Mid-ocean Ridge Basalt Mantle Research Articles

The helium and carbon isotopic signature of Ocean island basalts: Insights from Fogo volcano (Cape Verde archipelago)

Fluid inclusions (FI) entrapped in phenocrysts carried by Ocean Island Basalts (OIB) contain key information on volatiles’ abundance and origin in their mantle sources. Here, we add new piece of knowledge to our understanding of volatile geochemistry in global OIB magmas, by presenting new noble gas (He-Ne-Ar) and carbon (C) isotope results for olivine- and clinopyroxene-hosted FI from enclaves, lavas, tephra and volcanic gas samples from Fogo, the only frequently active volcano at the Cape Verde archipelago (eastern Atlantic Ocean). FI, together with crater fumaroles, constrain the Fogo 3He/4He signature at 7.14–8.44 Rc/Ra (where RC is the air-corrected 3He/4He isotope ratio, and Ra is the same ratio in air), which is within the typical MORB (Mid-Ocean Ridge Basalt) mantle. The carbon isotopic ratio (δ13C vs. Pee Dee Belemnite) of CO2 in FI and fumaroles range from -6.04 to -4.41 ‰. We identify systematic variations of δ13C and He/Ar* with FI entrapment pressure (estimated from a combination of host mineral barometry and FI microthermometry), from which we develop a model for volatile degassing in the mantle-to-crustal magma storage system. The model predicts a crustal-like signature for carbon (δ13C of -0.4 ± 1.0 ‰) in primary melts formed by mantle melting at ∼2200 MPa (∼77 km) and a source He/Ar* ratio of 0.90–0.24, which are indicative of variably depleted mantle metasomatized by carbon enriched melts/fluids from a crustal component. We also use our results to characterise regional (in the Cape Verde and Canary archipelagos) and global trends in C and He isotope composition from OIB. From a comparison with the few other OIB localities for which δ13C are available, we propose that a carbon enriched crustal component could be recurrent at a global scale in OIB magmatism, although often masked by isotope fractionation during magmatic degassing. We additionally find that, at regional scale, He isotopes in OIB scale inversely correlate with the degree of partial melting of the mantle beneath individual islands’ (inferred from the La/Yb ratio of erupted basalts). More widely, our results corroborate previously established global relationships between OIB He isotopic signature, plume buoyancy flux and overlying plate velocity. In this interpretation, the MORB-like 3He/4He (8 ± 1 Ra) at Fogo reflects a combination of (i) low to medium magma productivity, (ii) relatively low plume buoyancy flux (∼1.1 Mg/s), and (iii) slow average velocity (∼3 cm/yr) of the overlying plate.

The Volcanism of the Meseta del Lago Buenos Aires, Patagonia: the Transition from Subduction to Slab Window

Abstract The Meseta del Lago Buenos Aires (MLBA) in southern Patagonia, a volcanic plateau formed from ~12 Ma to present, provides an opportunity to investigate the temporal evolution in volcanism as this region transitions from the subduction of the Nazca plate to the formation of the slab window produced by the collision of the Chile Ridge and the Andean subduction zone. Here, we report new major, minor, and trace element contents, as well as Sr, Nd, Pb, and Hf isotopes of the MLBA lavas. Three distinct geochemical endmembers can be distinguished in the MLBA basalts: a subduction-influenced endmember, a transitional component similar to the South Atlantic enriched mid-ocean ridge basalts, and an enriched component akin to the EM1 mantle composition. Lavas older than ~1.5 Ma define a compositional continuum between the subduction-influenced and transitional endmembers; this trend is also present in many other southern Patagonian plateaus regardless of their distance to the trench, eruption age, and the composition of the continental blocks where they are located. In contrast, MLBA basalts younger than ~1.5 Ma uniquely define a transition into the EM1 mantle component at the time when this region was affected by the slab window. The estimated pressures and temperatures of mantle-melt equilibration for the MLBA basalts indicates an increase in both parameters after the formation of the slab window that roughly correlate with the changes in lava composition. The basalts’ composition from all southern Patagonia plateaus points to the presence of the South Atlantic mid-ocean ridge basalt mantle influenced by the Discovery, Shona, and Bouvet hotspots rather than the sub-slab mantle, as represented by the Chile Ridge basalts. This observation challenges the hypothesis that the sub-slab mantle within the slab window has had an important role in the composition of the erupted lavas. Instead, it suggests the presence of a South Atlantic mantle beneath southern Patagonia either within the mantle wedge, consistent with a long-lasting South Atlantic convection cell beneath South America, or in the subcontinental lithospheric mantle metasomatized before or just after the opening of the South Atlantic basin, as demonstrated by the composition of southern Patagonia mantle xenoliths. Although it is difficult to precisely distinguish the contributions of the asthenosphere from that of the metasomatized subcontinental lithospheric mantle beneath this region, our work suggests significant contributions from the latter in the composition of the MLBA lavas.

