冻融循环对黄土渗透系数各向异性影响的试验研究

为了反映冻融循环作用对原状黄土渗透各向异性及原状、 重塑黄土渗透差异的影响, 以西安Q₃黄土为研究对象, 通过三轴固结渗透试验对比分析了冻融前后水平、 竖直向原状黄土及重塑黄土的渗透系数随初始含水率、 围压、 冻融循环次数变化的规律。结果表明： 未冻融时各级围压下竖直向原状黄土的渗透系数为2×10^-6 ~ 18×10^-6 cm?s^-1, 水平向原状黄土和重塑黄土的渗透系数为0 ~ 4×10^-6 cm?s^-1; 经历冻融循环后, 水平、 竖直向原状黄土及重塑黄土的渗透系数与初始含水率的关系曲线分别呈现逐渐上升、 抛物线形式与变化平缓的不同特征, 而三者的渗透系数均随冻融循环次数的增加呈现数量级增大的趋势; 原状黄土的竖直 - 水平渗透系数比（k_v /k_h ）由冻融前的4.38逐渐减小到0.90, 可见冻融循环作用在显著提高黄土渗透性能的同时, 可以强烈弱化其各向异性特征。通过建立围压、 渗透系数与土体孔隙率的相关关系可知, 原状、 重塑黄土的孔隙率与围压存在极强的负线性相关性, 渗透系数随围压的增大呈典型指数衰减特征, 渗透系数与孔隙率具有相似的变化趋势, 因此冻融循环过程中土体孔隙率的改变是导致其渗透性质变化的主要原因。     

黄土边坡盐蚀剥落病害特征调查及其水盐迁移规律研究

通过现场调查及室内含水量和易溶盐分析试验开展了黄土边坡盐蚀剥落病害特征的研究。结果表明:盐蚀剥落病害按剥落形态可分为条带状、片块状及凹腔状盐蚀剥落;按地层岩性可分为马兰黄土、离石黄土、古土壤层及人工夯实黄土盐蚀剥落。试样含水量随着距边坡表层垂直深度增加而增大,含盐量随着距边坡表层垂直深度增加迅速降低且很快趋于稳定;阴离子CO₃2-和HCO₃-含量随距边坡表层垂直深度增加无明显变化规律,Cl-和SO₄2-含量随距边坡表层垂直深度增加迅速减小且很快趋于稳定;阳离子K+、Mg2+、Ca2+含量随距边坡表层垂直深度增加无明显变化规律,Na+随距边坡表层垂直深度增加迅速减小且趋于稳定;阴离子以SO₄2-和Cl-为主,阳离子以Na+为主,边坡表层易溶盐主要成分为Na₂SO₄和NaCl。边坡表层富集易溶盐Na₂SO₄和NaCl,在气候变化条件下的反复溶解和结晶作用是黄土边坡盐蚀剥落病害的主要诱因。     

Influence of microstructure of loess on triaxial shear strength

Vicksburg loess is characterized by preferred orientation of constituent grains, which on the average dip 4° toward the west (N 80–85°W). This investigation was undertaken to study, quantitatively, the relationship between natural fabric anisotropies of Vicksburg loess and the orientation of applied stress distribution Results of the study indicate the fabric anisotropies in Vicksburg loess are reflected by definite variation in triaxial shear strength of dry and moist specimens.

In two series of triaxial tests, ultimate strength of the loess is maximum where σ₁ is perpendicular to grain orientation, and it is reduced where the principal stresses are 45° to the fabric plane. In this respect, Vicksburg loess may serve as a structural model for granular earth materials in illustrating the influence of fabric on ultimate strength. Moreover, the Mohr-Coulomb fracture line consists of two line segments, with an increase in slope at higher confining pressure. This characteristic suggests that poorly-cemented sands, or sandstones, and silts, or siltstones, may undergo two failures: one at small strains where cement bonds are disrupted and the other at larger strains where internal shearing resistance of granular components is exceeded.     

