Calcium isotope systematics of altered oceanic crust at IODP site 1256: Insights into the hydrothermal alteration

• Published in: Lithos Vol. 438-439; p. 106994
• Main Authors: Hu, Ling-Zhi, Kang, Jin-Ting, Qi, Yu-Han, Gao, Yong-Jun, Nan, Xiao-Yun, Huang, Jian, Huang, Fang
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.2023
• Subjects: Altered oceanic crust; Calcium isotopes; Hydrothermal alteration; Subduction recycling; Subduction recycling; Calcium isotopes; Hydrothermal alteration; Altered oceanic crust
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2022.106994

To investigate the Ca isotope systematics in seafloor hydrothermal systems and further understand the deep Ca cycle, this study reports the Ca isotope compositions of altered oceanic crust (AOC) from Integrated Ocean Drilling Program (IODP) site 1256. These rocks were sampled from a drilling hole that penetrates from the top through a volcanic section, a lava-dike transition zone, a sheeted dike complex, and a plutonic section forming at a fast-spreading rate. Rocks in the volcanic section show Ca-depletion and indistinguishable Ca isotope compositions (δ44/40Ca = 0.81 ± 0.07‰, 2SD, n = 7) with the fresh MORB (0.85 ± 0.09‰), suggesting that the dissolution of primary silicates will not fractionate Ca isotope. Rocks in sheeted dike complex and plutonic section have more scattered δ44/40Ca (0.89 ± 0.14‰, 2SD, n = 12) and lower δ18O (3–4.6‰) than rocks in the volcanic section (δ18O = 4.6–8.6‰). Such a Ca-O isotope signature can not be explained by magmatic processes, primary silicates dissolution, secondary phases precipitation, or albitization. Alternatively, it may reflect the interaction between lower crusts and evolved fluids with heavy Ca and light oxygen isotope compositions. A simple mixing between fluids and crusts cannot explain the Ca and O isotope variations as manifested by the poorly defined δ18O-δ44/40Ca trend. We apply reactive-transport modeling and find that the Ca-O isotope signature may be controlled by the spatial effect during fluid-rock interaction. We estimate the average Ca isotope composition of the fast-spreading AOC to be 0.91 ± 0.24‰, which is slightly heavier but overlap the value of fresh MORB (0.85 ± 0.09‰). Our study suggests that the recycling of the fast-spreading AOC may not cause significant Ca isotope variations in the mantle. •Rocks in the volcanic section of AOC from IODP 1256 site have a MORB-like Ca isotope composition.•Rocks in sheeted dike complexes and plutonic sections show slightly heavier Ca isotope compositions.•The water-rock reaction may cause Ca isotope variations in AOC.•The average Ca isotope composition of fast-spreading AOC is estimated to be 0.91 ± 0.24‰.