# Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: A case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman

Adriana Currin, Paul Eric Wolff, Juergen Koepke, Renat R. Almeev, Chao

Zhang, Barbara Zihlmann, Benoit Ildefonse, Damon A. H. Teagle

► To cite this version:

Adriana Currin, Paul Eric Wolff, Juergen Koepke, Renat R. Almeev, Chao Zhang, et al.. Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: A case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman. Lithos, 2018, 323, pp.125-136. 10.1016/j.lithos.2018.09.015. hal-01975594

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1 Chlorine-rich amphibole in deep layered gabbros as evidence for
2 brine/rock interaction in the lower oceanic crust: a case study from
3 the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman

4 Adriana Currinª, Eric P. Wolffª, Juergen Koepkeª, Renat R. Almeevª, Chao Zhangª, Barbara Zihlmannᵇ,

5 Benoit Ildefonseᶜ, Damon A.H. Teagleᵇ

6 · Institut für Mineralogie, Leibniz Universität Hannover, Callinstr. 3, D-30167 Hannover, Germany

7 ᵈ Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton,
Southampton, SO14-3ZH, United Kingdom

9 · Géosciences Montpellier, CNRS, University of Montpellier, Université des Antilles, Montpellier, France

10

11 **Abstract**

12 Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the
13 Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep
14 oceanic crust, and these features record interactions between rock and high temperature seawater-derived
15 fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition
16 zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies
17 grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by
18 hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical,
19 isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional
variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich
ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of
formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between
rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and
cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at
variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies
phase separation and brine/rock interactions. *Sr*·*Sr* values of 0.7031 to 0.7039 and stable δ*O* isotopic
compositions of 4.1 to 5.6 ‰ of the different amphibole types suggest a rock-dominated environment, i.e.
with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate
there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides
new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence
of a seawater-derived fluid.

32 **Keywords:** Cl-bearing amphibole, high-temperature hydrothermal circulation, lower oceanic crust,
33 Samail Ophiolite, phase separation, brine-rock interactions.

34

## 1. Introduction

Fluid/rock interactions play important roles in mineral genesis and hydrothermal alteration at mid-ocean ridges, and are major contributors to heat and mass transfer control in the oceanic crust. Hydrothermal circulation occurs in the oceanic crust through the percolation of seawater into joints and fault zones in the seafloor (Manning & MacLeod, 1996; Manning et al., 2000). Several investigations argue that hydrothermal circulation is not restricted to the upper oceanic crust of fast-spreading mid-ocean ridges, but probably
reaches beyond the sheeted dyke complex and down into the gabbro section (Fig. 1) (Gregory & Taylor, 1981; Zhang et al., 2014; 2017b; Harris et al., 2017). This is backed up by different types of studies, including field research on ophiolites (Gregory & Taylor, 1981; Bosch et al., 2004; Nicolas et al., 2003; Abily et al., 2011); analysis of drill cores obtained from the recent oceans (Alt & Bach, 2006; Coogan et al., 2006; 2007), and is also suggested by geophysical modelling (Hasenclever, 2014).

Bosch et al. (2004) and Nicolas et al. (2003) (and references therein) describe an interconnected seawater-derived hydrothermal system in the Samail Ophiolite (Sultanate of Oman), made up of gabbroic dykelets (recharge) and hydrothermal veins (discharge). Hydrothermal alteration occurs over a range of temperatures from very high temperatures (900-1020 °C) involving hydrous partial melting, down to lower temperature hydrothermal reactions at greenschist facies conditions (Bosch et al., 2004). In this study, we focus on the formation of amphiboles by hydrothermal fluid/rock interactions at different temperatures between 500 and 1030 °C, through the detailed study of hydrothermal minerals formed in gabbros that crop out in the Wadi Wariyah, Wadi Tayin Massif, Samail Ophiolite (Sultanate of Oman).

Amphiboles are volatile-bearing minerals, and an important phase in the inventory of the partial hydration of the oceanic crust. They have been proven to be an important Cl reservoir in altered oceanic crust (e.g., Ito et al., 1983; Straub et al. 2003; Zhang et al., 2017a), since the same structural site that can host (OH) groups can accommodate Cl anions (Vanko et al, 1986; Chan et al., 2016; Oberti et al., 1993). Amphiboles are stable over a wide range of temperatures, and their compositional changes provide useful information on the temperature of fluid-rock interactions that resulted in their formation (e.g., Ernst and Liu, 1998). The occurrence of Cl-bearing amphibole in high temperature hydrothermal veins hosted in deep layered gabbros may suggest the presence of seawater-derived aqueous fluids and brines flowing down to the base of the oceanic crust (e.g., Manning et al., 1996; Coogan et al., 2001; Gillis et al., 2001; 2002; Bosch et al., 2004;
Tribuzio et al. 2014). Consequently, by investigating such Cl-rich amphiboles and associated minerals, we
can gain insight into the hydrothermal processes occurring in the lower oceanic crust.

The passage of fluids through different lithologies of the oceanic crust results in reactions with the host
rock minerals. In addition, variations of composition, temperature, and pressure of the fluid will lead to
changes in fluid salinity, solute composition and fluid oxygen fugacity.

