The incorporation of ferric iron in mantle silicates stabilizes different crystal structures and changes phase transition conditions, thus impacting seismic wave speeds and discontinuities. Recent experiments of MgSiO3-Fe2O3 mixtures indicate the coexistence of fully oxidized iron-rich (Mg0.5Fe0.53+)(Fe-0.53+Si-0.5)O-3 with Fe-poor silicate (wadsleyite or bridgmanite) and stishovite at 15 to 27 GPa and 1773 to 2000 K, conditions relevant to subducted lithosphere in the Earth's transition zone and uppermost lower mantle. X-ray diffraction (XRD) shows that (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O-3 recovered from these conditions adopts the R3c LiNbO3-type structure, which transforms to the bridgmanite structure again between 18.3 and 24.7 GPa at 300 K. XRD data are used to obtain the equation of state of the LiNbO3-type phase up to 18.3 GPa. Combined with multi-anvil experiments, these observations suggest that the stable phase of (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O-3 is bridgmanite at 15-27 GPa, which transforms on decompression to LiNbO3-type structure. Our calculation revealed that ordering of the ferric ion reduces the kinetic energy barrier of the transition between (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O-3 LiNbO3 structure and bridgmanite relative to the MgSiO3 akimotoite-bridgmanite system. A dense Fe3+-rich bridgmanite structure is thus stable at substantially shallower depths than MgSiO3 bridgmanite and would promote subduction.

Nitrogen has been proposed to be stored within planetary cores, but its effects on the structure and density of molten Fe-alloys have not been explored experimentally. Using energy-dispersive X-ray diffraction, we determined the structure of Fe-N(-C) liquids at core conditions (1-7 GPa and 1700-1900 degrees C) within a Paris-Edinburgh press. Variation of N up to 7 wt.% and C up to 1.5 wt.% results in near-linear changes in Fe-Fe atom distances and structure factor with increasing light element content. We did not observe a significant pressure-driven structural transition in Fe-N(-C) liquids. We model the expansion of the Fe-Fe bonds using a modified Birch-Murnaghan equation of state. With this model, we demonstrate that N or C contamination could lead to an overestimation of the Fe-Fe distances of pure Fe. We observe that the incorporation of 1 wt.% N or C into Fe results in a change in Fe-Fe distances that is twice as significant as the effect of 1 GPa. By approximating the change in volume, we infer that N and C incorporated in liquid iron could contribute to the density deficit observed in the cores of terrestrial bodies.

Ringwoodite is an important mineral in the mantle transition zone, and its cationic disorder can profoundly affect its physicochemical properties, but there is currently much controversy about this disorder. In this study, we investigate the cation disorder states of pure Mg2SiO4-ringwoodite and defective ringwoodite under mantle transition zone conditions through DFT calculations and thermodynamic models. Two stable endmembers are seen, one with normal ringwoodite structure and the other with inverted structure (its Si atoms and half of its Mg atoms have swapped sites). Our results indicate that pure ringwoodite does not invert (swap Mg and Si cations) under normal mantle temperatures but the introduction of a Si-excess, Mg-deficient defect induces a swap at normal mantle temperatures and this swap is likely induced by a wide range of defects including water. Thus, in the presence of such a defect or similar defects the olivine phase transition sequence may then go from olivine to wadsleyite to inverse ringwoodite, and then normal ringwoodite. We calculate the seismic properties of normal and inverse ringwoodite and find significantly slower wave speeds in inverted ringwoodite. Due to this difference the presence of inverse ringwoodite may provide a potential explanation for the discontinuous interface of seismic waves at the depth of ∼560 km.

