Graphitization of carbonaceous material in sedimentary rocks on short geologic time-scales: An example from the Kinsho-zan area, central Japan

摘要

Initially amorphous carbonaceous material becomes more crystalline with heating. The structural change depends not only on the maximum attained temperature but also the time-scale of heating. Raman spectroscopy of natural samples that have been heated for time-scales of 10(5) years or greater show that the degree of crystallinity has reached steady-state. In contrast, laboratory studies show very little change in crystallinity of carbonaceous material (CM) after heating at 1000 degrees C for a time of 3.5 weeks. Better constraints on the time-scale for crystallization require experiments on time-scales of years to thousands of years; such long time-scales can only be derived from natural examples of CM-bearing rocks that have been heated for a known length of time. Thermal modeling of contact metamorphism developed around a 13m dike within the Akasaka Limestone in Gifu Prefecture shows the time-scale of heating is of the order of 1-100 years. Raman spectroscopy reveals a significant increase in the crystallinity of the CM in a region within 3m from the dike. A comparison between the temperature predicted for the contact aureole and the degree of crystallinity of the carbonaceous material shows that even close to the dike the CM has not reached steady-state. This change began at over 550 degrees C (modeled temperature) for a time-scale of heating of a few years. Attaining steady-state in the crystallization of CM under natural geological condition requires heating on time-scales greater than about one hundred years. This study shows the utility of using natural laboratory studies to determine the kinetics of CM crystallization in rocks.