PALAEOPROTEROZOIC TRAVERTINES OF VOLCANIC AFFILIATION FROM A 13C-RICH RIFT LAKE ENVIRONMENT

Abstract

The ca. 2.2 Ga Kuetsjarvi Sedimentary Formation (KSF) of the Pechenga Greenstone Belt, the north-east Fennoscandian Shield, contains the oldest travertines so far reported in the literature. The travertine-hosting succession is composed of flat-laminated, dolomitic stromatolites, micritic and allochemical dolostones, all formed in coastal plains of a rift-related playa lake system in an arid/semiarid climate. The travertines are categorised as thermal, autochtonous, spring travertine crust and spring orifice travertine mounds formed from hot water springs issuing at the playa lakeshore. The travertines are represented by crusts and small-scale mounds, both composed of a series of 1-5-mm thick dark-light couplets of fibrous dolomite crystals with compromise boundaries and radiating dolomite crystals. Travertines are composed of dolomite (Mg/Ca = 0.42-0.47), exhibiting a general lack of replacement textures. Travertines exhibit several phases of growth separated by dissolution processes and accompanied by precipitation of SiO2. The upper surface of crusts and mounds are often affected by desiccation joints and covered with thicker SiO2 veneer (delta O-18 = +18.6 parts per thousand). The host dolostones are characterised by delta C-13(carb) of +6.1-+8.8 parts per thousand (V-PDB) and delta O-18 values ranging from 11 parts per thousand to 19 parts per thousand (V- SMOW). Microcored samples of travertine dolomite exhibit delta C-13 and delta O-18 values ranging from -6.1 parts per thousand to + 7.7 parts per thousand and from 12.0 parts per thousand to 21.5 parts per thousand, respectively. The C-13-rich nature of the background stratified dolostones is caused by the 2.4-2.06 Ga global shift in C-13/C-12 carbonate carbon, associated with enhanced accumulation of C-org and perturbation of the global carbon cycle. The source of CO2 for the travertines is considered to be of volcanogenic origin with an initial delta C-13 value of ca. - 6.0 parts per thousand. Several processes contributed to carbon and oxygen isotope composition of travertine dolomite: (i) an initial CO2 loss at the spring orifice, (ii) water cooling and rapid CO2 loss during water transit, (iii) persistent evaporation from thermal water ponds/pools, and (iv) mixing with CO2 of ambient lake water enriched in C-13. Silica is thought to have been dissolved by groundwater from underlying sediments and been transported upwards by a mechanism of capillary rise due to dry climatic condition.