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Continental flood basalts derived from the hydrous mantle transition zone

Abstract

It has previously been postulated that the Earth’s hydrous mantle transition zone may play a key role in intraplate magmatism, but no confirmatory evidence has been reported. Here we demonstrate that hydrothermally altered subducted oceanic crust was involved in generating the late Cenozoic Chifeng continental flood basalts of East Asia. This study combines oxygen isotopes with conventional geochemistry to provide evidence for an origin in the hydrous mantle transition zone. These observations lead us to propose an alternative thermochemical model, whereby slab-triggered wet upwelling produces large volumes of melt that may rise from the hydrous mantle transition zone. This model explains the lack of pre-magmatic lithospheric extension or a hotspot track and also the arc-like signatures observed in some large-scale intracontinental magmas. Deep-Earth water cycling, linked to cold subduction, slab stagnation, wet mantle upwelling and assembly/breakup of supercontinents, can potentially account for the chemical diversity of many continental flood basalts.

Introduction

Continental flood basalts (CFBs) form part of large igneous provinces (LIPs) erupted onto continental crust and are characterized by anomalously high rates of mantle melting, representing the largest volcanic events in Earth’s history. Such events have significant implications for understating Earth dynamics1,2 and global environmental catastrophes3. CFBs are conventionally attributed to decompressional melting of hot mantle, driven by mantle plumes—hot upwellings originating at the core–mantle boundary2. However, the chemical compositions of most CFBs are dissimilar to ocean island basalts (OIBs)4 that are associated with hot spots—the surface expressions of mantle plumes5. Furthermore, although CFBs are generally several hundred kilometres from plate boundaries (subduction zones), some exhibit subduction-like geochemical signatures4,6. Petrological and geochemical evidence from the Karoo7, Etendeka8, Siberian9 and Central Atlantic LIPs6 suggests that generation of CFBs is ultimately related to fluid/melt derived from subducted slabs. However, the question of how fluid is released from deep subducted oceanic slabs and effects or controls partial melting of the mantle in an intracontinental setting is unclear.

The mantle transition zone (MTZ) has been considered to act as a gigantic water tank in deep-Earth fluid cycling10,11,12,13,14,15,16,17 due to water-enriched wadsleyite and ringwoodite at depths between 410 and 660 km (refs 11, 13, 16, 17), which can host enough water to comprise up to 2.5% of the weight of these minerals13,14. This implies that the MTZ may play a key role in terrestrial magmatism and plate tectonics12,18. As an oceanic plate descends into the upper mantle, any fluid retained within the subducting plate beyond the depths of magma generation constitutes a return flux to the Earth’s interior15. Subducted oceanic slabs tend to be subhorizontally deflected or flattened as ‘stagnant slabs’ above, across or below the 660-km discontinuity11. Hydrous phases in the MTZ would eventually decompose into anhydrous phases by releasing fluids to produce wet upwellings that pool within the MTZ11,19. However, direct observations for the water content of the MTZ are spare and mainly inferred from geophysics16,17. With the exception of a single 10 μm ringwoodite grain from one location13, there is no geochemical evidence that the MTZ is water rich. Thus, finding confirmatory evidence of the origin of primary mantle melts from a hydrous MTZ is an important step in understanding deep-Earth water cycling and identifying contributions from non-plume processes in the generation of CFBs.

This study suggests that oxygen isotopes, 39Ar/40Ar ages, and other geochemical features of the Chifeng CFBs of East Asia are correlated with distance to the western edge of the stagnant Pacific slab within the MTZ. We demonstrate that the lavas immediately above the western edge of the stagnant Pacific slab are characterized by being oversaturated in silica, having the oldest eruptive ages, the lowest olivine δ18O values and possibly the highest fluid contents. The combination of clearly recycled oceanic crust-derived signatures and their correlation with the inferred position of stagnant Pacific slab within the hydrous MTZ therefore provides an important constraint on the depth of the mantle source. These observations lead us to propose an alternative thermochemical model that may be more widely applicable, whereby slab-triggered wet upwelling produces large volumes of melt that may rise from the hydrous MTZ. This model links deep-Earth water cycling, slab stagnation, wet mantle upwelling, large-scale intraplate magmatism and assembly/ breakup of supercontinents into a self-organised system.

Results

Hydration of the MTZ by stagnant Pacific slab

Central–East Asia is an ideal area to study deep-Earth water recycling on a large intracontinental scale, because Cenozoic volcanic rocks in this region (Fig. 1) are associated with stagnant slabs in the MTZ20,21. The subducted slab of the Pacific plate extends horizontally over a distance of 800 to >1,000 km in the MTZ at the 660-km discontinuity11,21,22. Such a stagnant slab has the ability to add a large amount of subducted fluid into the mantle, with the potential for subsequent triggering of wet mantle upwelling21,23,24,25.

