Phase Transitions In Minerals Research Articles

Kinetic and Thermodynamic Transition Pathways of Silica by Machine Learning: Implication for Meteorite Impacts

AbstractRocks falling to Earth from space may generate pressure and temperature approaching Earth's deep mantle, but such meteorite impact only persists for a very short period. Under these extreme conditions, kinetical factors largely control mineral phase transitions, in which the resultant phase may deviate from those at thermal equilibrium. Here, we focus on the phase transitions of silica during meteorite impact, and have elucidated multiple pathways from low‐coordinated silica to seifertite, the densest known silica found in meteorite samples. Utilizing a high‐dimensional neuro‐network potential specifically designed for silica, we exhaustively map the potential energy landscape through stochastic surface walking and uncover low‐barrier transition pathways toward seifertite at pressures far away from thermal equilibrium. These kinetic‐driven transitions are then characterized by first‐principles simulations, revealing narrow transition windows of pressure, with seifertite becoming more kinetically favored over stishovite at pressures in the vicinity of 10 and 25 GPa. Our results suggest that meteorite impacts should have reached such target pressures to overcome the thermodynamic limit of forming seifertite. The presence of seifertite may provide key information in constraining the relevant dynamic compression conditions.

Pressure-driven processes explain the decreasing magnetization of the subducting oceanic crust in the Japan Trench

Despite the decreases in temperature and permeability of oceanic plates with increasing age, hydrothermal circulation (HC) can be rejuvenated in the 130-Ma old Pacific plate in the vicinity of the Japan Trench, substantially affecting the thermal structure and remaining amount of magnetization (RAM). To decipher the roles of HC in the thermal structure and the RAM, the vigor and extent of HC in the vicinity of the Japan Trench should be quantitatively evaluated. Here we numerically show that HC is rejuvenated in the outer-rise zone but ceases after subduction owing to permeability evolution. The calculated thermal structure explains the measured heat flow evolution but negates the HC-driven thermal demagnetization, which was thought to decrease the RAM after subduction. Instead, we propose that the pressure-driven processes decrease the RAM after subduction through the demagnetizations of titanomaghemite and magnetite and the mineral phase transitions from maghemite to hematite.

Mixed mode I-II fracture mechanism of sandstone samples after thermal treatment: Insights from optical monitoring and thermal analysis

The complex response of rocks to high temperatures and mixed stress poses a significant challenge in deep rock engineering. Sandstone samples with varying notch offsets were subjected to thermal treatment and then tested under mixed mode I-II loading. The effect of notch offset and temperature on rock fracture mechanism is the focus. By employing digital image correlation (DIC) technology, the complete process of rock fracture was scrutinized and quantitatively characterized. The results indicate that thermal treatment has a negative effect on the peak load, fracture toughness, and fracture energy of sandstone. Specifically, sandstone samples treated at 700 °C had a fracture toughness that was less than half that of the samples tested at room temperature. Furthermore, the fracture toughness and fracture energy of sandstone exhibited significant increases with increasing notch offset. In particular, the crack tip displacement (CTD) and crack mouth displacement (CMD) curves of the samples with 72 mm notch offset, compared to the pure mode I loading, show a clear infection point near the peak load. The fracture process zone (FPZ) demonstrates an approximately linear increase with both temperature and notch offset. Additionally, thermal analysis was performed on the sandstone samples, revealing the impact of high temperature on the microstructure and mineral phase transition.

Mineral phase transition characteristics and its effects on the stabilization of heavy metals in industrial hazardous wastes incineration (IHWI) fly ash via microwave-assisted hydrothermal treatment

Toxic heavy metals in industrial hazardous waste incineration (IHWI) fly ash can be effectively stabilized by using microwave-assisted hydrothermal technology. However, few works have focused on the relationship between mineralogical conversion and stability of heavy metals of fly ash during hydrothermal process. This study investigated the effect of mineral phase transition process on the stabilization and migration behavior of heavy metals in IHWI fly ash using coal fly ash as silicon‑aluminum additive. Mineral composition analysis reveals that after microwave-assisted hydrothermal treatment (MAHT) of IHWI fly ash, zeolite-like minerals (e.g., tobermorite, katoite and sodalite), secondary aluminosilicate minerals (e.g., prehnite and anorthite) and other newly-formed minerals (e.g., wollastonite, pectolite and larnite) were found. The leaching concentrations of heavy metals (Cr, Ni, Cu, Zn, Cd and Pb) in IHWI fly ash decrease sharply after MAHT with the most obvious decreases in Cu, Pb and Zn. Spearman correlation analysis show significantly negative correlation between the content of zeolite-like minerals and the leaching concentrations of most heavy metals (e.g., Ni, Cu, Zn, Cd and Pb). These results suggest that the immobilization effects of heavy metals in IHWI fly ash can be effectively enhanced by promoting the formation of zeolite-like minerals during the MAHT. This study is expected to further promote the development of IHWI fly ash harmless treatment technology.

