首页|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
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NSTL
Elsevier
Lithium isotopes (delta Li-7) in coals have been shown to increase with thermal maturity, suggesting preferential release of Li-6 from kerogen to porefluids. This has important implications for paleoclimate studies based on delta Li-7 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 VRo 0.68% (T-max = 104 degrees C), contained <1.5 mu g/g Li with an average vitrinite delta Li-7 of -28.4 +/- 1.6 parts per thousand, while liptinite and inertinite were heavier, averaging -15.4 +/- 3.6 parts per thousand and - 10.5 +/- 3.7 parts per thousand, respectively. The contact metamorphosed vitrinite/coke showed the greatest change with temperature with delta Li-7 18 to 37 parts per thousand heavier than the unmetamorphosed vitrinite. The Dutch Creek coal, buried to VRo 1.15% (T-max = 147 degrees 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 mu g/g, and the vitrinite in metamorphosed coal had delta Li-7 values 8 to 21 parts per thousand 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 mu g/g) Li-contents near the dike, suggesting possible Li incorporation from dike fluid into metamorphic silicates. A negative correlation was observed between Li-content and C-12(+)/Si-3(0)+ count ratios, indicating that at metamorphic temperatures Li becomes concentrated in silicates.
Lithium isotopesCoalKerogenMaceralContact metamorphismSecondary Ion Mass Spectrometry (SIMS)ORGANIC-MATTERVITRINITE REFLECTANCEIGNEOUS INTRUSIONBITUMINOUS COALMINERAL MATTERBASINGEOCHEMISTRYCARBONSOILLI
Teichert, Zebadiah、Eble, Cortland F.、Bose, Maitrayee、Williams, Lynda B.
NSTL
Elsevier
