# High salinity fluid inclusions formed from recycled seawater in deeply subducted alpine serpentinite

Marco Scambelluri, Giuseppe Piccardo, Pascal Philippot, Angela Robbiano, Laura Negretti

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Marco Scambelluri, Giuseppe Piccardo, Pascal Philippot, Angela Robbiano, Laura Negretti. High salinity fluid inclusions formed from recycled seawater in deeply subducted alpine serpentinite. Earth and Planetary Science Letters, 1997, 148 (3-4), pp.485-499. 10.1016/S0012-821X(97)00043-5. hal-03791459

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<figure><img src="image_1.png" /></figure>
High salinity fluid inclusions formed from recycled seawater in
deeply subducted alpine serpentinite

Marco Scambelluri<sup>a,*</sup>, Giovanni B. Piccardo<sup>a</sup>, Pascal Philippot<sup>b</sup>, Andrea Robbiano<sup>a</sup>, Laura Negretti<sup>a</sup>

The origin of high-pressure brines has been investigated in the Erro-Tobbio peridotite (Western Alps), a mantle slice that was exposed to the seafloor of the Mesozoic Ligurian-Piedmontese Tethys and was later involved in Alpine subduction. Hydrothermal alteration by seawater-derived fluids led to replacement of the mantle assemblage by Cl-bearing serpentine (0.35 wt% Cl), brucite (0.2 wt%), Cl- and alkali-bearing phyllosilicates (0.2 wt% Cl; 0.2-0.55 wt% Na₂O; 1-5 wt% K₂O). Relics of these hydrous phases occur in olivine + titanian clinohumite + antigorite assemblages formed at 2.5 GPa and 550-600°C during partial devolatilization and veining of the hydrothermally altered peridotite. The high-pressure phases lack chlorine and alkalis and are coeval with fluid inclusions trapped in the syn-eclogitic veins. The inclusions are salt-saturated and contain up to 50 wt% Cl, Na, K, Mg and Fe. High fluid chlorinity was probably achieved during rehydration of relict mantle minerals and deposition of hydrous phases in the veins. The data presented suggest that the seafloor hydrothermal signature was inherited by the eclogitic fluid due to partitioning of chlorine and alkalis into the fluid phase. The presence of salty brines in eclogitized hydrous peridotites can indicate deep recycling of seawater-derived fluids. Hydrous ultramafic systems can therefore act as large-scale carriers of seawater into the mantle.

Keywords: ultramafic rocks; sea floor alteration; subduction; fluid inclusions; chlorine; Alps

# 1. Introduction

The studies of the water content of oceanic basalt glasses [1] and of nominally anhydrous minerals [2] demonstrate that at least twice the amount of water in the oceans is stored in the Earth's mantle as both

primordial and surface-recycled water [3]. Subduction zones are estimated to reflux into the mantle from one to six times the water that is released to the surface by magmatism [3-5] and to recycle water and associated elements deeper than 200 km to the transition zone at 650 km [3,6-8]. In this scenario importance is given to hydrous ultramafites, where talc, chlorite and antigorite survive to great depths [7-9]. Fully serpentinized peridotites can deliver to subduction zones 10-13 wt% water, which is 6-10

<footnote><sup>a</sup> Dipartimento di Scienze della Terra, University of Genova, C.so Europa 26, 16132, Genova, Italy</footnote>

<footnote><sup>b</sup> Laboratoire de Petrologie, CNRS URA, Université Paris 7, Paris, France</footnote>

<footnote><sup>*</sup> Corresponding author. Tel.: +39 10 3538315. Fax: +39 10 352169. E-mail: msca@hpg30.dister.unige.it</footnote>
times the water content of wet mafic oceanic crust [5]. Hydrothermal alteration of the oceanic lithosphere also causes uptake of chlorine from seawater-derived fluids; subduction of chlorine-enriched crust allows recycling of chlorine from the surface reservoir into the mantle [10]. The importance of serpentinites has recently grown after the discovery that they compose over 23% of the ocean floor surface [11]. Strong tectonization of such areas enhances water infiltration and enlarges the overall volume of serpentinized oceanic mantle. This process controls the volatile budget for subduction and makes serpentinites candidates for major transporters of seawater into the Earth's mantle. Although slab and crustal components characterize arc magmas [4] and research is increasingly aimed at defining the nature and behaviour of subduction-zone fluids [12–15], little is known about the origin of such fluids. Stable isotope studies [16,17] suggest that the primary source of subducted volatiles is the hydrothermally altered oceanic crust. However, direct natural evidence of ocean water recycling into the mantle is still lacking, because the link between the compositions of oceanic mineral assemblages and the chemistry of high-pressure fluids is still missing. Such examples are lost within orogens recording the subduction history of old oceanic lithosphere. The Alpine chain is suitable for this investigation because it includes subducted Jurassic ophiolites resembling the Atlantic basement [18].

Through the study of a serpentinized lherzolite slice we have documented the uptake of water, chlorine and alkalis into mineral assemblages formed during seafloor alteration of mantle rocks. Breakdown of these phases during subduction generated Mg-, Fe- and alkali-rich brines, which were trapped as fluid inclusions inside high-pressure vein minerals. The hydrothermal signature from seafloor environments retained by the inclusion fluids demonstrates recycling of original seawater-derived fluids in deep levels of the subduction zone.

## 2. Geological outline and metamorphic history

The Erro-Tobbio peridotite of the Alpine Penninic Domain (Italian Western Alps) represents a fragment of subcontinental mantle emplaced at high

crustal levels during rifting and opening of the Ligurian Mesozoic Tethys [19]. Closure of the oceanic basin during alpine convergence was accompanied by peridotite subduction and recrystallization under eclogite-facies conditions [20]. Shear localization during convergence produced serpentinite-mylonite shear zones (high-strain domains) surrounding volumes of serpentinized lherzolite unaffected by the alpine plastic deformation (low-strain domains) which preserve the mantle evolutionary stages (Fig. 1). In the low-strain domains, granular spinel lherzolites (clinopyroxene up to 10–15% by volume) are cut by spinel tectonites, plagioclase and hornblende mylonites (Fig. 1) [19]. The peridotite mylonites display stability of plagioclase- and hornblende-bearing assemblages. Hornblende forms during syntectonic breakdown of pyroxene and contains about 0.1 wt% K₂O, 2.5–3.1 wt% Na₂O and 0.2–0.4 wt% Cl: it has been related to the infiltration of crustal-derived aqueous fluids into the shear zones [19]. The above mantle structures are cut by MORB gabbroic and basaltic dikes and the mantle metamorphic path indicates exhumation of lithospheric mantle to shallow crustal environments [19]. Seafloor hydrothermal alteration caused serpentinization of the peridotites and local rodingitization of the mafic dikes. The mantle and the hydrothermal assemblages were patchily overgrown by alpine olivine, antigorite, titanian clinohumite, magnetite and diopside. The peridotite volumes recrystallized into this latter assemblage are cut by olivine + titanian clinohumite-bearing veins (Fig. 1, inset A). Veining was coeval with development of an eclogitic foliation in the mafic dikes: this indicates the high-pressure origin of the olivine-bearing assemblage in the peridotites [20].