Geodynamic predictions of seismic structure and discontinuity topography of the mantle transition zone

SUMMARYThe mantle transition zone (TZ) is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions and an increase in viscosity with depth. Still, neither its seismic structure nor its dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth (‘410’ and ‘660’) are classically associated with phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. Resulting discontinuity topography is seismically visible and can thus potentially provide insight on temperature and phase composition at depth. Besides the olivine phase changes, the disassociation of garnet may additionally impact the 660 at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified. This study presents hypothetical realizations of TZ seismic structure and major discontinuities based on the temperature field of a published 3-D mantle circulation model for a range of relevant mineralogies, including pyrolite and mechanical mixtures (MM). Systematic analysis of these models provides a framework for dynamically informed interpretations of seismic observations and gives insights into the potential dynamic behaviour of the TZ. Using our geodynamic-mineralogical approach we can identify which phase transitions induce specific topographic features of 410 and 660 and quantify their relative impact. Areal proportions of the garnet transition at the 660 are ∼3 and ∼1 per cent for pyrolite and MM, respectively. This proportion could be significantly higher (up to ∼39 per cent) in a hotter mantle for pyrolite, but remains low (<2 per cent) for MM. In pyrolite, both slabs and plumes are found to depress the 660—with average deflections of 14 and 6 km, respectively—due to the influence of garnet at high temperatures indicating its complex dynamic effects on mantle upwellings. Pronounced differences in model characteristics for pyrolite and MM, particularly their relative garnet proportions and associated topography features, could serve to discriminate between the two scenarios in Earth.

Mineral matter transition in lignite during ashing process: A case study of Early Cretaceous lignite from the Hailar Basin, Inner Mongolia, China

• NME are involved in the mineral transformation with the ashing temperature increasing. • The Ca for calcite formation in ash (500 °C) was sourced from the non-mineral Ca in coal. • The increase of ashing temperature causes the change of mineral crystallinity in coal. • NME are altered into crystalline minerals during the ashing process affected by temperature. Investigation of mode of occurrence and chemical transformation of non-mineral elements (NME) is of benefit to solve sintering and slagging during utilization of coal. To better understand the mineral phase transition and thermal behavior of NME during ashing process, lignite in Hailar basin and its ash obtained at different ashing temperatures are analyzed by the sequential selective leaching (SSL), X-ray fluorescence (XRF), X-ray diffraction (XRD) and field emission-scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDS). The results show that the phases of minerals in coals changed and the NME are involved in the mineral phase transformation with the increase of ashing temperature. In particular, non-mineral calcium presented as sulfate minerals (gypsum, bassanite or anhydrite) in the ashes of 120 °C, 350 °C and 815 °C, while changed into calcite in the 500 °C ashes. It may be due to the carbon dioxide generated by the oxidation of organic matter reacted with the non-mineral calcium that gradually released in coal under the high temperature (≥500 °C) and pressure. The transformation mechanism of non-mineral calcium implies an interaction between organic matter and mineral matter in the ashing process.

Slab extension and normal faulting in a low-angle subduction-related environment: An example of the Makran subduction zone (Iran-Pakistan)

• Mineral phase transition causes the density jump of the slab head. • Slab tail extension is caused by the suction of the ultra-dense slab head. • Greater density of the slab tail compared to the mantle imposes negative buoyancy. • Normal faulting occurs by the vertical downward movement of the extended slab tail. • The Eastern Makran dual slab is generated by the slab tail subduction . The top interface of the Makran slab can be recognized by the earthquake’s hypocenters. However, transition of focal mechanisms from thrust to normal creates ambiguity about the mechanical characteristics of the subduction. This study presents a well-constrained density structure of the upper mantle beneath the Makran through an integrated geophysical-petrological modeling. Moreover, the high-resolution Vp tomography (to determine the slab geometry), receiver function and reflection data are combined to set the upper mantle structure including cold slab. In light of accessing density structure, mechanical characteristics of descending oceanic lithosphere are inferred. Suction of the ultra-dense slab head (∼3750 kg/m 3 ), due to mineral phase transition, extends the shallow part of the slab, namely slab tail. Extension and thermal erosion by the asthenosphere close beneath the lithosphere reduces the slab thickness into half (ca. 50 km). Steep slab descends into the mantle slowly as its density contrast compared to mantle is only ∼30 kg/m 3 , while ultra-dense slab head speeds up and detaches from the steep slab. In the absence of slab head suction, considerable density contrast (∼60 kg/m 3 ) of the extended slab compared to asthenosphere supports the negative buoyancy and downward movement of the slab tail resulting in the present normal faulting. Finally, the slab tail sinks into the mantle to generate a dual subduction. In W-E elongation, the Bazman, Taftan and Kuh-Sultan slabs represent different phases of the Makran subduction prior to, simultaneous with and posterior the slab break-off, respectively.