Figure 1: Schematic map of the late Cenozoic intraplate volcanic province in Central–East Asia.
figure1

Digital topography of Central and East Asia (original data from National Oceanic and Atmospheric Administration/National Geophysical Data Center website: http://www.ngdc.noaa.gov/mgg/image/). The dark grey colour indicates the location of late Cenozoic volcanism in Central–East Asia. The thick solid white lines represent plate boundaries. The late Cenozoic intraplate volcanic province is composed of eastern (#1−#4), central (#5−#8) and western (#10−#13) units. The eastern unit is not typically intraplate due to its close proximity to the plate boundary. The western and central units are more representative of typical intraplate volcanism. Inset figure shows the tomographic map of the inferred stagnant paleo-Pacific slab within the mantle transition zone along latitude 43° (ref 22; highlighted by the grey dashed horizontally line). Numbers 3 and 7 in the inset figure represent the Changbaishan and Jining-Hannuoba volcanic rock units, respectively. The red dashed line indicates the western edge of the present-day stagnant Pacific slab21.

The Chifeng CFBs are located far from plate boundaries, but were erupted directly above the inferred edge of the Pacific stagnant slab (red dashed line in Fig. 1), and extend about 200 km further to the west. They were mainly (>90% volume) erupted between 23.8 and 6.1 Myr ago (Supplementary Figs 1 and 2), producing 100–450-m-thick sheet-like lava flows that cover a total area of 3,000 km2 (ref 26). Eighty-five samples were collected in this study from across the Chifeng CFB province to investigate the role of deep water cycling in their generation. The rocks include quartz tholeiite, olivine tholeiite and alkali basalt, all with high-sodium contents (Supplementary Figs 3−5) and depletion of heavy rare earth elements (Supplementary Fig. 5). All the samples show prominent positive spikes for Ba, K, Pb and Sr in the primitive mantle normalized diagram (Supplementary Fig. 5), a typical characteristic of hydrated mantle melts6,20,27,28.

The most striking observation is that oxygen isotopes, 39Ar/40Ar ages, and other geochemical features of the Chifeng CFBs (SiO2, Nb, Sm/Nd, Ce/Pb, K/Ce, Tb/Yb and Sr/Ce) are correlated with distance to the western edge of the stagnant Pacific slab (Fig. 2; Supplementary Fig. 6). The western termination of the Pacific slab is located at 119° 30′ E (ref 22) (red dashed line in Fig. 1), with the Chifeng CFBs extending westward to around 117° E. In a westward direction, the lavas decrease in age, are depleted in silica, and have lower Sm/Nd ratios, whereas they show progressive enrichment in Nb and have higher Ce/Pb ratios. Importantly, the olivine δ18O values of the basalts increase westward away from the stagnant slab (Fig. 2a). Thus, the lavas immediately above the western edge of the stagnant Pacific slab are characterized by being oversaturated in silica, having the oldest eruptive ages, the lowest olivine δ18O values and possibly the highest fluid contents (high Ce/Pb, K/Ce and Sr/Ce ratios; Fig. 2a,e and Supplementary Fig. 6a,c).

Figure 2: Covariations of geochemistry as a function of eruptive longitude.
figure2

(a) olivine δ18O, (b) Nb, (c) Sm/Nd, (d) SiO2, (e) Ce/Pb, and (f) 39Ar/40Ar age versus longitude (°). Data points represent an average of analysed whole-rock or olivine samples from individual Chifeng CFBs (for detailed sample distribution see Supplementary Dataset 1 and Supplementary Fig. 1); error bars represent 1 s.d. for each group of lava or olivine. r, correlation coefficient.

Variations in oxygen isotopes in the mantle source are ultimately related to surface water–rock interaction29,30,31. The oxygen isotopes of primary melts are therefore sensitive to contributions from hydrothermally altered recycled components. Partial melts of unaltered mantle peridotites record a relatively uniform value of δ18Oolivine=5.18±0.28‰, corresponding to δ18Owhole-rock=5.5±0.2‰ (refs 29, 31). In contrast, melts of subducted sediments and the upper oceanic crust display higher δ18O values (≥7‰), whereas melts of subducted lower oceanic crust and altered ultramafic rocks from deeper in the ocean lithosphere are characterized by lower δ18O values (mostly ranging from 5‰ to 3‰)30. Thus, oxygen isotopes are crucial for identifying any contribution from the stagnant Pacific slab in generating the Chifeng CFBs.

