REE patterns of microbial carbonate and cements from Sinemurian (Lower Jurassic) siliceous sponge mounds (Djebel Bou Dahar, High Atlas, Morocco)

Marine microbialites serve as robust seawater rare earth element and yttrium (REE+Y) proxies through many intervals of Earth history, but questions remain about the partitioning of REEs into different coeval carbonate phases, potential syn-depositional contaminants and elemental redistribution during diagenesis. Microbial carbonates, cements and background sediments were analysed for stable isotopes (O, C) and trace element geochemistry in Sinemurian (Lower Jurassic) mid- to outer ramp siliceous sponge microbial mounds from the High Atlas, Morocco. Trace elements were analysed using laser ablation-inductively coupled plasma-mass spectrometry. Microbialites, non-luminescent radial/radiaxial fibrous (RF) cement and well-preserved brachiopods have stable isotope values similar to published Early Jurassic marine values. Luminescent blocky sparite (BS) cements have lighter stable isotope values consistent with burial diagenesis. Early marine RF cement has shale-normalised (subscript sn) REE+Y patterns with characteristics of oxygenated seawater, whereas cavity-occluding BS cement has high concentration bell-shaped (REE+Y)sn patterns, very unlike seawater. Allomicrite has relatively high REE concentrations with flatter (REE+Y)sn patterns. Microbialites include three subclasses distinguished on the basis of petrography and (REE+Y)sn patterns. Clotted peloidal microbialites (MC1) have (REE+Y)sn patterns broadly consistent with seawater, but with variable Ce anomalies and higher concentrations and slightly less LREE depletion relative to RF cements. Other clotted peloidal to leiolitic microbialites (MC2) have flatter (REE+Y)sn patterns and variable Ce anomalies, whereas leiolitic microbialites (MC3) have patterns similar to allomicrite. Hence, MC1 microbialites and early marine RF cements preserved seawater-like REE+Y patterns despite subsequent diagenesis, confirming that Early Jurassic marine REE distributions were similar to late Palaeozoic, Late Jurassic and Holocene distributions. Importantly, LREE enrichment in allomicrite and some microbialites (MC2, MC3) highlights the occurrence of LREE-enriched components that may represent marine particulate matter that preferentially scavenged LREEs from the water column prior to sedimentation. The Sinemurian siliceous sponge microbial mounds accumulated in well-oxygenated settings rather than on the edge of an oxygen minimum zone. Some of the high Ce contents in the microbialites may reflect redistribution of Ce during earliest diagenesis in suboxic pore fluids, or incorporation of LREE enriched particles or LREE uptake in the growing microbialite consistent with scavenging on organic ligands in the biofilm itself. This study demonstrates how various sedimentary sources and diagenetic processes can significantly affect otherwise robust marine REE patterns in microbial mounds within a relatively siliciclastic-free carbonate environment.