Deep Mantle Research Articles

Ultrafast x-ray detection of low-spin iron in molten silicate under deep planetary interior conditions.

The spin state of Fe can alter the key physical properties of silicate melts, affecting the early differentiation and the dynamic stability of the melts in the deep rocky planets. The low-spin state of Fe can increase the affinity of Fe for the melt over the solid phases and the electrical conductivity of melt at high pressures. However, the spin state of Fe has never been measured in dense silicate melts due to experimental challenges. We report detection of dominantly low-spin Fe in dynamically compressed olivine melt at 150 to 256 gigapascals and 3000 to 6000 kelvin using laser-driven shock wave compression combined with femtosecond x-ray diffraction and x-ray emission spectroscopy using an x-ray free electron laser. The observation of dominantly low-spin Fe supports gravitationally stable melt in the deep mantle and generation of a dynamo from the silicate melt portion of rocky planets.

Sublithospheric diamond ages and the supercontinent cycle

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300–700 km depths1,2 and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb–Sr, Sm–Nd, U–Pb and Re–Os) applied to Fe-sulfide and CaSiO3 inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic–Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth—with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust—could have enhanced supercontinent stability.

A Comparison of Gravimetric, Isostatic, and Spectral Decomposition Methods for a Possible Enhancement of the Mantle Signature in the Long-Wavelength Geoidal Geometry

A long-wavelength geoidal geometry characterizes the most pronounced features of the Indian Ocean geoid low and the West Pacific and North Atlantic geoid highs. These large geoid undulations (globally roughly within ±100 m) are mainly attributed to a deep mantle structure and large lithospheric density and geometry variations (such as the African superswell), while maximum geoid modifications by a topographic relief of Himalaya and Tibet are up to ~30 m. To enhance the mantle signature in a long-wavelength geoidal geometry, gravimetric, isostatic, and spectral decomposition methods can be applied. In this study, we demonstrate that isostatic schemes yield isostatic geoid models that closely resemble a long-wavelength geoidal geometry. The gravimetric method, on the other hand, modifies the mantle geoid significantly. Further modifications of the mantle geoid by removing gravitational contributions of lithospheric mantle density and lithospheric thickness variations should (optimally) enhance the signature of the deep mantle in the sub-lithospheric mantle geoid. Our results confirm this assumption by revealing (large-scale) positive anomalies in the Central Pacific and along the Atlantic Ocean that are coupled by two negative anomalies in the East Pacific and South Eurasia. A gravimetric method thus better enhances the mantle signature in the geoidal geometry than isostatic and spatial decomposition methods. Nonetheless, our results also indicate the presence of possibly large errors in geoid modelling results that limit their full implementation in gravimetric studies of the Earth’s mantle density structure without using tomographic images of the mantle and additional geophysical, geothermal, and geochemical constraints.

Equations of State and Phase Boundaries of SiO2 Polymorphs Under Lower Mantle Conditions

AbstractSiO2 is an important modally abundant component of terrestrial planets, while there are few studies that quantitatively determine the properties of silica under simultaneously high‐temperature and high‐pressure conditions. In this study, thermal properties of stishovite, CaCl2‐type SiO2 and seifertite have been obtained by using extensive first‐principles simulations. The quasi‐harmonic approximation and intrinsic anharmonic effects were considered, and a generalized rescaling method was adopted to eliminate systematic errors in the simulation results. Based on the simulation data, we have established new equations of state for the SiO2 polymorphs and determined their phase boundaries. Anharmonicity has been demonstrated to show noticeable effects on their thermal properties and phase boundaries. With the new results, we found that SiO2 polymorphs show increasing buoyancy in the subducted basaltic slabs at depths greater than ∼800–900 km. In the normal lower mantle, they are readily stable over a broad range of depths approximately from 1,500 km to 2,600 km. This implies that SiO2 polymorphs may contribute to the lateral heterogeneity in the deep lower mantle as observed by small‐scale seismic scatterers.

