# Mn-carbonate deposition in a seafloor hydrothermal system (CLAM field, Iheya Ridge, Okinawa Trough): Insights from mineralogy, geochemistry and isotope studies

V.M. Dekov, K. Yasuda, G. Kamenov, B. Guéguen, A. Kano, T. Yoshimura, T. Yamanaka, L. Bindi, T. Okumura, D. Asael, et al.

► To cite this version:

V.M. Dekov, K. Yasuda, G. Kamenov, B. Guéguen, A. Kano, et al. Mn-carbonate deposition in a seafloor hydrothermal system (CLAM field, Iheya Ridge, Okinawa Trough): Insights from mineralogy, geochemistry and isotope studies. Marine Geology, 2023, 460, pp.107055. 10.1016/j.margeo.2023.107055. hal-05273119

HAL Id: hal-05273119

https://hal.univ-brest.fr/hal-05273119v1

Submitted on 22 Sep 2025

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Distributed under a Creative Commons Attribution 4.0 International License
1 Mn-carbonate deposition in a seafloor hydrothermal system

2 (CLAM field, Iheya Ridge, Okinawa Trough): Insights from

3 mineralogy, geochemistry and isotope studies

4

5 V.M. Dekov<sup>a,*</sup>, K. Yasuda<sup>a</sup>, G. Kamenov<sup>b</sup>, K. Yasukawa<sup>c,d</sup>, B. Guéguen<sup>e,f</sup>, A.

6 Kano<sup>g</sup>, T. Yoshimura<sup>h</sup>, T. Yamanaka<sup>a</sup>, L. Bindi<sup>i</sup>, T. Okumura<sup>j</sup>, D. Asael<sup>k</sup>, D.

7 Araoka<sup>l</sup>, Y. Kato<sup>c,d,m,n</sup>

8

9 <sup>a</sup> Department of Ocean Sciences, Tokyo University of Marine Science and Technology, 4-5-7

10 Konan, Minato-ku, Tokyo 108-8477, Japan

11 <sup>b</sup> Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville,

12 FL 32611, USA

13 <sup>c</sup> Frontier Research Center for Energy and Resources, School of Engineering, The University of

14 Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

15 <sup>d</sup> Department of Systems Innovation, School of Engineering, The University of Tokyo, 7-3-1

16 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

17 <sup>e</sup> CNRS, Univ Brest, UMR 6538 Laboratoire Géosciences Océan, F-29280 Plouzané, France

18 <sup>f</sup> CNRS, Univ Brest, UMS 3113, F-29280 Plouzané, France

19 <sup>g</sup> Department of Earth and Planetary Science, The University of Tokyo, Hongo 7-3-1, Tokyo 113-

20 0033, Japan

21 <sup>h</sup> Biogeochemistry Program, Research Institute for Marine Resources Utilization, Japan Agency

22 for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka,

23 Kanagawa 237-0061, Japan

24 <sup>i</sup> Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via Giorgio La Pira 4, I-

25 50121, Firenze, Italy

26 <sup>j</sup> Center for Advanced Marine Core Research, Kochi University, 200 Monobe-Otsu, Nankoku,

27 Kochi, 783-8502, Japan

<footnote>* Corresponding author.
E-mail address: vdekov0@kaiyodai.ac.jp (V.M. Dekov)</footnote>
28 k Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA

29 l Geological Survey of Japan (GSJ), National Institute of Advanced Industrial Science and Technology (AIST), Central-7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan

31 m Ocean Resources Research Center for Next Generation, Chiba Institute of Technology, 2-17-1
Tsudanuma, Narashino, Chiba 275-0016, Japan

32 n Submarine Resources Research Center, Research Institute for Marine Resources Utilization,
Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho,
Yokosuka, Kanagawa 237-0061, Japan

36

37

38 A B S T R A C T

39

40 A seafloor hydrothermal system located at the Iheya Ridge (Okinawa Trough), named CLAM,
deposits Mn-carbonate chimneys that have no analogue found so far on the seafloor. The
41 chimneys are composed of Mn-calcite and Ca-rhodochrosite. The crystallographic differences
42 between these carbonates appear to control the rare earth elements (REE) partitioning between
43 them that results in enrichment of the Ca-rhodochrosite in middle and heavy REE, and
44 enrichment of the Mn-calcite in light REE. Chemistry of the CLAM hydrothermal fluids
45 suggests: (1) low water/rock ratio of the hydrothermal system; (2) phase separation and
46 dominance of low-chlorinity vapor phase; (3) sub-seafloor formation of Na-rich alteration
47 minerals during fluid/rock reactions; (4) removal of some elements from the seawater to the host
48 rocks during the seawater/rock interaction; (5) high Mn/Ca ratio of the basement rocks is
49 responsible for the high Mn concentration in the hydrothermal fluids. C-O-isotope compositions
50 of the CLAM Mn-carbonates suggest they precipitated through binary mixing of end-member
51 hydrothermal fluid and seawater accompanied by progressive degassing and cooling of the fluid.
52 Mn-calcite precipitated from almost pure end-member hydrothermal fluid, whereas Ca-
53 rhodochrosite precipitated from seawater-dominated vent fluid. Mg-isotope fractionation during
54 Mn-carbonate precipitation is assumed to depend on carbonate growth conditions and resulting
55 carbonate mineralogy. S-isotope composition of the CLAM Mn-carbonates suggests that the Ca-
56 rhodochrosite precipitated in oxic conditions through rapid mixing of hydrothermal fluid and
57 seawater, whereas the Mn-calcite precipitated in reduced conditions (thermochemical or
58
microbial sulfate reduction) through slow mixing of hydrothermal fluid and seawater. Sr-isotope composition of the CLAM hydrothermal fluids is close to that of Okinawa Trough deep seawater. In contrast, Sr-isotopes in the CLAM Mn-carbonates are more variable, indicating that Sr was derived from seawater, local lavas and sediments. Nd-isotope composition of the Mn-carbonates indicates that Nd was derived from the local lavas and sediments. Pb in the majority of the CLAM Mn-carbonates is of sedimentary origin (Pb isotope data), but involvement of anthropogenic Pb in the hydrothermal system is inferred for some Mn-calcite samples. Stability phase diagram modeling coupled with C-O-S-Sr-Nd-Pb isotopes; Mn-carbonates; Okinawa Trough; Seafloor hydrothermal activity

Keywords: CLAM hydrothermal field; C-O-Mg-S-Sr-Nd-Pb isotopes; Mn-carbonates; Okinawa Trough; Seafloor hydrothermal activity

# 1. Introduction

Carbonate chimney- and mound-like structures form as a result of three different processes at three different seafloor geologic settings.

A common occurrence of carbonate chimneys and mounds at the seafloor is related to the burial of dead organic matter in the sediments and its subsequent degradation mediated by a consortium of microorganisms under oxic to anoxic conditions (Díaz-del-Río et al., 2003; De Boever et al., 2006; Chen et al., 2007; Takeuchi et al., 2007; Nyman et al., 2010; Bayon et al., 2013). In the zone of anoxia ([O₂] = 0) within the sediment the organic matter degradation results in methane (CH₄) formation. The released CH₄ encounters downward diffusing seawater sulfate (SO₄²⁻), it is oxidized anaerobically by the microorganisms that use sulfate as an oxidant, and produces sulfide and carbonate alkalinity according to the reaction:

$$CH_4 + SO_4^{2-} \rightarrow HCO_3^- + HS^- + H_2O$$

A significant portion of the bicarbonate (HCO₃⁻) produced through anaerobic oxidation of CH₄ precipitates as carbonate (aragonite or calcite) at (or below) the seafloor:
$$HCO_3^- + Ca^{2+} \rightarrow CaCO_3 + H^+$$

Thus, the continental margins are zones of high CH₄ flux and carbonate precipitation due to the tremendous amount of dead organic matter accumulation. Morphologically, the precipitated carbonates form chimney- and mound-like structures that cluster at the seafloor of the continental margins (Díaz-del-Río et al., 2003; De Boever et al., 2006; Chen et al., 2007; Takeuchi et al., 2007; Nyman et al., 2010; Bayon et al., 2013).

Carbonate chimneys and mounds form also as a result of serpentinization processes at the low-temperature (T<100°C) seafloor hydrothermal systems operating at the slow-spreading mid-ocean ridges (MOR) and oceanic fore-arcs (Launay and Fontes, 1985; Fryer et al., 1999; Kelley et al., 2001; Okumura et al., 2016). At these settings crustal blocks bounded by faults lift up and expose the lower crust and upper mantle rocks to shallow depths. Seawater reacts with the basic and ultrabasic rocks of the lower crust – upper mantle and alters them. Olivine and pyroxene from these rocks alter (hydrate) to serpentine and hydrogen (H₂) is released as a by-product. Subsequent reactions lead to precipitation of carbonates:

$$4H_2 + CO_2 \rightarrow CH_4 + 2H_2O$$

$$CH_4 + SO_4^{2-} \rightarrow HCO_3^- + HS^- + H_2O$$

$$HCO_3^- + Ca^{2+} \rightarrow CaCO_3 + H^+$$

Thus, tall carbonate chimneys and large carbonate mounds form at certain sites at the MOR and fore-arcs (Launay and Fontes, 1985; Fryer et al., 1999; Kelley et al., 2001; Okumura et al., 2016).

High-temperature (T>200°C) hydrothermal fluids, which circulate through spreading centers whose valleys are covered by sediment (sediment-hosted hydrothermal systems) react with both the basement rocks and the sediment. They dissolve carbonates and organic matter from the sediment and, as a result, become alkaline (pH > 7). In an alkaline environment the dissolved HCO₃⁻ reacts with the dissolved Ca²⁺ and forms CaCO₃ (calcite or aragonite):

$$HCO_3^- + Ca^{2+} \rightarrow CaCO_3 + H^+$$

This third type of seafloor carbonate chimneys and mounds is known from only three sites at the seafloor: Guaymas Basin, Gulf of California (Koski et al., 1985; Peter and Scott, 1991), southern Pescadero Basin, Gulf of California (Paduan et al., 2018) and Iheya Ridge, Okinawa Trough (Tanaka et al., 1989; Sakai et al., 1990a; Izawa et al., 1991). Whereas the hydrothermal carbonate deposits from the Guaymas and southern Pescadero Basins are composed of Ca-
carbonates (Koski et al., 1985; Peter and Scott, 1991; Paduan et al., 2018), the carbonate chimneys and mounds at the Iheya Ridge are composed of Mn-carbonates (Tanaka et al., 1989; Izawa et al., 1991).

The only known seafloor hydrothermal Mn-carbonate deposits sampled at the Iheya Ridge were studied at a little detail after their recovery (Tanaka et al., 1989; Izawa et al., 1991). The hydrothermal fluids discharged from these Mn-carbonate chimneys were investigated in separate works (Sakai et al., 1990a; Gamo et al., 1991). After these initial studies no other seafloor hydrothermal Mn-carbonate deposit has been found, neither have the Iheya Ridge hydrothermal Mn-carbonates been studied in further detail. Knowledge on hydrothermal precipitation of Mn-carbonates is important from economic geology point of view. Mn-carbonate deposits are known to be metasomatic (Hariya and Hasegawa, 1979), metamorphic (Nakagawa et al., 2009), diagenetic (Kuleshov, 2013), sedimentary (Du et al., 2020) and hydrothermal (Osaki, 1973; Chow et al., 1996; Nitta and Inoue, 2011). However, there is no work that describes recent precipitation of Mn-carbonates and their parent fluids.

Challenged by the limited knowledge on the seafloor hydrothermal Mn-carbonate deposition, we undertook a mineralogical-geochemical study of a set of Mn-carbonate chimney samples and paired vent fluids from the Iheya Ridge with the aim to understand the conditions of Mn-carbonate precipitation in a seafloor hydrothermal system.

139

## 2. Geologic setting

141

Okinawa Trough is located behind the Ryukyu Trench-Arc system (Fig. 1 A) along the eastern margin of the Eurasian tectonic plate and extends from the Japanese mainland to Taiwan (Ishibashi et al., 2015). It is an intra-continental back-arc basin at initial stage of rifting (Kimura et al., 1986) that is characterized by slow rate of extension (3.7 ± 0.06 cm/year; Kotake, 2000) and lack of oceanic lithosphere (Nagumo et al., 1986). The extensive intra-trough volcanism in the middle Okinawa Trough (Fig. 1 A) is regarded as a result of volcanic arc migration towards the back-arc basin (Sibuet et al., 1987). The Iheya Ridge is a northeast-southwest trending back-arc axial rift in the middle Okinawa Trough (Fig. 1 B) characterized by bimodal volcanism (Yamasaki, 2018; Zhang et al., 2020), anomalously high heat flow (Yamano et al., 1986) and active hydrothermal fields (Ishibashi et al., 2015). Frequent magma mixing in multi-chamber

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151
storage beneath the Iheya Ridge is supposed to play an essential role in the origins of diverse volcanism (basaltic, andesitic, rhyolitic) in the Iheya area (Zhang et al., 2020).

In order to explore the hydrothermal activity in the middle Okinawa Trough deep-tow TV camera, side-scan sonar, and dive surveys with the manned submersible *Shinkai 2000* (R/V *Natsushima*) were performed at the Iheya Ridge (Fig. 1 A,B) in 1988 and 1989. These investigations discovered an active hydrothermal field (Tanaka et al., 1989) named CLAM in a later study (Sakai et al., 1990a). The studies found that the Iheya Ridge was covered by thin sediment, which becomes thicker north and south of the ridge (Tanaka et al., 1989). Integrated Ocean Drilling Program (IODP) Expedition 331 at the Iheya North hydrothermal system (north from the Iheya Ridge) drilled interbedded hemipelagic and strongly pumiceous volcaniclastic sediment, along with volcanogenic breccias that are variably hydrothermally altered and mineralized (Expedition 331 Scientists, 2011; Takai et al., 2012; Totsuka et al., 2019; Nozaki et al., 2021a).

The CLAM hydrothermal field (Fig. 1 B) is confined within a narrow flat zone on the northwest slope of the eastern part of the Iheya Ridge. Basaltic pillow lavas, little hills of breccia, and volcanic sands cemented with amorphous silica, carbonates, anhydrite and sulfides cover the hydrothermally active area (Tanaka et al., 1989). Mounds and chimneys populated by typical vent biota and emitting hydrothermal fluids ($T \le 220^{\circ}\text{C}$; Tanaka et al., 1989; Sakai et al., 1990a) are scattered within the field (Tanaka et al., 1989).

171

## 3. Samples

173

In our attempt to investigate the mineralogy and geochemistry of the CLAM hydrothermal deposits we selected three carbonate chimney samples (Table 1; Fig. 2 A-C) collected by the manipulator of the manned submersible *Shinkai 2000* (R/V *Natsushima*) from the CLAM hydrothermal site in 1988 and stored in the JAMSTEC sample repository. From macroscopically different layers of the samples (Fig. 2 A-C) we collected sub-samples (~2 g) using a mini drill device with diamond bit. The sub-samples were ground to fine powders in an agate mortar for further mineralogy and geochemistry analyses. Thin polished sections from the same samples were prepared for optical microscopy observations and electron microscope (EMP) analyses.
Six hydrothermal fluid samples [sampling described by Ishibashi et al. (1995)] were selected for geochemical investigations from a set of hydrothermal fluids collected by the manned submersible *Shinkai 2000* (R/V *Natsushima*) equipped with a 6-port revolver-type Ti water sampler (Sakai et al., 1990b; Tsunogai et al., 1994) from the CLAM active chimneys in 1989 (Table 2). The fluid samples were filtered and acidified (pH<2) immediately after recovery and stored for further analyses. The reader needs to have in mind that the possible mineral precipitation (dregs) in the sampler had not been considered (e.g., Trefry et al., 1994; Rouxel et al., 2008) at the time of sampling of these vent fluids. Neither had the formation of dregs been considered in the hydrothermal fluids [Von Damm (1990) and Klinkhammer et al. (1994) for Guaymas Basin fluids; Butterfield et al. (1994) for Middle Valley fluids; Von Damm et al. (2005) and Klinkhammer et al. (1994) for Escanaba Trough fluids; Gamo (1995) and Hongo et al. (2007) for Okinawa Trough fluids; Klinkhammer et al. (1994) for Mariana back-arc basin fluids] [excluding the fluids from Manus Basin; Reeves et al. (2011), Craddock et al. (2010)] whose chemistry we compared to that of the studied CLAM vent fluids. Nonetheless, the motivation to investigate the CLAM hydrothermal fluids was based on the notion that the activity of this unique hydrothermal system (with no analogue in the modern ocean) may cease and we may have no chance to study such a system for a long time.

For comparison, we used carbonate deposits from similar seafloor geological setting, hydrothermal systems at sedimented spreading centers: Guaymas Basin and southern Pescadero Basin. For southern Pescadero Basin hydrothermal carbonate deposits we used the published data for an active flange (D757-R1) from the Auka hydrothermal field (Paduan et al., 2018). Among the published data for the Guaymas Basin hydrothermal deposits (Koski et al., 1985; Peter and Scott, 1991) there is no data for samples composed predominantly of carbonate minerals. Therefore, we investigated one flange sample (T529-G4A B; latitude 27°00'28"N, longitude 111°24'36"W, water depth 1999 m) (Fig. 2 D) from the Guaymas Basin collected by the ROV *Tiburon* deployed from the R/V *Western Flyer* during a research cruise supported by the Monterey Bay Aquarium Research Institute (MBARI) in 2003. Sample T529-G4A B is a slabby flange fragment with slightly weathered flat surfaces collected from near the base of the Toad Stool active mound. We collected one sub-sample (~2 g) from a white layer in the flange (Fig. 2 D) using the mini drill device with diamond bit. The sub-sample was ground to fine powder in an
212 agate mortar for further mineralogy and geochemistry analyses. A thin polished section from the
213 same sample was prepared for optical microscopy observations and EMP analyses.

214

215 **4. Methods of investigation**
216

218

217 **4.1. Mineral identification and mineral chemistry studies**
218

219

Initially, the CLAM chimney and Guaymas flange samples were observed with a
220 stereomicroscope (Nikon SMZ745T) and pictures of the major minerals that constitute the
221 chimneys and flange were acquired in natural light.

The mineral composition and texture of the chimneys and flange were investigated with
222 optical microscope (thin polished sections observations in transmitted and reflected light; Nikon
223 ECLIPSE LV100N POL polarizing microscope).

Secondary electron images (SEI) and energy dispersive X-ray spectra (EDS) were obtained
225 on small (~1x1 cm) sub-samples from each sample using a Hitachi TM3030 Plus tabletop
226 scanning electron microscope (SEM) (V=15.0 kV, I=1850 mA, electron beam diameter of 2 µm),
227 after mounting of the sub-samples on aluminum stubs using carbon tape and coating them with
228 carbon (The University of Tokyo).

Bulk mineral composition of the selected sub-samples (finely powdered) (Fig. 2) was
230 determined by X-ray diffraction (XRD) analysis of random mounts using Rigaku Ultima IV X-
231 ray diffractometer with monochromatic Cu K<sub>α</sub> radiation with X-ray scans from 4 to 70 °2θ, with
232 0.05 °2θ step, and 4 s/step (Tokyo University of Marine Science and Technology). XRD patterns
233 were interpreted by using the MacDiff software. For measuring the unit-cell parameters of the
234 main minerals that compose the studied samples we selected macroscopically monomineralic
235 sub-samples from each sample and ground them to powders in an agate mortar. The powders
236 were analyzed by Bruker D8 Advance X-ray diffractometer working with monochromatic Cu K<sub>α</sub>
237 radiation with X-ray scans from 5 to 70 °2θ, with 0.010 °2θ step, and 0.5 s/step (University of
238 Florence, Italy). The acquired XRD patterns were indexed on the basis of the calcite-
239 rhodochrosite structure (Maslen et al., 1995). The least-squares refinements, based on the Pawley
240 method, gave the unit-cell parameters of the studied minerals.
241

241
Mineral chemistry (point analyses) and element distribution within the minerals (X-ray maps in major elements detected: Fe $K_α$, Si $K_α$, Mn $K_α$, Ca $K_α$, and Mg $K_α$ lines) were analyzed on the same thin polished sections after coating them with carbon and using JEOL JXA-8230 electron microprobe (V=15 kV, I=1.2 nA, electron beam diameter of 2 µm) (Tokyo University of Marine Science and Technology). Standards used were: $Fe_2O_3$ (Fe $K_α$), $SiO_2$ (Si $K_α$), $MnSiO_3$ (Mn $K_α$), $CaCO_3$ (Ca $K_α$), $MgO$ (Mg $K_α$), $BaSO_4$ (Ba $L_α$), $SrSO_4$ (Sr $L_α$), and the detection limits (in wt.%) were 0.01 for Fe, 0.01 for Si, 0.01 for Mn, 0.01 for Ca, 0.01 for Mg, 0.03 for Ba, 0.01 for Sr.

250

## 4.2. Bulk chemistry of hydrothermal deposits

252

Concentrations of Mn, Ca, Fe, Mg, Na, Zn, Pb, Cu, As, Ni, Co, V, Mo, Ba, Sr, Al, Ti, P, K, Rb, Cs, B, Sc, U, Th, Hf, Zr, Nb, Ta, Y, and rare earth elements (REE) in the CLAM hydrothermal deposit samples were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) (Thermo Fisher Scientific i-CAP Q; The University of Tokyo). The samples were digested according to an analytical protocol originally described by Kato et al. (1998, 2002) with some modifications (Kato et al., 2005; Yasukawa et al., 2014; Fujinaga et al., 2016; Nozaki et al., 2016; Yasukawa et al., 2019). After drying at 110°C for ~12 hours, ~0.050 g of sample powder was decomposed by $HNO_3-HF-HClO_4$ in tightly closed, 10 mL Savillex® perfluoroalkoxy alkane (PFA) screw-cap beaker on a hot plate at 130°C for 2 hours. The dissolved sample was progressively evaporated at 110°C for 12 hours, at 160°C for 6 hours, and at 190°C until dryness. The residue was subsequently dissolved by 2 mL of *aqua regia* on a hot plate at 90°C for 6 hours. The digested sample was progressively evaporated at 120°C for 2 hours, then at 160°C until complete dryness. Subsequently, the residue was dissolved by 10 mL of 2 wt.% mixed acids of $HNO_3-HCl:HF = 20:5:1$ on a hot plate at 90°C for 3 hours. The final solution was diluted to 1:10000 by mass using the same 2 wt.% $HNO_3-HCl-HF$ acid mixture. The spectral overlaps from oxides and hydroxides ($^{44}$Ca$^{16}$O on $^{60}$Ni, $^{47}$Ti$^{16}$O on $^{63}$Cu, $^{50}$Ti$^{16}$O on $^{66}$Zn, $^{137}$Ba$^{16}$O on $^{153}$Eu, $^{141}$Pr$^{16}$O and $^{140}$Ce$^{16}$O$^1$H on $^{157}$Gd, $^{143}$Nd$^{16}$O on $^{159}$Tb, $^{147}$Sm$^{16}$O and $^{146}$Nd$^{16}$O$^1$H on $^{163}$Dy, $^{149}$Sm$^{16}$O on $^{165}$Ho, and $^{150}$Nd$^{16}$O, $^{150}$Sm$^{16}$O on $^{166}$Er, and $^{165}$Ho$^{16}$O on $^{181}$Ta) were corrected following the method described by Aries et al. (2000). The analyses were checked with the reference materials JB-2, JB-3 [basalt; Makishima and Nakamura (2006)] and
JMS-2 [deep-sea clay; Terashima et al., 2002; Takaya et al. (2014))] issued by the Geological Survey of Japan. The analytical precision in this study is comparable with previous studies in the same laboratory (Kato et al., 2011; Yasukawa et al., 2020).

Concentrations of Ca, Mg, Al, Ti, Na, K, Fe, Mn, P, Ba, Sr, and Zr in the Guaymas Basin hydrothermal flange sample T529-G4A B were determined by ICP-MS after fusing the sample powder with lithium metaborate ($LiBO_2$) in a graphite crucible (United States Geological Survey). Concentrations of Zn, Pb, Cu, As, Ni, Co, V, Mo, Ti, K, Rb, Cs, Sc, U, Th, Nb, Y, La, and Ce in the same sample were measured by ICP-MS after total sample digestion in a mixture of HCl, $HNO_3$, $HClO_4$, and HF acids (United States Geological Survey). Details of both analytical techniques can be found elsewhere (Taggart, 2002).

### 4.3. Chemistry of the hydrothermal fluids

Sub-samples of the hydrothermal fluid samples were diluted by ~50 times with ultrapure 2.5% $HNO_3$ before measurement of the major element concentrations. Indium was added as an internal standard for drift correction of the signal during measurements on the mass spectrometer. The concentrations of Ca, Mg, Si, and Sr were measured directly in the acid-diluted sub-samples by High Resolution-Inductively Coupled Plasma-Mass Spectrometry (HR-ICP-MS) (Element XR, ThermoFisher Scientific) at the Pôle de Spectrométrie Océan (IUEM/Ifremer, Brest, France). External standard (CASS-6; National Research Council, Canada) was used for concentration calibrations.

