Iron isotopic variations in basalts from oceanic crust due to low-temperature seawater alteration

Abstract

Iron (Fe) isotopic compositions of seafloor basalts can provide important insights into high-temperature processes in the upper mantle. However, low-temperature seawater alteration (i.e., seafloor weathering) may significantly affect the primary Fe isotopic compositions. In this paper, we report whole-rock Fe isotopes, major and trace elements, and SrNd isotopes, along with in situ elemental maps for slightly altered Site U1434 basalts collected on IODP Expedition 349 from the South China Sea. We use these data to investigate the influence of low-temperature seawater alteration on the Fe isotopic compositions of seafloor basalts. The basalts have δ57Fe values of +0.12‰ to +0.27‰. No correlation between δ57Fe values versus Sr or Nd isotopes that are source-sensitive and no correlation between δ57Fe values and La/Yb ratios that are melting-sensitive indicate the δ57Fe values have not been affected by mantle source heterogeneity and partial melting. The negative correlations of MgO versus CaO and Al2O3 contents suggest limited clinopyroxene and plagioclase fractionation during magmatic evolution. Possible influence of olivine fractionation on the Fe isotopic variations can further be ruled out by a positive correlation between MgO and SiO2 contents and a weak negative correlation between δ57Fe values and SiO2 contents because olivine fractionation can form basalts with increasing SiO2 and δ57Fe with decreasing MgO. The strong correlations between δ57Fe values and MgO, Al2O3, and TiO2 contents indicate the Fe isotopic variations were caused by low-temperature seawater alteration. This is also evident from in situ elemental mapping of representative Site U1434 basalt sample, which shows that low-temperature alteration was characterized by decreases in MgO and SiO2 contents, and increases in Al2O3 and TiO2 contents. We propose a two-stage alteration model to explain the Fe isotopic variations of the Site U1434 basalts. The alteration appeared to occur under oxidizing conditions: Fe2+-rich external fluid likely derived from the progressive alteration inside the oceanic crust was oxidized at the surface of the oceanic crust, which caused the alteration, Fe isotopic fractionation, and increase in δ57Fe values of the seafloor basalts. This study highlights how low-temperature seawater alteration under oxidizing conditions can modify the Fe isotopic compositions of slightly altered oceanic crust, with critical implications for understanding the Fe cycle in the oceans.