Time Scales of Magmatic Processes from Modeling the Zoning Patterns of Crystals

The zoning patterns of crystals are a valuable source of information that can be used to reconstruct the processes which have occurred in magmatic reservoirs and volcanic conduits. We stress that detailed analyses of the mineral zoning and modeling based on chemical diffusion laws also allows the determination of the durations and rates of some magmatic processes. This potential has been exploited by relatively few studies, but it is capable of providing data that contribute to progress in the understanding and quantification of magmatic systems. 2 Modeling and interpretation of zoning patterns can be anchored within the existing theoretical framework and experimental data for diffusion in solid silicates. There are still many unknowns with respect to the atomistic mechanisms of diffusion and the influences of some experimental variables. Nonetheless, the available information allows robust time estimates to be obtained for some systems (olivine and plagioclase mainly) and certain magmatic processes that characterize open systems. 3 The ranges of time scales that can be accessed from modeling mineral zoning patterns depend on the diffusion rates and on our capacity to resolve concentration gradients. As the ranges of diffusion coefficients may vary by more than 13 orders of magnitude in a single mineral (e.g., from CaAl - NaSi exchange in anorthite to Li diffusion in plagioclase) and we can measure concentration gradients in crystal zones ranging over about 6 orders of magnitude in length (some tens of nm to cm), the ranges of time scales that can be accessed is almost unlimited. The combination of measuring multiple elements in a single crystal, and in multiple crystals and minerals, offers the opportunity to retrieve a large number of time determinations from a single thin section. The availability and precision of diffusion coefficients will improve in the future, as will our capacity to analyze increasingly low concentrations and resolve increasingly smaller gradients. An increasing array of processes and their time scales will become accessible in the future. 4 The time scale determinations that have been performed so far on igneous systems focus on volcanic rocks: i.e., much interesting research remains to be done in modeling zoning patterns in plutonic crystals. Processes such as magma mixing or assimilation occur rapidly and may predate eruption by a few months to about 100 years. These times are - two to three orders of magnitude shorter than those obtained from radioactive isotopes. However, detailed analysis of the two data sets shows that the two methods are retrieving times associated with different magmatic processes or events. These results are more complementary than contradictory. More studies focussing on the same types of materials are needed to test the degree of difference and complementariness between diverse approaches. This will require precise determination of diffusion coefficients plus isotopic and element abundances on short length scales combined with insightful petrographic scrutiny.