The behavior of rare-earth elements, Zr and Hf during biologically-mediated deposition of silica-stromatolites and carbonate-rich microbial mats

Abstract

Venere Lake, in the Pantelleria Island thermal system (Central Mediterranean Sea) consists of a mix of seawater and hydrothermal volcanic fluids containing high levels of dissolved SiO 2 . Close to the lake's thermal springs, siliceous stromatolites are deposed under high bacterial activity conditions whereas roughly interlaminated Ca-carbonates and microbial mats are widely scattered in the lake. The dissolved REE speciation in lake waters is dominated by [REE(CO 3 ) 2 ] - , [REE(CO 3 )] + and [REE(H 3 SiO 4 )] 2+ complexes. On the contrary the most abundant Zr and Hf species are hydroxyl- and fluoride-complexes. The behavior of REE Zr and Hf in the Venere lake waters is controlled by precipitation of amorphous silica, carbonates and bacterial activity. Chondrite-normalized REE patterns in stromatolites are characterized by positive La anomalies, superchondritic Y/Ho ratios and negative Eu anomalies. The Eu depletion is also found in microbial mats where is associated to a "chondritic" Y/Ho signature. Zr and Hf differently behave in dissolved phase and in newly-formed siliceous and carbonatic material: Hf reacts easily into amorphous silica surfaces whereas Zr is preferentially partitioned onto phosphate binding sites on microbial cell surfaces similar to the observed behavior of heavy REE (HREE). We suggest that the Zr-Hf relationship reflect the interface processes between mineral phases and bacterial membranes or compounds produced by bacterial activity.