Depleted Mantle Research Articles

Subductability of continental lithosphere

Multiple lines of evidence from geological and geophysical observations indicate the deep subduction of continental lithosphere; however, the potential and driving forces of (self-sustained) continental subduction remain unclear. Here, systematic thermal-petrological models were conducted to quantitatively evaluate the subductability of continental lithosphere by analyzing its density structure and slab-pull evolution during subduction. The results indicate that the metamorphic densification of deeply subducted continental lithosphere (upper, middle, lower crust and lithospheric mantle) could provide considerable driving force for continental deep subduction. The numerical models further indicate that, if a Phanerozoic or Proterozoic continental lithosphere is dragged to a large depth of >300 km, then the continental slab pull overcomes the overall resistance force. Consequently, the continental subduction occurs self-consistently without any drag from the preceding oceanic slab. For Archean continental lithosphere, it is more difficult for self-sustained subduction to occur, due to the highly depleted mantle composition. In addition, we also systematically quantified the effects of multiple factors, including the scraping of continental crust, subduction velocity, subduction angle, and variable bulk-rock compositions of continental crust. Finally, a representative case study of the Himalayan orogen revealed that the slab pull of presently subducting Indian continental lithosphere ranges from 13 TN/m to 29 TN/m, providing a major contribution for the ongoing India-Asia collision.

Basalts record a limited extent of mantle depletion: cause and chemical geodynamic implications

Basalts record a limited extent of mantle depletion: cause and chemical geodynamic implications

Magma mingling in plagiogranites of the Oman ophiolite suggests an origin by fractional crystallisation

Late-stage plagiogranites in the Oman-UAE ophiolite are frequently associated with layered and massive gabbro bodies, intrusive into the mid-crustal section of the ophiolite. This study reports persuasive field evidence from five different localities for magma-mingling between plagiogranites and melts with a mafic and dioritic composition. It is argued from these relationships that mafic and felsic magmas coexisted in the same magma chamber. On this basis it is suggested that the range of melt compositions is related by a process of fractional crystallisation. The plagiogranites are interpreted as the end-product of this fractionation process and the associated gabbros are thought to be related cumulates. The hypothesis is supported with geochemical evidence which shows from the major element chemistry a continuum of compositions between the most mafic and felsic endmembers (47.6–80.6 wt% SiO2). These compositions ‘capture’ the fractionation process in operation. Modelling these variations using the major and trace element chemistry indicates that the fractionating assemblage comprised olivine-clinopyroxene-plagioclase+/−amphibole and that some plagiogranites contained cumulus plagioclase. The mafic rocks associated with late stage plagiogranite formation are derived from a highly depleted mantle source, which implies melting of an already-depleted mantle. This is consistent with previous models for the origin of the Oman ophiolite and implies a supra-subduction setting for this phase of ocean crust evolution in this ophiolite.

Earth’s Earliest Crust

The scarcity of rocks preserved from the first billion years (Gy) of Earth’s history hinders our ability to study the nature of the earliest crust. Rare >4.0-Gy-old zircons confirm that felsic crust was present within 500 million years of Earth’s formation. Given that most of that ancient crust has been destroyed, geochemical and isotopic tracers applied to rocks from the oldest sections of continents can be used to provide insights into the nature of the predecessor crust. Evidence from Earth’s oldest rocks and minerals suggests multiple early mantle depletion episodes, possibly linked to the formation of an initial, dominantly mafic, crust. This early crust was the precursor to evolved rocks that now constitute considerable portions of Earth’s oldest surviving crust.

Mars's Crustal and Volcanic Structure Explained by Southern Giant Impact and Resulting Mantle Depletion

AbstractMars features a crustal dichotomy, with its southern hemisphere covered by a thicker basaltic crust than its northern hemisphere. Additionally, the planet displays geologically recent volcanism only in its low latitude regions. Previous giant impact models coupled with simulations of mantle convection have shown that the crustal dichotomy can be explained by post‐impact melt crystallization that emplaced a thick crust in the southern hemisphere. In this study, we show that the depleted residue left behind by the original post‐impact crustal formation can spread laterally, potentially persisting beneath the northern hemisphere to the present‐day. Such a large‐scale mantle province would concurrently explain both the prevalence of long‐term magmatism on Mars and its strong preference for localized equatorial regions.

