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1.
Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO2 inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6 GPa and 900–1500 °C. We found that just below solidus K2O enters potassium feldspar or K2TiSi3O9 wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K0.90Na0.10)2CO3·60Ca0.55Mg0.24Fe0.21CO3 + 1.9 mol% SiO2 + 0.7 mol% TiO2 + 1.4 mol% Al2O3 at the solidus established near 1000 °C. Molecular CO2 liberates at 1100 °C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200 °C. This melt contains (mol/wt%): SiO2 = 57.0/52.4, TiO2 = 1.8/2.3, Al2O3 = 8.5/13.0, FeO = 1.4/1.6, MgO = 1.9/1.2, CaO = 3.8/3.2, Na2O = 3.2/3.0, K2O = 10.5/15.2, CO2 = 12.0/8.0, while carbonatite melt can be approximated as 24(K0.81Na0.19)2CO3·76Ca0.59Mg0.21Fe0.20CO3 + 3.0 mol% SiO2 + 1.6 mol% TiO2 + 1.4 mol% Al2O3. Both melts remain stable to at least 1500 °C coexisting with CO2 fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4–1.5 wt% K2O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali‑carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180–200 km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation. 相似文献
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Costijn Zwart Niels C. Munksgaard Alain Protat Naoyuki Kurita Dionisia Lambrinidis Michael I. Bird 《水文研究》2018,32(15):2296-2303
This work provides a comprehensive physically based framework for the interpretation of the north Australian rainfall stable isotope record (δ18O and δ2H). Until now, interpretations mainly relied on statistical relationships between rainfall amount and isotopic values on monthly timescales. Here, we use multiseason daily rainfall stable isotope and high resolution (10 min) ground‐based C‐band polarimetric radar data and show that the five weather types (monsoon regimes) that constitute the Australian wet season each have a characteristic isotope ratio. The data suggest that this is not only due to changes in regional rainfall amount during these regimes but, more importantly, is due to different rain and cloud types that are associated with the large scale circulation regimes. Negative (positive) isotope anomalies occurred when stratiform rainfall fractions were large (small) and the horizontal extent of raining areas were largest (smallest). Intense, yet isolated, convective conditions were associated with enriched isotope values whereas more depleted isotope values were observed when convection was widespread but less intense. This means that isotopic proxy records may record the frequency of which these typical wet season regimes occur. Positive anomalies in paleoclimatic records are most likely associated with periods where continental convection dominates and convection is sea‐breeze forced. Negative anomalies may be interpreted as periods when the monsoon trough is active, convection is of the oceanic type, less electric, and stratiform areas are wide spread. This connection between variability of rainfall isotope anomalies and the intrinsic properties of convection and its large‐scale environment has important implications for all fields of research that use rainfall stable isotopes. 相似文献
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Takuya Matsumoto Masahiko Honda Ian McDougall Igor Yatsevich Suzanne Y. O'Reilly 《地学学报》2004,16(1):23-26
Using a Rayleigh distillation fractionation model, we calculate that the maximum isotope fractionation potentially achievable is less than 5% during the early stages of gas release from a sample. Our calculation corrects the erroneous conclusions of Gautheron and Moreira (2003), who re‐interpreted the plume‐like neon isotopic compositions found in metasomatic apatite from a south‐eastern Australian xenolith (Matsumoto et al., 1997) to be the result of Rayleigh‐type isotope fractionation of originally MORB‐type neon during stepheating gas extraction. We stress that the modelling of neon isotopic fractionation by Gautheron and Moreira (2003) is incorrect, and that the finding of a plume‐like neon isotopic composition in the apatite by Matsumoto et al. (1997) remains a quite valid and robust conclusion. 相似文献
8.
Veined lithologies are formed by fracturing and sealing processes, with the veins representing former fluid conduits through the rock. Although detailed fieldwork and numerical simulations have provided a better understanding of vein growth, few studies have attempted to seal fractures and generate veins experimentally. In this pilot study, we subjected fractured quartzite to temperature gradients of 45–125 °C under hydrothermal conditions in a static fluid, with the aim of precipitating secondary quartz in the cooler portions of the fracture. Results show that secondary quartz precipitates due to the imposed temperature gradient, causing the initial fracture to seal locally. Although no systematic sealing pattern was observed along the fracture, samples subjected to higher temperatures exhibit a smaller fracture width and appear to have reacted more extensively. Electron microprobe mapping visualizes the spatial distribution of secondary quartz, which contains elevated concentrations of aluminium. 相似文献
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《Coastal Engineering》2006,53(7):589-611
The results of a series of 2DH numerical and 3D scaled physical modelling tests indicate that processes governing shoreline response to submerged structures, such as artificial surfing reefs, are different from those associated with emergent offshore breakwaters. Unlike the case of emergent offshore breakwaters, where shoreline accretion (salient development) is expected under all structural/environmental conditions, the principal mode of shoreline response to submerged structures can vary between erosive and accretive, depending on the offshore distance to the structure. The predominant wave incidence angle and structure crest level also have important implications on the magnitude of shoreline response, but not on the mode of shoreline response (i.e. erosion vs. accretion). Based on the results obtained here, a predictive empirical relationship is proposed as a preliminary engineering tool to assess shoreline response to submerged structures. 相似文献
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