Mid-ocean Ridge Basalt Research Articles

Syn-orogenic tectonomagmatic evolution of the Qilian Orogen: Insights from the Lumanshan gabbro–granite association in the Qilian Block, Northwest China

Gabbro–granite associations provide critical geological clues for unraveling crust–mantle interactions, regional tectonics and geodynamic processes. Many Late Ordovician–Early Devonian granitoids occur in the Qilian Block, display high-K calc-alkaline affinities but have less well-constrained geodynamic mechanisms. Here, we report the newly discovered Late Ordovician-Early Devonian Lumanshan (LMS) gabbro–granite association in the Qilian Block that is composed mainly of syenogranites and alkali-feldspar granites, with minor gabbro. Zircon U Pb dating yields crystallization ages of ca. 440 Ma for the gabbros and ca. 450 Ma and 414 Ma for the syenogranite and alkali-feldspar granite, respectively. The gabbros are calc-alkaline series and display enriched mid-ocean ridge basalt ( E -MORB)-like rare earth element (REE) patterns; U, Cs, and Rb enrichments; Nb, Ta, Zr, and Hf depletions; and depleted Sr–Nd–Hf isotope compositions. The results indicate that the gabbros were possibly derived from partial melting of an asthenospheric mantle wedge metasomatized by aqueous fluids. Two granite types within the LMS pluton are peraluminous and have a transitional geochemical affinity between S- and I-type, low Mg# values, and similarly evolved Sr Nd isotope compositions, which overlap with those of Neoproterozoic basement rocks exposed in the study area. The syenogranites show adakitic geochemical characteristics with high La/Yb and Nb/Ta ratios, whereas the alkali-feldspar granites are non-adakitic granites with flat heavy REE patterns and significant Ba, Sr, and Eu depletions, suggesting that they formed at lower and middle crustal levels, respectively. The LMS ca. 450–440 Ma gabbro and syenogranite were emplaced during a back-arc extensional regime associated with roll-back of the northward-subducting South Qilian Ocean slab. Synthesized data from this and previous studies suggest that the final closure of the South Qilian Ocean and continental collision between the Qilian and Qaidam blocks likely occurred at ca. 435 Ma, and the ca. 414 Ma LMS alkali-feldspar granites generated in syn -collisional setting related with exhumation of the subducted continental crust. The LMS alkali-feldspar granites also show decoupled Nd Hf isotopic systems, which should result from significant disequilibrium melting of metasedimentary materials with the apparent abundance of inherited zircons in their source due to a less intense thermal regime. • A gabbro–granite association in the Qilian Block formed in early–middle Paleozoic. • The ca. 472–435 Ma magmatism formed in an extensional setting related to slab roll-back. • The ca. 435–400 Ma magmatism occurred in syn-collisional setting.

Comparative Nd-Sr-Pb isotopes geochemistry of the eastern Circum-Rhodope belt ophiolitic mafic suites, Greece-Bulgaria

We report on the isotopic compositions of the Jurassic supra-subduction zone Evros ophiolite mafic rocks exposed in the eastern Circum-Rhodope Belt in the Thrace region of northeastern Greece. These mafic units consist of low-Ti gabbroic and basaltic rocks, which Nd-Sr-Pb isotopes are compatible with dominant mantle-derived Mid-Ocean Ridge Basalt (MORB) component mixed with a detectable amount of crustal material and/or sediment involved in their melt source in the subduction zone. These isotopic features are consistent with an intra-oceanic arc origin of the mafic ophiolite rocks, and the Evros ophiolite Nd and Pb isotopes are comparable to those of the counterpart mafic rocks from the Mandritsa Unit in the eastern Rhodope Massif of southern Bulgaria.

Geochronology and geochemistry of the Manxin ophiolitic mélange in the Changning‐Menglian Suture Zone, southwest China: Implications for the tectonic evolution of the Proto‐Tethys Ocean