Petrogenesis of Neoarchean granitic gneisses from the Daqishan area of the Dabie orogen and implications for the early crustal evolution of the Northern Yangtze Craton

ABSTRACT The Yangtze Craton is one of the crucial cratons constitued the eastern Asia continent. It is characterized by Archaean to Paleoproterozoic basement rocks, making it an excellent area to study the Precambrian geologic and tectonic regime. Recently, abundant Archaean rocks have been reported from the Dabie Orogen, which providing important information to understanding the early evolution of the Northern Yangtze Craton. In this contribution, we present petrological, whole-rock geochemical, and zircon U – Pb – Hf isotopic analyses on newly discovered Neoarchean granite in the Dabie Orogen. We identified 2744 ± 10 Ma and 2715 ± 11 Ma (~2.74 Ga) Na-rich granitic gneisses, 2691 ± 6 Ma and 2696 ± 8 Ma (~2.69 Ga) A2-type granitic gneisses, and 2664 ± 8 Ma, 2664 ± 6 Ma, 2672 ± 7 Ma, 2642 ± 10 Ma (~2.66 Ga) A1-type granitic/syenite gneisses from the Daqishan area of the Dabie Orogen. Geochemical and zircon Hf isotope data indicate that the ~ 2.74 Ga Na-rich granitic gneisses were partial melting products of thickened mafic crust with limited depleted mantle input, while the 2.69 Ga and 2.66 Ga A-type granitic gneisses represent felsic magmatism in a post-collisional to an extensional tectonic setting. Based on our research, we categorized the Northern Yangtze Craton into the Kongling domain and Dabie domain by comparing with the distribution of the zircon U-Pb magmatic ages and metamorphic events. We suggest that the initial unified basement of the northern proto-Yangtze Craton formed during the Nuna supercontinent cycle at ~ 2.0 Ga.

Petrogenetic and geochemical constraints on ca. 1.89–1.88 Ga Bastanar mafic dyke swarm, Bastar craton, India: Insights into MORB- and OIB-type contributions and interactions with metasomatized subcontinental lithospheric mantle

The Bastar craton of the Indian Shield hosts several generations of mafic dyke swarms of various trends, compositions, and ages, which span from ca. 2.7 Ga to 1.42 Ga. This study focuses on geochemical attributes of the ca. 1.89–1.88 Ga NNW-trending Bastanar swarm, aiming to address a perceived discrepancy between its arc-like geochemistry and the influence of a heterogeneous mantle source. To resolve the intra-swarm geochemical variations, we conducted a comprehensive geochemical characterization and petrogenetic interpretation of the ca. 1.89–1.88 Ga Bastanar swarm. The samples from this swarm are categorized into two distinct groups, primarily based on their geochemical composition. The Group 1 samples exhibit higher TiO2 (1.06–1.86 wt%), (La/Yb)N (7.2–8.6), (Gd/Yb)N (2- 2.27), Nb (14.6–16.6 ppm), Th (1.23–3.03 ppm) and Zr (104–118.72 ppm) concentrations than the Group 2 samples. Furthermore, rare-earth element patterns and variations in high-field strength element contents in the Group 1 samples suggest derivation from a deeper, less depleted mantle source resembling an OIB/less MORB-type. This inference is further supported by higher TiO2/Yb, Zr/Nb, and Nb/Y ratios. In contrast, the Group 2 samples indicate derivation from a shallower, more MORB/less OIB-type depleted mantle source, as evidenced by lower TiO2/Yb, Zr/Nb, and Nb/Y ratios. Variations in Dy/Yb and Gd/Yb ratios confirm the involvement of variable mantle sources, implying the derivation of the Group 1 and 2 samples from garnet-rich and spinel-rich lherzolite mantle sources, respectively. The absence of consistent negative Nb-Ta-Ti anomalies in the Group 1 samples suggests an uncontaminated nature, ruling out any role of crustal contamination. On the other hand, the Group 2 samples display negative Nb-Ta-Ti anomalies with enriched LREE and LILE patterns, indicating the involvement of crustal components in their genesis. A trace-element modelling suggests that the ca. 1.89–1.88 Ga mafic dyke swarm exhibits significant intra-swarm variability, with at least two distinct source components contributing to its genesis – a depleted MORB-type and an enriched OIB-type mantle. Notably, the geochemical characteristics of the Group 2 samples suggest interaction with a metasomatized mantle source, possibly enriched by fluids from an earlier subducted slab. Geochemical evidence presented in this work supports Archean subduction-related processes for the crustal growth of the Bastar craton and highlight the enduring influence of a metasomatized sub-continental lithospheric mantle on subsequent magmatism over millions of years.

