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The earliest mantle fabrics formed during subduction zone infancy
Earth and Planetary Science Letters 377–378 (2013) 106–113
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Earth and Planetary Science Letters www.elsevier.com/locate/epsl
The earliest mantle fabrics formed during subduction zone infancy Yumiko Harigane a,∗ , Katsuyoshi Michibayashi b , Tomoaki Morishita c , Kenichiro Tani d , Henry J.B. Dick e , Osamu Ishizuka a,d a
Institute of Geology and Geoinformation, Geological Survey of Japan/National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8567, Japan b Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan c Earth Science Course, School of Natural System, College of Science and Engineering, Kanazawa University, Kanazawa 920-1192, Japan d Institute for Research on Earth Evolution, Japan Agency for Marine–Earth Science and Technology, Kanagawa 237-0061, Japan e Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
a r t i c l e
i n f o
Article history: Received 15 December 2012 Received in revised form 11 June 2013 Accepted 24 June 2013 Available online 24 July 2013 Editor: Y. Ricard Keywords: harzburgite olivine crystal preferred orientation fore-arc mantle Izu–Bonin–Mariana (IBM) arc oceanic island arc
a b s t r a c t Harzburgites obtained from the oldest crust–mantle section in the Philippine Sea plate (∼52 Ma) along the landward slope of the southern Izu–Ogasawara Trench, preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The harzburgites have relatively fresh primary minerals despite of their heavy serpentinizations, and show inequigranular interlobate textures, and crystal preferred orientation patterns in olivine (001)[100] and Opx (100)[001]. The harzburgites have the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. We propose that the (001)[100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Fore-arcs provide key information on the initiation of magmatic and subduction zone processes during island arc formation (Bloomer et al., 1995; Stern and Smoot, 1998; Stern 2002, 2004). However, since fore-arc sections are exposed deep on the landward trench slope accessibility is limited. Thus only a few studies have considered the mantle structure in fore-arcs, in contrast to the many studies of the evolution of crustal structures in island arcs (e.g., Kodaira et al., 2010; Ishizuka et al., 2011). Furthermore, fore-arc mantle structures formed during the initial stages of subduction might be erased or modified during subsequent subduction-related tectonic events in mature and ancient arc accreted terrains (e.g., Arcay et al., 2005). For instance, many modern fore-arc regions are thought to be highly serpentinized mantle, infiltrated by slab-derived fluids in steady-state subduction systems (e.g., Bostock et al., 2002; Hyndman and Peacock, 2003; Hilairet and Reynard, 2009; Katayama et al., 2009; Boudier et al., 2010; Hirauchi et al., 2010). Here, we document for the first time the structure of immature fore-arc mantle at the time of subduction zone initiation as preserved in peridotites exposed on the deep
*
Corresponding author. Tel.: +81 29 849 1060; fax: +81 29 849 3765. E-mail address: [email protected] (Y. Harigane).
0012-821X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.06.031
seafloor along the landward slope of the southern Izu–Ogasawara Trench (Fig. 1). 2. Geological background Dive 7K417 of the ROV Kaiko 7000II during R/V Kairei cruise KR08-07, and Dredge 31 of R/V Hakuho-Maru cruise KH07-02, operated by the Japan Agency for Marine–Earth Science and Technology, explored the landward slope of the southern Izu–Ogasawara Trench (Fig. 1A). Outcrops of peridotite, gabbro, dolerite, and basalt were observed during the dive from 5336 to 5792 meters below sea level (mbsl) (Morishita et al., 2011) and 48 samples of 35 peridotites, 9 troctolites, pyroxenite, and 3 gabbros were dredged between 5293 and 5738 mbsl (Fig. 1B). It appears that these slopes expose a mantle-derived peridotite body developed in the Izu– Bonin–Mariana (IBM) arc (Morishita et al., 2011), whereas some doleritic samples have a fore-arc tholeiitic basalt signature that is considered to represent the earliest magmatism during subduction initiation (Fig. 1B; Ishizuka et al., 2011). 3. Microstructure The five harzburgite samples were selected for study: two from the dive (R9 and R19) and three from the dredge (D31-1, -3, and -10). The two harzburgites (R9 and R19) have visible foliations and
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Fig. 1. A: Bathymetric map of the Izu–Ogasawara arc region. Study area shown by the red star on the landward slope of the trench close to the Bonin ridge. Broken gray line indicates the Izu–Ogasawara volcanic front. B: Bathymetric map showing the sampling points and track (pink dashed line) for ROV dive KR08-07-7K417 and the approximate track for Dredge KH07-02-D31 (grey dashed line). Colored symbols show the typical lithologies collected at each ROV sampling location: green – harzburgite, light green – dunite, yellow – gabbro, orange – wehrlite. A doleritic rock with fore-arc tholeiitic basalt signature (FAB) is red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
lineations defined by the alignments of spinel and pyroxene grains on bleached and saw-cut samples (e.g., Michibayashi et al., 2009). Thin sections were cut in two samples perpendicular to the foliation and parallel to the lineation defined by pyroxene’s structure. However, the other three harzburgites (D31-1, -3, and -10) have no or very weak structures with their small sizes; thin sections of these samples were made in arbitrary sections. The main constituent minerals are olivine (15.6%), orthopyroxene (Opx; 13.1%) and spinel (0.5%), although serpentine is currently the most common mineral as more than 70% modal compositions within the five samples due to heavy serpentinization (Fig. 2). The harzburgites have coarse inequigranular interlobate (or protogranular) textures (e.g., Passchier and Trouw, 1996) consisting of largely coarse-grained olivine with serrated grain boundaries and coarse- to medium-grained Opx (Fig. 2A, B, C, D). They exhibit signs of intracrystalline deformation such as wavy extinction, and subgrain boundaries, but without intense grain-size reduction. Elongate Opx grains were dynamically recrystallized into mediumsized aggregates, and wavy extinction provides evidence for intracrystalline deformation (Fig. 2A, B, C, D). Harzburgite D31-10 is internally complex with a sharp-sutured contact between Opxrich and Opx-poor harzburgite, with finer grained pyroxene in the latter; a gabbro vein crosscuts the Opx-rich harzburgite with a razor sharp straight contact that is oblique to the Opx-rich–Opx-poor contact (Fig. 2E, F). Secondary serpentine shows the mesh texture in these harzburgites (Fig. 2), whereas we cannot observe the deformation microstructure of serpentine. Therefore, we argue that these harzburgites could preserve an original microstructure in the mantle structure at IBM region despite of their heavy serpentinizations.