To date, the evolutionary history of the Proto‐Tethys Ocean that separated the Gondwana and Laurasia continents remains ambiguous. The Changning‐Menglian Suture Zone assumingly represents the Proto‐ and Palaeo‐Tethys suture in the Sanjiang area (the Yunnan Province, southwestern China). The Palaeozoic Proto‐Tethys Ocean in the Manxin area in the southmost Changning‐Menglian Suture Zone is widely recognized, but the timing and formation mechanism of the Manxin ophiolitic mélange are poorly understood. In this study, we present petrographical, geochronological and geochemical data of the Manxin ophiolitic mélange situated in the Changning‐Menglian Suture Zone. We found that this ophiolitic mélange is composed of pillow basalt, metamorphic gabbro, diabase dike, and altered peridotite. The mafic rocks could be divided into two groups based on geochemical characteristics: (1) Mid‐ocean ridge basalt (MORB) [formed at 446.0 ± 3.2 Ma and exhibiting depleted light rare earth element (LREE) patterns and high ƐNd(t) (+5.7 to +8.4)] Trace element and isotope analyses indicated that the MORB was derived from a spinel‐facies depleted mantle. (2) Ocean island basalt (OIB) [crystallized at 460.2 ± 6.0 Ma]. ƐNd(t) (+5.7) and significant enrichments in large‐ion lithophile element (LILE) and light rare earth element (LREE) were identified, while the trace element and isotope systematics suggested that the OIB rocks originated from partial melting of the garnet‐spinel lherzolite mantle. Overall, we clearly constrained the timing and nature of the Manxin ophiolitic mélange using the proposed data, thus further supporting the notion that the Manxin ophiolites represent remnants of the Proto‐Tethys Ocean.

Hot or Fertile Origin for Continental Break-Up Flood Basalts: Insights from Olivine Systematics

Abstract The break-up of supercontinents is often temporally and spatially associated with large outpourings of basaltic magmas in the form of large igneous provinces (LIPs) and seaward dipping reflectors (SDRs). A widespread view is that the upwelling of hot mantle plumes drives both continental break-up and generation of associated LIPs. This is supported by petrologic estimates of the temperature from olivine-melt thermometers applied to basaltic magmas. These thermometers must be applied to a primary mantle-derived magma, requiring the selection of an appropriate primitive magma and an assumption of how much olivine is to be back-added to correct for fractional crystallization. We evaluated the effects of these assumptions on formation temperatures by compiling and analyzing a database of North Atlantic igneous province (NAIP) and Central Atlantic magmatic province (CAMP) lavas and olivines. Ni and FeOT systematics suggest that many picrite magmas have undergone olivine addition and are not true liquids, requiring careful selection of primitive magmas. The maximum amount of back-added olivine was determined by constraining mantle peridotite melt fractions for a range of possible mantle potential temperatures and continental lithosphere thicknesses. Using an empirical relationship between melting degree and forsterite (Fo) content, we show that the possible maximum olivine forsterite content in equilibrium with NAIP magmas is 90.9, which is lower than the maximum olivine forsterite content observed in the NAIP olivine population. We infer primary magmas that lead to mantle potential temperatures of 1420°C for the NAIP and 1330°C for CAMP. Using a similar approach for consistency, we estimate a mantle potential temperature of 1350°C for mid-ocean ridge basalts (MORB). Our results suggest that LIPs associated with continental break-up are not significantly hotter than MORB, which suggests that continental break-up may not be driven by deep-seated thermal plumes. Instead, we suggest that such voluminous magmatism might be related to preferential melting of fertile components within the lithosphere triggered by far-field extensional stresses.

