Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite

A widely used assumption of mantle geochemistry and the theory of partial melting at oceanic settings is the existence of isotopic equilibrium between mantle source and melt. Yet, recent diffusion studies and isotopic investigations of ophiolites, abyssal peridotites and associated MORBs have cast doubts on this assumption, by providing evidence for isotopic disequilibrium between residual peridotites and MORBs. Here we present Sr and Sm–Nd isotope data on mantle peridotites and gabbroic intrusions from the Mt. Maggiore (Alpine Corsica, France) Tethyan ophiolite, which document Nd isotopic homogeneity, implying isotopic equilibrium, on a 1-kilometer scale. The peridotites record multi-stage melt–rock interaction and melt intrusion occurring at different lithospheric depths. Samples studied are residual cpx-poor spinel lherzolites, reactive spinel harzburgites, impregnated plagioclase peridotites and related gabbronoritic veinlets, later gabbroic dykes. Strontium isotopes in peridotites and gabbros are highly variable, due to interaction with sea-water derived fluids, and cannot be used to test melt-residue isotopic equilibrium. In contrast, Nd isotopes are unaffected by sea-water alteration. Peridotites display present-day high 147Sm/144Nd (0.49–0.59) and 143Nd/144Nd (0.513367–0.513551) ratios, with no appreciable differences between residual and reactive spinel peridotites, and between spinel and plagioclase peridotites. Gabbroic dykes have present-day Nd isotopic compositions typical of MORB (143Nd/144Nd = 0.513122–0.513138). Internal (plag–whole rock–cpx) Sm–Nd isochrons for olivine gabbro dykes and a gabbronoritic veinlet yield Jurassic ages (162 ± 10 and 159 ± 15 Ma in ol-gabbros, 155 ± 6 Ma in gabbronorite), and initial εNd = 8.9–9.7 indicative of a MORB-type source. Sm–Nd isotopic compositions of peridotites conform to the linear array defined by the gabbroic rocks, and yield initial (160 Ma) εNd values of 7.6–8.9, again consistent with a MORB source. This Sm–Nd isotopic homogeneity may indicate a close approach to solid-melt equilibration on a 1-kilometer scale in the mantle, or it may represent an inherited isotopic homogeneity of the mantle source. In either case, melting and melt–rock interaction were the most likely processes to produce nearly complete equilibration between peridotite and melt. In contrast with recently accumulated evidence of apparent melt-source isotopic disequilibrium, generally on scales of several kilometers, here we show a setting where isotopic homogeneity between source rock and melt was attained. Whether this result can be applied elsewhere will depend on additional detailed studies in other regions.