Fe-Si-Mn-Oxyhydroxide Encrustations on Basalts at East Pacific Rise near 13?N: An SEM – EDS Study

WANG Xiaoyuan, ZENG Zhigang,, QI Haiyan, CHEN Shuai, YIN Xuebo, and YANG Baoju,

1) Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, P. R. China

2) University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Fe-Si-Mn-Oxyhydroxide Encrustations on Basalts at East Pacific Rise near 13?N: An SEM – EDS Study

WANG Xiaoyuan1), ZENG Zhigang1),*, QI Haiyan1), CHEN Shuai1), YIN Xuebo1), and YANG Baoju1),2)

1) Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, P. R. China

2) University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Fe-Si-Mn-oxyhydroxide encrustations at the East Pacific Rise (EPR) near 13?N were analyzed using the scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS). These encrustations are mainly composed of amorphous Fe-Si-Mn-oxyhydroxides forming laminated, spherical, porous aggregates with some biodetritus, anhydrite, nontronite, and feldspar particles. Anhydrite particles and nontronite crystals in the Fe-Si-Mn-oxyhydroxide encrustations imply that the Fe-Si-Mn- oxyhydroxide may have formed under relatively low- to high-temperature hydrothermal conditions. The Fe-Si-Mn-oxyhydroxide encrustations on pillow basalts are 1–2 mm thick. The growth rate of ferromanganese crusts in the survey area suggests that these encrustations are an unlikely result of hydrogenic deposition alone having a hydrothermal and (Fe/Mn ratio up to 7.7 and Fe/(Fe+Mn+Al) ratio exceeding 0.78) hydrogenic origin (0.22 Fe/Mn ratio close to the mean value of 0.7 for open-ocean seamount crusts). The varying Fe/Mn ratios indicate that the Fe-Si-Mn-oxyhydroxide encrustations have formed through several stages of seafloor hydrothermalism. It is suggested that, at the initial formation stage, dense Fe-Si-oxyhydroxides with low Mn content deposit from a relatively reducing hydrothermal fluid, and then the loose Fe-Si-Mn-oxyhydroxides deposit on the Fe-Si-oxyhydroxides. As the oxidation degree of hydrothermal fluid increases and Si-oxide is inhibited, Mn-oxide will precipitate with Fe-oxyhydroxides.

East Pacific Rise; Fe-Si-Mn encrustations; origin; pillow basalt

1 Introduction

Hydrothermal Fe-Si-Mn-oxyhydroxide deposits on the seafloor form in a range of geotectonic settings such as mid-ocean ridges (MOR) (e.g., Scott et al., 1974; Moore and Vogt, 1976; Corliss et al., 1979; Lonsdale et al., 1980; Grill et al., 1981; Hékinian et al., 1993; Nath et al., 1997; Dekov et al., 2010), volcanic arc-back-arc basins (e.g., Cronan et al., 1982; Moorby et al., 1984; Usui et al., 1986; Bolton et al., 1988; Hein et al., 1990; Herzig et al., 1990; Murphy et al., 1991; Binns et al., 1993; Sun et al., 2011, 2012; Zeng et al., 2012), intraplate submarine volcanoes and continental margins (e.g., Alt, 1988; Puteanus et al., 1991; Stoffers et al., 1993). The geotectonic settings occur as pavements coating volcanic and sedimentary substrates, as chimneys, as irregularly shaped edifices and mounds, as secondary alteration products of seafloor hydrothermal sulfide deposits, as primary precipitates from hydrothermal fluids, or as interstitial precipitates filling cracks between lava flows (Miriam and Rachel, 1986; Alt, 1988; Hannington and Scott, 1988; Herzig et al., 1991; Puteanus et al., 1991; Hékinian et al., 1993; Mills and Elderfield, 1995; Boyd and Scott, 1999; Bach et al., 2003; Benjamin and Haymon, 2006; Zeng et al., 2008). Major mineral phases in Fe-Si-Mn- oxyhydroxide crusts are todorokite, birnessite, vernadite, goethite, pyrolusite, and asbolane along with clay minerals such as nontronite and hisingerite (Varentsov et al., 1991; Mills et al., 2001; Glasby et al., 2006).