基于三轴试验的黄土K0固结特性研究

静止土压力系数(K₀)是岩土工程中计算侧向土压力的关键参数,原状和重塑黄土的K₀系数在固结过程中的发展规律明显区别于黏土或砂土。为研究围压和初始孔隙比对原状和重塑黄土的K₀系数的影响,采用加装高精度局部径向位移计(LVDT)的应力路径三轴仪开展了K₀固结试验研究,结果表明:(1)重塑黄土的K₀系数受初始孔隙比(e₀)的影响较为明显,当试样的初始孔隙比减小(从0.775变为0.503),其K₀系数的最大降幅为55%;然而原状黄土的K₀系数受有效围压(σ₃′)的影响较为明显,比如在σ₃′ < 350kPa时,K₀系数为0.77~0.85,在σ₃′>350kPa时,K₀系数为0.46~0.51;(2)在固结过程中,原状黄土试样和重塑黄土试样的K₀系数均发生陡降,具体表现为重塑黄土密样(e₀<0.6)的陡降发生在固结的起始阶段,重塑黄土松样(e₀ ≈ 0.77)的陡降发生在有效围压接近100kPa,而原状黄土试样的陡降发生在有效围压为350kPa附近;(3)在相同的有效围压和密度条件下,原状黄土的K₀系数大于重塑黄土。采用核磁共振技术(NMR)获取了重塑和原状黄土的孔隙分布,并辅以试样的超孔隙水压力、排水体积等数据进行了讨论,发现结构性是造成黄土K₀系数差异的主要原因。     

Strain parameters of discontinuities in rock for finite element calculation

The bulk strain induced by excavation of rooms for storage of highly radioactive waste is usually unimportant, but the change in aperture and shear displacement of permeable fractures may be of practical importance. Hence, the normal and shear of such discontinuities have to be considered and are commonly predicted by using special deformation moduli termed “normal” and “shear” stiffnesses in numerical calculations. For the use of some finite element methods, these have to be converted to compression and shear moduli ,which, in turn, requires that the thickness of the discontinuity is known. The normal and shear strain can be expressed in terms of soil mechanical parameters, yielding the compression modulus M and the shear modulus G, with the required form for finite element calculations. By definition they are functions of the normal and shear stiffness K_n and K_s. It is concluded that calculation of the normal and shear strain of discontinuities with clastic fillings by the use of finite element methods and deformation moduli derived from stiffness numbers is very uncertain except when the geometry of the weaknesses can be accurately defined.     

Experimental studies on the effects of fracture on the P and S wave velocity propagation in sedimentary rock (“Calcarenite del Salento”)

Experimental studies were carried out in laboratory in order to investigate the effects of fracture on compressional (P) wave and shear (S) wave velocity propagation and therefore the relations between seismic properties and rock mass parameters. The discontinuity index, I_d, fracture density parameter C, linear fracture parameter Γ and the rock quality designation (RQD) were used to describe the rock mass parameters. These parameters are analyzed and then related to the seismic properties. Four vertical aligned fractures were created on an intact calcarenite block, 0.6 m long, 0.4 m thick and 0.4 m width, by sawing. The measures were carried out in four different blocks of cacarenite, having the same physical properties, and in four different phases: in first block the fractures were filled with air; in the second block the fractures were filled with “terra rossa”; in third block the fractures were filled with wet “terra rossa” and in the fourth block the fractures were filled with clay. The test results were statistically analysed using the method of least squares regression and polynomial relationships with high correlation coefficient were found between the fractured rock parameters and P-wave, S-wave velocities and V_p/V_s ratio. The investigations suggest that the P-wave and S-wave velocities decrease with increasing the fracture parameters, while the V_p/V_s ratio increases with decreasing the fracture parameters.

Furthermore the results of the experimental studies were applied on the seismic refraction tomography data acquired in a great measurements campaign undertaken in the Adriatic salentina coast (south Italy) in order to monitor the coastal erosion.

The geophysical results, using the polynomial relationships between the fractured rock parameters and P-wave velocity, are in good agreement with the geomorphological and geological results.     

