Melting Of Peridotite Research Articles

Coexistence of Hainan Plume and Stagnant Slab in the Mantle Transition Zone beneath the South China Sea Spreading Ridge: Constraints from Volcanic Glasses and Seismic Tomography

Abstract The influence of Hainan mantle plume and subducting recycled oceanic crust beneath the spreading ridge of the South China Sea (SCS) have been widely proposed recently, but still controversial and ambiguous. Here, we present seismic tomographic evidence, new major and trace element, and Pb isotopic compositions of volcanic glasses from one International Ocean Drilling Program drill core (Site U1434) in the SCS spreading ridge. The volcanic glasses are relatively enriched in alkalis and light rare earth elements (LREEs) and depleted in heavy REEs (HREEs), exhibit slightly positive anomalies in Nb, Ta, Zr, and Hf as well as a positive Nb relative to La and Th, and show relatively high 207Pb/206Pb and 208Pb/206Pb isotopic ratios, suggesting ocean island basalt- (OIB-) type and enriched mantle 2- (EM2-) type geochemical features likely related to a mantle plume. These geochemical features are consistent with those of late Cenozoic volcanic rocks in Hainan and surrounding areas associated with a mantle plume, likely providing the influence of Hainan mantle plume beneath the spreading ridge of the SCS. The SCS primary-melt and volcanic glasses indicate that the source mantle involved 18.5% eclogite (dense, recycled oceanic crust from the stagnant subducted slab) and 46.1% garnet pyroxenite (produced by the reaction between the peridotite melt and recycled oceanic crust). The existence of Hainan mantle plume and stagnant subducted slab is further supported by geophysical evidence from a recent three-dimensional P-wave seismic tomographic model.

Eocene dike swarm and felsic stock in Central Iran: Roles of metasomatized mantle wedge and Neo-Tethyan slab

The Eocene felsic stock and dike swarm with intermediate composition from Central Iran (southwest of Jandaq) are studied to understand geodynamic evolution of the Central-East Iranian Microcontinent (CEIM). Field relationships show that the dikes, composed of trachyandesite and basaltic trachyandesite, cross-cut the felsic stock. Whole rock geochemical data reveal the calc-alkaline nature of stock and high-K calc-alkaline to shoshonitic nature of dikes. The felsic stock and intermediate dikes show LREE and LILE enrichment and HFSE (Ta, Nb and Ti) depletion, suggesting that they probably formed in a subduction zone and are similar to the volcanic arc rocks. The rock samples of felsic stock have geochemical characteristics of the high silica adakites (HSA) produced by melting of a subducted oceanic crust. On the other hand, the dikes have characteristics of low silica adakites (LSA) produced by partial melting of a lithospheric mantle peridotite previously metasomatized by slab-derived melts. The Eocene felsic stock and dikes probably formed by subduction of the CEIM-confining oceanic crusts beneath the CEIM. The field and geochemical studies indicate that the magma originated from melting of basic section (amphibolite) of the subducted slab (HSA) will arrive to the Earth surface before the magma formed by melting of mantle wedge peridotites (LSA).

Genesis and Tectonic Implications of the Kabr El‐Bonaya Ultramafic Rocks, Sinai Peninsula, Egypt: Constraints from Mineralogical and Geochemical Characteristics

AbstractThe Kabr El‐Bonaya mafic–ultramafic intrusion is exposed along the southeastern border of the Sinai Peninsula and the northernmost segment of the Arabian‐Nubian Shield (ANS). It occurs as an elliptical intrusive body that is located along the major NE–SW trending fracture zones that prevail in the Kid metamorphic complex. The ultramafic rocks in the complex comprise ultramafic cumulates of peridotites (dunite, harzburgite and wehrlite) and pyroxenite. These rocks are generally unmetamorphosed and have intrusive contacts with the country rock. Mineral chemistry and whole‐rock chemical compositions of these ultramafic rocks are mostly consistent with those of residual mantle peridotites from refractory supra‐subduction tectonic settings. Based on the variations of the major elements, the studied ultramafic rocks are consistent with those of a supra‐subduction zone mantle, as it seems to have melted at 1–2 GPa and 1300–1450°C. Linear variations of Al2O3, CaO, V and Ni with MgO, coupled with incompatible and rare‐earth‐element depletion and mineral compositions, suggest prior events of partial melting in both wehrlites and harzburgites. The LREE enrichment in the harzburgite, as well as the development of Cr‐rich spinel, is consistent with a history of melt–peridotite interaction. The calculated (Sm/Yb)N variations for the studied peridotites indicate a general increase in the addition of fluids with an increasing degree of melting from the wehrlite (∼13–15 wt% of fluid) in the source, after initial spinel peridotite melting to the harzburgite (∼20–25 wt% of fluid) in the same source, which is contrary to normal abyssal peridotites. The estimated equilibration temperature ranges from 1214 to 1321°C for the studied wehrlites and from 1297 to 1374°C for harzburgites. The Mg‐rich nature of the analysed olivines from the studied ultramafic rocks (Fo = 81.41 to 91.77) reflect their primary composition and are similar to olivines in Alaskan‐type ultramafic rocks. The Fo content of the analyzed olivines decrease slightly from the dunite to the harzburgite to the wehrlite and to pyroxenite, reflecting a fractional crystallization trend. The high Cr# and very low TiO2 contents (0.03–0.12 wt%) of the Cr‐spinels from the studied peridotites are mostly consistent with modern highly refractory fore‐arc peridotites, indicating that these peridotites developed in a supra‐subduction zone environment.