Major, trace elements and Sr–Nd–Pb isotopes systematics of mafic dykes from the Figuil (northern Cameroon) and Léré (southwestern Chad) areas

In this study, field descriptions of the dolerite dykes cross‐cutting the Pan‐African basement (DPB) and the dolerite and gabbroic dykes cross‐cutting the Cretaceous sedimentary basin (DGCSB) are used together with petrographic, bulk‐rock geochemistry, as well as Sr–Nd–Pb isotopic data to constrain the geodynamic setting and origin of these mafic (basanite, basalt and basaltic andesite in composition) rocks that outcrop in the Figuil–Léré area. The DPB dykes are alkaline with SiO2 of 46.63–49.03 wt% and MgO of 3.93–7.11 wt%. In contrast, the DGCSB dykes are subalkaline with SiO2 ranging between 50.24 and 53.16 wt% and MgO fluctuating between 5.22 and 7.90 wt%, comparable to dykes described in other regions of the Cameroon Line (CL). The rocks studied belong to the tholeiitic series and were emplaced in within‐plate basalts (WPB) and mid‐ocean ridge basalts (MORBs) setting. The variations in major and trace elements from the basanite to basaltic andesite compositions are dependable with fractional crystallization (FC) of different mineral phases such as opaque minerals, olivine, clinopyroxene, amphibole and alkali feldspar. Most samples exhibit negative Nb–Ta and positive Sr anomalies, which together with the negative correlation between MgO and 87Sr/86Sr, and isotopic ratio of 206Pb/204Pb <19.5 (18.27–19.49) suggests interactions of crustal components. Nevertheless, this contamination seems limited because there is a positive correlation between SiO2 and 143Nd/144Nd plot and low Ba contents (72–210 ppm) of the samples compared to the mid‐continental crust (259–628 ppm). Trace element modelling proposes a derivation of the Figuil–Léré dykes from parental melts generated by 4%–15% partial melting of a source containing garnet peridotite in the mixed lithospheric–asthenospheric mantle. The isotopic data of the dykes studied with hybrid magma composition are characterized by more radiogenic 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb isotopic ratios with a dominant mixing of depleted MORB mantle (DMM)–enriched mantle 1 (EM1)–enriched mantle 2 (EM2), like those of other mafic dykes of the CL.

Identification of recycled oceanic crust in the mantle source of sulfide-bearing lower crust of the Gangdese magmatic arc, Southern Tibet: Constraints from Sr–Nd–Li isotopes

Post-collisional porphyry copper deposit (PCD) is a new genetic type of PCDs discovered in the Himalayan–Tibetan orogenic belt. It is believed that the formation of ore-forming magmas results from the re-melting of a sulfide-bearing, thickened, juvenile mafic lower crust in a magmatic arc associated with the oceanic plate subduction. However, the material contribution of subduction processes to the formation of this particular arc magma remains unclear. In this paper, we attempted to present a study that includes petrology, mineral geochemistry, whole rock Sr–Nd–Li isotopes, of Late Cretaceous Lilong ultramafic igneous rocks, south Lhasa sub-terrane, Tibet. The target ultramafic igneous rocks are sulfide-bearing cumulates. They show relative depletion in whole rock Nd isotopic compositions, slightly elevated (87Sr/86Sr)i ratios (0.7043–0.7059), and variable δ7Li values (2.8–9.0‰). Simulation results showed that trace element of melts in equilibrium with cumulus hornblende grains exhibit typical arc-like trace element distribution patterns, indicating that these ultramafic igneous rocks crystallized from arc magma. Additionally, the modeled melts showed enrichment in melt-mobile elements such as Ba, Pb, Sr, Th, and light rare earth elements (LREE), and higher Th/Yb ratios, as compared with normal mid-ocean ridge basalt (MORB). These geochemical properties can be explained by the metasomatism of depleted MORB mantle peridotites by hydrous melts derived from subducted seafloor sediments. Whole rock Sr isotopes show correlations with Ba/Nb and Th/La ratios for the modeled melts, showing that fluids from the basaltic igneous crust, in addition to hydrous melts from the seafloor sediments, contribute to their mantle source. In addition, the heterogeneity of whole rock δ7Li values for ultramafic igneous rocks also indicates that their mantle sources have been metasomatized by terrigenous sediment- and altered oceanic crust-derived fluids. Depleted MORB mantle peridotites react with the mixed fluids at sub-arc depths to produce ultramafic metasomatites, and model calculations for trace element contents and Sr–Nd–Li isotopic compositions further validate that the geochemical compositions of the Lilong ultramafic rocks can be clarified by this process. The results show that the deeply subducted oceanic crust may have played an important role in the formation of sulfide-bearing juvenile mafic lower crust of this magmatic arc.

