When and how did plate tectonics begin？ Theoretical and empirical considerations

Plate tectonics is the horizontal motion of Earth’s thermal boundary layer (lithosphere) over the convecting mantle (asthenosphere) and is mostly driven by lithosphere sinking in subduction zones. Plate tectonics is an outstanding example of a self organizing, far from equilibrium complex system (SOFFECS), driven by the negative buoyancy of the thermal boundary layer and controlled by dissipation in the bending lithosphere and viscous mantle. Plate tectonics is an unusual way for a silicate planet to lose heat, as it exists on only one of the large five silicate bodies in the inner solar system. It is not known when this mode of tectonic activity and heat loss began on Earth. All silicate planets probably experienced a short-lived magma ocean stage. After this solidified, stagnant lid behavior is the common mode of planetary heat loss, with interior heat being lost by delamination and “hot spot” volcanism and shallow intrusions. Decompression melting in the hotter early Earth generated a different lithosphere than today, with thicker oceanic crust and thinner mantle lithosphere; such lithosphere would take much longer than at present to become negatively buoyant, suggesting that plate tectonics on the early Earth occurred sporadically if at all. Plate tectonics became sustainable (the modern style) when Earth cooled sufficiently that decompression melting beneath spreading ridges made thin oceanic crust, allowing oceanic lithosphere to become negatively buoyant after a few tens of millions of years. Ultimately the question of when plate tectonics began must be answered by informa- tion retrieved from the geologic record. Criteria for the operation of plate tectonics includes ophiolites, blueschist and ultra-high pressure metamorphic belts, eclogites, passive margins, transform faults, paleomagnetic demonstration of different motions of different cratons, and the presence of diagnostic geochemical and isotopic indicators in igneous rocks. This record must be interpreted individually; I interpret the record to indicate a progression of tectonic styles from active Archean tectonics and magmatism to something similar to plate tectonics at ~1.9 Ga to sustained, modern style plate tectonics with deep subduction——and powerful slab pull——beginning in Neoproterozoic time.     

Old EMI-type enriched mantle under the middle North China Craton as indicated by Sr and Nd isotopes of mantle xenoliths from Yangyuan, Hebei Province

Recent studies indicate that the Mesozoic litho- spheric thinning in North China was diachronous with that in west to the Taihangshan gravity lineament being later than in the eastern part of the North China Cra- ton[1―3]. During the Cenozoic, lithospher…     

Upper mantle SH velocity structure beneath Qiangtang Terrane by modeling triplicated phases

We constrain SH wave velocity structure for the upper mantle beneath western Qiangtang Terrane by comparing regional distance seismic triplicated waveforms with synthetic seismograms, based on an intermediate event （-220 km） recorded by the INDEPTH-Ⅲ seismic array. The ATIP model reveals a low-velocity anomaly with up to -4% variation at the depth of 190-270 km and a relatively small velocity gradient above the depth of 410 km in the upper mantle, which is in agreement with previous results. In combination with other geological studies, we suggest that the depth of top asthenosphere is 190 km and no large-scale lithosphere thinning occurs in western Qiangtang Terrane, besides, Qiangtang Terrane has the same kind of upper mantle structure as the stable Eurasia.     

Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean

The Earth's mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution.     

Distribution feature of terrestrial heat flow densities in the Bohai Basin,East China

Temperature logging curves at 8 boreholes and well-testing temperature data at 142 boreholes are used to determine geotemperature gradients in the Bohai Basin. The thermal conductivities of 86 rock samples are measured at laboratory and the effects of porosity and temperature are corrected to obtain conductivities in situ. Terrestrial heat flow densities at 76 wells are determined based on these data. The distribution of the heat flow indicates that the terrestrial heat flow in the Bohai Basin is relatively high with an average value of 65.8 mW/m². This characteristic is caused by the tectonic evolution of the basin. During Cenozoic, the lithosphere stretched intermittently and the crust thinned so that heat conducted from the mantle increased and formed thermal abnormity at depth beneath the basin.     

