Mineralogical and geochemical evidence for hydrothermal activity at the west wall of 12°50′N core complex (Mid-Atlantic ridge): A new ultramafic-hosted seafloor hydrothermal deposit?

Marine Geology 288 (2011) 90–102

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Mineralogical and geochemical evidence for hydrothermal activity at the west wall of 12°50′N core complex (Mid-Atlantic ridge): A new ultramafic-hosted seafloor hydrothermal deposit? Vesselin Dekov a,⁎, Tanya Boycheva b, Ulf Hålenius c, Kjell Billström d, George D. Kamenov e, Wayne C. Shanks f, Jens Stummeyer g a

Department of Geology and Paleontology, University of Sofia, 15 Tsar Osvoboditel Blvd., 1000 Sofia, Bulgaria Department of Mineralogy, Petrology and Economic Geology, University of Sofia, 15 Tsar Osvoboditel Blvd., 1000 Sofia, Bulgaria c Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden d Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden e Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611, USA f U.S. Geological Survey, 973 Denver Federal Center, Denver, CO 80225, USA g Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany b

a r t i c l e

i n f o

Article history: Received 21 October 2010 Received in revised form 7 September 2011 Accepted 9 September 2011 Available online 17 September 2011 Communicated by G.J. de Lange Keywords: Core complex Fe–Mn–oxyhydroxides Mid-Atlantic Ridge Seafloor hydrothermal deposit Sr–Nd–Pb isotopes Ultramafic-hosted

a b s t r a c t Dredging along the west wall of the core complex at 12°50′N Mid-Atlantic Ridge sampled a number of black oxyhydroxide crusts and breccias cemented by black and dark brown oxyhydroxide matrix. Black crusts found on top of basalt clasts (rubble) are mainly composed of Mn-oxides (birnessite, 10-Å manganates) with thin films of nontronite and X-ray amorphous FeOOH on their surfaces. Their chemical composition (low trace- and rare earth-element contents, high Li and Ag concentrations, rare earth element distribution patterns with negative both Ce and Eu anomalies), Sr–Nd–Pb-isotope systematic and O-isotope data suggest low-temperature (~ 20 °C) hydrothermal deposition from a diffuse vent area on the seafloor. Mineralogical, petrographic and geochemical investigations of the breccias showed the rock clasts were hydrothermally altered fragments of MORBs. Despite the substantial mineralogical changes caused by the alteration the Sr–Nd–Pb-isotope ratios have not been significantly affected by this process. The basalt clasts are cemented by dark brown and black matrix. Dark brown cement exhibits geochemical features (very low trace- and rare earth- element contents, high U concentration, rare earth element distribution pattern with high positive Eu anomaly) and Nd–Pb-isotope systematics (similar to that of MORB) suggesting that the precursor was a primary, high-temperature Fe-sulfide, which was eventually altered to goethite at ambient seawater conditions. The data presented in this work points towards the possible existence of high- and low-temperature hydrothermal activity at the west wall of the core complex at 12°50′N Mid-Atlantic Ridge. Tectonic setting at the site implies that the proposed hydrothermal field is possibly ultramafic-hosted. © 2011 Elsevier B.V. All rights reserved.

1. Introduction At many slow-spreading centers the tectonic processes dominate over magmatic spreading. Accretion is governed by two distinct modes of spreading (symmetrical and asymmetrical) reflected in the magmatic system, thermal structure and hydrothermal circulation at the spreading axis. Asymmetrical accretion is related to detachment faults along one of the ridge flanks accommodating about 50% of the plate separation (Escartín et al., 2008). Symmetrical accretion accommodates a lower proportion of the plate spreading by faulting (b20%) and is dominated by magmatic processes. The axial lithosphere is thicker and colder at asymmetrical sections of a given spreading center

⁎ Corresponding author. Fax: + 359 2 9446 487. E-mail address: [email protected]fia.bg (V. Dekov). 0025-3227/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2011.09.002

(Escartín et al., 2008). A central role in the accretion of slow-spreading oceanic lithosphere is played by detachment faulting (Escartín et al., 2008). Detachment faulting and core complex (CC) formation (Smith et al., 2006) expose lower crustal (gabbros) and upper mantle (peridotites) rocks at the seafloor. This setting hosts hydrothermal systems in which the upper-mantle ultramafics have substantial impact on the thermal regime, and on the hydrothermal fluid and deposit compositions (Rainbow, Douville et al., 2002; Lost City, Kelley et al., 2001; Logatchev, Douville et al., 2002). It has been suggested (Kelley et al., 2001) that the serpentinization (an exothermic process) of the uppermantle peridotites is capable of driving sub-seafloor hydrothermal cells. The observations at all types of spreading centers, however demonstrate (Baker, 2009) that nonmagmatic heat sources are insufficient to drive high-temperature hydrothermal activity. The high-temperature seafloor vent fields are almost universally associated with the presence or inference of magma near the surface. The high-temperature seafloor

V. Dekov et al. / Marine Geology 288 (2011) 90–102

hydrothermal activity at ridge segments with asymmetrical accretion is closely associated with detachment faults and the fluid circulation is focused along them (Boschi et al., 2006; McCaig et al., 2007). The number of known, well-characterized ultramafic-hosted hightemperature hydrothermal fields is still very limited. Three [Logatchev, Batuev et al., 1994; 13°30′N Mid-Atlantic Ridge (MAR), Beltenev et al., 2007; Ashadze, Beltenev et al., 2003] of all five (Logatchev; 13°30'N MAR; Ashadze; Rainbow, German et al., 1996; Nibelungen, Melchert et al., 2008) known fields occur along a segment of the MAR between the Fifteen-Twenty and Marathon fracture zones (FZ) (Fig. 1). The association of this type of hydrothermal activity with detachment faults and core complexes located at the ends of spreading segments (e.g., Rainbow and Logatchev fields) suggests a possible presence of hydrothermal activity at the core complex at 12°50′N MAR, located at the southern end of the spreading segment between the Fifteen-Twenty and Marathon FZ (Fig. 1). Here we report the results of an investigation on the deposits sampled at this promising site for core complex hydrothermal activity.

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2. Geological setting Detachment faults are dominant between the Fifteen-Twenty and Marathon FZs, with 70% of the ridge axis accreting asymmetrically (Escartín et al., 2008). The seismicity between these fracture zones is concentrated at segments shown to have active detachment faults: the Logatchev massif south of the Fifteen-Twenty FZ and the core complex at 12°50′N north of the Marathon FZ (Smith et al., 2006). High-temperature hydrothermal activity identified so far in the study region (Logatchev, 13°30′N MAR and Ashadze vent fields) is closely associated with asymmetrical accretion and detachment faulting. The dome-like corrugated massif at the inner corner of the intersection of the Marathon FZ with Mid-Atlantic Ridge was sampled by dredging during a cruise of the Russian R/V Yuzhmorgeologiya (St. Petersburg; October, 2000; Davydov et al., 2003). This core complex (Smith et al., 2006) named Fersman Seamount by Russian scientists (9th leg of R/V Akademik Nikolay Strahov; Raznicin et al., 1991) has a

Fig. 1. Bathymetry of Mid-Atlantic Ridge between Fifteen-Twenty FZ and Marathon FZ (by courtesy of J. Escartín, after Smith et al., 2006; Escartín et al., 2008; Smith et al., 2008) with locations of known hydrothermal fields (red stars) and dredge #28 site (yellow diamond; R/V Yuzhmorgeologiya cruise in 2000) at the west slope of 12°50′N CC (encircled). The inset shows the position (red solid circle) of the 12°50′N CC at the Mid-Atlantic Ridge. Color scale at the right-hand side indicates the depth to the seafloor.