Formation and Evolution of Chromitites in the Hongshishan Complex, Beishan Orogenic Collage, NW China: Constraints from Mineralogical Compositions, Re‐Os Isotopes and Platinum‐Group Element Geochemistry

AbstractThe Hongshishan chromitite deposits are situated to the north of the Beishan orogenic collage, in the southern part of the Central Asian Orogenic Belt. This study describes the mineral chemistry, Re‐Os isotopes and platinum‐group elements geochemistry of the Hongshishan chromitites for the purpose of constraining the origin, evolution and composition of their parental melts. The restricted ranges of Al2O3, Cr2O3 and Cr#‐Mg# variation of chromite‐cores and chromites fall within the field of the mid‐ocean ridge and ophiolitic podiform chromite settings. The (187Os/188Os)i ratios of the chromitites are in the range of 0.12449–0.12745 (average 0.12637) and the γOs are from –1.92 to –0.06 (average –0.83). In the Re‐Os isotope diagrams, all the samples fall in the field of chromitites and show a residual peridotitic trend. The range of Os isotopic compositions and γOs values indicate that they overlap the depleted MORB mantle (DMM) as well as being close to global Os isotopic data and γOs of ophiolite chromitites. The characteristics of the PGE contents can be roughly subdivided into two groups: podiform chromitites and Ural‐Alaskan type complexes. For the ferritchromite cores, the calculated Al2O3 concentrations of the parental melt are higher (average 16.65 wt%) in high‐Cr than high‐Al chromitite (average 16.17 wt%) and for the chromite, the calculated Al2O3 concentrations are even higher (average 16.48 wt%) in the high‐Cr than the high‐Al examples (average 15.38 wt%). In the (TiO2)melt vs. TiO2 diagrams, most high‐Al melts fall in the MORB, while the high‐Cr melts fall in the ARC field. The calculated FeO/MgO ratios for the parental melt show the closest resemblance to a MORB magma composition. The inferred parental melt composition for studied chromitites falls in the field of mid‐ocean ridge basalt (MORB) magmas and far away from boninite. The calculated degrees of partial melting producing the chromitites are 16%–22% (average 19%), which is around the range of those of the MORB magmas. The chromitites are suggested to have been formed in a MORB setting. The chromites and ferritchromite cores are mostly scattered along the MORB and SSZ harzburgite–dunite fields. Ferritchromite rims and ferritchromites with high YFes formed as a result of alteration during serpentinization. .

Zinc isotope fractionation during mid-ocean ridge basalt differentiation: Evidence from lavas on the East Pacific Rise at 10°30′N

Zinc (Zn) isotope data for mid-ocean ridge basalts (MORB) are scarce. The possible effects of MORB differentiation and mantle compositional heterogeneity on MORB Zn isotope compositions are poorly understood. In this paper, we report Zn isotope data of MORB lavas from the East Pacific Rise (EPR) at 10°30′N, which are understood to have been derived by a similar extent of melting from a compositionally uniform mantle source but manifest significantly varying extents of magma differentiation (MgO = 7.38–1.76 wt%) to demonstrate unequivocal Zn isotope fractionation during MORB differentiation. These lavas have a small δ66Zn (relative to the JMC-Lyon standard) range of 0.21–0.32‰, with an average of 0.26 ± 0.07‰ (2SD, N = 24), overlapping with literature data. However, δ66Zn values show significant correlations with indices of magma differentiation (e.g., MgO and CaO), with the more differentiated samples having isotopically heavier Zn. We can thus conclude that MORB differentiation can lead to ∼0.1‰ δ66Zn elevation, which is similar to that observed in Hawaiian Kilauea Iki lava lake samples. Fractional crystallization of olivine and clinopyroxene at MgO > ∼3.5 wt% and clinopyroxene and Fe–Ti oxides dominated by ilmenite at MgO < ∼3.5 wt% results in the observed Zn isotope fractionation. The δ66Zn values for primitive N-MORB and MORB source mantle are estimated to be ∼0.22‰ and ∼0.17‰, respectively. Furthermore, we observe a positive correlation between δ66Zn and [La/Sm]N in primitive (MgO = 7.58–9.01 wt%) literature MORB, indicating the existence of possible Zn isotope heterogeneity in the oceanic upper mantle. We thus predict heavy Zn isotope enrichment in mantle sources for E-MORB samples with high [La/Sm]N, which requires future testing.

The Phosphorus Budget of the Silicate Earth Based on an Updated Estimate of the P/Nd Ratio

AbstractCalculating the phosphorus (P) budget of the bulk silicate Earth (BSE) is necessary to understand the global P cycle and the origin and evolution of life. Covariations of P and neodymium (Nd) contents suggest that P is 1.1–1.4 times more incompatible than Nd during mantle melting and magmatic differentiation. As such, the P/Nd ratios of mantle sources can be estimated using inverse modeling, and the P budget of the BSE can be calculated from the P/Nd ratios of the continental crust, mid‐ocean ridge basalts (MORBs), and ocean island basalts (OIBs). The P/Nd ratios for the MORB and OIB mantle sources and continental crust are 62.5 ± 0.8 (2σm), 57.0 ± 0.5 (2σm), and 28.4 ± 12.0 (2σm), respectively. These reservoirs have different P/Nd ratios due to subduction, magmatic differentiation, and mineralogy. Subduction can significantly affect the global P and Nd cycles. Magmatic differentiation, especially in the continental crust, may redistribute P by changing the contents of garnet, apatite, and other minerals. The fractional contributions of these global reservoirs suggest the modern BSE has a P/Nd ratio of 57.7 ± 3.0 (2σm), equating to ∼3.1 × 1020 kg of P in the BSE.