Evidence for low Vp/Vs ratios along the eastern Romanche ridge-transform intersection in the equatorial Atlantic Ocean

Very long oceanic transform faults do not just offset mid-oceanic ridges by hundreds of kilometers, but they also induce variations in the lithospheric structure and affect mantle melting, and hence the crustal accretion at the ridge-transform intersection (RTI). However, the deep structure of large offset RTIs remains unknown. Here, we present three-dimensional seismic tomography results (P- and S-wave velocities, Vp, Vs) in the vicinity of the eastern Romanche transform and the Mid-Atlantic Ridge in the equatorial Atlantic Ocean, using arrival times from microearthquakes recorded by a recent temporary array of seafloor seismometers. We find that normal to low Vp/Vs ratios (∼1.7) are observed at the Mid-Atlantic Ridge, representing weak serpentinization and the dominant magmatic processes, whereas high Vp/Vs ratios (∼1.8-1.95) are observed west of the RTI, at the eastern termination of the 2016 Mw 7.1 earthquake. However, unusually low Vp/Vs ratios (∼1.6-1.7) are observed at 0-15 km depth and in the region of the 2016 Mw 7.1 earthquake rupture zone. We suggest that the unusually low Vp/Vs ratios result from the presence of thick cracks at shallow depths (0-8 km) and/or the enrichment of orthopyroxene in the deep mantle (8-15 km).

Effect of a micro-scale dislocation pileup on the atomic-scale multi-variant phase transformation and twinning

In this paper, we perform concurrent atomistic–continuum (CAC) simulations to assess the contribution of the internal stress induced by the microscale dislocation pileup at an atomically structured interface to the atomic-scale phase transformations (PTs), reverse PTs, and twinning. The main novelty of this work is to unify the atomistic description of the interface and the coarse-grained (CG) description of the lagging dislocations away from the interface within one single framework. Our major findings are: (a) the interface dynamically responds to a pileup by forming steps/ledges, the height of which is proportional to the number of dislocations arriving at the interface; (b) the pileup-induced internal stress concentration profile follows neither the classical Eshelby model nor the super-dislocation model alone, but a combination of them; (c) when the pre-sheared sample is compressed, a direct square-to-hexagonal PT occurs ahead of the pileup tip and eventually grows into a wedge shape. The two variants of the hexagonal phases form a twin with respect to each other; (d) upon a further increase of the loading, part of the newly formed hexagonal phase transforms back to the square phase. The square product phase resulting from this reverse PT forms a twin with respect to the initial square phase. All phase boundaries (PBs) and twin boundaries (TBs) are stationary and correspond to zero thermodynamic Eshelby driving forces; and (e) the microscale dislocation pileup-induced internal shear stress and the structural change at the atomic-scale interface reduces the stress required for initiating a PT by a factor of 5.5, comparing with that in the sample containing no dislocations. This work is the first characterization of the behavior of PTs/twinning resulting from the reaction between a microscale dislocation slip and an atomically structured interface. The gained knowledge will advance our understanding of how the multi-phase material behaves in many complex physical processes, such as the synthesis of multi-phase high-entropy alloys or superhard ceramics under high-pressure torsion, deep mantle earthquakes in geophysics, and so on, which all involve dislocation slip, PTs, twinning, and their interactions across from the atomistic to the microscale and beyond.

Kerguelen carbonatite tuff sourced from deep recycled material

Carbonatites are mantle-derived rocks associated with rare earth element deposits and their origin is debated. The majority of carbonatites are found in proximity to old continental crust and generally exhibit more geochemically enriched Sr and Nd isotopes relative to the prevalent mantle, with only a few known carbonatite occurrences found in intra-oceanic settings such as that in Cape Verde and Canary Island in the Atlantic Ocean. These oceanic carbonatites are moderately depleted with isotopic signatures similar to the prevalent mantle, indicating that Sr and Nd isotopic enrichment observed in continental carbonatites is controlled by the continental lithospheric mantle, metasomatized by melts and fluid over billions of years. Here we report a newly discovered intra-oceanic dolomite tuff unit with carbonatitic affinity, from the Kerguelen Archipelago, the emerged part of the long-lived Cretaceous Kerguelen Plateau. The Kerguelen dolomite tuff erupted at 24.1 ± 0.4 Ma after the main building phase of the northern Kerguelen Plateau and was fed by the Kerguelen mantle plume. The dolomite tuff shows strong Sr and Nd isotopic enrichments consistent with the presence of recycled material in the deep mantle source of the Kerguelen plume, possibly enriched during the assembly and breakup of the supercontinent Pangaea. These new data show a significant role of geodynamic processes in controlling deep material recycling, location of mantle plumes, and composition of carbonatitic melt.

