Possibility of titanium transportation within a mantle wedge : formation process of titanoclinohumite in Fujiwara dunite in Sanbagawa belt , Japan

Possibility of titanium transportation within a mantle wedge: formation process of titanoclinohumite in Fujiwara dunite in Sanbagawa belt, Japan S. Ishimaru and S. Arai Guraduate School of Science and Technology, Department of Earth and Environmental Sciences, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860–8555, Japan Earth Science Course, School of Natural System, College of Science and Engineering, Kanazawa University, Kakuma, Kanazawa 920–1192, Japan


Introduction
Titanium (Ti) is representative of high field strength elements (HFSE), and is believed to be immobile during aqueous fluid related processes (e.g., Tilley and Eggleton, 2005).In this context, most of HFSE in a subducting slab are transported along the base of mantle wedge to the deep mantle, which in part explains the HFSE depletion in Introduction

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Full both arc mantle and magmas.However, Iizuka and Nakamura (1995) synthesized titanoclinohumite through interaction between metabasite and olivine at 850 • C and 8 GPa, with an orthopyroxene layer in between.Their experimental results indicated that Ti is mobile by aqueous fluid at least in a short range under high-P conditions.This implies a possibility of the presence of HFSE-enriched part within the mantle wedge.
Titanoclinohumite, one of mantle minerals (e.g., Aoki et al., 1976), contains both Ti and volatiles, and its genesis in peridotites is potentially important to consider the Ti mobility through metasomatism in the mantle.The simplified chemical formula of titanoclinohumite is Ti 0.5 Mg 8.5 Si 4 O 17 (OH,F) and is thought as a kind mixture of olivine and brucite with minor amounts of TiO 2 (Jones et al., 1969).The TiO 2 content of titanoclinohumite is up to 6 wt % (Trommsdorff and Evans, 1980;Scambelluri et al., 1995;L ópez S ánchez-Vizcaíno et al., 2005).Fluorine substitutes (OH) of titanoclinohumite and this substitution stabilizes titanoclinohumite at higher-temperature ( 800• C or more) conditions (Engi and Lindsley, 1980).Titanoclinohumite has been occasionally reported from kimberlites (Aoki et al., 1976), peridotite xenoliths (Smith, 1979), and serpentinized peridotite bodies closely associated with high-P metamorphic rocks.Experiments of Engi and Lindsley (1980) indicated that the titanoclinohumites could be formed during metamorphism at crustal conditions: they are unstable above 475 • C at 0.35 GPa and 675 • C at 2.1 GPa.Ishibashi et al. (1978) found titanoclinohumite in serpetinized dunites of Fujiwara ultramafic-gabbro complex in the Sanbagawa metamorphic belt, central Shikoku, Japan (Fig. 1).The ultramafic part of the Fujiwara complex is composed of dunite and wehrlite presumed as cumulates (e.g., Enami, 1980;Kunugiza, 1984).The Fujiwara dunite suffered deserpentinization during progressive Sanbagawa metamorphism and subsequent serpentinization during the exhumation.Ishibashi et al. (1978) described their petrographical features and interpreted the formation of titanoclinohumite was after recrystallization of olivine because of the replacive character (Ishibashi et al., 1978).The Fujiwara serpentinite-metagabbro complex has been studied by Onuki et al. (1978) and Enami (1980) to more constrain the Sanbagawa metamorphism.Introduction

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Full However, no papers have thoroughly explained the petrogenesis of Fujiwara dunite and Ti source for the formation of titanoclinohumite.
In this paper, we describe the details of chemical features of Fujiwara titanoclinohumite, and discuss the Ti source and the formation process of titanoclinohumite in the dunite.It is quite important to understand what kind of fluid was involved in mobilization and concentration of HFSE.We can expect interaction between the Fujiwara complex and the Sanbagawa metamorphic rocks, especially an invasion of fluids released from Sanbagawa schists into the Fujiwara dunite during progressive metamorphism.This interaction possibly mimics slab/mantle wedge interaction; The titanoclinohumite formation in the Fujiwara dunite is, therefore, highly relevant to the mobility Ti within the mantle wedge.

