| Citation: |
YU Jing, SU Xiaopeng, SHI Zhaotong, LI Yan, WANG Chaowen, ZHU Shukui, WANG Yamei. Comparative Metabolomic Reveals Chemotaxonomic Markers of Resin Fossils for Identification of Botanical Origins[J]. Journal of Gems & Gemmology, 2024, 26(S1): 77-80.
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The terpene compounds in amber are secondary metabolites secreted by higher plants. Due to the different secondary metabolites secreted by various plants, these abundant terpene compounds can serve as chemical fingerprints for interpreting the ancient plant sources of amber[1-2]. However, the existing chemotaxonomy of fossil resins is limited in its ability to distinguish them at phylum, family, and genus levels using small amount of samples[3-6]. Herein an advanced head-space solid phase microextraction-comprehensive two-dimensional gas chromatography- time-of-flight mass spectrometer platform was established to analyze the complex compositions of resin fossils derived from ancient gymnosperms (e.g., Eocene Cupressaceae, Cretaceous Araucariaceae) and angiosperms (Miocene Dipterocarpaceae and Miocene Hymenaea)[7-9]. The secondary metabolites including 83 monoterpenes, 186 sesquiterpenes, 84 diterpenes, and 43 n-alkanes, were annotated and then chemically fingerprinted for the first time. Prior to conducting chemical composition classification, this study tested the vitrinite reflectance of resin-bearing rocks and maximum pyrolysis peak temperature (Tmax) of the ambers to demonstrate their similar thermal maturity. After that, chemotaxonomic markers were disclosed using multivariate statistical analysis to identify the botanical origins of resin fossils, and four identification rules were concluded. Gymnosperm and angiosperm resins can be distinguished by the relative content of monoterpenes and diterpenes; Eocene Cupressaceae and Cretaceous Araucariaceae resins can be identified by ten chemotaxonomic markers which were screened out by orthogonal partial least-squares regression; Miocene Hymenaea and Dipterocarpaceae resins have different combinations of chemical superclasses, with sesqui- and di-terpenes being the major compositions of Hymenaea resins, and mono- and sesqui- terpenes accounting for most of the compositions of Dipterocarpaceae resins; Specific chemotaxonomic markers can identify Hymenaea mexicana and Hymenaea protera resins. As the chemical components of fossil resins were determined by the genes present in their botanical origins, the chemotaxonomic markers revealed in this study have a genetic basis[10-12]. These findings provide a comprehensive database of secondary metabolites from ancient resin plants, which can be utilized for further research on their corresponding paleoenvironment and to clarify their phylogeny.
