Heterogeneous Mantle Research Articles

An entropy method for geodynamic modelling of phase transitions: capturing sharp and broad transitions in a multiphase assemblage

SUMMARY Phase transitions play an important role for the style of mantle convection. While observations and theory agree that a substantial fraction of subducted slabs and rising plumes can move through the whole mantle at present day conditions, this behaviour may have been different throughout Earth’s history. Higher temperatures, such as in the early Earth, cause different phase transitions to be dominant, and also reduce mantle viscosity, favouring a more layered style of convection induced by phase transitions. A period of layered mantle convection in Earth’s past would have significant implications for the secular evolution of the mantle temperature and the mixing of mantle heterogeneities. The transition from layered to whole mantle convection could lead to a period of mantle avalanches associated with a dramatic increase in magmatic activity. Consequently, it is important to accurately model the influence of phase transitions on mantle convection. However, existing numerical methods generally preclude modelling phase transitions that are only present in a particular range of pressures, temperatures or compositions, and they impose an artificial lower limit on the thickness of phase transitions. To overcome these limitations, we have developed a new numerical method that solves the energy equation for entropy instead of temperature. This technique allows for robust coupling between thermodynamic and geodynamic models and makes it possible to model realistically sharp phase transitions with a wide range of properties and dynamic effects on mantle processes. We demonstrate the utility of our method by applying it in regional and global convection models, investigating the effect of individual phase transitions in the Earth’s mantle with regard to their potential for layering flow. We find that the thickness of the phase transition has a bigger influence on the style of convection than previously thought: with all other parameters being the same, a thin phase transition can induce fully layered convection where a broad phase transition would lead to whole-mantle convection. Our application of the method to convection in the early Earth illustrates that endothermic phase transitions may have induced layering for higher mantle temperatures in the Earth’s past.

Differentiation between carbonate and silicate metasomatism based on lithium isotopic compositions of alkali basalts

Abstract Carbonate and silicate metasomatism occurring in subduction zones is an important mechanism underlying mantle heterogeneity and compositional diversity of mantle-derived rocks. However, distinguishing between the two kinds of metasomatism is often difficult. Lithium (Li) and its isotopes have great potential in this regard because of the different Li isotopic compositions of recycled marine carbonate and silicate components. We report Li isotopic data from Cenozoic and Mesozoic alkali basalts of the West Qinling orogen in central China. Relative to those for normal basalts, very high δ7Li values (up to +11.2‰) were observed for the Cenozoic alkali basalts, but significantly and systematically lower values (as low as −3.3‰) were estimated for the Mesozoic alkali basalts. Their abnormal Li isotopic compositions, combined with major- and trace-element contents and Sr-Mg isotope ratios, indicate that the Cenozoic and Mesozoic alkali basalts originated from carbonated and silicated mantle sources, respectively, reflecting metasomatism of the mantle by slab-derived carbonate and silicate melts during Paleotethyan oceanic subduction. Interactions of such melts with the mantle peridotite in subduction channels can account for the elemental and isotopic differences of the studied alkali basalts. The present study demonstrates an effective way to distinguish between carbonate and silicate metasomatism in subduction zones by studying Li isotopic compositions of alkali basalts and highlights the prospect of Li isotopes in tracing the deep carbon cycle.

Highly heterogeneous mantle caused by recycling of oceanic lithosphere from the mantle transition zone

Geochemical heterogeneities observed in the mantle are usually attributed to recycling of oceanic lithosphere through subduction. However, it remains hotly debated where recycled material stagnates, and how quickly it can be liberated back to surface. This knowledge gap hinders our understanding of mantle circulation and the chemical evolution of the Earth. Here we address these questions using a combination of geochronology and geochemistry from South China Sea (SCS) seamounts. The Shixingbei seamount lavas formed during active seafloor spreading at c. 19.1 Ma show limited geochemical variability, whereas the Zhenbei-Huangyan seamount chain formed during the post-spreading stage at c. 7.8 Ma and displays a wide range of compositions. However, melt inclusions in olivine and plagioclase from the Zhenbei-Huangyan basalts show considerably greater isotopic variability than seen in the whole rock compositions of both the SCS syn- and post-spreading lavas. A previously unidentified third mantle source component (FOZO) revealed by olivine-hosted melt inclusions along with both depleted (DMM) and enriched (EMII) mantle components is required in the source region to explain the observed isotopic and chemical variability. On the basis of our results, the age of the recycled ocean crust and sediments in this region are estimated to be c. 120 – 350 Ma. We infer that these enriched components in the SCS lavas come from the mantle transition zone. Variations in mantle source heterogeneity coupled with melting process control spatial–temporal (spreading vs. post-spreading stage) geochemical variations of lavas from the SCS and surrounding areas. Together with the results from published studies, we propose that marginal basins are one of the major locations on Earth where oceanic and/or continental lithosphere is transferred into the upper mantle and transition zone, representing an important source of upper mantle heterogeneity. We provide a simple conceptual model linking plate subduction and upper mantle heterogeneity and the volcanism in the SCS and surrounding areas.

