T253 Celebrating Dr. John W. Valley's Contributions to Isotope Geochemistry and Beyond, from the Hadean to the Holocene (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

MAGNESIUM ISOTOPIC EVIDENCE FOR ANCIENT SUBDUCTED OCEANIC CRUST IN LOMU-LIKE POTASSIUM-RICH VOLCANIC ROCKS

Paper number: T253-13
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

  • Yang Sun Presenting Author
    Institute of Earth Sciences, Academia Sinica, Taipei
  • Fangzhen Teng
    University of Washington, Seattle, WA
  • Ji-Feng Ying
    Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing
  • Ben-Xun Su
    Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing
  • Yan Hu
    University of Washington, Seattle, WA
  • Qi-Cheng Fan
    Institute of Geology, Chinese Earthquake Administration, Beijing
  • Xin-Hua Zhou
    Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing

Abstract

Continent-derived sediments which were recycled into the deep mantle through subduction event have been considered as a crucial component in the source region of potassium-rich volcanic rocks globally. However, little attention has been paid to the fate of the subducted oceanic crust. Whether the recycled oceanic crust also contributes to the genesis of potassium-rich magmas or not, and how it influences the source signatures remain unclear. Here, we present Mg isotope data for a set of well-characterized samples from Northeast China, including ultrapotassic lavas, potassic lavas, transitional lavas, and sodic lavas, with the goal of examining the role of subducted oceanic crust in the potassium-rich magmatism that has a LOMU-like source signature. These Cenozoic lavas overall are lighter in Mg isotopic compositions than the normal mantle (δ26Mg = -0.25 ± 0.07, [1]), and display considerable Mg isotopic variations, with δ26Mg ranging from -0.61 to -0.23. The covariation of δ26Mg with TiO2 in these rocks suggests their light Mg isotopic compositions were derived from recycled oceanic crust in the form of carbonated eclogite in the source region. The strong correlation between δ26Mg and (Gd/Yb)N as well as Sr-Pb isotopes also indicates a multicomponent and multistage origin of these rocks. Our studies here find, for the first time, the presence of recycled oceanic crust in the source region of LOMU-like potassium-rich magmas, indicating that Mg isotopes can be used as a tracer of carbonated eclogites in the magma source. These novel findings also provide a better understanding of the surficial carbon recycle at depth via subduction, and its return to surface through the volcanism.

[1] Teng et al. (2010) GCA 74, 4150-4166.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-304699

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