Chemical and textural equilibration of garnet during amphibolite facies metamorphism: The influence of coupled dissolution–reprecipitation

Metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low‐temperature amphibolite facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite‐isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. The 3D imaging of garnet porphyroblasts in staurolite‐bearing schists reveals a good crystal shape and little evidence of marginal dissolution; however, there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite‐rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion‐rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg‐rich, Mn‐poor composition and is interpreted to have formed during a coupled dissolution–reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite‐bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite‐producing reaction appears to be substantially overstepped during the relatively high‐pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution–reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re‐equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition, the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However, the partial re‐equilibration of the porphyroblasts during coupled dissolution–reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions.     

A numerical study of mechanical behavior of a cement paste under mechanical loading and chemical leaching

Based on relevant experimental data of a petroleum cement paste under mechanical loading and chemical leaching, an elastic‐plastic model is first proposed by taking into account plastic shearing and pore collapse. The degradation of mechanical properties induced by the chemical leaching is characterized by a chemical damage variable which is defined as the increase of porosity. Both elastic and plastic properties of the cement paste are affected by the chemical damage. The proposed model is calibrated from and applied to describe mechanical responses in triaxial compression tests respectively on sound and fully leached samples. In the second part, a phenomenological chemical model is defined to establish the relationship between porosity change and calcium dissolution process. The dissolution kinetics is governed by a diffusion law taking into account the variation of diffusion coefficient with calcium concentration. The chemical model is coupled with the mechanical model, and both are applied to describe mechanical response of cement paste samples subjected to progressive chemical leaching and compressive stresses. Comparisons between experimental data and numerical results are presented.     

Using monazite and zircon petrochronology to constrain the P–T–t evolution of the middle crust in the Bhutan Himalaya

The growth and dissolution behaviour of accessory phases (and especially those of geochronological interest) in metamorphosed pelites depends on, among others, the bulk composition, the prograde metamorphic evolution and the cooling path. Monazite and zircon are arguably the most commonly used geochronometers for dating felsic metamorphic rocks, yet crystal growth mechanisms as a function of rock composition, pressure and temperature are still incompletely understood. Ages of different growth zones in zircon and monazite in a garnet‐bearing anatectic metapelite from the Greater Himalayan Sequence in NW Bhutan were investigated via a combination of thermodynamic modelling, microtextural data and interpretation of trace‐element chemical ‘fingerprint’ indicators in order to link them to the metamorphic stage at which they crystallized. Differences in the trace‐element composition (HREE, Y, Eu_N/Eu*_N) of different phases were used to track the growth/dissolution of major (e.g. plagioclase, garnet) and accessory phases (e.g. monazite, zircon, xenotime, allanite). Taken together, these data constrain multiple pressure–temperature–time (P–T–t) points from low temperature (<550 °C) to upper amphibolite facies (partial melting, >700 °C) conditions. The results suggest that the metapelite experienced a cryptic early metamorphic stage at c. 38 Ma at <550 °C, ≥0.85 GPa during which plagioclase was probably absent. This was followed by a prolonged high‐T, medium‐pressure (~600 °C, 0.55 GPa) evolution at 35–29 Ma during which the garnet grew, and subsequent partial melting at >690 °C and >18 Ma. Our data confirm that both geochronometers can crystallize independently at different times along the same P–T path and that neither monazite nor zircon necessarily provides timing constraints on ‘peak’ metamorphism. Therefore, collecting monazite and zircon ages as well as major and trace‐element data from major and accessory phases in the same sample is essential for reconstructing the most coherent metamorphic P–T–t evolution and thus for robustly constraining the rates and timescales of metamorphic cycles.     

Dissolved organic matter in surface runoff in the Loess Plateau of China: The role of rainfall events and land-use

