Primordial and recycled helium isotope signatures in the mantle transition zone
Have you ever wondered what our planet looks like from the inside? What happens to the rocks and minerals that experience the superhot temperatures and superhigh pressures in Earth’s mantle? Although there is consensus that Earth’s structure is made up of a solid core, viscous mantle, and a silicate crust, it has been extremely challenging to get information from the deepest regions of our planet. So far, scientists have identified several chemical reservoirs in the mantle region. This research article looks at how we can use diamonds as time capsules to extract information from the oldest and most undisturbed parts of Earth.
Isotope compositions of basalts provide information about the chemical reservoirs in Earth’s interior and play a critical role in defining models of Earth’s structure. However, the helium isotope signature of the mantle below depths of a few hundred kilometers has been difficult to measure directly. This information is a vital baseline for understanding helium isotopes in erupted basalts. We measured He-Sr-Pb isotope ratios in superdeep diamond fluid inclusions from the transition zone (depth of 410 to 660 kilometers) unaffected by degassing and shallow crustal contamination. We found extreme He-C-Pb-Sr isotope variability, with high 3He/4He ratios related to higher helium concentrations. This indicates that a less degassed, high-3He/4He deep mantle source infiltrates the transition zone, where it interacts with recycled material, creating the diverse compositions recorded in ocean island basalts.
Wide ranges of Sr-Nd-Hf-Pb isotopic compositions for ocean island basalts (OIBs) and continental plume–related basalts indicate the existence of different geochemical reservoirs in the mantle (1, 2). This is also evident from their widely variable helium (He) isotope composition of 4.3 to 50 (1, 3, 4), with 3He/4He ratios expressed as R/Ra, the ratio of 3He/4Hesample to the 3He/4Heair standard (1.4 × 10−6). The chemical evolution, nature, and scale of these different reservoirs remain problematic. By contrast, mid–ocean ridge basalts (MORBs) are formed by shallow melting of the upper mantle and show much more uniform helium isotope compositions [8 ± 1 R/Ra (SD)] (5) and more homogeneous Sr-Nd-Hf-Pb isotope compositions. The higher R/Ra values of plume-related basalts relative to MORBs are taken as key evidence of a primordial undegassed reservoir with high 3He/4He ratios present in Earth’s lower mantle (6–10). However, the preservation of such a reservoir over the Earth’s history has been questioned on the basis of geophysical evidence of slab subduction into the lower mantle and mantle convection models (11). An upper mantle location for the high-3He/4He reservoir has also been suggested on the basis of seismic anomalies, heterogeneities sampled by small degrees of melt, and modeled low–U-Th/3He domains formed through melt depletion (1, 12–17). Resolving the existence and location of a long-term preserved, primordial, undegassed, high-3He/4He reservoir, and where it interacts with other reservoirs in the mantle, is key to understanding the evolution of Earth and deep mantle convection.
Diamonds are physically and chemically robust, allowing retention of He isotope signatures that reflect their formation environment (18–20). Here, we present helium isotope data of fluid inclusions from sublithospheric diamonds; these data provide an unparalleled perspective on He isotopes from transition zone depths (410 to 660 km). We studied 24 diamonds (1.3 to 6 mm in size) from the Juina-5 and Collier-4 kimberlites and São Luiz River (Juina, Brazil). The Juina area is renowned for the unusually predominantly sublithospheric origin of its diamonds (21–23). Our diamonds have physical features and properties that are consistent with other diamonds from Earth’s transition zone (410 to 660 km depth), including dislocations or diffuse growth zones (database S1) and no detectable nitrogen (N) or fully aggregated N defects (database S2) (24). The diamonds contain mineral inclusions seen in other transition zone diamonds (21, 25, 26), including breyite (database S3 and fig. S1, A and B), coesite, carbonates, sulfides, and amorphous silicate (fig. S1, C and D). Although only diamond C4-106 has a confirmed superdeep origin, on the basis of its breyite mineral inclusions, all diamonds show typical sublithospheric features. After establishing the sublithospheric origin for our diamonds, we measured helium isotopes of the fluid inclusions. We combined these data with picogram analyses of Pb-Sr isotopes of fluid inclusions, trace-element patterns of fluid inclusions, and carbon isotope values of the diamond hosts (24).
Helium is trapped in submicrometer fluid inclusions (database S1), as the Juina diamonds are relatively young, likely formed less than 500 million years ago (23, 27), and He diffusion through a diamond is extremely slow (19). Preservation of He and its isotope signatures in diamond is supported by He heterogeneities within individual diamonds (18, 20). We removed the outer rim of the diamond to eliminate any 4He implantation [up to 30 μm from the diamond surface (19)] from the surrounding matrix, either from the mantle or the kimberlite. The potential impact of production of radiogenic 4He within the diamond generally results in an R/Ra shift of <0.8, based on low U-Th-Sm concentrations measured on 10 of the studied diamonds (database S2) and assuming a diamond formation age of 500 million years ago (fig. S2). Cosmogenic production of 3He at Earth’s surface may also modify He isotope compositions, which is a concern for diamonds from alluvial sources (São Luiz River). The highest R/Ra value for our diamonds was from the Juina-5 kimberlite, and it coincides with the R/Ra of 49.8 from Baffin Island picrites (4), the highest observed in basalts. Diamonds recovered from kimberlite pipes are less likely to have been exposed to cosmic rays, but we cannot exclude exposure entirely, as the diamonds from Collier-4 and Juina-5 kimberlite pipes were likely recovered from the near-surface oxidized yellow ground kimberlite material. Thus, our studied diamonds provide the most direct and undegassed evidence of the variation in helium isotope compositions in Earth’s transition zone.
