Magnesium isotopic composition of the oceanic mantle and oceanic Mg cycling
Dick, Henry J. B.
MetadataShow full item record
KeywordAbyssal peridotite; Magnesium isotope; Mantle heterogeneity; Magnesium cycling; Seafloor alteration
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 δ26Mg varying from −0.30 to −0.12‰ (n = 7), whereas enstatite and diopside have δ26Mg varying from −0.27 to −0.16‰ (n = 7), and from −0.23 to −0.09‰ (n = 6), respectively. Whole-rock δ26Mg values range from −0.24 to 0.03‰ with an average of −0.12 ± 0.13‰ (2SD, n = 32). Strongly serpentinized peridotites have lower average δ26Mg values (δ26Mg = −0.19 ± 0.07‰, 2SD, n = 7) than weathering-dominated ones (δ26Mg = −0.10 ± 0.12‰, 2SD, n = 25). Calculated Mg isotopic compositions of fresh mantle peridotites vary from −0.29 to −0.13‰, beyond the previously reported range of the subcontinental lithospheric mantle (−0.25 ± 0.04‰) and the analytical uncertainty (±0.07‰, 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.
© The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 206 (2017): 151-165, doi:10.1016/j.gca.2017.02.016.
Suggested CitationPreprint: Liu, Ping-Ping, Teng, Fang-Zhen, Dick, Henry J. B., Zhou, Mei-Fu, Chung, Sun-Lin, "Magnesium isotopic composition of the oceanic mantle and oceanic Mg cycling", 2017-02, https://doi.org/10.1016/j.gca.2017.02.016, https://hdl.handle.net/1912/9088
Showing items related by title, author, creator and subject.
Marschall, Horst R.; Wanless, V. Dorsey; Shimizu, Nobumichi; Pogge von Strandmann, Philip; Elliott, Tim; Monteleone, Brian D. (2017-03-17)A global selection of 56 mid-ocean ridge basalt (MORB) glasses were analysed for Li and B abundances and isotopic compositions. Analytical accuracy and precision of analyses constitute an improvement over previously published ...
Helium isotope geochemistry of oceanic volcanic rocks : implications for mantle heterogeneity and degassing Kurz, Mark D. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1982-06)The concentrations and isotopic compositions of helium have been measured in a number of mantle derived oceanic basalts. The goal of this research is to use the helium isotopic systematics to constrain the nature and ...
Examination of precipitation chemistry and improvements in precision using the Mg(OH)2 preconcentration inductively coupled plasma mass spectrometry (ICP-MS) method for high-throughput analysis of open-ocean Fe and Mn in seawater Saito, Mak A.; Schneider, David L. (2006-02-14)The chemistry of magnesium precipitation preconcentration of Fe, Mn and Co from seawater was investigated, and this analytical technique was adapted for use with the Element-2 inductively coupled plasma mass spectrometer ...