Stability and crystal chemistry of iron-bearing dense hydrous magnesium silicates

Dense hydrous magnesium silicates (DHMS) are supposed to be key phases in planetary water cycles because of their ability to carry water to deep mantle regions in subduction slab environments. In order to understand water cycles in iron-enriched planetary systems such as Mars knowledge of the water content and stability of iron-bearing DHMS is required. Iron-bearing DHMS were synthesized based on two starting compositions, MgFeSiO₄ + 9.5 wt% H₂O system and a simple hydrous Martian mantle composition containing Fe, Mg, Al and Si + 12.35 wt% H₂O (hydrous FMAS system). Compared to literature data on phase D, iron-bearing phase D shows analogous variations in water contents as Mg-phase D but appears to be stable at higher temperatures than Mg-phase D for both starting compositions used in this study. Iron-bearing superhydrous phase B contains up to 7 wt% H₂O and shows an extended thermal stability in the hydrous FMAS system. The high-temperature stability of iron-bearing DHMS with a Mars-like bulk composition indicates that these hydrous phases could host significant amounts of water at core-mantle boundary conditions (1500 °C and 23 GPa) in a hydrous Martian mantle.     

中国显生宙造山带麻粒岩相高级变质岩石的地质特征、变质时代、P-T轨迹及其形成的大地构造背景

本文重点介绍我国显生宙造山带中麻粒岩的地质特征、岩石类型、P-T轨迹、变质时代及其形成的大地构造背景。我国显生宙造山带主要包括阿尔泰造山带、南天山-西南天山造山带、西昆仑造山带、东昆仑造山带、阿尔金-柴北缘造山带、北秦岭造山带、南秦岭勉略造山带、东秦岭-桐柏-大别造山带、班公湖-怒江造山带和喜马拉雅中东段造山带。这些造山带中麻粒岩的围岩有许多为蛇绿岩套或蛇绿混杂岩带,部分为副片麻岩和花岗质片麻岩,并一起经历了麻粒岩相变质改造,造山带中大多出现一种高压麻粒岩,有的与榴辉岩并存,但少数造山带中(例如阿尔泰造山带)多种压力类型麻粒岩并存,既有低-高压泥质麻粒岩、中低压基性麻粒岩、高压基性和长英质麻粒岩,又有高温-超高温泥质麻粒岩。变质时代除个别为新元古代晚期外,变质时间多为加里东、海西、印支、燕山、喜山期。麻粒岩的P-T轨迹除西天山木札尔特河低压麻粒岩具逆时针轨迹,反映大陆弧构造环境外,其它都是具有等温降压(ITC)特点的顺时针轨迹,形成的大地构造环境大部分为洋陆俯冲碰撞环境,少部分为陆-陆碰撞环境。目前显生宙造山带中麻粒岩的研究大多数尚在起步阶段,少数研究较详细,不少造山带中麻粒岩的类型和变质时代以及形成的构造背景还不清楚,有待深入研究,新的麻粒岩产地有待发现。     

Olivine intergranular plasticity at mantle pressures and temperatures

The ductile behavior of olivine-rich rocks is critical to constrain thermal convection in the Earth's upper mantle. Classical olivine flow laws for dislocation or diffusion creep fail to explain the fast post-seismic surface displacements observed by GPS, which requires a much weaker lithosphere than predicted by classical laws. Here we compare the plasticity of olivine aggregates deformed experimentally at mantle pressures and temperatures to that of single crystals and demonstrate that, depending on conditions of stress and temperature, strain accommodated through grain-to-grain interactions – here called intergranular strain – can be orders of magnitude larger than intracrystalline strain, which significantly weakens olivine strength. This result, extrapolated along mantle geotherms, suggests that intergranular plasticity could be dominant in most of the upper mantle. Consequently, the strength of olivine-rich aggregates in the upper mantle may be significantly lower than predicted by flow laws based on intracrystalline plasticity models.     

