Continental Lithospheric Mantle Research Articles

The Redox State of the Asthenospheric Mantle and the Onset of Melting Beneath Mid‐Ocean Ridges

AbstractThe redox state of the convective asthenospheric mantle governs the speciation of volatile elements such as carbon and, therefore, influences the depth at which (redox) melting can occur, with implications for seismic signals. Geophysical observations suggest the potential presence of carbonatite melts at a depth of 200–250 km. However, thermodynamic models indicate that the onset of (redox) melting would occur at 100–150 km for a mantle with 3%–4% of Fe3+/∑Fe. Here, we present a new oxybarometer that is based on the V/Sc exchange coefficient between olivine and the melt, which is insensitive to surficial alteration, volatile degassing, electron exchange reactions and fractional crystallization. By applying this method to primary mid‐ocean ridge basalts (MORBs) from Southwest Indian Ridge and East Pacific Rise, we demonstrate that the average oxygen fugacity (fo2) of MORBs corrected for the depth of formation is 0.78 ± 0.26 (1σ) log units above the fayalite‐magnetite‐quartz (FMQ) buffer, which is slightly more oxidized than previously estimated (near FMQ buffer). Our findings indicate that the convective asthenospheric mantle exhibits higher oxygen fugacity than the continental lithospheric mantle. Along an adiabat, carbonatitic melts can form from a CO2‐bearing source at a depth of 200–250 km, explaining the asthenospheric mantle's electrical conductivity and seismic velocity anomaly.

Gondwanan flood basalts linked seismically to plume-induced lithosphere instability

Delamination of the continental lithospheric mantle is well recorded beneath several continents. However, the fate of the removed continental lithosphere has been rarely noted, unlike subducted slabs reasonably well imaged in the upper and mid mantle. Beneath former Gondwana, recent seismic tomographic models indicate the presence of at least 5 horizontal fast-wavespeed anomalies at ~600 km depths that do not appear to be related to slab subduction, including fast structures in locations consistent with delamination associated with the Paraná Flood Basalt event at ~134 Ma and the Deccan Traps event at ~66 Ma. These fast-wavespeed anomalies often lie above broad slow seismic wavespeed trunks at 500 to 700 km depths beneath former Gondwana, with slow wavespeed anomalies branching around them. Numerical experiments indicate that delaminated lithosphere tends to stagnate in the transition zone and mid-mantle above a mantle plume where it shapes subsequent plume upwelling. For hot plumes, the melt volume generated during plume-influenced delamination can easily reach ~2 to 4 × 106 km3, consistent with the basalt eruption volume at the Deccan Traps. This seismic and numerical evidence suggests that observed high-wavespeed mid-mantle anomalies beneath the locations of former flood basalts are delaminated fragments of former continental lithosphere, and that lithospheric delamination events in the presence of subcontinental plumes induced several of the continental flood basalts associated with the multiple breakup stages of Gondwanaland. Continued upwelling in these plumes can also have entrained subcontinental lithosphere in the mid-mantle to bring its distinctive geochemical signal to the modern mid-ocean spreading centers that surround southern and western Africa.

Water Speciation and Storage Capacity of Olivine under the Reduced Fluid—Peridotite Interaction

The key features of the interaction between peridotites of the continental lithospheric mantle and reduced hydrocarbon-rich fluids have been studied in experiments conducted at 5.5 GPa and 1200 °C. Under this interaction, the original harzburgite undergoes recrystallization while the composition of the fluid changes from CH4-H2O to H2O-rich with a small amount of CO2. The oxygen fugacity in the experiments varied from the iron-wustite (IW) to enstatite-magnesite-olivine-graphite/diamond (EMOG) buffers. Olivines recrystallized in the interaction between harzburgite and a fluid generated by the decomposition of stearic acid contain inclusions composed of graphite and methane with traces of ethane and hydrogen. The water content of such olivines slightly exceeds that of the original harzburgite. Redox metasomatism, which involves the oxidation of hydrocarbons in the fluid by reaction with magnesite-bearing peridotite, leads to the appearance of additional OH absorption bands in the infrared spectra of olivines. The water content of olivine in this case increases by approximately two times, reaching 160–180 wt. ppm. When hydrocarbons are oxidized by interaction with hematite-bearing peridotite, olivine captures Ca-Mg-Fe carbonates, which are products of carbonate melt quenching. This oxidative metasomatism is characterized by the appearance of specific OH absorption bands and a significant increase in the total water content in olivine of up to 500–600 wt. ppm. These findings contribute to the development of criteria for reconstructing metasomatic transformations in mantle rocks based on the infrared spectra and water content of olivines.

