Precipitation of Calcite Veins in Serpentinized Harzburgite at Tianxiu Hydrothermal Field on Carlsberg Ridge (3.67°N), Northwest Indian Ocean: Implications for Fluid Circulation

0. INTRODUCTION

Mantle rocks are commonly exposed on the seafloor at the slow- and ultraslow-spreading ridges due to tectonic exhumation (Mével, 2003). They are normally extensively altered by fluid, resulting in serpentinization and carbonation and the formation of hydrous silicates, carbonates, oxides, and sulfides (Wang et al., 2019; Boschi et al., 2009; Kelemen and Matter, 2008; Hansen et al., 2005; Jedrysek and Halas, 1990; Frost, 1985; Barnes et al., 1973). The alteration of peridotite has profound consequences for the composition, structure, and geophysical signature of the oceanic lithosphere as well as the geochemical budgets of the ocean. Carbonates can form in several ways in the oceanic lithosphere, including replacement of silicate and hydroxide minerals via mineral carbonation or precipitation from fluids (Andreani et al., 2009; Eickmann et al., 2009). Most studies of carbonation from the altered ultramafic rock have been focused on its potential of carbon sequestration (e.g., Power et al, 2013; Kelemen et al., 2011; Boschi et al., 2009; Alexander et al., 2007). Mineral carbonation represents a potentially significant sink for carbon in the shallow oceanic lithosphere (Grozeva et al., 2017).

Carbonate veins, which precipitate during the conductive heating of seawater, conductive cooling of hydrothermal fluid, or mixing of seawater with hydrothermal fluid (Klein et al., 2015; Eickmann et al., 2009), are a common low- to moderate- temperature feature in altered ocean lithosphere. Previous studies have shown that the chemical and isotopic composition of carbonate veins may record the infiltration of seawater and its evolution as a hydrothermal fluid under a wide range of conditions (Guo et al., 2019; Schroeder et al., 2015; Bach et al., 2011; Coggon et al., 2004). Thus, carbonate veins can be used to track the information regarding the origin and nature of precipitating fluid. An assemblage of rock samples consisting of peridotite, gabbro, and basalt were collected from the seafloor at a depth of 3 500 m using a TV-guided grab (Fig. 1) in the vicinity of the active ultramafic-hosted Tianxiu hydrothermal field at 3.67°N/63.83°E on the Carlsberg Ridge, Northwest Indian Ocean during the Chinese DY33 cruise in 2015 (Han et al., 2015). Most of the peridotite samples underwent extensive serpentinization and calcite veins were observed.

Figure  1.  Bathymetric map exhibiting the locations of the Tianxiu hydrothermal field and samples. The distance between these two locations is less than 400 m. The bathymetry data were collected by ship-based EM120 during the DY24 cruise in 2012.

DownLoad: Full-Size Img PowerPoint

In this study, detailed petrological, mineralogical, and geochemical analyses were carried out on the harzburgite samples, and carbon and oxygen isotope compositions of calcite veins were also studied to constrain the alteration processes and reveal the origin and nature of calcite precipitating fluid.

1. SAMPLES AND PETROLOGY

Petrographic observations were carried out on 12 peridotites from the station (33I-TVG07). The samples are extensively serpentinized with relics of olivine ((Mg, Fe)₂SiO₄), orthopyroxene ((Mg, Fe)₂Si₂O₆), spinel (MgAl₂O₄), and minor clinopyroxene (Ca(Mg, Fe)Si₂O₆). Harzburgite samples that are mainly composed of olivine and orthopyroxene with clinopyroxene less than 5 vol.%, are selected for analyses in this study. Two types of calcite (CaCO₃) veins are recognized in serpentinized harzburgite samples according to their distributions. Calcite veins Ⅰ occur in the fractures that cut through the mesh texture; calcite veins Ⅱ precipitate within the mesh texture (Fig. 2).