Geological background
We examined titanoclinohumite-bearing dunites from the Fujiwara complex in the Sanbagawa metamorphic belt, central Shikoku, Japan (Fig. 1).The Sanbagawa belt is a regional metamorphic belt, of high-P intermediate metamorphic grade, extending approximately 800 km from east (Kanto district, north to Tokyo) to west (Saganoseki Peninsula, Kyushu) (Fig. 1a).The timing of peak metamorphism of the Sanbagawa belt was determined as 132-112 Ma (e.g., Okamoto et al., 2004).Abundant amphibolites, serpentinite and serpentinized ultramafic rocks and their complexes are distributed in the Sanbagawa metamorphic belt (e.g., Kunugiza 1984), and many studies have been published on P-T trajectories of the peridotites in the context of the subduction/exhumation history of the Sanbagawa metamorphic complex (e.g., Enami et al., 2004;Mizukami and Wallis, 2005).Maximum pressure recorded in garnet pyroxenites at Gongen area in the Sanbagawa metamorphic belt is 2.9-3.8GPa at 700-810 • C (Enami et al., 2004).Based on petrological and geochemical features, some of the serpentinites were interpreted as fragments of the mantle wedge incorporated into the subduction complex (e.g., Hattori et al., 2010).On the other hand, some others were Introduction

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Full originally in the oceanic lithosphere before incorporation into the subduction complex (e.g., Isozaki et al., 1990).The Fujiwara metagabbro-ultramafic complex (less than 200 × 400 m in plan view) is situated within the Sanbagawa metamorphic belt on the central Shikoku Island, Japan (Fig. 1; Onuki et al., 1978).This is one of the representatives of a subducting slab complex, and will give us information about the mantle wedge above the slab (e.g., Scambelluri et al., 2001;Khedr and Arai, 2009).Host metamorphic rocks around the Fujiwara complex are mainly pelitic schists of the Ojoin Formation of the Sanbagawa belt, which belong to the high-P part of biotite zone to garnet biotite transition zone (Enami, 1980).The foliation of the Fujiwara complex is consistent with the schistosity of the surrounding pelitic schists (Enami, 1980;Onuki et al., 1987).The Fujiwara complex is composed of dunite, wehrlite and metagabbros, and wehrlitic lenses were observed within the gabbro (Fig. 1c; Onuki et al., 1978).The relationship between the dunite and gabbros is not clear, but the latter are possibly enclosed as a lens (<50 m in thickness) within the former (Onuki et al., 1978).Based on the detailed observations by Onuki et al. (1978), wehrlite is only found at the central part of the Fujiwara complex, and the relationship between the dunite and wehrlite is also unclear.Titanoclinohumite was only observed in the dunite part of the Fujiwara complex.The dunite is overall massive except for the western and eastern marginal parts of the body, where it is brecciated (Onuki et al., 1978).Unfortunately, the main part of the Fujiwara complex has been covered with a dam lake for hydroelectric generation (the Fujiwara dam), and cannot be directly observed anymore.Titanoclinohumites were first described from the Fujiwara complex by Ishibashi et al. (1978), who ascribed the production of titanoclinohumites and associated minerals to thermal metamorphism by some hidden thermal source (attributed to granitic intrusions).Introduction

