Infrared spectroscopic investigation of hydroxyl in β-(Mg,Fe)2SiO4 and coexisting olivine: Implications for mantle evolution and dynamics

Wadsleyite (β-(Mg,Fe)₂SiO₄) is a major constituent of the Earth's transition zone and is known to accommodate OH. The portion of the transition zone between 400–550 km could be an important source or sink for hydroxyl in plumes and slabs intersecting this region. Micro-infrared spectroscopy has been carried out on the β-phase and coexisting metastable olivine synthesized in a multianvil apparatus at 14 GPa and 1550–1650 K under hydrous conditions. Single-crystal and polycrystal specimens of both phases were analyzed in the 1800–8500 cm^?1 frequency region to determine the speciation, abundances, and partitioning behavior of the hydrous components in coexisting β-phase and olivine. β-phase spectra consistently show three distinct OH bands at 3329, 3580, and 3615 cm^?1. OH concentrations range from 10000–65000 H/10⁶ Si. A strong positive correlation of grain size and extent of transformation with OH concentration in the β-phase indicates that grain-growth and transformation rates are enhanced in a hydrous environment. Olivine spectra are variable, but consistently show a prominent broad-band absorbance representing molecular H₂O, consistent with the infrared signature of the starting material. OH concentrations in olivine range from <300–1400 H/10⁶ Si. The highest OH concentrations measured for olivine and the β-phase may represent solubility limits, in which case the OH solubility ratio between these two phases is approximately 1∶40. Where both phases coexist and are undersaturated with OH, the partitioning ratio of OH between them is about 1∶100. The large solubility contrast between olivine and the β-phase suggests a mechanism for hydrating the transition zone via olivine carried down in subducting slabs. Plumes impinging on an OH-rich upper transition region could cause H₂ or H₂O to be released upon transformation of the β-phase to olivine, resulting in initiation of secondary upwellings. If dissolution of OH weakens the β-phase, and if OH is present in the mantle, the region between 400–550 km could be a zone of low viscosity.     

Heterogeneous distribution of phosphorus in olivine from otherwise well-equilibrated spinel peridotite xenoliths and its implications for the mantle geochemistry of lithium

The major- and trace-element abundances of the coexisting phases of four metasomatized spinel peridotite xenoliths from the Anakies locality (SE Australia) were determined by electron microprobe and laser-ablation ICP-MS. The compositions of all phases are remarkably homogeneous, with the exception of phosphorus (P), lithium (Li) and sodium (Na) in olivine. These three elements are enriched in large parts of most olivine crystals due to a second metasomatic episode. Apart from these elements, all phases are in mutual equilibrium with respect to both their major- and trace-element compositions. Li and Na show a strong correlation with P in olivine, although molar Li + Na are an order of magnitude less than molar P, indicating that the substitution mechanism of these elements is more complex than the simple charge-balanced coupled exchange ^IVSi⁴⁺ + ^VI(FeMg)²⁺ = ^IVP⁵⁺ + ^VI(LiNa)⁺. We suggest that Li and Na are decorating octahedral-site cation vacancies formed by the original incorporation of P. Elemental maps revealed that the P zoning patterns are concentric in a few large olivine porphyroblasts, but form irregular patches in most crystals. This distribution of P is proposed to be the result of a two-stage process, whereby the initial concentric zoning, caused by its exceptionally sluggish diffusion after metasomatic influx, is broken up by extensive sub-solidus deformation and recrystallization, attesting to large grain-scale strains even within the lithosphere. Such strains must be an efficient means of ensuring trace-element equilibrium during partial melting. The association of Li with P in olivine may help to explain the variability of Li abundances in mantle minerals and to interpret Li diffusion experiments and Li isotopic fractionation.     

