Oceanic Crust Research Articles

A slab window in the south rim of the Parece-Vela Basin

Slab windows represent regions within the mantle that are largely devoid of slab material, facilitating direct communication between the mantle above and below the subducting slab. This unprecedented interaction disrupts the conventional material-energy exchange mechanisms between the subducted slab and mantle wedge, giving rise to anomalous heat flow, distinct magmatism, metamorphism, and geophysical features. Geochemical analyses of samples collected from the southern margin of the Parece-Vela Basin have illuminated the magmatic processes associated with a slab window. C20-3 is indicative of a hot and dry mantle upwelling that occurs through the slab window. On the other hand, C20-1 signifies an adakitic rock, which originates from the melting of oceanic crust at the periphery of the slab window. Slightly east of the slab window, C21-1 reveals evidence of a heated depleted mantle, influenced by the hot and dry mantle upwelling through the slab window. Additionally, a common Island Arc Basalt (IAB), denoted as C21-2, was also observed in the island arc region. Topographical data highlights the irregular distribution of numerous seamounts between the West Mariana Ridge and the Parece-Vela Basin spreading center. Seismological records reveal a preponderance of strike-slip earthquakes in the southern seamount region, alongside exceptionally high heat flow measurements at the northwest extremity of this area. The shape of the slab window can be roughly modeled by analyzing the distribution of earthquakes. Collectively, these observations lead us to postulate the existence of a slab window beneath the southern rim of the Parece-Vela Basin, likely attributed to the tearing of the subducting Pacific Plate along the strike-slip fault situated between the Ulithi and Fais Atolls.

Petrochronology of High‐Pressure Veins Reveals the Evolution of Fluid Sources in Subducted Oceanic Crust (Rocciavrè Eclogites, W. Alps)

Petrochronology of High‐Pressure Veins Reveals the Evolution of Fluid Sources in Subducted Oceanic Crust (Rocciavrè Eclogites, W. Alps)

Do we really need to drill through the intact ocean crust?

We must persevere to drill through the intact ocean crust to fully address fundamental questions towards completion of the plate tectonics theory. The primary questions include: what is the ocean crust made up of, how thick is it and what is the petrological nature of the crust-mantle boundary (i.e., Mohorovičić discontinuity or Moho)? These questions may sound naive because they are widely believed to be well-understood facts, but they are not. Correctly, our current knowledge remains incomplete, and some popular misperceptions come from interpretations based on convenient assumptions. One assumption is that the ocean crust inferred from seismic data is of magmatic origin. Testing this assumption is a principal motivation of Project Mohole (1957–1966), attempting to drill intact ocean crust across the Moho into the mantle. Project Mohole failed because of its high cost, engineering challenges and insufficient tries, but the technologies developed made subsequent ocean drilling successful. However, answers to the original questions remain unsatisfactory. For example, seismic crust interpreted to be of magmatic origin is shown to have globally uniform thickness of 6.0 ± 1.0 km, but crust with such thickness at many slow-spreading ridge segments is dominated by serpentinized mantle peridotites exposed on seafloors. Therefore, the popular view on ocean ridge magmatism must be re-examined, which needs intact ocean crust drilling into the mantle. Drilling at geologically simple sites in the fast-spreading Pacific seafloor is most promising.The US-led D/V JOIDES Resolution that has well served the scientific ocean drilling since 1985 is to retire by the end of 2024, but timely the Chinese geoscience community wishes to continue this international endeavor using the purpose-built D/V Meng Xiang to be in service in 2025. The international community is to gather in November 24–27, 2024, Guangzhou, China, to discuss strategies on where and how to successfully drill intact ocean crust across the Moho in coming years