Geochronology, geochemistry and petrogenesis of the Neoarchean magmatism in the Jiefangyingzi area, northern North China Craton: Implications for crustal growth and tectonic affinity

The Bayan Obo-Duolun-Chifeng-Kaiyuan fault is considered as the craton-orogen boundary between the northern North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB). However, locations of this boundary from Duolun, Chifeng to Kaiyuan have not been well constrained. The Jiefangyingzi area northeast to Chifeng is located at the junction of the Bainaimiao arc belt and northern NCC, and has long been regarded as part of the Bainaimiao arc belt of the CAOB. This paper presents new LA-ICP-MS zircon U-Pb geochronological, whole-rocks geochemical and Nd-Hf isotopic data of some recently identified Neoarchean magmatic rocks from the Jiefangyingzi area. These rocks consist mainly of amphibolites, dioritic gneisses, quartz dioritic gneisses, tonalitic gneisses and trondhjemitic gneisses formed at 2569–2536 Ma and minor amphibole plagioclase gneisses formed at ca. 2640 Ma, and are distributed in an east–west belt that is about 13 km long and 1.5 km wide. Geochemical and Nd-Hf isotopic compositions suggest that the amphibolites were derived from partial melting of a depleted mantle that was metasomatized by subduction-derived fluids with significant crustal contamination, and the dioritic gneisses were generated by partial melting of mantle peridotite metasomatized by slab-melts. The quartz dioritic and tonalitic gneisses originated from high degree partial melting of the subducted oceanic crust with garnet-bearing amphibolite facies in the residue, while the trondhjemitic gneisses originated from low degree partial melting of the thickened mafic lower crust with eclogite facies in the residue due to underplating of mafic magmas. The occurrences of Neoarchean basement rocks, combined with previous Nd-Hf isotopic data of the late Paleozoic magmatic rocks, suggest that the Jiefangyingzi area has a tectonic affinity with the Eastern Block of the NCC rather than the CAOB, and the craton-orogen boundary between the northern NCC and the CAOB should be situated in places north to the Jiefangyingzi area. Our results support that the late Neoarchean (2.6–2.5 Ga) is also a period of crustal growth as well as extensive reworking of the 2.9–2.7 Ga ancient crust in the northwestern margin of the Eastern Block of the NCC. In addition, the transition from tonalite-trondhjemite-granodiorite (TTG) and other potassium-poor granitoid gneisses to crustal-derived potassium-rich granitoid gneisses at the end of Neoarchean (ca. 2.53 Ga) marked the initial cratonization of the NCC and ca. 2.53 Ga is a critical transformation period of maturation of the continental crust in the NCC.

Antigorite dehydration fluids boost carbonate mobilisation and crustal CO2 outgassing in collisional orogens