Effects of non-stoichiometry on the spinel structure at high pressure: Implications for Earth’s mantle mineralogy

A non-stoichiometric sample of spinel with composition ^T(Mg_0.4Al_0.6)^M(Al_1.8□_0.2)O₄ was investigated by single-crystal X-ray diffraction in situ up to about 8.7 GPa using a diamond anvil cell. The P(V) data were fitted using a third-order Birch-Murnaghan equation of state and the unit-cell volume V₀, the bulk modulus K_T0 and its first pressure derivative K′ were refined simultaneously providing the following coefficients: V₀ = 510.34(6) Å³, K_T0 = 171(2) GPa, K′ = 7.3(6). This K_T0 value represents the lowest ever found for spinel crystal structures. Comparing our data with a stoichiometric and natural MgAl₂O₄ (pure composition) we observe a decrease in K_T0 by about 11.5% and a strong increase in K′ by about 33%. These results demonstrate how an excess of Al accompanied by the formation of significant cation vacancies at octahedral site strongly affects the thermodynamic properties of spinel structure. If we consider that the estimated mantle composition is characterized by 3-5% of Al₂O₃ this could imply an Mg/Al substitution with possible formation of cation vacancies. The results of our study indicate that geodynamic models should take into account the potential effect of Mg/Al substitution on the incompressibility of the main mantle-forming minerals (olivine, wadsleyite, ringwoodite, Mg-perovskite).     

Melt migration and melt-rock reaction in the Alpine-Apennine peridotites:Insights on mantle dynamics in extending lithosphere

The compositional variability of the lithospheric mantle at extensional settings is largely caused by the reactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments.The Alpine-Apennine(A-A)ophiolites are predominantly constituted by mantle peridotites and are widely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition and slow-to ultraslow-spreading settings.Structural and geochemical studies on the A-A mantle peridotites have revealed that they preserve significant compositional and isotopic heterogeneity at variable scale,reflecting a long-lived multi-stage melt migration,intrusion and melt-rock interaction history,occurred at different lithospheric depths during progressive uplift.The A-A mantle peridotites thus constitute a unique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreading environments.In this work,we review field,microstructural and chemical-isotopic evidence on the major stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discuss their consequences in creating chemical-isotopic heterogeneities at variable scales and enhancing weakening and deformation of the extending mantle.Focus will be on three most important stages:(i)old(pre-Jurassic)pyroxenite emplacement,and the significant isotopic modification induced in the host mantle by pyroxenite-derived melts,(ii)melt-peridotite interactions during Jurassic mantle exhumation,i.e.the open-system reactive porous flow at spinel facies depths causing bulk depletion(origin of reactive harzburgites and dunites),and the shallower melt impregnation which originated plagioclase-rich peridotites and an overall mantle refertilization.We infer that migrating melts largely originated as shallow,variably depleted,melt fractions,and acquired Si-rich composition by reactive dissolution of mantle pyroxenes during upward migration.Such melt-rock reaction processes share significant similarities with those documented in modern oceanic peridotites from slow-to ultraslow-spreading environments and track the progressive exhumation of large mantle sectors at shallow depths in oceanic settings where a thicker thermal boundary layer exists,as a consequence of slow-spreading rate.     

Possible mantle phase transitions by the formation of SiO<Subscript>2</Subscript> peroxides: Implications for mantle convection

On the basis of quantum–chemical calculations of the linear to isomeric bent transition of the SiO₂ molecule, it is suggested that the bent to linear transition of SiO₂ forms can occur in melted mantle minerals of the lower mantle. This may be important for the formation of the peculiarities of mantle convection and origination of plumes.     

辽东中—新生代玄武岩的橄榄石斑晶和捕虏晶氧同位素组成及其岩石圈地幔演化启示

华北克拉通东部岩石圈地幔性质在中—新生代时期发生了重大转变,但细节还不清楚。本文对辽东半岛早白垩世(克拉通破坏峰期)小岭组玄武岩和第四纪(克拉通破坏后)宽甸玄武岩中橄榄石斑晶/捕虏晶进行了主量元素和氧同位素组成研究。早白垩世小岭组玄武岩斑晶橄榄石Fo为79～88,CaO>0.1%,具有高Ni/Mg(0.4～1.2)、低Mn/Fe(1.3～1.6)和低Ca/Fe比值(0.2～1.5)的特征,指示岩浆源区是辉石岩和橄榄岩混合的岩石圈地幔;高于正常地幔橄榄石的δ¹⁸O(4.77‰～5.96‰,平均值5.4‰)显示低温热液蚀变洋壳熔体/流体组分对地幔源区的影响。第四纪宽甸玄武岩捕虏晶橄榄石Fo值为88～92(平均值90),具有高NiO(0.3%～0.4%)和低CaO(<0.1%)、MnO(0.1%～0.2%)含量,显示主体饱满、与少量过渡型和难熔型并存的地幔组成特征;其δ¹⁸O(4.58‰～5.38‰,平均值5.3‰)与正常地幔值接近。结合华北其他地区地幔橄榄石氧同位素数据,发现早白垩世破坏峰期有较多俯冲洋壳来源的熔/流体交代岩石圈地幔,...     