Calcium isotope constraints on OIB and MORB petrogenesis: The importance of melt mixing

Lithological and isotopic heterogeneity in the mantle archive the mantle's melting history and fingerprint crustal recycling processes coupling the deep Earth with the terrestrial biosphere. Oceanic basalts, consisting of ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs), are invaluable tools for studying the chemical evolution of the mantle because they sample mantle heterogeneity at a variety of depths and length scales. In this study, we probe mantle heterogeneity by exploring the systematics of stable Ca isotopes in OIBs and MORBs. We find that OIBs from Iceland have δ44/40Ca values (∼0.85‰) that are indistinguishable from MORBs, while OIBs from Mangaia, the Azores, the Canary Islands and Hawaii have an average δ44/40Ca value ∼0.08‰ lower than the MORB-Iceland group. Moreover, MORB and Iceland δ44/40Ca values are relatively invariable, while OIBs range from 0.70‰ to 0.85‰ and correlate negatively with La/Lu and Nb/Y. The Ca isotope signature of the MORB-Iceland group is consistent with equilibrium fractionation during high-degree melting of average spinel/garnet peridotite with bulk silicate Earth (BSE) δ44/40Ca. Calcium-isotope variability in OIBs requires two-component mixing of melts with distinct δ44/40Ca values, one of which is a high-degree melt consistent with derivation from average peridotite with BSE δ44/40Ca. The melt-mixing endmember with isotopically light Ca (≤0.70‰) must be a deep, low-degree melt. This low-degree melt cannot be derived from a peridotite source with BSE δ44/40Ca, but instead requires either a source with lower-than-BSE δ44/40Ca or a different source mineralogy. The isotopically light, low-degree melt is consistent with derivation from a pyroxene- and garnet-rich source, but the effects of mineralogical heterogeneity in the source regions of OIBs cannot be easily distinguished from the possible influence of Ca isotope heterogeneity in the mantle. Regardless, mixing of melts from a shallow peridotite source and a deep source appears to be a ubiquitous part of OIB petrogenesis.

Crustal and Uppermost Mantle Heterogeneities Across the Ailaoshan Red River Shear Zone, SE Tibet: Implications for Cenozoic Magmatic Activity

AbstractThe Ailaoshan Red River shear zone (ARSZ) was formed in the Mesozoic as a suture zone between the Indochina block and the Yangtze craton. Since the Cenozoic, block extrusion due to the Indo‐Asian collision has reactivated the fault zone and caused large‐scale shearing. Affected by the Cenozoic orogeny, a large volume of magmatic and metamorphic rocks developed in the ARSZ, forming many orogenic gold deposits. However, the source and the geodynamic process of these magmatic activities are still unclear. To gain a basic understanding of the subsurface magmatic activity, we deployed a dense array of 24 broadband seismic stations across the Daping and Chang'an gold deposits at the southern end of the ARSZ. Receiver function analysis, common conversion point stacking, and a joint inversion of receiver functions and surface wave dispersions are performed to image the detailed structure of the crust and uppermost mantle. Low‐velocity zones in the mid‐lower crust and thinned lithosphere (∼70 km) are imaged under the ARSZ. The observed subsurface structures are verified by 3D numerical modeling with the SEM‐FK method. We speculate that the mantle upwelling caused by lithospheric delamination has provided the main source of the mantle component in the magmatic rocks since ∼35 Ma; afterward, high temperatures produced partial melting in the lower crust, which was emplaced along active shear zones.

Nd-Hf isotopic systematics of the arc mantle and their implication for continental crust growth

The 143Nd/144Nd and 176Hf/177Hf values of depleted mid-ocean ridge basalt (MORB) mantle have long been used to calculate the Nd and Hf model ages and constrain the growth rate of the continental crust. However, because continental crust forms principally at subduction zones, the appropriate 143Nd/144Nd and 176Hf/177Hf for use in model age calculations are those of the mantle wedge that underlies the juvenile magmatic arcs. In this study, we compile the Nd and Hf isotopic compositions of the modern island arc volcanic rocks. The average 143Nd/144Nd value obtained for the arc mantle is 0.512959 (εNd = +6.3), with upper and lower 95% confidence uncertainties of 0.513115 (εNd = +9.3) and 0.512613 (εNd = −0.5), respectively. The average obtained for 176Hf/177Hf is 0.283144 (εHf = +13.2) with upper and lower limits of 0.283272 (εHf = +17.7) and 0.282927 (εHf = +5.5), respectively, at the 95% confidence level. The Nd and Hf isotopic compositions of the modern island arc volcanic rocks can be divided into three populations, the high, transitional and low ε arc mantle populations, that reflect a combination of mantle heterogeneity and additions of different slab-derived components to the mantle wedge. The high ε arc mantle (HAM) population (εNd > +5.7 and εHf > +10) and the transitional ε arc mantle (TAM) population (εNd < +5.7 and εHf > +10) were metasomatized by the transfer of unradiogenic Nd and Hf from pelagic sediments, and Fe-Mn crusts and nodules, to the mantle wedge, the HAM by fluid dominate process and the TAM by a combination of sediment melts and fluids. In contrast, melts derived from subducted sediments, particularly zircon-rich turbidites, play a more important role in the low ε arc mantle population (LAM, εNd < +5.7 and εHf < +10). The average of both Nd and Hf isotopic data for the arc mantle, together with the 95% confidence limits, are used to define the arc mantle growth curves and their uncertainties. We suggest that these arc mantle growth curves should be used to calculate Nd and Hf model ages and constrain the growth rate of the continental crust.