Synthetic ANaB(Na x Li1 − x Mg1)CMg5Si8O22(OH)2 (with x = 0.6, 0.2 and 0) P21/m amphiboles at high pressure: a synchrotron infrared study

The high-pressure behavior of three synthetic amphiboles crystallized with space group P2₁/m at room conditions in the system Li₂O–Na₂O–MgO–SiO₂–H₂O has been studied by in situ synchrotron infrared absorption spectroscopy. The amphiboles have compositions ^ANa ^B(Na _x Li_1 − x Mg₁) ^CMg₅ Si₈ O₂₂(OH)₂ with x = 0.6, 0.2 and 0.0, respectively. The high-P experiments up to 32 GPa were carried out on the U2A beamline at Brookhaven National Laboratory (NY, USA) using a diamond anvil cell under non-hydrostatic or quasi-hydrostatic conditions. The two most intense absorption bands in the OH-stretching infrared spectra can be assigned to two non-equivalent O–H dipoles in the P2₁/m structure, bonded to the same local environment ^M1M3Mg₃–OH–^ANa, and pointing toward two differently kinked tetrahedral rings. In all samples these bands progressively merge to give a unique symmetrical absorption with increasing pressure, suggesting a change in symmetry from P2₁/m to C2/m. The pressure at which the transition occurs appears to be linearly correlated to the aggregate B-site dimension. The infrared spectra collected for amphibole ^B(Na_0.2Li_0.8Mg₁) in the frequency range 50 to 1,400 cm⁻¹ also show a series of changes with increasing pressure. The data reported here support the inference of Iezzi et al. (Am Miner 91:479–482, 2006a) regarding a new high-pressure amphibole polymorph.     

喜马拉雅造山带的变质作用与部分熔融

喜马拉雅造山带的核心由高级变质岩系和淡色花岗岩构成,是研究碰撞造山作用和板块构造的天然实验室。本文评述了喜马拉雅造山带变质作用和部分熔融研究取得的新进展和存在的争议,主要内容包括:(1)造山带核部具有"三明治"结构,高级变质和部分熔融的高喜马拉雅系列(GHS)夹持在较低级变质的特提斯喜马拉雅系列(THS)和低喜马拉雅系列(LHS)之中,GHS的变质作用程度具有向上和向下部构造层位降低的特征。高喜马拉雅系列主要由高压麻粒岩相到榴辉岩相的变质岩组成,具有1.2~1.6GPa和700~800℃峰期变质条件,顺时针型变质作用P-T轨迹,其进变质以增温增压为特征,退变质早期为近等温或增温降压过程,晚期为降温降压和近等压降温过程;(2)在造山带西段,紧邻缝合带产出的超高压变质岩具有4.4~4.8GPa和560~760℃的峰期变质条件和顺时针型P-T轨迹,并在退变质中期出现加热过程;(3)尽管造山带的高压和超高压变质岩形成在中、高温条件下,但岩石中的石榴石都保存有明显的主量和微量元素生长成分环带特征;(4)造山带变质核下部发育反转的中、高压型变质序列;(5)在造山带核部,变泥质和长英质麻粒岩的强烈部分熔融主要是增压、增温进变质过程中的白云母和黑云母脱水熔融,和近等温或增温降压过程中的黑云母脱水熔融,可以形成花岗质和英云闪长质熔体。加厚下地壳的高变质温度足以使各种成分岩石(包括基性岩)发生深熔,而不需要外来热源;(6)造山带变质核经历了长期的变质演化过程,其进变质始于~47Ma,峰期变质发生在~25Ma,退变质持续到~15Ma。这些岩石也记录了持续的(超过20Myr)高温变质和部分熔融过程。在造山带西段的超高压变质岩具有~46Ma的峰期变质年龄和~40Ma的退变质年龄,所以经历了一个快速俯冲与折返过程;(7)印度大陆西缘与岛弧的碰撞(造山带西段)和印度大陆东缘与大陆弧的碰撞时间一致,为~50Ma;(8)在造山带西段,印度大陆的深和陡俯冲形成了超高压变质岩;而在造山带中段,印度大陆的平缓俯冲形成了中高压变质岩;(9)构造变质不连续在变质核中广泛存在。多重有序逆冲和无序逆冲导致的岩片叠置控制着造山带的地壳结构;(10)现有的构造模型,包括楔形挤出、隧道流、临界楔和构造楔模型,都不能全面合理地解释造山带变质核部的折返机制。     

