[S-CG09_28PM1] Deep Carbon Cycle

[SCG09-06] Chemical composition of nano-inclusions in supe-deep diamonds and implications to the growth condition

*鍵 裕之¹、石橋 秀巳²、大藤 弘明³、Zedgenizov Dmitry⁴、Shatsky Vladislav⁴、Ragozin Alex⁴ (1.東京大学大学院理学系研究科、2.静岡大学大学院理学研究科、3.愛媛大学地球深部ダイナミクス研究センター、4.ロシア科学アカデミー)

キーワード：diamond, nano inclusion, X-ray fluorescence analysis, super deep diamonds

Superdeep diamonds originating from the mantle transition zone and the lower mantle were found from alluvial deposits of Sao-Luis river (Juina, Brazil). We investigated carbon isotopic variations and chemical compositions of nano-inclusions in the superdeep diamonds which can give a clue for the growth condition.The dominant inclusions in diamonds from studied here are CaSi-perovskite and AlSi-phases. MgSi- and CaTi-perovskites, ferropericlase, native iron, coesite and zircon have also been found. We found syngenetic inclusions of superdeep paragenesis from 59 diamond samples from Sao-Luis. Our SIMS analysis showed the wide variations of carbon isotopic compositions ranging from 2.7 to -25.3 ‰ in δ¹³C. Some samples contained microinclusions and FTIR analyses showed that water and carbonates are not major components of diamond-forming fluids. To identify the microinclusions, TEM observations were carried out on a foil of carbonado (0.1 micron thick) made from a polished diamond specimen after Au-coating. The foil was fabricated with a Ga ion beam using a focused ion beam (FIB) instrument (JEOL JEM-9310FIB). The foil was observed with a TEM (JEOL JEM-2010) under an accelerating voltage of 200 kV. We first found out euhedral inclusions of several tens nanometers in size. At present, the chemical composition or mineral species of these nano-inclusions are not clarified.Nanometer-sized inclusions were found in sevearal samples. TEM observation revealed that the nano-inclusions have a negative cystal shape suggesting the syngenetic origin directly related to the diamond growth. Chemical composition obtained from synchrotron X-ray fluoresncence analysis clarified that the nano-inclusions contain S, Cr, Mn, Fe, Co, Ni, Cu Zn, and so on. The present study suggests that the growth media of the superdeep diamonds are composed of sufilde melt.