Magnesium isotopic composition of the oceanic mantle and oceanic Mg cycling

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Liu, Ping-Ping, Teng, Fang-Zhen, Dick, Henry J. B., Zhou, Mei-Fu, Chung, Sun-Lin
• Other Authors: Liu, PP (reprint author), Acad Sinica, Inst Earth Sci, 128 Sec 2,Acad Rd, Taipei 11529, Taiwan., Peking Univ, Sch Earth & Space Sci, Key Lab Orogen Belts & Crustal Evolut, Beijing 100871, Peoples R China., Chinese Acad Sci, Inst Geol & Geophys, State Key Lab Lithospher Evolut, Beijing 100029, Peoples R China., Acad Sinica, Inst Earth Sci, 128 Sec 2,Acad Rd, Taipei 11529, Taiwan., Univ Washington, Dept Earth & Space Sci, Isotope Lab, Seattle, WA 98195 USA., Woods Hole Oceanog Inst, Dept Geol & Geophys, Woods Hole, MA 02543 USA., Univ Hong Kong, Dept Earth Sci, Pokfulam Rd, Hong Kong, Hong Kong, Peoples R China.
• Format: Journal/Newspaper
• Language: English
• Published: GEOCHIMICA ET COSMOCHIMICA ACTA 2017
• Subjects: Abyssal peridotite; Magnesium isotope; Mantle heterogeneity; Magnesium cycling; Seafloor alteration; NORTH CHINA CRATON; PLASMA-MASS SPECTROMETRY; MID-ATLANTIC RIDGE; PERIDOTITE XENOLITHS; ABYSSAL PERIDOTITES; CARBONATITE METASOMATISM; GAKKEL RIDGE; ARCTIC-OCEAN; THIN CRUST; FRACTIONATION; Arctic; Arctic Ocean; Gakkel Ridge; Indian; Mid-Atlantic Ridge
• Online Access: https://hdl.handle.net/20.500.11897/473106; https://doi.org/10.1016/j.gca.2017.02.016

To constrain the Mg isotopic composition of the oceanic mantle, investigate Mg isotope fractionation of abyssal peridotites during seafloor alteration, and assess Mg budget in the oceans, a suite of 32 abyssal peridotite samples from the Gakkel Ridge and Southwest Indian Ridge (SWIR) was, for the first time, selected for high-precision Mg isotope analyses. Although most of these samples are extensively altered, largely by serpentinization and weathering, primary olivine, diopside and enstatite grains are preserved in some samples. Olivine grains from the least altered samples have delta Mg-26 varying from -0.30 to -0.12 parts per thousand (n = 7), whereas enstatite and diopside have delta Mg-26 varying from -0.27 to -0.16% (n = 7), and from -0.23 to -0.09 parts per thousand (n = 6), respectively. Whole-rock delta Mg-26 values range from -0.24 to 0.03 parts per thousand with an average of -0.12 +/- 0.13% (2SD, n = 32). Strongly serpentinized peridotites have lower average delta Mg-26 values (delta Mg-26 = -0.19 +/- 0.07 parts per thousand, 2SD, n = 7) than weathering-dominated ones (delta Mg-26 = -0.10 +/- 0.12 parts per thousand, 2SD, n = 25). Calculated Mg isotopic compositions of fresh mantle peridotites vary from -0.29 to -0.13 parts per thousand, beyond the previously reported range of the subcontinental lithospheric mantle (-0.25 +/- 0.04 parts per thousand) and the analytical uncertainty (+/- 0.07 parts per thousand, 2SD). Our study therefore indicates that the oceanic mantle may have similar but slightly heterogeneous Mg isotopic compositions to that of subcontinental lithospheric mantle. Secondary serpentinization does not fractionate Mg isotopes of abyssal peridotites, whereas low-T weathering and formation of clay can result in the enrichment of heavy Mg isotopes in abyssal peridotites. This study also demonstrates that fluid-rock interaction does not necessarily produce rocks with intermediate Mg isotopic compositions. Magnesium isotopes of the rocks thereafter are dependent on the secondary minerals formed. We also conclude that the release of light Mg isotopes into the ocean during alteration of abyssal peridotites can be an important influx of Mg for the seawater Mg budget. Abyssal peridotites with a heavy Mg isotopic signature can be recycled into the mantle in subduction zones and may thus result in heterogeneous Mg isotopic compositions of the oceanic mantle and heavy Mg isotopic compositions of arc magmas. (C) 2017 Elsevier Ltd. All rights reserved. National Science Foundation of China [41473038, 41503010]; China Postdoctoral Science Foundation [2015M570145]; National Science Foundation [EAR-1056713, EAR-1340160]; US National Science Foundation [OCE-1434452]; [MOST104-2745-M-002-001-ASP] SCI(E) ARTICLE 151-165 206