Mo isotopes archive oceanic sediments in post-orogenic lithospheric mantle

The super-chondritic Mo isotopic composition of the continental crust has been attributed to contributions from subducted oceanic slabs and/or sediments, which introduce isotopically light Mo to the mantle. It has been hypothesized that dehydrated oceanic slabs subduct into the deep mantle and contribute to the genesis of ocean island basalts (OIBs). However, OIBs have similar average δ98/95Mo values (−0.22‰± 0.04‰; 2SE; n = 87) to the depleted mantle. Subducted oceanic sediments may create a light-δ98/95Mo reservoir in the subcontinental lithospheric mantle, but this has not been investigated in detail. Understanding the Mo isotopic systematics of the continental crust and mantle requires an understanding of the Mo isotopic composition of lithospheric mantle-derived volcanic rocks. The lithosphere in the Tibetan region has experienced multiple stages of oceanic subduction and is thus ideal for studying magmas derived from sediment-modified lithospheric mantle. Mafic ultrapotassic volcanic rocks (UPVRs) from the Sailipu area on the southern Tibetan Plateau have high Mg# values (67.1–72.5) and enriched Sr–Nd isotopic compositions and are primitive magmas derived from the lithospheric mantle. These rocks have higher Ce/Mo ratios (225–422) and lighter δ98/95Mo values (−0.45‰ to −0.13‰) than the depleted mantle, mid-ocean ridge basalts (MORBs), and OIBs, as well as high εHf(t) values at given εNd(t) values compared with the terrestrial array (ΔεHf = 4.59–5.73). The involvement of Ca-poor pelagic sediments rather than the lower continental crust and hydrated oceanic slab best accounts for the high Ce/Mo ratios and light δ98/95Mo values of the studied rocks. The Sailipu UPVRs also have higher Nb/Ta (18.9–20.8) and Th/La (2.6–3.4) ratios than arc-related igneous rocks and MORBs, suggesting that their source was phlogopite-bearing lithospheric mantle with a metasomatic vein component (rutile + monazite ± lawsonite). We propose a two-stage geodynamic process to explain the formation of the Sailipu UPVRs: (1) a metasomatic vein component with high Th/La and Ce/Mo ratios formed in the lithospheric mantle during multiple stages of oceanic subduction; (2) high mantle temperatures related to convective thinning of the lithosphere resulted in melting of the vein component and the ambient mantle, which produced hybrid melts of various mantle domains during the post-orogenic stage. The Sailipu UPVRs have enriched Sr–Nd isotopic compositions and high Th/La and Ce/Mo ratios, similar to lamproites that are widespread in the Tethyan orogenic realm from the Mediterranean to the southern Tibetan Plateau. Given that UPVRs with light δ98/95Mo values have also been identified in the Western Alps, we propose that the isotopically light Mo component is widespread in Tethyan lithospheric mantle. This isotopically light Mo in the lithospheric mantle may be a complementary end-member to the isotopically heavier continental crust. The recycling of subducted oceanic sediments has a key role in the deep recycling of Mo.

Widespread phosphorous excess in olivine, rapid crystal growth, and implications for magma dynamics

Trace element zoning is often used to unravel the crystallization history of phenocrysts in magmatic systems, but interpretation requires quantifying the relative importance of equilibrium versus disequilibrium. Published partition coefficients for phosphorous (P) in olivine vary by more than a factor of ten. After considering kinetic effects, a new equilibrium partition coefficient was extrapolated from a re-examination of natural and experimental systems, indicating that P partition coefficients in olivine are significantly over-estimated. These new partitioning constraints allow us to establish a theoretical P Equilibrium Fractionation Array (PEFA) for mid-ocean ridge basalts (MORBs), revealing that most olivines from MORBs have excess P (2–15 times PEFA) and are thus in disequilibrium. Using an independent case study of natural dendritic olivines, we show that such P enrichments can be explained by diffusion-limited incorporation of P during rapid crystal growth. If growth rate can be related to cooling, the rapid growth rates of olivines have implications for magma system dynamics, such as the size of magma bodies or where crystallization occurs within the body.

Iron and silicon isotope fractionation in silicate melts using first-principles molecular dynamics

The direct determination of silicate melts iron and silicon isotopes signature remains a major challenge of high-temperature isotope geochemistry. For this reason, melts are often approximated by silicate glasses. Calculation of precise equilibrium Si and Fe isotopes fractionation factors between minerals and melt would indeed allow us to distinguish equilibrium fractionation from diffusion-driven kinetic fractionation involved in the iron and silicon isotopes signatures of Earth and other planets. In this study, we use for the first time, first-principles molecular dynamics based on density functional theory to determine iron and silicon isotope compositions of different silicate melts, namely: iron-rich basalt, iron-depleted basalt, basanite, trachyte and phonolite. The 57Fe/54Fe reduced partition function ratios (β-factors) of the different melts span over a 1.1 ‰ range at 1000 Kelvin (K) while 30Si/28Si β-factors are much less influenced by the melt composition with a 0.5 ‰ fractionation range at the same temperature. The main parameter controlling iron isotope fractionation in silicate melts having similar iron oxidation state is, after temperature, the average Fe-O bond length. The chemical environment around iron (e.g. Fe-Fe distances) is suggested to contribute to Fe isotope fractionation as well. Silicon isotopes fractionation seems also affected, but to a lesser extent, by its local chemical composition with decreasing Si-Fe distances leading to slightly higher Si β-factor in the melt. From these melts Fe and Si β-factors, a new set of equilibrium fractionation factors between a variety of minerals and melts has been calculated. These new Δ57Femin-melt and Δ30Simin-melt sets allow us to discuss whether processes such as fractional crystallization, partial melting and diffusion could be responsible for the documented Fe and Si isotopes variations in igneous rocks. Our results suggest that: 1) fractional crystallization may explain at least part of the Fe and Si isotopic evolution during magmatic differentiation, for values up to δ57Fe = 0.65 ‰ and δ30Si = -0.1 ‰, respectively, 2) partial melting of the upper mantle can produce the Mid-Ocean Ridge Basalts (MORB) iron isotopes signature. Finally, we calculated that olivine at equilibrium with a basaltic melt could display an iron isotope composition down to −0.1 ‰ for δ57Fe. Hence, the lower isotopic compositions (δ57Fe < -0.1 ‰) observed in natural olivines are most likely due to diffusion-driven kinetic fractionation.

