Implications of the mineral assemblages in the carbonatitic xenoliths

The co-existence of diamond, ordered and disordered graphites provides potential records of pressure, temperature and redox conditions for the mantle source, whereas moissanite and native metals may track the emplacement conditions experienced by the xenoliths. Natural diamond generally forms at pressures >5-6 GPa and temperatures in the range of 900–1400 °C, but may form at depths as shallow as 120 km along subduction geotherms29. Experiments have shown that diamond formation via carbonate-silicate interaction can occur at pressures of 6-7 GPa and temperatures of 1350–1800 °C30. Thus, the preservation of tiny diamond crystals in the carbonatitic xenoliths indicates that they spent time at depths in excess of at least 120 km and possibly 150 km. The D band of 1334 cm−1 (the typical Raman shift of diamond) in the disordered graphite most likely arises from the distortion of the diamond structure during its transformation to graphite31. This indicates that the ordered and disordered graphites distributed widely through the carbonatitic xenoliths are transformation products from diamond during the upward migration of the carbonatitic melt. Although some fine-grained garnets were found in the carbonatitic xenoliths, they are not equilibrated with the pyroxene and thus cannot be used to estimate pressure. The low temperature estimated from Cpx-Opx pairs in the carbonatitic xenoliths (702–790 °C) indicate interaction with carbonate melt, which is stable to about 150 °C lower than that of garnet-free peridotite (T = 820–1029 °C). Considering the coexistence with spinel lherzolite xenoliths (T = 784–1046 °C) and referring to the geotherm of the northern margin of NCC32, it appears that the carbonatitic melt pockets could have solidified and been stored in the shallow lithospheric mantle (40–60 km) for a short while before they were sampled and brought to the surface by the Neogene volcanics.

The carbonatitic xenoliths have much lower contents of most trace elements than do igneous carbonatites, which are characterized by enrichments in incompatible trace elements (e.g., Ba, Sr, Nb, Th, U and LREE) and steep light REE-enriched patterns33. Their trace element patterns also differ greatly from those of mantle-derived continental34 and oceanic carbonatites35, but are similar to sedimentary carbonate rocks (Fig. 4b). The heavy oxygen isotopic compositions (δ18O SMOW = 20.7–21.5) fall in the range of Cretaceous to Palaeoproterozoic limestones (δ18O SMOW = 17.5–30.936) and are far removed from the δ18O SMOW = 5–12 typical of igneous carbonatites37. In the SiO 2 -MgO-CaO plot, the carbonatitic xenoliths vary along a mixing trend between argillaceous limestone and peridotite (Fig. 4a). The melts that led to the crystallization of these xenoliths were primarily derived from argillaceous limestone subducted to at least 120 km rather than from partial melting of carbonated eclogites or peridotites.

Although CaO contents in the bulk xenoliths correlate well with Ni, Cr and CaO contents in the matrix, the proportion of carbonate correlates well with Ni, but not with Cr (Fig. 5). Both Ni and Cr are enriched in mantle peridotite but depleted in sedimentary carbonate rocks. The differing behaviour of Ni and Cr between the carbonatitic xenoliths and matrix carbonate confirms that (1) the carbonatitic xenoliths were formed by carbonate melt-peridotite interaction, rather than just olivine consumption and (2) olivines were removed during the carbonate melt-peridotite interaction (probably partially replaced by pyroxene), as suggested by the absence of olivine in most carbonatitic xenoliths. Using average peridotite xenoliths and limestone as end members, mass balance calculations indicate that proportions of peridotite involved in the melt-peridotite interaction range from 1% to 30% (Fig. 5). Peridotite-melt interaction in the mantle wedge may have diluted REE, LILE and HFSE, resulting in lower contents in the carbonatitic xenoliths than in sedimentary carbonate rocks (Fig. 4b). The compatibilities of REE, LILE and HFSE in clinopyroxene are much higher than those in olivine and opx and the concentrations of these elements in olivine and opx are much lower than those in sedimentary carbonate rocks (Fig. 4b). Therefore, replacement of Ca-carbonate + olivine by Ca-Mg carbonate ± cpx ± opx is consistent with gradual depletion of these elements in carbonatitic melt. Unlike major and trace elements, δ18O SMOW of the xenoliths cluster in a very small range (21.1 ± 0.3 [1σ, n = 9]) and are slightly lower than the grand average of sedimentary limestone (24.7 ± 3.0 (1σ, n = 102)). This implies (1) that oxygen isotopic diffusion within the carbonatitic melt is much quicker than that between the melt and peridotite and (2) that the melt quickly solidified in the mantle so that only limited oxygen isotopic modification could occur by diffusion between peridotite and solidified carbonatitic melt pockets.

