Partitioning of trace elements and the effects of diagenesis and provenance in the heavy mineral fractions of Stanley Formation shales, Ouachita Mountains, Oklahoma and Arkansas.

摘要

The origin of the Ouachita Mountains and the provenance of the Carboniferous flysch deposits, that include the Stanley formation of Oklahoma and Arkansas, have been the focus of significant debate for decades. Previously collected geochemical data on the Stanley Shales suggests a mixed provenance. The same data seem to indicate the presence of heavy minerals in the Stanley that if, separated from the light minerals, could further our understanding of the origin of the Ouachitas. The primary impetus to this study was furthering the understanding of the provenance of the Stanley formation using heavy minerals.; This study is essentially a heavy-mineral investigation in which it was necessary to develop a technique to separate, identify, and determine the variability of heavy minerals in shale samples. Prior to this study, provenance determinations by heavy-mineral variations and direct measurement of the control of trace elements by heavy minerals in shales had not been possible because of the difficulties in separating heavy minerals from the obscuring mass of the clay matrix. The method developed is nondestructive and works well with shales of variable induration and maturation from shales to phyllites. The methodology involves the use lithium meta-tungstate, a non-organic heavy liquid, for density separations of bulk samples and a combination of optical and scanning electron microscopy (SEM) for identification and quantification of heavy-mineral proportions.; Heavy minerals were separated from the clay and light mineral matrices of a suite of shale samples as well as some interbedded sandstones from the Stanley formation of Arkansas and Oklahoma. Heavy minerals are as plentiful and diverse in the Stanley Shales as they are in the interbedded sandstones. Heavy minerals included variable amounts of Fe, Ti, Fe-Mn, Ba-Mn, and Cr-Fe-oxides, Fe-rich micas, zircon, tourmaline, rutile, apatite, barite, and monazite.; Monazite is ubiquitous in trace amounts in every sample, and is the probable site for much of the REE's in the heavy mineral fraction and the whole-rock. The occurrence of monazite almost exclusively in sialic igneous rocks implies that Sm/Nd isotopic signatures are not sensitive to sediment input from more mafic sources. In our samples, chromite and Mn-oxides were identified, and are associated with the more mafic component.; Mass balance calculations suggest that the heavy mineral fraction is an important site for many trace elements. In particular, the calculated effects of monazite on whole rock trace-element concentrations agree with the observation of monazite in the shale samples, and suggests that monazite is a dominant site of REE in these shales. Microprobe analyses of the heavy mineral fraction supports these calculations. Relative to the whole-rock analyses a small, but significant proportion of Ti, was observed in the heavy mineral portion.; These findings stress the importance of determining the mineralogical sites of trace elements in shales, and may improve interpretations based on whole-rock chemistry and increase our understanding of analytical variation in trace-element contents in single shale samples. The results of this study emphasize the importance of determining the mineralogic sites of trace elements, and realization of specific mineralogic contributions from mafic or sialic tectonic provenances. The heavy-mineral fraction of these shales can sequester many of the trace-elements used in whole-rock studies of provenance.