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.

Metamorphic Evolution and Orogenic Process Related to the Eastern Paleo-Tethyan Warm Subduction and Indochina–South China Collision

Abstract The eastern Paleo-Tethyan Jinshajiang–Ailaoshan–Song Ma orogenic belt in the Southeast Tibetan Plateau represents the tectonic boundary between the Indochina and South China blocks. Regional Cenozoic lithospheric strike-slip movements caused by the India–Asia collision have modified many of the pristine geological records associated with the Indosinian orogeny. The lack of reliable petrological evidences, particularly of high-pressure (HP) metamorphism, has hindered the tracing of the evolutionary history of the Paleo-Tethyan orogenic belt. We report the mineralogy, geochemistry, geochronology and thermodynamic modelling of eclogite lenses/blocks and their host garnet–phengite schists from the Song Ma Suture Zone of Northwest Vietnam and reveal their protolith origins and metamorphic evolution. The eclogites are geochemically similar to mid-ocean ridge basalt, showing weak depletions in high field-strength elements, positive to slightly negative whole-rock ɛNd(t) ratios (−1.69 to +5.15) and slightly high 87Sr/86Sr(t) ratios (0.70466–0.70834). The igneous protolith of the eclogites formed in the late Silurian (425.4 ± 3.6 Ma; igneous zircon dating, 638–459 Ma TDM dating), which makes them the oldest known mid-ocean ridge mafic rocks in an ancient Paleo-Tethyan Ocean. Various eclogites preserve similar peak mineral assemblages (garnet + omphacite + phengite + rutile + quartz ± epidote/clinozoisite ± kyanite ± winchite) under indistinguishable peak conditions. Their prograde P–T trajectories show a uniform pattern of progressive heating and weak compression along a geothermal gradient of 8°C–10°C/km, indicating a warm subduction setting at a low angle that contrasts with the cold oceanic subduction recorded in the adjacent Paleo-Tethyan suture zones. Combined with detrital U–Pb ages from previous studies, our geochronological data indicate that the host schists originated from continental crust within the South China Block and underwent a pervasive Triassic HP metamorphism related to the Indosinian orogeny. The eclogites and host schists yielded HP metamorphic ages of 239–234 Ma and retrogressive amphibolite-facies metamorphic ages of 231–229 Ma, suggesting rapid cooling (14.0°C–21.6°C/Myr) and exhumation (6.4 km/Myr) from the mantle to the crust. The Song Ma eclogites and their host schists formed when the final ocean closure transitioned to the initial Indochina–South China collision. The Song Ma eclogites and their host schists provide essential information on the opening and closure of the eastern Paleo-Tethys and the subsequent amalgamation of Southeast Asian continental fragments.