The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent findings suggest that carbon release along subduction geothermal gradients may well be more relevant than previously thought; however, it has remained difficult to achieve steady state atmospheric CO2 conditions over geological time scales based on current volatile cycle models. Here, we report on meta-ophicarbonate rocks in the contact metamorphic aureole of the Bergell intrusion, Val Malenco, European Alps, that reached T-P conditions of at least 650 °C at 0.35 GPa. We demonstrate by combined field evidence, geochemistry, and closed-system thermodynamic modelling that over 50% of the rock carbonate has been mobilised locally in response to reactive fluid flow upon progressive isobaric heating from 350 to >650 °C. Despite complete mineral transformation at the antigorite + calcite devolatilisation reaction, the resulting tremolite-ophicarbonate preserves the original ophicarbonate texture by olivine-chlorite clasts, representing the former antigorite-serpentinite fraction, embedded in a monomineralic tremolite matrix formed from the calcite fraction that often exceeded 50 vol%. Closed system thermodynamic modelling based on an ophicarbonate composition of 80 wt% serpentinite +20 wt% calcite reveals that rock-buffered fluid XCO2 values evolve from <0.09 to as high as ∼0.16, and the total H2O-CO2 fluid fraction released may be as high as 14 wt%; values that are highly sensitive to bulk ophicarbonate composition.Major to trace element geochemistry reveals a history of peridotite melt depletion followed by ocean floor hydration and ophicarbonate formation, consistent with the ocean continent transition (OCT) setting of the mantle rocks in the Mesozoic. This is demonstrated by positive anomalies in B, U, As, Sb, Bi, and W, and by bulk rock and notably carbonate primitive mantle normalised REE patterns that are basically identical to that of calcite precipitated from Jurassic seawater except for its negative Eu anomaly. Prograde metamorphic tremolite and diopside possess enrichments notably in Sr and REE inherited from reactant calcite, as is also supported by a good match between measured and modelled silicate mineral compositions. Specific geochemical characteristics of the prograde meta-ophicarbonate rocks imply fluid mediated, open system processes. Antigorite dehydration fluid ingress, likely produced in neighbouring antigorite-serpentinites, may have shifted bulk tremolite ophicarbonate rock compositions towards higher SiO2/MgO ratios along with an increase in B contents and depletion in fluid mobile elements such as As, Sb, and Sr.The massive, contact metamorphic CO2 mobilisation documented here occurred at P-T conditions that are similar to those that can be achieved in collisional orogenic settings, whose clockwise P-T-t paths are often characterised by further heating upon initial decompression. They thus evolve at large angle to devolatilisation reactions and silicate-carbonate rock-buffered fluid XCO2 isopleths, thus fostering carbon mobilisation towards metamorphic peak temperatures at moderate to low pressures (in the region of between >700 °C – 1.0 GPa and >600 °C – 0.3 GPa). Barrovian metamorphism, too, can reach T-P conditions of 800 °C – 1 GPa. Amagmatic CO2 mobilised this way will eventually reach the atmosphere via dispersion in the groundwater table and diffuse outgassing. Despite the huge uncertainties associated with quantification of such metamorphic carbon fluxes, geological time scale global carbon cycling models should more rigorously explore the significance of these contributions.

High-Mg andesitic rocks formed through crustal magmatic differentiation

High-Mg andesitic rocks are enriched in both compatible (Mg, Cr and Ni) and incompatible elements (Si, Ba and Sr), which indicates the complexity of their petrogenesis. Models involving hybridization of the mantle sources and input of adakitic melts have been invoked, but struggle to explain the low Mg# values (<0.65) of some low Sr/Y (<40) high-Mg andesitic rocks, because these rocks cannot represent primary melts (Mg# ≥ 0.72) produced by the partial melting of mantle peridotite. This study presents a petrological and geochemical analysis of the Xinsi diorite (443 ± 3 Ma) and Tiechang gabbro (440 ± 3 Ma) from the Early Paleozoic Wuyi-Yunkai Orogen in South China, and demonstrates the role of magmatic differentiation in the petrogenesis of high-Mg andesitic rocks. These rocks contain high SiO2, MgO, Ba and Sr concentrations with low Mg# values (≤0.69) and Sr/Y ratios (≤16.7), resembling typical low Sr/Y high-Mg andesitic rocks. Amphiboles and clinopyroxenes in the Xinsi diorite contain variable contents of Ba and Sr, both of which negatively correlate with Mg#. This indicates that their parental melts had low Ba and Sr and underwent significant enrichment of Ba and Sr through the fractionation of Mg-rich minerals within upper crust (3.1–4.4 kbar). Clinopyroxene, plagioclase and biotite in the Tiechang gabbro all contain lower Ba and Sr than those in the Xinsi diorite, suggesting that their parental melts were also initially low in Sr and Ba and experienced enrichment of these elements during magma evolution at middle crustal levels (5.7–6.3 kbar). The low Ba and Sr concentrations and Sr/Y ratios for parental melts of the Xinsi diorite and Tiechang gabbro suggest their derivation from the partial melting of mantle sources with input from slab melts at shallow depths without involvement of abundant garnet in petrogenesis. This study demonstrates the importance of fractional crystallization in the genesis of low Sr/Y high-Mg andesitic rocks and is conflict with past views that their geochemical features were related to the involvement of adakitic melts in their mantle sources. Therefore, this study presents an alternative model for the production of low Sr/Y and high Sr/Y high-Mg andesitic rocks, particularly in the Late Archean, which reflects the intense melting of subducted slab at different depths in the hotter mantle regime at that time.

