The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif

Abstract We have analysed the Li and Mg isotope ratios of a suite of samples from the Horoman Peridotite Massif. Our results show that most Li and all Mg isotopic compositions of the Horoman peridotites are constant over 100 metres of continuous outcrop, yielding values for pristine mantle of δ 7 Li = 3.8 ± 1.4‰ (2SD, n  = 9), δ 25 Mg = −0.12 ± 0.02‰ and δ 26 Mg = −0.23 ± 0.04‰ (2SD, n  = 17), in keeping with values for undisturbed mantle xenoliths. However, there are also some anomalously low δ 7 Li values (−0.2‰ to 1.6‰), which coincide with locations that show enrichment of incompatible elements, indicative of the prior passage of small degree melts. We suggest Li diffused from infiltrating melts with high [Li] into the low [Li] minerals and kinetically fractionated 7 Li/ 6 Li as a result. Continued diffusion after the melt flow had ceased would have resulted in the disappearance of this isotopically light signature in less than 15 Ma. In order to preserve this feature, the melt infiltration must have been a late stage event and the massif must have subsequently cooled over a maximum of ∼0.3 Ma from peak temperature (950 °C, assuming the melts were hydrous) to Li closure temperature (700 °C), likely during emplacement. The constant δ 26 Mg values of Horoman peridotites suggest that chemical potential gradients caused by melt infiltration were insufficient to drive associated δ 26 Mg fractionation greater than our external precision of 0.03‰.