Rare earth element distribution in the acetic acid soluble fraction of combusted coals: Its implication as a proxy for the original coal-forming plants

The precise concentrations of rare earth elements (REEs) in Japanese Ashibetsu and Taiheiyo coals, in Indonesian Ambalut coal, and in Australian boghead coal have been determined, in order to compare the REE contents of the coals with those of plants and also to obtain further knowledge of the original plants from which these coals were formed. By leaching the coals after ashing using a 10% acetic acid solution, each coal sample was separated into two fractions: an acetic acid soluble fraction, and an acetic acid insoluble fraction. Our results show that three different types of chondrite-normalized REE patterns are observed in the acetic acid soluble fraction of these coals. The acetic acid soluble fractions of the Ashibetsu, the Taiheiyo, and some of the Ambalut coals showed REE patterns identical to those of peat vegetation. Those of other Ambalut coals showed an REE pattern that was similar to that of tree trunks. The boghead coal showed a different REE pattern that was similar to algae, an origin that is also suggested from a maceral study. The REE patterns of the acetic acid soluble fraction can therefore be a useful proxy for determining the original plants that formed the coals.