Mutual replacement reactions in alkali feldspars II: trace element partitioning and geothermometry

摘要: Perthitic alkali feldspar primocrysts in layered syenites the Klokken intrusion South Greenland, underwent dissolution–reprecipitation reactions a circulating post-magmatic aqueous fluid at ~450°C, and are to large degree pseudomorphs. These ‘mutual replacement’ provide perfect natural experiment with which study trace element partitioning between sodium potassium feldspars growing simultaneously. The reactant ‘phase’ was cryptoperthitic consisting of low albite microcline coherent sub-μm ‘braid’ intergrowth product phases were ‘strain-free’ incoherent subgrains forming microporous patch perthites on scales up 200 μm. driving force for reaction reduction coherency strain energy. mechanisms this process described Part I. Five mixed braid perthite–patch perthite crystals analysed major elements using laser ablation-inductively coupled plasma mass spectrometry 19 μm beam diameter. This gave bulk analyses texture, range Ab73–54Or45–27An4.3–0.8, but could resolve Ab- Or-rich patches perthite. compositions retained during replacement reactions. Major components plot tielines Ab–Or–An ternary system that pass through or very close parent composition indicate local equilibrium scale few tens mm. Many elements, including REE, lost deuteric reactions, effect is only Fe Ti. Cs, Pb Sr added some crystals. Plots log distribution coefficient D Rb, Ba, Pb, Eu2+, La Ce Or- Ab-rich against ionic radius straight lines, assuming eightfold coordination, first approximation independent charge. K also lies these smaller ions Na Ca lie them. best linear fits obtained radii [8]K [8]Ca, there ambiguity as whether [7]Na [5]Na most appropriate. relationship shows listed M-site rather than inclusions. Tl M although an exact not be obtained. Cs ion partitions strongly into phase its value appears less predicted by extrapolation. near-linearity arises because occurring two solids sites have similar Young’s moduli, so parabolas normally represent liquids (which negligible shear strength) approximately cancel out. Ga Be T-sites, well Ti present, part oxide site Sc unclear, if structural it likely T. Partitioning M-sites potential geothermometer effective size irregular defined (Na + Ca) contents, controlled solvus relationships, calibration conventional two-feldspar geothermometers. Trace may however useful means confirming pairs equilibrium, recognising intergrowths produced non-isochemical exsolution. Two-feldspar geothermometry low-albite gives temperatures above stability microcline, markedly correction made Si–Al ordering. probably current geothermometers too sensitive concentrations An ordered feldspars. interpretation supported assemblages known geothermal systems sedimentary basins.