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Survival of majoritic garnet in diamond by direct kimberlite ascent from deep mantle
Author(s) -
Nishi Masayuki,
Kubo Tomoaki,
Kato Takumi,
Tominaga Aiko,
Shimojuku Akira,
Doi Naoko,
Funakoshi Kenichi,
Higo Yuji
Publication year - 2010
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2010gl042706
Subject(s) - kimberlite , mantle (geology) , geology , diamond , transition zone , nucleation , mantle convection , mineralogy , geochemistry , materials science , thermodynamics , paleontology , physics , tectonics , subduction , composite material
Majoritic garnets discovered as diamond inclusion in several localities originated from the deep upper mantle or the mantle transition zone. Some of them show evidence of partial transformation to low‐pressure phases of pyropic garnet and clinopyroxene during transportation to the Earth's surface. The degree of partial transformation can be used potentially as a unique speedometer on the average ascent rate of diamond through the entire upper mantle. Here we present experimental results on the back transformation kinetics of majoritic garnet into pyropic garnet and clinopyroxene by time‐resolved in‐situ synchrotron X‐ray diffraction measurements. We observed that back transformation occurs by grain‐boundary nucleation and diffusion‐controlled growth at 6.5–7.5 GPa and 1020–1300°C. The transformation mechanisms are similar to those observed in natural diamond. The temperature dependence of diffusion‐controlled growth rates were determined from the kinetic data obtained, which was used to constrain the survival time of majoritic garnet in diamond. We found that the ascending velocity in the normal mantle convection is far too slow to satisfy the time limitation for the survival of majoritic garnet, indicating that such diamonds have been transported directly from the deep mantle by the rapid kimberlite magma within a few days.