靖远地区大厚度黄土地基浸水湿陷过程及土中竖向应力特征试验研究

为了研究靖远大厚度黄土在浸水条件下的水分入渗规律和自重湿陷变形特征,在中兰铁路沿线的靖远北站黄土自重湿陷场地进行了不打注水孔的现场浸水试验,监测并分析了地表及地下湿陷变形、试坑周围裂缝、含水率和土中竖向应力变化情况,对水分扩散规律、自重湿陷特性和土中竖向应力变化规律进行了研究,并对地区修正系数β0值和浸润角进行了探讨。结果表明：体积含水率变化分为浸水稳定（2个）、快速增加（1个）和缓慢增加（1个）共4个阶段;浸水过程中,水分在21m处竖向入渗加快、径向扩散减缓,湿润峰最终形态呈现为椭圆状。根据探井和钻孔含水率测试结果,推算出浸润角最大为41°。该场地黄土自重湿陷过程历经剧烈湿陷、缓慢湿陷和固结稳定3个阶段。试验结束时共计发展了13圈环状裂缝,裂缝最远处距试坑边缘26m。根据室内试验和现场测试结果,建议地区修正系数沿土层深度进行修正,0～10m内β0值取1.05,10～27 m内β0值取0.95。在地表至21 m深度范围内,地基土浸水饱和且湿陷充分,土中竖向应力沿深度呈线性增加,土中竖向应力接近饱和自重应力,21m以下的地基土未能充分湿陷,土中竖向应力逐渐减小。该研究成果可应用于中兰铁路...     

Evolution of metamorphic fluids in shear zones: The record from the emeralds of Habachtal, Tauern Window, Austria

Fluid inclusions in emeralds from the Habachtal, Central Tauern Window, have been studied by microthermometry. Results allow a detailed reconstruction of trapping history and evolution of the metamorphic fluids during the Middle Alpine Tauernkristallisation metamorphic event and some of the subsequent cooling period. Five different types of fluid inclusions, corresponding to at least five trapping periods, have been distinguished. In general, the earliest primary (type 1) inclusions, which occur as negative crystals or thin long tubes, are represented by low salinity ( < 10 wt. % NaCl equivalent) aqueous fluids with or without CO₂ with up to X_CO2 ≈ 0.04. Later primary type 2 inclusions are distinguished by different morphologies and distribution patterns. Lower salinity CO₂-free brines and CO₂-bearing denser inclusions with higher CO₂ contents (up to X_CO2 ≈ 0.11) are characteristic of this stage. The type 2 inclusions may also occur as pseudosecondary arrays. The effects of necking have been studied, and found to be considerable in the type 1 primary inclusions. This mechanism has occasionally resulted in the appearance of almost pure CO₂ fluids. The possibility of fluid immiscibility has been examined, and rejected, for the apparent “coexistence” of primary brine and CO₂-bearing inclusions. Instead, mixing of fluids which fluctuated between two different compositions is proposed. The fluctuation was probably due to the sequence of hydration reactions during the Tauernkristallisation. Maximum trapping pressures (3.6 kbar) obtained for stage 1 of the Tauernkristallisation are thought to represent a situation where sublithostatic fluid pressures exested in shear zones during the crystallisation period of many of the emerald cores and coexisting biotite and actinolite. Maximum fluid pressures of 7 kbar were obtained from the type 2 inclusions. This is similar to pressure estimates obtained from mineral equilibria. At least four phases of deformation are indicated by the trapping history. A pressure-temperature-time path for the Tauernkristallisation and the subsequent cooling/uplift period has been constructed for the Habachtal area, using the maximum pressure estimates obtained in this work together with previously existing data. In the cooling period, fluid pressures lower than the lithostatic load again prevailed. This difference, about 1–2 kbar, was probably due to late stage fracturing and/or the development of an open system. At least two more phases of minor deformation and three more stages of entrapment have been defined for this period. During this time, fluids gradually evolved towards more CO₂-poor, and less saline compositions. The present work shows that the possibility of fluctuations in fluid pressures must be considered seriously when attempting to define the PT cooling path from fluid inclusions in metamorphic rocks, especially those in shear zones. Postulations of retrograde PT paths based on fluid inclusions alone may result in pressure estimates which are too low.     

Characteristics of asbestiform and non-asbestiform calcic amphiboles

In terms of morphology there are four major types of calcic amphibole; massive, prismatic, finely acicular and asbestos. Representatives of each of these types have been examined by optical microscopy, X-ray diffraction, scanning and transmission electron microscopy, and electron probe microanalysis. Massive specimens (nephrite) consist of randomly oriented clusters of fine, roughly lath-shaped, sub-microscopic crystals; within each cluster the lath lengths (z) are approximately aligned but neighbouring laths are rotated with respect to one another. Finely acicular specimens (“byssolites”) have well-formed crystals bounded mainly by {110} (100) and (010) faces and characteristically have striations parallel to their lengths. Asbestiform varieties range from finer (flexible) to coarser (more brittle) specimens and many specimens contain a mixture of fine and coarse fibrils. The fibrils in a bundle are aligned parallel to z but are in a range of azimuthal orientations. It is inferred that they are formed by multiple independent nucleation and growth parallel to z rather than through parting or cleavage on {110} planes. (100) defect or twin planes, or on (010) planar defects.