The role of subducted carbonate in mantle processes above the subduction zone

Recycling of limestone into the deep mantle has been previously suggested based on the discovery of coesite- and diamond-bearing marbles from orogenic belts10,38,39. Furthermore, mineral inclusions in diamonds indicate that sediments and carbonates could have been subducted as deep as the upper part of the lower mantle13. Mantle rocks become progressively more reduced with depth and the lower reaches of the upper mantle may be metal saturated15,16. Diamond should be the dominant host for carbon at depths below 150 km, although carbides are also possible hosts in the deep mantle40. Thus, the carbonate in subducted limestone could potentially be partially reduced to form a carbon-saturated mixture with highly reduced phases when it moves into the deep mantle wedge (Fig. 6). Carbides crystallized under reducing conditions are known from polycrystalline diamonds, which are interpreted to have formed by reduction and redox freezing of carbonate melts41. KCl, which is found in the calcite matrix and as inclusions in clinopyroxene, can destabilize carbonate, allowing greater solubility and diffusion of carbon42 and is of great importance in forming diamonds43. The preservation of KCl in the carbonatitic xenoliths (Fig. 2b) indicates that the carbonatitic melt was saturated in halogens, which could have acted as a solvent catalyst for diamond growth.

Figure 6 Deep mantle recycling of sedimentary limestone: S1 = subduction of crust with sedimentary limestone. S2 = diapirism of argillaceous limestone in the deep subduction zone (120–150 km) into the hot mantle wedge. S3 (or S3′) = rapid ascent of the carbonatitic melt. S4 = carbonated peridotite and carbonatitic xenoliths carried to the surface by the Neogene picrobasalt. The curves of hot subduction and intermediate subduction represent depth-temperature trajectories for the hottest and average slab-top conditions. Boundaries (Cc ss -out) for carbonated pelites from T12, 3–7 GPa59 and TS08, 2.5–5.0 GPa11 and GS11, ≥5.5 GPa60. The graphite–diamond reaction equilibrium is taken from Kennedy and Kennedy61. Full size image

Computed phase equilibria indicate that CO 2 -loss is negligible for carbonate-bearing marine sediments from depths of 80 to 180 km along low-temperature subduction geotherms8. Thomsen and Schmidt11 experimentally confirmed that >70–80% of the subducted carbonate will bypass the source region for volcanic arc melts and become transported to greater depths. This raises the question as to if and how the deeply subducted carbonate could be returned to the surface. Although deeply subducted carbonate could be tectonically exhumed to the surface as known from UHP orogenic belts, this may not account for melting to form carbonatites.

Rohrbach and Schmidt17 advocated a role for ‘redox freezing’ and ‘redox melting’ in the deep carbon cycle. If carbonatite melts were produced in deeply subducted lithosphere, they would be unstable when infiltrating the mantle wedge at depths greater than 250 kilometres and be reduced to diamond and thus immobilized. This redox freezing mechanism may form diamonds quickly, resulting in polycrystalline aggregates of diamond in which diamond is the major rock-forming mineral41,44. When such mantle packets, now carbon-enriched, upwell to higher levels, diamond will inevitably react with more oxidized mantle at shallower levels, oxidizing the diamond to carbonate and initiating redox melting to form deep-seated carbonatite melts. This coupled ‘redox freezing and melting’ process would transform the geochemical signature of the primarily REE-poor sedimentary carbonate into that of REE-rich carbonatite by low degree melting of carbonated peridotite45, because re-melting of the diamond aggregates together with surrounding peridotite on a larger scale equates to bulk melting of carbon-rich peridotite. This is a mechanism for transportation of carbon, but not for most of the other elements in the original carbonate rock. It may, however, be a precursor process for the formation of magmas seen at the surface as kimberlites and ultramafic lamprophyres46.

Origin and emplacement of the carbonatitic xenoliths

These ‘redox freezing’ and ‘redox melting’ processes do not apply to the Dalihu carbonatite xenoliths, because the trace element and oxygen isotope characteristics of the limestone would not be transmitted to the carbonatite pods. Instead, they must have been produced by high-degree melting of argillaceous limestone. Although no data is available for the solidus of impure limestone at high pressures, the isobaric diagram for the system Na 2 CO 3 -CaCO 3 at 1 kbar47 implies that the melting point of impure limestone is higher than the temperature at the surface of subducted slabs. Thus, limestones will not form high-degree melts at the top of the slab, but they may move in the form of buoyant diapirs due to their lower density than the surrounding peridotite. In other words, the mantle wedge likely undergoes carbonate fluxing by the diapiric rise of a marble mélange zone14 at temperatures below the melting point.