Exploring the tectono-magmatic evolution of intraoceanic fore-arc setting during subduction initiation: perspectives from trace and platinum group element systematics of the Jijal ultramafic arc system, NE Pakistan

ABSTRACT The Jijal Complex is considered one of the largest Neo-Tethyan remnants in the intra-oceanic Cretaceous–Palaeogene Kohistan Arc of Pakistan. This complex largely consists of gabbros, peridotites, and chromitites. Previous studies documented the genesis of gabbros and chromitites in detail; however, detailed geochemistry and magma genesis were lacking. In this investigation, we present whole-rock geochemical and platinum group elements (PGEs) data of Jijal peridotites (dunites and harzburgites). The objectives of this study are to elucidate magmatic processes and tectonic regimes involved in the formation of peridotites, factors influencing the concentration and dispersion of PGEs in them, and their tectonic evolution. The investigated peridotite samples are depleted in magmaphile major elements (Al2O3: 0.23–0.57, TiO2: 0.01–0.04, and CaO: 0.08–0.69 wt%), relative to the primitive mantle values, reflecting melt-rock interaction during their magmatic evolution. In addition, rare-earth element (REE) contents in harzburgites are relatively higher compared to the dunites; however, both rock types exhibit depletion with respect to the chondrite-normalized values. The obvious negative anomalies of Nb, Zr, and enriched LREE and large ion lithophiles collectively substantiate their association with depleted arc-type mantle components, metasomatism, and melt/fluid-rock reactions after partial melting episodes. Bulk-rock and geochemical modelling of PGEs data of the peridotites suggest their shallow depth spinel-bearing depleted heterogeneous mantle source and generation from refractory mantle residuum along with boninitic signatures in a supra-subduction zone (SSZ). The PGE’s geochemistry additionally reveals that partial melting, fractionation, metasomatism, and sulphur under-saturation were the key factors that controlled the concentration and distribution of PGEs in the studied rocks. The boninitic features of the Jijal peridotites are equated with intraoceanic fore-arc followed by subduction initiation, and melt-rock reactions in a hydrated forearc mantle, indicating robust evidence of mantle depletion and pervasive refertilisation in an embryonic Neo-Tethyan arc system. Tectonically, the investigated rocks encapsulate vestiges of the Cretaceous Tethyan Ocean and record multifaceted history from boninitic to slab-proximal island arc affinity in compliance with an intraoceanic SSZ fore-arc regime coherent with the subduction inception.

Mantle depletion recorded by olivine and plagioclase megacrysts in oceanic basalts

Mantle depletion recorded by olivine and plagioclase megacrysts in oceanic basalts

The role of high oxygen fugacity on genesis of the late Cenozoic Abaga basalts in Xilin Gol League, Inner Mongolia, China