Tectono-magmatic evolution during the extensional phase of a Wilson Cycle: a review of the Alpine Tethys case and implications for Atlantic- type margins

Despite the recent progress in seismic imaging and dynamic modelling, the tectono-magmatic evolution during late orogenic extension and rifting is at present little understood. Answering questions such as when and where magma forms, how it percolates and how it is emplaced requires access to magmatic rocks from different crustal and mantle levels, which is impossible in modern rift systems. An alternative, presented in this study, is to use remnants of fossil extensional systems exposed in collisional orogens. Here we review the Permian to Early Cretaceous tectono-magmatic evolution recorded in the Alpine realm of Western Europe. While most of the previous studies reviewed the geochemical and petrological characteristics of the magmatic rocks, here we rely on a holistic approach to link the magmatic evolution to extensional processes on a large scale. We synthesize our observations on the timing and location of magma production in a Wheeler diagram. Our study shows that two major magmatic events can be defined: the first one took place during the Permian and was linked to post- orogenic extension of the Variscan belt. It affected a large portion of Western Europe, where it was responsible for significant sub-aerial volcanic activity, for the emplacement of massive magmatic additions at the base and within the crust, and for the depletion of the lower subcontinental lithospheric mantle. The second included several pulses of magmatic activity related to the multiphase rifting event that led to the successive opening of the Meliata-Vardar, Alpine Tethys and southern North Atlantic (proto-)oceanic domains. In the Alpine Tethys, the first rift-related magmatic activity was linked to melt infiltration within the subcontinental lithospheric mantle during the so-called necking stage. Basalts were emplaced only later, during and after mantle exhumation. Their chemical signature reflects the interaction between melts derived from Depleted Mid-oceanic ridge basalt Mantle (DMM) and the previously refertilized/infiltrated subcontinental mantle. Late alkaline magmatic activity post- dated breakup. A main result of this study is that the tectono-magmatic evolution of extensional systems cannot be explained by simple models using ß-factors or mantle temperatures alone but also needs to integrate the inheritance acquired during previous tectonic phases.

Low-degree melt metasomatic origin of heavy Fe isotope enrichment in the MORB mantle

Studies of mid-ocean ridge basalts (MORB) show a variable Fe isotope composition of the oceanic upper mantle. To test a recent hypothesis that heavy Fe isotope enrichment in the MORB mantle results from the same process of incompatible element enrichment, we conduct an Fe isotope study of well-characterized MORB samples from a magmatically robust segment (OH-1) of the Mid-Atlantic Ridge (MAR) at ∼ 35°N. The data show large Fe isotope variation (δ56Fe = +0.03 to +0.18‰) that correlates well with the abundances and ratios of more-to-less incompatible elements and with Sr-Nd-Hf isotopes. Our findings in support of the hypothesis can be detailed as follows: (1) the oceanic upper mantle has a heterogeneous Fe isotope composition on varying small spatial scales with isotopically heavy Fe (high-δ56Fe) preferentially associated with pyroxenite lithologies; (2) such lithologies, which are also enriched in the progressively more incompatible elements, are of low-degree (low-F) melt metasomatic origin; (3) with all the conceivable processes considered, the low-F melt metasomatism takes place at the lithosphere-asthenosphere boundary (LAB) beneath ocean basins through crystallization of incipient (Low-F) melt in the seismic low velocity zone (LVZ) at the base of the growing oceanic lithosphere (i.e., LAB) over the Earth's history since the onset of plate tectonics, forming composite lithologies with geochemically enriched pyroxenite veins dispersed in the depleted peridotite matrix; (4) such mantle of composite lithology when transported to beneath the present-day ocean ridges will undergo decompression melting and produce MORB melts with geochemical trends of “melting-induced mixing” as observed at the MAR and global MORB; (5) we predict all this to be a globally common process and widespread.

The heterogeneous mantle massif in south Tibetan ophiolites and its implication for the tectonic evolution of Neo-Tethys

Ophiolitic mantle peridotites show large variations in their geochemical compositions, which provide important information for the histories of mantle melting, melt metasomatism and prior melt depletion in the context of tectonic evolution of ancient oceans. Here we report new petrological and geochemical data for mantle peridotites from the Zhagabu ophiolite in south Tibet, which could represent fossil upper mantle of the Mesozoic Neo-Tethys oceanic lithosphere. The Zhagabu mantle peridotites, including spinel lherzolites and spinel harzburgites, have highly heterogeneous geochemical compositions, with whole-rock Al2O3 of 0.3–2.0 wt% and CaO of 0.3–2.2 wt%, and spinel Cr# [Cr/(Cr + Al)] of 0.21–0.74 and Mg# [Mg/(Mg + Fe2+)] of 0.44–0.74. The clinopyroxenes from the studied samples show extremely depletion in light rare earth element compositions, some of which however are elevated by post-melting melt-rock interactions. Spinel and clinopyroxene compositions collectively suggest that the Zhagabu mantle peridotites have experienced ~9–20% of fractional melting from a depleted mid-ocean ridge basalt mantle source. Trace element modeling of clinopyroxenes further indicates that some of the studied samples follow the refertilization-hydrous melting trends with variable fluid fluxes. Therefore, the heterogeneous compositions of the Zhagabu mantle peridotites are not comparable to those of either abyssal peridotites or forearc peridotites, but most probably record a dynamic transition from the anhydrous condition beneath the Neo-Tethyan mid-ocean ridge to the hydrous condition above the subducted slab. Considering that a forearc spreading is in contradiction with the low magma budget of Yarlung Zangbo ophiolites, the Neo-Tethyan subduction re-initiation was suggested to be achieved along oceanic detachment faults at ultraslow-spreading ridges. The Zhagabu mantle peridotites represent the very heterogeneous mantle massif in south Tibetan ophiolites, and provide an excellent window to investigate the complex mantle dynamics and tectonic evolution of the Neo-Tethys Ocean.

