河南鹤壁幔源捕虏体中橄榄石的矿物谱学特征

Mineralogical and Spectroscopic Characteristics of Olivine in Mantle- Derived Xenoliths from Hebi, Henan Province, China

  • 摘要: 以河南鹤壁市上峪火山口尖晶石方辉橄榄岩捕虏体中的橄榄石为研究对象,采用偏光显微镜观察、红外光谱、激光拉曼光谱和电子探针等测试方法,分析了其谱学、包裹体与成分特征。结果表明,鹤壁橄榄石样品呈淡黄绿色-黄绿色,玻璃光泽,透明至半透明;橄榄石具扭折带结构,属镁橄榄石,与顽辉石、透辉石和铬尖晶石共生;主量元素组成较均匀,MgO质量分数为49.80%~51.08%,SiO2为41.01%~41.97%,FeO为7.14%~8.07%;其较高的Mg#值(91.76~92.70)指示鹤壁岩石圈地幔为难熔型及局部过渡型,橄榄石-尖晶石稳定温度为1 110~1 122 ℃。与吉林、河北产地橄榄石相比,鹤壁橄榄石Mg#值最高,暗示其形成于更古老的难熔地幔环境;其Mg#值落入优质宝石级橄榄石的成分区间,表明其具备宝石潜力;但受地幔深处的高温塑性变形及后期构造挤压作用,晶体破碎、裂隙发育及透明度降低,未能达到宝石级。

     

    Abstract: This study focuses on olivine in spinel harzburgite xenoliths from the Shangyu volcanic cone in Hebi, Henan Province, China. Using polarizing microscopy, infrared spectroscopy, laser Raman spectroscopy, and electron probe microanalysis, the spectroscopic, inclusion, and compositional characteristics of the olivine samples from Hebi were systematically investigated. The results show that the olivine samples are light yellowish-green to yellowish-green in colour, with a vitreous luster, and are transparent to translucent. The olivine exhibits kink band structures and is identified as forsterite, coexisting with enstatite, diopside, and Cr-spinel. The major element compositions of the olivine samples are relatively homogeneous, with MgO contents ranging from 49.80% to 51.08%, SiO2 from 41.01% to 41.97%, and FeO from 7.14% to 8.07%. The high Mg# values (91.76-92.70) indicate that the lithospheric mantle beneath the Hebi area is predominantly refractory with minor transitional characteristics, with olivine-spinel equilibrium temperatures ranging from 1 110 ℃ to 1 122 ℃. Comparison with olivines from Jilin and Hebei provinces, the olivine from Hebi shows the highest Mg# values, suggesting its formation in a more ancient and refractory mantle environment. The Mg# values of Hebi olivine fall well within the compositional range of high-quality gem-grade olivine, indicating its potential as a gem material. However, the primary reasons it has not reached gem-grade are high-temperature plastic deformation within the deep mantle and subsequent tectonic compression during rapid ascent, which have resulted in crystal fragmentation, extensive fissure development, and reduced transparency.

     

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