Ocean Island Basalts Research Articles

Carbonatite, aillikite and olivine melilitite from Zandkopsdrift, Namaqualand, South Africa: Constraints on the origin of an unusual lamprophyre-dominated carbonatite complex and the nature of its mantle source

We present a chemical and stable and radiogenic isotope data set for carbonatite, aillikite (ultramafic lamprophyre) and olivine melilitite from the Zandkopsdrift carbonatite complex in Namaqualand, South Africa. This carbonatite complex is associated with the ≈500-km-long, NE-SW oriented, age-progressive (40–80 Ma) Namaqualand-Bushmanland-Warmbad ultramafic alkaline magmatic lineation that consists mainly of olivine melilitite and kimberlite. Zandkopsdrift has a radiometric age of 55 ± 4 Ma and is unusual among southern African carbonatites because the dominant rock type exposed is aillikite (a variety of carbonated ultramafic lamprophyre), with carbonatite occurring mainly as a network of subvertical dykes in the southwest of the complex and olivine melilitite occurring as meter-sized blocks of float derived from the subsurface. Calcite and ferroan dolomite carbonatites are present but Mg-rich dolomitic carbonatite varieties are lacking. All rock types appear to have experienced at least moderate differentiation and all show elevated δ18OSMOW values for carbonate minerals, whereas phlogopite and clinopyroxene from these rock types fall near mantle values (with ranges of +13.3 to +21.8‰ and + 4.9 to +5.4‰ for carbonates and silicates, respectively), indicative of interaction with hydrous fluids at low-temperature. Sr-Nd-Pb isotope compositions indicate moderate to strong HIMU signatures (e.g., 206Pb/204Pbi = 19.4 to 21.0) with limited SrNd isotope variation and subvertical NdHf isotope arrays extending well below the ocean island basalt field and terrestrial array. Uncertainty regarding the differentiation history of the three rock types complicates estimation of primary magma compositions but strong chemical similarities between the carbonatites and aillikites, plus their close physical intermingling, strongly suggests that they are related by fractional crystallization to a common parental magma, whereas their relationship to the melilitites is less straightforward. The aillikites and melilitites have compositions of the REE and other elements similar to near-primary versions of these rock types from other alkaline igneous provinces globally, which have been explained as the products of low degrees of partial melting at 4–7 GPa (for aillikites) and slightly higher degrees of melting at 3–3.5 GPa (for melilitites). Lower pressures of generation and distinct trace element characteristics of the Zandkopsdrift melilitites could be explained by these originating as melts of a garnet wehrlite halo around the magmatic conduit formed by metasomatic reaction between carbonated melts and lithospheric peridotite. Most Zandkopsdrift carbonatites have high Lu/Hf ratios which, combined with their relatively low 176Hf/177Hf values, suggests that their mantle sources could not have formed more than 30–50 Myr prior to emplacement of the complex. This implies that a mantle plume had a role in their formation, either as a direct source of melts or as a source of heat with which to melt pre-existing metasomatic heterogeneities.

Geochemistry and new zircon U–Pb geochronology of Mesoproterozoic Punugodu granite pluton, SE India: implications for anorogenic magmatism along the western margin of Nellore Schist Belt, India

AbstractWe report a new zircon U–Pb age of 1257 ± 6 Ma for the Punugodu granite (PG) pluton in the Eastern Dharwar Craton (EDC), Southern India. The Mesoproterozoic PG is alkali feldspar hypersolvus granite emplaced at shallow crustal level, as evident from the presence of rhyodacite xenoliths and hornfelsic texture developed in the metavolcanic country rocks of the Neoarchaean Nellore Schist Belt (NSB). Geochemically, the PG is metaluminous, ferroan and alkali-calcic, and is characterized by high SiO2 and Na2O + K2O, Ga/Al ratios >2.6, high-field-strength elements and rare earth element (REE) contents with low CaO, MgO and Sr, indicating its similarity to anorogenic, alkali (A-type) granite. The highly fractionated REE patterns with negative europium anomalies of PG reflect its evolved nature and feldspar fractionation. Mafic (MME) to hybrid (HME) microgranular enclaves represent distinct batches of mantle-derived magmas that interacted, mingled and undercooled within the partly crystalline PG host magma. Felsic microgranular enclaves (FME) having similar mineralogical and geochemical characteristics to the host PG most likely represent fragments of marginal rock facies of the PG pluton. The PG appears to be formed from an oceanic island basalt (OIB)-like source in an anorogenic, within-plate setting. The emplacement of PG (c. 1257 Ma) in the vicinity of Mesoproterozoic Kanigiri Ophiolite (c. 1334 Ma) shows an age gap of nearly 77 Ma, which probably suggests PG emplacement in an extensional environment along a terrain boundary at the western margin of the Neoarchaean NSB in the EDC.

