An evaluation of the origin and post-depositional modification of coal mineral matter using rare earth elements and neodymium isotopes

This study was conducted in western Pennsylvania on the Lower Kittanning Coal bed to address the issue of coal mineral matter origin and depositional environment. Channel samples of the coal, the underlying clay (paleosol) unit, and the overlying shale were retrieved over a 170 km east-west range of sampling sites. Analytical techniques applied to the samples include the megascopic description of coal lithotypes, proximate and ultimate analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), coal petrography, instrumental neutron activation analysis (INAA), and thermal ionization mass spectrometry (TIMS).; Major cation concentrations in the coal are generally consistent with depositional environment interpretations made on the overlying shale (e.g., epigenetic mineralization). Titanium and shale overburden-normalized plots of the major cation data show that the coal bed is enriched in Fe and Ca compared to the overlying shale. Magnesium, sodium and potassium were depleted in the coal relative to the shale.; Rare earth element (REE) concentrations and neodymium (Nd) isotopes were used to identify sources of coal mineral matter and processes of alteration. A high degree of similarity exists between chondrite normalized REE plots of the coal, the shale overburden and the North American Shale Composite.; The Nd isotopic data from the Lower Kittanning Coal bed show a range of ϵ_Nd(t) (at time of deposition) from −8.4 to −9.9 at the study sites. The Nd isotopic data from the overlying shale and the underlying clay unit show a range from ϵ_Nd(t) values = −8.2 to −10.2. These data are interpreted to indicate a single mineral source throughout the deposition of all three units. That source appears to be clastic matter derived the Appalachian Mountains. An isochron-style plot of the isotopic data appears to indicate Sm-Nd fractionation occurred at the approximate age of the coal bed. This is interpreted as evidence of mobilization of the Sm and Nd. The mobilization of Sm, Nd and other REE may have been caused by the dewatering of the peat from Lower Kittanning early in the coalification process, the role of plant matter in preferential uptake or retention of heavy REE, or selective sorting of coal mineral matter during deposition.