Wet Plumes Research Articles

The Combination of Simulation and Heat Pump Technology for the Treatment and Reconstruction of “Wet Plume” in fossil-fuel power station

The saturated wet flue gas discharged from the power plant chimney after wet desulfurization often forms a thick “white plume” at the chimney’s mouth. This “wet plume” phenomenon has a very bad impact on the lives of local people, and it will also cause corrosion damage to surrounding equipment while polluting the environment. This paper simulates the “wet plume” phenomenon caused by flue gas emissions under different flue gas parameters and environmental parameters by using the CFD simulation method and the new technology of “smoke tower integration” of power plant emissions. Through the simulation data, the “wet plume” pollution change is studied, which provides a reliable theoretical basis for the design of new power plants and the reconstruction of old power plants. At the same time, through the simulation data, the combination of flue gas indirect condensation technology and heat pump technology is selected to realize the recovery and reuse of flue gas waste heat and realize the dual embodiment of social value and practical value.

An open GIS based 3D simulation software to predict cooling tower drift diffusion

This paper developed XJCT-3D, a simulation software for cooling tower wet plume dispersion. By coupling it with the Open GIS component Dotspatial, we have achieved geospatial visual representation of the calculation results, which has solved the problems of low calculation efficiency and insufficient visual representation of the traditional CFD software in the calculation of cooling tower wet plume dispersion. In order to verify the validity of the XJCT-3D software simulation results, we have conducted tracer experimental data from the ChalkPoint power plant. XJCT-3D accurately models wet plume deposition during cooling tower operation. From the XJCT-3D calculation results, we have observed that the maximum value of the cooling tower thermal plume wet deposition occurs near 610 m with a maximum value of 6.9E−07 kg/m2 s. This finding suggests that the cooling tower emissions carry a significant load of particles or droplets that have settled on surfaces at this particular altitude. It provides insights into potential environmental and human health impacts and helps in identifying and assessing areas at relatively higher risk of deposition, such as nearby ecosystems, farmland, or urban areas. This information can contribute to the development of effective mitigation strategies and the implementation of appropriate measures to minimize the impact of cooling tower emissions.

Convective heat transfer enhancement by corona discharge in a wire–cylinder electrostatic precipitator with the water-cooling system

Aiming at the current wet plume problem, we present a novel technique to integrate electrostatic precipitator (ESP) and heat exchanger in order to simultaneously increase heat exchange efficiency, collect water and control particle emission from flue gases. Unlike the conventional ESP, a lab-scale wire–cylinder type ESP with the collection electrode cooling by water is used for investigations in this paper. Our research indicates that the heat transfer coefficient of the ESP rises with reducing gaseous velocity, increasing applied voltage, corona current or gas temperature. The maximum improvement of the total heat transfer coefficient of 271% was achieved at 0.15 m/s, 80 °C and 16 kV of the negative discharge. Moreover, the presence of Particle matters can enhance the heat transfer coefficient by 9%–16%. The ionic wind, which is quantitatively expressed by electro-hydrodynamic number, plays a key role in modifying the gas flow patterns and consequently improving the heat exchange coefficient. For lowering the overall energy consumption, it is suggested that the ESP should be operated at a specific mode of low voltage and high current.

Analysis on Feasibility of Applying Low-Temperature Emission to Coal-fired Power Plant in Cold Weather

After the ultra-low emission(ULE) transformation, the emissions of ULE coal-fired power plants in China have been reduced greatly. However, considering the total amount of emissions, coal-fired power plants are still among the largest sources. And there may still have great potential in the removal of colored plume and some unconventional pollutants such as condensable particles matter (CPM) and sulfur trioxide (sulfuric acid mist). Though in cold days, hazes occur frequently. Besides, low-temperature condensation is conducive to the condensation and removal of vapor, CPM and sulfur trioxide in the flue gas. In this paper, a new method of low-temperature flue gas emission is proposed for coal-fired power plants in severe cold areas. The flue gas is cooled by ambient air, and discharged without reheating, and remarkable effects can be achieved in removing the pollutants. The technical feasibility and environmental compliance of applying this method to a power plant in cold weather have been analyzed. The results show that low-temperature emission of flue gas for coal-fired power plants in winter is effective in water-saving, emission reduction of pollutants and elimination of wet plume.