The latest geodynamics in Asia: Synthesis of data on volcanic evolution,lithosphere motion,and mantle velocities in the Baikal-Mongolian region

From a synthesis of data on volcanic evolution,movement of the lithosphere,and mantle velocities in the Baikal-Mongolian region,we propose a comprehensive model for deep dynamics of Asia that assumes an important role of the Gobi,Baikal,and North Transbaikal transition-layer melting anomalies.This layer was distorted by lower-mantle fluxes at the beginning of the latest geodynamic stage(i.e.in the early late Cretaceous) due to avalanches of slab material that were stagnated beneath the closed fragments of the Solonker,Ural-Mongolian paleoceans and Mongol-Okhotsk Gulf of Paleo-Pacific.At the latest geodynamic stage,Asia was involved in east-southeast movement,and the Pacific plate moved in the opposite direction with subduction under Asia.The weakened upper mantle region of the Gobi melting anomaly provided a counterflow connected with rollback in the Japan Sea area.These dynamics resulted in the formation of the Honshu-Korea flexure of the Pacific slab.A similar weakened upper mantle region of the North Transbaikal melting anomaly was associated with the formation of the Hokkaido-Amur flexure of the Pacific slab,formed due to progressive pull-down of the slab material into the transition layer in the direction of the Pacific plate and Asia convergence.The early—middle Miocene structural reorganization of the mantle processes in Asia resulted in the development of upper mantle low-velocity domains associated with the development of rifts and orogens.We propose that extension at the Baikal Rift was caused by deviator flowing mantle material,initiated under the moving lithosphere in the Baikal melting anomaly.Contraction at the Hangay orogen was created by facilitation of the tectonic stress transfer from the Indo-Asian interaction zone due to the low-viscosity mantle in the Gobi melting anomaly.     

H<Subscript>2</Subscript>O storage capacity of olivine and low-Ca pyroxene from 10 to 13 GPa: consequences for dehydration melting above the transition zone

The onset of hydrous partial melting in the mantle above the transition zone is dictated by the H₂O storage capacity of peridotite, which is defined as the maximum concentration that the solid assemblage can store at P and T without stabilizing a hydrous fluid or melt. H₂O storage capacities of minerals in simple systems do not adequately constrain the peridotite water storage capacity because simpler systems do not account for enhanced hydrous melt stability and reduced H₂O activity facilitated by the additional components of multiply saturated peridotite. In this study, we determine peridotite-saturated olivine and pyroxene water storage capacities at 10–13 GPa and 1,350–1,450°C by employing layered experiments, in which the bottom ~2/3 of the capsule consists of hydrated KLB-1 oxide analog peridotite and the top ~1/3 of the capsule is a nearly monomineralic layer of hydrated Mg# 89.6 olivine. This method facilitates the growth of ~200-μm olivine crystals, as well as accessory low-Ca pyroxenes up to ~50 μm in diameter. The presence of small amounts of hydrous melt ensures that crystalline phases have maximal H₂O contents possible, while in equilibrium with the full peridotite assemblage (melt + ol + pyx + gt). At 12 GPa, olivine and pyroxene water storage capacities decrease from ~1,000 to 650 ppm, and ~1,400 to 1,100 ppm, respectively, as temperature increases from 1,350 to 1,450°C. Combining our results with those from a companion study at 5–8 GPa (Ardia et al., in prep.) at 1,450°C, the olivine water storage capacity increases linearly with increasing pressure and is defined by the relation C_{\textH2 \textO}^\textolivine ( \textppm ) = 57.6( ±16 ) ×P( \textGPa ) - 169( ±18 ). C_{{{\text{H}}_{2} {\text{O}}}}^{\text{olivine}} \left( {\text{ppm}} \right) = 57.6\left( { \pm 16} \right) \times P\left( {\text{GPa}} \right) - 169\left( { \pm 18} \right). Adjustment of this trend for small increases in temperature along the mantle geotherm, combined with experimental determinations of D_{\textH2 \textO}^{\textpyx/olivine} D_{{{\text{H}}_{2} {\text{O}}}}^{\text{pyx/olivine}} from this study and estimates of D_{\textH2 \textO}^{\textgt/\textolivine} D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{gt}}/{\text{olivine}}}} , allows for estimation of peridotite H₂O storage capacity, which is 440 ± 200 ppm at 400 km. This suggests that MORB source upper mantle, which contains 50–200 ppm bulk H₂O, is not wet enough to incite a global melt layer above the 410-km discontinuity. However, OIB source mantle and residues of subducted slabs, which contain 300–1,000 ppm bulk H₂O, can exceed the peridotite H₂O storage capacity and incite localized hydrous partial melting in the deep upper mantle. Experimentally determined values of D_{\textH2 \textO}^{\textpyx/\textolivine} D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{pyx}}/{\text{olivine}}}} at 10–13 GPa have a narrow range of 1.35 ± 0.13, meaning that olivine is probably the most important host of H₂O in the deep upper mantle. The increase in hydration of olivine with depth in the upper mantle may have significant influence on viscosity and other transport properties.     

