Nachtrag zu dem Aufsatz: Zur Morphologie der Flußläufe

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Sulphur uptake by salt grasses,mangroves, and seagrasses in anaerobic sediments

Stable sulfur isotope analyses show that rooted estuarine plants growing in anoxic sediments incorporate substantial amounts of ³⁴S-depleted sulfides, or oxidation products thereof. In roots, this incorporation predominates over sulfate uptake from interstitial porewater. Either the plants incorporate normally toxic sulfides, or they are creating and using a specialized nutrient pool of oxidized sulfides at the root-sediment interface.     

Pressure-induced incipient amorphization of α-quartz and transition to coesite in an eclogite from Antarctica: a first record and some consequences

Monocrystalline quartz inclusions in garnet and omphacite from various eclogite samples from the Lanterman Range (Northern Victoria Land, Antarctica) have been investigated by cathodoluminescence (CL), Raman spectroscopy and imaging, and in situ X‐ray (XR) microdiffraction using the synchrotron. A few inclusions, with a clear‐to‐opalescent lustre, show ‘anomalous’ Raman spectra characterized by weak α‐quartz modes, the broadening of the main α‐quartz peak at 465 cm^?1, and additional vibrations at 480–485, 520–523 and 608 cm^?1. CL and Raman imaging indicate that this ‘anomalous’α‐quartz occurs as relicts within ordinary α‐quartz, and that it was preserved in the internal parts of small quartz inclusions. XR diffraction circular patterns display irregular and broad α‐quartz spots, some of which show an anomalous d‐spacing tightening of ～2%. They also show some very weak, hazy clouds that have d‐spacing compatible with coesite but not with α‐quartz. Raman spectrometry and XR microdiffraction characterize the anomalies with respect to α‐quartz as (i) a pressure‐induced disordering and incipient amorphization, mainly revealed by the 480–485 and 608‐cm^?1 Raman bands, together with (ii) a lattice densification, evidenced by d‐spacing tightening; (iii) the cryptic development of coesite, 520–523 cm^?1 being the main Raman peak of coesite and (iv) Brazil micro‐twinning. This ‘anomalous’α‐quartz represents the first example of pressure‐induced incipient amorphization of a metastable phase in a crustal rock. This issue is really surprising because pressure‐induced amorphization of metastable α‐quartz, observed in impactites and known to occur between 15 and 32 GPa during ultrahigh‐pressure (UHP) experiments at room temperature, is in principle irrelevant under normal geological P–T conditions. A shock (due to a seism?) or a local overpressure at the inclusion scale (due to expansion mismatch between quartz and its host mineral) seem the only geological mechanisms that can produce such incipient amorphization in crustal rocks. This discovery throws new light on the modality of the quartz‐coesite transition and on the pressure regimes (non‐lithostatic v. lithostatic) during high‐pressure/UHP metamorphism. In particular, incipient amorphization of quartz could favour the quartz‐coesite transition, or allow the growth of metastable coesite, as already experimentally observed.     

Metamorphic evolution of a subduction complex, South Shetland Islands, Antarctica

A subduction complex composed of ocean floor material mixed with arc-derived metasediments crops out in the Elephant Island group and at Smith Island, South Shetland Islands, Antarctica, with metamorphic ages of 120–80 Ma and 58–47 Ma, respectively. Seven metamorphic zones (I–VII) mapped on Elephant Island delineate a gradual increase in metamorphic grade from the pumpellyite–actinolite facies, through the crossite–epidote blueschist facies, to the lower amphibolite facies. Geothermometry in garnet–amphibole and garnet–biotite pairs yields temperatures of about 350 °C in zone III to about 525 °C in zone VII. Pressures were estimated on the basis of Si content in white mica, Al₂O₃ content in alkali amphibole, Na^M4/Al^IV in sodic-calcic and calcic amphibole, Al^VI/Si in calcic amphibole, and jadeite content in clinopyroxene. Mean values vary from about 6–7.5 kbar in zone II to about 5 kbar in zone VII. Results from the other islands of the Elephant Island group are comparable to those from the main island; Smith Island yielded slightly higher pressures, up to 8 kbar, with temperatures estimated between 300 and 350 °C. Zoned minerals and other textural indications locally enable inference of P–T –t trajectories, all with a clockwise evolution. A reconstruction in space and time of these P–T –t paths allows an estimate of the thermal structure in the upper crust during the two ductile deformation phases (D₁ & D₂) that affected the area. This thermal structure is in good agreement with the one expected for a subduction zone. The arrival and collision of thickened oceanic crust may have caused the accretion and preservation of the subduction complex. In this model, D₁ represents the subduction movements expressed by the first vector of the clockwise P–T–t path, D₂ reflects the collision corresponding to the second vector with increasing temperature and decreasing pressure, and D₃ corresponds to isostatic uplift accompanied by erosion, under circumstances of decreasing temperature and pressure.     

