Sm-Nd disequilibrium in high-pressure, low-temperature Himalayan and Alpine rocks

In order to decipher the causes of Sm-Nd isotopic disequilibrium in high-pr essure, low-temperature rocks, Sm-Nd isotopic analyses were carried out on minerals from four Himalayan (Tso Morari unit) and four Alpine (Dora-Maira, Monte Viso, Sesia Lanzo) eclogitic rocks of different lithologies and diff erent intensities of post-eclogitic metamorphism. In most of these samples, garnets show the following striking features: either Sm-147/Nd-144 ratios lower than whole-rock, or Nd-143/Nd-144 ratios lower than whole-rock for hi gher Sm-147/Nd-144 ratios. In both cases, no age can be calculated. Two hyp otheses can be proposed to explain the strong isotopic disequilibrium in ga rnet separates: (1) the preservation of the isotopic signature of the plagi oclase from which garnet formed, (2) the occurrence in garnet of inherited sub-microscopic crustal inclusions with low Sm-147/Nd-144 and Nd-143/Nd-144 ratios. Knowing the Lu-Hf eclogitisation age for two samples, these two hy potheses are simulated: (1) the composition of plagioclase before eclogitiz ation is recalculated from the measured garnet composition. The plagioclase /whole-rock age obtained with this simulation is geologically meaningless, showing that this hypothesis is invalid, (2) the Nd-143/Nd-144 ratio of sub -microscopic inclusions is calculated considering that the garnet separate, which was analysed, is a mixture between inclusions with low Sm-147/Nd-144 ratios (0.05-0.1) and pure garnet with a Sm-147/Nd-144 of 2. The inclusion /whole-rock ages recalculated for both samples suggest that inclusions have an old, crustal origin. The hypothesis of sub-microscopic inclusions in ga rnet can therefore explain the observed isotopic disequilibrium provided th at the inclusions have a low Sm/Nd ratio, like allanite, apatite (and monaz ite), and that they are inherited from crustal contamination processes. In the case of a magmatic protolith, contamination can be achieved through cru stal assimilation or a first stage of circulation of crustal fluid. In the case of Dora-Maira, the possible sedimentary protolith contains components from an old contaminant crust. In the Himalayan samples, the inverse correl ation between the degree of retrogression in blueschist and then amphibolit e facies, and the extent of disequilibrium between garnet and other mineral phases suggest a second stage of fluid circulation associated with retrogr ade metamorphism. This fluid circulation would result in the dissolution of these sub-microscopic crustal inclusions and partial leaching of their rar e-earth elements (REEs) from the rock. which would scavenge the crustal Nd- 143/Nd-144 isotopic signature from the rock. (C) 2001 Elsevier Science B.V. All rights reserved.