Our study focuses on high temperature hydrothermal veins and gabbroic dykelets hosted in the layered
gabbro section of the Samail Ophiolite that contain different types of amphibole. We explore the influence
of fluids on amphibole genesis at depth, as well as determine the origin of the fluids reacting with the lower
oceanic crust by using Sr and O isotopic analysis. Electron back-scatter diffraction (EBSD) analyses are
integrated with chemical data to determine orientation differences between magmatic amphiboles and Cl-
rich and Cl-poor hydrothermal amphiboles.

76

77

## 2. Analytical techniques

Polished thin sections of the rock samples were made of the hydrothermal veins and dykelets and areas
around them. Mineral major element compositions were determined in situ using a Cameca SX100
electron microprobe equipped with 5 spectrometers and the “Peak Sight” software at the Institute of
Mineralogy of the Leibniz University Hannover, Germany. A focussed beam current of 15nA and an
acceleration voltage of 15kV were used to analyse all minerals. Counting time for minerals was 10s for
peak and 5s for background, but was longer for Cl and F (30s at peak). Elemental measurements were
calibrated using the following natural and synthetic standards: albite (Na), wollastonite (Ca and Si),
orthoclase (K), Durango apatite (P), Al₂O₃ (Al), Mn₂O₃ (Mn), TiO₂ (Ti), MgO (Mg), Fe₂O₃ (Fe), NaCl (Cl),
and $SrF_2$ (F). In order to monitor analytical precision, each measurement was checked against the following international mineral standards (reference materials): Lake County plagioclase (USNM 115900), Kakanui hornblende (USNM 143965), Kakanui augite (USNM 122142), San Carlos Olivine (NMNH 111312-44), Jarosewich et al. 1980). Detection limits (in wt. %) in amphibole were: $SiO_2$: 0.03; $TiO_2$: 0.02; $Al_2O_3$: 0.03; $FeO$: 0.09, $MnO$: 0.05; $MgO$: 0.04; $CaO$: 0.04, $NaO$: 0.05; $K$: 0.015; $Cr$: 0.07; $Cl$: 0.007; F: 0.04. Representative values for measured amphiboles are listed in table 1. The amphibole formula was calculated using 13 cations excluding Ca, Na and K. Classification scheme used is that of Leake et al. 1997.

$^{40}$Sr-$^{80}$Sr radiogenic isotope analyses of amphiboles, whole vein and whole dykelet samples were performed at the University of Southampton by thermal ionisation mass spectrometry (TIMS) using a ThermoFisher Scientific TritonPlus. Mineral separates were crushed and digested using a standard $HF-HNO_3$ acid attack, mother solutions were prepared (Zohlmann et al., this issue) and bulk trace element compositions were measured from diluted daughter solutions, using a ThermoScientific X-Series 2 ICP-MS adjusted for REE measurements prior to isotope analysis. Strontium spec™ columns were then used to separate Strontium from dissolved samples (Harris et al., 2015). The Sr concentrate was loaded onto degassed tantalum filaments for analysis by TIMS. Good internal precision was ensured by measuring 150 ratios per sample and presented as given as two standard errors. The $^{40}$Sr/$^{80}$Sr ratio of standard reference material NBS987 was measured as a monitor of external precision. Over the duration of the analyses returned 0.710247 ± 0.000023 (2σ); n = 53).

The oxygen isotopic compositions of the same mineral separates as used for Sr-isotopes were determined by Actlabs (Activation Laboratories Ltd., Ancaster, Ontario, Canada). Oxygen was extracted from 5 mg samples at 550-600 °C according to the conventional $BrF_5$ procedure of Clayton and Mayeda (1963) and analyzed via dual inlet on a Thermo-Finnigan DeltaPlus XP Isotope-Ratio Mass Spectrometer (IRMS).
Results are calibrated to certified reference materials, relative to the VSMOW and VSLAP international standards. Precision and accuracy (standard deviation) are of 0.1‰.

Electron backscattered diffraction (EBSD) analyses were performed at the facilities of Montpellier University, France using a CamScan X500FE CrystalProbe with an Oxford/HKL EBSD system. This equipment allows for high resolution EBSD measurements (10-50 nm) and includes Energy Dispersive Spectrometry (EDS). The EBSD software used is AZtecHKL by Oxford Instruments for data acquisition and the Matlab-based code MTEX for analysis (see Ferrando et al., 2018).

