The effect of grain size on the distribution and fractionation of the major chemical trace and rare earth elements in shale, silt and sand rocks of the Muqdadia formation in north of Iraq

Abstract

(17) samples were collected from Muqdadia formation from various areas in Erbil governorate and Aqra district north of Iraq, which were represented by the sections (A, B, D, R, W). (10) samples were used in analyzing the element using Plasma–Mass Spectrometry (ICP–MS) Analysis and the Inductive Couple Plasma–Mass Spectrometry (ICP–MS) Analysis for four granule sizes after these granules were separated from each sample. The elements included were Al, Fe, Mg, Ca, Na, K, Ti, P, Rb, Cs, Sr, Ba, Au, Mo, Pb, Cu, Zn, As, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Cr, Ni, Co, Mn, Sc, V, Zr, Hf, Ta, Nb, Th, U, Ga, La, Dy, Ho, Er, Tm, Yb, Lu, Y.The geochemical study revealed that there is no significant variation in the content of the main, trace and rare earth elements in the four sizes of shale, silt, fine sand and coarse sand. Also, it was found that the sizes of silt and fine sand contain the highest portion of the trace elements and the rare earth element.It was also, noticed that the patterns of the rare earth elements in the granule sizes are similar in general with some differences, as these differences were reflected in some changes in the portions of these elements.In general, it was observed that the total content of the rare earth elements in the various sizes is relatively small and this might be due to two main factors; the high decrease by calcite (calcium carbonate) and the free silica (quartz) and the other reason is that the mineral that contain REEs are mostly derived from rock sources (mafic or ultra-mafic), which contain these elements with low concentrations.Patterns of REE in the study samples and the four sizes are similar to the pattern of (PAAS) with slight differences like the presence of LREE and on a lower level in HREE and also the presence of negative anomaly Ce*/Ce in the size of shale, silt, fine sand and coarse sand.Although there are variations in the values of anomaly Eu* /Eu, the values are not far from the acceptable PAAS values and they are, generally, higher than PAAS due to the presence of the mafic and ultra-mafic component in the sediments of the current study. In all the sizes, except the shale size, it was found that ∑ REE varies and increases with the increase of the contents of Al2O3 and Fe2O3.As for the shale size, it was found that the relationship between REE∑, Mn and P2O5 is negative and this is because the shale were sedimented in river environments that don’t permit them to absorb sufficient amounts of REE as is the case with the shale that are sedimented in sea environments where there is a direct contact with the seawater that although contains slight concentrations of REE but they are effective when they are adsorbed on shale particularly when the average sedimentation of the shale is low. But in the current study, the shale is river sediments and the river contains insignificant concentrations of REE and the sediment in the river is faster compared to the sea environment and this case doesn’t permit the shale to adsorb large portions of REE.