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Sample descriptions
We examined titanoclinohumite-bearing dunite samples from the northern end of marginal dunite of 7 samples of Fujiwara complex (cf.Onuki et al., 1976).The Fujiwara dunites examined in this study are highly serpentinized (cf.Fig. 2a-d) and mainly composed of olivine, chromian spinel and titanolinohumite with minor amount of chlorite, brucite, carbontates (dolomite and Ca carbonate), clinopyroxene, ilmenite and titanite, except for antigorite serpentine.The titanoclinohumite-bearing dunites show a peculiar breccia-like texture (cf., Ishibashi et al., 1978): a coarse-grained matrix includes strongly to completely serpentinized dunite clasts (Fig. 2a-b, e-f).Most of titanoclinohumite grains are exclusively observed in the coarse-grained matrix of olivine (Fig. 2a, b).The coarse-grained matrix exhibits strong deformation; both olivine and titanoclinohumite grains are kinked (cf.Fig. 2b).Olivines as well as titanoclinohumites are characterized by minute inclusions of magnetite as well as colorless fluids (Fig. 2g, h).The dunite clasts mainly comprise antigorite laths and brucite aggregates.Penetration of antigorite laths form ragged grain boundaries of relict olivine grains in the matrix part (Fig. 2g).The dunite is abundant in euhedral chromian spinels, both in the dunite clast and in the matrix, which are 4 %, on average, in volume (Fig. 2a, c, i-k).Spinels form composite grains; chromian spinel cores were thinly rimmed by ferritchromite and magnetite outward, each boundary being quite sharp (Fig. 2j, k).Both olivine and titanoclinohumite grains are in direct contact with the magnetite rim of the composite spinel grains in the matrix part (Fig. 2j-m).Chlorite is characteristically observed as coarse discrete grains (Fig. 2n), not always associated with altered chromian spinel as usually observed in metamorphosed peridotites (cf.Khedr and Arai, 2010).Magnetite is disseminated in whole part of the thin sections, and also in olivine as minute inclusions (Fig. 2a, g-h).Brucite is usually observed as veinlets crosscutting antigorite aggregates (Fig. 2i).
As stated above, titanoclinohumite grains are almost exclusively associated with olivine, as inclusions, veinlets replacing olivine and lamella-like thin intergrowths Introduction

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The Ti-bearing ludwigite shows fibrous shapes and forms aggregates with magnetite surrounded by brucite.

Major-element compositions
We determined major-element compositions, using the electron microprobe (EPMA) at Kanazawa University.For quantitative analysis of major elements, we used JXA-8800R (JEOL), and the accelerating voltage, beam current and probe diameter was 20 kV, 20 nA, and 3 µm, respectively.For map analysis, we used the same accelerating voltage with 50 nA beam current and 0 µm probe diameter.Counting time for each elements on quantitative analysis, we took 30 s and 15 s for peak and background for Ni, respectively, and 20 s and 10 s, respectively, for other elements (cf.Table 1).
Ferrous and ferric iron contents of chromian spinel were calculated assuming spinel stoichiometry, while all iron was assumed to be Fe 2+ in silicates.Mg# is Mg/(Mg + Fe 2+ ) atomic value, and Cr# is Cr/(Cr + Al) atomic value.Fluorine is an important component of titanoclinohume, but we did not measure the F content quantitatively, because there were no perceptible peaks for F in a preliminary qualitative analysis.Introduction

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Full The olivines show a wide range of Fo content even within a single specimen, from 88 to 96 (Fig. 3).The MnO content of olivine is high, up to 1.1 wt %, and shows a negative correlation with the Fo content (Fig. 3a).The CaO content of olivine is generally low <0.2 wt %.The NiO content of olivine varies from 0.2 to 0.5 wt %, being characterized by remarkably low content (0.2 to 0.3 wt %) even at high Fo content (≈95) (Fig. 3b).
The Cr# of spinel core (cf.Fig. 2j) is 0.5-0.6, and the TiO 2 content varies from 0.8 to 3.5 wt % (Fig. 4).The TiO 2 content of spinel is negatively correlated with Fe 3+ /(Cr + Al + Fe 3+ ) atomic value, which varies from 0.3 to 0.2 (Fig. 4c).The TiO 2 content is lower than 1 wt % in ferritchromite and marginal magnetite (Fig. 4c).The MnO content is around 1 wt % in the spinel core, less than 1 wt % in the marginal magnetite, and relatively high, from 2 to 3 wt %, in ferritchromite (Table 1).It is noteworthy that the chromian spinel tends to be more severely altered to magnetite, i.e. becomes Ti-poorer, when it is associated with titanoclinohumite than in serpentine aggregates (Fig. 2k).
Titanoclinohumites show a wide variation of chemical composition (Table 1; Fig. 5).The oxide total in microprobe analysis ranges from around 99 to 95 wt %, being negatively correlated with the TiO 2 content, 2 to 8 wt % (Fig. 5a, b).The result of qualitative analysis indicates that the anion of titanoclinohumite in the Fujiwara dunite is mainly hydroxyl, which is consistent with the result of precursory work by Ishibashi et al. (1978).In addition, Jones et al. (1969) argued that the high-Ti character (TiO 2 > 1.9 wt %) of clinohumite is associated with low-F concentrations because of its crystallographic constraints.The Fujiwara titanoclinohumites are quite TiO 2 -rich (up to 8 wt %) when they are in contact with ilmenite or perovskite (cf.Fig. 2q).Titanoclinohumites are basically magnesian, and their Mg# varies from 0.92 to 0.96, (Fig. 5a-d).Their NiO and MnO contents are overall low, <0.5 wt % (mostly around 0.2 wt %) and 0.2 to 0.6 wt %, respectively (Fig. 5c, d).For the Ti-rich varieties the Ti content is negatively correlated with the Si content, possibly indicating a substitution of Ti for Si (Fig. 5e).Ti is negatively correlated with (Fe + Mn + Mg + Ni) for the main cluster of titanoclinohumites, indicating substitution of Ti for M 2+ (divalent cations) (Fig. 5f).Introduction