## 3. Results

### 3.1 Outcrop description

The rocks cropping out in Wadi Wariyah (N 22°58′51.49", E 58°15′53.98"), previously described by Bosch et al. (2004) and Wolff (2014), mainly consist of layered gabbro, crosscut by hydrothermal veins and cm-scale gabbroic dykelets, and are representative of deep oceanic crust, with rocks sampled ~ 100 m above the Moho transition zone (MTZ) (Fig. 2). Magmatic layering in the gabbros is approximately perpendicular to the strike of the overlying sheeted dykes in the area. The vein system displays a large variety of gabbroic dykelets and hydrothermal veins with different mineral fillings, including dykelets of several cm in width, mostly cutting the gabbro layering at high angles. These dykelets connect to each other in a system of branches (branched connectivity), and contacts, although sharp, do not show chilled margins. A second type of feature are thin, black to dark green veins up to 4 mm wide that probably formed from the filling of microcracks, parallel to the gabbroic dykelets. These veins have sharp contacts to the host gabbro. The gabbroic dykelets and dark veins are clustered in several cm-wide swarms with
single dykelets and veins spaced every few cm within the swarms, and swarms are spaced up to several
metres apart. Both gabbroic dykelets and dark veins are roughly perpendicular to the gabbro layering and
subparallel to each other. A third type of hydrothermal veins are white veins dominated by
epidote/clinozoisite and/or prehnite that formed at lower temperatures. These white veins cross-cut the
aforementioned dark veins and gabbroic dykelets. For details of this outcrop including images see Wolff
(2014) and Bosch et al. (2004), and more details on the system of veins and dykelets see Nicolas et al.
(2003). In this study we present data from dark veins (2 samples; Fig. 3A, C) and gabbroic dykelets (5
samples; Fig. 3B, D). Samples were collected within an area of 50 metres wide by 20 metres high outcrop
of layered gabbro.

139 **3..2 Petrography and amphibole composition (major and minor elements)**

140 The host rock is a layered olivine gabbro olivine, clinopyroxene, and plagioclase, with small amounts of
141 Fe-Ti oxides and orthopyroxene. The magmatic foliation of the gabbro is defined by orientated olivine
142 and plagioclase crystals. The average grain size of the host rock minerals is ~1.5 mm. The dark veins and
143 gabbroic dykelets contain different types of amphiboles (pargasite, magnesiohastingsite,
144 magnesiohornblende, actinolite, hastingsite and ferropargasite), and some of them are highly enriched in
145 chlorine (Table 1).

146 Specimens WA02 and WA05 were sampled from a high temperature hydrothermal vein containing mainly
147 high-Ti pargasite (Ti >0.45 a.p.f.u.). Some pargasite grains are partially replaced (multiple zoning, Fig. 3A)
148 and display the transition from high-Ti pargasite via replacive magnesiohastingsite and green
149 magnesiohornblende to actinolite within a single grain, and locally, areas with greenish-blue Cl-rich
150 ferropargasite and hastingsite with chlorine contents of up to 1.5 a.p.f.u. (atoms per formula unit), forming
151 complex replacive textures (Figs. 4 & 5). In sample WA02, the grain size of the vein minerals is an average
152
153 of 2 mm. The magmatic minerals that make up the host layered gabbro mainly have large grain sizes of
154 about 2 to 3 mm. However, zoning within amphibole can be as small as ~5 µm.

155 Specimens WA42, WA04, WA33, WA46, and WA32A are samples of gabbroic dykelets. These samples
156 display poikilitic to fine-grained granular textures. Typically anhedral, mm-sized brown pargasitic
157 amphibole forms poikilitic clusters, enclosing lath-shaped plagioclase chadacrysts and uncommon granular
158 olivine, commonly associated with FeTi-oxides. Some plagioclase laths in the groundmass show
159 swallowtail-shapes indicating a rapid cooling during formation. The amphiboles in these dykelets are of
160 different types: Brown high-Ti pargasite and magnesiohastingsite (commonly poikilitic), green amphibole
161 (mainly magnesiohornblende) replacing brown pargasite, and local occurrences of a distinctive bluish-
162 green Cl-rich amphibole (WA04 & WA42: Cl-rich pargasite; WA32A: Cl-rich hastingsite). Microveins of
163 green amphibole and amphibole overgrowths (hornblende and actinolite) in these dykelets are common.
164 Sample WA42 displays chlorine-rich amphibole grains concentrated along two elongated and parallel
165 amphibole-rich, high-Cl zones, formed oblique to the margins of the dykelet (Fig. 3). The high-Ti pargasites
166 can have grain sizes of up to 150 µm whereas the other amphiboles have grain sizes between 10 to 50 µm.
167 Serpentinite minerals occur locally as secondary phases reflecting a later stage of hydrothermal alteration,
168 as seen from cross-cutting relationships between serpentine veins and the amphiboles.

169 Amphiboles measured in both veins and dykelets display similar compositional characteristics, with their
170 main differences being textural with the gabbroic dykelets generally having finer grained amphibole. The
171 analysis of different amphibole types including magmatic and hydrothermal amphibole shows a large
172 range of compositions (Table 1). High-Ti pargasites and magnesiohastingsites of magmatic origin have
173 high ⁷Al contents (1.5 to 1.9 a.p.f.u). A-site occupancy (Na + K) in this group of amphiboles ranges from
174 0.37 to 0.74, with total Na of 0.5 to 0.9 a.p.f.u. and K < 0.1 a.p.f.u. Ti contents are between 0.24 a.p.f.u.
175 and 0.5 a.p.f.u., and Cl contents range from 0.01 to 0.04 a.p.f.u., with no apparent correlation between Cl
176 content and ⁷Al, Fe, and K (Fig. 4). Formation temperatures calculated for these amphiboles using Ti-in-
amphibole thermometry (Ernst and Liu, 1998) range from 850 to 1030 °C. This thermometer is semi-quantitative, i.e. temperatures given here are only an approximation. The compositional heterogeneities between amphiboles indicate conditions of chemical disequilibrium, and thus the use of quantitative thermometers was not successful.

