Constraints on Earth’s atmospheric evolution from a gas-aqueous partition of fluid inclusion volatiles

Abstract

Recent attention has been paid to fluid inclusions in surficial minerals for their ability to capture and preserve aliquots of ancient atmospheric gas. Through mechanical or thermal decrepitation, the volatiles trapped in these multiphase inclusions can be analyzed by mass spectrometry, providing direct constraints on the composition of the Earth’s ancient atmosphere. It is often assumed that this measured gas composition reflects directly the atmosphere under which the minerals precipitated. However, when the effects of gas solubility are neglected, the interpreted atmosphere is likely to be erroneous, reflecting a mixture of gas and brine. Here, we present a novel technique and computer code, MAGPI (Method for Atmospheric Gas Partitioning from fluid Inclusions), to partition the atmospheric volatiles between the gas and aqueous phases present at the time of inclusion formation and volatile entrapment. The N2/40Ar ratios of the released gases are used to calculate the volume fractions of the gaseous and aqueous phases present at the time of entrapment, which allows the observed gas ratios to be corrected to accurately reflect the composition of the atmosphere under which they formed. We validate our method on contemporary halite fluid inclusions, and then apply it to existing data from a suite of Tonian (815 Ma) halite and gypsum evaporites from the Browne Formation, Australia, and the Minto Inlet Formation, Canada. The results of our partition indicate that the Tonian atmosphere contained 92.83 ± 0.70 % N2, 6.62 ± 0.71 % O2, 0.47 ± 0.01 % Ar, and 0.08 ± 0.07 % CO2, which is consistent with other proxy and model reconstructions of the Neoproterozoic atmosphere. These results demonstrate the importance of phase chemistry in fluid inclusion gas analyses and provide a fundamental framework for future studies of Earth’s atmospheric evolution through inclusion gases.