Exploring the chemical characterization of dissolved organic matter (DOM) is important for understanding the fate of laterally transported organic matter in watersheds. We hypothesized that differences in water-extractable organic matter (WEOM) in soils of varying land uses and rainfall events may significantly affect the quality and the quantity of stream DOM. To test our hypotheses, characteristics of rainfall-runoff DOM and WEOM of source materials (topsoil from different land uses and gullies, as well as typical vegetation) were investigated at two adjacent catchments in the Loess Plateau of China, using ultraviolet–visible absorbance and excitation emission matrix fluorescence with parallel factor analysis (PARAFAC). Results indicated that land-use types may significantly affect the chemical composition of soil WEOM, including its aromaticity, molecular weight, and degree of humification. The PARAFAC analysis demonstrated that the soils and stream water were dominated by terrestrial/allochthonous humic-like substances and microbial transformable humic-like fluorophores. Shifts in the fluorescence properties of stream DOM suggested a pronounced change in the relative proportion of allochthonous versus autochthonous material under different rainfall patterns and land uses. For example, high proportions of forestland could provide more allochthonous DOM input. This study highlights the relevance of soils and hydrological dynamics on the composition and fluxes of DOM issuing from watersheds. The composition of DOM in soils was influenced by land-use type. Precipitation patterns influenced the proportion of terrestrial versus microbial origins of DOM in surface runoff. Contributions of allochthonous, terrestrially derived DOM inputs were highest from forested landscapes.     

Controls on metamorphic equilibration: the importance of intergranular solubilities mediated by fluid composition

A dramatic demonstration of the role of intergranular solubility in promoting chemical equilibration during metamorphism is found in the unusual zoning of garnet in pelitic schist exposed at Harpswell Neck, Maine, USA. Many garnet crystals have irregular, patchy distributions of Mn, Cr, Fe and Mg in their inclusion‐rich interiors, transitioning to smooth, concentric zoning in their inclusion‐poor outer rims; in contrast, zoning of Ca and Y is comparatively smooth and concentric throughout. We re‐assess the disputed origin of these zoning features by examining garnet growth in the context of the thermal and structural history of the rocks, and by evaluating the record of fluid–rock interaction revealed in outcrop‐scale veining and fluid‐inclusion assemblages. The transition in the character of garnet zoning correlates with the onset of a synkinematic, simple‐shear‐dominated phase of garnet growth and with a shift in the composition of the intergranular fluid from CO₂‐rich to H₂O‐rich. Compositional variations in garnet are therefore best explained by a two‐stage growth history in which intergranular diffusive fluxes reflect differences in the concentration of dissolved species in these two contrasting fluids. Interiors of garnet crystals grew in the presence of a CO₂‐rich fluid, in which limited solubility for Mn and Cr (and perhaps Fe and Mg) produced patchy disequilibrium overprint zoning, while appreciable solubility for Ca and Y permitted their rock‐wide equilibration. Rims grew in the presence of an H₂O‐rich fluid, in which high intergranular concentrations for all elements except Cr enabled diffusion over length scales sufficient for rock‐wide equilibration. This striking example of partial chemical equilibrium during reaction and porphyroblast growth implies that thermal effects may commonly be subsidiary in importance to solubilities in the intergranular medium as determinants of length scales for metamorphic equilibration.     

Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage‐Colton Mylonite Zone,Adirondack Mountains,NY, USA

Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen‐bearing minerals and the dependence of mineral ¹⁸O/¹⁶O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10‐μm scale by secondary ion mass spectrometry. The patterns of δ¹⁸O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain‐size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage‐Colton Mylonite Zone (CCMZ), a long‐lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090–1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW‐dipping protomylonitic fabric (S₁) is crosscut by near‐vertical ultramylonitic shear zones (S₂), which are locally reoriented by a NNW‐dipping mylonitic fabric (S₃). Texturally early titanite (pre‐S₂) shows diffusion‐dominated δ¹⁸O zoning that records cooling from peak Ottawan, granulite‐facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed δ¹⁸O profiles for cooling rates of 50 ± 20 °C Ma^?1, which are considerably faster than the 1–5 °C Ma^?1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn‐S₂) has higher overall δ¹⁸O and shows a transition from diffusion‐dominated to recrystallization‐dominated δ¹⁸O zoning, indicating infiltration of elevated‐δ¹⁸O fluids along S₂ shear zones and continued shearing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The texturally latest titanite (post‐S₃) has growth‐dominated δ¹⁸O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.     

