Early diagenetic modification of the trace and rare earth elements record of Devonian carbonates: Maïder Basin conical mounds (Eastern Anti-Atlas, Morocco)

Carbonate mounds are "mud-dominated deposits with topographic relief and few or no stromatolites, thrombolites or in place skeletons" (Riding, 2002). Despite their fine-grained texture and the absence of a rigid framework of invertebrate skeletons, mounds can develop into large structures with a high angle of accumulation. The diagenetic effects that trigger the early lithification of the steep sides of the conical mounds are still far from understood. Here we present new data set concerning major and trace element (including rare earth element, REE) analyses of the Middle Devonian carbonates from the Maïder Basin (eastern Anti-Atlas, Morocco). Aim of this work is to use the REE and trace elements chemistry as a tool for assessing the early diagenesis processes that involved widespread geological bodies such as the carbonate mounds. A variety of different limestone facies, including wackestone, vein filling and cement phases display a coherent normal marine REE pattern characterized by fractioned LREE content (Pr/Yb < 1), slightly negative Ce anomaly, and positive Gd anomaly. Nevertheless anomalous pattern with positive Ce anomaly (up to 2.23) or lack of any Ce anomaly have been found, suggesting variable palaeoredox conditions during the formation of these mounds. The co-variation of trace elements, such as Mo and U, reinforces the chemical constrains given by the REE patterns. Few samples show high positive Ce anomaly coupled with high MoEF/UEF ratio revealing a clear, and probable intermittent, reducing condition. This scenario may be complicated by the dissolution of organic matter during the early diagenetic phases. Consumption of organic matter might indeed result in a "false" positive Ce-anomaly within the pore water precipitates and the contextual consumption of Mo and U. We found a good correlation between Fe contents and ?REE in the studied carbonates. Furthermore, the average [?REE/Fe] ratio around 0.015 (±3) is consistent in all the samples except for those from vein filling which shift from this average. Fe contents and LREE/HREE ratios correlation and homogeneous [?REE/Fe] ratios may suggest a major effect of early diagenesis on the overall carbonates chemistry. Our results suggest that the precipitation of carbonates within the vein system has been probably triggered by activity of microbial consortium. Evidences of the microbial activity are the peloidal and clotted fabrics preserved within the fracture filling carbonates. Whereas, the bulks of the mound carbonates have probably undergone a different diagenesis, which strongly affected the distribution of redox sensitive elements and partially overprinted the original geochemical signature of these carbonates. We should therefore consider that the assumption that REE pattern is stable within carbonatic lattice (e.g. Banner et al., 1988) is probably imprecise and that Fe-bearing burial carbonatic phases could have influenced consistently the overall carbonate REE pattern.