Composition and processes of the mantle lithosphere in northeastern Brazil and Fernando de Noronha: evidence from mantle xenoliths

Spinel–peridotite facies mantle xenoliths in Cenozoic alkali basalts of the Pico Cabuji volcano (Rio Grande do Norte State, Northeast Brazil) and the adjacent South Atlantic oceanic island of Fernando de Noronha are studied for: (1) the information they provide on the composition of the lithospheric component in the erupted basalt geochemistry, and (2) to check the effects of the Fernando de Noronha plume track on the mantle lithosphere. Xenoliths from Pico Cabuji are protogranular lherzolites and porphyroclastic harzburgites recording average equilibrium temperatures of 825 ± 116 and 1248 ± 19 °C, respectively. Pressure in the porphyroclastic xenoliths ranges from 1.9 to 2.7 GPa (Ca-in-olivine geobarometer). Both groups show major element chemical variation trends in whole-rock and Ti and HREE (Er, Yb) variations in clinopyroxene consistent with fractional melting and basalt extraction. REE (rare earth element) profiles of clinopyroxenes vary from LREE (La, Ce) enriched (spoon shaped) to LREE depleted in the protogranular group, whereas they are slightly convex upward in most porphyroclastic clinopyroxenes. HFSE (Ti and Zr) negative anomalies are in general modest in the clinopyroxenes of both groups. Xenoliths from Fernando de Noronha have textural variations similar to those of Pico Cabuji. Protogranular and porphyroclastic samples have similar temperature (1035 ± 80 °C) and the pressure is 1–1.9 and 2.3 GPa, respectively. Whole-rock chemical variation trends overlap and extend further than those of Pico Cabuji. The trace element profiles of the clinopyroxenes of the porphyroclastic xenoliths are enriched in La up to 30 × PM and are smoothly fractionated from LREE to HREE, with deep, negative, Zr and Ti anomalies. The geochemical heterogeneities of the xenoliths from both localities are interpreted in terms of reactive porous percolation. The porphyroclastic xenoliths from Pico Cabuji represent the lower part of a mantle column (the head of a mantle diapir, at the transition conductive–adiabatic mantle), where OIB infiltration triggers melting, and the protogranular xenoliths the top of the mantle column, chromatographically enriched by percolation at a low melt/rock ratio. This interpretation may also apply for Fernando de Noronha, but the different geochemical signature recorded by the clinopyroxenes requires a different composition of the infiltrated melt. Nd and Sr isotopes of the Pico Cabuji porphyroclastic clinopyroxenes (¹⁴³Nd/¹⁴⁴Nd= 0.51339–0.51255, ⁸⁷Sr/⁸⁶Sr=0.70275–0.70319) and of Fernando de Noronha (¹⁴³Nd/¹⁴⁴Nd=0.51323–0.51285, ⁸⁷Sr/⁸⁶Sr=0.70323–0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to low Nd–low Sr (EMI) and low Nd–high Sr (EMII), respectively. Correlation of the isotope variation with geochemical parameters indicates that the isotopic variation was induced by the metasomatic component, of EMI type at Pico Cabuji and of EMII type at Fernando de Noronha. These different components enriched a lithosphere isotopically similar to DMM (depleted MORB mantle) at both localities. At Fernando de Noronha, the isotopic signature of the metasomatic component is similar to that of the ∼ 8 Ma old lavas of the Remedios Formation, suggesting that this is the age of metasomatism. At Pico Cabuji, the mantle xenoliths do not record the high ⁸⁷Sr/⁸⁶Sr component present in the basalts. We speculate that the EMII component derives from a lithospheric reservoir, which was not thermally affected during mantle metasomatism at Pico Cabuji, but was mobilized by the hotspot thermal influence at Fernando de Noronha. This interpretation provides a plausible explanation for the presence of distinct metasomatic components at the two localities, which would be difficult to reconcile with their genetic relationship with the same plume. Received: 12 June 1999 / Accepted: 13 December 1999     

