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| Titel |
Olivine CPO in non-deformed peridotite due to topotactic replacement of antigorite |
| VerfasserIn |
Takayoshi Nagaya, Simon Wallis, Hiroaki Kobayashi, Katsuyoshi Michibayashi, Tomoyuki Mizukami, Yusuke Seto, Akira Miyake, Megumi Matsumoto |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090914
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| Publikation (Nr.) |
 EGU/EGU2014-5175.pdf |
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| Zusammenfassung |
| Olivine crystallographic preferred orientation (CPO) is thought to be the main cause of
seismic anisotropy in the mantle, and its formation is generally considered to be the result of
plastic deformation of mantle by dislocation creep. Olivine CPO has been reproduced in
laboratory deformation experiments and considerable success has been achieved in
understanding the deformation conditions (e.g. stress, temperature and water content) under
which different olivine CPO patterns develop. This opens the possibility of mapping
conditions in the mantle using seismic anisotropy and has been the subject of considerable
study. Here we report an alternative mechanism for olivine CPO without the need for
deformation. This process may be important in understanding the seismic properties of
mantle in convergent margins.
Metamorphic studies show peridotite in the Happo area, central Japan, formed
by the dehydration of antigorite-schist related to contact metamorphism around a
granite intrusion. Both field and microstructural observations suggest the olivine
has not undergone strong plastic deformation. This was confirmed by TEM work
that shows the olivine has very low dislocation densities and lacks low angle tilt
boundaries. Such tilt boundaries are general stable even after annealing. These
features show that peridotite in the Happo area formed in the absence of solid-state
deformation.
The olivine of the Happo peridotite formed dominantly by the dehydration breakdown of
antigorite schist. We propose that the olivine CPO formed as a result of topotactic
replacement of antigorite by the newly formed olivine. EBSD measurements in samples
where both antigorite and new olivine are present and in contact show a very close
crystallographic relationship between the two minerals: the a-axes are parallel, and the b- and
c-axes are perpendicular. We conclude the strong olivine CPO in the Happo area was
inherited from the original CPO of the antigorite. Such a process is likely to also occur in
subduction zones where serpentinite is dragged down by plate movement. Topotactic growth
of olivine may be an important cause of mantle anisotropy in convergent margins. |
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