The return of stagnant slab recorded by intraplate volcanism

Subducted plates often stagnate in the mantle transition zone (MTZ), and the fate of the stagnant slabs is still debatable. They may sink into the lower mantle, or remain partially trapped in the MTZ, but it is uncertain whether they can return to the upper mantle. We report geochemical evidence of late-Miocene (~6 Ma) basalts from, and upper mantle seismic evidence beneath Shuangyashan, an area above the slab tear of the stagnant Pacific plate in eastern Asia, to show how the slab returns to the upper mantle from the MTZ. Remarkably high δ57Fe, Gd/Yb and low δ26Mg, Ti/Eu, CaO/Al2O3 values of Shuangyashan basalts suggest that the subducted Pacific oceanic crust had been entrained to their upper mantle source. Therefore, the return of oceanic crust from the MTZ to the upper mantle appears to have been driven by upwelling triggered by tearing of the stagnant slab right beneath this area. Meanwhile, local shear splitting measurements reveal a circular pattern of anisotropy in the upper mantle with delay times diminishing toward the slab tear, suggesting that the slab-fragment-bearing upwelling subsequently turned into lateral flows in the upper mantle, and contributed to a wider intraplate magmatism above the stagnant slab. This finding, together with other widespread intraplate volcanism from eastern Asia, extending for approximately 6,000 km, demonstrates that a subduction-induced tear can lead to the destruction and partial return of stagnant slab material, and thus lead to the cycling of subducted crustal materials and the generation of subduction-induced intraplate magmatism.

Identification and Geological Significance of Late Cambrian OIB-Type Volcanic Rocks in the Nailenggeledaban Area, Northern Yili Block

Paleozoic igneous rocks exposed in the northern Yili Block are thought to have resulted from the subduction of the North Tianshan oceanic crust. However, the exact timing of the transition of the northern margin of the Yili Block from a passive to an active continental margin remains unknown. In this paper, the petrological and geochemical features, zircon U-Pb chronology, Lu-Hf isotopes, and Sr-Nd isotopes of volcanic rocks in the Nailenggeledaban area on the northern margin of the Yili Block were studied. Zircon U-Pb dating results show that the crystallization ages of the volcanic rocks in the Nailenggeledaban area on the northern margin of the Yili Block are 491 ± 2 Ma and 500 ± 2 Ma, suggesting they were formed during the Late Cambrian. Geochemical features show that the volcanic rocks are alkaline basalts with rare earth and trace element distribution patterns similar to OIB, although they exhibit some degree of Zr and Hf depletion. The εHf(t) values of alkaline basalts in the Nailenggeledaban area at the northern Yili Block range from −3.48 to −1.00, with a TDM1 age of 1152 to 1263 Ma. The εNd(t) values range from −3.53 to −0.96, with a TDM1 age of 1471 to 2162 Ma. Combined with geochemical data, the alkaline basalt magma in the Nailenggeledaban area on the northern margin of the Yili Block may be derived from the Mesoproterozoic enriched lithospheric mantle. The composition of the mantle source area is potentially garnet lherzolite, and the magma appears to have been either unaffected or only minimally contaminated by crustal materials during the ascending process. On the basis of the research results of the Early Paleozoic tectonic evolution in the northern margin of the Yili Block, this paper proposes that the volcanic rocks in the Nailenggeledaban area, located on the northern margin of the Yili Block, were formed in a back-arc extensional environment resulting from the subduction of the North Tianshan Ocean (or Junggar Ocean) beneath the northern margin of the Yili Block during the Late Cambrian.

Where and when the Palaeo-Asian Ocean was closed: new evidence from the Tuquan ophiolite

The Solonker Suture marks the location of the final collision in the southeastern Central Asian Orogenic Belt, but is obscured at the western margin of the Songliao Basin. The recently discovered Tuquan ophiolite is an important fragment within the Solonker Suture. The analysis of samples from the Tuquan ophiolite using laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating gave chronological data with weighted mean 206 Pb/ 238 U ages from 279 ± 6 to 265 ± 5 Ma, which we interpret to represent the formation age of the Tuquan ophiolite from the late Cisuralian to Guadalupian. The geochemistry of the Tuquan ophiolite shows the characteristics of a supra-subduction zone. We suggest that it represents an important bidirectional subduction event involving the oceanic crust of the Palaeo-Asian Ocean beneath the Mongolia Block and the North China Craton during the early to late Permian. Integrating these findings with previous research indicates that the final subduction of the Palaeo-Asian Ocean persisted until at least the late Permian. Our study provides further insights indicating that the Tuquan ophiolite could be regarded as the eastern extension of the Solonker Suture, providing new constraints on the evolution of the southeastern Central Asian Orogenic Belt.