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Full Irrespective of the mode of occurrences, chlorite shows almost constant Mg# and Al 2 O 3 content, around 0.95 and 11.0-12.7 wt %, respectively (Table 1).Carbonates are dolomite and Mg-bearing Ca carbonate, of which MgO content is up to 4 wt %.Brucite has high Mg# (0.97-0.98) and contains a small amount of FeO and NiO, 2.5-3.7 wt % and 0.24-0.28wt %, respectively (Table 1).The TiO 2 content and Mg# of Ti-bearing ludwigite varies from 2.7 to 3.8 wt % and 0.39 to 0.41, respectively (Table 1).

Trace-element compositions
We used a laser-ablation system inductively coupled plasma mass spectroscopy (LA-ICP-MS) at Kanazawa University to determine trace-element characteristics of minerals; the laser-ablation system and mass spectrometer was GeoLas Q-Plus (MicroLas) and Agilent 7500S (Yokogawa Analytical Systems), respectively.We used a laser energy density of 8 J/cm 2 operating at a frequency of 5 Hz.The laser spot size was 40 µm for ilmenite and petovskite, and 100 µm for olivine and titanoclinohumite.The NIST 612 standard glass was used for calibration with Si as an internal standard.Details of the analytical methods have been described by Ishida et al. (2004) and Morishita et al. (2005).
We checked incompatible trace-element concentrations of major Ti-rich phases (titanoclinohumite, ilmenite and perovskite) and olivine, to find which kind of elements were mobile with Ti.REE patterns of titanoclinohumites, normalized to primitive-mantle values of McDonough and Sun (1995), are quite similar to those of olivine except for positive spikes at HFSE of titanoclinohumites (Fig. 6a, b).Ilmenite is quite rich in Ti but shows almost the same concentrations as titanoclinohumites for other HFSE (Fig. 6b,  c).Pervoskite shows, however, quite enriched patterns with strong negative spikes at Pb, some LILE (Rb, Ba and Sr) and HFSE (Zr and Hf) (Fig. 6c).HFSE variations in the Fujiwara titanoclinohumites are shown in titanoclinohumites and the Fujiwara olivines (Fig. 7).The chondrite-normalized HFSE ratio (Ti, Nb and Zr) of titanoclinohumite shows their Ti/Nb variations with low Zr/Ti and Zr/Nb ratios (<0.1) (Fig. 7c).Titanoclinohumites from other localities, China (Zhang et al., 2007), Almirez (Garrido et al., 2005) and western Alps (Scambelluri et al., 2001), also show low Zr and variable Ti/Nb ratios.The titanoclinohumite from China shows a low Ti/Nb ratio due to the extremely high Nb concentration (84.8 ppm) (Zhang et al., 2007).

