Recycled Crustal Material Research Articles

Upper mantle scale enrichment of Cenozoic intraplate magmatism in Northeast Asia: He-Sr-Nd-Pb-O isotope geochemistry of the basalts around the Korean peninsula

The Earth’s mantle is considered to be geochemically heterogeneous, which is reflected by the diverse composition of oceanic island basalts (OIB). The mantle enrichment resulting in this is attributed primarily to the influx of recycled crustal materials into the mantle through subduction. Additionally, the sub-continental lithospheric mantle (SCLM) complicates the elucidation of mantle heterogeneity. From this perspective, Northeast Asia, where the Pacific stagnant slab in the mantle transition zone and the SCLM distribution are presented, is the suitable site for examining the upper mantle scale enrichment. Here we report He-Sr-Nd-Pb-O isotope values of basalts that erupted around the Korean Peninsula to illustrate the source lithology and components that led to mantle heterogeneity. Our measured helium isotope ratios ranging from 5.7 to 7.3 Ra (3He/4He ratio of air, Ra = 1.39 x 10-6) are mostly within the SCLM range (6.1 ± 0.9 Ra) but lower than the mid-ocean ridge basalt range (MORB; 8 ± 1 Ra). The Sr-Nd-Pb isotope compositions of the basalts generally display a mixture of depleted MORB mantle (DMM), enriched mantle 1 (EM1), and enriched mantle 2 (EM2) components. In addition, the basalts have δ18Oolivine (vs. V-SMOW) values ranging from 4.7 to 5.7 ‰ that deviate from the DMM range (δ18Oolivine = 5.1 ± 0.2 ‰). Our isotopic analysis results highlight the role of the pyroxenite source in the metasomatized SCLM in the genesis of basalts. The low 3He/4He range of the basalts indicates a significant role in the SCLM. Moreover, the delaminated cratonic SCLM and asthenosphere-lithosphere interaction are candidate scenarios for the low 3He/4He ratios. Therefore, we propose that mixing of DMM (high 3He/4He ratio; 7 to 9 Ra) and the metasomatized SCLM (low 3He/4He ratio; 5 to 7 Ra) allowed enrichment within the upper mantle scale for the Cenozoic intraplate magmatism in Northeast Asia.

Global mantle isotopic anomalies linked to supercontinent-superocean evolution for the past 700 million years

Earth’s mantle isotopic composition is highly heterogeneous, with enriched and depleted regions1–3 shaped by a complex history of depletion and re-enrichment over 4.5 billion years. The spatial configuration of large-scale heterogeneity in the mantle is not random4,5, but the timing and mechanisms that caused such heterogeneity are still debated2,6,7. We compiled radiogenic isotope data of mid-ocean ridge and plume-induced basalts from both present-day and past oceans for the past 900 million years of the relatively isotopically enriched African mantle domain. Our analysis shows that oceanic rocks of the African mantle domain exhibit a relatively homogenous composition similar to the prevalent mantle estimates (PREMA8) until ca. 350 million years ago when plume-derived rocks started becoming isotopically enriched with maximum enrichment reached in the mid-Cretaceous. From that point in time, plume-derived rocks shift toward depletion, getting closer to the prevalent mantle value. The enrichment trend is interpreted to have resulted from the recycling of crustal material during the assembly and the early stage of a break-up of Gondwana and Pangea, whereas the subsequent depletion trend is due to the outward retreat of the subduction girdle during the break-up of Pangea, thus both are legacies of geodynamic processes related to the supercontinent cycle.

Constraints on the Growth and Evolution of Continental Crust in the Northeast Cathaysia Block: Insights from Detrital Zircon U-Pb Age and Hf-O Isotope Analysis in Tingjiang Sediments

Fluvial detrital zircons provide crucial insights into the early Precambrian crustal evolution. In this study, we utilize U-Pb geochronology and Hf-O isotopic compositions of detrital zircons from the Tingjiang River to comprehend the provenance and continental crustal evolution of the northeast Cathaysia Block. Zircon U-Pb ages display seven major populations at 105-177 Ma, 200-262 Ma, 376-520 Ma with a striking peak at ca. 422 Ma, 606-775 Ma, 836-1457 Ma, 1710-2042Ma, and 2300-2661Ma, corresponding to the documented tectonic-thermal events in the South China Block. Episodic magmatism was the response to the formation and breakup of the Columbia and Rodinia supercontinent. The episode of 422 Ma matches with the ages of widely spread early Paleozoic intraplate orogenic events in the South China Block. The isotopic signatures of Hf and O in the detrital zircons indicate that the majority of Precambrian zircons likely originated from the re-melted ancient continental crust, with contributions from juvenile mantle-derived magmas. Phanerozoic zircons, on the other hand, were formed through partial melting of recycled crustal material. Analysis of crustal growth rates, based on the two-stage Hf model ages, suggests a significant presence of juvenile crustal growth during the Paleoproterozoic era. Specifically, it is estimated that approximately 40% and 90% of the present crust within the northeastern Cathaysia Block were formed around 2.3 Ga and 1.6 Ga, respectively.