Roles of partial hydration of mantle transition zone and wadsleyite–olivine phase transition in wet plume development: Origin of Quaternary intraplate volcanoes in Korea

The Quaternary intraplate volcanoes in Korea have a sparser distribution (~330–563 km) than that of Japanese arc volcanoes (~80 km), geochemistry indicating mixing of components from the stagnant slab and mantle transition zone, and a longer elapsed time (~10–20 Myr) of the Quaternary eruption after the stagnation of the Pacific Plate. Although volcanism can be explained by wet plumes originating from the hydrous layer of the stagnant slab, previous numerical studies could not consistently explain the spatiotemporal and geochemical observations because of the neglected partial hydration of the mantle transition zone and wadsleyite–olivine phase transition at the 410-km discontinuity, both of which affect the viscosity and buoyancy structures. Thus, we constructed a series of two-dimensional numerical models by considering the partial hydration of the mantle transition zone and phase transition at the 410-km discontinuity. The results showed that the wet plumes are retarded at the 410-km discontinuity by the phase transition and merge into batches forming hybrid plumes in the asthenosphere, explaining the mixing of components from the hydrous slab and mantle transition zone, evidenced by the geochemical observations. The spacing (~485 km) and elapsed time (~24 Myr) of the hybrid plumes beneath the lithosphere are consistent with the spatiotemporal observations. Furthermore, the degree of mixing of the components from the hydrous slab and mantle transition zone found in the hybrid plumes explains the spatial distribution of water in the mantle transition zone beneath Korea.

Direct Observation of Sulfate Explosive Growth in Wet Plumes Emitted From Typical Coal‐Fired Stationary Sources

AbstractThe origins of atmospheric sulfate production have previously been explained by focusing on air quality models and complex chemical reaction processes. Here, we first report direct observations of sulfate production in stack plumes discharged from coal‐fired power plants, industrial boilers, and sintering plants equipped with wet desulfurization systems. Less than one third of the particulate SO42− in plumes is attributed to dust‐SO42− and SO3 measured in stacks. The SO2 aqueous‐phase oxidation process is critical in explaining the unknown sulfate formation in plume droplets with pH values ranging from 2.3 to 2.8. When the rapidly formed sulfate in wet plumes is included, a notable amount of underestimated sulfate (∼0.24 Tg in 2017) is emitted from industrial stacks in China and can partially explain the “missing sulfate” on driving most particle pollution episodes. Policy‐making targeting particulate emissions is suggested to substantially reduce sulfate emissions for further air quality improvement.

Numerical simulation study on the influence of power plant flue gas factors on the formation of white wet plume of flue gas

When the wet flue gas of a gas power plant is discharged, a white wet plume of flue gas will be produced, causing environmental and visual pollution. Local governments require the treatment of white wet Plume of flue gas. This paper establishes a model of the process of flue gas emission to produce white wet plume of flue gas, and the numerical method is used to solve it. The effects of factors such as flue gas emission temperature and flue gas moisture content on the generation of white wet plume of flue gas are studied respectively. The results show that the numerical simulation method has a good simulation effect for studying the generation conditions and factors of the white wet Plume of flue gas. It is possible to obtain the limit emission temperature and the emission moisture content of the flue gas emission without producing white wet Plume of flue gas. It is possible to study the generation of white wet plume of flue gas under the conditions of different flue gas emission temperature and emission moisture content, and provide reference and guidance for the engineering practice of power plant flue gas reduction or elimination of white wet plume of flue gas.

Influence of particle aggregation on the tephra dispersal and sedimentation from the October 8, 2016, eruption of Aso volcano

The phreatomagmatic eruption at Aso volcano on October 8, 2016, released PDC and fallout up to 320 km from the vent, providing an opportunity to study the dispersal and sedimentation of tephra bearing wet and dry aggregates. We investigated fresh air-fall tephra and conducted both dry- and wet-sieving to assess the size distribution of both aggregate-bearing deposits on the ground and particles in the plume prior to aggregation. The tephra was asymmetrically distributed in the ENE direction, with a relatively wide distribution and small maximum grain sizes north of the depositional axis, reflecting different wind directions at each altitude. The secondary accumulation maxima at 4–8 km NE from the vent was characterized by the segregation of lapilli followed by accretionary pellets that were rapidly created by aggregation in the wet plume, thereby enhancing the removal of airborne ash. Coalesced aggregates possibly arose from salt precipitation processes. For dry aggregates fallen in medial-to-distal regions, observed deposition duration and fall velocity calculations concluded that fine ash was deposited not as individual particles but as aggregates. Most grain-size distributions along the dispersal axis based on the two sieving methods used were multimodal, with the coarser mode fining according to distance from the vent and the finer mode stabilizing at around 4ϕ; this is interpreted as resulting from the mixture of grain-size distributions with different origins, i.e., the normal settling of individual clasts and particle aggregation. The wet-sieved total grain-size distribution included 20.4 wt% fine ash compared with only 2.4 wt% fine ash in the dry-sieved distribution, suggesting that aggregated particles accounted for 18 wt% of the deposit. Total grain-size distributions based on wet-sieving showed extremely poor sorting that could be explained by the following causes: the release of coarse-grained ejecta originating from the host rock of the crater, fine particles due to fragmentation from a phreatomagmatic eruption, the supply of water and salt from the crater to the plume, and aggregation enhancement of the fine ash on the ground, reflecting the effect of the crater lake as external water source.