Figure  2.  Photographs of altered harzburgites, with sample E06 on the left and sample E22 on the right. (a) a photograph of a hand specimen that is almost completely altered; (b) a microphotograph of sample E06 under a polarizing microscope with plane polarized light. Olivine has been severely altered with the development of serpentine veins; (c) calcite veins occur in the fractures that cut through altered olivine and orthopyroxene; (d) a photograph of a hand specimen of sample E22, with widely distributed white veins; (e) a microphotograph of sample E22 under a polarizing microscope with perpendicular polarized light. Olivine has been severely altered, forming serpentine, talc, and minor magnetite. Orthopyroxene has been altered, forming bastite; (f) backscatter electron image of an enlarged area in (e). Calcite veins occur within the mesh texture. L1–L2 represents the line analyses that moved across veins formed during alteration (refer to Fig. 3 for details). Bas. bastite; Mag. magnetite; Tlc. talc; Ol. olivine; Opx. orthopyroxene; Cpx. clinopyroxene; Spl. spinel; Srp. serpentine; Cal. calcite (Kretz, 1983).

DownLoad: Full-Size Img PowerPoint

The hand specimen of the sample (E06) containing calcite veins Ⅰ is yellowish-brown with serpentinization degree close to 100%, which was estimated by the microscope observation (Fig. 2a). Olivine is entirely replaced by serpentine (Mg₃Si₂O₅(OH)₄) with opaque minerals occurring among the mesh texture. Orthopyroxene exhibiting exsolution lamellae is altered into bastite (Mg₃Si₂O₅(OH)₄). Serpentine veins cutting through mesh texture (Fig. 2b) were observed, 30–80 μm in width. Pseudomorphic replacements after olivine and orthopyroxene allow the mineral modal abundances to be estimated. The primary minerals include olivine (≈75 vol.%), orthopyroxene (≈20 vol.%), and clinopyroxene (≈5 vol.%). Calcite veins in the sample are approximately 13 μm in width and mainly line the walls of open fractures/cavities next to serpentine veins (Fig. 2c). The fractures filled by veins are irregular and jagged, indicating formation under pure tension.

The hand specimen of the sample (E22) containing calcite veins Ⅱ is approximately 80% serpentinized with visible pseudomorphic mesh and bastite textures (Fig. 2d). Relic olivine forms subhedral grains less than 0.2 mm in size. Relic orthopyroxene grains surrounded by bastite usually contain clinopyroxene exsolution lamellae. Magnetite (Fe₃O₄) grains occur within the serpentine mesh texture (Fig. 2e). Relict spinel has a translucent reddish-brown color with a fresh interior and subhedral shapes. The primary minerals include olivine (≈80 vol.%), orthopyroxene (≈15 vol.%), clinopyroxene (≈5 vol.%), and minor spinel (< 1 vol.%). Calcite veins in the sample vary from 5–22 μm in width, appearing within the serpentine- dominated veins and along the margin of olivine among the mesh texture (Fig. 2f). Approximately less than 2 vol.% of the rock volume is occupied by calcite veins according to the estimation using the dot-counting method in both samples.

2. METHOD

Thin sections of both samples were prepared for petrology identification using a reflected and transmitted light-polarizing microscope and underwent further mineral chemistry analyses using a JEOL JXA-8100 Electron Probe Microanalyzer (EPMA). Due to their different degrees of serpentinization, the EPMA analysis was only conducted on samples E22, at an acceleration voltage of 20 kV and beam current intensity of 1 nA with a 5 μm beam diameter. The precision of analyses for the reported elements is greater than ±10%. Samples (E6 and E22) were also pulverized for mineralogical analyses, which were conducted using a Panalytical X'Pert PRO X-ray Diffractometer (XRD) under a voltage of 45 kV and a current intensity of 40 mA. All of the analyses mentioned above were conducted in the Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Hangzhou, China.