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steep west wall with terrace-like structures and flat summit with 2 conical structures (Davydov et al., 2003). Dredging along the west wall of the core complex (Fig. 1) brought fragments of basalts, dolerites and breccias (Davydov et al., 2003). Basalts were MORB tholeiites of Nand P-type (porphyric and aphyric). A previous dredging of the lower part of west footwall of this core complex revealed gabbros and serpentinized ultramafics (Raznicin et al., 1991). A major hydrothermal field (Ashadze) consisting of a cluster of active hydrothermal sites and associated with ultramafic rocks (Cherkashov et al., 2008) is located at ~4200 m depth and about 40 km N-NW from the site of the current investigations (Fig. 1). The proximity of this hydrothermal field to the 12°50′N CC implies that the west wall of this core complex may be influenced by precipitation of Fe- and Mn-oxyhydroxides from the Ashadze hydrothermal plumes. 3. Materials and methods We studied 4 samples from a haul (#28; start: 12°49.96′N, 44°47.799′W, 3280 m depth; end: 12°49.931′N, 44°47.381′W, 2920 m depth) dredged across the west wall of the 12°50′N CC (Fig. 1). Samples from the same collection were previously studied and interpreted by Davydov et al. (2003). Here we have used a range of new approaches and novel analytical tools that give some new dimensions of the preliminary investigation by Davydov et al. (2003). After a preliminary macroscopic description of the 4 samples we divided them into 10 sub-samples (based on their color and texture), which were further analyzed. The X-ray diffraction data (powdered specimens) were collected with a Philips PW1710 diffractometer using graphite-monochromatised CuKα-radiation (PW1830 generator operated at 40 kV and 40 mA) in the range 2–65°2θ. Peak positions were determined with the X'Pert Graphics and Identify program. Samples in which clay minerals were identified were additionally X-rayed as oriented mounts to emphasize clay mineral peaks, as air-dry and glycolated (overnight in ethylene glycol atmosphere at 60 °C) specimens. They were analyzed in the same diffractometer under the same instrument conditions, in the range 2–40 °2θ, at 0.02 °2θ steps and 10 s/step. Randomly oriented specimens were prepared to identify di- and tri-octahedral clay minerals from the 060 diffraction peaks (57–64 °2θ scans, steps of 0.02 °2θ, at 120 s/step). Specimens (≤25.4 mm across) from all samples were prepared as polished sections and investigated with Olympus BX60 polarizing microscope. One specimen from a hydrothermal breccia sample was prepared as thin section and studied with Leica DM 2500 P polarizing microscope. Secondary electron images (SEI) and energy dispersive X-ray spectra (EDS) were obtained on small (~1 × 1 cm) pieces of natural samples dried at lab temperature (~20 °C), mounted on aluminum stubs using carbon tapes and coated with C using Hitachi S4300 scanning electron microscope (SEM) (V= 10–20 kV, I = 5–8 μA, electron beam diameter of 1 μm) with a solid state Si(Li) detector using the INCA software package (Oxford Instruments). Micro analyses of elements with Z ≥ 9 were acquired on C-coated polished sections using Cameca SX50 electron microprobe (EMP) (V= 20 kV, I = 15 nA, electron beam diameter of 2 μm). The ZAF-based PAP software package (Cameca) was applied for corrections of measured raw data. Standard samples used were: albite (Na), orthoclase (K), Al2O3 (Al), MgO (Mg), apatite (P), wollastonite (Ca, Si), vanadinite (Cl), hematite (Fe), MnTiO3 (Mn, Ti), native copper (Cu), native nickel (Ni), native cobalt (Co) and sphalerite (Zn). Mössbauer spectra were obtained at room temperature (ca. 295 K) using a constant acceleration system working in conjunction with a 1024 Multi Channel Analyzer. A nominal 50 mCi 57Co/Rh-source and a gas-filled proportional counter were used as source and detector in these experiments. The sample absorbers consisted of self-supporting pressed discs of sample powders mixed with polymeric transoptic powder. In order to minimize texture effects, all spectra were recorded with the absorber at an angle of 54.7° to the incident γ-rays (Ericsson and

Wäppling, 1976). The obtained raw data were folded and fitted using a computer program (Jernberg and Sundqvist, 1983) assuming resonance absorption lines of Lorentzian shape and equal intensity and line width of the components of each quadrupole doublet. The velocity range of the sample spectra was calibrated against metallic iron (α−Fe) at room temperature. The bulk chemical composition of the samples was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) (Ti, Al, Fe, Mn, Mg, Ca and Ag) and by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) [Ba, Bi, Cd, Co, Cr, Cs, Cu, Ge, Hf, Li, Mo, Nb, Ni, Pb, Rb, Sb, Sc, Se, Sn, Sr, Th, Tl, U, V, Y, Zn, Zr and rare earth elements (REE)]. All chemicals used in the ICP-OES analyses were of suprapure quality. 0.1 g sample was weighed in a PTFE microwave system vessel. 2 ml 48% HF and 5 ml 65% HNO3 were added and after a reaction time of 1 h the vessel was closed. Up to 8 vessels are heated in a microwave system to a temperature of 205 °C. Temperature left constant for 30 minutes at elevated pressure. After 30 minutes of heating the vessels were cooled down to room temperature before opening. The open vessels were heated again to remove excessive acid. When only 1–2 ml solution was left, 1 ml conc. HClO4 was added and the samples were heated nearly to dryness. The residue was dissolved in diluted HNO3 to an end-volume of 50 ml. The ICP-OES instrument [Horriba Jobin Yvon spectrometer (model JY 166 ULTRACE) combining both sequential and simultaneous operation mode with axial plasma view] was calibrated with commercial multi-element standard solutions. The instrument calibration was validated daily by measurement of laboratory reference materials (e.g., acid solutions of USGS reference standard materials). All reagents used for ICP-MS sample preparation were Optima-grade and sample preparation was performed in a class 500 clean lab, equipped with class 10 laminar flow hoods. Trace element analyses were performed on an Element2 HR-ICP-MS with Re and Rh used as internal standards. Quantification of results was done by external calibration using a combination of USGS rock standards following the procedure described in Kamenov et al. (2008). The instrument was tuned to minimum oxide levels and all measurements were performed in medium resolution mode. REE problematic interferences (e.g., BaO on Eu) are easily resolvable in medium resolution mode. External long-term reproducibility for all trace elements is b5%, based on multiple analyses of AGV-1 USGS standard. Oxygen isotopic analyses were performed using a Finnigan MAT 252 mass spectrometer. The oxygen isotopic composition of the sample was determined on CO2 prepared from manganese oxides using the BrF5 technique (Clayton and Mayeda, 1963). Data are reported on the per mil (‰) scale relative to the VSMOW standard with analytical precision of approximately ±0.2‰. Oxygen isotope equilibration temperature was calculated assuming isotopic equilibrium with seawater with δ 18O = 0‰ using the theoretical fractionation equation of Zheng (1991) for MnO2. The chemical preparation for the radiogenic isotope analyses (Sr, Nd, Pb) of samples followed standard routines described by De Ignacio et al. (2006), including sample dissolution and subsequent isolation of elements of interest by ion exchange techniques. A Thermo-Finnigan Triton (TIMS) instrument was used for the Sr and Nd isotope analyses and data were normalized to 88Sr/86Sr=0.1194 and 146Nd/144Nd =0.7219. The accuracy of the measurements was monitored by running a series of BCR-2, NBS 987 (Sr), and La Jolla (Nd) standards. The Pb isotope analyses were accomplished by a Micromass IsoProbe multi-collector (ICP-MS) instrument using an internal Tl tracer to correct for mass bias. 4. Results 4.1. Sample description The main part of the collected samples are fragments of hard, black crusts: up to 10 cm across and 4 cm thick (samples #12°50′N CC, St. 28 1, -2, -5; Table 1). The crust surfaces were coated with greenish-orange