ABSTRACT The Abaga basalts, which erupted within an intraplate background in the late Cenozoic, are situated in eastern central Inner Mongolia. Previous studies have explored the genesis of these basalts, which were influenced by the subduction of slab. Although trace element ratios demonstrate that altered oceanic crust (AOC) and sediments have contributed to the genesis of the basalts, limited information about the detailed percentages of mixing among sediments, AOC, and depleted mantle (DM) based on Sr‒Nd‒Pb isotopic values has been reported. In this study, we modelled the possible mixing percentages among AOC, DM, and sediments. The calculated results illustrate that a proportion of 80% DM, 16% AOC, and 4% sediments is appropriate for interpreting the Sr‒Nd‒Pb isotopic variation of the basalts. In addition, the associated major and trace elements of olivine and the partition coefficient of V between olivine and melt are used to estimate the crystallization temperatures of the olivine phenocrysts and oxygen fugacity of the basalts. The calculated results display that the olivine mainly crystallized between 1248 and 1293°C (standard error estimate: 51°C), and the fO2 values of the Abaga basalts range from FMQ + 0.14 to FMQ + 1.46 (FMQ, fayalite–magnetite–quartz buffer). The estimated oxygen fugacity values plot in the range of island arc basalts (from FMQ to FMQ + 2) but are higher than those of both mid-ocean ridge basalt (MORB) (from FMQ-0.4 to FMQ + 0.5) and Abaga xenoliths (from FMQ-1.55 to FMQ + 0.53), which indicates that the basaltic melts were more oxidized than those of MORB and the Abaga lithospheric mantle. Furthermore, the primary magma and the corresponding mantle source were more oxidized than MORB. The Ba/Th and Ce/Pb ratios of the basalt present positive correlations with fO2 values, demonstrating that the addition of sediments and oceanic crust into the mantle source may have contributed to increasing the fO2 values of the basalts.

Geochemical dichotomy of Tethyan oceanic mantle and the supercontinent connection: Insights from Os isotopic signatures of mantle peridotites from Naga Hills ophiolite

Naga Hills Ophiolite (NHO) represents a thrusted section of relict Neo-Tethyan oceanic lithosphere comprising distinct crustal and upper mantle lithologies that preserve imprints of different stages of evolution of ancient ocean basins and subsequent accretion onto continental margins during late Cretaceous-Eocene collision involving Indian and Eurasian Plates. This study presents bulk geochemistry and Re-Os isotopic compositions for the mantle peridotites of NHO to address their petrogenetic aspects, implications for Neo-Tethyan upper mantle heterogeneity and thermo-tectonic evolution of Neo-Tethyan oceanic lithosphere through subduction-accretion-collision processes. Geochemical characteristics of the mantle peridotites, as reflected from their chondrite normalized REE patterns, binary relationship between fractionation depletion indices, low Re/Os, suggest that these are mantle residues resulting from ∼ 5–20 % of melt extraction from a spinel peridotite source that experienced decompression melting in a fore-arc extensional setting in response to subduction initiation. Isotopically, these mantle peridotites are subchondritic with 187Os/188Os (0.1218–0.1266) and γOs values of −3.73 to −0.09 comparable with the depleted MORB source. LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns demonstrated by the studied samples with Re addition collectively corroborate multistage petrogenetic processes in compliance with geodynamic transition from extensional to compressional regime in a suprasubduction zone environment and partial melting of a chemically heterogeneous source mantle. This compositional heterogeneity can be equated with mantle depletion by melt extraction and refertilization by influx of slab-dehydrated fluids and percolation of boninitic melts. The non-radiogenic Os isotope signatures, negative γOs values and wide spectrum of melt extraction ages for these mantle peridotites correspond to (i) multiple melt production and melt extraction events prior to the opening of Neo-Tethyan seaway (ii) detachment and incorporation of ancient, depleted SCLM fragments into oceanic mantle concurrent with subduction driven ocean basin closure-congregation of Gondwana Supercontinent and emergence of Neo-Tethyan embryonic ocean in response to disintegration of Gondwana Supercontinent.

Vestiges of late Cambrian arc-related magmatism within the Variscan belt of Europe: Zircon geochronological and whole-rock geochemical data from the Leszczyniec Metaigneous Complex, Sudetes, Poland