Laboratory models of the thermal evolution of the mantle during rollback subduction

The subduction of oceanic lithosphere plays a key role in plate tectonics, the thermal evolution of the mantle and recycling processes between Earth's interior and surface. Information on mantle flow, thermal conditions and chemical transport in subduction zones come from the geochemistry of arc volcanoes, seismic images and geodynamic models. The majority of this work considers subduction as a two-dimensional process, assuming limited variability in the direction parallel to the trench. In contrast, observationally based models increasingly appeal to three-dimensional flow associated with trench migration and the sinking of oceanic plates with a translational component of motion (rollback). Here we report results from laboratory experiments that reveal fundamental differences in three-dimensional mantle circulation and temperature structure in response to subduction with and without a rollback component. Without rollback motion, flow in the mantle wedge is sluggish, there is no mass flux around the plate and plate edges heat up faster than plate centres. In contrast, during rollback subduction flow is driven around and beneath the sinking plate, velocities increase within the mantle wedge and are focused towards the centre of the plate, and the surface of the plate heats more along the centreline.     

Process and mechanism of mountain-root removal of the Dabie Orogen—Constraints from geochronology and geochemistry of post-collisional igneous rocks

Systematical studies of post-collisional igneous rocks in the Dabie orogen suggest that the thickened mafic lower crust of the oro- gen was partially melted to form low-Mg# adakitic rocks at 143-131 Ma. Delamination and foundering of the thickened mafic lower crust occurred at 130 Ma, which caused the mantle upwelling and following mafic and granitic magmatic intrusions. Mig- matite in the North Dabie zone, coeval with the formation of low-Mg# adakitic intrusions in the Dabie orogen, was formed by partial melting of exhumed ultrahigh-pressure metamorphic rocks at middle crustal level. This paper argues that the partial melting of thickened lower and middle crust before mountain-root collapse needs lithospheric thinning. Based on the geothermal gradient of 6.6~C/km for lithospheric mantle and initial partial melting temperature of ~1000~C for the lower mafic crust, it can be estimated that the thickness of lithospheric mantle beneath thickened lower crust has been thinned to 〈45 km when the thickened lower crust was melting. Thus, a two-stage model for mountain-root removal is proposed. First, the lithospheric mantle keel was partially removal by mantle convection at 145 Ma. Loss of the lower lithosphere would increase heat flow into the base of the crust and would cause middle-lower crustal melting. Second, partial melting of the thickened lower crust has weakened the lower crust and increased its gravity instability, thus triggering delamination and foundering of the thickened mafic lower crust or mountain-root collapse. Therefore, convective removal and delamination of the thickened lower crust as two mechanisms of lithospheric thin- ning are related to causality.     

A correlation between mid-ocean-ridge basalt chemistry and distance to continents

To fully understand the structure and dynamics of the Earth's convecting mantle, the origins of temperature variations within the mantle need to be resolved. Different hypotheses have been proposed to account for these temperature variations: for example, heat coming from the decay of radioactive elements or heat flowing out of the Earth's core. In addition, theoretical studies suggest that the thermal properties of continental masses can affect mantle convection, but quantitative data that could allow us to test these models are scarce. To address this latter problem, we have examined the chemistry of mid-ocean-ridge basalt--which reflects the temperature of the source mantle--as a function of the distance of the ridge from the closest continental margin. No correlation is observed for oceanic ridges close to subduction zones or hotspots; subduction zones probably inhibit thermal transfer between the mantle beneath continents and ocean, whereas hotspots influence the major-element chemistry of ridge basalts, which makes their interpretation with respect to mantle temperature more difficult. However, we do observe a significant correlation for mid-oceanic basalts from the Atlantic and Indian oceans. From this, we conclude that the location of continental masses relative to active ridges influences the large-scale thermal structure of the mantle and we estimate that the mantle cools by 0.05 to 0.1 degrees C per kilometre from the continental margins.     

Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt

A substantial uncertainty in the Earth's global geochemical water cycle is the amount of water that enters the deep mantle through the subduction and recycling of hydrated oceanic lithosphere. Here we address the question of recycling of water into the deep mantle by characterizing the volatile contents of different mantle components as sampled by ocean island basalts and mid-ocean-ridge basalts. Although all mantle plume (ocean island) basalts seem to contain more water than mid-ocean-ridge basalts, we demonstrate that basalts associated with mantle plume components containing subducted lithosphere--'enriched-mantle' or 'EM-type' basalts--contain less water than those associated with a common mantle source. We interpret this depletion as indicating that water is extracted from the lithosphere during the subduction process, with greater than 92 per cent efficiency.     