Subsurface structures around the subducting seamount illuminated by local earthquakes at the off-Ibaraki region, southern Japan Trench

The off-Ibaraki region is a convergent margin at which a seamount subducts in the southern Japan Trench. At this off-Ibaraki region, an Ocean Bottom Seismometer (OBS) experiment was conducted from before to after the 2011 Tohoku-oki earthquake. In this study area, a source region of the largest aftershock (Mw7.9) of the Tohoku-oki earthquake occurred and many subsequent aftershocks were recorded in the OBSs. An intensive event location was performed around the subducting seamount to reveal the spatiotemporal variation of the seismicity and the regional seismotectonics of this region. By applying a migration-based event location method to an Ocean Bottom Seismic network record of both P- and S-waves, over 20,000 events were determined in the off-Ibaraki region below ~ M4. The seismicity showed clear spatiotemporal patterns enough to identify the seismicity changes and geometry of the seismic interface. At the updip side, the shallow tectonic tremors and earthquakes are shown to be spatially complementary bounded by an updip limit of the seismogenic zone. At the downdip side, a semicircular low-seismicity zone was identified, which is possibly a rupture area of the Mw7.9 event. The event depth profile exhibited a gently sloped planar downdip interface subparallel to the subducting slab. This seismic plane is located about 8 km deeper than the plate interface and appears to be stably active from 2008 to 2011, although this study could not reveal its mechanism. A comparison with the active source seismic survey profiles exhibits that this planar downdip interface is several kilometers deeper than the top of the oceanic crust. After the Mw7.9 event, a high-angle downdip seismic interface was activated above the planar interface. Further, below the planar downdip interface, broadly scattered events occurred with a swarm manner. We successfully illuminated the complicated subsurface structures around the subducting seamount. It is suggested that most of the events occur along or below the plate interface as the top of the oceanic crust which could not be explained by the existing seamount subduction models. Further study in the future is needed to validate the conclusion of this study and then to construct a mechanism for this peculiar seismic interface.Graphical

Were early Archean carbonate factories major carbon sinks on the juvenile Earth?

Abstract. Paleoarchean carbonates in the Pilbara Craton (Western Australia) are important archives for life and environment on early Earth. Amongst others, carbonates occur in interstitial spaces of ca. 3.5–3.4 Ga pillow basalts (North Star, Mount Ada, Apex, and Euro Basalt, Dresser Formation) and are associated with bedded deposits (Dresser and Strelley Pool Formation, Euro Basalt). This study aims to understand the formation and geobiological significance of those early Archean carbonates by investigating their temporal–spatial distribution, petrography, mineralogy, and geochemistry (e.g., trace elemental compositions, δ13C, δ18O). Three carbonate factories are recognized: (i) an oceanic crust factory, (ii) an organo-carbonate factory, and (iii) a microbial carbonate factory. The oceanic crust factory is characterized by carbonates formed in void spaces of basalt pillows (referred to as “interstitial carbonates” in this work). These carbonates precipitated inorganically on and within the basaltic oceanic crust from CO2-enriched seawater and seawater-derived alkaline hydrothermal fluids. The organo-carbonate factory is characterized by carbonate precipitates that are spatially associated with organic matter. The close association with organic matter suggests that the carbonates formed via organo-mineralization – that is, linked to organic macromolecules (either biotic or abiotic), which provided nucleation sites for carbonate crystal growth. Organo-carbonate associations occur in a wide variety of hydrothermally influenced settings, ranging from shallow marine environments to terrestrial hydrothermal ponds. The microbial carbonate factory includes carbonate precipitates formed through mineralization of extracellular polymeric substances (EPSs) associated with microbial mats and biofilms. It is commonly linked to shallow subaquatic environments, where (anoxygenic) photoautotrophs might have been involved in carbonate formation. In the case of all three carbonate factories, hydrothermal fluids seem to play a key role in the formation and preservation of mineral precipitates. For instance, alkaline Earth metals and organic materials delivered by fluids may promote carbonate precipitation, whilst soluble silica in the fluids drives early chert formation, delicately preserving authigenic carbonate precipitates and associated features. Regardless of the formation pathway, Paleoarchean carbonates might have been major carbon sinks on the early Earth, as additionally suggested by carbon isotope mass balances indicating a carbon flux of 0.76–6.5 × 1012 mol yr−1. Accordingly, these carbonates may have played an important role in modulating the carbon cycle and, hence, climate variability on the early Earth.

