TRACE ELEMENT PROFILES ACROSS ZONED TITANITE CRYSTALS FROM THE HALF DOME GRANODIORITE, YOSEMITE NATIONAL PARK, CALIFORNIA

Titanite is an important accessory mineral in silicic magmas owing to its high partition coefficients for important trace elements such as REE, Y, Zr, Th, and U. In particular, partition coefficients of 100-1000 and slow diffusion of REE mean that titanite crystallization should rapidly deplete magmatic liquid of those elements via Rayleigh fractionation. We are examining trace element variations in large (up to 1 cm across) titanite crystals from the Late Cretaceous Half Dome Granodiorite to test this hypothesis.

Backscattered electron images reveal several types of complex zoning: oscillatory, sawtooth, sharply discontinuous, patchwork, and sector. Electron microprobe (EMPA) and LA-ICPMS analyses reveal elemental variations that are similarly complex. Backscatter brightness images REE concentration rather than Fe content. Light REE reach concentrations of 10,000x chondrite, declining to ~1000x chondrite for heavy REE; most patterns have a negative Eu anomaly, and concentrations vary by a factor of 2 over a few μm. REE concentrations are highly variable but generally decline by a factor of 2 from core to rim. However, many crystals have dark, REE-depleted zones at the core. Many crystals also have discrete zones containing ilmenite, anatase, and other oxides.

If Rayleigh fractionation is the process of crystallization, there should be a dramatic decrease in the concentration of trace elements from core to rim in titanite. The sawtooth variations of REE follows the concentric zoning pattern of the titanite crystal, while also decreasing in overall concentration towards the rim. However, there is an oscillation superimposed on a decrease in concentration. Complex zoning, REE variability, and this oscillation from core to rim suggests that there are other mechanisms besides Rayleigh fractionation in which titanite becomes enriched in trace elements as it crystallizes.