The serpentinite mylonites forming the high-strain domains display an antigorite-bearing, olivine-free, foliation cut by olivine-bearing veins. The foliation and the veins are, in turn, deformed by olivine + antigorite shear bands (Fig. 1, inset B). Boudins of high-pressure metamoringites and eclogites line the antigorite foliation cut by the above olivine-bearing structures. These features document a competence contrast between rigid mafic rocks and ductile serpentinite at eclogitic conditions and indicate that the olivine-bearing assemblage represents the highest grade achieved at the end of prograde subduction-
<figure><img src="image_2.png" /></figure>

Fig. 1. Schematic sketch of the structural relationships between the different types of lherzolites in the Erro-Tobbio peridotite. 1 = granular spinel peridotite; 2 = spinel tectonite; 3 = peridotite mylonites; 4 = mafic dikes and boudinaged mafic rocks; 5 = high-pressure peridotite with olivine veins. Inset A: high-pressure recrystallization in low-strain domains. Inset B: structural features of the high-strain domains. Data after [9,19,20].

zone metamorphism [9]. The eclogitic assemblages of metagabbros developed at 2–2.5 GPa and 550–600°C [21]. Eclogitization was accompanied by partial dewatering of the serpentinite precursors, due to the brucite-consuming reaction: brucite + serpentine = olivine + water; antigorite survived this episode of fluid release, high-pressure recrystallization and veining [9].

# 3. Analytical methods

## 3.1. Electron microprobe analysis

Mineral analyses were made at the Institut fur Mineralogie und Petrographie, ETH Zentrum, Zurich, using a Cameca SX-50 microprobe equipped with five crystal spectrometers, an accelerating potential of 15 kV and a beam current of 20 nA. A ZAF-type correction was applied to the data. Chlorine was measured using a scapolite standard (1.85 wt% Cl) on a PET crystal, with counting times of 30 s on the peak and 15 s on the background on both sides of the peak. The Cl detection limits were 0.05 wt%. The

relative error due to counting statistics (2σ) for Cl was 11% for a Cl concentration of 0.05 wt% (±0.0055 wt%), and 7.6% for a Cl concentration of 0.2 wt% (±0.02 wt%). For other elements natural and synthetic oxides and silicates were used. The detection limits of Na₂O and K₂O were 0.08 wt% and 0.07 wt%, respectively. The 2σ for Na₂O is 6% for a concentration of 0.5 wt% (±0.03 wt%). The 2σ for K₂O is 2.2% for a concentration of 4 wt% (±0.09 wt%).

## 3.2. Scanning electron microscopy of the salt solid phases in fluid inclusions

Scanning electron microscopy and analyses of the salt solid phases in fluid inclusions were made at the Dipartimento di Scienze della Terra, University of Genova, in energy dispersive mode by means of a Philips SEM 515 electron microscope. The accelerating potential was 15 kV, the beam current 20 nA, the counting time 100 s. Chlorine was measured using a pure NaCl standard (60.66 wt% Cl); for other elements natural mineral standards were used. The detection limits for Cl were 0.05 wt%; the 2σ was
12.5% for a Cl concentration of 0.4 wt% (±0.05 wt%). The vein samples containing primary fluid inclusions were analyzed following Shepherd et al. [22]. In order to minimize dissolution of the salt crystals, petroleum distillate was used during sample preparation. Fluid inclusions in some sections prepared for microthermometry were thermally decreased and the salt precipitates were analyzed with the SEM [23].

### 3.3. Microthermometry

Microthermometry was performed at the Dipartimento di Scienze della Terra, University of Genova using a USGS heating-freezing stage and 40 µm thick, doubly polished thin sections. The stage was calibrated on the melting point of pure CO₂ (−56.6°C) and H₂O (0.0°C), and on the critical point of pure water (371.4°C) using synthetic fluid inclusion standards. The error range is ±0.2°C for low-temperature measurements, and ±1.0°C at high

temperature. The heating rate during analytical runs was constantly kept at 0.1°C s⁻¹.

## 4. The seafloor hydrothermal alteration

### 4.1. Textures and mineral compositions

The hydrothermal minerals survived the alpine metamorphism in the low-strain domains of the peridotite, where lherzolites display different intensities of static alteration of mantle minerals to serpentine (lizardite and chrysotile), chlorite, magnetite brucite and mixed-layer phyllosilicates. Patchy hydration of the lherzolites gave rise to the association of highly altered and less altered volumes (the most widespread). In the highly altered lherzolites (ETA 92), olivine and pyroxenes are overgrown by fine-grained serpentine ± magnetite ± brucite. Spinel is replaced by magnetite and chlorite, locally associated with minor green mixed-layer phyllosilicate. Plagio-

<figure><img src="image_3.png" /><figcaption>Fig. 2. Scanning electron micrograph of mantle spinel (sp) replaced by a double corona with an inner ring of chlorite (chlA, light grey) and an outer rim of mixed layer phyllosilicate (phy, dark grey). The corona is coeval with serpentinization of pyroxene (upper right) and olivine; it appears overgrown by high-pressure chlorite (chlB, in black) stable with antigorite and olivine. Scale bar 60 µm.</figcaption></figure>
clase is replaced by fine-grained chlorite ± minor epidote. In the less serpentinized lherzolites (ET 11/27, ETA 20, ETA 90, PP11 and PP7) olivine is frequently preserved and is cut by mesh chrysotile ± magnetite. Olivine and pyroxenes are pseudomorphed by fine-grained lizardite + minor brucite; plagioclase is partly chloritized. Altered mantle spinels display coronitic microstructures with a corona of white Cr-chlorite touching spinel, and an outer corona of a green mixed-layer phyllosilicate (Fig. 2). This texture has been observed in samples primarily lack-

ing the mantle hornblende. In the hornblende-bearing mylonites (PP11 and PP7), lizardite ± tremolite ± diopside replace hornblende and clinopyroxene; olivine is cut by mesh chrysotile + magnetite.

Compositions of the hydrothermal phases are listed in Table 1. Based on the Al₂O₃ content, two groups of serpentine can be distinguished (Fig. 3A): one group containing 0–1.2 wt% Al₂O₃, and another group with 2.3–6 wt% Al₂O₃. Such a difference is due to the presence of both Al₂O₃-rich lizardite and of Al₂O₃-poor chrysotile, as confirmed by X-ray