We present major oxides, trace elements, Sr-Nd-Hf isotope and zircon age data for rock samples from the Leszczyniec Metaigneous Complex (LMC) in the Polish Sudetes. The LMC comprises low/medium-grade metamorphic mafic rocks of basaltic composition that are intimately associated with felsic and amphibole-bearing gneisses. Magmatic precursors to all varieties of gneisses were emplaced between c. 515 Ma and 495 Ma. The age of the metabasite remains unconstrained, but the calculated two-stage Nd model age (0.54 Ga) of the sample with highest radiogenic Nd value may in this case represent a maximum age. This is consistent with the oldest single zircon ages produced for the gneisses that may represent inheritance. The most prominent geochemical feature of the studied samples is a depletion in the HFSE, most notably Nb and Ta, which can be observed in all studied lithologies. The high abundance of basic rocks with troughs at Nb in primitive mantle-normalized multi-elemental diagrams, combined with a limited degree of REE fractionation and highly radiogenic Nd and Hf isotopic compositions favour an intraoceanic volcanic arc tectonic setting for emplacement of the LMC protoliths. The derivation of compositionally diverse magmatic precursors to gneisses can be explained by the combination of slab sediment melts at relatively low-pressure/temperature (high Nb/Y felsic gneiss), heterogeneous mixing of those melts and slab derived fluids with a highly depleted mantle (amphibole-bearing gneiss/metagabbro), and low-degree partial melting of metabasite of a former back-arc domain (low Nb/Y felsic gneiss). This study demonstrates that late Cambrian magmatism, widespread across terranes included in the Variscan belt, cannot be unequivocally attributed to continental rifting. Metaigneous suites belonging to the Variscan Middle and Upper Allochthon, such as the LMC, were presumably derived from marginal basins developed within a convergent tectonic setting outboard of Gondwana or Baltica, which may have remained active until the end of the Cambrian.

Heterogeneous mantle source compositions for boninite from Bonin and Troodos, evidence from iron isotope variations

Boninite has been widely accepted to be the melting product of highly refractory mantle with the addition of slab-derived fluids. Previous studies have found that boninite, like other subduction-related magmas (e.g., mature arc basalts), has generally consistent chondrite-like iron isotope composition (c. 0.03‰), which is on average lighter than those of oceanic basalts. However, it remains unclear whether the lighter iron isotope composition of subduction-related magmas is inherited from the depleted mantle source or caused by the addition of slab-derived oxidized fluids. Moreover, boninite has highly varied element patterns (e.g., Yb, Zr), reflecting varying degrees of mantle depletion and different contributions of slab-derived fluids. Given the limited data for iron isotope composition of boninite so far available, it is thus required to systematically study iron isotope composition of boninite with different compositions to further understand the petrogenesis of boninite and the iron isotope variation of subduction-related magmas. Here we report iron isotope compositions of boninite from two classic suites on Earth, i.e., those from Bonin islands and submarine forearc (low-Ca boninite; CaO/Al2O3 mainly <0.75) and Troodos ophiolite complex, Cyprus (high-Ca boninite: CaO/Al2O3 = c. 0.85). The δ56Fe values of the Bonin boninite vary from −0.046 ± 0.003‰ to 0.078 ± 0.031‰ (2SD, SD = standard deviation of 4 times repeated analysis; with an average of 0.03 ± 0.03‰, n = 14). The δ56Fe value of the Troodos boninite is relatively constant, i.e., from 0.046 ± 0.010‰ to 0.091 ± 0.016‰ with an average of 0.06 ± 0.01‰ (n = 7). Bonin forearc basalts (FABs) were also analyzed for comparison with δ56Fe of ~0.10‰, except for those FABs with higher Th/U ratio and Cu contents (δ56Fe = 0.01–0.05‰). Together with previously reported δ56Fe data for boninite from New Caledonia (low-Ca boninite with CaO/Al2O3 = ~ 0.4, δ56Fe = ~ +0.03‰), the lower δ56Fe values of the Bonin boninite than Bonin FAB and Troodos boninite reflect the lighter iron isotope composition of the more depleted mantle source. Based on relationships between different indicators for various fluids (e.g., Ba/La, Zr/Sm) and modeling, Bonin boninite is likely produced by partial melting of the refractory mantle with an addition of ~5–10% melts from sediments and oceanic crust, while the formation of Troodos boninite can be ascribed to the melting of less refractory mantle, which has been previously metasomatized by ~5% fluids derived from oceanic crust and sediments.

