Rock-forming processes

MINERAL- AND ROCK-FORMING PROCESSES - CRYSTALLIZATION AND PRECIPITATION... 

The Rare Earth Elements (REE-Lanthanides and Y) are commonly used to unravel rock-forming processes because of their simiiar chemicai properties, typicaiiy iow soiubiiities and assumed resistance to fractionation in crustai and surface environments. However, under some weathering conditions REE are significantiy mobiiised and fractionated (e.g., Nesbitt 1979 Duddy 1980 Sharma Rajamani 2000). 

Elements with ionic species commonly associated with quartz and mica minerals such as Na, Zr, and Hf are more likely to co-vary with those minerals rather than hematite. The ratio of these latter elements to Fe-substituting elements would be dependent on the relative amounts of hematite and other minerals in the sample rather than just die rock-forming processes. For this reason, elements that commonly substitute into quartz and other abundant minerals in the samples were excluded from the discriminant analysis. Some rare earth elements may also play a role in fingeiprinting the sources, and their importance was tested by comparing discriminant analysis models with and without La, Sm, and Th. 

Primary porosity The porosity preserved from some time between sediment deposition and the final rock-forming process (e.g., spaces between grains of sediment). 

Experimental studies of phase equilibria at elevated temperatures and pressures have had a tremendous impact on deciphering petrogenetic processes during the last three decades. It was soon realized, however, that complete understanding of rock-forming processes can hardly be achieved by empirical experiments alone. 

Stdffler, D. (1972), Deformation and Transformation of Rock-Forming Minerals by Natural and Experimental Shock Processes, I, Fortschr. Miner. 49, 50-113. 

Life evolved soon after Earth s formation, befiare any continents were present, during a time when the oceans were chemically and thermally controlled by tectonic processes. Thus, it has been proposed that the life evolved in hot anaerobic submarine environments similar to present-day hydrothermal vent systems. This hypothesis is supported by the observation of structures, thought to be the remains of protocells, in rocks formed by hydrothermal processes 3.5 to 3.8 billion years ago. Thus, the first organisms on Earth were probably anaerobic hyperthermophiles. Hydrothermal vent habitats probably offered an additional benefit by providing a stable environment relatively isolated from the catastrophic effects of bolide impacts. In other words, submarine hydrothermal vents coifld have acted as refugia enabling survival of early life forms. 

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