Multiple Mantle Sources of High‐Magnesium Terranes on Mercury

AbstractThe composition and interior structure of Mercury's mantle are poorly known but critical to understanding its magma ocean differentiation, mantle dynamics, and crustal petrogenesis. Various crustal geochemical terranes are identified on Mercury based on their Mg/Si and Al/Si values, but element abundances exhibit a weak correlation with surface reflectances. The high‐magnesium region (HMR), which includes end‐members for both reflectance spectral units (high‐reflectance plains and low‐reflectance materials (HRP and LRM)) and geochemical terranes (Mg and Al contents), contains important information about Mercury's mantle composition and dynamics. We propose that the northwestern boundary of HMR can be extended to the Sobkou basin and its ejecta deposits. Within the resolution limit of element maps, we notice that mantle materials beneath the Sobkou basin have an elevated content of magnesium, and a large area of HRP‐like materials in HMR exhibit Mg/Si greater than 0.5. Our results suggest that HMR was formed by partial melting of heterogeneous mantle sources in terms of compositions, supporting the existence of stratified and/or lateral geochemical diversities in the mantle. While typical HRP on Mercury, including the Borealis Planitia and Caloris interior plains formed by effusive volcanism around 3.7 Ga, exhibit a lower Mg/Si than the global average, the Rachmaninoff interior plains emplaced by volcanism at 1.7 Ga exhibit a Mg/Si greater than 0.5. This observation indicates that local thermal anomalies may be prolonged in the mantle, inducing a higher degree of partial melting. We also report a weak correlation between the global LRM and magnesium content in the crust.

Mineralogy and geochemistry of lavas from the submarine lower caldera walls of Santorini Volcano (Greece)

Santorini Volcano, located in the central sector of the South Aegean volcanic arc, is one of the most active and potentially dangerous volcanoes in Europe. It has hosted Plinian eruptions over the last 350 ka of which at least four eruptions were accompanied by caldera collapses. Even though Santorini Volcano is considered a major threat, the main focus of research, thus far, has been on the comparatively young and subaerial deposits, whereas older stages of volcanism have been poorly studied. Our study focusses on samples from the submarine inner caldera walls and gives new insights into the early evolutionary stages of Santorini Volcano, contributing to a better understanding of its eruptive history and thus potential risks. The submarine lava successions were sampled along the inner caldera wall by a remotely operated vehicle (ROV) during R/V POSEIDON cruise 511 in 2017.The recovered lavas comprise two magmatic series, a low-K basaltic series overlain by a medium- to high-K series, which includes basaltic andesites, andesites and occasional dacites to trachytes. Major and trace element compositions and mineral zonation patterns suggest that fractional crystallisation and periodic magma replenishment were the dominant processes controlling magma evolution of both magmatic series. In addition, repeated magma mixing events played an important role, as indicated by zonation patterns in plagioclase and clinopyroxene ante- and phenocrysts. The thickness of the submarine lava successions (≥100 m) and the occurrence of similarly zoned plagioclase throughout indicate long-lived magma plumbing systems characterised by repeated differentiation and magma mixing. Furthermore, the incompatible element and radiogenic isotope ratios indicate a heterogeneous mantle source for Santorini magmas, which reflects the relative contribution of subduction (sedimentary) input and mantle wedge influx. A lava sample from the northern submarine caldera wall probably represents a deep level of the original Peristeria volcano. 40Ar/39Ar dating of andesite lavas from the southern submarine caldera produced ages of ~255 ka, whereas one basalt lava produced an age of 309 ± 30 ka. Interestingly, the new ages of both groups fall within an age gap in the volcanic history of Santorini Volcano. Even though it was not possible to unequivocally correlate the sampled submarine lava series to known subaerial units, the major and trace element compositions, and Sr-Nd-Pb isotopic compositions of our intermediate lavas show a strong similarity to subaerial lavas of Peristeria volcano, the second oldest major stage of Santorini volcanism. It however seems more likely that we have sampled a thus far unknown stage of volcanism at Santorini Volcano.

