Elsevier

Geochimica et Cosmochimica Acta

Volume 72, Issue 22, 15 November 2008, Pages 5475-5504
Geochimica et Cosmochimica Acta

The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere

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Abstract

Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budget of these elements we have studied samples of highly serpentinized (>50%) spinel harzburgite drilled at the Mid-Atlantic Ridge (Fifteen–Twenty Fracture zone, ODP Leg 209, Sites 1272A and 1274A). In-situ analysis by secondary ion mass spectrometry reveals that the B, Li and Be contents of mantle minerals (olivine, orthopyroxene, and clinopyroxene) remain unchanged during serpentinization. B and Li abundances largely correspond to those of unaltered mantle minerals whereas Be is close to the detection limit. The Li contents of clinopyroxene are slightly higher (0.44–2.8 μg g−1) compared to unaltered mantle clinopyroxene, and olivine and clinopyroxene show an inverse Li partitioning compared to literature data. These findings along with textural observations and major element composition obtained from microprobe analysis suggest reaction of the peridotites with a mafic silicate melt before serpentinization. Serpentine minerals are enriched in B (most values between 10 and 100 μg g−1), depleted in Li (most values below 1 μg g−1) compared to the primary phases, with considerable variation within and between samples. Be is at the detection limit. Analysis of whole rock samples by prompt gamma activation shows that serpentinization tends to increase B (10.4–65.0 μg g−1), H2O and Cl contents and to lower Li contents (0.07–3.37 μg g−1) of peridotites, implying that—contrary to alteration of oceanic crust—B is fractionated from Li and that the B and Li inventory should depend essentially on rock–water ratios. Based on our results and on literature data, we calculate the inventory of B and Li contained in the oceanic lithosphere, and its partitioning between crust and mantle as a function of plate characteristics. We model four cases, an ODP Leg 209-type lithosphere with almost no igneous crust, and a Semail-type lithosphere with a thick igneous crust, both at 1 and 75 Ma, respectively. The results show that the Li contents of the oceanic lithosphere are highly variable (17–307 kg in a column of 1 m × 1 m × thickness of the lithosphere (kg/col)). They are controlled by the primary mantle phases and by altered crust, whereas the B contents (25–904 kg/col) depend entirely on serpentinization. In all cases, large quantities of B reside in the uppermost part of the plate and could hence be easily liberated during slab dehydration. The most prominent input of Li into subduction zones is to be expected from Semail-type lithosphere because most of the Li is stored at shallow levels in the plate. Subducting an ODP Leg 209-type lithosphere would mean only very little Li contribution from the slab. Serpentinized mantle thus plays an important role in B recycling in subduction zones, but it is of lesser importance for Li.

Introduction

At mid-ocean ridges, plates spread and new oceanic lithosphere is produced. At slow spreading-ridges, the magmatic section is often reduced and the oceanic mantle is exhumed at the ocean floor (e.g. Cannat, 1993, Michael et al., 2003, Escartin et al., 2003). The two currently debated models to explain mantle exhumation in slow-spreading environments invoke either spreading, and focused magmatism at the center of a ridge segment (Cannat, 1993, Ghose et al., 1996, Dijkstra et al., 2001) or detachment faulting (Tucholke et al., 1998, Escartin et al., 2003, Boschi et al., 2006, Ildefonse et al., 2007). In both cases, hydrothermal fluids transform olivine, orthopyroxene and clinopyroxene into serpentine, chlorite, tremolite, brucite or magnetite (e.g. Bach et al., 2004), depending on temperature and pressure conditions (e.g. Ulmer and Trommsdorff, 1995).

In the last years, the systematics of the light elements Li, B, and Be have been used to chemically trace low-temperature alteration processes in different geological environments (e.g. James and Palmer, 2000, Chan et al., 2002, Pistiner and Henderson, 2003, Rudnick et al., 2004, Kisakürek et al., 2004). Studies on fresh mid-ocean ridge basalts (MORB; Ryan and Langmuir, 1987, Ryan and Langmuir, 1993) and on altered MORB (e.g. Thompson and Melson, 1970, Bouman et al., 2004, Elliott et al., 2006) showed that during alteration Li and B may be enriched by a factor of 2 and 30, respectively.

