Basalt Source Research Articles

Mantle sources of ocean islands basalts revealed from noble gas isotope systematics

Noble gas isotope systematics, particular those of He, have been fundamental in showing that some ocean island basalts (OIB) were sourced from deep mantle plumes. Relationships between He, W, Os, Sr, Nd and Pb isotopes in Hawaiian, Samoan, Galapagos, and Icelandic lavas have been suggested to reflect contributions from less degassed lower mantle sources, and perhaps even materials advected from the core-mantle boundary. This study reviews the noble gas (He-Ne-Ar-Xe) isotope systematics of major OIB suites. Important in this evaluation are considerations of spatial and temporal variations, including the sample media (glass, minerals, hydrothermal gases and fluids) used for analyzing noble gases, as well as the degree of partial melting experienced to produce OIB. Limited availability of gas-rich samples means patchy coverage in definition of OIB Ne, Ar, Xe isotope compositions. Additionally, low-degree partial melting will lead to preferential sampling of more fusible, generally more enriched components which could affect noble gas isotope systematics. OIB with low- (<8RA) to MORB-like 3He/4He (8 ± 2RA) dominantly sample convecting mantle domains and can also contain some relatively undegassed (solar) components. Their range in Sr-Nd-Os-Pb isotope compositions reflect a strongly recycled crustal and/or lithospheric heritage. Intermediate (>10RA) to high-3He/4He (>25RA) OIB (Loihi, Hawaii; Iceland; Fernandina, Galapagos; Ofu, Samoa) sample a reservoir that has been relatively isolated since ~4.5 billion years as shown by noble gases (He, Ne, Ar, Xe), and by W isotopes. However, this reservoir is not pristine and these OIB show evidence for containing depleted and enriched recycled components from Xe isotopes as well as lithophile-siderophile radiogenic isotope systematics. Linking the highest-3He/4He OIB to a putative Focus Zone (FOZO) reservoir is also problematic; FOZO is not a reservoir least affected by recycling of crust, rather a mantle reservoir that contains recycled components, including depleted lithosphere, and that has been oxidized. This distinction in definition means that, while the FOZO reservoir is almost certainly sampled by the deepest, hottest mantle plumes, its status as a primitive reservoir is unsubstantiated. Models of mantle convection can satisfy seismic constraints on Earth's deep mantle and reveal extensive mixing throughout with enriched and depleted lithologies formed by partial melting processes at Earth's surface. Ancient deep isolated reservoirs are likely to be relatively minor present-day mantle features. More remarkable is the predicted abundance of strongly refractory mantle material. Focus on the noble gas, radiogenic and stable isotope attributes of such refractory reservoirs are required for a fuller understanding of mantle processes.

Spatial variation of subduction zone fluids during progressive subduction: Insights from Serpentinite Mud Volcanoes

Geological processes at subduction zones control seismicity, plutonism and volcanism, and geochemical cycling between the oceans, crust, and mantle. The down-going plate experiences metamorphism, and the associated dehydration and fluid flow alters the physical properties of the plate interface and mantle wedge, as well as controlling the composition of material descending into the mantle. Any direct study of slab evolution during subduction is inhibited by the prohibitive depths at which these processes occur. To examine these processes we use serpentinite mud volcanoes in the Mariana forearc, that permit sampling of serpentinite materials and their pore waters that ascend from the subduction channel. We present new pore water chemical data from the summit and flanks of three serpentinite mud volcanoes that were drilled during International Ocean Discovery Program Expedition 366 which are reflective of reactions within the crust and mantle during the early, shallow (<20 km) stages of subduction. We show, via thermodynamic modelling, that our new data on the evolution of pore water chemical compositions reflect mineralogical characteristics of a predominately basaltic source from the downgoing Pacific Plate. However, a component from sedimentary sources is likely, especially for those mud volcanoes near the trench. Other potential slab-derived constituents, such as lithospheric serpentinite, carbonate-rich sediments, or seamount basalts with an intraplate geochemical character, are not required to explain our results. Our results indicate that with progressive subduction the lawsonite-epidote mineral transformation boundary at ∼250 °C may help drive slab carbonate destabilisation, despite its apparent thermodynamic stability at such temperatures and projected pressures (∼300 °C and ∼0.6 GPa). New dissolved gas data also point to primary thermodynamic controls over methane/ethane production within the subduction channel as depths-to-slab increase. Our findings provide direct evidence for the progressive mineralogical and chemical evolution of a subducting oceanic plate, which liberates a progressively evolving fluid phase into the subduction channel.