Extremely enriched lithospheric mantle–derived magmas in the central Lhasa Terrane, southern Tibet

ABSTRACT Plate tectonics plays a crucial role in the evolution of Earth’s continents and oceans. However, the impact of oceanic subduction and continental collision on the continental lithospheric mantle within collisional orogenic belts is still a matter of debate. To address this issue, we performed a thorough analysis of mafic rocks in the Nixiong area within the central Lhasa Terrane, southern Tibet. The mafic rocks are composed of diabases and basalts formed during syn-collisional period (~50 Ma). The diabases have low SiO2 and moderate MgO contents, and are enriched in large ion lithophile elements and depleted in high field strength elements. They show high 87Sr/86Sri (0.711451–0.712084) and low εNd(t) (−7.9 to −6.44), as well as enriched Hf-Pb isotope compositions (εHf(t) = -7.58 to −6.59; 206Pb/204Pbi = 18.59–18.71; 207Pb/204Pbi = 15.71–15.72; 208Pb/204Pbi = 39.33–39.53). Geochemical modelling suggests that the diabases were formed through the partial melting of peridotite at high temperatures and shallow depths, with significant addition of subduction-related components to the primary melts. The diabases probably originated from a metasomatized lithospheric mantle due to subduction of Neo-Tethyan slab. The basalts display similar SiO2 but lower MgO contents compare with the diabases. They are also characterized by extremely high 87Sr/86Sri (0.713332–0.713448), low εNd(t) (−10.32 to −9.96), and significantly enriched Hf-Pb isotope compositions (εHf(t) = -12.30 to −12.05; 206Pb/204Pbi = 18.81–18.85; 207Pb/204Pbi = 15.74–15.75; 208Pb/204Pbi = 39.81–39.88), corresponding to an extremely enriched lithospheric mantle. Integrated with the previous studies, the Nixiong mafic rocks exhibit the most enriched isotopic compositions among the arc- or syn-collision-related mafic rocks in southern Tibet. These extremely isotopically enriched components were most likely derived from the ancient continental lithospheric mantle beneath the central Lhasa Terrane. Our results provided unambiguous evidence that the pre-existing ancient continental lithospheric mantle could persist locally during the prolonged oceanic subduction and subsequent continental collision.

Apatite U–Pb geochronology and whole rock, Sr–Nd–Pb isotopic geochemistry of XV mafic‐ultramafic intrusion, Bafq, Central Iran: Implications for petrogenesis and tectonic setting

AbstractThe XV mafic‐ultramafic intrusion is located in the western part of the Posht‐e‐Badam Block (PBB) within the Central Iranian Micro‐Continent (CIMC). Petrographically, the intrusion is composed of gabbro and pyroxenite. Apatite U–Pb dating has established the crystallization age of this intrusion to be 363 ± 67 Ma. The XV intrusive rocks are tholeiitic to slightly calc‐alkaline in nature and are characterized by an enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and Heavy Rare Earth Elements (HREE), respectively. The major oxide elements display continuous trends relative to SiO2. The 87Sr/86Sr(i) ratios range from 0.7045 to 0.7056, and the εNd(i) values range from 2.63 to 3.30. In addition, the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios exhibit a narrow range, varying from 18.68 to 18.70, 15.67 to 15.71, and 38.84 to 38.99, respectively. The geochemical and isotopic characteristics suggest that the parental magma was derived from a Sub‐ Continental Lithospheric Mantle (SCLM) that was modified by oceanic slab‐derived components. The locations of the XV intrusive rocks in εNd(i) versus TDM (Ga) and Nb/La versus discrimination diagrams further support this conclusion. Fractional crystallization is identified as the dominant process influencing the formation of distinct lithological units within the XV intrusive rocks. Our newly presented isotopic and geochronological data, when considered in the regional context, suggest that the XV intrusive rocks were formed in an extensional tectonic setting. In this scenario, upwelling from the asthenospheric mantle induced heating, leading to the melting of previously subduction‐modified SCLM. Comparative analysis with previously published ages indicates that extensional magmatism in the PBB continued into the Middle Paleozoic.

