|Title||Magnesium isotopic composition of altered oceanic crust and the global Mg cycle|
|Publication Type||Journal Article|
|Year of Publication||2018|
|Authors||Huang K.J, Teng F.Z, Plank T., Staudigel H., Hu Y., Bao Z.Y|
|Journal||Geochimica Et Cosmochimica Acta|
|Type of Article||Article|
|Keywords||continental basalts; fast-spreading ridge; geochemical cycles; Geochemistry & Geophysics; Hydrothermal alteration; Low-temperature alteration; Magnesium isotopes; mantle heterogeneity; marine-sediments; Mg cycle; mineral formation; oceanic crust; odp site-801; peridotite xenoliths; plasma-mass spectrometry; seawater chemistry|
To investigate the behavior of Mg isotopes during low-temperature alteration of oceanic crust and to further under- stand its role in the global Mg cycle, we measured the Mg isotopic compositions (Mg-25/Mg-24 and Mg-26/Mg-24) of a set of samples of altered oceanic crust (AOC) recovered from the Ocean Drilling Program Hole 801C, the reference site for old crust (similar to 170 Ma) subducting in the Pacific. The measured delta Mg-26 values range from -1.70 parts per thousand to 0.21 parts per thousand, deviating from that of pristine oceanic basalts (-0.25 +/- 0.07 parts per thousand). Composite samples of volcanoclastic breccia that have experienced relatively intense alteration have larger variation in delta Mg-26 values (-1.01 parts per thousand to 0.15 parts per thousand) than composite samples of massive basaltic flows (-0.53 parts per thousand to -0.04 parts per thousand), indicating significant Mg isotope fractionation during low-temperature alteration of the oceanic crust. Moreover, the upper off-axis basement has on average lower delta Mg-26 values (-1.70 parts per thousand to -0.04 parts per thousand) than the lower on-axis basement (-0.16 parts per thousand to 0.21 parts per thousand). These findings, combined with the co-variations between MgO content and Fe O-x/CaO ratio and between delta Mg-26 and FeOx/CaO ratio, suggest that formation of Mg-bearing minerals (i.e., saponite and calcite) during low-temperature alteration of the oceanic crust accounts for the highly variable delta Mg-26 of AOC. Early formation of saponite under anoxic condition preferentially takes up heavy Mg isotopes and accounts for Mg enrichment and relatively high delta Mg-26 in the on-axis basement. Subsequent precipitation of carbonates results in the dilution of Mg and relatively low delta Mg-26 in the off-axis basement. In addition, accumulation of carbonate-rich interflow sediments in the upper basement may contribute further to the low delta Mg-26. A weighted average delta Mg-26 value of 0.00 +/- 0.09 parts per thousand is estimated for the AOC at Site 801, implying that low-temperature alteration of oceanic crust drives the ocean to a lighter Mg isotopic composition, and thus requires additional carbonate precipitation to maintain a steady-state Mg isotopic composition of seawater. A mass balance calculation suggests that the Mg output flux due to low-temperature alteration of the oceanic crust equals similar to 12% of the annual Mg riverine input, indicating that AOC is a significant sink of Mg in seawater. Our study further highlights that recycling of AOC with highly variable delta Mg-26 m along with overlying marine sediments into the mantle through subduction may generate Mg isotopic heterogeneity in the mantle at small scales. (C) 2018 Elsevier Ltd. All rights reserved.