Another set of sub-samples from the hydrothermal fluid samples were subjected to a single-step separation procedure for isolation of the trace elements (Fe, Mn, Co, Ni, Cu, Zn, Cd, Mo, Pb, Ag, Sn, U, V, Y and REE) from the fluid saline matrix. The employed procedure was adapted from previously described methods (Sohrin et al., 2008; Minami et al., 2015) and its details were already published (Dekov et al., 2021). In short, 2 mL of fluid sample were weighted, and the fluid pH was adjusted to ~6 prior to column purification. The samples were then loaded on columns filled with NOBIAS Chelate-PA1 resin (with ethylenediaminetriacetic acid and iminodiacetic acid functional groups; Hitachi High-Technologies) and the elutions were collected in Savillex® PFA beakers for concentration measurement with HR-ICP-MS. In more detail, the pH of the fluid samples was adjusted with a buffer at pH~9 prepared by mixing 30 g of acetic
acid, 70 g of 18.2 MΩ water, and 50 g of 20% NH₃ (Minami et al., 2015). The columns were loaded with 0.5 mL of clean NOBIAS Chelate-PA1 resin. Cleaning of the resin included rinsing with 30 mL 18.2 MΩ water, 0.5 mL 3 M HNO₃, and then 10 mL 18.2 MΩ water. Before the sample load the resin was conditioned with a mixture of 2.5 % HNO₃ and buffer at final pH~6. The sample matrix was eluted with 10 mL 18.2 MΩ water and trace elements were eluted with 23 mL 3 M HNO₃. Prior to storage of the columns, the resin was rinsed with 20 mL 18.2 MΩ water.

The eluted solutions were evaporated to dryness and re-dissolved in 2 mL ~0.28 M HNO₃ with indium (In) as an internal standard for correcting the drift of the signal during HR-ICP-MS measurements. Concentrations of the trace elements in these solutions were measured with a HR-ICP-MS Element XR (ThermoFisher Scientific) at the Pôle de Spectrométrie Océan (IUEM/Ifremer, Brest, France), and the samples were calibrated using external calibration standards. A referenced seawater standard (CASS-6; National Research Council, Canada) and one blank were also processed following the above protocol and analyzed with the same HR-ICP-MS instrument.

Concentrations of Na, K and Li were measured by an ion chromatograph (IC) Metrohm 930 Compact IC Flex (Metrohm AG) at the JAMSTEC. The samples were eluted through a Metrohm Metrosep C6-250/4.0 column with 8 mM ultrapure HNO₃ (TAMAPURE AA-100) at a flow rate of 0.9 mL min⁻¹. The column temperature was set at 35°C throughout the analysis. Detection of Na, K and Li cations was accomplished with an electrical conductivity detector.

In order to get insight into the conditions of mineral precipitation in the studied chimneys we modeled Eh vs. pH, and log a vs. log a phase diagrams for the mineral-precipitating fluids using the Geochemist's Workbench 8.0 software based on the "thermo_minteq" database (Bethke, 2008). Physical and chemical parameters of the fluids venting from the CLAM vents (our data and data from previous studies) were used in our calculations.

## 4.4. C and O isotope composition of the hydrothermal deposits

Approximately 0.3 mg of powdered carbonate chimney bulk samples were reacted with 0.02 mL of purified H₃PO₄ in a 12-mL glass vial filled with 1-atom helium, to generate CO₂ at 60°C for more than 2 days. δ¹³C and δ¹⁸O were measured using a mass spectrometer (Thermo Finnigan Delta Plus) installed with a gas separation system (GAS-BENCH II) at the University of
Tokyo. The δ¹³C and δ¹⁸O values of the carbonate samples are expressed as ‰ versus Vienna PeeDee Belemnite (VPDB) standard by calibrating with the results of in-house standard. The measurement error (2σ) is typically less than 0.15‰ for both δ¹³C and δ¹⁸O values. Details of the measurements are described in Hori et al. (2009).

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## 4.5. Mg isotope composition of the hydrothermal deposits

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The powdered carbonate chimney samples were dissolved in concentrated HNO₃ then evaporated to dryness in an ISO6 clean bench. Re-dissolved solutes were purified by ion chromatograph (Metrohm 930, Metrohm AG) coupled to an Agilent 1260 Infinity II (Agilent) with the fraction collector (IC-FC) system for Mg isotope analysis (Yoshimura et al., 2018; Araoka and Yoshimura, 2019) at the JAMSTEC. The dissolved samples were eluted through a Metrohm Metrosep C6-250/4.0 column with 8 mM ultrapure HNO₃ (TAMAPURE AA-100) at a flow rate of 0.9 mL min⁻¹. Magnesium fraction was collected in 7-mL Savillex® PFA beakers and then evaporated to dryness in an ISO6 clean bench for isotope analysis. The Mg fractions were re-dissolved in 8 mM HNO₃ and loaded on the IC column three times to remove the high matrix of Mn in the samples (Mn-carbonates).

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The Mg isotopic composition (δ²⁶Mg) was determined with a Multi-Collector-Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS) (NEPTUNE plus; ThermoFisher Scientific) at the AIST (Geological Survey of Japan), using standard-sample bracketing methods. Samples and standards (IAPSO and Cambridge-1) were prepared as Mg solutions (100 ppb) in 0.3 M HNO₃. Solutions (samples and standards) were introduced with a nebulizer (PFA MicroFlow, 100 µL min⁻¹, ESI, and Omaha) attached to a quartz dual cyclonic spray chamber operated in free aspiration mode. We performed Mg isotope analysis with a standard X-skimmer cone in low-mass resolution mode. Samples and standards were analyzed 30 times with an integration time of 4 s per cycle. Background signal intensities were determined by analyzing ultrapure 0.3 M HNO₃ 10 times with an integration time of 4 s per cycle; baselines were subsequently subtracted from standard/sample signal intensities. The reported Mg isotopic compositions are relative to measured IAPSO (δ²⁵Mg = -0.39‰, 2SD = 0.05‰; δ²⁶Mg = -0.76‰, 2SD = 0.04‰) and Cambridge-1 (δ²⁵Mg = -1.36‰, 2SD = 0.05‰; δ²⁶Mg = -2.63‰, 2SD = 0.06‰) Mg isotope values.
## 4.6. *S* isotope composition of the hydrothermal deposits

For sulfur isotope analysis, the different types of S-containing minerals were separated from the powdered carbonate chimney samples using selective chemical dissolution procedures. First, the samples were treated with 6 M HCl at 80°C. If the samples contain sphalerite, pyrrhotite and galena they will be dissolved and H₂S gas will be liberated. During this treatment there was no smell of H₂S. Hence, the samples did not contain these sulfide minerals. The HCl treatment would also dissolve anhydrite if the samples contain it. Therefore, we filtered the solutions (nuclepore filters; pore size 0.4 µm) and the potential SO₄²⁻ liberated from anhydrite in the filtrate was recovered as BaSO₄ precipitate after adding 0.5 M BaCl₂ solution. I.e., the samples contained anhydrite. Then, the residual solids were reacted with Na₂CO₃ solution at 95°C for extracting barite following the method described by Breit et al. (1985). No barite was recovered. After barite removal step the residual solids that may have possibly contained chalcopyrite, isocubanite, pyrite, tennantite, and native sulfur (which do not react with 6 M HCl) were washed with CS₂ solvent for removing of (possible) native sulfur, and then oxidized to SO₄²⁻ in a mixture solution of Br and HNO₃. The liberated SO₄²⁻ (from dissolution of possible chalcopyrite, isocubanite, pyrite and tennantite) was recovered as BaSO₄ precipitate after adding 0.5 M BaCl₂ solution. XRD analysis of the residual solids collected after each of the above steps indicated that they contained anhydrite, native sulfur, and pyrite and chalcopyrite. Thus, we could measure the sulfur isotope composition of separated sulfates (i.e., anhydrite), native S and sulfides (i.e., pyrite + chalcopyrite).

Measurements of the S-isotope compositions of the selected S-containing mineral fractions were performed using IsoPrime EA. The δ³⁴S values are reported relative to Canyon Diablo Troilite (CDT). Analytical errors of the S-isotope ratios were less than ±0.3‰.

## 4.7. *Sr-Nd-Pb* isotopes of the hydrothermal deposits and fluids

All reagents used for the sample preparation (solids and fluids) for Sr, Nd, and Pb isotopes were Optima-grade. The sample preparation was performed under a clean lab environment at the Department of Geological Sciences at the University of Florida. 0.5 mL of the hydrothermal fluid
samples were transferred to acid-cleaned Savillex® PFA beakers and evaporated to dryness on a hot plate. After evaporation, the residues were dissolved in 3.5 *M* HNO₃ and loaded on cation exchange columns packed with Sr-spec resin (Eichrom Technologies, Inc.) to separate Sr from other ions.

The carbonate chimney samples were dissolved with a mixture of HCl and HNO₃. After digestion, the solutions were evaporated to dryness. The dry residue was dissolved in 1 *M* HBr and loaded on columns packed with Dowex 1X-8 resin to separate Pb for isotope analysis. The sample was washed three times with 1 mL of 1 *M* HBr and the Pb fraction was collected in 1 mL of 3 *M* HNO₃. The 3 mL of 1 *M* HBr wash was collected for subsequent Sr and Nd separation, as Sr and Nd are not adsorbed on the Dowex resin. Sr and Nd were further purified for isotope analysis following standard chromatographic methods. Sr, Nd, and Pb isotopes were determined on a “Nu Plasma” MC-ICP-MS, following methods described in Kamenov et al. (2008). The reported ⁸⁷Sr/⁸⁶Sr ratios are relative to NBS 987 ⁸⁷Sr/⁸⁶Sr = 0.71024 (+/-0.00003, 2σ). The Nd isotopic compositions are relative to JNdi-1 ¹⁴³Nd/¹⁴⁴Nd = 0.512115 (±0.000017, 2σ). Pb isotopic compositions were determined with Tl normalization and the data are relative to NBS 981 values of ²⁰⁶Pb/²⁰⁴Pb = 16.937 (±0.004, 2σ), ²⁰⁷Pb/²⁰⁴Pb = 15.490 (±0.004, 2σ), ²⁰⁸Pb/²⁰⁴Pb = 36.695 (±0.009, 2σ).

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## 4.8. Stability phase diagrams modeling

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In order to get insight into the conditions of mineral precipitation in the studied chimneys we modeled Eh vs pH and log₁₀*a* vs log₁₀*a* phase diagrams for the calculated average mineral-precipitating fluids using the Geochemist's Workbench 8.0 software based on the “thermo_minteq” database (Bethke, 2008). Physical and chemical parameters of the fluids venting from the CLAM site were used in our calculations.

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# 5. Results

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## 5.1. Mineralogy of the CLAM chimneys

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Macroscopically, the hand specimens of the studied CLAM chimneys were of two types: (1) porous grayish-white (Fig. 2 A, C), and (2) dense and hard pinkish-white (Fig. 2 B). The porous grayish-white samples are composed of alternating pinkish-white and grey layers (Fig. 2 A, C) whereas the dense and hard pinkish-white sample has rare light-grey thin layers and irregular in shape zones (Fig. 2 B).

The optical microscope observations of thin polished sections revealed that the hard pinkish-white sample (Fig. 2 B) was composed of rhombohedral crystals densely packed, transparent, with high relief, perfect rhombohedral cleavage along two directions (Supplementary Fig. 1 A), with fifth-order interference colors (Supplementary Fig. 1 B) and optical extinction symmetrical to the cleavage planes typical for carbonates of the calcite group.

The porous greyish-white samples (Fig. 2 A, C) were composed of skeletal rhombohedral carbonate crystals (Supplementary Fig. 1 C, D) showing alternation of zones with different transparency (Supplementary Fig. 1 C, D). The carbonate crystals were coated by thin, colloform and transparent film (at plain polarized light; Supplementary Fig. 1 E), which appears to be optically isotropic (becomes black at crossed polarized light; Supplementary Fig. 1 F) and seems to be amorphous silica.

The XRD studies revealed that the macroscopically different layers (sub-samples; Fig. 2) of the studied chimney samples are composed of different carbonates (Table 3; Supplementary Fig. 2). Sample 2K#366-St.2 is composed of rhodochrosite with traces of witherite (Table 3; Supplementary Fig. 2 A). The composition of sample 2K#366-St.3 is dominated by calcite, but it contains mixed carbonates like calcite-kutnohorite and ankerite-kutnohorite as well (Table 3; Supplementary Fig. 2 B). Sample 2K#366-St.3C has the most complex composition. It is rhodochrosite-dominated, but contains kutnohorite-ankerite, calcite, ankerite, gypsum, and dolomite (Table 3; Supplementary Fig. 2 C) as well.

Pinkish-white rhombohedral rhodochrosite crystals (Supplementary Fig. 3 A) were observed in the cavities and pores of the porous greyish-white chimney samples. They show skeletal growth and rare black spots on the surfaces (Supplementary Fig. 3 A). In general, the carbonate minerals, which are the principal constituent of the studied chimneys, have typical rhombohedral crystal forms (Supplementary Fig. 3 B). They were almost always coated by thin colloform film (Supplementary Fig. 3 C), which according to the EDS data (not shown) was most probably composed of X-ray amorphous silica. Often, bacteria-like filaments (10-40 µm length, 2
µm in diameter) composed of X-ray amorphous silica were observed on the silica coated surface of the carbonate crystals (Supplementary Fig. 3 E). Rarely, the carbonate crystals were coated by thin botryoidal film of Mn-oxides (Supplementary Fig. 3 D). Occasionally, worm-like structures presumably coated by clays and Mn-oxides were observed on the surface of the carbonate crystals (Supplementary Fig. 3 F).

Sulfides and sulfates were minor minerals observed in the carbonate chimneys. Pseudo-hexagonal plate-like galena and pyrrhotite crystals surrounded by X-ray amorphous silica were rarely observed. Clusters of rhombic anhydrite were found on the surface of the carbonate crystals (EDS and SEM data; available on request).

Mineral chemistry investigations (EMP analyses) revealed that the studied CLAM chimneys are composed of three major minerals: Mn-calcite, Ca-rhodochrosite and X-ray amorphous silica (Table 4).

Two major minerals compose samples 2K#366-St.2 and 2K#366-St.3C: Ca-rhodochrosite and X-ray amorphous silica (Table 4). Ca-rhodochrosite always contains traces of Fe and has average empirical formulae varying from $(Ca_{0.26}Mn_{0.73}Fe_{0.01})CO_3$ (sample 2K#366-St.2) to $(Ca_{0.20}Mn_{0.79}Fe_{0.01})CO_3$ (sample 2K#366-St.3C) (Table 4). The chemistry of Ca-rhodochrosite deviates from the stoichiometry of pure rhodochrosite. The EMP analyses of the X-ray amorphous silica gave totals around 95 wt.% (2K#366-St.2: average of 5 analyses in wt.%, $SiO_2$ = 91.90, MnO = 1.48, CaO = 0.57, SrO = 0.27, FeO = 0.04, MgO = 0.02, BaO = 0.05; 2K#366-St.3C: average of 5 analyses in wt.%, $SiO_2$ = 93.55, MnO = 1.25, CaO = 0.18, SrO = 0.25, FeO = 0.03, MgO = 0.01, BaO = 0.04) likely a result of the missing adsorbed $H_2O$ ($SiO_2$ $nH_2O$), which has not been analyzed.

Major constituent of the sample 2K#366-St.3 is Mn-calcite with an average empirical formula $(Ca_{0.90}Mn_{0.10})CO_3$ (Table 4). The chemistry of calcite from the Guaymas Basin is close to the stoichiometry: $(Ca_{0.99}Mn_{0.01})CO_3$ (Table 4).

The unit-cell parameters ($a$ and $c$) of the studied rhodochrosite and calcite (Supplementary Table 1) show clear correlation to the Mn content (Supplementary Fig. 4). The Guaymas Basin calcite (T529-G4A B), which contains negligible amount of Mn (Table 4) has unit-cell parameters close to those of the pure stoichiometric calcite (Supplementary Fig. 4). The little amount of Mn in the Mn-calcite crystal lattice (2K#366-St.3; Table 4) results in decrease in the unit-cell parameters (Supplementary Table 1; Supplementary Fig. 4). The increase of Mn in the
489 Ca-rhodochrosite (2K#366-St.2 – 2K#366-St.3C; Table 4) leads to further shrinking of the unit-
490 cell parameters towards those of pure stoichiometric rhodochrosite (Supplementary Table 1;
491 Supplementary Fig. 4).

492 Mapping of the concentration distributions of the elements present in the studied samples
493 (in characteristic lines) showed that sample 2K#366-St.3C is composed of rhombic crystals with
494 zonal texture (Supplementary Fig. 5 A-E). The crystals are built of alternating concentric zones
495 composed of Mn-Ca-carbonates and X-ray amorphous silica (Supplementary Fig. 5 B-D). The
496 amorphous silica mostly coats the surfaces of the carbonate crystals, but it is also finely mixed
497 with the carbonates in the carbonate-dominant layers (Supplementary Fig. 5 B). The carbonates
498 are Mn-carbonates (Supplementary Fig. 5 C) with minor Ca (Supplementary Fig. 5 D) and traces
499 of Mg (Supplementary Fig. 5 E). The Fe distribution within these crystals is irregular
500 (Supplementary Fig. 5 F).

501 The principal mineralogy of the sample T529-G4A B from the Guaymas Basin is calcite
502 (abundant), amorphous silica (minor), pyrrhotite (minor), and sphalerite (minor) (Table 3).

5.2. *Geochemistry of the CLAM chimneys*

506 Manganese concentrations of the studied chimneys are one to two orders of magnitude
507 higher than those of the Guaymas and southern Pescadero Basins carbonate chimneys (Table 5).
508 Calcium content of the CLAM Mn-calcite chimney (2K#366-St.3) is lower than, but close to that
509 of the Guaymas and southern Pescadero Basins carbonate chimneys, whereas the Ca content of
510 the Ca-rhodochrosite chimneys (2K#366-St.2 and 2K#366-St.3C) is up to 20 times lower than
511 that of the Guaymas and southern Pescadero Basins carbonate chimneys (Table 5). Iron
512 concentrations of the studied chimneys are similar (2K#366-St.2 and 2K#366-St.3) to or higher
513 (2K#366-St.3C) than those of the Guaymas and southern Pescadero Basins carbonate chimneys
514 (Table 5). Magnesium concentrations are similar in all studied samples and are within the Mg
515 concentration range of the Guaymas and southern Pescadero Basins carbonate chimneys (Table
516 5). Ca-rhodochrosite chimneys are richer in Na, K, Zn, Pb, Cu, As, V, Mo, Cs, U, and Y than
517 both the CLAM Mn-calcite chimney, and the Guaymas and southern Pescadero Basins carbonate
518 chimneys (Table 5). Studied Mn-carbonate chimneys are richer in Ni, but poorer in Co, Ba and Sr
519 than the Guaymas and southern Pescadero Basins carbonate chimneys. Ca-rhodochrosite
chimneys are richer in Al, Ti, P, Rb, B, Th, Hf, Zr, Ta, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu, and
poorer in La, Ce, Pr, Nd, Gd and ΣREE than the Mn-calcite chimney (Tables 5, 6). The REE
distribution patterns of all studied samples show negative Ce anomaly (Ce/Ce*<1), strong
positive Eu anomaly (Eu/Eu*>1) and enrichment of light REE over heavy REE (LaCN/LuCN>1)
(Fig. 3 A,B,C; Table 6).

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5.3. Geochemistry of the CLAM vent fluids

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In order to compare the chemical composition of the studied CLAM hydrothermal fluids (Table 7) to those of the hydrothermal fluids from similar hydrothermal fields on the same basis we have to use the chemistry of end-member hydrothermal fluids ([Mg] = 0 mmol/kg) (Von Damm, 1990). Studying the CLAM hydrothermal fluid collection part of which is the set of fluids studied in this work, Gamo et al. (1991) inferred that it would not be correct to calculate the characteristics of the CLAM end-member hydrothermal fluid by extrapolating the mixing lines to zero-Mg fluid, because the CLAM end-member fluid contains significant amount of SO₄. This means that the Mg content in the CLAM end-member fluid will also not be zero. It was concluded (Gamo et al., 1991) that various small hydrothermal convective cells with diverse end-member compositions operate at the CLAM hydrothermal field. Therefore, we have not calculated the chemistry of the CLAM end-member hydrothermal fluid (Tables 7, 9) extrapolating to zero-Mg fluid, but compare the chemistry of the average CLAM venting fluid to that of the end-member hydrothermal fluids of sedimented ridges and back-arc basins (Tables 8, 10).

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The concentrations of alkali elements (Li, Na, K) in the CLAM hydrothermal fluids do not follow coherent patterns when compared to the hydrothermal fluids from similar seafloor settings and seawater (Table 8). CLAM hydrothermal fluids along with those from the other Okinawa Trough hydrothermal fields (JADE and South Ensei) show the highest Li content when compared to any other hydrothermal fluid from sedimented ridges and back-arc basins (Table 8). Potassium concentrations in the CLAM fluids are higher than that of the seawater, similar to those in the Middle Valley hydrothermal fluids, but lower than those in the other seafloor hydrothermal systems from sedimented ridges and back-arc basins (Table 8). Sodium content of the CLAM
fluids is similar to that of the Middle Valley fluids, but lower than Na content of seawater and the
other hydrothermal systems from sedimented ridges and back-arc basins (Table 8).

Alkaline earths (Mg, Ca, Sr) concentrations in the CLAM hydrothermal fluids also do not show coherent trends relative to the other seafloor hydrothermal fluids and seawater (Table 8). It is known that Mg is nearly completely removed from the hydrothermal fluids during high-temperature reactions with rocks and sediments (Bischoff and Dickson, 1975; Mottl and Holland, 1978). We already noted (see first paragraph of this section) that according to the previous studies the CLAM end-member fluid is not supposed to be a zero-Mg fluid. On the other hand, we cannot completely rule out mixing of the CLAM end-member fluid with ambient seawater during sampling. Therefore, although the reported Mg concentration of the CLAM fluid is lower than that of the seawater it is still well above the zero-Mg concentrations of the other seafloor end-member hydrothermal fluids (Table 8) and we have to consider it with caution. Although the Ca concentration of the CLAM hydrothermal fluid is slightly higher than that of the seawater it is on the low end of the range for all hydrothermal fluids from sedimented ridges and back-arc basins (Table 8). Strontium concentrations in the CLAM fluids are not only the lowest among all the fluids from sedimented ridges and back-arc hydrothermal systems, but also they are lower than that of the seawater (Table 8).

Most of the measured transition metals (Fe, Cu, Zn, Co, Pb, Cd) are present in concentrations lower than those of the hydrothermal fluids from sedimented ridges and back-arc basins (Table 8). Manganese concentration alone is higher than that measured in fluids from any other sedimented ridge hydrothermal system, but lower than the Mn concentration in the Manus back-arc basin hydrothermal fluids (Table 8). Molybdenum, U, and Cd contents in the CLAM hydrothermal fluids are lower than those in the seawater (Table 8).

The REE concentrations of the CLAM hydrothermal fluids (Table 9) are lower than those of both Manus and Okinawa back-arc basins (Table 10). Compared to the fluids from the sedimented Guaymas Basin and Escanaba Trough hydrothermal systems the CLAM hydrothermal fluids are depleted in La and Ce, and enriched in Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu (Table 10). The same trend is also observed for the Mariana back-arc basin hydrothermal fluids (Table 10). The REE distribution patterns of the CLAM hydrothermal fluids show negative Ce anomaly (Ce/Ce*<1), positive Eu anomaly (Eu/Eu*>1) and enrichment of light REE over heavy REE (LaCN/LuCN>1) (Fig. 3 D; Table 9).
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## 5.4. C and O isotope composition of the CLAM chimneys

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Carbon and O isotope composition of the studied chimneys varies in a wide range: $\delta^{13}C_{VPDB} = -6.75 - +1.48\‰$; $\delta^{18}O_{VSMOW} = +9.56 - +27.88\‰$ (Table 11). Both isotope ratios become heavier with increasing Mn and decreasing Ca contents in the Mn-carbonates (Fig. 4; Table 5). In general, C and O isotope compositions of the CLAM Mn-carbonate chimneys are heavier than those of the carbonate chimneys from the Guaymas and southern Pescadero Basins (Fig. 4; Table 11).

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## 5.5. Mg isotope composition of the CLAM chimneys

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We were able to measure the Mg isotope composition of four chimney samples only (Table 11) because of the low Mg concentrations in most of the samples. Our measurements showed that Mn-calcite (sample 2K#366-St.3-c) has the lightest Mg isotopic composition ($\delta^{26}Mg = -2.18\‰$) among all samples analyzed (Table 11). Ca-rhodochrosite (samples 2K#366-St.3C-c, 2K#366-St.3C-f, 2K#366-St.3C-i) exhibits slightly heavier Mg isotopic composition ($\delta^{26}Mg$ ranging from -1.50 to -1.07‰; Table 11) than Mn-calcite.

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## 5.6. S isotope composition of the CLAM chimneys

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Trace amounts of sulfides and native S were found in the CLAM Ca-rhodochrosite chimneys, but were not detected in the calcite deposits of either CLAM or Guaymas (Table 12). The S-isotope compositions of these sulfides and native S are within the S-isotope composition range of the sulfide ($H_2S$) of the CLAM hydrothermal fluids and close to the S-isotope composition of the terrestrial mantle sulfide (Fig. 5).

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The sulfates of the CLAM chimneys partition into two groups in respect to their S-isotope composition: (1) sulfates from the Ca-rhodochrosite chimneys with S-isotope composition close to that of the terrestrial mantle sulfide; and (2) sulfates from the calcite chimneys (CLAM and Guaymas) with S-isotope composition heavier than that of the seawater sulfate (Table 12; Fig. 5).