Petrology of gabbroic rocks from the lower crust of a back-arc basin: an example from the ophiolitic Liuyuan Complex, northwestern China

Abstract The 290-282 Ma ophiolitic Liuyuan Complex in northwestern China preserves a section of oceanic crust formed in a back-arc basin during the final stages of the evolution of the Central Asian Orogenic Belt. We present results of detailed field work, petrography, whole rock, and mineral chemistry of the plutonic rocks of the Liuyuan Complex. These include: troctolite, melatroctolite, olivine gabbro, podiform olivine gabbro, podiform hornblende gabbro and intrusive hornblende gabbro. Major element data, petrography, low incompatible trace element concentrations, and positive Eu-Sr anomalies suggest that basal troctolite and melatroctolite are accumulations of olivine + plagioclase primocrysts, with varying proportions of interstitial, ophitic, and poikilitic clinopyroxene forming from trapped melt. Olivine gabbro dominates the plutonic crust between the troctolites and sheeted dykes. Troctolites and olivine gabbros have weak paleo-horizontal foliations and show systematic upward cryptic mineral-chemical trends of decreasing clinopyroxene Mg#, plagioclase An, and olivine Fo and NiO contents. Trace element modelling suggests low trapped melt fractions in the cumulates (5-15%) and implies clinopyroxene was a minor cumulus phase in the olivine gabbros. The cryptic mineral-chemical evolution and textures imply most troctolites and olivine gabbros formed as sequential accumulations from an evolving melt. Melatroctolite layers (~5m thick) in troctolite may represent fossil replenishment events. Immediately below the sheeted dyke unit, there are meter-scale pods of cumulate olivine gabbro containing abundant hornblende (25-45%), as well as decimeter- to meter-scale pods of quasi-liquid olivine gabbro with compositions similar to overlying dykes and lavas. Isolated intra-plutonic dykes of hornblende gabbro also have basaltic compositions. In the podiform hornblende gabbro, high-temperature magnesio-hornblende likely formed as a reaction product between clinopyroxene and a hydrous pore melt. Trace element inversion models indicate that these diverse plutonic rock units formed from a tholeiitic parental melt cogenetic with the overlying basalts. The Liuyuan ophiolitic complex represents a coherent slice of oceanic crust formed in a back-arc basin.

PROBLEMS OF NUMERICAL MODELING OF LARGE-SCALE MANTLE CONVECTION IN THE SUBDUCTION ZONE

The article provides a review of modern models of large-scale mantle convection in the zone of a heavy cold oceanic plate (slab) subduction into the upper mantle. The formal approximation of the upper mantle for the present case is an incompressible Newtonian fluid with variable viscosity. It is assumed that the plate subduction is preceded by the stage of regime formation for thermo-gravitational convection in the mantle, which is caused by temperature and buoyancy of the lightweight hot substance. Important in this situation is the problem of quantitative formal modeling of phase transitions in the plate itself, as a result of which it becomes compacted due to thermal compression, removal of a part of lightweight mobile components of its original sediments and, consequently, overall weighting of the residual components of its material. It is also important to take into account the impact of mantle currents on the plate, which leads to its geometric distortion. Emphasis should also be placed on representing this plate/slab as an object of numerical modeling, since in the case of its representation as a thin elastic plate, adopted by Gustav Kirchhoff, the current hypotheses of normal remaining normal to the deformed middle surface of the plate and an unchanging thickness are violated.The aim of the work is to construct a large-scale 2D numerical model of mantle convection in the subduction zone, which takes into account the thermal gravity regime for the upper mantle and the plate, initiated by plate subduction, the influence thereon of mantle flows (mantle wind), and phase transitions in the plate. Based on smoothed particles hydrodynamics (SPH), there was constructed a computational scheme of the slab dynamics. To verify the model, there have been performed a number of computational experiments, the results of which are generally consistent with the seismotomographically identified structure of mantle flows in the subduction zone. Thus, the model appears to show fragmentary nature of the process of subduction being due to the interaction between the subducting plate and the part that remains on the surface, which leads to deformation of the descending plate.