Origin and evolution of Neoproterozoic metaophiolitic mantle rocks from the eastern Desert of Egypt: Implications for tectonic and metamorphic events in the Arabian-Nubian Shield

The mantle rocks from Kadaboura and Madara areas represent sections of dismembered ophiolitic complexes developed during the Neoproterozoic in the Eastern Desert of Egypt, which is located in the northwestern corner of the Arabian–Nubian Shield. The Kadaboura mantle rocks comprise serpentinites and serpentinized dunites, whereas those of the Madara consist of serpentinites and serpentinized pyroxenites.Despite the serpentinization of the studied mantle rocks, few relicts of primary chromite, olivine and pyroxene are preserved. Chromite is partly altered having unaltered Al-rich chromite cores surrounded by Fe-rich chromite and Cr-rich magnetite rims. The unaltered Al-rich chromite cores show compositions equilibrated at temperatures mostly below ~500-600°C, which is a temperature comparable to that estimated for primary chromite in greenschist up to lower amphibolite facies rocks. The high Cr# [100×Cr/(Cr+Al)= 47-76] of the unaltered chromite cores and the Mg-rich nature of the olivine relicts (Fo91–94) indicate that the studied mantle rocks were produced from a highly depleted mantle that experienced high degrees of melt extraction (mostly ~30-40%). This range of melt extraction resembles that estimated for supra-subduction zone peridotites, but higher than that in abyssal and passive margin peridotites. Furthermore, the clinopyroxene relicts show compositions comparable to those from the Mariana forearc peridotites. Bulk-rock geochemistry also reflects derivation from an extremely depleted and a highly refractory mantle source. Modelling of rare-earth elements suggests that the studied mantle rocks were possibly formed by the interaction of their highly depleted harzburgitic mantle precursors with subduction-related melts/fluids during their evolution in a fore-arc basin of the supra-subduction zone.The proposed geodynamic model suggests that the oceanic lithosphere generated during the seafloor spreading of the Mozambique Ocean was emplaced in the upper plate of the intra-oceanic subduction zone, in which the formely depleted Neoproterozoic mantle of the Arabian-Nubian Shield experienced mature phases of hydrous melting, extreme depletion and enrichment.

Flat subduction in the Early Earth: The key role of discrete eclogitization kinetics

Flat (shallow) subduction is mainly proposed as an Archean-Paleoproterozoic process, contributing to the growth of continental lithosphere. However, numerical models of Precambrian subduction commonly show steep subduction. Here we investigate the effects of lithology-dependent eclogitization (minor for gabbroic lower crust, strong for basaltic upper crust) of oceanic crust during subduction. We performed 2D petrological-thermomechanical modeling of subduction at potential mantle temperatures up to 250 °C warmer than present, using a numerical approach that accounts for buoyancy effects from both mantle depletion and eclogitization. The modeling reveals that increases in mantle potential temperature and the related thickness of oceanic crust and depleted mantle may induce transition to the flat subduction regime at ΔT ≥ 150 °C but only when a delayed eclogitization of gabbroic compared to basaltic crust is taken into account. Otherwise, subduction operates in the steep slab regime. Flat Precambrian subduction models show episodic bimodal magmatism within arcs located > 400 km from the trench, caused by short-lived (1–4 Myr) subduction transients consisting of three consecutive stages: (1) twisting downward of the eclogitized slab portion, (2) subvertical descent and break-off of this twisted portion, (3) rising of the remnant non-eclogitised slab tip and continuation of flat subduction. Limited formation of TTG-series granitoids is caused by partial melting of the subducting basaltic layer but becomes less pronounced at ΔT = 250 °C. Flat subduction produces an overthickened, layered keel-like continental mantle root, with an intermediate layer of subducted oceanic crust. However, construction of long-lived mantle keels by this mechanism might be limited by peeling off of the gradually eclogitizing oceanic crust and the underlaying mantle when plate convergence ends. At elevated mantle temperatures flat subduction produces voluminous hydration of the mantle wedge, forming a subcrustal serpentinite mélange layer that becomes a potential source of fluids in the subsequent evolution of the overlying continental crust.

