Carbon Isotopic Composition of Graphite Inclusion in Black Nephrite Jade from Xinjiang
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摘要:
新疆地区是中国最重要的和田玉成矿区带，赋存大量优质原生和次生和田玉矿床。其中，新疆和田墨玉可以划分为两种典型类型，一种以阳起石为主要矿物，另一种则主要以透闪石和石墨为主要矿物，后者颜色通常由石墨包裹体导致。众所周知，石墨在有机和无机碳循环过程中发挥重要作用，对科学揭示地球深部碳循环具有重要科学指示意义，而和田墨玉中石墨包裹体则是研究新疆地区复杂背景下碳聚集、固定及循环的有利对象。但以往的研究重点关注了和田墨玉成因类型、形成机制等方面，针对石墨包裹体的碳质来源，尚未被厘定，不利于和田玉成矿规律和构造演化的完整科学认识。因此，本研究采用显微岩相学、电子探针、碳稳定同位素等技术手段，对新疆和田墨玉样品的矿物组成、石墨包裹体碳同位素特征进行了系统研究。结果表明，新疆和田墨玉的主要矿物组成为透闪石，副矿物或次要矿物组成为石墨、方解石、透辉石和磷灰石等；石墨碳稳定同位素揭示，新疆和田墨玉中石墨包裹体的δ13CPDB处于-21.3‰~-8.5‰，平均值为-13.76‰，具有相对较重的碳同位素组成，指示石墨碳质来源具有有机和无机双重成因。结合区域地质环境，我们认为在古生代中酸性岩浆侵入到前寒武纪白云石大理岩过程中携带幔源重同位素碳，导致石墨和透闪石共同结晶或沉淀。本文研究为和田玉矿床成矿过程及地球深部碳循环方面提供独特视角。
Abstract:Xinjiang is the most important metallogenic belt of nephrite jade in China, with a large number of high quality primary and secondary nephrite jade deposits. Two typical types of black nephrite jade from Xinjiang are classified. One is mainly composed of actinolite, and the other one is mainly composed of tremolite and graphite with its colour generally caused by graphite inclusions. It is well known that graphite plays an important role in the processes of organic and inorganic carbon cycle, which exhibits important scientific significance for the deep carbon cycle of the Earth, and the graphite inclusion is acted as a favorable object for the study of carbon accumulation, fixation and cycling under the complex background of Xinjiang. However, previous studies mainly focused on the genetic types and formation mechanisms of nephrite jade, and the carbon sources of graphite inclusions have not been determined, which is not conducive to the complete scientific understanding of the metallogenic regularity and tectonic evolution of nephrite jade. In this study, the mineral components and carbon isotope characteristics of graphite inclusions in black nephrites from Xinjiang were studied by means of microscopic petrography, electron probe microscopy, and carbon stable isotopes. The results show that the main mineral component of black nephrite jade is tremolite, and the accessory or secondary minerals are graphite, calcite, diopside, and apatite, etc. The carbon stable isotopes in graphite reveal that the δ13CPDB values of graphite inclusions in black nephrite jade range from -21.3‰ to -8.5‰, with the average value -13.76‰. The δ13CPDB of graphite inclusions exhibits relatively heavy carbon isotopic compositions, indicating that the carbon sources of graphite are both organic and inorganic. Combined with the regional geological environments, we infer that the relative heavy isotopic carbon derived from the mantle source was carried during the processes of intrusion of the Paleozoic intermediate-felsic magma into the Precambrian dolomite marble, leading to the crystallization or precipitation of graphite and tremolite. This study provides a new perspective for the ore-forming processes of the nephrite jade deposits and deep carbon cycle of the Earth.