The {110} cleavage in amphiboles is well reported but (100) features are rarely mentioned in the literature. Our observations reveal the importance of (100) as a cleavage or parting as well as the tendency in nephrites, byssolites and asbestos towards a lath-like (parallel to z) morphology with flattening on (100). In the latter varieties therefore, the y-direction is that of second fastest crystal growth, after z.

When subjected to moderate grinding, the comminution of asbestos fibres proceeds more by separation of fibrils and less by fracturing to shorter lengths as compared with prismatic and byssolite specimens. Prolonged grinding does, however, shorten lengths of even the least brittle asbestos.

Transmission electron microscopy revealed extensive sub-grain boundaries and dislocation networks (suggesting a deformation history) in all prismatic and nephrite specimens. Fine multiple (100) twinning was observed in asbestos but not in other varieties. Although chain-width defects [on (010)], with visibility enhanced by beam damage, were most abundant in nephrites and fibrous tremolites, there appears to be no completely consistent relationship between such features and morphological type.

Electron probe analyses showed that specimens that contain more than a very small amount of aluminium do not have asbestiform habit. Asbestos specimens also have lower contents of Mn, Na and K and have formulae closer to the ideal Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂. Small departures from this in asbestos involve Na in the A site compensated by Na for Ca rather than Al for Si whereas the reverse is true in byssolites. Chemical substitutions in prismatic specimens are much less constrained.

The characteristics of the four morphological sub-groups correlate reasonably well with what is known of their geological environments.     

Effects of temperature on strength and compressibility of sand-bentonite buffer

Dense sand-bentonite buffer (γ_d = 1.67Mg/m³) has been proposed in Canada as one of several barriers for isolating nuclear fuel waste. The buffer will be required to function under conditions of high total pressures and elevated temperatures approaching 100°C. Summary results are presented from two test programs: (1) isothermal consolidated undrained triaxial (CIU¯) tests; and (2) isothermal drained constant-p′ (CID) triaxial tests. Specimens were consolidated at effective stresses up to 9.0 MPa and temperatures up to 100°C.

The results indicate parallel hardening lines at systematically lower values of specific volume at elevated temperatures. In shear, increased temperatures produced lower values of maximum deviator stressq_f, and higher pore water pressure changesΔu_f. The net result is curved peak strength envelops in plots ofq_f versusp_f′ that are higher at elevated temperatures, even though the strengths,q_f of individual specimens are lower. The critical state strength envelope is curved inq, p′-planes.

The effect of drained heating on buffer to 100°C is not marked. Compressibilities, stiffnesses, strengths, and pore water pressure generation are all affected, but none of the changes are great.     

裂隙黄土边坡三维稳定性极限分析

黄土边坡中竖直裂隙的发育往往会对边坡稳定产生影响。相对于平面应变机制,建立三维破坏机制下边坡稳定性分析方法更能接近实际边坡失稳情况。基于塑性极限分析上限法,考虑预先存在竖直裂隙的三维黄土边坡不同破坏机制（坡面破坏、坡脚破坏和坡底破坏）,建立能量平衡方程及其无量纲临界高度值γH/c表达式,采用随机搜索法得到了临界高度的上限解。分析了约束宽度、边坡坡度、内摩擦角以及裂隙深度对三维竖直裂隙黄土边坡临界高度值的影响。结果表明：对于坡脚破坏机制,临界高度值随着裂隙深度的增加而减小,减小至临界裂隙深度 (δ /H)_min后,裂隙深度的增加不再影响临界高度值;临界裂隙深度随着坡度β 的增大而增大,随着内摩擦角φ 的增大而减小。当约束宽度B/H<0.8时,大多数破坏机制为坡面破坏。当约束宽度B/H=0.8、内摩擦角φ =10° 及约束宽度B/H=0.6、内摩擦角φ =15° 时,边坡的破坏从坡面破坏机制逐渐过渡到坡脚破坏机制。存在竖直裂隙的黄土边坡比完整边坡具有更小的临界高度,约束宽度及内摩擦角会对三维黄土边坡破坏机制产生影响。     

A constitutive thermomechanical model for saturated clays

The structural deformation in clays results from microscopic phenomena involving the mechanical contact-stress change, the physico-chemical variation of repulsive forces in expansive clays, and thermal dilatancy of macropores. These textural strains are associated to three plastic mechanisms represented by respectively the yield surfacesf_Tm, f_R-A andf_T. Under a thermal cycle, the sizes of interlamellar spaces between clay platelets are not modified, hence the temperature cycle is expected to have no effect on repulsive forces and thus the second mechanism is not affected by temperature changes.