V. Dekov et al. / Marine Geology 288 (2011) 90–102 Table 1 Mineralogical composition (XRD data) of studied samples. Sample # 12°50′N CC, St. 28 1, greenish-orange 1, black 2 4, dark brown 4, black 4, green clast 4, 5, 5, 5,

brownish-white black rusty-brown brownish-white

Minerals smectite, 10-Å manganate birnessite, smectite, 10-Å manganate? birnessite, 10-Å manganate, quartz goethite, 10-Å manganate, talc? birnessite, 10-Å manganate, smectite, quartz anorthite, smectite, chlorite, lizardite, calcite, chrysotile?, quartz calcite, 10-Å manganate, birnessite birnessite, 10-Å manganate X-ray amorphous FeOOH, smectite, quartz? calcite, smectite, kaolinite?, quartz

(lower surface; Fig. 2A, B) to brownish-white and rusty-brown (upper surface; Fig. 2D) thin (b4 mm) layers. In cross section the crusts exhibited fine (1–6 mm), parallel layers with earthy to shiny black to grayishblack color (Fig. 2B). These layers were often cross-cut by thin (~1 mm), brown veins. The other type of samples was breccias (sample #12°50′N CC, St. 28 4; Table 1) composed of grayish-green, angular rock clasts cemented with black and dark brown finely porous to cavernous (cavities up to 3 mm) matter (Fig. 2C). The black and dark brown cement formed distinct layers (zones). While the cement was hard, the green rock clasts were very fragile obviously due to the fine net of cracks passing through them (Fig. 2C).

4.2. Mineralogy The black layers of the crusts are composed mainly of Mn-oxides (birnessite and 10-Å manganates) and traces of smectite and quartz (Table 1; XRD patterns not shown). The thin films on the surfaces of the crusts are composed of smectite, X-ray amorphous FeOOH and biogenic calcite (foraminiferal tests; microscope observations), and traces of 10-Å manganates and quartz (Table 1).

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Mn-oxides that form the black crusts are globular aggregates (Fig. 3A) of botryoids (Fig. 3B) composed of lamellar Mn-(Si)-oxide crystals (Fig. 3C). The lamellar Mn-oxide crystals are sometimes curly (Fig. 3D) or form rosettes on top of the botryoids (Fig. 3E). Often, the Mn-oxides (+ SiO2am.) encrust dense net of bacterial-like filaments (Fig. 3F). At cross section (Fig. 4A, B) the colloform Mnoxide aggregates show thin, parallel layers with varying gray color. The XRD patterns of oriented mounts of the greenish-orange coating of the crusts' surfaces showed 00 l peaks characteristic for smectite: 14.8-Å 001 peak expands to 16.2 Å after ethylene glycolation. The 060 reflection of 1.518 Å indicates a dioctahedral smectite, namely nontronite. Nontronite forms aggregates (lumps) of fine lamellar crystals (Fig. 5A) or encrusts dense bacterial-like (filaments up to 20 μm long, 2 μm thick) mats (Fig. 5B). On some crusts it was observed as casts (after foraminiferal tests, Fig. 5C) composed of lamellar crystals (Fig. 5D). In addition to the nontronite and X-ray amorphous FeOOH (XRD data) the surface films of the black crusts contain akaganeite (β-FeOOH) (Mössbauer studies, Table 2; Fig. 6). Grayish-green clasts of the breccias are composed of plagioclase, smectite and chlorite with minor lizardite, chrysotile (?), calcite and quartz (Table 1). Petrographic studies (thin section microscopic observations in transmitted light) showed the clasts were fragments of highly altered basalt. Original volcanic glass is scarce and along with the primary mafic minerals it is almost completely altered to chloriteand smectite-group minerals. The only survivors of the alteration are prismatic to lath-shaped, small plagioclase crystals (andesine to labradorite) and small, skeletal crystals of magnetite. Some of the magnetite crystals are altered to Fe-oxyhydroxides. Rare, small quartz and calcite crystals occur in the veins. The black cement of the clasts is composed mainly of Mn-oxides (birnessite, 10-Å manganates), while the dark brown cement is composed of goethite (Table 1). The brownish-white coating on the surface of some breccia fragments contains calcite (biogenic), 10-Å manganates and birnessite (Table 1). Little quantity of light brown matter was selected from within the cement. The XRD data showed it was trioctahedral smectite (d060 = 1.521 Å). Structural formula of this smectite was estimated on the basis of 22 oxygen

Fig. 2. Macrophotographs of: (A) Mn-oxide crust (black; seen at the periphery) coated with smectite (greenish-orange) (sample 12°50′N CC, St. 28 1; lower surface); (B) Mn-oxide crust (side view) (sample 12°50′N CC, St. 28 1); (C) hydrothermal breccia: grayish-green rock clasts cemented with Mn-oxides (black) and Fe-oxyhydroxides (dark brown) (sample 12°50′N CC, St. 28 4; cross-cut); (D) Mn-oxide crust coated with smectite (brownish-white) and Fe-oxyhydroxides (rusty-brown) (sample 12°50′N CC, St. 28 5). Scale bars in cm.

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Fig. 3. SEM photomicrographs (SEI) of: (A) globular aggregates of Mn-oxides (sample 12°50′N CC, St 28 1, black); (B) Mn-oxide botryoid (sample 12°50′N CC, St. 28 1, black); (C) lamellar Mn-(Si)-oxide masses forming lamellar botryoides (sample 12°50′N CC, St. 28 2); (D) curly lamellar Mn-oxide blades (sample 12°50′N CC, St. 28 1, black); (E) rosette of fine lamellar Mn-oxide blades on top of Mn-oxide botryoid (sample 12°50′N CC, St. 28 1, black); (F) bacterial-like structures encrusted by SiO2am. + Mn-oxides (sample 12°50′N CC, St. 28 2).

Fig. 4. Photomicrographs of: (A), (B) colloform Mn-oxides: thin parallel bands with varying gray color (optical microscope, reflected light on polished section; sample 12°50′N CC, St. 28 1, black). Scale bars equal to 200 μm.

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Fig. 5. SEM photomicrographs (SEI) of: (A) lumps of fine lamellar crystals of nontronite (sample 12°50′N CC St. 28 5, black); (B) bacterial filaments encrusted by nontronite (sample 12°50′N CC, St. 28 5, black); (C) foraminiferal (Orbulina universa) cast of nontronite on top of a black crust (sample 12°50′N CC, St. 28 2); (D) close-up of lamellar nontronite from the foraminiferal cast shown at (C) (sample 12°50′N CC, St. 28 2).

atoms using the EMP data (Table 3): (Si3.16Al0.84)IV(Al0.10Mg2.06 Fe III0.61Mn0.01Ti0.02)VI(Ca0.08Na0.30K0.02)int.O10(OH)2. Chemical compositions of some of the minerals/mineral associations that constitute the studied samples are summarized in Table 3. Due to the very fine-grained nature of the samples, single phase EMP-analyses were rarely possible to obtain. However, the analyses confirm the presence of hydrated Fe- and Mn-oxides, smectite-group clays, SiO2am. as well as labradorite plagioclase. In addition, microprobe analyses indicate remnants of calcic clinopyroxene and amphibole.

4.3. Geochemistry The black (Mn-oxide) layers of the crusts have high contents of Mn and Fe (in varying proportions) and Fe/Mn ratios less than or about 1 (Table 4). They have low concentrations of Al, Mg and Ca, high-field-strength elements (HFSE) (Ti, Zr, Nb, Hf, Th), rare earth elements (REE) and of a number of micro elements considered to be scavenged either from seawater or from hydrothermal fluid (Cd, Co, Cr, Cu, Ni, Pb, Zn, V, Mo, Se, etc.) (Table 4). They are, however, enriched in Ag and Li. Their REE distribution patterns show negative to slightly positive Ce anomalies, weak negative Eu anomaly, and little fractionation between light and heavy REE (Fig. 7; Table 4).

