Reaction kinetics and plastic deformation in mantle rocks
Final Report Abstract
Experiments were carried out at high pressure and temperature in order to investigate the influence of concurrent deviatoric stresses and plastic deformation on the progress of metamorphic reactions involving the formation of garnet in simplified upper mantle lithologies. Results show that deviatoric stresses lower the kinetic barrier for the formation of new garnets causing a faster reaction rate compared to static phase transformations. The absolute amount of this increase is however dependent on the degree of overstepping the phase boundary as well as the microstructure and grain size of the reacting rock material. The amount of product phases in the deformed samples was consistently higher than that in the statically transformed samples, however with increasing time (decreasing strain rates and stresses) the difference became relatively small, indicating that the effects of plastic deformation through microstructural effects remained comparatively negligible. The nucleation and growth features of the newly forming garnet are more complex than traditionally assumed in petrological models. We observed that initially garnet may grow along grain or phase boundaries effectively forming rather large amoeboid shaped grains which finally enclose their educt phases. This kind of formation deviates strongly from the often used concentric growth model and may already induce chemical heterogeneities in the initial stages of garnet formation. A crucial role in this process is the grain size of the educt material, whereby small grain size favor concentric growth whereas larger grain size induce (at least initially) growth along boundaries. The results of this project show that controlled deformation experiments at high pressure (ca. 10 GPa) and temperature (1200°C) are feasible and reproducible over long durations (up to nearly 100 hours) with the employed six‐anvil press. The proposed in‐situ stress measurements could not be carried out successfully partly because of technical difficulties (piezometer method), partly because of infrastructure problems (diffraction method with neutrons or synchrotron X‐rays). Nevertheless we are optimistic that both of these methods can be employed in the near future. Additional (and slightly unexpected) complications in this project arose from the production of the starting materials. Producing a polyphase material with a reasonable and reproducable grain size (e.g. reaction 1) or the correct mineralogical composition (reaction 2) turned out to be more difficult or at least time‐consuming than expected from previous studies, such that one part of the study (reaction 2) was abandoned. The transformation experiments turned out to be more sensitive to grain size than expected such that only samples with the same grain size distribution in the starting material could be compared. Due to these technical problems the experiments were overall more time‐consuming than initially planned.
Publications
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(2013): Fluid‐assisted fracturing, cataclasis, and resulting plastic flow in mylonites from the Moresby Seamount detachment, Woodlark Basin. Journal of Structural Geology 56, 156‐171
R. Speckbacher, M. Stipp, J.H. Behrmann & F. Heidelbach
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(2013): Inherited fabric in an omphacite symplectite: reconstruction of plastic deformation under ultra‐high pressure conditions. Microscopy and Microanalysis 19, 942‐949
F. Heidelbach & M.P. Terry
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(2013): The effect of aluminum and water on the development of deformation fabrics in orthopyroxene. Contributions to Mineralogy and Petrology 165, 495‐505
M.A.G.M. Manthilake, N. Miyajima, F. Heidelbach, V. Soustelle & D.J. Frost
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(2014): Spherulitic omphacite in pseudotachylytes: Microstructures related to fast crystal growth from seismic melt at eclogite‐facies conditions. Chemie der Erde/Geochemistry 74, 407‐418
K. Pollok, F. Heidelbach, T. John & F. Langenhorst
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(2015): Extreme thermal stability of a hydrous aluminosilicate and H2O in the lower mantle. Nature Geoscience 8, 75‐79
M.G. Pamato, R. Myhill, T. Boffa Ballaran, D.J. Frost, F. Heidelbach & N. Miyajima
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(2016): Deformation inside a subduction channel ‐ Insights from microstructures and crystallographic preferred orientations of eclogites from the Tauern Window, Austria. Journal of Structural Geology 82, 60‐79
R. Keppler, M. Stipp, J.H. Behrmann, K. Ullemeyer & F. Heidelbach
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(2016): High temperature deformation of ortho‐ and protoenstatite. Journal of Geophysical Research 121
M. Bystricky, J. Lawlis, P. Raterron, F. Heidelbach & S. Mackwell