Lithospheric weakening by a small-scale plume and its geodynamic implications

The process of small-scale mantle plume emplacement within the continental lithosphere influences the dynamic topography and architecture of the lithosphere-asthenosphere boundary (LAB). Furthermore, plume emplacement alters the rheology of the crust and lithospheric mantle through melt-driven, mechanical and thermal weakening. Previous studies of plume emplacement within the lithosphere have described these outcomes, albeit without considering plume-driven weakening. In this study, we quantitatively evaluated the effects of small-scale plume-driven predefined weakening of the continental lithospheric mantle and crust on dynamic topography and LAB depth changes using a series of two-dimensional numerical models. Model calculations showed that small plume-driven predefined weakening led to short-wavelength dynamic topography and LAB depth reduction through both mechanical and thermal erosion of the lithospheric mantle by the plume. An increase in the degree of weakening decreased the lithospheric strength and reduced the LAB depth, thereby transforming the short-wavelength topography from depressions to elevations as the extensional kinematics shifted to compression. Our model findings explain the topography, lithospheric strength, and LAB depth constrained from natural small-scale plume emplacement zones.

Petrogenesis of the Late Eocene to Early Oligocene Yao'an Shoshonitic Complex, Southeastern Tibet: Partial Melting of an Ancient Continental Lithospheric Mantle beneath the Yangtze Block

AbstractCenozoic potassic–ultrapotassic igneous rocks are widespread in the southeastern Tibetan Plateau. Their petrogenesis and magmatic processes remain subject to debate in spite of numerous publications. Almost all of the Cenozoic extrusive and intrusive rocks in the Yao'an area, western Yunnan Province, SW China, are geochemically shoshonitic, collectively termed here the Yao'an Shoshonitic Complex (YSC). The YSC is located in the (south) easternmost part of the ENE–WSW‐trending, ∼550 km‐long and ∼250 km‐wide Cenozoic magmatic zone; the latter separates the orthogonal and oblique collision belts of the India–Eurasia collision orogen. Previously published geochronological and thermochronological data revealed that the rocks of the YSC were emplaced over a short timespan of 34–32 Ma. This and our new data suggest that the primary magma of the YSC likely was formed by partial melting of ancient continental lithospheric mantle beneath the Yangtze Block. This part of the continental lithospheric mantle had likely not been modified by any oceanic subduction. Fractionation crystallization of an Mg‐ and Ca‐bearing mineral and Ti‐Fe oxides during the magmatic evolution probably account for the variable lithologies of the YSC.

Chlorine isotope evidence for Farallon-derived metasomatism of the North American lithospheric mantle

Metasomatism of the sub-continental lithospheric mantle (SCLM) can result in enrichment in volatile elements introduced via subduction of seawater-altered oceanic lithosphere. Subduction-related metasomatism can introduce chlorine isotope heterogeneity to the SCLM inherited from the subducted source, which can be used to characterize the source of metasomatism. This study presents Cl isotope compositions (δ37Cl) of variably metasomatized continental lithospheric mantle xenoliths from the Colorado Plateau and southern Rio Grande Rift (southwestern United States). Fertile mantle xenoliths that represent asthenosphere that has recently been emplaced at the base of the North American lithosphere have δ37Cl values of 0.0 ‰ to +0.1 ‰, suggesting that the convecting mantle has an average δ37Cl value similar to seawater and chondrites. In contrast, metasomatized xenoliths span a range of δ37Cl values from +0.1 ‰ to +1.9 ‰, extending above the average convecting mantle value. The range in δ37Cl values can be explained by mixing between a subduction-related fluid with δ37Cl values between +1.0 ‰ and +1.9 ‰ and a shallow crustal fluid with a δ37Cl value less than +0.3 ‰. The most likely source of the high δ37Cl subduction-related signature is the lower crust in the subducted Farallon plate. Lower crustal Cl likely entered the SCLM either through direct dehydration of lawsonite ± phengite in the deeply subducted Farallon plate or from a mixed serpentinite-oceanic crust source in the subducted Farallon. Regardless of the mechanism, this study provides evidence for subduction of crustal Cl into the mantle, which can introduce isotopic heterogeneity into the SCLM and has implications for the volatile cycling in the HIMU mantle and SCLM.