4. Mineral chemistry The chemical composition of primary minerals in three harzburgite samples (D31-1, -3, and -10) were analyzed using a JEOL JCXA733 electron probe micro-analyzer (Shizuoka University, Japan). Analyses were made with a probe current of 12 nA, accelerating voltage of 15 kV, and a correction procedure after Bence and Albee (1968). The harzburgite samples have high olivine forsterite (90.6–92.1 mol.%) and NiO (∼0.4 wt%) contents (Table 1), low Opx Al2 O3 (< ∼1.5 wt%) and Na2 O (<0.03 wt%) (Table 2), and high spinel Cr# (65–67) (Table 3). These data is similar to that for two harzburgites (KR08-07-7K417R9 and R19) in Morishita et al. (2011). These mineral compositions lie largely outside the range for abyssal peridotites from mid-ocean ridges (Fig. 3; e.g. Dick and Bullen, 1984), indicating that the harzburgites are refractory consistent with an origin in a supra-subduction zone mantle wedge (Morishita et al., 2011). 5. Crystal fabric analysis The crystallographic preferred orientations (CPOs) of olivine and Opx grains in highly polished thin sections were analyzed with a JEOL JSM6300 SEM and a HITACHI S-3400N SEM, equipped for electron back-scattered diffraction (EBSD with HKL Channel5), at Shizuoka University, Japan. We measured the crystal orientations of 91 to 233 olivine grains and 59 to 171 Opx grains in 5 harzburgites, visually checking the computerized indexation of each diffraction pattern. We then calculated the J -index (Mainprice et al., 2000; Michibayashi and Mainprice, 2004; Michibayashi et al., 2006) to determine the fabric strength and distribution densities of the principal crystallographic axes. The J -index has a value of 1
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Fig. 2. Microphotographs of 5 harzburgites studied in this paper. (A) KR08-07-7K417R19 and (B) KR08-07-7K417R9 in the section cutting perpendicular to the foliation and parallel to the lineation. (C) KH07-02-D31-3 and (D) KH07-02-D31-1 in arbitrary sections, since they show very weak structures with their small sizes. (E) KH07-02-D31-10 in the whole thin section. This sample shows internally complex microstructures with a sharp-sutured contact between Opx-rich and Opx-poor harzburgite (solid white line), with finer grained pyroxene in the latter. A gabbro vein also crosscuts the harzburgite (broken white line). (F) Enlargement of the area outlined by the yellow rectangle in E. All images were taken under cross-polarized light. Ol: Olivine; Opx: Orthopyroxene; Sp: Spinel. The secondary serpentine occurs more than 70% of modal compositions in all thin sections. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
for random distributions and a value of infinity for single crystals. The index increases with increasing axial strain based on the experimental data of Nicolas et al. (1973), and thus can be used as a plastic-strain indicator to a first approximation (Ben Ismaïl and Mainprice, 1998).
Olivine has mainly (001)[100] CPO patterns in three of our samples, with a strong alignment of the [100] axis along the lineation, and the [001]-axis perpendicular to the foliation (Fig. 4A). These olivine CPOs have a high J -index value (Fig. 4A; J = 11.50–12.96). We note that the olivine CPO and Opx CPO patterns in each sample
Y. Harigane et al. / Earth and Planetary Science Letters 377–378 (2013) 106–113
Table 1 Representative composition of olivine in KH07-02-D31 harzburgites. Sample No.
D31-1OL14
D31-1OL18
wt% SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O Cr2 O3 NiO V2 O3 Total
39.93 0 0 7.98 0.13 51.88 0.03 0 0.01 0.36 0 0.01 100.32
40.40 0.01 0.01 8.04 0.10 51.51 0.02 0 0 0.40 0.02 0.04 100.55
Cations\O
4
4
4
4
4
Si Ti Al Fe2+ Mn Mg Ca Na K Cr Ni V Total Mg# *
0.971 0 0 0.162 0.003 1.881 0.001 0 0 0.007 0 0 3.025 92.1
0.979 0 0 0.163 0.002 1.862 0.001 0 0 0.008 0 0.001 3.016 92.0
0.981 0 0.001 0.161 0.004 1.863 0.001 0 0 0 0.008 0 3.018 92.0
0.979 0 0 0.167 0.002 1.863 0.001 0 0 0 0.008 0 3.020 91.8
0.976 0 0 0.189 0.003 1.846 0.001 0 0 0 0.008 0 3.024 90.7
*
D31-3OL16 39.98 0 0.02 7.86 019 50.91 0.02 0 0 0 0.38 0 99.36
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Table 2 Representative composition of orthopyroxene in KH07-02-D31 harzburgites.
D31-3OL43
D31-10OL66
D31-10OL71
Sample No.
D31-1OPX13
D31-1OPX23
D31-3OPX17
D31-3OPX24
D31-10DPX62
D31-10OPX72
39.87 0 0.01 8.12 0.09 50.89 0.03 0 0 0 0.41 0 99.42
40.06 0.02 0.01 9.57 0.14 49.08 0.04 0 0 0.01 0.42 0.00 99.34
39.69 0 0.01 9.17 0.15 50.35 0.04 0 0 0 0.41 0.01 99.81
wt% SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O Cr2 O3 NiO V2 O3 Total
55.90 0.01 1.48 5.43 0.11 34.14 1.25 0.01 0 0.11 0 0.74 99.18
56.30 0.01 1.45 5.24 0.11 34.69 1.31 0.01 0 0.15 0 0.64 99.90
56.17 0.02 1.39 5.28 0.14 34.20 1.32 0 0 0.59 0.09 0.02 99.21
55.84 0 1.43 5.40 0.14 34.18 1.25 0 0 0.69 0.10 0.01 99.04
55.21 0.01 1.44 7.25 0.19 33.14 1.38 0.03 0 0.59 0.11 0.01 99.37
55.86 0.01 1.52 6.56 0.17 33.71 1.45 0.02 0 0.60 0.08 0 99.97
4
Cations\O
6
6
6
6
6
6
0.974 0 0 0.193 0.003 1.847 0.001 0 0 0 0.009 0 3.026 90.6
Si Ti Al Fe2+ Mn Mg Ca Na K Cr Ni V Total Mg# *
1.946 0 0.061 0.158 0.003 1.772 0.046 0 0 0.003 0 0.021 4.011 91.8
1.945 0 0.059 0.151 0.003 1.786 0.049 0.001 0 0.004 0 0.018 4.015 92.2
1.953 0 0.057 0.153 0.004 1.773 0.049 0 0 0.016 0.002 0.001 4.010 92.0
1.948 0 0.059 0.158 0.004 1.777 0.047 0 0 0.019 0.003 0 4.013 91.9
1.938 0 0.060 0.213 0.006 1.734 0.052 0.002 0.000 0.016 0.003 0 4.025 89.1
1.941 0 0.062 0.191 0.005 1.746 0.054 0.001 0 0.016 0.002 0 4.020 90.2
Mg/(Mg + Fe) ratio.