This paper suggests a formulation of a model of thermo-elasto-plastic behaviour of non-expansive saturated clays characterised by two plastic mechanisms. The mechanical yield surfacef_Tm of the contact-stress mechanism is based on a modified cam-clay model; the thermal softening yield surfacef_T is a plane separating two thermal domains. In normally consolidated conditions, the resulting response to an increase of temperature is compressive. However, in highly overconsolidated conditions, a small irreversible dilative volumetric strain is observed when the temperature is above a threshold value. In intermediate conditions, the material starts with an expansion and tends to a compression.

The constitutive model combines thermo-mechanical hardening, predominant in normally consolidated states (NCS) and absent in overconsolidated states (OCS) where the thermal softening occurs. The characterisation of the model requires information about rheological parameters obtained from oedometric and triaxial paths. Lastly, some numerical simulations of thermo-mechanical tests onremoulded Boom, ‘Bassin Parisien’ andPontida clays are presented, which show satisfactory agreement between experiments and model predictions.     

Structures in the Canyon Mountain Ophiolite indicate an island-arc intrusion

Lithosphere created in an interarc basin is expected to be characterized by features distinguishing it from “normal” oceanic lithosphere. Apart from island-arc geochemical affinities and from the occurrence of hydrous high-T parageneses in the mantle and deep crustal sequences, it is expected that due to a low rate of spreading, vertical transport prevails over lateral drifting.

The Canyon Mountain complex located in an island-arc environment of Permo-Triassic age offers a remarkable illustration of these expected geological characteristics. In particular, mantle diapirism is deduced from the structural study. Smaller diapirs are formed in crustal formations. The intrusions took place at variable temperatures (1300°–800°C) and were accompanied by multistage melting in hydrous conditions.     

基于数字图像分析的土工合成材料加筋砂土拉拔试验研究

由于加筋土界面作用的复杂性,加筋土工程建设中铺设土工格栅时往往采用经验的方法,很大程度上造成了土工格栅的浪费及工程安全隐患,理清不同填料筋土界面作用的影响范围,有助于确定加筋土结构的合理加筋间距。为了揭示不同填料筋土界面作用的影响范围,采用4种不同类型的砂土与格栅在不同法向应力下进行了一系列的拉拔试验,并结合数字图像量测技术,分析了不同类型砂土下界面剪切带厚度、颗粒位移矢量、格栅拉拔阻力峰值及应变等演变规律。研究表明：界面剪切带厚度H随法向应力σ_v与砂土平均粒径d₅₀的增加而增大,通过多变量拟合的方法,得到了H、σ_v与d₅₀三者之间的函数表达式;格栅在拉拔过程中,砂土颗粒位移矢量以土工格栅为界有着显著的差别,格栅上部的颗粒位移矢量明显大于下部颗粒,且在格栅上下一定范围内会形成颗粒位移矢量集中带;拉拔阻力峰值随σ_v及d₅₀的增加而增大;不同类型砂土各区段的格栅应变均表现出由前向后依次递减的趋势。     

Seismic activity in the McNaughton Lake area, Canada

Filling of McNaughton Lake, with a capacity at full load of25 · 10⁹m³ and maximum depth 191 m, was initiated on March 29, 1973. An earthquake swarm of 747 events (M_L > 0) with largest eventM_L = 4.7 occurred within 17 km of the reservoir just prior to loading. Subsequent to this, three swarms of 194, 292 and 22 events with maximumM_L = 4.1 occurred in the same region; however, no earthquakes have occurred between the reservoir and the swarm area. The level of regional seismic activity is similar to that observed prior to loading. The distribution of this activity, excluding swarm events, exhibits a spatial pattern similar to that recorded earlier by regional seismic stations, except that several events appear to be associated with the fault underlying the Rocky Mountain Trench in which the lake is formed. During a loading-unloading cycle in which the maximum water depth varied from 98 m to 171 m to 131 m, the change ofv_p was less than 2%. This indicates that no significant change in dilatancy or degree of water saturation occurred in the upper crustal layer during this cycle.     

