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

Abstract

The 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 pores were occluded and permeability dropped. As with fossil bone, at the outer margin of the phosphate interval, this allowed for fractionation and preferential substitution of Ca cations in the CFA lattice by light REEs of similar ionic radius. Heavier REEs and actinides of smaller ionic radius diffused further into margin of the phosphate interval.