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The S-isotope composition of the latter is at the high end of the δ³⁴S range of the CLAM hydrothermal fluid sulfates and is even heavier than it (Fig. 5).

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## 5.7. Sr-Nd-Pb isotope composition of the CLAM chimneys and vent fluids

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CLAM vent fluids showed relatively narrow range in ⁸⁷Sr/⁸⁶Sr, from 0.70893 to 0.70914 (Table 13). These Sr isotope compositions are very similar to previously published ⁸⁷Sr/⁸⁶Sr data for CLAM vent fluids (Table 13). Mn-calcite chimney samples (2K#366-St.3) showed lower ⁸⁷Sr/⁸⁶Sr values (0.7082 to 0.7083) compared to those of the Ca-rhodochrosite samples 2K#366-St.2 (0.70896 to 0.7094) and 2K#366-St.3C (0.7083 to 0.7108) (Table 13). Nd isotopic compositions of the Mn-carbonate chimneys showed a range from 0.512506 to 0.51281. The lowest and highest ¹⁴³Nd/¹⁴⁴Nd values were observed in 2K#366-St.3C (Ca-rhodochrosite; Table 13). The lowest Nd isotopes (0.512506) and the highest Sr isotopes (0.7108) were observed in sample 2K#366-St.3C-c. Ca-rhodochrosite samples (2K#366-St.2 and 2K#366-St.3C) showed very similar ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios (Table 13). In contrast, Mn-calcite samples (2K#366-St.3) showed much wider variations in ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios (Table 13).

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## 5.8. Stability phase diagrams modeling

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In the modeling of Eh vs pH and log₁₀a vs log₁₀a phase diagrams for the stability of the major cations of interest (Mn and Ca) and their precipitates we used the physical and chemical parameters of calculated average CLAM vent fluid (Supplementary Table 2).

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# 6. Discussion

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## 6.1. Mn-carbonates of the CLAM chimneys

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Mn-calcite and Ca-rhodochrosite are the two major constituents of the CLAM chimneys (Tables 3, 4). CLAM chimney calcite contains Mn [(Ca₀.₉₀Mn₀.₁₀)CO₃] and its unit-cell parameters (a and c) are smaller than those of the Guaymas Basin hydrothermal calcite, which is

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close to stoichiometry [(Ca₀.₉₉Mn₀.₀₁)CO₃] (Tables 4, 5; Supplementary Fig. 4). Ca-rhodochrosite chemistry [(Ca₀.₂₆Mn₀.₇₃Fe₀.₀₁)CO₃ – (Ca₀.₂₀Mn₀.₇₉Fe₀.₀₁)CO₃; Table 4] deviates from that of the stoichiometric rhodochrosite and its unit-cell parameters are larger than those of pure MnCO₃ (Supplementary Table 1; Supplementary Fig. 4). Unit-cell parameters of the CLAM hydrothermal Ca-Mn-carbonates decrease with increasing Mn content (in atoms per formula unit; Supplementary Fig. 4). Progressive replacement of Ca²⁺ [with ionic radius 1.08 Å in six-fold coordination (Whittaker and Muntus, 1970) like that of calcite] with the smaller Mn²⁺ [with ionic radius 0.75 Å (low spin) to 0.91 Å (high spin) in six-fold coordination; Whittaker and Muntus (1970)] explains the shrinking of the unit-cell parameters from Mn-calcite to Ca-rhodochrosite.

Ca-rhodochrosite from sample 2K#366-St.3C [(Ca₀.₂₀Mn₀.₇₉Fe₀.₀₁)CO₃] is richer in Mn than the Ca-rhodochrosite from sample 2K#366-St.2 [(Ca₀.₂₆Mn₀.₇₃Fe₀.₀₁)CO₃] (Table 4). This is in contrast to the bulk Mn concentrations in these two samples: sample 2K#366-St.3C is poorer in Mn than sample 2K#366-St.2 (Table 5). This discrepancy can be explained with presence of the “excess” Mn in sample 2K#366-St.2 in non-carbonate minerals like Mn-oxides (Supplementary Fig. 3 D).

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## 6.2. Crystallography control on REE partitioning between rhodochrosite and calcite

659

Manganese along with Ca is a major cation in the CLAM carbonates and this is the principle difference between the CLAM hydrothermal system and the hydrothermal systems at the Guaymas and southern Pescadero Basins: deposition of Ca-rhodochrosite and Mn-calcite (Tables 3, 4) instead of pure Ca-carbonates (Koski et al., 1985; Peter and Scott, 1991; Paduan et al., 2018). Iron and Mg (major isomorphic replacements of Ca and Mn in the CaCO₃-MnCO₃ sequence) show similar concentrations in the carbonates of all the three sediment-hosted hydrothermal systems (Iheya Ridge, Guaymas Basin and southern Pescadero Basin) and likely the supply of these two elements (e.g., concentrations and activities in the depositing fluid) had not played essential role in the hydrothermal carbonate precipitation.

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The precipitation of Mn-Ca-carbonates close to the two end-members CaCO₃ and MnCO₃ results in crystallographically controlled partitioning of the REE in the CLAM carbonates: enrichment in Tb, Dy, Ho, Er, Tm, Yb and Lu, and depletion in La, Ce, Pr and Nd of the Ca-rhodochrosite chimneys relative to the Mn-calcite chimney (Tables 5, 6). Indeed, the ionic radii
of trivalent Tb (1.00 Å), Dy (0.99 Å), Ho (0.98 Å), Er (0.97 Å), Tm (0.96 Å), Yb (0.95 Å) and Lu (0.94 Å) are close to that of divalent Mn (0.97 Å) whereas those of trivalent La (1.13 Å), Ce (1.09 Å), Pr (1.08 Å), and Nd (1.06 Å) are close to that of divalent Ca (1.08 Å) (all in six-fold coordination; Whittaker and Muntus, 1970). This suggests relatively easy isomorphic replacement (and enrichment) of Tb, Dy, Ho, Er, Tm, Yb and Lu for Mn in the Ca-rhodochrosite crystal lattice and isomorphic replacement (and enrichment) of La, Ce, Pr and Nd for Ca in the Mn-calcite crystal lattice. The weak negative Ce anomaly and strong positive Eu anomaly in the REE distribution patterns of all chimney samples (Fig. 3 A,B,C; Table 6) are likely inherited from the REE distribution patterns of the parent hydrothermal fluids (Fig. 3 D). The origin of these anomalies is discussed elsewhere (Klinkhammer et al., 1994).

Higher concentrations of Zn, Pb, Cu, As, and Mo in the CLAM Ca-rhodochrosite chimneys than in the calcite hydrothermal deposits of the Guaymas and southern Pescadero Basins (Table 5) is likely a result of the presence of trace amounts (<4 wt.%, the detection limits of XRD) of sulfides, sulfates or sulfosalts of these elements in the CLAM deposits (SEM observations; see 5.1).

## 6.3. Vent fluid geochemistry constraints on CLAM chimney formation

We are not aware of any data on the Li concentrations in the Iheya Ridge rocks (rhyolites, andesites, basalts) and sediments, which are supposed to be the main source of elements to the CLAM hydrothermal fluids. Therefore, in order to explain the enrichment of Li in the CLAM vent fluids (and in the vent fluids of JADE and South Ensei, Okinawa Trough) relative to the hydrothermal fluids from other sedimented ridges and back-arc basins (Table 8) we will consider the average Li concentrations in the upper continental crust (UCC) (35 ppm) and oceanic crust (OC) (10 ppm) (Teng et al., 2004). As the Okinawa Trough is an intra-continental back-arc basin at initial stage of rifting it is plausible that rocks with average chemical composition close to that of the UCC are source of elements to the CLAM hydrothermal system. On the other hand, rocks of the OC are the source of elements to the hydrothermal systems of the compared hydrothermal fields (Table 8). Thus, the Li concentrations in the CLAM source rocks that are higher than those in the source rocks of the compared hydrothermal fields (at other sedimented ridges and back-arc basins) may be responsible for the Li enrichment in the CLAM hydrothermal fluids. Lithium may
be considered an indicator of the water/rock ratio, with high Li concentrations resulting from low water/rock ratios (Butterfield et al., 1994). Therefore, Li concentration data indicate that the CLAM hydrothermal system has a low water/rock ratio.

Sodium depletion of the CLAM hydrothermal fluids (similar to that of the Middle Valley fluids) relative to seawater (Table 8) may be due to either (1) boiling and phase separation of the hydrothermal fluid into vapor and brine phases, or (2) the formation of Na-rich alteration minerals (e.g., albite; Berndt et al., 1989) below the seafloor. The depletion of the main alkali element (Na) in the hydrothermal fluids correlates with the depletion of chlorine, the main anion in these fluids ([Cl]~CLAM~ = 513 mmol/kg, [Cl]~seawater~ = 536 mmol/kg; Gamo et al., 1991). The chlorinity variations in the seafloor hydrothermal systems could be accounted for by mixing of seawater, hydrothermal fluid and the vapor and brine phases produced during phase separation of the hydrothermal fluid (Von Damm, 1990; Butterfield et al., 1994; Von Damm et al., 2005). The chlorinity and Na concentration of the CLAM fluids suggest that these fluids might have undergone phase separation and contain a substantial proportion of low-chlorinity vapor. The molar ratios of these two ions [(Na/Cl)~CLAM~ = 0.77; (Na/Cl)~seawater~ = 0.87] show Na depletion relative to seawater and indicates that some process has removed Na (relative to Cl) from the mother fluid (seawater). Formation of Na-rich alteration minerals like albite during fluid/rock reactions is a plausible explanation of this observation.

Although K is enriched in the CLAM fluids with respect to the seawater value [(K/Cl)~CLAM~ = 0.04, (K/Cl)~seawater~ = 0.02; Table 8] its concentration in these fluids is lower than that in the other seafloor hydrothermal systems from sedimented ridges and back-arc basins (with the exception of the Middle Valley hydrothermal fluids, which have similar K concentrations). The lower K concentration in the CLAM fluids relative to the compared hydrothermal systems may be (1) due to the lower K supply from the source rocks (sediments) or (2) a result of K sequestration from the fluids (e.g., K-feldspar formation, incorporation in the interlayer space of clay minerals; Nozaki et al., 2021b).

The depletion (in absolute and Cl-normalized concentrations) in Ca and Sr of the CLAM hydrothermal fluids relative to (1) the hydrothermal fluids from other sedimented ridges and back-arc basins [Ca: (Ca/Cl)~CLAM~ = 0.03, (Ca/Cl)~seawater~ = 0.02] and (2) seawater [Sr: (Sr/Cl)~CLAM~ = 0.0001, (Sr/Cl)~seawater~ = 0.0002] (Table 8) can be reasonably explained with Ca and Sr removal from the hydrothermal fluids upon precipitation of the Mn-Ca-carbonates.
The net removal of Mo, U and Cd from the seawater to the host rocks during the seawater/rock interaction (German and Von Damm, 2003) is most probably responsible for the depletion (in absolute and Cl-normalized concentrations) in Mo [$(Mo/Cl)_{CLAM} = 0.3 \times 10^{-7}$, $(Mo/Cl)_{seawater} = 2 \times 10^{-7}$], U [$(U/Cl)_{CLAM} = 0.05 \times 10^{-7}$, $(U/Cl)_{seawater} = 0.3 \times 10^{-7}$] and Cd [$(Cd/Cl)_{CLAM} = 0.005 \times 10^{-7}$, $(Cd/Cl)_{seawater} = 0.02 \times 10^{-7}$] of the CLAM hydrothermal fluids relative to the seawater (Table 8).

Elevated Mn concentrations of the CLAM hydrothermal fluids relative to those of the other sedimented ridge hydrothermal systems (Table 8) and their orders of magnitude higher values (absolute and Cl-normalized) than that of seawater [$(Mn/Cl)_{CLAM} = 3 \times 10^{-4}$, $(Mn/Cl)_{seawater} < 0.00002 \times 10^{-4}$] imply the Mn concentration of the CLAM fluid in the reaction zone is very high. The higher $(Mn/Ca)_{mol}$ of the CLAM hydrothermal fluids than that of the other sedimented ridge hydrothermal fluids (Table 8) suggests that although certain amount of Mn is removed from the CLAM fluids to Mn-carbonates its content in the fluids is still high. One of the major controls on the Mn concentration in the hydrothermal fluids is the Mn budget of the source rocks. Although the source rocks at the Iheya Ridge and Guaymas Basin are different (from basic to acidic at the Iheya Ridge and basic at the Guaymas Basin), the Mn concentrations in them are comparable (Supplementary Table 3). However, in a system (e.g., $MnCO_3$ – $CaCO_3$) in which the precipitation towards one or the other end-member depends on the competition between the major cations (Mn and Ca) the important is the concentration ratio of the competing cations. Higher Mn/Ca ratio of the Iheya Ridge rocks than that of the Guaymas Basin rocks (Supplementary Table 3) seems to be responsible for the higher Mn concentration in the CLAM hydrothermal fluids than that in the Guaymas hydrothermal fluids.

The lower concentrations of the transition metals (Fe, Cu, Zn, Co, Pb, Cd) in the CLAM hydrothermal fluids than those in the hydrothermal fluids from sedimented ridges and back-arc basins (Table 8) may be a result of: (1) low concentrations of these elements in the end-member CLAM fluid; (2) sub-surface precipitation of these elements (e.g., as sulfides). We do not have any proof for either of these two possibilities and will not speculate further.

### 6.4. *C and O* isotope constraints on CLAM chimney formation
δ¹³C and δ¹⁸O values of the CLAM Mn-carbonates show linear positive correlation (Fig. 4).
This correlation can be explained by two different processes: (1) Mn-carbonate precipitation due to mixing of two fluids with different alkali-chloride concentrations, or (2) Mn-carbonate precipitation from a H₂CO₃-dominant fluid due to a temperature effect coupled with either CO₂ degassing or fluid/rock interaction (Zheng and Hoefs, 1993). We calculated the C-O-isotope variations of calcite precipitated as a result of mixing between CLAM hydrothermal fluid and seawater (Fig. 4) based on the mass balance equations of Zheng and Hoefs (1993). For the C-O-isotope composition of the CLAM end-member hydrothermal fluid, we used δ¹³CₗCO₂ (VPDB) = -4.4‰ of the CLAM vent fluid (Sakai et al., 1990a) and δ¹⁸OₗH₂O (VSMOW) = +1.5‰ that is the average O-isotope composition of the hydrothermal fluids venting at the adjacent JADE (+1.8‰; Ishibashi et al., 2014) and Iheya North (+1.2‰; Kawagucci, 2015) sites because of the lack of δ¹⁸OₗH₂O (VSMOW) data for the CLAM fluid. Temperatures, pH, and dissolved inorganic carbon (DIC) species at each mixing rate were calculated with the computer program PHREEQC version 3 (Parkhurst and Appelo, 2013) by using the compositions of the CLAM vent fluid and seawater (Supplementary Table 2). Our data do not lie on the theoretical mixing line between the reconstructed CLAM end-member hydrothermal fluid and seawater although they follow the mixing trend from samples precipitated from almost pure end-member hydrothermal fluid (2K#366-St.3) to samples with seawater-dominated vent fluid (2K#366-St.2) (Fig. 4). The sample sequence 2K#366-St.3 – 2K#366-St.3C – 2K#366-St.2 (Fig. 4) seems to have precipitated from a fluid that had progressively been degassed and cooled in that order in addition to the progressive mixing of the hydrothermal fluid with seawater. The variations of the C and O isotope values within each sample may reflect composition and/or temperature fluctuations in the fluid that precipitated the carbonates. Thus, the precipitation of the CLAM Mn-carbonates can partly be explained by simple binary mixing of CLAM hydrothermal fluid and seawater. The deviation of the C-O-isotope composition of the CLAM carbonates from the calculated mixing line (Fig. 4) may be a result of (1) the difference between the inferred hydrothermal end-member composition and that of the real end-member fluid, and (2) a temperature effect coupled with either CO₂ degassing or fluid/rock interaction.

The C-O-isotope composition of the CLAM Mn-carbonates is heavier than that of both Guaymas and southern Pescadero Basins carbonate chimneys (Fig. 4). The lighter O-isotope composition of the carbonate chimneys from the Guaymas and southern Pescadero Basins may
reflect the higher temperature (up to ~300°C) of their hydrothermal fluids, whereas their lighter C-isotope composition may reflect the lighter $\delta^{13}C_{DIC}$ of the hydrothermal fluids. The lighter $\delta^{13}C_{DIC}$ may have resulted from incorporation of $C_{org}$ from the sediments through which the Guaymas and southern Pescadero hydrothermal fluids pass. The broad $\delta^{13}C$ and $\delta^{18}O$ variations in the CLAM Mn-carbonates may have resulted from wide variations of the fluid composition precipitating the carbonates.

## 6.5. Mg isotope constraints on CLAM chimney formation

The Mg isotopic compositions of the CLAM hydrothermal fluids ($\delta^{26}Mg = -0.81$ and $-0.80\text{‰}$) are indistinguishable (within analytical error) from the seawater $\delta^{26}Mg = -0.83 \pm 0.07\text{‰}$ (Eom et al., 2020). Therefore, simple binary mixing of CLAM end-member hydrothermal fluid with seawater cannot explain the observed Mg isotope compositions of the CLAM Mn-carbonate chimneys (Table 11). The $\delta^{26}Mg$ values of the CLAM Mn-carbonate chimneys (Table 11) are $-0.24\text{‰}$ to $-1.35\text{‰}$ lower than the seawater value. Magnesium isotope fractionation ($\Delta^{26}Mg$) between authigenic carbonates and $Mg^{2+}_{(aq)}$ of precipitating solution generally depends on the carbonate mineralogy and the strength of the Mg-O bond in carbonate. The $\Delta^{26}Mg$ values of natural carbonates and theoretical calculations of Mg isotope fractionation generally follow the order: aragonite < dolomite < magnesite < calcite (Saenger and Wang, 2014). The Mg isotope fractionation factor for calcite at 216°C (maximum temperature measured in the CLAM hydrothermal fluids; Gamo et al., 1991) will be about $-1.1$ to $-1.5\text{‰}$ lower than the precipitating solution according to density-functional electronic structure calculations by Rustad et al. (2010). Thus, the $\delta^{26}Mg$ of calcite precipitating from the CLAM hydrothermal fluid would be $-2.0\text{‰}$, which is consistent with the measured $\delta^{26}Mg$ value of sample 2K#366-St.3-c ($-2.18\text{‰}$; Table 11).

Much uncertainty remains regarding equilibrium $\Delta^{26}Mg$ for co-precipitation of $Mg^{2+}_{(aq)}$ with Mn-carbonates. We are not aware of any published fractionation factor for Mg incorporation into $MnCO_3$ and suppose this is probably due to the analytical challenges in the course of Mg separation from high-Mn matrix. Nevertheless, we may assume similar fractionation factors for $MnCO_3$ and $MgCO_3$ (both of trigonal crystal system) since $Mn^{2+}$ have similar ionic radius [0.75 Å in octahedral coordination; Whittaker and Muntus (1970)] to that of $Mg^{2+}$ [0.80 Å in octahedral coordination; Whittaker and Muntus (1970)]. $\delta^{26}Mg$ for $MgCO_3$ at 216°C extrapolates
827 to about -0.5‰ [see Figure 6 in Saenger and Wang (2014)]. From this point of view, the δ²⁶Mg
828 difference between the two Mn-carbonates in the CLAM chimneys, Mn-calcite (2K#366-St.3-c)
829 and Ca-rhodochrosite (2K#366-St.3C-c, -f, -i), seems consistent: strong influence of calcite for
830 2K#366-St.3-c. We realize that this is a speculation because we do not know the actual isotope
831 difference between the direct Mg incorporation into MgCO₃ and Mg co-precipitation into
832 MnCO₃.

833 Still, the proportion of mixed Ca-Mn-Mg-Fe-carbonates is likely to be the primary control
834 on their Mg isotopic compositions. We identified high correlations between δ²⁶Mg and δ¹³C (R²
835 = 0.98), and δ²⁶Mg and δ¹⁸O (R² = 0.97), suggesting that Mg isotope fractionation during Mn-
836 carbonate precipitation is also sensitive to their growth conditions and resulting different
837 carbonate mineralogy (see 6.4).

838 The ⁸⁷Sr/⁸⁶Sr values of the studied Mn-carbonate chimneys showed no correlation with
839 δ²⁶Mg, suggesting the mineralogical difference played a dominant influence on Mg isotope
840 composition compared to fluid mixing.

841 Although our Mg isotope data are scarce this study provides the first insights into the use of
842 Mg isotopes for how Mg is accommodated into high-temperature hydrothermal Mn-carbonates.

843
844 6.6. *S* isotope constraints on CLAM chimney formation
845

845
846 S-isotope composition of the sulfides and native S in the CLAM chimneys is similar to the
847 S-isotope composition of the CLAM hydrothermal fluid sulfide (Table 12; Fig. 5), suggesting
848 that it is likely inherited from the CLAM fluids. Obviously, the sulfur in these two minerals
849 groups (although in trace amounts in the chimneys) is dominated by basement rock S with a little
850 proportion of seawater S (Shanks, 2001).

851
852 The dual behavior of the S-isotope composition of the CLAM chimney sulfates (Fig. 5) is
853 interesting and deserves discussion. It is known that SO₄²⁻ reduction in a closed system causes an
854 increase of δ³⁴S of the residual SO₄²⁻ (removal of the light sulfide during SO₄²⁻ reduction) with
855 decreasing SO₄²⁻ concentration (Ohmoto and Rye, 1979; Shanks et al., 1981). Thus, the S-isotope
856 compositions of the sulfates from the CLAM chimney Mn-calcite and Guaymas flange calcite
857 that are heavier than that of the seawater sulfate can be interpreted as results of sulfate reduction.
857 The S-isotope composition of the sulfates from the Ca-rhodochrosite chimneys (as low as that of
the terrestrial mantle sulfide; Table 12, Fig. 5) likely reflects the oxidation of H₂S during hydrothermal venting and subsequent CaSO₄ precipitation (e.g., Goodfellow and Blaise, 1988; Peter and Shanks, 1992). According to previous studies (Peter and Shanks, 1992), sulfide oxidation or sulfate reduction depend on the rate of mixing between the hydrothermal fluid and seawater. Sulfide oxidation is inferred to dominate when the hydrothermal fluid mixes quickly with seawater. Peter and Shanks (1992) estimated that this process occurs without isotopic fractionation and contributes isotopically light S from H₂S in the hydrothermal fluid to the sulfate reservoir. Slow mixing of the hydrothermal fluid and seawater in the chimney walls or within a mound favors sulfate reduction. This sulfate reduction results in S isotope fractionation approaching equilibrium values and ³⁴S-enriched SO₄²⁻ resulting from a reservoir effect, as ³²S is removed to the sulfide reservoir (Peter and Shanks, 1992).

A recent study (Aoyama et al., 2014) showed that the reduction of seawater sulfate diffusing downward the sediment at the Iheya North hydrothermal field (located north of the CLAM field) was caused rather by microbial than by thermochemical processes. This microbial sulfate reduction resulting in ³⁴S-enriched sulfate happens in the sediment pore-waters at some depth within the sediment. This process may be assumed to happen in the CLAM sediments and eventually, the resulting heavy-S sulfate pore-waters to be entrained within the ascending hydrothermal fluids and to influence the S-isotope signature of the CLAM chimneys and venting fluids.

Overall, the S-isotope composition of the sulfates in the CLAM Mn-carbonate deposits suggests that (1) Ca-rhodochrosite likely precipitated in oxic conditions through rapid mixing of hydrothermal fluid and seawater; (2) Mn-calcite precipitated in reduced conditions (thermochemical or microbial sulfate reduction) presumably through slow mixing of hydrothermal fluid and seawater.

## 6.7. *Sources of Sr, Nd and Pb in CLAM hydrothermal fluids and deposits: isotope constraints*

When compared to the Okinawa Trough deep seawater, lavas and sediments, the ⁸⁷Sr/⁸⁶Sr ratio of the CLAM vent fluids (including the end-member hydrothermal fluid) is very close to that of modern deep seawater in the southern Okinawa Trough (Fig. 6). This in turn suggests that the sampled CLAM vent fluids have not extracted significant amount of Sr from the underlying
volcanic rocks or sediments. In contrast to the vent fluids, the Mn-carbonate chimneys show more variable Sr isotopes (Table 13; Fig. 6). Therefore, the sampled CLAM vent fluids are not entirely representative of the hydrothermal fluids that precipitated the Mn-carbonate chimneys. Majority of the chimney samples, particularly those of the Mn-calcite chimney (2K#366-St.3), show lower Sr isotope ratios ($^{87}$Sr/$^{86}$Sr < 0.7089; Table 13), compared to the range observed in the vent fluids (Table 13; Fig. 6), indicating that higher proportion of their Sr was derived from the local volcanic rocks. Overall, Sr isotopes in all, but two of the chimney samples show $^{87}$Sr/$^{86}$Sr ratios that can be explained by deposition from circulating hydrothermal fluids that extracted minor amount of Sr from the underlying volcanic rocks (Fig. 6). However, two of the chimney samples (2K#366-St.2-a and 2K#366-St.3C-c, both Ca-rhodochrosite) show $^{87}$Sr/$^{86}$Sr higher than that of the local seawater, indicating also involvement of sediment-derived Sr in the hydrothermal system. Similarly, Sr isotope data for the Hakurei and JADE seafloor hydrothermal deposits show a range from 0.70905 to 0.7095, also indicative of some sediment-derived Sr in the hydrothermal systems (Marumo and Hattori, 1999; Noguchi et al., 2011). The much more variable Sr isotopes in the Mn-carbonate chimneys compared to the vent fluids indicate involvement of seawater, lavas, and sediments in the CLAM hydrothermal system. This in turn implies that the CLAM hydrothermal system has been more dynamic compared to the observed narrow range of Sr isotopes in the current venting fluids. The sampled vent fluids most likely represent a short time interval of the system life and are not entirely representative of the CLAM hydrothermal activity that formed the Mn-carbonate chimneys. Similar information is evident in the Nd isotopic compositions of the chimneys. Some of the chimneys show similar $^{143}$Nd/$^{144}$Nd to the local lavas, but others extend to lower values indicating that part of the Nd was derived from the local sediments (Fig. 7).