The green clasts of the breccias contain high concentrations of HFSE (Zr, Nb, Hf), Cr, V, Sc and Ni, and low concentrations of largeion lithophile elements (LILE) (Ba, Cs, Sr, Rb), Sb, Cd, Tl and U relative to the other studied samples (Table 4). Relative to N-MORB, the green clasts have higher abundances of some LILE (Pb, Rb, Ba) and HFSE (Th, U, Nb, La) (Fig. 8). They are slightly depleted in the rest of REE, Co, Ni, Cu, Sn, some HFSE (Zr, Nb, Hf) and Sr (LILE). The green clasts show almost flat REE patterns with no Ce, weak negative Eu anomalies and very little fractionation among light and heavy REE (Fig. 7; Table 4). The chemical composition of the dark brown cement of the breccias is very different from that of the black cement (Table 4). The dark brown cement has high concentrations of Ba and U, and low concentrations of a number of elements: Cr, Cs, Hf, Nb, Sc, REE, Th, Zr, Li, Cu, Ni, Pb, Tl, V and Zn (Table 4). The black cement has Ba and U content less than that of the dark brown cement and Cd, Cu, Li, Ni, Pb, Sb, Tl, V, Zn, Cr, Hf, Nb, Sc, REE, Th and Zr content higher that that of the dark brown cement (Table 4). The REE distribution pattern of the dark brown cement shows a very weak negative Ce anomaly and a well-pronounced positive Eu anomaly, whereas the REE distribution pattern of the black cement has a negative Ce anomaly and a weak negative Eu anomaly (Fig. 7; Table 4). The dark brown cement is slightly enriched in heavy REE (LaN/LuN b 1), whereas the black cement is slightly enriched in light REE (LaN/LuN N 1) (Fig. 7; Table 4).

Table 2 Room temperature Mössbauer parameters of studied hydrothermal deposits. Sample #

FeI3+ I (%)

W (mm/s)

CS (mm/s)

dQ (mm/s)

FeII3+ I (%)

W (mm/s)

CS (mm/s)

dQ (mm/s)

Possible minerals

12°50′N CC, St. 28 1, greenish-orange 1, black

60.0 60.9

0.43 0.39

0.35 0.36

0.50 0.58

40.0 39.1

0.43 0.38

0.36 0.35

0.92 0.98

β-FeOOH β-FeOOH

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5. Discussion

Fig. 6. Representative 57Fe Mössbauer spectrum (at 293 K) of studied hydrothermal deposits: sample 12°50′N CC, St. 28 1, greenish-orange. Squares = experimental data; red line = sum of fitted components; green line = doublet 1; blue line = doublet 2.

4.4. O–Sr–Nd–Pb isotopes O-isotope analyses of samples #12°50′N CC, St. 28 1, black and #12°50′N CC, St. 28 2 (both with main mineral birnessite) gave δ 18O values of 16.1 and 10.2‰, respectively (Table 5). Oxygen isotope fractionation data for this mineral have not been measured experimentally or calculated theoretically, but Zheng (1991) calculated mineral–water isotope fractionation for MnO2. Using the Zheng (1991) theoretical fractionation curve and assuming the studied Mn-oxide minerals precipitated in seawater with δ 18O = 0‰, we calculate a temperature of formation of 20 °C for sample #12°50′N CC, St. 28 2 and of −8 °C for sample #12°50′N CC, St. 28 1, black. Although the breccia clasts have gained some Pb and lost Sr and Nd (Fig. 8) during the alteration, this process has obviously not affected the radiogenic isotope ratios: the Sr–Nd–Pb-isotope data are consistent with a MORB-type source (Table 5; Figs. 9, 10). The black crust layers, thin surface crust films, and breccia cement (dark brown and black), show more radiogenic Sr and Pb, and less radiogenic Nd isotopic compositions relative to the breccia clasts (Table 5). In detail, Nd isotope data of hydrothermal precipitates vary quite significantly, whereas the Sr isotope signature is more constant and close to modern seawater values (Fig. 9). In addition, a weak linear trend is distinguished in Pb–Pb space (Fig. 10).

Preliminary study (Davydov et al., 2003) of samples from the same collection presents general mineralogical and geochemical data and suggests low-temperature hydrothermal activity at this site. Since large part of the reported results is average values we find it hard to compare our results to those obtained in this earlier investigation. The well-studied ultramafic-hosted Logatchev hydrothermal field is located in a setting very similar to that at 12°50′N CC: along a detachment fault at the west wall of a core complex (Petersen et al., 2009). Hydrothermal activity at the Logatchev field is hosted in debris flows consisting of heterogeneous ultramafic and mafic intrusive rocks (Petersen et al., 2009). The two types of deposits we studied at the 12°50′N CC were precipitated either among or on top of rock clasts (breccias and crusts, respectively). This, along with the assumption of the preliminary study (low-temperature hydrothermal activity may have existed at this site; Davydov et al., 2003) implies that the deposits at the 12°50′N CC may have hydrothermal origin. However, their mineralogical composition is dominated by Mn- and Fe-oxyhydroxides, which can be either hydrogenous (hydrogenetic or diagenetic) or hydrothermal (Bonatti et al., 1972; Hein et al., 2008). Precipitation from sediment pore fluids (diagenetic origin) can be ruled out since the Mn-oxide crusts and breccias have not been found in any contact with sediment. Hydrothermal Mn-oxide deposits have been found at all tectonic settings with hydrothermal systems such as spreading ridges, volcanic arcs, back-arc basins and hot spot volcanoes (Rona, 2008). Hydrogenetic and hydrogenetic–hydrothermal Mn-oxide deposits are also ubiquitous at the seafloor (Hein et al., 2008). Therefore we need to find unambiguous proofs for the origin of the studied Mn-oxide deposits. Birnessite (one of the Mn-oxides found in our samples) is a 7-Å phyllomanganate (consists of sheets of [MnO6] octahedra) having a single layer of H2O. The 10-Å manganates are subdivided into phyllomanganates (buserite, asbolane, asbolane-buserite mixed-layer) and tektomanganates with tunnel-structure (todorokite) (Turner and Buseck, 1981; Burns et al., 1983; Chukhrov et al., 1985). The 10-Å phyllomanganates have an expandable and contractible sheet [double layer of H2O (Potter and Rossman, 1979)], which causes contraction of the characteristic 10 Å spacing to 7 Å on dehydration (at ~100 °C in air) and its expansion to 25 Å on saturation (e.g., with dodecylammonium; Arrhenius and Tsai, 1981). Although this behavior allows 10-Å phyllomanganates (asbolane, buserite and asbolane-buserite) to be distinguished from todorokite (Usui and Mita, 1995) the identity of 10-Å manganates continues to be a subject of controversy (Burns et al., 1985; Giovanoli, 1985). We have not performed any investigations additional to the XRD in order to unambiguously distinguish the type(s) of 10-Å manganates in our samples. However, since all the 10-Å manganates have

Table 3 Electron microprobe analyses (mean values) of selected minerals/associations. Sample #

Number of analyses

SiO2 (wt.%)

TiO2

Al2O3

Fe2O3a

Mn2O3a

MgO

CaO

Na2O

K2O

Cl

P2O5

SUM

Mineral/Association

12°50′N CC, St. 28 1

7 7 1 1 2

0.43 28.88 52.20 46.21 8.09

n.d.b n.d. 0.07 2.05 n.d.

n.d. 0.02 27.77 5.57 0.15

2.46 53.44 1.09 12.72 9.99

72.56 0.19 0.24 0.59 61.35

2.17 0.78 0.36 11.28 2.09

0.59 0.99 12.86 20.43 0.91

0.74 0.58 4.04 0.18 1.35

0.25 0.30 0.06 n.d. 0.54

0.15 0.44 n.d. n.d. 0.08

0.02 0.34 n.d. n.d. 0.02

79.37 85.96 98.70 99.03 84.57

12°50′N CC, St. 28 4

2 1 3 1 11 1

52.42 100.34 45.57 50.70 0.88 35.09

0.08 n.d. 0.40 1.43 n.d. n.d.

