The infiltration of fluids into continental lithospheric mantle is a key mechanism for controlling abrupt changes in the chemical and physical properties of the lithospheric root, as well as diamond formation, yet the origin and composition of the fluids involved are still poorly constrained. Such fluids are trapped within diamonds when they form and so diamonds provide a unique means of directly characterizing the fluids that percolate through the deep continental lithospheric mantle. Here we show a clear chemical evolutionary trend, identifying saline fluids as parental to silicic and carbonatitic deep mantle melts, in diamonds from the Northwest Territories, Canada. Fluid-rock interaction along with in situ melting cause compositional transitions, as the saline fluids traverse mixed peridotite-eclogite lithosphere. Moreover, the chemistry of the parental saline fluids - especially their strontium isotopic compositions - and the timing of host diamond formation suggest that a subducting Mesozoic plate under western North America is the source of the fluids. Our results imply a strong association between subduction, mantle metasomatism and fluid-rich diamond formation, emphasizing the importance of subduction-derived fluids in affecting the composition of the deep lithospheric mantle.
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Acknowledgements Y.W. acknowledges his Lamont postdoctoral fellowship and National Science Foundation grant no. 1348045. We thank T. Stachel and D. Walker for discussions and J. J. Gurney, J. Carlson, T. Nowicki and BHP Minerals/Dominion Diamonds for access to diamonds from the Ekati mine. J.M. was funded by a scholarship from the Diamond Trading Company at Durham University. D.G.P. completed this work under tenure of a Canada Excellence Research Chair, with support from the Deep Carbon Observatory (Sloan Foundation). Y.W. thanks Israel Science Foundation grant number 435/12 for funding the EPMA and Fourier transform infrared (FTIR) analyses at the Hebrew University. D. E. Jacob, M. Santosh and M. Walter made excellent suggestions that greatly improved this paper. This is Lamont–Doherty Earth Observatory contribution number 7908.
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