The Mn-carbonate chimneys show distinct Pb isotopes when compared to the Okinawa lavas (Fig. 8) indicating that the underlying volcanic basement was not a major source of Pb to the CLAM hydrothermal system. Most of the CLAM Mn-carbonates show similar Pb isotopic compositions to Okinawa sediments indicating that Pb was mostly derived from the later (Fig. 8). Furthermore, majority of the CLAM chimney samples show very similar Pb isotopes to the Hakurei hydrothermal field galena and JADE hydrothermal field ores (Fig. 8). Lead isotopes of the Hakurei and JADE hydrothermal precipitates have also been interpreted to be mostly of sedimentary origin (Halbach et al., 1997; Totsuka et al., 2019). In contrast, galena samples from
the Iheya North field, located in volcanic complex, show Pb isotopes (Totsuka et al., 2019) indicative of Pb with volcanic origin (Fig. 8).

Although the majority of the Mn-carbonates show overlapping Pb isotopes indicating sediment-derived Pb, three of the Mn-calcite samples (2K#366-St.3-a, -c, -e) show distinct Pb isotopic compositions (Fig. 8). Sample 2K#366-St.3-c shows elevated Pb isotopes and plots in general direction expected for Pb derived from sedimentary rocks (Fig. 8). The other two samples (2K#366-St.3-a and 2K#366-St.3-e) show lower ²⁰⁶Pb/²⁰⁴Pb and higher ²⁰⁷Pb/²⁰⁴Pb and plot away from the local volcanic rocks or sediments (Fig. 8). As can be seen on the Figure 8, none of the possible natural sources or mixing between them, can explain the isotopic compositions of these two samples. These two samples also show much lower Pb concentrations compared to the majority of the Mn-carbonates (Table 5) and it is quite possible that they were precipitated during waning, lower-temperature stages of the CLAM hydrothermal system. Pb in low-temperature hydrothermal precipitates is controlled by the local seawater Pb, not by hydrothermal leaching from underlying sediments or volcanic rocks (Kamenov et al., 2009). The unusual Pb isotopic compositions of these two samples (2K#366-St.3-a and 2K#366-St.3-e) potentially can be explained by involvement of anthropogenic Pb in the system. It is well documented that Pb in surficial marine sediments, corals, and modern seawater, in particular during the leaded gasoline usage era of the 20th century, was controlled by anthropogenic input (Shen and Boyle, 1987; Hamelin et al., 1990; Wu and Boyle, 1997). During the waning stages of the hydrothermal activity the hydrothermal fluid is not capable of extracting significant amount of Pb from the underlying rocks and as a result recent hydrothermal precipitates reflect the anthropogenic Pb present in the local seawater (Kamenov et al., 2009). Therefore, it is possible that local anthropogenic Pb, carried by the circulating Okinawa seawater, became incorporated in the Mn-calcite samples 2K#366-St.3-a and 2K#366-St.3-e. This in turn suggests that these two carbonate samples precipitated recently, potentially during the 20th century period of leaded gasoline usage. The rest of the Mn-carbonates indicate sediment-derived Pb and have much higher Pb concentrations (Table 5), potentially precipitated from a higher temperature hydrothermal fluid capable of leaching significant amount of Pb from the local sediments. Only sample 2K#366-St.3-d shows low Pb concentration and sediment-derived Pb, suggesting that this sample was formed under different conditions or different time period compared to the other two low-Pb samples (2K#366-St.3-a and 2K#366-St.3-e). Overall, the sedimentary origin of Pb in the
majority of the CLAM hydrothermal Mn-carbonates is similar to observations in other seafloor
hydrothermal systems located in areas with significant sediment cover. For example, Pb in
metalliferous sediments from the Atlantis II Deep (Red Sea) is also derived from the underlying
sediments, not from local volcanic rocks (Dupre et al., 1988). Similarly, Pb isotopes in
hydrothermal deposits from the sediment-covered Escanaba Trough also show Pb derivation from
the local sediments (LeHuray et al., 1988).

### 6.8. Hydrothermal carbonate precipitation: stability phase diagram constraints

In the average CLAM vent fluid (Supplementary Table 2) the rhodochrosite is stable in a
narrow Eh-pH range ($6<pH<10$; Eh reduced to slightly oxic) (Fig. 9 A) and in a wide range of
[Mn] and [Ca] activities (Fig. 9 C). At the same conditions, the stable Ca-minerals are anhydrite
and mixed Ca-Mg-carbonates (dolomite and huntite) (Fig. 9 B). Anhydrite is stable in wide range
of [Mn] and [Ca] activities (Fig. 9 D). Calcite can precipitate from the average CLAM vent fluid
only if $[Mg] = 0$ (Fig. 9 E). This means that the Mn-calcite in the studied carbonate chimneys had
likely precipitated from a hydrothermal fluid with close to zero Mg concentration. In other words,
from a close to end-member hydrothermal fluid (with a low proportion of mixed seawater). In
fact, our C-O-isotope data confirm exactly that: the Mn-calcite samples (2K#366-St.3)
precipitated from a fluid close to the end-member hydrothermal fluid (Fig. 4). Similarly, the Sr
isotope data for Mn-calcite samples (2K#366-St.3) show lower $^{87}\text{Sr}/^{86}\text{Sr}$ ratios compared to those
of the Ca-rhodochrosite samples (Table 13), also indicating lower proportion of seawater
contributing to the Mn-calcite chimney formation. Available Sr isotope data for the southern
Pescadero Basin hydrothermal calcite and anhydrite show a range from 0.7062 to 0.7079, also
indicating relatively lower proportion of seawater Sr in the hydrothermal system (Paduan et al.,
2018). At the same time, however, a single Pb isotope analysis of galena from southern
Pescadero Basin chimneys showed elevated ratios suggesting sedimentary Pb input (Paduan et
al., 2018): i.e., low seawater, but visible sedimentary influence. In contrast to Sr isotopes
(implying minor seawater influence), Nd isotopes for the CLAM Mn-calcite samples show a
spread suggesting involvement of sediment-derived Nd. Similarly, Pb isotopes in the CLAM Mn-
calcite samples are also controlled by either sedimentary or seawater (potentially anthropogenic)
Pb, as discussed above (see sub-section 6.7). With increasing seawater component, evident in
higher Sr isotope ratios (Table 13) and heavier both C and O isotope compositions (Fig. 4), the carbonates that precipitate are rhodochrosite and mixed Ca-Mn-Mg-Fe-carbonates (2K#366-St.3C; Table 3, Fig. 4) to almost pure rhodochrosite (2K#366-St.2; Table 3, Fig. 4). The S-isotope composition of the CLAM Mn-carbonates puts additional constraints on the most probable areas (conditions) of their precipitation. Sulfur-isotope composition of the Ca-rhodochrosite suggests precipitation in oxic conditions (Eh>0), whereas that of the Mn-calcite suggests precipitation in reduced conditions (Eh<0) (see sub-section 6.6). Thus, the areas of the highest stability of the Ca-rhodochrosite (Fig. 9 A) and Mn-calcite (Fig. 9 E) in the CLAM chimneys become further restricted. It is important to note that in the stability fields of the above chemical species one mineral is the most stable phase, but other minerals may co-exist to a lesser extent.

993

## 7. Summary

995

Carbonate chimneys from the CLAM hydrothermal field located at the Iheya Ridge (Okinawa Trough) are composed of Mn-calcite and Ca-rhodochrosite. These mixed Mn-Ca-carbonates have varying unit-cell parameters (a and c) depending on the mineral chemistry: the more Mn in the crystal lattice the smaller a and c. The crystallography differences between Mn-calcite and Ca-rhodochrosite control the isomorphic replacement of REE for Ca and Mn and result in partitioning of the REE between these carbonates: enrichment in Tb, Dy, Ho, Er, Tm, Yb and Lu, and depletion in La, Ce, Pr and Nd of the Ca-rhodochrosite chimneys relative to the Mn-calcite chimney.

CLAM hydrothermal fluids are enriched in Li relative to hydrothermal fluids from other sedimented ridges and back-arc basins that indicates a low water/rock ratio of the CLAM hydrothermal system. Sodium and Cl concentrations and molar ratios in the CLAM hydrothermal fluids suggest (1) these fluids have undergone phase separation and contain a substantial proportion of low-chlorinity vapor, and (2) sub-seafloor formation of Na-rich alteration minerals (e.g., albite) during fluid/rock reactions. CLAM hydrothermal fluids are depleted in Ca and Sr, likely a result of Ca and Sr removal upon precipitation of the Mn-Ca-carbonates. The removal of Mo, U and Cd from the seawater to the host rocks during the seawater/rock interaction is most probably responsible for the depletion in Mo, U and Cd of the CLAM hydrothermal fluids
relative to the seawater. Although certain amount of Mn is removed from the CLAM fluids to Mn-carbonates upon hydrothermal fluid/seawater mixing, its content in the hydrothermal fluids is still high. Higher Mn/Ca ratio of the Iheya Ridge rocks than that of the rocks from the other sedimented ridges seems to be responsible for the higher Mn concentration in the CLAM hydrothermal fluids compared to the hydrothermal fluids from the other sedimented ridge systems.

Carbon and O isotope compositions of the CLAM Mn-carbonates suggest they precipitated through simple binary mixing of end-member hydrothermal fluid and seawater with Mn-calcite precipitation from almost pure end-member hydrothermal fluid and Ca-rhodochrosite precipitation from seawater-dominated vent fluid. In addition to this mixing (hydrothermal fluid – seawater) the fluid had progressively been degassed and cooled.

Simple mixing between hydrothermal fluid and seawater without isotope fractionation cannot explain the Mg isotope composition of the CLAM Mn-carbonates. We suppose that Mg isotope fractionation during Mn-carbonate precipitation depends on their growth conditions and resulting different carbonate mineralogy.

The S-isotope composition of the CLAM Mn-carbonate chimneys suggests that the Ca-rhodochrosite precipitated in oxic conditions through rapid mixing of hydrothermal fluid and seawater, whereas the Mn-calcite precipitated in reduced conditions (thermochemical or microbial sulfate reduction) through slow mixing hydrothermal fluid and seawater.

Strontium isotope composition of the CLAM hydrothermal fluids is close to that of modern deep seawater in the southern Okinawa Trough and suggests that the currently venting CLAM fluids have not extracted significant amount of Sr from the underlying volcanic rocks or sediments. In contrast, Sr isotope compositions of most of the CLAM chimneys indicate higher amount of Sr derived from the underlying volcanic rocks compared to the sampled vent fluids. In addition, Sr isotopes in two of the chimney samples indicate involvement of sediment-derived Sr in the hydrothermal system. Neodymium isotope composition of the CLAM Mn-carbonate chimneys also provides similar information: Nd was derived from the local lavas and sediments.

Pb isotope data suggests that Pb in the majority of the CLAM Mn-carbonates is of sedimentary origin similar to that of Pb in other sedimented ridge hydrothermal systems. Involvement of anthropogenic Pb in the hydrothermal system can explain the unusual Pb isotope compositions of two Mn-calcite samples.
Stability phase diagram modeling along with C-O-S-Sr-isotope data show that in the average CLAM vent fluid the rhodochrosite is stable in a narrow Eh-pH range (6<pH<10; Eh slightly oxic) and in a wide range of [Mn] and [Ca] activities, whereas calcite can only precipitate from a close to the end-member hydrothermal fluid in reduced conditions (Eh<0).

1048

1049 **Data statement**

1050

Supplementary data to this article can be found online at Mendeley Data: http://dx.doi.org/10.17632/8xhvhhx2kb.1.

1053

1054 **Acknowledgements**

1055

The successful completion of this work would not be possible without the valuable help of a number of colleagues. Our sincere thanks go to: Prof. T. Gamo (The University of Tokyo) and Dr. T. Tomiyama (JAMSTEC) for providing the hydrothermal fluid and chimney samples (respectively) from the CLAM field and Dr. R.A. Koski (U.S. Geological Survey) for providing a flange sample (and some unpublished data on it) from the Guaymas Basin for this study, Prof. Y. Takahashi (The University of Tokyo) for giving us the possibility to perform our SEM studies in his lab, and Mrs. M. Otsuki (Tokyo University of Marine Science and Technology) for helping us with the EMP analyses. We greatly appreciate the efforts of J.-I. Ishibashi, T. Nozaki and an anonymous reviewer to read this work and make valuable comments and suggestions that substantially improved it.

1066

1067

1068 **References**

1069

Aoyama, S., Nishizawa, M., Takai, K., Ueno, Y., 2014. Microbial sulfate reduction within the Iheya North subseafloor hydrothermal system constrained by quadruple sulfur isotopes. Earth Planet. Sci. Lett. 398, 113-126.

Araoka, D., Nishio, Y., Gamo, T., Yamaoka, K., Kawahata, H., 2016. Lithium isotopic systematics of submarine vent fluids from arc and back-arc hydrothermal systems in the western Pacific. Geochem. Geophys. Geosyst. 17, 3835-3853.
1076 Araoka, D., Yoshimura, T., 2019. Rapid purification of alkali and alkaline-earth elements for isotope analysis (δ⁷Li, δ²⁶Mg, ⁸⁷Sr/⁸⁶Sr, and δ⁸⁸Sr) of rock samples using borate fusion followed by ion chromatography with a fraction collector system. *Anal. Sci.* 35, 751-757.

1079 Aries, S., Valladon, M., Polvé, M., Dupré, B., 2000. A routine method for oxide and hydroxide interference corrections in ICP-MS chemical analysis of environmental and geological samples. *Geostand. Newslett.* 24, 19-31.

1082 Bayon, G., Dupré, S., Ponzevera, E., Etoubleau, J., Chéron, S., Pierre, C., Mascle, J., Boetius, A., de Lange, G.J., 2013. Formation of carbonate chimneys in the Mediterranean Sea linked to deep-water oxygen depletion. *Nat. Geosci.* 6, 755-760.

1085 Berndt, M.E., Seyfried, W.E., Jr., Janecky, D.R., 1989. Plagioclase and epidote buffering of cation ratios in mid-ocean ridge hydrothermal fluids: experimental results in and near the supercritical region. *Geochim. Cosmochim. Acta* 53, 2283-2300.

1088 Bethke, C.M., 2008. Geochemical and Biogeochemical Reaction Modeling. Cambridge University Press, New York, 547 p.

1090 Bischoff, J.L., Dickson, F.W., 1975. Seawater-basalt interaction at 200°C and 500 bars: implications for origin of seafloor heavy metal deposits and regulation of seawater chemistry. *Earth Planet. Sci. Lett.* 25, 385-397.

1092 Breit, G.N., Simmons, E.C., Goldhaber, M.B., 1985. Dissolution of barite for the analysis of strontium isotopes and other chemical and isotopic variations using aqueous sodium carbonate. *Chem. Geol.* 52, 333-336.

1094 Butterfield, D.A., McDuff, R.E., Franklin, J., Wheat, C.G., 1994. Geochemistry of hydrothermal vent fluids from Middle Valley, Juan de Fuca Ridge. In: Mottl, M.J., Davis, E.E., Fisher, A.T., Slack, J.F. (Eds.), *Proceedings of the Ocean Drilling Program, Scientific Results* 139, 395-410.

1097 Chen, J.H., Wasserburg, G.J., Von Damm, K.L., Edmond, J.M., 1986. The U-Th-Pb systematics in hot springs on the East Pacific Rise at 21°N and in the Guaymas Basin. *Geochim. Cosmochim. Acta* 50, 2467-2479.

1099 Chen, Y., Matsumoto, R., Paull, C.K., Ussler III, W., Lorenson, T., Hart, P., Winters, W., 2007. Methane-derived authigenic carbonates from the northern Gulf of Mexico - MD02 Cruise. *J. Geochem. Explor.* 95, 1-15.

1101 Chow, N., Morad, S., Al-Aasm, I.S., 1996. Origin of authigenic carbonates in Eocene to Quaternary sediments from the Arctic Ocean and Norwegian-Greenland Sea. In: Thiede, J., Myhre, A.M., Firth, J.V., Johnson, G.L., Ruddiman, W.F. (Eds.), *Proceedings of the Ocean Drilling Program, Scientific Results*, 151, 415-434.

1104 Craddock, P.R., Bach, W., Seewald, J.S., Rouxel, O.J., Reeves, E., Tivey, M.K., 2010. Rare earth element abundances in hydrothermal fluids from the Manus Basin, Papua New Guinea: indicators of sub-seafloor hydrothermal processes in back-arc basins. *Geochim. Cosmochim. Acta* 74, 5494-5513.

1107 De Boever, E., Swennen, R., Dimitrov, L., 2006. Lower Eocene carbonate cemented chimneys (Varna, NE Bulgaria): Formation mechanisms and the (a)biological mediation of chimney growth? *Sediment. Geol.* 185, 159-173.

1110 Dekov, V.M., Guéguen, B., Yamanaka, T., Moussa, N., Okumura, T., Bayon, G., Liebetrau, V., Yoshimura, T., Kamenov, G., Araoka, D., Makita, H., Sutton, J., 2021. When a mid-ocean ridge encroaches a continent: Seafloor-type hydrothermal activity in Lake Asal (Afar Rift). *Chem. Geol.* 568, 120126.
1113 Díaz-del-Río, V., Somoza, L., Martínez-Frias, J., Mata, M.P., Delgado, A., Hernandez-Molina, F.J., Lunar, R.,
1114 Martín-Rubí, J.A., Maestro, A., Fernández-Puga, M.C., León, R., Llave, E., Medialdea, T., Vázquez, J.T.,
1115 2003. Vast fields of hydrocarbon-derived carbonate chimneys related to the accretionary wedge/olistostrome
1116 of the Gulf of Cádiz. Mar. Geol. 195, 177-200.

1117 Dou, Y., Yang, S., Liu, Z., Shi, X., Li, J., Yu, H., Berne, S., 2012. Sr-Nd isotopic constraints on terrigenous
1118 sediment provenances and Kuroshio Current variability in the Okinawa Trough during the late Quaternary.
1119 Palaeogeogr. Palaeoclimatol. Palaeoecol. 356-366, 38-47.

1120 Du, S., Wen, H., Liu, S., Qin, C., Yan, Y., Yang, G., Feng, P., 2020. Mineralogy and metallogenesis of the Sanbao
1121 Mn-Ag (Zn-Pb) deposit in the Laojunshan ore district, SE Yunnan province, China. Minerals 10, 650.

1122 Dupre, B., Blanc, G., Boulegue, J., Allegre, C.J., 1988. Metal remobilization at a spreading center studied using lead
1123 isotopes. Nature 333, 165-167.

1124 Eom, J., Yoshimura, T., Araoka, D., Gamo, T., Kawahata, H., 2020. Magnesium isotopic composition of submarine
1125 vent fluids from arc and back-arc hydrothermal systems in the western Pacific. Chem. Geol. 551, 119767.

1126 Expedition 331 Scientists, 2011. Expedition 331 summary. In: Takai, K., Mottl, M.J., Nielsen, S.H., and the
1127 Expedition 331 Scientists, Proc. IODP, Tokyo (Integrated Ocean Drilling Program Management International,
1128 Inc.), 331, 39 p.

1129 Fouquet, Y., Cambon, P., Etoubleau, J., Charlou, J.L., Ondréas, H., Barriga, F.J.A.S., Cherkashov, G., Semkova, T.,
1130 Poroshina, I., Bohn, M., Donval, J.P., Henry, K., Murphy, P., Rouxel, O., 2010. Geodiversity of hydrothermal
1131 processes along the Mid-Atlantic Ridge and ultramafic-hosted mineralization: A new type of oceanic Cu-Zn-
1132 Co-Au volcanogenic massive sulfide deposit. In: Rona, P.A., Devey, C.W., Dyment, J., Murton, B.J. (Eds.),
1133 Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges, Geophysical Monograph Series 188,
1134 321-367.

1135 Friedman, I., O'Neil, J.R., 1977. Compilation of stable isotope fractionation factors of geochemical interest. In:
1136 Fleischer, M. (Ed.), Data of Geochemistry, 6th edition, Geol. Surv. Prof. Paper 440-KK, 61 p.

1137 Fryer, P., Wheat, C.G., Mottl, M.L., 1999. Mariana blueschist mud volcanism: Implications for conditions within the
1138 subduction zone. Geology 27, 103-106.

1139 Fujinaga, K., Yasukawa, K., Nakamura, K., Machida, S., Takaya, Y., Ohta, J., Araki, S., Liu, H., Usami, R., Maki,
1140 R., Haraguchi, S., Nishio, Y., Usui, Y., Nozaki, T., Yamazaki, T., Ichiyama, Y., Ijiri, A., Inagaki, F.,
1141 Machiyama, H., Iijima, K., Suzuki, K., Kato, Y., KR13-02, MR13-E02 Leg 2 and KR14-02 Cruise members,
1142 2016. Geochemistry of REY-rich mud in the Japanese Exclusive Economic Zone around Minamitorishima
1143 Island. Geochem. J. 50, 575-590.

1144 Gamo, T., 1995. Wide variation of chemical characteristics of submarine hydrothermal fluids due to secondary
1145 modification processes after high temperature water-rock interaction: a review. In: Sakai, H., Nozaki, Y.
1146 (Eds.), Biogeochemical Processes and Ocean Flux in the Western Pacific. Terra Scientific Publishing
1147 Company, Tokyo, pp. 425-451.

1148 Gamo, T., Sakai, H., Kim, E-S., Shitashima, K., Ishibashi, J.-I., 1991. High alkalinity due to sulfate reduction in the
1149 CLAM hydrothermal field, Okinawa Trough. Earth Planet. Sci. Lett. 107, 328-338.
1150 German, C.R., Von Damm, K.L., 2003. Hydrothermal processes. In: Turekian, K.K., Holland, H.D. (Eds.), Treatise on Geochemistry, Vol. 6 The Oceans and Marine Geochemistry, Elsevier, Oxford, pp. 181-222.

1151 Goodfellow, W.D., Blaise, B., 1988. Sulfide formation and hydrothermal alteration of hemipelagic sediment in Middle Valley, northern Juan de Fuca Ridge. Can. Mineral. 26, 675-696.

1152 Halbach, P., Hansmann, W., Köppel, V., Pracejus, B., 1997. Whole-rock and sulfide lead-isotope data from the hydrothermal JADE field in the Okinawa back-arc trough. Miner. Deposita 32, 70-78.

1153 Hamelin, B., Grousset, F., Sholkovitz, E.R., 1990. Pb isotopes in surficial pelagic sediments from the North Atlantic. Geochim. Cosmochim. Acta 54, 37-47.

1154 Hariya, Y., Hasegawa, K., 1979. Contact metasomatic zone and massive ore body in manganese epithermal deposits of the Jokoku mine, Hokkaido, Japan. J. Fac. Sci., Hokkaido Univ., Ser. IV, 19, 265-272.

1155 Hongo, Y., Obata, H., Gamo, T., Nakaseama, M., Ishibashi, J., Konno, U., Saegusa, S., Ohkubo, S., Tsunogai, U., 2007. Rare earth elements in the hydrothermal system at Okinawa Trough back-arc basin. Geochem. J. 41, 1-15.

1160 Honma, H., Kusakabe, M., Kagami, H., Iizumi, S., Sakai, H., Kodama, Y., Kimura, M., 1991. Major and trace element chemistry and D/H, ¹⁸O/¹⁶O, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of rocks from the spreading center of the Okinawa Trough, a marginal back-arc basin. Geochem. J. 25, 121-136.

1161 Hori, M., Kawai, T., Matsuoka, J., Kano, A., 2009. Intra-annual perturbations of stable isotopes in tufas: Effects of hydrological processes. Geochim. Cosmochim. Acta 73, 1684-1695.

1162 Huang, K.-F., You, C.-F., Chung, C.-H., Lin, I.-T., 2011. Nonhomogeneous seawater Sr isotopic composition in the coastal oceans: A novel tool for tracing water masses and submarine groundwater discharge. Geochem. Geophys. Geosyst. 12, Q05002.

1163 Ishibashi, J., Sano, Y., Wakita, H., Gamo, T., Tsutsumi, M., Sakai, H., 1995. Helium and carbon geochemistry of hydrothermal fluids from the Mid-Okinawa Trough Back Arc Basin, southwest of Japan. Chem. Geol. 123, 1-15.

1164 Ishibashi, J.-I., Noguchi, T., Toki, T., Miyabe, S., Yamagami, S., Onishi, Y., Yamanaka, T., Yokoyama, Y., Omori, E., Takahashi, Y., Hatada, K., Nakaguchi, Y., Yoshizaki, M., Konno, U., Shibuya, T., Takai, K., Inagaki, F., Kawagucci, S., 2014. Diversity of fluid geochemistry affected by processes during fluid upwelling in active hydrothermal fields in the Izena Hole, the middle Okinawa Trough back-arc basin. Geochem. J. 48, 357-369.

