Chlorine in wadsleyite and ringwoodite: An experimental study

Mathilde Roberge a, Hélène Bureau a,∗, Nathalie Bolfan-Casanova b, Caroline Raepsaet c, Suzy Surble c, Hicham Khodja c, Anne-Line Auzende d, Patrick Cordier e, Guillaume Fiquet a
aInstitut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Universités – UPMC Univ. Paris 06, CNRS UMR 7590, MuséumNational d’Histoire Naturelle, IRD UR 206, 4 place Jussieu, 75252 Paris Cedex 05, France
bLaboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, France
cLEEL, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
dUniversité Grenoble Alpes, ISTerre, UMR5275 F-38041 Grenoble cedex 9, France
eUnité Matériaux et Transformations, Université Lille 1, 59655 Villeneuve d’Ascq, France

We report concentrations of Chlorine (Cl) in synthetic wadsleyite (Wd) and ringwoodite (Rw) in the system NaCl–(Mg,Fe)2SiO4under hydrous and anhydrous conditions. Multi-anvil press experiments were performed under pressures (14–22GPa) and temperatures (1100–1400◦C) relevant to the transition zone (TZ: 410–670 km depth). Cl and H contents were measured using Particle Induced X-ray Emission (PIXE) and Elastic Recoil Detection Analysis (ERDA) respectively. Results show that Cl content in Rw and Wd is significantly higher than in other nominally anhydrous minerals from the upper mantle (olivine, pyroxene, garnet), with up to 490ppm Cl in anhydrous Rw, and from 174 to 200ppm Cl in hydrous Wd and up to 113ppm Cl in hydrous Rw.
These results put constrains on the Cl budget of the deep Earth. Based on these results, we propose that the TZ may be a major repository for major halogen elements in the mantle, where Cl may be concentrated together with H2Oand F (see Roberge et al., 2015). Assuming a continuous supply by subduction and a water-rich TZ, we use the concentrations measured in Wd (174ppm Cl) and in Rw (106 ppm Cl) and we obtain a maximum value for the Cl budget for the bulk silicate Earth (BSE) of 15.1 ×1022g Cl, equivalent to 37 ppm Cl. This value is larger than the 17ppm Cl proposed previously by McDonough and Sun(1995)and evidences that the Cl content of the mantle may be higher than previously thought. Comparison of the present results with the budget calculated for F (Roberge et al., 2015)shows that while both elements abundances are probably underestimated for the bulk silicate Earth, their relative abundances are preserved. The BSE is too rich in F with respect to heavy halogen elements to be compatible with a primordial origin from chondrites CI-like (carbonaceous chondrites CC) material only. We thus propose a combination of two processes to explain these relative abundances: a primordial contribution of different chondritic-like materials, including EC-like (enstatite chondrites), possibly followed by a distinct fractionation of F during the Earth differentiation due to its lithophile behaviorcompared to Cl, Br and I.

http://www.sciencedirect.com/science/article/pii/S0012821X17301619