<table><caption>Table 1
Representative compositions of mineral phases pertaining to the seafloor (A) and to the high-pressure assemblages (B)</caption><thead><tr><th rowspan="2">A</th><th colspan="6">B</th></tr><tr><th>1chr</th><th>2liz</th><th>3brc</th><th>4phy</th><th>4phl</th><th>4chl</th><th>atg</th><th>chl</th><th>*Ticl rim</th><th>*Ticl core</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>39.23</td><td>37.33</td><td>1.54</td><td>38.97</td><td>20.43</td><td>18.54</td><td>42.59</td><td>35.17</td><td>35.31</td><td>35.91</td></tr><tr><td>TiO<sub>2</sub></td><td>0.00</td><td>0.13</td><td>0.00</td><td>0.02</td><td>0.02</td><td>-</td><td>0.00</td><td>0.00</td><td>4.75</td><td>4.83</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.02</td><td>0.56</td><td>0.00</td><td>0.02</td><td>-</td><td>0.05</td><td>0.00</td><td>1.12</td><td>0.06</td><td>0.03</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>0.53</td><td>2.37</td><td>0.00</td><td>11.32</td><td>5.77</td><td>5.55</td><td>1.86</td><td>13.12</td><td>0.13</td><td>0.14</td></tr><tr><td>FeO</td><td>7.97</td><td>9.92</td><td>24.43</td><td>4.31</td><td>-</td><td>4.31</td><td>3.82</td><td>4.56</td><td>13.30</td><td>13.35</td></tr><tr><td>MnO</td><td>0.09</td><td>0.10</td><td>0.69</td><td>0.02</td><td>-</td><td>0.02</td><td>0.02</td><td>0.00</td><td>0.31</td><td>0.29</td></tr><tr><td>MgO</td><td>39.29</td><td>36.70</td><td>44.26</td><td>27.44</td><td>13.71</td><td>13.73</td><td>38.97</td><td>33.90</td><td>43.15</td><td>42.75</td></tr><tr><td>CaO</td><td>0.08</td><td>0.08</td><td>0.27</td><td>0.01</td><td>-</td><td>0.01</td><td>0.00</td><td>0.00</td><td>0.05</td><td>0.04</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.01</td><td>0.00</td><td>0.00</td><td>0.55</td><td>0.55</td><td>-</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>K<sub>2</sub>O</td><td>0.01</td><td>0.00</td><td>0.00</td><td>4.50</td><td>4.50</td><td>-</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>Cl</td><td>0.18</td><td>0.36</td><td>0.23</td><td>0.07</td><td>0.07</td><td>-</td><td>&lt; 0.05</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>H<sub>2</sub>O</td><td>12.40</td><td>12.31</td><td>28.56</td><td>12.50</td><td>-</td><td>-</td><td>12.73</td><td>12.30</td><td>3.16</td><td>3.09</td></tr><tr><td>Total</td><td>99.84</td><td>99.86</td><td>99.98</td><td>99.76</td><td>-</td><td>-</td><td>99.99</td><td>100.17</td><td>100.22</td><td>100.43</td></tr><tr><td>N. ox</td><td>7</td><td>7</td><td>1</td><td>-</td><td>11</td><td>14</td><td>7</td><td>14</td><td>17</td><td>17</td></tr><tr><td>Si</td><td>1.89</td><td>1.80</td><td>0.00</td><td>-</td><td>3.00</td><td>3.77</td><td>2.00</td><td>3.32</td><td>4.00</td><td>4.06</td></tr><tr><td>Ti</td><td>0.00</td><td>0.00</td><td>0.02</td><td>-</td><td>0.00</td><td>-</td><td>0.00</td><td>0.00</td><td>0.40</td><td>0.41</td></tr><tr><td>Cr</td><td>0.00</td><td>0.02</td><td>0.00</td><td>-</td><td>-</td><td>0.01</td><td>0.00</td><td>0.08</td><td>0.00</td><td>0.00</td></tr><tr><td>Al</td><td>0.03</td><td>0.13</td><td>0.00</td><td>-</td><td>1.00</td><td>1.33</td><td>0.10</td><td>1.46</td><td>0.02</td><td>0.02</td></tr><tr><td>Fe</td><td>0.32</td><td>0.38</td><td>0.23</td><td>-</td><td>-</td><td>0.73</td><td>0.15</td><td>0.36</td><td>1.26</td><td>1.26</td></tr><tr><td>Mn</td><td>0.00</td><td>0.00</td><td>0.01</td><td>-</td><td>-</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.03</td><td>0.03</td></tr><tr><td>Mg</td><td>2.82</td><td>2.64</td><td>0.75</td><td>-</td><td>3.00</td><td>4.16</td><td>2.74</td><td>4.77</td><td>7.28</td><td>7.21</td></tr><tr><td>Ca</td><td>0.00</td><td>0.00</td><td>0.00</td><td>-</td><td>-</td><td>-</td><td>0.00</td><td>0.00</td><td>0.01</td><td>0.00</td></tr><tr><td>Na</td><td>0.00</td><td>0.00</td><td>0.00</td><td>-</td><td>0.16</td><td>-</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>K</td><td>0.00</td><td>0.00</td><td>0.00</td><td>-</td><td>0.84</td><td>-</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>Cl</td><td>0.02</td><td>0.03</td><td>0.00</td><td>-</td><td>0.02</td><td>-</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>OH</td><td>3.98</td><td>3.97</td><td>1.99</td><td>-</td><td>1.98</td><td>-</td><td>4.00</td><td>8.00</td><td>1.19</td><td>1.16</td></tr><tr><td>XMg</td><td>0.92</td><td>0.87</td><td>0.76</td><td>-</td><td>1.00</td><td>0.69</td><td>0.95</td><td>0.93</td><td>0.85</td><td>0.85</td></tr></tbody></table>

Mineral abbreviations: chr = chrysotile; liz = lizardite; brc = brucite; phy = mixed-layer phyllosilicate; phl = phlogopite in the mixed-layer phyllosilicate; chl = chlorite in the mixed layer phyllosilicate; atg = antigorite; chl = chlorite; ticl = titanian clinohumite. Ticl analyses marked core and rim represent two end points of a compositional profile on a vein titanian clinohumite containing multi-phase inclusions. In starred analyses fluorine was checked and not found. 4Phl and 4chl are recalculated from analysis 4. The amount of phlogopite present in the mixed-layer phyllosilicate was estimated by constructing an ideal phlogopite (SiO₂ 43.2; Al₂O₃ 12.21; MgO 28.99; K₂O 11.29; H₂O 4.3 wt%) and by attributing all K₂O, Na₂O, TiO₂ and Cl measured to the phlogopite component. Estimates of the FeO content of phlogopite were not possible and all measured iron has been attributed to chlorite. The ratio K₂O<sub>measured</sub>/K₂O<sub>ideal phlogopite</sub> was multiplied for the wt% concentration of each single oxide in the ideal phlogopite in order to obtain the amount of these oxides that is due to phlogopite in the mixed-layer phyllosilicate analyzed.
diffraction on some samples. Serpentine composition
can depend on the precursor mineral, as the olivine-
derived pseudomorphs contain lower Al₂O₃ and
Cr₂O₃ compared to serpentines replacing pyroxenes
and hornblende (Fig. 3B,C). Lizardite after horn-
blende has lower X Mg than serpentines after olivine
and pyroxene (Fig. 3C); it may also contain up to 0.1
wt% Cl and up to 0.5 wt% K₂O. Serpentine, brucite
and the mixed-layer phyllosilicates can contain chlo-
rine (Table 1, Figs. 3 and 4). The chlorine concentra-

tion in hydrothermal serpentines ranges from < 0.05
wt% to 0.35 wt% and is irrespective of the nucle-
ation site. Cl shows an inverse correlation with X<sub>Mg</sub>
(Fig. 3D), which is better highlighted if single sam-
ples, or groups of samples with similar bulk compo-
sitions and assemblages, are considered (e.g.
ET11/27, ETA90 and PP11, PP7; Fig. 3D). Fig. 3D
shows that the hydrothermal lizardite and chrysotile
are able to store up to three times the chlorine hosted
by later, high-pressure antigorite. In the mixed-layer

<figure><img src="image_4.png" /><figcaption>Fig. 3. (A) Cl versus Al₂O₃ of serpentine minerals. The shaded area represents the detection limit for Cl and the bars indicate the error range of Cl. (B) Cr₂O₃ versus X_Mg of hydrothermal serpentines. (C) Al₂O₃ versus X_Mg of hydrothermal serpentines. (D) Cl versus X_Mg for oceanic serpentines and high-pressure antigorite.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4. (A) Cl versus K₂O of oceanic mixed-layer phyllosilicate and chlorite coronas compared with the high-pressure chlorite. The shaded area represents the detection limit for Cl and the bars indicate the error range of Cl determination. (B) Compositional profile along the phyllosilicate corona of Fig. 2. The sketch in lower part of the figure reports the analytical points (dots) made on the coronal layers around spinel.</figcaption></figure>

phyllosilicate, K₂O varies from 1 to 5 wt%, Na₂O from 0.15 to 0.6 wt% and chlorine from < 0.05 to 0.21 wt% (Fig. 4). The phyllosilicate compositions can be recast as binary mixtures of chlorite and phlogopite following the procedure reported in Table 1: the resulting phlogopite/chlorite ratios range from 1:1 to 1:4. However, it is reasonable to assume that part of the FeO measured enters the phlogopite component and that the phyllosilicate actually corresponds to mixed layers of chlorite and biotite. K₂O variability of this mineral can reflect either heterogeneity of the chlorite-biotite mixture on a scale smaller than the volume excited by the microprobe beam, or chemical zoning of the coronas. The profile in Fig. 4B shows that alkalis progressively increase towards the outermost part of the phyllosilicate corona (chlorine is variable and is unrelated to the location of the analytical points and to variations in alkalis) and suggests that the compositional heterogeneity is controlled by the distribution of alkalis at the reaction site.