High-C content and CO2/Ba ratio of the Earth’s enriched upper mantle

One of the fundamental constraints on the carbon content in the Earth’s mantle comes from depleted mid-ocean ridge basalts (D-MORB) that experienced minimal CO2 degassing. The CO2/Ba ratio of 120 ± 20 in these D-MORB have been used to estimate the CO2 content in more enriched MORB (e.g., N- and E-MORB) that have lost CO2 through degassing. However, it has been unclear as to how constant the CO2/Ba ratio is in D-, N-, and E-MORB, which are known to derive from partial melting of a chemically and isotopically heterogeneous upper mantle. In this study, we constrained the CO2/Ba ratio of the E-MORB mantle source in the Pacific by studying the compositions of MORB glass samples from the southern East Pacific Rise (on– and off-axial SEPR and Garrett transform fault) as well as volcanic glass samples from the Pukapuka ridges and Rano Rahi seamounts that extend to the west of the SEPR. Some of the submarine glasses from the Rano Rahi seamounts are D-MORB with highly depleted CO2 and incompatible trace element contents, but with isotopic composition pointing to a long-term enriched mantle source. These lavas were generated by re-melting of the residual Pacific E-MORB mantle source that had undergone a recent melting episode responsible for the Pukapuka ridge magmatism. Some Rano Rahi seamount lavas experienced limited CO2 degassing during eruption, and we consider that their most reliable pre-degassing CO2/Ba ratio is the maximum value of 241 ± 30, measured in the most trace element depleted but isotopically enriched sample. While we show that this high CO2/Ba ratio is in part due to fractionation during the earlier melting event that generated the Pukapuka ridges, the estimated initial CO2/Ba ratio of the Pacific E-MORB mantle source is 177-52+71, still higher on average than that of the D-MORB mantle source (120 ± 20). We use this CO2/Ba ratio for the Pacific E-MORB mantle source to provide an estimate of its CO2 content of 391-180+455 ppm. We speculate that the high CO2/Ba and C-enriched nature of the Pacific E-MORB mantle source could originate from either (1) recycling of oceanic lithosphere at subduction zones or (2) foundering of metasomatized continental lithospheric mantle into the convecting mantle.

Sulfur recycling in subduction zones and the oxygen fugacity of mafic arc magmas

Sulfur is important to a range of subduction zone processes, but the factors controlling the sulfur content of primitive arc magmas are poorly understood. In particular, uncertainties about the oxidation state of primary melts in the sub-arc mantle hinder efforts to understand the behavior of sulfur in arc magmas. Here, we use olivine-hosted melt inclusion data from 32 arc segments globally to characterize sulfur contents of magmas from a variety of subduction zones and identify the key processes that control the sulfur content of arc magmas. Average primary magma sulfur contents estimated from these data range from 466±220ppm (Mariana Forearc) to 4,264±819ppm (Ecuador). Primitive and more evolved magmas in both hot- and cold-slab subduction zones commonly have higher sulfur contents than mid-ocean ridge basalt (MORB) magmas. In most cases, these elevated sulfur contents require a subduction-modified mantle source with more sulfur than MORB-source mantle. Correlations between magma S and Cl concentrations and S/Dy and Th/Yb ratios in the global data set confirm that sulfur is partially sourced from the subducting slab. By comparing melt inclusion sulfur contents to sulfur solubility limits imposed by sulfide saturation, we find that 88% of arc magmas in our compilation require conditions more oxidizing than the quartz-fayalite-magnetite buffer (QFM). The relationship between S/Dy and calculated magmatic oxygen fugacity (fO2) suggests that slab-derived sulfate is responsible for the higher oxidation state of arc magmas compared to MORB. Model calculations of redox equilibrium between S and Fe species in basaltic melts when sulfate is added to the mantle wedge demonstrate the feasibility of this causal link, which can explain the common range of arc magma fO2 values observed for subduction zones globally. Correlations between magma sulfur contents, fO2, and the proportion of added slab-derived material inferred from trace elements can drive associations between high Sr/Y magmas and fertile porphyry ore deposits, as well as the MORB-like Cu contents of arc magmas.

Geochemistry of volcanic rocks and dykes from the Remeshk-Mokhtarabad and Fannuj-Maskutan Ophiolites (Makran Accretionary Prism, SE Iran): New constraints for magma generation in the Middle East neo-Tethys

