Nd and Sr systematics in an eclogite xenolith from Tanzania: Evidence for frozen mineral equilibria in the continental lithosphere

Abstract

Combined isotopic (Nd, Sr) and petrographic analyses are reported for an eclogitic xenolith from the Vissury Mine, Tanzania. An exceedingly large difference in measured ϵ Nd values in garnet and clinopyroxene (>200 ϵ units) suggest a major mineral equilibration at 1.75 ± 0.014 Ga. Nd isotopic heterogeneities are demonstrated in different generations of garnet and zonation in Sr and Nd isotope compositions is present within the clinopyroxene. The variation in measured Nd isotope compositions in different generations of garnet corresponds to 16 ϵ units. Exsolution of garnet from clinopyroxene occurred in an essentially closed system with respect to Sr and Nd. Petrographically different generations of garnet are present. One garnet with rutile inclusions as well as the rims of clinopyroxene are clearly off the isochron. Features like this can originate either if a system is not closed ( e.g. affected by a migrating fluid) or if a remanent isotopic heterogeneity survived the homogenisation at 1.75 Ga. The subsolidus partitioning of Sm and Nd between garnet and clinopyroxene results in higher fractionation of the Sm Nd ratio in garnet than in the liquid-crystal partitioning. Since the closure temperature for the Sm-Nd exchange between garnet and clinopyroxene may be around 850°C in a dry system, many of the lithospheric ultramafic rocks may exhibit frozen mineral equilibria. The isotopic data suggest that this eclogite was emplaced at 1.75 Ga into the lithosphere mantle, at temperatures of about >1040°C, and then cooled as a closed system at a rate of 6–50°C/my down to 840°C, where all the exchange reactions were efficiently frozen in at 1.75 Ga ago. The coincidence of the emplacement of granites (1.7–1.8 Ga) into the central part of the Tanzanian craton with the age of the eclogite suggests that lower crustal melting was triggered by a thermal event in the upper lithosphere.