are not parallel each other regardless of its identified foliation and lineation. The [100] maxima in the olivine is approximately 20◦ oblique to the [001] maxima in the Opx (Fig. 4A). Opx CPOs in the oriented harzburgites have (100)[001] patterns with [001] parallel to the lineation and (100) normal to foliation, although these CPOs are relatively weak (Fig. 4A). Opx in harzburgite D31-3 also has a (100)[001] CPO matching those in oriented harzburgites, though its relationship to the macroscopic mineral shape fabrics is unknown. The remaining two harzburgites have unclear CPO fabrics, because of a small number of the measured grains (Fig. 4B). Sample D31-1 have a weak concentration of olivine [100] axis but the scatter distribution of olivine [010] and [001] axes, which CPO pattern does not match to any known olivine fabrics (Fig. 4B; cf. Karato et al., 2008; Mainprice, 2007). Both olivine and Opx CPO patterns of Sample D31-10 are quite difficult to interpret their relationships, since their CPO patterns are so weak (Fig. 4B). This sample involves a gabbro vein (Fig. 2B) and therefore might be possible evidence of late-stage melt impregnation as it has anomalous low forsterite olivine content compared to the other harzburgites (Fo90.7 vs Fo91.8–92.1 , Table 1). 6. Discussion Stern and Bloomer (1992) proposed a tectonic model for island arc formation in the IBM region in which they discussed for the earliest stages of its evolution. Ar–Ar dating of volcanic and U–Pb zircon dates for dolerite and gabbro (Ishizuka et al., 2006, 2011) further support this model. They suggest that the first basaltic magmatism occurred in the fore-arc region due to decompression melting of the mantle during subduction initiation, which occurred at 51–52 Ma based on basalt Ar–Ar dates and gabbro zircon U–Pb dates (Ishizuka et al., 2011). Arc formation then began at ∼45–48 Ma with boninitic volcanism and fore-arc extension,
*
Mg/(Mg + Fe) ratio.
which occurred contemporaneously along the entire length of the IBM system. The boninitic melts resulted from the flux of water into mantle wedge harzburgite from the Pacific plate as it sank into the asthenosphere during this stage (Stern and Bloomer, 1992; Ishizuka et al., 2006). This produced high Cr-spinel refractory harzburgites (Fig. 3) lying outside the range for abyssal peridotites (Dick and Bullen, 1984; Parkinson and Pearce, 1998). Morishita et al. (2011) argued that the Dive 7K417R15 dunite (spinel Cr# ∼82; Fig. 3) formed due to extraction of the boninitic melts from the harzburgites during the boninitic volcanism stage of subduction initiation, while the lower Cr-spinel dunites formed during the earlier extraction of the basalts. Based on this analysis, the Dive 7K417 and Dredge 31 dunites and harzburgites likely preserve the signatures of the earliest mantle structures formed at the initial stages of subduction in an oceanic island arc. None of the Dive 7K417 and Dredge 31 harzburgites have visible porphyroclastic or fine-grained secondary textures with variable CPO patterns and weak J -indices consistent with lowtemperature deformation (e.g., Nicolas, 1986; Suhr, 1993; Michibayashi and Mainprice, 2004; Michibayashi et al., 2007, 2009). Therefore, the coarse-grained textures and strong crystal fabrics in these peridotites could be established at near-solidus temperatures during asthenospheric flow consistent with the pyroxene foliations and lineations. The three Dredge 31 and Dive 7K417 harzburgites with mainly [100](001) olivine CPOs with a strong alignment of the [100] axis along the lineation, and the [001]-axis perpendicular to the foliation represent the E-type fabric of Katayama et al. (2004) (Fig. 4A). Michibayashi and Mainprice (2004) interpreted [100](001) olivine fabrics in Oman ophiolite harzburgites as due to mechanical weakening under the low temperatures in the presence of preexisting mechanical mantle anisotropy in the lithosphere. Mehl et al. (2003), however, interpreted natural E-type olivine patterns in Alaskan Talkeetna arc harzburgites as due to the effect of water on
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Table 3 Representative composition of spinel in KH07-02-D31 harzburgites. Sample No.
D31-1SP15
D31-1SP17
D31-3SP18
D31-3SP30
D31-10SP55
D31-10SP67
wt% SiO2 TiO2 Al2 O3 Fe2 O3 FeO MnO MgO CaO Na2 O K2 O Cr2 O3 NiO V2 O3 Total
0.02 0.04 16.64 2.89 17.60 0.08 10.99 0 0.03 0.00 51.30 0.05 0.17 99.8
0.04 0.02 16.91 2.63 17.75 0.26 11.05 0 0 0 51.89 0.04 0.17 100.7
0.02 0.02 16.85 2.27 17.20 0.21 11.10 0 0 0 51.34 0.09 0.20 99.3
0.01 0.01 16.88 3.04 16.99 0.22 11.20 0.02 0 0 50.64 0.10 0.23 99.3
0.02 0 17.58 5.13 16.87 0.23 11.69 0 0 0.01 49.15 0.08 0.18 100.9
0.02 0.03 17.06 6.20 18.59 0.24 10.30 0 0 0 47.39 0.12 0.15 100.1
Cations\O
4
4
4
4
4
4
Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K Cr Ni V Total Mg# * Cr# **
0.001 0.001 0.627 0.070 0.471 0.002 0.524 0 0.002 0 1.297 0.001 0.004 3.000 49.2 67.4
0.001 0.001 0.631 0.063 0.470 0.007 0.522 0 0 0 1.300 0.001 0.004 2.999 49.5 67.3
0.001 0.001 0.637 0.055 0.461 0.006 0.531 0 0 0 1.302 0.002 0.005 3.000 50.7 67.1
0 0 0.637 0.073 0.455 0.006 0.535 0.001 0 0 1.283 0.003 0.006 3.000 50.3 66.8
0.001 0 0.652 0.121 0.444 0.006 0.548 0 0 0 1.222 0.002 0.005 3.000 49.25 65.2
0.001 0.001 0.644 0.150 0.498 0.006 0.492 0 0 0 1.201 0.003 0.004 3.000 43.17 65.1
* **
Mg/(Mg + Fe) ratio. Cr/(Cr + Al) ratio.