The Accuracy Assessment of Lithospheric Density Models

The Earth’s synthetic gravitational and density models can be used to validate numerical procedures applied for global (or large-scale regional) gravimetric forward and inverse modeling. Since the Earth’s lithospheric structure is better constrained by tomographic surveys than a deep mantle, most existing 3D density models describe only a lithospheric density structure, while 1D density models are typically used to describe a deep mantle density structure below the lithosphere-asthenosphere boundary. The accuracy of currently available lithospheric density models is examined in this study. The error analysis is established to assess the accuracy of modeling the sub-lithospheric mantle geoid while focusing on the largest errors (according to our estimates) that are attributed to lithospheric thickness and lithospheric mantle density uncertainties. Since a forward modeling of the sub-lithospheric mantle geoid also comprises numerical procedures of adding and subtracting gravitational contributions of similar density structures, the error propagation is derived for actual rather than random errors (that are described by the Gauss’ error propagation law). Possible systematic errors then either lessen or sum up after applying particular corrections to a geoidal geometry that are attributed to individual lithospheric density structures (such as sediments) or density interfaces (such as a Moho density contrast). The analysis indicates that errors in modeling of the sub-lithospheric mantle geoid attributed to lithospheric thickness and lithospheric mantle density uncertainties could reach several hundreds of meters, particularly at locations with the largest lithospheric thickness under cratonic formations. This numerical finding is important for the calibration and further development of synthetic density models of which mass equals the Earth’s total mass (excluding the atmosphere). Consequently, the (long-to-medium wavelength) gravitational field generated by a synthetic density model should closely agree with the Earth’s gravitational field.

Geochemistry of sulfide in mafic enclaves from Gushan granite: Implications for the role of contemporaneous mafic magma in the genesis of the large-scale Jiaodong gold province, eastern China

Studies have revealed the key role of deep-seated, ore-forming fluids and metals in the generation of the Early Cretaceous large-scale gold deposits in the Jiaodong gold province of eastern China. However, how the ore-forming materials were transported to shallow crustal levels remains unclear. Here, we investigate trace elements and sulfur isotopes of pyrite within mafic microgranular enclaves (MMEs) of Gushan granite to evaluate the role of the syn-mineralization of mafic enclaves in the transportation of ore-forming materials. Zircon U-Pb and molybdenite Re-Os dating indicate that the magmatic-hydrothermal event in the potassic-altered Gushan granite occurred at ca. 120 Ma, which is contemporaneous with the Jiaodong gold mineralization. The texture and geochemical compositions of pyrite indicate that pyrite grains hosted by Gushan MMEs are of deuteric hydrothermal origin and precipitated during or shortly after magma mixing. The distribution of elements (i.e., Co, Cu, Ni, Zn, As, Ag, Au, Pb, and Bi) and the sulfur isotope (4.14‰−8.8‰) data for pyrites from Gushan MMEs are quite identical to those of the Xiejia diorite intrusion (DI) and other ores from the Jiaodong gold province, which indicates a common source of these pyrites. The common origin of the pyrites, combined with evidence from previous work, suggest that the ore-forming fluids and materials originated from the metasomatized lithospheric mantle, which was the repository of water, sulfur, and volatiles from the subducted Paleo-Pacific plate, rather than by direct release from the subducted sediments of the Paleo-Pacific plate. Our results collectively show that the arc-like mafic magmas derived from the metasomatized lithospheric mantle at ca. 120 Ma were the intermediary that transported the gold and other ore-forming components from the deep mantle to the shallow crustal levels where gold and ore-related material were injected into the Weideshan granitic suite during magma mixing. Thus, the Weideshan granitic suite may have played a critical role by continuously transferring gold to the shallow crustal faults where it precipitated. Therefore, future research or deep-drilling exploration programs in the area should emphasize the Weideshan granitic suite rather than the Xiejia DI.