C- and O-isotope analysis was conducted at the Queen's Facility for Isotope Research, Geological Sciences and Geological Engineering Department, Queen's University, Canada. Calcite was carefully selected from the coarsely crushed rock fragments of the two harzburgite samples and then ground into a 200-mesh powder. CO₂ was released by reaction with 100% H₃PO₄ at 72 ℃ for 4 hours in a heated aluminum block. CO₂ was isotopically analyzed by a Thermo-Finnigan GasBench coupled to a Thermo-Finnigan MAT Delta Plus XP continuous- flow isotope-ratio mass spectrometer (CF-IRMS). δ¹⁸O and δ¹³C ratios were determined against a working calcite standard and are expressed in the common δ-notation relative to Vienna Standard Mean Ocean Water (V-SMOW) for O and Vienna Pee Dee Belemnite (PDB) for C. The precision for δ¹⁸O and δ¹³C was 0.5‰ and 0.1‰ (2δ), respectively.

The geochemical modeling approach is useful for interpreting alteration processes for rocks whose mineral composition is known and providing valid information on fluid composition. The reaction path models were constructed using EQ3/6 (Wolery, 1992) to predict the assemblages that would develop in harzburgite fluxing with CO₂-enriched seawater-derived fluids at water/rock (w/r) ratios from 0.2 to 10 in a titration system. The thermodynamic database, including equilibrium constants (logK) for dissolution of minerals, dissociation of aqueous species, and redox reactions, was generated by the software code DBCreate (Kong et al., 2013) to support the models. Dissolution kinetics of silicate minerals in the peridotite was not considered in the model (McCollom and Bach, 2009).

3. RESULTS

3.1 The Geochemical Characteristics of Serpentinized Harzburgite

The electron microprobe analysis (EMPA) data for olivine, orthopyroxene, clinopyroxene, spinel, and serpentine-group minerals of the sample E22 are listed in Table 1. Olivine (Fo_90–91) relicts contain small amounts of Cr (≈0.03 wt.% Cr₂O₃) and significant Ni contents (≈0.36 wt.% NiO). Orthopyroxene (En_90–91) compositions correspond to Al- and Cr-bearing enstatite, with Mg^# (=100×Mg²⁺/(Mg²⁺+Fe²⁺)) similar to that of olivine. Clinopyroxenes (En_50–53, Wo_43–45, Fs_5–6) are poor in Ti (≈0.10 wt.% TiO₂) and rich in Cr (≈1.28 wt.% Cr₂O₃). Spinels generally have high Mg^# (≈68) and low Cr^# (=100×Cr³⁺/(Cr³⁺+ Al³⁺); ≈30) values. Bastite after orthopyroxene is higher in Al (≈1.59 wt.% Al₂O₃) and Cr (≈0.92 wt.% Cr₂O₃) and lower in Ca (≈0.08 wt.% CaO) and Ni (≈0.10 wt.% NiO) than serpentinite after olivine (Table 1). The line-scanning of the EPMA analysis on sample E22 shows that the calcite veins are characterized by the high relative intensity of Ca and are almost free of Mg and Fe (Fig. 3).

Table  1.  Average and ranges of electron microprobe analyses of minerals in harzburgite