The olivine oxygen isotope compositions of the Chifeng CFBs reveal three prominent features: (1) a wide range of δ18O values varying from 3.0‰ to 5.8‰ (Fig. 2a; Supplementary Figs 7 and 8), which plot significantly beyond the range of pristine upper mantle olivine (5.18±0.28‰); (2) significant depletion of 18O (370 of 478 total analyses have δ18O values <5.18‰), varying from +3‰ to 5‰, similar to those of hydrothermally altered gabbros30; and (3) systematic spatial variation of average olivine δ18O values that are tightly correlated with eruptive site (highlighted by green band in Fig. 2a). Because the above-mentioned features are tightly correlated with the position of the stagnant paleo-Pacific slab within the MTZ, this implies that slab stagnation, dehydration and wet upwelling may have played an important role in the generation of the Chifeng CFBs.

Differentiation of the Chifeng CFBs is dominated by olivine and clinopyroxene fractionation (Supplementary Fig. 9). Crystallization of olivine will increase both the silica content32 and the δ18O29 value in differentiated magmas and thus would generate positive δ18O-SiO2 and negative δ18O-Fo correlations, respectively. This is in stark contrast with our observations (Fig. 3a,c). Crustal materials are generally characterized by high SiO2 contents and high La/Sm, but low Sm/Nd, ratios, whereas asthenospheric mantle-derived melts generally have typical mantle oxygen isotopes (olivine δ18O of 5.18±0.28‰), lower La/Sm and higher Sm/Nd ratios compared with crustal materials. Thus, if the low δ18O signatures were produced by contamination of asthenospheric mantle melts by crustal materials, the δ18O values should positively correlate with SiO2 and Sm/Nd (Fig. 3a,b) and negatively correlate with La/Sm (Fig. 4b). Such predicted crustal contamination trends are in distinct contrast with the observations. Furthermore, the negative SiO2-La/Sm and positive SiO2-Sm/Nd correlations are also inconsistent with predicted crustal contamination trends (Fig. 5c,f). Thus, contributions from assimilation and fractional crystallization processes in producing the trends observed in Figs 2, 3, 4 can be excluded. In summary, the correlations between olivine δ18O and other geochemical parameters reflect the nature of the primary melts and their source region.

Figure 3: Correlation of olivine oxygen isotopes and corresponding whole-rock major trace elements.
figure3

Covariation of high-field-strength elements (HFSEs, for example, Nb) and fluid sensitive trace element ratio (Ce/Pb). Pristine upper mantle olivine oxygen isotope composition31 is also shown. Mixing model endmembers in a and b are δ18O=+5.2‰ and +3.5‰, SiO2=44 and 54 wt%, Sm/Nd=0.2 and 0.34, Nb=70.4 and 4.4 p.p.m. (0.1 recycled sediment+0.9 recycled slab), and Ce/Pb=24.4 ((oceanic-island basalts (OIB)-type melt, blue diamond in d) and 5.3 (0.1 recycled sediment+0.9 recycled slab)66, respectively. The average values of alkalic OIB67 and normal mid-ocean ridge basalts (N-MORB)67 are also shown. AFC, assimilation fractional crystallization; sed, sediment; Fo, forsterite content of olivine in c. Solid red lines with small diamonds represent the mixing of two endmembers. Each diamond on the mixing curves represents a 10% increment of end-member melts. The detailed chemical compositions of analysed olivine are presented in Supplementary Fig. 13.

Figure 4: Correlation between average oxygen isotope values and major trace elements.
figure4

δ18O signatures versus (a) Sr*/Sr, (b) La/Sm, (c) Al2O3 and (d) Lu/Hf. Sr*/Sr=2 × SrN/(CeN+NdN), where subscript N indicates primitive mantle67 normalized values. Because crustal materials are characterized by high La/Sm ratios, if the low δ18O signatures resulted from crustal contamination of normal asthenospheric mantle melts, a negative δ18O-La/Sm trend is expected as highlighted by the blue line with arrow in b, whereas a positive δ18O-La/Sm correlation is observed. This implies that the low δ18O signature is a primary feature of the melts.

Figure 5: Whole-rock geochemical covariation diagrams of the Chifeng CFBs.
figure5

(a) Zr/Hf, (b) Nb/La, (c) SiO2, (d) Lu/Hf, and (e) Al2O3/TiO2 versus Sm/Nd, and (f) SiO2 versus La/Sm. Continental crustal materials68 and normal mid-ocean ridge basalt (N-MORB)67 are also shown. The contamination of asthenospheric mantle melts (N-MORB) by crustal materials, such as UCC, would result in increasing SiO2 and La/Sm, but decreasing Sm/Nd in the evolved magmas, producing a positive SiO2-La/Sm, and a negative SiO2-Sm/Nd. These predicted crustal contamination trends are inconsistent with the trends observed in the Chifeng CFBs in c and f. These trends demonstrate that the effects of crustal contamination can be ignored. BCC, bulk continental crust; LCC, lower continental crust; UCC, upper continental crust.

