SURVIVAL OF ORGANIC MATTER AFTER HIGH TEMPERATURE EVENTS (METEORITE

Introduction: A potential consequence of the high temperatures involved in impact events is that organic matter in the target is destroyed. Where the temperatures are so high that rocks are volatilized, destruction of the organic matter is intrinsic, although conceivably some methane generated by the process could become entrapped in adjacent rocks. Where rocks are simply heated and deformed by the impact, some organic matter may survive, but heating above 200 to 300 oC is likely to leave this matter as a refractory solid from which little biomolecular data can be extracted. However a small fraction of organic material may be entrapped within fluid inclusions in rocks, where it has a greater chance of survival. Valuable lessons about the fate of organic matter can be gained from the response to igneous intrusions: intrusions and impacts both involve temperatures of several hundred degrees and higher, sudden exposure for a geologically instantaneous period, and cross-cutting of existing formations where organic matter resides. The preservation of organic matter after impacts is important, as this gives us an insight into the likely survival of life/fossil remains on heavily cratered planets like Mars and the early Earth. Preservation of organic matter within fluid inclusions: Fluid inclusions are micron-scale volumes of fluid entrapped during mineral growth which in sedimentary rocks may be during cementation or during healing of microfractures. They are therefore representative of the ambient fluid present during these processes. The preservation of evidence for life inside fluid inclusions could be in the form of trapped microbes or, more likely, biochemical signatures in the water. Microbes are found in terrestrial fluid inclusions, particularly in rock salt (halite) where the inclusions can be relatively large, up to millimetre-scale [1, 2]. In some cases, RNA extracted from salt deposits in the geological record is interpreted to represent fossil organic material, which is presumed to be located particularly in fluid inclusions [3]. The water in inclusions represents a potential source of organic molecules that may be a more direct signature of former life [4]. The metabolism and degradation of bacteria and algae contribute to all of the major groups of organic compounds that are dissolved in natural waters, including hydrocarbons, carbohydrates, carboxylic acids, amino acids and humic acids [5]. A few of these products have longterm stability. Natural waters therefore contain traces of compounds that reflect the occurrence of microbial life in the environment. There is therefore a substantial value in trying to develop the capability to extract such compounds from inclusion fluids, both in terrestrial settings and in the surface/near-surface deposits of Mars [6]. A range of surface environments could include mineral precipitates [7] that contain fluid inclusions representing the ambient water and any accompanying traces of life. Fluid inclusions have a potential for survival over billions of years. Terrestrial studies of fluid inclusions of Archean age show that they have preserved organic compounds for over 2 billion years, including abiotic organic compounds [8] and oils derived from Archean microbial matter [9]. Meteorite data similarly shows that inclusion waters can be preserved over this time scale [10]. Survival of organic matter in inclusions during heating: There are important parallels with the preservation of fluid inclusions, including hydrocarbon inclusions, in terrestrial settings where petroleum systems have encountered temperatures and pressures higher than normally encountered in sedimentary basins. Inclusions containing higher hydrocarbons are preserved in sites where microthermometry indicates temperatures of up to 200 oC or higher [11]. Hydrocarbon inclusions are regularly recorded in ore deposits which have been formed by anomalous hydrothermal activity [12, 13]. In one of the most extreme examples documented, hydrocarbons up to C33 size have been extracted from fluid inclusions in an ore deposit that experienced temperatures of 500-600 °C [14]. Hydrocarbons also survive the intrusion of magmatic bodies into organic-rich sedimentary rocks. There are a range of responses to such intrusion (Fig. 1, [15]): particular aspects that have parallels in impact craters are (1) a quite limited zone of thermal alteration, (2) devolatilization of organic matter to a refractory state within this zone, (3) instigation of hydrothermal circulation, and (4) extensive adjacent fracturing. The latter two aspects cause the entrainment of organic materials in hydrothermal systems, evidenced by hydrocarbon fluid inclusions [15].