The data presented document that chlorine and alkalis were hosted by hydrous phases formed during hydrothermal alteration of the lherzolite. Such minerals either developed at the expense of precursor mantle phases which already contained the above

elements (e.g. hornblende), or in lherzolite domains originally lacking chlorine- and alkali-bearing phases.

# 5. The high-pressure recrystallization

## 5.1. Textures and mineral compositions

The hydrothermal minerals appear directly overgrown by the high-pressure phases. Antigorite, chlorite and olivine overprint lizardite and brucite; chlorite and antigorite replace the mixed-layer phyllosilicates (Fig. 2). Complete eclogitic recrystallization produced fine-grained olivine, titanian clinohumite, magnetite, antigorite, chlorite and diopside. Mantle clinopyroxenes are preserved even in domains extensively recrystallized at eclogite-facies conditions whereas hornblende is absent in these volumes. The veins cutting the eclogitized domains contain the same high-pressure assemblage as the host rocks. Veins with crystal fibres have been observed in high-strain domains, where they are frequently transposed along high-pressure shear bands (Fig. 1, inset B). Antigorite has Al₂O₃ between 1 and 2.5 wt% and Cl < 0.1 wt% (Fig. 3A; Table 1). Compared to early lizardite and chrysotile, antigorite has the high-
est $X_{Mg}$ and the lowest Cl (Fig. 3D). The high-pressure chlorite replacing the mixed layer phyllosilicate lacks chlorine and potassium (Fig. 4A). The vein titanian clinohumites are homogeneous and are halogen free. The differences between the seafloor and the high-pressure mineral phases indicate that alkalis and chlorine are no longer retained by the high-pressure assemblage.

## 5.2. Fluid inclusion occurrence and compositions

Fluid inclusions have been found inside vein-filling diopside, olivine and titanian clinohumite. The inclusions are randomly distributed and do not occur in planar arrays or in secondary microfractures. Fibrrous vein minerals deformed along high-pressure shear bands contain tubular fluid inclusions which parallel the long axis of minerals and follow the deformation pattern of the fibres. These geometric relationships indicate that inclusion entrapment precedes high-pressure deformation and is primarily related to vein deposition.

<table><caption>Table 2
Microthermometry of the primary multi-phase inclusions</caption><thead><tr><th>Tmi<br>(°C)</th><th>Tmf<br>(°C)</th><th>Th<sub>v-1</sub><br>(°C)</th><th>Tdiss<sub>NaCl</sub><br>(°C)</th><th>Salinity<br>(wt% NaCl eq)</th><th>Density<br>(g/cm<sup>3</sup>)</th></tr></thead><tbody><tr><td>-33.9</td><td>-</td><td>228.8</td><td>311.0</td><td>39.03</td><td>1.17</td></tr><tr><td>-41.2</td><td>-26.2ice</td><td>233.0</td><td>352.0</td><td>42.56</td><td>1.20</td></tr><tr><td>-36.8</td><td>-25.0ice</td><td>240.7</td><td>354.3</td><td>42.80</td><td>1.19</td></tr><tr><td>-38.5</td><td>-25.2ice</td><td>240.6</td><td>361.0</td><td>43.44</td><td>1.20</td></tr><tr><td>-25.6</td><td>-</td><td>248.2</td><td>352.3</td><td>42.62</td><td>1.18</td></tr><tr><td>-27.2</td><td>-</td><td>283.8</td><td>332.3</td><td>40.81</td><td>1.13</td></tr><tr><td>-24.6</td><td>-</td><td>273.5</td><td>356.8</td><td>43.04</td><td>1.17</td></tr><tr><td>-38.0</td><td>-</td><td>293.3</td><td>372</td><td>44.52</td><td>1.16</td></tr><tr><td>-24.8</td><td>-</td><td>269.0</td><td>351.5</td><td>42.54</td><td>1.16</td></tr><tr><td>-36.2</td><td>-</td><td>257.2</td><td>361.7</td><td>43.51</td><td>1.19</td></tr><tr><td>-38.6</td><td>-</td><td>287.6</td><td>352.6</td><td>42.65</td><td>1.15</td></tr><tr><td>-36.2</td><td>-</td><td>245.7</td><td>359.8</td><td>43.33</td><td>1.19</td></tr><tr><td>-31.8</td><td>-</td><td>201.4</td><td>420</td><td>49.68</td><td>1.29</td></tr></tbody></table>

The primary inclusions are mostly 2–20 µm long tubular inclusions containing liquid + vapour + a solid salt ± ilmenite ± magnetite

<figure><img src="image_6.png" /><figcaption>Fig. 5. Primary multi-phase inclusions containing salt (S) + liquid + vapour bubble + additional ilmenite and magnetite (black solids on both sides of the bubble). Scal bar 40 µm.</figcaption></figure>
(Fig. 5). The multi-phase inclusions are associated with subordinate two-phase metastable aqueous inclusions (liquid + vapour) < 10 µm in size showing nucleation of a salt crystal during freezing runs. Uniform liquid/vapour ratios of the multi-phase inclusions suggest that they were not affected by post-entrapment necking-down. The initial melting temperatures are lower than the NaCl-H₂O eutectic (-20.8°C) and range between -24°C and -40°C (Table 2, Fig. 6A), thus indicating the presence of additional KCl, MgCl₂ and FeCl₂ [22]. Total homogenization of the inclusions occurs through halite disappearance at about 350°C, homogenization to liquid of the vapour bubble takes place between

200°C and 300°C (Fig. 6B). Based on halite dissolution [24,30], salinities of the multi-phase inclusions range from 40 to 50 wt% NaCl equivalents (Fig. 6B). The total homogenization temperatures indicate re-equilibration of the fluid inclusion densities at pressures much lower than the eclogitic peak. Absence of decrepitation textures and the observation that the highest homogenization temperatures are from the largest inclusions (Fig. 6C), suggest that fluid inclusion stretching during exhumation accomplished the change in the fluid inclusion density and allowed preservation of the original fluid compositions [25].

In order to evaluate the approximate ratios of salts

<caption>Table 3</caption>

Analyses of salt phase (D1 and D2) from opened multi-phase inclusions and of thermally induced decrepitation rims (SR) around the inclusions. The compositions of the host silicate phases olivine (ol) and titanian clinohumite (ticl) are reported for comparison