Geochemical evidence for a widespread Paleoproterozoic continental arc-back-arc magmatism in the Lesser Himalaya during the Columbia supercontinent assembly

The geodynamic evolution of the proto-northern Indian continental margin during Paleoproterozoic is widely debated. The proposed models include (i) a mantle plume triggered rift-related tectonic setting at a passive continental margin and (ii) an Andean type active continental margin. A widespread quasi-continuous belt of Paleoproterozoic (∼1.9–1.8 Ga) granitoids and orthogneisses, as well as mafic volcanic and intrusive rocks intercalated with metasedimentary rocks, occur all along the strike of Lesser Himalaya. A geochemical re-investigation is presented here using the existing geochemical data on mafic volcanic and intrusive rocks from the Lesser Himalayan Sequence of Himachal, Garhwal, and Nepal sectors to address the tectonic affiliation of these rocks and to infer the geodynamic setting of the proto-northern continental margin of the Indian Shield. These mafic magmatic rocks are classified as subalkaline basalt and basaltic andesite with calc-alkaline magmatic affinity to transitional character between calc-alkaline and tholeiites. Broad initial ɛNd range reveals a heterogeneous mantle source developed by mixing between a depleted mantle and a recycled component. Mantle source enrichment of these rocks is attributed to the interaction between the siliceous partial melt from the subducted sediment-laden oceanic crust and the mantle wedge, which is further supported by compositional similarity with experimental hybrid partial melts of eclogite-peridotite reaction. Estimated mantle potential temperature from the composition of these mafic rocks is comparable to that of mid-oceanic ridge basalts and does not invoke a mantle plume in their genesis. Geochemical proxies based on the major oxide abundances reveal the generation of these rocks from olivine-poor lithologies. It is suggested that these rocks were generated in a back-arc setting during the assembly of the Columbia supercontinent at the proto-northern continental margin of the Indian Shield. Distinct Nd isotopic systematics in these rocks compared to the Paleoproterozoic metamafic rocks of Cathaysia Block of South China Craton questions their purported linkage in the Columbia configuration.

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.

Petrology of ophiolites of Memel, Nsimè-Kellé and Mapan (Yaoundé group): Evidence of the geodynamic evolution of the Pan-African orogeny in South Cameroon

Ophiolites are the remnants of ancient oceanic crust that can serve as tools to reconstruct the evolutionary history of fossil oceans. At Boumnyebel, the western part of the Yaoundé group, the sole of the lower unit (magnesian mica-schists and gneiss) of the Pan-African nappes is made up of ophiolitic rocks (talc schists, chlorite schists, serpentinites, amphibolites, pyroxenites, hornblendites). Structurally, the lower unit has a flat-lying schistosity with some recumbent folds while the underlying TTG gneisses are marked by a high-dipping foliation at Mapan. The contact between the Pan-African mica schists and the basement formations (Congo craton) is therefore discordant. In ophiolitic rocks, amphibole is actinolite, tremolite or magnesio-riebeckite. Chlorite compositions correspond to clinochlore. Talc shows notable substitution of Mg by Fe whereas substitution of Mg by Fe and Al prevails in serpentine. Spinel compositions are either Cr-rich with high FeO/Fe2O3 values (peridotitic spinels) or Cr-poor with low FeO/Fe2O3 values (melt-derived or magmatic spinels). Main mineral features indicate a metamorphic retromorphosis of the original lherzolites, harzburgites, dunites and pyroxenites to hornblendites and serpentinites. This is followed by hydrothermal alteration (at various temperatures: 250–570 °C and low pressure: < 2 kbar) that yielded chlorite schists and talc schists. Chemically, talc schists are made up of SiO2, MgO and minor FeO and Al2O3, while chlorite schists and hornblendites contain notable amounts of Al2O3 and CaO, respectively. All the ultramafic rocks display the same range of high Ni, Cr, Co contents with various LREE enrichments but flat HREE patterns. Multi-element patterns show that most talc schist are depleted in almost all elements. All the ultramafic rocks are marked by deep Nb, Ta, K, Pb, Sr, Ti, Zr and Hf troughs suggesting that the ultramafic rocks possibly derived from heterogeneous mantle sources varying from fertile to refractory peridotites. Amphibolites are generated from N-MORB, IAT or CA geotectonic settings. The compositions of spinel and ultramafic whole rocks reveal an arc setting that has evolved from divergent plate setting (oceanisation, N-MORB) to convergent plate (IAT, CA, SSZ D-MORB) settings. In summary, ophiolites emplaced in various geotectonic settings and were amalgamated during the Pan-African collision.