Highly variable H2O/Ce ratios in the Hainan mantle plume

Mantle plume-generated basalts have long been utilized to assess the large-scale heterogeneity of the deep mantle. Their chemical and isotopic enrichment has been thought to result from the incorporation of recycled sediments or oceanic crust in their sources. Variable extents of dehydration or rehydration of recycled materials during subduction can induce different H2O/Ce ratios in plume-related basalts. The Hainan plume has been regarded as the cause of the massive Cenozoic basaltic volcanism in southeastern Asia. In this study, we measured bulk-rock major and trace element compositions of basalts from a volcano at Heishanling Hill, northern Hainan Island, and made FTIR analyses of water in clinopyroxene phenocrysts to determine magmatic water contents. In addition, we determined Sr-Nd isotope compositions of basalts at Heishanling Hill and other three localities.Water contents of Cpx phenocrysts vary from 20 to 254 ppm, which were used to calculate the water contents in the corresponding equilibrated melts, yielding values from 0.22 to 2.42 wt% with an average of 1.07 ± 0.47 wt%. Considering the effect of magma differentiation on water contents, the primary melts of Heishanling basalts were estimated to have a range of 0.67–1.40 wt% for individual Heishanling basalts, with the average of 1.03 ± 0.33 wt%. The calculated water contents in the mantle source of the Heishanling basalts vary from 279 to 582 ppm, falling in the range of global OIBs.Combined with the results previously reported, H2O/Ce ratios of Hainan basalts were estimated to vary from 50 to 425, covering the reported range from EM-type OIBs to NMORBs. The enriched Sr-Nd isotopic compositions of the studied samples resemble those of other Hainan plume-related basalts, and their 143Nd/144Nd ratios are well correlated with incompatible trace element ratios (e.g., Th/La, Ba/Nb). Bulk-rock compositions are consistent with a contribution of pyroxenite in the mantle source of the Hainan basalts and mixing modeling suggests coupled contribution of recycled sediment and oceanic crust. H2O/Ce displays a negative correlation with 143Nd/144Nd and a positive correlation with (Rb/Nb)n and (Th/La)n, meaning that an increasing amount of recycled oceanic crust and sediment in the source increases its H2O/Ce. The recycled material incorporated in the Hainan plume source may have undergone variable degrees of dehydration or deep rehydration by fluids released from subcrustal hydrous minerals at grater depths during subduction.

Etudes géochimiques et geomorphologie lacustre du bassin du lac Yakhi en Mongolie orientale

This study presents the landscape evolution of Lake Yakhi basin in eastern Mongolia where the lake plays an important role in the surface water resources. The multiple paleoshoreline features indicate wave-cut terraces, whereas the modern-day shorelines show that Lake Yakhi is at rapid northward regression. The paleo- and modern-day shorelines imply that the paleo-Lake Yakhi may have covered an extensive area with different hydrological conditions in the past. Lake Yakhi shows a decreasing trend in an area with a loss of ~62.2 km2 or 63.9% of the total area during the past five decades. The hydrological dynamics of Lake Yakhi during the last 50 years show its shift into a playa lake coinciding with the local climate changes in warming since 1987 and drying since 1992. In the lake's margin, mafic igneous provenance indicates a derivation from weathered granite and gneiss terrains on the oceanic island arc. However, in the lake's center, felsic igneous and quartzose sedimentary provenances show a derivation from the pre-existing sedimentary terrain on the active continental margin. Lake Yakhi sediments show the granite (felsic) and granite basalt (felsic to mafic) source rocks with mostly shale and arkose. The A-CN-K ternary plot shows that granites, granodiorites, and tonalities dominated in the lake sediments may have been transported from the source rocks enriched in plagioclase and feldspars. The weathering trend in the source area shows a low degree of chemical weathering and the chemical maturity shows a climatic shift from semiarid to arid climatic conditions in the Lake Yakhi basin.

