Meeting room "Glova", 4 floor, Integrated Research Building
Abstract
Majoritic garnet is a stable mineral in the Earth's deep mantle at depth of 400-700 km, which is discovered as diamond inclusions in several localities. The presence of majoritic garnet in diamonds indicates that they have been formed in such extremely deep regions. Majoritic garnet should transform to low-pressure phases during transportation to the Earth's surface because it is unstable at depth shallower than 400 km. Therefore, diamonds containing majoritic garnet should have been transported quickly before the majoritic garnet inclusions were converted to other low-pressure phases. Some of them show evidence of partial transformation to its low-pressure phases of pyropic garnet and clinopyroxene during their transportation to the Earth's surface. A degree of partial transformation can be used potentially as a unique speedometer on the delivery of diamond through the entire upper mantle. Here we present the 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. The transformation mechanisms are similar with those observed in the natural diamond. The temperature dependence of the transformation rate was determined from the kinetic data obtained, which was used to constrain the survival time of majoritic garnet in diamond. We found that the velocity of the typical convective mantle flow (1-20 cm/year) is far too slow to satisfy the time limitation for the survival of majoritic garnet, indicating that such diamonds with majoritic garnet inclusions have been transported directly from the deep mantle by the rapid movement of the kimberlite magma.
For inquiry：Taku Tsuchiya TEL:（089）927-8198
E-mail takut@sci.ehime-u.ac.jp