我国砂岩型铀矿分带特征研究现状及存在问题

作为一种重要的国家战略资源,砂岩型铀矿床是当今世界上最重要的铀矿床类型之一。本文详细地介绍了砂岩型铀矿在国内外的分布特征及占比情况,并对外生地质作用矿床类型中表生流体作用形成的层间渗透砂岩型和潜水渗透砂岩型铀矿床进行了讨论,发现层间渗透砂岩型铀矿床在外表颜色、矿物组合以及地球化学等方面均具有明显的氧化-还原分带现象,此外,矿床内部还具有细菌分带现象。颜色分带在氧化带、氧化-还原过渡带以及还原带之间具有明显不同的特征;矿物组合在不同分带之间各不相同;地球化学分带表现为U、TOC含量以及Fe~(2+)/Fe~(3+)、Th/U比值在各分带之间差异较大。此外,硫酸盐还原菌、硫杆菌、铁细菌及硝化菌等细菌在不同分带之间的数量相差悬殊,而且硫酸盐还原菌数量与TOC呈明显正相关性。通过矿化带内的碳、硫同位素分析,发现硫酸盐还原菌参与了成矿过程,推测其可能是导致碳、硫同位素分馏的主要因素。总体来看,颜色分带、矿物分带、地球化学分带以及细菌分带均与氧化-还原分带呈耦合关系。本文通过总结层间渗透砂岩型和潜水渗透砂岩型铀矿床的成矿模式和当前分带研究中存在的问题,提出了由细菌、地球化学反应参与的砂岩型铀矿床成矿机理,以及未来亟需解决的若干关键科学问题。典型砂岩型铀矿床的分带现象在物、化、探、遥等领域的异常响应对寻找砂岩型铀矿床具有重要的指导意义。     

河南萑香洼金矿地球化学分带模型研究

河南萑香洼金矿床作为典型的构造蚀变岩型金矿床,通过研究分析该矿床F985矿化带的Ⅰ号矿体各类元素组分在不同地质体的含量变化特征,原生晕异常分带特征和地球化学轴向分带特征,建立了该矿床完整的地球化学异常分带理想模型,得出矿床地球化学异常轴向分带序列确定为:烃类、Sb(前缘晕)→As、Hg(矿头)→(矿中晕)Au、Ag、Cu、Pb、Zn、W、Mn→(矿尾晕)Mo、Co、Ni、Sn,并总结出了找矿预测标志,为该矿区深部找矿提供了一定的科学理论依据。     

兴蒙造山带—华北克拉通地球化学走廊带沉积物主成分的含量与空间分布研究

选取兴蒙造山带—华北克拉通地球化学走廊带上沉积物的SiO_2、Al_2O_3、CaO、MgO、Na_2O、K_2O、TFe_2O_3、FeO、TiO_2、MnO、P_2O_5、CO_2、H_2O~+、pH十四个指标,研究其在不同二级构造单元、地理景观、土壤类型、降雨量等级下的含量与空间分布特征,并讨论该地球化学走廊带上化学蚀变指数。结果表明:在兴蒙造山带—华北克拉通地球化学走廊带上,作为沉积物主体的SiO_2、Al_2O_3两者含量为明显负相关而空间分布表现出此消彼长的特征;CaO、MgO、CO_2高含量分布区则与碳酸盐岩地层或含碳酸盐矿物的土壤有关,而MgO的高含量还与走廊带上镁铁质基性-超基性岩有一定关系,表明了这三个指标的高含量受特定岩性或矿物的影响较大;Na_2O、K_2O含量除了受继承的基岩影响外,还受到后期的风化作用和气候及自身地球化学性质等复杂因素综合影响,对Na_2O来说尤为如此;TFe_2O_3、MnO、TiO_2、P_2O_5相对于在地质背景复杂地段,经过冲积平原的沉积物混匀后,含量差异更小;相对干冷的内蒙古半干旱草原的气候条件有利于FeO存在,使得其相对燕山地区含量差异较TFe_2O_3要小,在章丘以南相对温暖湿润地区更易被氧化而导致其含量整体较低;北方干冷气候条件下,沉积物H_2O~+含量普遍较低,局部高含量位于碳酸盐岩地层或第四系分布区;走廊带上的pH值反映了沉积物偏碱性的特征,而在章丘以南的地区随着降雨量的增加,相对温暖湿润的气候条件,沉积物pH值表现为中性-偏弱酸性;兴蒙造山带—华北克拉通地球化学走廊带沉积物的CIA值反映出在寒冷、干燥气候条件下低等的化学风化程度,兴蒙造山带沉积物平均风化程度相对华北克拉通沉积物的平均风化程度差异变化相对要小,显示兴蒙造山带的降雨量与温度等风化影响因素变化较小。作为反映元素的地球化学亲和性的量化指标,离子电位可以更好地帮助理解沉积物元素的含量和空间分布差异。