Experimental studies of trace-element partitioning applicable to igneous petrogenesis - Sedona 16 years later

In the 16 years since the Sedona Conference on the behaviour of trace elements in silicate systems, numerous studies providing new data have filled many of the gaps in knowledge of trace-element partitioning evident at that conference. The advent of new microbeam techniques for in situ trace-element analysis has provided great impetus for this work. For example, values for large ion lithophile element (LILE) and high field strength element (HFSE) partitioning between olivine, pyroxene, garnet, amphibole and titanate minerals and silicate liquids have been determined. When plotted on mantle normalizing geochemical diagrams, partition coefficients for the main mantle silicate minerals show steeply inclined patterns (over several orders of magnitude) from LILE to heavy rare-earth elements (HREE). Amphibole, however, has a relatively flat pattern (though still favouring HREE over LILE by about an order of magnitude). Also, there is a notable flattening of the patterns for HREE in pyroxenes and garnet relative to olivine. The effects of pressure, temperature and melt composition on trace-element partition coefficients have been evaluated, as well as crystal-chemical controls on the substitution of trace elements in minerals. This has led to formulation of relationships between mineral compositions and trace-element partition coefficients for olivine, low-Ca pyroxene and calcic pyroxene. These studies have been motivated by the need of geochemists for partition coefficient data to apply to models of igneous petrogenesis and mantle evolution. Overall, the new data show systematic and consistent behaviour, as determined in different laboratories. Also, partitioning relationships for key elements between selected mantle minerals and H₂O-rich fluids have been established experimentally, although further work exploring the effect of variable fluid composition, for a wider range of mantle minerals is needed. Future work will allow refinement and more precise "fingerprinting" of geochemical processes, including the role of fluids and trace element-enriched accessory minerals in metasomatic events. Also, experiments are needed to extend the partition coefficient determinations to much greater pressure, to resolve controversies over the geochemical evolution of the mantle at depths greater than ≈ 130 km.