The non-traditional Mg, Ca and Fe isotopes have proven to be useful tools for investigating the petrogenesis of the mantle- and crust-derived rocks. Individual non-traditional isotopes in continental basalts are well-studied, particularly in subduction settings. In contrast, there is a significant lack of comprehensive research on various non-traditional stable isotopes related to magmatic processes within continental collision zones, especially during post-collisional stage. To address this knowledge gap, this study presents stable Mg-Ca-Fe isotopic data for the Miocene post-collisional magmatic rocks including ultrapotassic, potassic, and adakitic rocks from the Lhasa Terrane in southern Tibet. The ultrapotassic rocks exhibit slightly lower delta 26Mg (-0.28 +/- 0.07%0, 2SD, n = 18) and significantly lower delta 44/40Ca (0.69 +/- 0.03%0, 2SD, n = 18) values than the depleted mantle. The high CaO/Al2O3 and CaO/TiO2 ratios of the ultrapotassic rocks are typical characteristics of carbonate metasomatism, suggesting that the light Mg and Ca isotopic composition may be attributed to the involvement of recycled Carich sedimentary carbonate in the mantle source. Additionally, these ultrapotassic rocks display delta 56Fe values (0.026-0.163%0, 2SD, n = 18) ranging from the normal peridotite mantle to much higher values, suggesting a likely origin from a pyroxenite-bearing lithospheric mantle source. These findings imply that subducted oceanic crust and overlying Ca-rich sediments plausibly account for the formation of a mantle source characterized by distinctive Mg-Ca-Fe isotope compositions. The adakitic rocks exhibit the highest delta 26Mg (-0.13 +/- 0.10%0, 2SD, n = 15) and delta 56Fe (up to 0.304%0) values, along with the lowest delta 44/40Ca values (0.60 +/- 0.07%0, 2SE, n = 15) among these post-collisional magmatic rocks. They also demonstrate strong correlations between delta 56Fe and (Gd/ Lu)N ratios, indicating a significant role of garnet in their source, consistent with their origin from the thickened lower crust associated with the collision between India and Asia continents. Given the consistent delta 56Fe values (0.102 +/- 0.067%0, 2SD, n = 5) observed in the adakitic rocks from the western Lhasa Terrane, we posit that it reflects the Fe isotopic composition of the thickened lower crust in southern Tibet. The potassic rocks display lighter Ca isotopes (delta 44/40Ca = 0.67 +/- 0.03%0, 2SE, n = 5) than the mantle, consistent with a carbonate metasomatized source. They exhibit varying delta 26Mg (-0.31 +/- 0.07%0 to -0.04 +/- 0.07%0, 2SD) and relatively uniform delta 56Fe (0.186 +/- 0.051%0, 2SE, n = 6) values, falling within the range of nearly contemporaneous ultrapotassic and adakitic rocks. This suggests that potassic rocks may be formed through the mixing of mantle materials resembling ultrapotassic rocks and thickened crustal components resembling adakitic rocks in the lower crust. Building upon these findings, we propose that the distinctive geochemical characteristics, as well as the spatial and temporal distribution of post-collisional magmatic rocks in southern Tibet, are the outcome of partial melting within a carbonated pyroxenite-bearing lithospheric mantle, coupled with the involvement of thickened crust and their subsequent interactions.

Elastic and plastic properties of Fe-light element alloys and compounds are needed to determine the compositions and dynamics of planetary cores. Elastic strength and plastic deformation mechanisms and their relationship to electronic properties of epsilon-Fe7N3 and gamma'-Fe4N mixture were investigated by x-ray diffraction and x-ray emission spectroscopy in the diamond anvil cell from 1 bar up to 60 GPa. X-ray diffraction shows that epsilon-Fe7N3 reaches a pressure of 15-20 GPa before undergoing bulk plasticity at a differential stress of 4.4-10.4 GPa. epsilon-Fe7N3 is stronger than gamma'-Fe4N and hcp-Fe which achieve a flow stress of 1.5-3.6 GPa at 10-15 GPa and 2-3 GPa at similar to 20 GPa, respectively. X-ray emission spectroscopy shows that a decrease in electronic spin moment begins before and completes after plastic flow onset for each nitride, suggesting that pressure-driven changes in electronic arrangement do not trigger a plastic response although they may modify the strength and plastic behavior of Fe-N compounds. Plastic deformation in epsilon-Fe7N3 and hcp-Fe results in a preferred orientation of (0001) normal to maximum compression, while gamma'-Fe4N develops a maximum in the (110). These observations may be combined with measurements of elasticity to model seismic properties of cores of small planetary bodies such as Mars, Mercury, and the Moon.