MORB-like δ56Fe values unveil the effect of subduction on the South China Sea basalts

Clarifying the genesis of backarc basin basalts (BABBs) is of great significance for understanding the cycling of subducted materials. Here we show MORB-like Fe isotopic signatures (δ56Fe) of basalts recovered from the South China Sea (SCS), a well-developed backarc basin, as an indicator to investigate the influence of subduction on the BABBs. Whole-rock chemical compositions and olivine chemistry indicate the presence of pyroxenitic components in the peridotitic mantle. Melting of a pyroxenite-bearing peridotite indicates an addition of ~10–40% of pyroxenitic melts to the peridotitic mantle in the SCS, which should result in higher δ56Fe values than that of global MORBs. It is inconsistent with the MORB-like δ56Fe values we observed in the SCS basalts. Therefore, a component with low δ56Fe signatures may exist in the source of the SCS basalts, which is most likely derived from subducted materials. Such a scenario can be well corroborated by mixing depleted MORB mantle with subducted fluids and pyroxenitic melts. This study emphasizes that the MORB-like Fe isotopic signatures in backarc basin basalts can be caused by multi-factors, especially in the case of heterogeneous mantle source.

Genetic Link between Podiform Chromitites in the Mantle and Stratiform Chromitites in the Crust: A Hypothesis

No genetic link between the two main types of chromitite, stratiform and podiform chromitites, has ever been discussed. These two types of chromitite have very different geological contexts; the stratiform one is a member of layered intrusions, representing fossil magma chambers, in the crust, and the podiform one forms pod-like bodies, representing fossil magma conduits, in the upper mantle. Chromite grains contain peculiar polymineralic inclusions derived from Na-bearing hydrous melts, whose features are so similar between the two types that they may form in a similar fashion. The origin of the chromite-hosted inclusions in chromitites has been controversial but left unclear. The chromite-hosted inclusions also characterize the products of the peridotite–melt reaction or melt-assisted partial melting, such as dunites, troctolites and even mantle harzburgites. I propose a common origin for the inclusion-bearing chromites, i.e., a reaction between the mantle peridotite and magma. Some of the chromite grains in the stratiform chromitite originally formed in the mantle through the peridotite–magma reaction, possibly as loose-packed young podiform chromitites, and were subsequently disintegrated and transported to a crustal magma chamber as suspended grains. It is noted, however, that the podiform chromitites left in the mantle beneath the layered intrusions are different from most of the podiform chromitites now exposed in the ophiolites.

A Thermodynamic Model for the Subsolidus Evolution and Melting of Peridotite

Abstract We present a structural update to the thermodynamic model for calculating peridotite phase relations and melt compositions at 0·01 to 60 kbar and from 600 °C to the peridotite liquidus in the system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–TiO2–Fe2O3–Cr2O3 (KNCFMASTOCr), based on the model of Holland et al., 2018 [Melting of Peridotites through to Granites: A Simple Thermodynamic Model in the System KNCFMASHTOCr. Journal of Petrology 59, 881–900]. The new model is better able to predict the phase relations and melting of ultramafic rocks, in particular the abundance of orthopyroxene in the residue and the concentration of silica in the melt. In addition, improvements in modelling Cr-spinels mean that the model is now able to reproduce Cr-content of garnet and spinel above and below the solidus without modification to the knorringite free energy. Model calculations indicate that, for peridotite composition KR4003, the spinel to garnet transition intersects the solidus at 22·1–24·8 kbar and orthopyroxene disappears from the solidus at 29·1 kbar. Below the solidus, the model is able to reproduce the abundances and compositions of phases in experimental studies and natural samples spanning a range of compositions, allowing it to be used for investigating subsolidus equilibration during mantle cooling and pressurisation/decompression. The liquid model provides a good fit to experimental data and is able to replicate the position of the solidus and the composition of both melt and residue at and above the solidus for a range of peridotite compositions. The model may, therefore, be used to investigate fractional mantle melting and basalt generation in modern geodynamic regimes, and also to explore equilibrium mantle melting in the early Earth. The model can also be used to explore liquid and residue compositions for melting of non-pyrolitic mantle, for which there is a paucity of experimental data. We demonstrate the scope of the model using two case studies investigating the subsolidus evolution and melting of a silica-rich cratonic peridotite from the Kaapvaal craton.

