[S-GC37] Volatiles in the Earth - from Surface to Deep Mantle

[SGC37-02] From Slab to Mantle: Phengite’s Role in Deep Earth Fluorine and Chlorine Delivery

Yingzhuo Liu^1,2, *Yu Wang^1,2, Chunfei Chen³ (1.State Key Laboratory of Deep Earth Processes and Resources, 2.Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 3.China University of Geosciences (Wuhan))

Keywords:Experimental petrology, Fluorine and chlorine cycling, Deep subduction , Phengite stability

The mechanism by which fluorine (F) and chlorine (Cl) are transported deeper into the mantle by subducting slabs remains a critical unresolved issue. To investigate the potential for F and Cl transport to the deep mantle and evaluate phengite as an effective carrier, we conducted high-temperature and high-pressure experiments using synthetic materials designed to represent altered oceanic crust. The starting materials contained 0.33 wt.% F and 0.21 wt.% Cl, with added water to promote phengite growth. Experiments were performed at temperatures of 900-1200℃ and pressures of 5-11 GPa. We examined phengite’s capacity to incorporate F and Cl, its stability limits, partition coefficients with melt, and whether it decomposes into other high-pressure phases capable of further transporting F and Cl into the deep mantle. Our results show that phengite remains stable in this bulk composition at pressures of 5-11 GPa and temperatures of 900-1050℃, coexisting with a phase assemblage of garnet, clinopyroxene, coesite/stishovite, rutile, melt, and minor apatite and titanite. Phengite melting/dissolution occurs at 950-1200℃, and it dehydrates to form K-hollandite and KMgF₃ at 11 GPa and 900℃. The F content in phengite ranges from 1.2 to 2 wt.%, while Cl content varies from 0.22 to 0.68 wt.%. Our findings indicate that phengite in the F-bearing system can remain stable up to ~330 km depth (~11 GPa), representing the maximum depth to which F and Cl can be transported by phengite. Phengite serves as the dominant host for F and Cl at depths beyond the sub-arc region, while apatite and titanite, though capable of incorporating F and Cl, are limited as carriers due to their relatively low abundance.