The Remeshk-Mokhtarabad and Fannuj-Maskutan ophiolites represent two major ophiolitic units in the North Makran Domain (Makran Accretionary Prism). Volcanic rocks and dykes of these ophiolites mainly consist of basalts and rare basaltic andesites, andesites and dacites. No chemical distinction can be seen in basalts from these two ophiolitic units, or between volcanic rocks and dykes. Basaltic rocks show a broad MORB-type nature but variable chemical composition (e.g., SiO2= 42.64–52.63 wt%; TiO2= 0.98–2.43 wt%; Mg# = 71–50). They show both N-MORB (Type 1) and E-MORB (Type 2) compositions (MORB: mid-ocean ridge basalt; N: normal; E: enriched). Type 1 rocks are very rare in both ophiolitic units, whereas Type 2 rocks are predominant. Type 1 rocks show low Th (0.10–0.16 ppm), Nb (1.86–2.82 ppm), Ta (0.09–0.17 ppm) abundance and low (La/Yb)N (0.50–0.75), (La/Sm)N (0.48–0.72) ratios. Compared to N-MORBs, Type 2 basalts show slight enrichment in Th (0.42–1.60 ppm), Nb (6.09–14.6 ppm), and Ta (0.227–0.792 ppm), as well as (La/Yb)N and (La/Sm)N ratios >1 like those observed in typical E-MORB. Trace element petrogenetic models indicate that primitive basalts derived from partial melting of a heterogeneous sub-oceanic mantle variably metasomatized by plume-type (OIB-) components. Type 1 basalts derived from partial melting of mantle regions with no enrichment in OIB-type components, whereas Type 2 basalts derived from partial melting of DMM sources variably enriched by OIB-components. These rocks formed in an oceanic basin that was strongly affected by mantle plume activity and different extents of plume-ridge interaction.

Magnesium isotopic composition of back-arc basin lavas and its implication for the recycling of serpentinite-derived fluids

Dehydrated fluids expelled from the serpentinised mantle in subducted slabs contribute significantly to arc magmatism and element cycling on Earth. However, the recycling mechanism of serpentinite-derived fluids at various subduction zone depths remains unclear, as does their contribution to the genesis of back-arc basin lavas. In this study, we investigated magnesium (Mg) isotopic compositions of lavas from the Okinawa Trough (OT) and the Lau Basin, because Mg isotopes have shown great potential in tracing the dehydration of slab serpentinites in recent years. After excluding dubious samples affected by alteration, lavas from the OT and the Lau Basin have heavier Mg isotopic compositions than mid-ocean ridge basalt (MORB) mantle, which could be attributed to the involvement of slab serpentinite-derived fluids rather than crustal assimilation or subducted sediment inputs, as indicated by the isotopic modelling results. The δ26Mg values of the southern OT (SOT) and the southern Lau Basin lavas are generally higher than those in the middle OT (MOT) and the northern Lau Basin, respectively, with an average of −0.09 ± 0.02‰ (2SD, n = 6) for the SOT, −0.18 ± 0.10 ‰ (2SD, n = 9) for the MOT, −0.13 ± 0.08 ‰ for the southern Lau Basin (2SD, n = 5), and − 0.19 ± 0.06 ‰ (2SD, n = 10) for the northern Lau Basin. The binary modelling results show that various quantities of serpentinite-derived fluids could explain the variations in Mg isotopic compositions observed in the OT and the Lau Basin. We suggest that the thermal structure of the inter-subduction zone controls the signal of Mg-rich serpentinite-derived fluids, based on both reported δ26Mg values in subduction zones and our findings. In contrast, the contributions of these fluids to different segments in a specific subduction zone may depend on the slab depth beneath sites of magmatic activity.

Redox evolution of crystallizing magmas with C-H-O-S volatiles and its implications for atmospheric oxygenation

The evolution of Earth’s atmospheric O2 levels on long timescales is modulated in part by the composition of magmatic gases, specifically their oxidation state and speciation. Here, we examine how magma differentiation affects the oxygen fugacity and composition of magmatic gases. Variations in temperature, pressure, composition, and redox conditions of evolving magmatic systems result in complexities, which have not been fully treated previously. Taking all these variables into account, we develop a multipronged approach to a comprehensive understanding on the role of magmatic gases on atmospheric oxygenation. This includes a compilation of petrological observations on the redox systematics of mid-ocean ridge basalts, a geochemical model for magma crystallization in oxygen-closed systems with variable initial redox conditions, and a thermodynamic framework for gas–melt systems with multi-component C-H-O-S volatiles.By considering magmatic differentiation at variable depths, we show that magma crystallization progressively changes the redox state of residual melts and coexisting gases due to differences in iron speciation between the melt and cumulate minerals. In thick crusts, high pressures of magma differentiation suppress magnetite saturation but promote garnet crystallization, increasing magmatic oxygen fugacity by ∼4–8 orders of magnitude and thereby resulting in oxidized gases. In thin crusts, the prevalence of magnetite prevents significant oxidation of the magma, yielding reduced gas emission. Hence, magmatic gases from thick crusts are much less effective O2 sinks, which does not appear to be affected by gas decompressional transport or polybaric degassing from the deep crustal magmatic zone to the atmosphere. Depending on how crustal thickness during active magmatism varies with time, magmatic gases may play an important role in modulating atmospheric O2 levels on million-year timescales. Rapid growth of continental crust in the late Archean appears to have been associated with the first evidence for significant crustal thickening during magmatism. If so, this would have led to a substantial decrease in O2 sink, possibly permitting atmospheric O2 to rise to a new state.