Some Fe-Si-Mn oxyhydroxide crusts are formed by a combination of hydrogenetic and hydrothermal processes (Glasby, 1988; Varentsov et al., 1991; Gibbs et al., 1993; Hein et al., 1997; Usui and Someya, 1997; Van de Flierdt et al., 2004). Growth rates of these deposits are large, up to 105 mm Myr-1(Hein et al., 1997). The Fe/Mn ratio and trace element content of hydrothermal Fe-Si-Mn deposits vary widely. The latter reflects local hydrothermal inputs and complex particle-scavenging reactions that occur near hydrothermal vents (Hein et al., 1997). In general, hydrothermal Fe-Mn deposits have lower trace metal contents and larger varying Fe/Mn ratios than their hydrogenous counterparts (Hein et al., 1997). It has been suggested that exhalative Fe-Si-Mn oxyhydroxide accumulations in seafloor hydrothermal fields are protoliths for Fe and/orMn-rich exhalites in ancient volcanic-hosted massive sulfide (VHMS) deposits (Heath et al., 2000). Understanding the mineralogy, geochemistry and formation of Fe-Si-Mn oxyhydroxides in seafloor geological environments is important for mineral exploration of ancient VHMS deposits.

In order to shed more light on the origin of Fe-Si-Mnoxyhydroxide encrustations on MOR basalts and evaluate the role that hydrothermal and hydrogenetic processes play in the formation of these encrustations, basalt samples from the seafloor at the EPR near 13?N by SEMEDS are studied in the present work.

2 Geological Setting

The study area lies on the fast-spreading EPR with a spreading rate of 10–12 cm yr-1(Hekinian et al., 1983) between 12?30?N and 13?N (Fig.1). The mid-ocean ridge here consists of an axial graben structure striking 345?±5?. The graben is 200–600 m wide and 20–50 m deep; the average water depth in the graben is 2630 m (Hékinian et al., 1983). The bottom of the graben is flat with many fissures at the center and the faults near the axis. These fissures are filled by fresh basaltic sheet lava flows (Gente et al., 1986).

About twenty active and more than sixty inactive hydrothermal vents had been found within a narrow graben averaging about 300 m in width along a 20 km long segment of the ridge crest (Hékinian et al., 1983). Most hydrothermal activities occur in three structural environments: 1) axial graben, 2) graben fault, and 3) off-axis segment (Fouquet et al., 1996). Active and inactive vents and extensive mounds of mature sulfides are distributed in the central part of the graben, the graben faults, the marginal high and the SE seamount (Fouquet et al., 1996). The active hydrothermal vents range from low-temperature vents to high-temperature (up to 380℃) ‘black smo kers’ with polymetallic sulfide deposits (Charlou et al., 1991; Zeng et al., 2010) and hydrothermal Fe and Si oxyhydroxides associated with or occurring close to sulfide formation on axial and off-axial structures (Hékinian et al., 1993; Zeng et al., 2008).

On the EPR near 13?N, the older ‘fissural domain’hosts inactive hydrothermal deposits rich in Fe-Mn- oxyhydroxides and is characterized by altered pillow and massive basaltic flows (Moss and Scott, 1996). Extinct hydrothermal chimneys and formless deposits made up essentially of Fe- and Si-rich hydrothermal products are frequently found (Hékinian and Fouquet, 1985). There are small (<30 cm in height), purple-red low-temperature Fe oxyhydroxide chimneys that are partly covered by located lava flows (Hékinian et al., 1993). Yellow coral-like structures are growing through sediment or on less than 1 mhigh Fe oxide mounds resulting from the oxidation of sulfide blocks. In the sulfide area, Mn-oxide is represented only by a thin layer (<1 mm thick) on the surface of sulfides and oxides, and at the top of the seamount on the EPR near 13?N，whereas deposits are represented by thick (up to 6 cm) Mn crusts growing directly on the basalt or around a nontronitic core (Hékinian et al., 1983; Fouquet et al., 1988).

Fig.1 a, Bathymetric map of the EPR segment near 13?N. Sampling site is indicated by a solid dot with dredge site number. Depth contours are in meters. b, The hydrothermal vents near E11 station (the solid dot) on EPR. The active hydrothermal vents are marked with open triangles, and the inactive hydrothermal vents are marked with open circles. The locations of hydrothermal vents are from Fouquet et al. (1988).