Oceanic and Continental Lithospheric Mantle in the 1.95-Ga Jormua Ophiolite Complex, Finland: Implications for Mantle and Crustal Evolution

ABSTRACT The ca. 1.95-Ga Jormua Ophiolite Complex (JOC), Finland, is a rare Paleoproterozoic ophiolite that preserves a record of diverse upper mantle materials and melting processes. Meter-scale grid sampling of four JOC outcrops, as well as non-grid samples, permits evaluation of meter- to kilometer-scale mantle heterogeneity within the JOC. Significant heterogeneity is observed between the four grids, and also among a number of the non-grid samples examined. Variations in the concentrations of fluid-mobile elements are particularly large among different samples and locations. New whole-rock major, lithophile trace, and highly siderophile element data (HSE: Os, Ir, Ru, Pt, Pd, Re), including 187Re–187Os isotopic data, for serpentinized harzburgites indicate the presence of two distinct compositional types and probable modes of origin within the JOC. This is consistent with prior findings. Type 1 is similar to modern refractory abyssal-type mantle. Type 2 is more highly refractory than Type 1, and most likely represents samples from sub-continental lithospheric mantle (SCLM). Type 1 mantle is moderately heterogeneous with respect to major and trace element and Os isotopic compositions at both the meter and kilometer scales. By contrast, Type 2 mantle is considerably more homogeneous than Type 1 grids at the meter scale, but is more heterogeneous at the kilometer scale. The median initial γOs value for Type 1 mantle, calculated for 1.95 Ga, is ~ −2.0 (where γOs is the % deviation in 187Os/188Os relative to a chondritic reference calculated for a specified time). This isotopic composition is consistent with a moderate, long-term decrease in Re/Os relative to the estimate for primitive mantle, prior to JOC formation. The similarity in this γOs value to the value for the modern abyssal mantle, as well as the initial values for several Phanerozoic ophiolites, suggests that the upper mantle achieved a Re/Os ratio similar to the chondritic reference by ~2 Ga, then evolved along a subparallel trajectory to the chondritic reference since then. For this to occur, only limited Re could have been permanently removed from the upper mantle since at least the time the JOC formed. A localized secondary metasomatic event at ~2 Ga, concurrent with the estimated obduction age for the JOC and subsequent Svecofennian Orogeny, affected the HSE systematics of some Type 1 samples. By contrast, late Archean Os TRD model ages for Type 2 rocks indicate a depletion event superimposed upon the long-term Re depletion of the abyssal mantle. This event was established no later than ~2.6 Ga and may have occurred during a period of significant, well-documented crustal production in the Karelia craton at ~2.7 Ga.

Co-evolution of continental lithosphere and deep mantle dynamics

Co-evolution of continental lithosphere and deep mantle dynamics

Petrogenesis of alkaline magmas across a continent to ocean transect, northern Ross Sea, Antarctica

The West Antarctic Rift System (WARS) is one of the world's largest active rift systems and alkaline magmatic provinces. The alkaline magmatism crosscuts oceanic and continental lithosphere but maintains generally similar Sr-Nd-Pb isotope signatures. Here we present new major-, trace- and highly siderophile-element (Os, Ir, Ru, Pt, Pd, Re) abundances and Sr-Nd-Pb-Os isotope data for a Miocene to recent WARS alkaline basalt suite from a continental-oceanic transect in the northwest Ross Sea. The samples are geochemically enriched similar to ocean island basalts, with oceanic lavas having higher absolute incompatible trace element abundances than continental lavas. The suite exhibits a large range in measured 187Re/188Os (1.7 to 1305) and age-corrected 187Os/188Os (0.1254 to 1.055) that has a positive, highly curved relationship with measured 87Sr/86Sr (0.70287–0.70331), but a negative, highly curved relationship with both measured 143Nd/144Nd (0.51280–0.51300) and 206Pb/204Pb (19.237–20.237). These data suggest that contamination by a mafic continental lithospheric component has occurred to varying degrees and exerts some control on the geochemistry of the continental lavas. Primitive lavas from the oceanic Adare Basin that have experienced negligible contamination are used to constrain the primary source of northwest Ross Sea alkaline lavas and to assess previously proposed hypotheses regarding the origin of magmatism. Explanations for the origin of WARS alkaline lavas range from plume magmatism to partial melting of continental lithospheric mantle metasomatized by subduction fluids prior to cessation of subduction of the Phoenix Plate underneath the Gondwana margin. Samples from the continental-oceanic transect, however, neither exhibit classic arc-lava geochemical signatures nor provide any evidence to support an active mantle plume source. Rather, they are geochemically enriched magmas generated through partial melting of previously subducted oceanic lithospheric mantle concomitant with continental lithospheric thinning during the Cenozoic.