黑龙江依兰地区蓝片岩的变质演化P-T-t轨迹

黑龙江省依兰地区出露了一套经高压变质形成的蓝片岩、云母片岩、长英质糜棱岩和少量超基性岩及大理岩并经历了强变形作用改造,一直被认为是佳木斯地块与其西侧松嫩地块碰撞拼贴所形成的缝合带产物。本次研究,通过岩相学与矿物化学分析,根据矿物组合的不同,识别出研究区内三种不同类型的蓝片岩;并获得了绿帘蓝闪石片岩(T=350~550℃,P=10~14kbar)、蓝闪霓辉石片岩(T=350~550℃,P=10~14kbar)和石榴石冻蓝闪石片岩(T=500~540℃,P=10~12kbar)的峰期变质温压条件。结合前人发表的地质年代学数据,建立了变质演化P-T-t轨迹。其中,绿帘蓝闪石片岩具"发卡式"变质演化P-T-t轨迹;而蓝闪霓辉石片岩与石榴石冻蓝闪石片岩具顺时针的演化轨迹。我们推测其成因可能为不同类型的蓝片岩形成于不同的俯冲阶段,由动力学机制的变化所造成。该研究成果为进一步探索佳木斯地块与松嫩地块碰撞拼贴的构造演化过程提供重要的依据。     

A new high-pressure phase of strontium carbonate

High-pressure and high-temperature experiments conducted in a laser-heated diamond-anvil cell with a synchrotron X-ray diffraction method have revealed a phase transformation in the aragonite-type SrCO₃ at pressures above 10 GPa. The new phase has an orthorhombic symmetry and was confirmed to remain stable to 32 GPa. The Birch-Murnaghan equation of state for new phase was determined from the experimental unit cell parameters, with K₀ = 101 (± 16) GPa, K₀ = 4 (constrained value), and V₀ = 111.9 (± 2.2). This transformation in SrCO₃ is different from that in BaCO₃ as reported in previous studies. After decompression at ambient pressure, the high-pressure phase transforms to a metastable structure, which has an orthorhombic symmetry. This result should also resolve a dispute regarding the stable high-pressure phases in BaCO₃, which is an analog material of CaCO₃ and SrCO₃.This revised version was published in February 2005 with corrections to the Introduction and to the References.     