Deciphering the origin of a basanite-alkali basalt-tholeiite suite using Zn isotopes

Basanite-alkali basalt-tholeiite suites have unique chemical and isotopic signatures associated with recycled crustal materials. However, the relative contribution of recycled carbonates and silicates is unclear owing to the inability of traditional isotopic approaches (e.g., Sr, Nd, Pb, and O isotopes) to fully identify trace amounts of recycled crustal materials. On the other hand, the identification of the types of recycled components would help decipher origin of these magmatic suites. Since the zinc isotopic ratios (reported as the per mil deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard, δ66Zn) are distinct between marine carbonates and silicate reservoirs (e.g., mantle, oceanic mafic crust, and siliciclastic sediments), they could be used to identify the different source endmembers in magmatic rocks. In this work, we used the Zn and Sr-Nd isotopic ratios of Cenozoic continental basalts from the Central North China Craton (also known as Trans-North China Orogen, TNCO) to identify the nature of the recycled materials present in the mantle source of these basanite-alkali basalt-tholeiite suite. We found that, basanites and alkali basalts have high δ66Zn values (ranging from 0.32‰ to 0.46‰), while tholeiites have low δ66Zn values (0.28 ± 0.04‰, 2sd) similar to mid-oceanic ridge basalts (MORB) (δ66Zn = 0.27 ± 0.05‰, 2sd). Given the limited extent of Zn isotopic fractionation (<0.1‰) during crystallization and <20% partial melting, the elevated δ66Zn values observed in the basanites and alkali basalts reflect the involvement of recycled marine carbonates (which have an average δ66Zn of ∼0.91‰) in the mantle sources. Moreover, the δ66Zn values of these basalts show clear correlations with carbonate metasomatism indexes (such as, Ti/Eu and Zr/Hf ratios). Mixing models for δ66Zn values vs. 87Sr/86Sr ratios and εNd(t) values demonstrate that the mantle sources of basanites, alkali basalts, and tholeiites were metasomatized by different proportions of carbonates and siliceous sediments. Furthermore, both isotopic and geochemical compositions exhibit symmetrical variations from the central zone towards the south and the north along the TNCO, that is, δ66Zn values and 143Nd/144Nd and Ce/Pb ratios decrease, while SiO2 content and 87Sr/86Sr ratios increase. We suggest that more carbonates and fewer siliceous sediments are released from the subducted slab, with an increase in the depth of subduction. We further propose that this process is one of the most important causes of compositional variations in the basanite-alkali basalt-tholeiite suite.

RETRACTED: Experimental determination of ferric iron partitioning between pyroxene and melt at 100 kPa

Pyroxene is the principal host of Fe 3+ in basalt source regions, hosting 79 and 81% of the Fe 3+ in spinel and garnet lherzolite, respectively. In spinel peridotite, orthopyroxene (opx) and clinopyroxene (cpx) host 48% and 31%, respectively, of the total Fe 3+ . Yet the relationship between mantle mineralogy, pyroxene chemistry, and the oxygen fugacity ( f O2 ) recorded by mantle-derived basalts remains unclear. To better understand partitioning of Fe 3+ between pyroxene and melt we conducted experiments at 100 kPa with f O2 controlled by CO-CO 2 gas mixes between ∆QFM −1.19 to +2.06 in a system containing andesitic melt saturated with opx or cpx only. To produce large (100–150 μm), homogeneous pyroxenes, we employed a dynamic cooling technique with a 5–10 °C/h cooling rate, and initial and final dwell temperatures 5–10 °C and 20–30 °C super and sub-liquidus, respectively. Resulting pyroxene crystals have absolute variation in Al 2 O 3 and TiO 2 < 0.05 wt% and < 0.02 wt%, respectively. Fe 3+ /Fe T in pyroxenes and quenched glass were measured by Fe K -edge XANES. We used a newly developed XANES calibration for cpx and opx by selecting spectra with X-rays vibrating on the optic axial plane at 45 ± 5° to the crystallographic c axis. Values of D Fe3+ cpx/melt increase from 0.03 to 0.53 as f O2 increases from ∆QFM −0.44 to +2.06, while D Fe3+ opx/melt remains unchanged at 0.26 between ∆QFM −1.19 to +1.37. In comparison to natural peridotitic pyroxenes, Fe 3+ /Fe T in pyroxenes crystallized in this study are lower at similar f O2 , presumably owing to lower Al 3+ contents. Comparison to thermodynamic models implemented in pMELTS and Perple_X suggest that these over-predict the stability of Fe 3+ in pyroxenes, causing these models to underpredict the f O2 of spinel peridotites under conditions of basalt genesis.