Plate tectonics plays a crucial role in the evolution of Earth's continents and oceans. However, the impact of oceanic subduction and continental collision on the continental lithospheric mantle within collisional orogenic belts is still a matter of debate. To address this issue, we performed a thorough analysis of mafic rocks in the Nixiong area within the central Lhasa Terrane, southern Tibet. The mafic rocks are composed of diabases and basalts formed during syn-collisional period (similar to 50 Ma). The diabases have low SiO2 and moderate MgO contents, and are enriched in large ion lithophile elements and depleted in high field strength elements. They show high Sr-87/Sr-86(i) (0.711451-0.712084) and low epsilon(Nd)(t) (-7.9 to -6.44), as well as enriched Hf-Pb isotope compositions (epsilon(Hf)(t) = -7.58 to -6.59; Pb-206/Pb-204(i) = 18.59-18.71; Pb-207/Pb-204(i) = 15.71-15.72; Pb-208/Pb-204(i) = 39.33-39.53). Geochemical modelling suggests that the diabases were formed through the partial melting of peridotite at high temperatures and shallow depths, with significant addition of subduction-related components to the primary melts. The diabases probably originated from a metasomatized lithospheric mantle due to subduction of Neo-Tethyan slab. The basalts display similar SiO2 but lower MgO contents compare with the diabases. They are also characterized by extremely high Sr-87/Sr-86(i) (0.713332-0.713448), low epsilon(Nd)(t) (-10.32 to -9.96), and significantly enriched Hf-Pb isotope compositions (epsilon(Hf)(t) = -12.30 to -12.05; Pb-206/Pb-204(i) = 18.81-18.85; Pb-207/Pb-204(i) = 15.74-15.75; Pb-208/Pb-204(i) = 39.81-39.88), corresponding to an extremely enriched lithospheric mantle. Integrated with the previous studies, the Nixiong mafic rocks exhibit the most enriched isotopic compositions among the arc- or syn-collision-related mafic rocks in southern Tibet. These extremely isotopically enriched components were most likely derived from the ancient continental lithospheric mantle beneath the central Lhasa Terrane. Our results provided unambiguous evidence that the pre-existing ancient continental lithospheric mantle could persist locally during the prolonged oceanic subduction and subsequent continental collision. [GRAPHICS] .

Extensively developed Early Cretaceous intermediate-felsic rocks in the northern North China Craton (NCC), offer an opportunity to unravel the nature of Paleo-Pacific subduction and associated geodynamic processes by investigating their spatio-temporal characteristics and petrogenesis. Here we present geochemical and geochronological results of Early Cretaceous (118-115 Ma) trachyandesites and rhyolites from the eastern Hebei, northern NCC. The trachyandesites show low MgO (0.74-2.65 wt%) and Mg# (17-38), as well as enriched whole-rock Nd-Hf isotopic compositions (epsilon(Nd)(t) =-17.1 to-13.3, epsilon(Hf)(t) =-12.9 to-10.8), suggesting a probable origin of the enriched mafic lower crust. They also exhibit relative enrichment in LREE and flat in HREE. Trace element geochemical modeling results indicate that the trachyandesites likely formed through partial melting of the mafic lower crust at 790-820 degrees C and ca. 10 kbar (30-33 km). The rhyolites, characterized by higher SiO2 contents and similar epsilon(Nd)(t) (-17.4 to-15.5) and zircon epsilon(Hf)(t) values (-14.4 to-7.6) compared to the trachyan-desites, probably represent the magmatic derivates of the trachyandesites. MELTS modeling results suggest that the rhyolites likely formed through fractional crystallization of the trachyandesitic magmas at similar to 775 degrees C and 1-3 kbar. Integrating with previous studies, our study confirms that the Early Cretaceous intermediate-felsic magmatism across the northern NCC became younger from northwest to southeast. We propose that the change of the migration direction of these Early Cretaceous intermediate-felsic rocks is primarily attributed to slab rollback of the subducting Paleo-Pacific Plate.

