差应力与岩石熔融性状关系的实验研究

为考察不同应力状态下岩石应变与岩石熔融作用的关系,利用英国Instron公司生产的电液伺服实验系统,配以围压和加温系统设计了动、静态不同条件下细粒闪长岩的对比熔融实验,力图找到差应力与岩石熔融程度及其熔体成分的关系。通过岩石动、静态熔融实验结果的对比得出：差应力的存在能够导致细粒闪长岩的熔融,随着差应力（应变速率）的增加,熔体的量增加,熔体成分向富Si、Al方向转变。     

超基性岩中锆石年龄的特征和意义——以西南天山UHP蛇纹岩中锆石U-Pb年龄为例

超基性岩本身难以生长锆石的特性,使得研究其中的锆石需要特别谨慎。超基性岩中的锆石虽然具有多解性,但是锆石也携带了很多演化信息。产出不同地质背景的超基性岩,其中的锆石特征不同。本文总结现有的研究实例表明:(1)经历高温高压变质作用的石榴橄榄岩通常通过交代作用获得锆石,且锆石能够记录峰期变质时代,其中的继承锆石较少,可能在高温高压条件下,继承锆石发生分解重结晶;(2)大洋蛇绿岩型超基性岩和地幔岩捕掳体中通常具有年龄分布很广的锆石年龄特征,锆石年龄峰值通常与区域上构造事件相吻合,为捕掳晶锆石。接下来本文以西南天山超高压(UHP)蛇纹岩为例,对其锆石年龄进行解释。西南天山蛇纹岩为经历过超高压变质作用的大洋蛇绿岩型超基性岩,2个蛇纹岩样品中锆石的阴极发光图像分析和SIMS U-Pb定年分析结果显示,西南天山UHP蛇纹岩中的锆石包含捕掳晶锆石和变质锆石,捕掳晶锆石的年龄为2.1~1.0Ga,对应该区变泥质岩中碎屑锆石记录的年龄峰值。409~537Ma可能代表了蛇纹岩原岩结晶时代。区域上的变质压力峰期年龄(~320Ma)在蛇纹岩中没有记录,仅有1颗锆石记录了309±5Ma的近峰期时代。270~155Ma的退变质时代在西南天山蛇纹岩中出现较广,这与榴辉岩中出现的退变年龄相吻合,代表了折返过程中较为普遍的后期热液事件。基于对超基性岩中锆石特征的初步了解,结合西南天山蛇纹岩的研究实例,认为通过研究锆石的年代学,结合锆石矿物化学、包体矿物学、同位素地球化学等特征,不仅可以提供年代学信息,还可以对超基性岩的来源和演化过程进行解析。     

铁含量对滑石脱水动力学的影响及其地质意义

在常压下研究了2种不同铁含量滑石的原位X射线衍射高温脱水反应。选取粒径2~5μm的2种不同铁含量的滑石样品,在常压、空气氛围下进行了同步辐射原位X射线衍射脱水实验。实验结果表明,铁含量高的滑石脱水温度明显偏低,2个滑石样品在常压下发生明显脱水反应的温度相差达127℃以上。滑石在常压下的脱水动力机制为随机成核和生长机制,符合Avrami方程。将实验数据拟合Avrami方程得出:n=1.669。由实验结果可以推测,不同铁含量的滑石脱水深度可能有几十到上百千米的差别,研究铁含量与滑石脱水动力的相关性对于了解俯冲带浅-中源地震的成因机制具有重要意义。     

东昆仑清水泉蛇纹岩中铬铁矿环带成因

东昆仑造山带清水泉超基性岩已蚀变成蛇纹岩,部分浸染状铬铁矿颗粒具有环带结构,这将为研究铬铁矿形成后所经历的变质过程及其寄主岩体的构造演化提供可靠的信息.通过对铬铁矿进行显微结构观察和电子探针分析得出:所研究铬铁矿的环带结构从核部到边缘依次为铝铬铁矿、高铁铬铁矿和铬磁铁矿,被绿泥石所包裹.从铝铬铁矿到高铁铬铁矿,Cr2O3,Al2O3和MgO含量下降,Fe2O3和FeO含量升高;Cr#,TiO2含量,YFe值以及Fe2+#值明显具有升高的趋势,而Mg#值却急剧下降.以上变化规律表明本文所研究的铬铁矿经历了由高温到低温,同时氧逸度上升的过程,并伴随有蛇纹石化作用、热液流体以及区域变质作用等改造,最终形成环带结构.此过程与铬铁矿寄主岩体经历的由地幔抬升至浅部地壳以及相关的变质作用过程相对应.     