Petrology and age of the Lepreau River Dyke, southern New Brunswick, Canada: source of the end-Triassic Fundy Group basalts

A large dyke of quartz tholeiitic gabbronorite has been mapped for 59 km in southern New Brunswick, Canada, between Lepreau River in the northeast and Indian Island in the southwest. Scattered outcrops occur along a positive aeromagnetic lineament, providing a dyke strike of N42°E overall (segments N30°E to N72°E), dips of 80° to 90°NNW, and widths of 4 to 30 m. A new 40Ar/39Ar plagioclase age of 201.67 ± 0.35 Ma for the Lepreau River Dyke is similar to dates for the massive North Mountain Basalt in the Fundy Basin to the east. The dyke is associated with the Ministers Island and Christmas Cove dykes, which are indistinguishable in chemistry, petrology, and probable age, and we regard them as segments of the same co-magmatic dyke system. In addition, their petrology is similar to that of the basalts of the adjacent Early Mesozoic Fundy and Grand Manan basins. We propose that the Lepreau River and associated dykes were sources for the regional basin basalts, which in turn are part of the Central Atlantic Magmatic Province (CAMP) that overlaps the Triassic–Jurassic boundary and associated mass extinction event.

Low H2O/Ce ratios and δ18O values for continental basalts in eastern China: Geochemical evidence for involvement of the dehydrated crustal component in the mantle source

Crustal components derived from subducted oceanic crust have been increasingly identified in the mantle sources of intraplate basalts. The geochemical processes in transferring the crustal signatures into the mantle sources of these basalts has been documented to be similar to those for the origin of island arc basalts. However, the dehydration melting of subducting oceanic slabs is assumed to occur at postarc depths, releasing less water than the dehydration that occurs at subarc depths. This study examines this assumption using a combination of mineral H2O/Ce and O isotope ratios for Cenozoic continental basalts in eastern China, an area where the western Pacific plate is subducting beneath its continental margin. Clinopyroxene phenocrysts from these continental basalts show H2O/Ce ratios of 108–204 for their magma and δ18O values of 3.9‰-5.0‰ for the clinopyroxene, both of which are lower than the values of the depleted MORB mantle. This suggests that the basalts were derived from a mantle source enriched by the dehydrated lower oceanic crust-derived component. Mineral H2O/Ce thermometry yields high temperatures of 1055 °C–1267 °C that represent the conditions of chemical metasomatism at the slab-mantle interface, significantly higher than those in generating the mantle sources of island arc basalts. On the other hand, lithochemical thermometry of the basalts yields much higher temperatures of 1456 °C–1522 °C that are indicative of the condition of mantle melting. While the substantial differences between these two temperatures indicates two-stage processes for origin of the continental basalts, the considerably higher metasomatic temperatures testify the dehydration melting of subducting oceanic slabs at postarc depths. These interpretations are verified by quantitative modeling of geochemical differentiation and mixing in an oceanic subduction zone. Therefore, the origin of intraplate basalts generally involves dehydration melting of subducting oceanic slabs at much greater depths than that of island arc basalts.

Early Cretaceous basalts record the modification of the North China Craton lithospheric mantle: implications for lithospheric thinning

ABSTRACT The modification of the Archaean lithospheric mantle root beneath the eastern North China Craton (NCC) has been noticed. However, the degree of modification and the characteristics of metasomatic agents for the NCC lithospheric mantle are still unclear. Here, we compile the geochemistry and 87Sr/86Sr and 143Nd/144Nd isotope ratios of Cretaceous primitive basalts (Yixian, Sihetun, Fangcheng and Feixian) from the eastern NCC and estimate the inputs of subduction-related elements into the Archaean lithospheric mantle beneath the NCC. These basalts show primitive (high MgO, Cr and Ni contents) and arc-like geochemical features (enrichments in LILEs and depletions in HFSEs) that indicate that the mantle sources were modified by fluids related to subducted crustal materials. Substantial proportions of subduction-mobile elements (e.g. ~95% Ba and ~90% Th) transferred to the basalt sources via hydrous melts. Thus, a large volume of fluids transferred into the lithospheric mantle. The data support the model that the NCC Archaean lithospheric mantle was weakened by hydrous melts, which resulted in a fusible weakened lithospheric mantle. Preliminary lithospheric thinning was induced by the extension of the NCC resulting from trench retreat of the Paleo-Pacific plate at ~140-120 Ma. Then, decompression melting of the lithospheric mantle caused pervasive melts in the weakened lithospheric mantle, resulting in the lithospheric mantle having a low viscosity comparable to the asthenospheric mantle, which principally reduced the lithospheric mantle.