Experimental studies of sodic microperthites from the Loch Ailsh syenite

Solvus curves under both ‘dry’ and ‘hydrothermal’ (P_H2O = 1 kbar) conditions have been determined for two microperthitic alkali feldspar starting materials from the Loch Ailsh syenite complex, Assynt, Scotland. One sample (Ab₇₄ Or₂₄ An₂) possessed a K-phase showing dominant monoclinic symmetry to both powder and single-crystal X-ray methods, the other (Ab₈₈Or₃₀An₂) had maximum microcline as K-phase. These materials gave essentially identical solvi for both types of experiment, interpreted as indicating similar degrees of local order in the K-phase, but with differing domain size, leading to the different X-ray properties. The ‘dry’ solvus curves confirm the determination of the microcline-low albite solvus of Bachinski & Müller. The hydrothermal curves are used to estimate P_H1O <3.5 kbars, T > 755 °C for emplacement of the Loch Ailsh syenites.     

Crack damage stress as a composite function of porosity and elastic matrix stiffness in dolomites and limestones

Twenty-five uniaxial compression tests were performed to determine stress at onset of dilation, referred to herein as “the crack damage stress,” in heterogeneous dolomites and limestones. A simplified model for crack damage stress (σ_cd) is developed here using porosity, elastic modulus, Poisson's ratio and three empirical coefficients. The model shows that when porosity decreases and elastic modulus increases, σ_cd rapidly increases and approaches its maximum value. On the other hand, when porosity increases and elastic modulus decreases, σ_cd rapidly decreases and approaches its minimum value. The proposed model is validated for six heterogeneous limestone and dolomite formations which are widely distributed in Israel.     

Reactivated strike–slip faults: examples from north Cornwall, UK

Several strike–slip faults at Crackington Haven, UK show evidence of right-lateral movement with tip cracks and dilatational jogs, which have been reactivated by left-lateral strike–slip movement. Evidence for reactivation includes two slickenside striae on a single fault surface, two groups of tip cracks with different orientations and very low displacement gradients or negative (left-lateral) displacements at fault tips.

Evidence for the relative age of the two strike–slip movements is (1) the first formed tip cracks associated with right-lateral slip are deformed, whereas the tip cracks formed during left-lateral slip show no deformation; (2) some of the tip cracks associated with right-lateral movement show left-lateral reactivation; and (3) left-lateral displacement is commonly recorded at the tips of dominantly right-lateral faults.

The orientation of the tip cracks to the main fault is 30–70° clockwise for right-lateral slip, and 20–40° counter-clockwise for left-lateral slip. The structure formed by this process of strike–slip reactivation is termed a “tree structure” because it is similar to a tree with branches. The angular difference between these two groups of tip cracks could be interpreted as due to different stress distribution (e.g., transtensional/transpressional, near-field or far-field stress), different fracture modes or fractures utilizing pre-existing planes of weakness.

Most of the d–x profiles have similar patterns, which show low or negative displacement at the segment fault tips. Although the d–x profiles are complicated by fault segments and reactivation, they provide clear evidence for reactivation. Profiles that experienced two opposite slip movements show various shapes depending on the amount of displacement and the slip sequence. For a larger slip followed by a smaller slip with opposite sense, the profile would be expected to record very low or reverse displacement at fault tips due to late-stage tip propagation. Whereas for a smaller slip followed by larger slip with opposite sense, the d–x profile would be flatter with no reverse displacement at the tips. Reactivation also decreases the ratio of d_max/L since for an original right-lateral fault, left lateral reactivation will reduce the net displacement (d_max) along a fault and increase the fault length (L).

Finally we compare Crackington Haven faults with these in the Atacama system of northern Chile. The Salar Grande Fault (SGF) formed as a left-lateral fault with large displacement in its central region. Later right-lateral reactivation is preserved at the fault tips and at the smaller sub-parallel Cerro Chuculay Fault. These faults resemble those seen at Crackington Haven.     