Collisional orogenic belts serve as excellent places for investigating geodynamic process and growth mechanism of the continental crust. As one of the largest collisional orogenic belts worldwide, the Tibetan Plateau is formed by the amalgamation of multiple tectonic terranes. However, the nature and composition of the crustal basement beneath the northern Lhasa Terrane, located in central Tibetan Plateau, is still a matter of debate. Here, we investigated a series of Early Cretaceous granites in the Beila and Dongga areas, northern Lhasa Terrane. Zircon U-Pb dating reveals that the Beila and Dongga granites were emplaced at ca. 118 Ma and 110 Ma, respectively. These granites are characterized by high SiO2 and K2O, low TiO2 and P2O5 contents and low Mg#. They exhibit enrichment in light rare earth elements, significant negative Eu anomalies, and depletion in Ba and Sr. These geochemical features suggest that they belong to highly fractionated I-type granites. Both the Beila and Dongga granites display similar and enriched whole-rock Sr-Nd and zircon Hf isotope compositions, implying a likely common ancient crustal origin. Given the temporal sequences and redox conditions during their formation, it is highly probable that the Beila and Dongga granites formed during the syn-collision stage between the Lhasa and Qiangtang Terranes and the breakoff of the Bangong-Nujiang oceanic slab, respectively. Integrated with coeval magmatism in the central Tibetan Plateau, our new data reveal the presence of a regionally preserved ancient basement beneath the northern Lhasa Terrane, analogous to the equivalent basement beneath the central Lhasa Terrane.

To constrain the water solubility of coesite (Coe) at typical temperatures of subduction zones, a series of Coe coexisting with an aqueous fluid was synthesized at 3–6 GPa and at 600–800 °C. Most experiments were performed in the system SiO2-H2O, and some were performed with small amounts of boron addition. With very long heating durations (120–336 h), all these experiments successfully produced Coe crystals of large grain size, ranging from ∼100 to 1300 μm. For every experimental product, multiple unpolarized FTIR spectra were collected on randomly-selected Coe crystals. We have found that type-I hydrogarnet substitution (Si4+(Si2) + 4O2− = [4]□(Si2) + 4OH−) is the major water-incorporation mechanism whereas B-related defect (H+ + B3+ ↔ Si4+) makes no much contribution. The water solubility of Coe, ranging from 3(1) to 47(12) wt ppm, positively correlates with both P and T. It can be well described by the empirical equation cH2O = −49(17) + 6.0(21) × P + 0.06(2) × T (cH2O representing water content in wt ppm, P pressure in GPa and T temperature in °C) and by the thermodynamic expression cOH=exp∆S1barRfH2O2exp−∆H1bar−∆VsolidPRT (cOH representing water content in H/106 Si, fH2O water fugacity in GPa, R the gas constant, P pressure in GPa, T temperature in K, ∆S1 bar reaction entropy as 18.5(509) J/mol/K, ∆H1 bar reaction enthalpy as −10.7(516) kJ/mol, and ∆Vsolid the volume change of Coe during hydroxylation as 23.6(42) cm3/mol). Consequently, the water solubility of Coe in subduction zones should be ∼0–123 wt ppm. When Coe with deep origin becomes metastable (i.e., approaching the P-T locus of the Coe-quartz (Qz) reaction), its water content is likely less than ∼10 wt ppm. This trace water may be quickly lost along with further exhumation process, and metastable Coe becomes completely dry, as observed in the Coe discovered in all exhumed ultrahigh-pressure (UHP) metamorphic rocks. Since structurally-bonded water substantially speeds up the Coe-to-Qz phase transition, zero water in metastable Coe may be the key to the preservation of Coe in the UHP metamorphic rocks.

Subsurface imaging is key to understanding the origin of intraplate volcanoes. The Changbaishan volcano, located about 2,000 km away from the western Pacific subduction zone, has several debated origins. To investigate this, we compared regional seismic tomography with the electrical resistivity results and obtained high‐resolution 1D and quasi‐2D velocity‐depth profiles. We show that the upper mantle is characterized by two anomalies exhibiting distinct features which cannot be explained by the same mechanism. We document a localized low‐velocity anomaly atop the 410‐km discontinuity, where the P‐wave velocity is reduced more than that of the S‐wave (i.e., lower Vp/Vs). We propose that this anomaly is caused by the reduction of the effective moduli during the phase transformation of olivine. The other anomaly, located between 300 and 370 km depth, reveals a significant reduction of the S‐wave velocity (i.e., higher Vp/Vs), associated with a reduction of the electrical resistivity, altogether consistent with partial melting.