Early zircon saturation in adakitic magmatic differentiation series and low Zr content of porphyry copper magmas

Porphyry copper ore-forming magmas worldwide are chemically distinguished from ordinary arc granitoids by lower Zr and by higher Sr/Zr ratios at equivalent SiO2 contents. Low ppm Zr in zircon-saturated melts and high whole-rock Sr/Zr in granitoid samples retaining igneous plagioclase are useful discriminants of Cu-fertile intrusive complexes. These and other chemical discriminants of porphyry-copper-forming melts of tonalite-granodiorite-adamellite composition cannot develop during crystallisation-differentiation at upper-crustal pressures. They indicate unusually high dissolved H2O (≥ 9 wt%) in residual melts at depths near the base of the crust. We compare Zr behaviour during mafic-to-felsic magmatic differentiation in orogenically deforming, copper-mineralised arc segments with behaviour during differentiation in non-orogenic, unmineralised arc segments laterally adjacent along the same subduction zones. In orogenically deforming segments, horizontal compressive stress provides resistance to opening of tensile dyke fractures and promotes entrapment of mantle-derived basaltic melts in magma chambers embedded in hot country rock at Moho-vicinity depths, where magmas cool slowly and residual melts tend to last long enough to experience intermittent chamber replenishment by basaltic melts from the deeper mantle. Over several cycles of replenishment and fractional crystallisation, residual melts acquire high concentrations of inherited chemical components that were largely excluded from cumulus minerals—H2O, Cl, CO2, SO3, etc. Accumulating H2O re-orders the high-pressure crystallisation sequence of igneous minerals in successive cycles and leads to early and prolific production of hornblende and early zircon saturation in mafic melts (< 60 wt% SiO2) of later cycles and leads to low Zr contents of zircon-saturated intermediate-composition residual melts.

Slab dehydration and potassium-lithium recycling in the forearc indicated by potassium and lithium isotope compositions of exhumed metabasites

Slab dehydration at subduction zones has broad implications for arc magmatism, mantle heterogeneity and elemental cycling. We present the petrography, mineral chemistry and whole-rock potassium (K) and lithium (Li) isotopic compositions of subducted metabasites exhumed from serpentinite mud volcanoes from the Mariana forearc that were recovered during International Ocean Discovery Program Expedition 366, to evaluate the capability of K and Li isotopes for tracking slab dehydration and the evolution of the slab-derived fluids at shallow subduction zones. The results show that the δ41K values of HIMU-type alkali metabasites recovered from the shallow-sourced site (i.e., Fantangisña, −0.34 ± 0.06‰, 2SD, n = 3) are slightly higher than those from the deep-sourced site (i.e., Asùt Tesoru, −0.42 ± 0.20‰, 2SD, n = 8). In addition, all the HIMU-type metabasites have homogeneous radiogenic isotope compositions (e.g., Sr, Nd and Hf), and the clinopyroxenes therein show similar trace elements patterns, indicating they have a common mantle source. These observations together with the positive correlation between δ41K and δ11B suggest that K isotope fractionation most likely resulted from metamorphic dehydration. In contrast, no such correlation was observed for Li isotopes (δ7Li ranging from +3.7‰ to +6.6‰). The Li isotope compositions can be modeled by low-temperature fluid/rock interactions with varying water/rock ratios, implying that the Li isotopic system was mainly controlled by fluid metasomatism within the subduction channel. Notably, EM-II-type metabasites have the lightest K isotope compositions (−0.72‰ to −0.67‰) and heaviest Li isotope compositions (+8.1‰ to +8.3‰) in this study. These anomalous signatures were accompanied with the lowest Nb/Th, εNd(t) and εHf(t), indicating that the K and Li isotope data of EM-II-type metabasites were indicative of their mantle source (e.g., EM-II mantle reservoir). Rayleigh fractionation modeling illustrates that the deeply subducted metabasites are characterized by high K and Li concentrations, and low δ41K, but MORB-like δ7Li. Thus, they may transfer large amounts of fluid-mobile elements into the deep mantle that contributes to the K isotopic heterogeneity in the mantle.

