Oceanic and super-deep continental diamonds share a transition zone origin and mantle plume transportation

Rare oceanic diamonds are believed to have a mantle transition zone origin like super-deep continental diamonds. However, oceanic diamonds have a homogeneous and organic-like light carbon isotope signature (δ13C − 28 to − 20‰) instead of the extremely variable organic to lithospheric mantle signature of super-deep continental diamonds (δ13C − 25‰ to + 3.5‰). Here, we show that with rare exceptions, oceanic diamonds and the isotopically lighter cores of super-deep continental diamonds share a common organic δ13C composition reflecting carbon brought down to the transition zone by subduction, whereas the rims of such super-deep continental diamonds have the same δ13C as peridotitic diamonds from the lithospheric mantle. Like lithospheric continental diamonds, almost all the known occurrences of oceanic diamonds are linked to plume-induced large igneous provinces or ocean islands, suggesting a common connection to mantle plumes. We argue that mantle plumes bring the transition zone diamonds to shallower levels, where only those emplaced at the base of the continental lithosphere might grow rims with lithospheric mantle carbon isotope signatures.


Results
We observe a striking correlation between the occurrence of oceanic diamonds in either present-day oceanic rocks 13,14 or ancient ophiolites 15,16 , and the oceanic mantle plume record 33 (Fig. 1; Table 1). The correlation is most obvious when only considering the modern oceanic diamonds, as both the Hawaiian and Malaita islands examples ( Fig. 1) are known to be of plume origin. Diamond inclusions are found in garnet-bearing xenoliths from the Malaita islands, the exhumed portion of the southwest Ontong Java oceanic plateau. Both seismic data and mantle xenolith studies there point to a mantle lithosphere thicker than 130-140 km [55][56][57][58] . Such a thickened oceanic lithosphere, together with the highly depleted nature of the mantle lithosphere in these two regions, is consistent with their plume origin 59 .
From the ophiolite record (Table 1; Fig. 1), the Tibetan ophiolites from the Yarlung-Zhanbo belt, which are the remnants of the lithospheric mantle of Tethyan oceanic plateau(s) 60,61 (Fig. 3), provide the most frequent www.nature.com/scientificreports/ occurrences of oceanic diamonds (Fig. 1). The Sartohay (part of the Darbut ophiolitic melange) and Hegenshan ophiolites are associated with accreted OIBs in the Central Asian Orogenic Belt (CAOB) in the West Jungaar suture zone 62 and the Inner Mongolia-Daxinganling orogenic belt 63 , respectively, both exhibiting O-LIP characteristics 33 . The Pozanti-Karsanti (also known as Aladag) ophiolite in Turkey is part of the eastern Tauride belt, and is characterised as an accreted OIB or oceanic plateau 64 similar to the Mirdita ophiolite nappe 65 .
Although the original authors interpreted the Ray-Iz and Myitkynia ophiolites to be of suprasubduction origin without plume involvement, we found that the mafic and ultramafic rocks from these two ophiolites share similar geochemical features to the plume-modified oceanic lithosphere ( Fig. 3 and Figures S3 and S4). Diamonds from these two ophiolites also share similar features as the O-LIP/OIB-related diamonds: the presence of ultra-high pressure minerals and highly-reduced phases, and very low δ 13 C (i.e. − 30 to − 20‰ for the Ray-Iz diamonds) ( Fig. 2) 27,66 . We thus consider them to be plume-related diamond-bearing ophiolites as well.
In addition, the magmatic ages of diamond-bearing ophiolites (Table 1) also coincide with the peaks of oceanic mantle plume activities for the last 500 million years at 430, 395, 165, 125 and 95 million years ago (Fig. 4) 33 . These peaks reflect an increase in mantle plume activity with time, interpreted to be the result of global mantle dynamics driven by the supercontinent cycle 70,71 . This observation further reinforces a plume connection for oceanic diamonds. A similar general correlation between continental plume record and diamondiferous kimberlites suggest the same connection for continental diamonds 72,73 .