3 Samples and Methods

The samples studied were obtained from station E11 (103?57?W, 12?50?N, 2626 m) on the EPR near 13?N (less than 1 km away from the nearest vent) during the DY105-12 cruise (R/V DAYANG YIHAO, November 4, 2003) (Fig.1). They include fragments of pillow basalts (up to30 cm across) covered with thin (up to 1–2 mm) tan encrustations (Fig.2).

Fig.2 A dark brown Fe-Si-Mn-oxyhydroxide coating is on pillow basalt from station E11.

Petrographic analysis of the encrustations was performed on polished thin sections under an optical microscope (Nikon). Then the polished thin sections were investigated with a scanning electron microscope (SEM): TESCAN VEGA 3 LMH SEM with an Oxford INCA XMax energy dispersive spectrometer (EDS). Olivine, pyroxene, enstatite, diopside, anorthoclase, basaltic glass, calderite and anhydrite were used for standardization, and the results were corrected by the XPP method.

4 Results

The analysis of the thin sections by optical microscope shows that all samples contain phenocrysts of olivine, clinopyroxene and plagioclase, basaltic glass and Fe-Si-Mn-oxyhydroxides. The Fe-Si-Mn-oxyhydroxide encrustations occur as fine layers, micro-veins and coatings around minerals (Figs.3a, b and c) with globular particles scattered on the surface (Figs.3d, e and f). The encrustations are composed mainly of amorphous Fe-Si-Mn- oxyhydroxides (Fig.4), with scarce crystals of feldspar, quartz, anhydrite and nontronite, and biogenic debris (Fig.4, Figs. 5a–e). The Fe-Si-Mn-oxyhydroxides also have a framboidal structure (Fig.5f) with the major contents of Fe, Si, and Mn, and the trace contents of Na, K, Ca, Al, Mg, Ti, P, S, Cl, Cu, Zn, Co, Ni, and Cr (EDS studies).

Two types of oxyhydroxides have been distinguished according to their chemical composition: Fe-Si-oxyhydroxides with low Mn content (Table 1) and Fe-Si-Mnoxyhydroxides. Fe-Si-oxyhydroxides coat basaltic glass and plagioclase crystals (Fig.6b, Fig.7), and some fill microcracks in the basaltic glass (Fig.6a). Fe-Si-Mnoxyhydroxides occur near Fe-Si-oxyhydroxides and those with Cu and Ni occur between Fe-Si-Mn oxyhydroxides (Figs.6b, c). Fe-Si-Mn-oxyhydroxide encrustations have a laminated texture (Figs.8a, c) with some layers having the minor contents of Ni, Cu and Zn (Figs.8b, d). Concentric layers are composed of Fe-Si- and Fe-Si-Mn-oxyhydroxides that constitute the dense core, and the loose and porous margin of the layers, respectively (Figs.9a, b).

Fig.3 a, Fe-Si-Mn-oxyhydroxide (FSM) encrustations covering the surface of basaltic glass (Gl). b, Fe-Si-Mn-oxyhydroxide vein filling micro-crack of basaltic glass. c, Fe-Si-Mn-oxyhydroxide coatings on pyroxene (PX). d, Globular particles on the surface of Fe-Si-Mn-oxyhydroxide encrustations. e, Apophysis on the surface of Fe-Si-Mn-oxyhydroxide encrustations. f, EDS spectrum of globular particles.

Fig.4 XRD pattern of Fe-Si-Mn-oxyhydroxide encrustation on pillow basalt.

Fig.5 a, b, Anhydrite (An) in Fe-Si-Mn-oxyhydroxide encrustations. c, honeycomb texture formed by flakes of nontronite (No) crystals in Fe-Si-Mn-oxyhydroxide encrustation. d, e, Biogenic debris in Fe-Si-Mn-oxyhydroxide encrustations. f, Feldspar micro-particles and framboidal Fe-Si-Mn oxyhydroxides.

