C-S-Fe relationships and the isotopic composition of pyrite in the New Albany Shale of the Illinois Basin, U.S.A (original) (raw)

1987, Geochimica et Cosmochimica Acta

Ah&act-The rek~tionship hetweett pyritic solfur cotttettt (S& and organic carbon content (0 of shales analyzed from the New Albany Group depends upon C,. For samples of <6 wt.% &, s, and c, ate stron8ly correlated (r = 0.85). For C&"rich" shales (>6 wt.%), no !&-Cm correlation is apparent. The degne of Fe pyritization (DGP) shows similar relationships to C,. These C-S-Fe relationships su88est that pyrite formation was limited hy the avaihthihty of metaholizahle organic carhott in samples when C, < 6 wt.96 and hy the avaii&ihty of reactive Fe for samples where C, > 6 wt.%. Apparent sulfur isotope hattionatiotts relative to ~n~rn~~~~ seawater sulfate (A%) for pyrite fo~ation avera@ -40% for noncaicareous shaies and -25%~ for cakamous shales. A?? valuea hecome smaller with incmasin8 C,, f&, and DOP for all &-'w (~6 wt.%) and some Cm-"rich" (~6 wt.%) shalea. These trends stt8@st that pyrite formation occmred in a cktsed system or that instantaneous bacterial fractionation for sol&e redttction decmamdinma@tudewith inueasin8 organic carbon content. The isotopic trends oheerved in the New Albany Group are not neceskly repremntative of other shales having a contparahle range of oqanic carbon content, e.g. Cmtaceous shales and mudstones from the western interior of North America (GA~JTIW, 1986). A?S values in the remainder of the &-rich New Albany Group shahs are mhttivety Fargo (-38 to -47%) and independent of C,, &, and DOP, which sugaests that pyrite in them shaks formed mostly at or ahove the sediment-water interface hy precipitation from an isotopically uniform reservoir of dissolved i&S.