28.72 0.21 11.47 14.93 0.40 15.51

1.02 0.39 11.65 10.79 0.36 17.01

0.04 0.00 0.22 0.13 73.85 0.46

0.30 0.27 19.90 7.30 1.34 22.22

13.05 0.06 1.11 11.46 0.43 0.32

3.96 0.00 2.26 2.52 0.75 0.00

0.05 n.d. 0.19 0.13 0.12 n.d.

n.d. n.d. 0.04 n.d. 0.18 0.09

n.d. n.d. n.d. n.d. n.d. n.d.

99.64 101.25 92.81 99.40 78.32 90.70

hydrous Mn-oxides Fe-oxyhydroxides + SiO2am. plagioclase pyroxene hydrous Mn-oxides + Fe-oxyhydroxide + SiO2am. plagioclase SiO2am. trioctahedral smectite amphibole hydrous Mn-oxides trioctahedral smectite + chlorite (?)

a b

Fe- and Mn-oxide concentrations based on trivalent cations. n.d. = not detected.

Table 4 Bulk chemical composition (ICP-OES and ICP-MS) of 12°50′N CC hydrothermal deposits. Sample # 12°50′N CC, St. 28

1, 1, 2 4, 4, 4, 4, 5,

greenish-orange black dark brown black green clast brownish-white black

AGV1, measured AGV1, reference datad a b c

Al

Fe

Mn

Mg

Ca

Fe/Mn

Ag (ppm)

Ba

Bi

Cd

Co

Cr

Cs

Cu

Ge

Hf

Li

Mo

Nb

Ni

Pb

Rb

Sb

Sc

Se

b.d.l.c 0.02 0.09 b.d.l. -

1.2 0.6 1.5 0.5 -

42.0 17.8 2.74 18.8 -

0.09 21.4 19.7 18.2 -

0.88 1.1 1.2 0.7 -

0.55 1.35 2.50 1.20 -

467 0.83 0.14 1.03 -

b.d.l. 40 43 40 -

20 608 127 6298 590 77 184 538 1221 1200

b.d.l. 0.01 0.02 0.01 0.01 0.01 0.13 0.01 -

0.01 0.94 0.95 0.56 1.11 0.04 0.15 0.49 -

2.1 16 22 21 17 35 12 12 15 15.2

4.2 12 51 7.1 46 199 40 10 10 9.4

0.60 0.25 0.25 0.12 0.18 0.05 1.47 0.35 1.2 1.3

50 81 32 26 75 52 101 25 59 58

8.0 2.8 0.5 2.1 0.9 1.1 0.6 2.1 -

0.01 0.14 0.61 0.05 0.56 2.47 1.06 0.08 5.1 5.1

6.3 314 373 2.9 527 35.3 30.5 95.7 10.6 10.7

b.d.l. b.d.l. b.d.l. b.d.l. -

b.d.l. 0.31 1.72 0.05 1.45 8.78 4.93 0.05 14.7 14.6

6.6 62 38 15 63 82 42 9.1 16 15.5

0.7 1.0 1.6 0.5 1.6 0.9 10 0.4 36.5 37.4

38.1 8.3 6.4 6.4 5.2 2.6 24.4 11.7 67.2 66.6

0.05 2.86 0.41 0.26 2.40 0.07 0.58 0.18 -

2.2 3.3 9.6 2.0 8.8 32.8 7.7 2.9 11.9 12.3

b.d.l. b.d.l. b.d.l. b.d.l. -

Ce/Ce* = 2CeN/(LaN + PrN). Eu/Eu* = 2EuN/(SmN + GdN). b.d.l. = below detection limit. GeoReM (http://georem.mpch-mainz.gwdg.de/).

V. Dekov et al. / Marine Geology 288 (2011) 90–102

d

Ti (wt.%)

Table 4 (continued) Sn (ppm)