A similar reasoning can be applied to the underlying mantle lithosphere. Estimates of the light element content of the primitive mantle (McDonough and Sun, 1995, Lyubetskaya and Korenaga, 2007) and of the depleted mantle (Salters and Stracke, 2004) imply very low light element abundances in whole rock samples and major mantle phases (around 1 μg g−1 for Li and B). These whole rock data are consistent with C1 chondrite concentrations (Anders and Grevesse, 1989, Lyubetskaya and Korenaga, 2007). In contrast, elevated Li (up to 13.7 μg g−1, Decitre et al., 2002, Niu, 2004) and high B whole rock contents (up to 85 μg g−1, Bonatti et al., 1984, Spivack and Edmond, 1987) were found in dredged abyssal peridotites. High Li (<19.5 μg g−1, Decitre et al., 2002) and B abundances (∼47 μg g−1, Scambelluri et al., 2004) were found in serpentine minerals.

In addition, high but variable whole rock B (0.4–79.0 μg g−1; Benton et al., 2001, Savov et al., 2005, Wei et al., 2005) and Li concentrations (0.5–18.9 μg g−1; Parkinson and Pearce, 1998, Benton et al., 2004, Savov et al., 2005, Zanetti et al., 2006, Savov et al., 2007) were measured in serpentinized peridotites from forearc settings.

Beryllium (Be) data are scarce, and concentrations are extremely low in primitive mantle (0.054 μg g−1 or 0.068 μg g−1; McDonough and Sun, 1995, Lyubetskaya and Korenaga, 2007) and in depleted mantle (0.025 μg g−1; Salters and Stracke, 2004), highly variable in dredged abyssal peridotites (0.001–0.212 μg g−1, Niu, 2004), and elevated in forearc peridotites (0.07–0.16 μg g−1, Zanetti et al., 2006).

Experimentally determined mineral/melt and mineral/fluid partition coefficients are in line with these observations and show that B, Li and Be are incompatible in mantle minerals (e.g. Brenan et al., 1998, Taura et al., 1998). However, there are only a few experiments concerning the light element contents of hydrothermally precipitated minerals. Seyfried and Dibble (1980) performed seawater–peridotite experiments at 300 °C and 500 bar. During the experiment, the B content in the solution (pH <5) remained constant, but during quenching the B content decreased from 4.01 to 1.75 μg g−1. It was proposed that B is incorporated into serpentinized peridotite at low temperatures (<300 °C). Bonatti et al. (1984) suggested that the maximum B uptake in peridotite is related to temperature and to the serpentine polymorph, and measured a maximum value of 110 μg g−1 B in serpentine minerals (orthochrysotile, clinochrysotile and lizardite). Reversing the alteration process by heating up altered basalt to 350 °C at 500 bar showed that Li and B are incorporated in low-temperature alteration phases and are liberated into the fluid phase upon heating (Seyfried et al., 1998).

The light element content of the oceanic mantle is important in the context of recycling of these elements in subduction zones. Previous studies have shown that the magmatic and sedimentary oceanic crust can be a major host for Li and B. Oceanic sediments are enriched in B (up to 100.2 μg g−1, Leeman et al., 2004) and Li (up to 74.3 μg g−1, Bouman et al., 2004, Leeman et al., 2004). Even after metamorphism at high pressure and moderate temperature (∼400 °C) in a subduction zone, they retain up to 66.7 μg g−1 Li, 2.32 μg g−1 Be, and 93.9 μg g−1 B (Marschall et al., 2006). Altered oceanic basalt may contain up to 36.5 μg g−1 Li (Bouman et al., 2004). Subducted oceanic crust, nowadays eclogites, was found to contain up to 37.4 μg g−1 Li (Zack et al., 2002, Marschall et al., 2006), and up to 4.77 μg g−1 B (Marschall et al., 2006). However, the crustal section of a subducting oceanic lithosphere is thin compared to the mantle part and magmatic crust can be largely absent in lithosphere produced at slow-spreading ridges. Therefore, serpentinized mantle should play a key role in light element recycling. Experiments on serpentinite breakdown (simulation of a subducting plate) by Tenthorey and Hermann (2004) suggest that serpentinized mantle should be the major sink of B in subduction zones. The extreme enrichment in B of the Mariana forearc mantle wedge supports this theory (Savov et al., 2007).