1165 Ishibashi, J.-I., Ikegami, F., Tsuji, T., Urabe, T., 2015. Hydrothermal activity in the Okinawa trough back-arc basin: Geological background and hydrothermal mineralization. In: Ishibashi, J.-I., Okino, K., Sunamura, M. (Eds.), Subseafloor Biosphere Linked to Hydrothermal Systems: TAIGA Concept. Springer, Tokyo, pp. 337-359.

1166 Ishizuka, H., Kawanobe, Y., Sakai, H., 1990. Petrology and geochemistry of volcanic rocks dredged from the Okinawa Trough, an active back-arc basin. Geochem. J. 24, 75-92.

1167 Izawa, E., Motomura, Y., Tanaka, T., Kimura, M., 1991. Hydrothermal carbonate chimneys in the Iheya Ridge of the Okinawa Trough. JAMSTEC Deepsea Res., 185-192.

1168 Kamenov, G.D., Perfit, M.R., Mueller, P.A., Jonasson, I.R., 2008. Controls on magmatism in an island arc environment: study of lavas and sub-arc xenoliths from the Tabar-Lihir-Tanga-Feni island chain, Papua New Guinea. Contrib. Mineral. Petrol. 155, 635-656.
1188 Kamenov, G.D., Dekov, V.M., Willingham, A.L, Savelli, C., Belluci, L.G., 2009. Anthropogenic Pb in recent
1189 hydrothermal sediments from the Tyrrhenian Sea: implications for seawater Pb control on low-temperature
1190 hydrothermal systems. Geology 37, 111-114.

1191 Kato, Y., Ohta, I., Tsunematsu, T., Watanabe, Y., Isozaki, Y., Maruyama, S., Imai, N., 1998. Rare earth element
1192 variations in mid-Archean banded iron formations: Implications for the chemistry of ocean and continent and
1193 plate tectonics. Geochim. Cosmochim. Acta 62, 3475-3497.

1194 Kato, Y., Nakao, K., Isozaki, Y., 2002. Geochemistry of Late Permian to Early Triassic pelagic cherts from
1195 southwest Japan: implications for an oceanic redox change. Chem. Geol. 182, 15-34.

1196 Kato, Y., Fujinaga, K., Suzuki, K., 2005. Major and trace element geochemistry and Os isotopic composition of
1197 metalliferous umbers from the Late Cretaceous Japanese accretionary complex. Geochem. Geophys. Geosyst.
1198 6, Q07004.

1199 Kato, Y., Fujinaga, K., Nakamura, K., Takaya, Y., Kitamura, K., Ohta, J., Toda, R., Nakashima, T., Iwamori, H.,
1200 2011. Deep-sea mud in the Pacific Ocean as a potential resource for rare-earth elements. Nat. Geosci. 4, 535-
1201 539.

1202 Kawagucci, S., 2015. Fluid geochemistry of high-temperature hydrothermal fields in the Okinawa Trough. In:
1203 Ishibashi, J.-I., Okino, K., Sunamura, M. (Eds.), Subseafloor Biosphere Linked to Hydrothermal Systems:
1204 TAIGA Concept. Springer, pp. 387-403.

1205 Kelley, D.S., Karson, J.A., Blackman, D.K., Früh-Green, G.L., Butterfield, D.A., Lilley, M.D., Olson, E.J., Schrenk,
1206 M.O., Roe, K.K., Lebon, G.T., Rivizzigno, P. and the AT3-60 Shipboard Party, 2001. An off-axis
1207 hydrothermal vent field near the Mid-Atlantic Ridge at 30°N. Nature 412, 145-149.

1208 Kimura, M., Kaneoka, I., Kato, Y., Yamamoto, S., Kushiro, I., Tokuyama, H., Kinoshita, H., Isezaki, N., Masaki, H.,
1209 Ishida, A., Uyeda, S., Hilde, T.W.C., 1986. Report on DELP 1984 Cruise in the Middle Okinawa Trough-5:
1210 topography and geology of the central grabens and their vicinity. Bull. Earth. Res. Inst. Univ. Tokyo 61, 269-
1211 310.

1212 Klinkhammer, G.P., Elderfield, H., Edmond, J.M., Mitra, A., 1994. Geochemical implications of rare earth element
1213 patterns in hydrothermal fluids from mid-ocean ridges. Geochim. Cosmochim. Acta 58, 5105-5113.

1214 Koski, R.A., Lonsdale, P.F., Shanks, W.C., Berndt, M.E., Howe, S.S., 1985. Mineralogy and geochemistry of a
1215 sediment-hosted hydrothermal sulfide deposit from the Southern Trough of Guaymas Basin, Gulf of
1216 California. J. Geophys. Res. 90, 6695-6707.

1217 Kotake, Y., 2000. Study on the tectonics of western Pacific region derived from GPS data analysis. Bull. ERI Univ.
1218 Tokyo 75, 229-334.

1219 Kuleshov, V.N., 2013. Manganese rocks and ores: Isotope geochemistry, origin, and ore genesis evolution. Nauchniy
1220 Mir, Moscow, 508 p. (in Russian)

1221 Labidi, J., Cartigny, P., Birck, J.L., Assayag, N., Bourrand, J.J., 2012. Determination of multiple sulfur isotopes in
glasses: A reappraisal of the MORB δ³⁴S. Chem. Geol. 334, 189-198.

1222 Launay, J., Fontes, J.C., 1985. Les sources thermales de Prony (Nouvelle Calédonie) et leurs précipités chimiques,
1223 exemple de formation de brucite primaire. Geol. France 1, 83-100.
1225 Makishima, A., Nakamura, E., 2006. Determination of major, minor and trace elements in silicate samples by ICP-
1226 QMS and ICP-SFMS applying isotope dilution-internal standardisation (ID-IS) and multi-stage internal
1227 standardisation. Geostand. Geoanal. Res. 30, 245-271.

1228 Marumo, K., Hattori, K.H., 1999. Seafloor hydrothermal clay alteration at Jade in the back-arc Okinawa Trough:
1229 Mineralogy, geochemistry and isotope characteristics. Geochim. Cosmochim. Acta 63, 2785-2804.

1230 Maslen, E.N., Streltsov, V.A., Streltsova, N.R., Ishizawa, N., 1995. Electron density and optical anisotropy in
1231 rhombohedral carbonates. III. Synchrotron X-ray studies of CaCO3, MgCO3 and MnCO3. Acta Crystallogr.
1232 B51, 929-939.

1233 McDonough, W.F., Sun, S.-s., 1995. The composition of the Earth. Chem. Geol. 120, 223-253.

1234 Minami, T., Konagaya, W., Zheng, L., Takano, S., Sasaki, M., Murata, R., Nakaguchi, Y., Sohrin, Y., 2015. An off-
1235 line automated preconcentration system with ethylenediaminetriacetate chelating resin for the determination
1236 of trace metals in seawater by high-resolution inductively coupled plasma mass spectrometry. Anal. Chim.
1237 Acta 854, 183-190.

1238 Mottl, M.J., Holland, H.D., 1978. Chemical exchange during hydrothermal alteration of basalt by seawater. I.
1239 Experimental results for major and minor components of seawater. Geochim. Cosmochim. Acta 42, 1103-
1240 1115.

1241 Nagumo, S., Kinoshita, H., Kasahara, J., Ouchi, T., Tokuyama, H., Asanuma, T., Koresawa, S., Akiyoshi, H., 1986.
1242 Report on DELP 1984 cruises in the middle Okinawa Trough Part 2: seismic structural studies. Bull. ERI
1243 Univ. Tokyo 61, 167-202.

1244 Nakagawa, M., Santosh, M., Maruyama, S., 2009. Distribution and mineral assemblages of bedded manganese
1245 deposits in Shikoku, Southwest Japan: Implications for accretion tectonics. Gondwana Res. 16, 609-621.

1246 Nitta, M., Inoue, A., 2011. Carbon and oxygen isotope compositions of calcite and rhodochrosite from geothermal
1247 exploratory drills TH-4 and TH-6 near the Toyoha deposit, Hokkaido, Japan. Resour. Geol. 61, 159-173.

1248 Noguchi, T., Shinjo, R., Ito, M., Takada, J., Oomori, T., 2011. Barite geochemistry from hydrothermal chimneys of
1249 the Okinawa Trough: insight into chimney formation and fluid/sediment interaction. J. Mineral. Petrol. Sci.
1250 106, 26-35.

1251 Nozaki, T., Ishibashi, J.-I., Shimada, K., Nagase, T., Takaya, Y., Kato, Y., Kawagucci, S., Watsuji, T., Shibuya, T.,
1252 Yamada, R., Saruhashi, T., Kyo, M., Takai, K., 2016. Rapid growth of mineral deposits at artificial seafloor
1253 hydrothermal vents. Sci. Rep. 6, 22163.

1254 Nozaki, T., Nagase, T., Ushikubo, T., Shimizu, K., Ishibashi, J.-I., and the D/V Chikyu Expedition 909 Scientists,
1255 2021a. Microbial sulfate reduction plays an important role at the initial stage of subseafloor sulfide
1256 mineralization. Geology 49, 222-227.

1257 Nozaki, T., Nagase, T., Takaya, Y., Yamasaki, T., Otake, T., Yonezu, K., Ikehata, K., Totsuka, S., Kitada, K.,
1258 Sanada, Y., Yamada, Y., Ishibashi, J.-I., Kumagai, H., Maeda, L., and the D/V Chikyu Expedition 909
1259 Scientists, 2021b. Subseafoor sulphide deposit formed by pumice replacement mineralization. Sci. Rep. 11,
1260 8809.

1261 Nozaki, Y., 1992. Trace elements in seawater: Their mean concentrations and North Pacific profiles. Chikyukagaku
1262 (Geochemistry) 26, 25-39. (in Japanese with English abstract)
1263 Nyman, S.L., Nelson, C.S., Campbell, K.A., 2010. Miocene tubular concretions in East Coast Basin, New Zealand: 1264 Analogue for the subsurface plumbing of cold seeps. Mar. Geol. 272, 319-336.

1265 Ohmoto, H., Rye, R.O., 1979. Isotopes of sulfur and carbon. In: Barnes, H.L. (Ed.), Geochemistry of Hydrothermal 1266 Ore Deposits, Wiley, New York, pp. 509-567.

1267 Okumura, T., Ohara, Y., Stern, R.J., Yamanaka, T., Onishi, Y., Watanabe, H., Chen, C., Bloomer, S.H., Pujana, I., 1268 Sakai, S., Ishii, T., Takai, K., 2016. Brucite chimney formation and carbonate alteration at the Shinkai Seep 1269 Field, a serpentinite-hosted vent system in the southern Mariana forearc. Geochem. Geophys. Geosyst. 17, 1270 3775-3796.

1271 Osaki, S., 1973. Carbon and oxygen isotopic compositions of hydrothermal rhodochrosites from Oe, Inakuraishi and 1272 Jokoku mines, Hokkaido, Japan. Geochem. J. 6, 151-162.

1273 Paduan, J.B., Zierenberg, R., Clague, D.A., Spelz, R.M., Caress, D.W., Troni, G., Thomas, H., Glessner, J., Lilley, 1274 M.D., Lorenson, T., Lupton, J., Neumann, F., Rosa-del Rio, M.A.S., Wheat, C.G., 2018. Discovery of 1275 hydrothermal vent fields on Alarcón Rise and in southern Pescadero Basin, Gulf of California. Geochem. 1276 Geophys. Geosyst. 19, 4788-4819.

1277 Paris, G., Sessions, A.L., Subhas, A.V., Adkins, J.F., 2013. MC-ICP-MS measurement of δ³⁴S and Δ³³S in small 1278 amounts of dissolved sulfate. Chem. Geol. 345, 50-61.

1279 Parkhurst, D.L., Appelo, C.A.J., 2013. Description of input and examples for PHREEQC version 3—A computer 1280 program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. 1281 U.S.G.S. Techniques and Methods 6-A43, 497 p.

1282 Peter, J.M., Scott, S.D., 1991. Hydrothermal mineralization in the Guaymas Basin, Gulf of California. AAPG Mem. 1283 47, 721-741.

1284 Peter, J.M., Shanks III, W.C., 1992. Sulfur, carbon, and oxygen isotope variations in submarine hydrothermal 1285 deposits of Guaymas Basin, Gulf of California, USA. Geochim. Cosmochim. Acta 56, 2025-2040.

1286 Reeves, E.P., Seewald, J.S., Saccocia, P., Bach, W., Craddock, P.R., Shanks, W.C., Sylva, S.P., Walsh, E., Pichler, 1287 T., Rosner, M., 2011. Geochemistry of hydrothermal fluids from the PACMANUS, Northeast Pual and 1288 Vienna Woods hydrothermal fields, Manus Basin, Papua New Guinea. Geochim. Cosmochim. Acta 75, 1088- 1289 1123.

1290 Rouxel, O., Shanks III, W.C., Bach, W., Edwards, K.J., 2008. Integrated Fe- and S-isotope study of seafloor 1291 hydrothermal vents at East Pacific Rise 9-10°N. Chem. Geol. 252, 214-227.

1292 Rustad, J.R., Casey, W.H., Yin, Q.-Z., Bylaska, E.J., Felmy, A.R., Bogatko, S.A., Jackson, V.E., Dixon, D.A., 2010. 1293 Isotopic fractionation of Mg²⁺(aq), Ca²⁺(aq), and Fe²⁺(aq) with carbonate minerals. Geochim. Cosmochim. Acta 1294 74, 6301-6323.

1295 Saenger, C., Wang, Z., 2014. Magnesium isotope fractionation in biogenic and abiogenic carbonates: implications 1296 for paleoenvironmental proxies. Quat. Sci. Rev. 90, 1-21.

1297 Sakai, H., Gamo, T., Kim, E-S., Shitashima, K., Yanagisawa, F., Tsutsumi, M., 1990a. Unique chemistry of the 1298 hydrothermal solution in the mid-Okinawa trough backarc basin. Geophys. Res. Lett. 17, 2133-2136.

1299 Sakai, H., Yamano, M., Tanaka, T., Gamo, T., Kim, E.-S., Ishibashi, J., Shitashima, K., Matsumoto, T., Oomori, i. 1300 T., Yanagisawa, F., Tsutsumi, M., 1990b. Geochemical studies of the hydrothermal system at the Izena
1301 Cauldron using "Shinkai 2000" - Reports on dive numbers 413 and 415, and on the liquid CO₂ bubbles and
1302 hydrate collected these during dive number 424. JAMSTEC Tech. Rep., Deep Sea Res. 6, 69-85. (in Japanese,
1303 with English abstract)

1304 Saunders, A.D., Fornari, D.J., Joron, J.-L., Tarney, J., Treuil, M., 1982. Geochemistry of basic igneous rocks, Gulf of California, Deep Sea Drilling Project Leg 64. In: Curray, J.R., Moore, D.G., et al., Init. Repts. DSDP, 64, Pt. 1: Washington (U.S. Govt. Printing Office), 595-642.

1307 Shanks III, W.C., 2001. Stable isotopes in seafloor hydrothermal systems: Vent fluids, hydrothermal deposits, hydrothermal alteration, and microbial processes. In: Valley, J.W., Cole, D.R. (Eds.), Stable Isotope Geochemistry. Rev. Mineral. Geochem. 43, pp. 469-525.

1310 Shanks III, W.C., Bischoff, J.L., Rosenbauer, R.J., 1981. Seawater sulfate reduction and sulfur isotope fractionation in basaltic systems: Interaction of seawater with fayalite and magnetite at 200-350°C. Geochim. Cosmochim. Acta 45, 1977-1995.

1313 Shen, G.T., Boyle, E.A., 1987. Lead in corals: reconstruction of historical industrial fluxes to the surface ocean. Earth Planet. Sci. Lett. 82, 289-304.

1315 Shinjo, R., Kato, Y., 2000. Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin. Lithos 54, 117-137.

1317 Shinjo, R., Chung, S.L., Kato, Y., Kimura, M., 1999. Geochemical and Sr-Nd isotopic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: Implications for the evolution of a young, intracontinental back arc basin. J. Geophys. Res. Solid Earth 104, 10591-10608.

1320 Shu, Y., Nielsen, S.G., Zeng, Z., Shinjo, R., Blusztajn, J., Wang, X., Chen, S., 2017. Tracing subducted sediment inputs to the Ryukyu arc-Okinawa Trough system: Evidence from thallium isotopes. Geochim. Cosmochim. Acta 217, 462-491.

1323 Sibuet, J.-C., Letouzey, J., Barbier, F., Charvet, J., Foucher, J.-P., Hilde, T., Kimura, M., Ling-Yun, C., Marsset, B., Muller, C., Stephan, J.-F., 1987. Back arc extension in the Okinawa Trough. J. Geophys. Res. 92, 14041-14063.

1326 Sohrin, Y., Urushihara, S., Nakatsuka, S., Kono, T., Higo, E., Minami, T., Norisuye, K., Umetani, S., 2008. Multielemental determination of GEOTRACES key trace metals in seawater by ICPMS after preconcentration using an ethylenediaminetriacetic acid chelating resin. Anal. Chem. 80, 6267-6273.

1329 Taggart, J.E., 2002. Analytical methods for chemical analysis of geologic and other materials, U.S. Geological Survey. U.S. Geological Survey Open-File Report 02-223.

1331 Takai, K., Mottl, M.J., Nielsen, S.H.H., and the IODP Expedition 331 Scientists, 2012. IODP Expedition 331: Strong and expansive subseafloor hydrothermal activities in the Okinawa Trough. Sci. Drill. 13, 19-27.

1333 Takatani, Y., Enyo, K., Iida, Y., Kojima, A., Nakano, T., Sasano, D., Kosugi, N., Midorikawa, T., Suzuki, T., Ishii, M., 2014. Relationships between total alkalinity in surface water and sea surface dynamic height in the Pacific Ocean. J. Geophys. Res. Oceans 119, 2806-2814.

1336 Takaya, Y., Hiraide, T., Fujinaga, K., Nakamura, K., Kato, Y., 2014. A study on the recovery method of rare-earth elements from REY-rich mud toward the development and the utilization of REY-rich mud. J. MMIJ 130, 104-114. (In Japanese with English abstract)
1339 Takeuchi, R., Matsumoto, R., Ogihara, S., Machiyama, H., 2007. Methane-induced dolomite "chimneys" on the Kuroshima Knoll, Ryukyu islands, Japan. J. Geochem. Explor. 95, 16-28.

1341 Tanaka, T., Mitsuzawa, K., Hotta, H., 1989. "SHINKAI 2000" diving surveys in the east of Iheya Small Ridge in the Central Okinawa Trough. JAMSTEC Deepsea Res., 267-281.

1343 Teng, F.-Z., McDonough, W.F., Rudnick, R.L., Dalpé, C., Tomascak, P.B., Chappell, B.W., Gao, S., 2004. Lithium isotopic composition and concentration of the upper continental crust. Geochim. Cosmochim. Acta 68, 4167-4178.

1346 Terashima, S., Imai, N., Taniguchi, M., Okai, T., Nishimura, A., 2002. The preparation and preliminary characterisation of four new Geological Survey of Japan geochemical reference materials: Soils, JSO-1 and JSO-2; and marine sediments, JMS-1 and JMS-2. Geostand. Newslett. 26, 85-94.

1349 Totsuka, S., Shimada, K., Nozaki, T., Kimura, J.-I., Chang, Q., Ishibashi, J.-I., 2019. Pb isotope compositions of galena in hydrothermal deposits obtained by drillings from active hydrothermal fields in the middle Okinawa Trough determined by LA-MC-ICP-MS. Chem. Geol. 514, 90-104.

1352 Trefry, J.H., Butterfield, D.B., Metz, S., Massoth, G.J., Trocine, R.P., Feely, R.A., 1994. Trace metals in hydrothermal solutions from Cleft segment on the southern Juan de Fuca Ridge. J. Geophys. Res., Solid Earth 99, 4925-4935.

1355 Tsunogai, U., Ishibashi, J., Wakita, H., Gamo, T., Watanabe, K., Kajimura, T., Kanayama, S., Sakai, H., 1994. Peculiar features of Suiyo seamount hydrothermal fluids, Izu-Bonin Arc: Differences from subaerial volcanism. Earth Planet. Sci. Lett. 126, 289-301.

1358 Von Damm, K.L., 1990. Seafloor hydrothermal activity: Black smoker chemistry and chimneys. Annu. Rev. Earth Planet. Sci. 18, 173-204.

1360 Von Damm, K.L., Parker, C.M., Zierenberg, R.A., Lilley, M.D., Olson, E.J., Clague, D.A., McClain, J.S., 2005. The Escanaba Trough, Gorda Ridge hydrothermal system: Temporal stability and subseafloor complexity. Geochim. Cosmochim. Acta 69, 21, 4971-4984.

1363 Whittaker, E.J.W., Muntus, R., 1970. Ionic radii for use in geochemistry. Geochim. Cosmochim. Acta 34, 945-956.

1364 Wu J. and Boyle E. A. (1997) Lead in the western North Atlantic Ocean: Completed response to leaded gasoline phaseout. Geochim. Cosmochim. Acta 61, 3279-3283.

1366 Yamano, M., Uyeda, S., Kinoshita, H., Hilde, T.W.C., 1986. Report on DELP 1984 cruises in the middle Okinawa Trough Part 4: heat flow measurements. Bull. ERI Univ. Tokyo 61, 251-267.

1368 Yamasaki, T., 2018. The role of bimodal magmatism in seafloor massive sulfide (SMS) ore-forming systems at the Middle Okinawa Trough, Japan. Ocean Sci. J. 53, 413-436.

1370 Yasukawa, K., Liu, H., Fujinaga, K., Machida, S., Haraguchi, S., Ishii, T., Nakamura, K., Kato, Y., 2014. Geochemistry and mineralogy of REY-rich mud in the eastern Indian Ocean. J. Asian Earth Sci. 93, 25-36.

1372 Yasukawa, K., Ohta, J., Miyazaki, T., Vaglarov, B.S., Chang, Q., Ueki, K., Toyama, C., Kimura, J.-I., Tanaka, E., Nakamura, K., Fujinaga, K., Iijima, K., Iwamori, H., Kato, Y., 2019. Statistic and isotopic characterization of deep-sea sediments in the Western North Pacific Ocean: Implications for genesis of the sediment extremely enriched in rare earth elements. Geochem. Geophys. Geosyst. 20, 3402-3430.
1376 Yasukawa, K., Kino, S., Azami, K., Tanaka, E., Mimura, K., Ohta, J., Fujinaga, K., Nakamura, K., Kato, Y., 2020.
1377 Geochemical features of Fe-Mn micronodules in deep-sea sediments of the western North Pacific Ocean:
1378 Potential for co-product metal extraction from REY-rich mud. Ore Geol. Rev. 127, 103805.

1379 Yoshimura, T., Araoka, D., Tamenori, Y., Kuroda, J., Kawahata, H., Ohkouchi, N., 2018. Lithium, magnesium and
1380 sulfur purification from seawater using an ion chromatograph with a fraction collector system for stable
1381 isotope measurements. J. Chromatogr. A 1531, 157-162.

1382 Yoshimura, T., Wakaki, S., Ishikawa, T., Gamo, T., Araoka, D., Ohkouchi, N., Kawahata, H., 2020. A aystematic
1383 assessment of stable Sr isotopic compositions of vent fluids in arc/back-arc hydrothermal systems: Effects of
1384 host rock type, phase separation, and overlying sediment. Front. Earth Sci. 8, 591711.

1385 Zhang, Y.-X., Zeng, Z.-G., Gaetani, G., Zhang, L., Lai, Z.-Q., 2020. Mineralogical constraints on the magma mixing
1386 beneath the Iheya Graben, an active back-arc spreading centre of the Okinawa Trough. J. Petrol. 61, egaa098.

1387 Zheng, Y.-F., Hoefs, J., 1993. Carbon and oxygen isotopic covariations in hydrothermal calcites. Miner. Deposita 28,
1388 79-89.

1389
1390
1391
Figure captions
1392

1393 **Fig. 1.** (A) Map of the Ryukyu Trench-Arc system and Okinawa Trough with location of the
1394 studied area (red square). (B) Detail map of the Iheya Ridge with location of the CLAM
1395 hydrothermal site (red star).

1396
1397 **Fig. 2.** Photographs of the studied samples (cross-sectional slabs) from the CLAM carbonate
1398 chimneys (Iheya Ridge): (A) sample 2K#366-St.2; (B) sample 2K#366-St.3; (C) sample 2K#366-
1399 St.3C; and Guaymas Basin flange: (D) sample T529-G4A B. Closed dotted lines and lower case
1400 letters indicate the areas of sub-sampling for mineralogy, chemistry and isotope analyses.

1401
1402 **Fig. 3.** CI chondrite-normalized (McDonough and Sun, 1995) REE distribution patterns of the
1403 studied carbonate chimneys and vent fluids samples (CLAM hydrothermal field, Iheya Ridge):
1404 (A) 2K#366-St.2 (carbonate chimney); (B) 2K#366-St.3 (carbonate chimney); (C) 2K#366-St.3C
1405 (carbonate chimney); (D) vent fluids. Lower case letters in the legends of A, B and C indicate the
1406 sub-sample IDs (for sub-sample positions see Figure 2).

