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Rutile-bearing Eclogite: One of the Possible Reservoirs Balancing the Nb-depleted Silicate Earth

Liang Jinlong,Shi Zeming, Xu Jinyong,Gao Ying   

  1. Department of Geochemistry, Chengdu University of Technology, Chengdu 610059, China
  • Received:2011-12-17 Revised:2012-10-09 Online:2012-10-10 Published:2012-10-10

Liang Jinlong,Shi Zeming, Xu Jinyong,Gao Ying. Rutile-bearing Eclogite: One of the Possible Reservoirs Balancing the Nb-depleted Silicate Earth[J]. Advances in Earth Science, DOI: 10.11867/j.issn.1001-8166.2012.10.1094.

The mass imbalance of Nb-Ta in the silicate Earth is a significant geochemical issue which has long been concerned as a paradox. Weather is rutile-bearing eclogite the bridge of continental crust and the depleted mantle with regard to the Nb depletion,and how to explain the controversy between the oceanic basalt partial melting results (rutile favors Ta over Nb) and the naturally observed results (rutilebearing eclogites commonly have superchondritic Nb/Ta ratios)? The present work analyzed the trace elements of minerals in eclogites from the main hole of Chinese Continental Scientific Drilling Project (CCSD) and some outcrops nearby by LA-ICP-MS. The results showed that: (1) Rutile predominantly controls the distribution of Nb, Ta in eclogite, with extremely minor concentrations of the two elements in other minerals; (2) Significant fractionation between Nb and Ta presents in eclogites (Nb/Ta =5.3 ~96.2), with a overall superchondritic ratio (the mean Nb/Ta is 25.9); (3) The ratios of Nb/Ta in amphibole and phengite are unexpectedly high, 48.6 and 21.8 respectively, exhibiting stronger fractionation between Nb and Ta in the two minerals; (4) Both the concentrations of Nb, Ta and the ratios of Nb/Ta in the minerals such as garnet, omphacite, apatite, phengite, epidote, etc., in eclogite are extremely low. Based on these results, we argued it is likely that Ti-bearing minerals, such as pargasite and phengite fractionated Nb from Ta significantly before rutile occurring steadily, and the results of fractionation were fixed and inherited by rutile subsequently because of its absolutely control for Nb and Ta. Therefore, we propose that rutile-bearing eclogite be one of the reservoirs in deep mantle which can balance the depletion of Nb in the silicate Earth.

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