Tsutomu Ota et al. (2008) Boron cycling by subducted lithosphere; insights from diamondiferous tourmaline from the Kokchetav ultrahigh-pressure metamorphic belt, Geochimica et Cosmochimica Acta, 72, 3531–3541, doi: 10.1016/j.gca.2008.05.002. < 20120712092835-962-785 > pub

Map on Earth or stone. Places and analyses are shown.

  • abstract:

    Subduction of lithosphere, involving surficial materials, into the deep mantle is fundamental to the chemical evolution of the Earth. However, the chemical evolution of the lithosphere during subduction to depth remains equivocal. In order to identify materials subjected to geological processes near the surface and at depths in subduction zones, we examined B and Li isotopes behavior in a unique diamondiferous, K-rich tourmaline (K-tourmaline) from the Kokchetav ultrahigh-pressure metamorphic belt. The K-tourmaline, which includes microdiamonds in its core, is enriched in 11B relative to 10B (δ11B = −1.2 to +7.7) and 7Li relative to 6Li (δ7Li = −1.1 to +3.1). It is suggested that the K-tourmaline crystallized at high-pressure in the diamond stability field from a silicate melt generated at high-pressure and temperature conditions of the Kokchetav peak metamorphism. The heavy isotope signature of this K-tourmaline differs from that of ordinary Na-tourmalines in crustal rocks, enriched in the light B isotope (δ11B = −16.6 to −2.3), which experienced isotope fractionation through metamorphic dehydration reactions. A possible source of the heavy B-isotope signature is serpentine in the subducted lithospheric mantle. Serpentinization of the lithospheric mantle, with enrichment of heavy B-isotope, can be produced by normal faulting at trench-outer rise or trench slope regions, followed by penetration of seawater into the lithospheric mantle. Serpentine breakdown in the lithospheric mantle subducted in subarc regions likely provided fluids with the heavy B-isotope signature, which was acquired during the serpentinization prior to subduction. The fluids could ascend and cause partial melting of the overlying crustal layer, and the resultant silicate melt could inherit the heavy B-isotope signature. The subducting lithospheric mantle is a key repository for modeling the flux of fluids and associated elements acquired at a near the surface into the deep mantle.

  • doi: 10.1016/j.gca.2008.05.002
  • modified at 16 h ago
62ede20 thumb A6-ts < 20120712090232-360-303 >pub
  • classification: metamorphic
  • physical-form: thin section
  • status: (unknown)
  • description: Thin section of a diamondiferous rock (sample name, A6) from Kokchetav UHPM belt, Kazakhstan
  • modified at 2019-12-05
Img 5000 thumb T225 < 20120106094514-584-723 >pub
  • classification: metamorphic
  • physical-form: thin section
  • status: (unknown)
  • modified at 2019-12-05
Img 5003 thumb T40 < 20120106094532-340-329 >pub
  • classification: metamorphic
  • physical-form: thin section
  • status: (unknown)
  • modified at 2019-12-05
Img 5008 thumb K111R < 20120106094610-146-183 >pub
  • classification: metamorphic
  • physical-form: thin section
  • status: (unknown)
  • modified at 2019-12-05
Img 5005 thumb A377 < 20120106094552-402-121 >pub
  • classification: metamorphic
  • physical-form: thin section
  • status: (unknown)
  • modified at 2019-12-05
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caption ID assembly stone
Table 1. Representative analyses of the diamond-grade K-tourmaline and lower-grade Na-tourmalinesa from the Kokchetav belt pub 20191205174625-498419 major-tour (Ota et al., 2008) 32
manual
  • name
  • Tsutomu Ota
  • Katsura Kobayashi
  • Takuya Kunihiro
  • Eizo Nakamura