Pargasite, magnesiohastingsite, and edenite occurring in the gabbroic dykelets and veins are light brown to green. This group of amphiboles has $^{w}$Al contents ranging from 0.7 to 1.6 a.p.f.u. A-site occupancy (Na + K) ranges from 0.12 to 0.61, with total Na of 0.26 to 0.7 a.p.f.u. and K < 0.1 a.p.f.u. These amphiboles have Ti contents between 0.07 and 0.28 a.p.f.u. Cl concentrations range from 0 to 0.25 a.p.f.u. There's an overall trend of increasing $^{w}$Al, Fe$^{2+}$ and K with increasing Cl from a.p.f.u. 0.05 onwards. Below this Cl concentration there is no correlation between Cl content and $^{w}$Al, Fe$^{2+}$ and K (Fig. 4). Temperatures calculated for these amphiboles range from 650 °C to 850 °C (Ti in amphibole thermometer) corresponding to the transition between magmatic and amphibolite facies metamorphic conditions.

Blue-green amphiboles have high concentrations of Cl (contents above 0.25 a.p.f.u.) and occur as rims around other amphibole grains or as fine-grained crystals. They are low in Si and high in $^{w}$Al compared to other amphiboles with lower Cl contents. Tetrahedral Al contents range from 1.2 to 2.3 a.p.f.u. They are mainly classified as Cl-bearing pargasite, Cl-bearing magnesiohastingsite, Cl-bearing hastingsite and Cl-bearing ferropargasite (Fig. X). A-site occupancy (Na + K) in the measured amphiboles ranges from 0.17 to 1 a.p.f.u., with a Na contents between 0.16 and 0.85 a.p.f.u., and K contents between 0.01 and 0.24 a.p.f.u. Ti contents are below 0.16 a.p.f.u. Cl contents in this group of amphiboles ranges from 0.25 up to 1.5 a.p.f.u. Cl. There is an overall trend of increasing $^{w}$Al, Fe$^{2+}$ and K with increasing Cl. Formation temperatures for these amphiboles, calculated using Ti in amphibole thermometry, range from 484 to 765 °C (Ernst and Liu, 1998). The highest Cl contents (>1 a.p.f.u. Cl) are measured in amphibole grains formed at temperatures below 650 °C (see Table 1), at amphibolite to greenschist facies conditions.
Green actinolite, tremolite and magnesiohornblende form overgrowths over pargasite and magnesiohastingsite. These overgrowths are high in Si and low in wAl compared to other amphiboles described in this section. Tetrahedral Al contents range from 0 to 1.6 a.p.f.u. Total Na ranges from 0 to 0.7 a.p.f.u., with K < 0.04 a.p.f.u. Ti contents are found below 0.11 a.p.f.u. Maximum Cl content is of 0.25 a.p.f.u. There is an overall trend of increasing wAl, Fe<sup>2+</sup> and K with increasing Cl from a.p.f.u. 0.05 onwards. Below this value, there is no apparent trend (Fig. 4). Formation temperatures for these amphiboles are estimated between 484 °C and 646 °C from Ti in amphibole thermometry (Ernst and Liu, 1998), indicating hydrothermal alteration at amphibolite to greenschist facies conditions.

Fig. 5 shows a typical situation of amphibole grain, displaying different zones of amphiboles formed under different conditions, with a compositional profile through a zoned amphibole grain from a hydrothermal vein hosted in layered gabbro (sample WA02). This profile shows (A) a Cl-hastingsite zone (Cl contents up to 0.9 a.p.f.u.) with low silica content and high aluminum, as well as noticeably high total Fe and low Ti, corresponding to metamorphic conditions (amphibolite to greenschist facies); (B) a low-temperature actinolite zone (greenschist facies); (C) a magnesiohornblende zone with intermediate Cl contents (0.06 to 0.25 a.p.f.u.) with higher Si content and poor in Ti (amphibolites facies); and (D) a zone of brown high-Ti pargasite formed at.

216

## 3.4 EBSD analyses

To determine whether Cl-rich and Cl-poor amphiboles have the same orientation, we obtained EBSD maps of areas containing amphiboles with different compositions, and compared the misorientations in a chosen area with the Cl-concentration map (EDS) of the same area (Fig. 6). Misorientations observed in zoned amphibole in relation to a point selected in a Cl-rich area (>0.7 a.p.f.u. Cl) are of 1 to 2° (Fig. 6,
222 Mis2Mean map). Area 1 in Fig. 6D shows a misorientation boundary line with the same shape as the
223 boundary between different domains of high and low Cl concentrations (Fig. 6C). This is also the case of
224 the area circled in figures 6A and B, where changes in Cl content coincide with misorientations.