<table><thead><tr><th></th><th>ol host</th><th>1D1</th><th>2D2</th><th>ticl host</th><th>3D1</th><th>4SR</th><th>5D1</th><th>6D1</th><th>7D2</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>40.67</td><td>30.91</td><td>13.50</td><td>36.49</td><td>35.13</td><td>35.70</td><td>33.40</td><td>33.94</td><td>20.36</td></tr><tr><td>TiO<sub>2</sub></td><td>0.07</td><td>0.00</td><td>0.02</td><td>4.08</td><td>3.84</td><td>3.78</td><td>3.44</td><td>3.93</td><td>3.30</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.08</td><td>0.02</td><td>0.12</td><td>0.13</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>FeO</td><td>12.88</td><td>8.98</td><td>2.37</td><td>12.35</td><td>11.61</td><td>11.13</td><td>10.41</td><td>10.57</td><td>6.34</td></tr><tr><td>MnO</td><td>0.31</td><td>0.37</td><td>0.57</td><td>0.33</td><td>0.00</td><td>0.45</td><td>0.33</td><td>0.39</td><td>0.89</td></tr><tr><td>MgO</td><td>46.74</td><td>35.79</td><td>15.60</td><td>45.11</td><td>42.95</td><td>44.25</td><td>41.09</td><td>41.74</td><td>25.05</td></tr><tr><td>CaO</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.14</td><td>0.00</td><td>0.00</td><td>0.13</td></tr><tr><td>NaCl</td><td>0.00</td><td>8.18</td><td>nd</td><td>0.00</td><td>3.51</td><td>2.92</td><td>4.67</td><td>2.34</td><td>nd</td></tr><tr><td>KCl</td><td>0.00</td><td>0.00</td><td>nd</td><td>0.00</td><td>2.24</td><td>0.00</td><td>0.00</td><td>0.00</td><td>nd</td></tr><tr><td>FeCl<sub>2</sub></td><td>0.00</td><td>7.62</td><td>nd</td><td>0.00</td><td>0.00</td><td>1.26</td><td>1.23</td><td>2.54</td><td>nd</td></tr><tr><td>MgCl<sub>2</sub></td><td>0.00</td><td>3.80</td><td>nd</td><td>0.00</td><td>0.00</td><td>0.00</td><td>1.90</td><td>4.76</td><td>nd</td></tr><tr><td>Si/Fe</td><td>2.07</td><td></td><td></td><td>1.93</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Si/Mg</td><td>0.63</td><td></td><td></td><td>0.63</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Fe<sub>host</sub> wt%</td><td></td><td>6.98</td><td>3.05</td><td></td><td>9.02</td><td>8.65</td><td>8.09</td><td>8.22</td><td>4.93</td></tr><tr><td>Mg<sub>host</sub></td><td></td><td>21.58</td><td>9.41</td><td></td><td>25.90</td><td>26.49</td><td>24.78</td><td>25.17</td><td>15.11</td></tr><tr><td>Fe<sub>d</sub> wt%</td><td></td><td>1.72</td><td>13.10</td><td></td><td>0.00</td><td>0.65</td><td>0.44</td><td>1.09</td><td>16.03</td></tr><tr><td>Mg<sub>d</sub></td><td></td><td>1.86</td><td>28.31</td><td></td><td>0.00</td><td>0.00</td><td>0.60</td><td>1.20</td><td>11.43</td></tr><tr><td>Na<sub>d</sub></td><td></td><td>3.29</td><td>0.00</td><td></td><td>1.38</td><td>1.12</td><td>1.82</td><td>0.99</td><td>0.00</td></tr><tr><td>K<sub>d</sub></td><td></td><td>0.00</td><td>0.00</td><td></td><td>1.20</td><td>0.15</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>Cl<sub>d</sub></td><td></td><td>12.05</td><td>1.74</td><td></td><td>3.07</td><td>2.15</td><td>5.18</td><td>4.84</td><td>4.30</td></tr><tr><td>Fe (mol)</td><td></td><td>0.03</td><td>0.23</td><td></td><td>-</td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.28</td></tr><tr><td>Mg (mol)</td><td></td><td>0.08</td><td>1.16</td><td></td><td>-</td><td>-</td><td>0.02</td><td>0.05</td><td>0.47</td></tr><tr><td>Na (mol)</td><td></td><td>0.14</td><td>-</td><td></td><td>0.06</td><td>0.05</td><td>0.08</td><td>0.04</td><td>-</td></tr><tr><td>K (mol)</td><td></td><td>-</td><td>-</td><td></td><td>0.03</td><td>0.004</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Cl (mol)</td><td></td><td>0.34</td><td>0.05</td><td></td><td>0.09</td><td>0.06</td><td>0.15</td><td>0.14</td><td>0.19</td></tr></tbody></table>

Analyses D1, D2 and SR computed recasting the oxide wt% into element wt%. All chlorine and alkalis measured have been assigned to the salt phases. The olivine and titanian clinohumite components of the mixed analyses were computed assuming that all the silica measured in the salt phases was due to the host mineral. The Si/Fe and Si/Mg ratios of the host silicates have been used to evaluate the amount of Fe and Mg that in the mixed analyses is due to interference of the host phases according to: $Fe_{\text{salt}} = Fe_{\text{tot}} - Fe_{\text{host}}$; $Fe_{\text{host}} = Si_{\text{tot}}/(Si_{\text{host}}/Fe_{\text{host}})$. The Na, K, Fe, Mg and Cl of the salt phases have been converted to moles and combined to form NaCl, KCl, FeCl₂ and MgCl₂. The latter have been recast in wt% by multiplying the number of moles for the molecular weight of the different chlorides. Discrepancies in the salt stoichiometry can be due to contributions from the mineral substrate and to analytical errors, which can be significant for this geometry.
<figure><img src="image_7.png" /><figcaption>Fig. 6. (A) frequency histogram of the initial melting temperatures (Tmi). (B) Histogram of vapour to liquid homogenization and halite dissolution temperatures. (C) Total homogenization versus salinity as a function of the fluid inclusion size, showing that the highest homogenization temperatures pertain to the largest inclusions.</figcaption></figure>

in the NaCl-KCl-(Mg,Fe)Cl₂ brine, the solid phases in opened multi-phase fluid inclusions and a few salt rims formed after decrepitation of the fluid inclusions were analyzed with a scanning electron microscope [22,23]. The solid phases consist of magnetite,

ilmenite and mixed chloride salts. Analyses of the salt phases and of the vein minerals hosting the opened inclusions are reported in Table 3. The results obtained are mixed analyses of host minerals and salts, due to the larger size of the excited volume compared to the salt crystals. Considering that the host olivine and titanian clinohumite lack alkalis and chlorine, the salt compositions have been approximated following the procedure reported in Table 3. Two types of salt phases can be distinguished: (1) salts and decrepitation rims (respectively D1 and SR of Table 3) containing alkalis + Mg and/or Fe, resulting in almost stoichiometric mixtures of NaCl-(Fe,Mg)Cl₂ (MgCl₂ up to 50 mol%) and of NaCl-KCl (KCl up to 33 mol%); (2) Cl-bearing solid phases lacking alkalis and showing FeO and MgO contents much higher than the host silicates (D2; Table 3), they may correspond to non-stoichiometric (Mg,Fe)Cl₂ salts. Presence of NaCl-KCl solid solutions (analysis 3 D1; Table 3) indicates that salts were precipitated at T > 500°C, since at lower temperatures halite and sylvite are immiscible [26]. This argument further supports the high-pressure origin of the fluid inclusions analyzed because the temperature of eclogitization was 550-600°C.

## 6. Discussion and conclusions

### 6.1. Uptake of chlorine and alkalis during hydrothermal alteration of lherzolite

Hydrothermal alteration of the Erro-Tobbio peridotite produced chlorine- and alkali-bearing hydrous phases below 300°C, as indicated by the presence of chrysotile [27]. Textures and compositions of the alteration products suggest that uptake of these elements was governed by the composition of precursor minerals and by the interaction with Cl- and alkali-bearing fluids. The compositions of pseudomorphic lizardite and chrysotile indicate that the mantle minerals controlled the chemistry of hydrothermal serpintines: the chlorine of lizardite after mantle hornblende was probably inherited from this latter mineral. On the other hand, formation of Cl-bearing serpentine and of Cl- and alkali-bearing phyllosilicates in lherzolites originally lacking hornblende,
suggest interaction with a fluid phase bringing these elements. Experiments indicate that potassium is taken up by rocks from seawater below 150–200°C [28] and suggest that lherzolite alteration occurred at 150–300°C. Since the halogen content of biotite allows calculation of the fugacity of halogen acid of the coexisting fluid at known temperature and pressure [29,30], the log (fH₂O/fHCl) of the hydrothermal fluid at 150°C and 200°C has been estimated in the range of 3.2–3.8 and 3–3.6 using the phlogopite component of the mixed-layer phyllosilicates and applying the revised expression of Munoz [30]. These values represent rough approximations of fH₂O/fHCl of the alteration fluid, due to the complexity of the mixed-layer phyllosilicates and due to the fact that the phyllosilicate analyses have been recast considering a pure phlogopite component with X<sub>Mg</sub> = 1. Interaction with an acid, HCl-bearing hydrothermal fluid contrasts with the presence of brucite, which generally forms from basic, water-rich fluids [31]. Studies on Cl-bearing serpentines coexisting with brucite suggest that serpentinization is enhanced by infiltration of acid to neutral fluids and causes precipitation of chlorine from the fluid, thus shifting the fluid pH to more basic compositions [32]. This effect was reproduced during experiments with olivine and brucite, where solutions with pH 8 were formed after initially acid solutions (pH 3–5) reacted with olivine in presence of Cl [31]. Cl-bearing serpentine associated with brucite may therefore reflect fluctuations of the fluid pH during the alteration process.