Partial melting caused by subduction of young, hot oceanic crust in shallow high-temperature and low-pressure environments: Indications from Middle and Late Jurassic oceanic plagiogranite in Shiquanhe, Central Tibet

The oceanic plagiogranites that intruded into the ophiolite suite have received a great deal of attention from researchers. Some of these plagiogranites were controlled by mid-ocean ridge spreading and were formed via crystallization of basaltic magma or partial melting of the mafic gabbroic oceanic crust. The other plagiogranites were controlled by subducted oceanic crust slabs, and their formation mechanism is similar to that of the oceanic island arcs. The identification of different types of oceanic plagiogranites is of great significance for understanding the evolution of ancient ocean basins. In the Shiquanhe–Namtso mélange zone in central Tibet, scholars have recently identified many rock sequences related to intra-oceanic arcs, including MORB-like basalts, boninites, high-Mg andesites, and multiple series of granitic rocks. To further explore the formation mechanism of felsic rocks in oceanic island arcs and their role in the growth of continental crustal material, we sampled a series of Middle and Late Jurassic granitoids exposed in the ophiolitic mélange belt in the Shiquanhe area. Using zircon U-Pb dating, zircon Lu-Hf isotopic testing, whole-rock Sr-Nd isotopic testing and whole-rock major and trace geochemical testing, we determined the magmatic genesis and tectonic history of these samples. According to the test results and our field survey, we divided our samples into three groups. The first group consists of low-K plagiogranites (LKGs). These samples have ages that fall between 163 and 162 Ma. With their low K2O/Na2O ratios, low La/Yb ratios, low Sr/Y ratios, low Gd/Yb ratios, weak negative Eu anomalies and their significantly depleted zircon isotopic compositions (εHf(t) = 10.0–12.5; εNd(t) = 0.64–1.10), we propose that these samples are the products of partial melting of oceanic gabbro and turbidite at shallow depths in low-pressure and high-temperature conditions. The second group consists of samples from dioritic enclaves within the LKGs. These samples, which are slightly older than the LKG samples (175–173 Ma), have isotopic characteristics that are similar to those of the LKG samples (εHf(t) = 10.4–13.5; εNd(t) = 0.51–0.83). The samples in the second group may be homologous captives that formed during the early crystallization of LKG homologous magmas. The third group consists of samples with compositions of high-K granodiorites (HKGs) and ages similar to those of the LKG group (~163 Ma). With high K2O/Na2O ratios, relatively high La/Yb ratios, low Sr/Y ratios, low Gd/Yb ratios, negative Eu anomalies, significantly enriched isotopic compositions (εHf(t) = −11.1 to −15.4; εNd(t) = −13.29 to −13.43). Considering the basement exposure and known magmatic characteristics in the Shiquanhe area, we propose that HKGs samples are the result of interaction between continental subduction sediment melts and heterogeneous mantle material in high-temperature and low-pressure conditions.By comparing the oceanic plagiogranites in the Shiquanhe–Namtso mélange zone with those developed in the young, hot intra-oceanic subduction zone, we determined that the oceanic plagiogranites in the Shiquanhe–Lhaguotso–Asa area are forearc granites developed in the forearc area. These plagiogranites were controlled by the young, hot subducting plate and were formed by partial melting of a multi-component source area in a high-temperature and low-pressure environment.

Exploring small-scale recycled mantle components with intraplate continental twin volcanoes

Primitive magmas with distinct geochemical signatures that record mantle heterogeneity are obscured in continental intraplate volcanic settings owing to crustal interaction. Here, we report new geochronological and geochemical data for two adjacent Late Neogene primitive intraplate volcanoes in Northeast China: the Great and Lesser Wangqing River twin volcanoes (GWR/LWR). Our results suggest that the GWR and LWR volcanics are controlled by olivine crystal sorting and that crustal contamination is negligible, which implies these twin volcanoes provide a rare window into mantle components. The LWR tephrite is marked by moderate enrichment of light rare earth elements (REEs) with significant positive Sr anomalies. It is also enriched in radiogenic Sr-Nd-Hf-Pb isotopes but exhibits 187Os/188Os values that are comparable to those of the primitive upper mantle. The GWR volcanics (picrite, basanite and tephrite) are characterised by a steeper REE pattern and a lack of Sr anomalies, while being isotopically more radiogenic. Combined with olivine chemistry, which reveals a hybrid source lithology, we propose that the LWR and GWR sampled recycled lower and upper oceanic crust components, respectively. Both components have high 87Sr/86Sr and low 206Pb/204Pb ratios, akin to an enriched mantle I (EM1) source, indicating that the EM1 components can be formed by recycling oceanic crust with limited sediment.