The effect of the degree of partial melting on trace element ratios of Sm/Nd, Nb/La, La/Sm and Lu/Hf at low to moderate melting fractions (0.1–0.3; Supplementary Fig. 10) is insignificant compared with the large variation of these ratios in the Chifeng CFBs. Thus, by excluding crustal contamination, the correlations as shown in Fig. 5 clearly suggest two end-member melts were involved in the generation of the Chifeng CFBs. The correlations in Figs 2, 3, 4, 5 and Supplementary Figs 11 and 12 show that one end-member melt has an enriched OIB-like incompatible element composition, pristine upper mantle-like olivine δ18O values (5.18±0.28‰,) and low silica content (44%). After correction for olivine fractional crystallization by adjusting to equilibrate with Fo90 (ref. 32), the OIB-like end-member melt has a chemical composition similar to that of the incipient partial melt of garnet peridotite33, with SiO2=44−46 wt%, TiO2=2.0−2.5 wt%, Al2O3=9.4−10 wt%, MgO=17 wt% and FeOtotal=10−11 wt%; Sm/Nd=0.20, Zr/Hf≥44, Nb/La≥1.4, Lu/Hf≤0.04, Al2O3/TiO2=4–5 and La/Sm≥4.0. The evidence from low 3He/4He ratios shows that they are similar to melts generated in the shallow mantle, but significantly lower than those generated in the primordial lower mantle28. The lack of lower mantle-rooted low-velocity seismic structure21, the alkalic nature of the dominant phase and no evidence for pre-eruption uplift34 argue strongly against a deep plume origin for the OIB-like end-member melt35. The large-scale doming in the Hangai–Hövsgöl region of central Mongolia has been cited as key evidence for the presence of a mantle plume below Central Asia in the late Cenozoic36. However, a combined gravity- and topography-based geophysical investigation37 demonstrated that a high-heat-flux mantle plume was unlikely to be involved because upper mantle dynamics are in reasonable agreement with the gravity, tomography, volcanic and xenolith data from the area. Recent geochemical and petrological studies35,38 have demonstrated that an OIB-like enriched mantle component can form within the upper part of the mantle by enrichment with fluid or melt released from depth, possibly originating from the MTZ as we propose.

The other end-member melt is characterized by low olivine δ18O values (3–4‰), high silica (Fig. 3a), depleted incompatible trace element compositions (Figs 2c and 3b) and subduction slab-like Ce/Pb ratios (Fig. 3d). Furthermore, the depletion of 18O is coupled with depletion in high-field-strength elements (for example, Nb; Fig. 2c) and other incompatible elements, as evidenced by decreasing La/Sm ratios (Fig. 4b). More importantly, the low δ18O end-member melt also has characteristics of melts typical of recycled oceanic crust that is dominated by gabbroic components, such as high-positive Sr anomalies (Fig. 4a), high Al2O3 (Fig. 4c) and SiO2 (Fig. 3a) contents, and high Fe/Mn (Supplementary Fig. 8f), Sm/Nd (Fig. 3b) and Lu/Hf (Fig. 4d) ratios, but low La/Sm (Fig. 4b) and Tb/Yb (Supplementary Fig. 6b) ratios. This is confirmed by the studies of olivine-hosted melt inclusions39 from the south-eastern extremity of the Chifeng CFBs (119° 15′ E) erupted directly above the western edge of the stagnant Pacific slab. The melt inclusion data imply that these CFBs were mainly derived from an olivine-free pyroxenite-dominated source with high water content (>450 p.p.m.). These observations show that the low δ18O end-member melt most likely originated from hydrothermally altered oceanic gabbros30,32. Furthermore, from the systematic regional trend presented in Fig. 2, hydrothermally altered oceanic gabbro was most likely present in the stagnant Pacific slab. Thus, the combination of clearly recycled oceanic gabbro-derived signatures and their correlation with the inferred position of stagnant Pacific slab within the hydrous MTZ provides an indirect, but important, constraint on the depth of the mantle source.