Sr

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

ΣREE

(Ce/Ce*)a

(Eu/Eu*)b

LaN/LuN

Th

Tl

U

V

Y

Zn

Zr

0.00 b.d.l. 0.19 b.d.l. 0.21 0.59 0.88 b.d.l. -

167 379 220 813 283 99 1129 297 659 660

3.54 4.98 3.37 1.45 5.83 4.67 16.6 1.90 39 38.2

6.68 5.92 8.77 2.78 7.03 11.8 28.4 4.93 69 67.6

1.02 1.26 1.05 0.35 1.60 1.78 3.87 0.65 8.1 8.3

5.68 5.87 4.53 1.66 6.96 8.81 14.9 3.35 32.3 31.7

1.83 1.59 1.33 0.46 1.82 2.84 3.21 1.02 6.0 5.72

0.63 0.54 0.40 0.55 0.54 0.95 0.76 0.41 1.67 1.58

3.58 2.47 1.48 0.81 2.04 3.48 3.00 1.73 5.1 4.7

0.78 0.47 0.28 0.15 0.38 0.65 0.51 0.35 0.71 0.69

5.94 3.62 1.79 1.21 2.47 4.32 2.92 2.77 3.6 3.55

1.70 0.91 0.40 0.35 0.54 0.94 0.59 0.71 0.68 0.68

5.57 2.96 1.19 1.23 1.59 2.71 1.65 2.28 1.79 1.82

0.95 0.47 0.18 0.21 0.25 0.43 0.25 0.38 0.30 0.28

6.47 3.13 1.19 1.47 1.58 2.70 1.50 2.57 1.69 1.63

1.12 0.56 0.21 0.32 0.27 0.43 0.22 0.46 0.26 0.24

45.5 34.8 26.2 13 32.9 46.5 78.4 23.5 -

0.85 0.56 1.13 0.93 0.55 1.00 0.84 1.08 -

0.74 0.83 0.87 2.73 0.85 0.92 0.74 0.94 -

0.34 0.95 1.72 0.49 2.31 1.16 8.09 0.44 -

0.01 0.20 0.51 0.13 0.41 0.57 3.36 0.09 6.44 6.40

0.05 1.82 0.37 0.08 1.78 0.08 0.78 0.39 -

0.90 1.58 1.57 7.70 2.03 0.52 0.76 2.05 1.94 1.93

5.8 58 67 25 87 214 76 22 119 119

44.7 31.5 11.9 19.7 17.1 26.5 17.7 23.2 20.2 19.0

24 62 43 37 76 74 54 17 88 87

b.d.l. 4.0 21.3 0.4 19.3 88.0 37.3 2.0 226 231

97

98

V. Dekov et al. / Marine Geology 288 (2011) 90–102

been found to occur in both hydrogenous (hydrogenetic and diagenetic) and hydrothermal Fe-Mn-oxyhydroxide deposits (Chukhrov et al., 1981; 1983; Siegel and Turner, 1983; Glasby et al., 2000; Hein et al., 2008) their precise identification will not add genetic information to the study. The chemical composition of the black crusts and black cement of breccias (low trace element contents) is characteristic of a hydrothermal origin. In this respect, the ternary diagram by Bonatti et al. (1972) is very effective in differentiating Fe–Mn deposits. The data for black Mn-oxide crusts and their Fe-oxyhydroxide-dominated surface films plot entirely in the hydrothermal field of this diagram (Fig. 11). The unusually high Li and Ag concentrations of the studied Mn-oxide deposits also support a hydrothermal origin (e.g., Von Damm, 1990). Total REE contents are very low, also consistent with a hydrothermal origin (Hekinian et al., 1993). Their REE distribution patterns in general resemble that of the deep seawater (Fig. 7) and suggest low-temperature hydrothermal deposition (Hekinian et al., 1993; Dubinin, 2004). The weak positive Ce anomaly of some of the Mn-oxide crusts points to a hydrogenetic imprint on the primary low-temperature hydrothermal signature. The O isotope studies revealed a temperature of formation of 20 °C for one of the black crusts that is reasonable for deposition from a low temperature diffuse vent area on the seafloor. The formation temperature of − 8 °C calculated for the other studied black crust sample is unrealistic for the seafloor setting of these deposits where bottom seawater temperature is about 2–4 °C. This spurious value could be due to contributions by other minor minerals (smectite, amorphous SiO2?) in the sample (Table 1). Hydrothermal Mn-oxides may be either deposited as fallout from hydrothermal plume, or precipitated from a diffuse flow through the debris pile at the sampling site. Manganese in the hydrothermal plume is thought to have been rapidly transported away from the vent area by bottom currents and not deposited locally (Lilley et al., 1995). We do not have any direct observation for hydrothermal activity at 12°50′N CC and the nearby Ashadze hydrothermal field is a possible candidate for Mn-oxide supply to the sampling site. In any case [local or distal (Ashadze field) delivery] the hydrothermal plume fallout presumes: (1) precipitation of Mn-oxides in the plume at ambient seawater temperatures (2–4 °C); (2) enhanced and characteristic trace element concentrations of the Mn-oxides due to scavenging from the seawater (Baker et al., 1995). The very low trace element content of our Mn-oxides and their temperature of precipitation (20 °C; our very limited measurements) suggest they are not plume fallout deposits, but rather low-temperature precipitates from a local diffuse flow. The precipitation of nontronite observed as foraminiferal casts at the surface of some Mn-oxide crusts (Fig. 5C) is important from a genetic point of view. The original calcitic foraminiferal shells obviously provided a micro-environment for nontronite precipitation, but are now dissolved. Calcite dissolves in deep-sea environment [beneath the calcite compensation depth (CCD)] due to the aggressive, cold, deep seawater. The position of the CCD at the study area is at 4800– 5000 m (Tucker et al., 1990). The possible deepest location of any of our samples is 3280 m (see 3.), which is well above the CCD. This suggests that the dissolution of the foraminiferal tests holding the nontronite casts is not caused by the deep (below the CCD) seawater aggressive to calcite. Obviously, the foraminiferal tests dissolution had occurred after the nontronite precipitation and might be caused by the fluids depositing nontronite. According to the previous investigations (Harder, 1976; 1978) nontronite forms at a pH = 7–10 and low temperatures (Dekov et al., 2007). We may speculate that the fluids precipitating the nontronite inside the foraminiferal tests had been neutral to slightly acidic (pH ~ 7) and low-temperature, and have dissolved the calcite tests after nontronite precipitation. The foraminiferal tests could also be dissolved by seawater with pH lowered due to weathering of pre-existing sulfides in the vicinity of the sampling site. The seawater can penetrate into the porous sulfides at the seafloor, react with them and produce strongly acid pore-fluids

through a series of sulfide-oxidation reactions (e.g., Nordstrom, 1982). These acidic solutions mix with the ambient seawater and lower its pH so it becomes capable to dissolve calcite at the seafloor around. The occurrence of low-temperature hydrothermal Mn-oxides does not guarantee the presence of associated, high-temperature sulfide deposits at the 12°50′N CC. Petrography, mineralogy and geochemistry evidence suggests the green clasts of the breccias are fragments of altered MORBs, which have lost some rare-earth and trace elements (Cu, Co, Ni) during the alteration (Fig. 8). The low-temperature (presumably ~20 °C) Mn-oxide cement implies that the alteration of breccia clasts has at least been low-temperature hydrothermal process. The chemistry of the dark brown cement gives additional clue for the character of the hydrothermal process at the studied site. The key point is that its REE distribution pattern has a high positive Eu anomaly (Fig. 7), which resembles that of a high-temperature hydrothermal fluid (e.g., from the closest vent field with investigated fluids, Logatchev; Schmidt et al., 2007). The main mineral in the dark brown cement, goethite is a low-temperature phase stable at ambient seawater conditions (e.g., Hekinian et al., 1993). Goethite might be either a primary (low-temperature) hydrothermal precipitate or an alteration product after unstable under ambient seawater conditions (high-temperature?) Fe-containing minerals (e.g., Fe-sulfides). Primary hydrothermal goethite might either precipitate from plume fallout (and eventually composes metalliferous sediment) or form chimneys at the seafloor. In both cases it commonly mimics the seawater chondrite-normalized REE distribution pattern (with negative both Ce and Eu anomalies) (Hekinian et al., 1993; Dekov et al., 2010b) due to the high seawater-vent fluid mixing ratio. However, it has been observed that low-temperature (~46–100 °C) diffuse hydrothermal fluids representing a mixture of high-temperature vent fluids and seawater as well as conductively cooled vent fluids have positive Eu anomaly interpreted to be retained of the parent high-temperature fluid (James and Elderfield, 1996). The Fe-oxyhydroxide precipitates of these fluids have REE distribution pattern similar to that of the parent diffuse fluid: with a positive Eu anomaly (Mills et al., 1996). Therefore, solely on the REE data we cannot distinguish if the dark brown cement of the studied breccias is a lowtemperature diffuse flow precipitate, or was it formed as an alteration product after primary, high-temperature hydrothermal minerals unstable at ambient seawater. The isotope data discussed below suggest that the dark brown cement is likely formed after alteration of a high-temperature mineral phase. It is important to point out that either possibility (even the cooling or mixing of a high-temperature hydrothermal fluid to a diffuse fluid) indicates the presence of high-temperature venting in the area. Most common high-temperature hydrothermal minerals at seafloor are Fe-, Cu- and Zn-sulfides. The very low concentrations of Cu and Zn in the dark brown cement (Table 4) suggest that the primary, high-temperature hydrothermal mineral that might have precipitated in the open spaces of the basalt clast flow could be Fe-sulfide: pyrrhotite, pyrite, or marcasite. Any of them is unstable in seawater and converts into Fe-oxyhydroxides (goethite) with time, but would retain the REE pattern of the parent high-temperature vent fluid with positive Eu anomaly. The dark brown cement has U concentration (7.7 ppm) distinctly higher than in all the other samples whereas another sample with dominantly Fe-oxyhydroxide composition (12°50′N CC, St. 28 1, greenish-orange) has relatively low U content (0.9 ppm) (Table 4). It has been recognized (Veeh and Boström, 1971) that the seawater is the main source of U in the seafloor Fe-oxyhydroxide hydrothermal deposits and coprecipitation of U with Fe is the mechanism of U enrichment in these deposits. Plume-derived Fe-oxyhydroxide deposits have U concentrations generally between 1 and 5 ppm (in average) (Veeh and Boström, 1971; Gurvich, 2006; Dekov et al., 2009, 2010a). Therefore sample 12°50′N CC, St. 28 1, greenish-orange might be interpreted as a plumederived precipitate whereas the dark brown breccias cement is rather

V. Dekov et al. / Marine Geology 288 (2011) 90–102

1000

2 3 4

o

o

o

o

o

o

o

o

10

o

5

1

o

sample/chondrite

o

o

o

o o

100

1

6 7

0.1

8 9

0.01 0.001 0.0001

0.00001

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 7. C1 chondrite-normalized (Sun and McDonough, 1989) REE distribution patterns of 12°50′N CC hydrothermal deposits. 1 = 12°50′N CC, St. 28 1, greenish-orange; 2 = 12°50′N CC, St. 28 1, black; 3 = 12°50′N CC, St. 28 2; 4 = 12°50′N CC, St. 28 4, dark brown; 5 = 12°50′N CC, St. 28 4, black; 6 = 12°50′N CC, St. 28 4, brownishwhite; 7 = 12°50′N CC, St. 28 4, green clast; 8 = 12°50′N CC, St. 28 5, black; 9 = Atlantic deep seawater x10 (Douville et al., 1999); grey-shadowed strip = endmember hydrothermal fluid of the Logatchev field (Schmidt et al., 2007).

