READING THE RECORD: MINERAL AND WHOLE ROCK GEOCHEMISTRY IN SULFIDIC, GRAPHITIC SCHISTS AS INDICATORS OF BLACK SHALE DEPOSITIONAL ENVIRONMENTS

Black shales are currently the subject of great interest in the Earth Sciences. The study of sulfidic and graphitic schists, the metamorphosed equivalents of black shales, reveals important information about the original depositional environment of sediments, as well as on diagenetic and metamorphic processes. The challenge lies in deconvoluting these signatures. What geochemical signatures would be preserved through diagenesis and metamorphism? We wish to develop techniques that may be applied to ancient, former black shales.

Using rocks from the extensive Silurian Smalls Falls Formation in Maine, New Hampshire, and Massachusetts, we have studied the geochemistry of specific minerals and whole rock samples in order to “read through” high grade regional metamorphism. Our goal here is to understand more about the narrowing Silurian depositional basin that existed in the interval immediately preceding the Devonian Acadian Orogeny in New England.

In this study we focus on the behavior of trace elements V, Cr, Ni, As, Y and U, as well as major elements Fe, Mn and S. Whole rock V/(V+Ni) ratios in our samples demonstrate considerable spatial variability in bottom water oxicity. (V+Cr) enriched micas show probable anoxic conditions in the southwestern portion of our study area. Notable concentrations of As in pyrite-pyrrhotite bearing rocks in the central portion of our study area document regional concerns about As in groundwater. Rocks in this region are also enriched in Y and U, as shown by the occurrence of the rare mineral yttrobetafite. Preliminary δ15N data on ammonia-bearing micas show that as expected, total N decreases with increasing metamorphic grade. However, we report the surprising result that regional metamorphism has not fractionated this isotope, suggesting that prevailing values represent those established by marine organisms.