Complex magmatic processes recorded by clinopyroxene phenocrysts in a magmatic plumbing system: A case study of mafic volcanic rocks from the Laiyang Basin, southeastern North China Craton

Understanding the evolution of magmatic plumbing systems requires an understanding of the mechanisms of magma ascent from the magma source to the surface. We undertook textural and in situ geochemical studies of clinopyroxene phenocrysts in Early Cretaceous mafic volcanic rocks from the Laiyang Basin, China. Four distinct clinopyroxene populations were recognized based on their textures and zoning patterns in the mafic volcanic rocks (types 1, 2, 3, and 4). Type 1 is reversely zoned and contains rounded, irregularly resorbed, low-Mg cores (Mg# = 60–75) surrounded by clear, high-Mg mantles (Mg# = 75–90). Type 2 phenocrysts are typically normally zoned and characterized by relatively high-Mg cores (Mg# > 85) and thin lower Mg# rims (Mg# = 70–85). Type 3 comprises strong oscillatory zoned crystals (Mg# = 72–85) that are generally normally zoned and, in some cases, exhibit substantial sector zoning. Type 4 crystals are euhedral and unzoned phenocrysts. The textural and compositional features of the clinopyroxenes record complex magmatic processes including fractional crystallization, recycling of early formed crystals, magma convection, and magma recharge and subsequent mixing. Clinopyroxene–melt equilibrium pairs in the volcanic rocks yielded a relatively wide range of temperatures (984–1264°C) and pressures (0.1–10.1 kbar). The P–T array defines two main magma storage zones: (1) a deep, high-T, magma reservoir (~22–28 km), and (2) a shallow, low-T, magma reservoir (~8–19 km). At pressures of >3 kbar, the amount of water in melts in equilibrium with the clinopyroxene is in the range 1.5–3.0 wt.%, but at pressures of <3 kbar, the melt water contents decrease to 0.1 wt.%, indicative of extensive magma degassing. The clinopyroxene phenocrysts have a wide range of 87Sr/86Sr ratios from 0.70683 to 0.71366. The broad range of 87Sr/86Sr ratios (0.70796–0.71171) for the most primitive clinopyroxene (Mg# > 89) might have been inherited from a heterogeneous mantle source. However, some type 1 crystals with more radiogenic Sr isotopic compositions may record assimilation of crustal material during evolution of the mantle-derived magma. The textural and compositional features of the studied clinopyroxene phenocrysts suggest that Laiyang volcano is fed by a vertically extensive magmatic plumbing system that extends from shallow to deep levels, in which geochemically distinct magmas crystallized and mixed under variable physicochemical conditions.

Co-Occurrence of HIMU and EM1 Components in a Single Magellan Seamount: Implications for the Formation of West Pacific Seamount Province

ABSTRACT Our general understanding of mantle composition and dynamics mainly comes from the composition of lavas from oceanic intraplate volcanoes. They are generally accepted to originate from deep, relatively stationary mantle plumes. Many groups of seamounts comprising the West Pacific Seamount Province (WPSP), however, do not form long-lived, narrow and continuous chains of volcanoes with clear age progression; thus, their origin does not seem to fit this general model. Here we show a wide compositional spectrum of lavas from the Pako guyot within the Magellan seamount trail (MST), an age-progressive but short-lived volcanic chain in the WPSP. For the first time, both extreme high μ = 238U/204Pb mantle (HIMU)- and enriched mantle 1 (EM1)-like mantle plume components occur in a single seamount in the Pacific Ocean. Based on alteration resistant trace element and Sr–Nd–Pb–Hf isotopic compositions, the Pako lavas fall into three distinct groups. Group 1 has a distinctive HIMU composition with high (206Pb/204Pb)i ratios (20.41–20.94) similar to the Arago (also known as ‘Young Rurutu’ or ‘Atiu’) hotspot composition, suggesting derivation from a HIMU mantle source. Groups 2 and 3 EM1-like lavas comprise the dominant rock type and display the largest Sr–Nd–Pb–Hf isotopic variations that can be best explained by mixing between melts of focal zone (FOZO) and EM1-like mantle components. Additionally, olivine phenocrysts from Group 3 EM1-like lavas have high Ni contents, Fe/Mn and Mn/Zn ratios and low Zn/Fe*10 000 ratios, which can be explained through fractional crystallization of high-pressure partial melts from fertile peridotite. The EM1-like Groups 2 and 3 lavas are compositionally similar to the Rarotonga hotspot composition, suggesting that the MST is possibly an old trace of the Rarotonga hotspot, which is most likely a long-lived hotspot generated above a deep mantle plume. Combined with existing geochemical and tectonic data, we propose that the Magellan seamounts were likely derived from partial melting of a heterogeneous mantle plume containing HIMU, FOZO and EM1-like components. Alternatively, they could have been derived from the Arago and Rarotonga mantle plumes, each having its distinct compositional signature. The occurrence of HIMU- and EM1-like mantle plume components in a single volcano suggests that the superposition of compositionally different hotspot volcanic trails in the South Pacific could have played an important role in the generation of the WPSP.

A wet heterogeneous mantle creates a habitable world in the Hadean.