Composition and provenance analysis of beach sands in an almost isolated sedimentary system – A field study of the Galápagos Archipelago

The Galápagos Archipelago is the surface expression of an active hotspot or long-lived mantle plume. The Archipelago consists of a group of 13 main islands which are located in the eastern central Pacific Ocean about 1,000 km west of the northern edge of the South American continent, east of the East Pacific Rise and south of the Galápagos spreading center. Because of the large distance to the nearest continental land mass, Galapagos can be seen as an almost isolated sedimentary system. A provenance study conducted on samples collected from seventeen beaches on eleven islands, demonstrates that mineral grains and particles were derived from weathering of predominantly basaltic rocks and were transported within the islands, between the islands or inside the coastal area around the Archipelago. The exclusion of external sources allows advanced studies about erosion processes, transport pathways of particles and the accumulation of autochthonous sediments. The combined usage of optical particle size and shape analysis with RAMAN spectroscopy allows a successful spatial delimitation of host rocks and a reconstruction of transport pathways. The analyzed samples can be subdivided into three groups: 1) Type-A sediments: fine-grained and sampled on beaches of the oldest islands in the eastern part of Galápagos. The composition of volcanic minerals corresponds to the alkaline character of the basaltic source rocks. 2) Type-B: well sorted sediments characterized by medium-grained olivine, pyroxene, plagioclase and even a small amount of quartz grains. The islands of this group are located in the central region of the Archipelago. 3) Type-C samples: olivine and pyroxene are the predominant volcanic minerals. These samples indicate bimodal, coarse-grained size distributions and large proportions of pumice and are found in Floreana in the south and the youngest islands Isabela and Fernandina in the west of Galápagos.

Pre-Variscan granitoids with adakitic signature at west Getic basement of the South Carpathians (Romania): constraints on genesis and timing based on whole-rock and zircon geochemistry

Research on two strata-like intrusions from Slatina-Timiş (STG) and Buchin (BG) at West Getic Domain of the South Carpathians (Semenic Mountains) identified granitoids with adakitic signature in a continental collision environment. Whole-rock geochemical composition with high Na2O, Al2O3 and Sr, depleted Y (18ppm) and HREE (Yb 1.8ppm) contents, high Sr/Y (40), (La/Yb)N (10) ratios and no Eu anomalies overlaps the High-Silica Adakites (HSA) main characteristics, though there are differences related to lower Mg#, heavy metal contents and slightly increased 87Sr/86Sr ratios. Comparison with HSA, Tonalite-Trondhjemite-Granodiorite (TTG) rocks and melts from experiments on basaltic sources suggests partial melting at pressures exceeding 1.25GPa and temperatures of 800-900ºC (confirmed by calculated Ti-in zircon temperatures) as the main genetic process, leaving residues of garnet amphibolite, garnet granulite or eclogite type. The adakitic signature along with geochemical variations observed in the STG-BG rocks indicate oceanic source melts affected by increasing mantle influence and decreasing crustal input that may restrict the tectonic setting to slab melting during a subduction at low angle conditions. An alternative model relates the STG-BG magma genesis to garnet-amphibolite and eclogite partial melting due to decompression and heating at crustal depth of 60-50km during syn-subduction exhumation of eclogitized slab fragments and mantle cumulates. The granitoids were entrained into a buoyant mélange during collision and placed randomly between two continental units. U-Pb zircon ages obtained by LA-ICP-MS and interpreted as Ordovician igneous crystallization time and Variscan recrystallization imprint are confirmed by trace-element characteristics of the dated zircon zones, connecting the STG-BG magmatism to a pre-Variscan subduction-collision event. The rich zircon inheritance reveals Neoproterozoic juvenile source and older crustal components represented by Neoarchean to Paleoproterozoic zircons.

Helium isotopes and olivine geochemistry of basalts and mantle xenoliths in Jeju Island, South Korea: Evaluation of role of SCLM on the Cenozoic intraplate volcanism in East Asia