Monitoring subseafloor temperature and fluid pressure in sealed ODP/IODP boreholes to constrain in-situ hydrological state and processes in igneous oceanic crust

We review the contributions to our understanding of the hydrogeology of the oceanic crust as gained by monitoring subseafloor temperatures and pressures in CORK (“Circulation Obviation Retrofit Kit”) sealed hole observatories that penetrate through marine sediments into igneous basement. CORKs were installed during 1991-2012 in 17 holes drilled by the Ocean Drilling Program (ODP) and Integrated Ocean Drilling Programs (IODP) in igneous oceanic crust (aged 0.4 to 24 Ma) in the thickly sedimented eastern Pacific Ocean and in the thinly sedimented north Atlantic Ocean. Results indicate that the igneous crust in all these settings is highly transmissive and supports vigorous lateral fluid flow and/or convection beneath the sediment cover driven by very small horizontal pressure gradients. Inter-CORK experiments suggest that the horizontal permeability of the young oceanic basement at these sites may be anisotropic, with higher values in the direction parallel to the axis of spreading and tectonic fabric and lower values in a ridge-normal cross-strike direction. Tracer experiments applied to ridge-parallel flow suggest that only a small fraction of the crust is well connected. CORKs also record the response of subseafloor formation fluid pressures to seafloor tidal loading, and, once the tidal responses are filtered out, response to tectonic strain events. Interpretations based on these responses and original two-dimensional modelling of ridge-normal fluid circulation in the crust suggest a scale dependence of permeability of igneous oceanic crust (higher values at larger scales up to tens of km), although recent three-dimensional models suggest a more limited scale effect. CORKs have also provided important geochemical and microbiological results from the igneous oceanic crust, although they are not reviewed in this paper.

Primary Sulfur Isotopes of Intraplate Basalts and Implications for Deep S Recycling of Altered Oceanic Crust

AbstractAltered oceanic crust (AOC) is the largest contributor to the subducted sulfur (S) budget and its recycling modulates the redox evolution and S distribution in the mantle. However, the role of AOC in the deep cycling of S remains poorly constrained. Here we probe the primary S isotopes of Cenozoic intraplate basalts in eastern China by investigating sulfide inclusions in magmatic clinopyroxene megacrysts. These basalts were derived from the deep mantle metasomatized by melts derived from recycled AOC but show MORB‐like S isotopes (−0.9–0.9‰), suggesting that AOC‐derived melts transfer negligible sulfate and hardly change the δ34S and redox state of the deeper mantle. This contrasts with the generally high δ34S values of mantle wedge peridotites and primary arc magmas that reflect the slab addition of sulfate, indicating that S species and isotopes released from the subducted slab and associated fO2 are not constant and vary with subduction depth.

Potassium alkaline volcanism of Alaid volcano, Kuril Islands: the role of subduction melange in magmogenesis

New data are presented on the contents of major elements and trace elements, Sr-Nd-Pb isotopes in the Holocene high-potassium basic lavas of the Alaid volcano, located in the north of the Kuril island arc in the junction zone with the Kamchatka volcanic segment. According to petrochemical criteria, two groups of coeval rocks are distinguished: Ne-normative shoshonites and high-potassium subalkaline basalts, which are similar to each other in a number of geochemical characteristics. Chondrite-normalized REE distribution spectra show enrichment in LREE, with flat HREE distribution spectra, and the absence of Eu and Ce anomalies. MORB-normalized incoherent element concentrations show LILE enrichment and a well-defined negative Ta-Nb-Ti anomaly typical of suprasubduction volcanics. High K2O/Rb and Rb/Sr ratios indicate the presence of biotite and amphibole in the magmatic source, while low Sr/Y ratios and flat distribution spectra of medium and heavy lanthanides indicate the absence of garnet in the restite paragenesis. Significant variations in the contents of macro- and microcomponents at similar MgO concentrations indicate a heterogeneous magmatic source, and taking into account linear mixing trends in isotope and discrimination diagrams, experimental data, suggest the involvement in magmogenesis of not only the peridotite mantle, but also amphibole-clinopyroxene mineral paragenesis. An analysis of the literature data shows that in «cold» island arcs, manifestations of potassium alkaline magmatism are often, if not in all cases, associated with local extension zones. Since such zones are associated with the adiabatic rise of a hot and plastic asthenosphere, it can be assumed that subduction melange formed along the boundary of the slab and supra-subduction mantle, consisting of hydrated fragments of ultrabasites and metamorphosed oceanic crust transformed into amphibole-bearing pyroxenites, was involved in the melting. This mechanism makes it possible to logically explain the geochemical and isotopic features of the anomalous alkaline magmatism of the Kuril island arc and the connection with the anomalous tectonics of its northern segment. The results obtained may be important in discussing the genesis of potassium alkaline magmas manifested in subduction geodynamic settings.