<figure><img src="image_8.png" /><figcaption>Fig. 7. Chlorine content of hydrothermal serpentines from abyssal peridotites [33–37] and from the Erro-Tobbio peridotite.</figcaption></figure>

Oceanic peridotites generally consists of high-temperature mantle fabrics overgrown by serpentine and other phyllosilicates [33–37]. The chlorine concentrations (<1 wt%) of serpentine from abyssal peridotites reflect interaction with marine fluids and closely match those of the Erro-Tobbio hydrothermal serpentine (Fig. 7). Alkali-bearing phyllosilicates comparable with the mixed-layer phyllosilicates presented here, have been found in peridotites from the Tyrrhenian Sea and have been interpreted as products of retrograde seawater infiltration [38]. The above data indicate that uptake of water, chlorine and alkalis in ultramafic rocks during interaction with seawater-derived fluids is a widespread feature affecting both modern and fossil oceanic mantle.

## 6.2. Cl-bearing fluids formed from seawater and element recycling at eclogite-facies conditions

The hydrothermal phases were overprinted by eclogitic minerals without formation of intermediate (prograde) assemblages. The high-pressure minerals lack Cl and alkalis and their crystallization was accompanied by fluid production and veining. Moreover, Cl- and alkali-rich fluid inclusions (up to 50 wt% Cl, Na, K. Mg and Fe) were trapped in the vein minerals. The question as to whether the highly saline vein fluid reflects infiltration of externally derived fluids, or relates to some internal processes confined to the Erro-Tobbio metaperidotite is crucial to assess the fluid-rock regime active during high-pressure metamorphism. Evidence suggests in situ fluid production because

1. Veins and host rocks display the same mineralogy, reflecting control by the country rocks on the solute material transported by the fluid.

2. Recent work shows that the Erro-Tobbio country peridotites and the high-pressure veins display comparable bulk REE patterns; the absolute REE concentrations of the veins are governed by the modal abundance of metamorphic diopside [39]. The vein diopside analyzed by an ion microprobe (CSCC, University of Pavia, Italy) displays light REE-depleted patterns, and medium to heavy REE absolute concentrations up to 10–20 times those of chondrite. Such compositions are quite similar
to those of the mantle clinopyroxene relics forming the host rocks ([39], and unpubl. data). These features suggest rock control of the vein fluid and equilibration between the country rocks and the fluid.

3. The presence of MgCl₂, FeCl₂, ilmenite and magnetite in the inclusions indicate high Fe, Mg and Ti concentrations in the fluid solute and present strong evidence that the fluid was rock controlled. Moreover, the mantle and hydrothermal minerals were important sources of the chlorine and alkalis forming the brine. Relevant carriers of sodium were the mantle hornblende (< 3.1 wt% Na₂O) and clinopyroxene (< 0.5 wt% Na₂O) [19], whose breakdown products at high-pressure were sodium free. The above relationships suggest that the chlorine and alkalis hosted by hydrothermal and mantle minerals were not counter-balanced by the high-pressure assemblages and were taken up by the dehydration fluid.

Alternatively, the analyzed brines could be the result of fluid infiltration from the surrounding lithologies and equilibration of this fluid with the peridotite. Widespread findings of saline fluids in eclogite-facies rocks of the Alps [12–14,40] suggests that brines formed over large rock volumes at the

peak of subduction. As a consequence, the hypothesis of salty fluids infiltrating the Erro-Tobbio peridotite from the surrounding lithologies cannot be excluded. However, the studies on Alpine eclogite-facies rocks and fluids show that the mineralogies of eclogitic veins and the mineral suites inside the fluid inclusions are indicative of fluid migration over local scales and of compositional control of the fluid by the host rocks [12–14]. Strong evidence in favour of short-range fluid migration is represented by centimetre scale isotopic heterogeneities (in δ¹⁸O, δD and δ¹³C) in eclogitic rocks, requiring fluid buffering by the host rocks and suggesting that large-scale circulation of homogeneous fluids was not attained at the pressure and temperature of formation of eclogite-facies rocks of the Alps [16,17,41]. According to literature and to the data presented here, in situ production of a Cl-bearing fluid which evolved into the briny inclusions trapped in the high-pressure veins appears more reasonable than massive, large-scale infiltration of externally derived fluids.

High salinity of the vein fluid suggests that the devolatilization fluid was internally recycled and interacted with the host rocks. The pressure–temperature diagram of Fig. 8 reports the breakdown reactions of serpentine and brucite [27,42], the stability field of antigorite [7] and the eclogitic peak in the Erro-Tobbio peridotite [9]. Fluid production at high-pressure is governed by reactions 1 and 2 (Fig. 8), which consumed brucite (less abundant than serpentine) and produced olivine, chlorite and fluid stable with antigorite. Contemporaneously with the dewatering of highly altered domains, the less serpentinized domains probably underwent hydration reactions, due to the instability of relict mantle orthopyroxene and olivine in the presence of excess fluid. These reacted to produce antigorite (Fig. 8) and such reactions increased the solute content of the residual fluid. Moreover, crystallization of Cl-free hydrous phases in the veins might have further increased fluid chlorinity. We therefore propose that fluid-consuming processes broadly coeval with the main devolatilization event might have been responsible for the high chlorinity of the residual vein fluid. The model proposed requires the survival of the oceanic phases until high pressures. These could be preserved as stable phases, since the stability field of mixed-layer phyllosilicates and chlorine-bearing ser-

<figure><img src="image_9.png" /><figcaption>Fig. 8. Pressure–temperature diagram reporting the eclogite-facies equilibration of the Erro-Tobbio peridotite (shaded area) [9], the stability field of antigorite [7] and the breakdown reactions of serpentine + brucite (1 and 2) [27,42].</figcaption></figure>
pentine is actually unknown, and thermodynamic calculations by Chernosky et al. [42] predict stability of lizardite + brucite to very high pressures. Alternatively, fast subduction rates may account for metastable preservation of oceanic assemblages at depth due to slow reaction kinetics during cold subduction.

In the Erro-Tobbio peridotite, despite formation of a Cl-rich brine, the associated minerals are not chlorine enriched due to Cl-OH exchange with the fluid. Rather, the data suggest that chlorine behaved as an incompatible element. The understanding of chlorine behaviour is hampered by the lack of knowledge of its partitioning between solids and fluids at high pressure. Chlorine replaces the OH groups in hydrous phases and the available data show that chlorine partitioning between fluids and minerals at low pressure is affected by parameters such as $X_{Mg}$, pressure, temperature, mineral structure, paragenesis and fluid pH [29,43,44]. Positive correlations between $Fe^{2+}$ and Cl documented in amphibole and biotite indicate that the incorporation of chlorine on a hydroxyl site is dependent on the $Fe^{2+}$ content of the neighbouring octahedral site [43,44]. Fig. 3D shows that, in the Erro-Tobbio, peridotite transition from hydrothermal serpentines to high-pressure antigorite takes place with a decrease in Cl and an increase in the $X_{Mg}$. Wicks and Plant [45] showed that chlorine can be present in early-stage serpentine formed during cooling of high-temperature peridotites and their data are reinforced by the finding of chlorine in serpentine from oceanic peridotites (Fig. 7). On the other hand, chlorine is absent in prograde antigorite formed during recrystallization of low-temperature serpentine [45]. Antigorite can form after pre-existing chrysotile with increasing metamorphic grade [46] and is the high-density serpentine form constituting high-pressure ultramafic rocks: under such metamorphic conditions the crystal structures are compact and are characterized by dense atomic packing. Although crystallographic data on Cl-bearing serpentines are lacking, from a crystal-chemical point of view it is conceivable that the large chlorine anions are incompatible with compact mineral structures like that of antigorite (Mellini, pers. commun.). This reasoning suggests that antigorite was unable to retain Cl because of compositional and structural features: it may ex-

plain the formation of Cl-bearing fluids as the result of the breakdown of hydrothermal serpentines and crystallization of antigorite-bearing assemblages.