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中国玉文化具有十分悠久的历史。不管是“林有朴樕，野有死鹿，白茅纯束，有女如玉”还是“君子温润如玉”，均可以看出玉在中国人心中美好且重要的地位。和田玉作为中国传统名玉之一，自开发利用以来便被广泛应用于各类家具、器皿等，更是亚洲大中华文化圈中备受追捧的玉石品种之一，深受中国人民的青睐[1]。
目前，国内外针对和田玉的研究产出了较为丰硕的成果，主要集中在致色机理、产地判别、成因类型及成矿机制等方面[2-7]，但对其具体成矿过程的研究仍然存在较大争议[8]。鉴于此，本文以和田墨玉中石墨包裹体为主要研究对象，详细表征石墨中C同位素特征，精细刻画石墨的成因机制，从而进一步明确和田玉成因模式，对深入了解玉石构造演化历史具有重大指示意义。
1. 地质背景与取样
1.1 区域地质背景
中国新疆和田玉主要产自新疆南部西昆仑-阿尔金山和田玉矿带，该地区矿床分布呈现出NW-EW-NE向线状构造，自塔身库尔干一直延伸至若羌县，在长达1 300 km的山脉中有大量和田玉原生矿产出，由西向东可划分为莎车-叶城、和田-于田、且末-若羌3个典型成矿带，区域内河流众多，是中国和田玉最为重要的成矿带。除此之外，玉龙喀什河(白玉河)与喀拉喀什河(墨玉河)产出高品质和田玉籽料，是著名的砂矿分布区[9-11]。
1.2 样品描述
本文研究手标本主要为和田墨玉籽料，主要矿物组成为透闪石、石墨以及碳酸盐类矿物等，外观呈白色和黑色，致密块状构造，微透明，油脂光泽；石墨包裹体多呈点状、星点状、浸染状及条带状分布(图 1a和图 1b)。
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图 1 代表性和田墨玉手标本(a-b)和偏光镜下岩相学观察(c-d)Figure 1. Representative black nephrite jade samples (a-b) and petrographic observation using polarizing microscope (c-d)Gr-Graphite; Tr-Tremolite2. 测试方法
在山东省地质科学研究院(SIGS)使用JEOLJXA-8230型号微探针对和田墨玉样品中矿物主量成分进行电子探针分析(EMPA)。测试条件: 加速电压15 kV，加速电流2×108 A，光束光斑直径为0.5 μm。采用ZAF法对数据进行校正，分析精度优于1%。
石墨碳稳定同位素测试在北京燕都中实测技术有限公司完成。采用同位素质谱法，检测依据为GB/T 18340.2-2010，结果以与PDB标准相关的传统符号δ13C表示，分析误差在±0.2‰以内。
3. 结果与讨论
3.1 显微岩相学分析
显微岩相学观察显示，和田墨玉样品的主要矿物组成为透闪石，次要矿物或副矿物为石墨、方解石、磷灰石等。其中，透闪石的光性特征大致相同：在单偏光下呈无色，正中突起，中等解理；在正交偏光下干涉色最高为Ⅱ级蓝绿，其主要结构为毛毡状隐晶质变晶结构、显微纤维变晶结构等。石墨为不透明矿物，在正交/单偏光镜下均呈现为黑色，反射光下则呈现为褐黄色(图 1c和图 1d)。
3.2 电子探针显微分析
电子探针显微结果表明，新疆和田墨玉样品的主要矿物组成为透闪石，其主要化学成分为SiO2、MgO及CaO，其含量与标准透闪石理论值接近[8-11]；另外，FeO整体含量较低，表明其颜色主要由石墨包裹体导致；Mg/(Fe+Mg)比值范围在0.991~1.000，平均值为0.996，含量较高，进一步指示样品的主要组成矿物为透闪石。而次要矿物/副矿物组成为透辉石、方解石、磷灰石及白云石等，与偏光显微镜下观察、鉴定的结果一致。
3.3 碳稳定同位素地球化学分析
碳稳定同位素地球化学显示，石墨δ13CPDB值具有比较宽的变化范围(δ13CPDB=-21.3‰~-8.5‰)，平均值为-13.76‰，主要由重同位素组成，少量轻同位素表明和田玉在变质交代过程中可能混入少量有机碳源。结合区域地质背景，认为中酸性岩浆(花岗岩、花岗闪长岩)侵入到白云石大理岩过程中携带地幔岩浆碳源[12]，在相对还原条件下，石墨和透闪石共同结晶或沉淀。
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图 2 自然界中δ13C分布情况及本研究中δ13C值Figure 2. Natural distributions of δ13C values and the δ13C values in this study4. 结论
(1) 新疆和田墨玉中石墨包裹体的碳质主要源于幔源岩浆碳，并可能混入少量有机碳。
(2) 新疆和田玉矿床成因机制与俯冲-碰撞-造山运动密切相关，而和田墨玉中石墨包裹体碳同位素组成为研究和田玉矿床构造、热演化历史提供了崭新思路。
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图 1 代表性和田墨玉手标本(a-b)和偏光镜下岩相学观察(c-d)
Figure 1. Representative black nephrite jade samples (a-b) and petrographic observation using polarizing microscope (c-d)
Gr-Graphite; Tr-Tremolite
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