Survival of majoritic garnet in diamond by direct kimberlite ascent from deep mantle
Survival of majoritic garnet in diamond by direct kimberlite ascent from deep mantle
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DOI：
10.1029/2010gl042706
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发表时间：
2010-05
影响因子：
5.2
通讯作者：
M. Nishi;T. Kubo;Takumi Kato;A. Tominaga;A. Shimojuku;N. Doi;K. Funakoshi;Y. Higo
中科院分区：
文献类型：
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作者：
M. Nishi;T. Kubo;Takumi Kato;A. Tominaga;A. Shimojuku;N. Doi;K. Funakoshi;Y. Higo
在多个地区发现的金刚石包裹体状的石榴子石，起源于上地幔深部或地幔过渡带。其中一些显示了在向地球表面运输过程中部分转化为低压相的火成石榴石和单斜辉石的证据。部分转化的程度可以作为衡量金刚石在整个上地幔中平均上升速率的一个速度计。在这里，我们提出的实验结果的回转变动力学的贵橄榄石石榴石到火石石榴石和单斜辉石的时间分辨原位同步辐射X射线衍射测量。我们观察到，在6.5-7.5 GPa和1020-1300°C下，通过晶界成核和扩散控制生长发生反向转变。其转化机制与天然金刚石中观察到的相似。从获得的动力学数据确定扩散控制生长速率的温度依赖性，其用于约束金刚石中的多晶石榴石的存活时间。我们发现，正常地幔对流中的上升速度太慢，不足以满足多晶石榴石的生存时限，表明这类金刚石是由快速的金伯利岩岩浆在几天内直接从地幔深部搬运过来的。
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.