Geochemical constraints on the origin of late Cenozoic basalts in the Mt. Changbai volcanic field, NE China: evidence for crustal recycling

ABSTRACT We conducted a systematic geochemical study of the Mt. Changbai basalts (MCBs) in NE China to better constrain their mantle sources and petrogenesis. The MCBs display ocean island basalt-like trace element patterns (e.g., positive Nb–Ta anomalies) and variable MgO contents (3.2–10.0 wt.%), trace element ratios (e.g., Ba/Nb and Rb/Nb), and Sr–Nd–Pb–Hf isotopic ratios (87Sr/86Sr(i) = 0.704497–0.705629; εNd(i) = – 2.8 to +3.3; 206Pb/204Pb(i) = 16.9–18.1; εHf = −1.2 to +6.2), which indicate derivation from a heterogeneous mantle source. The least evolved samples (MgO > 7 wt.%), with moderate Fe/Mn and FC3MS (defined as FeOT/CaO – 3MgO/SiO2) values, require a hybrid pyroxenite–peridotite mantle source. The isotopic compositions indicate the mantle source is a mixture between depleted mid-ocean ridge basalt mantle and two enriched mantle components. Our geochemical and published geophysical data indicate that these two enriched components are recycled ancient sediments and Pacific oceanic crust, which were derived from the mantle transition zone. An upwelling wet plume from the mantle transition zone transported these two enriched components into the asthenospheric mantle. These enriched components were then transformed into pyroxenite and/or eclogite through melt–rock reaction or mechanical mixing with asthenospheric mantle peridotite. We further propose that partial melting of this heterogeneous asthenospheric mantle containing fusible pyroxenite and/or eclogite (i.e., the enriched components) and refractory peridotite produced the MCBs.

Wet plume atop of the flattening slab: Insight into intraplate volcanism in East Asia

Geophysical observations imply the intraplate volcanism in East Asia is related to dehydration of slab stagnating in the transition zone. To better understand the dynamics of such process, a thermochemical mantle convection model is constructed to simulate numerically the thermal evolution of slab and the transportation of water in the process of slab downgoing, flattening and stagnation. Equation of water transfer is included, and water effects on density and viscosity are considered. Model results indicate the warming of slab by surrounding mantle is rather slow. Water could be successfully dragged into the transition zone if the reference viscosity of the hydrous layer (with initial water of 2wt%) is higher than 1017Pas and that of mantle is 1021Pas. Wet plumes could then originate in the flat-lying part of the slab, relatively far from the trench. Generally, the viscosity of the hydrous layer governs the initiation of wet plume, whereas the viscosity of the overlying mantle wedge controls the activity of the ascending wet plumes – they are more active in the weaker wedge. The complex fluid flow superposed by corner flow and free thermal convection influences greatly the water transport pattern in the upper mantle. Modeling results together with previous modeling infer three stages of water circulation in the big mantle wedge: 1) water is brought into the mantle transition zone by downward subducting slab under some specific thermo-rheological conditions, otherwise water is released at shallow depth near wedge tip; 2) wet plume generates from surface of the flattening slab warmed by surrounding mantle, and 3) water spreads over the big mantle wedge. Wet plume from the flattening Pacific Plate arrives at the lithospheric base and induces melting, which can explain the intraplate Cenozoic volcanoes in East Asia.