Titanian clinohumite is the other phase which in principle may incorporate chlorine due to Cl-OH exchange with the fluid. Cl was not detected in the titanian clinohumite analyzed and published chemical analyses show that Cl is absent in titanian clinohumites from high- and very high-pressure metaperdotites, the only halogen measured being fluorine [20,47–51]. The main substitutions taking place in titanian clinohumite are represented by $Mg + 2(OH) = Ti + 2(O)$ and by $(OH) = F$ [48]. These stabilize the titanian clinohumite structure by reducing the number of H atoms, and therefore the strong H-H repulsion which inevitably takes place if more than one OH per formula unit is present. Unpublished crystallographic work shows that the dense titanian clinohumite structure may incorporate fluorine but not chlorine, due to its over-large ionic radius (Un-garetti, pers. commun.). Additional information about chlorine behaviour is given by thermodynamic modelling of chlorine redistribution between fluids and micas at low-pressures (max. 2 kbar), suggesting that an increase in pressure favours Cl partitioning into the fluids [29]. The chlorine behaviour at low pressures cannot be applied straightforwardly to ultramafic systems at high pressure; however, the documented variations in $X_{Mg}$ in serpentine minerals, the structural control of chlorine uptake into minerals and the pressure control on chlorine partitioning between minerals and fluids can be taken to explain the chlorine partitioning into the fluids at high pressure, as observed in the Erro-Tobbio peridotite.

One main result of our study is that the brine produced by the subducted serpentinite was generated by recycling of seawater-derived fluids stored in hydrous minerals during ocean floor metamorphism of original mantle peridotites. Delivery of chlorine and alkalis from seafloor assemblages into the dehydration fluid allowed inheritance by the eclogite-facies fluid of the hydrothermal signature acquired during oceanic alteration. At 2.5 GPa alkalis and chlorine were transferred to the metamorphic fluid, whereas large amounts of water were still stored in the mineral assemblages. The metamorphic redistribution of water, chlorine and alkalis at mantle depths demonstrates that subducted serpentinites can play a
role in the genesis of deep metasomatic fluids and in
the Earth's redistribution of chlorine [51,52].

6.3. *The role of serpentinite in the large-scale subduction of water*

Antigorite, chlorite and titanian clinohumite survived eclogitization of the Erro-Tobbio metaperidotite. These rocks therefore represent an intermediate stage of transformation between the end-member cases of serpentinized abyssal peridotites [11], wet ultramafic rocks containing dense hydrous silicates [8] and garnet peridotites [49]. This status is common to many alpine ultramafites, where antigorite serpentinites are associated with high- and very high-pressure rocks [9]. The fact that serpentinites have been found over large areas of the ocean floor, that serpentine is stable to considerable depth and the recognition that seawater is recycled from hydrous peridotites in deep levels of subduction zones, identifies these rocks as the major transporters of surface water into the mantle on a large scale. This fact dramatically increases the amount of water recycled into the mantle.

Acknowledgements

We wish to thank Volkmar Trommsdorff and Othmar Muentener for microprobe facilities at the ETH-Zentrum of Zurich. Comments by Larryn Diamond, Othmar Muentener, Elisabetta Rampone and Peter Ulmer on a preliminary version of the manuscript have been much appreciated. We also wish to acknowledge the help of Marcello Mellini and Luciano Ungaretti for discussion on chlorine compatibility in antigorite and titanian clinohumite and for sharing some unpublished results of their research. The paper benefited from thoughtful and constructive reviews by Michael J. Mottl, Joris Gieskes and Bruce W. Yardley. Financial support was provided by the Italian CNR, MURST and by the University of Genova. [MK]

References

[1] J.G. Moore, Water content of basalt erupted on the ocean floor, Contrib. Mineral. Petrol. 28 (1970) 272-279.

[2] D.R. Bell, G.R. Rossman, Water in Earth's mantle: The role

of nominally anhydrous minerals, Science 255 (1992) 1391-1397.

[3] P. Gillet, L'eau du manteau terrestre, Recherche 24 (1993) 676-685.

[4] J. Gill, Orogenic Andesites and Plate Tectonics, Springer, Heidelberg, 1980.

[5] S.M. Peacock, Fluid processes in subduction zones, Science 248 (1990) 329-337.

[6] S. Poli, M.W Schmidt, Water transport and release in subduction zones: Experimental constraints on basaltic and andesitic systems, J. Geophys. Res. 100 (1995) 22299-22314.

[7] P. Ulmer, V. Trommsdorff, Serpentine stability to mantle depths and subduction-related magmatism, Science 268 (1995) 858-861.

[8] A.B. Thompson, Water in the Earth's upper mantle, Nature 358 (1992) 295-302.

[9] M. Scambelluri, O. Muentener, J. Hermann, G.B. Piccardo, V. Trommsdorff, Subduction of water into the mantle: History of an alpine peridotite, Geology 23 (1995) 459-462.

[10] E. Ito, D.M. Harris, A.T. Anderson, Alteration of oceanic crust and geologic cycling of chlorine and water, Geochim. Cosmochim. Acta 47 (1983) 1613-1624.

[11] M. Cannat, C. Mevel, M. Mala, C. Deplus, C. Durand, P. Gente, P. Agrinier, A. Belarouchi, G. Dubuisson, E. Humler, J. Reynolds, Thin crust, ultramafic exposures, and rugged faulting patterns at the Mid Atlantic Ridge (22°-24° N), Geology 23 (1995) 49-52.

[12] P. Philippot, Fluid-melt-rock interaction in mafic eclogites and coesite-bearing metasediment: Constraints on volatile recycling during subduction. Chem. Geol. 108 (1993) 93-112.

[13] P. Philippot, J. Selverstone, Trace-element rich brines in eclogitic veins: Implications for fluid compositions and transport during subduction, Contrib. Mineral. Petrol. 106 (1991) 417-430.

[14] J. Selverstone, G. Franz, S. Thomas, Fluid heterogeneities 2 GPa eclogites: implications for element recycling during subduction, Contrib. Mineral. Petrol. 112 (1992) 341-357.

[15] T. Andersen, A. Austrheim, E.A.J. Burke, S. Evevold, N₂ and CO₂ in deep crustal fluids: Evidence from the Caledonides of Norway, Chem. Geol. 108 (1993) 113-132.

[16] S. Nadeau, P. Philippot, F. Pineau. Fluid inclusion and mineral isotopic compositions (H-C-O) in eclogitic rocks as tracers of local fluid migration during high pressure metamorphism, Earth Planet. Sci. Lett. 114 (1993) 431-448.

[17] A.C. Barnicoat, I. Cartwright, Focussed fluid flow during subduction: Oxygen isotope data from high-pressure ophiolites of the Western Alps. Earth Planet. Sci. Lett. 132 (1995) 53-61.

[18] Y. Lagabrielle, M. Cannat, Alpine Jurassic ophiolites resemble the modern central Atlantic basement, Geology 18 (1990) 319-322.