In this study, we investigated serpentinized oceanic peridotite drilled from near the Mid-Atlantic Ridge (MAR, ODP Leg 209, Sites 1272A and 1274A). We analyzed minerals and whole rock samples for major elements, Li, Be and B contents. Our aims were to identify the host minerals of the light elements, to understand light element systematics, to establish a mass balance and to discuss the calculations in the framework of light element recycling at subduction zones.

Section snippets

Geological setting

ODP Leg 209 was drilled at the MAR, north and south of the Fifteen–Twenty Fracture zone (Fig. 1, Kelemen et al., 2004a, Paulick et al., 2006, Kelemen et al., 2007). A full spreading rate of 25 mm/year was determined (Fujiwara et al., 2003), corresponding to a slow-spreading ridge. The half-spreading rates (E and W) show little long-term asymmetry. This inspired Fujiwara et al. (2003) and later Escartin et al. (2003) to propose a model of shallow detachment faults to explain the geomorphologic

Site 1272A

Site 1272A was drilled at a water depth of 2560 m at the ridge (Fig. 1). The drilled material consists of ∼55 m diabase in the upper part, and 75 m of harzburgite containing rare dunite in the lower part (Fig. 2; Kelemen et al., 2004a). The major mineral phase in the mantle rocks is lizardite, and a downhole decrease in serpentinization can be observed (Kelemen et al., 2004a). Modal mineralogy and petrographic features of the analyzed spinel harzburgites are summarized in Table 1 and Fig. 2.

The

Analytical techniques

The major element compositions of olivine, clinopyroxene, orthopyroxene, spinel, magnetite and serpentine minerals were determined by electron microprobe analysis (EPMA) using a JEOL JXA-8200 (University of Bern, and ETH Zürich, Switzerland), a Cameca SX-51 (University of Heidelberg, Germany) and a SX-100 (University of Freiburg, Germany), all equipped with wavelength-dispersive systems. An accelerating potential of 15 kV, a beam current of 20 nA and counting times of 30 s for Si, Al, Mg and Na

Major and light element contents of minerals

Concentrations of major and light elements are given in Table 3, Table 4, respectively. Primary olivine is homogeneous with an average Mg# of 0.91 (Mg# = 100 × Mg/[Mg + Fetot], all iron assumed as divalent), and NiO contents of ∼0.4 wt% (Fig. 4). Magmatic olivine in sample OD 42 (Fig. 3f) has a Mg# of 0.80 and NiO contents of 0.1–0.2 wt%. Average light element contents of primary olivine are 1.04 μg g−1 for Li, 0.022 μg g−1 for B, and below the critical value for Be. They are higher in magmatic olivine of

Discussion

In the following, we will discuss the nature of the protoliths, including chemical and textural evidence of melt infiltration. Then, we will examine the impact of serpentinization on the B and Li contents of whole rock samples and present a model of the light element budget of the oceanic lithosphere.

Summary and conclusions

The results of our study of serpentinized peridotites from the Mid-Atlantic Ridge show that the B, Li and Be contents of the mantle minerals olivine, orthopyroxene and clinopyroxene remain unchanged during serpentinization and still reflect older processes such as melt infiltration. As serpentine has high B and fairly low Li contents, the process of serpentinization increases the B (and H2O and Cl) and lowers the Li contents of the whole rock. These findings are in line with the composition of

Acknowledgments

This research used samples provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc.. Funding for this research was provided by the Swiss National Science Foundation (Grants 200021-10361 and 200021-103479/1). We thank E. Reusser (ETH Zürich, Switzerland), E. Gnos and A. Berger (Bern, Switzerland), H.-P. Meyer (Heidelberg, Germany) and H.

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