Jeju Island, South Korea, is a Cenozoic intraplate volcano located in East Asia. The Jeju basalts display ocean island basalt (OIB)-like trace element patterns and enriched mantle type 2 (EM2) radiogenic isotope compositions, but the source that enriched the mantle remains unclear. Here we report new geochemical compositions of basalts and xenoliths, including the first helium isotope and element analysis of olivine phenocrysts to constrain the source of basalt. Olivines in the Jeju basalts have 3He/4He ratios of 3.5 to 7.3 Ra while olivines in the mantle xenoliths are 2.9 to 6.3 Ra. They provide no evidence of a lower mantle plume but overlap the range of the regional subcontinental lithospheric mantle (SCLM) and OIBs with low 3He/4He values. Olivine phenocrysts included in the basalts show variations in Mn, Ni, and Ca concentrations with forsterite (Fo) contents, similar to the trend for olivines crystallized from the pyroxenite-derived melts. Elemental ratios of Ca-Fe-Ni-Mg-Mn for the olivines also indicate the pyroxenite contribution. Considering our results, it is suggested that the main source of the Jeju basalts is the SCLM containing pyroxenite. In addition, the tomography image shows the low-velocity zone extending to the asthenosphere mantle beneath the central Jeju Island. This implies that the localized asthenospheric upwelling is caused by edge-driven convection, and the high-velocity zones exist around Jeju island indicating a thick cratonic lithosphere. Therefore, we propose that the interaction between the SCLM component containing pyroxenite and the rising asthenosphere is the main mechanism to generate enriched basaltic magmas for the Cenozoic intraplate volcanism in East Asia, including Jeju Island.

The Chicken and Egg Dilemma Linking Dunites and Chromitites in the Mantle–Crust Transition Zone beneath Oceanic Spreading Centres: a Case Study of Chromite-hosted Silicate Inclusions in Dunites Formed at the Top of a Mantle Diapir (Oman Ophiolite)

AbstractThe mantle–crust boundary beneath oceanic spreading centres is a major chemical and thermal interface on Earth. Observations in ophiolites reveal that it is underlined by a dunitic transition zone (DTZ) that can reach a few hundred meters in thickness and host abundant chromitite ore bodies. The dunites have been deciphered as essentially mantle-derived in most ophiolitic massifs; that is, reactional residues of interactions between peridotite and percolating melt(s). Although both dunite and chromitite in ophiolites have been the focus of many studies, the reasons for their systematic association remain unclear. In this study we have explored the inclusion content of the chromite grains disseminated in the dunites from the DTZ exposed in the Maqsad area of the Oman ophiolite where a former asthenospheric diapir is exposed. Similarly to chromite in chromitite ore bodies, disseminated chromite grains in dunites contain a great diversity of silicate inclusions. Based on the major and minor element composition of 1794 single silicate inclusions in chromites from 285 samples of dunite and associated rocks in the DTZ, we infer that the disseminated chromites formed by a similar ‘metallogenic’ process to the chromitites, and that, as a whole, dunites from the DTZ actually represent the low-grade end-member of a single, giant ore body. The nature of the silicate inclusions (amphibole and mica among others) enclosed in chromite grains in dunites from the Maqsad DTZ precludes their crystallization from an anhydrous primitive basaltic melt, and rather calls for a crystallization from a melt hybrid between common mafic melts and more exotic Si-, Na- and volatile-rich fluids. The hybrid parent medium of both dunites and chromitites results from the interaction between an asthenospheric diapir (the mid-ocean ridge basalt source), and a colder, altered lithospheric lid and hydrothermal fluids responsible for this alteration. The excess silica in the hybrid melt is provided by the incongruent dissolution of enstatite from mantle harzburgite and/or from moderate degree of partial melting of the altered gabbroic crust. The chemical composition of the silicate inclusions is more variable when enclosed in the disseminated chromites than in the chromitites, suggesting a greater variability of melt and/or fluid fractions involved in the genesis of dunites than of chromite ores. Finally, the DTZ can be viewed as a metamorphic contact aureole between episodically rising asthenospheric diapirs and formerly accreted axial lithospheric lids. Our conclusion about the chicken and egg dilemma linking dunites and chromitites beneath oceanic spreading centres (i.e. do the chromitites form in response to the formation of dunites or conversely?) is that the mantle dunitization itself is a potential process for the release of Cr and its re-concentration as chromite ores, and that in turn the competition between orthopyroxene (± plagioclase) and chromite fractionation during this fluid–melt–peridotite reaction process is responsible for the great mineralogical and chemical variability of the DTZ dunites.