Constraints on the source of Siberian Trap magmas from Mo isotope evidence

The relative contributions of asthenospheric mantle, lithospheric mantle, and continental crust in the genesis of the Siberian Traps Large Igneous Province (ST-LIP) remain poorly constrained. Most models invoke partial melting of asthenospheric mantle within a mantle plume with an inventory of recycled crustal material, with or without melting of subcontinental lithospheric mantle, and crustal contamination during ascend through the continental lithosphere. A greater understanding of this topic is of fundamental importance because the ST-LIP basalts are associated with the large Permian-Triassic extinction and the world’s largest magmatic Ni-Cu-platinum group element sulfide resources (Ni-Cu-PGE), the Norilsk-Talnakh mining camp. The ∼250 ± 2 Ma Siberian Traps at Norilsk contain a classic sequence of basaltic rocks that provide a spatial and stratigraphic context for changing chemistry of the eruptive products. We present a detailed geochemical and Sr-Nd-Hf-Mo isotopic investigation of ST-LIP volcanic rocks and associated sedimentary rocks, including coal and anhydrite, from the Norilsk area. The 98Mo/95Mo isotope ratios (reported as δ98Mo ratio relative to NIST SRM 3134) vary significantly from −0.62 to 0.07 ‰ for the older basalt formations and −0.41 to 0.03 ‰ for the younger basalt formations. The range of δ98Mo and its correlation with the other geochemical tracers cannot be explained by post-magmatic alteration, magmatic differentiation, or sulfide fractionation as Mo behaves as a lithophile element under these magmatic conditions. We suggest that the δ98Mo range can be explained by interaction of the plume with subcontinental lithospheric mantle modified by subduction processes. This is particularly prominent in the earlier Ivakinsky to Gudchikhinsky formations where light δ98Mo values coupled with low Mo/Ce can be explained by contributions from a dehydrated eclogitic component, whereas more rare heavy δ98Mo values and high Mo/Ce likely require contributions from a fluid metasomatized mantle source. Later magmas of the Nadezhdinsky formation show clear evidence of crustal contamination in their combined Mo-Sr-Nd-Hf isotope and trace element systematics, while the later more voluminous Morongovsky type magmas are shallower, large degree melts with limited crustal interaction. Our data shows the usefulness of Mo isotopes in deciphering magma sources of large igneous provinces eruptions.

Deep recycling of crustal materials by the Hainan mantle plume: evidence from Zn–Sr–Nd–Pb isotopes of Hainan Island basalts

Deep recycling of crustal materials by the Hainan mantle plume: evidence from Zn–Sr–Nd–Pb isotopes of Hainan Island basalts

Stable Zirconium Isotopic Compositions of the Metasomatized Mantle

AbstractMantle metasomatism, which affects the geochemical and lithological properties of the Earth, is crucial to the evolution and modification of the Earth's interior. Variations in chemical proxy of metasomatism, including high‐field strength elements (HFSE, e.g., Zr‐Ti isotopes and isovalent Zr‐Hf), have been commonly utilized as a tool to unravel the processes associated with the evolution of the Earth. However, the impact of mantle metasomatism by diverse agents on the Zr isotopic compositions of metasomatized mantle remains poorly understood. Here we carried out high‐precision double‐spike δ94/90Zr measurements for a suite of basalts, which were derived from the mantle sources metasomatized by typical metasomatic agents of fluid, silicate melt and carbonate melt originating from the deep mantle or recycled crustal materials. Mantle metasomatism results in lithological diversities within the mantle, ranging from hybridized peridotites to pyroxenite/eclogite and carbonated eclogite, which can significantly fractionate the isovalent HFSEs. However, our data show that these basalts have primitive mantle‐like δ94ZrNIST values (−0.033–0.043 ‰), regardless of their superchondritic Zr/Hf. The primitive mantle‐like δ94ZrNIST, irrelevant to any proxy of metasomatism (e.g., Sr‐Nd isotopes, Zr/Hf and La/Sm), indicate that mantle metasomatism and subsequence melting produce no measurable Zr isotopic variations in metasomatized mantle‐derived basalts. The δ94ZrNIST of basalts thus can be representative of the mean metasomatized mantle compositions. Combined with previously reported komatiites and oceanic basalts, we show that the Earth's mantle displays consistent δ94ZrNIST (−0.013 ± 0.053 ‰), even if it has a secular dynamic evolution with the interaction between Earth's exterior and interior.