Tectonically assisted emplacement of oceanic intraplate volcanoes: The Bathymetrists Seamounts, central Atlantic

The origin of broad intraplate volcanic provinces has been related to deep mantle plumes or shallow lithospheric processes, e.g., underlying large fault systems. One example of an understudied intraplate volcanic province is the Bathymetrists Seamounts (BSM) in the central-eastern Atlantic, an area with dense and extended fracture zones, but the BSM has been associated with a mantle plume origin. Extensive bathymetric mapping and seafloor sampling show that most BSM-seamounts resemble flat-topped guyots capped by carbonate platforms. Vesicular, volcanoclastic samples imply that their tops formed near sea level, followed by reef formation during cessation of volcanism and crustal subsidence. Erosion determines the seamount irregularity proportional to their sizes. Strong ellipticity of some seamounts is related to multiple vents that erupted along fractures. The orientation of the volcanoes, carbonate platforms, and morphological lineaments of the BSM show particular trends that reveal information on their origin and formation mechanisms. Geomorphological analyses indicate a structural control on volcano emplacement related to underlying lithospheric faults resembling a Riedel shear pattern. The stress field corresponding to their orientations is related to a NE-SW tensional setting, fitting to the prevalent tectonic setting ~56–38 Ma years ago that coincides with the BSM formation and an increase in spreading rates. A change in movement of the African plate during this time, together with the reactivation of fracture zones of the strongly sheared equatorial Atlantic, created pathways in the lithosphere and possibly enhanced magmatism. The seamounts do not show distinct differences in erosion state, morphology, carbonate platform depth, or Mn-crust thickness, in contrast to what would be expected for an age progression within the seamount chain. Our observations, therefore, do not support a plume mantle source. While the magmatic source remains undefined, we show that tectonic pathways determined the shape of the seamounts and enabled the broad emplacement of the Bathymetrists seamount chain.

The role of subduction in the formation of Pangaean oceanic large igneous provinces

Abstract Large igneous provinces (LIPs) have been linked to both surface and deep mantle processes. During the formation, tenure and break-up of the supercontinent Pangaea, there is an increase in emplacement events for both continental and oceanic LIPs. There is currently no clear consensus on the origin of LIPs, but a hypothesis relates their formation to crustal emplacement of hot plume material originating in the deep mantle. The interaction of subducted slabs with the lowermost mantle thermal boundary and subsequent return flow is a key control on such plume generation. This mechanism has been explored for LIPs below the interior of a supercontinent (i.e. continental LIPs). However, a number of LIPs formed exterior to Pangaea (e.g. Ontong Java Plateau), with no consensus on their formation mechanism. Here, we consider the dynamics of supercontinent processes as predicted by numerical models of mantle convection and analyse whether circum-supercontinent subduction could generate both interior (continental) and exterior (oceanic) deep mantle plumes. Our numerical models show that subduction related to the supercontinent cycle can reproduce the location and timing of the Ontong Java Plateau, Caribbean LIP and potentially the Shatsky Rise by linking the origin of these LIPs to the return flow that generated deep mantle exterior plumes.

The Volcanic and Radial Expansion/Contraction History of the Moon Simulated by Numerical Models of Magmatism in the Convective Mantle

AbstractTo understand the evolution of the Moon, we numerically modeled mantle convection and magmatism in a two‐dimensional polar rectangular mantle. Magmatism occurs as an upward permeable flow of magma generated by decompression melting through the convecting matrix. The mantle is assumed to be initially enriched in heat‐producing elements (HPEs) and compositionally dense ilmenite‐bearing cumulates (IBC) at its base. Here, we newly show that magma generation and migration play a crucial role in the calculated volcanic and radial expansion/contraction history. Magma is generated in the deep mantle by internal heating for the first several hundred million years. A large volume of the generated magma ascends to the surface as partially molten plumes driven by melt buoyancy; the magma generation and ascent cause a volcanic activity and radial expansion of the Moon with the peak at 3.5–4 Gyr ago. Eventually, the Moon begins to radially contract when the mantle solidifies by cooling from the surface boundary. As the mantle is cooled, the activity of partially molten plumes declines but continues for billions of years after the peak because some basal materials enriched in the dense IBC components hold HPEs. The calculated volcanic and radial expansion/contraction history is consistent with the observed history of the Moon. Our simulations suggest that a substantial fraction of the mantle was solid, and there was a basal layer enriched in HPEs and the IBC components at the beginning of the history of the Moon.

