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The ultramafic-hosted Logatchev Hydrothermal Field (LHF) at 15°N on the Mid-Atlantic Ridge and the Arctic Gakkel Ridge (GR) feature carbonate precipitates (aragonite, calcite, and dolomite) in voids and fractures within different types of host rocks. We present chemical and Sr isotopic compositions of these different carbonates to examine the conditions that led to their formation. Our data reveal that different processes have led to the precipitation of carbonates in the various settings. Seawater-like 87Sr/86Sr ratios for aragonite in serpentinites (0.70909 to 0.70917) from the LHF are similar to those of aragonite from the GR (0.70912 to 0.70917) and indicate aragonite precipitation from seawater at ambient conditions at both sites. Aragonite veins in sulfide breccias from LHF also have seawater-like Sr isotope compositions (0.70909 to 0.70915), however, their rare earth element (REE) patterns show a clear positive europium (Eu) anomaly indicative of a small (< 1%) hydrothermal contribution. In contrast to aragonite, dolomite from the LHF has precipitated at much higher temperatures (~ 100 °C), and yet its 87Sr/86Sr ratios (0.70896 to 0.70907) are only slightly lower than those of aragonite. Even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine–talc fault schists form north of the LHF. These calcites show unradiogenic 87Sr/86Sr ratios (0.70460 to 0.70499) indicative of precipitation from evolved hydrothermal fluids. A simple mixing model based on Sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation. Dolomite precipitated from a mixture of 97% seawater and 3% hydrothermal fluid that should have had a temperature of approximately 14 °C assuming that no heat was transferred. The much higher apparent precipitation temperatures based on oxygen isotopes (~ 100 °C) may be indicative of conductive heating, probably of seawater prior to mixing. The hydrothermal calcite in the fault schist has precipitated from a mixture of 67% hydrothermal fluid and 33% seawater, which should have had an isenthalpic mixing temperature of ~ 250 °C. The significantly lower temperatures calculated from oxygen isotopes are likely due to conductive cooling of hydrothermal fluid discharging along faults. Rare earth element patterns corroborate the results of the mixing model, since the hydrothermal calcite, which formed from waters with the greatest hydrothermal contribution, has REE patterns that closely resemble those of vent fluids from the LHF. Our results demonstrate, for the first time, that (1) precipitation from pure seawater, (2) conductive heating of seawater, and (3) conductive cooling of hydrothermal fluids in the sub-seafloor all can lead to carbonate precipitation within a single ultramafic-hosted hydrothermal system. 相似文献
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《地学前缘(英文版)》2022,13(2):101324
The causes of the global mass extinction that occurred around the Permian–Triassic boundary have been widely studied through the geological record and in various locations. The results show that volcanic activity was a key factor in initiating the crisis during the Late Permian. Compared to other thermal events triggered by volcanic activity, pyrolysis of petroleum in Pre-Permian reservoirs has rarely been suggested as a significant source of the greenhouse gases that caused the mass extinction. In this study, geochemical analysis is carried out of a huge paleo-oil reservoir in the Yangtze Block (YB), South China. The detection of mineral inclusions and pyrobitumens