Is the transition zone a deep reservoir for fluorine?

It is now recognized that the transition zone (TZ) is a significant repository for water. This means that other volatile species may also be stored in this region such as halogen elements. We have measured the solubility of fluorine in wadsleyite (Wd) and ringwoodite (Rw) under hydrous and anhydrous conditions at different pressures and temperatures, relevant for the transition zone. F contents are similar in Wd (665 to 1045 ppm F, up to 956 ppm H2O) and in Rw (186 to 1235 ppm F, up to 1404 ppm H2O). This suggests that F may be incorporated in the same manner as water in the major nominally anhydrous minerals of the TZ: ringwoodite and wadsleyite and that the transition zone could be a major reservoir for fluorine. In the framework of the “water filter model” proposed by Bercovici and Karato (2003), the contrast of volatile element contents between a depleted upper mantle and an enriched transition zone could be maintained over geological time scales. Previous estimates of the fluorine content of the Bulk Silicate Earth (BSE), such as 25 ppm by mass (McDonough and Sun, 1995), have assumed a homogeneous mantle. Although we do not know whether the TZ is F saturated or not, we used our new experimental data and estimates of the lower mantle F content from ocean island basalts to estimate a maximum BSE fluorine content of 59 ppm by mass for a hydrous, F-saturated TZ. This upper bound on the range of possible BSE F content emphasizes the challenges when explaining the origin of volatile elements in the Earth from a carbonaceous chondrite late veneer.