REACTION RATE LIMITING MECHANISMS DURING CONTACT METAMORPHISM

Porphyroblast formation in metamorphic rocks has recently received increased attention, since an understanding of their growth and nucleation kinetics would provide an independent measurement of duration and rates of metamorphic processes. Application of laboratory based rate kinetics is only possible if the rate is determined by material properties of individual minerals (e.g. surface kinetics, solid state diffusion). Grain boundary transport properties of rocks obtained in laboratory experiments are difficult to apply to reacting natural rocks, since grain boundary properties change during growth. Controlling mechanisms need to be identified in field studies for which contact aureoles are ideal model systems. 3-D textural evidence, stable isotope geochemistry of reactants, and textural models provide clear evidence for grain boundary diffusion limited growth of olivines in the outer olivine zone of the Ubehebe Peak contact aureole. Calcite precipitated by this reaction surrounds tabular olivine crystals. It was difficult to displace since only small (or no) gradients in its chemical potential existed. It hinders olivine growth. In contrast, fluid advection in the inner part of the olivine zone led to a change in olivine morphology (Roselle&Baumgartner, 1997). Increased grain boundary diffusion might have resulted in a surface kinetics controlled growth of these round olivines. Obvious diffusion limited structures are reaction rims of Ca- and Mg-silicates surrounding quartz concretions in carbonates (e.g. Joesten&Fisher, 1988). These segregations can serve as case studies to understand the behavior of stable isotopes during grain boundary diffusion limited mineral growth. Different models need to be invoked to explain stable isotope data on closed-system wollastonite growth: instantaneous isotopic mass balance and zone refining models. Rayleigh type models do not describe the observed patterns. In summary, we propose that most net transfer mineral reactions are transport limited in contact aureoles. Surface kinetics limited reactions occur only in fluid-rich environments.

Joesten&Fisher, 1988, Am.Bul. v. 100, 5; Roselle&Baumgartner, 1997, Geology, v. 25, 9.