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In the current plate tectonic regime, thermal modeling, petrology, and seismology show that subsurface portions of cold slabs carry some of their volatiles into the deep upper mantle, mantle transition zone, and uppermost lower mantle avoiding the devolatilization occurring with normal arc and wedge subduction. Slab crustal remnants at these depths can melt by intersecting their carbonated solidus whereas slab mantle remnants can devolatilize by warming and metamorphosing to ‘dryer’ mineral assemblages. Since fluid release and earthquake production (“dehydration embrittlement”) operates down to ~300 km depths in all subduction zones, we propose, that deep-focus earthquakes trace the places of fluid release at deeper levels (350 to 750 km). Fluids in faults related to earthquake generation will become diamond-forming as they react with mantle rocks along the fault walls. Diamonds thus formed will record deformation produced by mantle convection and slab buckling during mantle storage. Lithospheric diamonds, stored in static ancient continental keels, lack the connection to this type of geodynamic regime that is evident for sublithospheric diamonds. However, a comparison between the two diamond types suggests a geologic model for lithospheric diamond formation in the ancient past. Lithospheric diamonds and sublithospheric diamonds both contain evidence for the recycling of sediments or surficial rocks that have equilibrated at low temperatures with seawater. The known way to inject these materials into diamond-forming regions is slab subduction. Hence both diamond types may have formed by variants of this same process that differ in depth and style over geologic time. Lithospheric diamonds are different from sublithospheric diamonds in critical ways: higher average N content, ages extending into the Paleoarchean, inclusion assemblages indicating formation at lower pressure, and lack of ubiquitous deformation features. Nitrogen content is the key to relating lithospheric diamonds to the subducting slab. Nitrogen occurs in clays and sediments at the slab surface or uppermost crust. Regardless of whether the slab is hot or cold during subduction, nitrogen will be removed into a mantle wedge if one exists. Additionally, diamonds will not survive in the melts/fluids generated in the wedge under oxidizing conditions. For sublithospheric diamonds, their low to non-existent nitrogen content occurs because they are derived from slab fluids/melts once nitrogen has been largely removed or from rocks deeper in the slab where nitrogen is scarce. The much higher nitrogen in lithospheric diamonds suggests that they formed from fluids/melts derived near the slab surface that contained N. In the Archean, such slabs must have subducted close to the nascent mantle keel with no mantle wedge so the fluids could be directly reduced by the mantle keel. We propose a gradual temporal change from shallow, keel-adjacent, mantle-wedge-poor subduction that produced lithospheric diamonds starting in the Paleoarchean to wedge-avoiding, cold and deep subduction that produced sublithospheric diamonds in the Paleozoic. This temporal change is consistent with many geologic features: an early stagnant lid and a buoyant Archean oceanic lithosphere; the slab-imbrication, advective thickening, and diamond-richness of portions of mantle keels; and anomalously diamond-rich ancient eclogites. 

Over the past decades, dating inclusions in lithospheric diamonds has advanced from analysing tens of pooled inclusions to single sulphide Re-Os analyses and single silicate Sm-Nd analyses, resulting in a fair global coverage of lithospheric diamond ages (Smit et al. 2022) and linking these to tectonomagmatic events. On the other hand, dating of inclusions in sublithospheric diamonds is incredibly limited, mainly due to the rarity of sulphide inclusions and complex retrogressed Ca-silicate phases in already scarce sublithospheric diamonds. Yet, sublithospheric diamonds are important from both a scientific and economic perspective, representing the deepest pristine samples of Earth’s mantle and some of the most valuable diamonds recovered. Understanding their chemical signatures in a broader geological context requires dating such samples. Here we will review the recent progress in dating inclusions in sublithospheric diamonds and discuss their link to the supercontinent cycle and emplacement history. 

Characterizing compositional heterogeneity in Earth’s lower mantle is critical to understanding its dynamics. Three low-nitrogen diamonds from Koffiefontein (South Africa), containing inclusion assemblages of ferropericlase ± orthopyroxene ± magnesite, constrain diamond formation in an Mg-rich lower-mantle environment. Ferropericlase inclusions have Mg# 82.7–88.5 and orthopyroxene inclusions (retrogressed bridgmanite) have Mg# 95.0–95.1 and mantle-like δ18O of +5.6‰ ± 0.2‰. Magnesite included in one diamond implicates carbonated fluids in diamond formation. High Mg# and low Ca, Al, and Na of the assemblage indicate a melt-depleted meta-harzburgitic environment, in contrast to more fertile compositions expected for primitive lower mantle. Extremely low Ca in orthopyroxene inclusions may reflect a combination of melt depletion and low equilibration temperatures at the time of trapping. Inclusion compositions implicate subducted oceanic slab meta-harzburgite as the host for diamond growth. Mantle-like δ18O of the orthopyroxene inclusions indicates unaltered oceanic lithosphere. Similar melt-depleted characteristics in lower-mantle inclusion assemblages worldwide support that meta-harzburgite is the dominant host of lower-mantle diamonds.

 

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300–700 km depths^1,2and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb–Sr, Sm–Nd, U–Pb and Re–Os) applied to Fe-sulfide and CaSiO₃inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic–Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry^3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth—with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust—could have enhanced supercontinent stability.

 

Tschauner et al . (Reports, 11 November 2021, p. 891) present evidence that diamond GRR-1507 formed in the lower mantle. Instead, the data support a much shallower origin in cold, subcratonic lithospheric mantle. X-ray diffraction data are well matched to phases common in microinclusion-bearing lithospheric diamonds. The calculated bulk inclusion composition is too imprecise to uniquely confirm CaSiO 3 stoichiometry and is equally consistent with inclusions observed in other lithospheric diamonds. 

Abstract The origin of the eclogites that reside in cratonic mantle roots has long been debated. In the classic Roberts Victor kimberlite locality in South Africa, the strongly contrasting textural and geochemical features of two types of eclogites have led to different genetic models. We studied a new suite of 63 eclogite xenoliths from the former Roberts Victor Mine. In addition to major- and trace-element compositions for all new samples, we determined 18O/16O for garnet from 34eclogites. Based on geochemical and textural characteristics we identify a large suite of Type I eclogites (n = 53) consistent with previous interpretations that these rocks originate from metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, crystallised from melts of depleted mid-ocean ridge basalt (MORB) mantle. We identify a smaller set of Type II eclogites (n = 10) based on geochemical and textural similarity to eclogites in published literature. We infer their range to very low δ18O values combined with their varied, often very low zirconium-hafnium (Zr-Hf) ratios and light rare earth element-depleted nature to indicate a protolith origin via low-pressure clinopyroxene-bearing oceanic cumulates formed from melts that were more depleted in incompatible elements than N-MORB. These compositions are indicative of derivation from a residual mantle source that experienced preferential extraction of incompatible elements and fractionation of Zr/Hf during previous melting.