Geochemistry of olivine-hosted melt inclusions in the Baekdusan (Changbaishan) basalts: Implications for recycling of oceanic crustal materials into the mantle source

We determined the major and trace element concentrations of olivine-hosted melt inclusions in basalts from the active Baekdusan volcano situated on the border between China and North Korea in order to understand better the nature of the mantle source and the geodynamic processes that gave rise to volcanism at this site. Rehomogenized melt inclusions (after ‘Fe-loss’ correction) can be divided into three groups: a low-Si alkaline group, a high-Si alkaline group, and a high-Si sub-alkaline group. The low-Si group is composed of picrobasalt to basanite, and the high-Si group consists of (trachy)basalt to basaltic (trachy)andesite. The low-Si group has generally higher TiO2, CaO, and P2O5, but lower Al2O3, Na2O, and K2O contents at a given MgO concentration compared with those in the high-Si group. The CaO and P2O5 contents are positively correlated, indicating the presence of a calcium phosphate, probably tuite [γ-Ca3(PO4)2], in the source. The melt inclusions are enriched in light rare earth elements [(La/Yb)N=7.8–30.4]. On a primitive-mantle-normalized incompatible element plot, the low-Si group has positive anomalies in Ba and P compared with typical oceanic island basalt (OIB). On the other hand, the high-Si group exhibits remarkable positive anomalies in Eu, Ba, Rb, K, Pb, P, and Ti, implying that K-hollandite and tuite are essential phases in the source. The high-Si subalkaline group has lower abundances of incompatible elements than the high-Si alkaline group, reflecting different degrees of partial melting from the same source. In contrast to OIB, Baekdusan magmatism is characterized by positively fractionated (Zr/Hf)N ratios, and is best approximated by admixture of partial melts derived from both clinopyroxene-rich eclogite and garnet peridotite. Intraplate volcanism in northeastern Asia is closely associated with deep subduction of the Pacific plate. The subducting Pacific slab flattens and stagnates in the mantle transition zone under northeastern Asia, and this zone may yield a wet plume. Focused mantle upwelling through a gap in the stagnant slab may also be accompanied to the plume responsible for the Baekdusan magmatism. Thermal decomposition of K-hollandite within recycled continent-derived sediments is likely to metasomatize the ambient mantle peridotite above the stagnant slab. As the plume ascends through the upper mantle, the metasomatized mantle and recycled oceanic crustal materials (tuite-bearing eclogite or garnet pyroxenite) entrained by the plume may undergo partial melting, resulting in the volcanism observed at Baekdusan.

Geochemical fingerprint of the primary magma composition in the marine tephras originated from the Baegdusan and Ulleung volcanoes

The intraplate Baegdusan (Changbai) and Ulleung volcanoes located on the border of China, North Korea, and East/Japan Sea, respectively, have been explained by appeals to both hotspots and asthenospheric mantle upwelling (wet plume) caused by the stagnant Pacific plate. To understand the origin of the Baegdusan and Ulleung volcanism, we performed geochemical analyses on the tephra deposits in the East/Japan Sea basins originating from the Baegdusan and Ulleung volcanoes. The volcanic glass in the tephra from the Baegdusan and Ulleung volcanoes ranged from alkaline trachyte to peralkaline rhyolite and from phonolite to trachyte, respectively. The tephra from the two intraplate volcanoes showed highly enriched incompatible elements, such as Tb, Nb, Hf, and Ta, distinct from those of the ordinary arc volcanoes of the Japanese islands. The straddle distribution of the Th/Yb and Ta/Yb ratios of the tephra deposits from the Baegdusan volcano may originate from the alkali basaltic magma resulting from mixing between the wet plume from the stagnant Pacific plate in the transition zone and the overlying shallow asthenospheric mantle. In contrast, the deposits from the Ulleung volcano show a minor contribution of the stagnant slab to the basaltic magma, implying either partial melting of a more enriched mantle, smaller degrees of partial melting of a garnet-bearing mantle source, or a combination of both processes as the magma genesis. Our study indicated that the Baegdusan and Ulleung volcanoes have different magma sources and evolutionary histories.