Granitoid intrusions at the periphery of Kursk block as a part of Paleoproterozoic silica large igneous province at the Eastern Sarmatia

At the eastern border of the Archean Kursk block of Sarmatia the Paleoprotorozoic 2.04–2.08 Ga diorite-granodiorite magmatism is widespread. The intrusive massifs granitoids are metaluminous calc-silica I-type rocks enriched with incoherent elements (LILE and LREE) with negative Ti, P and Nb anomalies. They have wide variety of εNd(T) values in rocks and εHf(T) values in zircons, and vary greatly in melt origin depths with the heterogenous Archean lower crust mafic sources. The diorites have the less radiogenic ancient crustal sources. Granodiorites have Paleo- and Mesoarchean and more juvenile Neoarchean sources. The reason of intensive 2.06 Ga magmatism was the astenospheric mantle uplift during the breakdown of pushed oceanic plate due to flat subduction. The plate breakdown and the mafic underplating lead to intracrustal melting in the upper plate consisting of different age Archean and Paleoproterozoic crustal fragments which were joint as a result of previous accretion. Diorite-granodiorite magmas were formed at melting points of different depths in the ancient Archean crust at the periphery of Kursk block with involvement of the Eastern Sarmarian orogen Paleoproterozoic lithosphere fragments into the melting sources.

Mantle xenoliths in Northeast China record deep carbon cycling triggered by subduction of the Pacific Plate

Earth science studies have focused on the deep mantle carbon cycle and its geodynamic effects. However, the way in which carbonated silicate melts modify the mantle remains poorly constrained. In this study, we report petrographic, mineral major and trace element, and Sr isotopic data for a suite of peridotite and pyroxenite xenoliths from the Shulan and Yitong areas of Northeast China. These data are used to investigate how carbonated silicate melts modify the mantle. The olivine, orthopyroxene, and clinopyroxene in the Shulan harzburgite and lherzolite xenoliths have relatively high Mg# values (90−93), and the clinopyroxenes have low (La/Yb)N ratios (0.49−0.61), implying that they may be post-partial melting residues of mantle previously modified by silica-rich melts, which is consistent with the light rare earth element (LREE)-depleted REE patterns of the clinopyroxenes. The olivine, orthopyroxene, and clinopyroxene in the Shulan (olivine) websterite xenoliths also have high Mg# values (90−94) that are indicative of a mantle origin. However, the clinopyroxenes have high (La/Yb)N (1.43−65.8), Ti/Eu (3504−5255), and Ca/Al (5.13−9.59) ratios, a positive correlation between Sm/Hf and Zr/Hf ratios, and high LREE contents, which suggest carbonated silicate metasomatism. This inference is also consistent with the replacement of olivine and orthopyroxene by clinopyroxene. In contrast, the Mg# values (78−86) of the olivines in the Yitong orthopyroxenite, wehrlite, websterite, and clinopyroxenite xenoliths are lower than those of the corresponding minerals from peridotites. This result, combined with the variably LREE-enriched and high field strength element (HFSE)−depleted patterns (e.g., Nb, Ta, Zr, and Hf) of the clinopyroxenes, suggest that these pyroxenites could have crystallized from mantle-derived melts. Compared with the Shulan peridotites, the Yitong xenoliths record higher temperatures (1028−1310 °C) and higher (La/Yb)N (1.05−5.30) and Ti/Eu (2624−6567) ratios, and a positive correlation between La and Sr, which reflect the occurrence of carbonated silicate melts in the lithospheric mantle. This is also supported by the estimated equilibrium melt compositions obtained from clinopyroxene. Thus, we propose that the different types of Yitong xenoliths are the crystallized products of silica-rich to carbonated silicate melts. The low 87Sr/86Sr ratios (0.70345−0.70488) of the clinopyroxenes in the wehrlites and pyroxenites from Shulan and Yitong, along with the timing of formation of the Northeast Asian big mantle wedge (ca. 20 Ma) and geochemical characteristics of the Cenozoic basalts and high-Mg andesites in Northeast China, indicate that the carbonated silicate melts were derived from the partial melting of recycled carbonatized oceanic crust and ancient recycled crustal material in the mantle transition zone. This implies that the lithospheric mantle along the Yilan−Yitong faults experienced not only crystallization and the accumulation of silica-rich and carbonated silicate melts that formed the Yitong wehrlite and pyroxenite xenoliths, but also subsequent modification by carbonated silicate melts that formed the Shulan (olivine) websterite xenoliths. These mantle xenoliths record deep carbon cycling triggered by the subduction of the Pacific Plate.

