PETROLOGICAL MODELLING – APPLICATIONS AND LIMITATIONS (Invited Presentation)

Metamorphic and igneous processes leave footprints that vary from micro to macro scales. Recent analytical advances allow us to collect large quantities of increasingly precise geochemical and geochronological data from metamorphic and igneous systems. These large datasets can be interrogated with sophisticated statistical techniques—including artificial intelligence approaches—that generate conceptual models of crustal processes and Earth’s long-term evolution. Testing these ensuing hypotheses requires linking geochemical datasets to underlying petrological processes. These processes have many controlling variables; the relative importance of each variable is not always clear. This results in a major gap between geochemical trends and understanding the dominant petrological controls. Petrological modelling is an exploratory tool that can test the wide variety of metamorphic and igneous controls that yield specific geochemical signatures. Here, I present two case studies.

First, I explore the petrological controls on the anomalies in redox-sensitive rare earth elements (europium and cerium) in rocks and accessory minerals. Europium and cerium anomalies are a complex function of the interplay between oxygen fugacity and the residual mineral assemblage, which both vary with pressure and temperature. Such anomalies are unlikely to be effective thermometers and barometers in crustal rocks.

Second, I present a cautionary tale on the limits of an equilibrium approach to petrochronology. I use a suite of ultra-high temperature granulites from southwest Peru to illustrate how monazite with similar ages have variable trace element concentrations depending on microstructural setting. Petrological modelling of trace element compositions in monazite does not reproduce measured values. These results suggest that whole-rock equilibration of trace elements between residual minerals (including accessory minerals) and melt may be unlikely. Microstructural setting is a dominant control on trace element concentrations in accessory minerals even during ultra-high temperature metamorphism. This raises doubts about the utility of equilibrium-based petrological modelling to accessory mineral behaviour in some natural systems.