Kerguelen carbonatite tuff sourced from deep recycled material

Carbonatites are mantle-derived rocks associated with rare earth element deposits and their origin is debated. The majority of carbonatites are found in proximity to old continental crust and generally exhibit more geochemically enriched Sr and Nd isotopes relative to the prevalent mantle, with only a few known carbonatite occurrences found in intra-oceanic settings such as that in Cape Verde and Canary Island in the Atlantic Ocean. These oceanic carbonatites are moderately depleted with isotopic signatures similar to the prevalent mantle, indicating that Sr and Nd isotopic enrichment observed in continental carbonatites is controlled by the continental lithospheric mantle, metasomatized by melts and fluid over billions of years. Here we report a newly discovered intra-oceanic dolomite tuff unit with carbonatitic affinity, from the Kerguelen Archipelago, the emerged part of the long-lived Cretaceous Kerguelen Plateau. The Kerguelen dolomite tuff erupted at 24.1 ± 0.4 Ma after the main building phase of the northern Kerguelen Plateau and was fed by the Kerguelen mantle plume. The dolomite tuff shows strong Sr and Nd isotopic enrichments consistent with the presence of recycled material in the deep mantle source of the Kerguelen plume, possibly enriched during the assembly and breakup of the supercontinent Pangaea. These new data show a significant role of geodynamic processes in controlling deep material recycling, location of mantle plumes, and composition of carbonatitic melt.

Mapping the Structure and Metasomatic Enrichment of the Lithospheric Mantle Beneath the Kimberley Craton, Western Australia

AbstractThe lithology, geochemistry, and architecture of the continental lithospheric mantle (CLM) underlying the Kimberley Craton of north‐western Australia has been constrained using pressure‐temperature estimates and mineral compositions for >5,000 newly analyzed and published garnet and chrome (Cr) diopside mantle xenocrysts from 25 kimberlites and lamproites of Mesoproterozoic to Miocene age. Single‐grain Cr diopside paleogeotherms define lithospheric thicknesses of 200–250 km and fall along conductive geotherms corresponding to a surface heat flow of 37–40 mW/m2. Similar geotherms derived from Miocene and Mesoproterozoic intrusions indicate that the lithospheric architecture and thermal state of the CLM has remained stable since at least 1,000 Ma. The chemistry of xenocrysts defines a layered lithosphere with lithological and geochemical domains in the shallow (<100 km) and deep (>150 km) CLM, separated by a diopside‐depleted and seismically slow mid‐lithosphere discontinuity (100–150 km). The shallow CLM is comprised of Cr diopsides derived from depleted garnet‐poor and spinel‐bearing lherzolite that has been weakly metasomatized. This layer may represent an early (Meso to Neoarchean?) nucleus of the craton. The deep CLM is comprised of high Cr2O3 garnet lherzolite with lesser harzburgite, and eclogite. The peridotite components are inferred to have formed as residues of polybaric partial mantle melting in the Archean, whereas eclogite likely represents former oceanic crust accreted during Paleoproterozoic subduction. This deep CLM was metasomatized by H2O‐rich melts derived from subducted sediments and high‐temperature FeO‐TiO2 melts from the asthenosphere.

