THE GROWTH KINETICS IN SECOND PHASE AFFECTED SYSTEMS: THE STATIC CASE

Under contact metamorphic conditions, carbonate rocks in the direct vicinity of the Adamello pluton reflect grain coarsening of the matrix calcite (Dcc) from 5µm to 1cm with increasing temperature. Despite this large-scale trend, a considerable scatter in calcite grain size occurs at the different locations. This is due to Zener drag of the calcite grain boundaries by second phase particles, whose grain sizes (dp) range from the nano- to the micron-scale. Nano-scale particles are dominantly kerogens, while the micro-scale particles are sulfides and/or micas. The volume fraction (fp) of the second phases varies (<10vol%) inducing variability in calcite grain size of a factor of 10. In contrast to the calcite grain size, the size of the micron-scale particles does not change systematically with temperature. Nevertheless, the Zener parameter (Z=dp/fp) correlates with the variability in calcite grains via (1) Dcc=C1 Z^m and therefore the growth-kinetics at given temperature can be described as: (2) dD/dt=K * (F-D), where (F-D) is the net driving force of grain growth consisting of the driving force (F) and the retardation force (D) which is caused by dragging and pinning of grain boundaries by second phases. During growth, the driving force (F) continuously decreases with increasing grain size, whereas the retardation force depends on the quantity and size of second phase particles. Hence the term (F-D) is a temperature independent geometric term, whereas the constant K incorporates the growth kinetics which is temperature dependent. Thus, the final grain-size at any given T-t path depends on driving/retardation forces as well as on the kinetic parameters. Incorporating now the effect of Zener drag (1) into equation (2) yields: (3) Dcc=C2* exp(Q*/RT) * Z^m*, where C2 is a constant, Q* is a activation energy, T the temperature, and m* the exponent of the Z. Using data from the investigated contact-metamorphic carbonates and data from Mas & Crowley (1996), a frist guess on C2 and Q* can be made. However, more data are needed to obtained reliable kinetic parameters.

Mas, D. L. & Crowley, P. D., 1996. Journ. Metam. Geology, 14, 155-162