Raman spectroscopy
We performed Raman spectroscopy to identify fluid inclusion phases, using a micro-Raman system LabRAM HR800 (HoRIBA Jobin Yvon) equipped with a 532 nm Nd:YAG laser J100GS-16 (Showa Optronics CO., Ltd.).The laser beam was narrowed and focused through a 100 µm entrance pinhole and a 100× objective lens MPLN100x, NA = 0.90, (Olympus).We basically used a 600 lines/mm grating, but for specific analysis, we used 1800 lines/mm grating.Before measurements, the spectrometer was calibrated by using the Si 520 cm −1 peak.It was verified that the peak centers of neon lines are reproduced with precisions lower than 0.5 cm −1 .Details of the methods are in Miura et al. (2011).
We checked minute fluid inclusions (micro-inclusions) both in olivine and titanoclinohumite (Fig. 8).They are characteristically irregular, sometimes ameboid, in shape (Fig. 8), and distributed at random within grains, possibly indicating a primary character.The inclusions are colorless and homogeneous in appearance under the microscope (Fig. 8).Raman spectra from the micro-inclusions show a very strong peak at around 2907 cm −1 , varying from 2906 to 2910 cm −1 (Fig.

Chemical characteristics of the Fujiwara titanoclinohumite
The Fujiwara titanoclinohumites display quite high TiO 2 contents (up to 8 wt %), showing a negative correlation with the oxide total by microprobe (Fig. 5a).This indicates that the Ti substitution in titanoclinohumite is not only with M (divalent cations), although Ti usually substitutes for their Mg(OH) 2 (brucite) component (Jones et al., 1969).To check the Ti substitution in the Fujiwara titanoclinohumites, we examined relations of the atomic ratio between Ti and M (divalent cations) and Ti and Si.Most of our analyses are plotted on the ideal substitution line of Ti-M with some exceptions (Fig. 5e-f).According to the ordinary stoichiometry of titanoclinohumite, the cation ratio of Ti should be below 0.49 on the basis of O = 17, although some Fujiwara titanoclinohumites exceed this value (up to 0.67) (Fig. 5f).This high-Ti character indicates the presence of Mg(OH,F) 2 -TiO 2 layers replacing Mg 2 SiO 4 layers (i.e., substitution of Ti(OH) with SiO 2 ), suggested by Bradshaw and Leake (1964), in the Fujiwara titanoclinohumites.
If so, the titanoclinohumites will be non-stoichiometric when the Mg(OH,F) 2 -TiO 2 layer exists, that is, when they are Ti-rich.This is consistent with the low oxide total for the TiO 2 -rich titanoclinohumites (Fig. 5a, e-f).This possibly indicates the low availability of Si during the formation of titanoclinohumite.

Ti source for the formation of titanoclinohumite
As mentioned above, there have been several reports on titanoclinohumite-bearing ultramafics, of which the original lithology is mostly lherzolite or pyroxenites (e.g., Zhang et al., 1995;Garrido et al., 2005).Trommsdorff and Evans (1980)  is also an important container of TiO 2 in peridotites, especially when clinopyroxene is absent.It occasionally shows high TiO 2 contents, up to 12 wt % in cumulative dunites from oceanic hotspots (Arai et al., 2011).Chromian spinel was possibly a source of Ti for titanoclinohumite in the Fujiwara dunite, which had originally contained high-TiO 2 (up to 3.5 wt %) chromian spinel (Fig. 4c).Release of Ti from the primary chromian spinel during metamorphism is clear;s the spinel core decreases its TiO 2 content towards the magnetite rim (<0.5 wt %) through the ferritchromite (around 1.0 wt %) in between (Figs. 2k, 4).The rugged outline of the magnetite rim indicates later addition of magnetite around primary chromian spinel from surrounding olivine on serpentinization (e.g., Barnes, 2000).Ferritchromite was formed by modification of primary chromian spinel during serpentinization due to subtraction of the MgAl 2 O 4 spinel component by forming chlorite and/or other hydrous minerals, although chromian spinel with chlorite is not so common.(Fig. 2e, i).The degree of conversion of chromian spinel to magnetite is high even though it is huge in size (≈500 µm) when closely associated with titanoclinohumite (Fig. 2l, m).In Contrast, chromian spinel is well preserved in the dunite clast, where titanoclinohumite has not been found (Fig. 2e, f).We also examined ilmenite and/or perovskite coexisting with titanoclinohumite as a possible source of Ti.Elemental distribution maps of titanoclinohumite with ilmenite or perovskite show no systematic Ti enrichement towards them (Fig. 2q).The TiO 2 content is just averaged value (≈3 wt %) in some titanoclinohumite grains in contact with these Ti-rich phases.Assemblage of ilmenite and olivine can form titanoclinohumite at relatively high-P and low-T conditions (e.g., 600 • C and >14 kbar) (Engi and Lindsley, 1980).We did not find, however, any textures indicating reaction relation between them (Fig. 2q): the shape and size of ilmenite are euhedral and ≈120 µm, respectively.Perovskite also has euhedral shapes even in the wide titanoclinohumite vein.Chromian spinel is the most probable source of Ti to form titanoclinohumite and these Ti-rich phases in the Fujiwara dunites.We examined the Ti budget for three thin sections.We measured modal amounts of titanoclinohumite, and opaque minerals by image processing.We assumed that Introduction