Calcium isotopic compositions of eclogite melts and negligible modification during reaction with lithospheric mantle

Subduction-driven recycling of crustal materials plays a substantial role in the generation of geochemical and lithological heterogeneities in the Earth’s mantle. Because the distinct compositions and/or mineral-specific equilibrium isotopic fractionation effects are present in eclogitized crustal materials, stable isotopes of major elements that exert significant impacts on geochemical and petrologic properties of the mantle may serve as powerful tracers for linking the geochemical anomaly to lithological heterogeneity within the Earth’s interior. Here we present high-precision Ca isotope data, combined with previously reported Mg-Fe isotopes, for a suite of intraplate basalts with a large contribution from recycled crustal material in their mantle sources. The δ44/40Ca in these potassic basalts are consistently lower (0.66–0.78 ‰) than those of mid-ocean ridge basalt (MORBs; 0.85 ± 0.09 ‰), with an average of 0.72 ± 0.09 ‰. The co-variations of major elements with radiogenic isotopes (e.g., MgO-ɛNd) reflect the mixing of two distinct endmembers, one of which is the low-MgO primitive melts and the other is the lithospheric mantle. The low-MgO, SiO2-rich primitive melts are characterized by low CaO/Al2O3 (<0.4) and high Dy/Yb (>5). Remarkably, the primitive melts exhibit low δ44/40Ca (ca. 0.70 ‰)-δ26Mg (ca. –0.60 ‰), and elevated δ57Fe (ca. 0.30 ‰) relative to MORBs. These features are consistent with their origin as partial melts of eclogitic crustal materials. During ascending to the surface, these melts reacted with the surrounding lithospheric peridotite. This process resulted in a noticeable shift of the Mg-Fe isotopes towards the typical mantle values but very limited δ44/40Ca modification. These results suggest that Ca isotopes largely hold the signature of primary melts and are a promising tracer of eclogite in the mantle source of basalts. We further show that the combined isotopes of Ca-Mg-Fe, which are stoichiometrically incorporated into mantle minerals, offer substantial potential to establish the links between lithological and geochemical heterogeneity in the mantle.

O–Hf isotope ratios of Alvand S-type granite, western Iran, reveal crustal melting in an extensional regime

There are a few intrusive bodies in the central Sanandaj-Sirjan Zone (C-SaSZ), which is parallel to the Zagros suture zone, in western Iran. The Alvand batholith in the Hamedan area is one of these bodies and consists of rock types with large variation, in particular, two-mica garnet-bearing granite with an S-type signature. Zircon UPb dating gives crystallization ages of 170–163 Ma for the S-type granite. The 176Hf/177Hf ratios in the zircon grains vary from 0.282001 to 0.282780 (rim), with low 176Lu/177Hf ratios (0.002 to 0.018). The εHf(t) values (rim) are clustered into two main groups: a negative εHf(t) group (−6.9 to 0.0, n = 39), with some highly negative εHf(t) values (−25.8 to −21.9, n = 3), and slightly positive εHf(t) values (+0.3 to +2.9, n = 14) of inherited grains. The δ18O values for the zircon grains are positive and range from +10.4‰ to +11.8‰. The high δ18O (positive) and mostly negative εHf(t) values are more consistent with continental crustal sources for these rocks, and the Hf model ages (TDM2) show a wide range from 2.7 to 0.8 Ga, suggesting the heterogeneity of the crustal components. The long incubation time (2089 to 544 Ma) indicates a long history of crustal residence and crustal reworking. The incubation age of the zircon grains, >300 Ma, confirms the recycling of crustal material by partial melting. The simultaneous occurrence of S-type granites with mafic bodies, such as gabbro and diorite members with tholeiitic signatures, suggests that the injection of mafic magma increased the geothermal gradient from the bottom and sides along the major faults, which was the trajectory for the injection of hot mafic melts to shallow depths. These processes abnormally increased the geothermal gradients locally, and the partial melting of crustal and supracrustal materials produced S-type granites and migmatization among the hot blade-shaped mafic bodies. Therefore, the partial melting of crustal material in an extensional tectonic regime and/or in a thin continental crust occurred in a continental rift and/or in the Neo-Tethys passive margin. The present work concludes that the passive margin can be suggested as the tectonic setting for the generation of S-type granites, without the collisional regime that has been widely considered for the sources of this type of granite in the world.