For the Chifeng CFBs, the hydrous melts considered to have originated in the MTZ do not record any high δ18O values (>6‰). This new finding implies that water (possibly including other fluid phases) was transported into the MTZ in hydrothermally altered gabbros and possibly ultramafic rocks from the interior part of the subducted oceanic lithosphere. In contrast, the upper part of the subducted slab that would have higher δ18O values most likely underwent breakdown during the subduction process and may have generated high δ18O fluid via arc or back arc volcanism in the subduction channel. This is perhaps why high δ18O (>6‰) samples have not been identified in this study. Another possibility is that any high δ18O signatures would be diluted by low δ18O components, implying that the upper part of the subducted slab may play only a minor role in deep-Earth water cycling, at least in this region.

In summary, the source of the CFBs is dominated by two end-member reservoirs: the stagnant Pacific slab and fertile peridotite. Mixing of melts from these two end-members can explain the major element, incompatible trace element, and oxygen isotope compositions of the Chifeng CFBs (Fig. 3a,b,d). The two-end-member mixing calculation shows that the source of low Ce/Pb lavas may also contain minor subducted sediments (Fig. 3d; Supplementary Figs 11 and 12). This is consistent with the coexistence of low Ce/Pb and normal olivine δ18O values (5.0‰) in individual samples. Direct or indirect contribution from stagnant slabs therefore significantly affects the upper mantle thermochemical state and has played an important role in generating the Chifeng CFBs23,40.

MTZ wet upwellings produce the Chifeng CFBs

These observations lead us to propose an alternative model for the Chifeng CFBs, whereby slab-triggered wet upwelling and upward percolation/hydration, which originated at the MTZ, produced large volumes of melt (Fig. 6). The slow kinetics of the pyroxene–garnet transformation41 and slab viscosity42 within the MTZ enables the Pacific slab to reside for a long time in this zone (over 108 year) and hence lose water by dehydration reactions within the MTZ11. Because feedbacks between trench backward migration and slab deformation are integrated during the slab stagnation42, the declining rate of Pacific-Eurasia convergence from a late Cretaceous convergence rate of 120–140 mm year to a minimum in the Eocene of 30–40 mm year43 may also have contributed to the Pacific slab stagnation. After completion of the pyroxene–garnet transformation41, the stagnant Pacific slab is likely to eventually fall into the lower mantle (slab avalanche) and release most of its water by dehydration melting when it penetrates the 660-km discontinuity19. The viscosity reduction due to slow grain growth may also induce slab penetration42. Combining geophysical and experimental studies has therefore demonstrated the presence of extensive dehydration within the MTZ beneath this region25. Experiments44 and seismic investigations45 indicate that hydrous melt or supercritical fluid is gravitationally stable atop the 410-km discontinuity and may possibly spread laterally. The density contrast between melts and surrounding solids determines the direction of material transport in the Earth and therefore the density of silicate melts plays a very important role in controlling the chemical differentiation of the Earth46,47,48,49,50. In this model, the density contrast between hydrous melts and coexisting minerals is a critical parameter that affects hydrous melting and consequent upwelling. Hydrous melt is denser than the associated minerals if the water content is low, whereas when the water contents increase to higher than the threshold values, the hydrous melts will be buoyant50. This aspect has been investigated48,49 and it is confirmed that a hydrous melt formed at relatively low temperatures will be buoyant because the melt should have a large water content, but that a hydrous melt formed at high temperature will be denser. Thus, large-scale wet upwelling may be caused by a combination of slab avalanches and the positive buoyancy generated by accumulation of less-dense fluid phases. The wet upwellings will fertilize the shallow mantle (highlighted by yellow colour in Fig. 6) and decrease its solidus temperature, resulting in large-scale partial melting51. The temporal-spatial distribution of the Chifeng CFBs may thus be controlled by variations in slab stagnation and its final penetration of the 660-km discontinuity into the lower mantle.

Figure 6: Effects of slab stagnation and water cycling on the upper mantle thermochemical state.
figure6

Water cycling refers to wet upwelling, upward percolation and refertilization. This model is mainly based on the strong spatial correlation of geochemical features with distance of eruptive lavas relative to the edge of the stagnant slab. This is based on water partitioning in the Earth’s mantle18, behaviour of slab-triggered wet upwelling and upward percolation23,40, hydrous mantle melting51, the mantle wedge model21, and upwelling from the hydrated mantle transition zone24.