225 Point 2 in Fig. 6D shows a misorientation boundary line that coincides with a change in Cl content in
226 Fig. 6C. In contrast, Point 3 in Fig. 6D shows only a gradual change in misorientation, but a more
227 pronounced change in Cl content in Fig. 6C. Points 4 and 5 highlight areas where there is no relationship
228 between the misorientation map (Fig. 6D) and distribution of Cl concentrations (Fig. 6C).

229

230

231 **3.5 Isotopic data**

232 **3.5.1 Sr isotopes**

233 ⁹Sr-⁹Sr radiogenic isotope analyses were carried out on amphiboles from both gabbroic dykelets and dark
234 veins. We divided our samples into three groups: dark brown amphiboles (high-Ti pargasite and
235 magnesiohastingsite) formed at magmatic temperatures >850 °C, green and blue-green amphiboles
236 (magnesiohornblende and actinolite, and Cl-rich ferropargasite and hastingsite) formed at subsolidus
237 temperatures between 550 and 750 °C, and whole rock samples (dark vein and gabbroic dykelet samples)
238 (Fig. 7, Table 2).

239 The dark brown amphiboles contain 60 to 150 ppm Sr and show ⁹Sr-⁹Sr values between 0.7031 and 0.7035
240 (Table 2), similar to the green and blue-green amphiboles in the second group (Sr between 80 to 105 ppm;
241 ⁹Sr-⁹Sr = 0.7032 and 0.7036). Whole vein and dykelet analyses yield ⁹Sr-⁹Sr ratios between 0.7031 and
242 0.7039 (Sr concentration from 111 to 160 ppm).
The *Sr/*Sr ratios of all the amphiboles measured are only slightly elevated compared to the range of fresh Oman rocks (*Sr*/*Sr* ~0.7030; McCulloch et al., 1981) and there are no significant differences between the different amphibole groups. The slight increase in *Sr/*Sr indicates only a minor exchange with seawater-derived Sr even for amphiboles with high Cl concentrations, suggesting fluid-rock exchange with seawater-derived hydrothermal fluids that had already reacted with a large amount of rock at low fluid/rock ratios (Fig. 7).

249

250 **3.5.2 O isotopes**

251 δ¹⁰O values obtained for amphiboles fall in a narrow range between 4.1 and 5.6 ‰ except for one sample
252 with value of 7.3 ‰ (Fig. 7, Table 2). Values obtained from veins fall between 4.1 and 4.7, and values
253 from gabbroic dykelets range from 4.9 to 7.3. Using the amphibole formation temperatures estimated
254 from semi-quantitative Ti-in-amphibole thermometry as starting point, we calculated the oxygen isotopic
255 composition of the hydrothermal fluid in equilibrium with these minerals using the amphibole-water
256 fractionation equation of Zheng (1993; Table 2). If the formation temperatures are reasonable and the
257 amphiboles have retained their original oxygen isotopic compositions and not been subject to low
258 temperature isotopic exchange, they are in equilibrium with hydrothermal fluids with high δ¹⁰O between
259 6.9 and 7.5 ‰. Given the semi-quantitative nature of the thermometer used, and the heterogeneity of the
260 amphiboles used for isotope measurements, these values are only an approximation. This is very different
261 from Late Cretaceous seawater (δ¹⁰O = -1 ‰; Gregory and Taylor, 1981) or the composition of ancient
262 black smoker fluids that are typically 1 to <4 ‰ higher than co-existing seawater (see Shanks et al.,
263 1995). This suggests that oxygen isotopic exchange occurred in a very rock dominated system at low
264 water/rock ratios. Alternatively, if the fluids are similar to Late Cretaceous seawater or a black smoker-
265 like fluid derived from seawater, the analysed amphibole δ¹⁰O suggests formation temperatures between
266 90 and 130 °C (Appendix A1).

267

268 Discussion: episodes of fluid/rock interaction

269 In addition to textural observations (poikilitic amphibole) and semi-quantitative geothermometry,
270 magmatically formed amphiboles and hydrothermal amphiboles formed at subsolidus temperatures from
271 hydrothermal fluids can be distinguished by their Ti, Na, K, Si contents and Mg number, with
272 hydrothermal amphiboles exhibiting higher Si contents and Mg numbers but lower Ti, Na and K contents
273 (Gillis et al., 2003). The occurrence of different amphibole types from supra- and subsolidus reactions in
274 an alteration parageneses in gabbros is also documented in other studies of oceanic (e.g. Vanko, 1986;
275 Silantyev et al., 2008; Coogan et al., 2001) and continental rocks (Liu et al., 2009; Enami et al., 1999;
276 McCormick & McDonald, 1999), and from experimental investigations (e.g., Currin et al., this issue;
277 suprasolidus experiments: Khodorevskaya & Aranovich, 2016; Koepke et al., 2004; 2007; Chan et al.,
278 2016). The passage of hydrothermal fluids through the crust in episodes of different temperatures and
279 salinities is recorded by the formation of different amphibole types, equilibrating at progressively lower
280 temperatures. This may indicate the progressive cooling of the lower oceanic crust by involvement of
281 hydrothermal fluids (Fig. 8).

