Serpentinites form by hydration of ultramafic lithologies in a range of seafloor and shallow subduction zone settings. Serpentinites are recognised as major reservoirs of fluid mobile elements and H₂O in subducting oceanic lithosphere, and together with forearc serpentinites formed in the mantle wedge, provide critical information about shallow-level volatile fluxes during subduction. The current study provides new Cl, as well as the first comprehensive Br, I and noble gas analyses reported for seafloor and forearc chrysotile–lizardite serpentinites. The samples were recovered from IODP drilling campaigns of mid-ocean ridge, passive margin and forearc settings (n=17), and ophiolites in the Italian Alps and Apennines (n=10). The aims of this study were to determine the compositional variability of noble gases and halogens in serpentinites entering subduction zones and evaluate the efficiency of gas loss during the early stages of serpentinite subduction. The chrysotile–lizardite serpentinites and serpentised peridotites contain 43-2300 ppm Cl and 3×10⁻¹³-2×10⁻¹¹ mol g⁻¹³⁶Ar, with the concentrations of these elements broadly related to the estimated degree of serpentinisation. The serpentinites have extremely variable Br/Cl and I/Cl ratios with many samples preserving compositions similar to organic-rich sedimentary marine pore fluids. Serpentinites from the Marianas Forearc have very high I concentrations of up to 45 ppm I and I/Cl ratios of ~14,000 times the seawater value that is even higher than the maximum I/Cl enrichment observed in sedimentary marine pore fluids. The serpentinites have ¹³⁰Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar ratios that are mostly close to or above seawater values, and ²⁰Ne/³⁶Ar ratios that range from seawater to lower values. The serpentinites contain 200 km, imply that serpentinites could dominate the deep recycling budgets of both the heavy halogens and atmospheric noble gases.11 page(s

Diamond-bearing kimberlites were born from similar melts in the deep Earth and variably modified during ascent.