RARE EARTH ELEMENT FRACTIONATION DURING PHOSPHATE NODULE DIAGENESIS

Major element and some trace element compositions including the REE compositions of marine and non-marine phosphate nodules from the Mississippian age of central Arkansas and Oklahoma regions have been analyzed.

Differences in the major and trace element chemistry between the rims and the cores of these nodules reflect competing influences such as diagenesis, weathering, and aqueous geochemistry of certain elements. These nodules contain, in their cores, exceptionally well preserved fossils, which can be attributed to the REE chemistry and the patterns of these phosphate nodules itself.

There is a difference in the cores and the rims of the phosphate nodules collected from the possibly non-marine to the marginally marine shales in Arkansas and that of the deeper marine facies in Arkansas and Oklahoma. The cores are enriched in the MREE in the deeper facies whereas the opposite trend is observed in the coastal nodules with the rims enriched in the middle rare earth elements. The relative loss of the MREE is generally not accompanied by a commensurate loss in the light or heavy REE.

The non-marine phosphate nodules can indeed be justified as non-marine nodules rather than marine ones due to the evidence such as in-situ plant fossils in root casts that are closely associated with the phosphate concretions. Delicate plant material such as fern foliage that are preserved in the phosphate concretions would certainly not survive transport from a terrestrial to a marine environment.

Several factors may explain the trend observed in the ICP-MS based data. Mineralogical differences between the cores and the rims of both the marine and non-marine concretions may control the fractionation of the MREE. Rims in the near–shore nodules are generally enriched in silicate material than their marine counterparts. The higher electronegativity of the clays may encourage the movement of MREE away from the cores in the non-marine nodules. However, in the marine nodules, the movement of MREE may be towards the core. High phosphate content may characterize clay-poor nodules, which probably favors the higher overall REE abundance. Although many of these hypothesized factors may favor enhanced fossil preservation, further exploration of the chemical and mineralogical differences both within and among these phosphate nodules is required.