Search

Abstract: The Cl, Br and I contents of Earth are depleted by a factor of 10 relative to predicted values from chondritic and solar abundances. Possible explanations for the apparent discrepancy include (1) unrecognized sequestration of Cl in the core, (2) a much higher nebular volatility than normally presumed or (3) a preferential loss of the heavy halogens during planetary accretion. We tested the first assumption by conducting high pressure–temperature equilibration experiments between silicate and metal. At 15GPa and 1900°C, the D Cl(metal–silicate) value for Cl is less than 0.007, indicating that the core is not a significant reservoir for Cl. The concentration of Cl in all chondritic classes follows a depletion trend very similar to that of Na and Mn, arguing against a low condensation temperature for Cl. Instead, we propose that the depletion of the heavy halogens is due to their unique hydrophilic behavior. Almost half of Earth's Cl and Br inventory resides in the ocean and evaporites, demonstrating the unique affinity for aqueous solutions for these elements. During planetary accretion, there would have been a strong sequestration of halogens into the crustal reservoir. ‘Collisional erosion’ during planetary accretion provides a mechanism that would uniquely strip the heavy halogens out of an accreting Earth. Had such loss not occurred, the salinity of the oceans would be 10× the present value, and complex life would probably never have evolved. [Copyright &y& Elsevier]

Abstract: Wei et al. [Wei, W., Kastner, M., Spivack, A., 2008. Chlorine stable isotope and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean. Earth Planet. Sci. Lett. 266, 90–104] published chlorine isotope data for pore fluids and serpentinites from three subduction zones and two obducted serpentinites. The measured δ 37Cl values of serpentinites ranged from 1.2 to 6.0‰ (vs. SMOC), far higher than all other published data for serpentinites. The explanation for these high δ 37Cl values defies known fractionation mechanisms. Instead, the anomalous δ 37Cl values are almost certainly due to analytical artifacts known to exist for the method the authors employed. Wei et al. [Wei, W., Kastner, M., Spivack, A., 2008. Chlorine stable isotope and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean. Earth Planet. Sci. Lett. 266, 90–104] concluded that if high δ 37Cl serpentinites (∼6‰) were subducted into the mantle, then a change in the δ 37Cl value of the crustal reservoir would be observed in the evaporite record over the last 200Ma. Because the Cl isotope composition of the crustal reservoirs has been constant over this time period, as constrained by evaporites, they argue that Cl in serpentinites is recycled back to the oceans. This argument is flawed for two reasons. First, the δ 37Cl values of serpentinites is close to 0‰, not 6‰ as suggested in their paper. Second, the δ 37Cl value of the crustal reservoir would change immeasurably over 200Ma even if serpentinites of 6‰ were subducted into the mantle. In fact, it has been shown that the crust, mantle and chondritic reservoir all have δ 37Cl values close to 0‰ [Sharp, Z.D., Barnes, J.D., Brearley, A.J., Fischer, T.P., Chaussidon, M., Kamenetsky, V.S., 2007. Chlorine isotope homogeneity of the mantle, crust and carbonaceous chondrites. Nature 446, 1062–1065]. No secular variation in the δ 37Cl value of the crust is expected, a conclusion that is supported by chlorine isotope data on sediments extending back over 3.5Ga [Sharp, Z.D., Barnes, J.D., Brearley, A.J., Fischer, T.P., Chaussidon, M., Kamenetsky, V.S., 2007. Chlorine isotope homogeneity of the mantle, crust and carbonaceous chondrites. Nature 446, 1062–1065]. [Copyright &y& Elsevier]

Abstract: Dissolved cation and anion species were measured in the water-soluble fraction of massive seafloor serpentinites leached with distilled H2O. Soluble chloride contents ranged from 0.09 to 0.51 wt.% (average=0.26 wt.%), SO42− to 0.09 wt.%, Mg2+ to 0.16 wt.% and Na+ to 0.09 wt.%. The concentration of evolved ions was the same in samples equilibrated with water at 25 and 230 °C, demonstrating that a significant amount of Cl present in serpentinites occurs as water-soluble salts. A nearly equal amount of Cl exists in the leached serpentinite, likely sited in serpentinite itself. The formation of intergranular salts in the serpentinite can be explained by seawater hydrothermal alteration of peridotite. Hydration reactions and H2O-incorporation in the serpentine leave the residual fluid with ever-increasing salinity and ultimately lead to precipitation of water-soluble chlorides. Chlorine isotope ratios in the water-soluble fraction average −1.49‰ (vs. standard mean ocean chlorine, SMOC) from a core at the Iberian Abyssal Plain, and +0.16‰ from a much younger core at Hess Deep. The δ37Cl value of insoluble Cl is ∼0.33‰ higher than the soluble fraction. The nonzero δ37Cl values demonstrate that the Cl in the serpentinites is not an artifact of seawater incorporation during drilling. The following conclusions are presented: (1) Serpentinite may be the major conduit for surficial chlorine transfer to mantle depths and an important part of the global chlorine cycle. The Cl flux in serpentinites into the mantle is larger than all other previously identified fluxes. (2) Cl is sited in both a water-soluble and -insoluble component. (3) A high-salinity fluid ‘plume’ is evolved during serpentinite dehydration of a subducting slab. The high-salinity fluid affects metamorphic reactions and melting properties above the slab and can cause intense metasomatism. The unusual bulk composition of the whiteschist from Dora Maira can be explained by channeled passage of a high-salinity fluid. (4) High-salinity fluids reduce the stability field of serpentine, causing its dehydration to occur over a broad interval, which may partly explain the broad locus of intermediate-depth earthquakes. [Copyright &y& Elsevier]