Focal mechanism dependence of static stress triggering of earthquakes

We perform a global statistical investigation into the problem of Coulomb stress triggering of earthquakes by using the Harvard CMT catalogue of shallow earthquakes from 1976 to 2001. We consider ‘earthquake pairs’, i.e., pairs of successive earthquakes occurring near each other with similar focal mechanisms, and address the problem of whether or not the change of the Coulomb failure stress (CFS) produced by the first earthquake of the pair ‘encourages’ the occurrence of the second one. An interesting feature is that such a Coulomb stress-triggering phenomenon has an apparent focal mechanism dependence: for thrust earthquakes, a more evident triggering effect can be observed.     

构造物理化学的思路、研究和问题

构造物理化学是研究地壳物质受构造作用产生的物理和化学变化相互关联的领域。构造力可以分解为两部分一部分是均应力,指各向相等的应力,它叠加在原有压力之上,并且影响着各种化学反应的平衡,也是成岩、成矿和变质作用的影响因素。另一部分是差应力,固体中受外力作用普遍产生差应力,它引起地壳物质变形,产生各种构造形迹。构造物理化学特别关注构造作用产生或引起的压力、温度及其他的物理化学条件的变化,研究这些构造附加参量对各种化学平衡的影响,逐渐发展成为独立的学科研究领域。     

Metastability of granulites and processes of eclogitisation in the UHP region of western Norway

The Flatraket Complex, a granulite facies low strain enclave within the Western Gneiss Region, provides an excellent example of metastability of plagioclase‐bearing assemblages under eclogite facies conditions. Coesite eclogites are found <200 m structurally above and <1 km below the Flatraket Complex, and are separated from it by amphibolite facies gneisses related to pervasive late‐orogenic deformation and overprinting. Granulites within the Flatraket Complex equilibrated at 9–11 kbar, 700–800°C. These predate eclogite facies metamorphism and were preserved metastably in dry undeformed zones under eclogite facies conditions. Approximately 5% of the complex was transformed to eclogite in zones of fluid infiltration and deformation, which were focused along lithological contacts in the margin of the complex. Eclogitisation proceeded by domainal re‐equilibration and disequilibrium breakdown of plagioclase by predominantly hydration reactions. Both hydration and anhydrous plagioclase breakdown reactions were kinetically linked to input of fluid. More pervasive hydration of the complex occurred during exhumation, with fluid infiltration linked to dehydration of external gneisses. Eclogite facies shear zones within the complex equilibrated at 20–23 kbar, 650–800°C, consistent with the lack of coesite and with the equilibration conditions of external HP eclogites. If the complex experienced pressures equivalent to those of nearby coesite eclogites (> 28 kbar), unprecedented metastability of plagioclase and quartz is implied. Alternatively, a tectonic break exists between the Flatraket Complex and UHP eclogites, supporting the concept of a tectonic boundary to the UHP zone of the Western Gneiss Region. The distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability, and is a function of the protolith.     

新疆阿尔泰南缘乌吐布拉克铁矿成矿机制研究

乌吐布拉克中型铁矿床赋存于上志留统-下泥盆统康布铁堡组变质火山-沉积岩系中,矿体呈似层状、透镜状,矿体及其周围发育大量矽卡岩矿物组合。早期矽卡岩阶段包裹体均一温度为256~534℃,盐度为11.90%~＞73.96% NaCleqv,密度为0.56~0.96g/cm³,表明成矿流体为高-中温、高-中盐度、高-中密度的NaCl-H₂O体系;退化蚀变阶段包裹体均一温度为188~313℃,盐度为12.30%~＞39.76% NaCleqv,密度为0.83~1.05g/cm³,表明成矿流体为中温、中-低盐度、高-中密度的NaCl-H₂O体系。石英-硫化物-碳酸盐阶段包裹体均一温度为162~320℃,盐度为2.90%~15.57% NaCleqv,密度为0.70~1.02g/cm³,成矿流体为NaCl-H₂O-CO₂±CH₄或N₂型流体。石榴子石氢氧同位素表明早期矽卡岩阶段成矿流体主要来源于岩浆水,石英及方解石的氢氧同位素暗示石英-硫化物-碳酸盐阶段存在低温、低盐度的大气降水的加入。方解石的碳、氧同位素表明流体中碳主要来自深部岩浆。硫化物硫同位素表明硫来源于岩浆硫。成矿机制可能为早三叠世岩浆热液交代上志留-下泥盆统康布铁堡组火山岩形成矽卡岩矿物,在矽卡岩退化蚀变过程中形成铁矿体。