not. Fe-oxyhydroxide hydrothermal chimneys may have fairly elevated U concentrations (up to 18.7 ppm; Dekov et al., 2010b) and this suggests the dark brown cement might be a direct precipitate after mixing of the vent fluid with seawater in between the breccias clasts. On the other hand, it has been revealed (Mills et al., 1994) that the oxidative alteration of primary hydrothermal pyrite leads to U enrichment in the alteration product (Fe-oxyhydroxides). Bacterial mechanism has been proposed to explain the observed U enrichment. High U concentration of the dark brown cement implies that it might be the alteration product of a primary, high-temperature hydrothermal Fe-sulfide. Heavy isotope data for the studied samples (excluding the breccias clasts) suggest precipitation during interaction between a hydrothermal fluid (carrying elements leached from the underlying magmatic rocks) and seawater. The breccia clasts have mostly preserved their original Atlantic MORB-like character, evident in their isotopic similarity to local basalts (Figs. 9, 10). In contrast, the isotope systematics of the other samples are strongly influenced by seawater, and resemble the Nd–Sr-isotope relationships observed in peridotites from the Atlantis Massif (30°N, MAR; Delacour et al., 2008) serpentinized during substantial seawater circulation. With the exception of one sample (12°50′N CC, St. 28 2), all of the black crust layers appear to have formed almost in

sample/MORB

10

1.0

0.1

Ba U La Pb Sr Zr Sm Gd Dy Ho Tm Lu Cu Ni Rb Th Nb Ce Pr Nd Hf Eu Tb Y Er Yb Sc Co

Fig. 8. N-MORB-normalized (Hofmann, 1988) trace element pattern for green rock clasts of the breccias (sample #12°50′N CC, St. 28 4, green clast).

99

equilibrium with seawater (Fig. 9) as shown by their Sr signatures that is close to that of modern seawater (87Sr/86Sr = 0.70917) (e.g., Faure et al., 1965). Nd data reveal a difference between different crusts that probably relates to their mode of formation. The black crusts and the black cement of the breccias are supposed to be of a low-temperature hydrothermal origin and their Nd-isotope signatures are likely reflecting a variable input of Nd from ambient seawater. As can be seen in Fig. 8 these samples form a trend towards Atlantic seawater Nd isotopic values. The dark brown breccias cement (12°50′N CC, St. 28 4, dark brown), which is inferred to be an alteration product of primary hydrothermal sulfides formed at more elevated temperatures shows a MORB-like Nd-isotope composition (Fig. 9). With regard to the Pb-isotope data (Fig. 10), there is a difference between the relatively unradiogenic compositions of the black and dark brown cement in the breccia sample (12°50′N CC, St. 28 4) and the isotopically more evolved black crusts (samples #12°50′N CC, St. 28 1, -2, -5). Recent studies show that the Pb-isotope systematics of low-temperature hydrothermal precipitates are dominated by Pb scavenged from the ambient seawater (Frank et al., 2006; Kamenov et al., 2009). As can be seen in Fig. 10, the 12°50′N CC samples form a trend extending from the MORB field to the ambient Atlantic seawater, represented by WNADW and ENADW, with a number of samples (black crusts) plotting within the fields of Atlantic pelagic and biogenic sediments and Mn-nodules. As discussed below, there is no significant sedimentary cover in the area, therefore, the Pb isotopic systematics are best explained by progressive scavenging of Pb from the ambient seawater. Therefore, Mn-oxides of the black cement apparently were deposited among the breccia clasts during the lowtemperature waning stage of the hydrothermal system, evident in their isotopic systematics. In contrast to low-temperature studies (Frank et al., 2006; Kamenov et al., 2009), direct samples from modern high-temperature fluids at MORs show Pb isotopic compositions similar to the local MORB, clearly indicating derivation from the latter (Chen and Wasserburg, 1987). Similarly, Pb isotopic studies of high-temperature hydrothermal precipitates also show Pb derivation from local MORB sources (Godfrey et al., 1994; Andrieu et al., 1998; Stuart et al., 1999; Dias et al., 2008). Exceptions are the sediment-hosted Guyamas and Escanaba hydrothermal fields, where Pb-isotope systematics of hydrothermal precipitates show significant incorporation of local sedimentary component due to the thick (~500 m) sedimentary cover present at both fields (LeHuray et al., 1988). No significant sedimentary cover is present along the axial zone of the Mid-Atlantic Ridge between Fifteen-Twenty and Marathon FZs, therefore, input of sedimentary Sr, Pb, or Nd components to the local hydrothermal system is highly unlikely. This is also supported by the lack of mineralogical and geochemical evidence for the presence of a substantial detrital component in studied samples. Therefore, typical seafloor massive sulfides in the studied area are expected to be isotopically similar to associated basalts, as shown by data from the TAG hydrothermal field (German et al., 1993; Mills et al., 1993; Andrieu et al., 1998). The Pb isotopic similarity between the dark brown cement data and those of NA-MORB and TAG massive sulfides (Fig. 10) supports the assumption that the dark brown cement was a primary sulfide altered by the ambient seawater to goethite. The Nd–Pb-isotope difference between the dark brown and black cement of the breccia can be explained by high hydrothermal fluid/seawater ratio at the high-temperature stage of the system, during the precipitation of primary sulfides, later altered to Fe-oxyhydroxides. The Sr-isotope signal is clearly swamped by seawater Sr, which most probably occurred during the alteration of the primary sulfides to Feoxyhydroxides. We may assume that the positive Eu anomaly observed in the lowtemperature diffuse fluids of the TAG mound is not retained from the parent vent fluid as interpreted by James and Elderfield (1996), but is a result of low-temperature sub-seafloor dissolution of anhydrite [with retrograde solubility, i.e. it is more soluble at T b 150 °C (Haymon,

100

V. Dekov et al. / Marine Geology 288 (2011) 90–102

Table 5 Isotopic composition of the 12°50′N CC hydrothermal deposits. Sample #

87

12°50′N CC, St. 28 1, greenish-orange 1, black 2 4, dark brown 4, black 4, green clast 5, black BCR-2, measured BCR-2, reference dataa

0.709092 ± 11 0.709089 ± 6 0.708569 ± 12 0.709090 ± 6 0.709086 ± 6 0.703198 ± 15 0.709082 ± 9 0.704995 ± 7 0.70501 ± ?

a

Sr/86Sr

143

Nd/144Nd

ε Nd

ε Sr

206

Pb/204Pb

0.512994 ± 16 0.512610 ± 6 0.512711 ± 5 0.513109 ± 14 0.512454 ± 5 0.513135 ± 5 0.513041 ± 13 0.512660 ± 10 0.512640

6.9 − 0.5 1.4 9.2 − 3.6 9.7 7.9

65.2 65.1 57.8 65.2 65.1 − 18.5 65.0

18.578 18.743 18.843 18.485 18.434 18.321 18.659 18.746 18.757

207

Pb/204Pb

208

15.565 15.632 15.650 15.545 15.618 15.501 15.652 15.611 15.624

Pb/204Pb

δ18OVSMOW (‰)

38.346 38.641 38.822 38.282 38.327 37.870 38.597 38.675 38.723

16.1 10.2

Collerson et al. (2002).

