@article{HalamaKonradSchmolke2015,
  author    = {Halama, Ralf and Konrad-Schmolke, Matthias},
  title     = {Retrograde metasomatic effects on phase assemblages in an interlayered blueschist-greenschist sequence (Coastal Cordillera, Chile)},
  series = {Lithos : an international journal of mineralogy, petrology, and geochemistry},
  volume    = {216},
  journal   = {Lithos : an international journal of mineralogy, petrology, and geochemistry},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0024-4937},
  doi       = {10.1016/j.lithos.2014.12.004},
  pages     = {31 -- 47},
  year      = {2015},
  abstract  = {Interlayered blueschists and greenschists of the Coastal Cordillera (Chile) are part of a Late Palaeozoic accretionary complex. They represent metavolcanic rocks with oceanic affinities based on predominantly 01B-type REE patterns and immobile trace element ratios. Both rock types have similar mineralogies, albeit with different mineral modal abundances. Amphibole is the major mafic mineral and varies compositionally from glaucophane to actinolite. The presence of glaucophane relicts as cores in zoned amphiboles in both blueschists and greenschists is evidence for a pervasive high-pressure metamorphic stage, indicating that tectonic juxtaposition is an unlikely explanation for the cm-dm scale interlayering. During exhumation, a retrograde greenschist-facies overprint stabilized chlorite + albite + winchitic/actinolitic amphibole + phengitic white mica +/- epidote +/- K-feldspar at 0.4 +/- 0.1 GPa. Geochemical variability can be partly ascribed to primary magmatic and partly to secondary metasomatic processes that occurred under greenschist-facies conditions. Isocon diagrams of several adjacent blueschist-greenschist pairs with similar protolith geochemistry were used to evaluate metasomatic changes due to retrograde fluid-rock interaction. The most important geochemical changes are depletion of Si and Na and addition of water in the greenschists compared to the blueschists. Transition metals and LILE are mobilized to varying degrees. The unsystematic deviations from magmatic fractionation trends suggest open system conditions and influx of an external fluid. Pseudosection and water isopleth calculations show that the rocks were dehydrating during most of their exhumation history and remained at water-saturated conditions. The mineralogical changes, in particular breakdown of blue amphibole and replacement by chlorite, albite and calcic/sodic-calcic amphibole, are the prime cause for the distinct coloring. Pseudo-binary phase diagrams were used as a means to link bulk rock geochemical variability to modal and chemical changes in the mineralogy. The geochemical changes induced by fluid-rock interaction are important in two ways: First, the bulk rock chemistry is altered, leading to the stabilization of higher modal proportions of chlorite in the greenschists. Second, the retrograde overprint is a selective, layer-parallel fluid infiltration process, causing more intense greenschist-facies recrystallization in greenschist layers and therefore preferential preservation of blue amphibole in blueschist layers. Hence, the distinct colors were acquired by a combination of compositional variability, both primary magmatic and secondary metasomatic, and the different intensity of retrograde fluid infiltration. (C) 2014 Elsevier B.V. All rights reserved.},
  language  = {en}
}
@misc{KonradSchmolkeHalama2014,
  author    = {Konrad-Schmolke, Matthias and Halama, Ralf},
  title     = {Combined thermodynamic-geochemical modeling in metamorphic geology: Boron as tracer of fluid-rock interaction},
  series = {Lithos : an international journal of mineralogy, petrology, and geochemistry},
  volume    = {208},
  journal   = {Lithos : an international journal of mineralogy, petrology, and geochemistry},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0024-4937},
  doi       = {10.1016/j.lithos.2014.09.021},
  pages     = {393 -- 414},
  year      = {2014},
  abstract  = {Quantitative geochemical modeling is today applied in a variety of geological environments from the petrogenesis of igneous rocks to radioactive waste disposal. In addition, the development of thermodynamic databases and computer programs to calculate equilibrium phase diagrams has greatly advanced our ability to model geodynamic processes. Combined with experimental data on elemental partitioning and isotopic fractionation, thermodynamic forward modeling unfolds enormous capacities that are far from exhausted. In metamorphic petrology the combination of thermodynamic and trace element forward modeling can be used to study and to quantify processes at spatial scales from mu m to km. The thermodynamic forward models utilize Gibbs energy minimization to quantify mineralogical changes along a reaction path of a chemically open fluid/rock system. These results are combined with mass balanced trace element calculations to determine the trace element distribution between rock and melt/fluid during the metamorphic evolution. Thus, effects of mineral reactions, fluid-rock interaction and element transport in metamorphic rocks on the trace element and isotopic composition of minerals, rocks and percolating fluids or melts can be predicted. Here we illustrate the capacities of combined thermodynamic-geochemical modeling based on two examples relevant to mass transfer during metamorphism. The first example focuses on fluid-rock interaction in and around a blueschist-facies shear zone in felsic gneisses, where fluid-induced mineral reactions and their effects on boron (B) concentrations and isotopic compositions in white mica are modeled. In the second example, fluid release from a subducted slab, the associated transport of B as well as variations in B concentrations and isotopic compositions in liberated fluids and residual rocks are modeled. We compare the modeled results of both examples to geochemical data of natural minerals and rocks and demonstrate that the combination of thermodynamic and geochemical models enables quantification of metamorphic processes and insights into element cycling that would have been unattainable if only one model approach was chosen. (C) 2014 Elsevier B.V. All rights reserved.},
  language  = {en}
}