Discussion
Super-deep continental diamonds and oceanic diamonds share some characteristics. However, the extremely variable carbon isotopic composition of super-deep continental diamonds (δ 13 C ranges from − 28 to 3‰, average of − 8 ± 9‰) 8,10,12 from one locality to another (Fig. 2b) is in stark contrast to the relatively homogenous carbon isotopic composition of oceanic diamonds (δ 13 C range from − 28 to − 19‰, average of − 25 ± 4‰; Fig. 2c), leading previous researchers to believe that they are of distinct carbon sources and therefore geneses and origins. A detailed examination reveals that some individual diamonds from the Juina-5 10 and Sao Luis (Brazil) 8 continental super-deep diamonds exhibit carbon isotopic zonation, featuring very light C isotope fractions in their cores (δ 13 C from − 28 to − 20‰) and heavier, mantle-like C isotope compositions in the rims (δ 13 C − 15‰ to − 5‰) 8,10 (Fig. 2b). The Kankan diamonds represent a rare exception with dominantly >− 5‰ C isotope (Fig. 2b). This general trend of very light isotope in the cores and heavy isotope in the rims is generally coupled with distinct cathodoluminescence colours between the cores and rims, interpreted as indicating pulses of diamond growth 10 .
Previously proposed models to explain the variability of carbon isotopic composition in super-deep continental diamonds include (1) primordial isotopic variability inherited from Earth's accretion 76 , (2) distinct carbon sources for the cores and rims (organic and inorganic) [49][50][51] , and (3) isotopic fractionation of carbon in the mantle 77,78 .
The fact that (1) the highly negative carbon isotope values for the cores of the super-deep continental diamonds from Juina-5 and Sao Luis (δ 13 C − 24 ± 6‰), and the overall homogeneous negative values for the superdeep diamonds from Jagersfontein (δ 13 C up − 20 ± 4‰) 11 and Monastery (δ 13 C up − 17 ± 1‰) 11 , are comparable to those of the relatively homogeneous carbon isotopic composition of oceanic diamonds (δ 13 C − 25 ± 4‰) ( Fig. 2), and (2) they are believed to be of an organic origin from subducted slabs, can rule out the possibility of primordial origin and variability. The model involving fractionation processes during degassing of CO 2 (enriched in 13 C) and nitrogen is not supported either due to the lack of correlation between δ 13 C and nitrogen in various growth zones 8 . This leads us to hypothesize that the very light cores of Juina-5 and Sao Luis super-deep diamonds, and superdeep diamonds from Jagersfontein and Monastery, share a common origin with the oceanic diamonds, as reflected by their carbon isotope signatures (Fig. 2b,c). Furthermore, the organic matter-like (δ 13 C between − 30 and − 20‰ 49,50 ) very light isotopic composition of such diamonds (Fig. 2b,c) from the mantle transition  www.nature.com/scientificreports/ zone argues for a common organic carbon origin. Most of the carbon (90%) in the oceanic lithosphere is stored in the altered crust, while organic matter represents only a small fraction (< 10%) of the available carbon 79,80 . In view of the distinct organic carbon signature exhibited by the transition zone diamonds, it appears that organic carbon might be the dominant carbon available in the transition zone, and the transition zone likely plays a critical role in carbon cycles 81 . Some mantle plumes are rooted from the lower mantle, whereas others could have a root near the transition zone, possibly as secondary plumes 82,83 , all above the large low-shear-velocity provinces (LLSVPs) in the lower mantle 82,84,85 . We envisage that mantle upwellings, caused by plumes 86,87 , entrain microdiamonds formed in the transition zone and transport them (and other associated ultra-high pressure minerals) to shallower levels [88][89][90][91][92][93] . In contrast, mantle convection around normal mid-ocean ridges, away from mantle plumes, do not contain such diamonds and ultra high-pressure minerals (Fig. 5). The high degree of melt extraction induced by mantle plumes is responsible for the formation of thicker (100-140 km) 55,56 and highly depleted (in iron and other incompatible elements) oceanic lithospheric mantle 59 . Buoyancy caused by such depletion, along with plume-induced thermal buoyancy and the