Magnesium isotopic constraints on the lithospheric mantle metasomatism beneath the craton margin

The magnesium (Mg) isotopic distinction between surficial carbonate and mantle rock makes the Mg isotopes a potential tracer for recycled crustal materials (especially the carbonate-bearing materials) within the mantle. However, whether or to what extent recycled carbonate-bearing crustal materials could influence the Mg isotopes of the lithospheric mantle is still in debate. Here, we report the Mg isotope data for three well-characterized alkaline complexes from the northern margin of the North China Craton, i.e., the Hekanzi, Yaojiazhuang and Saima alkaline complexes. These alkaline rocks were all derived from enriched lithospheric mantle sources beneath the craton margin that were variably metasomatized by carbonate-bearing recycled crustal materials. However, they have relatively homogeneous and normal mantle-like δ26Mg values, i.e., −0.23 to −0.25‰ for the Hekanzi, −0.20 to −0.30‰ for the Yaojiazhuang and − 0.25 to −0.30‰ for the Saima alkaline rocks, respectively. These Mg isotopic characteristics were unrelated to shallow crustal processes (i.e., surface alteration, fractional crystallization and crustal assimilation) but inherited from the mantle sources. The mantle-like Mg isotopes suggest that the recycled carbonate-bearing crustal materials did not change the Mg isotopic compositions of the lithospheric mantle overlying the subduction zone, which mainly ascribes to the large contrast of MgO contents between the recycled crustal materials and the lithospheric mantle peridotite. The MgO contents of calcite-rich carbonates in the metasomatic agents released from the subducted crustal materials are too low to induce Mg isotopic anomaly in the MgO-rich peridotite. Our work reveals that Mg isotopes might not be a sensitive tracer of carbonate melt/fluid metasomatism in the lithospheric mantle beneath the craton margins.

Fe–Mg Isotopes Trace the Mechanism of Crustal Recycling and Arc Magmatic Processes in the Neo‐Tethys Subduction Zone

AbstractThere has been intense debate on the mechanism of crustal recycling in subduction zones, and Fe–Mg isotopes may provide new constraints on this issue. This study reports Fe–Mg isotope data for mafic plutonic rocks from the eastern and central Gangdese arc, along with their associated trench sediments in southern Tibet. The δ26Mg values of the eastern Gangdese arc rocks show negative correlations with (87Sr/86Sr)i and (206Pb/204Pb)i values, but positive correlations with εNd(t) and εHf(t) values. Conversely, the δ56Fe values of the eastern Gangdese arc rocks show positive correlations with (87Sr/86Sr)i and (206Pb/204Pb)i values, but negative correlations with εNd(t) and εHf(t) values. The Mg and Fe isotopic compositions of the central Gangdese arc rocks are comparable with those of the eastern ones, but they do not correlate with Sr–Pb–Nd–Hf isotopic compositions. Notably, the Fe–Mg isotopic compositions of most arc rocks fall between those of local trench sediments and the mantle wedge. Combined qualitative analyses and quantitative simulations suggest that: (a) the Fe–Mg isotope variations observed in the eastern Gangdese arc rocks highlight the important role of source mixing between sediment‐derived melts and peridotite, whereas (b) the Fe–Mg isotope variations observed in the central Gangdese arc rocks reflect the superposition of carbonated serpentinite‐derived Mg‐rich fluids‐peridotite source mixing and source melting. The strong correlations between Fe–Mg isotope ratios and traditional geochemical tracers provide further evidence for the recycling of crustal materials in subduction zones via various types of slab‐derived fluids and melts.

Modern ocean island basalt–like 182W signature in Paleoarchean mafic rocks: Implications for the generation, preservation, and destruction of early mantle heterogeneities