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Full (1) all opaque grains (>100 µm) are spinel group minerals, (2) the volume of primary chromian spinel is equal to that of the present opaque grain (overgrown magnetite is negligible), (3) the degree of conversion of chromian spinel to magnetite is the same in every grain, and (4) the TiO 2 content of all chromian spinel and magnetite is 2.2 wt % and 0.45 wt %, respectively (averaged values) (cf.Fig. 4).The total modal amount of opaque minerals varies from 2 to 8 vol.%, and the averaged degree of conversion of chromian spinel to magnetite is almost constant (≈50 %).The sample containing 7 vol.% opaque minerals (T4-4) can supply an amount of TiO 2 sufficient to crystallize 0.6 vol.% titanoclinohumite.The other two samples (T4-1 and T4-3) contain too small amount of chromian spinel to provide titanoclinohumite with sufficient amounts of Ti.
This indicates that, although chromian spinel is possibly the source of Ti, an external source for Ti is indispensible for formation of titanoclinohumite within a limited volume of the sample.
Comparing with titanoclinohumites from other localities (China, Almirez and western Alps), the Fujiwara titanoclinohumites is apparently low in Nb and Ta (Fig. 7a).On the other hand, Zr and Hf contents of the Fujiwara titanoclinohumites are higher than those from Almirez (Fig. 7b).These differences of the HFSE concentrations possibly reflect the difference of the source of HFSE to form titanoclinohumite.For example, residual clinopyroxenes of spinel lherzolite have high concentrations of Zr and Ti relative to secondary clinopyroxenes in Erro-Tobbio peridotites (Scambelluri et al., 2001).
Additional data of HFSE concentrations in chromian spinel would be needed for further discussion.

Titanoclinohumite formation process and HFSE mobility
The primary Fujiwara dunites are characterized by the high-Ti chromian spinel and relatively low Fo content of olivine (Fo 87 ;Enami, 1980) or bulk-rock Mg# (0.85-0.87;Onuki et al., 1978).They were possibly cumulates from an intra-plate basalt (Arai et al., 2011) (cf.Figs. 3 and 4).The Fujiwara dunites are possibly of deserpentinization origin.The highly variable Fo content of olivine is one of characteristics of deserpentinized Introduction