deformation. Katayama et al. (2004), subsequently experimentally produced the E-type olivine fabrics by deformation in the presence of water (200 < COH < 1000 H/106 Si) at moderate to low stress (<400 MPa). Our three harzburgites with E-type olivine fabrics are distinct from those found in some abyssal peridotites (e.g.: Achenbach et al., 2011), which have a strong [010] maximum sub-perpendicular to foliation, and less strongly developed [100] and [001] maxima parallel to the foliation plane. These abyssal peridotites, however, have Opx fabrics similar to our samples, but stronger, with a strong [001] Opx maximum parallel to foliation, and a weaker [100] maximum sub-perpendicular to foliation. This fabric is consistent with relatively low strain rate operating on olivine (010)[100] and the Opx (100)[001] slip systems at ∼1250 ◦ C. Achenbach et al. (2011) conclude this reflects high-temperature mantle deformation beneath the Mid-Atlantic Ridge, a tectonic setting that contrasts sharply to the arc setting of our samples. The other two harzburgites (D31-1 and -10) do not show any known olivine fabric type because of no or weak visible structures. Opx fabric of D31-1 may show a weak (100)[001] pattern despite the fact that its olivine fabric does not show any typical CPO patterns, indicating that a complex deformation process may cause the formation of each CPO pattern in D31-1 at mantle wedge. Furthermore, D31-10 is also very weak olivine and Opx patterns, suggesting that D31-10 may have been modified by late-stage melt flow suggested by the Opx-poor–Opx-rich contact which is consistent with channelized melt flow through the finergrained, pyroxene-poor, portion of the sample. The latter characteristics may be consistent with partial dissolution of Opx during melt migration as the phase field of olivine expands with decreasing pressure (Kushiro, 1969). As a consequence, we exclude these
Fig. 3. Relationships between spinel Mg# and Cr#’s modified from Morishita et al. (2011) by adding the three Dredge 31 harzburgites (green-filled triangles). Additional colored symbols are for dunites (light green-filled circles) and harzburgites (green-filled squares) from the same dive (7K417), and for additional IzuBonin Trench dunites (small grey-filled circles, averaged from Ishii et al., 1992, and Parkinson and Pearce, 1998). Compositional fields for abyssal peridotites – black solid line (Dick et al., 2010) and fore-arc peridotites – black broken line (Ishii et al., 1992). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
two harzburgites from our discussion, since their structural and fabric data are too obscure to examine in any more detail. Moderately chromian spinel reported by Morishita et al. (2011) in several dunites from Dive 7K417 (Fig. 3) is similar to those in MORB and abyssal peridotites and dunites (e.g.: Dick and Bullen, 1984), likely representing the early Stage 1 basalt melt extraction (Morishita et al., 2011). The TiO2 content of these spinels is lower than that found in most abyssal dunites, consistent with the low TiO2 content of IBM fore-arc basalts (Reagan et al., 2010). Our microstructural evidence for high-temperature mantle flow in peridotites from the Izu–Ogasawara fore-arc region are con-
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Fig. 4. Crystallographic preferred orientation (CPO) data for olivine and Opx in the harzburgites are plotted on equal-area, lower-hemisphere projections. Contours are multiples of uniform density. N is the number of analyzed olivine and Opx grains. Foliation and lineation are shown E–W in line drawings and pole figures. A: CPO data of the three harzburgites. CPO data for harzburgite sample D31-3 rotated to show it matches that in the foliated harzburgites. B: CPO data of the two harzburgites.
sistent with the initial stage of the tectonic models for island arc formation as proposed by Stern and Bloomer (1992), Ishizuka et al. (2006, 2011), and Morishita et al. (2011) as discussed above (Fig. 5A). Consequently, dehydration from the subducting slab into the fore-arc mantle during the initial stages of subduction may favor asthenospheric flow under a low flow stress in the mantle wedge by decreasing rocks strength (e.g., Arcay et al., 2005), resulting in the development of the E-type olivine patterns (Fig. 4A). Mehl et al. (2003) and Tommasi et al. (2006) report (010)[100] olivine patterns (the A-type pattern of Katayama et al., 2004) in the refractory harzburgites in ancient island arc settings. They suggest that A-type olivine patterns, which are similar to those reported by Achenbach et al. (2011), developed at high-temperature in anhydrous conditions, developed during lateral and/or upwelling asthenospheric flow into the mantle wedge during subduction initiation (Fig. 3A). The upwelling flow due to slab sinking may alternatively result in the development of {0kl}[100] olivine patterns (i.e. Katayama et al., 2004, D-type pattern) under high stress and anhydrous conditions (Fig. 4A).
Even though there are a small number of samples, it possibly represents an importance as a natural sample of the fore-arc region of the IBM. To understand the process for the development of the mantle wedge from the immature to mature subduction zone, further sampling of mantle wedge pieces from the fore-arc region in IBM is necessary. We believe that this finding from in situ island arc is an important step for practical understanding of the whole section of the mantle wedge. 7. Conclusion Although the mantle wedge in a mature oceanic island forearc can be highly serpentinized mantle (e.g., Fig. 5B, and Iwamori, 1998; Bostock et al., 2002; Hyndman and Peacock, 2003; Maekawa et al., 2001, 2004; Karato et al., 2008; Hilairet and Reynard, 2009; Katayama et al., 2009; Boudier et al., 2010; Hirauchi et al., 2010), our fabrics were not developed in this situation. The olivine patterns we find were likely transformed from A-type or D-type olivine patterns originally developed under anhydrous condition to (100)[001] (i.e., C-type) or E-type olivine patterns under hydrous condition with a low to moderate flow stress during the subduc-
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Fig. 5. Schematic illustration of the evolution of an oceanic island arc system modified after Ishizuka et al. (2006). A: Stage 1 and 2 Infant arc development where hightemperature flow associated with upwelling and/or lateral asthenospheric flow (A-type or D-type?) occurs under low stress in the mantle wedge. This is enhanced by water from dehydration of the sinking plate, resulting in the development of the E-type olivine patterns (graded grey area) in the wedge. B: Subsequent stable subduction regime with mature arc volcanism. Subducting slab provides the mantle wedge with additional hydrous fluid, resulting in the development of C or E-type olivine patterns. The formation of serpentinite in the subduction zone hanging wall (grey area) occurs over the subducting slab high up the wedge.
tion zone infancy (Fig. 5A). These patterns were then preserved at the tip of mantle wedge where they would be relatively isolated from later events associated with the mature oceanic arc system (Fig. 5B). The differences in the patterns in our sample suit also uniquely tie the boninitic phase of volcanism to mantle roll over beneath the nascent arc. These structures therefore assist us in understanding the development of the mantle wedge from the immature to mature subduction zone, including the circulation of water in the mantle wedge (e.g., Iwamori, 1998), changes in deformation mechanisms (e.g., Hirth and Kohlstedt, 2003; Karato et al., 2008), and variations in seismic anisotropy (e.g., Nakajima and Hasegawa, 2004; Katayama et al., 2009; Long and Becker, 2010). Acknowledgements We are thankful for the support and cooperation of the science parties during cruises KH07-02 and KR08-07, and the captains and crews of the R/V Hakuho-Maru, the R/V Kairei, and the remotely operated vehicle Kaiko 7000II. We appreciate the help of Akihiro Tamura-Hasebe in discussing an early version of this paper. Figures for the CPO were made using the interactive programs of David Mainprice of Université Montpellier II, France. This study made use of analytical instruments housed at the Center for Instrumental Analysis (JEOL JCXA-733 electron probe micro-analyzer and JEOL JSM6300 SEM) and Michibayashi Laboratory (HITACHI S-3400N SEM), Shizuoka University. We thank Yosuke Kondo for their assistance in operating the HITACHI S-3400N SEM at Shizuoka University. We also thank Hideki Mori for technical assistance in preparing thin sections. We greatly appreciate constructive comments by Sylvie Demouchy, and an anonymous reviewer, as well as careful editorial handling by Yanick Ricard. This study was supported by research grants to Y.H. (24740344), K.M. (22244062), T.M. (21403010) and O.I. (22540473 and 25287133) from the Japan Society for the Promotion of Science (JSPS). This work was also supported by the JSPS Institutional Program for Young Researcher Overseas Visits.