大陆碰撞造山带的流体成分与演化:东喜马拉雅构造结南迦巴瓦岩群高压麻粒岩的流体包裹体研究

位于喜马拉雅东构造结的南迦巴瓦岩群经历了高压麻粒岩相、中压麻粒岩相和角闪岩相三期变质作用.在高压麻粒岩中含有复杂的流体包裹体类型,按照捕获先后顺序有:H2O-CO2±CH4包裹体(Ⅰ型);CO2±CH4±N2包裹体(Ⅱ型);高盐度多相包裹体(Ш型);中.低盐度H2O包裹体(Ⅳ型)和极低密度气体包裹体或"空"包裹体(Ⅴ型).在基性麻粒岩中,被石榴石包裹石英中孤立分布的H2O-CO2 4-CH4包裹体,以及部分沿石榴石晶内裂隙分布的H2-CO2±CH4和H2O包裹体轨迹未穿过围绕石榴石的辉石 斜长石后成合晶冠状体,表明它们有可能是在麻粒岩相变质阶段捕获的.然而,所有流体包裹体的等容线均从麻粒岩相变质峰期P-T区间下方通过,说明麻粒岩相变质峰期捕获的包裹体均受到了不同程度的改造,包括部分爆裂、渗漏和流体-矿物相互作用等.现存的富CO2流体包裹体均具有较低密度,并且往往含有明显数量CH4和N2组分,不可能是麻粒岩相变质峰期捕获的包裹体.根据富CO2包裹体与具有不同相比的H2-CO2包裹体共存推测,大部分CO2包裹体是通过H2O-CO2包裹体中H2O的选择性泄漏而形成的.Ⅲ型高盐度盐水包裹体很可能是角闪岩相退变质过程中捕获的,因其等容线与退变质轨迹近于平行,这些包裹体很可能保存了其在角闪岩相阶段捕获时的原生物理化学特征.沿矿物颗粒裂隙分布的大量Ⅳ型和Ⅴ型包裹体,应该是角闪岩相或更晚期形成的次生包裹体,代表了浅成(近地表)环境的循环流体.与世界许多地区麻粒岩相岩石普遍舍高密度纯CO2流体包裹体不同,南迦巴瓦岩群高压麻粒岩以富含H2O-CO4±CH4和H2O包裹体为特征,这可能与高压麻粒岩与高温麻粒岩产出于不同的构造环境和经历的退变质轨迹有关.     

胶北高级变质基底中高压基性麻粒岩的地球化学特征及其成因

胶北高级变质基底中高压基性麻粒岩主要由石榴基性麻粒岩、石榴紫苏麻粒岩和石榴斜长角闪岩所组成,并主要以不规则透镜体或变形岩墙的形式赋存于TTG质片麻岩或花岗质片麻岩之中。胶北高压基性麻粒岩在变质过程中,以大离子亲石元素(K、Na、Sr、Rb)为代表的活动元素发生了显著的改变;而高场强元素(Th、Nb、Zr、Ti)和稀土元素基本无变化,保持稳定。胶北高压基性麻粒岩属于拉斑玄武质岩石系列,其SiO2集中变化于44.04%～53.54%,Mg#值集中变化于35～60之间;稀土配分曲线不仅存在总量较低的平坦型(ΣREE=21.13×10-6～78.49×10-6,(La/Yb)CN=1.03～2.86),也有轻稀土相对富集且含量相对较高的右倾型(92.74×10-6～133.5×10-6,(La/Yb)CN=2.93～4.56),Eu异常不明显(Eu/Eu*=0.93～1.04)。高压基性麻粒岩Cr、Ni含量变化较大,但与MgO含量具有很好的相关性。与显生宙岛弧拉斑玄武岩一样,胶北高压基性麻粒岩几乎所有样品皆具有Nb、Zr、Ti负异常,且εNd(t)为正值(+2.70～+4.77)。综合分析认为,胶北高压基性麻粒岩具有岛弧拉斑玄武质岩石的地球化学特征,其原岩可能为弧后扩张背景下侵入的辉长岩或辉绿岩,以及相应喷出的基性火山岩。     

Eutectic temperatures and melting relations in the Fe–O–S system at high pressures and temperatures

We have investigated melting relations in the Fe–O–S ternary system in the pressure range of 15–27 GPa and 1873 K. Subsolidus phase relations are Fe, Fe₃S₂, and FeO up to 17 GPa and Fe, Fe₃S, and FeO above this pressure. The eutectic temperature slightly decreases from ambient pressure to 17 GPa, whereas increases above this pressure. The eutectic temperature in this study is 100 K lower than that in the Fe–S binary system. The oxygen content in the Fe–O–S eutectic liquid drops when the coexisting solid phases changes from FeS to Fe₃S₂. The cotectic lines in the ternary phase diagram lie close to the Fe–FeS binary axis. The isothermal sections indicate that oxygen solubility in the Fe–O–S liquid increases with increasing temperature, and with increasing sulfur content. The solubility of sulfur in the solid Fe has a maximum value at the eutectic temperature, and decreases with increasing temperature. Our results could have important implications for formation and composition of the Martian core.