Autori: Ganguly J, Cheng W, Ducea M

Editorial: Geochimica et Cosmochimica Acta, 67 (18), p.116, 2003.

Rezumat:

Diffusion in minerals could induce or modify compositional zoning, and also reset mineral ages during the thermal evolution of rocks. These diffusion modified properties provide important records of the time scales of metamorphic processes. We critically summarize here the available diffusion data in garnet, which is the most commonly used mineral to recover these time scales from diffusion-kinetic modeling, along with new experimental data from our on-going research program. These data are applied to retrieve the time scales of pro- and retro-grade metamorphic processes and exhumation velocities.
Recent developments on the theory of closure temperatures and closure ages [1,2], which build on Dodson?s seminal works on this subject, permit determination of cooling rate of metamorphic rocks from the extent of resetting of bulk mineral age or from the difference between the core and bulk ages of a mineral, as determined by a single decay system. This method has been applied to a garnet sample from the Valhalla complex, British Columbia, using the Sm-Nd decay system, to simultaneously retrieve both the peak metamorphic age and the high temperature cooling rate. The latter is compatible with the cooling rate derived from the retrograde Fe-Mg zoning of the same crystal and earlier geochronological data [3].
We have recently determined the Fe-Mg interdiffusion in spinel [4] by diffusion couple experiments, and have now extended the program to study the coupled Fe-Mg and Cr-Al interdiffusion kinetcs. Cr/(Cr+Al) ratio is found to strongly enahance Fe-Mg interdiffusion, whereas Cr-Al interdiffusion is much slower than the latter. These data permit modeling of compositional zonings in natural spinels, especially in the ultramafic rocks, to retrieve their cooling rates, which are illustrated by applications to selected samples.

Cuvinte cheie: metamorphic rates, geochronology, Nd isotopes