One of the most fundamental questions for humanity is how life emerge on Earth and in this context how were the first prebiotic organic compounds formed and distributed. Therefore it is necessary to find and investigate environmental archives that retained information from the early Earth.
The analysis of fluid iclusions (FIs) is a widely used geochemical tool to determine thermobaric and chemical evolution of geologic systems. Most naturally grown crystals contain inclusions of other minerals, melts, fluids and gases, which were originally present at the mineral surfaces during crystal growth and then included. FIs are important archives and their investigation allows drawing conclusions on their original fluid composition. Several generations can be present and it is crucial to differentiate between primary FIs formed during mineral growth and secondary and pseudo-secondary FIs formed during later geological processes. These mineralogical aspects are essential when attributing ages to the rocks hosting FIs.
FIs also represent archives for organic matter e.g. in the form of hydrocarbons and biomarkers from which their biological origin can be inferred. The analysis of oil and natural gas in FIs has become an important tool in petroleum exploration to investigate porosity evolution, thermal history, source regimes and migration pathways alongside formation mechanisms, product types and quality.
Gas-liquid inclusions can be found in all types of rock. Aside from stable inorganic gases like N2, Ar, CO2, O2). FIs often contain methane and its homologues. A few concepts of the abiotic origin of CH4 and longer-chained hydrocarbons also exist and will be outlined. Sherwood Lollar studied the isotopic signatures of C1-C4 hydrocarbons in field samples pointing to CH4 as a precursor to forming longer-chained hydrocarbons.
Next to the classification of hydrocarbon formation into bacteriogenesis and thermogenesis further abiotic CH4 production pathways have to be considered (see Fig.1). The abiotic field can be subdivided in the more enriched 13C- and 2H values from high temperature volcanic-hydrothermal systems and serpentinised ultramafic rocks and the more depleted 13C- and 2H-values from crystalline igneous rocks and present day serpentinisation seeps.
Fig.1: δ13C and δ2H isotopic composition of biotically formed CH4 in subsoil petroleum systems (red: thermogenic; green: biogenic) and of abiotically formed CH4 (black-rimmed: abiotic) (Schreiber et al.).
Thermogenic processes also produce a variety of volatile organohalogens (VOX), mostly chlorinated and fluorinated alkanes, alkenes and alkynes, but also cyclic and aromatic halogenated compounds. Halogenated methanes are the prevailing molecules emitted by volcanoes. The studies of Harnisch and Eisenhauer as well as Harnisch et al. are ground-breaking in VOX analysis from FIs of rocks and minerals. They demonstrated that CF4 and SF6 are commonly present in natural fluorites and granites. Additionally, they detected CF2Cl2 and CFCl3 from a number of natural samples and CF3Cl, CHF3 and NF3 from one fluorite sample. A multitude of organohalogens have additionally been detected by Mulder at al. in FIs from rocks and minerals after grinding.
U. Schreiber, C. Mayer, O.J. Schmitz, P. Rosendahl, A. Bronja, M. Greule, F. Keppler, I. Mulder, T. Sattler, H.F. Schöler: Organic compounds in fluid inclusions of Archean quartz –
Analogues of prebiotic chemistry on early Earth. PLoS ONE (2017) 12(6) e0177570
I. Mulder, S.G. Huber, T. Krause, C. Zetzsch, K. Kotte, S. Dultz, H.F. Schöler: A new purge and trap headspace technique to analyze low volatile compoundsfrom fluid inclusions of rocks aand minerals
Chemical Geology (2013) 358:148–155