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Earth and Planetary Science Letters www.elsevier.com/locate/epsl
The earliest mantle fabrics formed during subduction zone infancy Yumiko Harigane a,∗ , Katsuyoshi Michibayashi b , Tomoaki Morishita c , Kenichiro Tani d , Henry J.B. Dick e , Osamu Ishizuka a,d a
Institute of Geology and Geoinformation, Geological Survey of Japan/National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8567, Japan b Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan c Earth Science Course, School of Natural System, College of Science and Engineering, Kanazawa University, Kanazawa 920-1192, Japan d Institute for Research on Earth Evolution, Japan Agency for Marine–Earth Science and Technology, Kanagawa 237-0061, Japan e Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
a r t i c l e
i n f o
Article history: Received 15 December 2012 Received in revised form 11 June 2013 Accepted 24 June 2013 Available online 24 July 2013 Editor: Y. Ricard Keywords: harzburgite olivine crystal preferred orientation fore-arc mantle Izu–Bonin–Mariana (IBM) arc oceanic island arc
a b s t r a c t Harzburgites obtained from the oldest crust–mantle section in the Philippine Sea plate (∼52 Ma) along the landward slope of the southern Izu–Ogasawara Trench, preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The harzburgites have relatively fresh primary minerals despite of their heavy serpentinizations, and show inequigranular interlobate textures, and crystal preferred orientation patterns in olivine (001)[100] and Opx (100)[001]. The harzburgites have the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. We propose that the (001)[100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Fore-arcs provide key information on the initiation of magmatic and subduction zone processes during island arc formation (Bloomer et al., 1995; Stern and Smoot, 1998; Stern 2002, 2004). However, since fore-arc sections are exposed deep on the landward trench slope accessibility is limited. Thus only a few studies have considered the mantle structure in fore-arcs, in contrast to the many studies of the evolution of crustal structures in island arcs (e.g., Kodaira et al., 2010; Ishizuka et al., 2011). Furthermore, fore-arc mantle structures formed during the initial stages of subduction might be erased or modified during subsequent subduction-related tectonic events in mature and ancient arc accreted terrains (e.g., Arcay et al., 2005). For instance, many modern fore-arc regions are thought to be highly serpentinized mantle, infiltrated by slab-derived fluids in steady-state subduction systems (e.g., Bostock et al., 2002; Hyndman and Peacock, 2003; Hilairet and Reynard, 2009; Katayama et al., 2009; Boudier et al., 2010; Hirauchi et al., 2010). Here, we document for the first time the structure of immature fore-arc mantle at the time of subduction zone initiation as preserved in peridotites exposed on the deep
*
Corresponding author. Tel.: +81 29 849 1060; fax: +81 29 849 3765. E-mail address: [email protected] (Y. Harigane).
0012-821X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.06.031
seafloor along the landward slope of the southern Izu–Ogasawara Trench (Fig. 1). 2. Geological background Dive 7K417 of the ROV Kaiko 7000II during R/V Kairei cruise KR08-07, and Dredge 31 of R/V Hakuho-Maru cruise KH07-02, operated by the Japan Agency for Marine–Earth Science and Technology, explored the landward slope of the southern Izu–Ogasawara Trench (Fig. 1A). Outcrops of peridotite, gabbro, dolerite, and basalt were observed during the dive from 5336 to 5792 meters below sea level (mbsl) (Morishita et al., 2011) and 48 samples of 35 peridotites, 9 troctolites, pyroxenite, and 3 gabbros were dredged between 5293 and 5738 mbsl (Fig. 1B). It appears that these slopes expose a mantle-derived peridotite body developed in the Izu– Bonin–Mariana (IBM) arc (Morishita et al., 2011), whereas some doleritic samples have a fore-arc tholeiitic basalt signature that is considered to represent the earliest magmatism during subduction initiation (Fig. 1B; Ishizuka et al., 2011). 3. Microstructure The five harzburgite samples were selected for study: two from the dive (R9 and R19) and three from the dredge (D31-1, -3, and -10). The two harzburgites (R9 and R19) have visible foliations and
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Fig. 1. A: Bathymetric map of the Izu–Ogasawara arc region. Study area shown by the red star on the landward slope of the trench close to the Bonin ridge. Broken gray line indicates the Izu–Ogasawara volcanic front. B: Bathymetric map showing the sampling points and track (pink dashed line) for ROV dive KR08-07-7K417 and the approximate track for Dredge KH07-02-D31 (grey dashed line). Colored symbols show the typical lithologies collected at each ROV sampling location: green – harzburgite, light green – dunite, yellow – gabbro, orange – wehrlite. A doleritic rock with fore-arc tholeiitic basalt signature (FAB) is red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
lineations defined by the alignments of spinel and pyroxene grains on bleached and saw-cut samples (e.g., Michibayashi et al., 2009). Thin sections were cut in two samples perpendicular to the foliation and parallel to the lineation defined by pyroxene’s structure. However, the other three harzburgites (D31-1, -3, and -10) have no or very weak structures with their small sizes; thin sections of these samples were made in arbitrary sections. The main constituent minerals are olivine (15.6%), orthopyroxene (Opx; 13.1%) and spinel (0.5%), although serpentine is currently the most common mineral as more than 70% modal compositions within the five samples due to heavy serpentinization (Fig. 2). The harzburgites have coarse inequigranular interlobate (or protogranular) textures (e.g., Passchier and Trouw, 1996) consisting of largely coarse-grained olivine with serrated grain boundaries and coarse- to medium-grained Opx (Fig. 2A, B, C, D). They exhibit signs of intracrystalline deformation such as wavy extinction, and subgrain boundaries, but without intense grain-size reduction. Elongate Opx grains were dynamically recrystallized into mediumsized aggregates, and wavy extinction provides evidence for intracrystalline deformation (Fig. 2A, B, C, D). Harzburgite D31-10 is internally complex with a sharp-sutured contact between Opxrich and Opx-poor harzburgite, with finer grained pyroxene in the latter; a gabbro vein crosscuts the Opx-rich harzburgite with a razor sharp straight contact that is oblique to the Opx-rich–Opx-poor contact (Fig. 2E, F). Secondary serpentine shows the mesh texture in these harzburgites (Fig. 2), whereas we cannot observe the deformation microstructure of serpentine. Therefore, we argue that these harzburgites could preserve an original microstructure in the mantle structure at IBM region despite of their heavy serpentinizations.