Table 1 Contents of Si, Mn and Fe in Fe-Si-Mn-oxhydroxide encrustations

Fig.6 a, Micro-vein of Fe-Si-oxyhydroxides (FS). b, Fe-Si-oxyhydroxides and Fe-Si-Mn oxyhydroxides (FSM) close to the basaltic glass (Gl). Nos. 1, 2, 3, and 4 are the points where EDS analyses are conducted. c, EDS spectrum of Fe-Si-Mn-oxyhydroxides with the trace contents of Cu and Ni at point 4 (Fig.6b).

Fig.7 Fe-Si-oxyhydroxides (FS) and Fe-Si-Mn-oxyhydroxides (FSM) close to the plagioclase crystals (Pl). Nos. 1, 2, 3 and 4 are the points where EDS analyses are conducted.

5 Discussion

5.1 Thickness of Fe-Si-Mn-Oxyhydroxide Encrustations

The estimated age of the basalt samples is about 10 kyr, according to the distance from the axial central graben to the sampling site (nearly 1 km) and the spreading rate of about 10–12 cm yr-1(Hékinian et al., 1983). The growth rate of ferromanganese crusts (hydrogenetic) is about 15–27 mm Myr-1in the survey area on EPR 13?N (Manheim and Lane- Bostwick, 1988). Therefore the Fe-Si-Mn- oxyhydroxide encrustations (hydrogenetic) should be 0.15–0.27 mm thick. However, the thickness of the Fe-Si- Mnoxyhydroxide encrustations is 1–2 mm thick based on the samples. So it is unlikely that these encrustations have been a result of hydrogenetic deposition alone and there must be other factors dominating the formation of Fe-Si-Mn-oxyhydroxide encrustations.

5.2 Anhydrite and Nontronite in Fe-Si-Mn-Oxyhydroxide Encrustations

Generally, there are two mechanisms for anhydrite precipitation under seafloor conditions: 1) mixing of Caenriched hydrothermal fluid with sulfate-enriched seawater at temperatures higher than 150℃ (Teagle et al., 1998; Amini et al., 2008), and 2) increasing of seawater temperature to 150℃ (Blount and Dickson, 1969; Bischoff and Seyfried, 1978). Both scenarios suggest that anhydrite is formed at temperatures above 150℃. The anhydrite particles inter-grown with the studied Fe-Si-Mnoxyhydroxides (Figs.5a, b) indicate that the Fe-Si-Mnoxyhydroxide encrustations probably form under hydro-thermal conditions.

Nontronite is often formed by direct precipitation from hydrothermal fluids above the seafloor (Keeling et al., 2000) such as in the hydrothermal systems in the Red Sea, Galapagos Rift, Mariana Trough, Juan de Fuca Ridge, and Manus Basin (Cole and Shaw, 1983; Murnane and Clague, 1983; Singer and Stoffers, 1987; Kohler et al., 1994; Zeng et al., 2012). At an oxidation potential (Eh) range between -0.1 and -0.8 V, and a pH of 7–10, nontronite could be synthesized by co-precipitation of Feoxyhydroxide and silica from solutions that contain Fe2+and Si at a temperature lower than 96℃. A higher temperature will inhibit nontronite formation in favor of Fe-oxyhydroxide (Harder, 1978; de Carlo et al., 1983). In the studied Fe-Si-Mn-oxyhydroxide encrustations, a fine honeycomb texture formed by flakes of nontronite crystals (Fig.5c) implies that the encrustations may have formed under reduced hydrothermal conditions (T<96℃).

Fig.8 a, c, Laminated Fe-Si-Mn-oxyhydroxide encrustations. The points analyzed by EDS are marked by crosses. b, EDS spectrum of the point marked by the cross in (a). d, EDS spectrum of point 5 in (c).

Fig.9 a, Concentric texture of Fe-Si-oxyhydroxides. Nos. 1, 2, 3, 4, 5 and 6 are the points where EDS analyses are conducted. b, Fe-Si-Mn oxyhydroxides close to the concentric texture of Fe-Si-oxyhydroxides.

