PIXE and microthermometric analyses of fluid inclusions in hydrothermal quartz from the 2.2 Ga Ongeluk Formation, South Africa: Implications for ancient seawater salinity

• We carried out PIXE and microthermometric analyses for individual fluid inclusion.
• Analyses were conducted for primary and secondary inclusions, separately.
• Primary inclusions have endmembers of two hydrothermal fluids and seawater.
• 2.2 Ga Ongeluk seawater salinity was estimated to be ca. six times the modern value.

The Ongeluk Formation mainly consists of submarine volcanics (pillow lavas and sheet flows) composed of basaltic andesites that are between underlying and overlying glaciogenic deposits (i.e., the Makganyene diamictite and a dropstone layer at the base of the Hotazel Formation, respectively). The stratigraphic position of the Ongeluk Formation indicates that the Ongeluk volcanism occurred during a period of global glaciation. The Ongeluk volcanic rocks are host to subseafloor hydrothermal quartz deposits as drainage cavities and interpillow voids. The hydrothermal quartz contains many primary (Type 1) and secondary (Type 2) liquid–vapor fluid inclusions, as well as inclusions that are randomly distributed without a trace of secondary healed cracks (Type 3). All these fluid inclusions types were individually analyzed with microthermometry and particle-induced X-ray emission (PIXE) methods. The results show that Type 1 fluid inclusions are highly saline, whereas Type 2 fluid inclusions are relatively less saline. Type 3 fluid inclusions have bimodal peaks of salinity corresponding to those of Type 1 and Type 2 fluid inclusions and thus appear to represent mixtures between latter two inclusion types. Among the various fluid inclusion salinities of fluid inclusions, a wide range in Na/Ca values was identified in the high-salinity fluid inclusions (i.e., Type 1 and a subset of Type 3) which are thought to represent subseafloor fluids circulated by the Ongeluk submarine volcanism. The wide range in Na/Ca values can be explained by a mixing process between Na-rich and Ca-rich fluids associated with albitization of the host basaltic andesites. In the albitization process, Na is removed from the fluids and fixed by the host rocks; by contrast Ca is released from the host rocks into the fluids. PIXE analysis also showed two distinct trends (i.e., vertical and horizontal) on variation diagram of Ca/Cl versus Cu/Cl and positive correlations between Mn/Cl, Cu/Cl, Zn/Cl and Pb/Cl. These data are best explained by the presence of other mixing processes between; (1) a Na-rich, Ca- and Cu-poor fluid, (2) a Ca-rich, Na- and Cu-poor fluid, and (3) a Na- and Cu-rich, Ca-poor fluid. The Cu-, Mn-, Zn- and Pb-rich fluid likely represents a high-temperature hydrothermal fluid from a deep reaction zone in the Ongeluk subseafloor hydrothermal system. By contrast, the (1) Na- (and K-) rich, Ca- and Cu-poor endmember for the high-salinity primary inclusions is considered to represent the composition of 2.2 Ga Ongeluk seawater. We propose the estimation of 2.2 Ga Ongeluk seawater composition is >∼2237 mmol/kg of Na, 200–1000 mmol/kg of K, <135 mmol/kg of Ca, ∼3230 mmol/kg of Cl (average values), and ∼400–500 of Cl/Br.