Rare-earths in size fractions and sedimentary rocks of Pennsylvanian-Permian age from the mid-continent of the U.S.A.

The REE (rare-earth) contents of sixty-three <2 μ fractions of Pennsylvanian and Permian platform sediment from the mid-continent of the U.S.A. vary considerably (ΣREE = 46–439 ppm;La/ Lu = 5.2–15.7; correlation coefficient of REE with La/Lu = 0.89), but the Eu/Sm ratios are nearly constant even in reducing environments that concentrate U (0.16–0.22). There is no correlation of REE content to clay mineralogy.Lower Permian <2 μ fractions from continental to nearshore marine sediment in Oklahoma have higher REE content (244–261 ppm) than marine facies in Kansas (46–140ppm), but <2μ Upper Permian fractions in an evaporite basin have constant but high REE content (288–281 ppm; one = 153—ppm). All Pennsylvanian <2 μ fractions from Oklahoma have high REE content (209–439 ppm), and fractions from Kansas cyclothems have variable REE content (86–438 ppm). REE content in the <2 μ fractions is inherited from the provenance, but is modified by ion exchange during weathering, transportation, or deposition. Exchangable REE tend to be concentrated in clay minerals in basic environments, but removed in acid environments.Sand and gravel-size fractions consist mostly of quartz or chert so their REE content is low (7.9–40.6 ppm) although heavy minerals may contribute a large fraction of the REE content. Unexpectedly, silt-size fractions have REE contents (74–355 ppm) that are usually lower but similar to their <2 μ fractions, and the REE contents do not correlate to clay mineral/quartz ratios. The interpretation of REE content in sedimentary rocks needs to be done cautiously due to the above factors.