无机生油假说及其在中国的应用前景

无机生油假说认为,原油和天然气和近地表的生物物质没有根本联系,它们是生成于地幔内的非生物来源的碳氢化合物。因而油气不是一个不可再生资源,而是一个可再生资源。无机生油假说得到地质学、物理学和化学等三个基本学科的支持。在地质观察上,发现全球许多大油田的油气储藏与原始生物物质之间数量上有巨大落差,难于解释它们是由生物生成的。此外,有许多地区在结晶基底或变质基底内,或直接位于其上的沉积岩中发现石油。从生物生油假说来说,也是无法理解的。在化学上,早在二战期间,德国已由人工合成石油(费托合成),并生产了占德国战争中用油的9%的石油。无可争辩地说明,无机可以生成石油。根据化学(物理学)热力学理论分析确认,甲烷是唯一一种在标准温压条件(温度为298.15 K;压力为101325 Pa)下稳定的碳氢化合物,从甲烷形成正常烷属烃只有在压力>3×106kPa、温度>700°C时(相当于地下深度约100 km)才有可能。在地壳内的温压条件下由生物变质形成石油的假说,与化学热力学的基本原则相抵触。从氧化的有机分子,如碳水化合物(C6H12O6)形成较高的碳氢化合物在任何条件下都是不可能的。根据我国长期对深部构造的研究,笔者认为在中国东部及西太平洋蘑菇云岩石圈地幔发育的地区是寻找巨型无机油气田的有利地区,建议在发育蘑菇云岩石圈地幔地区开展无机油气田的勘探,并在无机油气田远景地区布置超深参数钻,以评价含油气远景。另外建议加强物探工作,尤其是研究地震勘探处理基底内三维含油气构造的技术。     

Slab-fluid metasomatism in the Early Paleozoic forearc mantle deduced from the Motai serpentinites,South Kitakami Belt,northeast Japan

The present study examines the petrology and geochemistry of the Early Paleozoic Motai serpentinites, the South Kitakami Belt, northeast Japan, to reveal the subduction processes and tectonics in the convergent margin of the Early Paleozoic proto-East Asian continent. Protoliths of the serpentinites are estimated to be harzburgite to dunite based on the observed amounts of bastite (orthopyroxene pseudomorph). Relic chromian spinel Cr# [=Cr/(Cr + Al)] increases with decreasing amount of bastite. The compositional range of chromian spinel is similar to that found in the Mariana forearc serpentinites. This fact suggests that the protoliths of the serpentinites are depleted mantle peridotites developed beneath the forearc regions of a subduction zone. The Motai serpentinites are divided into two types, namely, Types 1 and 2 serpentinites; the former are characterized by fine-grained antigorite and lack of olivine, and the latter have coarse-grained antigorite and inclusion-rich olivine. Ca-amphibole occurs as isolated crystals or vein-like aggregates in the Type 1 serpentinites and as needle-shaped minerals in the Type 2 serpentinites. Ca-amphibole of the Type 1 serpentinites is more enriched in LILEs and LREEs, suggesting the influence of hydrous fluids derived from slabs. By contrast, the mineral assemblage, mineral chemistry, and field distribution of the Type 2 serpentinites reflect the thermal effect of contact metamorphism by Cretaceous granite. The Ca-amphibole of the Type 1 serpentinites is different from that of the Hayachine–Miyamori Ophiolite in terms of origin; the latter was formed by the infiltration of melts produced in an Early Paleozoic arc–backarc system. Chemical characteristics of the Ca-amphibole in the ultramafic rocks in the South Kitakami Belt reflect the tectonics of an Early Paleozoic mantle wedge, and the formation of the Motai metamorphic rocks in the forearc region of the Hayachine–Miyamori subduction zone system, which occurred at the Early Paleozoic proto-East Asian continental margin.     

Trace element compositions of jadeite (+omphacite) in jadeitites from the Itoigawa-Ohmi district, Japan: Implications for fluid processes in subduction zones

Abstract   Trace-element compositions of jadeite (±omphacite) in jadeitites from the Itoigawa-Ohmi district of Japan, analyzed by a laser-ablation inductively coupled plasma mass spectrometry technique showed chemical zoning within individual grains and variations within each sample and between different samples. Primitive mantle-normalized patterns of jadeite in the samples generally showed high large-ion lithophile element contents, high light rare earth element/heavy rare earth element ratios and positive anomalies of high field strength elements. The studied jadeitites have no signatures of the protolith texture or mineralogy. Shapes and distributions of minerals coupled with chemical zoning within grains suggest that the jadeitites were formed by direct precipitation of minerals from aqueous fluids or complete metasomatic modification of the precursor rocks by fluids. In either case, the geochemical characteristics of jadeite are highly affected by fluids enriched in both large-ion lithophile elements and high field strength elements. The specific fluids responsible for the formation of jadeitites are related to serpentinization by slab-derived fluids in subduction zones. This process is followed by dissolving high field strength elements in the subducting crust as the fluids continue to circulate into the subducting crusts and serpentinized peridotites. The fluids have variations in chemical compositions corresponding to various degrees of water–rock interactions.     

