of secondary minerals. Specifically, bowl-shaped parent-rock-normalised patterns in the Cheras saprolites appear to be a result of apatite dissolution. Moreover, moderate weathering evident in lower Mineralogical Indices of Alteration (MIA) at Cheras has conserved parent rock REE patterns and fractionation factors in the saprolites. By contrast, more intense weathering observed in Rawang profiles has produced abundant kaolinite group minerals that have preferentially retained LREE, which consequently display high LREE/HREE fractionation. This study provides important insights into the factors controlling REE mobility during tropical weathering, and its potential as an indicator of weathering intensity.

Homogeneous and heterogeneous phase relationships in the alkali feldspars are reviewed, and behaviour diagrams developed. A1,Si ordering is almost certainly continuous and higher order in both albite and potassium feldspar and has been established reversibly or nearly so down to below 500 ~ in albite and possibly to ~ 200 ~ in potassium feldspar. The degree of order in intermediate albite changes strongly over a range of ~ 75-150 ~ depending on pressure, low albite being stable up to about 620-650 ~ and high albite above about 725 ~ at low pressure. Symmetry is broken at ~ 980 ~ mainly by a cooperative shearing of the whole framework and not by A1,Si ordering alone; there is a thermal crossover near 700~ shearing being dominant above (high albite) and ordering dominant below (intermediate albite).In potassium feldspar symmetry is broken by A1,Si ordering at a temperature of about 500 ~ The change in degree of order with respect to temperature has been followed easily and reversibly in sanidine from ~ 1075 to ~ 550 ~ and to a lesser extent in microcline from 450 to 200 ~ Ordering rates in sanidine down to 500 ~ and ordering rates in microcline between 450 and 200 ~ are almost as fast as in albite. Ordering in sanidine at 500 ~ and below slows and then stops with the development of the tweed orthoclase domain texture. The tweed texture acts as a barrier to further order because the strain energy associated with the (incipient) twin domain texture balances or nearly balances the free energy decrease resulting from ordering. Ordering stops not because of the kinetics of A1,Si diffusion, but because the total driving force is very small or nil. Ordering can readily proceed to completion, with the formation of low microcline, only if the domain-texture barrier is overcome by processes involving fluids or strong external stresses. There is no barrier in albite.The symmetry-breaking process in alkali feldspar changes with composition from mainly shearing in albite to ordering in potassium feldspar. Symmetry is broken equally at a compositional crossover (metastable with respect to exsolution) near Abso 75 at low pressure and progressively displaced towards Or at higher pressures. Ordering in pure albite occurs by a (nearly) one-step path which progressively becomes two-step with substitution of Or. Diagrams showing the near-equilibrium variation of the order parameters at low pressure with composition and T are given, as well as two extreme phase and behaviour diagrams for complete coherent and complete incoherent (strain-free) relationships, These diagrams can be used to understand feldspar relationships and microtextures in hypersolvus and subsolvus rocks, the occurrence of orthoclase, and of intermediate and low microcline.

Scanning Electron Microscopy and Transmission Electron Microscopy show that normal, slightly turbid alkali feldspars from many plutonic rocks contain high concentrations of micropores, from ∼1 µm to a few nm in length, typically 0.1 µm. There may be 109 pores mm−3 and porosities as high as 4.75 vol.% have been observed, although ∼1% is typical. Only ‘pristine’ feldspars, which are dark coloured when seen in the massive rock, such as in larvikite and some rapakivi granites, are almost devoid of pores. Weathering enlarges prexisting pores and exploits sub-regularly spaced edge dislocations which occur in semicoherent microperthites, but the underlying textures which lead to skeletal grains in soils are inherited from the high temperature protolith. Most pores are devoid of solid inclusions, but a variety of solid particles has been found. Although the presence of fluid in pores cannot usually be demonstrated directly, crushing experiments have shown that Ar and halogens reside in fluids. Some pores are ‘negative crystals’, often with re-entrants defined by the {110} Adularia habit, while others have curved surfaces often tapering to thin, cusp-shaped apices. The variable shape of pores accounts for the ability of some pores to retain fluid although the texture is elsewhere micropermeable, as shown by 18O exchange experiments.Apart from rare, primary pores in pristine feldspar, pore development is accompanied by profound recrystallization of the surrounding microtexture, with partial loss of coherency in cryptoperthites. This leads to marked ‘deuteric coarsening’ forming patch and vein perthite, and replacement of ‘tweed’ orthoclase by twinned microcline. The Ab- and Or-rich phases in patch perthite are made up of discrete subgrains and the cuspate pores often develop at triple-junctions between them. Coarsened lamellar and vein perthites are composed of microporous subgrain textures. These ‘unzipping’ reactions result from fluid-feldspar interactions, at T <450°C in hypersolvus syenites and T < 350°C in a subsolvus granites, and are driven by elastic strain-energy in coherent cryptoperthites and in tweed textures. Further textural change may continue to surface temperatures. In salic igneous rocks there is a general connection between turbidity and the type of mafic mineral present; pristine alkali feldspars occur in salic igneous rocks with a preponderance of anhydrous mafic phases.Because alkali feldspar is so abundant (and larger, 10 μm pores have previously been described in plagioclase), intracrystal porosity is a non-trivial feature of a large volume of the middle and upper crust. The importance of pores in the following fields is discussed: 39Ar/40Ar dating and ‘thermochronometry’; oxygen exchange; Rb and Sr diffusion; weathering; experimental low-temperature dissolution; development of secondary porosity and diagenetic albitization; leachable sources of metals; nuclear waste isolation; deformation; seismic anisotropy; electrical conductivity. Important questions concern the temperature range of the development of the textures and their stability during burial and transport into the deeper crust.