Effects of contact metamorphism on the lithium content and isotopic composition of kerogen in coal
Effects of contact metamorphism on the lithium content and isotopic composition of kerogen in coal
复制标题
接触变质作用对煤干酪根锂含量及同位素组成的影响
DOI：
10.1016/j.chemgeo.2022.120885
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发表时间：
2022
期刊：
影响因子：
3.9
通讯作者：
Teichert, Z.
中科院分区：
文献类型：
--
作者：
Teichert, Z.
煤中锂同位素（δ 7 Li）随热成熟度的增加而增加，表明6Li从干酪根中优先释放到孔隙流体中。这对基于埋藏海相碳酸盐岩的δ 7 Li的古气候研究具有重要意义，这些碳酸盐岩可能在重结晶过程中从孔隙流体中掺入Li。在这里，锂含量和同位素组成的显微组分从两个煤层侵入岩脉，作为温度的函数进行了研究，通过热梯度到未变质煤。在科罗拉多（美国）从Vermejo Fm采集样品。利用二次离子质谱（西姆斯）原位测定了煤显微组分的锂含量和锂同位素组成，并与荷兰溪2号煤层（长英斑岩脉）进行了对比。Vermejo煤样品的显微组分，埋深为VRo0.68%（Tmax= 104 °C），含Li <1.5 μg/g，平均镜质组δ 7 Li为−28.4 ± 1.6‰，而脂质组和惰质组较重，平均值分别为−15.4 ± 3.6‰和− 10.5 ± 3.7‰。接触变质镜质组/焦炭随温度变化最大，δ 7 Li比未变质镜质组重18 ~ 37‰。荷兰溪煤在岩墙就位前被埋藏至VRo 1.15%（Tmax= 147 °C），可能在埋藏过程中释放了Li，因为在接触变质和未变质显微组分之间观察到的同位素变化较小。总体而言，Li含量< 1 μg/g，变质煤镜质组δ 7 Li值比未变质煤重8 ~ 21‰。对岩脉附近显微组分的西姆斯测量没有显示出Li含量的增加，表明Li来自岩脉流体，然而，先前包括硅酸盐的大量测量显示出岩脉附近的Li含量略高（2-3 μg/g），这表明Li可能从岩脉流体掺入变质硅酸盐中。Li含量与~（12）C +/~（30）Si+计数比值呈负相关，表明在变质温度下Li富集于硅酸盐中。
Lithium isotopes (δ7Li) in coals have been shown to increase with thermal maturity, suggesting preferential release of6Li from kerogen to porefluids. This has important implications for paleoclimate studies based on δ7Li of buried marine carbonates, which may incorporate Li from porefluids during recrystallization. Here, the Li content and isotopic composition of macerals from two coal seams intruded by dikes, were studied as a function of temperature across a thermal gradient into the unmetamorphosed coal. Samples were collected in Colorado (USA) from a Vermejo Fm. coal seam intruded by a mafic-lamprophyre dike and compared to a Dutch Creek No.2 coal seam intruded by felsic-porphyry dike; a potential source of Li-rich fluids.The Li-content and Li-isotope compositions of coal macerals were measured in situ by Secondary Ion Mass Spectrometry (SIMS). The macerals of the Vermejo coal samples, buried to VRo0.68% (Tmax= 104 °C), contained <1.5 μg/g Li with an average vitrinite δ7Li of −28.4 ± 1.6‰, while liptinite and inertinite were heavier, averaging −15.4 ± 3.6‰ and − 10.5 ± 3.7‰, respectively. The contact metamorphosed vitrinite/coke showed the greatest change with temperature with δ7Li 18 to 37‰ heavier than the unmetamorphosed vitrinite.The Dutch Creek coal, buried to VRo 1.15% (Tmax= 147 °C), prior to dike emplacement, may have released Li during burial, as less isotopic change was observed between contact metamorphosed and unmetamorphosed macerals. Overall, Li contents were < 1 μg/g, and the vitrinite in metamorphosed coal had δ7Li values 8 to 21‰ heavier than the unmetamorphosed coal. SIMS measurements on macerals near the dike did not show an increase in Li-content indicative of Li derived from dike fluids, however previous bulk measurements that included silicates showed slightly higher (2-3 μg/g) Li-contents near the dike, suggesting possible Li incorporation from dike fluid into metamorphic silicates. A negative correlation was observed between Li-content and12C+/30Si+count ratios, indicating that at metamorphic temperatures Li becomes concentrated in silicates.