The Hadean eon, following the global-scale melting of the mantle1-3, is expected to be a dynamic period, during which Earth experienced vastly different conditions. Geologic records, however, suggest that the surface environment of Earth was already similar to the present by the middle of the Hadean4,5. Under what conditions a harsh surface environment could turn into a habitable one remains uncertain6. Here we show that a hydrated mantle with small-scale chemical heterogeneity, created as a result of magma ocean solidification, is the key to ocean formation, the onset of plate tectonics and the rapid removal of greenhouse gases, which are all essential to create a habitable environment on terrestrial planets. When the mantle is wet and dominated by high-magnesium pyroxenites, the removal of carbon dioxide from the atmosphere is expected to be more than ten times faster than the case of a pyrolitic homogeneous mantle and could be completed within 160 million years. Such a chemically heterogeneous mantle would also produce oceanic crust rich in olivine, which is reactive with ocean water and promotes serpentinization. Therefore, conditions similar to the Lost City hydrothermal field7-9 may have existed globally in the Hadean seafloor.

Effects of Hotspot‐Induced Long‐Wavelength Mantle Melting Variations on Magmatic Segmentation at the Reykjanes Ridge: Insights From 3D Geodynamic Modeling

AbstractSpatial variations in mantle melting induced by the Iceland hotspot have strong effects on meso‐scale mantle upwelling and crustal production along the slow‐spreading Reykjanes Ridge. The ridge‐hotspot interaction has been recorded by diachronous V‐shaped ridges and troughs extending away from Iceland, as well as by changes in ridge segmentation since 37 Ma. The origins of V‐shaped structures are widely debated, while the causes of the gradual erasion of ridge segments bounded by transform faults are rarely investigated. Through 3D time‐dependent geodynamic modeling, this study investigates how the hotspot‐induced regional mantle melting variations affect ridge segmentation. Periodic temperature perturbations were initially imposed beneath the melting zone to trigger buoyant upwelling cells, which corresponded to the offset ridge segments at the Reykjanes Ridge. Iceland hotspot‐induced long‐wavelength mantle melting variations were generated by applying a regional linear temperature gradient at the bottom of the model domain. Modeling reveals a two‐stage evolution of the buoyant upwelling cells that characterizes the segmentation transition at the Reykjanes Ridge. In Stage 1, the regional mantle melting variations trigger along‐axis pressure‐driven mantle flow, which alters the segment‐scale mantle upwelling and promotes the propagation of segment boundaries away from the region with relatively higher mantle temperature. In Stage 2, buoyant upwelling cells are destroyed progressively as along‐axis mantle flow dominants, leaving V‐shaped diachronous boundaries between the segmented and unsegmented crust. These results advance our understanding of the effects of long‐wavelength mantle melting variations induced by regional mantle heterogeneities on ridge segment evolution at slow‐spreading ridges.

The Role of Syn-Extensional Lamprophyre Magmatism in Crustal Dynamics—the Case of the Menderes Metamorphic Core Complex, Western Turkey

Abstract The Menderes Massif in Turkey represents one of the largest metamorphic core complexes in the world. It is regarded as a section of lower continental crust exhumed along low-angle detachment faults in the Late Miocene during a period of extension that affected the entire Aegean province. Syn-extensional magmatic activity within the Menderes metamorphic core complex is predominantly felsic forming several plutons, whereas mantle-derived magmatism has not been known so far. Here, we present a detailed study of the petrology and geochemistry of previously unreported mafic to intermediate lamprophyres within the Menderes Massif and assess their role in the geodynamic evolution of the core complex. The Menderes lamprophyres are mostly kersantites, with 49–60 wt % SiO2, 3.2–8.4 wt % MgO, 100–360 ppm Cr, 32–132 ppm Ni and Mg# of 37–50. Positive Pb and negative Ti–Nb–Ta anomalies suggest a clear orogenic affinity. Isotopes of Sr and Pb are relatively radiogenic (87Sr/86Sr = 0.70609–0.71076; 206Pb/204Pb = 18.88–19.03, 207Pb/204Pb &amp;gt; 15.71), while Nd is unradiogenic (εNd = −1.4 to −3.2). Most phenocrysts are sharply zoned with a primitive core (Mg# 77–85, up to 0.95 wt % Cr2O3 in clinopyroxene; Mg# 72–76 in amphibole) and a more evolved rim (Mg# 68–74, &amp;lt;0.25 wt % Cr2O3 in clinopyroxene; Mg# 69–71 in amphibole). Trace element ratios between different cores may vary significantly (e.g. Dy/Yb 2–5 in amphiboles), whereas rims show less variation but are more enriched than the cores. U–Pb dating of zircons provides an age of 15 Ma for the lamprophyres, coeval with the syn-extensional granite magmatism. The Hf isotopic composition of magmatic zircons is variably unradiogenic (176Hf/177Hf15Ma = 0.28248–0.28253, εHf15Ma = −8.6 to −10.5), while zircon xenocrysts with dominantly Cadomian and older ages show highly variable Hf isotopic signatures at the time of lamprophyre emplacement (εHf15Ma = −7.6 to −46.7). The orogenic geochemical signature of the lamprophyres’ parental melts is similar to nearby orogenic lavas from the West Anatolian Volcanic Province. Variation in bulk-rock εNd and in Dy/Yb ratios of phenocryst cores reflect moderate mantle heterogeneity. The chemical heterogeneity of phenocrysts and zircon εHf values implies intense hybridisation of proto-lamprophyre melts with felsic crustal melts, most probably derived from the melting of augen gneisses of the Menderes basement. We propose that fluid released from the lamprophyre primary melt had a decisive impact on crustal melting and the formation of granitic plutons within the Menderes core complex.