The Mechanical Nature of the Lithosphere Beneath the Eastern Central Atlantic Hotspots

AbstractThe Eastern Central Atlantic (ECA) region includes the Azores, Canary, Cape Verde, Great Meteor, and Madeira hotspots. These hotspots exhibit a large variety of characteristics and are rooted in the lithosphere ranging in age from newly created at the Mid Atlantic Ridge to Jurassic at the NW Africa Atlantic margin. Therefore, the ECA region represents an excellent scenario to investigate in an integrated way the effects of hotspots on the mechanical structure of oceanic lithosphere. Here, we calculate the effective elastic thickness (Te) of the lithosphere from an analysis of gravity and topography. Azores hotspot is characterized by a Te &lt; 10 km, whereas the Great Meteor, Cape Verde, and Madeira hotspots have intermediate Te (15–30 km) values. In contrast, the Canary hotspot is characterized by a much higher Te (&gt;50 km), forming the largest and most prominent mechanical feature in the ECA. All the hotspots except Canary show standard elastic thickness values when compared to average values for the same age lithosphere and to other oceanic areas in the world. The high strength of the Canary hotspot may be related to the highly depleted mantle composition in the area. The comparison between the elastic thickness distribution and the upper mantle seismic velocity structure shows no correlation between the Te estimated at the ECA hotspots (with the exception of Azores) and the presence of low shear‐wave velocity anomalies in the underlying mantle. This lack of correlation suggests a negligible effect of upper mantle temperature anomalies on the flexure of the ECA region.

Land Fraction Diversity on Earth-like Planets and Implications for Their Habitability.

A balanced ratio of ocean to land is believed to be essential for an Earth-like biosphere, and one may conjecture that plate-tectonics planets should be similar in geological properties. After all, the volume of continental crust evolves toward an equilibrium between production and erosion. If the interior thermal states of Earth-sized exoplanets are similar to those of Earth-a straightforward assumption due to the temperature dependence of mantle viscosity-one might expect a similar equilibrium between continental production and erosion to establish, and hence a similar land fraction. We show that this conjecture is not likely to be true. Positive feedback associated with the coupled mantle water-continental crust cycle may rather lead to a manifold of three possible planets, depending on their early history: a land planet, an ocean planet, and a balanced Earth-like planet. In addition, thermal blanketing of the interior by the continents enhances the sensitivity of continental growth to its history and, eventually, to initial conditions. Much of the blanketing effect is, however, compensated by mantle depletion in radioactive elements. A model of the long-term carbonate-silicate cycle shows the land and the ocean planets to differ by about 5 K in average surface temperature. A larger continental surface fraction results both in higher weathering rates and enhanced outgassing, partly compensating each other. Still, the land planet is expected to have a substantially dryer, colder, and harsher climate possibly with extended cold deserts in comparison with the ocean planet and with the present-day Earth. Using a model of balancing water availability and nutrients from continental crust weathering, we find the bioproductivity and the biomass of both the land and ocean planets to be reduced by a third to half of those of Earth. The biosphere on these planets might not be substantial enough to produce a supply of free oxygen.

The Kataevo Island Arc System of the Paleoasian Ocean (Transbaikalia): Composition, Age, Paleomagnetism, and Formation Geodynamic Settings

Abstract—Based on new data on the geology, composition, U–Pb isotopic age, and paleomagnetism of the metavolcanic rocks of the Kataevo Formation, we consider the geodynamic conditions of their formation and alteration. The Kataevo Formation metavolcanic rocks belong to the K–Na-high-alumina andesite–andesibasalt–basalt volcanic series. Results for U–Pb analysis of magmatic zircon (SHRIMP II, 8 spots) from a metaandesibasalt sample of the stratotype section on Ungo River yielded and age of 852 ± 9 Ma. Isotope systems for Sm–Nd yield a positive εNd(852) = +9.29, which indicates a juvenile magmatic source, close to depleted mantle (DM), with a Neoproterozoic protolith TNd(DM) model age. The content of the less mobile HFSE and REE (ppm) is consistently low for Nb (8–15), Ti (7074–12,410), Ta (0.32–0.93), Eu (1.80–2.29), Се (50–79), Y (21–25), Yb (2.1–2.8), Rb (10–24) and elevated for Sr (1000–1500), Zr (170–270), La (25–41), and Ba (600–800). All studied parameters place the metavolcanic rocks close to the contemporary Kurile–Kamchatka type of developed island arcs. Paleomagnetic analysis of the section of metavolcanic rocks shows a complete remagnetization ca. 120 Ma. This is synchronous with manifestations of intraplate basaltoid magmatism in the studied region, the most typical example of which is the Lower Cretaceous Khilok Formation.

