HR: 15:05h
AN: U13C-06    [Abstracts]
TI: The Magnesium isotopic composition of the ocean and its consequences on chemical geodynamics
AU: * Galy, A
EM: albert00@esc.cam.ac.uk
AF: Department of Earth Science University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom
AB: It has been long recognised that the input of Mg in the ocean by river is removed by precipitation of Mg-rich bearing phases, either directly from the ocean such as dolomite or through hydrothermal circulation in the oceanic crust. By subduction, a leakage of Mg towards the mantle becomes a part of the global geochemical cycle and unlike the large-ion lithophile elements, carbon or sulfur, Mg is poorly stripped off from the subducting slab. The fluxes (either riverine input or heat budget and hydrothermal output) suggest a residence time of Mg in the ocean in the order of 10 Ma but the removal of Mg from the whole mantle at ridges is far greater (20 to 50 times) than the re-incorporation of Mg through subduction zones. Nevertheless, none of these fluxes will have a significant impact of the Mg inventory of the mantle since less than 2% of Mg has seen the surface during the whole Earth's history. Therefore, any Mg isotopic heterogeneity in mantle rock could be viewed has primordial or the result of processes related to the production and/or the transfer of these rocks to the surface, including late stage metasomatism. The recent report of a range of 4‰ in δ²⁶Mg for mantle material is rather surprising and great care has to be taken when considering Mg-isotopic composition of silicate material because of unaccounted chemical bias during the analysis and related inaccuracy of the measurements. Nevertheless, 2 viable mechanisms could account for such a range in mantle rocks. First, a late stage metasomatism and associated large kinetic isotopic fractionation, as suggested by Pearson et al. (doi:10.1016/j.chemgeo.2005.09.029). However, Mg is a major element and it has to be replaced for mass/charge balance purpose by the diffusion of Fe in the opposite direction. The lack of relationship between Forsterite content and δ²⁶Mg in mantle olivine suggests that late stage metasomatism and kinetic fractionation associated with diffusion are unlikely to induce large Mg isotopic heterogeneity in the mantle, but this mechanism should be more precisely investigated. Second, the introduction of fractionated Mg from the ocean. We find the Mg isotope composition of seawater to be constant, with a δ²⁶Mg = -0.82±0.10 ‰ relative to the DSM3 standard, about 0.5‰ lighter than chondrite. The re-incorporation of Mg by the formation of serpentine occurs in the top 10% of the oceanic crust. In addition, dolomites are even further depleted in heavy isotopes than seawater. Considering the modern production rate of oceanic crust and the Mg isotopic oceanic budget (e.g. Tipper et al., doi:10.1016/j.epsl.2006.07.037), an anomaly of up to -0.4‰ in δ²⁶Mg is carried by the top of the slab. Since magmatic differentiation has been ruled out as a process introducing Mg isotopic fractionation (Teng et al., doi:10.1016/j.epsl.2007.06.004) and that the range in Mg isotopes in chondritic material is small, the recycling of the oceanic crust and overlying sediment is likely to be the dominant mechanism responsible for variation in the mantle.
DE: 1030 Geochemical cycles (0330)
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
SC: Union [U]
MN: 2007 Fall Meeting