1407
**Fig. 4.** C and O isotopic compositions of the carbonate chimneys from the CLAM hydrothermal field (Iheya Ridge). C-O-isotope compositions of the Guaymas Basin (Peter and Shanks, 1992) and southern Pescadero Basin (Paduan et al., 2018) carbonate chimneys are plotted for comparison. For details about the construction of the mixing line between the inferred CLAM end-member hydrothermal fluid and seawater see the text and Supplementary Table 2. VPDB = Vienna Pee Dee Belemnite; VSMOW = Vienna Standard Mean Ocean Water.

**Fig. 5.** Sulfur-isotope composition (range) of the CLAM chimneys (sulfates, sulfides and native S) and CLAM hydrothermal fluids (sulfates and sulfides; Gamo et al., 1991). References: terrestrial mantle (Labidi et al., 2012) and seawater (Paris et al., 2013). V-CDT = Vienna-Canyon Diablo Troilite.

**Fig. 6.** CLAM Mn-carbonate chimneys Sr isotopic compositions (Table 13) compared to those of CLAM vent fluids [Table 13; end-member hydrothermal fluid is the average of the calculated CLAM end-member fluid by Araoka et al. (2016) and Yoshimura et al. (2020), D426 RV-6ª from Araoka et al. (2016), D426 RV-6ᵇ from Yoshimura et al. (2020), D427 RV-6ª from Araoka et al. (2016), D427 RV-6ᵇ from Yoshimura et al. (2020)], and Okinawa Trough deep seawater (station 679-S1, depth 800 m; Huang et al., 2011), sediments (core DGKS9604; Dou et al., 2012), and lavas [basalts (Honma et al., 1991; Shinjo et al., 1999; Shu et al., 2017), andesites (Shinjo and Kato, 2000), rhyolites (Shinjo and Kato, 2000), dacites (Honma et al., 1991)]. The sediments and lavas ⁸⁷Sr/⁸⁶Sr are average values calculated from the cited references. The extent of the gray fields represent the Sr isotope variations observed in each of the three chimney samples (Table 13). Note, the ⁸⁷Sr/⁸⁶Sr range for the chimney samples is much wider compared to the vent fluids indicating that the later are not entirely representative of the hydrothermal fluids that formed the Mn-carbonates. For more discussion see the text.

**Fig. 7.** Sr and Nd isotopes for the CLAM Mn-carbonate chimneys (Table 13) compared to Mid Okinawa lavas and sediments, data from the references cited in Figure 6.

**Fig. 8.** Pb isotopic compositions of the CLAM Mn-carbonate chimneys compared to possible Pb sources in the region, Mid Okinawa Trough. Majority of the Mn-carbonate samples show Pb
isotopes similar to Hakurei hydrothermal field galena and JADE hydrothermal field ores and are
overall closer isotopically to Mid Okinawa sediments suggesting Pb derivation from the later.
Data sources: Iheya North galena (Totsuka et al., 2019), Hakurei galena (Totsuka et al., 2019),
JADE ores (Halbach et al., 1997), Mid Okinawa lavas [basalts (Halbach et al., 1997; Shu et al.,
2017), rhyolites (Halbach et al., 1997), dacites (Halbach et al., 1997)] and sediments (Halbach et
al., 1997).

**Fig. 9.** Stability phase diagrams of the chemical species (dissolved and solid) in the average vent fluid from the CLAM hydrothermal field (Supplementary Table 2): (A) Eh-pH diagram calculated for Mn species; (B) Eh-pH diagram calculated for Ca species; (C) log₁₀(a[Ca]) – log₁₀(a[Mn]) diagram calculated for Mn species; (D) log₁₀(a[Ca]) – log₁₀(a[Mn]) diagram calculated for Ca species; (E) same as B, but with [Mg] = 0. Hatched areas = most probable areas of precipitation (highest stability) of rhodochrosite (A) and calcite (E) in the studied chimneys according to the S isotopes.

Supplementary figure captions

**Fig. 1.** Microphotographs (optical microscope, thin polished sections) of carbonates from the carbonate chimneys of the CLAM hydrothermal field (Iheya Ridge): (A) rhombohedral carbonate crystals (transmitted plane polarized light, 2K#366-St.3); (B) the same as at (A) (transmitted crossed polarized light); (C) skeletal carbonate crystals (reflected plane polarized light, 2K#366-St.3C); (D) skeletal carbonate crystals (transmitted plane polarized light, 2K#366-St.3C); (E) carbonates (Carb.) coated with amorphous silica (SiO₂am) (transmitted plane polarized light, 2K#366-St.3C); (F) the same as at (E) (transmitted crossed polarized light).

**Fig. 2.** Selected XRD patterns of the studied samples: (A) 2K#366-St.2-a (rhodochrosite); (B) 2K#366-St.3-a (calcite); (C) 2K#366-St.3C-e (ankerite). Numbers above the peaks = d-spacings in Å.
**Fig. 3.** Photomicrographs of minerals from the studied carbonate chimneys (CLAM hydrothermal field, Iheya Ridge): (A) rhodochrosite rhombohedral crystals (stereo microscope; sample 1469 2K#366-St.3C-a); (B) the same as at (A) (SEM SEI; 2K#366-St.3C-i); (C) close up of the 1470 botryoidal surface (amorphous silica according to the EDS spectra, not shown) of the 1471 rhodochrosite crystals shown at (B) (SEM SEI; 2K#366-St.3C-i); (D) botryoidal coating of Mn- 1472 oxides (EDS spectra, not shown) on the surface of rhodochrosite crystals (SEM SEI; 2K#366- 1473 St.3C-b); (E) bacteria-like filaments coated with amorphous silica (EDS spectra, not shown) on 1474 the surface of the rhodochrosite crystals (SEM SEI; 2K#366-St.3C-i); (F) worm-like structures 1475 coated by clays (Al, Si, and O at the EDS spectrum, not shown) and Mn-oxides (Mn and O at the 1476 EDS spectrum, not shown) on the surface of the rhodochrosite crystals (SEM SEI; 2K#366- 1477 St.3C-c). 1478

**Fig. 4.** Unit-cell parameters (a and c) correlation with Mn content (a.p.f.u.) in the studied calcite-rhodochrosite (CLAM hydrothermal field, Iheya Ridge). 1480 1481

**Fig. 5.** Image in back-scattered electrons (BSE) and X-ray maps collected at cross-cut rhodochrosite (thin polished section) from a studied carbonate chimney (CLAM hydrothermal field, Iheya Ridge) (2K#366-St.3C): (A) BSE image; (B) X-ray scan in Si $K_α$; (C) X-ray scan in Mn $K_α$; (D) X-ray scan in Ca $K_α$; (E) X-ray scan in Mg $K_α$; (F) X-ray scan in Fe $K_α$. rh = rhodochrosite; $SiO_{2am.}$ = amorphous silica. Color scales (right-hand side), elemental concentrations. 1482 1483

1490 1491
122°E

124°

126°

128°E

A

28°N

<figure><img src="image_2.png" /></figure>

26°

24°N

126°52′E

126°56′

127°00′

127°04′E

B

<figure><img src="image_3.png" /></figure>

27°36′N

27°32′

27°28′N

3000

2000

1000

1800

1200

600

Water depth (m)

Water depth (m)
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>
sample/chondrite

A

<figure><img src="image_8.png" /></figure>

C

<figure><img src="image_9.png" /></figure>

B

<figure><img src="image_10.png" /></figure>

D

<figure><img src="image_11.png" /></figure>
$\delta^{18}\text{O}_{\text{VSMOW}} (\text{‰})$

<figure><img src="image_12.png" /></figure>

CLAM end-member fluid
<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /></figure>

CLAM vent fluids
<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /></figure>
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>
<table><caption>**Table 1**</caption><caption>Studied hydrothermal deposit samples from the CLAM hydrothermal field (Iheya Ridge).</caption><thead><tr><th>Sample ID</th><th>Cruise ID</th><th>Sample type</th><th>Latitude</th><th>Longitude</th><th>Water depth (m)</th><th>Sampling device</th></tr></thead><tbody><tr><td>2K#366-St.2</td><td>NT88-06</td><td>chimney</td><td>27°32.65′N</td><td>126°58.13′E</td><td>1410</td><td>manned submersible Shinkai 2000</td></tr><tr><td>2K#366-St.3</td><td>- “ -</td><td>- “ -</td><td>- “ -</td><td>- “ -</td><td>- “ -</td><td>- “ -</td></tr><tr><td>2K#366-St.3C</td><td>- “ -</td><td>- “ -</td><td>- “ -</td><td>- “ -</td><td>1400</td><td>- “ -</td></tr><tr><td>T529-G4A B<sup>a</sup></td><td></td><td>flange</td><td>27°00′28″N</td><td>111°24′36″W</td><td>1999</td><td>ROV Tiburon</td></tr></tbody></table>

<footnote><sup>a</sup> Flange, Guaymas Basin.</footnote>
**Table 2**

Studied hydrothermal fluid samples from the CLAM hydrothermal field (Iheya Ridge)ª.

<table><thead><tr><th>Sample ID</th><th>Cruise ID</th><th>Latitude (N)</th><th>Longitude (E)</th><th>Water depth (m)</th><th>Treatment after recovery</th><th>pH (°C)</th><th>Titration alkalinity (25°C)</th><th>Sampling device</th></tr></thead><tbody><tr><td>D426 RV-3</td><td>NT89-04</td><td>27°33'</td><td>126°58'</td><td>1390</td><td>filtered and acidified</td><td>-</td><td>-</td><td>6-port revolver-type Ti water sampler</td></tr><tr><td>D426 RV-6</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>90</td><td>5.33</td><td>7.51</td><td>-</td></tr><tr><td>D427 RV-1</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>100</td><td>5.42</td><td>9.18</td><td>-</td></tr><tr><td>D427 RV-2</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>100</td><td>5.23</td><td>9.38</td><td>-</td></tr><tr><td>D427 RV-3</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>100</td><td>5.42</td><td>9.31</td><td>-</td></tr><tr><td>D427 RV-6</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>100</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

<footnote>ª Data from Gamo et al. (1991) and Gamo (1995).</footnote>
<caption>**Table 3**</caption>

Mineralogy (XRD) of the studied chimney samples (CLAM hydrothermal field, Iheya Ridge).

<table><thead><tr><th>Sample ID</th><th>Mineralogy</th></tr></thead><tbody><tr><td>2K#366-St.2-a</td><td>rhodochrosite</td></tr><tr><td>2K#366-St.2-b</td><td>rhodochrosite, witherite</td></tr><tr><td>2K#366-St.2-c</td><td>rhodochrosite</td></tr><tr><td>2K#366-St.2-d</td><td>rhodochrosite, witherite</td></tr><tr><td>2K#366-St.3-a</td><td>calcite</td></tr><tr><td>2K#366-St.3-b</td><td>calcite</td></tr><tr><td>2K#366-St.3-c</td><td>calcite-kutnohorite, ankerite-kutnohorite</td></tr><tr><td>2K#366-St.3-d</td><td>calcite</td></tr><tr><td>2K#366-St.3-e</td><td>calcite</td></tr><tr><td>2K#366-St.3C-b</td><td>kutnohorite-ankerite</td></tr><tr><td>2K#366-St.3C-c</td><td>rhodochrosite, calcite, gypsum</td></tr><tr><td>2K#366-St.3C-e</td><td>ankerite</td></tr><tr><td>2K#366-St.3C-f</td><td>dolomite, ankerite, rhodochrosite</td></tr><tr><td>2K#366-St.3C-g</td><td>rhodochrosite</td></tr><tr><td>2K#366-St.3C-i</td><td>rhodochrosite</td></tr><tr><td>T529-G4A B<sup>a</sup></td><td>calcite, amorphous silica, pyrrhotite, sphalerite</td></tr></tbody></table>

<footnote><sup>a</sup> Flange, Guaymas Basin.</footnote>
<table><caption>**Table 4**

Mineral chemistry (EMP) and empirical formulae of carbonates from the studied chimney samples (CLAM hydrothermal field, Iheya Ridge).</caption><thead><tr><th>Sample ID</th><th>Number of point analyses</th><th>Mineral</th><th>CaO<br>(wt.%)</th><th>MnO</th><th>FeO</th><th>MgO</th><th>SrO</th><th>BaO</th><th>Total</th><th>CO<sub>2</sub><sup>a</sup></th><th>Empirical formulae<sup>b</sup></th></tr></thead><tbody><tr><td>2K#366-St.2</td><td>16</td><td>Ca-rhodochrosite</td><td>11.7</td><td>40.7</td><td>0.31</td><td>0.11</td><td>0.02</td><td>0.01</td><td>52.9</td><td>47.1</td><td>(Ca<sub>0.26</sub>Mn<sub>0.73</sub>Fe<sub>0.01</sub>)CO<sub>3</sub></td></tr><tr><td>2K#366-St.3</td><td>14</td><td>Mn-calcite</td><td>46.5</td><td>6.10</td><td>0.12</td><td>0.17</td><td>0.02</td><td>0.03</td><td>52.9</td><td>47.1</td><td>(Ca<sub>0.90</sub>Mn<sub>0.10</sub>)CO<sub>3</sub></td></tr><tr><td>2K#366-St.3C</td><td>16</td><td>Ca-rhodochrosite</td><td>8.87</td><td>43.8</td><td>0.53</td><td>0.07</td><td>0.01</td><td>&lt;LD<sup>c</sup></td><td>53.3</td><td>46.7</td><td>(Ca<sub>0.20</sub>Mn<sub>0.79</sub>Fe<sub>0.01</sub>)CO<sub>3</sub></td></tr><tr><td>T521-G4A B<sup>d</sup></td><td>18</td><td>calcite</td><td>52.3</td><td>0.52</td><td>0.05</td><td>0.04</td><td>0.07</td><td>&lt;LD</td><td>53.0</td><td>47.0</td><td>(Ca<sub>0.99</sub>Mn<sub>0.01</sub>)CO<sub>3</sub></td></tr></tbody></table>

<footnote><sup>a</sup> Calculated as difference to 100 wt.% from the analytical totals.</footnote>

<footnote><sup>b</sup> Calculated on the basis of (Ca+Mn+Fe+Mg+Sr+Ba) = 1 a.p.f.u.</footnote>

<footnote><sup>c</sup> <LD = below limits of detection.</footnote>

<footnote><sup>d</sup> Flange, Guaymas Basin.</footnote>
<table><caption>**Table 5**

Chemical composition (ICP-MS) of the studied chimney samples (CLAM hydrothermal field, Ileya Ridge).</caption><thead><tr><th>Sample ID</th><th>Mn<br>(wt.%)</th><th>Ca</th><th>Fe</th><th>Mg</th><th>Na</th><th>Zn<br>(ppm)</th><th>Pb</th><th>Cu</th><th>As</th><th>Ni</th><th>Co</th><th>V</th><th>Mo</th><th>Ba</th><th>Sr</th><th>Al</th></tr></thead><tbody><tr><td>2K#366-St.2-a</td><td>27.6</td><td>2.44</td><td>0.50</td><td>0.13</td><td>0.31</td><td>1887</td><td>2000</td><td>1205</td><td>404</td><td>58.2</td><td>0.04</td><td>18.5</td><td>0.65</td><td>31.9</td><td>9.30</td><td>43.6</td></tr><tr><td>2K#366-St.2-b</td><td>12.2</td><td>0.72</td><td>0.88</td><td>0.04</td><td>0.30</td><td>4859</td><td>3957</td><td>2001</td><td>651</td><td>215</td><td>0.10</td><td>15.0</td><td>5.85</td><td>21.2</td><td>7.90</td><td>251</td></tr><tr><td>2K#366-St.2-c</td><td>30.6</td><td>3.24</td><td>0.48</td><td>0.12</td><td>0.39</td><td>2040</td><td>1668</td><td>958</td><td>106</td><td>42.5</td><td>0.16</td><td>21.4</td><td>0.34</td><td>7.58</td><td>6.69</td><td>48.3</td></tr><tr><td>2K#366-St.2-d</td><td>7.20</td><td>0.33</td><td>0.89</td><td>0.07</td><td>0.51</td><td>5325</td><td>4480</td><td>3033</td><td>354</td><td>19.2</td><td>0.12</td><td>21.3</td><td>18.4</td><td>23.6</td><td>20.2</td><td>239</td></tr><tr><td><strong>average</strong></td><td><strong>19.4</strong></td><td><strong>1.68</strong></td><td><strong>0.69</strong></td><td><strong>0.09</strong></td><td><strong>0.38</strong></td><td><strong>3528</strong></td><td><strong>3026</strong></td><td><strong>1799</strong></td><td><strong>379</strong></td><td><strong>83.7</strong></td><td><strong>0.11</strong></td><td><strong>19.1</strong></td><td><strong>6.30</strong></td><td><strong>21.1</strong></td><td><strong>11.0</strong></td><td><strong>146</strong></td></tr><tr><td>2K#366-St.3-a</td><td>6.40</td><td>33.4</td><td>0.12</td><td>0.10</td><td>0.02</td><td>172</td><td>38.3</td><td>236</td><td>2.63</td><td>40.0</td><td>0.19</td><td>1.38</td><td>0.94</td><td>14.8</td><td>147</td><td>5.24</td></tr><tr><td>2K#366-St.3-b</td><td>8.96</td><td>28.3</td><td>0.66</td><td>0.12</td><td>0.11</td><td>1979</td><td>1275</td><td>123</td><td>17.1</td><td>270</td><td>0.31</td><td>0.96</td><td>1.03</td><td>28.5</td><td>119</td><td>45.8</td></tr><tr><td>2K#366-St.3-c</td><td>11.9</td><td>23.4</td><td>0.68</td><td>0.08</td><td>0.07</td><td>2309</td><td>1640</td><td>204</td><td>19.9</td><td>31.8</td><td>0.09</td><td>0.71</td><td>0.54</td><td>66.5</td><td>81.8</td><td>5.58</td></tr><tr><td>2K#366-St.3-d</td><td>5.05</td><td>36.1</td><td>0.09</td><td>0.10</td><td>0.03</td><td>66.3</td><td>24.0</td><td>64.1</td><td>1.82</td><td>13.4</td><td>0.06</td><td>0.41</td><td>0.07</td><td>20.0</td><td>146</td><td>6.28</td></tr><tr><td>2K#366-St.3-e</td><td>4.60</td><td>37.4</td><td>0.09</td><td>0.12</td><td>0.01</td><td>134</td><td>16.1</td><td>199</td><td>1.72</td><td>8.35</td><td>0.07</td><td>0.28</td><td>0.09</td><td>15.7</td><td>139</td><td>105</td></tr><tr><td><strong>average</strong></td><td><strong>7.38</strong></td><td><strong>31.7</strong></td><td><strong>0.33</strong></td><td><strong>0.10</strong></td><td><strong>0.05</strong></td><td><strong>932</strong></td><td><strong>599</strong></td><td><strong>165</strong></td><td><strong>8.63</strong></td><td><strong>72.6</strong></td><td><strong>0.14</strong></td><td><strong>0.75</strong></td><td><strong>0.53</strong></td><td><strong>29.1</strong></td><td><strong>127</strong></td><td><strong>33.6</strong></td></tr><tr><td>2K#366-St.3C-b</td><td>19.0</td><td>10.1</td><td>1.09</td><td>0.09</td><td>0.17</td><td>3835</td><td>2339</td><td>235</td><td>549</td><td>14.8</td><td>0.24</td><td>1.44</td><td>2.26</td><td>14.6</td><td>13.7</td><td>155</td></tr><tr><td>2K#366-St.3C-c</td><td>10.8</td><td>1.34</td><td>3.71</td><td>0.24</td><td>1.22</td><td>-</td><td>17879</td><td>1502</td><td>1853</td><td>159</td><td>0.44</td><td>96.1</td><td>169</td><td>492</td><td>93.1</td><td>659</td></tr><tr><td>2K#366-St.3C-e</td><td>19.0</td><td>9.80</td><td>1.84</td><td>0.10</td><td>0.25</td><td>5639</td><td>3781</td><td>672</td><td>605</td><td>1148</td><td>0.40</td><td>1.59</td><td>2.65</td><td>74.7</td><td>17.5</td><td>177</td></tr><tr><td>2K#366-St.3C-f</td><td>16.8</td><td>4.44</td><td>1.17</td><td>0.05</td><td>0.19</td><td>3919</td><td>2620</td><td>302</td><td>295</td><td>340</td><td>0.12</td><td>0.23</td><td>0.39</td><td>33.3</td><td>6.08</td><td>343</td></tr><tr><td>2K#366-St.3C-g</td><td>16.3</td><td>2.99</td><td>2.57</td><td>0.06</td><td>0.23</td><td>4173</td><td>3547</td><td>971</td><td>1057</td><td>182</td><td>0.08</td><td>0.32</td><td>1.05</td><td>25.0</td><td>4.84</td><td>136</td></tr><tr><td>2K#366-St.3C-i</td><td>18.2</td><td>2.78</td><td>1.16</td><td>0.06</td><td>0.28</td><td>3090</td><td>2780</td><td>525</td><td>678</td><td>96.1</td><td>0.08</td><td>2.72</td><td>1.76</td><td>43.6</td><td>6.40</td><td>431</td></tr><tr><td><strong>average</strong></td><td><strong>16.7</strong></td><td><strong>5.24</strong></td><td><strong>1.92</strong></td><td><strong>0.10</strong></td><td><strong>0.39</strong></td><td><strong>4131</strong></td><td><strong>5491</strong></td><td><strong>701</strong></td><td><strong>839</strong></td><td><strong>323</strong></td><td><strong>0.22</strong></td><td><strong>17.1</strong></td><td><strong>29.5</strong></td><td><strong>114</strong></td><td><strong>23.6</strong></td><td><strong>317</strong></td></tr><tr><td>Standards</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>JB-2 (measured)</td><td>0.17</td><td>6.89</td><td>9.67</td><td>2.68</td><td>1.38</td><td>104</td><td>4.88</td><td>221</td><td>1.71</td><td>12.8</td><td>36.4</td><td>578</td><td>0.89</td><td>208</td><td>162</td><td>7.30<sup>a</sup></td></tr><tr><td>JB-2 (reference)</td><td>0.16</td><td>6.68</td><td>9.58</td><td>2.74</td><td>1.49</td><td>107</td><td>4.88</td><td>219</td><td>2.87</td><td>15.4</td><td>33.0</td><td>613</td><td>0.96</td><td>215</td><td>177</td><td>7.62</td></tr><tr><td>JB-3 (measured)</td><td>0.15</td><td>6.89</td><td>8.14</td><td>2.93</td><td>1.88</td><td>100</td><td>4.92</td><td>191</td><td>1.50</td><td>36.0</td><td>34.9</td><td>390</td><td>1.04</td><td>224</td><td>376</td><td>8.19</td></tr><tr><td>JB-3 (reference)</td><td>0.14</td><td>6.93</td><td>8.39</td><td>3.13</td><td>2.03</td><td>114</td><td>4.85</td><td>179</td><td>1.84</td><td>37.0</td><td>35.1</td><td>384</td><td>1.12</td><td>239</td><td>414</td><td>9.26</td></tr><tr><td>JMS-2 (measured)</td><td>1.63</td><td>3.47</td><td>7.23</td><td>1.89</td><td>3.92</td><td>150</td><td>68.4</td><td>438</td><td>36.8</td><td>295</td><td>219</td><td>173</td><td>24.1</td><td>1794</td><td>404</td><td>6.64</td></tr><tr><td>JMS-2 (reference)</td><td>1.78</td><td>3.43</td><td>7.65</td><td>2.02</td><td>4.37</td><td>167</td><td>77.5</td><td>472</td><td>35.6</td><td>315</td><td>215</td><td>183</td><td>22.9</td><td>1799</td><td>435</td><td>7.66</td></tr><tr><td>Detection limits (ppb)</td><td>0.45</td><td>266</td><td>7.26</td><td>2.05</td><td>18.8</td><td>0.78</td><td>0.06</td><td>0.09</td><td>0.17</td><td>0.32</td><td>0.04</td><td>0.60</td><td>0.03</td><td>0.19</td><td>0.83</td><td>9.96</td></tr><tr><td>T529-G4A B<sup>b</sup></td><td>0.70</td><td>37.8</td><td>0.79</td><td>0.05</td><td>0.05</td><td>689</td><td>646</td><td>331</td><td>71</td><td>10.7</td><td>0.70</td><td>3.00</td><td>0.45</td><td>1520</td><td>914</td><td>600</td></tr><tr><td>D757-R1<sup>c</sup></td><td>0.50</td><td>38.1</td><td>0.41</td><td>0.20</td><td>0.18</td><td>920</td><td>808</td><td>417</td><td>22.1</td><td>20</td><td>1.6</td><td>-</td><td>7</td><td>620</td><td>913</td><td>200</td></tr></tbody></table>