Decoupled Zn-Sr-Nd isotopic composition of continental intraplate basalts caused by two-stage melting process

Ocean island basalts (OIBs) with Zn isotopic ratios higher than the normal mantle (δ66Zn = 0.17 ± 0.08‰) or mid-ocean ridge basalts (MORBs; δ66Zn = 0.27 ± 0.06‰) generally also have an enriched Sr-Nd isotopic signature, suggesting carbonate-bearing eclogites, whose protolith is inferred to be subducting altered oceanic crust, in their mantle source. On the contrary, continental intraplate basalts with high δ66Zn usually show depleted Sr-Nd isotopic signatures (i.e., decoupled Zn-Sr-Nd isotopic composition). To elucidate the origin of the decoupled Zn-Sr-Nd isotopic composition in continental intraplate basalts, we report the discovery of both coupled and decoupled Zn-Sr-Nd isotopic data for a suite of Cenozoic continental intraplate basalts from the Zhejiang province, Southeast China. These basalts display clear spatial and temporal geochemical variations, with early-stage inland low-silica samples presenting moderately enriched Sr-Nd isotopic signatures and high δ66Zn (coupled Zn-Sr-Nd isotopic composition, similar to OIBs), and later-stage coastal high-silica samples that display a pronounced δ66Zn decrease with increasing SiO2 and 87Sr/86Sr and with decreasing alkali contents and 143Nd/144Nd (decoupled Zn-Sr-Nd isotopic composition). The early-stage basalts with coupled high Zn-Sr-Nd isotopic signatures are also more enriched in incompatible elements than any other basalts from eastern China reported so far. We explain the spatial and temporal geochemical variations of these basalts as the result of two main melting events: 1) the low-silica early-stage magmatism mostly occurs inland and results from high-pressure partial melting of a carbonated eclogite-bearing asthenospheric mantle. Because of the presence of a thick lithosphere limits the melting of the depleted mantle component, the signature of the Zn-Sr-Nd isotopically enriched, and more fusible carbonated eclogite is preserved. 2) At the later stage, magmatism mostly occurs on the coast where the subcontinental lithosphere is thinner. Hence, decompression melting progresses to shallower depth, resulting in an increase of the contribution from the depleted peridotite matrix and a dilution of the signal from the isotopically enriched fusible component. Further upwelling and in-situ melting at the base of the subduction-modified sub-continental lithospheric mantle (SCLM) explains both the decoupled Zn-Sr-Nd isotopic signature of the coastal basalts and their major and trace element variability. We further propose that decompression melting is driven by small-scale convection resulting from variations of lithospheric thickness. Our data highlight the importance of dynamic melting of carbonated eclogite-bearing asthenosphere and subsequent lithospheric melting in preservation and destruction of the coupled enriched Zn-Sr-Nd isotopic signature of carbonated eclogite component and generation of the apparent decoupled Zn-Sr-Nd isotopic signal commonly observed in continental intraplate basalts.

Compositional signatures of ophiolitic rocks from the Dongco massif: Novel insights into the evolution of the central Tibetan Meso-Tethyan oceanic plateau

The role of the Meso-Tethyan oceanic plateau in the growth of the Tibetan Plateau is debated. In this paper, we report coexistence of large-scale mid-ocean ridge basalt (MORB)-like and enormous ocean island basalt (OIB)-type mafic rocks from the Dongco ophiolitic rocks of the Bangong–Nujiang Suture Zone. The MORB-like rocks host tholeiitic geochemical characterizes, and have supra-subduction zone affinity such as enrichments in large ion lithophile elements (LILEs), depletions in high field strength elements, high εNd(t) values (+6.5 to +9.5) and low initial 87Sr/86Sr ratios (0.7033–0.7050). The OIB-type rocks have an alkaline affinity and are characterized by enrichments in LILEs and light rare earth elements, strongly negative Sr anomalies, variable εNd(t) values of −3.7 to +7.3, and low initial 87Sr/86Sr ratios ranging from 0.7034 to 0.7050. Metamorphic zircons from amphibolites in the sole of the ophiolitic rocks yielded UPb ages of 104–100 Ma. Detrital zircons from the upper Cretaceous sandstone of the Abushan Formation (K2ab) yielded young UPb ages of 100–86 Ma. Considering the regional geology, we suggest that the Dongco MORB-like rocks were derived from a depleted mantle source and formed from an intra-oceanic subduction-related setting, representing remnants of normal oceanic crust, whereas the OIB-type rocks were derived from an enriched mantle source representing remnants of a Meso-Tethyan oceanic plateau. Our preferred hypothesis is that the Meso-Tethyan oceanic plateau experienced a phase of accretion at 104–100 Ma, which probably increased local subduction erosion of the upper plate and providing provenance for the deposition of the K2ab sandstone. The maximum depositional ages of K2ab sandstone constraint the minimum ages for closure of the Bangong–Nujiang Tethys Ocean most likely at 100–86 Ma.

A quantitative framework for global variations in arc geochemistry

Arc magmas are generated by partial melting of a mantle wedge modified by inputs from subducting sediment and igneous ocean crust. Despite this uniform tectonic process, global arc-front stratovolcano compositions exhibit trace element abundances that can vary by more than an order of magnitude. All incompatible elements correlate well, including those that are thought to be wedge derived, “fluid-mobile”, and sediment derived. It has been suggested that global variations in arc trace element abundances occur due to the addition of variable slab components to a depleted mantle wedge, though quantitative global models that include both trace elements and isotopes have been lacking. Here we present an alternative framework in which a relatively constant proportion of slab material, consisting of melts both of altered ocean crust and sediment, is added to ambient mantle wedge that varies from more depleted than MORB to as enriched as “EM1” ocean island basalt sources. The modified mantle melts to varying extents, with melting controlled primarily by the thickness of the overlying lithosphere. Thick lithosphere leads to lower extents of melting in the garnet stability field in most continental arcs. Continental arcs also have enriched ambient mantle, likely because of contributions from enriched sub-continental lithospheric mantle. High degrees of melting of more depleted sources cause oceanic arcs to have low incompatible element abundances. Quantitative modeling of these processes shows that this framework can account for the global arc systematics exhibited by arc-front stratovolcanoes and provides a tool to examine regional variations and more unusual magma compositions. Variations in slab temperature and amount of slab input remain important for second-order variations and certain element ratios.