Steep subduction of the Indian continental mantle lithosphere beneath the eastern Himalaya revealed by gravity anomalies

Steep subduction of the Indian continental mantle lithosphere beneath the eastern Himalaya revealed by gravity anomalies

A Devonian Shoshonitic Appinite–Granite Suite in the North Qinling Orogenic Belt: Implications for Partial Melting of a Water-Fluxed Lithospheric Mantle in an Extensional Setting

AbstractAppinite–granite suites commonly occur in the final stage of collisional orogenic processes, providing a unique opportunity to reveal the properties of continental lithospheric mantle and crust–mantle geodynamics. In this paper, we present a systematic study of the petrology, mineral chemistry, whole-rock geochemistry and geochronology of the Xiong’erling pluton and adjacent appinite dikes in the northern margin of the North Qinling orogenic belt. The pluton is mainly composed of diorites, quartz monzonites and minor granites. The diorites and appinites have LA-ICP-MS zircon U–Pb ages of c. 389 Ma with variable εHf(t) values of −5.58 to +3.36 and TDM1 model ages peaking at c. 1133 Ma. These rocks belong to the shoshonitic series with high Ba–Sr content and were emplaced in an intraplate extensional environment. The quartz monzonites and granites are oxidized A-type granites and have zircon U–Pb ages of c. 396 and c. 378 Ma with negative εHf(t) values of −11.99 to −1.18 and TDM2 model age peaks at 1434, 1720, and 1888 Ma. The in situ87Sr/86Sr ratios of plagioclase from the mafic and felsic rocks are 0.70565-0.70752 and 0.70695-0.71008, respectively. The Xiong’erling pluton and the coeval appinite dikes represent a Devonian appinite–granite suite in the North Qinling orogenic belt. Asthenospheric upwelling during the intraplate extension triggered partial melting of a phlogopite- and amphibole-bearing garnet lherzolite mantle source that had been previously metasomatized by the subducted oceanic slab, leading to the formation of a primitive hydrous mafic magma. The rapid ascent of the water-rich magma along deep-seated active faults with fast crystallization of amphibole resulted in emplacement of the c. 389 Ma appinite dikes. Replacement of anhydrous minerals by amphibole and biotite via hydrous reaction during magma cooling resulted in the formation of the coeval diorites. The mafic magma underplating triggered episodic remelting of a late Paleoproterozoic to early Mesoproterozoic crustal source, leading to the generation of the c. 396–378 Ma quartz monzonites and granites. Combining our results with existing data, we identify a sequence of (1) northward subduction of the Shangdan oceanic crust beneath the Qinling block at c. 524–438 Ma resulting in island-arc calc-alkaline magmatism, (2) closure of the Shangdan Ocean indicated by collision between the Qinling block and the South Qinling terrane and slab failure magmatism at c. 438–410 Ma, and (3) post-collisional to intraplate extension with alkaline magmatism at c. 410–370 Ma. The Devonian extensive intraplate magmatism marks the end of the Paleozoic orogenesis in the North Qinling belt.

Magmatism during the continent – ocean transition

As continents break apart, the dominant mechanism of extension transitions from faulting and lithospheric stretching to magma intrusion and oceanic crust formation in a new ocean basin. A common feature of this evolution preserved at magmatic rifted margins worldwide are voluminous lava flows that erupted close to sea level during the final stages of development of the continent-ocean transition (COT). The mechanisms responsible for the generation of the melts that contribute to these voluminous flows, the so-called seaward dipping reflectors (SDR), and their significance in the context of COT development, are relatively poorly understood; they lie deep below post-rift strata along submarine rifted margins where they cannot be studied directly. Extensive coring of the Afar Stratoid Series - an areally-extensive sequence of Pliocene-aged basalts and intercalated sediments that lie atop the developing COT in the sub-aerial Afar Depression, northern Ethiopia - offers fresh scope to address this issue. We present a numerical model simulating the formation of enriched metasomes within the continental lithospheric mantle by the passage of magmas resembling modern axial basalts. Thermal destabilization of the metasome, caused by plate stretching, initiates melt formation within the metasome. These melts, when mixed with a depleted lithospheric mantle component, closely match the range of compositions of the Afar Stratoid Series lavas in this study. Metasomatic re-enrichment and subsequent melting of the lithospheric mantle during the COT may contribute to further plate thinning. These results demonstrate a novel mechanism by which large-volume flows may be erupted during the COT.

