中国辽宁金刚石中高硅钙铁榴石(Majorite)等超高压矿物包裹体的发现及地质意义

在中国辽宁金刚石中获得的高硅钙铁榴石(Majorite)为一单晶碎片包裹体,与其共存的金刚石包裹体还有刚玉、碳化钛、红色金刚石碎片、钙钛矿、二氧化硅等。通过电子探针成分分析,确定该高硅钙铁榴石(Majorite)成分超硅高钙缺镁,8个分析点平均值计算的矿物分子式为(Ca2.35Fe0.49Mn0.15Mg0.02)3.01(Al1.08Fe0.48Si0.44)2.00(SiO4)3,根据Kenneth等(2000)提出的计算压力的公式得到该包裹体高硅钙铁榴石(Majorite)形成的压力为14GPa,估算形成深度达400km。用四圆单晶衍射仪测定了该石榴石的晶体结构,a=1.195 15(4)nm,求得了各原子的座标、占位度和各向异性温度因子,用I>2σ(I),计算得到R1=0.077 9,WR2=0.141 6,Goodness-of-fit(F2)=1.382。在该高硅钙铁榴石包裹体中还存在微米级二氧化硅(呈四边形断面)和氧化铁(含钠)的包裹体(析离体),它们可能是斯石英和方铁矿(或似沂蒙矿)。从高硅钙铁榴石(Majorite)的成分判断,其物质来源具壳源性质,由此推断的大陆壳俯冲深度要超过400km,这与地球物理探测郯庐断裂已切穿了上地幔、进入软流圈的看法相一致。

Discovery and Geological Significance of Majorite Inclusion in Diamond from Liaoning Province,China

The high calcium silicate iron garnet(Majorite) in diamond from Liaoning of China is the inclusion of single crystal fragments with coexistence corundum,titanium carbide,red diamond fragments,perovskite and silica inclusions.The composition of the Majorite was displayed ultra-silicon high calcium and magnesium deficiency by electron probe microanalysis and its mineral formula based on 8 analysis points is(Ca2.35Fe0.49Mn0.15Mg0.02)3.01(Al1.08Fe0.48Si0.44) 2.00(SiO4)3.X-ray diffraction peaks is obtained from single crystal diffractometer demonstrated that the crystal structure of the Majorite is a=1.195 15(4) nm,and using I>2σ(I),got R1= 0.077 9,WR2=0.141 6,Goodness-of-fit(F2)=1.382.Micron silica and iron oxide exist in the Majorite inclusion and may be the Sri Lankan quartz and square iron ore.According to Kenneth proposing the calculation formula of pressure,the formation pressure of the Majorite was of 14 GPa and depth of 400 km.The presence of Majorite inclusion as ultra-high pressure mineral in diamond may be exemplified the view of geophysical Tanlu fault cut through the upper mantle into the asthenosphere.