Unradiogenic lead isotopic signatures of the source mantle beneath the southernmost segment of the Central Indian Ridge

Numerous studies have shown that the mid-ocean-ridge basalts (MORBs) distributed in the southern Central Indian Ridge (CIR) exhibit the DUPAL anomaly; yet, the corresponding temporal variations of this signal remain underexplored in this region. Here, we report new Sr–Nd–Pb isotope data for the MORB which forms the southernmost segment of the CIR (CIR-1) and interpret these together with previously reported elemental data. Our results indicate that the MORB from the current spreading axis of the CIR-1 segment is produced by a mixture of the partial melt of the depleted MORB mantle and melt derived from the lower continental crust (LCC). This interpretation is consistent with the previously proposed model for Indian MORBs that exhibit the DUPAL anomaly. In contrast, the MORB from the off-axis area shows a depleted trace element composition, particularly for highly incompatible elements. This can be explained by the mixture of the partial melt of a highly trace element-depleted mantle that has not been previously defined in this area and melt from the LCC and C component. Furthermore, high 87 Sr/ 86 Sr, Δ8/4, and Δ7/4 values indicate that the source mantle of the MORB in the off-axis area of the CIR-1 segment has undergone at least two melting events, including the formation of the current MORB. The first melting event occurred approximately 500 Ma and resulted in lower parent/daughter ratios (Th/Pb and U/Pb) in the residual mantle, yielding an unradiogenic lead isotope composition. Overall, our results suggest that a highly depleted mantle domain that underwent an ancient melting event lies under the northern CIR-1 segment, contributing to the formation of MORBs with an unradiogenic lead isotopic signature. • We sampled on- and off-axis mid-ocean-ridge basalt (MORB) from CIR-1. • Depleted mantle partial melt and LCC melt mix to form the on-axis MORB. • In contrast, the off-axis MORB is derived from trace element-depleted mantle. • Trace element-depleted melt could be mixed with melts of the LCC and C component. • A highly depleted mantle domain that underwent ancient melting is suggested.

Noble gas isotope systematics in the Canary Islands and implications for refractory mantle components

Noble gas isotope systematics of ocean island basalts (OIB) provide evidence for relatively undegassed and primitive mantle sources. These OIB sources partly derive from the deep mantle by virtue of their distinctiveness from mid-ocean ridge basalts (MORB), which dominantly sample upper mantle. New helium, neon and argon isotope data are presented for Canary Islands lavas, carbonatites and cumulate and mantle xenoliths confirming 3He/4He ratios that are the same or lower than MORB, but that are heterogeneous (∼3–9.5RA) within and between islands in the archipelago. Neon and Ar isotope systematics for lavas are mostly within the range of air compositions. Harzburgite xenoliths from Lanzarote, which are interpreted to represent ancient refractory mantle residues, have distinct He-Ne-Ar isotope systematics from lavas or cumulate xenoliths. The harzburgites are characterized by forsteritic olivine (>Fo91) with uniformly low-3He/4He (6.6 ± 0.2RA), high 40Ar/36Ar (630–4900), and have Ne isotope compositions that range between air values or that are similar or more nucleogenic than depleted MORB mantle (DMM). Similar refractory mantle peridotites have been discovered as xenoliths at other OIB localities and are likely to be distinct from continental lithospheric mantle. Refractory mantle (RM), which by virtue of its high melting temperature is difficult to partially melt, may be a significant component in the convecting mantle and has potential to impart a cryptic noble gas signature to partial melts in intraplate, divergent and convergent margin settings. In this sense, RM may represent a ‘sixth mantle component’ after DMM, high-µ (high 238U/204Pb; HIMU), enriched mantle endmembers (EMI, EMII) and the ‘focus zone’ (FOZO). An RM-type component may be presented in the older eastern Canary Islands and possibly in recent rejuvenated volcanism from Teide (Tenerife). Canary Island intraplate volcanism samples multiple mantle components, confirming that the most gas-rich mantle sources involved in magmatism dominate OIB noble gas compositions.