1983)]: a mineral with crystallographically-controlled positive Eu anomaly (Mills and Elderfield, 1995). In other words, it does not seem likely to expect low-temperature hydrothermal fluids with positive Eu anomaly (as in many seafloor low-temperature hydrothermal deposits; Hekinian et al., 1993) without subsurface dissolution of anhydrite: i.e., without previous high-temperature (T N 150 °C) hydrothermal activity. In any case, the positive Eu anomaly in the hydrothermal deposits suggests for (at least previous) high-temperature hydrothermal activity. Substantial involvement of seawater in the vent fluid (evolution from end-member fluid towards seawater) would swamp the original MORB Pb signal. Therefore, our isotope data rule out the possibility of low-temperature precipitation of the dark brown cement of the breccia from diffuse hydrothermal fluids.

6. Conclusions Presented data in this work suggest the existence of high- and low- temperature hydrothermal activity (presumably recent) leading to precipitation of sulfides within the debris flows and oxyhydroxides on the seafloor at the west wall of the core complex at 12°50′N MAR. Tectonic setting at this site suggests the proposed hydrothermal vent field might be hosted in ultramafic lithologies. The investigated deposits are of two types: (1) black oxyhydroxide crusts on top of rock clasts; (2) breccias cemented by dark brown and black cement. Black

crusts are mainly composed of Mn-oxides (birnessite and 10-Å manganates) and have geochemical (low trace- and rare earth-element contents, high Li and Ag concentrations, REE distribution patterns with negative Ce and Eu anomalies) and O–Sr–Nd–Pb-isotope features that suggest low-temperature (~20 °C) hydrothermal precipitation from diffuse venting through debris flows. Breccias are composed of altered MORB basalt clasts cemented by dark brown and black cement. Although the mineralogy of the basalt clasts has been substantially altered the chemical mass transfer (element gains and losses) and Sr–Nd–Pbisotope ratios' changes have been negligible during the alteration processes. The black cement of the breccias has mineralogical, geochemical and Sr–Nd–Pb-isotope signatures that suggest it is a low-temperature hydrothermal precipitate. In contrast, the dark brown cement of the breccias shows very low trace- and rare earth-element contents, high U concentration, REE distribution pattern with high positive Eu anomaly and Nd–Pb-isotopic similarity to MORB and other seafloor massive sulfides that suggest its precursor had been a primary, high-temperature Fe-sulfide, which has altered to goethite at background seawater conditions. The apparent preservation of the primary Pb isotopic signal during alteration of the primary hydrothermal sulfide can possibly be used as exploration tool to distinguish whether a given Fe-Mn-oxyhydroxide

15.8 Pelagic and biogenic sediments ENADW

207Pb/204Pb

5b

+

5b 4db

NA-MORB

1 g-o

4 2

0

4b

2 1b

15.6 1 g-o 4db 4gc

15.5

Lucky Strike basalts

+

NA-MORB

TAG sulfides and metalliferous sediments

1b

15.4

-4

MAR serpentinites

4b

N

H

R

L

εNd

*

MAR gabbros

4gc

8

Mn-nodules

15.7

12

WNADW

15.3

-8

16

17

18

19

20

21

206Pb/204Pb

-12

-16 0.701

Atlantic seawater

0.703

0.705

0.707

0.709

0.711

0.713

87

Sr/86Sr

Fig. 9. Nd vs. Sr isotopic diagram. Sample abbreviations follow the listing in Table 5. The field for North Atlantic MORB (NA-MORB) is outlined using data from Dosso et al. (1991); the Nd isotope span of Atlantic seawater is from Piepgras et al. (1979); data for the MAR gabbros and serpentinites are from Delacour et al. (2008).

Fig. 10. 207Pb/204Pb vs. 206Pb/204Pb isotope diagram. The Northern Hemisphere Regression Line (NHRL) as defined by Hart (1984) is shown for reference. The fields for Mnnodules, pelagic and biogenic sediments are adopted after Andrieu et al. (1998); data for TAG (MAR) sulfides and metalliferous sediments are from German et al. (1993), Mills et al. (1993) and Andrieu et al. (1998); data for Lucky Strike (MAR) basalts are from Dosso et al. (1999) and Ferreira (2006); present-day data for Western North Atlantic Deep Water (WNADW) and Eastern North Atlantic Deep Water (ENADW) as reported by Abouchami et al. (1999). Note that the 12°50′N CC samples lay on a trend from MORB to Atlantic water, consistent with scavenging of Pb from ambient seawater, for more discussion see the text.

V. Dekov et al. / Marine Geology 288 (2011) 90–102

HYDROTHERMAL

Fe

DIA GE NE TIC

HY

DR

O

G

EN

ET

IC

(Co+Ni+Cu)x10

Mn

Fig. 11. Mn-Fe-(Co+ Ni + Cu)x10 ternary diagram of seafloor Fe-Mn deposits (after Bonatti et al., 1972): closed circle =12°50′N CC, St. 28 1, greenish-orange; open circle= 12°50′N CC, St. 28 1, black; open square =12°50′N CC, St. 28 2; closed diamond =12°50′N CC, St. 28 5, black.

deposit is “primary” precipitate or “secondary” (formed by oxidation of primary sulfides). Acknowledgments This study was supported by SYNTHESYS (SE-TAF-5065; EC Research Infrastructure Action, FP6 Structuring the European Research Area) grant to T. Boycheva and Resumption of the Research Fellowship (Alexander von Humboldt Foundation) grant to V. Dekov, which are greatly appreciated. Sincere thanks goes to M. Davydov (VNIIOkeangeologia, St. Petersburg) who kindly provided the samples for this study, to J. Escartín (Institut de Physique du Globe de Paris) who made available the bathymetry map of the studied area, and to J. Cuadros (Natural History Museum, London) who helped with the clay minerals interpretations. We also thank two anonymous reviewers and the editor Gert J. De Lange for their helpful comments which significantly improved our manuscript. References Abouchami, W., Galer, S.J.G., Koschinsky, A., 1999. Pb and Nd isotopes in NE Atlantic Fe–Mn crusts: proxies for trace metal paleosources and paleocean circulation. Geochimica et Cosmochimica Acta 63, 1489–1505. Andrieu, A.S., Honnorez, J.J., Lancelot, J., 1998. Lead isotope compositions of the TAG mineralization, Mid-Atlantic Ridge 26°08'N. In: Herzig, P.M., Humphris, S.E., Miller, D.J., Zierenberg, R.A. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 158, pp. 101–109. Arrhenius, G., Tsai, A.G., 1981. Structure, phase transformation and prebiotic catalysis in marine manganate minerals. Scripps Institution of Oceanography Reference Series 81, 1–19. Baker, E.T., 2009. Relationships between hydrothermal activity and axial magma chamber distribution, depth, and melt content. Geochemistry Geophysics Geosystems 10. doi:10.1029/2009GC002424. Baker, E.T., German, C.R., Elderfield, H., 1995. Hydrothermal plumes over spreading-center axes: Global distributions and geological inferences. In: Humphris, S., Zierenberg, R., Mullineaux, L., Thomson, R. (Eds.), Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions. : Geophysical Monograph, 91. American Geophysical Union, Washington, D.C, pp. 47–71. Batuev, B.N., Krotov, A.G., Markov, V.E., Cherkashev, G.A., Krasnov, S.G., Lisitsin, Y.D., 1994. Massive sulfide deposits discovered and sampled at 14°45'N, Mid-Atlantic Ridge. BRIDGE Newsletter 6, 6–10. Beltenev, V., Nescheretov, A., Shilov, V., Shagin, A., Stepanova, T., Cherkashev, G., Batuev, B., Samovarov, M., Rozhdestvenskaya, I., Andreeva, I., Fedorov, I., Davydov, M., Romanova, L., Rumyantsev, A., Zaharov, V., Luneva, N., Artemseva, O., 2003. New discoveries at 12°58'N, 44°52'W, MAR: Professor Logatchev-22 cruise, initial results. InterRidge News 12, 13–14.

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