Halogens and noble gases in serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon
Résumé
Ophiolitic serpentinites and secondary peridotites formed by serpentinite dehydration were investigated to improve constraints on the fates of noble gases and halogens during subduction zone metamorphism. The work extends previous studies to encompass F and four stages of serpentinization and serpentinite dehydration including: (i) oceanic serpentinites preserving the features of seafloor serpentinization; (ii) subducted high grade (olivine bearing) antigorite-serpentinites; (iii) spinifex and granofels textured chlorite harzburgites; and (iv) a garnet peridotite.
Serpentinites and secondary peridotites from different ophiolites are shown to have characteristic ranges of 40Ar/36Ar: chrysotile and antigorite serpentinites from Erro Tobbio (Western Alps) have 40Ar/36Ar of ∼ 296–390; antigorite serpentinites and chlorite harzburgites from Cerro del Almirez (Betic Cordillera) have 40Ar/36Ar of ∼ 340–600, and chlorite harzburgites and garnet peridotites from Cima di Gagnone (Swiss Alps) have 40Ar/36Ar of ∼ 600–1100. The variation of 40Ar/36Ar is unrelated to metamorphic grade at each locality but is broadly correlated with variation in other radiogenic isotopes (206Pb/204Pb and 87Sr/86Sr) between localities. This suggests excess 40Ar was derived from terrigenous sediments with characteristic ranges of 40Ar/36Ar and 87Sr/86Sr in different subduction zones.
The secondary chlorite harzburgites have 20Ne/36Ar ratios of greater than seawater, contain parentless (or excess) 4He, and have higher F concentrations than any of the serpentinites investigated. The 20Ne/36Ar is broadly correlated with 40Ar/36Ar in samples from Cerro del Almirez suggesting derivation of excess 40Ar and atmospheric 20Ne from a common source. The chlorite is shown to have higher concentrations of F, Ne and other noble gases than coexisting olivine and enstatite, which contain fluid-related inclusions. The high F content and high 20Ne/36Ar ratios of the chlorite harzburgites are ascribed to fluxing of dehydrating serpentinites with F-, 40Ar-, 4He- and 20Ne-rich fluids derived from metasediments in the subducting slab, and an inferred high compatibility of F and Ne in chlorite.
The garnet peridotite from Cima di Gagnone records the final and complete dehydration of serpentinite. Based on the analysis of mineral separates minimally affected by retrogression (marked by garnet breakdown and the appearance of Cl-rich hornblende), nominally anhydrous garnet peridotite retains Cl, Br, I and non-radiogenic noble gas concentrations up to an order of magnitude higher than average depleted mantle.
The data are consistent with serpentinised lithosphere and related secondary peridotites as major sources of deeply subducted seawater-derived volatiles in the Earth’s mantle. The data also demonstrate that the relative abundances of volatiles subducted into the mantle are controlled by multiple factors including: original seafloor alteration, the relative compatibilities of different noble gases and halogens in minerals forming during different stages of subduction and chemical exchange between different lithologies during subduction. The combination of these processes has produced elevated 20Ne/36Ar in chlorite harzburgites from two unrelated localities. This suggests that subduction of atmospheric Ne could be significantly more efficient than previously realised, which has implications for interpretation of the mantles primordial 20Ne/22Ne ratio and how the Earth accreted.