Minerals	(wt.%)	SiO2	TiO2	Al2O3	Cr2O3	MgO	CaO	MnO	FeO	NiO	Total	Mg#
Ol (n=25)	Average	40.99	0.01	0.02	0.03	49.77	0.06	0.13	8.98	0.36	100.31	90.81
	Min	40.85	< l.o.d	< l.o.d	< l.o.d	48.53	0.03	0.06	8.46	0.28	98.50	91.09
	Max	41.81	0.02	0.04	0.06	50.60	0.10	0.18	9.65	0.46	101.98	90.34
Opx (n=7)	Average	54.79	0.06	4.05	0.93	32.05	2.23	0.14	5.86	0.09	100.19	90.70
	Min	54.54	0.00	3.29	0.81	31.31	1.43	0.12	5.63	0.04	99.72	90.84
	Max	55.14	0.09	4.25	1.06	32.83	3.45	0.18	6.27	0.14	100.83	90.32
Cpx (n=6)	Average	51.31	0.10	4.79	1.28	17.38	21.62	0.11	3.06	0.06	99.72	91.01
	Min	50.42	0.06	4.49	1.18	16.35	18.48	0.05	2.63	< l.o.d	98.35	91.72
	Max	52.04	0.13	5.08	1.34	19.38	23.20	0.14	4.04	0.10	100.21	89.53
Spl (n=3)	Average	0.04	0.06	41.48	26.36	17.33	0.01	0.20	14.34	0.25	100.09	68.30
	Min	< l.o.d	0.05	40.88	25.95	16.98	0.00	0.16	14.03	0.15	99.95	68.33
	Max	0.07	0.07	41.88	27.03	17.65	0.04	0.25	14.64	0.30	100.23	68.24
Srp (n=2)	Average	41.59	0.05	0.69	0.07	37.12	0.15	0.09	4.51	0.29	84.46	94.62
	Min	41.48	< l.o.d	0.69	0.06	36.89	0.06	0.08	4.48	0.11	83.91	93.62
	Max	41.71	0.05	0.70	0.08	37.35	0.30	0.10	4.55	0.47	85.01	93.60
Bas (n=4)	Average	40.43	0.04	1.59	0.92	36.02	0.08	0.15	5.65	0.10	84.74	91.91
	Min	39.86	0.02	1.30	< l.o.d	34.87	0.01	0.10	5.19	0.09	83.85	92.29
	Max	41.26	0.05	1.79	0.99	37.23	0.13	0.19	6.33	0.15	85.84	91.29
Averages are reported for analyses above the limit of detection (l.o.d); Mg#=100×Mg2+/(Mg2++Fe2+); Ol. olivine; Opx. orthopyroxene; Cpx. clinopyroxene; Spl. spinel; Srp. serpentine; Bas. bastite.

 | Show Table

DownLoad: CSV

Figure  3.  EMPA line analyses results (see Fig. 2f). (a)–(b) EMPA line analyses results across the veins in serpentinized harzburgite sample E22. The carbonate veins are distributed among the mesh texture. They are characterized by the high Ca relative intensity and are almost free of Mg and Fe in the middle of the vein (the relevant data are given in supplementary Table B1).

DownLoad: Full-Size Img PowerPoint

XRD analysis of the whole rock powder of the sample E06 confirmed the presence of secondary minerals of serpentine (lizardite), magnetite, hematite, calcite, bornite, pyrrhotite, and pentlandite. By contrast, XRD analysis of the whole rock powder of the sample E22 confirmed the presence of serpentine (lizardite), magnetite, hematite, pyrrhotite, and galena (Fig. 4).

Figure  4.  The XRD profiles of serpentinized harzburgite samples from the Tianxiu hydrothermal field. Lz. lizardite; Gn. galena; Bn. bornite; Pn. pentlandite; Po. pyrrhotite; Hem. hematite; Cc. chalcocite; Cal. calcite (Kretz, 1983).

DownLoad: Full-Size Img PowerPoint

3.2 Carbon and Oxygen Isotope Chemistry

The δ¹³C_PDB and δ¹⁸O_PDB signals of the calcite veins Ⅰ are +0.58‰ and +4.46‰, respectively. By contrast, those of the calcite veins Ⅱ are +0.54‰ and -16.67‰. The equation proposed by Coplen et al. (1983) is used to convert δ¹⁸O_V-SMOW to δ¹⁸O_PDB (R1). These two calcite samples have similar δ¹³C_PDB signals, but their δ¹⁸O_PDB signals are very different. Similar isotope signals of calcite veins were also reported by Schroeder et al. (2015) (-4.66‰ to +3.29‰ δ¹³C_PDB; -19.34‰ to +5.45‰ δ¹⁸O_PDB) during their studies on an oceanic core complex near the 15°20'N fracture zone (ODP Leg 209) along the Mid-Atlantic Ridge (MAR).

