Atmospheric gas in modern and ancient halite fluid inclusions: A screening protocol

Abstract

Abstract Halite possesses great potential for hosting and storing information vital to the reconstruction of Earth s ancient climate, seawater chemistry and evolving atmosphere. Here, we propose a screening protocol that not only distinguishes between primary and secondary halite, but also identifies fluid inclusions that carry original gas trapped during the primary crystallization process. An integrated multi-analytical protocol is presented for sample preparation, petrographic evaluation, analytical measurements and distinguishing original gases and contents from contaminants. The screening protocol starts with the visual inspection of halite for primary chevron and hopper features and/or milky appearance. Then, petrography is used to distinguish between primary and secondary (diagenetic) crystal fabrics and inclusions. The trace element and isotope geochemistry speak directly to the composition of the depositional and/or diagenetic fluids and to the formation in marine, non-marine or diagenetic settings. Furthermore, rare earth elements and redox sensitive elements may address the redox conditions of the salt pan/flat brines, whereas microthermometry helps characterize the depositional/diagenetic environment s temperature. Finally, gas extracted by quadrupole mass-spectrometer from gas bubbles in fluid inclusions is screened with concomitant CH4, CO2 and Ar contents to distinguish and quantify organic matter decomposition contributions and with 40Ar/36Ar to monitor modern atmospheric gas diffusion/leakage issues, and thus, contamination of the primary gas contents. Our test case shows that halite collected from cores is well-suited in maintaining its original mineralogical texture, chemistry and gas content. Our integrated screening protocol suggests that the 815 Myr old halite from the Tonian Browne Formation of the Officer Basin, Australia formed under atmospheric conditions of about 10.9% pO2 (back calculation) or 9.6% pO2 (mean calculation). The coeval Neoproterozoic gypsum collected in outcrop from Minto Inlet, Victoria Island, Canada underwent some mineralogical alteration. However, experiments suggest that depositional gypsum from outcrop/core may have potential of retaining vestiges of original gas in fluid inclusions.