An Intra-Oceanic Subduction System Influenced by Ridge Subduction in the Diyanmiao Subduction Accretionary Complex of the Xar Moron Area, Eastern Margin of the Central Asian Orogenic Belt

This study focuses on the geology, geochemistry, Sr-Nd isotopes and their tectonic settings of three types of basalts in Diyanmiao ophiolite in the Xar Moron area located on the eastern margin of the Central Asian Orogenic Belt. Type I basalts are oceanic tholeiites with a depleted light rare earth element (LREE) pattern, which are similar to the typical N-mid-oceanic ridge basalt (MORB) and suggests that they were formed at a mid-oceanic ridge. The initial 87Sr/86Sr ratios of Type I basalts range from 0.703 966 to 0.705 276 and the eNd(t) values are from 16.49 to 17.15, indicating that they were derived from a depleted mantle source. Type II basalts belong to the medium-potassium calc-akaline series and have the geochemical characteristics of Nb-enriched basalt (NEB) with high Nb content (14.5 ppm) and strong enrichment in LREEs, implying that they were created by the partial melting of mantle wedge peridotite that previously metasomatized by slab melts. Type III basalts are high-Al basalt (HAB) with high-Al contents (Al2O3=16.75 wt.%–18.00 wt.%), distinct Nb depletion and high Th/Yb ratios. Thus they were likely generated in a normal island-arc setting. Therefore, the association of MORB, NEB, and HAB in the study area may be due to the subduction of a mid-oceanic ridge, and the Diyanmiao ophiolite is proposed to be formed in the forearc setting of a mid-oceanic ridge subduction system.

Geochemistry and geochronology of Pengco subduction‐related ophiolites, Tibet: Implications for Dongkaco microcontinent in the Bangong–Nujiang suture zone

The identification of a microcontinent in Bangong–Nujiang suture zone (BNSZ) is important to understanding the tectonic evolution of Bangong–Nujiang Tethys Ocean (BNTO). Subduction‐related ophiolite is one of the keys to discriminate a microcontinent in suture zones, which can be generated during the oceanic subduction beneath the microcontinent. Generally, ophiolites in the adjacent area of a microcontinent would show characteristics of a volcanic arc, while ophiolites in adjacent area of an ocean show characteristics of a fore‐arc. North Pengco ophiolites (NPOs) and South Pengco ophiolites (SPOs) were collected in the central section of BNSZ, and dated at 115.0–111.5 Ma by SHRIMP II. According to their trace element characteristics, NPOs were determined to have formed at a setting of subduction‐related volcanic arc and affected by slab‐derived fluids or melts. The diagram based on the Ce versus Ce/Y shows that NPOs originated from the partial melting of spinel peridotites. On the other hand, SPOs were generated from the partial melting of ultra‐depleted mantle at a setting of subduction‐related fore‐arc on the basis of extraordinarily low REEs summation and HFSEs (e.g., Zr, Ti, Nb and Ta), relatively high concentrations of compatible elements (e.g., Cr, Ni), large ratios of CaO/TiO2 (37.67–437.50) and Al2O3/TiO2 (20.78–424.50), which were also affected by slab‐derived fluids. Based on the relative location of NPOs and SPOs, it reasonably proposed that a northward subduction of a minor oceanic basin had occurred at the Early Cretaceous, trigged by the Dongkaco microcontinent (DMC). In reverse, these ophiolites provide evidence for the existence of DMC in BNTO. Combining with other subduction‐related ophiolites developed in BNSZ, BNTO might have comprised of several minor oceanic basins separated by some blocks like DMC in the central and west sections, rather than a unified ocean basin.

Petrogenesis and dynamic implications of the Cenozoic alkali basalts from the Jingpohu Volcanic Field, NE China

Abstract The Cenozoic alkali basalts are widely exposed in the Jingpohu Volcanic Field, NE China. Previous volcanology and geochronology research has revealed that they were formed in three periods in the Miocene ( c. 29.23–13.59 Ma), Pleistocene ( c. 83.7 ka) and Holocene ( c. 5.5–5.2 ka BP). The Miocene and Pleistocene basalts consist of alkali olivine basalts, while the Holocene basalts are composed of alkali olivine basalts and leucite tephrites. Petrogenetic studies reveal that the primary magmas of the Miocene and Pleistocene alkali olivine basalts originated from partial melting of EM2-like garnet peridotites, and those of the Holocene alkali olivine basalts were derived from melting of EM1- and EM2-like garnet peridotites with higher garnet proportions. In contrast, the primary magmas of the Holocene leucite tephrites were derived from the melting of eclogites and peridotites. Combined with previous research, we suggest that melting of the mantle source region to generate Jingpohu alkali basalts was triggered by decarbonization and dehydration of the slabs stagnated in the mantle transition zone.