<footnote><sup>a</sup> Concentrations of Al in all standards in wt.%.</footnote>

<footnote><sup>b</sup> Flange, Guaymas Basin.</footnote>

<footnote><sup>c</sup> Active flange, Auka hydrothermal field, Pescadero Basin (Paduan et al., 2018).</footnote>
<table><caption>Table 5 (Continued)</caption><thead><tr><th>Sample ID</th><th>Ti<br>(ppm)</th><th>P</th><th>K</th><th>Rb</th><th>Cs</th><th>B</th><th>Sc</th><th>U</th><th>Th<br>(ppb)</th><th>Hf</th><th>Zr</th><th>Nb</th><th>Ta</th><th>Y</th></tr></thead><tbody><tr><td>2K#366-St.2-a</td><td>2.38</td><td>14.7</td><td>160</td><td>0.29</td><td>0.32</td><td>19.6</td><td>0.19</td><td>1.93</td><td>0.79</td><td>2.19</td><td>77.6</td><td>-</td><td>0.90</td><td>681</td></tr><tr><td>2K#366-St.2-b</td><td>0.99</td><td>109</td><td>314</td><td>1.04</td><td>1.13</td><td>24.4</td><td>0.17</td><td>1.54</td><td>1.82</td><td>1.93</td><td>85.8</td><td>-</td><td>0.84</td><td>416</td></tr><tr><td>2K#366-St.2-c</td><td>1.69</td><td>6.21</td><td>200</td><td>0.29</td><td>0.34</td><td>32.5</td><td>0.36</td><td>1.27</td><td>0.30</td><td>1.97</td><td>54.9</td><td>-</td><td>0.66</td><td>1166</td></tr><tr><td>2K#366-St.2-d</td><td>0.94</td><td>357</td><td>483</td><td>1.12</td><td>1.61</td><td>24.0</td><td>0.33</td><td>1.88</td><td>5.73</td><td>4.20</td><td>240</td><td>3.83</td><td>0.75</td><td>517</td></tr><tr><td><strong>average</strong></td><td><strong>1.50</strong></td><td><strong>122</strong></td><td><strong>289</strong></td><td><strong>0.68</strong></td><td><strong>0.85</strong></td><td><strong>25.1</strong></td><td><strong>0.26</strong></td><td><strong>1.65</strong></td><td><strong>2.16</strong></td><td><strong>2.57</strong></td><td><strong>115</strong></td><td><strong>3.83</strong></td><td><strong>0.79</strong></td><td><strong>695</strong></td></tr><tr><td>2K#366-St.3-a</td><td>6.29E-16</td><td>-</td><td>10.7</td><td>-</td><td>0.01</td><td>9.78</td><td>0.08</td><td>0.05</td><td>0.88</td><td>0.70</td><td>40.5</td><td>1.67</td><td>0.93</td><td>86.3</td></tr><tr><td>2K#366-St.3-b</td><td>1.62</td><td>2.03</td><td>61.9</td><td>0.07</td><td>0.02</td><td>20.7</td><td>0.09</td><td>0.05</td><td>0.37</td><td>1.20</td><td>26.7</td><td>11.7</td><td>1.21</td><td>559</td></tr><tr><td>2K#366-St.3-c</td><td>6.17E-16</td><td>-</td><td>93.4</td><td>0.08</td><td>0.04</td><td>11.0</td><td>0.02</td><td>0.06</td><td>0.20</td><td>0.79</td><td>12.8</td><td>1.47</td><td>0.78</td><td>8.48</td></tr><tr><td>2K#366-St.3-d</td><td>5.83E-16</td><td>-</td><td>14.0</td><td>0.01</td><td>0.01</td><td>9.16</td><td>0.08</td><td>0.05</td><td>0.18</td><td>0.59</td><td>3.17</td><td>-</td><td>0.28</td><td>184</td></tr><tr><td>2K#366-St.3-e</td><td>6.11E-16</td><td>3.35</td><td>24.1</td><td>-</td><td>&lt;0.001</td><td>10.9</td><td>0.05</td><td>0.06</td><td>0.31</td><td>0.32</td><td>17.8</td><td>-</td><td>0.38</td><td>159</td></tr><tr><td><strong>average</strong></td><td><strong>0.32</strong></td><td><strong>2.69</strong></td><td><strong>40.8</strong></td><td><strong>0.05</strong></td><td><strong>0.02</strong></td><td><strong>12.3</strong></td><td><strong>0.06</strong></td><td><strong>0.05</strong></td><td><strong>0.39</strong></td><td><strong>0.72</strong></td><td><strong>20.2</strong></td><td><strong>4.95</strong></td><td><strong>0.72</strong></td><td><strong>200</strong></td></tr><tr><td>2K#366-St.3C-b</td><td>1.56</td><td>-</td><td>180</td><td>0.35</td><td>0.58</td><td>31.5</td><td>9E-05</td><td>1.37</td><td>0.66</td><td>0.61</td><td>34.0</td><td>7.06</td><td>1.39</td><td>64.5</td></tr><tr><td>2K#366-St.3C-c</td><td>7.68</td><td>1442</td><td>1525</td><td>2.26</td><td>1.78</td><td>139</td><td>0.09</td><td>1.27</td><td>15.6</td><td>14.3</td><td>1917</td><td>69.2</td><td>1.59</td><td>2728</td></tr><tr><td>2K#366-St.3C-e</td><td>2.31</td><td>-</td><td>202</td><td>0.45</td><td>0.64</td><td>15.0</td><td>-</td><td>0.36</td><td>0.61</td><td>0.51</td><td>30.8</td><td>7.08</td><td>1.09</td><td>30.3</td></tr><tr><td>2K#366-St.3C-f</td><td>0.89</td><td>0.68</td><td>240</td><td>0.56</td><td>1.09</td><td>43.4</td><td>-</td><td>0.13</td><td>0.17</td><td>0.16</td><td>28.2</td><td>-</td><td>0.31</td><td>28.4</td></tr><tr><td>2K#366-St.3C-g</td><td>5.92E-16</td><td>-</td><td>186</td><td>0.36</td><td>0.49</td><td>71.6</td><td>-</td><td>0.32</td><td>0.03</td><td>0.31</td><td>43.9</td><td>3.83</td><td>0.46</td><td>28.8</td></tr><tr><td>2K#366-St.3C-i</td><td>4.02</td><td>57.5</td><td>284</td><td>0.92</td><td>1.19</td><td>12.0</td><td>-</td><td>0.26</td><td>0.24</td><td>0.44</td><td>38.9</td><td>12.8</td><td>0.79</td><td>22.9</td></tr><tr><td><strong>average</strong></td><td><strong>2.74</strong></td><td><strong>500</strong></td><td><strong>436</strong></td><td><strong>0.82</strong></td><td><strong>0.96</strong></td><td><strong>52.1</strong></td><td><strong>0.04</strong></td><td><strong>0.62</strong></td><td><strong>2.88</strong></td><td><strong>2.72</strong></td><td><strong>349</strong></td><td><strong>20.0</strong></td><td><strong>0.94</strong></td><td><strong>484</strong></td></tr><tr><td><em>Standards</em></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>JB-2 (measured)</td><td>0.66<sup>d</sup></td><td>0.04<sup>d</sup></td><td>0.32<sup>d</sup></td><td>5.72</td><td>0.84</td><td>-</td><td>54.3</td><td>0.16</td><td>0.25</td><td>1.53</td><td>44.7</td><td>0.43</td><td>0.04</td><td>22.3</td></tr><tr><td>JB-2 (reference)</td><td>0.70</td><td>0.04</td><td>0.35</td><td>6.12</td><td>0.77</td><td>-</td><td>53.3</td><td>0.16</td><td>0.26</td><td>1.45</td><td>42.9</td><td>0.46</td><td>0.03</td><td>20.6</td></tr><tr><td>JB-3 (measured)</td><td>0.81</td><td>0.15</td><td>0.58</td><td>13.3</td><td>0.98</td><td>-</td><td>34.1</td><td>0.51</td><td>1.27</td><td>2.82</td><td>89.1</td><td>1.86</td><td>0.14</td><td>24.6</td></tr><tr><td>JB-3 (reference)</td><td>0.82</td><td>0.13</td><td>0.66</td><td>14.0</td><td>0.93</td><td>-</td><td>34.2</td><td>0.48</td><td>1.30</td><td>2.65</td><td>86.2</td><td>1.87</td><td>0.11</td><td>23.1</td></tr><tr><td>JMS-2 (measured)</td><td>0.77</td><td>0.56</td><td>1.95</td><td>58.3</td><td>3.40</td><td>-</td><td>25.7</td><td>2.33</td><td>11.7</td><td>4.63</td><td>205</td><td>28.6</td><td>1.53</td><td>241</td></tr><tr><td>JMS-2 (reference)</td><td>0.84</td><td>0.58</td><td>2.28</td><td>62.2</td><td>3.26</td><td>-</td><td>25.8</td><td>2.77</td><td>14.2</td><td>4.88</td><td>212</td><td>28.5</td><td>1.42</td><td>246</td></tr><tr><td>Detection limits (ppb)</td><td>5.55E-17</td><td>52.8</td><td>24.3</td><td>0.06</td><td>0.001</td><td>17.8</td><td>0.14</td><td>8E-04</td><td>0.003</td><td>0.002</td><td>0.02</td><td>0.02</td><td>0.0003</td><td>0.01</td></tr><tr><td>T529-G4A B</td><td>&lt;100</td><td>100</td><td>200</td><td>1.70</td><td>0.37</td><td>-</td><td>0.10</td><td>0.20</td><td>&lt;200</td><td>-</td><td>&lt;10000</td><td>100</td><td>-</td><td>200</td></tr><tr><td>D757-R1</td><td>-</td><td>-</td><td>-</td><td>1.1</td><td>0.3</td><td>-</td><td>12.7</td><td>0.2</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

<footnote>d Concentrations of Ti, P and K in all standards in wt.%.</footnote>
<table><caption>**Table 6**
REE concentrations (ICP-MS) of the studied chimney samples (CLAM hydrothermal field, Iheya Ridge).</caption><thead><tr><th>Sample ID</th><th>La<br>(ppb)</th><th>Ce</th><th>Pr</th><th>Nd</th><th>Sm</th><th>Eu</th><th>Gd</th><th>Tb</th><th>Dy</th><th>Ho</th><th>Er</th><th>Tm</th><th>Yb</th><th>Lu</th><th>ΣREE</th><th>(Ce/Ce*)<sup>a</sup></th><th>(Eu/Eu*)<sup>b</sup></th><th>La<sub>CN</sub>/Lu<sub>CN</sub></th></tr></thead><tbody><tr><td>2K#366-St.2-a</td><td>990</td><td>1434</td><td>178</td><td>635</td><td>124</td><td>5147</td><td>101</td><td>17.4</td><td>103</td><td>18.3</td><td>54.0</td><td>8.36</td><td>47.6</td><td>6.51</td><td>8864</td><td>0.77</td><td>136</td><td>15.8</td></tr><tr><td>2K#366-St.2-b</td><td>544</td><td>837</td><td>105</td><td>348</td><td>77.2</td><td>2625</td><td>64.2</td><td>12.2</td><td>64.4</td><td>10.4</td><td>36.9</td><td>5.86</td><td>32.7</td><td>3.42</td><td>4766</td><td>0.80</td><td>110</td><td>16.5</td></tr><tr><td>2K#366-St.2-c</td><td>1968</td><td>2817</td><td>319</td><td>1144</td><td>291</td><td>5878</td><td>204</td><td>28.9</td><td>181</td><td>31.0</td><td>92.1</td><td>12.5</td><td>76.9</td><td>9.71</td><td>13054</td><td>0.78</td><td>69.8</td><td>21.0</td></tr><tr><td>2K#366-St.2-d</td><td>485</td><td>601</td><td>97.9</td><td>343</td><td>76.8</td><td>1638</td><td>87.3</td><td>13.0</td><td>88.4</td><td>17.7</td><td>57.3</td><td>8.60</td><td>56.2</td><td>7.63</td><td>3577</td><td>0.63</td><td>60.7</td><td>6.60</td></tr><tr><td><strong>average</strong></td><td><strong>997</strong></td><td><strong>1422</strong></td><td><strong>175</strong></td><td><strong>618</strong></td><td><strong>142</strong></td><td><strong>3822</strong></td><td><strong>114</strong></td><td><strong>17.9</strong></td><td><strong>109</strong></td><td><strong>19.4</strong></td><td><strong>60.1</strong></td><td><strong>8.83</strong></td><td><strong>53.4</strong></td><td><strong>6.82</strong></td><td><strong>7565</strong></td><td><strong>0.74</strong></td><td><strong>94.1</strong></td><td><strong>15.0</strong></td></tr><tr><td>2K#366-St.3-a</td><td>2096</td><td>1803</td><td>134</td><td>351</td><td>32.1</td><td>1143</td><td>42.6</td><td>2.24</td><td>13.3</td><td>2.62</td><td>4.98</td><td>0.62</td><td>4.64</td><td>0.25</td><td>5630</td><td>0.57</td><td>94.3</td><td>860</td></tr><tr><td>2K#366-St.3-b</td><td>10616</td><td>10815</td><td>804</td><td>2256</td><td>255</td><td>1299</td><td>380</td><td>21.6</td><td>97.6</td><td>13.8</td><td>39.1</td><td>3.91</td><td>26.2</td><td>2.82</td><td>26630</td><td>0.66</td><td>12.7</td><td>390</td></tr><tr><td>2K#366-St.3-c</td><td>462</td><td>363</td><td>26.2</td><td>60.9</td><td>7.95</td><td>656</td><td>4.68</td><td>0.88</td><td>2.48</td><td>0.38</td><td>1.88</td><td>0.40</td><td>1.15</td><td>0.05</td><td>1588</td><td>0.53</td><td>302</td><td>885</td></tr><tr><td>2K#366-St.3-d</td><td>4447</td><td>4155</td><td>320</td><td>863</td><td>80.6</td><td>1721</td><td>91.3</td><td>8.36</td><td>34.6</td><td>5.83</td><td>14.0</td><td>2.68</td><td>10.4</td><td>1.55</td><td>11755</td><td>0.61</td><td>60.9</td><td>298</td></tr><tr><td>2K#366-St.3-e</td><td>5502</td><td>4728</td><td>342</td><td>810</td><td>83.3</td><td>1404</td><td>124</td><td>4.75</td><td>33.8</td><td>5.06</td><td>8.89</td><td>1.93</td><td>8.74</td><td>0.63</td><td>13056</td><td>0.57</td><td>42.1</td><td>907</td></tr><tr><td><strong>average</strong></td><td><strong>4625</strong></td><td><strong>4373</strong></td><td><strong>325</strong></td><td><strong>868</strong></td><td><strong>91.8</strong></td><td><strong>1245</strong></td><td><strong>128</strong></td><td><strong>7.57</strong></td><td><strong>36.3</strong></td><td><strong>5.53</strong></td><td><strong>13.8</strong></td><td><strong>1.91</strong></td><td><strong>10.2</strong></td><td><strong>1.06</strong></td><td><strong>11732</strong></td><td><strong>0.59</strong></td><td><strong>102</strong></td><td><strong>668</strong></td></tr><tr><td>2K#366-St.3C-b</td><td>1797</td><td>1557</td><td>119</td><td>290</td><td>31.1</td><td>2388</td><td>27.7</td><td>2.61</td><td>12.8</td><td>1.58</td><td>6.07</td><td>0.92</td><td>4.07</td><td>0.65</td><td>6238</td><td>0.57</td><td>243</td><td>288</td></tr><tr><td>2K#366-St.3C-c</td><td>1675</td><td>736</td><td>287</td><td>1204</td><td>241</td><td>912</td><td>336</td><td>51.6</td><td>360</td><td>79.9</td><td>281</td><td>40.0</td><td>266</td><td>45.3</td><td>6514</td><td>0.24</td><td>9.76</td><td>3.84</td></tr><tr><td>2K#366-St.3C-e</td><td>652</td><td>567</td><td>40.2</td><td>117</td><td>12.3</td><td>1837</td><td>13.4</td><td>1.41</td><td>5.10</td><td>1.05</td><td>2.82</td><td>0.52</td><td>1.13</td><td>0.04</td><td>3251</td><td>0.58</td><td>435</td><td>1549</td></tr><tr><td>2K#366-St.3C-f</td><td>1172</td><td>1105</td><td>82.0</td><td>224</td><td>24.9</td><td>1375</td><td>42.8</td><td>1.89</td><td>11.0</td><td>1.57</td><td>4.38</td><td>0.59</td><td>0.81</td><td>0.48</td><td>4045</td><td>0.62</td><td>127</td><td>253</td></tr><tr><td>2K#366-St.3C-g</td><td>451</td><td>421</td><td>32.4</td><td>86.5</td><td>8.22</td><td>1903</td><td>18.4</td><td>0.49</td><td>5.67</td><td>0.62</td><td>1.30</td><td>0.50</td><td>0.95</td><td>0.05</td><td>2930</td><td>0.61</td><td>457</td><td>875</td></tr><tr><td>2K#366-St.3C-i</td><td>359</td><td>330</td><td>26.4</td><td>62.4</td><td>8.33</td><td>638</td><td>6.55</td><td>0.63</td><td>5.66</td><td>0.95</td><td>2.09</td><td>0.40</td><td>1.67</td><td>0.10</td><td>1442</td><td>0.60</td><td>254</td><td>359</td></tr><tr><td><strong>average</strong></td><td><strong>1018</strong></td><td><strong>786</strong></td><td><strong>97.8</strong></td><td><strong>331</strong></td><td><strong>54.3</strong></td><td><strong>1509</strong></td><td><strong>74.1</strong></td><td><strong>9.8</strong></td><td><strong>66.7</strong></td><td><strong>14.3</strong></td><td><strong>49.5</strong></td><td><strong>7.15</strong></td><td><strong>45.8</strong></td><td><strong>7.77</strong></td><td><strong>4070</strong></td><td><strong>0.54</strong></td><td><strong>254</strong></td><td><strong>555</strong></td></tr><tr><td>Standards</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>JB-2 (measured)</td><td>2.26</td><td>6.67</td><td>1.16</td><td>6.74</td><td>2.27</td><td>0.81</td><td>3.27</td><td>0.60</td><td>4.03</td><td>0.86</td><td>2.66</td><td>0.40</td><td>2.54</td><td>0.38</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>JB-2 (reference)</td><td>2.14</td><td>6.39</td><td>1.10</td><td>6.32</td><td>2.19</td><td>0.82</td><td>3.20</td><td>0.58</td><td>4.01</td><td>0.87</td><td>2.58</td><td>0.38</td><td>2.49</td><td>0.39</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>JB-3 (measured)</td><td>8.81</td><td>22.0</td><td>3.29</td><td>16.4</td><td>4.33</td><td>1.30</td><td>4.72</td><td>0.75</td><td>4.68</td><td>0.94</td><td>2.80</td><td>0.40</td><td>2.55</td><td>0.38</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>JB-3 (reference)</td><td>8.12</td><td>20.9</td><td>3.14</td><td>15.9</td><td>4.17</td><td>1.31</td><td>4.77</td><td>0.74</td><td>4.66</td><td>0.95</td><td>2.69</td><td>0.38</td><td>2.50</td><td>0.38</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>JMS-2 (measured)</td><td>142</td><td>168</td><td>36.9</td><td>154</td><td>35.5</td><td>8.68</td><td>39.5</td><td>6.02</td><td>38.5</td><td>7.92</td><td>23.4</td><td>3.22</td><td>20.1</td><td>3.05</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>JMS-2 (reference)</td><td>135</td><td>159</td><td>37.4</td><td>150</td><td>35.9</td><td>9.00</td><td>40.1</td><td>6.12</td><td>38.6</td><td>8.07</td><td>23.3</td><td>3.22</td><td>20.3</td><td>3.15</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Detection limits (ppb)</td><td>0.005</td><td>0.006</td><td>0.002</td><td>0.004</td><td>0.005</td><td>0.0004</td><td>0.002</td><td>0.0004</td><td>0.003</td><td>0.0003</td><td>0.001</td><td>0.0002</td><td>0.001</td><td>0.001</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>T529-G4A B<sup>c</sup></td><td>1000</td><td>1200</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>D757-R1<sup>d</sup></td><td>1200</td><td>1000</td><td>-</td><td>-</td><td>-</td><td>200</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1200</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

<footnote>a Ce/Ce*=2Ce<sub>CN</sub>/(La<sub>CN</sub>+Pr<sub>CN</sub>).</footnote>

<footnote>b Eu/Eu*=2Eu<sub>CN</sub>/(Sm<sub>CN</sub>+Gd<sub>CN</sub>).</footnote>

<footnote>c Flange, Guaymas Basin.</footnote>

<footnote>d Active flange, Auka hydrothermal field, Pescadero Basin (Paduan et al., 2018).</footnote>
<caption>**Table 7**</caption>

Chemical composition (ICP-MS and IC) of the vent fluids from the CLAM hydrothermal field (Iheya Ridge).

<table><thead><tr><th>Sample ID</th><th>Ca<br>(ppm)</th><th>Mg</th><th>Sr</th><th>Na</th><th>K</th><th>Li</th><th>Mn</th><th>Fe<br>(ppb)</th><th>Cu</th><th>Zn</th><th>Pb</th><th>Mo</th><th>Ni</th><th>Co</th><th>V</th><th>U</th><th>Cd</th><th>Ag</th><th>Sn</th><th>Y</th></tr></thead><tbody><tr><td>D426 RV-3</td><td>200</td><td>513</td><td>3.44</td><td>4709</td><td>200</td><td>1.0</td><td>0.12</td><td>341</td><td>2.44</td><td>7.32</td><td>4.60</td><td>2.89</td><td>7.24</td><td>0.16</td><td>0.56</td><td>0.49</td><td>0.03</td><td>0.18</td><td>0.13</td><td>0.01</td></tr><tr><td>D426 RV-6</td><td>695</td><td>1197</td><td>8.72</td><td>12566</td><td>1104</td><td>11.8</td><td>7.11</td><td>166</td><td>2.40</td><td>18.0</td><td>0.87</td><td>3.95</td><td>1.71</td><td>0.05</td><td>0.29</td><td>1.57</td><td>0.04</td><td>0.08</td><td>0.39</td><td>0.36</td></tr><tr><td>D427 RV-1</td><td>682</td><td>919</td><td>7.65</td><td>10813</td><td>1120</td><td>12.9</td><td>10.5</td><td>119</td><td>2.06</td><td>13.2</td><td>0.73</td><td>0.77</td><td>0.69</td><td>0.02</td><td>0.26</td><td>0.41</td><td>0.02</td><td>0.25</td><td>0.30</td><td>0.42</td></tr><tr><td>D427 RV-2</td><td>348</td><td>484</td><td>3.92</td><td>5626</td><td>583</td><td>7.1</td><td>9.08</td><td>76.9</td><td>0.79</td><td>4.03</td><td>0.33</td><td>0.28</td><td>0.48</td><td>0.01</td><td>0.10</td><td>0.25</td><td>&lt;LD<sup>a</sup></td><td>0.15</td><td>0.19</td><td>0.20</td></tr><tr><td>D427 RV-3</td><td>644</td><td>877</td><td>7.13</td><td>10199</td><td>1070</td><td>12.8</td><td>14.2</td><td>101</td><td>2.45</td><td>13.4</td><td>1.68</td><td>0.32</td><td>0.51</td><td>0.01</td><td>0.10</td><td>0.31</td><td>0.03</td><td>0.18</td><td>0.16</td><td>0.37</td></tr><tr><td>D427 RV-6</td><td>651</td><td>895</td><td>7.20</td><td>10596</td><td>1133</td><td>13.4</td><td>12.7</td><td>115</td><td>1.61</td><td>13.2</td><td>0.90</td><td>0.47</td><td>0.66</td><td>0.01</td><td>0.18</td><td>0.40</td><td>0.04</td><td>0.42</td><td>0.18</td><td>0.41</td></tr><tr><td>average</td><td><strong>537</strong></td><td><strong>814</strong></td><td><strong>6.34</strong></td><td><strong>9085</strong></td><td><strong>869</strong></td><td><strong>9.84</strong></td><td><strong>8.95</strong></td><td><strong>153</strong></td><td><strong>1.96</strong></td><td><strong>11.5</strong></td><td><strong>1.52</strong></td><td><strong>1.45</strong></td><td><strong>1.88</strong></td><td><strong>0.04</strong></td><td><strong>0.25</strong></td><td><strong>0.57</strong></td><td><strong>0.03</strong></td><td><strong>0.21</strong></td><td><strong>0.22</strong></td><td><strong>0.30</strong></td></tr><tr><td>CASS-6 (measured)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.002</td><td>2.07</td><td>0.52</td><td>1.14</td><td>0.03</td><td>4.81</td><td>0.44</td><td>0.06</td><td>0.45</td><td>2.67</td><td>0.03</td><td>0.01</td><td>0.02</td><td>0.015</td></tr><tr><td>CASS-6 (reference)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.002</td><td>1.53</td><td>0.52</td><td>1.24</td><td>0.01</td><td>8.96</td><td>0.41</td><td>0.07</td><td>0.49</td><td>2.86</td><td>0.02</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

<footnote><sup>a</sup> <i>&lt;LD</i> = below limits of detection.</footnote>
<table><caption>**Table 8**