Origin of Fe‐Ca‐Metasomatism in Exhumed Mantle Rocks at the MARK Area (23°N, ODP Leg 153) and Implications on the Formation of Ultramafic‐Hosted Seafloor Massive Sulfide Deposits

AbstractAt Mid‐Ocean Ridges, hot, reduced, acidic, and metal‐rich fluids are responsible for the formation of ultramafic‐hosted seafloor massive sulfide deposits (UM‐SMSs), where mantle exhumation efficiently operates. As UM‐SMSs display great structural, mineralogical, and geochemical variabilities from site to site, a simple genetic model cannot be applied. Notably, fluid circulation and Fe‐Ca metasomatism are reported in ultramafic‐hosted hydrothermal deposits exposed in ophiolites, suggesting it might have genetic implications on the formation of mineralized systems. Similar Fe‐Ca metasomatism was reported in drilled mantle rocks at the Mid Atlantic Ridge Kane (MARK) area, offering access to the vertical dimension beneath an exhumed oceanic core complex to provide an integrative study of the nature and geometry of deep magmatic and hydrothermal processes. At MARK, mantle rocks underwent complex processes of melt‐rock and fluid‐rock interactions. Magma channeling and interactions with surrounding rocks enriched mantle silicates in Fe, Co, and Zn. There, subsequent hydrothermal alteration allowed to stabilize Fe‐rich silicates. Mineralogy and geochemistry of hydrothermal phases at MARK suggest mineral crystallization under temperatures from ∼830° down to 350°C during early mantle exhumation at a depth &lt;6.5 km below seafloor, followed by serpentinization of the massif during progressive mantle denudation. Considering the lithological heterogeneity at (ultra)slow‐spreading ridges, metal enrichment in deep mantle rocks during melt‐rock interactions may be a widespread process. In ultramafic‐dominated environments where extensional tectonics allow fluid flows to these deep zones, fluids may leach and transport metals to the surface, accounting for metal entrapment in UM‐SMSs.

Interpretation of Geothermal Magnetic Depths, Physicochemical Parameters and Heavy Metals Determination of Lamurde Hot-spring in the North-Eastern Benue Trough, Nigeria

An integrated interpretation of the magnetic data and the analysis of physicochemical parameters and heavy metals from Lamurde hot spring has been done for the first time. The temperature of the hot spring was measured to be at 44 °C while the normal surface temperature is 25 °C. Three prominent magnetic anomalies were found at bottom depths of 11.379 ± 0.253 km, 13.015 ± 2.120 km and 9.161 ± 1.200 km suggesting that the Lamurde hot-spring is heated at deep mantle (13.015 ± 2.120 km) and issued to the surface through either local or regional fault system or both. The physicochemical analysis gave pH (7.93 ± 0.06), total chlorine (125.0 ± 2.89 µg/L), total dissolved solids TDS (202.0 ± 2 ppm), hardness (64 ± 1.73 µg/L), alkalinity (210 ± 4 µg/L) and nitrate (1.3 ± 0.3 µg/L). In comparison with that of WHO standards, the results indicated that all the physicochemical parameters studied except for alkalinity fall within the permissible limits. Heavy metals were identified and analyzed in the sampled water: lead (0.1501 ± 0.0007 mg/L), chromium (0.0729 ± 0.0007 mg/L), nickel (0.1987 ± 0.1476 mg/L), cadmium (0.0115 ± 0.0003 mg/L), copper (0.0442 ± 0.0008 mg/L), arsenic (0.0456 ± 0.0003 mg/L), iron (0.8016 ± 0.0005 mg/L), cobalt (0.0274 ± 0.0004 mg/L), selenium (undetectable), manganese (5.31 ± 0.0361 mg/L) and zinc (0.1629 ± 0.0004 mg/L). The concentrations of heavy metals observed in the area with the exception of Zn, Cu and Se are all above the standards for drinking water. Their concentrations are in the order of Mn &gt; Fe &gt; Ni &gt; Zn &gt; Pb &gt; Cr &gt; As &gt; Cu &gt; Co &gt; Cd &gt; Se. These results suggest that the Lamurde hot spring is a potential zone for Mn, Fe, Ni, Zn and Pb deposit.