282 Regarding microstructure, the subtle intragrain misorientations illustrated in Fig. 6 are not large enough to
283 account for different orientations at the time of mineral growth. Thus, the Cl-rich ferropargasite and
284 hastingsite may be epitactic with the other amphibole compositional zones. This may suggest that low-Ti
285 amphiboles formed from the interaction of pre-existing magmatic amphibole with an aqueous Cl-rich
fluid. Small orientation changes may be related to structural changes in the amphibole lattice due to Cl incorporation, or may be unrelated to mineral composition – possibly due to deformation of unknown origin. In some amphibole grains the misorientation boundary (see Fig. 6C) is not exactly coincident with the change in Cl content. This may be due to the fact that only higher Cl contents cause significant changes in the amphibole lattice (as present in the right hand side of Fig. 6). Consequently, the misorientation of the amphibole structure due to Cl incorporation is only visible at higher Cl contents.

### a) Suprasolidus:

Considering the fine-grained granular texture of the aforementioned gabbroic dykelets, and considering previous experimental work done on the hydrous partial melting of gabbro (Koepke et al., 2004; 2005; Wolff et al., 2013), we consider that these dykelets were formed as products of hydrous partial melting of the host gabbro. The crystallization of brown high-Ti pargasite with poikilitic textures also imply that the magma they crystallised from was water-bearing (Fig. 8A), and their formation marks the onset of hydrothermal fluid-rock interactions at very high subsolidus to suprasolidus temperatures (850 – 1020 °C) in the described lithologies. *Sr/*Sr values only slightly above 0.703 indicate Sr-isotopic exchange with a very rock dominated fluid with only a hint of seawater-derived Sr.

### b) Subsolidus:

In contrast, the green amphibole zones appear to have formed from high temperature fluid-rock reactions occurring at upper amphibolite facies conditions (650 – 850 °C. In some samples, these dark veins retain relict amphibole cores that formed during the magmatic precursor stage (Wolff, 2014).
307 Epitactic growth of Ti-poor and Cl-rich amphiboles (Fig. 8 B to D) suggests the occurrence of sub-solidus
308 fluid-rock interactions at amphibolite facies.

309 The occurrence of Cl-rich amphibole domains (8D) record the reaction between high Cl hydrothermal
310 fluids or brines and pre-existing amphibole, albeit at low water/rock ratios. Finally, further fluid-rock
311 exchange at lower amphibolite to greenschist facies temperatures is deduced from the presence of
312 actinolite overgrowths (8E). This sequence of processes identified at the mm-scale illustrates the
313 transition from the suprasolidus regime to the subsolidus regime in the presence of hydrothermal fluids
314 percolating through the lower oceanic crust.

315 Correlations between Cl concentrations in amphibole and wAl, Fe+ and K contents are observed in Cl-rich
316 amphiboles (up to 1.5 a.p.f.u. Cl, Fig. 4E), due to constraints in the amphibole structure and the larger
317 size of the Cl anion with respect to OH (Oberti et al., 1993). For natural amphiboles from Wadi Wariyah,
318 this correlation starts to be apparent for Cl contents greater than 0.1 a.p.f.u. In amphiboles containing less
319 than 0.1 a.p.f.u. Cl, the contents of wAl, Fe+ and K do not seem to be influenced by Cl incorporation. This
320 also holds true for amphiboles obtained experimentally (Currin et al. (this issue)) that do not show any
321 correlation between the aforementioned cations and Cl incorporation, since these amphiboles contain <
322 0.1 a.p.f.u. Cl (Fig. 9). This lack of correlation between Cl and certain cations (e.g. Al, Fe, K) when small
323 amounts of Cl are measured in the amphibole has also been reported in other natural Cl-bearing
324 amphiboles (Kullerud, 1996; Enami, 1999), as well as from experimental studies (Chan et al., 2016).

325 The occurrence of Cl-rich amphiboles is linked to the participation of brines in fluid/rock interaction
326 (Manning and Aranovich, 2014). This has also been shown experimentally. Experiments performed with
327 an NaCl with the aim of producing Cl-rich amphibole require supersaturated brines to succeed in
328 producing amphiboles with Cl contents greater than 0.1 a.p.f.u. Cl (Chan et al., 2016). This may suggest
329 that amphiboles formed in nature require reaction with a hydrothermal fluid that is saturated in Cl in order
330 to reach high Cl contents, and this may be applied to the fluid that equilibrated with the rocks outcropping
331 in Wadi Wariyah. Decrease of fluid/rock ratio is caused by the uptake of OH- of the host rocks to form
332 amphibole (amphibolitisation) and other hydrous minerals, as well as other processes elsewhere in the
333 crust prior to reaching lower crustal gabbros (e.g. previous serpentinisation or greenstone alteration,
334 Aranovich et al., 2010), with the subsequent concentration of Cl in the seawater-derived hydrothermal
335 fluid, leading to the formation of brines that react with the host rock to form Cl rich amphiboles.