Oxidized mantle sources of HIMU- and EM-type Ocean Island Basalts

Oxygen fugacity (fO2) is a fundamental variable in igneous petrology with utility as a potential tracer of recycled high Fe+3/ΣFe surficial materials in the sources of mantle-derived lavas. It has been postulated that ocean island basalts (OIB) have elevated fO2 relative to mid-ocean ridge basalts (MORB) owing to higher average Fe+3/ΣFe in their source regions. To examine this issue, trace-element systematics of olivine grains are reported from OIB lavas with HIMU (high-μ; Mangaia, Canary Islands), enriched mantle (EM; Samoa; São Miguel in the Azores Islands) and depleted MORB mantle (DMM; Pico in the Azores Islands) Sr-Nd-Pb-Os isotope signatures, in order to constrain the fO2 of each magmatic system. Despite sampling distinct mantle reservoirs based on radiogenic isotope systematics, these OIB suites show similar fO2, ranging from +0.8 to +2.6 ΔFMQ, with an average of +1.5 ± 0.8 ΔFMQ, largely higher than MORB using the same oxybarometry method at +0.6 ± 0.2 ΔFMQ. The studied OIB show no correlation between fO2 and bulk rock isotopic ratios or calculated parental magma compositions. The lack of correlations with isotopic signatures likely results from radiogenic isotope signatures being hosted in volumetrically minor trace element enriched mantle lithologies, while Fe+3/ΣFe ratios that act as the primary control on fO2 are controlled by the volumetrically dominant mantle component. Higher fO2 in many OIB relative to MORB implies that they sample a uniformly high Fe+3/ΣFe plume source mantle that may be the result of either a common oxidized oceanic crust-rich reservoir parental to many modern plume lavas, or preservation of un-degassed and oxidized mantle domains formed shortly after core formation.

Geochemistry of basaltic blueschists from the Deyader Metamorphic Complex (Makran Accretionary Prism, SE Iran): New constraints for magma generation in the Makran sector of the Neo-Tethys

• The Deyader Complex in the Makran includes a blueschist HT-LP metaophiolitic unit. • Blueschist volcanic protoliths show N-MORB, E-MORB, and P-MORB affinities. • Volcanic protoliths were erupted in mid-ocean ridge featuring plume-ridge interaction. • Blueschists show geochemical features similar to those of the Makran ophiolites. The North Makran Domain in the Makran Accretionary Prism consists of an imbricate stack of Mesozoic Neo-Tethyan ophiolitic and metaophiolitic units. The Deyader Complex is an important metamorphic unit of this Domain and includes tectonic slices of HP-LT blueschists derived from upper oceanic crust protoliths. The volcanic protoliths consist of basalts with Mg#=75.1–62.7, SiO 2 = 41.59–48.02 wt%, and TiO 2 = 0.79–1.53 wt%. Based on incompatible and REE contents and ratios, three geochemical types can be distinguished. Group 1 basalts show trace element compositions, and La N /Yb N and La N /Sm N ratios <1 resembling those of normal-type (N-) mid-ocean ridge basalts (MORB). Compared to N-MORBs, Group 2 basalts show slight enrichment in Th, Ta, Nb, as well as La N /Yb N and La N /Sm N ratios = ∼1 similar to those observed in enriched-type (E-) MORB . Group 3 basalts show a transitional-alkaline nature (Nb/Y = 0.7–1.0), marked Th, Ta, Nb enrichment and high La N /Yb N (4–7) and La N /Sm N (2–3.5) ratios resembling those of plume-type (P-) MORB. Trace element and REE petrogenetic models show that N-MORB protoliths were generated from a depleted MORB mantle source (DMM), whereas E-MORB and P-MORB protoliths were generated from partial melting of a DMM source that was metasomatized to variable extents by OIB-type chemical components. The Deyader blueschists protoliths show close geochemical and petrogenetic similarities with all the North Makran ophiolites, suggesting that they were formed in a large Late Jurassic – Cretaceous oceanic basin that was strongly affected by mantle plume activity and different extents of plume-ridge interaction.

RETRACTED: Fe3+ partitioning between clinopyroxene and silicate melt at 1–2.5 GPa: Implications for Fe3+ content of MORB and OIB source mantle