abnormal thickness of such plume-modified oceanic lithosphere 94 , makes it more resistant to subduction 95 , leading to components of it being accreted onto arcs and preserved in orogenic belts 96 . Subduction fluids modify the original chemical signature of the lithospheric mantle by melt-rock interactions at shallow depth during both accretion/obduction and exhumation in the spinel stability field (< 80 km), leading to the formation of large podiform chromite bodies, typical of subduction zones 16 . During the accumulation processes of podiform chromite, oceanic diamonds are incorporated in newly formed high-Cr chromite 97 . Such  www.nature.com/scientificreports/ ultra-high-pressure minerals and chromite, but also the subduction origin of the high-Cr podiform chromite. Our model (Fig. 5) also provides a possible explanation for the extremely variable carbon isotopic composition of the super-deep continental diamonds (Fig. 2b) and the contrasting carbon isotope signatures of the three types of diamonds (Fig. 2). According to our model, mantle plumes bring the same super-deep microdiamonds, with homogeneous carbon isotopic composition, from the transition zone to the lithospheric levels of both the continental and oceanic realms (Fig. 5a) where they can potentially grow. Diamond growth is governed by conditions including the quantity of carbon available (CO, CO 2 , CH 4 ), the pressure-temperature condition of the ambient environment (P > 130-150 km for T > 1000 °C), the redox condition (ΔlogfO 2 (oxygen fugacity) <−2) which controls the speciation of carbon and its precipitation 98 , and the time available for the growth to occur. The P-T-fO 2 of the lithospheric mantle beneath the oldest continents (aka cratons) is known to favor the growth of diamond 99 . Such lithosphere is typically thicker (up to 300 km), colder (< 900-1000 °C) and reduced (down to ΔlogfO 2 <−4) 100 , contains sufficient amounts of carbon 101 , and is able to survive for a sufficiently long time. In such an environment, "purely" continental diamonds (white diamonds in Fig. 5) can grow to gemstone sizes with a homogeneous and predominantly lithospheric carbon isotope signature (δ 13 C − 5‰) (Fig. 2a). Super-deep continental diamonds carried up by plumes (Fig. 5) can also grow rims there that share the same lithospheric carbon isotope signature (δ 13 C − 10 to 0‰), yet their cores, of super-deep origin, retain their original lighter carbon isotopic signature which is the same as that of the oceanic diamonds (δ 13 C − 25 to − 20‰) (Fig. 2).
The P-T-fO 2 and duration of the oceanic plateau and ocean island lithosphere, on the other hand, are rather different although some parameters are still poorly constrained (e.g., a total lack of data on the redox state). The lithospheric mantle of oceanic plateaus and ocean islands is believed to be thinner (< 140 km) 55,58,102 , hotter (1000-1200 °C) 59 , more oxidised (ΔlogfO 2 from − 3 to − 1 according to limited data on mid-ocean ridge peridotites) 103 , with less carbon available 79 , and recycled quickly through Wilson cycles. Oceanic microdiamonds, once incorporated in the thickened oceanic lithosphere by plumes, are thus suppressed from growth, or even totally frozen in size, shape (cubo-octahedral) and low aggregation state (Type Ib), due to such unfavorable conditions (Fig. 5) 30 . They therefore still retain their original narrow range and homogeneous carbon isotopic composition (Fig. 2c).
Our model thus provides a processes for the formation and emplacement of three major types of diamonds. Our model differs from that of Yang's group 17 in that their model envisages widespread oceanic diamonds in the upper mantle, including in mid-ocean ridge environments, whereas in our model the occurrence of all three major types of diamonds are restricted to rocks linked to mantle plumes as our observations demonstrate ( Fig. 1; Table 1). If correct, future work can use oceanic diamonds as a tracer for past oceanic mantle plume records in ophiolites formed through Earth's history, and to test competing geodynamic models 33,70 . Further testing of our model requires an improved mantle oxidation dataset from oceanic plateaus and ocean islands, and more stable isotope ratio and age data for oceanic diamonds.