Multigrain X-ray diffraction for the study of deformation and phase transformation microstructures at deep mantle pressures and temperatures

Multigrain X-ray diffraction for the study of deformation and phase transformation microstructures at deep mantle pressures and temperatures

No Late Cretaceous true polar wander oscillation and implications for stability of Earth relative to the rotation axis

Knowledge of Earth's stability with respect to the spin axis sets boundary conditions for understanding the deep mantle and interpreting records of past climate. Recently it has been argued that the solid Earth rotated by 12∘, and then rotated back, 86 to 78 million years ago. Herein we reanalyze the paleomagnetic data and report new rock magnetic analyses, from the Scaglia Rossa limestone of Italy on which this true polar wander (TPW) was based. We find that these data record an unrecognized secondary magnetization carried by authigenic hematite. This overprint has a differential angular effect on the normal and reversed polarity primary magnetizations, creating biased directions. This bias, together with probable tectonic and sedimentary slide rotations, creates false polar wander signals. The recognition of these artifacts serves as a cautionary tale and indicates that the null hypothesis of no large TPW since the Late Cretaceous cannot be rejected. This stability is fundamentally different from that of other planetary bodies which have undergone large polar wander, and provides the framework for Earth's climate history. Because of uncertainties in paleolongitude and geomagnetic field morphology, the unambiguous definition of terrestrial TPW deeper in time remains elusive.

Carbonate-rich crust subduction drives the deep carbon and chlorine cycles.

The flux balances of carbon and chlorine between subduction into the deep mantle and volcanic emissions into the atmosphere are crucial for the habitability of our planet1,2. However, pervasive loss of fluids from subducting slabs has been thought to cut off the delivery of both carbon and chlorine to the deep mantle owing to their high mobility under hydrous conditions3,4. Our new high-pressure experiments show that most carbonates (>75 wt%) in carbonate-rich crustal rocks-one of the main subducting carbon reservoirs-survive devolatilization and hydrous melting in cold and warm subduction zones, indicating that their subduction has driven the deep carbon cycle since the Mesoproterozoic. We found that KCl and NaCl, respectively, become stable phases crystallizing from hydrous carbonatite melts with low chlorine solubility in warm and hot subduction zones, resulting in the sequestration of chlorine in the solid residue in downwelling slabs. Accordingly, the subduction of carbonate-rich rocks facilitated highly effective recycling of both chlorine and carbon into the deep mantle at intermediate stages of Earth's history and led to declining atmospheric pCO2 and the formation of carbon-rich and chlorine-rich mantle reservoirs since the Mesoproterozoic. This period of optimal carbon and chlorine subduction may explain the ages of eclogitic diamonds and the formation of the HIMU mantle source.

Early Jurassic lithospheric delamination in the Amdo microcontinent, central Tibet: Inferred from coeval OIB- and MORB-like gabbros

The Bangong–Nujiang suture zone (BNSZ) is the remnants of the Meso-Tethyan main oceanic basin in the Tibetan plateau. It is noted that several microcontinents, e.g., Amdo and Jiayuqiao, occur within the Meso-Tethyan ocean basin. These microcontinents were strongly reworked associated with the subduction and closure of this ancient ocean, thus they play a key role in understanding the evolution of this ocean. In this contribution, we report newly documented Early Jurassic gabbros (ca. 182–175 Ma) from the Amdo microcontinent in the middle segment of the BNSZ. The gabbros can be subdivided into two groups, i.e., OIB (ocean island basalt) and MORB (mid-ocean ridge basalt) groups. The OIB-group gabbros are characterized by highly enriched NdHf isotopic characteristics (zircon εHf(t) = −11.7 to −8.9; whole-rock εNd(t) = −11.0 to −10.1). They are geochemically similar to OIB and are interpreted to be products of the partial melting of a relatively deep mantle source (> 85 km) and subsequent continental crustal inputting. The MORB-group gabbros belong to the tholeiitic series and have similar compositions to those of MORB. They were most likely derived from a depleted mantle source (zircon εHf(t) = +3.7 to +16.5; whole-rock εNd(t) = +4.9 to +8.1) in a relatively shallow depth (< 85 km). Integrating previous studies with the data presented in this contribution, we suggest that the generation of coeval OIB- and MORB-like gabbros are related to lithospheric delamination after the Qiangtang-Amdo collision and the closure of the Bangong-Nujiang ocean basin. This result will give new constraint on the evolution of the Meso-Tethyan Ocean.