The redox state of subduction zones: insights from arc-peridotites

Spinel peridotites from a variety of island arcs have been utilised to calculate the redox state of the mantle wedge above subduction zones. Oxygen fugacities (f_O2 values) calculated from the ferric iron content of spinels, measured by Electron Microprobe (EMP) using secondary standards [Wood, B.J., Virgo, D., 1989. Upper mantle oxidation state: ferric iron contents of lherzolite spinels by ⁵⁷Fe Mössbauer spectroscopy and resultant oxygen fugacities. Geochim. Cosmochim. Acta, 53, 1277–1291.], yield values which range from 0.3 to 2.0 above the fayalite–magnetite–quartz (FMQ) buffer. These data provide further evidence that the mantle wedge is ubiquitously oxidised relative to oceanic and ancient cratonic mantle. There is no correlation between f_O2 values and the presence of hydrous phases and, in fact, the most oxidised samples contain no hydrous phases. Within individual suites there is no correlation between f_O2 and degree of depletion as indicated by spinel Cr#, except for a suite of reacted forearc-peridotites. However, when the data is viewed as a whole there is broad a positive correlation between f_O2 and spinel Cr# suggesting that partial melting processes may influence the redox state of the mantle wedge. We suggest that the ultimate source of the oxygen which oxidises the mantle wedge is from the subducted slab. It is not clear whether this oxidising agent is a solute-rich hydrous fluid or a water-bearing silicate melt. However, our data does indicate that silicate melts are effective oxidisers of the depleted shallow upper mantle. Simple mass balance calculations based on the ferric iron content of primitive subduction zone magmas indicates that the source region must contain 0.6–1.0 wt.% Fe₂O₃. This amount of Fe₂O₃ in a fertile spinel peridotite yields an oxygen fugacity of 0.5–1.7 log units above FMQ in the IAB source. If water is the sole oxidising agent in the mantle wedge then 0.030–0.075 wt.% H₂O is required which is considerably less than the 0.25% H₂O envisaged by Stolper and Newman [Stolper, E.M., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett., 121, 293–325.], suggesting water is not necessarily an efficient oxidising agent. Alternatively, ferric iron may be added to the mantle wedge by addition of a ferric iron-rich sediment melt or more likely as a solute-rich hydrous fluid. This model would produce spinel, orthopyroxene or amphibole in the wedge with only a slight increase in f_O2 of the source region. Although it is unclear which model is correct the maximum f_O2 of the fertile mantle wedge is unlikely to be above FMQ+2 and therefore some decompression melting in the mantle wedge is required to explain the higher f_O2 values of primitive arc lavas than arc-peridotites.     

Spinel peridotite xenoliths from the Tariat Depression, Mongolia. II: Geochemistry and Nd and Sr isotopic composition and their implications for the evolution of the subcontinental lithosphere

A suite of spinel peridotite xenoliths from the Shavaryn-Tsaram volcano, Tariat Depression (central Mongolia) represents (for major elements) fertile to moderately depleted subcontinental lithosphere. Part of the variation of moderately incompatible trace elements is ascribed to small-scale mineralogical heterogeneities caused by processes like metamorphic differentiation accompanying partial melting or by mechanical segregation. Several bulk lherzolites show a high relative enrichment of the LREE over HREE which can be traced to a grain boundary phase genetically linked to, but not directly representing, the host basanitoid. In Nd and Sr isotopic composition the anhydrous peridotites cover the field of oceanic basalts (¹⁴³Nd/¹⁴⁴Nd = 0.5128-0.5133, ⁸⁷Sr/⁸⁶Sr = 0.7020-0.7039). In contrast, a phlogopite peridotite has a high ⁸⁷Sr/⁸⁶Sr and also a less radiogenic ¹⁴³Nd/¹⁴⁴Nd. The majority of “dry” lherzolites have Nd and Sr “bulk earth” model ages around 2 Ga. They may be interpreted as dating a small-degree (< ˜5%) melting event which would not have severely affected the major element chemistry of the xenoliths. The ˜2 Ga model ages may indicate a genetic relation between the lithospheric mantle and the stabilization of the continental crust in Mongolia at that time. Alternatively, if the peridotites are unrelated to the overlying crust, they may be pieces of a young asthenospheric diapir. Coexisting ortho-and clinopyroxenes are in Nd isotopic equilibrium for Iherzolites having equilibrated at temperatures around 950°C at mantle pressures. Disequilibrium melting models of mantle rocks are not supported by our data because for medium to coarse-grained mantle spinel peridotite the Rb-Sr and Sm-Nd isotopic systems close with respect to diffusional exchange at temperatures around 900°C, as indicated by recently published diffusion experiment results and supported by our data.