Increased metamorphic conditions in the lower crust during oceanic transform fault evolution

Abstract. Oceanic transform faults connect the segments of active spreading ridges that slide past each other. In a classical view, transform faults are considered conservative, where no material is added or destroyed. Recent studies, however, suggest that the crust in the transform fault region is deformed during different episodes and is therefore non-conservative. We combine high-resolution 3D broadband seismic data with shipborne potential field data to study ancient oceanic fracture zones in Albian–Aptian aged oceanic crust in the eastern Gulf of Guinea offshore São Tomé and Príncipe. The crust in this region is characterized by a thin, high-reflective upper crust, underlain by a thick, almost seismically transparent lower crust. At the paleo-transform faults, the lower crust, however, comprises reflectors, which dip towards the transform fault and were previously interpreted as extrusive lava flows at an extensionally thinned inside corner. The lower crust therefore defines the target area for inversion and forward modeling of the potential field data. The chosen seismic horizons are used as geometrical boundaries of the crustal model. First, we perform a lateral parameter inversion for the lower crust, which provides vertical columns of density and magnetic susceptibility. Second, we sort the estimated values using a clustering approach and identify five groups with common parameter relationships. Third, we use the clustered lower-crustal domains to define a consistent 3D model of the study area that aligns with the seismic structure and geological concepts, which is preferred over the simple inversion of the first step. The final model generally shows anomalous low susceptibility and medium to high densities close to the buried fracture zones, which reflects increasing pressure and temperature as the transform faults evolved. This is accompanied by a change in metamorphic facies from prehnite-pumpellyite to greenschist. Our model indicates evolving extension and a second magmatic phase during juxtaposition against the trailing ridge segment. These results are in line with recent studies and strengthen the impressions of a widespread non-conservative character of transform faults.

Geotourism Potensial on Geoheritage in Natuna Geopark

Abstract Natuna is an archipelago and the name of a regency located in the Riau Islands Province. Bunguran Island, the largest island, has a unique geological diversity, where the rocks are derived from continental and oceanic crusts, including old river deposits. Natuna is rich in biological and cultural diversity of local communities. The diversity of the Earth’s heritage made Natuna designated as a national geopark in 2018. One of geopark’s goals is to foster a local economic value, which is usually approached through geotourism activities. The purpose of this research is to assess the potential of geotourism, especially from geological heritage aspect. The research method used were literature review, field data sampling, including analysis and classification. It was identified that five geoheritage sites are feasible to be developed into geotourism objects and attractions.

Localized Ultra‐Low Velocity Zone as a Strong Scatterer at the Core‐Mantle Boundary Beneath Central America

AbstractSmall‐scale scatterers in the lower mantle contain key information on the recycling process in the whole mantle. Resolving the fine structures of these scatterers is crucial for understanding the dynamic evolution in the deep Earth. Here, we search for scatterers in the lower mantle beneath Central America and the northwestern Pacific Ocean by events in South America and PKP recordings in Japan. In this work, we perform migration of PKP precursors on the source and receiver sides by raytracing to obtain the distribution map of scatterers, after considering 3D heterogeneous structures. Furthermore, we locate a distinct scatterer at the core‐mantle boundary (CMB) beneath Central America that generates strong PKP precursors with unique slowness and back‐azimuth by array processing, which is compatible with the migration result. Based on waveform modeling, this scatterer is essentially a localized ultra‐low velocity zone (ULVZ) with approximately P‐wave velocity reduction of 6% and a lateral extent of 3°. Heated up of the subducted lithosphere by the core or partial melting of the subducted oceanic crust are plausible sources of the localized ULVZ at the CMB we detect here in the subduction region.