Komatiites and picrites generated by high degrees of mantle partial melting serve as potential probes of Earth’s deep mantle. Tungsten (W) isotopes in these rocks offer a rare chance to better understand early differentiation, late accretion, core-mantle interaction, and subsequent evolution of Earth’s mantle. We present new W isotope data for Archean komatiites and basalts from the Barberton (South Africa) and Suomussalmi (Finland) Greenstone Belts and Permian picrites from the Emeishan large igneous province (China). The Paleoarchean samples from the Barberton Greenstone Belt have modern ocean island basalt (OIB)−like μ182W values ranging from −20.4 to +5.6, whereas the Mesoarchean komatiites from the Suomussalmi Greenstone Belt show μ182W values of −2.2 to +11.3. The Permian Emeishan picrites give μ182W values of −7.1 to +3.1. Our data, combined with the published global data set, show that W isotope heterogeneity in the mantle has existed throughout Earth’s history, with positive μ182W values transitioning to near-zero in the upper mantle by the end of the Archean. The negative μ182W values of Paleoarchean samples in the Barberton Greenstone Belt and modern OIBs likely result from either early differentiation or core-mantle interaction. The incorporation of a plume-delivered negative μ182W component and enhanced mantle mixing is a viable mechanism to explain the transition of μ182W values in the upper mantle from positive to near-zero, while recycling of crustal materials into the mantle would result in a shift of negative μ182W values of the lower mantle closer to zero since the onset of plate tectonics. The latter process could possibly explain the slightly negative to near-zero μ182W values of the Emeishan picrites and some kimberlites. The well-resolved negative μ182W anomalies observed in this study provide important insights into the generation, preservation, and obliteration of W isotope heterogeneities in the lower mantle.

Chromium isotope system of intraplate basaltic lavas: Implication for recycling materials into mantle

To explore the mantle source properties and the magmatic evolution process for intraplate basaltic lavas, as well as to investigate the potential of Cr isotopes to trace recycled crustal materials, we present Cr isotope compositions for 72 well-characterized samples of Cenozoic intraplate basaltic lavas from eastern China. These basalts display a resolvable δ53Cr range of −0.24‰ to −0.03‰ (n = 72), comparable to MORB and OIB. With respect to the evolved lavas, their δ53Cr scatter can be interpreted as a result of fractional crystallization of predominant olivine (or/and clinopyroxene) with 1% to 3% spinel from heterogeneous magma sources under different oxygen fugacity conditions. Combined with Sr-Nd-Zn-Fe isotope data, the least evolved basalts with MgO of ∼10% display a large δ53Cr range, indicating the presence of different magma component end-members, including one low-Si and two high-Si components. Two high-Si enriched components have δ53Cr values lower than those of primitive N-MORB and OIB magmas, consistent with their derivations most likely from relatively enriched mantle pyroxenites. Notably, the low-Si nephelinitic component could not be a simple end-member as proposed before. Correlations of δ53Cr with εNd(t)-87Sr/86Sr(i)-Fe/Mn-Ti/Ti*-Hf/Hf*-δ66Zn-δ56Fe [Ti/Ti* = TiN/(NdN-0.055 × SmN0.333 × GdN0.722), Hf/Hf* = HfN/(SmN × NdN)0.5. N represents the primitive mantle-normalized values] for nephelinites from both the North China Block and the South China Block suggest mixing between partial melts of carbonated peridotite and pyroxenite sources. The carbonated peridotite derived melt is characterized by depleted SrNd isotopes, low Fe/Mn, Ti/Ti* and Hf/Hf* ratios, and heavier than BSE's Cr-Fe-Zn isotope compositions. The highly oxidized environment induced by carbonatization effectively reduces Cr isotope fractionation between melts and residues during partial melting of the carbonated peridotite, resulting in the heavy Cr isotope feature of melts being inherited from the carbonated mantle source, consistent with the heavy Cr isotope compositions of carbonated mantle xenoliths obtained previously. Thus, this work highlights that Cr isotopes might act as a potential tracer for subducted sedimentary carbonates.

Geochemical characterization, U–Pb apatite geochronology, and geodynamic significance of olivine minette dykes from the Julian Alps, NE Italy

AbstractWe investigated olivine minette dykes from the Rio Colan Valley of the Julian Alps (NE Italy) to provide new constraints on the mantle geochemistry underneath this extreme sector of the Italian Southern Alps. Petrographic observations, high Mg#s, high Cr and Ni contents, low Dy/YbCN ratios and flat heavy rare earth element (REE) profiles imply these are primary magmas derived from a depleted peridotite with olivine and garnet as possible residual phases. However, high K2O and incompatible trace element contents, coupled with superchondritic Nb/Ta ratios, suggest that the source was modified into a rutile–phlogopite-bearing carbonated peridotite by multiple metasomatic events such as recycling of crustal material and carbonatitic metasomatism related to an old orogenic event and rutile-rich metasomatism linked to the Pangaea break-up. Laser ablation – multi-collector – inductively coupled plasma – mass spectrometer (LA-MC-ICP-MS) U–Pb geochronology of apatites from two dykes yielded differences between unforced and forced discordia ages up to ∼45 Ma, likely due to the initial 207Pb/206Pb correction adopted. Nonetheless, because forced discordia and 206Pb/238U weighted mean ages are overall similar within the uncertainty, our preferred interpretation is that olivine minettes intruded and crystallized at ∼67 Ma. The age and anorogenic signature of these magmas are consistent with the regional extensional tectonics in the Julian Alps due to the advance of the external Dinaric front following Adria plate eastward subduction. Eventually, we show also that Rio Colan magmas are geochemically hybrid products between lamprophyres and lamproites compared with similar Italian and Western Mediterranean volcanics.