336 Phase separation in saline fluids occurs below the critical curve that is found at different pressures for a
337 given temperature – e.g. around 1.3 kbar for 750 °C and 20 wt. % NaCl (Driesner & Heinrich, 2007). By
338 crossing this curve towards lower pressures, a fluid will undergo phase separation into a NaCl-free vapour
339 and a NaCl-concentrated fluid (i.e. brine). Pressures in the lower oceanic crust (ca. 6 km deep in the crust)
340 are estimated at 2 kbar (Iturrino et al., 2002), and may vary due to exhumation and faulting of the oceanic
341 crust, affecting fluid circulation and changing fluid behaviour, locally resulting in phase separation.

342 Large measured Cl quantities in amphibole from Wadi Wariyah (< 1.5 a.p.f.u.) confirms reaction with a
343 seawater-derived fluid, and the isotopic signature of the amphibole (being relatively close to fresh Oman
344 gabbro) suggests the environment was mostly rock-dominated and water/rock ratios were low during
345 equilibration between fluid and amphibole. This is consistent with the findings of Bickle & Teagle (1992)
346 for the Troodos ophiolite, and Gregory and Taylor (1981) for the Samail ophiolite, who also report on
347 fluid-rock exchange at low water/rock ratios near the base of the crust, and Rospabé et al. (2017) who
348 inferred the involvement of hydrothermal fluids in the mantle-crust transition in the Samail ophiolite.

349

350
351

**4. Concluding remarks**

352

353

354

355

356

357

358

359

360

361

362

363

364

365

366

367

Amphiboles found in samples of hydrothermal veins and gabbroic dykelets from Wadi Wariyah, Samail

Ophiolite, record a complex formation history involving fluid/rock interaction. These amphiboles are
hosted within a hydrothermal microcrack system (Nicolas et al., 2003) and show a wide range of
compositions from poikilitic high-Ti pargasite, via replacive magnesiohastingsite and

magnesiohornblende, to Cl-rich hastingsite and Cl-poor actinolite, suggesting different episodes of
hydrothermal activity at different temperatures. Variations in Cl content (from 0.02 to 1.5 a.p.f.u. Cl) and

differences in Si, ⁴Al, Mg#, Na and K in amphibole, indicate equilibration of the host rock with fluids of
different salinities at different points in time. In addition, EBSD analyses show compositional differences

within single amphibole grains, confirming alteration of pre-existing magmatic amphibole by interaction
with hydrothermal fluids. Near MORB-like ⁴²Sr/⁴⁰Sr ratios (0.7031 to 0.7039) and high δ¹⁸O isotopic

compositions (4.1-5.6) suggest the influence of a high-temperature seawater-derived fluid from magmatic
(suprasolidus) to hydrothermal (subsolidus) temperatures, albeit in rock-dominated low fluid-rock ratio

conditions, consistent with the presence of a highly saline fluid or brine. Our findings provide new

petrological data on high temperature hydrothermal circulation present in the deep oceanic crust near fast-

spreading ridges, and suggest an inhomogeneous distribution of hydrothermal fluid pathways in the lower
oceanic crust, with stepwise fluid infiltration, and variations in fluid temperature and salinity.

368

**Acknowledgements**

369

370

371

372

The research leading to these results has received funding from the People Programme (Marie Curie

Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant
agreement n°608001. Many thanks to Julian Feige (Leibniz Universität Hannover) for thin section

preparation, to Fabrice Barou (Géosciences Montpellier, CNRS) for help and technical support during
373 EBSD analyses, to James Andy Milton (National Oceanography Centre Southampton) for assistance with
374 the LA-ICP-MS, Matt Cooper (National Oceanography Centre Southampton) for assistance in the clean
375 lab and operating the TIMS, and Aurélien Beaumais (National Oceanography Centre Southampton) for
376 support in the clean lab.

377
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515

516

517 **Figure captions**
**Figure 1.** Model showing the magma supply system at fast spreading MOR and paths of hydrothermal circulations (blue arrows) involving also the deep crust modified after Zhang et al. (2014). See Zhang et al. (2014) for details.

**Figure 2.** Map of sampling location in Wadi Wariyah (star), in Wadi Tayin Massif, Samail Ophiolite, Sultanate of Oman, modified after Nicolas et al. (2000).

**Figure 3.** **A:** BSE image showing the area marked in red in thin section C (sample WA02) showing a zoned amphibole. This is the same area selected for a compositional profile (Fig. 5). 1: plagioclase, 2: high-Ti pargasite, 3: magnesiohornblende to actinolite, 4: Cl-rich ferropargasite, 5: secondary calcite, 6: Fe-Ti oxides.
**B:** BSE image showing the area marked in blue in thin section D (sample WA42). 1: pargasite, 2: plagioclase, 3: Cl-pargasite, 4: Fe-Ti oxides.
**C:** thin section of layered olivine gabbro crosscut by an amphibole-rich hydrothermal vein (sample WA02).
**D:** thin section of layered olivine gabbro crosscut by a fine-grained amphibole-rich gabbroic dykelet that is, in turn, crosscut by two oblique Cl-rich pargasite zones (sample WA42).

