Rare Earth Elements of Methane-Derived Authigenic Carbonates and Its Constraints on Redox Condition in the Gulf of Cadiz

Abstract:

The mineral composition, stable carbon and oxygen isotopes, and the rare earth elements (REE) concentrations in methane-derived authigenic carbonates collected from the Gulf of Cadiz were used to trace fluid sources and provide information on the associated biogeochemical processes during their formation. These samples are composed by a detrital fraction (mainly composed by quartz and clays) cemented by authigenic carbonates: aragonite and Mg-calcite, pure Mg-calcite, or dolomite and Mg-calcite. The δ¹³C values of the samples vary between -45.78‰ and -9.72‰ VPDB and δ¹⁸O values range from 3.67‰ to 6.92‰ VPDB. The δ¹³C composition is indicative of the methane-derived (probably from thermogenic gas or a mixed of thermogenic and biogenic gas) source of these carbonates. The total REE content (ΣREE) of these seep carbonates range from 13 to 31 ppm and average ΣREE values of 21 ppm, with most of samples lower than the typical marine carbonate value of ~28 ppm. This result suggests that the REE composition of the methane-derived authigenic carbonates is controlled primarily by the migrated methane-rich fluids from which they have precipitated. The results that the REE enrichment of the samples containing dolomite and Mg-calcite minerals is higher than samples with aragonite also suggest that the authigenic mineral composition and the formation setting are important factors on the REE concentration. The shale-normalized REE patterns of the seep carbonates show no abnormality or slight positive Ce anomalies, suggesting that the formation of these seep carbonates occurs in anoxic condition. The correlation results of Ce/Ce* and La_N/Sm_N, Ce/Ce* and Dy_N/Sm_N, Ce/Ce* and ΣREE have also suggested that the REE characteristics of most seep carbonate samples preserve the original redox conditions of their formation and that late diagenesis has little effect on the REE. Therefore, it is feasible to assume that these methane-derived authigenic carbonates represent primary carbonate phases and that they have not undergo significant diagenesis and that their geochemical and isotopic signatures reflect mainly their formation processes.