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

Siliceous sponge mounds from Sinemurian mid- to outer ramp setting (High Atlas, Morocco) were investigated for stable isotope and trace element geochemistry. Trace elements were analysed on thin sections using laser ablation-inductively coupled plasma-mass spectrometry demonstrating that the various carbonate components yield important differences in REE concentrations. Mound automicrites (biologically induced precipitated micrite) have stable isotope mean values (δ18O -2.78‰; δ13C 2.34‰) comparable to non luminescent, early marine, radial fibrous (RF) cement (δ18O -1.6‰; δ13C 2.29), to brachiopods (δ18O -2.70‰; δ13C 2.23‰) and to published Early Jurassic marine values. Automicrite precipitation occurred in equilibrium with seawater without major enzymatic fractionation. Luminescent blocky sparite (BS) cement of burial origin has mean δ18O -5.54‰ and δ13C 1.77‰. Allomicrite has δ18O -3.78‰ and δ13C 1.99‰ suggesting burial diagenetic alteration. RF cement has MUQ shale-normalized (REE+Y)sn patterns comparable to modern seawater with: light REE (LREE) depletion (mean (Nd/Yb)sn=0.19); superchondritic Y/Ho=82.33; positive La anomalies; negative Ce anomalies consistent with oxygenated waters. BS cement has: bell-shaped, MREE-enriched REE+Y patterns (mean (Nds/Yb)sn=1.77); low Y/Ho ratios (36-64); mostly negative Ce anomalies; mostly positive La anomalies. Allomicrite has relatively high REE concentrations (ΣREE=12-26ppm) with seawater-like, but LREE enriched, flat (REE+Y)sn patterns (mean (Nds/Yb)sn=0.80); Y/Ho=37-49; negative Ce anomalies; positive La anomalies. Their reduced LREE depletion and high ΣREE cannot be explained by shale contamination and may reflect incorporation of particulate matter that scavenged LREE from the overlying water column. Three samples of automicrites have seawater patterns: mean (Nds/Yb)sn=0.28; Y/Ho=54-90; positive La anomalies but variable Ce anomalies. Most of the automicrites, however, have reduced LREE depletion and more variable anomalies. Four automicrites (leiolitic to clotted peloidal micrites with sponge spicules) have low ΣREE and a flatter pattern (mean (Nd/Yb)sn=0.57; Y/Ho=43-61) with mostly positive Ce and La anomalies. A second group of automicrites, evidently mixed with allomicrite, has higher ΣREE and a flat pattern (mean (Nd/Yb)sn=0.85; Y/Ho=33-39; mostly positive Ce and La anomalies). Automicrite REEs were variably contaminated by LREE-enriched allomicrite and early diagenetic processes. The positive Ce anomalies may reflect precipitation in dysoxic surface or early diagenetic microenvironments. Hypothetical mixing lines between RF and MUQ shales show that shale contamination cannot explain the elevated LREEs. The stability of REE patterns in RF cements probably reflects their lower porosity relative to the microporous automicrites and lack of particulate matter. This study confirms that pre-Cenozoic (Early Jurassic) REE patterns were comparable to modern ones.