Magnesium isotopic composition of altered oceanic crust and the global Mg cycle. (1st October 2018)

• Record Type: Journal Article
• Authors: Huang, Kang-Jun; Teng, Fang-Zhen; Plank, Terry; Staudigel, Hubert; Hu, Yan; Bao, Zheng-Yu
• Is Part Of: Geochimica et cosmochimica acta. Volume 238(2018)
• Page Start: 357
• Page End: 373
• Publication Date: 2018-10-01
• Subjects: Magnesium isotopes -- Oceanic crust -- Low-temperature alteration -- Mg cycle -- Mantle heterogeneity; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2018.07.011
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: To investigate the behavior of Mg isotopes during low-temperature alteration of oceanic crust and to further understand its role in the global Mg cycle, we measured the Mg isotopic compositions ( 25 Mg/ 24 Mg and 26 Mg/ 24 Mg) of a set of samples of altered oceanic crust (AOC) recovered from the Ocean Drilling Program Hole 801C, the reference site for old crust (∼170 Ma) subducting in the Pacific. The measured δ 26 Mg values range from −1.70‰ to 0.21‰, deviating from that of pristine oceanic basalts (−0.25 ± 0.07‰). Composite samples of volcanoclastic breccia that have experienced relatively intense alteration have larger variation in δ 26 Mg values (−1.01‰ to 0.15‰) than composite samples of massive basaltic flows (−0.53‰ to −0.04‰), indicating significant Mg isotope fractionation during low-temperature alteration of the oceanic crust. Moreover, the upper off-axis basement has on average lower δ 26 Mg values (−1.70‰ to −0.04‰) than the lower on-axis basement (−0.16‰ to 0.21‰). These findings, combined with the co-variations between MgO content and FeO ∗ /CaO ratio and between δ 26 Mg and FeO ∗ /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 δ 26 Mg of AOC. Early formation of saponite under anoxic condition preferentially takes up heavy Mg isotopes and accounts for Mg enrichment and relatively high δ 26 Mg in the on-axis basement. Subsequent precipitation of carbonates results in the dilution of Mg and relatively low δ 26 Mg in the off-axis basement. In addition, accumulation of carbonate-rich interflow sediments in the upper basement may contribute further to the low δ 26 Mg. A weighted average δ 26 Mg value of 0.00 ± 0.09‰ 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 ∼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 δ 26 Mg along with overlying marine sediments into the mantle through subduction may generate Mg isotopic heterogeneity in the mantle at small scales.