Decarbonation reaction of magnesite in subducting slabs at the lower mantle

High-pressure and temperature experiments (28–62 GPa, and 1,490–2,000 K, corresponding to approximately 770–1,500 km depth in the mantle) have been conducted on a MgCO₃ + SiO₂ mixture using a laser-heated diamond anvil cell combined with analytical transmission electron microscope observation of the product phases to constrain the fate of carbonates carried on the subducting basalt into the lower mantle. At these conditions, the decarbonation reaction MgCO₃ (magnesite) + SiO₂ (stishovite) → MgSiO₃ (perovskite) + CO₂ (solid) has been recognized. This indicates that above reaction takes place as a candidate for decarbonation of the carbonated subducting mid ocean ridge basalts in the Earth’s lower mantle.     

Slab breakoff: a model for Caledonian, Late Granite syn-collisional magmatism in the orthotectonic (metamorphic) zone of Scotland and Donegal, Ireland

None of the existing models for calc-alkaline “Late Granite” (Siluro–Devonian) genesis in the metamorphic Caledonian orogenic belt of Ireland and Scotland fully explains their spatial, age or chemical character. A consistent model must involve the closure of Iapetus Ocean, where slab breakoff is a natural consequence of attempted subduction of continental crust. Expected outcome is a long linear belt of high-K, calc-alkaline magmas, some with characteristic trace element signatures, specifically high Ba, Sr and Zr. Other features include the critical magmatic association of coeval appinite and granite, rapid uplift, erosion and the low-grade regional metamorphism in the Southern Uplands. The linear heat pulse on breakoff is spatially, intensity and time limited producing small volume melts emplaced as separated plutons, over a short time span. Magmatism in the Caledonian metamorphic belt is accurately accounted for by slab breakoff on collision of Baltica with the Scoto–Greenland margin during the Scandian orogeny, following Iapetus Ocean closure. The two chemically, isotopically and areally distinctive suites in the metamorphic belt in Scotland, viz. the Argyll and Cairngorm Suites, can be modelled by reference to the Donegal granites where sequential partial melting of new, lamprophyric underplated crust, then shallower old crust, as heat conduction moved up through the crust on slab breakoff, produced magmas characteristic of the two suites.     

Pull-down basin in the central part of Japan due to subduction-induced mantle flow

The driving force for the basin subsiding against isostatic balance in and around Lake Biwa in the Kinki district, Japan is discussed. The lake region is characterized by strong negative Bouguer anomalies, especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake. The large negative anomaly (>50 mgal) cannot be explained by low-density sediments beneath it. A down-warping structure extending to the Moho depth should be taken into account. This conjecture has been strongly supported by a short-period receiver function imaging, which shows a clear offset of about 8 km for the Moho discontinuity under the steep gravity gradient zone.A question arises as to what is the driving force to create such a large down-warping structure. We consider that the subduction of the shallow-dipping slab under the region (Philippine Sea Slab) may cause crustal deformation by dragging the viscous mantle downward. In order to verify this model, we simulated the induced mantle flow due to the subduction of the Philippine Sea Slab and the pressure distribution on the crust–mantle boundary. This numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction. This offset of the slab is consistent with plate models deduced from hypocentral distributions and Sp phases of the deep-focus earthquakes.     

圣安德烈斯断层门多西诺三联点迁移的构造热效应

由于圣安德烈斯断层门多西诺三联点向北迁移，或北美板块的相对向南运动，驱动角落流使地幔热物质向板片窗上涌，造成地幔温度升高和部分熔融．本文通过二维暂态有限单元法计算表明，当北美盖层厚度为20km时，计算预测的地热分布、部分熔融百分比、岩浆岩活动的时空分布特征等都与实际地热、地震和地质观测很好地吻合，确认了角落流机制和它引起的暂态热过程的重要性．     

A new view of Italian seismicity using 20 years of instrumental recordings

In this paper, we show the seismicity of the past 20 years that occurred in Italy and surrounding regions. Hypocentral locations have been obtained by using P- and S-wave arrival times from the INGV national and several regional permanent seismic networks. More than 48,000 events, selected from an original data set of about 99,780, are used to reconstruct the most complete seismic picture of the Italian region so far. The seismicity distribution allows inference on seismotectonics of this complex region of subduction versus continental collision. Our results clearly reveal the geometry of the Adria and the Ionian subduction and a continuous normal fault belt in the upper crust, following the Apennines mountain range. The depth of the seismogenic layer is computed from the cut-off of seismicity at depth and shows large variations along and across the seismic active regions. Earthquakes are generated by the different velocity of slab retreat and the subsequent asthenospheric upwelling.     

