Diagenetic sequestration of rare earths and actinides in phosphatic oil shale from the lacustrine Green River Formation (Eocene), Utah, USA: an SEM and LA-ICP-MS study

摘要

AbstractThe upper Green River Formation of the eastern Uinta Basin of Utah comprises variably organic-rich mudrock deposited in a stratified, alkaline lake that fluctuated in size and in hydrology from open to evaporitic. Beds of oil shale, representing periods of deep lake, suboxic-anoxic deposition include examples with anomalously high phosphate and toxic trace-metal levels, as determined by whole rock X-ray diffraction, scanning electron microscopy (SEM), and both solution and laser-ablation (LA) inductively coupled plasma-mass spectrometry (ICP-MS). The SEM identifies an early, pre-compaction, diagenetic succession of euhedral rhombic dolomite and high-Mg-calcite, nodular pyrite, blocky low-Mg calcite, and microcrystalline carbonate fluorapatite (CFA). LA-ICP-MS indicates the CFA intervals have distinct lower and upper margins where P content rises from ~0.079 to ~3.2% across 2–3?cm of section. Enrichments of rare earth elements (REEs) and actinides (Th, U) show a complex correlation with the phosphatic margins. Light REEs average a tenfold enrichment at the outer margins with progressively heavier REEs exhibiting up to 20-fold enrichment toward the inner margin (Lu from 0.15 to 2.63?ppm). Actinide abundances also increase tenfold (U from ~5 to ~80?ppm) toward the inner margin. There is greatest similarity between Th and the Ho distributions and between U and Lu. This is a similar distribution to what elsewhere has been found in fossil bone and teeth. The microcrystalline CFA is interpreted to have precipitated (or recrystallized from a precursor phosphate) when the rate of dissolved P being generated from organic-matter breakdown exceeded P diffusion rates into the water column, allowing for P supersaturation. As the crystallites grew, diffusion of REE and actinide ions and complexes from surrounding porewaters into the concreting phosphate occurred only until por