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Full peridotites (cf.Arai, 1975); Fujiwara dunites are a mixture of olivines of various origins.Inclusion of magnetite in olivines is also one of petrographical characteristics of deserpentinized peridotites (Arai, 1975).The lamellar intergrowth of titanoclinohumite in high-Mg# olivines indicates their simultaneous crystallization upon deserpentinization.
Although the behavior of Ti during the precursory serpentinization process is not clear, the titanoclinohumite and other high-Ti phases (perovskite and ilmenite) were formed at the expense of the released Ti from primary Ti-rich spinels.
The presence of borate (high-Ti ludwigite) is also consistent with the serpentinization-deserpentinization imposed on the Fujiwara dunites.Serpentine minerals are one of the repositories of B in hydrous ultramafic rocks (Tenthorey and Hermann, 2004), and B is expected to be released during the breakdown process of serpentine.Antigorite, titanoclinohumite and chlorite actually contain quite high amounts of B (up to 11 000 ppm) (cf.Table 1), which are rather high compared with marine pelagic clays (230 ppm; Li, 1991).Boron has been available from outside of the Fujiwara dunite body, from, for example, the surrounding Sanbagawa pelitic schists.Breakdown of serpentinite and progressive metamorphism of the Sanbagawa metamorphic rocks produced high-B aqueous solutions, which affected the petrografic and chemical characteristics of the Fujiwara dunites.The solution was possibly rich in hydrocarbons; fluid inclusions in both titanoclinohumite and olivine are methane with brucite and/or serpentine (Fig. 8).The presence of brucite and/or serpentine in methane-rich fluid inclusions is interpreted as a reaction product between the host olivine or titanoclinohumite with the H 2 O-CH 4 fluid as follows: Methane was involved in these reactions and remained as the main phase of the fluid inclusions.It has been well known that Ti is mobile through hydrocarbon-rich fluids even at low temperature conditions (e.g., Pointer et al., 1989;Rasmussen and Glover, 1994;Cornu et al., 1999;Parnell 2004).The hydrocarbon-rich fluids when Introduction

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Full in contact with Ti-bearing phases scavenged Ti and other transitional metal elements to transport as hydrocarbon complexes to other places (Parnell, 2004).Carbonates (dolomite and Ca carbonate), frequently coexisting with Fujiwara titanoclinohumite, are possibly a reaction product of the hydrocarbon-rich fluid and serpentinite.Calcium to form Ca carbonate and perovskite was also derived from outside of the Fujiwara dunite, especially from associated gabbros (cf.Fig. 1).The presence of methane as fluid inclusions in both metamorphic olivines and titanoclinohumites, combined with the vein-like occurrences of titanoclinohumite, suggests mobility of Ti via hydrocarbon-rich aqueous solutions.The texture of the titanolinohumite-bearing dunites possibly indicates a breccia as their protolith: dunite clasts with matrix rich in carbonaceous materials or bitumen (cf.Parnell, 2004).The brecciation may have occurred before subduction near a trench where an intra-plate volcano was deeply disrupted after transportation by an oceanic plate.The condition for formation of the Fujiwara titanolinohumite-bearing dunite should be bracketed by two reactions; brucite + antigorite = forsterite + H 2 O (Evans, 1977) and titanoclinohumite = forsterite + ilmenite + H 2 O (Engi and Lindsley, 1980) (Fig. 9).The protolith for the Fujiwara dunite, i.e. partially serpentinized (brecciated) dunite, was dragged to depth to be deserpentinzied mainly within the stability field of titanoclinohumite (Fig. 9).Based on the discussion of Enami (1980), the lower limit of Sanbagawa metamorphism recorded in the Fujiwara complex is 0.6-0.7 GPa and 450 • C, and the lack of talc and breakdown texture of titanoclinohumite in Fujiwara duntie implies a P-T trajectory which is almost the same as the Bessi unit (Fig. 9).Although the detailed P-T trajectory for the Fujiwara dunite body has not been known, it was derived from a depth far shallower than that of the Higashi-akaishi ultramafic complex (Fig. 9; Mizukami and Wallis, 2005).

