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• Altered seafloor basalts preferentially incorporate F relative to altered gabbros, which contain more Cl and Br. • The efficiency of halogen devolatilization from subducting ocean crust is Cl ≈ Br > I ≫ F. • At high P-T, lower crustal gabbros can be converted to halogen-enriched eclogites. • Subducted ocean crust represents the largest flux of F, Cl, Br, and I to the mantle. • Approximately 50% of the initial Cl and 95% of initial Br and I is lost before the arc. In order to constrain the geochemical cycling of halogens (F, Cl, Br, I) during subduction of altered ocean crust (AOC), this study compares bulk halogen concentrations from seven seafloor AOC drill cores (n = 21) and exhumed eclogites (n = 22) from three paleo-subduction settings (Raspas Complex, Ecuador; Zambezi Belt, Zambia; Cabo Ortegal, Spain). As ocean crust undergoes hydrothermal alteration, basalts and gabbros preferentially incorporate different halogens. Fluorine is predominantly hosted in basalts (averaging 155 μg/g); Cl and Br are enriched in gabbros (averaging 241 μg/g and 225 ng/g, respectively). During prograde metamorphism of AOC, F behaves compatibly and is decoupled from the heavy halogens (Cl, Br, I). Eclogite samples retain F in concentrations (30–160 μg/g) similar to seafloor values (20–190 μg/g). Chlorine and Br are strongly coupled and effectively mobilized during prograde metamorphism, with high-pressure (HP) samples containing between 5–15 μg/g Cl and 17–69 ng/g Br. Elevated F/Cl, Br/Cl, and I/Cl in eclogites relative to MORB and AOC ratios support the decoupling of halogens during metamorphism as Cl is most efficiently expelled from the slab; the mobility of halogens in subducting AOC is as follows: Cl ≈ Br > I ≫ F. In conjunction with published values of halogen abundances in seafloor serpentinites, marine sediments, and HP metaserpentinites and metasediments, the results of this study are used to estimate global halogen fluxes. Mass balance estimates indicate that up to 2% of the initial F, 50% of the initial Cl, 93% of the initial Br, and 97% of initial I entering subduction zones is lost between the trench and eclogite facies. Subducted AOC represents a substantial halogen reservoir to arc magma depths and may represent the most significant carrier of halogens to the deep mantle. [ABSTRACT FROM AUTHOR]

A greater quantity of halogens (F, Cl, Br, and I) appear to be subducted globally than are erupted at magmatic arcs, requiring either an increase in the mantle halogen budget through time or an additional output pathway or reservoir. The sub-continental lithospheric mantle (SCLM) is one such reservoir. SCLM can be enriched in volatile elements after metasomatism by fluids or melts from subducting oceanic lithosphere. We analyzed the bulk halogen content (F, Cl, Br, and I) of 28 variably metasomatized spinel and garnet peridotite xenoliths from the Navajo Volcanic Field (NVF; Central Colorado Plateau) to constrain the effects of the subducting Farallon plate-derived metasomatism on the halogen content of the western North American SCLM. We also analyzed 33 anhydrous spinel peridotites from various localities on and around the Colorado Plateau, as well as xenoliths from Oahu, Hawaii, and Eifel, Germany to compare them to the modally hydrated NVF xenoliths. Cl, Br, and I are enriched in NVF xenoliths relative to the depleted mantle, correlating with olivine oxygen isotope values and indices of metasomatism. F is less enriched, indicating preferential retention in the slab residue. Br/Cl and I/Cl are similar to partially dehydrated serpentinite, indicating that the source of these fluids is consistent with serpentinized oceanic lithosphere of the Farallon plate. Anhydrous xenoliths from the southwestern United States have more depleted mantle-like Cl and F contents but have highly enriched Br contents similar to the NVF xenoliths. Similar to NVF xenoliths, anhydrous xenolith Br/Cl and I/Cl suggest a Farallon derived serpentinite source of halogen enrichment. These results indicate that the Farallon slab sequestered halogens in the North American SCLM of the central and south Colorado Plateau and the southern Rio Grande Rift. Therefore, halogen sequestration in the SCLM may be a critical component in the global halogen cycle. [ABSTRACT FROM AUTHOR]

Lithium and chlorine concentrations and isotopic compositions of well-characterized metasedimentary suites from the Schistes Lustrés and Lago di Cignana (Western Alps) and the Catalina Schist (southern California, USA) were compared to determine Li and Cl behavior during prograde subduction-zone metamorphism. New data were obtained for Cl from both suites and for Li from the Western Alps. The Catalina Schist contains lawsonite-albite to amphibolite grade rocks recording peak temperatures of 200 to 750 °C and depths of 15 to 45 km, with the higher-grade units representing relatively warm subduction P-T paths. Bulk Cl concentrations decrease from ~100–500 μg/g to ~10–25 μg/g across metamorphic grades, largely due to Cl loss from chlorite and white mica, with most of this loss occurring at the lower grades. This loss mirrors the previously documented decreases in N, B, Cs, As, and Sb concentrations, with relatively little loss at higher metamorphic grade, and contrasts with the behavior of Li showing only modest loss and change in δ7Li values across all grades. Metapelitic rocks from the Western Alps representing a relatively "cool" geothermal gradient of ~8 °C/km, record peak temperatures of 350 to 550 °C and depths of up to 90 km. In this suite, both Cl and Li concentrations are uniform across grade, showing no obvious change in concentration. Lithium concentrations range to somewhat higher values compared to those previously measured in the Catalina Schist. Chlorine isotope compositions of the Schistes Lustrés overlap with those measured in the Catalina Schist, whereas Li isotope compositions of the Schistes Lustrés samples range to somewhat higher values than those from the Catalina Schist. Both suites show significant isotopic heterogeneity within single metamorphic grades and no systematic change in δ37Cl or δ7Li values with increasing grade. These heterogeneities are interpreted to be inherited from the protolith. Lithium isotope compositions correlate well with the chemical index of alteration (CIA), an observation also supporting minimal metamorphic alteration of the isotopic composition reflecting the protolith. These data demonstrate that, although some Cl is retained to great depths during subduction, most Cl is lost from metasedimentary rocks at shallow depths, similar behavior to that of B, Cs, As, and Sb. In contrast, Li loss is minimal across the full range of grade, more similar in behavior to Rb, Ba, and K. • Li loss from metasedimentary rocks is minimal across metamorphic grades. • Cl loss from metasedimentary rocks is significant at low grade. • Some Cl (≤ ~ 20 μg/g) is retained at high metamorphic grade. • Li and Cl isotope values reflect the protolith composition regardless of grade. [ABSTRACT FROM AUTHOR]