Tracing the composition, fluid source and formation conditions of the methane-derived authigenic carbonates in the Gulf of Cadiz with rare earth elements and stable isotopes

Abstract

The mineral composition, stable carbon and oxygen isotopes, and rare earth element (REE) concentrations in methane-derived authigenic carbonates collected from the Gulf of Cadiz were used to trace fluid sources and examine the controlling factors on REE composition during carbonate formation. These authigenic carbonate samples are composed of a detrital fraction (mainly quartz and clay) cemented by authigenic carbonate minerals: aragonite and Mg-calcite, pure Mg-calcite, or dolomite and Mg-calcite. The δ13C values of the whole rock samples vary between −45.8‰ and −9.7‰ VPDB and δ18O values range from 3.7‰ to 6.9‰ VPDB. The δ13C values are indicative of a methane-derived source for these carbonates, most likely resulting from thermogenic gas or a mixture of thermogenic and biogenic gas. The total REE content (ΣREE) of these authigenic carbonate minerals range from 13 to 31 ppm; all but four of the subsamples are lower than a typical marine carbonates (∼28 ppm). This suggests that the REE composition of methane-derived authigenic carbonates is controlled primarily by the methane-rich fluids. The results also show that the REE contents in carbonate mineral subsamples containing dolomite and Mg-calcite is higher than those subsamples with aragonite, suggesting that the authigenic mineral composition and the formation environment are important factors. The shale-normalized REE patterns of the seep carbonates show no or slightly positive Ce anomalies, suggesting that the formation of these seep carbonates occurred under anoxic conditions and associated with rapid fluid seepage. The correlation results of Ce/Ce* and LaN/SmN, Ce/Ce* and DyN/SmN, and Ce/Ce* and ΣREE suggest that the REE characteristics of most seep carbonate samples preserve the original redox conditions of their formation. Late diagenesis or post-oxidation of the samples by their exhumation from the sediments into seawater has little effect on REEs. Therefore, it can be assumed these methane-derived authigenic carbonates represent primary carbonate phases; they have not undergone significant diagenesis and their geochemical and isotopic signatures mainly reflect their formation processes.