TY - JOUR
T1 - Dehydration breakdown of antigorite and the formation of B-type olivine CPO
AU - Nagaya, Takayoshi
AU - Wallis, Simon R.
AU - Kobayashi, Hiroaki
AU - Michibayashi, Katsuyoshi
AU - Mizukami, Tomoyuki
AU - Seto, Yusuke
AU - Miyake, Akira
AU - Matsumoto, Megumi
N1 - Funding Information:
We thank M. Enami and other members of the petrology group of Nagoya University for discussions and comments about this study. We also thank D. Mainprice for advice on EBSD installation and providing the software to analyze the CPO data and T. Nozaka for providing his original U-stage measurements. This work was supported by JSPS grants-in-aid Nos. 21244083 , 23340169 , 24244079 and 25000199 .
Copyright:
Copyright 2020 Elsevier B.V., All rights reserved.
PY - 2014/2/1
Y1 - 2014/2/1
N2 - Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). The lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form as a result of solid-state deformation. Instead, the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable. The Happo Ol CPO has a strong a-axis concentration perpendicular to the lineation and within the foliation-commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the mantle wedge of these regions. Experimental work has shown that B-type CPO can form by dislocation creep under hydrous conditions at relatively high stresses. There are, however, several discrepancies between the characteristics of natural and laboratory samples with B-type Ol CPO. (1) The formation conditions (stress and temperature) of some natural examples with B-type CPO fall outside those predicted by experiments. (2) In deformation experiments, slip in the crystallographic c-axis direction is important but has not been observed in natural examples of B-type CPO. (3) Experimental work suggests the presence of H2O and either high shear stress or relatively low temperatures are essential for the formation of B-type CPO. These conditions are most likely to be achieved close to subduction boundaries, but these regions are also associated with serpentinization, which prevents strong olivine CPO patterns from forming. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of foliated serpentinite. These results resolve the discrepancies between experimental and natural examples of B-type CPO and show the need to rethink the formation process of olivine CPO in convergent margins. Topotactic growth of olivine after antigorite can account for the inferred distribution of B-type Ol CPO in the mantle wedge more successfully than dislocation creep.
AB - Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). The lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form as a result of solid-state deformation. Instead, the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable. The Happo Ol CPO has a strong a-axis concentration perpendicular to the lineation and within the foliation-commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the mantle wedge of these regions. Experimental work has shown that B-type CPO can form by dislocation creep under hydrous conditions at relatively high stresses. There are, however, several discrepancies between the characteristics of natural and laboratory samples with B-type Ol CPO. (1) The formation conditions (stress and temperature) of some natural examples with B-type CPO fall outside those predicted by experiments. (2) In deformation experiments, slip in the crystallographic c-axis direction is important but has not been observed in natural examples of B-type CPO. (3) Experimental work suggests the presence of H2O and either high shear stress or relatively low temperatures are essential for the formation of B-type CPO. These conditions are most likely to be achieved close to subduction boundaries, but these regions are also associated with serpentinization, which prevents strong olivine CPO patterns from forming. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of foliated serpentinite. These results resolve the discrepancies between experimental and natural examples of B-type CPO and show the need to rethink the formation process of olivine CPO in convergent margins. Topotactic growth of olivine after antigorite can account for the inferred distribution of B-type Ol CPO in the mantle wedge more successfully than dislocation creep.
KW - Antigorite
KW - B-type olivine CPO
KW - Microstructure
KW - Subduction zones
KW - Topotaxy
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U2 - 10.1016/j.epsl.2013.11.025
DO - 10.1016/j.epsl.2013.11.025
M3 - Article
AN - SCOPUS:84889586733
VL - 387
SP - 67
EP - 76
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
SN - 0012-821X
ER -