Barium isotope evidence for recycled crustal materials in the mantle source of continental basalts

To investigate the potential of using Barium (Ba) isotopes to trace recycled crustal materials, we report high-precision Ba isotopic compositions for 22 well-characterized samples of Cenozoic basalts from the Taihang Mountains, North China Craton. The basalts have δ138/134Ba [= (138Ba/134Basample/138Ba/134BaSRM3104a – 1) × 1000] of −0.06‰ to +0.03‰(±0.05‰，2SD), lower than the depleted upper mantle (+0.03‰ to +0.15‰). Because Ba isotope fractionation during basaltic magmatic differentiation is negligible, the light Ba isotopic compositions likely reflect the involvement of isotopically light crustal materials in the mantle source. The Ba isotopic variations, combined with the data of elements and SrNd isotopes of the basalts, indicate that the light Ba isotopic compositions mainly resulted from marine sediments rather than altered oceanic crust. The quantitative modeling using Sr-Nd-Ba isotopic compositions suggests that about 0.5% to 3% of marine sediments were involved in generating the continental basalts. Our study here highlights the potential of using Ba isotopes to trace recycled materials in the mantle.

Combined Lithophile‐Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources

AbstractDetection of Hadean isotopic signatures within modern ocean island basalts (OIB) has greatly influenced understanding of Earth's earliest history and long‐term dynamics. However, a relationship between two isotopic tools for studying early Earth processes, the short‐lived 146Sm‐142Nd and 182Hf‐182W systems, has not been established in this context. The differing chemical behavior of these two isotopic systems means that they are complementary tracers of a range of proposed early Earth events, including core formation, magma ocean processes, and late accretion. There is a negative trend between 142Nd/144Nd and 182W/184W ratios among Réunion OIB that is extended by Deccan continental flood basalts. This finding is contrary to expectations if both systems were affected by silicate differentiation during the lifetime of 182Hf. The observed isotopic compositions are attributed to interaction between magma ocean remnants and Earth's core, coupled with later assimilation of recycled Hadean mafic crust. The effects of this scenario on the long‐lived 143Nd‐176Hf isotopic systematics mirror classical models invoking mixing of recycled trace‐element enriched (sedimentary) and depleted (igneous) domains in OIB mantle sources.If the core provides a detectible contribution to the tungsten element budget of the silicate Earth, this represents a critical component to planetary‐scale tungsten mass balance. A basic model is explored that reconciles the W abundance and isotopic composition of the bulk silicate Earth resulting from both late accretion and core‐mantle interaction. The veracity of core‐mantle interaction as proposed here would have many implications for long‐term thermochemical cycling.

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.

Mare basalt flooding events surrounding Chang'e-4 landing site as revealed by Zhinyu crater ejecta

Von Kármán crater's floor was flooded with mare basalts during the Imbrian period. This site is the target of China's ongoing Chang'e-4 mission that includes the Yutu-2 rover and its instrumental payload. The Zhinyu crater, one of the largest craters within a few tens of kilometers from the landing site, is the product of a fresh impact that excavated subsurface, basaltic materials onto the surface. The compositional characteristics of the continuous ejecta around the crater vary radially with distance, suggesting possible mineral heterogeneity at depth. At least three main mare flooding phases could have occurred within Von Kármán, producing a basalt layer of at least 320 m in thickness. However, a broadly consistent olivine composition (Fo#: 55) of the three proposed basalt layers suggests that the sources of the mare basalts would have been similar in composition, perhaps even consisting of a single magma chamber where magma had time to evolve. The Yutu-2 rover that is edging its way westwards, could in principle test this hypothesis, thus further constraining the thermal history of the Von Kármán crater.

U–Pb–Hf–O–Nd isotopic and geochemical constraints on the origin of Archean TTG gneisses from the North China Craton: Implications for crustal growth