Ancient and recycled sulfur sampled by the Iceland mantle plume

Stable sulfur isotope ratios of mid-ocean ridge and ocean island basalts (MORBs and OIBs) preserve unique information about early Earth processes and the long-term volatile cycles between Earth's mantle and the surface. Icelandic basalts present ideal material to examine the oldest known terrestrial mantle reservoir, accessed through a deep-rooted mantle plume, but their multiple sulfur isotope systematics have not been explored previously. Here, we present new sulfur concentration (30–1570 ppm) and isotope data (δ34S = −2.5 to +3.8‰ and Δ33S = −0.045 to +0.016‰; vs. Canyon Diablo Troilite) from a sample suite (n = 62) focused on subglacially erupted basaltic glasses obtained from Iceland's neovolcanic zones. Using these data along with trace element systematics to account for the effects of magmatic processes (degassing and immiscible sulfide melt formation) on δ34S, we show that primitive (MgO > 6 wt.%), least degassed glasses accurately record the δ34S signatures of their mantle sources. Compared to the depleted MORB source mantle (DMM; δ34S = −1.3±0.3‰), the Iceland mantle is shown to have a greater range of δ34S values between −2.5 and −0.1%. Similarly, Icelandic basalts are characterized by more variable and negatively shifted Δ33S values (−0.035 to +0.013‰) relative to DMM (0.004±006‰). Negative δ34S-Δ33S signatures are most prominent in basalts from the Snæfellsnes Volcanic Zone and the Kverkfjöll volcanic system, which also have the lowest, most MORB-like 3He/4He (8–9 R/RA, where RA is the 3He/4He of air) and the highest Ba/La (up to 12) in Iceland. We propose that subduction fluid-enriched, mantle wedge type material, possibly present in the North Atlantic upper mantle, constitutes a low-δ34S-Δ33S component in the Icelandic mantle. This suggests that volatile heterogeneity in Iceland, and potentially at other OIBs, may originate not only from diverse plume-associated mantle components, but also from a heterogeneous ambient upper mantle. By contrast, a set of samples with high 3He/4He (up to 25.9 R/RA) and negative μ182W anomalies define an ancient lower mantle reservoir with a near-chondritic Δ33S and δ34S signature of ∼0‰. The difference between DMM and the high high-3He/4He mantle may reflect separate conditions during core-mantle differentiation, or a previously unidentified flux of sulfur from the core to the high-3He/4He reservoir.

Episodic continental extension in eastern Gondwana during the mid-late mesozoic: insights from geochronology and geochemistry of mafic rocks in the Tethyan Himalaya

ABSTRACT Mesozoic extension and rifting processes of the Gondwana continent are critical for understanding the opening and formation of the Indian Ocean. Here, we report petrological, geochemical, zircon U–Pb age, and Lu–Hf isotopic data of mafic dikes in the central Tethyan Himalaya to reveal the timing and mechanism of the eastern Gondwana rifting. These mafic rocks exhibit two groups in terms of TiO2 and MgO contents (Group I with TiO2 < 2.0 wt.%, whereas Group II with TiO2 > 2.0 wt.%), but (1) they have similar trace elements patterns and exhibit enriched Light Rare Earth Element (LREE) patterns without Eu anomalies and (2) they have a wide range of zircon εHf(t) values. Both the above geochemical characteristics are similar to those of the Oceanic Island Basalt (OIB). However, they also show typical features of continental crust input. Compared to Group II, Group I has higher MgO, Cr, and Ni abundances (more primitive) and more depleted Nb-Ta contents, which are similar to those of lower TiO2 Dala mafic rocks, suggesting that the continental crust signature was mainly inherited from the Greater India subcontinental lithospheric mantle (GI-SCLM). Therefore, the mafic rocks here were mainly derived from a hybrid source from both OIB-like enriched mantle and GI-SCLM. Furthermore, two groups of zircon U–Pb ages have been identified: the first group with a weight mean age of 139.9 ± 0.2 Ma, whereas the second group with a weight mean age of 163.2 ± 0.9 Ma. The Middle Jurassic zircons show similar characteristics to those of intermediate to acidic igneous rocks in the continental setting, and thus they might record a stage of magmatism associated with a tectonic extension event in the Indian passive continental margin. The Early Cretaceous ages represent the crystallization timing of the mafic dikes, which are coeval with most mafic rocks in the Tethyan Himalaya. Based on these observations and the literature data in the Tethyan Himalaya, we proposed that the Kerguelen plume was incubating underneath the Tethyan Himalaya at ca. 140 Ma, but it was not located in the triple junction among the Antarctic, Australian and Indian plates. Therefore, the Kerguelen plume might play a synergistic role in the break-up of eastern Gondwana, which had experienced at least two stages of tectonic extension during the Mid-Late Mesozoic.