Chapter 4.1b Antarctic Peninsula: petrology

Abstract Scattered occurrences of Miocene–Recent volcanic rocks of the alkaline intraplate association represent one of the last expressions of magmatism along the Antarctic Peninsula. The volcanic rocks were erupted after the cessation of subduction which stopped following a series of northward-younging ridge crest–trench collisions. Volcanism has been linked to the development of a growing slab window beneath the extinct convergent margin. Geochemically, lavas range from olivine tholeiite through to basanite and tephrite. Previous studies have emphasized the slab-window tectonic setting as key to allowing melting of peridotite in the asthenospheric void caused by the passage of the slab beneath the locus of volcanism. This hypothesis is revisited in the light of more recent petrological research, and an origin from melting of subducted slab-hosted pyroxenite is considered here to be a more viable alternative for their petrogenesis. Because of the simple geometry of ridge subduction, and the well-established chronology of ridge crest–trench collisions, the Antarctic Peninsula remains a key region for understanding the transition from active to passive margin resulting from cessation of subduction. However, there are still some key issues relating to their tectonomagmatic association, and, principally, the poor geochronological control on the volcanic rocks requires urgent attention.

Sulfur content at sulfide saturation of peridotitic melt at upper mantle conditions

Abstract The concentration of sulfur that can be dissolved in a silicate liquid is of fundamental importance because it is closely associated with several major Earth-related processes. Considerable effort has been made to understand the interplay between the effects of silicate melt composition and its capacity to retain sulfur, but the dependence on pressure and temperature is mostly based on experiments performed at pressures and temperatures below 6 GPa and 2073 K. Here we present a study of the effects of pressure and temperature on sulfur content at sulfide saturation of a peridotitic liquid. We performed 14 multi-anvil experiments using a peridotitic starting composition, and we produced 25 new measurements at conditions ranging from 7 to 23 GPa and 2173 to 2623 K. We analyzed the recovered samples using both electron microprobe and laser ablation ICP-MS. We compiled our data together with previously published data that were obtained at lower P-T conditions and with various silicate melt compositions. We present a new model based on this combined data set that encompasses the entire range of upper mantle pressure-temperature conditions, along with the effect of a wide range of silicate melt compositions. Our findings are consistent with earlier work based on extrapolation from lower-pressure and lower-temperature experiments and show a decrease of sulfur content at sulfide saturation (SCSS) with increasing pressure and an increase of SCSS with increasing temperature. We have extrapolated our results to pressure-temperature conditions of the Earth’s primitive magma ocean, and show that FeS will exsolve from the molten silicate and can effectively be extracted to the core by a process that has been termed the “Hadean Matte.” We also discuss briefly the implications of our results for the lunar magma ocean.

MAGLAB: A computing platform connecting geophysical signatures to melting processes in Earth's mantle

Decompression melting of the upper mantle produces magmas and volcanism at the Earth's surface. Experimental petrology demonstrates that the presence of CO2 and H2O enhances peridotite melting anywhere within the upper mantle down to approximately 200–300 km depth. The presence of mantle melts with compositions ranging from carbonate-rich to silicate-rich unavoidably affects the geophysical signals retrieved from Earth's mantle. Geochemical investigations of erupted intraplate magmas along with geophysical surveys allow for constraining the nature and volume of primary melts, and a sound formalism is required to integrate these diverse datasets into a realistic model for the upper mantle including melting processes. Here, we introduce MAGLAB, a model developed to calculate the composition and volume fraction of melts in the upper mantle, together with the corresponding electrical conductivity of partially molten mantle peridotites at realistic pressure-temperature conditions and volatile contents. We use MAGLAB to show how the compositions of intraplate magmas relate to variations in lithosphere thickness. Progressive partial melting of a homogeneous peridotitic mantle source can in theory create the diversity of compositions observed among the spectrum of intraplate magma types, with kimberlite melts beneath thick continental shields, alkaline magmas such as melilitite, nephelinite and basanite beneath thinner continents and relatively old plus thick oceanic lithospheres, and ‘regular’ basalts beneath the youngest and thinnest oceanic lithospheres as well as beneath significantly thinned continental lithospheres. MAGLAB calculations support recent experimental findings about the role of H2O in the upper mantle on producing primary kimberlitic melts in addition to CO2. We demonstrate the robustness of MAGLAB calculations by reproducing the compositions of erupted melts as well as associated mantle electrical conductivities beneath the Society hotspot in the Pacific Ocean. A comparison of our simulations with magnetotelluric surveys at various oceanic settings shows that the heterogeneities in electrical conductivity of Earth's upper mantle are related to variations in volatile content via the presence of small (generally <<1 wt%) and heterogeneously distributed fractions of CO2-H2O-bearing melts.

