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 root1,2, as well as diamond formation3, yet the origin and composition of the fluids involved are still poorly constrained. Such fluids are trapped within diamonds when they form4,5,6,7 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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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.
The authors declare no competing financial interests.
Sample metadata have been archived in the System for Earth Sample Registration (SESAR) with associated International GeoSample Numbers (IGSNs). The data set can be found in the EarthChem library (http://doi.org/10.1594/IEDA/100540).
Extended data figures and tables
Extended Data Figure 1 Photomicrographs and cathodoluminescence images of three diamonds from the Fox kimberlite.
a, b, Diamond E154; c, d, diamond E217; e, f, diamond E142. The location of the microinclusions analysed by EPMA is imposed on the cathodoluminescence images.
Absorbance bands of the diamond itself, nitrogen in the diamond lattice, and carbonate and water in the microinclusions are observed in all diamonds. In diamond E221 olivine absorption due to the presence of olivine microinclusions is also observed, while in diamonds E142 and E11014 the presence of pyroxene (omphacite) is detected. The nitrogen in all Fox fibrous diamonds is aggregated in 100% A centres. The nitrogen defect structure in diamonds evolves over time, from individual nitrogen atoms replacing single carbon atoms (C centres) when the diamond forms, to pairs of adjacent nitrogen atoms (A centres), to four nitrogen atoms tetrahedrally arranged around a vacancy (B centres). At the mantle temperatures at which most diamonds form (that is, 950–1,250 °C)42, the full transformation from C to A centres (100% A centres) will form over a timescale of hundreds of thousands to millions of years, while A centres merge to form B centres over billions of years43,44.
Extended Data Figure 3 Microinclusion spatial location and MgO and Cl composition in diamond ON-DVK-294 (refs 7, 45).
a, The data point locations are superimposed on the original figure 3 from ref. 45 (reproduced with permission). The shift in the position of a few of the data points is related to errors in the position of fiducial marks. b, The position of each data point was radially normalized to allow plotting of the compositional change of the fluids along a uniform diamond profile. Clear chemical differences between the inner and outer parts of this diamond exist; however, the transition between the two zones in the diamond shows a gradual evolution rather than an abrupt change.
Extended Data Figure 4 Total N in parts per million (ppm) versus percentage of B centres for fibrous diamonds from the central Slave lithosphere.
Isochrons are calculated for ambient temperatures of 930 °C (dotted red lines) and 1,010 °C (solid black lines) as described previously43,44. Fibrous diamonds in the Slave lithosphere have 600–1,600 ppm nitrogen and are essentially of pure IaA type (that is, %B = 0). The presence of a very small quantity of B centres (≪1) is difficult to evaluate from FTIR spectroscopy but such centres are necessary for calculating a possible age based on nitrogen aggregation. The geothermometry results for mineral inclusions in fibrous diamonds from the Panda kimberlite yielded low equilibrium temperatures of 930–1,010 °C (ref. 5). Applying these temperatures to the amount of nitrogen in the central Slave diamonds and %B ≈ 0 (grey area), as indicated from FTIR spectroscopy, reasonable mantle residence times for these diamonds vary between 1–210 Ma, before the kimberlite eruption age of 55 Ma (ref. 46).
Extended Data Figure 5 MgO and SiO2 against Cl content, all in wt%, water- and CO2-free composition, of HDF microinclusions in fibrous diamonds from the central Slave craton and diamond ON-KAN-383 from Kankan, Guinea.
Intermediate compositions between saline to silicic are not unique to the PAN4 diamond5. The composition of 21 microinclusions with similar composition in an eclogitic zoned coated diamond from Kankan, Guinea (ON-KAN-383) was previously reported7. The saline-to-silicic inclusions in diamond ON-KAN-383 are restricted to the core–coat boundary of this diamond and develop, progressing towards silicic compositions (inner, middle) that finally evolve to low-Mg carbonatitic HDFs close to the rim of the diamond (outer). The paragenesis of this diamond and its analogous HDF compositions to eclogitic diamonds from the central Slave craton provide strong evidence for broader geographical involvement of saline fluids in the formation of silicic melts in eclogite, as well as for the continuous evolution (shaded red arrow) from saline through silicic to low-Mg carbonatitic composition when saline HDFs intersect with hydrous-carbonated eclogite lithology in the CLM root.
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Weiss, Y., McNeill, J., Pearson, D. et al. Highly saline fluids from a subducting slab as the source for fluid-rich diamonds. Nature 524, 339–342 (2015). https://doi.org/10.1038/nature14857
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