New orthorhombic phases on the join NiAl2O4 (spinel analog)-Ni2SiO4 (olivine analog): Stability and implications to mantle mineralogy

Three new crystalline phases differing in Si/Al ratio have been synthesized from compositions along the join NiAl₂O₄-Ni₂SiO₄. Four reversible univariant equilibria involving these new phases plus Ni₂SiO₄ (olivine) have been located within the P-T region studied (1 atm–40 kb, 1000–1700° C); an invariant point occurs near 22 kb, 1150°C.All three new phases are orthorhombic. Precession photographs and electron microprobe analyses yield the following information:Phase I: 5NiO·3Al₂O₃·SiO₂ = 3NiAl₂O₄·Ni₂SiO₄, Pmma, a=5.67, b=11.51, c=8.10 (Å)Phase II: 7NiO·3Al₂O₃·2SiO₂ = 3NiAl₂O₄· 2Ni₂SiO₄, Imma, a=5.66, b=17.32, c=8.11Phase III: 3NiO· Al₂O₃· SiO₂ = NiAl₂O₄·Ni₂SiO₄, Imma, a=5.68, b=11.49, c=8.12Comparison with known structures suggests that these three phases plus NiAl₂O₄ spinel and high pressure Ni₂SiO₄ spinel belong to a homologous series based on a cubic close oxygen packing of the formula: M_2n O _n}-1 (T _n O_3n+1) where M and T are octahedrally and tetrahedrally coordinated cations, respectively. When n=1 the formula for spinel is obtained; n = 2 for phase I and phase III, both similar to the beta-phase of orthosilicates; and n = 3 for phase II which is related to the manganostibite structure.Similar phase equilibria and structural relations may occur on other joins of the aluminateorthosilicate type. Furthermore, the occurrence of such structural modifications between the spinel (aluminate) and olivine (orthosilicate) compositions suggests that there could be a corresponding polymorphic series between the olivine and spinel forms of orthosilicates.     

Crustal zircons and mantle sulfides: Archean to Triassic events in the lithosphere beneath south-eastern Sicily

Three types of zircon coexist in an unusual lower crustal xenolith from the Valle Guffari diatreme (Hyblean Plateau, Sicily): igneous Type 1 (near-euhedral, weakly zoned; Ce/Ce > 1); partially recrystallised Type 2 (ovoid, structureless; weak Ce anomaly); hydrothermal Type 3 (sugary, spongy-textured, probably related to F-rich aqueous fluids). U–Pb dating by LAM-ICPMS, supported by in situ Hf-isotope analysis, suggests that both Type 1 and Type 2 zircons were originally Archean (ca 2.7 Ga), though many of these grains have experienced severe Pb loss. The U–Pb ages of the hydrothermal zircons cluster around 246 Ma, interpreted as the timing of the hydrothermal event. Their ε_Hf (+ 8.5 to − 1.2) indicates the mixing of old crustal components and material from a juvenile source.

In situ Os-isotope analyses of sulfides hosted in peridotite xenoliths from Valle Guffari show Paleoproterozoic–Archean T_RD minimum ages, corresponding to the age of the oldest zircon grains in the crustal xenolith. Other peaks of T_RD ages suggest that multiple metasomatic events have affected the lithospheric mantle.

These observations suggest that the lower crust and the upper part of the lithospheric mantle beneath the Hyblean Plateau represent the northernmost portion of the African Plate. These two units have coexisted since at least late Archean time, and have remained linked through several episodes of crustal modification, including the Permo-Triassic hydrothermal event, which was probably related to the onset of rifting in the Ionian Basin.     