The partitioning of Cu and Ag between minerals and silicate melts during partial melting of planetary silicate mantles

Geochemical and cosmochemical reservoirs show Cu/Ag variations from 500 to 40,000. To understand magmatic Cu–Ag fractionation and origins of such large Cu/Ag variations, we have measured the partition coefficients of Cu and Ag between minerals and silicate melts (DCu,AgMin/SM) relevant for partial melting of the silicate mantles of Earth, Mars, and the Moon. The experiments were conducted at 1–3 GPa, 1300–1600 °C, and oxygen fugacity ∼2–5 log units below the FMQ buffer. The results show that DCuMin/SM are 0.034–0.109 for olivine, 0.011–0.044 for orthopyroxene, and 0.046–0.092 for clinopyroxene; DAgMin/SM are 0.003–0.012 for orthopyroxene and 0.037–0.093 for clinopyroxene. These DCu,AgMin/SM values increase mainly with increasing pressure and are oxygen fugacity-independent. DAgMin/SM are 0.0005–0.0017 for olivine, which do not show measurable dependence on P–T, oxygen fugacity, or silicate composition. One pair of DCu,AgMin/SM values are 0.0037 and 0.0004 for garnet, respectively; 0.15 and 0.005 for spinel, respectively. These results indicate that Ag is more incompatible than Cu in all mantle minerals. Our DCu,AgMin/SM combined with previous sulfide–silicate melt partition coefficients of Cu and Ag imply that although sulfides host large fractions of planetary mantle Cu and Ag, silicate minerals can be also important Cu and Ag reservoirs, particularly when sulfides occur as monosulfide solid solutions. The application of our DCu,AgMin/SM to various melting models demonstrates that Cu–Ag fractionation occurs during partial melting of the silicate mantles of Earth, Mars, and the Moon; however, the modeled Cu/Ag ratios in basalts agree with their mantle source Cu/Ag ratios within 50% relative. Therefore, the superchondritic but similar Cu/Ag ratios of ∼3000–4000 in mid-ocean ridge basalts, arc basalts, mantle plume-related oceanic island basalts and plateau basalts, and the bulk silicate Earth (BSE) reflect homogeneous distribution of Cu/Ag in Earth’s mantle and limited Cu–Ag fractionation during slab dehydration/melting in subduction zones. However, components with chondritic Cu/Ag ratios (∼600–2300) may exist in the mantle sources of Hawaii oceanic island lavas. The Cu/Ag ratios (∼500–1100) in the Martian basalts and bulk silicate Mars (BSMars) are chondritic, but the Cu/Ag ratio (∼15,000–40,000) in the bulk silicate Moon (BSMoon) is strongly superchondritic. Such largely different Cu/Ag ratios in different planetary reservoirs cannot be explained by magmatic Cu–Ag fractionation, solar nebular process, or core-formation process. We propose that different degrees of planetary melting and evaporation provide a solution because Ag is more volatile than Cu during silicate melt evaporation. The chondritic Cu/Ag ratios in the BSMars could be explained by limited evaporation of planetesimals that delivered Mars’ moderately volatile elements, and the superchondritic Cu/Ag ratios in the BSE may be due to Earth’s accretion of planetesimals with partial evaporative loss of Cu and Ag. However, high degrees of evaporative Cu and Ag loss during the Moon-formation are required to explain the strongly superchondritic Cu/Ag ratios in the BSMoon. Accordingly, Cu/Ag ratios in the bulk silicate planets may provide new insights into the nature of their building blocks.

The origin of nitrogen in Earth's mantle: Constraints from basalts 15N/14N and N2/3He ratios

Plate tectonics is thought to be a major driver of volatile redistribution on Earth. The budget of nitrogen in Earth's mantle has been suggested to be almost entirely surface-derived. Recycling would contribute nitrogen with relatively heavy 15N/14N isotope ratios to Earth's mantle. This could explain why the Earth's mantle 15N/14N isotope ratio is substantially higher than both solar gases and chondritic parent bodies akin to enstatite chondrites. Here, published nitrogen isotope data of mid-ocean ridge and ocean island basalts are compiled and used to evaluate the nitrogen subduction hypothesis. Nitrogen isotope ratios are used in conjunction with published N2/3He and K2O/TiO2 ratios on the same basalts. Assuming that 3He is not recycled, N2/3He ratios are argued to trace nitrogen addition to mantle sources via subduction. Various mantle source enrichments for basalts are tracked with K2O/TiO2 ratios: elevated K2O/TiO2 ratios are assumed to primarily reflect the contributions of recycled components in the basalts mantle sources. The main result of our data compilation is that for most basalts, δ15N and N2/3He remain constant across a vast range of K2O/TiO2 ratios. Mid-ocean ridge basalts have δ15N signatures that are lower than air by ~4‰ and an average N2/3He ratio of 3.7 (±1.2) x106 (95% confidence, n = 30). Published δ15N and N2/3He are invariant across K2O/TiO2 ratios that vary over a factor of ~20. Using estimates of slab K2O/TiO2 and [TiO2], the observed invariant δ15N and N2/3He may be fit with slabs containing ~0.1 ppm N. A mass balance shows that adding ~10% recycled slabs to the convective mantle only raises the N2/3He by <5%. Lavas from Iceland, Galapagos and Hawaii have high 3He/4He and 15N/14N ratios relative to the convective mantle. Only seven samples show nitrogen isotopic signatures that are unaffected by air contamination, although those samples are poorly characterized for N2/3He. The seven basalts show δ15N between −2 and 0‰ that do not vary systematically with K2O/TiO2 ratios that vary over a factor of ~5. The N2/3He ratios of these seven basalts is unknown, but the high 3He/4He mantle may be estimated by combining published N2/36Ar to 3He/36Ar ratios. This yields a N2/3He of 2.3 (±1.2) x 106 (1σ uncertainty). This is indistinguishable from the MORB estimate of 3.7 (±1.2) x 106. Invariant δ15N across variable degrees of mantle enrichments and MORB-like N2/3He for the high 3He/4He mantle are not consistent with nitrogen addition to plume sources with elevated 3He/4He ratios. A δ15N between −2 and 0‰ for plume sources, only marginally higher than MORB, could be a primordial feature of undegassed mantle reservoirs. Nonetheless, nitrogen subduction may have contributed to a specific array of mantle sources, as revealed by the few published data on basalts with low 3He/4He ratios. Lavas from the Society plume with low 3He/4He ratios show an enriched mantle source, and they have elevated δ15N ≥ +0.5‰ and N2/3He > 107. For those, the addition of slabs with concentrations of ~0.1 ppm N to a mantle source can account for the integrated dataset. To summarize, the published data suggest that nitrogen subduction may explain a sub-set of published N isotope data on basalts, but that N recycling has an overall more limited impact on mantle nitrogen than previously thought.