Sluggish lithium isotopic response of continental intraplate basalts to recycled sedimentary carbonate

Recognizing the signal of recycled carbonate in the deep mantle and associated basalts is crucial for depicting deep carbon cycling. It has long been recognized that marine sedimentary carbonates have much heavier lithium isotope compositions (δ7Li) than the depleted mantle. However, Li isotope systematics are rarely employed to trace recycled carbonate. To investigate basalt Li isotopic response to recycled carbonate, we carried out whole-rock geochemical and Li isotopic studies on a series of the Cenozoic continental intraplate basalts from eastern China that sample a typical carbonated mantle. The basalts have been reported to have light magnesium (Mg) isotope compositions, but their δ7Li values vary in a limited range (0.9–5.7‰) around the depleted mantle (~3.8 ± 1.4‰). The decoupled LiMg isotopic records are closely related to recycled Mg-rich carbonates (dolomite) that have lighter Mg isotope compositions than the depleted mantle but show mantle-like δ7Li values. After modeling the Li isotopic effects of carbonate sediment, silicate sediment and altered oceanic crust (AOC) recycling, we find that even calcium-rich carbonate recycling is inadequate to shift basalt Li isotope composition. Collectively, Li isotopic responses of intraplate basalts to all types of recycled sedimentary carbonates are more sluggish than expected. Our modeling also suggests that recycling of AOC and silicate sediments is likely responsible for the basalts with δ7Li values higher and lower than the depleted mantle, respectively. In addition, seawater δ7Li values probably exert a first-order role in the Li isotopic response. Low δ7Li values of paleo-seawater (< 20‰) for most of the Earth's history have generated AOC with δ7Li values indistinguishable from the depleted mantle, which goes against the emergence of high δ7Li values for intraplate basalts. Although some limitations exist in Li isotopic applications, we propose that a combination of Li isotopic data and whole-rock element ratios is effective in tracing recycled crustal materials hidden in the intraplate basalts.

Geological evolution of the Proterozoic Betul belt (∼2.16–0.95 Ga) of the Central Indian tectonic Zone: Its linkage to the assembly and dispersal of Columbia and Rodinia supercontinents

The Proterozoic Betul Belt is an important supracrustal unit that records the protracted history of crustal accretion and growth in the Central Indian Tectonic Zone (CITZ). Here, we present results from systematic geological mapping, petrological, geochemical (including Sm-Nd isotope) studies, and U-Th-Pb geochronology on rocks from the Betul belt, with the objective of reconstructing its Proterozoic evolution and its link with past supercontinents. The Betul belt comprises ca. 2167 ± 11 Ma granite gneisses, ca. 2051 ± 80 Ma mafic volcanics (pillowed lava), 1715 ± 10 Ma rhyolite, and older granitoids (1671 ± 29 Ma) that have undergone multi-stage deformation and amphibolite-facies regional metamorphism. The bulk rock geochemistry of gneisses, pillow lava, and mafic intrusions are characterized by arc-like signatures, whereas the rhyolite and granites are anorogenic and back-arc related. The whole-rock geochemical and Sr-Nd isotopic compositions of pillow lavas and mafic intrusions suggest the involvement of recycled crustal material in their source, possibly in the form of associated sediments. The Betul granite gneisses were emplaced at 2167 Ma in a collisional setting by melting crustal protoliths having an affinity with the Bundelkhand craton and may constitute the basement for the supracrustal sequence. The supracrustal units, i.e., mafic volcanics and rhyolites, occur as a bimodal suite formed in the arc and back-arc environment with penecontemporaneous sedimentation. Bulk rock geochemistry and age data indicate two phases of A-type granite magmatism at 1674 Ma and 1079 Ma, with post-collisional geochemical traits forming within an anorogenic and back-arc setting, respectively. Younger post-orogenic granites (Navegaon) were emplaced in a back-arc setting at ca 1079–954 Ma, synchronous with a collisional event in the Sausar belt. Spot ages from monazite in Hbl gabbro yield a weighted mean of 1026 ± 97 Ma (MSWD = 0.15), reflecting Grenville-age regional metamorphism in the CITZ. The geological events recorded in the rocks of the Betul belt between ca 2167 Ga and 954 Ma correlate well with events that affected Laurentia, Baltica, Australia, and East Antarctica within the frameworks of the Columbia (ca. 2.1–1.7 Ga) and Rodinia (ca.1.2–0.9 Ga) supercontinents.