中国东部及西秦岭地区新生代岩石圈地幔中的相转变带及其温压条件

五相（橄榄石＋斜方辉石＋单斜辉石＋石榴石＋尖晶石）共存的地幔橄榄岩捕虏体是来自岩石圈地幔相转变带的直接样品.中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的少量五相共存的地幔橄榄岩捕虏体为探讨这些地区新生代岩石圈地幔中相转变带提供了宝贵的样品.本文根据地幔橄榄岩捕虏体中石榴石和尖晶石的产出状况,将这些橄榄岩捕虏体分为三类：第一类橄榄岩中尖晶石为粒状残核,尖晶石外缘被石榴石的反应边包围.这种橄榄岩捕虏体代表尖晶石-石榴石相转变带的上限,故称为尖晶石带橄榄岩;第二类橄榄岩中尖晶石和石榴石以单颗粒零散分布为特征,二者共存但未见明显的相转变关系.这类橄榄岩多位于相转变带中部,拟称为尖晶石-石榴石过渡带橄榄岩;第三类橄榄岩中以石榴石为主,尖晶石和辉石等微晶构成石榴石反应边.这类橄榄岩代表尖晶石-石榴石相转变带的下限,故称为石榴石带橄榄岩.因此,根据不同类型橄榄岩捕虏体中矿物的组成,结合温度压力估算即可确定岩石圈地幔中相转变带的深度和厚度.本文通过对中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的尖晶石-石榴石二辉橄榄岩捕虏体的温度压力估算来进一步厘定中国东部新生代岩石圈地幔中的相转变带深度和厚度.     

南海复蘑菇状地幔低速柱结构及其地幔动力学

运用高分辨率天然地震面波层析成像和体波层析成像技术,研究东亚西太平洋地区及全球地幔三维速度结构时发现南海地区地幔存在深达2000km以上的巨型复蘑菇状地幔低速柱,结合地质、地球化学和地球物理相关标志,将复蘑菇状地幔低速柱称为南海复蘑菇状地幔柱。本文在论述南海复蘑菇状地幔柱的地质地球物理特征基础上,将地幔柱划分为柱头、柱体、柱尾、幔枝和热点等部分,建立起地幔柱三维几何结构模型,探讨了复蘑菇状地幔柱在南海海盆扩张过程中的主导作用以及欧亚板块、菲律宾海板块和印度洋板块相互作用对南海演化过程的影响。     

The role of water in the deep upper mantle and transition zone: dehydration of stagnant slabs and its effects on the big mantle wedge

Mineral physics data related to the deep dehydration of stagnant slabs are summarized. The hydrogen diffusion in minerals of the mantle transition zone is not fast enough to homogenize the transition zone on the geological time scale, and hydrogen is expected to be unevenly distributed there. The hydrous fluid formed in the transition zone tends to percolate into shallower depths to form gravitationally stable hydrous magmas at the base of the upper mantle. We need further studies on the relation of intraplate volcanism above the stagnant slab and deep dehydration, because we expect the geochemical fingerprints of deep dehydration to be quite different from those of shallow dehydration from the subducting slabs.     

Geophysical evidence on segmentation of the Tancheng-Lujiang fault and its implications on the lithosphere evolution in East China

The north–south trending Tancheng-Lujiang (Tanlu) fault belt extends from northeast China to the Dabie–Sulu orogenic belt, for a length of more than 3000 km. This fault belt probably has close links with the lithosphere evolution, seismic activity and mineral resource concentration in East China. Surface geological mapping and studies on sedimentation and basin formation have indicated segmentation at the southern, middle and northern domains of the fault. Here we employ geophysical constraints to evaluate these fault segments. Unlike previous geophysical studies focused on laterally varying crust/mantle seismic velocity structure across the fault, in this study we have integrated a variety of geophysical data sets, such as crustal P-wave velocity, earthquake occurrence and released seismic energy, seismogenic layer thickness, surface heat flow and geothermal field, to understand the deep structure and strength of the lithosphere along the Tanlu segmented fault belt. The results demonstrate remarkable crustal-scale north-to-south segmentation this major fault. The geophysical evidence and some geochemical constraints suggest that the Tanlu fault belt probably served as a channel for melt and fluid percolation, and exerted a significant control on the lithosphere evolution in East China.     