Stagnant slab, wet plumes and Cenozoic volcanism in East Asia

Data from seismology and mineral physics indicate the existence of low-velocity zones in the deep part of the upper mantle wedge and the mantle transition zone under East Asia (Zhao and Ohtani, 2009; Zhao et al., 2009). It suggests a link between the deep subduction (and stagnation) and deep dehydration of the Pacific slab below this area and the Cenozoic volcanism observed at the surface. Here we present a numerical model describing the dynamics of a hydrated layer covering a slab that is stagnating in the Earth⿨tm)s mantle transition zone. Specifically, our model includes the effects of water on density and viscosity of the mantle minerals. We conclude that the presence of water is likely to induce Rayleigh⿢Taylor type instabilities (wet plumes) that are able to transport water up to the base of the surficial lithosphere in a relatively short time scale (a few million years). Under initial conditions representative of the subducting Pacific slab under East Asia, melting is induced where wet plumes reach the base of the lithosphere. According to our results and analysis of the known composition of Cenozoic basalts from Southwest Japan (Iwamori, 1991, 1992), wet plumes and their melting induced by water related to the stagnant slab provides an appealing hypothesis to explain Cenozoic volcanism in East Asia.

Abrupt shifts between wet and dry phases of the 1875 eruption of Askja Volcano: Microscopic evidence for macroscopic dynamics

The eruption of Askja in 1875 was a powerful eruption with intervals of sustained activity, yet also with abrupt shifts in eruption style. Shifts from dry to wet fall, from buoyant to collapsing column conditions, and sharp fluctuations of eruption intensity occurred throughout the duration of the eruption. Deep ascent processes, including decompression, bubble nucleation and early growth of bubbles were similar among the four main phases, regardless of style or column conditions. Uniformly high vesicle number densities (8.0 × 10 8 to 2 × 10 9 cm − 3 ) suggest that magma degassing throughout the eruption was a disequilibrium process with very high nucleation rates; power law distributions of the subpopulations of small bubbles on cumulative number density plots suggest continuous nucleation of bubbles. Shallow processes of bubble growth and magma ascent were at times decoupled from deep ascent processes; at the onset of each phase, the erupted magma appears to have a mature vesicle signature, suggesting extended residence times in the shallow conduit, irrespective of decompression or magma ascent rate. During each phase, the textural and density data suggest more uniform conditions of ascent and shallow degassing. Two pauses in the eruption were accompanied by shifts in vent position and we suggest that magma during this eruption was intruded in an elongated dike-like geometry and vent migrations between phases led to the eruption of magma that had slightly greater residence time and hence more mature textures. The major yet reversible shifts between ‘dry’ or magmatic and ‘wet’ or phreatomagmatic eruption styles were driven by changing external conditions accompanying the shifts in vent position, the migration of the vent into and out of water sources rather than by changes of mass flux. The shift between buoyant phreatoplinian wet plume conditions to the production of wet density currents is inferred to be a function of vent widening during the high intensity phreatoplinian phase, thereby decreasing the exit velocity of the wet mixture. In terms of the role of external water in the phreatoplinian phase, our qualitative textural observations and quantitative data suggest that the magma was a foam prior to fragmentation. However presently we cannot resolve the exact role and timing of the interaction with the external water.

Seismic implications of mantle wedge plumes

We use a coupled petrological–thermomechanical model to investigate the dynamical effects of metamorphic reactions and melting on the seismic structure of thermal–chemical plumes beneath volcanic arcs. Plume generation is driven by the subduction of buoyant crustal rocks and expulsion of aqueous slab fluids that causes hydration and partial melting of the mantle wedge. The model demonstrates two chemically distinct types of plumes. Unmixed plumes initiate from the melting front within the mantle that arises as a consequence of infiltration of slab-derived water-rich fluids, whereas mixed plumes initiate from the slab itself and entrain both slab- and mantle-derived magmas. Mixed plumes explain magmas such as adakites with crustal signatures, while primitive arc tholeiites are attributed to unmixed plumes. As a consequence of the interplay between water content and temperature both positive and negative seismic velocity anomalies are associated with the plumes. Positive anomalies are prevalent close to the slab due to the lowered temperatures associated with regions of cold plume initiation. Negative seismic anomalies develop at shallower depth due to the partially molten rocks that form plume heads. Flat lying partially molten regions that form beneath volcanic arcs as a consequence of cold wet plumes are manifest by >20% variations in the local Poisson ratio, as compared to variations of ∼2% expected as a consequence of temperature variation within the mantle wedge. In contrast to models that attribute a purely thermal origin for mantle wedge seismic anomalies, the petrological–thermomechanical model is able to explain the strong seismic velocity variations existing beneath volcanic arcs.