Effects of sub-lithospheric small-scale convection with a Newtonian rheology on the seafloor topography and heat flux

Scafloor topography and heat flux show clear dependence on the age of seafloor. A half-space cooling (HSC)model can reproduce seafloor topography and heat flux data for younger seafloor, but for older seafloor the observations show reduced variations with the age in comparison with the HSC model predictions. The deviation was attributed to the sub-lithospheric small-scale (SSC) convection first by Parsons and McKenzie (1978). While there is little doubt that the SSC can enhance heat flux at relatively old seafloor, questions were raised as to whether or not the SSC can actually lead to a reduced topography. In this study, the effects of SSC on scafloor topography and heat flux are investigated by formulating a 2-D thermal convection model that is parallel to plate motion. Instead of using closed boundary conditions,which will bring large pressure effects because of return flow,a flow through boundary condition is adopted. The results show that although the SSC enhances the surface heat flux, it has little effects on topography for the fluids with a more realistic rheology. The reason for this is that the SSC transports the heat from the bottom to the top and cools down the whole fluids, and with the existence of a stagnant lid, the whole effects on topography are negligible.     

Some basic concepts and problems on the petrogenesis of intra-plate ocean island basalts

Basaltic magmatism that builds intra-plate ocean islands is often considered to be genetically associated with ＂hotspots＂ or ＂mantle plumes＂. While there have been many discussions on why ocean island basalts （OIB） are geochemically highly enriched as an integral part of the mantle plume hypothesis, our current understanding on the origin of OIB source material remains unsatisfactory, and some prevailing ideas need revision. One of the most popular views states that OIB source material is recycled oceanic crust （ROC）. Among many problems with the ROC model, the ocean crust is simply too depleted （e.g., [La/Sm]PM 〈1） to be source material for highly enriched （e.g., [La/Sm]pM 〉〉 1） OIB, Another popular view states that the enriched component of OIB comes from recycled continental crust （RCC, i.e.; terrigenous sediments）. While both CC and OIB are enriched in many incompatible elements （e.g., both have [La/Sm]PM 〉〉1）, the CC has characteristic enrichment in Pb and deletion in Nb, Ta, P and Ti. Such signature is too strong to be eliminated such that CC is unsuitable as source material for OIB. Plate tectonics and mantle circulation permit the presence of ROC and RCC materials in mantle source regions of basalts, but they must be volumetrically insignificant in contributing to basalt magmatism. The observation that OIB are not only enriched in incompatible elements, but also enriched in the progressively more incompatible elements indicates that the enriched component of OIB is of magmatic origin and most likely associated with low-degree melt metasomatism. H2O and CO2 rich incipient melt may form in the seismic low velocity zone （LVZ）. This melt will rise because of buoyancy and concentrate into a melt rich layer atop the LVZ to metasomatize the growing lithosphere, forming the metasomatic vein lithologies. Erupted OIB melts may have three components： （1） fertile OIB source material from depth that is dominant, （2） the melt layer, and （3） assimilation of the metasomatic vein lithologies formed earlier in the growing/grown lithosphere. It is probable that the fertile source material from depth may be （or contain） recycled ancient metasomatized deep portions of oceanic lithosphere. In any attempt to explain the origin of mantle isotopic end-members as revealed from global OIB data, we must （1） remember our original assumptions that the primitive mantle （PM） soon after the core separation was compositionally uniform/homogeneous with the core playing a limited or no role in causing mantle isotopic heterogeneity; （2） not use OIB isotopes to conclude about the nature and compositions of ultimate source materials without understanding geochemical consequences of subduction zone metamorphism; and （3） ensure that models and hypotheses are consistent with the basic petrology and major/trace element geochemistry.     

Coral reef ecosystems in the South China Sea as a source of atmospheric CO<Subscript>2</Subscript> in summer

