Extreme redox variations in a superdeep diamond from a subducted slab.

The introduction of volatile-rich subducting slabs to the mantle may locally generate large redox gradients, affecting phase stability, element partitioning and volatile speciation1. Here we investigate the redox conditions of the deep mantle recorded in inclusions in a diamond from Kankan, Guinea. Enstatite (former bridgmanite), ferropericlase and a uniquely Mg-rich olivine (Mg# 99.9) inclusion indicate formation in highly variable redox conditions near the 660 km seismic discontinuity. We propose a model involving dehydration, rehydration and dehydration in the underside of a warming slab at the transition zone-lower mantle boundary. Fluid liberated by dehydration in a crumpled slab, driven by heating from the lower mantle, ascends into the cooler interior of the slab, where the H2O is sequestered in new hydrous minerals. Consequent fractionation of the remaining fluid produces extremely reducing conditions, forming Mg-end-member ringwoodite. This fractionating fluid also precipitates the host diamond. With continued heating, ringwoodite in the slab surrounding the diamond forms bridgmanite and ferropericlase, which is trapped as the diamond grows in hydrous fluids produced by dehydration of the warming slab.

Sublithospheric diamond ages and the supercontinent cycle.

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 depths1,2 and 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 CaSiO3 inclusions 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 geobarometry3,4, 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.

Dense hydrated Mg-silicates in diamond: Implications for transport of H2O into the mantle.

Water in Earth's upper mantle is a minor and yet critically important component that dictates mantle properties such as strength and melting behavior. Minerals with stoichiometric water, such as those of the humite group, are important yet poorly characterized potential reservoirs for volatiles in the upper mantle. Here, we report observation of hydroxyl members of the humite group as inclusions in mantle-derived diamond. Hydroxylchondrodite and hydroxylclinohumite were found coexisting with olivine, magnesiochromite, Mg-bearing calcite, dolomite, quartz, mica, and a djerfisherite-group mineral in a diamond from Brazil. The olivine is highly forsteritic (Mg# 97), with non-mantle-like oxygen isotope composition (δ18O +6.2‰), and is associated with fluid inclusions and hydrous minerals-features that could be inherited from a serpentinite protolith. Our results constitute direct evidence for the presence of deserpentinized peridotitic protoliths in subcratonic mantle keels, placing important constraints on the stability of hydrous phases in the mantle and the origin of diamond-forming fluids.

Evidence for oxygen-conserving diamond formation in redox-buffered subducted oceanic crust sampled as eclogite.

Cratonic eclogite is the product of oceanic crust subduction into the subcontinental lithospheric mantle, and it also is a fertile diamond source rock. In contrast to matrix minerals in magma-borne xenoliths, inclusions in diamond are shielded from external fluids, retaining more pristine information on the state of the eclogite source at the time of encapsulation. Vanadium is a multi-valent element and a widely used elemental redox proxy. Here, we show that that xenolithic garnet has lower average V abundances than garnet inclusions. This partly reflects crystal-chemical controls, whereby higher average temperatures recorded by inclusions, accompanied by enhanced Na2O and TiO2 partitioning into garnet, facilitate V incorporation at the expense of clinopyroxene. Unexpectedly, although diamond formation is strongly linked to metasomatism and xenoliths remained open systems, V concentrations are similar for bulk eclogites reconstructed from inclusions and from xenoliths. This suggests an oxygen-conserving mechanism for eclogitic diamond formation, and implies that eclogite is an efficient system to buffer fO2 over aeons of lithospheric mantle modification by subduction-derived and other fluids.

Comment on "Discovery of davemaoite, CaSiO3-perovskite, as a mineral from the lower mantle".

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 CaSiO3 stoichiometry and is equally consistent with inclusions observed in other lithospheric diamonds.

Formation of diamond in the Earth's mantle.

The principal sources of natural diamonds are peridotitic (about 2/3 of diamonds) and eclogitic (1/3) domains located at 140-200 km depth in the subcratonic lithospheric mantle. There, diamonds probably form during redox reactions in the presence of melt (likely for eclogitic and lherzolitic diamonds) or under subsolidus conditions in the presence of CHO fluids (likely for harzburgitic diamonds). Co-variations of δ(13)C and the nitrogen content of diamonds suggest that two modes of formation may have been operational in peridotitic sources: (1) reduction of carbonates, that during closed system fractionation drives diamond compositions to higher δ(13)C values and lower nitrogen concentrations and (2) oxidation of methane, that in a closed system leads to a trend of decreasing δ(13)C with decreasing nitrogen. The present day redox state of subcratonic lithospheric mantle is generally too reduced to allow for methane oxidation to be a widespread process. Therefore, reduction of carbonate dissolved in melts and fluids is likely the dominant mode of diamond formation for the Phanerozoic (545 Ma-present) and Proterozoic (2.5 Ga-545 Ma). Model calculations indicate, however, that for predominantly Paleoarchean (3.6-3.2 Ga) to Mesoarchean (3.2-2.8 Ga) harzburgitic diamonds, methane reduction is the principal mode of precipitation. This suggests that the reduced present day character (oxygen fugacity below carbonate stability) of peridotitic diamond sources may be a secondary feature, possibly acquired during reducing Archean (>2.5 Ga) metasomatism. Recycling of biogenic carbonates back into the mantle through subduction only became an important process in the Paleoproterozoic (2.5-1.6 Ga) and diamonds forming during carbonate reduction, therefore, may predominantly be post-Archean in age. For eclogitic diamonds, open system fractionation processes involving separation of a CO(2) fluid appear to dominate, but in principal the same two modes of formation (methane oxidation, carbonate reduction) may have operated. Direct conversion of graphitized subducted organic matter is not considered to be an important process for the formation of eclogitic diamonds. The possible derivation of (12)C enriched carbon in eclogitic diamonds from remobilized former organic matter is, however, feasible in some cases and seems likely involved, for example, in the formation of sublithospheric eclogitic diamonds from the former Jagersfontein Mine (South Africa).