Spatiotemporal Distribution of Precipitates and Mineral Phase Transition During Biomineralization Affect Porosity–Permeability Relationships

Enzymatically induced calcium carbonate precipitation is a promising geotechnique with the potential, for example, to seal leakage pathways in the subsurface or to stabilize soils. Precipitation of calcium carbonate in a porous medium reduces the porosity and, consequently, the permeability. With pseudo-2D microfluidic experiments, including pressure monitoring and, for visualization, optical microscopy and X-ray computed tomography, pore-space alterations were reliably related to corresponding hydraulic responses. The study comprises six experiments with two different pore structures, a simple, quasi-1D structure, and a 2D structure. Using a continuous injection strategy with either constant or step-wise reduced flow rates, we identified key mechanisms that significantly influence the relationship between porosity and permeability. In the quasi-1D structure, the location of precipitates is more relevant to the hydraulic response (pressure gradients) than the overall porosity change. In the quasi-2D structure, this is different, because flow can bypass locally clogged regions, thus leading to steadier porosity–permeability relationships. Moreover, in quasi-2D systems, during continuous injection, preferential flow paths can evolve and remain open. Classical porosity–permeability power-law relationships with constant exponents cannot adequately describe this phenomenon. We furthermore observed coexistence and transformation of different polymorphs of calcium carbonate, namely amorphous calcium carbonate, vaterite, and calcite and discuss their influence on the observed development of preferential flow paths. This has so far not been accounted for in the state-of-the-art approaches for porosity–permeability relationships during calcium carbonate precipitation in porous media.

A novel process for recovery of scandium, rare earth and niobium from Bayan Obo tailings: NaCl-Ca(OH)2-coal roasting and acid leaching

Bayan Obo tailings from Baotou Steel were discharged into tailings pond. The advantages of symbiotic resources have not been fully played. A large number of accumulated tailings also have a great impact on the surrounding environment. Comprehensive utilization of valuable elements in tailings becomes key measure to solve the above problems. Extraction of valuable elements, such as scandium, rare earth and niobium could be achieved by roasting Bayan Obo tailings in NaCl-Ca(OH)2-coal, followed by hydrochloric acid and sulfuric acid leaching. The phase transitions of main minerals were investigated. In the roasting process, niobium minerals were converted to CaTiO3-type compounds. The original rare earth minerals turned into rare earth oxide or rare earth silicate. Iron in Sc-Fe-bearing silicate migrated to CaFeSiO4, FeSiO3 and Fe3O4. Meanwhile, the structure of silicate was destroyed completely, which facilitated for leaching scandium. In the acid leaching process, newly generated niobium minerals were dissolved in sulfuric acid as niobium sulfate or niobic acid. For Bayan Obo tailings, under the optimal roasting and leaching conditions determined in this research, 97.6% Sc2O3, 97.18% REO and 40.1% Nb2O5 entered hydrochloric acid. 51.67% Nb2O5 was leached by concentrated sulfuric acid. Sc2O3 and REO were enriched in hydrochloric acid.

