Halogen recycling initiated by sediment subduction: Insights from Mediterranean alkaline rocks

Slab subduction is the only efficient mechanism for recycling halogens from the Earth's surface into the interior, thereby regulating long-term ocean salinity and sustaining the Earth's habitability. Dehydration and melting are the two principal pathways by which subducted slabs transfer surface-derived halogens into the mantle. Previous constraints on halogen behavior in subduction zones have been broadly established based on fluid-dominated arc lavas and metamorphic rocks. However, the compositions and behavior of halogens during slab melting, and their role in subduction-zone halogen cycling, remain poorly constrained. Here we present bulk-rock halogen (F and Cl), C, and S data for two Mediterranean alkaline suites formed in a subduction-related setting–leucite-bearing rocks and orogenic lamproites–interpreted to be derived from the lithospheric mantle metasomatized predominantly by carbonate-rich and siliciclastic sediment melts, respectively. Both leucite-bearing rocks and high-Mg# (>75) lamproites exhibit higher F (1850–3210 and 660–4180 ppm) than N-MORB. In contrast, a subset of lamproites preserve near-primary Cl content (mean ~ 632 ppm), while all leucite-bearing rocks and some of the lamproites show significant Cl depletion due to volcanic degassing. Melting models show that the mantle sources of these alkaline rocks were strongly enriched in halogens (~800 ppm F and ~ 50 ppm Cl), with concentrations equivalent to ~50 times those of the depleted mantle. This suggests that substantial surface F and Cl can be transported into the mantle wedge through sediment subduction. The mantle sources of these alkaline rocks are characterized by high F/Cl (~4.7) and by F/Nd (~20.7) and Cl/Nb (~16.8) ratios that fall within the ranges of dehydrated subducted sediments and primary N-MORB. This contrasts sharply with dehydration-dominated arc lavas, which exhibit low F/Cl (~0.58), elevated F/Nd (~34.1), and extremely high Cl/Nb (~1315). These results indicate that sediment-melt metasomatism enriches F and Cl in the mantle without significantly modifying F/Nd or Cl/Nb, whereas slab-fluid metasomatism preferentially enriches Cl and strongly increases Cl/Nb of the mantle. Our results therefore demonstrate that combined F/Cl, F/Nd, and Cl/Nb systematics provide an effective means of distinguishing the relative contributions of melting and dehydration to halogen recycling in subduction zones.