4. DISCUSSION

4.1 Origin and Nature of the Carbonate-Precipitating Fluid

The geochemical isotope data for calcite samples provide important clues for understanding the origin of precipitating fluid. The δ¹³C values for calcite of both types (+0.54‰ and +0.58‰) are close to that of seawater (≈1‰; Kump, 1989), which implies that dissolved inorganic carbon in the seawater is the main source of calcite. The δ¹⁸O_V-SMOW values for carbonate reflect the combined effect of temperature and δ¹⁸O_V-SMOW of the precipitating fluid (Urey, 1947). If the precipitation temperature of calcite is known, the δ¹⁸O_V-SMOW of the fluid can be calculated based on the oxygen isotopic fractionation factor between calcite and water, and vice versa. Temperature calculations are performed using the composition of modern Indian Ocean deep seawater (-0.18‰ δ¹⁸O_V-SMOW; Schmidt et al., 1999) and the equation of Kim and O'Neil (1997) (R2).

The formation temperature of calcite with -16.67‰ and +4.46‰ δ¹⁸O_PDB is calculated to be 117 and -6 ℃, respectively. The negative temperature is inconsistent with the temperature of the seafloor bottom water, which can be attributed to the fact that its precipitation occurred under higher seawater δ¹⁸O_V-SMOW rather than modern values (δ¹⁸O_V-SMOW= -0.18‰), such as during a glacial maximum (Schroeder et al., 2015). The bottom water temperature of the sampling site (water depth: 3 500 m) is 2 ℃, according to the conductivity-temperature- depth investigation during the DY38th cruise (unpublished data). The seasonal variation of deep water temperature is insignificant. Considering that the samples were collected on the seafloor, we assume 2 ℃ as the precipitation temperature of the calcite vein with δ¹⁸O_PDB of 4.46‰. Utilizing the equation of Kim and O'Neil (1997), the equilibrium δ¹⁸O_V-SMOW of calcite- precipitating fluid is calculated to be 1.78‰.

The δ¹⁸O_V-SMOW values of hydrothermal vent fluids range from 0.28‰ to 2.39‰, with an average value of 1.00‰ according to the statistics of more than 150 vent fluid samples (supplementary Table B2; James et al., 2014; Schmidt et al., 2011; Gamo et al., 2001; Shanks, 2001; Jean-Baptiste et al., 1997). By contrast, vent fluids from the high-temperature and ultramafic- hosted hydrothermal systems, such as the Kairei, Logatchev, and Nibelungen sites along the Central Indian Ridge and the MAR normally have relatively higher δ¹⁸O_V-SMOW values, ranging from 1.23‰ to 2.39‰ (Schmidt et al., 2011; Gamo et al., 2001), with an average value of 1.74‰ (Table 2). This value is close to the calculated δ¹⁸O_V-SMOW value (1.78‰) of calcite-precipitating fluid in this study, which indicates that hydrothermal fluid exerted an influence. Hence, the formation temperature of calcite with -16.67‰ δ¹⁸O_PDB is inferred to be approximately 134 ℃. Moreover, calcite veins in the serpentinized harzburgite samples may also precipitate from the fluid with higher δ¹⁸O_V-SMOW values. The precipitation temperature of calcite veins would be higher by 6 ℃ if a fluid δ¹⁸O_V-SMOW of +2.39‰ was adopted, which corresponds to the most ¹⁸O-enriched fluids reported thus far (Schmidt et al., 2011; Table 2).

Table  2.  Endmember fluid oxygen isotopic composition from the Logatchev I, Nibelungen, and Kairei vent fields.

Vent field	Temperature (℃)	Sample ID	δ18O Endmember (‰)
Logatchev Ia	350b	253ROV-10	1.42
		255ROV-3	1.23
		255ROV-17	1.31
		259ROV-25	1.45
		271ROV-11	1.32
		275ROV-5	1.36
		275ROV-7	1.42
Nibelungena	372	314ROV-2	1.42
		314ROV-3	2.39
		314ROV-4	2.24
		314ROV-5	2.2
		314ROV-6	2.16
Kaireic	360	ROV-Kaiko	1.90
Average			1.74
aSchmidt et al. (2011); bSchmidt et al. (2007); cGamo et al. (2001).

 | Show Table

DownLoad: CSV

Significant differences in carbonate precipitation temperatures are found in various ultramafic-hosted hydrothermal systems (Schwarzenbach et al., 2013). Calcite veins in serpentinite from the Mid-Atlantic Ridge Kane (MARK) Fracture Zone area have formation temperatures between 1 and 235 ℃ (Alt and Shanks, 2003), while calcite veins near the 15°20'N Fracture Zone show temperatures from near ambient water to 175 ℃ in detachment fault rocks (Schroeder et al., 2015; Bach et al., 2011). Both hydrothermal systems are located along the MAR, where the young mantle has been exposed by detachment fault (Schroeder et al., 2015, 2007; Bach et al., 2011; Alt and Shanks, 2003; Shanks, 2001; Karson and Lawrence, 1997).