Scandium: Ore deposits, the pivotal role of magmatic enrichment and future exploration

Scandium (Sc), known as the 'miracle metal', has unique properties in material sciences, and is a critical resource in the world due to the low supply but wide applications in advanced technology. Despite not extraordinarily sparse in the Earth’s crust, Sc is often dispersed and does not form ore deposits. Nonetheless, Sc can be occasionally concentrated to form magmatic, hydrothermal or supergene deposits. Magmatic deposits are estimated to contribute about 90% of the global Sc resources, in which Sc is primarily hosted in clinopyroxene ± amphibole in mafic–ultramafic intrusions. Hydrothermal enrichment of Sc is often associated with W-Sn mineralization in quartz-vein systems. Supergene deposits mainly include regolith-hosted deposits formed from weathering of mafic–ultramafic protolith and marine sediment-hosted deposits. Magmatic enrichment of Sc is essential to form many of these different types of Sc deposits. Partial melting of anhydrous fertile spinel peridotites could produce Sc-rich primary melts, whereas melts derived from garnet peridotites are generally of low Sc concentrations. Involvement of water during partial melting may not increase Sc concentrations in the primitive melts but, during fractional crystallization, the high water content can sufficiently suppress the crystallization of orthopyroxene and the Sc scavenging by orthopyroxene. Thereby, Sc could be enriched in the melts and subsequently concentrate in clinopyroxene and amphibole. Subduction environment, at where hydrous mafic–ultramafic melts could be generated, is likely favorable locality for exploring Sc deposits, such as the Sc-rich Alaskan-type mafic–ultramafic intrusions. Under favorable supergene conditions such as tropical and sub-tropical climate, regolith-hosted Sc deposits could develop from weathering of these Sc-rich mafic–ultramafic complexes.

Simultaneous growth and reworking of the Lhasa basement: A case study from Early Cretaceous magmatism in the north-central Tibet

Magmatic rocks preserve a lot of information about the formation and evolution of continental crust, and they play an important role in understanding the growth and reworking of ancient continent. Cretaceous magmatic rocks occurring in the Lhasa terrane are a window to investigate magmatic processes and continental evolution of the Tibetan Plateau. In this study, we report zircon UPb ages and Hf isotopic data, as well as whole-rock major and trace elemental compositions for Early Cretaceous magmatic rocks, including andesite and dacite from the Zenong Group, and associated diorite and granite in the Xinji area, north-central Tibetan Plateau. Zircon from andesite and dacite yielded weighted-mean 206Pb/238U ages of 120–108 Ma and variable zircon εHf(t) values between −12.3 and +2.2. Geochemical compositions suggested that they both displayed high-K calc-alkaline features. Andesite was probably formed by mixing between crust- and mantle-derived melts, whereas dacite was most likely generated by partial melting of ancient crust with input of mantle-derived material. Dioritic and granitic intrusions were intruded into the Precambrian strata and Zenong Group, respectively. Zircon from diorite gave a weighted-mean 206Pb/238U age of ca. 116 Ma and zircon εHf(t) values of −0.5 to +7.3. Diorite displayed medium-K calc-alkaline series signatures and was most likely formed by partial melting of mantle wedge peridotite accompanied by crustal assimilation. Granite yielded zircon weighted-mean 206Pb/238U ages of 107–104 Ma, and positive zircon εHf(t) values of +1.5 to +6.7, indicating a major source of juvenile crust. Granitic samples show irregular REE distribution with variable Ce anomalies, which were probably inherited from subducted pelagic sediments. The Early Cretaceous magmatic rocks in the Xinji area records distinct formation processes involved partial melting of mantle, re-melting of crust, and mixing between mantle and crust. Such processes, however, had a complicated source from mantle, ancient and juvenile continental crust, and recycled pelagic sediments, suggesting simultaneous crustal growth and reworking of the Lhasa basement during the Early Cretaceous.

