CHEMISTRY AND ORIGIN OF TRAPPED MELTS IN OPHIOLITIC PERIDOTITES

Melt impregnation and peridotite-melt interaction are ubiquitous pheno mena in the oceanic-type lithospheric mantle. Nevertheless, the nature of the infiltrating melts is still poorly understood. We performed de tailed textural and chemical investigations (by means of electron and ion microprobe) on: (1) impregnated plagioclase-bearing ophiolitic per idotites from the Internal Ligurides (Northern Apennine. Italy) and Mt . Maggiore (Corsica), and (2) olivine cumulates (consisting of 85 vol% olivine plus interstitial plagioclase and rare poikilitic clinopyroxe ne) from magmatic pods intruded within the Mt. Maggiore peridotites. F ield evidence indicates that such cumulates crystallized from the melt s, which impregnated the surrounding peridotites. Melt impregnation in the peridotites is verified by the occurrence of peculiar microstruct ures: (1) plagioclase blebs and/or veins confined along grain boundari es or crosscutting mantle minerals; (2) partial dissolution of mantle clinopyroxene and replacement by orthopyroxene and plagioclase aggrega tes, which indicate disequilibrium between melts and host peridotites. interaction with melts also produces chemical modifications in mantle clinopyroxenes, i.e., Ti, M(middle)- to H(heavy)-REEs land, to a less er extent, Zr, Y, and Sc) enrichment coupled to depletion in Al. Miner als crystallized from the melts have depleted geochemical signature: p lagioclases are highly Anorthitic (An(88)-An(94) in the peridotites; A n(81)-An(85) in the cumulates), and show extremely low Sr(<26 ppm) and LREE(<1 x Cl; Ce-N/Sm-N = 0.13-0.50) concentrations. Interstitial cli nopyroxenes in the cumulates are characterized by high Mg values (90.6 -91.7): their REE spectra show significant LREE depletion (Ce-N/Yb-N = 0.027-0.039), high M- to H-REE concentrations (15-30 x Cl) and pronou nced negative Eu-N anomalies. Geochemical modeling indicates that the impregnating liquids probably consisted of unmixed depleted melt incre ments produced by 6-7% fractional melting. The results of this study t herefore suggest that the impregnating melts originated at deeper mant le levels and presumably represented the last melt increments of a fra ctional melting process. There is thus a higher probability that they will remain incorporated in the upper mantle. They subsequently ascend ed, partly crystallized as cumulate pods, and interacted with the stud ied peridotites dissolving mantle clinopyroxene and precipitating orth opyroxene. Such a process has been increasingly invoked in studies of melt/rock reaction zones from ophiolitic peridotites. Copyright (C) 19 97 Elsevier Science Ltd.