MISC

2018年1月1日

Recycled ancient ghost carbonate in the Pitcairn mantle plume

Proceedings of the National Academy of Sciences of the United States of America
  • Xiao Jun Wang
  • ,
  • Li Hui Chen
  • ,
  • Albrecht W. Hofmann
  • ,
  • Takeshi Hanyu
  • ,
  • Hiroshi Kawabata
  • ,
  • Yuan Zhong
  • ,
  • Lie Wen Xie
  • ,
  • Jin Hua Shi
  • ,
  • Takashi Miyazaki
  • ,
  • Yuka Hirahara
  • ,
  • Yuka Hirahara
  • ,
  • Toshiro Takahashi
  • ,
  • Toshiro Takahashi
  • ,
  • Ryoko Senda
  • ,
  • Ryoko Senda
  • ,
  • Qing Chang
  • ,
  • Bogdan S. Vaglarov
  • ,
  • Jun Ichi Kimura

115
開始ページ
8682
終了ページ
8687
DOI
10.1073/pnas.1719570115

© National Academy of Sciences. All rights reserved. The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ26Mg values. We therefore infer that the subnormal δ26Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate-silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source.

リンク情報
DOI
https://doi.org/10.1073/pnas.1719570115
URL
https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=85053629458&origin=inward

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