Barium isotope composition of depleted MORB mantle constrained by basalts from the South Mid-Atlantic Ridge (5–11°S) with implication for recycled components in the convecting upper mantle

Stable barium (Ba) isotopes are emerging as tracers for the recycling of crustal material into the mantle. Small but significant Ba isotope variations have been found in global MORB (δ138/134Ba values of −0.04‰ to 0.15‰), but the processes causing these Ba isotope variations remain poorly understood. In addition, uncertainties still exist in the estimate of the Ba isotope composition of the depleted upper mantle. Here, we present a systematic study of Ba isotopes for well-characterized MORB glass samples from the South Mid-Atlantic Ridge (SMAR) between 5 and 11°S, which span a wide range of radiogenic isotope ratios and trace element contents. Our results show that the northernmost segment A0 basalts with highly depleted radiogenic isotope compositions have δ138/134Ba values ranging from −0.02 to 0.05‰. In comparison, the δ138/134Ba values of the isotopically enriched basalts from the A1-A4 segments vary from 0.02 to 0.11‰. Thus, while small, the enriched components involved can be observed to modify the Ba isotope composition of the mantle source beneath A1-A4 segments. Combining our new measurements with literature data, this study places new constraints on the Ba isotope composition of the depleted upper mantle, as well as the origin of Ba isotope heterogeneity in MORB. The Ba isotope composition of the depleted upper mantle in the absence of recycled components is estimated to have a δ138/134Ba value of 0.03 to 0.05‰, which can be adopted as the baseline for using Ba isotope ratios as a tracer of mass transfer processes between the crust and mantle. In comparison, global MORB have δ138/134Ba values ranging from −0.04‰ to 0.15‰. No global correlation is observed between Ba isotope ratios and geochemical parameters that record mantle enrichment, indicating that the Ba isotope variations in MORB cannot be simply ascribed to the addition of some single component, such as sediment. Models for melting and mixing between recycled materials and depleted mantle suggest that the variable Ba isotope compositions of the enriched MORB require a contribution from recycled altered oceanic crust and crustal sedimentary materials. Therefore, these recycled components both play important roles in the chemical budget of the convecting upper mantle, especially for incompatible elements such as Ba.

Sulfur in Archean komatiite implies early subduction of oceanic lithosphere

Komatiite is a magnesium-rich volcanic rock that erupted almost exclusively during the Archean era, requiring high partial melting of Earth's upper mantle to form. Archean komatiites, therefore, provide the most reliable information on the mantle geochemistry of the early Earth. However, it remains unclear whether komatiite compositions reflect only remnants of an early deep mantle or may provide evidence of recycling of subducted components. Sulfur mass-independent fractionation (S-MIF) could be useful to distinguish the two scenarios because S-MIF is produced only by photochemistry in anoxic atmospheres. We report here multiple sulfur isotope analyses of the 2.7 Ga Belingwe komatiites and related volcanics. Some Belingwe komatiites show clear S-MIF signatures (Δ33S from −0.20‰ to −0.12‰). Petrology, trace element chemistry, and Sr-Nd-Pb isotope compositions demonstrate that the samples contain Ni-poor pristine igneous sulfides and crustal contamination is minimal. The observed S and initial Nd isotope covariation can be reconciled if the komatiite source involved an early depleted shallow mantle which acquired S-MIF from seawater (Δ33S < 0‰) via seafloor alteration. We suggest that the Belingwe komatiites were derived at least partly from a past subducted oceanic lithosphere that stagnated in the mantle from the Archean era to the present. Therefore, the sulfur isotopes in Archean komatiite provide new constraints on the history of subduction and the recycling of crustal material.

Formation of Continental Crust by Diapiric Melting of Recycled Crustal Materials in the Mantle Wedge

AbstractThe compositional similarity between high‐Mg andesite‐dacite from accretionary orogens and bulk continental crust (CC) provides an opportunity to unravel the CC formation paradox. Compositional data from a global compilation of Quaternary magmatic arcs indicate the presence of low‐Mg series (LMS) and high‐Mg series (HMS). The LMS show trends of crystal fractionation and can be subdivided into high Ba/Th and high La/Sm groups, which likely originate from fluid‐ and sediment melt‐modified mantle wedge, respectively. In contrast, the HMS have variably mixed compositions (e.g., high Mg#, Ba/Th, and La/Sm) and can be explained by partial melting of mélange diapirs rising into the mantle wedge, which are mixtures of subducted sediment, eroded arc crust or CC, buoyant oceanic crust, and peridotite. We, therefore, propose a two‐step process for creating CC involving extraction of LMS from the mantle followed by re‐melting of recycled LMS in the mantle to generate HMS and thus CC.

