AbstractsEarth & Environmental Science

Volatile Organohalogens from Fluid Inclusions of Rocks and Minerals

by Ines Mulder

Institution: Universität Heidelberg
Department: Chemie und Geowissenschaften
Degree: PhD
Year: 2015
Record ID: 1111845
Full text PDF: http://www.ub.uni-heidelberg.de/archiv/18184


Volatile organic halocarbons (VOX) play an important role in atmospheric processes. However, biogeochemical release mechanisms from terrestrial environments are complex, not well understood in most parts and a clear view of their relative importance is lacking. Previously, the lithospheric VOX formation potential was subject of only few studies. In the first part of this thesis the development of a new method for the analysis of VOX from rocks and minerals is reported in order to investigate terrigenic VOX formation potential. The purge and trap GC-MS system was optimized for the analyses of halogenated volatile organic compounds having boiling points as low as -128 °C for carbon tetrafluoride (CF4). The design of the U-shaped glass lined steel tube (GLT™) cold trap for sample preconcentration and the rapid desorption via resistant heating transferred the desorbed analytes directly onto the GC column via a deactivated capillary column retention gap made sample re-focussing unnecessary. Furthermore, a special air-tight grinding device was developed in which samples ranging from soft halite (hardness 2, Moh’s scale) to hard quartz (hardness 7) are effectively ground to average diameters of 1000 nm or below, thereby releasing gases from fluid inclusions of minerals. The gases are then purged from the grinding chamber with a He carrier gas flow. In the second part of this work, the newly developed method is applied to a set of various mineral and rock samples including fluorite, quartz and halite. The analytical results from GC-MS prove the presence of a wide spectrum of volatile compounds from FIs trapped in various minerals. SF6 and CF4 were released from fluorites. Methyl bromide, dichloroethene and dichloroethane were detected in quartz samples from the Archean Yilgarn craton in Australia. Methyl chloride (MeCl) has been detected from almost all samples, including halites, fluorites, quartz and dolerites. Initial heating experiments with halites using purge-and-trap GC-MS as well as pyrolysis-GC-MS demonstrated the important role of temperature in MeCl and VOX formation. Finally, in the last part of this dissertation a case study on one possible formation pathway for the volatile compounds MeCl and dimethylsulfide (DMS), via thermolytic degradation of the amino acid derivative methyl methionine is investigated. A fast response of MeCl and dimethylsulfide emission upon heating of freeze-dried samples at 40 °C was observed and made this a plausible abiotic volatile formation mechanism. Besides the mechanistic studies with methyl methionine and structurally related substances, the emission of MeCl and DMS from fluid inclusions, soil samples of terrestrial salt lakes and air sampled immediately above the salt lake surfaces indicated the relevance of this formation pathway for hypersaline environments.