High-Pressure Collaborative Access Team (HPCAT) is a synchrotron-based facility located at the Advanced Photon Source (APS). With four online experimental stations and various offline capabilities, HPCAT is focused on providing synchrotron x-ray capabilities for high pressure and temperature research and supporting a broad user community. Overall, the array of online/offline capabilities is described, including some of the recent developments for remote user support and the concomitant impact of the current pandemic. General overview of work done at HPCAT and with a focus on some of the minerals relevant work and supporting capabilities is also discussed. With the impending APS-Upgrade (APS-U), there is a considerable effort within HPCAT to improve and add capabilities. These are summarized briefly for each of the end-stations.

Pure sediment-derived granite, a recently recognized crustal geochemical end-member, is relatively rare but provide primary access to probe the process of continental crustal reworking. This paper presents geochemical and geochronological data for granites emplaced in the Asuo area within the central Lhasa Terrane of the Tibetan Plateau at ca. 158 Ma (zircon U - Pb isotopic dating). These granites are two mica-bearing and contain high SiO2 (75.40-79.11 wt%), high A/CNK (1.06-1.33), heterogeneous (Sr-87/Sr-86); (0.705519-0.710891), and relatively uniform and low epsilon(Nd)(t) (-13.18 to -14.81) values, which are analogous to typical S-type granites. The low epsilon(Hf)(t) (-13.98 to -12.08) and high delta O-18 (8.01 parts per thousand-8.85 parts per thousand) values of magmatic zircons, the similar of epsilon(Nd)(t) values between the Asuo granites and regional Jurassic slates, and the absence of mafic enclaves, verify that these granites were derived from a pure sedimentary source. The trondhjemitic features, low CaO/Na2O (<0.1) and molar CaO/(MgO + FeOT) (0.06-0.58), relatively constant and low Rb/Sr (0.04-0.32), as well as the heterogeneous (Sr-87/Sr-86); ratios, indicate that the Asuo granites formed via fluid-present muscovite-dehydration melting of metapelitic protoliths, with mica as the main residue. Combined with a synthesis of existing tectonomagmatic data on the Lhasa Terrane, we propose that the Jurassic Asuo granites were formed in a continental extensional setting associated with the northward subduction of the Neo-Tethyan lithosphere. The present study highlights that the inboard region of an active subduction zone is probably an important site of crustal reworking, and provides a geochemical end-member of pure sediment-derived melts for the central Lhasa Terrane.

Elastic and plastic properties of materials and phase transitions at extreme conditions vary with both hydrostatic pressure and deviatoric stress. To generate and measure controlled deviatoric stress at pressures beyond those accessible with large volume differential and rotational presses and optical access for spectroscopy, experiments tested the combination of diamond anvil cell and thin film technology. Thin films of polycrystalline Cr-doped Al2O3 ruby were prepared using pulsed laser deposition on single-crystal substrates of either Al2O3 sapphire or yttria-stabilized cubic zirconia for contrasting initial film stress, and loaded in diamond anvil cells for confining stress. The piezospectroscopic response of the ruby films demonstrates consistently higher deviatoric stress in the film on zirconia relative to the film on the control sapphire, and an increase in deviatoric stress with applied load. Complementary synchrotron X-ray diffraction of the zirconia substrate confirmed that no pressure-induced phase transitions impacted the stress state of the ruby film, but differences in compressibility of film and substrate result in changes in film stress analogous to thermal expansion mismatch. This technique may be applied to evaluate elastic and plastic response of thin films of a variety of materials under extreme stress.