Sikkimite: A New and Rare Suite of Potassic-perpotassic Syenites from South Sikkim, Lesser Himalaya, India

Abstract A new and rare suite of radioactive, plutonic-hypabyssal, nonperalkaline to peralkaline, potassic to perpotassic syenites have been reported as dyke-like and/or lensoid bodies (c. 100 m x 20 m to 5 m x 2 m) within the Buxa and Daling formations (Proterozoic) and Gondwana Group of rocks (Permo-Carboniferous) within the Ranjit tectonic window (RTW) and its peripheral parts in south Sikkim. These are christened in this work as ‘Sikkimites’ considering their unique field occurrences, petrography, mineralogy and geochemistry, unlike any known saturated potassic - perpotassic syenites from the plutonic environment from India or elsewhere. The modal (vol.%) mineralogical variations observed within the potassic syenites include orthoclase (57-65%) with subordinate to minor amounts of brown alkali amphibole (kataphorite, 3-32%), aegirine (1-20%), biotite (13% in one sample), opaque minerals (ilmenite and magnetite, 4.2-24.5%), quartz (traces to 12.9%) and minor to trace amounts (&amp;lt; 3%) of apatite, sphene, zircon, and monazite. Compositionally, the Sikkimite suite range from non-peralkaline to highly peralkaline types (with Mol. Na2O + K2O/Al2O3 varying from&amp;lt;1 to1.2 to 1.5), mildly-perpotassic (Mol. K2O/Al2O3&amp;gt;0.83 to 0.96) and perpotassic (Mol. K2O/Al2O3&amp;gt;1 and Wt.% K2O/Al2O3 = 2.52-23.9) and Wt.% K2O/Na2O(3.05-23.9). The potassic syenites shows wide variations in major, minor and trace elements with significant enrichments in TiO2, Ba, REE, Y, Zr and Nb. The petrogenetic model for these rare potassic syenite suite of rocks (termed here as Sikkimite) invokes a near-primary origin by partial melting of enriched, amphibole- and phlogopite-bearing heterogeneous mantle, having chemical signatures of both non-subduction and subduction-related metasomatism. Ar40-Ar39 dates on them indicate a Cretaceous age (100-90 Ma) suggesting their links to either the widespread igneous manifestation of the Kerguelen or the Marion plume of the Indian Ocean.. These rare potassic syenites were apparently brought to the near surface environments in south Sikkim due to the collision of Greater India with Asia and the ensuing Himalayan thrusts from the north (MCT1 and MCT2) which shifted the crystalline basement and the Gondwana rocks maximum to the south, unlike any other parts of the Himalaya.

The effects of discontinuity topography in the mantle transition zone on global geodynamic observables and mantle heterogeneity

SUMMARYDespite progress in tomographic imaging of Earth's interior, a number of critical questions regarding the large-scale structure and dynamics of the mantle remain outstanding. One of those questions is the impact of phase-boundary undulations on global imaging of mantle heterogeneity and on geodynamic (i.e. convection-related) observables. To address this issue, we developed a joint seismic-geodynamic-mineral physical tomographic inversion procedure that incorporates lateral variations in the depths of the 410- and 660-km discontinuities. This inversion includes S-wave traveltimes, SS precursors that are sensitive to transition-zone topography, geodynamic observables/data (free-air gravity, dynamic surface topography, horizontal divergence of tectonic plates and excess core-mantle boundary ellipticity) and mineral physical constraints on thermal heterogeneity. Compared to joint tomography models that do not include data sensitivity to phase-boundary undulations in the transition zone, the inclusion of 410- and 660-km topography strongly influences the inference of volumetric anomalies in a depth interval that encompasses the transition zone and mid-mantle. It is notable that joint tomography inversions, which include constraints on transition-zone discontinuity topography by seismic and geodynamic data, yield more pronounced density anomalies associated with subduction zones and hotspots. We also find that the inclusion of 410- and 660-km topography may improve the fit to the geodynamic observables, depending on the weights applied to seismic and geodynamic data in the inversions. As a consequence, we find that the amplitude of non-thermal density anomalies required to explain the geodynamic data decreases in most of the mantle. These findings underline the sensitivity of the joint inversions to the inclusion of transition-zone complexity (e.g. phase-boundary topography) and the implications for the inferred non-thermal density anomalies in these depth regions. Finally, we underline that our inferences of 410- and 660-km topography avoid a commonly employed approximation that represents the contribution of volumetric heterogeneity to SS-wave precursor data. Our results suggest that this previously employed correction, based on a priori estimates of upper-mantle heterogeneity, might be a significant source of error in estimating the 410- and 660-km topography.