In addition to calcite veins, minerals in harzburgite could also give insights into the possible formation conditions. Marques et al. (2007) suggested that spinels with Mg/Fe > 1 and Al/Cr > 1 indicated mineralized conditions through the studies on spinels from early and evolved stockwork, steatite, and semi-massive sulfide samples. The ratios of Mg/Fe and Al/Cr in spinels of the sample E22 are higher than 1, which indicates that the harzburgite locates in the mineralized zone. Hence, the calcite veins and spinel in the harzburgite samples may indicate the possible influence of hydrothermal fluids from the discharge zone below the Tianxiu hydrothermal field, although the samples are approximately 400 m away from the vent site.

4.2 Models of Water-Rock Interaction and Fluid Circulation

4.2.1   Geochemical reaction path modeling of calcite precipitation

Considering that the sample E06 was highly altered, the primary mineral components of the sample E22 were chosen as the reactant. According to the petrology and geochemical analyses, the reactant contains 80 vol.% olivine (Fo₉₀), 15 vol.% orthopyroxene (Mg^#≈90), and 5 vol.% clinopyroxene (Mg^#≈90). Hence, the geochemical models were conducted under 35 MPa and 135 ℃, which correspond to the sampling depth and the formation temperature of calcite in E22, respectively. The predicted mineral assemblages and fluid compositions are shown in Fig. 5. The mineral assemblage of reaction path models is stable and almost consistent with the petrological observation. Serpentine, magnetite, and olivine appear under various w/r ratios, while calcite only occurs at w/r ratios ≤3 (Fig. 5a). It is suggested that the instability of calcite under higher w/r ratios is caused by the low Ca²⁺ and HCO₃^- concentrations in the fluid (Fig. 5b). Actually, the concentrations of Ca²⁺, Fe²⁺, SiO₂(aq) all decrease toward higher w/r ratios. This is consistent with the decreasing amount of rocks in the reaction. The increase of Mg²⁺ toward higher w/r ratios is attributed to the instability of olivine at higher w/r ratios. Alteration of harzburgite drives down the fluids' oxygen fugacity and increases pH, thereby decreasing the carbonate solubility toward lower w/r ratios. The mineral assemblage of magnetite, galena, bornite, pyrrhotite, and pentlandite in both samples indicates a low oxygen fugacity and reduced condition (Fouquet et al., 2010). This result suggests that the variations in mineral assemblage may be due to local variations in the degree of fluid fluxing and rock interaction (Schroeder et al., 2015). Our model supports the idea that lower w/r ratios, alkalinity and reduced conditions promote the precipitation of calcite. This idea also supplements the conclusion given by Schroeder et al. (2015), who adopted troctolite and CO₂-enriched fluid as the reactant and noted that the compositional effects may influence the mineral assemblage of the reaction. Additionally, although calcite precipitation temperatures of the MARK area and the 15°20'N Fracture Zone reach up to 235 and 175 ℃ (Schroeder et al., 2015; Bach et al., 2011; Alt and Shanks, 2003) respectively, their median value is only 59 and 3 ℃, with most of the temperatures below 100 ℃. Studies conducted by Allen and Seyfried (2004), Proskurowski et al. (2006) and Klein et al. (2009) have shown that pH values for fluid from serpentinization reactions range from alkaline under moderate to low temperatures to acid under high temperatures. This provides a possible explanation for why high- temperature carbonate minerals in serpentinized ultramafic rocks are seldom discovered.