If the carbonate pockets migrate upward in the solid state, any diamonds they contain would inevitably be transformed to graphite. The survival of tiny diamonds and their disordered graphite breakdown products thus argues for rapid transportation into the shallow lithospheric mantle and cooling just shortly after segregation from the source. This would leave insufficient time for the complete elimination of diamond (Fig. 6). Melting of the carbonate-rich diapir probably occurred at high lithospheric levels during this process, leaving restricted time for assimilation and re-equilibration.

The abundant cavities and irregular blebs (Fig. 1c,d) may have formed by a volatile dissolution process similar to that suggested for the emplacement of kimberlites48. When carbonate diapirs rise through the overlying mantle wedge from the colder subduction zone, the combined effects of heating, decompression and SiO 2 addition due to reaction of the carbonate with surrounding peridotite initiates partial melting, causing a catastrophic drop in the solubility of CO 2 , resulting in fluid expulsion48. This may facilitate rapid propagation of crack tips, promoting melt transportation in the lithospheric mantle, enhancing rapid and accelerating ascent of the carbonatitic melt into the shallow mantle, where it quickly cools to <800 °C as recorded by the Cpx + Opx pairs in the carbonatitic xenoliths. Melting of the diapir probably first occurred at high lithospheric levels, leaving restricted time for assimilation and re-equilibration.

The origin of ultra-reduced phases

The origin of phases such as carbides and metal alloys in diamonds and kimberlites appear to indicate formation in extremely reducing conditions well below those typical of the upper mantle49,50. This has been controversial for many years. More recently, similar occurrences have been found in deeply subducted and exhumed ophiolites and serpentinites51,52. Since the maximum stability of moissanite in the upper mantle is several orders of magnitude fO 2 below that for saturation in Ni-Fe metal49 and reduction of SiO 2 to Si-metal requires even lower oxygen fugacities ( fO 2 < IW-9 if αSiO 2 = 1), appropriate conditions seem unattainable in the normal upper mantle40. The occurrence of carbides and alloys in the carbonatitic xenoliths requires either very local extremely reducing conditions, possibly related to vapor phase reactions53, or that the sedimentary carbonate rock was subducted into the lower half of the upper mantle where such reducing conditions may be prevalent before returning to the surface.

Under conditions of SiC stability, silicates must be essentially Fe-free due to the reduction of all Fe2+ to Fe, which is not observed. Carbonate melts, if present, should be rapidly reduced to diamond at depths greater than 250 km17, exhausting the melts. This appears to rule out large-scale equilibrium processes, favouring extremely local (millimetre-scale) redox variations in disequilibrium conditions at a late stage.

Shiryaev and Gaillard53 proposed that the extremely reducing conditions required for formation of moissanite, Si° and iron carbides may be achieved by SiC deposition from the gas phase at pressures <100 bars. However, this exact mechanism is difficult to envisage for the Dalihu rocks as it translates to a depth of less than 300 metres at 1300 °C. The host magma at the Dalihu locality is a picritic alkali basalt with low Mg/(Mg + Fe) of 0.58, indicating that some fractionation had occurred at upper mantle pressures. Experiments on a similar alkali-basalt from Skye indicate temperatures closer to 1200 to 1300 °C54, which would restrict the necessary strong reduction reactions to even closer to the surface or eliminate them completely.

However, KCl may have promoted and catalyzed the formation of carbides and metals on very local scales. Also, the reaction 3C + SiO 2 → SiC + 2CO suggested by Shiryaev and Gaillard53 may be replaced by 2C + SiO 2 → SiC + CO 2 because CO concentrations are negligible at pressures ≥1 kbar55. SiO 2 saturation is indicated by the presence of quartz and feldspar in the carbonatitic xenoliths.

There appear to be two possible scenarios for the formation of SiC and native Si: (1) they formed during rapid ascent of the carbonate-rich diapir from the top of the slab to the shallow mantle, which melted at a late stage before sampling by the basaltic melt; or (2) they formed during later transportation of carbonatitic xenoliths within the basaltic melt. We prefer the former because the massive degassing prompted by silicate assimilation48 could provide SiO 2 and promote the above reaction, explaining the association of metal and carbide occurrences with cavities. Dusty inclusions of moissanite + quartz in diamonds from Fuxian kimberlite56 provide evidence for the same reaction in the deep mantle. Also, the breakdown of diamond to form two types of graphite would not have had time to occur in a process as close to the surface as 300 m.

Our work provides the first direct evidence for the recycling of sedimentary limestone to at least >120 km and for its rapid return through the mantle wedge in the form of a reactive carbonate-rich diapir and then carbonatite melt into the shallow lithosphere (Fig. 6). Such processes may have played an important role in changing the chemical composition of the mantle and in global carbon recycling since plate tectonics became operative.