4. Mineral chemistry The chemical composition of primary minerals in three harzburgite samples (D31-1, -3, and -10) were analyzed using a JEOL JCXA733 electron probe micro-analyzer (Shizuoka University, Japan). Analyses were made with a probe current of 12 nA, accelerating voltage of 15 kV, and a correction procedure after Bence and Albee (1968). The harzburgite samples have high olivine forsterite (90.6–92.1 mol.%) and NiO (∼0.4 wt%) contents (Table 1), low Opx Al2 O3 (< ∼1.5 wt%) and Na2 O (<0.03 wt%) (Table 2), and high spinel Cr# (65–67) (Table 3). These data is similar to that for two harzburgites (KR08-07-7K417R9 and R19) in Morishita et al. (2011). These mineral compositions lie largely outside the range for abyssal peridotites from mid-ocean ridges (Fig. 3; e.g. Dick and Bullen, 1984), indicating that the harzburgites are refractory consistent with an origin in a supra-subduction zone mantle wedge (Morishita et al., 2011). 5. Crystal fabric analysis The crystallographic preferred orientations (CPOs) of olivine and Opx grains in highly polished thin sections were analyzed with a JEOL JSM6300 SEM and a HITACHI S-3400N SEM, equipped for electron back-scattered diffraction (EBSD with HKL Channel5), at Shizuoka University, Japan. We measured the crystal orientations of 91 to 233 olivine grains and 59 to 171 Opx grains in 5 harzburgites, visually checking the computerized indexation of each diffraction pattern. We then calculated the J -index (Mainprice et al., 2000; Michibayashi and Mainprice, 2004; Michibayashi et al., 2006) to determine the fabric strength and distribution densities of the principal crystallographic axes. The J -index has a value of 1
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Fig. 2. Microphotographs of 5 harzburgites studied in this paper. (A) KR08-07-7K417R19 and (B) KR08-07-7K417R9 in the section cutting perpendicular to the foliation and parallel to the lineation. (C) KH07-02-D31-3 and (D) KH07-02-D31-1 in arbitrary sections, since they show very weak structures with their small sizes. (E) KH07-02-D31-10 in the whole thin section. This sample shows internally complex microstructures with a sharp-sutured contact between Opx-rich and Opx-poor harzburgite (solid white line), with finer grained pyroxene in the latter. A gabbro vein also crosscuts the harzburgite (broken white line). (F) Enlargement of the area outlined by the yellow rectangle in E. All images were taken under cross-polarized light. Ol: Olivine; Opx: Orthopyroxene; Sp: Spinel. The secondary serpentine occurs more than 70% of modal compositions in all thin sections. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
for random distributions and a value of infinity for single crystals. The index increases with increasing axial strain based on the experimental data of Nicolas et al. (1973), and thus can be used as a plastic-strain indicator to a first approximation (Ben Ismaïl and Mainprice, 1998).
Olivine has mainly (001)[100] CPO patterns in three of our samples, with a strong alignment of the [100] axis along the lineation, and the [001]-axis perpendicular to the foliation (Fig. 4A). These olivine CPOs have a high J -index value (Fig. 4A; J = 11.50–12.96). We note that the olivine CPO and Opx CPO patterns in each sample
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Table 1 Representative composition of olivine in KH07-02-D31 harzburgites. Sample No.
D31-1OL14
D31-1OL18
wt% SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O Cr2 O3 NiO V2 O3 Total
39.93 0 0 7.98 0.13 51.88 0.03 0 0.01 0.36 0 0.01 100.32
40.40 0.01 0.01 8.04 0.10 51.51 0.02 0 0 0.40 0.02 0.04 100.55
Cations\O
4
4
4
4
4
Si Ti Al Fe2+ Mn Mg Ca Na K Cr Ni V Total Mg# *
0.971 0 0 0.162 0.003 1.881 0.001 0 0 0.007 0 0 3.025 92.1
0.979 0 0 0.163 0.002 1.862 0.001 0 0 0.008 0 0.001 3.016 92.0
0.981 0 0.001 0.161 0.004 1.863 0.001 0 0 0 0.008 0 3.018 92.0
0.979 0 0 0.167 0.002 1.863 0.001 0 0 0 0.008 0 3.020 91.8
0.976 0 0 0.189 0.003 1.846 0.001 0 0 0 0.008 0 3.024 90.7
*
D31-3OL16 39.98 0 0.02 7.86 019 50.91 0.02 0 0 0 0.38 0 99.36
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Table 2 Representative composition of orthopyroxene in KH07-02-D31 harzburgites.
D31-3OL43
D31-10OL66
D31-10OL71
Sample No.
D31-1OPX13
D31-1OPX23
D31-3OPX17
D31-3OPX24
D31-10DPX62
D31-10OPX72
39.87 0 0.01 8.12 0.09 50.89 0.03 0 0 0 0.41 0 99.42
40.06 0.02 0.01 9.57 0.14 49.08 0.04 0 0 0.01 0.42 0.00 99.34
39.69 0 0.01 9.17 0.15 50.35 0.04 0 0 0 0.41 0.01 99.81
wt% SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O Cr2 O3 NiO V2 O3 Total
55.90 0.01 1.48 5.43 0.11 34.14 1.25 0.01 0 0.11 0 0.74 99.18
56.30 0.01 1.45 5.24 0.11 34.69 1.31 0.01 0 0.15 0 0.64 99.90
56.17 0.02 1.39 5.28 0.14 34.20 1.32 0 0 0.59 0.09 0.02 99.21
55.84 0 1.43 5.40 0.14 34.18 1.25 0 0 0.69 0.10 0.01 99.04
55.21 0.01 1.44 7.25 0.19 33.14 1.38 0.03 0 0.59 0.11 0.01 99.37
55.86 0.01 1.52 6.56 0.17 33.71 1.45 0.02 0 0.60 0.08 0 99.97
4
Cations\O
6
6
6
6
6
6
0.974 0 0 0.193 0.003 1.847 0.001 0 0 0 0.009 0 3.026 90.6
Si Ti Al Fe2+ Mn Mg Ca Na K Cr Ni V Total Mg# *
1.946 0 0.061 0.158 0.003 1.772 0.046 0 0 0.003 0 0.021 4.011 91.8
1.945 0 0.059 0.151 0.003 1.786 0.049 0.001 0 0.004 0 0.018 4.015 92.2
1.953 0 0.057 0.153 0.004 1.773 0.049 0 0 0.016 0.002 0.001 4.010 92.0
1.948 0 0.059 0.158 0.004 1.777 0.047 0 0 0.019 0.003 0 4.013 91.9
1.938 0 0.060 0.213 0.006 1.734 0.052 0.002 0.000 0.016 0.003 0 4.025 89.1
1.941 0 0.062 0.191 0.005 1.746 0.054 0.001 0 0.016 0.002 0 4.020 90.2
Mg/(Mg + Fe) ratio.