[19] E.H. Hoogerduijn Strating, E. Rampone, G.B. Piccardo, M.R. Drury, R.L.M. Vissers. Subsolidus emplacement of mantle peridotites during incipient oceanic rifting and opening of the Mesozoic Tethys (Voltri Massif NW Italy), J. Petrol. 34 (1993) 901-927.
[20] M. Scambelluri, E.H. Hoogerduijn Strating, G.B. Piccardo, R.L.M. Vissers, E. Rampone, Alpine olivine and titanian clinohumite bearing assemblages in the Erro-Tobbio peridotite, J. Metamorph. Geol. 9 (1991) 79-91.

[21] B. Messiga, M. Scambelluri, G.B. Piccardo, Chloroitoid-bearing assemblages in mafic systems and eclogite-facies hydration of alpine Mg-Al metagabbros (Erro-Tobbio Unit), Ligurian Western Alps, Eur. J. Mineral. 7 (1995) 1149-1167.

[22] T.J. Shepherd, A.H. Rankin, D.H.M. Alderton, A Practical Guide to Fluid Inclusion Studies. Blackie, 1985, 239 pp.

[23] F.M. Haynes, S.M. Sterner, R.J. Bodnar, Synthetic fluid inclusions in natural quartz. IV. Chemical analyses by SEM/EDA: evaluation of method, Geochim. Cosmochim. Acta 52 (1988) 969-978.

[24] N.B. Keevil, Vapor pressures of aqueous solutions at high temperatures, J. Am. Chem. Soc. 64 (1942) 841-850.

[25] S.M. Sterner, R.J. Bodnar, Synthetic fluid inclusions. VII. Re-equilibration of fluid inclusions in quartz during laboratory-simulated metamorphic burial and uplift, J. Metamorph. Geol. 7 (1989) 243-260.

[26] V. Trommsdorff, G. Skippen, P. Ulmer, Halite and sylvite as solid inclusions in high-grade metamorphic rocks, Contrib. Mineral. Petrol. 28 (1985) 24-29.

[27] B.W. Evans, J. Johannes, H. Oterdoom, V. Trommsdorff, Stability of chrysotile and antigorite in the serpentine multi-system, Schweiz. Mineral. Petrogr. Mitt. 56 (1976) 79-93.

[28] M.J. Motl, H.D. Holland, Chemical exchange during hydrothermal alteration of basalt by seawater, Geochim. Cosmochim. Acta 42 (1978) 1103-1115.

[29] C. Zhu, D.A. Sverjensky, Partitioning of F-Cl-OH between minerals and hydrothermal fluids, Geochim. Cosmochim. Acta 55 (1991) 1837-1858.

[30] J.L. Munoz, Calculation of HF and HCL fugacities from biotite compositions: revised equations, Geol. Soc. Am. Abstr. Progr. 24, A221.

[31] B. Poty, H.D. Holland, M. Borcsik, Solution-mineral equilibria in the system MgO-SiO₂-H₂O-MgCl₂ at 500°C and 1 kbar, Geochim. Cosmochim. Acta 26 (1972) 1101-1113.

[32] J.C. Rucklidge, G.C. Patterson, The role of chlorine in serpentinization, Contrib. Mineral. Petrol. 65 (1977) 39-44.

[33] P. Agrinier, C. Mevel, J. Girardeau, Hydrothermal alteration of the peridotites cored at the ocean/continental boundary of the Iberian margin: Petrologic and stable isotope evidence, Proc. ODP Sci. Results 103 (1988) 225-234.

[34] P.F. Sciuto, G. Ottonello, Water-rock interaction on Zabargad Island, Red Sea—A case study: I. Application of the concept of local equilibrium, Geochim. Cosmochim. Acta 59 (1995) 2187-2206.

[35] R. Hebert, A.C. Adamson, S.C. Komor, Metamorphic petrology of ODP Leg 109, hole 670A serpentinized peridotites: serpentinization processes at a slow spreading ridge environment. Proc. ODP Sci. Results 103 (1990) 106-109, 113-115.

[36] K. L. Kimball, F.S. Spear, H.J.B. Dick, High temperature alteration of Abyssal ultramafics from the Islas Orcadas Fracture Zone. South Atlantic. Contrib. Mineral. Petrol. 91 (1985) 307-320.

[37] C. Mevel, C. Stamoudi, Hydrothermal alteration of the upper-mantle section at Hess Deep, Proc. ODP Sci. Results 147 (1996) 293-309.

[38] E. Bonatti, M. Seyler, J. Channel, J. Girardeau, G. Mascle, Peridotite drilled from the Thyrrenian sea. ODP Leg 107. Proc. ODP Sci. Results 107 (1990) 37-47.

[39] M. Scambelluri, E. Rampone, G.B. Piccardo, Fluid-rock interaction and seawater recycling in eclogite-facies serpentinite, Goldschmidt Conference, J. Conf. Abstracts 1 (1996) 535.

[40] P. Philippot, P. Chevallier, C. Chopin. J. Dubessy, Fluid composition and evolution in coesite-bearing rocks (Dora-Maira Massif, Western Alps): implications for element recycling during subduction. Contrib. Mineral. Petrol. 121 (1995) 29-44.

[41] S.R. Getty, J. Selverstone, Stable isotopic and trace element evidence for restricted fluid migration in 2 Gpa eclogites, J. Metamorph. Geol. 12 (1994) 747-760.

[42] J.V. Chernosky, R.G. Berman, L.T. Bryndzia, Stability, phase relations and thermodynamic properties of chlorite and serpentine group minerals, In: S.W. Bailey (Ed.), Hydrous Phyllosilicates, Rev. Mineral. 19 (1988) 295-346.

[43] J. Morrison, Compositional constraints on the incorporation of Cl into amphibole, Am. Mineral. 76 (1991) 1920-1930.

[44] M. Volfinger, J.L. Robert, D. Vielzeuf, A.M.R. Neiva, Structural control of the chlorine content of OH-bearing silicates (micas and amphiboles), Geochim. Cosmochim. Acta 49 (1985) 37-48.

[45] F.J. Wicks, A.G. Plant. Electron microprobe and X-ray-microbeam studies of serpentinite textures, Can. Mineral. 17 (1979) 785-830.

[46] M. Mellini, V. Trommsdorff, R. Compagnoni, Antigorite polysomatism: behaviour during progressive metamorphism, Contrib. Mineral. Petrol. 97 (1987) 147-155.

[47] N.W. Jones, P.H. Ribbe, G.V. Gibbs, Crystal chemistry of the humite minerals, Am. Mineral. 54 (1969) 391-411.

[48] K. Fujino, Y. Tacheuchi, Crystal chemistry of titanian chondrodite and titanian clinohumite of high-pressure origin, Am. Mineral. 63 (1978) 535-543.

[49] B.W. Evans, V. Trommsdorff, Fluorine hydroxyl titanian clinohumite in alpine recrystallized garnet peridotite: compositional controls and petrologic significance, Am. J. Sci. 283 (A) (1983) 355-369.

[50] R.Y. Zhang, J.G. Liou, B.L. Cong, Talc, magnesite- and Ti-clinohumite-bearing, ultrahigh- pressure meta-mafic and ultramafic complex in the Dabie Mountains, China, J. Petrol. 36 (1995) 1011-1037.

[51] B. Ransom, A.J. Spivack, M. Kastner, Stable Cl isotopes in subduction-zone pore waters: implications for fluid-rock interactions and the cycling of chlorine, Geology 23 (1995) 715-718.

[52] A.J. Magenheim, A.J. Spivack, P.J. Michael, J.M. Gieskes, Chlorine stable isotope composition of the oceanic crust: implications for the Earth's distribution of chlorine. Earth Planet. Sci. Lett. 131 (1995) 427-432.