Chemical compositions of the average CLAM venting fluid and end-member hydrothermal fluids of sedimented ridges and back-arc basins.</caption><thead><tr><th>Elements</th><th>CLAM field</th><th>Guaymas Basin<sup>a</sup></th><th colspan="2">Middle Valley<sup>b</sup></th><th>Escanaba Trough<sup>c</sup></th><th colspan="2">Okinawa Trough<sup>d</sup></th><th>Manus Basin<sup>e</sup></th><th>Seawater<sup>f</sup></th></tr><tr><td></td><td></td><td></td><td>Dead Dog</td><td>Bent Hill</td><td></td><td>JADE</td><td>South Ensei</td><td></td><td></td></tr></thead><tbody><tr><td>Ca, mmol/kg</td><td>13.4</td><td>31.3</td><td>81</td><td>40</td><td>32.0-33.0</td><td>22</td><td>21-22</td><td>21.6</td><td>10.2</td></tr><tr><td>Mg</td><td>33.5</td><td>0</td><td>0</td><td>0</td><td>0 (4.6; 18.7)</td><td>0</td><td>0</td><td>16.5</td><td>52.2</td></tr><tr><td>Na</td><td>395</td><td>487</td><td>398</td><td>315</td><td>536-557</td><td>425</td><td>410-431</td><td>406</td><td>464</td></tr><tr><td>K</td><td>22.2</td><td>42.7</td><td>18.7</td><td>13.5</td><td>33.2-35.8</td><td>72</td><td>49-51</td><td>68.7</td><td>10.1</td></tr><tr><td>Fe</td><td>0.003</td><td>0.067</td><td>0.01-0.02</td><td>0.017</td><td>0.041-0.058</td><td>0.003</td><td>-</td><td>4.93</td><td>&lt;0.001</td></tr><tr><td>Li, µmol/kg</td><td>1417</td><td>892</td><td>430-550</td><td>370</td><td>1110-1260</td><td>2500</td><td>5400-5800</td><td>888</td><td>26</td></tr><tr><td>Sr</td><td>72.4</td><td>203</td><td>257</td><td>162</td><td>192-(701)</td><td>94</td><td>215-227</td><td>-</td><td>87</td></tr><tr><td>Mn</td><td>163</td><td>160</td><td>63</td><td>78</td><td>&gt;25-31.1</td><td>110</td><td>88-94</td><td>3262</td><td>&lt;0.001</td></tr><tr><td>Cu</td><td>0.031</td><td>-</td><td>0.6-1.3</td><td>0.3-1.3</td><td>3.2</td><td>0.003</td><td>-</td><td>-</td><td>0.007</td></tr><tr><td>Zn</td><td>0.176</td><td>11.3</td><td>0.9-1.7</td><td>0.7</td><td>11.3</td><td>7.6</td><td>-</td><td>-</td><td>0.012</td></tr><tr><td>Mo</td><td>0.015</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.104<sup>g</sup></td></tr><tr><td>Ni</td><td>0.032</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.012</td></tr><tr><td>Co</td><td>0.001</td><td>-</td><td>-</td><td>-</td><td>0.115</td><td>0.003</td><td>-</td><td>-</td><td>0.00003</td></tr><tr><td>V</td><td>0.005</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Sn</td><td>0.002</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Pb, nmol/kg</td><td>7.33</td><td>-</td><td>95-125</td><td>50</td><td>1560</td><td>36</td><td>-</td><td>-</td><td>0.01</td></tr><tr><td>U</td><td>2.40</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>13.5<sup>h</sup></td></tr><tr><td>Cd</td><td>0.28</td><td>-</td><td>-</td><td>-</td><td>115</td><td>1.3</td><td>-</td><td>-</td><td>1.0</td></tr><tr><td>Ag</td><td>1.93</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.02</td></tr><tr><td>Y</td><td>3.33</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.146<sup>g</sup></td></tr><tr><td>Mn/Ca</td><td>0.012</td><td>0.005</td><td>0.001</td><td>0.002</td><td>0.001</td><td>0.005</td><td>0.004</td><td>0.151</td><td>&lt;0.0000001</td></tr></tbody></table>

<footnote><sup>a</sup> Data from Von Damm (1990).</footnote>

<footnote><sup>b</sup> Data from Butterfield et al. (1994).</footnote>

<footnote><sup>c</sup> Data from Von Damm et al. (2005).</footnote>

<footnote><sup>d</sup> Data from Gamo (1995).</footnote>

<footnote><sup>e</sup> Data from Reeves et al. (2011).</footnote>

<footnote><sup>f</sup> Data from German and Von Damm (2003).</footnote>

<footnote><sup>g</sup> Data from Fouquet et al. (2010).</footnote>

<footnote><sup>h</sup> Data from Chen et al. (1986).</footnote>
<caption>**Table 9**</caption>

REE concentrations (ICP-MS) of the vent fluids from the CLAM hydrothermal field (Iheya Ridge).

<table><thead><tr><th>Sample ID</th><th>La<br>(ppb)</th><th>Ce</th><th>Pr</th><th>Nd</th><th>Sm</th><th>Eu</th><th>Gd</th><th>Tb</th><th>Dy</th><th>Ho</th><th>Er</th><th>Tm</th><th>Yb</th><th>Lu</th><th>ΣREE</th><th>(Ce/Ce*)<sup>a</sup></th><th>(Eu/Eu*)<sup>b</sup></th><th>La<sub>CN</sub>/Lu<sub>CN</sub></th></tr></thead><tbody><tr><td>D426 RV-3</td><td>0.014</td><td>0.020</td><td>&lt;LD<sup>c</sup></td><td>0.010</td><td>&lt;LD</td><td>0.011</td><td>&lt;LD</td><td>&lt;LD</td><td>&lt;LD</td><td>&lt;LD</td><td>&lt;LD</td><td>&lt;LD</td><td>&lt;LD</td><td>0.05</td><td>-</td><td>-</td><td>-</td></tr><tr><td>D426 RV-6</td><td>0.075</td><td>0.111</td><td>0.014</td><td>0.066</td><td>0.015</td><td>0.055</td><td>0.027</td><td>&lt;LD</td><td>0.031</td><td>0.008</td><td>0.024</td><td>&lt;LD</td><td>0.020</td><td>&lt;LD</td><td>0.45</td><td>0.77</td><td>8.19</td><td>1.94</td></tr><tr><td>D427 RV-1</td><td>0.079</td><td>0.127</td><td>0.016</td><td>0.079</td><td>0.018</td><td>0.048</td><td>0.032</td><td>0.005</td><td>0.038</td><td>0.010</td><td>0.030</td><td>&lt;LD</td><td>0.027</td><td>0.004</td><td>0.51</td><td>0.82</td><td>6.10</td><td>-</td></tr><tr><td>D427 RV-2</td><td>0.038</td><td>0.056</td><td>0.007</td><td>0.032</td><td>0.007</td><td>0.014</td><td>0.013</td><td>&lt;LD</td><td>0.017</td><td>&lt;LD</td><td>0.013</td><td>&lt;LD</td><td>0.012</td><td>&lt;LD</td><td>0.21</td><td>0.80</td><td>4.40</td><td>-</td></tr><tr><td>D427 RV-3</td><td>0.064</td><td>0.089</td><td>0.012</td><td>0.062</td><td>0.014</td><td>0.048</td><td>0.027</td><td>&lt;LD</td><td>0.032</td><td>0.008</td><td>0.025</td><td>&lt;LD</td><td>0.023</td><td>&lt;LD</td><td>0.40</td><td>0.72</td><td>7.45</td><td>-</td></tr><tr><td>D427 RV-6</td><td>0.069</td><td>0.108</td><td>0.015</td><td>0.072</td><td>0.017</td><td>0.049</td><td>0.029</td><td>0.005</td><td>0.035</td><td>0.009</td><td>0.028</td><td>&lt;LD</td><td>0.026</td><td>0.004</td><td>0.47</td><td>0.79</td><td>6.67</td><td>1.81</td></tr><tr><td>average</td><td><strong>0.057</strong></td><td><strong>0.085</strong></td><td><strong>0.013</strong></td><td><strong>0.053</strong></td><td><strong>0.014</strong></td><td><strong>0.038</strong></td><td><strong>0.025</strong></td><td><strong>0.005</strong></td><td><strong>0.031</strong></td><td><strong>0.009</strong></td><td><strong>0.024</strong></td><td><strong>&lt;LD</strong></td><td><strong>0.022</strong></td><td><strong>0.004</strong></td><td><strong>0.35</strong></td><td><strong>0.78</strong></td><td><strong>6.56</strong></td><td><strong>1.87</strong></td></tr><tr><td>CASS-6 (measured)</td><td>0.010</td><td>0.005</td><td>0.001</td><td>0.006</td><td>0.001</td><td>&lt;LD</td><td>0.002</td><td>&lt;LD</td><td>0.001</td><td>&lt;LD</td><td>0.001</td><td>&lt;LD</td><td>0.001</td><td>&lt;LD</td><td>-</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

<footnote>a Ce/Ce* = 2Ce<sub>CN</sub>/(La<sub>CN</sub>+Pr<sub>CN</sub>).</footnote>

<footnote>b Eu/Eu* = 2Eu<sub>CN</sub>/(Sm<sub>CN</sub>+Gd<sub>CN</sub>).</footnote>

<footnote>c <LD = below limits of detection.</footnote>
<caption>**Table 10**</caption>

REE concentrations of the average CLAM venting fluid and end-member hydrothermal fluids of sedimented ridges and back-arc basins.

<table><thead><tr><th>REE</th><th>CLAM field</th><th>Guaymas Basin<sup>a</sup></th><th>Escanaba Trough<sup>a</sup></th><th>Mariana back-arc basin<sup>a</sup></th><th>Manus back-arc basin<sup>b</sup></th><th>Okinawa back-arc basin<sup>c</sup></th></tr></thead><tbody><tr><td>La, pmol/kg</td><td>407</td><td>963</td><td>829</td><td>2140</td><td></td><td>4397 6617</td></tr><tr><td>Ce</td><td>608</td><td>953</td><td>983</td><td>1710</td><td></td><td>7965 8624</td></tr><tr><td>Pr</td><td>89.4</td><td>89.0</td><td>123</td><td>191</td><td></td><td>979 910</td></tr><tr><td>Nd</td><td>370</td><td>285</td><td>487</td><td>698</td><td></td><td>3873 3421</td></tr><tr><td>Sm</td><td>95.5</td><td>30.3</td><td>112</td><td>111</td><td></td><td>1051 793</td></tr><tr><td>Eu</td><td>247</td><td>673</td><td>165</td><td>1380</td><td></td><td>4197 3038</td></tr><tr><td>Gd</td><td>162</td><td>19.3</td><td>93.0</td><td>79</td><td></td><td>1026 969</td></tr><tr><td>Tb</td><td>32.4</td><td>4.00</td><td>17.0</td><td>6</td><td></td><td>161 146</td></tr><tr><td>Dy</td><td>188</td><td>14.0</td><td>79.5</td><td>58</td><td></td><td>943 898</td></tr><tr><td>Ho</td><td>52.5</td><td>3.33</td><td>16.0</td><td>13</td><td></td><td>137 207</td></tr><tr><td>Er</td><td>143</td><td>9.00</td><td>36.5</td><td>27</td><td></td><td>390 621</td></tr><tr><td>Tm</td><td>&lt;LD</td><td>-</td><td>-</td><td>-</td><td></td><td>- 80.4</td></tr><tr><td>Yb</td><td>126</td><td>-</td><td>-</td><td>-</td><td></td><td>298 429</td></tr><tr><td>Lu</td><td>23.5</td><td>-</td><td>-</td><td>-</td><td></td><td>- 63.7</td></tr></tbody></table>

<footnote><sup>a</sup> Average end-member fluid (Klinkhammer et al., 1994).</footnote>

<footnote><sup>b</sup> Average end-member fluid (Craddock et al., 2010).</footnote>

<footnote><sup>c</sup> Average venting fluid at Yonaguni Knoll IV hydrothermal field (Hongo et al., 2007).</footnote>
Table 11

<table><caption>C, O and Mg isotopic composition of the chimneys from the CLAM hydrothermal field (Iheya Ridge).</caption><thead><tr><th>Sample ID</th><th>δ<sup>13</sup>C<sub>VPDB</sub><br>(‰)</th><th>1SD<br>(‰)</th><th>δ<sup>18</sup>O<sub>VPDB</sub><br>(‰)</th><th>1SD<br>(‰)</th><th>δ<sup>18</sup>O<sub>VSMOW</sub><sup>a</sup><br>(‰)</th><th>1SD<br>(‰)</th><th>δ<sup>25</sup>Mg<br>(‰)</th><th>δ<sup>26</sup>Mg<br>(‰)</th></tr></thead><tbody><tr><td>2K#366-St.2-a</td><td>0.54</td><td>0.11</td><td>-3.70</td><td>0.11</td><td>27.04</td><td>0.11</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.2-b</td><td>-0.11</td><td>0.07</td><td>-5.94</td><td>0.07</td><td>24.74</td><td>0.07</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.2-b, duplicate</td><td>-0.14</td><td>0.10</td><td>-6.30</td><td>0.13</td><td>24.36</td><td>0.13</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.2-c</td><td>1.25</td><td>0.06</td><td>-2.89</td><td>0.06</td><td>27.88</td><td>0.06</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.2-d</td><td>1.48</td><td>0.08</td><td>-4.60</td><td>0.08</td><td>26.11</td><td>0.08</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-a</td><td>-6.74</td><td>0.07</td><td>-20.66</td><td>0.07</td><td>9.56</td><td>0.07</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-b</td><td>-6.22</td><td>0.10</td><td>-19.23</td><td>0.10</td><td>11.03</td><td>0.10</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-c</td><td>-5.66</td><td>0.02</td><td>-17.95</td><td>0.02</td><td>12.35</td><td>0.02</td><td>-1.15</td><td>-2.18</td></tr><tr><td>2K#366-St.3-c, duplicate</td><td>-5.61</td><td>0.20</td><td>-17.34</td><td>0.22</td><td>12.99</td><td>0.22</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-d</td><td>-6.46</td><td>0.06</td><td>-20.04</td><td>0.06</td><td>10.20</td><td>0.06</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-e</td><td>-6.75</td><td>0.04</td><td>-20.53</td><td>0.04</td><td>9.70</td><td>0.04</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3C-b</td><td>-1.75</td><td>0.07</td><td>-9.12</td><td>0.07</td><td>21.46</td><td>0.07</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3C-c</td><td>-3.43</td><td>0.06</td><td>-12.26</td><td>0.08</td><td>18.22</td><td>0.08</td><td>-0.80</td><td>-1.50</td></tr><tr><td>2K#366-St.3C-e</td><td>-3.32</td><td>0.06</td><td>-12.38</td><td>0.06</td><td>18.10</td><td>0.06</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3C-f</td><td>-2.62</td><td>0.05</td><td>-10.77</td><td>0.05</td><td>19.76</td><td>0.05</td><td>-0.64</td><td>-1.15</td></tr><tr><td>2K#366-St.3C-f, duplicate</td><td>-2.65</td><td>0.11</td><td>-10.21</td><td>0.12</td><td>20.33</td><td>0.12</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3C-g</td><td>-2.15</td><td>0.06</td><td>-9.46</td><td>0.06</td><td>21.11</td><td>0.06</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3C-i</td><td>-2.63</td><td>0.09</td><td>-10.78</td><td>0.09</td><td>19.75</td><td>0.09</td><td>-0.59</td><td>-1.07</td></tr><tr><td>T529-G4A B<sup>b</sup></td><td>-12.18</td><td>0.09<sup>c</sup></td><td>-23.39</td><td>0.12<sup>c</sup></td><td>6.74</td><td></td><td>-</td><td>-</td></tr></tbody></table>

<footnote>a Calculated according to Friedman and O'Neil (1977).</footnote>

<footnote>b Flange, Guaymas Basin.</footnote>

<footnote>c 2SD.</footnote>
<table><caption>**Table 12**
S isotopic composition of the chimneys from the CLAM hydrothermal field (Iheya Ridge).</caption><thead><tr><th rowspan="2">Sample ID</th><th colspan="3">Sulfates (anhydrite)</th><th colspan="3">Sulfides (pyrite+chalcopyrite)</th><th colspan="3">Native sulfur</th></tr><tr><th>concentration (wt.%)</th><th>δ<sup>34</sup>S (‰)</th><th>1σ (‰)</th><th>concentration (wt.%)</th><th>δ<sup>34</sup>S (‰)</th><th>1σ (‰)</th><th>concentration (wt.%)</th><th>δ<sup>34</sup>S (‰)</th><th>1σ (‰)</th></tr></thead><tbody><tr><td>2K#366-St.2</td><td>0.19</td><td>2.95</td><td>0.27</td><td>3.64</td><td>1.98</td><td>0.04</td><td>0.40</td><td>2.25</td><td>0.27</td></tr><tr><td>2K#366-St.2, duplicate</td><td></td><td>2.57</td><td></td><td></td><td>1.92</td><td></td><td></td><td>1.86</td><td></td></tr><tr><td>2K#366-St.3-a</td><td>0.54</td><td>27.4</td><td>0.06</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-a, duplicate</td><td></td><td>27.5</td><td></td><td></td><td>-</td><td></td><td></td><td>-</td><td></td></tr><tr><td>2K#366-St.3-b</td><td>0.38</td><td>25.9</td><td>0.06</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-b, duplicate</td><td></td><td>26.0</td><td></td><td></td><td>-</td><td></td><td></td><td>-</td><td></td></tr><tr><td>2K#366-St.3-c</td><td>0.21</td><td>24.9</td><td>0.19</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-c, duplicate</td><td></td><td>25.1</td><td></td><td></td><td>-</td><td></td><td></td><td>-</td><td></td></tr><tr><td>2K#366-St.3-d</td><td>1.03</td><td>27.6</td><td>0.36</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-d, duplicate</td><td></td><td>28.1</td><td></td><td></td><td>-</td><td></td><td></td><td>-</td><td></td></tr><tr><td>2K#366-St.3-e</td><td>0.85</td><td>28.3</td><td>0.06</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>2K#366-St.3-e, duplicate</td><td></td><td>28.4</td><td></td><td></td><td>-</td><td></td><td></td><td>-</td><td></td></tr><tr><td>2K#366-St.3C-e</td><td>0.37</td><td>0.69</td><td>0.28</td><td>1.63</td><td>0.82</td><td>0.21</td><td>0.42</td><td>2.66</td><td>0.02</td></tr><tr><td>2K#366-St.3C-e, duplicate</td><td></td><td>1.08</td><td></td><td></td><td>1.11</td><td></td><td></td><td>2.63</td><td></td></tr><tr><td>2K#366-St.3C-f</td><td>0.21</td><td>2.44</td><td>0.15</td><td>3.38</td><td>2.58</td><td>0.34</td><td>0.33</td><td>2.37</td><td>0.08</td></tr><tr><td>2K#366-St.3C-f, duplicate</td><td></td><td>2.66</td><td></td><td></td><td>2.10</td><td></td><td></td><td>2.48</td><td></td></tr><tr><td>2K#366-St.3C-g</td><td>0.20</td><td>1.91</td><td></td><td>4.92</td><td>2.46</td><td>0.35</td><td>0.29</td><td>2.62</td><td>0.06</td></tr><tr><td>2K#366-St.3C-g, duplicate</td><td></td><td></td><td></td><td></td><td>2.95</td><td></td><td></td><td>2.53</td><td></td></tr><tr><td>T529-G4A B<sup>a</sup></td><td>1.83</td><td>24.3</td><td>0.13</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>T529-G4A B, duplicate</td><td></td><td>24.1</td><td></td><td></td><td>-</td><td></td><td></td><td>-</td><td></td></tr></tbody></table>

<footnote><sup>a</sup> Flange, Guaymas Basin.</footnote>
<table><caption>**Table 13**
Sr-Nd-Pb isotopic composition of the chimneys and vent fluids from the CLAM hydrothermal field (Iheya Ridge).</caption><thead><tr><th>Sample ID</th><th><sup>87</sup>Sr/<sup>86</sup>Sr</th><th>error</th><th><sup>143</sup>Nd/<sup>144</sup>Nd</th><th>error</th><th>εNd</th><th><sup>206</sup>Pb/<sup>204</sup>Pb</th><th>error</th><th><sup>207</sup>Pb/<sup>204</sup>Pb</th><th>error</th><th><sup>208</sup>Pb/<sup>204</sup>Pb</th><th>error</th><th>Reference</th></tr></thead><tbody><tr><td colspan="14"><strong>Chimneys</strong></td></tr><tr><td>2K#366-St.2-a</td><td>0.709376</td><td>0.000024</td><td>0.512801</td><td>0.000017</td><td>3.2</td><td>18.549</td><td>0.000792</td><td>15.621</td><td>0.000713</td><td>38.755</td><td>0.00174</td><td>this study</td></tr><tr><td>2K#366-St.2-b</td><td>0.709006</td><td>0.000067</td><td>0.512733</td><td>0.000020</td><td>1.9</td><td>18.547</td><td>0.000792</td><td>15.619</td><td>0.000694</td><td>38.750</td><td>0.00180</td><td>this study</td></tr><tr><td>2K#366-St.2-c</td><td>0.708957</td><td>0.000022</td><td>0.512777</td><td>0.000013</td><td>2.7</td><td>18.550</td><td>0.000741</td><td>15.622</td><td>0.000678</td><td>38.758</td><td>0.00196</td><td>this study</td></tr><tr><td>2K#366-St.2-d</td><td>0.709118</td><td>0.000017</td><td>0.512710</td><td>0.000020</td><td>1.4</td><td>18.552</td><td>0.00119</td><td>15.623</td><td>0.00104</td><td>38.760</td><td>0.00258</td><td>this study</td></tr><tr><td>2K#366-St.3-a</td><td>0.708269</td><td>0.000014</td><td>0.512597</td><td>0.000059</td><td>-0.8</td><td>18.373</td><td>0.000691</td><td>15.613</td><td>0.000667</td><td>38.476</td><td>0.00174</td><td>this study</td></tr><tr><td>2K#366-St.3-b</td><td>0.708271</td><td>0.000016</td><td>0.512776</td><td>0.000009</td><td>2.7</td><td>18.545</td><td>0.00128</td><td>15.616</td><td>0.00110</td><td>38.745</td><td>0.00290</td><td>this study</td></tr><tr><td>2K#366-St.3-c</td><td>0.708303</td><td>0.000031</td><td>0.512695</td><td>0.000031</td><td>1.1</td><td>18.595</td><td>0.00151</td><td>15.669</td><td>0.00143</td><td>38.875</td><td>0.00382</td><td>this study</td></tr><tr><td>2K#366-St.3-d</td><td>0.708194</td><td>0.000011</td><td>0.512789</td><td>0.000041</td><td>2.9</td><td>18.528</td><td>0.00110</td><td>15.620</td><td>0.000924</td><td>38.723</td><td>0.00249</td><td>this study</td></tr><tr><td>2K#366-St.3-e</td><td>0.708159</td><td>0.000016</td><td>0.512765</td><td>0.000011</td><td>2.5</td><td>18.254</td><td>0.00117</td><td>15.641</td><td>0.00105</td><td>38.327</td><td>0.00281</td><td>this study</td></tr><tr><td>2K#366-St.3C-b</td><td>0.708272</td><td>0.00002</td><td>0.512760</td><td>0.000020</td><td>2.4</td><td>18.547</td><td>0.000898</td><td>15.618</td><td>0.000850</td><td>38.747</td><td>0.00204</td><td>this study</td></tr><tr><td>2K#366-St.3C-c</td><td>0.710834</td><td>0.000016</td><td>0.512506</td><td>0.000012</td><td>-2.6</td><td>18.548</td><td>0.00124</td><td>15.620</td><td>0.00112</td><td>38.752</td><td>0.00278</td><td>this study</td></tr><tr><td>2K#366-St.3C-e</td><td>0.708499</td><td>0.000022</td><td>0.512762</td><td>0.000043</td><td>2.4</td><td>18.547</td><td>0.000796</td><td>15.618</td><td>0.000751</td><td>38.748</td><td>0.00193</td><td>this study</td></tr><tr><td>2K#366-St.3C-f</td><td>0.708685</td><td>0.000022</td><td>0.512706</td><td>0.000029</td><td>1.3</td><td>18.551</td><td>0.000862</td><td>15.622</td><td>0.000738</td><td>38.756</td><td>0.00189</td><td>this study</td></tr><tr><td>2K#366-St.3C-g</td><td>0.708706</td><td>0.000029</td><td>0.512810</td><td>0.000058</td><td>3.4</td><td>18.551</td><td>0.000617</td><td>15.622</td><td>0.000589</td><td>38.761</td><td>0.00168</td><td>this study</td></tr><tr><td>2K#366-St.3C-i</td><td>0.709009</td><td>0.000019</td><td>0.512753</td><td>0.000066</td><td>2.2</td><td>18.550</td><td>0.000624</td><td>15.621</td><td>0.000584</td><td>38.758</td><td>0.00133</td><td>this study</td></tr><tr><td colspan="14"><strong>Vent fluids</strong></td></tr><tr><td>D426 RV-3</td><td>0.709142</td><td>0.000017</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>this study</td></tr><tr><td>D426 RV-6</td><td>0.709049</td><td>0.000009</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>this study</td></tr><tr><td>D426 RV-6</td><td>0.709003</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Araoka et al., 2016</td></tr><tr><td>D426 RV-6</td><td>0.709000</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Yoshimura et al., 2020</td></tr><tr><td>D427 RV-1</td><td>0.708939</td><td>0.000013</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>this study</td></tr><tr><td>D427 RV-2</td><td>0.708961</td><td>0.000017</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>this study</td></tr><tr><td>D427 RV-3</td><td>0.708936</td><td>0.000016</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>this study</td></tr><tr><td>D427 RV-6</td><td>0.708925</td><td>0.000011</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>this study</td></tr><tr><td>D427 RV-6</td><td>0.708962</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Araoka et al., 2016</td></tr><tr><td>D427 RV-6</td><td>0.708948</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Yoshimura et al., 2020</td></tr><tr><td>CLAM end-member fluid</td><td>0.7090</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Araoka et al., 2016</td></tr><tr><td>CLAM end-member fluid</td><td>0.70909</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Yoshimura et al., 2020</td></tr></tbody></table>