Tonalite–trondhjemite–granodiorite (TTG), an Archean dominant lithological assemblage within the cratonic provinces, is believed to have been formed by partial melting of the hydrous basalts in the garnet stability field. The origin of TTG is a window to decipher the evolution of Archean continental crust, whereas the petrogenesis is not well constrained. To solve this issue, we have carried out field observations, zircon U–Pb–Hf–O isotope, as well as whole-rock Nd–O isotope and geochemical analyses on the TTG and associated dioritic gneisses from the North China Craton. U–Pb isotopic dating using the LA–ICP–MS on zircons reveals that the granitoid gneisses in northern Liaoning were contemporaneously generated during ca. 2.56–2.51 Ga and immediately experienced a regional metamorphic event at 2.48–2.45 Ga. The dioritic gneisses occur as quenched inclusions with rounded to ellipsoidal globules in shape rather than lithospheric relics. Such occurrence, together with isotopic and geochemical data, suggests that the mafic enclaves and host TTG represent two co-existing, compositionally distinct magmas. The TTG gneisses are featured by elevated SiO2 (64.72–86.46 wt%) contents, (La/Yb)N (68 on average) and Sr/Y (119 on average) ratios, and low MgO (0.35–2.58 wt%) contents and Mg# values, with radiogenic Hf and Nd isotopes (εHf(t) = +3.5 to +9.0 and εNd(t) = +0.8 to +5.4), indicating the derivation from partial melting of the juvenile basaltic sources at lower crustal levels. Compared with the TTG gneisses, the dioritic gneisses have lower SiO2 contents, higher MgO contents and Mg# values, with similar Hf–Nd isotopes (εHf(t) = +3.6 to +9.5 and εNd(t) = +3.3 to +5.1), implying the depleted mantle source. A model is proposed whereby partial melts from juvenile basaltic sources evolved via mixing with mafic enclaves (viz. dioritic magma) and fractional crystallization, and finally were contaminated by the sedimentary rocks, forming the TTG magma. The granitoid gneisses in northern Liaoning witnessed the major vertical growth of continental crust at the end of the Neoarchean, which is supported by Hf–Nd–O isotopic data and zircon Hf–O isotopic modeling. Our results show that the geochemistry of TTG (such as elevated La/Yb and Sr/Y) might be affected by complex petrogenetic processes, i.e., partial melting of individual sources, magma mixing, fractional crystallization, and crustal contamination, rather than solely controlled by the P–T conditions of crustal anataxis as previously thought, which might have been also overlooked in other Archean cratons.

Appraisal for the environment, weathering and provenance of Upper Cretaceous-Lower Tertiary shales, Western Desert, Egypt

The combination of the mineralogical and geochemical composition of shales is considered a key to decipher their environment evolution, weathering, climatic conditions, and provenance. The Upper Cretaceous-Lower Tertiary succession is extensively dispersed in Egypt. The present work is devoted to studying the Dakhla shales of (Maastrichtian- Danian), Duwi (Campanian) and Quseir (pre-Campanian) formations. Chemical and mineralogical analyses were conducted by using seven representative surface sections from Dakhla Oasis. Dakhla shales are dominates by Smectite and kaolinite. The average percentages of SiO2, Al2O3, CaO, MgO, Na2O and K2O are subordinate values. In contrast, Al2O3 contents of Dakhla shale, TiO2, P2O5 and Fe2O3, contents are relatively higher than common shale composition. The CIA and CIW high values due to clay minerals and the absence of feldspars. This is also confirmed by the smectite domination and subordinate kaolinite in these formations. The ICV values for Dakhla and Duwi shales are 0.59 and 0.74 (ICV &lt; 1). The shale is mature and deposited in a quiescent environment. Whereas the ICV for Quseir shale is 1.24, it can be incidental that it is immature. The shale is wholly detrital and a product of moderate to intensive weathering. The provenance was constituted of granitic and basaltic source rocks, and the parent provenance is basalt. Consequently, the shale is deposited under fluvio-marine environments, and the prevailed condition was of alkaline chemical affinity probably passed through different environments varying from a fluvial, eolian and shallow marine.

Remnants of early Earth differentiation in the deepest mantle-derived lavas

The noble gas isotope systematics of ocean island basalts suggest the existence of primordial mantle signatures in the deep mantle. Yet, the isotopic compositions of lithophile elements (Sr, Nd, Hf) in these lavas require derivation from a mantle source that is geochemically depleted by melt extraction rather than primitive. Here, this apparent contradiction is resolved by employing a compilation of the Sr, Nd, and Hf isotope composition of kimberlites-volcanic rocks that originate at great depth beneath continents. This compilation includes kimberlites as old as 2.06 billion years and shows that kimberlites do not derive from a primitive mantle source but sample the same geochemically depleted component (where geochemical depletion refers to ancient melt extraction) common to most oceanic island basalts, previously called PREMA (prevalent mantle) or FOZO (focal zone). Extrapolation of the Nd and Hf isotopic compositions of the kimberlite source to the age of Earth formation yields a 143Nd/144Nd-176Hf/177Hf composition within error of chondrite meteorites, which include the likely parent bodies of Earth. This supports a hypothesis where the source of kimberlites and ocean island basalts contains a long-lived component that formed by melt extraction from a domain with chondritic 143Nd/144Nd and 176Hf/177Hf shortly after Earth accretion. The geographic distribution of kimberlites containing the PREMA component suggests that these remnants of early Earth differentiation are located in large seismically anomalous regions corresponding to thermochemical piles above the core-mantle boundary. PREMA could have been stored in these structures for most of Earth's history, partially shielded from convective homogenization.