Dehydration Breakdown of Antigorite and the Formation of B-Type Olivine CPO
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Earth and Planetary Science Letters 387 (2014) 67–76 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Dehydration breakdown of antigorite and the formation of B-type olivine CPO Takayoshi Nagaya a, Simon R. Wallis a, Hiroaki Kobayashi a, Katsuyoshi Michibayashi b, Tomoyuki Mizukami c, Yusuke Seto d, Akira Miyake e, Megumi Matsumoto d a Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan b Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan c Department of Earth Science, Graduate School of Environmental Studies, Kanazawa University, Kanazawa 920-1192, Japan d Department of Earth and Planetary Science, Kobe University 1-1, Rokkoudai, Nada-ku, Kobe 657-8501, Japan e Department of Earth and Planetary Science, Faculty of Science, Kyoto University, Kitashirakawa-Oiwake, Sakyo-ku, Kyoto 606-8502, Japan article info abstract Article history: Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from Received 23 April 2013 the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). The Received in revised form 4 October 2013 lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural Accepted 12 November 2013 evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form Available online 4 December 2013 as a result of solid-state deformation. Instead, the good correspondence between the original antigorite Editor: L. Stixrude orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic Keywords: growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction subduction zones zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer microstructure stable. The Happo Ol CPO has a strong a-axis concentration perpendicular to the lineation and within B-type olivine CPO the foliation—commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench antigorite are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the topotaxy 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction on olivine has revealed strong similarities between the CPO de- veloped by dislocation creep in laboratory samples and natural Rocks commonly show some degree of crystallographic pre- mantle rocks (e.g., Avé Lallemont and Carter, 1970; Zhang and ferred orientation (CPO) of their constituent minerals. This crystal- Karato, 1995; Jung and Karato, 2001a; Katayama and Karato, 2006; lographic alignment causes anisotropy to develop in the physical Skemer et al., 2012). These studies show that one of the dominant properties of the rock including, for instance, heat conduction and influences on the CPO is the kinematic framework of deformation. the polarization of transverse seismic waves. In many ductilely de- formed rocks—and in particular mantle tectonites—the dominant This means that if the CPO of a region of seismically anisotropic cause of CPO is assumed to be the lattice rotations caused by glide mantle is known or can be inferred, then measurements of seismic of dislocations and imposed grain shape changes, which lead to anisotropy can be used to infer the geometry of mantle flow (e.g., stable CPO patterns. In support of this idea, experimental work Kneller et al., 2005). 0012-821X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.11.025 68 T. Nagaya et al. / Earth and Planetary Science Letters 387 (2014) 67–76 Fig. 1. Geological sketch map of the Happo-O’ne complex in central Japan. The locations of the two samples as indicated by their mineral assemblages is shown bythe ellipsoidal areas including star symbols. The metamorphic zones are divided into Tremolite (Tr = olivine + tremolite), Diopside (Di = olivine + antigorite + diopside), and Talc (Tlc = olivine + talc + tremolite) zones. Serpentinite mylonites are dominantly exposed in the Diopside zone but are also locally present in the Tremolite zone. Modified after Nozaka (2005) (metamorphic zone boundary and trend of foliation from Nakamizu et al., 1989 and lithofacies distribution from Nakano et al., 2002). In this work we document field and microstructural character- XA-8800R) at Nagoya University, at 20 kV accelerating voltage and istics of mantle rock in the Happo region of central Japan and 12 nA specimen current on the Faraday cup. The beam diameter show that the olivine CPO is due to growth in a preferred orienta- was 2–3 μm. Minerals were also identified using a laser Raman tion rather than ductile deformation. This previously unrecognized spectrometer (Thermo Nicolet Almega XR) equipped with a confo- mechanism for CPO development in peridotite is potentially impor- cal microscope (Olympus BX51) at Nagoya University. The 25 mW, tant in subduction zone settings and can account for discrepancies 532 nm Nd–YAG laser produces an irradiation power of 10 mW between the microstructural characteristics of natural examples of on the samples. The Rayleigh scattering was removed using an subduction–related ‘B-type’ olivine CPO and those of laboratory ex- edge filter. The Raman signal was dispersed by a 2400 grooves − periments. mm 1 grating and detected by a Peltier-cooled CCD camera (Andor Technology, 256 × 1024 pixels). The room temperature was kept at ◦ 22 ± 1 C. The spectral resolution is about 1 μm, and a wavenum- 2. Previous studies on olivine CPO in the Happo area − ber accuracy of approximately ±0.3–0.4cm 1 was determined by −1 In the Happo region of the Hida Gaien belt in central Japan checking the position of the silicon Raman band at 520 cm be- olivine CPO has been reported from a peridotite body that formed fore each measurement. in a metamorphic aureole around a granite body due to the de- 3.1. HP5 (antigorite mylonite) hydration of foliated antigorite schist (Nozaka, 2005). There is no significant change in the orientation of the regional mesoscopic fo- HP5 is a strongly foliated antigorite schist containing olivine- liation and mineral lineation towards the granite contact implying rich patches and veins that cross cut the foliation. In addition to intrusion was not associated with significant deformation of the antigorite, the matrix also consists of porphyroclasts of olivine, surrounding rocks (Fig. 1). These field observations provide good tremolite and Cr-magnetite (Table 1). The veins and patches con- evidence that the peridotite around the granite in the Happo area sist of olivine + antigorite + talc + chlorite + Cr-magnetite + and its associated CPO formed in the absence of plastic deforma- tremolite (Fig. 2a–b; Table 1). The talc and chlorite are generally tion and some process other than dislocation creep is responsible developed along the grain boundaries (Fig. 2c–d). The presence of for the development of the CPO. The CPO reported by Nozaka fine-grained talc (<10 μm) was confirmed using EPMA and laser (2005) is a B-type CPO with the a-axis maximum perpendicular to Raman spectroscopy. the stretching lineation and within the regional foliation (Fig. 6a). For our studies it is important to distinguish between old Such B-type CPO patterns have been highlighted as important con- olivine with a porphyroclastic micrsotructure and younger post- tributors to seismic anisotropy in the mantle of convergent mar- deformation olivine. In HP5 neoblastic metamorphic grains of gins. olivine are clear with straight grain boundaries and occur in patches or veins cross cutting the antigorite foliation. In contrast, 3. Sample description the old partially serpentinized olivine has irregular outlines, is wrapped around by the foliation and is commonly cleaved (Fig. 2e). Here we present detailed descriptions of two key samples There is also a difference in chemistry: neoblastic olivine shows from the Happo area. The chemical compositions of minerals a distinctly higher Fo (forsterite) content (>92 mol.%) than the were analyzed using a electron-probe microanalyzer (EPMA) (JEOL porphyroclastic olivine (P-olivine) (88–90 mol.%) (Nozaka, 2005; T. Nagaya et al. / Earth and Planetary Science Letters 387 (2014) 67–76 69 Fig. 2. Photomicrographs of sample HP5 (antigorite schist) in sections cut parallel to the foliation (a–d and f) and in a section cut parallel to the mineral lineation and normal to the foliation (e). The arrow shows the direction of the lineation. (a) Olivine vein surrounded by antigorite schist (one-nicol).