MODELING OF RARE EARTH ELEMENT ZONATION IN GARNET FROM HIGH PRESSURE/LOW TEMPERATURE (HP/LT) LAWSONITE ECLOGITE FROM THE MOTAGUA FAULT ZONE IN CENTRAL GUATEMALA

Trace element concentration profiles of garnet from high pressure/low temperature (HP/LT) lawsonite eclogite from the Motagua Fault Zone in Guatemala display an unusual zonation pattern with a central peak for heavy rare earth elements (HREEs) and depletion towards the rim, but with an atypical additional peak of HREEs at the rim. Major element compositions (Fe, Mg, Ca, and Mn) also show typical chemical zonation from the core of the garnet to approximately halfway to the rim. After this point major element compositions, including Mn, show a marked compositional shift that is atypical for prograde garnet growth. The variations in major elements allow for multiple interpretations with these compositions being sensitive to changes in pressure, temperature, and fluid composition. Therefore, due to their higher sensitivity, REEs may serve as a better indicator to the processes experienced during growth.

While trace element concentration profiles of garnets have been modeled at higher temperatures, the behavior and zonation of REEs in garnets at lower temperatures, like those determined for the lawsonite eclogite found along the Motagua Fault Zone, have not been fully characterized. We have calculated kinetic models that attempt to reproduce the observed chemical zonation profiles using different growth rate laws and growth conditions. The different parameters for these calculations includes a temperature range of 300-520°C, as well as use of two different growth models for garnet, which include interface-limited growth, and diffusion-limited growth. Our results from these calculations indicate that garnet nucleation must occur at temperatures greater than 300°C, but lower than 400°C in order to produce a secondary peak of HREEs towards the rim of a garnet. Of the two growth models, our calculations show that the observed chemical zonation in our garnets was best reproduced using an interface-limited growth model, as that was the only way to produce the necessary broad central peak of HREEs. These results show that the chemical zonation for these HP/LT garnets can be explained with a simple growth history within its subduction zone, without fluid influx from an outside source, repeated burial with multiple episodes of crystallization or other complex thermal history.