Origin of low-MgO primitive intraplate alkaline basalts from partial melting of carbonate-bearing eclogite sources

Alkaline basalts occur widely in intraplate settings and carbonate-bearing mantle sources such as carbonated peridotites are increasingly regarded to play a key role in their formation. Carbonated eclogites, most likely the products of subducted carbonate-bearing altered oceanic crust, are probable alternative ingredients in the mantle sources of many intraplate alkaline basalts, highlighting the importance of the subduction-driven deep carbon cycle. However, this widely proposed hypothesis remains enigmatic because the recognition of low-MgO primitive alkaline basalts predicted by experiments is scarce. Here we show that Cenozoic continental intraplate alkaline basalts occurring above the stagnant oceanic slab in the mantle transition zone beneath the Hannuoba region, eastern China, display geochemical features consistent with their origin as low-degree partial melts of carbonate-bearing eclogites. Their MgO contents correlate positively with CaO, Ba/Th and Ti/Eu, but negatively with Dy/Yb and εNd. Remarkably, the most primitive alkaline basalts are characterized by low MgO (<5.25 wt.%), low heavy rare earth elements and Sc contents, low CaO/Al2O3 (<0.41), low Ti/Eu (<3380), but Dy/Yb (>7.1) higher than those of ocean island basalts (OIBs). These features cannot be ascribed to differentiation from high-MgO alkaline basalts because significant amounts of crystallization of clinopyroxene and garnet did not occur during ascent. Differentiation also cannot account for the correlations of time-integrated Sr-Nd isotopes with MgO, Dy/Yb and Ba/Th. Instead, the linear correlations mainly reflect strong interaction between ascending primitive alkaline melts and the lithospheric mantle. The compositions of primitive alkaline basalts reflect the key control of garnet and clinopyroxene in the mantle residue (eclogites), and the Ti, Zr and Hf anomalies further indicate the critical effect of carbonates in the eclogite source. Partial melting of the carbonate-bearing eclogites likely occurred in the uppermost asthenosphere. The production of alkaline basalts with low MgO contents by partial melting of carbonate-bearing eclogite below the peridotite solidus in an intraplate setting has been overlooked and the magmas were instead often considered to be highly evolved. Recycled altered oceanic crust thus may not only cause metasomatism of the deep mantle but may also serve as a direct source of mafic melts. These results on natural rocks support the experiment-based model for subducted altered oceanic crustal material and also indicate its diverse fate in the mantle.

Highly Oxidising Conditions in Volatile-Rich El Hierro Magmas: Implications for Ocean Island Magmatism

Abstract Recent studies investigating magmatic volatile contents indicate widespread enrichment of carbon, sulfur, and halogens in ocean island basalts (OIBs). At El Hierro in the Western Canary Islands, magmas with exceptionally high CO2 and S contents have been erupting throughout the Holocene. High S content of up to 5200 ppm requires an oxidised mantle source, but estimates of initial magmatic oxygen fugacity (fO2) are sparse. Here, we present estimates of fO2 and magmatic temperature for El Hierro together with a global mantle potential temperature dataset to evaluate redox and temperature conditions in the early stages of melt evolution for volatile-rich OIBs. Oxygen fugacities calculated using vanadium partitioning between melt inclusions (MIs) and their olivine hosts are &amp;gt;FMQ + 2.9 (2.9 log10 units above the fayalite-magnetite-quartz buffer), indicating that El Hierro magmas are highly oxidised. MI and matrix glass sulfur speciation data record fO2 between FMQ-1 to FMQ + 2; these values strongly depend on the position of the S2− to S6+ transition relative to the FMQ buffer. Nonetheless, glass sulfur speciation data record lower oxygen fugacity than V partitioning data, indicating MIs were able to maintain Fe3+/ΣFe and S6+/ΣS equilibrium with the surrounding melt during their evolution. The high fO2 of El Hierro magmas is coupled with an average mantle potential temperature estimate of 1443 ± 66°C (1σ, n = 17) for the broader Canary Islands, which is slightly higher than the average potential temperature estimated for adjacent mid-ocean ridge segments (1427 ± 33°C, 1σ, n = 474), albeit the two values are well within error. We find that ~98% of Canary Island rock compositions are not suitable for calculation of mantle potential temperatures using currently available methods. This is caused by the presence of substantial pyroxenite and volatile-enriched peridotite mantle domains under the Canary Islands. A wider compositional calibration of various petrological models is necessary to precisely determine mantle potential temperatures for volatile-rich alkali basalts. Our high oxygen fugacity estimates for El Hierro magmas reflect the fertile, fusible, and volatile-enriched nature of the mantle source beneath the Western Canary Islands.

