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Analyses of primary and early diagenetic fluid inclusions in the halite from the Late Ordovician Mallowa Salt, Canning Basin, Western Australia indicate a Ca-rich composition and high concentration of parent brines in the basin which were close to sylvite and carnallite precipitation. The salt-bearing series in the sampled interval was overheated up to 62 °C. The recorded differences in gas compositions result from the input of several gas sources including dispersed organic matter in the salt series and hydrocarbon deposits in the underlying rocks. The high concentration of the brines in fluid inclusions does not allow quantitative reconstruction of the chemical composition of Late Ordovician parent seawater. Using the information from Early Cambrian and Late Silurian basins as a proxy, however, the new data indicate that Late Ordovician seawater was undoubtedly Ca-rich and, in comparison with modern seawater, had a similar K content, considerably lower Mg content ( c. 30%), approximately three times the Ca content and one-third the SO4 content. [ABSTRACT FROM AUTHOR]

Abstract: Thirty nine samples of Lower Cambrian marine sedimentary halite with zoned primary fluid inclusions were analyzed using the UMCA method introduced by Petrichenko (1973) [Petrichenko, O.I., 1973. Metody doslidzhennya u mineralakh galogennykh porid. Naukova Dumka, Kyiv, 91 pp.] in order to determine the contents of K+, Mg2+, SO4 2− and Ca2+ ions and to interpret the composition of parent seawater. The analyzed samples were from the giant salt basin of eastern Siberia where five major phases of salt deposition can be distinguished, in the Late Vendian (Danilovo) and Early Cambrian (Usolye, Belsk, Angara, and Litvintsevo) basins. Our samples are all from Early Cambrian basins although the largest data set comes from the Angara Basin. The results indicate that Early Cambrian parent seawater was similar in all studied basins and was characterized by lower Mg2+ and SO4 2−, and higher Ca2+ concentrations relative to modern seawater; the concentration of K+ in Early Cambrian seawater was similar to that of modern seawater. Accordingly, the seawater in the entire Early Cambrian was Ca2+-rich and SO4 2−-poor. The change of composition of seawater–from SO4 2−-rich, Ca2+-poor in the latest Neoproterozoic to Ca2+-rich, SO4 2−-poor recorded throughout the Early Cambrian–probably occurred within a relatively short time span during the earliest Cambrian (Nemakit-Daldynian) time and caused a rise in Ca2+ concentrations in the shelf seas leading to the onset of biocalcification and then the Cambrian explosion. [Copyright &y& Elsevier]

The Late Proterozoic Pedro Leopoldo facies (Bambui Group) in the vicinity of Belo Horizonte, Brazil, compriss alternating laminated microsparitic limestones (10-35 mm thick beds) and fibrous limestones (10-55 mm thick). The latter are composed of a mosaic of sparry calcite crystals. These irregularly cross-cut rays and fans are composed of feathery precursor crystal bundles with squared-off growth zones. Ghosts of an original fibrous mineral, hexagonal in cross-section, are visible. The petrographic characteristics, very high strontium content and low magnesium content of the fibrous beds, as well as microspar beds, strongly argue for an original aragonitic mineralogy. The rays are interpreted as having formed by precipitation at the sediment-water interface, whereas the micrite was precipitated from the water column prior to deposition on the sea floor. The lack of emergence features suggests widespread aragonite precipitation under persistently subtidal conditions. [ABSTRACT FROM AUTHOR]

Lower Cambrian salt deposits in the eastern Officer Basin occur in troughs along the western and northwestern margin of the Gawler Craton, and are interpreted to have been deposited in marginal to restricted marine environments. The salt horizons in two drillholes [Manya 6 (373554E, 6938478N: GDA94) and Wilkinson 1 (263545E, 6693435N: GDA94)] were studied utilising multiple techniques including mineralogical investigation, fluid inclusion and isotopic analysis. New data on Br values of 19 salt samples provide evidence of marine origin for the salt. Abundant two-phase (gas – liquid) inclusions and poor preservation of primary inclusions indicate intensive post-depositional recrystallisation. Homogenisation studies of fluid inclusions in halite from Manya 6 suggest that the upper part of the salt-bearing strata (Ouldburra Formation) has been heated to 80°C, whereas the lower part reached even higher temperatures. This heating event may be related to the Devonian – Carboniferous Alice Springs Orogeny. The chemical composition of fluid inclusions in Manya 6 and Wilkinson 1 samples indicates that parent brines were Ca-rich and SO 4 -poor, whereas modern seawater is SO 4 -rich and Ca-poor. Such a composition is similar to that recorded in the Lower Cambrian halite from the East Siberia Basin. [ABSTRACT FROM AUTHOR]

Abstract: Data on chemical composition of brines in primary inclusions of marine halites and on mineralogy of marine evaporites and carbonates lead to the conclusion that during the Phanerozoic two long-term cycles of chemical composition of seawater existed. During each of those cycles, seawater dominantly a Na-K-Mg-Ca-Cl (Ca-rich) type changed to a Na-K-Mg-Cl-SO4 (SO4-rich) type. Recrystallised halite from the uppermost Neoproterozoic Salt Range Formation (ca. 545Ma) in Pakistan, contains solitary inclusions indicating SO4-rich brines. This supports the concept derived from the study on primary fluid inclusions from the Neoproterozoic Ara Formation of Oman; SO4-rich seawater existed during latest Neoproterozoic time (ca. 545Ma). In contrast, samples of recrystallised halite from the Bitter Springs Formation (840–830Ma) in Australia contain inclusion brines that are entirely Ca-rich, indicating that basin brines and seawater were Ca-rich during deposition of central Australian evaporites. These combined data supported by the timing of aragonite and calcite seas suggest that during the Proterozoic, significant oscillations of the chemical composition of marine brines, and seawater, occurred, which are similar to those known to exist during the Phanerozoic. It is suggested that Ca-rich seawater dominated for a substantial period of time (more than 200Ma), at 650Ma, this was replaced by SO4-rich seawater, finally returning to Ca-rich seawater at 530Ma. [Copyright &y& Elsevier]