Methods
Data compilation. We compiled the known occurrences of both present-day oceanic diamonds and those found in ophiolite belts (Table 1) 13,14,16 . The location of each occurrence is presented in Fig. 1. Diamond-bearing mantle xenoliths in present-day oceans provide the most recent (34-0.44 million years old, m.y.) occurrence of oceanic diamonds (Table 1). These modern oceanic diamonds are found in garnet-bearing xenoliths in the Salt Lake crater (0.44 m.y.) near Honolulu, Hawaii 14 , and from the alnoite pipe in the Malaita Islands (34 m.y., Solomon Islands) 13 (Fig. 1). Diamond-bearing ophiolites contain much older oceanic diamonds (420-95 m.y.) ( Table 1) 16  Diamond-bearing ophiolites. A majority of diamond-bearing ophiolites are dismembered/mélange ophiolites 60,104-108 , with the exceptions of the Mirdita and Pozanti-Karsanti ophiolites which are often described as Penrose-type ophiolites 107,109 . We compiled the geological and geochemical information for the mafic and ultramafic rocks (when available) of these ophiolites ( Fig. 3 and Table S1, Figures S3 and S4). The mafic rocks, representing the oceanic crust, and the ultramafic rocks, representing the oceanic lithospheric mantle, from all ophiolites share geochemical characteristics of deep melting products (Fig. 3). The mafic rocks are characterised by a garnet peridotitic source rock, illustrated by their high TiO2/Yb, Nb/Yb and Th/Yb (Fig. 3a and Figure S3), with major and trace element compositions similar to ocean island basalts and oceanic plateau basalts ( Fig. 3a; Figures S2 and S3).
The ultramafic rocks consist of harzburgite and subordinate lherzolite and dunite. The lherzolites and dunites likely represent the products of post-melting metasomatism and melt-rock interaction, and are therefore not representative of the unaltered lithospheric mantle 110 . The harzburgites, representing the lithospheric mantle, are characterised by high Mg# ([Mg/(Mg + FeOt)at] > 0.91), low Al 2 O 3 (1.5-0.2 wt%) and very minor SiO 2 enrichments (Fig. 3b). Despite evidence for metasomatic enrichment in some harzburgites (e.g. enrichment of FeOt to > 9 wt%, Fig. 3b and S4), the least affected, and most refractory samples indicate an anhydrous melting origin by at least 30% of melt extraction at depths > 3 GPa (Fig. 3b) 68 . Such a deep melting origin is supported by the absence of silica enrichment 111 ( Figure S4), an indicator for SSZ peridotites ( Figure S2). In addition, garnetbreakdown features (i.e., spinel symplectite texture) have been reported in ophiolites from the Yarlung Zhangbo belt 112  www.nature.com/scientificreports/ Diamond classification. In this study we classify diamonds based on (1) nitrogen and boron contents as well as their configuration in the diamond lattice to define the "type" classification system 113 , and (2) their inclusions 114,115 that defines their paragenesis 3 and ultimately their lithospheric or sub-lithospheric provenances 116,117 . Lithospheric diamonds are diamonds formed in the continental lithospheric mantle and have mineral inclusions of eclogite and peridotite typical of continental lithosphere mantle, including forsterite, pyrope, omphacite, diopside, enstatite, and sulfides. Lithospheric diamonds are commonly subdivided into "eclogitic" and "peridotitic", depending on the association of inclusions. For example, diamonds with almandine and omphacite inclusions are classified as eclogitic while diamonds with forsterite and pyrope are classified as peridotitic.
Superdeep (or sub-lithospheric) diamonds have mineral inclusions typical of the upper mantle, including ferro-periclase, CaSiO 3 -walstromite, jeffbenite, majoritic garnet and retrogressed bridgmanite. Superdeep diamond inclusions also indicate the depth of the diamonds, for example, diamonds with majoritic inclusions are believed to have come from the transition zone while diamonds with retro-morphosed bridgmanite are interpreted to have come from the lower mantle (a much rarer occurrence).

Data availability
Data needed to evaluate the conclusions in the paper are presented in the paper and/or the Supplementary Materials.