The oceanic lithosphere subduction has been attested to exert the first-order control on the formation of the continental crust, but the role of oceanic plateaus on the crustal growth is elusive. It is difficult to identify the component of the oceanic plateaus in the continental crust because of their disappearance after subduction into deep mantle or obduction to be a part of the ophiolite me acute accent langes in the ancient orogenic belts. Amdo micro -continent in the central Tibetan Plateau is an ideal place to assess the role of oceanic plateaus on crustal growth as it preserves the well-exposed arc magmatic rocks and fragments of the Meso-Tethyan oceanic plateaus to its south. Here, we present an integrated investigation of zircon U-Pb ages and Hf isotope, as well as whole-rock elements and Sr-Nd-Hf isotopes of a suit of dacites in the Amdo microcontinent. Zircon U-Pb dating indicates that the Amdo dacites formed at ca. 117 Ma, postdated the major formation period of the Meso-Tethyan oceanic plateaus. The Amdo dacites have relatively high SiO2 (63.45-66.82 wt%) contents and low Mg# values (20.0-36.1), belonging to calc-alkaline series. The zircon thermometer and oxybarometer give estimated tem-perature and oxygen fugacity of crystallization as ca. 700-720 degrees C and Delta FMQ = + 0.98, respectively. The Amdo dacites have low Y (5.51-6.61 ppm) and Yb (0.44-0.53 ppm) contents and high La/Yb ratios (28.4-30.3). Despite their low Sr/Y ratios (6.86-13.5) caused by a small degree of fractionation of plagioclase, the Amdo dacites have an adakitic geochemical affinity, which was most likely derived from the thickened continental lower crust. These dacites have more depleted Sr-Nd-Hf isotope compositions (whole-rock 87Sr/86Sri = 0.709796-0.710563, eNd(t) =-3.27 to-2.82, eHf(t) = 4.20-4.83 and zircon eHf(t) = 4.2-8.8) than the Neoproterozoic and Cambrian orthogneisses, as well as the Jurassic magmatic rocks. Simple binary mixing modeling reveals that the Amdo dacites could represent the hybrid melts of 50-60% of the subducted Meso-Tethyan oceanic plateau materials with 50-40% of the Amdo ancient continental crustal materials. The decoupled whole-rock Nd-Hf isotopes (Delta eHf(t) = 7.55-8.43) of the Amdo dacites could be explained by the addition of zircon-free sediments overlie on the oceanic plateaus. These results indicate that subduction of the Meso-Tethyan oceanic plateaus with/without the overlying sediments have made a substantial contribution to continental crustal growth. In conjunction with previous studies, we conclude that the subduction of the Meso-Tethyan oceanic plateaus may have intermittently occurred for around 37 million years, thus highlighting the significance of subduction of the Meso-Tethyan oceanic plateaus during the secular evolution of the central Tibetan Plateau.

Abstract Heat flux from the core to the mantle provides driving energy for mantle convection thus powering plate tectonics, and contributes a significant fraction of the geothermal heat budget. Indirect estimates of core‐mantle boundary heat flow are typically based on petrological evidence of mantle temperature, interpretations of temperatures indicated by seismic travel times, experimental measurements of mineral melting points, physical mantle convection models, or physical core convection models. However, previous estimates have not consistently integrated these lines of evidence. In this work, an interdisciplinary analysis is applied to co‐constrain core‐mantle boundary heat flow and test the thermal boundary layer (TBL) theory. The concurrence of TBL models, energy balance to support geomagnetism, seismology, and review of petrologic evidence for historic mantle temperatures supports QCMB ∼15 TW, with all except geomagnetism supporting as high as ∼20 TW. These values provide a tighter constraint on core heat flux relative to previous work. Our work describes the seismic properties consistent with a TBL, and supports a long‐lived basal mantle molten layer through much of Earth's history.