is evidence of rapid pyrolysis of accumulated oil in the Ediacaran reservoir. New evidence from hydrothermal minerals and the presence of domain mesophase in the pyrobitumen suggest that the pyrolysis process occurred abruptly and that greenhouse gases were rapidly released through venting pipes. The dating of such a complex geological event in this old and deeply buried reservoir is inevitably difficult and potentially unreliable. However, cross-validation of the multiple evidence sources, including hydrothermal minerals and domain mesophase, indicates that the rapid oil pyrolysis must have been driven by a major thermal event. Reconstruction of burial and thermal histories suggests that the thermal event was most likely to have been triggered by the Emeishan Large Igneous Province (ELIP), which was in a period of significant volcanic activity during the Late Permian. Massive volumes of gases, including methane, carbon dioxide, and possibly hydrogen sulfide, were released, causing a significant increase in greenhouse gases that may have contributed to global warming and the resulting mass extinction during the Late Permian Crisis (LPC). 相似文献
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罗河铁矿床位于长江中下游铁铜金成矿带庐枞矿集区,已探明资源量约10亿吨,是成矿带内最大的铁矿床。矿床主成矿阶段广泛发育热液成因榍石,以榍石-硬石膏-磷灰石组合为特征,在矿床-800~-900m和-1500~-1600m两个不同深度稳定发育,为确定成矿时代和成矿流体演化提供了良好的研究对象。本次工作对罗河铁矿床深部和浅部两层矿体中热液榍石开展了年代学及地球化学研究,结果表明榍石中存在(Al,Fe)~(3+)+(F,OH)~-=Ti~(4+)+O~(2-)置换反应,并富含Zr、Nb和REE等元素。榍石中Zr含量估算矿床成矿温度约700℃,指示玢岩型矿床比矽卡岩型铁矿床具有更高的成矿温度,高温条件有利于Ti元素的迁移。榍石的稀土元素配分图均呈现出明显右倾,并具较高的稀土元素总量以及明显的负Eu异常特征,轻稀土富集表明其结晶早于磷灰石和绿帘石等矿物,形成于高温热液环境,不同深度榍石的Eu异常变化表明成矿流体的氧逸度从深部到浅部有所升高。热液榍石原位微区LA-ICPMS U-Pb定年结果表明,上、下两层榍石形成时代分别为130.0±0.9Ma和129.1±0.8Ma~129.7±0.8Ma,表明罗河铁矿床两层矿体是同一成矿系统的在不同深度的产物。矿床成矿时代与区内隐伏闪长岩体成岩时代一致,结合矿床地质特征研究,本文认为闪长玢岩是矿床的成矿母岩。罗河铁矿床属于燕山晚期岩浆热液活动的产物,形成于区域早白垩世岩石圈减薄的环境。 相似文献
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采用应力跌落的简化应力-应变模型考虑土的应变软化特性,同时采用简化的体积应变?v与大主应变?1及大主应变?1与小主应变?3之间的相互关系反映土的剪胀特性,根据空间准滑动面(SMP)理论和平面应变轴对称问题的柱形孔扩张基本方程,推导并给出一般黏性土中柱形孔扩张问题的应力场、应变场、位移场、塑性区半径和孔扩张压力。通过算例分析,探讨了土的剪胀因素、软化特性对孔扩张问题的影响程度。为了反映中主应力的影响,将本文解与基于Mohr-Coulomb破坏准则的解答进行了比较。计算结果表明,土的剪胀性和软化特性及中主应力对孔扩张问题的影响是显著的,基于Mohr-Coulomb破坏准则的孔扩张解答往往偏于保守。 相似文献
18.
Franz May Stephan Hoernes Horst J. Neugebauer 《International Journal of Earth Sciences》1996,85(4):782-799
Three major, interdependent processes control the genesis and distribution of mineral and thermal waters in the Rhenish Massif,
Central Europe: (a) Magmatic processes in the upper mantle provide most of the CO2 to produce bicarbonate waters in shallow aquifers. (b) Extension of the brittle upper crust enables the ascent of sodium
chloride waters. (c) Uplift and erosion shape the massif's relief, which determines the extent of flow systems and the distribution
of thermal springs. The chemistry of mineral waters further depends on the aquifers' mineral composition. A comprehensive
set of hydrological, chemical, tectonic and geophysical data on the Rhenish Massif has been compiled. It was used to classify
the mineral waters and to map the spatial distribution of water properties. The composition of cuttings from several representative
wells producing different water types shows that the hydrothermal alteration of the aquifer rocks consists mainly of kaolinization
of chlorite and dissolution of feldspar. Numerical transport simulations favour two modes of groundwater flow: topography-driven
flow and the pressure-driven ascent of basement brines along active faults. Thermal convection is less important. 相似文献
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