Determination of the Activation Energies of Phase Transition for Calcium Orthophosphates Based on Powder X‐Ray Diffraction Data

AbstractKinetic studies of the transformation of calcium orthophosphates metastable precipitates are performed under different synthesis conditions. The phase composition and degree of crystallinity are investigated by X‐ray powder diffraction analysis. In acidic solution, precipitates of CaHPO 4 . 2H 2 O (DCPD) and CaHPO 4 (DCPA) are formed at the early stage of precipitation, with the degree of crystallinity at the range of 17–35%. Specifically, DCPD precipitates at 30 °C and anhydrous DCPA at 50 °C. In alkaline solution (pH 8–10), only amorphous forms of calcium orthophosphate are precipitated, which is explained by the high degree of supersaturation. The effect of pH on the composition of precipitates is illustrated by the solubility isotherms of pure calcium orthophosphates. It is determined the activation energy of phase transformation of calcium orthophosphate from X‐ray powder diffraction patterns. Based on this relationship developed, the activation energies for the recrystallization DCPD and DCPA are 10.2 and 13.1 kJ mol−1, respectively, and for the phase transition of DCPD to DCPA – 36.7 kJ mol−1. Further recrystallization to most thermodynamically stable hydroxyapatite occurs at the activation energy of 5.2 kJ mol−1. These findings are critical to the phase transition and transformation of calcium phosphate minerals.

Acoustic monitoring of laser-induced phase transitions in minerals: implication for Mars exploration with SuperCam

The SuperCam instrument suite onboard the Mars 2020 Perseverance rover uses the laser-induced breakdown spectroscopy (LIBS) technique to determine the elemental composition of rocks and soils of the Mars surface. It is associated with a microphone to retrieve the physical properties of the ablated targets when listening to the laser-induced acoustic signal. In this study, we report the monitoring of laser-induced mineral phase transitions in acoustic data. Sound data recorded during the laser ablation of hematite, goethite and diamond showed a sharp increase of the acoustic signal amplitude over the first laser shots. Analyses of the laser-induced craters with Raman spectroscopy and scanning electron microscopy indicate that both hematite and goethite have been transformed into magnetite and that diamond has been transformed into amorphous-like carbon over the first laser shots. It is shown that these transitions are the root cause of the increase in acoustic signal, likely due to a change in target’s physical properties as the material is transformed. These results give insights into the influence of the target’s optical and thermal properties over the acoustic signal. But most importantly, in the context of the Mars surface exploration with SuperCam, as this behavior occurs only for specific phases, it demonstrates that the microphone data may help discriminating mineral phases whereas LIBS data only have limited capabilities.

In-situ measurement of mineral phase transition and kinetics in roasting process of Bayan Obo mixed rare earth concentrate by sodium carbonate

In this study, the Bayan Obo rare earth concentrates mixed with Na2CO3 were used for roasting research. The phase change process of each firing stage was analyzed. The kinetic mechanism model of the continuous heating process was calculated. This study aims to recover valuable elements and optimize the production process to provide a certain theoretical basis. Using X-ray diffraction (XRD), Fourier infrared spectroscopy, scanning electron microscopy with energy dispersive spectrometry, the reaction process and the existence of mineral phases were analyzed. The variable temperature XRD and thermogravimetric method were used to calculate the roasting kinetics. The phase transition results show that carbonate-like substances first decompose into fine mineral particles, and CaO, MgO, and SiO2 react to form silicates, causing hardening. Further, REPO4 and NaF can directly generate CeF3 and CeF4 at high temperatures, and a part of CeF4 and NaF forms a solid solution substance Na3CeF7. Rare earth oxides calcined at a high temperature of 750 °C were separated to produce Ce0.6Nd0.4O1.8, Ce4O7, and LaPrO3+x. Then, BaSO4, Na2CO3, and Fe2O3 react to form barium ferrite BaFe12O19; the kinetic calculation results show that during the continuous heating process, the apparent activation energy E reaches the minimum in the entire reaction stage in the temperature range of 440–524 °C, and the reaction order n reaches the maximum, which indicates that the decomposition product REFO significantly impacts the reaction system and reduces the activation energy. The mechanism function is F(α) = [−ln (1−α)]1/3. The reaction order n reaches the minimum in the temperature range of 680–757 °C, and the apparent activation energy E is large. The difficulty of the reaction increases during the final stage. The reaction mechanism function is F(α) = [1−(1−α)1/3]2. Observing the entire reaction stage, the step of controlling the reaction rate changes from random nucleation to three-dimensional diffusion (spherical symmetry).