Previous studies of δ13C-δ18O diamond-mineral inclusion correlations (25) and major and trace-element abundances of mineral inclusions (23, 28) have established the presence of subducted material in the transition zone under the Juina region. The He isotopic signatures released from the sparse fluid inclusions vary from 0.7 to 49.9 R/Ra (Figs. 1 and 2). This range is larger than the variation found in plume-related basalts. Lead isotope ratios also bear similarities to plume-related basalts (Fig. 3, fig. S3, and database S2) and, along with trace-element patterns (Fig. 4 and fig. S4), provide evidence for the involvement of subducted material. Furthermore, the large range in 87Sr/86Sr ratios (0.7051 to 0.7260) (Fig. 3) suggests the involvement of old continental crust [enriched mantle (EMII) component]. Low Rb/Sr ratios that do not support the observed radiogenic Sr can be caused by subduction-related loss of Rb relative to Sr. A negative Nb anomaly characterizing the involvement of subducted material is present in all the trace-element patterns of our studied samples. This anomaly implies that a recycled crustal component, and not the ambient mantle, dominates the trace-element budget in the fluid inclusions. Furthermore, the negative Y/Ho, negative Sr, and positive Eu anomalies and the Zr-Hf depletion measured in our samples are all best explained by shallow (crustal) processes (24). Eu anomalies range from 0.01 to 3.2 Eu/Eu* and correlate positively with La/Nd ratios. We found no correlation of trace-element ratios with diamond host C or fluid He isotope compositions, indicating decoupling of volatile from lithophile elements.
The carbon isotope compositions of the diamonds form two arrays with the R/Ra values (Fig. 1). The large range in R/Ra values (trend A, Fig. 1) could be the result of mixing of various mantle reservoirs and oceanic crust–lithosphere with different R/Ra ratios (29) but with typical mantle carbon isotope values centered around −5‰. Similar ranges in He isotope composition were observed in a compilation of MORBs (5), OIBs (30), and two cloudy São Luiz diamonds (31). We also found diamonds that presented a negative correlation between δ13C and R/Ra (trend B, Fig. 1). Although unexpected, a high-3He/4He source dominating the isotope ratio could explain why the R/Ra values are higher than those found for MORBs. This would be most visible in rocks that have low helium concentrations and low U-Th-Sm contents, such as recycled pelagic sediments strongly depleted in almost all their helium and U-Th during subduction; indeed, the low δ13C values of some Juina diamonds have previously been related to subducted pelagic sediments (23). The diamonds that showed mantle-like carbon and low R/Ra values may be related to oceanic lithosphere, which would likely be more retentive of U-Th than sediments would, given their lower water content and diminished propensity to form melt. Mixing between fluids from oceanic lithosphere (mantle δ13C, high U-Th/3He, low R/Ra) and pelagic sediments contaminated by high-3He/4He material (low δ13C, low U-Th, high R/Ra) could explain trend B. Helium is trapped in fluid inclusions during diamond precipitation, likely caused by an interaction between a low-degree oxidized melt from subducted material and ambient reduced mantle or plume material (32). Several additional lines of evidence support a high-3He/4He source, potentially delivered by a mantle plume that may have originated from the African Large Low Shear Velocity Province (33). A clear increase in 3He concentrations and, to a lesser extent, in 4He concentrations, is accompanied by higher 3He/4He ratios for the Collier-4 and Juina-5 diamonds (Fig. 2, C and D). This observation requires that the high-3He/4He source has higher He abundances than reservoirs with low 3He/4He ratios and thus supports the presence of a primordial 3He plume. A similar conclusion was reached for fibrous lithospheric diamonds from Russia (34). Further, a high-3He/4He component could be related to a Cretaceous mantle plume in the Juina area. The contemporaneous formation of alkaline rocks and the Trindade plume track (32) with the young diamond formation age of a sublithospheric Collier-4 diamond [101 ± 7 million years (23)] along with the kimberlite eruption ages around 93 million years ago in the Juina area (35) all provide evidence for a deep high-3He/4He source delivered by a plume. Transportation of superdeep diamonds to shallower depths also has been suggested to occur by a mantle plume (22). Evidence from seismic tomography indicates that this plume-related mantle remains coupled to the lithosphere beneath Brazil today (36).
The complexity of isotope compositions observed in oceanic basalts has attracted development of a variety of models to explain them. Central to these models has been the problem of constraining the heterogeneous He isotope compositions of the OIB source, because these compositions seem to be largely decoupled from those of other radiogenic isotopes. Resolving this issue is critical, because He in particular has been used to define large-scale mantle structures (3) despite debate about the depth of the high-3He/4He source region tapped by OIB (1, 6–17). The He isotopic data for fluid inclusions in superdeep diamonds presented here resolve this issue by showing direct evidence that the high-3He/4He source must be present in the deep mantle, beneath a depth of 410 km. Further, the wide-ranging Sr-Pb-C isotope compositions of the superdeep diamond-forming fluids document the extreme variability in the Earth’s transition zone due to recycled crustal inputs. This recycled material is also recorded by C-N-O isotopes in other superdeep diamonds and their mineral inclusions (25, 37) and clearly has the potential to generate much of the isotopic variation found in OIBs. Our results show that the transition zone is an important heterogeneous reservoir sampled by ascending plumes, ultimately forming OIBs with less extreme isotope compositions due to mixing.
Materials and Methods
Figs. S1 to S5
Tables S1 to S4
Databases S1 to S4
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