Modification of the lithospheric mantle induced by recycled crustal components: Insights from Early Cretaceous appinites from the Liaodong Peninsula, NE China

AbstractThe term “appinites” refers to amphibolerich mafic rocks, as one of the most typical products of partial melting of a metasomatic lithospheric mantle. This paper presents results of integrated geochemical and isotopic analyses for six Early Cretaceous (125–119 Ma) appinitic plutons from the Liaodong Peninsula, northeastern North China Craton (NCC), to not only investigate the properties of the metasomatic lithospheric mantle induced by variable recycled crustal components but also to discuss a hydrous mantle source. The appinites originated from partial melting of sub-continental lithospheric mantle (SCLM), and can be geochemically categorized into three groups distinguished by the input of different recycled crustal components (e.g., terrigenous sediments and delaminated lower continental crust) on the basis of two distinct Hf-O isotopic arrays. Based on the occurrence of voluminous amphiboles (50–60 vol%) and minor clinopyroxene remnants, a petrogenetic model was proposed to interpret the formation of appinites. Asthenospheric upwelling caused by the subduction and roll-back of the Paleo-Pacific plate firstly resulted in partial melting of the SCLM to yield basaltic magmas. Subsequently, numerous hydration reactions between early anhydrous minerals (e.g., olivine and pyroxene) of basaltic magmas and hydrous melts derived from terrigenous sediments and delaminated lower continental crust occurred to produce the studied appinites. Ultimately, the recycled crustal materials can serve as the metasomatic agent not only to transfer the enriched isotopic signatures into the studied appinites but also to represent hydrous components to play the part of hydrous reactions. The high abundance of amphibole in the studied appinites is indicative of a hydrous lithospheric mantle beneath the eastern NCC.

New insights into the mantle source of a large igneous province from highly siderophile element and Sr-Nd-Os isotope compositions of carbonate-rich ultramafic lamprophyres

Despite being volumetrically minor components, carbonate-rich ultramafic magmas like aillikites represent good candidates to investigate the compositional variations in plume and/or lithospheric mantle sources because they represent low-degree melts which preferentially sample highly fusible components including recycled crustal material. To gain new insights into the composition of the plume-related magmas and, more broadly, the petrogenesis of ultramafic lamprophyres, we have undertaken the first comprehensive study of bulk rock and mineral (olivine and Ti-magnetite) highly siderophile element (HSE) abundances and Re-Os isotopes combined with in situ major-, trace-element and Sr-Nd isotope analyses of apatite and perovskite from the Permian Wajilitag aillikites of the Tarim large igneous province, China. The Wajilitag aillikites have high PPGE (Pt and Pd) contents relative to IPGE (Os, Ir and Ru), which can be ascribed to low-degree partial melting and/or fractionation of olivine and laurite. Measured 187Os/188Os ratios are moderately to highly radiogenic (0.186–0.313) with age-corrected γOs values up to +113. In situ Sr and Nd isotope analyses of apatite phenocrysts (87Sr/86Sr(i) = 0.70349–0.70384; εNd(i) = +1.3 to +4.9) and fresh perovskite grains (87Sr/86Sr(i) = 0.70340–0.70390; εNd(i) = +1.3 to +3.8) exhibit limited variability both within and across samples from different aillikite dykes and the only volcanic pipe in the area. These Nd isotopic values resemble those from bulk-rock samples (εNd(i) = +1.9 to +5.2), whereas Sr in apatite and perovskite extends to marginally less radiogenic values than the bulk-rock compositions (87Sr/86Sr(i) = 0.70362–0.70432). The moderately depleted Sr-Nd isotope compositions of magmatic apatite and perovskite, and the previously reported mantle-like C isotope values of these samples suggest that the aillikites and their carbon probably derived from a sub-lithospheric (plume) source with minimal contribution of deeply subducted material. Conversely, the radiogenic Os isotope compositions of the Tarim aillikites and separated minerals require some contribution from recycled crustal material in the plume source. Mass balance calculations suggest that the radiogenic Os isotopes and moderately depleted Sr-Nd isotopes can be reproduced by less than one third of eclogite component addition to a moderately depleted mantle source. We conclude that the combination of complementary isotopic systems can enlighten contributions from different components to mantle-derived magmas and, in this case, clarifies the occurrence of carbon-free subducted oceanic crust in the Tarim plume.