Mineral Phase Transitions of Jarosite Substituted by Oxyanions during the Reductive Dissolution Using Oxalate Solution

Mineral Phase Transitions of Jarosite Substituted by Oxyanions during the Reductive Dissolution Using Oxalate Solution

Inconsistency of changes in uniaxial compressive strength and P-wave velocity of sandstone after temperature treatments

It is generally accepted that the uniaxial compressive strength (UCS) and P-wave velocity of rocks tend to decrease simultaneously with increasing temperature. However, based on a great number of statistical data and systematic analysis of the microstructure variation of rocks with temperature rising and corresponding propagation mechanism of elastic wave, the results show that (1) There are three different trends for the changes of UCS and P-wave velocity of sandstone when heated from room temperature (20 °C or 25 °C) to 800 °C: (i) Both the UCS and P-wave velocity decrease simultaneously; (ii) The UCS increases initially and then decreases, while the P-wave velocity decreases continuously; and (iii) The UCS increases initially and then fluctuates, while the P-wave velocity continuously decreases. (2) The UCS changes at room temperature–400 °C, 400 °C–600 °C, and 600 °C–800 °C are mainly attributed to the discrepancy of microstructure characteristics and quartz content, the transformation plasticity of clay minerals, and the balance between the thermal cementation and thermal damage, respectively. (3) The inconsistency in the trends of UCS and P-wave velocity changes is caused by the change of quartz content, phase transition of water and certain minerals.

Intriguing minerals: photoinduced solid-state transition of realgar to pararealgar—direct atomic scale observation and visualization

This lecture text is aimed at teaching some insight into phase transitions of minerals. It summarizes the results of a detailed study of the reaction mechanism of photoinduced solid-state transformation of the mineral realgar (α-As4S4) to its distinct polymorph pararealgar by a combination of in situ single-crystal X-ray photodiffraction, Fourier transform infrared spectroscopy, and micro-Raman spectroscopy. The process of transformation takes place in four steps. The initiating photoreaction step requires oxygen and thereby the intermediate uzonite (As4S5) and arsenolite (As2O3) are obtained (step 1). The process continues through a set of cyclic reactions in which the sulfur atom released by the decomposition of As4S5 (step 2) reacts with a molecule of realgar to produce a molecule of pararealgar (step 3), whereupon a sulfur atom is released which continues the process (step 4). The photodiffraction technique provides direct atomic resolution evidence of formation of intermediate As4S5 phase in which half of the realgar molecule retains its envelope-type conformation, while the geometry of the other half is transformed by effective switching of positions of one sulfur and one arsenic atom. The migration (hopping) of sulfur atoms between the molecules of the single crystal of realgar is observed and visualized.

Development of novel formulation for high performance fire retardant cementitious mortars

A study has been made to develop Novel high-performance cementitious fire-retardant mortars. In situ mineral phase transition at high temperatures ranging above 1000 °C has been considered. Inorganic minerals like-Talc, Kaolinite Clay, Silicate binder, White Portland Cement, Gypsum etc. are mixed in different proportions, to get different combinations of mortars and were subjected to testing at high temperatures of above 1000 °C. Test results showed resistance to heat and fire. Various analytical studies such as X-ray diffraction (XRD), Raman spectroscopy and Fourier Transform Infra-Red spectroscopy (FTIR), have been carried out to study the chemical changes in the internal structural characteristics. The results obtained are presented and discussed.

Using the high-temperature phase transition of iron sulfide minerals as an indicator of fault slip temperature

The transformation of pyrite into pyrrhotite above 500 °C was observed in the Chelungpu fault zone, which formed as a result of the 1999 Chi-Chi earthquake in Taiwan. Similarly, pyrite transformation to pyrrhotite at approximately 640 °C was observed during the Tohoku earthquake in Japan. In this study, we investigated the high-temperature phase-transition of iron sulfide minerals (greigite) under anaerobic conditions. We simulated mineral phase transformations during fault movement with the aim of determining the temperature of fault slip. The techniques used in this study included thermogravimetry and differential thermal analysis (TG/DTA) and in situ X-ray diffraction (XRD). We found diversification between 520 °C and 630 °C in the TG/DTA curves that signifies the transformation of pyrite into pyrrhotite. Furthermore, the in situ XRD results confirmed the sequence in which greigite underwent phase transitions to gradually transform into pyrite and pyrrhotite at approximately 320 °C. Greigite completely changed into pyrite and pyrrhotite at 450 °C. Finally, pyrite was completely transformed into pyrrhotite at 580 °C. Our results reveal the temperature and sequence in which the phase transitions of greigite occur, and indicate that this may be used to constrain the temperature of fault-slip. This conclusion is supported by field observations made following the Tohoku and Chi-Chi earthquakes.

