Sphene (titanite): phase relations and role as a geochronometer

Abstract

Useful U–Pb isotopic data may be obtained from sphene (or titanite, CaTiSiO 5) because: (1) it is a widespread accessory mineral, (2) it can incorporate uranium in its structure, and (3) it has a high closure temperature. In igneous rocks, sphene is most abundant in relatively oxidized rocks, such as metaluminous rocks of intermediate composition. These rocks have the high Ca/Al ratios wherein sphene is stabilized relative to ilmenite+quartz or ilmenite+anorthite. In metamorphic rocks, sphene is stable to the highest temperatures in mafic and calc-silicate rocks. It is found mostly in greenschist, blueschist, and amphibolite facies, although in calcic rocks its stability may extend into granulite facies. Recent studies show that the closure temperature for sphene lies at the upper limit of amphibolite facies. Because sphene reacts readily during metamorphism, U–Pb sphene ages are likely to yield the age of metamorphic crystallization, rather than resetting by simple diffusion. For this reason, metamorphic sphene may yield complex U–Pb systematics that contain information on the whole metamorphic history of the rock. Sphene from igneous rocks and orthogneisses has initial U contents ranging from 10 to over 100 ppm and ratios of initial U to common Pb ranging from 10 to 1000, ratios that may potentially yield high precision U–Pb ages. Sphene in marbles, calc-silicates, and metagraywackes has a similar range in composition to that from igneous rocks, but sphene from metabasites may have initial U contents of less than 1 ppm and ratios of initial U to common Pb lower than 1, making them unsuitable for geochronology. These low-U sphenes are most commonly found in weakly metamorphosed metabasites. Strategies to extract age information from sphene with moderate initial U/common Pb ratios include estimation of common Pb isotopic composition of sphene from coexisting low-U phases, use of U–Pb and Pb–Pb isochron plots, and step-wise leaching methods to improve 206Pb/ 204Pb spread. By correlating sphene compositions to metamorphic or hydrothermal reactions, age determinations on sphene can be used to directly date metamorphism, deformation, and hydrothermal alteration.