A Profound Meteoric Water Influence on Genesis in the Permian Waterberg Coalfield, South Africa: Evidence From Stable Isotopes
A Profound Meteoric Water Influence on Genesis in the Permian Waterberg Coalfield, South Africa: Evidence From Stable Isotopes
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DOI：
10.1306/d4268171-2b26-11d7-8648000102c1865d
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发表时间：
1995-10
影响因子：
2
通讯作者：
K. Faure;C. Harris;J. P. Willis
中科院分区：
文献类型：
--
作者：
K. Faure;C. Harris;J. P. Willis
摘要 Waterberg 煤田 Grootegeluk 组的含煤沉积物含有在沉积和成岩作用过程中沉淀的各种形态的菱铁矿、铁白云石和方解石，并且具有高度可变的 d18O 和 d13C 值。球晶菱铁矿主要以早期成岩结核（直径 1-5 mm）形式存在于 Grootegeluk 组下部的煤和碳质泥岩中，具有相对较高的 d13C 值，高达 +8 (PDB)，d18O 值在 +9 和 +15 (V-SMOW) 之间。粒状菱铁矿（d13C 值（-l9 至 -14）和 d18O 值在 +12 和 +22 之间。铁白云石仅存在少量（d18O 值在 +16 和 +20 之间，d13C 值 » -12，仅存在于煤层中的割理填充方解石的氧和碳同位素值，其 d18O 值在 +12 和 +22 之间） d13C 值在 -15 和 -7 之间，浸染在碳质泥岩中并作为高岭石中的包裹体，仅存在于 Grootegeluk 地层的底部，并且 d13C 值在 -7 和 +3 之间，d18O 值 » +15。方解石透镜体被解释为同沉积岩，是从 d18O 值为-13. 早期成岩作用期间沉淀的球晶菱铁矿和浸染性方解石，主要是在较高温度下形成的粒状菱铁矿（割理填充方解石的 d13C 值（-12）表明碳的主要来源，但不稳定且范围广泛的 d18O 值无法区分沉积物隆起过程中的沉淀物或后来的沉淀物。碳酸盐的稳定同位素数据与大气降水一致，但对地层温度的估计并不可靠，因为埋藏过程导致孔隙水d18O高度耗尽。
ABSTRACT The coal-bearing sediments of the Grootegeluk Formation of the Waterberg Coalfield contain siderite, ankerite, and calcite of various forms that precipitated during deposition and diagenesis, and have highly variable d18O and d13C values. Spherulitic siderite, which is present predominantly as early diagenetic nodules (1-5 mm in diameter) in coal and carbonaceous mudrock in the lower part of the Grootegeluk Formation, has relatively high d13C values up to +8 (PDB) and d18O value between +9 and +15 (V-SMOW). Granular siderite (d13C values (-l9 to -14) and d18O values between +12 and +22. Ankerite is present only in minor amounts (d18O values between +16 and +20, and d13C values » -12, The oxygen and carbon isotopic values of cleat-filling calcite, which is present only in coal seams, has d18O values between +12 and +22 and d13C values between -15, and -7. Fine-grained calcite, disseminated in carbonaceous mudrock and as inclusions in kaolinite, is present only at the base of the Grootegeluk Formation and has d13C values between -7 and +3 and d18O values » +15. The calcite lenses are interpreted to be synsedimentary and to have precipitated from depositional waters with d18O value of -13. Spherulitic siderite and disseminated calcite precipitated during early diagenesis dominated by anaerobic microbial decomposition of organic matter. Granular siderite formed at higher temperatures (d13C values (-12) of the cleat-filling calcite indicate a predominantly organic source for the carbon, but the erratic and wide range of the d18O values do not allow distinction between precipitation daring burial or later during uplift of the sediments. Stable-isotope data of the carbonates are consistent with precipitation from meteoric water, but estimation of temperatures of formation are not reliable because burial processes result in highly d18O-depleted pore water.