Initial subduction of the Pacific oceanic plate under this part of Asia was inferred to take place at 130−120 Myr ago by integrating the drift history of the Pacific plate with tectonic evolution and gold mineralization in eastern China52. Combining the temporal distribution of volcanism and calculating the cumulative amount of total subduction within the MTZ with geophysical tomography in this region shows that collapse of stagnant slabs may have started about 25 Myr ago12, which coincides with the onset of eruption of the Chifeng CFBs (ca. 24 Myr ago; this study). Furthermore, because the completion of the pyroxene–garnet transformation within the MTZ will take 100 Myr ago41, the age of the stagnant Pacific slab is inferred to be 125 Myr ago, consistent with previous estimates52. Our study therefore leads to the proposition that a MTZ water-filtering model may have controlled water in the MTZ via stagnation of the Pacific oceanic plate. The contributions from water filtering19 and high water solubility18 result in hydration of the MTZ10,11,12.

Discussion

The hydrous MTZ model may also apply to CFBs associated with the breakup of supercontinents. Geological evidence shows that both the Pangea53 and Rodinia54 supercontinents were surrounded by circum—supercontinent subduction zones, a feature that seems to be true for other supercontinents and is likely to have important geodynamic implications53. Assembly of supercontinents therefore potentially leads to accumulation of a large volume of hydrated oceanic slabs within the MTZ11,12 and may possibly change the mode of mantle convection11. The time delay of 140–150 Myr between the final assembly and initial breakup of Pangea53 and Rodinia54 is in good consistent with the lifespan of stagnant slabs within the MTZ constrained by slab viscosity42 and phase transformation41. Such similarity may hint at a possible causal relationship between slab avalanche (and/or mantle overturn) and breakup of supercontinents. Slab avalanches within the hydrated MTZ would generate large-scale wet upwellings that hydrate the subcontinental lithospheric mantle (Fig. 6), resulting in an instability of the lowest part of the continental lithosphere55,56. More importantly, hydrous basaltic melts would prefer to pond at the boundary between the lithosphere and asthenosphere57,58, providing a lubricating mechanism for speeding-up plate motion. The slab avalanche-driven upwelling and mobility of hydrous basaltic melts would hence promote the breakup of supercontinents and may in turn facilitate slab avalanches by adjusting trench motions and changing the geometry of subducted slabs59. Recent studies have proposed that the strictures of classical physics combined with modern seismic imaging, require that convective circulation in the upper mantle is plate driven and that the principal response to plate subduction and delamination is large, wide, slowly upwelling regions of mantle rooted within the MTZ60. Thus, plate- and slab avalanche-driven upwelling originating from the hydrous MTZ may have played an important role in the breakup of supercontinents and the circulation of water (and possibly other fluid phases) between Earth’s surface and the MTZ.

The arc-like geochemical features of CFBs are traditionally attributed to continental lithosphere (both crust and mantle) input. The presence of strong arc-like geochemical signatures and hydrous phenocrysts in some high-magnesium basaltic rocks from the Siberia9 and Karoo7 LIPs show that crustal contamination cannot be the controlling factor. Detailed geochemical and isotopic analyses of basaltic rocks from the Siberia9, Karoo7 and Central Atlantic6 LIPs demonstrate that their generations involved water and possibly other fluid phases released from subducted slabs and that their source reservoirs were both isolated and hydrated over a considerable period of time. Because the hydration processes would result in instability of subcontinental lithospheric mantle56, hydrated subcontinental lithospheric mantle cannot remain isolated from the convective upper mantle. Geochemical studies and thermochemical modelling suggest that the massive degassing of volatiles (CO2 and HCl) in some CFBs mostly originates from recycled crust in the deep Earth61. These observations show that continental lithosphere cannot be the dominant reservoir that produces the observed hydrous and arc-like geochemical characteristics of these CFBs. Thus, finding a mechanism to explain the long-term hydration and isolated nature of the mantle reservoir beneath the lithosphere is crucial for understanding the origin of CFBs.

