Mantle chronology using Zircon grains

第一年，我们在西班牙南部的隆达橄榄岩区进行了为期四周的野外工作。采集了近500个岩石样品，详细观察了橄榄岩中辉石岩的产状。第二年，我们访问了南非和博茨瓦纳的钻石矿，以收集地幔结核。博茨瓦纳以世界上最好的钻石矿而闻名，但这些矿长期以来一直对地质学家关闭。在日本大使馆的努力下，我们幸运地获准在该国目前运营的三个矿山中采集岩石。我们在这样一个钻石矿山中使用锆石颗粒进行了测年。由广岛大学的佐野教授小组使用SHRIPM进行分析。对12个样品的U/Pb年龄测定结果为77 Ma。对采自博茨瓦纳矿区的橄榄岩样品进行了显微镜观察和微探针分析。结果表明，部分样品具有强烈的形变，可能是大气层的产物。用岩石学温压计测定了大陆下地幔的地温，并进行了高压实验，测定了高压下橄榄岩和玄武岩的矿物化学成分，压力高达27 GPa。这些结果对研究来自地球内部的结核的成因有一定的参考价值。

In the first year, we made a field work at Ronda peridotite massif in southern Spain for four weeks. About five hundred rock samples were collected, and the occurrence of pyroxenites in peridotite was observed in details. In the following year, we visited diamond mines in South Africa and Botswana in order to collect mantle nodules. Botswana is famous for world best diamond mines, but the mines have long been closed to geologists. According to the efforts by Japanese embassy, we were fortunately permitted to collect rocks in all of three mines currently operated in this country.We have done dating using zircon grains in such a diamond mine. Analyses were made by the group of Prof. Sano at Hiroshima University using SHRIPM. The U/Pb age of 77 Ma was obtained from the analyses of 12 grains. This age likely indicates the kimberite eruption age.Microscope observations as well as microprobe analyses were made on peridotite samples from mines in Botswana. Results show that part of the samples were highly deformed, which are possibly asenosphere origin. Using petrological thermobarometer, we determined the mantle geotherm beneath continent.We also made high-pressure experiments and determined the mineral chemistry in both peridotitic and basaltic compositions up to 27 GPa. These results could be a good reference to investigate the origins of nodules coming from the interior of the Earth.