EXTENDING THE RECORD OF SEAWATER CHEMISTRY BY ~300 MY: FLUID-INCLUSION RESULTS FROM THE MID-NEOPROTEROZOIC BROWNE FM, OFFICER BASIN, AUSTRALIA

The major ion composition (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, SO₄^2-, and HCO3^-) of seawater varied through the Phanerozoic Eon. Fluid-inclusion data from primary halite crystals in marine halite indicate that seawater had high Mg²⁺/Ca²⁺ ratios (>2) and relatively elevated Mg²⁺ and SO₄^2- concentrations during the Late Neoproterozoic, Permian, Triassic, and the last 40 Ma. During the Cambrian, Silurian, Devonian, Jurassic, and Cretaceous, seawater had relatively low Mg²⁺/Ca²⁺ ratios (<2) and Ca²⁺>SO₄^2-. K⁺ levels have remained stable throughout Phanerozoic time.

We present here the first analyses from primary fluid inclusions (determined via CRYO-SEM-EDS) from mid-Neoproterozoic marine halite, which extends our knowledge of seawater chemistry by ~300 My. The 840-830 Ma Cryogenian Browne Formation, from the Officer Basin, Australia consists primarily of dolomite, anhydrite, and halite. Bromide analyses of 73-122 ppm fall within the range expected for halite precipitated from seawater. The Browne Fm is correlated with the Gillen Member, Bitter Springs Fm, Amadeus Basin, Australia, both biostratigraphically using stromatolites and acritarchs, and chemostratigraphically using C and Sr isotopes (Grey et al. 2005).

The chemistry of latest Neoproterozoic seawater determined from fluid inclusions from the Ara Formation, Oman (544 Ma) is similar to the chemistry of modern seawater with Mg²⁺, K⁺ and SO₄^2- in relatively high concentrations and low Ca²⁺(Brennan et al. 2004). In contrast, primary fluid inclusions from the Browne Fm contain Ca²⁺, relatively lower Mg²⁺ and K⁺, and very low SO₄^2-. When Ca²⁺ exceeds SO₄^2-, SO₄^2- is removed from the evaporating brine during gypsum precipitation. Low sulfate levels were also interpreted from Neoproterozoic evaporite δ³⁴S_SO4 and carbonate-associated sulfate (Strauss 2004; Kah et al. 2004). The low K⁺ in the Browne Fm could result from its removal during precipitation of late-stage potash salts (after halite); however, there is no evidence of K-salts in the Browne Fm.

Fluid-inclusion data from recrystallized halite, Bitter Springs Fm, Amadeus Basin, yielded similar results (Kovalevych et al. 2006). Despite recrystallization, the similarity of results from the Bitter Springs Fm and the Browne Fm suggests a global “calcite-sea” seawater signal for the mid-Neoproterozoic.