The deep-Earth cycling of water thus requires a fresh perspective to explain long-term hydration and its isolation within the mantle. For example, water and other fluid phases released from stagnant slabs carry large-ion lithophile elements such as Ba, Pb, K and Sr, and light rare earth elements (La to Nd), but are not able to carry high-field-strength elements such as Nb, Ta and Ti9. Long-term hydration and isolation within the MTZ therefore provide a mechanism to produce a hydrated reservoir that is characterized by both low high-field-strength and high large-ion lithophile element abundances and enriched radiogenic isotopes—a characteristic of arc-like CFBs4,6,7,8,9. Water-driven wet mantle will rise by its own buoyancy and melt on crossing its wet solidus beneath supercontinents. Such a water cycle, linked to cold subduction, slab stagnation and downward fluid transfer in the MTZ by water filtering, is expected to produce arc-like CFBs4,6,7,8,9 with chemistry similar to that of island arcs, but at greater distances from the trench. These processes may account for the geochemical diversity of CFBs that makes them elementally and isotopically distinct from OIBs4. In this model, mantle upwelling is driven by deep-Earth water cycling rather than excess temperature, such as in a classical thermal plume model, which can cause about 0.8–1 km of broad surface uplift per 100 °C of plume excess temperature62. Because the wet upwelling of mantle requires no excess temperature in this model, there will be no thermal anomaly to result in pre-volcanic eruption surface uplift62. Also, some parts of the subducted slab may undergo dehydration melting and release water when it penetrates the 660-km discontinuity into the lower mantle19, whereas other subducted slab fragments may be entrapped in wet upwellings and returned to the upper mantle10,51. The latter provides a variant of the model that also accounts for the involvement of deep subducted slabs in the generation of large-scale intracontinental magmatism. In addition, numerical simulations23 and geophysical investigations11,21,22 have demonstrated that volcanism produced by wet upwelling of the mantle originating from a hydrated MTZ can be located between 600 and over 1,000 km from the trench. This can lead to CFB provinces being located at the margins of Precambrian cratons1. The evidence for a craton-accreted terrane setting is particularly well established for the Central Atlantic LIP, where Paleozoic island arcs and continental fragments were accreted onto the eastern margin of Laurentia during the assembly of Pangea4. Subsequently, the Central Atlantic LIP was extruded directly onto these terranes during the breakup of Pangea. The dominant arc-like, low-Ti-type basalts4,9 within the Siberian LIP have also been attributed to wet upwelling, driven by dehydration of the Mongolia–Okhotsk oceanic slab at the MTZ9. Finally, water and other fluid phases released from the hydrated MTZ may also lead over time to melt accumulating at the lithosphere–asthenosphere boundary57,58. Accumulated melts can escape to the surface through weak zones, such as pre-existing suture zones, within the lithosphere. This explains why CFB provinces are commonly located over suture zones1. Accumulated hydrous melts at the LAB have the potential to behave as a lubricant to continent rifting and thus enhance the breakup of supercontinents. This means that arc-like CFBs, such as the Karoo, Central Atlantic and Siberian LIPs, were prominent in the initial breakup of Pangea. Here we emphasize that the presence of a plume that originated at the core–mantle boundary is not a necessary condition for producing arc-like CFBs, especially those associated with rifting and initial breakup of supercontinents.

Methods

Whole-rock major and trace element analysis

Seventy-three fresh basalts samples were collected from the Chifeng-Keshiketengqi area (119° 30′ E−117° 00′ E, 42° 00′ N−43° 20′ N; Supplementary Fig. 1) for major element analysis (Supplementary Dataset 1), 45 of which were further analysed for whole-rock trace elements (Supplementary Dataset 2). Rock samples were sawn into slabs and the central parts (>200 g) were used for bulk-rock analyses. The slabs were crushed into small fragments (<0.5 cm in diameter) in a jaw crusher before being further cleaned and reduced to powder in a corundum mill. Bulk-rock major element oxides were analysed using X-ray fluorescence (Rikagu RIX 2,100) with analytical uncertainties better than 3% for SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O and K2O and better than 5% for TiO2, MnO and P2O5. Trace elements were analysed using inductively coupled plasma mass spectrometry at the Key Laboratory of Continental Dynamics, Northwest University, Xi’an, after acid digestion of samples in Teflon bombs. Repeated runs gave <3% relative standard deviation for most trace elements analysed.

Mount making

Eleven samples which were analysed for whole-rock major and trace elements were chosen to separate olivine grains for electron microscope chemical and in situ secondary-ion mass spectrometry (SIMS) oxygen isotope analyses (Supplementary Dataset 3). The separated olivine grains were mounted together with reference San Carlos olivine (δ18O=5.35‰; Eiler29) in the centre of epoxy resin mounts. The mounts were then ground and polished using different grain size Diamond Films to minimize any relief difference.

Electron microscope analysis

Chemical compositions of olivines were determined using a JEOL JXA-8,100 Superprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing. The operating conditions were: 15 kV accelerating voltage, 20 nA beam current, 3 μm beam diameter and 10 s peak counting time for most elements (30 s for Ca and Ni, 7 s for Na and 8 s for K). The data reduction was carried out using the atomic number, absorption, and characteristic fluorescence (ZAF) correction.