宝石的特殊光学效应可以增加其价值，在宝石学研究中具有重要意义。在特殊光学效应中，星光效应与猫眼效应最为常见[1]。具有星光和猫眼效应的宝石种类丰富，如斜方晶系的金绿宝石，三方晶系的刚玉与石英，六方晶系的绿柱石，等轴晶系的石榴石等[2-5]。具有猫眼效应的磷灰石很常见，如马达加斯加、坦桑尼亚、肯尼亚、纳米比亚等地区是这种宝石的重要产地[6-7]。此外，印度、缅甸、斯里兰卡也曾报道过磷灰石猫眼的产出[8]。磷灰石的猫眼效应通常由其内部定向包裹体导致，前人推测可能为针铁矿等含铁矿物、平行生长管道或纤维状白色矿物[6, 9-11]。近期，马达加斯加发现了一种含定向包裹体的磷灰石，这种磷灰石在此前未被报道过。本研究通过拉曼光谱仪、扫描电子显微镜、X射线能谱仪、磁性测试等测试方法对这种磷灰石样品进行内部特征观察与成分分析。
本研究磷灰石样品产自马达加斯加，原石晶体为黄绿色，具玻璃光泽；外表磨损严重，肉眼可见大量平行排列的深色包裹体，整体透明度较低(图 1a)。对原石定向切磨后可产生猫眼效应(图 1b)。包裹体形态观察在中国地质大学(武汉)珠宝学院的ZEISS Axio Imager 2 Pol显微镜上完成；拉曼光谱测试在中国地质大学(武汉)珠宝学院使用JASCO NSR-7500激光拉曼光谱仪完成；背散射电子相与EDS测试在中国地质大学(武汉)地球科学学院搭载Oxford Instrument能谱仪与背散射电子衍射系统的Tescan Mira-3扫描电镜上完成；样品的磁滞回线通过Quantum Design PPMS-9磁力计测得。
选取其中一颗原石样品平行c轴制成薄片。在偏光显微镜的高倍镜下，深棕色包裹体为细长的针状且平行分布，整体延伸方向平行于磷灰石c轴(图 2a)；包裹体的长度约5~100 μm，宽度约1~3 μm；此外，磷灰石内还含有丰富的气液二相包裹体(图 2b)，它们大多分布在晶体内的同一平面并平行磷灰石c轴延长。许多气液二相包裹体具有“颈缩”现象，表明晶体经历了溶解-重结晶过程[12]。
通过EDS面扫描测试分析包裹体的元素分布，结果显示包裹体的出露区域富含Fe与O元素(图 3)，推测其为一种铁氧化物。
在磷灰石的振动模式中，与[PO4]四面体相关的拉曼位移分为四种：(1)由O-P-O对称伸缩振动ν1产生的拉曼位移位于962~965 cm-1；(2)由O-P-O弯曲振动ν2产生的拉曼位移位于419~431 cm-1；(3)由O-P-O非对称伸缩振动ν3产生的拉曼位移位于1 040~1 049 cm-1；(4) 由O-P-O非对称弯曲振动ν4产生的拉曼位移位于575~ 593 cm-1[13]。磷灰石基底与定向包裹体出露位置的拉曼光谱(图 4)显示，最强的拉曼峰位于964 cm-1，为[PO4]的ν1振动所致；445 cm-1处的拉曼峰为[PO4]的ν2振动所致；1 033、1 059 cm-1处的弱拉曼峰归因为[PO4]的ν3振动所致；583、610 cm-1处的拉曼峰均为[PO4]的ν4振动所致；位于1 162 cm-1的拉曼峰不属于[PO4]四面体，推测是由取代[PO4]位置的[SO4]的ν3振动产生[14]。
在包裹体出露区域的拉曼光谱中出现了一个位于662 cm-1的弱振动峰。通过与RRUFF数据库中的标准样品(R080025)对比可知，该峰为磁铁矿的A1g振动峰。磁铁矿主要有位于662、535、297 cm-1处的三个拉曼峰[15]，经多次测试仍未见另外两个拉曼峰，可能由于样品本身尺寸太小，测试信号强度较低所导致。
不同物质的磁性类型由其内部磁矩和电子自旋排列决定，可以反映在磁滞回线上[16]。磷灰石与铁氧化物包裹体混合物所获得的磁滞回线表明，样品具有磁滞现象(图 5)。在高达1 500 Oe的外部磁场中，磁滞回线快速闭合并达到饱和。磁滞回线的形状表明，磷灰石中铁氧化物是具有高矫顽力的铁磁性物质，低矫顽力物质含量较低。结合X射线能谱与拉曼光谱测试结果，推测磷灰石内的针状包裹体为磁铁矿(Fe3O4)。
(1) 显微观察表明，马达加斯加磷灰石内部含有大量的针状包裹体与拉长的气液二相包裹体，这些包裹体整体沿磷灰石c轴方向延伸，表明与主晶之间存在一定的晶体学取向关系。
(2) X射线能谱测试结果表明，马达加斯加磷灰石内含物区域以铁、氧元素为主；拉曼光谱表明，包裹体具有磁铁矿的662 cm-1主峰；磁滞回线测试结果表明样品中的铁氧化物为铁磁性物质。综合以上结果，推断磷灰石中定向包裹体为磁铁矿。
(3) 马达加斯加磷灰石内部丰富的、具“颈缩”现象的气液二相包裹体表明，磷灰石晶体形成后可能经历了流体辅助的溶解-重结晶作用。气液二相包裹体与针状磁铁矿包裹体间具有相似的取向，推测磁铁矿包裹体的形成过程中也有流体参与。但这种流体作用是否与针状磁铁矿包裹体的形成直接相关还有待进一步验证。
该项研究受国家自然科学基金(42307510)，中央高校基础研究经费(CUG2106128)及湖北省珠宝工程研究中心((CIGTXM-03-202103) 资助。感谢南京古生物研究所王博研究员提供了福建漳浦的琥珀样品；感谢宁涛博士和陈品博士在分析测试过程中提供的帮助。
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