ПАЛЕОПРОТЕРОЗОЙСКИЕ УЛЬТРАМАФИТОВЫЕ ДАЙКИ КУН-МАНЬЁНСКОГО АРЕАЛА (ЮГО-ВОСТОК СИБИРСКОЙ ПЛАТФОРМЫ): СТРУКТУРНОЕ ПОЛОЖЕНИЕ, СОСТАВ И ОБСТАНОВКА ФОРМИРОВАНИЯ

Dykes of Paleoproterozoic ultramafic rocks from the Kun-Manie area were studied within the southeastern fringe of the Siberian Platform. At the end of the Paleoproterozoic, the processes of overthrust faulting and folding were widely manifested here, the latest of which were accompanied by the emplacement of mafic-ultramafic dykes and sills that control the location of Cu-Ni-PGE sulfide mineralization. The petrographic structure of ultramafic dykes suggests their emplacement in tectonic settings. The elevated concentrations of LREE relative to HREE, coupled with other data, allow them to be referred to high-Mg ultramafic rocks of the picrite-tholeiite series, the parent magmas of which could be not ultramafic, but picrobasaltic magmas. The level of REE concentrations in ultramafic rocks and the nature of the normalized spectra indicate the involvement of sufficiently enriched sources in the generation of primary melts from which rocks crystallized that radically differed from peridotites of ophiolite associations and typical komatiite melts. In terms of spectral shapes and contents of indicator elements (Nb, Yb and Th), they are close to the «enriched» mantle source of the E-MORB type differing in lower REE concentrations. The Zr/Y – Nb/Y ratio in ultramafic rocks suggests a non-plume source of their primary melts in the upper depleted mantle. Rocks derived from such a melt are characteristically enriched in fluid components (S, Cl, F, As, Te, Se, and H2O), which is evident in the presence of mineral phases rich in hydroxyl, that is, amphiboles and phlogopite of igneous origin. The ore-forming system evolved from Ni-bearing at the early magmatic stage during the formation of disseminated mineralization to Cu-enriched at the postmagmatic stage. The geological data indicate that they formed during the collision between the Paleoproterozoic island arc and the microcontinent. The combination of subduction and intraplate geochemical characteristics in magmatites is characteristic of the transform margin as a result of the oceanic (Verkhnemaya) and continental (Uchur) microplates sliding horizontally past each other.

Effects of variable slab components and tectonics on magma composition in the intra-oceanic Kermadec Arc-Backarc system

The Kermadec Arc-Backarc system (25–39°S) in the southwest Pacific is an important natural laboratory to examine the controls on arc magma compositions, because the physical parameters that are thought to influence subduction-related magmatism (the thickness and composition of the upper plate, mantle depletion and partial melting, and the slab components) change along the arc. Furthermore, the presence of back-arc and rear-arc volcanism allow insight into how mantle composition varies with distance to the trench. We present new major and trace element and Pb–Sr–Nd–Hf isotope data for 101 lava samples from 10 volcanoes along the Kermadec Arc, and combine these with published data for volcanic arc front, rear-arc, and back-arc lavas. The Kermadec Arc-Backarc magmas experienced similar fractional crystallization paths, and those erupted onto oceanic crust are not affected by crustal assimilation. The degree of partial melting varies along and across the arc, partly due to variations in the thickness of the lithosphere of the upper plate, but also due to variations in slab fluid input. We distinguish four different arc segments varying from 100 to 600 km wide, which erupt lavas with distinct compositions due to differences in along-arc slab input. All Kermadec arc and rear-arc lavas were derived from mantle sources to which the sediment melt and a fluid from subducted Pacific oceanic crust or the Hikurangi Seamounts had been added. Increasing sediment melt input into the mantle wedge southwards is reflected in increasing 87Sr/86Sr and Th/Nd, and decreasing 143Nd/144Nd southwards towards New Zealand. Some volcanoes from the central Kermadec Arc show with unusually radiogenic Pb isotope compositions that are probably derived from subducted alkaline rocks from seamounts built on the Hikurangi Plateau. The geochemical segmentation of the Kermadec Arc indicates that slab-derived components can be relatively homogeneous on large scales, but small features like seamounts cause complex additions to the slab component.