5.3 Laminated Fe-Si-Mn-Oxyhydroxide Encrustations

Ferromanganese oxyhydroxides can be divided into three types according to their origins: diagenetic, hydrogenetic and hydrothermal (Halbach, 1986). However, these processes seldom occur in isolation and each may play a key role in the precipitation of Fe-Mn oxyhydroxides at different stages (Varentsov et al., 1991). The Fe/ Mn and Fe/(Fe+Mn+Al) ratios of Fe-Mn- oxyhydroxides are good indicators of their origins. Hydrogenetic Fe-Mn crusts have a stable Fe/Mn ratio, and a mean value of 0.7 has been estimated for the open-ocean seamount crusts (Hein et al., 1997). Fe/(Fe+Mn+Al) ratio exceeding 0.78 indicates a hydrothermal input (Edmonds and German, 2004).

In this study the laminated Fe-Si-Mn-oxyhydroxide encrustations have varying Fe/Mn ratios (Fig.8c, Table 1). The maximum is up to 7.73 with a Fe/(Fe+Mn+Al) ratio of 0.87, indicating a hydrothermal input. The minimum Fe/Mn ratio is 0.22, which indicates a hydrogenic origin. The high Ni (about 1.8%), Cu (about 1.8%), and Zn (about 0.9%) contents in some layers (Figs.8b, d) also imply a hydrogenic influence, as Ni, Cu and Zn can be scavenged from seawater by adsorption during Fe-Si-Mn-oxyhydroxide formation (Krauskopf, 1956; Loganathan and Burau, 1973; Moore and Vogt, 1976; Varentsov et al., 1991; Koschinsky and Halbach, 1995; Hein et al., 1997; Koschinsky and Hein, 2003; Dekov et al., 2007). Thus, the laminated Fe-Si-Mn-oxyhydroxide encrustations have hydrothermal-hydrogenic origin, and the Fe-Si-Mn-oxyhydroxides can be contemporaneously deposited on the surface of the seafloor pillow basalts.

5.4 Initial Formation of Fe-Si-Mn-Oxyhydroxide Encrustations

During the formation process of Fe-Si-Mn- oxyhydroxide encrustations, it is obvious that Fe-Si-oxyhydroxides first deposited, and then relatively loose Fe-Si-Mn-oxyhydroxides settled on the top of Fe-Si-oxyhydroxides, either on the surface of basaltic glass (Fig.6b), or on the surface of feldspar particles (Fig.7), or forming a concentric ring (Fig.9a). Moreover, from Fe-Si-oxyhydroxides to Fe-Si-Mn-oxyhydroxides, the contents of Mn and Fe increase, whereas the Si contents decrease (Table 1). This suggests that the initial hydrothermal fluid is relatively reduced so that Fe-Si-oxyhydroxides are easy to precipitate (Krauskopf, 1957), and as the the degree of oxidation of hydrothermal fluid increases, Mn-oxides precipitate with Fe-oxyhydroxides while Si-oxides are inhibited.

6 Conclusions

The surfaces of pillow basalts from the EPR near 13?N are covered with Fe-Si-Mn encrustations that largely consist of amorphous Fe-Si-Mn-oxyhydroxides with anhydrite, nontronite, feldspar and biogenic debris. The laminated Fe-Si-Mn-oxyhydroxide encrustations have varying Fe/Mn ratios with relatively high Fe/(Fe+Mn+Al) ratios and high Ni, Cu, and Zn contents in some layers, which indicates a hydrothermal input with a hydrogenic origin. At the initial formation process, Fe-Si-oxyhydroxides first deposite from a relatively reduced hydrothermal fluid, and then Fe-Si-Mn-oxyhydroxides deposite on the Fe-Sioxyhydroxides. During the precipitation of Mn-oxides with Fe-oxyhydroxides, Si-oxides are inhibited.

Acknowledgements

We would like to thank the crew of the DY105-12 cruise for helping us collect samples. This work was supported by the National Key Basic Research Program of China (2013CB429700), the Shandong Province Natural Science Foundation for Distinguished Young Scholars (JQ200913), the National Natural Science Foundation of China (40830849), and the National Special Fund for the Eleventh Five-Year Plan of COMRA (DY125-12-R-02 and DY125-11-R-05).

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(Edited by Xie Jun)

(Received April 2, 2013; revised May 3, 2013; accepted February 13, 2014)

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2014

* Corresponding author. Tel: 0086-532-82898525

E-mail: zgzeng@qdio.ac.cn