Conclusions and implications
This study indicates formation of titanoclinohumites together with deserpentinized olivine in a progressively deserpentinized dunite, suggesting some mobility of Ti Introduction

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Full through the fluid/solution activated during metamorphism at at <1 GPa and ≈500 • C. Ti is possibly mobile via hydrocarbon-rich solutions as shown in some metamorphosed bitumen-rich sediments (e.g., Parnell, 2004).This is consistent with the vein-like concentrations of titanoclinohumite cutting olivine-rich matrix.Therefore, Ti can be liberated from the slab to the overlying mantle wedge via released hydrocarbon-rich fluids.
Ti fixed in minerals, such as chromian spinel, in the mantle wedge peridotite can be also mobilized by hydrocarbon-rich fluids released from the slab.The transported Ti can be concentrated as titanoclinohumite within the mantle peridotite at relatively low-T conditions (Engi and Lindsley, 1980).Therefore, HFSE, including Ti, are, however, immobile by aqueous fluids from the slab to the mantle wedge, and may not be incorporated in the arc magma source because titanoclinohumite is stable around the subducting slab.
In addition, if titanoclinohumite is unstable, HFSE sink will be succeeded to ilmenite or other Ti-rich phases, stable at higher pressure.Then, HFSE are concentrated in the subducting slab and/or mantle peridotite (or serpentinite) just above the slab.Introduction

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Full  Full  Full  Full olivine Fo olivine Fo mantle olivine array (Takahashi, 1986) olivine NiO wt% (b) olivine MnO wt% (a)    akaishi) and Bessi units and geotherm of continental crust and oceanic lithosphere are also from Mizukami and Wallis (2005) and references therein.Reaction curves 1, 2 and 3 are from Evans (1977), Ulmer andTrommsdorff (1995), andEngi andLindsley (1980), respectively.The Fujiwara complex was exhumed together with HA and Bessi units after the peak of the Sanbagawa metamorphism, although they were originated from the different depths.

Fig. 3 Ishimaru & Arai
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Fig. 7. Their Ta and Nb contents are quite high relative to the Fujiwara olivines, although they are fairly low relative to titanoclinohumites from Almirez (L ópez S ánchez-Vizcaíno et al., 2005).On the other hand, concentrations of Zr and Hf are fairly high relative to those in both of the Almirez Introduction Discussion Paper | Discussion Paper | Discussion Paper | 8), indicating fluid methane filling them.The methane is coexisting with serpentine and/or brucite irrespective of the host minerals.Some inclusions in titanoclinohumite are free of fluid phases and mainly comprise brucite with or without serpentine.Discussion Paper | Discussion Paper | Discussion Paper | 5 Discussion ascribed the Ti source for titanoclinohumite in the Malenco sepentinite to clinopyroxene in its protolith because of the strong association of the mineral with metamorphic clinopyroxene and chlorite.Clinopyroxene is important as a sink of Ti in mantle peridotites because of the strong preference of Ti to clinopyroxene (McDonough et al., 1992).Chromian spinel Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Ulmer, P. and Trommsdorff, V.: Serpentine stability to mantle depths and subduction-related magmatism, Science, 268, 858-861, 1995.Zhang, R. Y., Liou, J. G., and Cong, B. L.: Talc-, magnesite-and Ti-clinohumite-bearing ultrahigh-pressure mafic and ultramafic complex in the Dabie Mountains, China.J. Petrol.Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 3 .
Fig. 3. Major-element variations of olivines in Fujwara dunites.(a) Relationships between the Fo content and MnO content.Thick gray line shows the fractional crystallization trend of Satoand Banno (1983).(b) Relationships between the Fo content and NiO content.Thick gray line and light gray area show the fractional crystallization trend and mantle olivine array ofSato and Banno (1983) andTakahashi (1986), respectively.

Fig. 5 .Fig. 9 .
Fig. 5. Major-element variations of titanoclinohumites in Fujwara dunites.(a) Relationships between the oxide total in microprobe analysis and TiO 2 content.(b) Relationships between Mg# and TiO 2 content.(c) Relationships between Mg# and MnO content.(d) Relationships between Mg# and NiO content.(e) Relationships between the cation ratios of Ti and Si.(f) Relationships between the cation ratio of Ti and divalent cations (Fe, Mn, Mg and Ni).

Table 1 .
Representative major-element compositions of minerals in Fujiwara dunites.FeO and Fe 2 O 3 contents of spinels are calculated values (see text).core sp1 and core sp2 in T4-1 are the core and rim of a spinel grain, respectively (cf.Fig.2k).