In situ SIMS olivine oxygen isotope analysis

The sample mount was gold coated and oxygen isotope compositions of olivines were analysed with a CAMECA IMS-1,280 ion microprobe at the IGGCAS. The Cs+ primary beam was accelerated at 10 kV with an intensity of ca. 2 nA. The spot size was about 20 μm in diameter (10 μm beam diameter+10 μm raster). The normal incidence electron flood gun was used to compensate for sample charging during analysis. Secondary ions were extracted with a −10 kV potential. Oxygen isotopes were measured in multi-collector mode with two off-axis Faraday cups with a mass resolution of 2,500 (exit slit #1) in a circular focusing mode. Each analysis consisted of 20 cycles × 4 s counting time. Typical ion intensities of 2–4 × 106 counts per second obtained on the 18O peak yielded an internal 2σ uncertainty of better than ±0.2‰. Three unknown and two standard δ18O measurements were run in bracketed mode. Measured 18O/16O ratios were normalized using the Vienna Standard Mean Ocean Water composition (VSMOW, 18O/16O=0.0020052), and then corrected for instrumental mass fractionation (IMF) as follows:

The unknown olivine data were corrected for IMF and calculated as a function of time within the standard blocks, to derive an average between the standard measurements at the beginning and end of each cycle. The external reproducibility of the standard (San Carlos) for δ18O is 0.4‰ 2 s.d., which was set as the error for individual measurements. Many studies have proved that there is no obvious matrix effect for olivine with Fo number >70 and San Carlos olivine standard (Fo=91)63,64,65. However, olivine with Fo number <65 has been proven to show about 0.6 permil bias IMF with San Carlos olivine as the standard65. Considering the unknown olivine grains have Fo number ranging from 60 to 91, we selected only grains with Fo >70 for oxygen isotope analysis. Thus, the potential matrix effect need not be considered further.

40Ar/39Ar analysis

Twelve groundmass samples were dated using the step-heating 40Ar/39Ar method (Supplementary Dataset 4). 40Ar/39Ar measurements were performed at the IGGCAS, Beijing. To constrain the eruption age and avoid excess argon, the groundmass grains with grain size ranging from 0.2 to 0.3 mm were purified carefully under a binocular microscope to remove the visible phenocrysts and xenocrysts. The samples were cleaned with acetone followed by further cleaning with deionized water in an ultrasonic bath for three times, each for 40 min. The cleaned samples were then dried at 100 °C for 20 min. Groundmass wafers weighing 3–16 mg, and multiple samples of the 18.6±0.4 Ma neutron fluence monitor mineral Brione muscovite, were irradiated in vacuo within a cadmium-coated quartz vial for 5 h in position H8 of the Beijing Atomic Energy Research Institute reactor (49–2). The argon isotopes were analysed on a MM5400 mass spectrometer.

Additional information

How to cite this article: Wang, X.-C. et al. Continental flood basalts derived from the hydrous mantle transition zone. Nat. Commun. 6:7700 doi: 10.1038/ncomms8700 (2015).

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Acknowledgements

We thank Professors Zheng-Xiang Li (Curtin University) and Simon Turner (Macquarie University) for comments on an earlier draft. We are also grateful to X. M. Liu (Northwest University) and X.H. Li, F. Wang and L. K. Yang (Institute of Geology and Geophysics, Chinese Academy of Sciences) for their help during the geochemical and olivine oxygen analyses and 39Ar/40Ar dating. This work was supported by the Australian Research Council (ARC) Future Fellowship (FT140100826), the National Natural Science Foundation of China (Grant No. 41222023 and 40803010), and the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (Grant No. SKLIG-KF-13-07). This is TIGeR publication No. 627 and contribution 641 of the CCFS ( http://www.ccfs.mq.edu.au/).

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X.-C.W. had the idea for the study, wrote the manuscript, helped perform the chemical and isotopic analyses, and formulated the conclusions. S.W. evaluated the results and conclusions and assisted with manuscript preparation and writing. Q.-L.L and Y.-N.Y. performed in situ SIMS and electron microprobe analyses.

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Correspondence to Xuan-Ce Wang.

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Supplementary information

Supplementary Information

Supplementary Figures 1-13 and Supplementary References (PDF 2687 kb)

Supplementary Data 1

Major elements determined by X-ray fluorescence (XRF) spectrometry (XLSX 18 kb)

Supplementary Data 2

Trace elements (ppm) determined by inductively coupled plasma mass spectrometry (ICP-MS) (XLS 36 kb)

Supplementary Data 3

Oxygen isotope and chemical compositions of olivine phenocrysts from the Chifeng CFBs (XLS 132 kb)

Supplementary Data 4

Ar-Ar dating of the Chifeng continental flood basalts (XLS 53 kb)

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Wang, XC., Wilde, S., Li, QL. et al. Continental flood basalts derived from the hydrous mantle transition zone. Nat Commun 6, 7700 (2015). https://doi.org/10.1038/ncomms8700

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