Recycling of carbonates into the deep mantle beneath central Balkan Peninsula: Mg-Zn isotope evidence

Magnesium (Mg) and zinc (Zn) isotopes have been applied to trace whether surface carbonates have been recycled into the deep mantle beneath Balkan Peninsula, where a stagnant slab exists in the mantle transition zone. Here we investigate a suite of Cenozoic sodic alkaline basaltic rocks (Na2O/K2O > 1), called the East Serbian Mafic Alkaline Rocks (ESMAR) from East Serbia in central Balkan Peninsula, which is an important segment of the Tethyan orogenic belt. The ESMAR have lower δ26MgDSM-3 (−0.49‰ to −0.39‰; n = 21) and higher δ66ZnJMC 3-0749L (0.35‰ to 0.45‰; n = 21) than those of normal mantle-derived melts (e.g., mid-oceanic ridge basalts; MORBs; δ26Mg = −0.25 ± 0.06‰ and δ66Zn = 0.27 ± 0.06‰). After excluding the effects of low-T and magmatic processes, the Mg and Zn isotopic anomalies of the ESMAR are proposed to have been caused by recycled carbonates in their mantle sources. The significantly low Zn contents, Zn/Fe and Fe/Mn ratios of the ESMAR indicate a highly depleted peridotitic mantle source. Their major element compositions (e.g., low Ti2O and high CaO/Al2O3) are consistent with those of experimental partial melts of “peridotite + CO2”. The incremental batch melting and end-member mixing models suggest that ∼5–10% of Mg-rich carbonates (dolomite and magnesite) have been recycled into the mantle sources of the ESMAR. Our results provide evidence for deep recycling of carbonates related to subduction of a Tethyan oceanic slab, most likely into the mantle transition zone beneath the Balkan Peninsula. This contributes a significant part of global deep carbon cycling during the Cenozoic.

Textures Induced by the Coesite‐Stishovite Transition and Implications for the Visibility of the X‐Discontinuity

AbstractThe coesite‐stishovite phase transition is considered the most plausible candidate to explain the X‐discontinuity observed at around 300 km depth in a variety of tectonic settings. Here, we investigate the microstructure in SiO2 across the coesite‐stishovite transition in uniaxial compression experiments. We apply the multigrain crystallography technique (MGC) in a laser‐heated diamond‐anvil cell (LH‐DAC) to identify the seismic signature of the transition and the amount of SiO2 in the mantle. While coesite displays weak lattice‐preferred orientations (LPO) before the transition, stishovite develops strong LPO characterized by the alignment of [112] axes parallel to the compression direction. However, LPO has little effect on the impedance contrast across the transition, which is up to 8.8% for S‐waves in a mid‐ocean ridge basalt (MORB) composition at 300 km depth along a normal mantle geotherm, 10 GPa‐1700 K. Therefore, 10–50 vol.% of a MORB component, corresponding to 0.6–3.2 vol.% SiO2, mechanically mixed with the pyrolytic mantle would be required to explain the range of impedance (and velocity) contrasts observed for the X‐discontinuity. Based on the reflection coefficients computed for the coesite‐stishovite transition, we show that the incidence angle or epicentral distance is critical for the detection of silica‐containing lithologies in the upper mantle, with highest detection probabilities for small incidence angles. The intermittent visibility of the X‐discontinuity may thus be explained by the seismic detectability of the coesite‐stishovite transition rather than by absence of the transition or chemical heterogeneities in some specific tectonic settings.