are not parallel each other regardless of its identified foliation and lineation. The [100] maxima in the olivine is approximately 20◦ oblique to the [001] maxima in the Opx (Fig. 4A). Opx CPOs in the oriented harzburgites have (100)[001] patterns with [001] parallel to the lineation and (100) normal to foliation, although these CPOs are relatively weak (Fig. 4A). Opx in harzburgite D31-3 also has a (100)[001] CPO matching those in oriented harzburgites, though its relationship to the macroscopic mineral shape fabrics is unknown. The remaining two harzburgites have unclear CPO fabrics, because of a small number of the measured grains (Fig. 4B). Sample D31-1 have a weak concentration of olivine [100] axis but the scatter distribution of olivine [010] and [001] axes, which CPO pattern does not match to any known olivine fabrics (Fig. 4B; cf. Karato et al., 2008; Mainprice, 2007). Both olivine and Opx CPO patterns of Sample D31-10 are quite difficult to interpret their relationships, since their CPO patterns are so weak (Fig. 4B). This sample involves a gabbro vein (Fig. 2B) and therefore might be possible evidence of late-stage melt impregnation as it has anomalous low forsterite olivine content compared to the other harzburgites (Fo90.7 vs Fo91.8–92.1 , Table 1). 6. Discussion Stern and Bloomer (1992) proposed a tectonic model for island arc formation in the IBM region in which they discussed for the earliest stages of its evolution. Ar–Ar dating of volcanic and U–Pb zircon dates for dolerite and gabbro (Ishizuka et al., 2006, 2011) further support this model. They suggest that the first basaltic magmatism occurred in the fore-arc region due to decompression melting of the mantle during subduction initiation, which occurred at 51–52 Ma based on basalt Ar–Ar dates and gabbro zircon U–Pb dates (Ishizuka et al., 2011). Arc formation then began at ∼45–48 Ma with boninitic volcanism and fore-arc extension,
*
Mg/(Mg + Fe) ratio.
which occurred contemporaneously along the entire length of the IBM system. The boninitic melts resulted from the flux of water into mantle wedge harzburgite from the Pacific plate as it sank into the asthenosphere during this stage (Stern and Bloomer, 1992; Ishizuka et al., 2006). This produced high Cr-spinel refractory harzburgites (Fig. 3) lying outside the range for abyssal peridotites (Dick and Bullen, 1984; Parkinson and Pearce, 1998). Morishita et al. (2011) argued that the Dive 7K417R15 dunite (spinel Cr# ∼82; Fig. 3) formed due to extraction of the boninitic melts from the harzburgites during the boninitic volcanism stage of subduction initiation, while the lower Cr-spinel dunites formed during the earlier extraction of the basalts. Based on this analysis, the Dive 7K417 and Dredge 31 dunites and harzburgites likely preserve the signatures of the earliest mantle structures formed at the initial stages of subduction in an oceanic island arc. None of the Dive 7K417 and Dredge 31 harzburgites have visible porphyroclastic or fine-grained secondary textures with variable CPO patterns and weak J -indices consistent with lowtemperature deformation (e.g., Nicolas, 1986; Suhr, 1993; Michibayashi and Mainprice, 2004; Michibayashi et al., 2007, 2009). Therefore, the coarse-grained textures and strong crystal fabrics in these peridotites could be established at near-solidus temperatures during asthenospheric flow consistent with the pyroxene foliations and lineations. The three Dredge 31 and Dive 7K417 harzburgites with mainly [100](001) olivine CPOs with a strong alignment of the [100] axis along the lineation, and the [001]-axis perpendicular to the foliation represent the E-type fabric of Katayama et al. (2004) (Fig. 4A). Michibayashi and Mainprice (2004) interpreted [100](001) olivine fabrics in Oman ophiolite harzburgites as due to mechanical weakening under the low temperatures in the presence of preexisting mechanical mantle anisotropy in the lithosphere. Mehl et al. (2003), however, interpreted natural E-type olivine patterns in Alaskan Talkeetna arc harzburgites as due to the effect of water on
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Table 3 Representative composition of spinel in KH07-02-D31 harzburgites. Sample No.
D31-1SP15
D31-1SP17
D31-3SP18
D31-3SP30
D31-10SP55
D31-10SP67
wt% SiO2 TiO2 Al2 O3 Fe2 O3 FeO MnO MgO CaO Na2 O K2 O Cr2 O3 NiO V2 O3 Total
0.02 0.04 16.64 2.89 17.60 0.08 10.99 0 0.03 0.00 51.30 0.05 0.17 99.8
0.04 0.02 16.91 2.63 17.75 0.26 11.05 0 0 0 51.89 0.04 0.17 100.7
0.02 0.02 16.85 2.27 17.20 0.21 11.10 0 0 0 51.34 0.09 0.20 99.3
0.01 0.01 16.88 3.04 16.99 0.22 11.20 0.02 0 0 50.64 0.10 0.23 99.3
0.02 0 17.58 5.13 16.87 0.23 11.69 0 0 0.01 49.15 0.08 0.18 100.9
0.02 0.03 17.06 6.20 18.59 0.24 10.30 0 0 0 47.39 0.12 0.15 100.1
Cations\O
4
4
4
4
4
4
Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K Cr Ni V Total Mg# * Cr# **
0.001 0.001 0.627 0.070 0.471 0.002 0.524 0 0.002 0 1.297 0.001 0.004 3.000 49.2 67.4
0.001 0.001 0.631 0.063 0.470 0.007 0.522 0 0 0 1.300 0.001 0.004 2.999 49.5 67.3
0.001 0.001 0.637 0.055 0.461 0.006 0.531 0 0 0 1.302 0.002 0.005 3.000 50.7 67.1
0 0 0.637 0.073 0.455 0.006 0.535 0.001 0 0 1.283 0.003 0.006 3.000 50.3 66.8
0.001 0 0.652 0.121 0.444 0.006 0.548 0 0 0 1.222 0.002 0.005 3.000 49.25 65.2
0.001 0.001 0.644 0.150 0.498 0.006 0.492 0 0 0 1.201 0.003 0.004 3.000 43.17 65.1
* **
Mg/(Mg + Fe) ratio. Cr/(Cr + Al) ratio.