Analysis of change in the decorative properties of granites under thermal exposure

We investigated samples extracted in Ukraine from nine granite deposits experimentally. We performed tests of granite samples at high temperatures of 200, 400, 600, and 900 °C. All presented granites showed a change in the color of surface at a temperature of 200 °C and higher. The behavior of granites under heating depends on their mineral composition, structure and texture. Surfaces of all samples became lighter and some granite samples lost saturation of their color. The largest increase in L component (image of stone samples brightens) of the CIELab color system occurred on Cardinal Gray and Carpazi granite samples of natural stone under heating to 900 °C. The increase made up 42 and 44 %, respectively. The smallest increase in L component was on Gray Ukraine and L granites under heating to 900 °C. It made up 4 and 8.5 %, respectively. The effect of temperature was less visible on red granite, since both fresh and heated samples had a similar red color. Flower of Ukraine granite samples acquired a uniform violet-pink color at the temperature of 900 °C due to the content of apatite and fluorite. Red spots appeared on gray granites under heating. The red spots located mainly around mica and other minerals, which were rich in Fe. Reddish-brown spots appeared at the temperature of 200 °C on green Verde Oliva granite. Red spots occupied 67 % of the sample area under heating to 900 °C. We observed the greatest color change on granites, where the phase transition of dark-colored minerals (biotite and pyroxene) into polymorphic minerals took place. This gave granite samples a light color, as the minerals changed color from black to gray or white. Quartz provided the shades of white. White microcracks appeared under heating of quartz. Noticeable aesthetic damage appears at temperatures from 200 to 400 °C at the surfaces of natural stone samples. Thus, one can consider a fire with temperatures lower than this threshold as a “safe” fire in terms of aesthetic damage, if we take into consideration the heating coefficient of fire only and exclude ash and gases.

Phase transition and thermoelastic behavior of barite-group minerals at high-pressure and high-temperature conditions

Experimental studies on the phase transition and thermoelastic behavior of barite-group minerals are crucial to understand the recycle of sulfur in Earth’s interior. Here, we present a high-pressure and high-temperature (high P–T) study on two barite-group minerals—barite (BaSO4) and celestite (SrSO4) up to ~ 59.5 GPa 700 K and ~ 22.2 GPa, 700 K, respectively, using in situ synchrotron-based X-ray diffraction (XRD) combined with diamond anvil cells (DACs). Our results show that BaSO4 undergoes a pressure-induced phase transition from Pbnm to P212121 at ~ 20.3 GPa, which is different from the previous results. Upon decompression, the high-pressure phase of BaSO4 transforms back into its initial structure, which indicates a reversible phase transition. However, no phase transitions have been detected in SrSO4 over the experimental P–T range. In addition, fitting a third-order Birch–Murnaghan equation of state to the pressure–volume data yields the bulk moduli and their pressure derivatives of BaSO4 and SrSO4. Simultaneously, the thermal expansion coefficients of BaSO4 and SrSO4 are also obtained, by fitting the temperature-volume data to the Fei-type thermal equation of state. Furthermore, the compositional effects on the phase transformation and thermoelastic behavior of barite-group minerals are also discussed, and the results suggest that the bond length of (M=Ba, Sr, Pb) is an important factor that causes the phase transition pressure of SrSO4 to be the largest, PbSO4 is the second, and BaSO4 is the lowest.

A Sequence of up to 11 Seismic Discontinuities Down to the Midmantle Beneath Southeast Asia

AbstractThe Earth's mantle exhibits a layered structure seismically characterized by sudden velocity changes or strong gradients. Several seismic boundaries have been identified in the mantle, and a large number of theoretical calculations and laboratory experiments have contributed to the debate on their origin. We analyze P‐to‐S converted phases generated at such interfaces to image the velocity structure within the sublithospheric mantle beneath Indonesia. Our study confirms the existence of various layer boundaries in the upper and lower mantle revealing up to 11 consecutive discontinuities down to ~1,700‐km depth. We detect Ps phases from the Lehmann and the X discontinuities originating at ~245 and ~294 km, respectively, followed by the top of a low‐velocity layer (LVL‐410) at ~368 km. The transition zone discontinuities are imaged at average depths of 408 and 665 km, respectively, which indicates the absence of significant temperature anomalies. In the midmantle we find vague indications for another interface at ~970‐km depth. At ~1,220 km a negative phase is observed followed by a sequence of converting structures of unknown origin at ~1,320, ~1,460, and ~1,500 km. We interpret these interfaces as compositional anomalies related to persisting fragments of subducted lithosphere. A further boundary is observed at ~1,700 km. Even though different causes exist to explain the observed seismic discontinuities including mineral phase transitions, partial melt, and chemical changes, most of them require additional mineral components. Thus, our findings provide clear evidence for significant compositional alteration of the mantle beneath Indonesia as a result of recurring subduction.