The Early Paleozoic Subashi ophiolite in the West Kunlun Orogenic Belt (northwestern Tibetan Plateau): Implication for the tectonic evolution of the Proto-Tethys

The West Kunlun Orogenic Belt (W-KOB) in the northwestern Tibetan Plateau retains critical information about the tectonic evolution of the Proto- and Paleo-Tethys during the Early Paleozoic to Mesozoic. Although a trench-arc-back arc basin model was used to address the tectonic evolution of Proto-Tethys, records about the evolution of this ocean basin remain ambiguous, particularly no studies about ages and tectonic affinities of ophiolites in the W-KOB are available. In this paper, we present new results on whole-rock elemental and Sr-Nd isotopic composition of basalts, and U-Pb zircon ages of gabbro, diabase and basalt from the Subashi ophiolite. The basalts are characterized by low REE concentrations, depleted LREEs in the chondrite normalized REE patterns, and unfractionated HFSEs in the primitive mantle normalized trace element spider diagrams, resembling normal mid-ocean ridge basalts. They have constant 87Sr/86Sr(i) values of 0.70472–0.70590 and positive εNd values of +7.5 to +7.9. Accordingly, it can be concluded that the Subashi ophiolite was derived from a depleted spinel lherzolite mantle source in a mid-ocean ridge setting. Zircon grains separated from basalt, diabase and gabbro have 206Pb/238U ages of 455 ± 4 Ma, 451 ± 6 Ma, and 446 ± 6 Ma, respectively, which are taken to be the crystallization ages of these rocks. Together with previously published data, our new data propose that the Mazha-Kangxiwa-Subashi ophiolitic mélange zone marks the existence of the Proto-Tethys ocean basin during the Early Paleozoic.

No measurable calcium isotopic variations of back-arc lavas across the Okinawa Trough

Calcium (Ca) is an essential element closely connecting to the recycling of marine carbonate at subduction zones. Here, we report Ca isotopic compositions (denote as δ44/40Ca with respect to NIST SRM915a) of back-arc lavas collected across the Okinawa Trough. The δ44/40Ca values showed a narrow range with an average of 0.81 ± 0.08‰ (2SD, n = 13), almost identical to reported values of arc magmas, mid-ocean ridge basalts (MORBs), and other mafic rocks. The MORB-like δ44/40Ca values were dominantly attributed to the mantle partial melting. The accumulation of clinopyroxene and plagioclase has played a negligible role on the Ca isotopes, despite the collected lavas exhibited distinctive rock types ranging from basalt to dacite. The metasomatism by subduction components (slab-derived fluids, sediment melts) and/or crustal assimilation have imprinted trace elemental and HfNd isotopic signals on the Okinawa Trough lavas, while the Ca isotopes have not been significantly modified. Our work suggests that thermal structures and tectonic stages are not pivotal factors constraining the Ca isotopic characteristics of arc and back-arc magmas at subduction zones. The opportunity and possibility to discriminate carbonate Ca isotopic signals in subduction zones are obscured by the highly variable Ca isotopes and carbonate contents of subducted sediments, the homogenization effect of the mantle wedge, variable degrees of sediment melting, and the survival of sediment melts during petrogenesis.

Multiple volcanic episodes of the Kermanshah forearc basin, SW Iran: a record of the deactivation and re-initiation of Neotethyan subduction involving a mid-ocean ridge

Numerical simulations and theoretical analyses predict that forearc ophiolites probably record the end of subduction followed by re-initiation. We report here the results of a field study and new geochronological data that enable identification of the Kermanshah forearc basin, SW Iran. This basin is filled with two terrestrial volcaniclastic successions separated by an Upper Cretaceous reef facies limestone. Early Late Cretaceous ocean island basalts, early Eocene incompatible element-enriched mid-ocean ridge basalts/normal-type mid-ocean ridge basalts and late Eocene arc basalts of the basin indicate that Neotethyan subduction ended during the early Late Cretaceous and then re-initiated during the late Eocene. We suggest that the Neotethyan Ocean between the Iranian and Arabian continents consisted of leading and trailing oceanic plates separated by a mid-ocean ridge (MOR). Subduction of the leading plate generated the Jurassic–Early Cretaceous Sanandaj–Sirjan arc, which was ended by MOR–trench collision. The Zagros late Early Cretaceous forearc ophiolites represent the youngest/hottest segments of the leading plate emplaced during the MOR–trench collision. Subsequently, subduction of the MOR generated the forearc basin and basalts with diverse geochemistry. After c. 60 myr, the cooled trailing plate started to subduct, generating the late Eocene arc basalts. This model has significant implications for investigating and understanding other fossil subduction zones elsewhere in the world. Supplementary material: Supplementary material is available at https://doi.org/10.6084/m9.figshare.c.6086586

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.