Mantle Hg isotopic heterogeneity and evidence of oceanic Hg recycling into the mantle

The geochemical cycle of mercury in Earth’s surface environment (atmosphere, hydrosphere, biosphere) has been extensively studied; however, the deep geological cycling of this element is less well known. Here we document distinct mass-independent mercury isotope fractionation (expressed as Δ199Hg) in island arc basalts and mid-ocean ridge basalts. Both rock groups show positive Δ199Hg values up to 0.34‰ and 0.22‰, respectively, which deviate from recent estimates of the primitive mantle (Δ199Hg: 0.00 ± 0.10‰, 2 SD)1. The positive Δ199Hg values indicate recycling of marine Hg into the asthenospheric mantle. Such a crustal Hg isotope signature was not observed in our samples of ocean island basalts and continental flood basalts, but has recently been identified in canonical end-member samples of the deep mantle1, therefore demonstrating that recycling of mercury can affect both the upper and lower mantle. Our study reveals large-scale translithospheric Hg recycling via plate tectonics.

Geochemical and zircon U-Pb-Hf isotopic study of volcanic rocks from the Yaolinghe Group, South Qinling orogenic belt, China: Constraints on the assembly and breakup of Rodinia

The tectonic mechanism responsible for the Neoproterozoic magma generation at the northern margin of the Yangtze craton is crucial to understanding the assembly and breakup of the Rodinia supercontinent. Here, we present a systematic study of the petrology, whole-rock geochemistry and geochronology of mafic and felsic volcanic rocks from the Yaolinghe Group, which outcrops mainly in the Shangnan area (north) and the Wudang area (south) of the South Qinling orogenic belt along the northern margin of Yangtze craton. Basalts from the Shangnan area are tholeiitic and calc-alkaline and indicated an arc environment. They have a LA-ICP-MS zircon U-Pb age of 847 ± 5 Ma with positive εHf(t) values of + 6.93 to + 8.79 and TDM1 model ages of 1130–1053 Ma. The rhyolites from this area exhibit geochemical characteristics similar to S-type granites and have a zircon U-Pb age of 841 ± 7 Ma, with a wide range of εHf(t) values of + 1.02 to −11.40 and TDM2 model ages of 2453–1674 Ma. This is different from the rhyolites from the Wudang area, which reveal geochemical affinities similar to A2-type granites, with a zircon U-Pb age of 788 ± 28 Ma, uniform negative εHf(t) values of −8.75 to −0.15 and TDM2 model ages of 2214–1663 Ma. Both rhyolites were sourced from partial melting of Paleoproterozoic crust with minor input of mantle material, but were formed in an arc setting and a post-collision extension setting, respectively. The 663–644 Ma mafic volcanic rocks from the Wudang area are enriched in Th, U, Nb and Ta, similar to oceanic island basalts and have positive zircon εHf(t) values ranging from + 9.24 to + 4.89 with TDM1 model ages peaking at 1018 Ma. They were derived from partial melting of an asthenospheric mantle source that had been modified by slab-derived melts and erupted during late Neoproterozoic continental rifting. Combining our results with existing data, we suggest that the South Qinling orogenic belt resulted from bidirectional subduction of the South Qinling Ocean between the Douling block and the Yangtze craton at ca. 950–750 Ma, accompanied by the formation of a trench-arc-basin system along the Douling block and an Andean-type continental arc belt at the northern margin of the Yangtze craton. After accretion, the amalgamated Douling block and Yangtze craton were affected by oceanic subduction of the Mirovoi Ocean at ca. 750–680 Ma, and subsequent continental rifting at ca. 680–620 Ma. The model further poses that the Yangtze craton was assembled to the periphery of the Rodinia supercontinent.

Barium isotope fractionation during dehydration melting of the subducting oceanic crust: Geochemical evidence from OIB-like continental basalts

Continental basalts commonly show ocean island basalts (OIB)-like geochemical compositions, and their origin is geochemically linked to metasomatism of the big mantle wedge by the deeply subducting oceanic slab at postarc depths of >200 km. This linkage is tested by a combined study of Ba isotopes with major-trace elements and Sr–Nd isotopes in Cenozoic continental basalts from eastern China. These basalts show OIB-like trace element distribution patterns and depleted to weakly enriched Sr–Nd isotope compositions, suggesting incorporation of the dehydrated slab component into their mantle sources. Their δ138Ba values vary from −0.05‰ to +0.06‰, which are lower than the depleted mid-ocean ridge basalts (MORB) mantle but similar to island arc basalts, OIB and OIB-like continental flood basalts. The variation of δ138Ba values cannot be ascribed to fractional crystallization and crustal contamination of mantle-derived magmas, but it results from mixing of the depleted mantle with the recycled crustal material of low δ138Ba values. In other words, the geochemical deviations from the depleted MORB mantle are attributable to the source mixing at mantle depths rather than the crustal contamination at crustal depths. Although the δ138Ba values of the dehydrated oceanic crust would be lower than the initially subducting crust based on the lower coordination number of Ba in liquid phases relative to solid phases in the crust, the low δ138Ba recycled crustal material including igneous oceanic crust (IOC) and seafloor sediment is still required to produce the low δ138Ba values for the studied basalts. The similarity in δ138Ba to island arc basalts not only indicates the similarly low δ138Ba values for the recycled crustal components in these basalts, but also suggests the limited Ba isotope fractionation during dehydration melting of the subducting oceanic crust at postarc depths.