Geochronology, petrogenesis, and magmatic oxidation state of the Mangling intrusive complex, Northern Qinling Belt, Central China: Implications for magma fertility and tectonic setting

The Mangling intrusive complex has different dioritic to granitic phases and is spatially and temporally related to molybdenum deposits in the Qinling Orogen. Zircon U-Pb dating of the Mangling intrusive complex indicates that dioritic rocks (biotite diorite and biotite diorite enclave; ca. 150−147 Ma) formed earlier than granitic rocks (medium- to fine-grained and fine-grained monzogranite and K-feldspar granite; ca. 145−141 Ma). The Mangling dioritic rocks exhibit large ion lithosphere element (e.g., Rb) and light rare earth element enrichment and high field strength element (e.g., Nb, Ta, and Ti) depletion. They have low to moderate SiO2 (51.33−58.16 wt%), high MgO (3.10−4.75 wt%) and Mg# (48−60), and negative zircon εHf(t) values (−11.6 to −6.8), suggesting origination from the continental lithospheric mantle that may have been metasomatized by previous sediment-derived melts and slab-derived fluids constrained by high Nb/Y, Th/Yb, and Rb/Y ratios. The Mangling granitic rocks are I-type granites and have high SiO2 (67.90−81.88 wt%) and low MgO (0.16−0.74 wt%). They have low and negative zircon εHf(t) values (−18.7 to −1.9) and old zircon Hf two-stage model ages (2334−1287 Ma), as well as similar mineral fractionation (e.g., hornblende, biotite, sphene, and apatite) with the Mangling dioritic rocks, indicating that they were derived from the remelting of old crustal rocks (e.g., Xiong’er and Kuanping groups) by the evolved underplated mafic magma. Compared with the Taoguanping mineralized monzogranite in the Northern Qinling Belt, zircon geochemistry (e.g., Ce4+/Ce3+, Eu/Eu*, and ΔFMQ [relative fayalite-magnetite-quartz buffer]) indicates that magma of the Mangling intrusive complex (except the biotite diorite) has high oxygen fugacity and small fractionated granitic intrusions, which are coeval with the biotite diorite enclave or younger than the Mangling granitic rocks, may have potential for generating porphyry molybdenum mineralization. The combination of this study and previous studies corroborates that the Qinling Orogen underwent an intracontinental orogenic evolution in a post-collisional compression to extension transitional setting during the Late Jurassic to Early Cretaceous, affected by far-field Paleo-Pacific slab subduction.

Reaction between volatile-bearing eclogite and harzburgite as a function of degree of interaction: Experimental constraints at 4 GPa

Abstract The mantle is known to be heterogeneous, mainly composed of peridotite and eclogite. Eclogite-derived hydrous melts may interact with harzburgite at the slab-mantle interface in subduction zones or in the sub-continental lithospheric mantle. In this study, such interactions were simulated by performing hybridization experiments in which a layer of eclogite was juxtaposed to a layer of harzburgite in the presence of H2O-CO2 at 4 GPa and 1200 °C, conditions where eclogite is super-solidus while harzburgite is sub-solidus. A diamond trap was placed in between the two layers to trap the fluid or melt phase, allowing direct determination of their composition. The multi-anvil was rotated at different frequencies to examine the effect of increasing degree of interaction on the melt composition as well as the mineral compositions. The interaction of eclogite-derived hydrous melt and harzburgite results in a reaction layer at the interface between the two lithologies, composed of Opx and garnet. The harzburgite above the reaction layer is metasomatized, containing various amounts of olivine, Opx, Cpx, and garnet. The eclogitic melt is modified during this interaction. With increasing interaction, a thicker reaction layer is formed. Both the eclogitic and the peridotitic garnet compositions approach each other and become intermediate between the composition of the garnet in the eclogite+H2O+CO2 system and the garnet in the harzburgite+H2O+CO2 system at these conditions. The Mg# of the peridotitic olivine and Opx decreases with increasing interaction. The initial basaltic melt in equilibrium with eclogite is metaluminous, turning to a peralkaline melt with increasing interaction with the harzburgite. The metasomatizing effect of the eclogite-derived hydrous melt on the harzburgite is observed by increasing the mode of the peridotitic Opx, Cpx, and garnet at the expense of peridotitic olivine and eclogitic garnet. A slight increase in melt fraction occurs as well. This interaction also results in a gradient in the log fO2. Relatively more oxidizing conditions occur near the reaction layer, becoming more reduced into the peridotite, suggesting that the reaction zones act as partial barriers for the melt to travel through the peridotite. Increased interaction leads to higher log fO2 values. These experiments demonstrate the influence of the degree of interaction on the range of melt compositions found in volcanic arcs as well as the degree of metasomatism in the mantle found in the sub continental lithospheric mantle.

