Geochemistry of synsedimentary cements in Early Cambrian Reefs

Synsedimentary fibrous calcite cements in reefs representing different periods of Early Cambrian time from eastern Siberia, southern Australia, and eastern Canada were analyzed to determine whether they retained information about the chemical and isotopic compositions of the seawater from which they precipitated. Although petrographically similar, the Sr, O, and C isotopic compositions as well as the Sr and Mg contents of the cements from the three areas are different. Such differences can be attributed to varying degrees of diagenesis of the cements that correlate with the tectonic history of each area: cements from the Siberian craton are the least altered, cements from near the Appalachian Orogen are partially altered and cements from the Flinders fold and thrust belt are extensively altered. Strontium contents of the fibrous cements generally increase with Mg contents, although the correlation among these two elements is more variable than that reported for Devonian or Holocene cements. Those cements with SrMg ratios near 0.04, which include clear fibrous calcite, have the lowest 87Sr86Sr ratios and have been least altered. Strontium isotopic ratios in the least altered cements are lower than whole rock ratios and probably approach the original values of Early Cambrian seawater. δ18O values in the cements are variable because of exchange with diagenetic fluids, whereas δ13C values exhibit local variations that do not match temporal curves constructed from whole-rock data for the same regions. Thus, most of the original chemical and isotopic compositions of these cements have been variably changed through diagenesis, although limited information about the composition of Early Cambrian seawater, such as Sr and C isotopic compositions, have been retained in some samples.