Pyroxene is the chief reservoir of Fe 3+ in upper mantle peridotite, but experiments exploring pyroxene/melt Fe 3+ partitioning ( D Fe3+ pyx/liq ) have been restricted to 100 kPa and pyroxene with low alumina. Here we present Fe 3+ partitioning experiments between clinopyroxenes (cpx) and mafic melt at elevated pressures (1–2.5 GPa). Experiments were conducted with f O2 buffered and modulated by Ru + RuO 2 and Fe-Pt alloy capsules, respectively, between ΔQFM −2.7 and +5.1. Fe 3+ /Fe T of both cpx and melt were determined by Fe K -edge X-ray absorption near edge structure spectroscopy. The experimentally synthesized cpx compositions (Al 2 O 3 = 2.36–6.01 wt.%, CaO = 19.33–22.21 wt.%) approximate those expected in basalt source regions. We find that Fe 3+ is moderately incompatible in cpx and D Fe3+ cpx/liq correlates with cpx Al 2 O 3 content, increasing from 0.05 ± 0.09 to 0.81 ± 0.04. D Fe3+ cpx/liq also increases with increasing f O2 . Comparison between experimentally synthesized cpx with those from natural peridotites indicates influences of both temperature and composition on Fe 3+ /Fe T for cpx in spinel and garnet peridotites. The combined effects of decreased pyroxene Al 2 O 3 concentration and pyroxene mode with progressive partial melting of peridotite diminishes the bulk partition coefficients of Fe 3+ , leading to greater Fe 2 O 3 contents in high degree partial melts, and this accounts for an inverse relationship between Na 2 O and Fe 2 O 3 observed in mid-ocean ridge basalts (MORB). Comparison to numerical experiments with pMELTS and the model of Jennings and Holland (2015) show that these models overpredict D Fe3+ cpx/liq for partial melting of the mantle, and so they do not accurately determine the relationship between the f O2 and Fe 2 O 3 of peridotite in basalt source regions. To estimate the Fe 3+ /Fe T ratio of the mantle source of MORB, we modeled liquid Fe 2 O 3 during isentropic batch melting of peridotite at three potential temperatures (1320 °C, 1400 °C, and 1440 °C) for peridotitic sources with Fe 3+ /Fe T ratios between 0.02–0.06. A source with an Fe 3+ /Fe T ratio of 0.038 ± 0.007 matches most of the span of natural MORB. This ratio is similar to that typical of continental lithospheric mantle sampled by xenoliths, but lower than that surmised by several recent experimental and thermodynamic studies. Considering this source Fe 3+ /Fe T but extending the partial melting calculations to higher pressures (2.5–4 GPa) reveals that bulk D Fe3+ perid/liq significantly decreases for garnet peridotite relative to spinel peridotite because the cpx become significantly less aluminous with increasing pressure. This results in high pressure partial melts with greater liquid Fe 3+ /Fe T ratios. Therefore, elevated Fe 3+ /Fe T ratios observed from some oceanic island basalts (OIB), such as those from Hawaii and Iceland, reflect in part the differences in conditions of melting and may not require mantle source regions more oxidized than those that produce MORB.

Chromium isotope fractionation during magmatic processes: Evidence from mid-ocean ridge basalts

Chromium (Cr) isotopes represent a powerful tool for tracing the redox conditions during planetary magmatic evolution. However, so far, the systematic investigation of Cr isotope variation has been only performed on ocean island basalts (OIBs) in terrestrial magmatic rocks. Therefore, the Cr stable isotope compositions (expressed as δ53Cr relative to NIST SRM 979) of other magmatic rocks, formed under different oxygen fugacity, have remained unconstrained. In this paper, we present the first Cr stable isotopic data of mid-ocean ridge basalts (MORBs) from the Eastern Pacific Ocean ridge, the Indian ridge, and the Atlantic Ocean ridge. The oxygen fugacity of such basalts is different from that of the above-mentioned OIBs. The Rhyolite-MELTS model shows that the chemical composition variations in the studied basalts are induced by varying extents of fractional crystallization of olivine, clinopyroxene, plagioclase, and spinel. The δ53Cr values of the MORBs range from −0.27 ± 0.03‰ to −0.07 ± 0.02‰ (n = 28), and two distinct groups of basalts are identified based on the correlation between δ53Cr and MgO. On the one hand, the δ53Cr of group I basalts (−0.27 ± 0.03‰ to −0.14 ± 0.03‰; n = 24) are systematically lower than the established average value of the Bulk Silicate Earth (BSE) (δ53Cr = −0.12 ± 0.04‰; 2SD), which are positively correlated with their MgO and Cr concentrations, indicating that Cr isotopes are fractionated during magmatic differentiation. Moreover, Rayleigh fractionation modelling suggests that the crystallization of olivine, clinopyroxene, and spinel gives rise to the Cr depletion and thereby decreases δ53Cr values. On the other hand, group II basalts (−0.10 ± 0.03‰ to −0.07 ± 0.02‰; n = 4) exhibit higher δ53Cr values than group I with identical MgO and Cr concentrations. This is possibly associated with the crystallization of clinopyroxene under low pressure.The average Cr isotope composition (δ53Cr = −0.16 ± 0.02‰, n = 3) of the primitive basalts (MgO > 9%) represents that of the primary MORB melt, which is lighter than the average value of BSE. Using the non-modal melting equations, the δ53Cr of the MORB mantle source is estimated to be −0.12 ± 0.02‰ (2σ), which is consistent with that of BSE. Compared with the Cr isotopic data of OIBs from Fangataufa island, we find that the equilibrium fractionation factors (Δ53Crcrystal-melt = +0.04‰ to +0.13‰) of MORBs during fractional crystallization are larger than that of Fangataufa island lavas (Δ53Crcrystal-melt = 0.010 ± 0.005‰ for low-K suite, and 0.020 ± 0.010‰ for high-K suite; Bonnand et al., 2020a), indicating that the basalts from Fangataufa island have higher oxygen fugacity than those of MORBs analyzed in this study, which is strongly supported by their higher V/Sc ratios.

