Geochemical profile of rare earth elements, 87Sr/86Sr and stable isotope carbonate rocks from the active hydrothermal system at Grota da Fervedeira , Bodó, Rio Grande do Norte (RN).

Introduction

The rare earth elements and yttrium (REY) are present at trace levels at the environment, and their coherent distribution is associated with the geochemical processes of the carbonate rocks formation. These processes are associated with redox conditions, as well as the behavior of fluid ligands, and provenance indicator. In this context, this study aims to investigate the formation Recente carbonates, occuring along the Grota da Fervedeira creek, in Bodó, Rio Grande do Norte (RN), Brazil. The determination of trace concentrations of REY, associated with mineralogical and isotopic data (⁸⁷Sr^/86Sr isotopes, δ¹³C and δ ¹⁸O), can contribute to better understand the formation of carbonate system of Grota da Fervedeira.

Method

Twelve hand-samples were collected along the drainage system and were classified according to its facies using the Miall (1978) system. For REY determination, the samples were weighted at 100 mg, and digested with 50 mL acetic acid (Hac) at 1 mol/L concentration. The samples were the filtered with a syringe and cellulose acetate membranes (ø = 27 mm, 0,22 µm porosity). The samples were fully evaporated and redissolved in 5 mL 1,25 mol/L HCl, followed by a chromatographic column separation with AG50w-x8 resin. The samples were the evaporated and analyzed via ICP-MS. The ⁸⁷Sr^/86Sr isotopes, δ¹³C and δ ¹⁸O data are available elsewhere.

Results

The carbonate facies were classified as wavy laminated limestone (Wll), granular laminated limestone (Gll), opsolitic limestone (Ol), limestone tufa (Lt), laminated lithoclastic limestone (Lll), crystalline carbonátic crust (Ccc). The samples represented by CCc group presented no Ce positive anomaly, with the exception of one, with Ce/Ce* = 0.3. These variations implies that this sample can be affected by different processes, mainly of oxidative origin. In contrast, the samples presented a wide variation of total REY concentrations (0,6 mg/kg and 27,7 mg/kg). The REY profiles obtained for the samples representing Wll, Ol, Gll, Lll and Lt exhibited a positive Ce anomaly, varying the Ce/Ce* between 1.1 and 2.3. This positive anomaly could be associated to alkaline waters. The enrichment of LREE and HREE was  observed in sample many samples of these groups, with variations of Yb/Pr ratio throught 1.23 to 1.9. The variation of HREE and LREE enrichment could be associated to different degrees of fluid interaction with the main rock, in which the higher values of Yb/Pr may indicate more proximity of the source rock. The Y/Ho anomaly is >38.0 in most of samples, with values between 38.1 and 61.5 for the Wll group sample. Y/Ho anomalies with values <38.0 were observed in Ol and CCc group. In the same way, the Y/Ho anomaly is originated by the preferential scavenging of Y in relation to Ho for Fe and Mn oxyhydroxides. In this sense, the ratio Y/Ho is controlled by the ligands presents in the fluids.

Conclusion

The REY content of carbonate samples at Grota da Fervedeira system was determined. With this data, its possible to implie that Ccc facies can present a widely variation of REY profiles due to different depositional conditions of the carbonate, but none of them is related to strong alkaline waters since no positive Ce anomaly is found. In other hand, Wll, Gll, Lt, and Lll presented mild to strong positive Ce anomalies, related with chemical precipitated tufas, characterized by strong alkaline conditions.