Geochemistry of Khor Um-Safi ophiolitic serpentinites, central Eastern desert, Egypt: Implications for neoproterozoic arc-basin system in the Arabian-Nubian shield

Serpentinites in the Eastern Desert of Egypt are the most distinctive lithological unit in the Arabian–Nubian Shield (ANS) ophiolite sequence which associated with major suture zones. Khor Um-Safi (KUS) serpentinites represent dismembered fragments of ophiolitic rocks located in the central Eastern Desert (CED) of Egypt.KUS serpentinites exhibit affinity to the typical metamorphic peridotites with harzburgitic protolith compositions. Their opaque mineral assemblage (pentlandite, heazlewoodite and magnetite) is similar to that observed in oceanic serpentinites and implies serpentinization under highly reducing conditions. They have refractory major element compositions with Al2O3 contents comparable to oceanic and active margin peridotites as well as Pan-African serpentinites. The Cr and TiO2 contents reflect evolution within a supra-subduction zone (SSZ) environment. This implication is confirmed by the Al2O3/SiO2 and MgO/SiO2 ratios which akin to ANS ophiolitic peridotites in fore-arc setting. Their enrichment in compatible trace elements (Cr, Ni and Co) reveals a depleted mantle peridotite protolith.Modelling trace elements indicates that they represent the mantle residues from 15 to 20 % melting of spinel peridotite at oxygen fugacity conditions of the QFM + 1 buffer. This range of melt extraction is consistent with the typical range of SSZ peridotite. Oxygen fugacity estimation suggests evolution under more oxidizing regime similar to modern fore-arc basin system. Moreover, this implication indicates that the KUS mantle represents arc lithosphere interacted with arc melt.

Genesis of high-Ni olivine phenocrysts of the Dali picrites in the Central Emeishan large igneous province

AbstractThe Emeishan large igneous province (ELIP) in SW China is considered to be a typical mantle-plume-derived LIP. The picrites formed at relatively high temperatures in the ELIP, providing one of the important lines of argument for the role of mantle plume. Here we report trace-element data on olivine phenocrysts in the Dali picrites from the ELIP. The olivines are Ni-rich, and characterized by high (&gt;1.4) 100×Mn/Fe value and low (&lt;13) 10 000×Zn/Fe value, indicating a peridotite-dominated source. Since the olivine–melt Ni partition coefficient (KDNiol/melt) will decrease at high temperatures and pressures, the picrites derived from peridotite melting at high pressure, and that crystallized olivines at lower pressure, can generate high concentrations of Ni in olivine phenocrysts, excluding the necessity of a metasomatic pyroxenite contribution. Based on the Al-in-olivine thermometer, olivine crystallization temperature and mantle potential temperature (TP) were calculated atc. 1491°C andc. 1559°C, respectively. Our results arec. 200°C higher than that of the normal asthenospheric mantle, and are consistent with the role of a mantle thermal plume for the ELIP.

The chondritic neodymium stable isotope composition of the Earth inferred from mid-ocean ridge, ocean island and arc basalts

Accurate knowledge of the composition of Earth’s major chemical reservoirs is fundamental for constraining all modern geochemical cycles. Basaltic rocks provide a direct way of sampling the composition of Earth’s inaccessible interior. Here, we present the first comprehensive neodymium (Nd) stable isotope analyses for a global compilation of mid-ocean ridge, ocean island, continental intraplate and island arc basalts using a double-spike technique. In these primitive magma compositions magmatic differentiation has no resolvable effect on δ146/144Nd. Mid-ocean ridge basalts possess an extremely homogenous δ146/144Nd with an average composition of δ146/144Nd = −0.025 ± 0.013‰ (±2 s.d.; n = 33). Ocean island and continental intraplate magmas possess more variable compositions (δ146/144Nd = 62 ppm) that are related to the variable incorporation of recycled components in their source regions. Island arc basalts from New Britain (δ146/144Nd = 61 ppm) reflect the complex interplay between source composition, degree of melting and slab-fluid inputs. Variations are uncorrelated with indicators of magmatic differentiation or slab-fluid addition, rather increasing δ146/144Nd with slab depth is attributed to a higher proportion of metasomatized sub-arc mantle in the melting region. A partial melting model for Nd stable isotopes has been constructed using NdO force constants calculated using the Born-Lande approximation. Melting of typical mantle peridotite will induce no resolvable fractionations of Nd stable isotopes (Δ146/144Ndmelt-mantle < 0.003‰ at 1200 °C). The lack of fractionation upon partial melting means primitive magmatic rocks can be used to calculate the average composition of the bulk silicate Earth (BSE), which is δ146/144Nd = −0.024 ± 0.031‰ (±2 s.d.; n = 80). This BSE composition is indistinguishable at the 95 % confidence level from that of chondritic meteorites, the building blocks of Earth. Therefore, sequestration of significant quantities of Nd into the sulfide matte did not occur, this combined with recent experimental evidence for no Sm-Nd fractionation means the sulfide matte cannot be considered a plausible solution for the 142Nd/144Nd offset between the Earth and chondrites. Despite resolvable variations in δ146/144Nd from the canonical value being widespread in terrestrial materials, they are not large enough to generate the difference in radiogenic Nd isotope ratios between the BSE and chondrites.