Geochemistry of mantle source during the initial expansion and its implications for the opening of the South China Sea

The South China Sea (SCS) is the largest extensional basin in the western Pacific. Due to the lack of sampling appropriate seafloor rocks, few studies focused on magma evolution in the initial expansion of the SCS, which limits the understanding of the tecto-magmatic evolution of the SCS. Here, we analyzed whole-rock Sr-Nd-Pb-Hf isotopic compositions as well as major and trace element concentrations of basalts recovered from the International Ocean Discovery Program (IODP) Expedition 367 Site U1500B in the northern margin of the SCS, which represents the early-stage formed oceanic crust. Combining with published oceanic crust basalts (e.g., IODP Expedition 349 Site U1431 in the eastern sub-basin, Sites U1433 and U1434 in the southwestern sub-basin) formed in the late spreading, the mantle nature and its evolution during the expansion of the SCS were evaluated. Although Sr-Nd-Pb-Hf isotope compositions of these basalts from the SCS suggest an Indian-type mid-ocean ridge basalts (MORB) mantle, the northern margin basalts from Site U1500B show moderate Pb isotopic compositions between those from the eastern and southwestern sub-basin, suggesting a heterogeneous mantle source beneath the SCS. Modeling calculations based on Sr-Nd-Pb-Hf isotopic compositions show that different amounts of lower continental crust and Hainan plume can account for the heterogeneity of such Indian-type MORB mantle in the SCS. The difference in the involved enriched components in the mantle source might be related to the periodic expansion of the SCS. Based on the geochemical imprints of Hainan plume on the SCS basalts that formed in the initial and late spreading stage, we propose that Hainan plume has a great effect in triggering the opening of the SCS.

Petrogenesis of Lava from Christmas Island, Northeast Indian Ocean: Implications for the Nature of Recycled Components in Non-Plume Intraplate Settings

Lava samples from the Christmas Island Seamount Province (CHRISP) record an extreme range in enriched mantle (EM) type Sr-Nd-Pb-Hf isotope signatures. Here we report osmium isotope data obtained on four samples from the youngest, Pliocene petit-spot phase (Upper Volcanic Series, UVS; ~4.4 Ma), and four samples from the earlier, Eocene (Lower Volcanic Series, LVS; ~40 Ma) shield building phase of Christmas Island. Osmium concentrations are low (5–82 ppt) with initial Os isotopic values (187Os/188Osi) ranging from (0.1230–0.1679). Along with additional new geochemical data (major and trace elements, Sr-Nd-Pb isotopes, olivine δ18O values), we demonstrate the following: (1) The UVS is consistent with melting of shallow Indian mid-ocean ridge basalt (MORB) mantle enriched with both lower continental crust (LCC) and subcontinental lithospheric mantle (SCLM) components; and (2) The LVS is consistent with recycling of SCLM components related to Gondwana break-up. The SCLM component has FOZO or HIMU like characteristics. One of the LVS samples has less radiogenic Os (γOs –3.4) and provides evidence for the presence of ancient SCLM in the source. The geochemistry of the Christmas Island lava series supports the idea that continental breakup causes shallow recycling of lithospheric and lower crustal components into the ambient MORB mantle.

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.

Re-visiting barium isotope compositions of mid-ocean ridge basalts and the implications

Barium (Ba) isotopes can be used as potential tracers for crustal material recycling in the mantle. Determination of the Ba isotope composition of the depleted mantle is essential for such applications. However, Ba isotope data for mantle-derived basalts are still rare. In this study, we reported high-precision Ba isotope data of 30 oceanic basalts including 25 mid-ocean ridge basalts (MORBs) from geochemically and geologically diverse mid-ocean ridge segments and five back-arc basin basalts. The δ&lt;sup&gt;138/134&lt;/sup&gt;Ba values of these samples varied from −0.06‰ to +0.11‰, with no systematic cross-region variation. Together with published data, we constrained the average δ&lt;sup&gt;138/134&lt;/sup&gt;Ba of global MORBs to +0.05‰±0.09‰ (2 standard deviation, &lt;i&gt;n&lt;/i&gt; = 51). Based on depleted MORBs that have (La/Sm)&lt;sub&gt;N&lt;/sub&gt; &lt; 0.8, low &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr (&lt; 0.70263), and low Ba/Th &lt; 71.3, we estimated the average δ&lt;sup&gt;138/134&lt;/sup&gt;Ba of the depleted MORB mantle (DMM) as + 0.05‰ ± 0.05‰ (2SD, &lt;i&gt;n&lt;/i&gt; = 16) that is significantly lower than the DMM (≈ 0.14‰) reported previously. If a new estimation of the DMM is applied, it is unreasonable to infer that the Ba isotope signatures of the “enriched-type” MORBs (E-MORBs) could be attributed to pervasive sediment recycling in the upper mantle. We, therefore, conclude that the Ba isotope compositions of the E-MORBs could be sourced from the incorporation of subducted altered oceanic crust and/or sediments depending on the Ba isotope composition and other geochemical information of the local mantle.