锡的地球化学性质与华南晚白垩世锡矿成因

锡在地质过程中表现出亲氧、亲硫和亲铁三重特性。在地幔岩浆过程中,锡是一种中等不相容的金属元素。锡成矿主要与酸性岩浆活动有关,其地球化学性质决定了其成矿主要受源区性质、氧逸度以及挥发分含量、岩浆结晶分异等因素控制。高度结晶分异可使锡在岩浆中进一步富集,是锡成矿相关花岗岩的普遍特征;锡为变价元素,岩浆体系氧逸度影响源区中锡的迁移能力和分离结晶过程中锡的元素行为,还原性岩浆体系有利于锡富集成矿。富含F、Cl和B等挥发组分对锡元素的迁移和富集起积极的作用。全球锡矿床分布与俯冲带关系密切,特提斯和环太平洋构造域是主要蕴藏区。重要的锡成矿事件表现出区域性和阶段性的特征。结合锡的地球化学特性以及锡矿分布特征,我们认为最有利锡成矿的动力学机制是俯冲板片后撤机制。俯冲板片后撤引发深部软流圈地幔上涌,导致强烈的壳幔相互作用,形成低氧逸度、富F、Cl和B等花岗岩,有利于锡成矿。对于华南晚白垩世锡成矿事件,新特提斯洋俯冲板片发生后撤是其成矿地球动力学背景。     

印度大陆板片前缘撕裂与分段俯冲：来自冈底斯新生代火山-岩浆作用证据

青藏高原碰撞造山带不仅呈现南北不均一性,而且显示东西分段性。以横贯高原腹地的NNE向负磁异常带为界,将冈底斯分为三段。在宽约300km的负磁异常带为代表的中段,近SN向的裂谷和正断层系统、重要地震和现代热水活动、古新世林子宗火山岩系和中新世超钾质火山岩系、以及日喀则弧前盆地集中发育,伴有斑岩型Cu-Mo和成因独特的Au-Cu矿化;在85°E以西的西段,主要发育强烈逆冲推覆系、同碰撞期花岗岩和中新世钾质-超钾质火山岩系,伴有造山型Au矿化;而在90°E以东的东段,主要发育走滑断裂系、同碰撞期花岗岩和中新世埃达克质斑岩,伴有斑岩型Cu-Mo矿化。古新世林子宗火山岩的精细定年和地球化学特征揭示,印度大陆板片向北的俯冲-汇聚至少在50Ma前没有表现出明显的时间差异性。然而,中新世钾质-超钾质岩和大规模花岗岩基的时空分布和地球化学特征反映,印度大陆板片前缘可能发生撕裂,并发生分段式差异俯冲,西段(85°E以西)俯冲规模大,距离远,东段(90°E以东)俯冲规模小,可能未跨过雅江缝合带。沿着负磁异常带两侧的边界裂谷带,高SiO_2煌斑岩和念青唐古拉花岗岩基及相伴钾质火山岩的发育,揭示来源于软流圈地幔的岩浆和高热流穿过板片撕裂带并沿耦合上覆的裂谷带上涌,前者侵位和喷发,后者诱发地壳熔融。90°E与85°E之间的俯冲板片可能由于撕裂、断离和破碎,因而导致斜跨高原腹地的大面积通道式负磁异常带。     

A Simple Single-Layer Urban Canopy Model For Atmospheric Models: Comparison With Multi-Layer And Slab Models

We developed a simple, single-layer urban canopy model, and comparedit to both multi-layer and slab models. Our single-layer model has thefollowing features: (a) It is a column model of energy and momentumexchange between an urban surface and the atmosphere, (b) it includesthe influence of street canyons, which are parameterized to representthe urban geometry, (c) it includes shadowing from buildings andreflection of radiation, and (d) it estimates both the surfacetemperatures of, and heat fluxes from, three surface types: roof, wall,and road. In the simulation of the single-layer model, the roof washottest during the daytime, but coolest from midnight to early morning.This is consistent with output from the multi-layer model and fieldobservations at a residential area on a clear, summer day. The diurnalvariation of the energy budget from the single-layer model agrees wellwith that from the multi-layer model. Our single-layer model'sperformance is nearly that of a multi-layer model for studyingmesoscale heat islands. Nevertheless, it is simply parameterized,and thus easily included in larger-scale atmospheric models. The slabmodel has the largest nighttime cooling rate of the three models. Toovercome this, it needs more adjustments than for the canopy models.     