deformation. Katayama et al. (2004), subsequently experimentally produced the E-type olivine fabrics by deformation in the presence of water (200 < COH < 1000 H/106 Si) at moderate to low stress (<400 MPa). Our three harzburgites with E-type olivine fabrics are distinct from those found in some abyssal peridotites (e.g.: Achenbach et al., 2011), which have a strong [010] maximum sub-perpendicular to foliation, and less strongly developed [100] and [001] maxima parallel to the foliation plane. These abyssal peridotites, however, have Opx fabrics similar to our samples, but stronger, with a strong [001] Opx maximum parallel to foliation, and a weaker [100] maximum sub-perpendicular to foliation. This fabric is consistent with relatively low strain rate operating on olivine (010)[100] and the Opx (100)[001] slip systems at ∼1250 ◦ C. Achenbach et al. (2011) conclude this reflects high-temperature mantle deformation beneath the Mid-Atlantic Ridge, a tectonic setting that contrasts sharply to the arc setting of our samples. The other two harzburgites (D31-1 and -10) do not show any known olivine fabric type because of no or weak visible structures. Opx fabric of D31-1 may show a weak (100)[001] pattern despite the fact that its olivine fabric does not show any typical CPO patterns, indicating that a complex deformation process may cause the formation of each CPO pattern in D31-1 at mantle wedge. Furthermore, D31-10 is also very weak olivine and Opx patterns, suggesting that D31-10 may have been modified by late-stage melt flow suggested by the Opx-poor–Opx-rich contact which is consistent with channelized melt flow through the finergrained, pyroxene-poor, portion of the sample. The latter characteristics may be consistent with partial dissolution of Opx during melt migration as the phase field of olivine expands with decreasing pressure (Kushiro, 1969). As a consequence, we exclude these
Fig. 3. Relationships between spinel Mg# and Cr#’s modified from Morishita et al. (2011) by adding the three Dredge 31 harzburgites (green-filled triangles). Additional colored symbols are for dunites (light green-filled circles) and harzburgites (green-filled squares) from the same dive (7K417), and for additional IzuBonin Trench dunites (small grey-filled circles, averaged from Ishii et al., 1992, and Parkinson and Pearce, 1998). Compositional fields for abyssal peridotites – black solid line (Dick et al., 2010) and fore-arc peridotites – black broken line (Ishii et al., 1992). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
two harzburgites from our discussion, since their structural and fabric data are too obscure to examine in any more detail. Moderately chromian spinel reported by Morishita et al. (2011) in several dunites from Dive 7K417 (Fig. 3) is similar to those in MORB and abyssal peridotites and dunites (e.g.: Dick and Bullen, 1984), likely representing the early Stage 1 basalt melt extraction (Morishita et al., 2011). The TiO2 content of these spinels is lower than that found in most abyssal dunites, consistent with the low TiO2 content of IBM fore-arc basalts (Reagan et al., 2010). Our microstructural evidence for high-temperature mantle flow in peridotites from the Izu–Ogasawara fore-arc region are con-
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Fig. 4. Crystallographic preferred orientation (CPO) data for olivine and Opx in the harzburgites are plotted on equal-area, lower-hemisphere projections. Contours are multiples of uniform density. N is the number of analyzed olivine and Opx grains. Foliation and lineation are shown E–W in line drawings and pole figures. A: CPO data of the three harzburgites. CPO data for harzburgite sample D31-3 rotated to show it matches that in the foliated harzburgites. B: CPO data of the two harzburgites.
sistent with the initial stage of the tectonic models for island arc formation as proposed by Stern and Bloomer (1992), Ishizuka et al. (2006, 2011), and Morishita et al. (2011) as discussed above (Fig. 5A). Consequently, dehydration from the subducting slab into the fore-arc mantle during the initial stages of subduction may favor asthenospheric flow under a low flow stress in the mantle wedge by decreasing rocks strength (e.g., Arcay et al., 2005), resulting in the development of the E-type olivine patterns (Fig. 4A). Mehl et al. (2003) and Tommasi et al. (2006) report (010)[100] olivine patterns (the A-type pattern of Katayama et al., 2004) in the refractory harzburgites in ancient island arc settings. They suggest that A-type olivine patterns, which are similar to those reported by Achenbach et al. (2011), developed at high-temperature in anhydrous conditions, developed during lateral and/or upwelling asthenospheric flow into the mantle wedge during subduction initiation (Fig. 3A). The upwelling flow due to slab sinking may alternatively result in the development of {0kl}[100] olivine patterns (i.e. Katayama et al., 2004, D-type pattern) under high stress and anhydrous conditions (Fig. 4A).
Even though there are a small number of samples, it possibly represents an importance as a natural sample of the fore-arc region of the IBM. To understand the process for the development of the mantle wedge from the immature to mature subduction zone, further sampling of mantle wedge pieces from the fore-arc region in IBM is necessary. We believe that this finding from in situ island arc is an important step for practical understanding of the whole section of the mantle wedge. 7. Conclusion Although the mantle wedge in a mature oceanic island forearc can be highly serpentinized mantle (e.g., Fig. 5B, and Iwamori, 1998; Bostock et al., 2002; Hyndman and Peacock, 2003; Maekawa et al., 2001, 2004; Karato et al., 2008; Hilairet and Reynard, 2009; Katayama et al., 2009; Boudier et al., 2010; Hirauchi et al., 2010), our fabrics were not developed in this situation. The olivine patterns we find were likely transformed from A-type or D-type olivine patterns originally developed under anhydrous condition to (100)[001] (i.e., C-type) or E-type olivine patterns under hydrous condition with a low to moderate flow stress during the subduc-
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Fig. 5. Schematic illustration of the evolution of an oceanic island arc system modified after Ishizuka et al. (2006). A: Stage 1 and 2 Infant arc development where hightemperature flow associated with upwelling and/or lateral asthenospheric flow (A-type or D-type?) occurs under low stress in the mantle wedge. This is enhanced by water from dehydration of the sinking plate, resulting in the development of the E-type olivine patterns (graded grey area) in the wedge. B: Subsequent stable subduction regime with mature arc volcanism. Subducting slab provides the mantle wedge with additional hydrous fluid, resulting in the development of C or E-type olivine patterns. The formation of serpentinite in the subduction zone hanging wall (grey area) occurs over the subducting slab high up the wedge.
tion zone infancy (Fig. 5A). These patterns were then preserved at the tip of mantle wedge where they would be relatively isolated from later events associated with the mature oceanic arc system (Fig. 5B). The differences in the patterns in our sample suit also uniquely tie the boninitic phase of volcanism to mantle roll over beneath the nascent arc. These structures therefore assist us in understanding the development of the mantle wedge from the immature to mature subduction zone, including the circulation of water in the mantle wedge (e.g., Iwamori, 1998), changes in deformation mechanisms (e.g., Hirth and Kohlstedt, 2003; Karato et al., 2008), and variations in seismic anisotropy (e.g., Nakajima and Hasegawa, 2004; Katayama et al., 2009; Long and Becker, 2010). Acknowledgements We are thankful for the support and cooperation of the science parties during cruises KH07-02 and KR08-07, and the captains and crews of the R/V Hakuho-Maru, the R/V Kairei, and the remotely operated vehicle Kaiko 7000II. We appreciate the help of Akihiro Tamura-Hasebe in discussing an early version of this paper. Figures for the CPO were made using the interactive programs of David Mainprice of Université Montpellier II, France. This study made use of analytical instruments housed at the Center for Instrumental Analysis (JEOL JCXA-733 electron probe micro-analyzer and JEOL JSM6300 SEM) and Michibayashi Laboratory (HITACHI S-3400N SEM), Shizuoka University. We thank Yosuke Kondo for their assistance in operating the HITACHI S-3400N SEM at Shizuoka University. We also thank Hideki Mori for technical assistance in preparing thin sections. We greatly appreciate constructive comments by Sylvie Demouchy, and an anonymous reviewer, as well as careful editorial handling by Yanick Ricard. This study was supported by research grants to Y.H. (24740344), K.M. (22244062), T.M. (21403010) and O.I. (22540473 and 25287133) from the Japan Society for the Promotion of Science (JSPS). This work was also supported by the JSPS Institutional Program for Young Researcher Overseas Visits.
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