TRACE ELEMENT VARIABILITY IN TITANITE FROM DIVERSE GEOLOGIC ENVIRONMENTS

Titanite (ideally CaTiO[SiO₄]) is a widespread mineral in a variety of igneous, metamorphic and hydrothermal rocks. Among the common accessory phases, titanite is of particular geochemical interest because it can incorporate a diverse suite of trace elements (including U and Th, of geochronologic value, and Zr for geothermometry) that may be diagnostic of the local chemical and P-T conditions of its formation. To explore these compositional variations, we used the Sensitive High-Resolution Ion Microprobe (SHRIMP-RG) to measure the trace element contents of titanite from a reconnaissance collection of differentiated calc-alkaline granitoids, mid-ocean ridge (MOR) mafic intrusives, amphibolite facies orthogneisses and mylonites, and alkali-chloride metasomatic assemblages accompanying Fe-oxide-Cu-Au (IOCG) mineralization. The SHRIMP-RG is ideally suited for these measurements, coupling extreme mass resolution, necessary to eliminate most problematic molecular interferences, with high sensitivity and fine-scale spatial and depth resolution.

Analytical data confirm extreme variability in rare earth element (REE) abundances both among the different rock types and within sets of zoned crystals from individual samples. Chondrite-normalized REE patterns may show peak enrichments in the LREE (La-Nd, with a maximum typically at Pr or Nd), MREE (Sm-Dy) or HREE (Ho-Lu), or can be virtually flat. Concentrations of dominant REE approach 10000X chondritic values for titanite from many of the granitoid and hydrothermal samples, but among several of the metamorphic and MOR samples the least abundant REE may be less than the chondritic values. Eu anomalies (Eu/Eu*) may be negative or positive, but within a suite trend towards more positive values with decreasing total REE and concomitant decreasing temperature (T). Indeed, a detailed set of titanite analyses from two southern California granodiorites show progressive evolution in Eu/Eu* as well as complex systematic changes in the overall REE pattern curvature as functions of T (calculated by Zr-in-titanite geothermometry). Titanite trace element geochemistry promises to be a valuable complement to other petrologic tools in better understanding diverse geologic processes and environments.