Fluorine and chlorine in mantle minerals and the halogen budget of the Earth’s mantle, Contributions to Mineralogy and Petrology (B. M. Urann, V. Le Roux, K. Hammond, H. R. Marschall, C.‑T. A. Lee, B. D. Monteleone)
We have developed a technique to reliably measure the F and Cl contents of natural peridotite minerals by SIMS down to ≥0.4 μg/g F and ≥0.3 μg/g Cl. This work is the first extensive study of the distribution of F and Cl in coexisting natural peridotite minerals (olivine, orthopyroxene, clinopyroxene, and amphibole).
We support the hypothesis that F in olivine is controlled by melt polymerization, and that F in pyroxene is controlled by their Na and Al contents, with some effect of melt polymerization. We infer that Cl compatibility ranks as follows: amphibole > clinopyroxene > olivine ~ orthopyroxene, while F compatibility ranks as follows: amphibole > clinopyroxene > orthopyroxene ≥ olivine, depending on the tectonic context. In addition, we show that F, Cl, Be and B are correlated in pyroxenes and amphibole. F and Cl variations suggest that interaction with slab melts and fluids can significantly alter the halogen content of mantle minerals. In particular, F in oceanic peridotites is mostly hosted in pyroxenes, and proportionally increases in olivine in subduction-related peridotites. The mantle wedge is likely enriched in F compared to un-metasomatized mantle, while Cl is always low (<1 μg/g) in all tectonic settings studied here. The bulk anhydrous peridotite mantle contains 1.4–31 μg/g F and 0.14–0.38 μg/g Cl. The bulk F content of oceanic-like peridotites (2.1–9.4 μg/g) is lower than DMM estimates, consistent with F-rich eclogite in the source of MORB. Furthermore, the bulk Cl budget of all anhydrous peridotites studied here is lower than previous DMM estimates. Our results indicate that nearly all MORB may be somewhat contaminated by seawater-rich material and that the Cl content of DMM could be overestimated. With this study, we demonstrate that the halogen contents of natural peridotite
minerals are a unique tool to understand the cycling of halogens, from ridge settings to subduction zones.