Numerical models of ionic diffusion in one and three dimensions: application to dehydration of mantle olivine

The hydrogen content of nominally anhydrous minerals is of great interest, because it can influence many physical and mechanical properties of mantle rocks. Moreover, the hydrogen diffusion profiles can be used to constrain timescales related to magma eruptions. Here, we report models of ionic diffusion for trace elements in anisotropic crystals and apply them to hydrogen diffusing out of mantle-derived olivine. We first compare and discuss the characteristics of 1D and 3D models and show that only 3D anisotropic diffusion models can lead to diffusion profiles exhibiting non-equilibrium plateau at the center of the solid along the slowest axis, as measured in natural samples. In a second part, we discuss the differences between hydration and dehydration of olivine for diffusion that is linked to two different atomic sites involved in hydrogen mobility. Finally, we apply our 3D anisotropic model to previous results on mantle-derived olivine from Pali-aike to better characterize diffusion coefficients and their anisotropy that could be relevant for dehydration of olivine. Our results show that dehydration has to be strongly anisotropic, with a fast [100] axis and a significantly slower [001] axis.     

The effect of crystal field stabilization on the olivine→spinel transition in the system Mg2SiO4 - Fe2SiO4

Crystal field stabilization (CFS) plays a significant role in determining equilibrium phase boundaries in olivine→spinel transformations involving transition-metal cations, including Fe²⁺ which is a major constituent of the upper mantle. Previous calculations for Fe₂SiO₄ ignored pressure and temperature dependencies of crystal field stabilization enthalpies (CFSE) and the electronic configurational entropy (S _CFS). We have calculated free energy changes (ΔG _CFS) due to differences of crystal field splittings between Fe₂SiO₄ spinel and fayalite from: ΔG _CFS=?ΔCFSE?TΔS _CFS, as functions of P and T, for different energy splittings of t _2g orbital levels of Fe²⁺ in spinel. The results indicate that ΔG _CFS is always negative, suggesting that CFS always promotes the olivine→spinel transition in Fe₂SiO₄, and expands the stability field of spinel at the expense of olivine. Because of crystal field effects, transition pressures for olivine→spinel transformations in compositions (Mg_1?x Fe _x )₂SiO₄ are lowered by approximately 50x kbar, which is equivalent to having raised the olivine→spinel boundary in the upper mantle by about 15 km.     

大洋岩石圈拆沉与大陆下地壳拆沉：不同的机制及意义——兼评“下地壳＋岩石圈地幔拆沉模式”

拆沉作用（delamination）是地球科学中一个重要的科学问题。本文认为,大洋岩石圈拆沉和大陆下地壳拆沉是不一样的：（1）拆沉的物质不同。大洋岩石圈拆沉的物质包括大洋地壳、岩石圈地幔甚至一部分软流圈地幔,它们共同进入地幔深部;而大陆下地壳拆沉仅仅限制在下地壳,不包括岩石圈地幔。（2）拆沉的动力不同。大洋岩石圈拆沉是由板块俯冲引起的,是地幔对流的产物,因此是一种快速的主动的拆沉;而下地壳拆沉是由于下地壳加厚使下地壳密度增加引起的,还要求其下刚性的岩石圈地幔转变成塑性的软流圈地幔才有可能发生。因此下地壳拆沉要克服许多阻力才能实现,使拆沉成为一个漫长的过程,是慢速的和被动的拆沉。（3）拆沉的过程不同,大洋岩石圈拆沉是由板块俯冲触发的,俯冲导致碰撞,大洋岩石圈从根部断裂,拆沉进入地幔。大陆下地壳拆沉由地壳加厚开始,使下地壳转变为榴辉岩相;随后,岩石圈地幔减薄,直至全部转化为软流圈地幔;下地壳发生部分熔融,形成大规模的（埃达克质）岩浆,使下地壳榴辉岩的密度大于下伏的地幔,从而引发拆沉。大陆下地壳拆沉不大可能是整体进行的,可能是一块一块地被蚕食,被拆沉的。（4）拆沉后的效液压不同,大洋岩石圈地幔拆沉,使热的软流罪状地幔上涌,从而引发了一系列地质效应;如岩浆活动,地壳抬升,构造松弛以及随后的造山带垮塌等。而下地壳拆沉只引起地壳减薄,高原和山脉垮塌,并不伴有大规模的岩浆活动和地壳抬升等过程。（5）拆沉与岩浆活动的关系不同,主动拆沉导致大规模岩浆活动,而被拆沉是在大规模岩浆活动的基础上开始的。此外,文中还对“下地壳＋岩石圈地幔拆沉”模式提出了质疑,认为该模式有许多难以理解的问题和太多推测的成分,而且与现在保存的地质事实不符。