俯冲带硫的地球化学行为及硫循环SCIEI北大核心CSCD

俯冲带是全球最大的物质循环系统,控制着硫(S)在地球内部圈层及表层的循环,影响着大气圈、水圈、生物圈、岩石圈的稳定性以及地球的宜居性。厘清S在俯冲带中的地球化学行为和循环特征对理解地球各储库的氧化还原状态、岩浆作用与演化、成矿物质聚集、以及地球大气成分等具有重要意义。本文首先总结了进入俯冲带之前的大洋岩石圈的S结构模型,对S在大洋板片中的分布状态和地球化学特征进行了系统归纳。随后,系统阐述了俯冲带高压-超高压变质岩记录的板片变质及脱水过程中硫的地球化学行为。岩石学研究表明俯冲板片中的S多以硫化物相存在,硫酸盐矿物在弧前深度就已被释放或分解。相较于熔体,俯冲带流体中S的溶解度更高,是运移硫的更有效方式。DEW模型计算结果显示,流体中S含量总体较低,但在俯冲板片~90km处其含量有一个峰值(浓度0.5%~1.0%)。岩相学证据、地球化学测试结果、磷灰石S近边吸收结构(S-XANES)特征以及模拟结果都显示俯冲深部流体中S多以HS^(-)及H_(2)S形式存在,不含大量的SO_(4)^(2-)及硫酸盐;中f_(S_(2))流体有利于S迁移出俯冲板片,从而促进俯冲带大规模S循环,而高f_(S_(2))流体在流-岩交换过程沿流体通道发生S的锁固作用而不利于俯冲带S循环。质量平衡计算显示全球俯冲带S输入通量为4.65×10^(13)g/yr,弧下深度板片S输出通量为2.91×10^(12)g/yr,板片-岛弧S循环效率仅6.3%。俯冲板片在弧下深度可能存在一个短暂高效的S释放窗口,释放流体的δ^(34)S值为-2.1±3.0‰。基于高压-超高压变质岩中硫化物的研究,初步厘清了俯冲板片中S的地球化学行为,首次从板片角度全面、定量地限定了俯冲带的脱硫通量、效率、种型和同位素特征,提出俯冲带循环的S不是岛弧岩浆的氧化剂,与岛弧环境的正δ^(34)S值也无直接因果联系,对解析俯冲带S循环和理解地球长期的S循环具有重要意义。最后,本文还展望了俯冲带S循环的未来发展方向,应在俯冲带流体氧化还原性质(硫酸盐的命运)、俯冲沉积物对S循环的制约、俯冲带环境下多硫同位素的分馏效应、S循环与其它挥发分(如C等)循环之间的耦合关系、地球历史上深部S循环等方向做出探索,更深入地理解俯冲带及全球S循环过程。     

西南三江杂岩带景洪南部地区晚泥盆世火山岩的发现及意义

本文对西南三江地区景洪南部新发现的晚泥盆世英安质火山岩进行了系统的锆石U-Pb年龄、Hf-O同位素以及全岩地球化学研究。测年结果显示,该英安质火山岩形成于362.3±3.4Ma。火山岩的SiO₂含量为62.87%~66.29%,MgO含量为2.15%~2.49%,Mg^#值为44~47,富集Na₂O（4.77%~5.51%）,高Na₂O/K₂O比值（2.1~3.3）,低Y（12.5×10^-6~15.3×10^-6）和Yb（1.38×10^-6~1.70×10^-6）,具有高硅埃达克岩的地球化学特征,表明它们由俯冲的洋壳部分熔融而成。低的锆石ε_Hf（t）（+0.87~+3.27）,相对较高的锆石δ¹⁸O（6.31‰~7.64‰）,以及高的全岩Th/Yb（4.86~7.78）和（La/Sm）_N（3.62~4.56）比值,指示岩浆中混染了大洋沉积物或混入了大洋沉积岩起源的熔体。综合区域岩浆岩和沉积岩资料,本文认为南澜沧江带晚泥盆世时经历了板片的俯冲,该俯冲作用很可能代表的是由大中河、大平掌火山岩以及火山块状硫化物矿床（VHMS）指示的弧后洋盆的俯冲消减过程。