Field measurements of air-sea CO2 exchange in three coral reef areas of the South China Sea (i.e. the Yongshu Reef atoll of the Nansha Islands, southern South China Sea (SCS); Yongxing Island of Xisha Islands, north-central SCS; and Luhuitou Fringing Reef in Sanya of Hainan Island, northern SCS) during the summers of 2008 and 2009 revealed that both air and surface seawater partial pressures of CO2 (pCO2) showed regular diurnal cycles. Minimum values occurred in the evening and maximum values in the morning. Air pCO2 in each of the three study areas showed small diurnal variations, while large diurnal variations were ob-served in seawater pCO2. The diurnal variation amplitude of seawater pCO2 was ~70 μmol mol–1 at the Yongshu Reef lagoon, 420–619 μmol mol–1 on the Yongxing Island reef flat, and 264–579 μmol mol–1 on the reef flat of the Luhuitou Fringing Reef, and 324–492 μmol mol–1 in an adjacent area just outside of this fringing reef. With respect to spatial relations, there were large differences in air-sea CO2 flux across the South China Sea (e.g. ~0.4 mmol CO2 m–2 d–1 at Yongshu Reef, ~4.7 mmol CO2 m–2 d–1 at Yongxing Island, and ~9.8 mmol CO2 m–2 d–1 at Luhuitou Fringing Reef). However, these positive values suggest that coral reef ecosystems of the SCS may be a net source of CO2 to the atmosphere. Additional analyses indicated that diurnal variations of surface seawater pCO2 in the shallow water reef flat are controlled mainly by biological metabolic processes, while those of deeper water lagoons and outer reef areas are regulated by both biological metabolism and hydrodynamic factors. Unlike the open ocean, inorganic metabolism plays a significant role in influencing seawater pCO2 variations in coral reef ecosystems.     

Trace-element of Tuoyun clinopyroxene: Implication for the deep processes of lithospheric mantle beneath the southwest Tianshan, West China

Based on the LAM-ICPMS analytic results on the trace elements of clinopyroxenes in peridotitic xenoliths occurring in early-Cretaceous basalts from western Xinjiang, the properties and the deep processes, including partial melting and mantle metasomatism, of the subcontinental lithospheric mantle beneath the Tuoyun Basin are analyzed. In the northern edge of the Tarim Basin (southwest Tianshan), the Mesozoic subcontinental lithosphere which has experienced the effect of partial melting (<10%) and intricate mantle metasomatism is characterized by Phanerozoic ‘ocean-type’ mantle. The superposed influence of SiO₂-unsaturated silicate melt and carbonate melt probably results in the metasomatic medium which resembles the hydrous silicated carbonate melt in some aspects. By comparing Tuoyun mantle with Cenozoic main mantle beneath eastern China, the properties are similar, while the former shows finer grain and higher diopside content of the peridotites and more conspicuous modal metasomatism.     

Osmium isotope determination on mantle-derived peridotite xenoliths from Panshishan with N-TIMS

The¹⁸⁷Os/¹⁸⁸Os ratios of spinel lherzolite xenoliths from Panshishan determined with N-TIMS are lower than that of the primitive mantle, which shows depleted mantle characteristics. Their positive correlation with Al₂O₃/MgO suggests that the geochemical behavior of trace elements Re and Os is similar to that of Al₂O₃ and MgO respectively during the magmatic evolution and Re-Os isotopic system is largely immune to mantle metasomatism. A model age of 2.8–3.4 Ga obtained by¹⁸⁷Os/¹⁸⁸Os-Al₂O₃/ MgO correlation might represent the homogeneous age of the mantle lithosphere beneath the area.     

Crustal P-wave velocity structure in Lower Yangtze region： Reinterpretation of Fuliji-Fengxian deep seismic sounding profile

A deep seismic sounding profile in this paper, from Fuliji in Anhui Province to Fengxian of Shanghai City, is located at eastern China (Fig. 1). The field work was jointly accomplished by the Chinese Geological and Mineral Bureau, the China Seismological …     

He-Ar isotope geochemistry of the Yaoling-Meiziwo tungsten deposit,North Guangdong Province: Constraints on Yanshanian crust-mantle interaction and metallogenesis in SE China