Development of a Wet Plume Following Liquid Release along a Fault

To investigate unsaturated flow through a fault located within fractured welded tuff, we performed in situ field experiments in the Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of approximately 82000 L water for a period of 17 mo directly into a near-vertical fault under both constant positive head (at about 0.04 m) and decreasing fluxes. As water was released into the fault, changes in moisture content were monitored in the formation while a large cavity excavated below the test bed was visually inspected for seepage. We observed that water (introduced along the fault) maintained the fault as the primary vertical flowpath, while the adjacent fractured rock served to move water laterally and vertically. Further, unlike primary flowpaths along the fault, flow was not persistent along the secondary flowpaths. While this field experiment provided preliminary insights about the flow field associated with a fault, it also demonstrated the need to investigate the role of infill material and secondary fractures in diverting flow from gravity-driven fast flow toward flowpaths in which lateral flow may occur.

Development of a Wet Plume Following Liquid Release along a Fault

Development of a Wet Plume Following Liquid Release along a Fault

Assessment of the visibility impact of a plume emitted by a desulphuration plant

Abstract Wet plumes become visible when vapour condenses. In such a situation visible plumes may create some concerns in the population. A method to reduce the occurrence and the size of visible plumes is to increase the stack emission temperature. The mathematical model presented by Janicke and Janicke (Atmos. Environ. 35 (5) (2001) 887), using a revised entrainment formulation, was implemented in a computer code to deal with sequential records of meteorological conditions. The predictions of this model have been compared with experimental data giving satisfactory results. Then the model has been used for simulating the occurrence and the size of visible plumes from a stack with a scrubber and for three different emission temperatures.

Temperature and melting of a ridge-centred plume with application to Iceland. Part II: Predictions for electromagnetic and seismic observables

The dynamic and melting processes of a ridge-centred plume have been investigated in a companion paper by Ruedas et al. (hereafter referred to as Paper I) in a set of 3-D numerical fluid dynamic models with varying plume excess temperatures and melt extraction thresholds. In Paper I, the modelled thickness of the generated crust has been compared to observations of the Icelandic crust. Using the results of those plume models magnetotelluric (MT) transfer functions and seismic velocity anomalies are predicted in this paper. Together with Paper I, a dynamically consistent set of geophysical observables of a ridge-centred plume is presented and applied to Iceland. Temperature, partial melting and the connectivity of the melt phase influence the electrical conductivity of crust and mantle rocks. The temperature and melt fraction of our plume models are used to calculate 3-D conductivity models for MT modelling. For the melt geometry ellipsoidal inclusions with appropriate aspect ratios were assumed to control melt connectivity. The resulting transfer functions are compared to each other and to models not including a plume to separate signals from the ridge and the plume. They may be applied to observed MT measurements. If the plume head contains only 1 per cent of melt, the plume signal cannot be distinguished from the ridge signal, at least 3 per cent melt is needed for such distinction. The other predicted observables calculated from the different numerical models are seismic velocity anomalies. The temperature-induced VP and VS anomalies were estimated including anharmonic and anelastic effects as well as the water induced increase of dislocation mobility that lowers seismic velocities. Realistic melt geometries, as observed in laboratory experiments, were used to calculate the effect of partial melts on the seismic velocities. VS anomaly distributions are synthesized from the different plume models and compared to seismic observations. To reconcile seismic anomalies of the plume head and plume stem, a wet plume stem overlain by a partially molten, dehydrated plume head is favoured. The combined interpretation of available observations, crustal thicknesses (Paper I) and seismic results, with our dynamic plume models (Paper I) leads to a favoured plume model with 135 K excess temperature and a vertical velocity of approximately 13 cm yr−1 at 200 km depth, with 1 per cent melt extraction threshold, and a melting zone of approximately 500 km width and 100 km depth extent.

Hydrous solidus of CMAS‐pyrolite and melting of mantle plumes at the bottom of the upper mantle

We showed in previous experiments that the melting temperature of hydrous pyrolite, at the transition boundary between wadsleyite and olivine, is abruptly reduced by the presence of 2 wt.% H2O. In this paper we determine the apparent solidus for CaO‐MgO‐Al2O3‐SiO2‐pyrolite with lower and geologically more reasonable H2O contents (0.5 wt.%). Phase relations and melt compositions have been determined at pressures of 13.5–17.0 GPa and temperatures of 1600 to 2100°C. There was no abrupt decrease of solidus temperature along the phase boundary between olivine and wadsleyite in pyrolite with 0.5 wt.% H2O. However significant gradual decrease of the solidus temperature at pressures below 15–16 GPa still supports previous models for a hydrous origin of some ancient komatiites by dehydration melting of rising wet plumes at pressures of 4–10 GPa.