Contrasting rifted margin styles south of Greenland: implications for mantle plume dynamics

We present new and reprocessed seismic reflection data from the area where the southeast and southwest Greenland margins intersected to form a triple junction south of Greenland in the early Tertiary. During breakup at 56 Ma, thick igneous crust was accreted along the entire 1300-km-long southeast Greenland margin from the Greenland Iceland Ridge to, and possibly 100 km beyond, the triple junction into the Labrador Sea. However, highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest Greenland margin at the same time. Thus, a transition from a volcanic to a non-volcanic margin over only 100–200 km is observed. Magmatism related to the impact of the Iceland plume below the North Atlantic around 61 Ma is known from central-west and southeast Greenland. The new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction. The extent of initial plume-related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere. Plume mantle is channelled to great distances provided that significant melting does not take place. Melting causes cooling and dehydration of the plume mantle. The associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift. We further suggest that thick Archaean lithosphere blocked direct flow of plume material into the magma-starved southwest Greenland margin while the plume was free to flow into the central west and east Greenland margins. The model is consistent with a plume layer that is only moderately hotter, 100–200°C, than ambient mantle temperature, and has a thickness comparable to lithospheric thickness variations, 50–100 km. Lithospheric architecture, the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism.     

Regionality of deep low-frequency earthquakes associated with subduction of the Philippine Sea plate along the Nankai Trough, southwest Japan

The Fukuoka District Meteorological Observatory recently logged three possible deep low-frequency earthquakes (LFEs) beneath eastern Kyushu, Japan, a region in which LFEs and low-frequency tremors have never before been identified. To assess these data, we analyzed band-pass filtered velocity seismograms and relocated LFEs and regular earthquakes using the double-difference method. The results strongly suggest that the three events were authentic LFEs, each at a depth of about 50 km. We also performed relocation analysis on LFEs recorded beneath the Kii Peninsula and found that these LFEs occurred near the northwest-dipping plate interface at depths of approximately 29–38 km. These results indicate that LFEs in southwest Japan occur near the upper surface of the subducting Philippine Sea (PHS) plate. To investigate the origin of regional differences in the occurrence frequency of LFEs in western Shikoku, the Kii Peninsula, and eastern Kyushu, we calculated temperature distributions associated with PHS plate subduction. Then, using the calculated thermal structures and a phase diagram of water dehydration for oceanic basalt, the water dehydration rate (wt.%/km), which was newly defined in this study, was determined to be 0.19, 0.12, and 0.08 in western Shikoku, the Kii Peninsula, and eastern Kyushu, respectively; that is, the region beneath eastern Kyushu has the lowest water dehydration rate value. Considering that the Kyushu–Palau Ridge that is subducting beneath eastern Kyushu is composed of tonalite, which is low in hydrous minerals, this finding suggests that the regionality may be related to the amount of water dehydration associated with subduction of the PHS plate and/or differences in LFE depths. Notable dehydration reactions take place beneath western Shikoku and the Kii Peninsula, where the depth ranges for dehydration estimated by thermal modeling agree well with those for the relocated LFEs. The temperature range in which LFEs occur in these regions is estimated to be 400–500 °C.     

Diffusive dehydration and bubble resorption during open-system degassing of rhyolitic melts

Degassing through open paths such as bubble and/or fracture networks is considered to be the principal mode of degassing in silicic magmas. However, its detailed mechanisms remain unclear. To investigate the behavior of bubbles in a hypothetical open-system condition, we performed a series of vesiculation experiments on natural rhyolitic obsidian using a newly designed semipermeable cell, which artificially maintains a pressure difference between its inside and outside. The thick-wall cell maintains a constant volume within the sample chamber, while allowing water vapor to escape the cell during the experimental runs. The cells containing obsidian cores with ca. 0.66 wt.% initial water content were externally heated to 1000 °C for a period of 1–288 h. The run charges generally showed a zonal structure composed of two contrasting regions: a central region within which the bubbles were concentrated (bubble-rich core, BC) and a bubble-free melt region surrounding the BC (bubble-free margin, BFM). With increased heating duration, the thickness of the BFM increased via dissolution of the outermost bubbles in the BC. The water content was nearly uniform throughout the BC, whereas an outward-decreasing gradient was observed in the BFM. We found that diffusive dehydration occurred from the sample surface, and the bubbles were resorbed into the melt. Thus, the BFM–BC boundary moved inwards. These processes were modelled numerically, and the calculation results were in good agreement with the experimental data. If a “lifetime” of open paths is approximated as the relaxation time of a melt in a shallow volcanic environment, then the paths have to be pinched off quickly (1.2 h at maximum pressure difference between the open path and the melt) and thus the thickness of the bubble-free layer reaches at most ~ 0.1 mm. For the formation of bubble-free obsidian layers with a width of a few millimeters, which are often observed in natural obsidian flows, open paths should be maintained for at least a few hundred hours.