Vesicle paleobarometry in the Pongola Supergroup: A cautionary note and guidelines for future studies

Abstract

Abstract Earth’s global barometric pressure, currently 1 bar at sea level, may have changed over its 4.5-billion-year history. Proxy measurements, including N2/36Ar ratios in ~3.5 to 3.0 Ga hydrothermal quartz, ~2.7 Ga raindrop imprints, and ~2.7 Ga vesicle sizes in subaerial basalt lava flows indicate Archean air pressure could have been between 0.1 and 1.2 bar. However, some models argue air pressure in the Archean should have been much higher than now and could allow pressure broadening of greenhouse gas absorption lines to counteract the “Faint Young Sun”. Thus, additional paleobarometric measurements would be useful to further constrain Earth’s atmospheric evolution. We attempted to use vesicle sizes in lavas erupted near sea-level from the ~2.9 Ga Pongola Supergroup from Mahlangatsha and Mooihoek, eSwatini (formerly Swaziland) and the White Mfolozi River gorge of KwaZulu-Natal, South Africa to provide further Archean paleobarometric data. However, reliable results were unobtainable due to small and scarce amygdales, irregular vesicle morphologies and metamorphic mineralogical homogenization preventing the use of X-ray Computed Tomography for accurate vesicle size determination. Researchers attempting paleobarometric analysis using lava vesicle sizes should henceforth avoid these areas of the Pongola Supergroup and instead look at other subaerially emplaced lava flows. With this being only the second time this method has been used on Precambrian rocks, we provide a list of guidelines informed by this study to aid future attempts at vesicular paleobarometry.