**Figure 4.** Profile through an amphibole grain in sample WA02, ranging from magmatic high-Ti pargasite (D), through magnesiohornblende (C) and actinolite (B) to Cl-rich ferropargasite (A). Dashed and dotted black lines showing changes in amphibole zoning.

**Figure 5.** Compositional diagrams for measured amphiboles. **A:** (Na+K) vs. Al; **B:** Si a.p.f.u. vs. Mg# (Mg/(Mg+Fe+Al)), with (Na+K)>0.5, Ti<0.5; **C:** Si a.p.f.u. vs. Mg# (Mg/(Mg+Fe+Al)), with (Na+K)<0.5, Ca<0.5; **D:** Al₂O₃ vs. TiO₂; **E:** Cl a.p.f.u. vs. Al+Fe+K. Abbreviations after Whitney et al. (2010): Ts, tschermakite; Mhs, magnesiohastingsite; Prg, pargasite; Mhb, magnesiohornblende; Tr, tremolite; Act, actinolite; Fed, ferro-edenite; Fprg, ferropargasite; Hst, hastingsite; Fac, ferro-actinolite; Fts, ferrotschermakite; Fhb, ferrohornblende.
540 **Figure 8.** Chlorine EDS maps (A & C) and EBSD misorientation maps of areas in sample WA02.
541 Chlorine maps highlight the areas most enriched in Cl (A: purple, C: bright green). In contrast with EBSD
542 misorientation maps (B & D) we can figure out if there are differences in orientation, grain boundaries or
543 dislocations between Cl-rich areas and Cl-poor areas of the amphibole.

544 **Figure 9.** Initial ⁹⁰Sr/⁹⁸Sr isotopic ratios plotted against δ⁹⁰O for amphibole separates from high temperature
545 veins and gabbroic dykelets of the Wadi Wariyah, Sultanate of Oman, as well as whole vein and dyke
546 measurements. Values for Cretaceous seawater at 95 Ma are from McArthur et al. (2001), and MORB
547 mantle, from Lanphere et al. (1981). Blue line: mixing curve fresh Oman gabbro to seawater. Results are
548 compared to measurements done by Wolff (2014) and Bosch et al. (2004).

549 **Figure 10.** Sketch showing the evolution of amphibole in gabbroic dykelets and dark veins in Wadi
550 Wariyah. A: hydrous partial melting, formation of poikilitic magmatic amphibole – pargasite and
551 magnesiohastingsite – in gabbroic dykelets & dark veins – amphibole is part of a dykelet that cross-cuts
552 the host rock; red: represents the fine-grained matrix of the dykelets consisting of plagioclase, olivine,
553 clinopyroxene, orthopyroxene, Fe-Ti oxides; Wolff, 2014. B: Flushing of dykelet with the infiltration of a
554 high-T hydrothermal fluid, heat removal and metamorphism of vein and dykelet minerals. Formation of
555 magnesiohastingsite containing Cl > 0.2 wt. %. C: Flushing of dykelet with the infiltration of a
556 hydrothermal seawater-derived fluid, cooling & progressive concentration of Cl & lower water/rock
557 ratios. Formation of magnesiohornblende containing Cl < 1 wt. %. D: infiltration of highly concentrated
558 saline fluid. formation of hastingsite and ferropargasite containing Cl > 1 wt. % up to 5.4 wt. %. E: new
559 episode of fluid infiltration at lower temperature – Cl-poor fluid – and formation of actinolite
560 overgrowths.
**Figure 11.** Composition of amphiboles from Wadi Wariyah plotted against experimental results from Currin et al. (this issue). Abbreviations after Whitney et al. (2010): Ts, tschermakite; Mhs, magnesiohastingsite; Prg, pargasite; Mhb, magnesiohornblende; Tr, tremolite; Act, actinolite.

564

## 565 Tables

**Table 1.** Major elemental compositional data for amphiboles sampled in Wadi Wariyah, measured with EPMA.

**Table 2.** Sr and O isotopic composition of all amphibole and whole vein samples analysed from Wadi Wariyah.

## 570 Appendix

**Table. A1.** Fractionation factor between amphibole and water (Δ<sup>δ<sub>18</sub>O</sup> amphibole-water) was calculated for each of the amphibole samples analyzed for oxygen isotopes (magmatic and hydrothermal amphiboles), using the fractionation equation from the calibration of Zheng (1993) (A= 3.89, B= -8.56, C= 2.43) for oxygen isotope fractionation between hornblende and water, and inserting temperatures calculated from Ti-in-amphibole thermometry. The δ<sup>18</sup>O value for water was obtained by subtracting measured δ<sup>18</sup>O values for amphibole to calculated values for the fractionation factor (Δ<sup>δ<sub>18</sub>O</sup> amphibole-water). δ<sup>18</sup>O values calculated for water range between 6.9 and 7.5 ‰. The result of this calculation suggests that, assuming temperatures from thermometry to be true, the fluid that equilibrated with the analysed amphiboles was
579 very different from Cretaceous seawater (values of -1 ‰; Gregory & Taylor, 1981) or typical

580 hydrothermal fluids derived from Cretaceous seawater (0 to 3 ‰; e.g. Shanks et al., 1995).

581