Nitrogenous Altered Volcanic Glasses as Targets for Mars Sample Return: Examples From Antarctica and Iceland

AbstractMars exploration is focused on seeking evidence of habitable environments and microbial life. Terrestrial glassy basalts may be the closest Mars‐surface weathering analog and observations increasingly indicate their potential to preserve biogeochemical records. The textures, major and trace element geochemistry, and N concentrations and isotopic compositions of subaerial, subglacial and continental lacustrine hyaloclastites from Antarctica, Iceland, and Oregon, respectively, were studied using micro‐imaging and chemical methods, including gas‐source mass spectrometry. Alteration by meteoric‐sourced waters occurred in circum‐neutral, increasingly alkaline low‐temperature conditions of ∼60°C–100°C (Iceland) and ∼60°C–170°C (Antarctica). Incompatible large ion lithophile element (LILE) enrichments compared to mid‐ocean ridge basalt (MORB) are consistent with more advanced alteration in Antarctic breccias consisting of heulandite‐clinoptilolite, calcite, erionite, quartz, and fluorapophyllite. Granular and tubular alteration textures and radial apatite represent possible microbial traces. Most samples contain more N than fresh MORB or ocean island basalt reflecting enrichment beyond concentrations attributable to igneous processes. Antarctic samples contain 52–1,143 ppm N and have δ15Νair values of −20.8‰ to −7.1‰. Iceland‐Oregon basalts contain 1.6–172 ppm N with δ15Ν of −6.7‰ to +7.3‰. Correlations between alteration extents, N concentrations, and concentrations of K2O, other LILEs, and Li and B, reflect the siting of secondary N likely as NH4+ replacing K+ and potentially as N2 in phyllosilicates and zeolites. Although much of the N enrichment and isotope fractionation presented here is not definitively biogenic, given several unknown factors, we suggest that a combination of textures, major and trace element alteration and N and other isotope geochemical compositions could constitute a compelling biosignature in samples from Mars' surface/near‐surface.

Partitioning of noble gases (He, Ne, Ar, Kr, Xe) during Earth’s core segregation: A possible core reservoir for primordial noble gases

Noble gas isotopes in plume-influenced oceanic-island basalts (OIBs) clearly indicate the existence of distinct reservoirs somewhere in the deep Earth that are characterized by their more primitive noble gas isotopic signatures. As one of the largest differentiated layers on Earth, whether the Earth’s liquid outer core can preserve ‘primordial’ noble gases is fundamental for better understanding the Earth’s formation and the mantle evolution. Since Earth’s core is believed to have formed via liquid iron segregated from magma ocean during the planetary accretion stage, how much noble gases could have been incorporated into Earth’s core depends on the partitioning of noble gases between liquid metal and molten silicates. In this study, we perform thermodynamic integration (TI) calculations based on first-principles molecular dynamics (FPMD) simulation to investigate the partition coefficients of noble gases (NG: He, Ne, Ar, Kr, Xe) between liquid Fe and MgSiO3 melt. The simulations have been performed by adopting conditions that the pressures are from 10 ∼ 135 GPa and the temperatures from 2300 ∼ 5000 K to fully account for possible core-mantle equilibrium conditions. By analyzing the temperature and pressure dependence of the results, we suggest that the metal-silicate partitioning of noble gases (DNG) merely depends on the temperature, with extremely low DNG < 0.001 at temperatures < 2500 K (20 GPa), but increases to different degrees at high temperatures. The DNG tends to be more affected by temperature as the noble gas atomic mass increases, with the least temperature effect on the partition coefficient of He (DHe), which increases from ∼ 10−3 at 2300 K (10 GPa) to ∼ 10−1 (5000 K, 135 GPa) near the core-mantle boundary (CMB), and DNe = 10−6 ∼ 10−3, DAr = 10−6 ∼ 10−2, DKr = 10−7 ∼ 10−1 and DXe = 10−5 ∼ 100 from 2300 to 5000 K, respectively. Analysis of electronic interactions indicates that noble gases have positive chemical bonding with Fe, Mg, and Si atoms, and the bonding strengths increase with pressure as well as increasing noble gas atomic mass. Based on our calculated partition coefficients, we suggest that Earth’s core may have preserved significant amounts of noble gases considering the volatile-rich environment from which the core segregated.

Mafic-ultramafic rocks in the Buqingshan Complex of the East Kunlun Orogen, northern Tibetan Plateau: remnants of the Paleo-Tethys Ocean

ABSTRACT The Buqingshan Complex, the eastern part of the Muztagh–Buqingshan–Anemaqen ophiolitic mélange zone (MBAM), contains a series of oceanic rocks that represent the remnants of the Paleo-Tethys Ocean. Field investigations reveal that the mafic-ultramafic rocks consist mainly of serpentinite, diabase, massive basalt, and pillow basalt. The serpentinites show similar geochemical characteristics to abyssal peridotites, which is probably the residual part of the depleted upper mantle after 17–20% melt extraction. The mafic rocks have geochemical affinities similar to both normal mid-ocean ridge basalt (N-MORB) and oceanic island basalt (OIB). The rocks with N-MORB features (diabases, massive basalts, and pillow basalts) are associated with serpentinites and cherts, characterizing the sequence of MOR-type ophiolites. The OIB-type pillow basalts are characterized by enrichment of Nb, Ti, and LREEs. They are associated with massive bioclastic limestones, limestone breccias, basalt breccias, and matrix-supported conglomerates, resembling a typical seamount sequence of bottom pillow lavas–cap carbonate–slope facies deposits. Zircon U–Pb dating of the diabases yields a Late Carboniferous age at 303 ± 3 Ma. The reported biostratigraphic age data from the overlying limestone suggest the formation age of the basalt basement to be the Early Permian. Linking all the geological, geochemical, and geochronological data together, these pieces of evidence suggest that the Late Palaeozoic N-MORB- and OIB-type rock assemblages in the MBAM are most likely to be fragments of mid-ocean ridge oceanic lithosphere and seamount, respectively. Such new observations also represent the two main stages of the evolution of the Paleo-Tethys Ocean: the spreading of the Carboniferous mid-ocean ridge and the Early Permian plume magmatism.

