Depositional conditions and source of rare earth elements in carbonate strata of the Aptian-Albian Mural Formation, Pitaycachi section, northeastern Sonora, Mexico Jayagopal
Major, trace and rare earth elements (REE) concentrations in limestone beds of the Canova and El Caloso members of the Mural Formation in the Cerro Caloso Pitaycachi area were measured to understand the depositional conditions and source of REE. Contents of SiO2 and Al2O3 and concentrations of Zr, Y and Th are higher in the Canova member than in El Caloso member, whereas the content of CaO is comparable in both members. The sum of REE content however is low in both the Canova (10.4 ± 2.6, n=12) and El Caloso (3.4 ± 2.1, n=2) members. These values indicate that carbonate sedimentation of the Canova and El Caloso possess seawater-like shale-normalized REE+Y patterns with 1) light REE depletion (NdN/YbN = 0.74 ± 0.08, n=12; 0.58 ± 0.2, n=2; respectively), 2) both negative and positive Ce anomalies (Ce/Ce* : 0.81–1.10, 0.94 ± 0.10, n=12; Ce/Ce* : 0.60 to 0.91, 0.76 ± 0.22, n=2; respectively), and 3) superchondritic Y/Ho ratio (33.5 ± 4.0, n=12; 40.4 ± 8, n=2; respectively). The observed variations in Ce contents and Ce anomalies in the studied samples resulted from detrital input and scavenging processes. The limestones show positive Mn* values (0.439 to 0.850) and low contents of U (~0.35–1.70 ppm) and authigenic U (0.31–1.57 ppm) suggesting that they were deposited under oxygen-rich environment. The REE+Y patterns of the Canova samples are identical to those of Late Devonian carbonate sediments whereas the El Caloso samples are comparable to Holocene reefal microbalite with slight light REE depletion. This suggests that the limestones from Canova and El Caloso members possibly retained their original seawater-like REE patterns. The observed variability in REE content and REE+Y pattern is due to the presence of minor amounts of detrital materials in some samples. The present study reveals that the limestones still retain their original seawater-like pattern, provided that shale contamination was least (<5%), and they serve as a seawater proxy.