Isotopic abundances and ratios of He and Ar found in inclusion fluids in pyrites formed in the Yaoling-Meiziwo tungsten miner-alization epoch show that the concentration of 4He varies widely,from 1.54×10-7 cm3 STP/g to 2609×10-7 cm3 STP/g.3He is 0.759×10-12 cm3 STP/g-3.463×10-12 cm3 STP/g.3He/4He is 0.0043-4.362 Ra,varying from crustal to mantle values.The concen-tration of 40Ar ranges from 0.624×10-7 cm3 STP/g to 8.89×10-7 cm3 STP/g.The 40Ar/36Ar varies extensively,from 330 to 2952,between atmospheric and crustal or mantle radiogenic values.Mantle-derived He is present in ore-forming fluids and the calcu-lated average proportion of the mantle He is 22%;the maximum is 67%.Our research results show that mantle-derived fluids play a significant role in tungsten mineralization.The fractionation of He and Ar indicate that there was 4He-enriched air-saturated water(MSAW) in the ore-forming fluid.The ore-forming fluid was a mixture of mantle fluid,crustal magmatic fluid and MSAW.The occurrence of a mantle component in ore-forming fluid indicates the large-scale W and Sn mineralization,including Yaol-ing-Meiziwo,in southeastern China was the result of crust and mantle interaction.The underplating or intrusion of voluminous basaltic magma formed by partial melting of the upper mantle provided the necessary heat to cause partial melting of the crust and the generation of voluminous S-type granitic magmas.Crustal magmatic fluid and mantle fluid with high 3He/4He were released from magma crystallization and fractionation,mixed with the circulating modified air-saturated water,and filled the extensional tectonic fractures,leading to the formation of world-class W and Sn deposits in southeastern China.     

Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume

The compositional differences between mid-ocean-ridge and ocean-island basalts place important constraints on the form of mantle convection. Also, it is thought that the scale and nature of heterogeneities within plumes and the degree to which heterogeneous material endures within the mantle might be reflected in spatial variations of basalt composition observed at the Earth's surface. Here we report osmium isotope data on lavas from a transect across the Azores archipelago which vary in a symmetrical pattern across what is thought to be a mantle plume. Many of the lavas from the centre of the plume have lower 187Os/188Os ratios than most ocean-island basalts and some extend to subchondritic 187Os/188Os ratios-lower than any yet reported from ocean-island basalts. These low ratios require derivation from a depleted, harzburgitic mantle, consistent with the low-iron signature of the Azores plume. Rhenium-depletion model ages extend to 2.5 Gyr, and we infer that the osmium isotope signature is unlikely to be derived from Iberian subcontinental lithospheric mantle. Instead, we interpret the osmium isotope signature as having a deep origin and infer that it may be recycled, Archaean oceanic mantle lithosphere that has delaminated from its overlying oceanic crust. If correct, our data provide evidence for deep mantle subduction and storage of oceanic mantle lithosphere during the Archaean era.     

The Earth's mantle

Seismological images of the Earth's mantle reveal three distinct changes in velocity structure, at depths of 410, 660 and 2,700 km. The first two are best explained by mineral phase transformations, whereas the third-the D" layer-probably reflects a change in chemical composition and thermal structure. Tomographic images of cold slabs in the lower mantle, the displacements of the 410-km and 660-km discontinuities around subduction zones, and the occurrence of small-scale heterogeneities in the lower mantle all indicate that subducted material penetrates the deep mantle, implying whole-mantle convection. In contrast, geochemical analyses of the basaltic products of mantle melting are frequently used to infer that mantle convection is layered, with the deeper mantle largely isolated from the upper mantle. We show that geochemical, seismological and heat-flow data are all consistent with whole-mantle convection provided that the observed heterogeneities are remnants of recycled oceanic and continental crust that make up about 16 and 0.3 per cent, respectively, of mantle volume.     

Evidence for mantle plumes?

Geophysical hotspots have been attributed to partially molten asthenosphere, fertile blobs, small-scale convection and upwellings driven by core heat. Most are short-lived or too close together to be deeply seated, and do not have anomalous heat flow or temperature; many are related to tectonic features. Bourdon et al. investigate the dynamics of mantle plumes from uranium-series geochemistry and interpret their results as evidence for thermal plumes. Here we show why alternative mechanisms of upwelling and melting should be considered.     

Isotopic composition of helium in eclogite from the Dabie Mountains, central China and its geological significance

Isotopic geochemical characteristics of helium in garnet and omphacite of eclogite in the Dabie Mountains are discussed. Concentrations of³He and⁴He in garnet and omphacite are 3.9 × 10⁻¹⁴–24.0 × 10⁻¹⁴ and 0.48 × 10⁻⁷–9.42 × 10⁻⁷cm¹³.g⁻¹, respectively. Values of³He/⁴He have a range of (1.19–4.63) × 10⁻⁷. Helium In the eclogite is derived from both mantle and crust. Isotopic geochemical data of helium indicate that eclogite in the Dabie Mountains might be formed in depleted mantle and the age of the cologite would be Indo-China epoch.