缅甸克钦邦琥珀与共生煤的成因联系:多谱学与地球化学证据

The Genetic Link Between Amber and Associated Coal from Kachin State, Myanmar: Evidence from Multiple Spectroscopy and Geochemistry

  • 摘要: 缅甸克钦邦胡康河谷琥珀与煤层共同产于细粒碎屑岩中,表现出密切共生的关系。尽管关于缅甸琥珀的形成年代、宝石学特征和地质背景等研究已十分丰富,但从共生关系视角的探讨仍较缺乏。本研究通过常规宝石学测试以及红外光谱、激光拉曼光谱、扫描电子显微镜和激光剥蚀电感耦合等离子体质谱仪等测试方法,获取了10件缅甸琥珀与煤共生样品详细的宝石学特征、谱学特征、表面形貌特征和成分特征等,以期揭示树脂体(琥珀前身)与植物体(煤前身)在元素选择性吸收和地质保存机制上的差异。结果表明,琥珀与煤共生样品呈盘状,具层状结构和“煤包琥珀”结构,表面发育白色脉状杂质矿物;琥珀呈浅黄色和红棕色,树脂光泽,相对密度为1.07;煤为黑色至褐黑色,抛光后呈沥青光泽,条痕褐红色,横截面具孔隙结构;显微放大观察显示,琥珀与煤之间的接触边界并非直线连续,而表现出溶蚀破碎状和非均匀的特征;红外光谱揭示了二者不同的有机成分特征:在官能团区(4 000~1 300 cm-1)与指纹区(1 300~600 cm-1)均存在特征吸收峰;拉曼光谱表明,缅甸琥珀与煤中存在杂质矿物方解石、重晶石与黄铁矿;扫描电子显微镜下,琥珀与煤接触区域呈现相互交织现象,越接近琥珀区域,煤有机质颗粒粒径越小;此外,在琥珀和煤基底上散布大量无机黄铁矿微晶集合体(3~5 μm),指示琥珀与煤形成于缺氧及稳定的沉积环境;LA-ICP-MS测试结果显示,煤中富含微量元素,其中U元素的富集与成岩过程中的有机质还原背景密切相关。不透明琥珀、煤与透明琥珀三者之间存在显著的微量元素差异——不透明琥珀和煤中的微量元素种类及含量远多于透明琥珀。不透明琥珀成分受外来细小矿物脉侵入和元素替代等影响,煤与透明琥珀间元素差异则主要归因于继承母体与后期成岩过程中地球化学配位机制的不同。

     

    Abstract: Amber and coal seams from Hukawng Valley, Kachin State, Myanmar, co-occur within fine-grained clastic rocks, exhibiting a close symbiotic relationship. Although research on the formation age, gemmological characteristics, and geological background of amber from Myanmar has been extensive, investigations from the perspective of this symbiotic relationship remain relatively scarce. This study employs conventional gemmological testing, Fourier-transform infrared (FTIR) spectroscopy, laser Raman spectroscopy, scanning electron microscopy (SEM), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to obtain detailed gemmological, spectroscopic, surface micromorphological, and compositional characteristics of amber-coal symbiotic samples, aiming to reveal the differential elemental selective absorption and geological preservation mechanisms between resinous bodies (amber precursors) and plant matter (coal precursors). The results demonstrate that the amber and coal coexisting sample is disk-shaped, with a layered structure and a "coal-enclosed amber" structure, and white vein-like impurity minerals are developed on the surface. The amber shows light yellow to reddish-brown colour with a resinous luster and a relative density of 1.07; the coal is black to brownish-black, exhibiting a pitchy luster after polishing, with a brownish-red streak, and displays a porous structure in cross-section. Microscopic observation reveals that the contact boundary between amber and coal is not linear and continuous, but rather exhibits corrosion- erosion fragmentation features and heterogeneous characteristics. FTIR spectroscopy reveals distinct organic compositional features: characteristic absorption peaks are present in both the functional group region (4 000-1 300 cm-1) and fingerprint region (1 300-600 cm-1). Raman spectroscopy indicates the presence of impurity minerals (calcite, barite and pyrite) in both amber and coal from Myanmar. Under SEM, the contact zone between amber and coal shows interpenetrating phenomena; organic particles in the coal become progressively smaller when approaching the amber region. Additionally, numerous inorganic pyrite microcrystalline aggregates (3-5 μm) are dispersed on both amber and coal substrates, indicating formation in an anoxic and stable depositional environment. LA-ICP-MS data demonstrate that the coal is enriched in trace elements, with uranium (U) enrichment closely associated with organic matter reduction conditions during diagenesis. Significant trace element differences exist among opaque amber, coal, and transparent amber—the variety and concentration of trace elements in opaque zones and coal far exceed those in transparent regions. The opaque amber was affected by intrusion of exotic fine mineral veins and elemental substitution, and the distinction between the elements of coal and transparent amber was mainly attributed to differences in inherited parent material and geochemical coordination mechanisms during subsequent diagenetic processes.

     

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