Figure  5.  Reaction path model simulating the serpentinization of harzburgite at 135 ℃ and 35 MPa with w/r ratios ranging from 0.2 to 10 in a titration system. (a) predicted equilibrium mineral assemblages and pH during serpentinization under different w/r ratios; (b) predicted fluid composition and logfO₂ during serpentinization under different w/r ratios (see the abbreviations above).

DownLoad: Full-Size Img PowerPoint

4.2.2   A schematic model of alteration processes

Along the mid-ocean ridge, the formation of secondary minerals in ultramafic rocks is partly explained by the creation of fluid pathways through tectonic processes. The fractures caused by changes in the stress regime, faults during extensional unroofing, and mechanical weathering all provide pathways for fluid (Liu et al., 2018; Boschi et al., 2009; Andreani et al., 2007; Kelley et al., 2001). In the absence of external forces, two mechanisms have been proposed to explain the microfractures: anisotropic thermal contraction and reaction-driven cracking (Rouméjon and Cannat, 2014). The former is capable of producing microcracks in peridotite, particularly in its primary mineral constituent olivine at the slow-spreading ridge (Demartin et al., 2004). The latter produces intense self- propagating fractures (Kelemen and Hirth, 2012; Plümper et al., 2012; Jamtveit et al., 2009). During serpentinization, decomposition of olivine and pyroxene to serpentine is accompanied by a volume increase of up to 40% (Coleman, 1997), which creates microfractures and propagates cracks (Andreani et al., 2007; Macdonald and Fyfe, 1985; Martin and Fyfe, 1970). Thus, the ultramafic rocks are characterized by complex permeability structures and fluid pathways.

Based on the petrological and geochemical considerations above, we propose a schematic model to explain how the alteration processes occurred, and particularly addresses how low- and high-temperature calcite might form during the alteration (Fig. 6). It is assumed that harzburgite consists of olivine, orthopyroxene, and clinopyroxene (Fig. 6a). When the hydrothermal fluid is discharged toward the seafloor surface along the fractured-porous ultramafic rocks, it may mix with the infiltrated seawater, resulting in the alteration of orthopyroxene and olivine. Allen and Seyfried (2004) reported that the rate of serpentinization of olivine is lower than that of orthopyroxene when the temperature is greater than 300 ℃. Hence, it is inferred that the serpentinization of samples with relic olivine occurs at temperatures > 300 ℃. When hydrothermal fluid mixes with infiltrated cold seawater, the mixing fluid drives the alteration of olivine and promotes the precipitation of calcite (Kelley et al., 2001) at 134–140 ℃ along microfractures developed in olivine (Figs. 6b, 6c). This is also consistent with the studies that showed that the majority of carbonates in oceanic peridotite formed during the mixing of seawater and hydrothermal fluid (Klein et al., 2015; Bach et al., 2011; Eickmann et al., 2009). Moreover, the altered sample with high- temperature calcite veins might also indicate the role of tectonic denudation. The low-temperature calcite veins (≈2 ℃) that filled the fractures in the almost completely altered sample (Figs. 6d, 6e) indicate conducive cooling processes that hydrothermal fluid undergoes during the slow ascent from deeper sources along fractures or faults in the mid-ocean ridge. This model provides an insight into how the alteration processes occur and how the alteration minerals form under the fluid interaction with the samples from the Tianxiu hydrothermal field.

Figure  6.  Schematic representations of the alteration process of serpentinized harzburgite samples. (a) unaltered harzburgite with olivine, orthopyroxene, and clinopyroxene as the major constituents of two harzburgite types; (b) serpentinized harzburgite with magnetite in the middle of serpentine veins; (c) calcite veins appearing among mesh texture and along the margin of olivine, exhibiting the alteration of the sample E22; (d) totally serpentinized harzburgite with magnetite in the middle of serpentine veins of the sample E06; (e) serpentine and calcite veins filling the fractures in the sample (see the abbreviations above).