40Ar/39Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province

Seamounts are features generated by hot spots and associated intraplate volcanic activity. The geochemical characteristics of igneous rocks constituting seamounts provide evidence of important details of dynamic processes in the Earth, such as mantle magma source areas, and are key to understanding how mantle plume processes control the formation and evolution of seamounts and their resulting geochemical characteristics. The Pacific Ocean contains a large number of hitherto unstudied seamounts, whose ages and geochemical characteristics remain poorly known. This study presents the geochemical characteristics of six basalt samples from five seamounts in the Western Pacific and the 40Ar/9Ar ages of three samples are determined. The new analysis yielded 40Ar/39Ar ages for seamounts samples MP3D21, MP5D11, and MP5D15A of 95.43 ± 0.33, 62.4 ± 0.26, and 99.03 ± 0.4 Ma, respectively. The geochemical profiles of seamounts samples MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 are consistent with alkaline basalts, as evidence by alkali-rich, silicon-poor compositions along with high titanium concentrations. The primitive mantle normalized rare-earth elements and trace elements spider pattern are similar to those of ocean island basalts. The Ta/Hf and Nb/Zr ratios and La/Zr-Nb/Zr discriminant diagrams indicate that the six seamounts formed from magma that originated in the deep mantle.

Preserved ancient oceanic lithosphere within the Buem structural unit at the eastern margin of the West African Craton

This work presents new major and trace element geochemical data and the first SmNd and LuHf isotopic data for the mafic suite of the Buem structural unit (BSU) in Ghana in order to infer its origin and geodynamic setting, relationship to the Pharusian suture zone and significance in the Dahomeyide belt. The BSU, which occurs about 50 km to the west of the Pharusian suture zone, comprises well-preserved, weakly metamorphosed, mafic volcanic and mafic-ultramafic plutonic rocks. Two lava types can be distinguished: Type I and Type II lavas that are spatially separated and geochemically distinct. The Type I lavas are subalkaline and show trace element patterns akin to mid-ocean ridge basalt (MORB). Their εNd800Ma values of +5.3 to +6.2 and εHf800Ma values of +11.1 to +17.2 are indications of juvenile magma derived from a depleted mantle source in the spinel stability field. The Type II lavas have alkaline signatures with trace element patterns that resemble ocean island basalts. They formed by smaller degrees of disequilibrium partial melting at depth in which garnet likely played a role as a residual phase having εHf800Ma and εNd800Ma values of +3.2 to +5.1 and + 4.1 to +6.3 respectively. The gabbros show similar trace elements concentration and isotopic ratios as the BSU lavas, an indication of a similar mantle source and environment of formation. Because the geochemical features of the BSU mafic suites are comparable to the MORB-type high-pressure rocks of the Pharusian suture zone, both represent the remnant of the Pharusian oceanic crust, formed during the breakup of Rodinia. The BSU corresponds to the true oceanic crust of the West Gondwana Orogen continental margin-type ophiolite, sharing similar lithological and geochemical features to other continental margin-type ophiolites in the world.

Evidence of intraplate magmatism and subduction magmatism during the formation of Nagaland–Manipur Ophiolites, Indo–Myanmar Orogenic Belt, north‐east India

Mafic extrusive rocks (basalts) and intrusive rocks (gabbros) from the Nagaland–Manipur Ophiolite (NMO) of the Indo–Myanmar Orogenic Belt (IMOB), north‐east India, are investigated to understand their magmatic evolution in diverse tectonic environments. Basalts are distinguished into two types: basalt‐I and basalt‐II. Basalt‐I type shows the sub‐alkaline character with Nb/Y &lt; 0.50, low Nb/Th (2.36–7.94), and low to moderate La/Sm (1.00–4.12) indicating derivation from a slightly enriched mantle source and also supported by their enriched LREE pattern with flat HREE. They are depleted in HFSEs (Nb and Ti) but enriched in U and Pb, which is indicative of a typical subduction origin derived from an MORB‐type mantle source. Investigated samples of basalt‐II and gabbros have an equal composition with alkaline characteristics. They have Nb/Y &gt; 0.50, high Nb/Th (8.38–13.37), and highly enriched LREE (La/Sm = 4.41–6.35) pattern. They show typical Ocean Island Basalt (OIB) characters of a plume source. The two sets of basalts and gabbros found in this study have no sign of genetic relationship, and therefore, it strongly suggests that they were derived from two different mantle sources of a plume and a subduction zone mantle wedge. Our study supports the theory that the NMO has records of different magmatic episodes produced ranging from plume‐related magmatism, to divergent and convergent plate magmatism that were generated at diverse tectonic settings.