The origin and formation of the continental collision-related magmas remain elusive. The volcanic rocks erupted during India-Asia continental collision offer an ideal opportunity to explore their genesis and geodynamic process. Here, we report new zircon U-Pb dating results and Hf isotope, whole-rock element and Sr-Nd isotope data of volcanic rocks in Linzhou Basin, central Lhasa Terrane, southern Tibet. These volcanic rocks are mainly comprised of andesites and belong to Dianzhong Formation. The timing of their formation is ca. 63–66 Ma, coeval with the India-Asia continental initial collision in the central part of southern Tibet. All these rocks show an arc-like geochemical affinity and they have more depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sri = 0.705006–0.705963, εNd(t) = −1.78 to 3.52, zircon εHf(t) = 1.2–7.0) than the Cretaceous pre-collisional andesites and Eocene ancient lithospheric mantle-derived melts beneath the central Lhasa Terrane. The correlation between Mg# and CaO, TiO2, Al2O3, Sr/Y, 87Sr/86Sr, εNd(t) suggests that they were likely hybrid production between an isotopically depleted end-member and the enriched lithospheric mantle (ELM). The oceanic crust-derived melts would be the best candidate for the former end-member according to their depleted Sr-Nd isotopic compositions. The andesitic rocks of Dianzhong Formation in the central Lhasa Terrane were most likely stemmed from partial melts of altered Neo-Tethyan crust and then mixed with the ELM-derived melts. Given the Cretaceous-Tertiary upper crustal shortening, back-arc extension, and voluminous volcanism in the Lhasa Terrane, along with abrupt increasing in the magma temperature and the convergence rate between India and Asia during Palaeocene, the occurrence of the Dianzhong Formation volcanic rocks can be well explained by the Neo-Tethyan slab roll-back. The mixing between the oceanic crust- and the continental lithospheric mantle-derived melts induced by the oceanic slab roll-back would be a complimentary scenario for the formation of the syn-collisional magmatism in collisional orogeny belts.

The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (β-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its β-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller β-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.

Strength, texture, and equation of state of hexagonal tungsten monocarbide (WC) have been determined under quasi-hydrostatic and non-hydrostatic compression to 66 GPa using angle-dispersive X-ray diffraction in the diamond anvil cell. Quasi-hydrostatic compression in a Ne pressure medium demonstrates that nanocrystalline WC is slightly less incompressible than bulk-scale WC, with respective bulk moduli of K 0 = 377 ± 7 and 397 ± 7 GPa and pressure derivatives K 0 ’ = 3.8 ± 0.3 and 3.7 ± 0.3. This decrease in incompressibility with grain size is similar to behavior observed in other ceramics. Under nonhydrostatic compression, WC supports a mean differential stress of ∼12-15 GPa at plastic yielding, which occurs at ∼30 GPa. Strength in WC is anisotropic, with the (001) plane supporting 29-42% higher stress than stresses calculated from mean strain. Simulations using an Elasto-ViscoPlastic Self-Consistent model indicate that strength inferred from lattice strain theory may be overestimated due to effects of plastic deformation. Plastic deformation generates a texture maximum near 〈 2 ¯ 110 〉 in the compression orientation, initially through prismatic slip on the { 10 1 ¯ 0 } 〈 1 ¯ 2 1 ¯ 0 〉 and { 10 1 ¯ 0 } 〈 0001 〉 slip systems, followed by activation of pyramidal slip on { 10 1 ¯ 1 } 〈 2 ¯ 113 〉 at ∼40-50 GPa.

The stable forms of carbon in Earth's deep interior control storage and fluxes of carbon through the planet over geologic time, impacting the surface climate as well as carrying records of geologic processes in the form of diamond inclusions. However, current estimates of the distribution of carbon in Earth's mantle are uncertain, due in part to limited understanding of the fate of carbonates through subduction, the main mechanism that transports carbon from Earth's surface to its interior. Oxidized carbon carried by subduction has been found to reside in MgCO3 throughout much of the mantle. Experiments in this study demonstrate that at deep mantle conditions MgCO3 reacts with silicates to form CaCO3. In combination with previous work indicating that CaCO3 is more stable than MgCO3 under reducing conditions of Earth's lowermost mantle, these observations allow us to predict that the signature of surface carbon reaching Earth's lowermost mantle may include CaCO3.

Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O-3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mossbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mossbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10-50% Fe/total cations, 0-25% Al/total cations, 12-100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mossbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at similar to 70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mossbauer sites with center shift similar to 1 mm/s and quadrupole splitting 2.4-3.1 and 3.9 mm/s at similar to 70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to similar to 20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.