RARE EARTH ELEMENT PARTITIONING BETWEEN TITANITE AND GROUNDMASS IN SILICIC VOLCANIC SYSTEMS

High trace-element concentrations in titanite (CaTiSiO₅) make it an important accessory mineral in intermediate to silicic igneous systems. Knowing the magmatic conditions of titanite crystallization and the degree to which titanite partitions trace-elements will help us to better understand the evolution of and connection between plutonic and volcanic rocks. In this study we present data on the stability of titanite in silicic magmatic systems and the factors influencing its partitioning of trace-elements.

We studied the major and trace-element compositions of titanite and glass separates from seven Tertiary volcanic units in the western US: the Fish Canyon Tuff, Pagosa Peak Dacite, Wall Mountain Tuff, Bonanza Tuff, Ammonia Tanks Member of the Timber Mountain caldera, Tiva Canyon Member of the Oasis Valley caldera, and the Peach Springs Tuff. We also analyzed the major and trace-element composition of titanite grains from plutons in the Sierra Nevada and Mount Princeton batholiths. Titanite grains exhibit a range of REE concentrations, with Ce concentrations ranging from 8,000 to 14,000 ppm, and negative Eu anomalies of variable strength. Titanite/groundmass ratios range over an order of magnitude, with the lowest values belonging to the Wall Mountain Tuff (Ce/groundmass=120), and the highest belonging to the Peach Springs Tuff (Ce/groundmass=1080). We speculate that partitioning behavior is controlled mainly by the melt composition and the degree of melt polymerization, while temperature and pressure likely have little effect. Furthermore, a comparison of plutonic and volcanic titanite shows higher concentrations of REEs (except Eu) in most volcanic titanite grains, indicating crystallization from more polymerized melts with higher REE concentrations.