Subduction-stalled plume tail triggers Tarim large igneous province

Cold slab subduction and hot plume burst are generally envisaged as independent triggers for convergent margin and intraplate magmatisms, respectively. However, descending oceanic plates occasionally encounter ascending mantle plumes, leading to contrasting hypotheses that plumes interrupt subduction processes and/or slabs choke plume pathways. This study used 2-D numerical simulation to reproduce a Paleozoic scenario in Central Asia where a subduction-induced plume head is invoked to interpret the formation of the Tarim large igneous province (LIP). The model assumes a long-lived mantle plume beneath the South Tianshan oceanic plate adjacent to the trench. As subduction initiated, plume materials spread first under the moving oceanic lithosphere, which developed a sequence of seamounts. Subsequently, the continual subduction drove a strong downwelling flow that stalled or restricted plume ascent in the upper mantle and caused the accumulation of hot materials in the uppermost lower mantle. Ultimately, the slab break-off after collision provided an opening pathway allowing for the accumulated hot materials to reach the surface, resulting in the development of a concurrent plume head and the formation of LIP on the overriding Tarim craton. Bending and rollover of the subducted oceanic lithosphere beneath an implemented stationary trench may contribute slab components to the LIP source, which can reasonably explain the slab-like geochemical fingerprints of basaltic rocks. Our work offers a tentative interpretation for the paradox that seamount formation preceded the LIP eruption in Tianshan and highlights possible slab effects, where subduction can stall the plume tail, causing heat accumulation that triggers a LIP.

The Neoarchean granitoids in the Yanlingguan area, western Shandong, North China Craton record the transition from TTG to K-rich granitoids

The Neoarchean is an important period of continental crustal growth worldwide. Determining how the continental crust increases in size and was finally cratonized at the end of the Archean is therefore an important scientific issue in geology. This study involved field investigations, SHRIMP U-Pb zircon dating, zircon in situ Hf isotopic analysis, and whole-rock geochemistry of Neoarchean granitoids at Yanlingguan, a typical Neoarchean area in the western Shandong granite-greenstone belt, North China Craton. Three samples of diorite/TTG that occur as enclaves in monzogranite record the oldest magmatic zircon 207Pb/206Pb ages of 2738–2731 Ma. They have low ΣREE (total REE) contents (37.5–72.4 ppm), low (La/Yb)n (8.9–25.5) and variable Eu/Eu* (0.90–1.86). The Xinfushan trondhjemite has a weighted mean 207Pb/206Pb age of 2599 ± 4 Ma for magmatic concordant or near-concordant zircons from six samples. The trondhjemites have large REE variations, with ΣREE, (La/Yb)n, and Eu/Eu* being 36.3–150.9 ppm, 23.9–91.1 and 0.57–2.00, respectively. The Lianhuashan monzogranite has a weighted mean 207Pb/206Pb age of 2502 ± 6 Ma for magmatic concordant or near-concordant zircons from four samples. These rocks have ΣREE contents, (La/Yb)n and Eu/Eu* of 42.9–573.7 ppm, 2.4–89.1 and 0.31–1.20, respectively. The geochemisal data indicate that the 2.7 Ga diorite/TTG rocks formed by fractional crystallization of mafic magma under low-pressure conditions. In contrast, the 2.6 Ga trondhjemites were most likely formed by a mixture of magmas derived from partial melting of low-K mafic and the earlier tonalitic rocks; whereas the 2.5 Ga monzogranites were derived by partial melting of the earlier TTG rocks, with addition of some sedimentary material. These events commenced with the subduction and melting of early Neoarchean oceanic crust, likely influenced by mantle plume activity, through an increase in crustal melting and consequent increase in the K2O/Na2O ratio in the 2.6 Ga TTG, and culminated in the production of the widespread generation of K-rich monzogranites at 2.5 Ga. Western Shandong thus underwent an increase in crustal maturity as it transitioned from TTG-dominated magmatism to K-rich granite, marking cratonization of the North China Craton at the end of the Neoarchean.