Tungsten in the mantle constrained by continental lithospheric peridotites: Less incompatible and more abundant

Isotope dilution W whole-rock data, coupled with data for other trace elements in 42 peridotite xenoliths from Tariat (Mongolia), Yangyuan and Hannuoba (China), and Letlhakane (Botswana) are used to constrain the W abundance and behavior in the continental lithospheric mantle. Tungsten concentrations are >11 ng g−1, even in the most depleted peridotites, and higher than the W abundance estimate of the bulk silicate Earth (BSE) in all but one sample. Combined W-Th-U systematics reveal fractionation of W from Th and U with increasing degree of melt depletion, while incompatible element-enriched samples display no such fractionation. Integrated melt reactions and degrees of melt depletion are estimated based on either residual Th or residual Yb abundances for each sample, relative to a BSE precursor. This allows modelling of different partial melting scenarios that consider maximized trace element retention (Th-based model; where measured Th contents are decoupled from Al2O3 contents) or correction for metasomatic trace element contributions (Yb-based model; where measured Yb contents are well-coupled with Al2O3 contents). Modelled residual W concentrations reveal W excesses of 11–19 ng g−1, even when correcting for metasomatic W contributions. Combined with available in situ trace element mineral data for the rock-forming silicates and spinel, which all display W abundances much higher than expected, these findings suggest that either (i) W is less incompatible than previously thought, or that (ii) the peridotites experienced W metasomatism after the last melt depletion event with subsequent re-equilibration. The latter is unlikely, as similar geochemical patterns are observed for oceanic lithospheric mantle based on published data, requiring such W metasomatism to be a global phenomenon. We thus conclude that W is not as incompatible as Th and U during upper mantle melting and potential W-Th-U fractionation mechanisms are examined to explain near-constant W/Th and W/U in mantle melts. We propose revised W concentrations for the BSE of 27 ± 16 ng g−1 to 31 ± 14 ng g−1 and for the depleted mantle of 22 ± 13 ng g−1.

Evidence and significance of wehrlitization within continental lithospheric mantle

异剥橄榄岩化是地幔橄榄岩与硅不饱和熔体发生强烈相互作用的地幔过程。这一过程通常伴随地幔物理-化学性质的显著改变，与克拉通岩石圈减薄和破坏密切相关。本文梳理了前人对异剥橄榄岩化的驱动机理、结构和成分特征及其对大陆稳定性影响等方面的定性认识，并在此基础上通过热力学模拟，刻画了不同属性地幔与熔体在不同温度压力条件下的相互作用过程，目标是为定量限定异剥橄榄岩化的发生条件、物理-化学效应及地球动力学意义。研究显示，异剥橄榄岩化通常会出现磷灰石、角闪石、金云母和碳酸盐等标志性交代矿物，并以斜方辉石被单斜辉石替换为典型特征，还可发育海绵边、熔体囊（已冷却结晶为单斜辉石、橄榄石等细粒矿物）和矿物环带等非平衡结构。热力学模拟表明，熔岩反应过程受原岩、熔体性质以及熔/岩比例的共同控制，不同属性地幔（难熔方辉橄榄岩、饱满二辉橄榄岩）与贫硅（如霞石岩）熔体反应都能生成富单斜辉石橄榄岩，且在较高压力（如2.5GPa和4.0GPa）条件下更容易形成异剥橄榄岩；相比之下，不同性质地幔与富硅（如MORB）熔体反应在任何压力条件下均不能产生异剥橄榄岩。此外，两类熔体参与的熔岩反应均能引起围岩密度逐渐增加，并在高压条件下能导致饱满岩石圈地幔密度超过地幔参考值、达到岩石圈失稳的条件。因此，本文从热力学的角度定量约束了异剥橄榄岩化的发生条件和对岩石圈地幔的影响，为前人提出的橄榄岩-熔体反应能够造成岩石圈地幔减薄和破坏这一概念模型提供了理论实证，对评估大陆稳定性具有重要意义。