DownLoad: Full-Size Img PowerPoint

4.3 Calcite Veins as a Sink of CO₂

Recently, there has been increasing interest in serpentinization due to its potential for sequestering CO₂ through the formation of carbonate veins and replacive carbonate (Grozeva et al., 2017; Schroeder et al., 2015; Klein and McCollom, 2013; Kelemen and Matter, 2008). The developed microfractures could act as fluid pathways for the precipitation of secondary minerals or veins. However, the formation of serpentine and/or carbonate can be self-limiting. They potentially result in the closure of fluid pathways and reduce fluid migration, preventing further precipitation of secondary minerals (Bankole et al., 2019; Ma et al., 2018; Schwarzenbach, 2016; Plümper et al., 2012; Macdonald and Fyfe, 1985). This might partly explain the scarcity of calcite veins in our sample. The limited contribution of carbonate veins for carbon sequestration was reported by Bach et al. (2011), who suggested that the general scarcity of low-temperature aragonite veins in the serpentinized mantle indicates their minor role as a sink for CO₂. Similarly, the calcite veins in our samples probably do not indicate a globally important sink for CO₂ in the Tianxiu hydrothermal field, due to their scarcity in the samples. However, the estimates of CO₂ uptake may increase if fracture and carbonate veins are more developed in rocks beneath the area. This possibility can be verified by drilling holes.

5. CONCLUSION

The harzburgite samples collected from 400 m north to the Tianxiu hydrothermal field, Carlsberg Ridge were characterized by intensive serpentinization with the presence of calcite veins. Two types of calcite veins developed in serpentinized harzburgite samples. Low-temperature calcite veins Ⅰ cutting through the mesh text were present in highly serpentinized harzburgite samples. By contrast, high-temperature calcite veins Ⅱ precipitating within the mesh texture were present in relatively weaker serpentinized harzburgite samples. Both types exhibited similar δ¹³C_PDB (+0.54‰ and +0.58‰) but different δ¹⁸O_PDB (-16.67‰ and +4.46‰) signals. The similar δ¹³C_PDB values suggest that the calcite was derived from the same carbon source. A possible explanation for the δ¹⁸O_PDB discrepancy is due to the different precipitation temperatures of calcite veins during the fluids' slow upflow along fractures. Assuming calcite veins Ⅰ formed at the temperature of deep seawater (2 ℃), the equilibrium δ¹⁸O_V-SMOW of calcite-precipitating fluid was calculated to be 1.78‰, which is close to the average δ¹⁸O_V-SMOW of the hydrothermal fluids in the ultramafic-hosted systems. The formation temperatures of calcite veins Ⅱ were calculated to be 134–140 ℃, based on the adopted δ¹⁸O_V-SMOW signals. The geochemical modeling provides us with the possible conditions of calcite precipitation, which is favored by relatively low w/r ratios (≤3), as well as alkalinity and reduced conditions. Hence, the formation of calcite as described above may indicate the possible influence of hydrothermal fluids from the discharge zone at Tianxiu hydrothermal field. The spinels with Mg/Fe > 1 and Al/Cr > 1 in harzburgite support the influence of hydrothermal fluids as well.

The microfractures could act as fluid pathways for the precipitation of secondary minerals or veins, but this process can be self-limiting, and prevent further precipitation of secondary minerals. Hence, the scarcity of calcite veins due to the self-limiting process in the samples might assign a minor role to abyssal harzburgites as a sink for CO₂ in the oceanic ridge environment.

ACKNOWLEDGMENTS

This study was funded by the National Key Research and Development Program of China (No. 2018YFC0309903), the Scientific Research Fund of the Second Institute of Oceanography, MNR (No. QNYC 1701), the China Ocean Mineral Resources R & D Association Project (No. DY135-S2-1-02 & 05), and the National Science Foundation of China (No. 41976076). We would like to thank the captains, the crew, and the scientific parties onboard during the DY33rd cruise in 2015. We appreciate the valuable suggestions given by Thomas M. McCollom and Klischies Meike, which substantially improved the quality of the manuscript. We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-0876-y.

Electronic Supplementary Materials: Supplementary materials (Tables B1–B2) are available in the online version of this article at https://doi.org/10.1007/s12583-020-0876-y.