Chlorine in wadsleyite and ringwoodite: An experimental study
Introduction
Halogen elements (fluorine F, chlorine Cl, bromine Br, iodine I) are minor volatiles compared to hydrogen and carbon. Major halogens F and Cl have been mostly studied for their role in the shallowest Earth's reservoirs: lithosphere, crust, atmosphere, and hydrosphere, mostly because they are the most abundant halogens and because they have been shown to be strongly involved in volcanic and igneous processes. Indeed, Cl is an important constituent of volcanic fumaroles and plumes, and form individualized fluids such as brines and molten salts. These brines are strongly involved in hydrothermal systems and in ore forming processes (see the reviews after Pyle and Mather, 2009; Aiuppa et al., 2009). Cl is particularly used to trace igneous processes and ore-forming processes, and to track magmas from their genesis to their eruption Cl is known to affect magma properties (see Pyle and Mather, 2009), it is significantly degassed from subaerial volcanic activity (e.g. Aiuppa et al., 2009), and it can impact the stratosphere chemistry. Cl is enriched in sea water, it has been shown that oceanic subduction delivers fluids to the mantle through serpentinites related processes (serpentinization and deserpentinization), that are significantly Cl-rich (e.g. Ito et al., 1983, John et al., 2011, Kendrick et al., 2011), making this last one an important constituent in mantle metasomatism processes, that enriches the sources for arc magmatism (e.g. Tatsumi, 1989, Philippot et al., 1998, Scambelluri et al., 2004). Indeed magmas from subduction zones are among the most enriched in Cl (Ito et al., 1983, Straub and Layne, 2003, Kendrick et al., 2012).
During subduction, the interaction between seawater and rocks produces secondary minerals containing significant amounts of Cl (Ito et al., 1983, Pagé et al., 2016). It has thus been proposed that the subduction of oceanic lithospheric material at convergent plate boundaries would drive an annual global flux of 2.9–22 × 1012 g Cl to the Earth's interior (John et al., 2011). It has also been proposed that a part of the subducted Cl would reach high depths in the mantle (>200 km) and would possibly enrich the sources for Ocean Island Basalts (Kendrick et al., 2015, Joachim et al., 2015). Cl is significantly present in ultrahigh pressure metamorphic rocks (Scambelluri et al., 2004, Ottolini and Fèvre, 2008, Pagé et al., 2016), in inclusions of saline brines in diamonds (Weiss et al., 2014). Recent studies show that Cl is present in olivine, pyroxene, and garnet: the highest contents of Cl are measured in minerals formed by metamorphic dehydration of serpentine, olivines and pyroxenes (up to 400 ppm Cl, Scambelluri et al., 2004, Ottolini and Fèvre, 2008). In natural upper mantle nominally anhydrous minerals, the Cl contents are very low: up to 6.3 ppm in olivine (Beyer et al., 2012). Partitioning experiments performed for natural compositions however demonstrate the capability of upper mantle minerals to be the Cl carriers: up to 148 ppm Cl in orthopyroxene, 17 ppm Cl in clinopyroxene, 13 ppm in garnet, 5 ppm in plagioclase, up to 170 ppm in olivine (Dalou et al., 2012). To our knowledge, there is currently no evidence about the presence of Cl in the transition zone and in the lower mantle. Recently we have suggested that a strong link connects the water global cycle and that of fluorine at depth (Crépisson et al., 2014; Roberge et al., 2015). We have proposed that the transition zone can be a major reservoir for fluorine (Roberge et al., 2015). It could be similar for Cl.
This study aims at constraining the possible deep storage and cycling of Cl through the determination of Cl potential contents in wadsleyite (Wd) and ringwoodite (Rw), the major minerals of the transition zone (TZ). We use these data to discuss the Cl content of the bulk silicate Earth.
Section snippets
Starting materials an experimental strategy
The starting bulk composition was olivine Fo90 with a slight excess of silica ((Mg+Fe)/Si atomic ratio = 1.75), obtained with a mixture of (1) oxide powders of MgO, SiO2, FeO, or (2) natural Fo90 olivine with SiO2 powder. 5 wt% of Cl was added to the mixture as crushed NaCl powder. NaCl was chosen as the source of Cl because it is enriched in sea water and in subducted oceanic floor. For experiments under hydrous conditions, 2 wt% of water was added as brucite Mg(OH)2, an amount close to the
Synthesized samples
Ten samples were synthesized (see Table 1). Among these samples, two are volatile-free (no H2O and no Cl), two are anhydrous (Cl-enriched but no H2O), and six are Cl–H2O-enriched. Recovered mineralogical assemblages are described in Table 1. They correspond to the mineral assemblages expected for the pressure and temperature conditions of the transition zone. The minerals are in equilibrium with a saline and silica-rich melts mostly present at grain boundaries. SEM shows that wadsleyite and
Cl storage in the mantle transition zone's wadsleyite and ringwoodite phases
It is now recognized that the TZ is hydrated at least locally as shown by recent studies: (1) the significant water content of a hydrous ringwoodite trapped in a natural diamond exhumed from the transition zone (Pearson et al., 2014), (2) geophysical data (e.g. Huang et al., 2005). We consider hydrous Wd and Rw in the following sections: according to our measurements, ringwoodite contains ppm Cl and ppm ; wadsleyite can take up to ppm Cl and ppm . Such
Conclusion
The experimental determination of the Cl concentration in hydrous wadsleyite and ringwoodite shows that they can be major carriers for Cl in the deep Earth. The transition zone can thus be a deep significant repository for Cl. Cl contents are lower by a factor of 3 than those observed by Roberge et al. (2015) for F. For both Cl and F, the presence of water lowers their concentrations in Rw and Wd, but this storage capacity remains significant. It is thus likely that both halogen elements are
Acknowledgments
We acknowledge Daniel J. Frost and the BGI's Staff, particularly H. Schulze, for their constant availability and precious help with the high pressure experiments performed in Bayreuth. We also thank the LMV's staff for their help during the experiments carried out in Clermont Ferrand. We are grateful to H. Khodja and the LEEL staff during the analysis of the samples using the nuclear microprobe. We warmly thank J. C. Boulliard from the mineralogical collection of UPMC, I. Estève for her
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