GEOCHEMISTRY OF ULTRAPURE CARBONATES ACROSS THE FRASNIAN/FAMENNIAN BOUNDARY INTERVAL IN SOUTHEAST ASIA (XOM NHA FORMATION, CENTRAL VIETNAM)

The Late Devonian Kellwasser Crises generally exhibit characteristic black shales and/or black limestones in many successions around the globe. To develop a set of best practices for detecting ocean anoxia in ultrapure carbonates, Upper Devonian carbonates deposited through the Frasnian/Famennian (F/F) boundary within the Xom Nha Formation in Vietnam were studied using whole rock geochemistry and cathodoluminescence. The sedimentological record of the studied section shows a continuous lithology without black shales or black limestones, but rather pure carbonates instead. Microfacies analysis suggests a hemipelagic setting on an outer shelf or outer platform environment with low sedimentation rates.

The section is composed entirely of calcite with a maximum of 4 wt % SiO₂ and 1.04 wt % Al₂O₃ (with average values of 0.39 wt % SiO₂ and 0.87 wt % Al₂O₃), and there is no differentiation in mineralogy at the F/F boundary. During cathodoluminescence examination, minor fluid alteration was observed in some of the samples, but most samples do not show evidence of alteration. Trace element geochemical analysis shows remarkably stable signatures in all major, trace and rare earth elements, with the exception of the boundary area between the Palmatolepis rhenana conodont zone and the Palmatolepis linguiformis conodont zone, which contains excess barium and a cerium anomaly. These excursions correspond to the Kellwasser Crisis (it is not possible to differentiate whether it is the upper or lower Kellwasser Event). There is no mercury anomaly across this boundary, indicating that volcanism was not severe enough to result in a global Hg anomaly, and was thus not likely a factor in the F-F extinction. Although challenges remain in regards to interpretation of these two geochemical proxies, excess barium (associated with primary productivity changes) and cerium anomalies (associated with changes in redox conditions) become critical for providing information about changes in ocean chemistry in ultrapure carbonate rock units.