Seawater is constantly circulating through oceanic basement as a low-temperature hydrothermal fluid (<150°C). In cases when
ultramafic rocks are exposed to the fluids, for instance during the initial phase of subduction, ferromagnesian minerals are
altered in contact with the water, leading to high pH and formation of secondary magnesium hydroxide, among other – brucite,
that may scavenge borate and phosphate from seawater. The high pH may promote abiotic formation of pentoses, particularly
ribose. Pentoses are stabilized by borate, since cyclic pentoses form a less reactive complex with borate. Analyses have shown
that borate occupies the 2' and 3' positions of ribose, thus leaving the 5' position available for reactions like phosphorylation.
The purine coding elements (adenine, in particular) of RNA may be formed in the same general hydrothermal environments of
the seafloor. 相似文献
The waning stage(s) of the Tethyan ocean(s) in the Balkans are not well understood. Controversy centres on the origin and life‐span of the Cretaceous Sava Zone, which is allegedly a remnant of the last oceanic domain in the Balkan Peninsula, defining the youngest suture between Eurasia‐ and Adria‐derived plates. In order to investigate to what extent Late‐Cretaceous volcanism within the Sava Zone is consistent with this model we present new age data together with trace‐element and Sr–Nd–Pb isotope data for the Klepa basaltic lavas from the central Balkan Peninsula. Our new geochemical data show marked differences between the Cretaceous Klepa basalts (Sava Zone) and the rocks of other volcanic sequences from the Jurassic ophiolites of the Balkans. The Klepa basalts mostly have Sr–Nd–Pb isotopic and trace‐element signatures that resemble enriched within‐plate basalts substantially different from Jurassic ophiolite basalts with MORB, BAB and IAV affinities. Trace‐element modelling of the Klepa rocks indicates 2%–20% polybaric melting of a relatively homogeneously metasomatised mantle source that ranges in composition from garnet lherzolite to ilmenite+apatite bearing spinel–amphibole lherzolite. Thus, the residual mineralogy is characteristic of a continental rather than oceanic lithospheric mantle source, suggesting an intracontinental within‐plate origin for the Klepa basalts. Two alternative geodynamic models are internally consistent with our new findings: (1) if the Sava Zone represents remnants of the youngest Neotethyan Ocean, magmatism along this zone would be situated within the forearc region and triggered by ridge subduction; (2) if the Sava Zone delimits a diffuse tectonic boundary between Adria and Europe which had already collided in the Late Jurassic, the Klepa basalts together with a number of other magmatic centres represent volcanism related to transtensional tectonics. 相似文献
The Proterozoic rhyolitic volcanics constituting the foot-wall rocks in the Stollberg ore-field, Bergslagen, south-central Sweden, locally contain gedrite altered to chlorite and serpentine, biotite altered to chlorite and plagioclase altered to epidote.
The intergrowths between the host gedrite and the chlorite/serpentine inclusions are oriented with the a* of gedrite parallel to the c* of serpentine and chlorite. The biotite has been altered to chlorite by brucitization of both the K-interlayer and talc-like layer. In both cases the net change in volume during chloritization is small.
The assumption that Al is conserved during alteration of gedrite and biotite agrees very well with the micro-structures and orientation relations observed by transmission electron microscopy. Normalizing the chlorite to 1.00 mole, the overall chemical change that took place during the retrograde metamorphism of the Stollberg rocks can be written as: 0.84Ged+0.14Bio+0.65Mg+4.76H2O+0.42H=1.00Chl+0.57Alb+0.72Fe+0.01Na+0.12K+0.01Ti+0.05Mn+0.95H4SiO4 The reaction results in ca 9% increase in volume for the solid phases. Thus, a slightly acidic Mg-rich fluid started the reaction and, upon leaving the system, the metasomatic fluid was enriched in Na, Fe, K, and Si. 相似文献
Tephrochronological dating of postglacial volcanism in the Dyngjufjöll volcanic complex, a major spreading center in the Icelandic Rift Zone, indicates a high production rate in the millennia following deglaciation as compared to the present low productivity. The visible and implied evidence indicates that lava production in the period 10 000–4500 bp was at least 20 to 30 times higher than that in the period after 2900 bp but the results are biased towards lower values for lava volumes during the earlier age periods since multiple lava layers are buried beneath younger flows. The higher production rate during the earlier period coincides with the disappearance of glaciers of the last glaciation. Decreasing lithostatic pressure as the glacier melts and vigorous crustal movements caused by rapid isostatic rebound may trigger intense volcanism until a new pressure equilibrium has been established. 相似文献
The nature of the drainage system beneath ice sheets is crucial to their dynamic behaviour but remains problematic. An experimentally based theory of coupling between groundwater and major channel systems is applied to the esker systems in the area occupied the last ice sheet in Europe, which we regard as a fossil imprint of major longitudinal drainage channels. We conclude that the large-scale distribution and spacing of major eskers is consistent with the theory of groundwater control, in which esker spacing is partly controlled by the transmissivity of the bed.It is concluded that esker patterns reflect the large-scale organisation of the subglacial drainage pattern in which channel development is coupled to groundwater flow and to the ice sheet's dynamic regime. The theory is then used to deduce: basal meltwater recharge rates and their spatial variability from esker spacing in an area in which the ice sheet was actively streaming during its final retreat; patterns of palaeo-groundwater flow and head distribution; and the seasonally varying magnitude of discharge from stream tunnels at the retreating ice sheet margin. Major channel/esker systems appear to have been stable at least over several hundred of years during the retreat of the ice sheet, although major dynamic events are demonstrably associated with major shifts in the hydraulic regime.Modelling suggests: that glaciation can stimulate deep groundwater circulation cells that are spatially linked to channel locations, with groundwater flow predominantly transverse to ice flow; that the circulation pattern has the potential to create large-scale anomalies in groundwater chemistry; and that the spacing of channels will change through the glacial cycle, influencing water pressures in stream tunnels, subglacial hydraulic gradients and effective pressure. If the latter is reduced sufficiently, it could trigger enhanced bed deformation, thus coupling drainage to ice sheet movement. It suggests the possibility of distinctive phases of sediment deformation and drumlin mobilisation during a glacial cycle. 相似文献
This article presents a method for estimating chemical thermodynamic constants from experimental data using the two computer programs UCODE_2005 and PHREEQC. As an example, the conditional stability constant for lead (Pb) complexation by a remediation agent (carboxymethyl-beta-cyclodextrin) is estimated, but the method can be applied to estimate other thermodynamic parameters such as sorption constants and degradation rate constants. Advantages of this technique include estimation of uncertainties associated with estimated parameters, evaluation of information content of observations, statistical evaluation of the appropriateness of the conceptual model, and statistical-based comparison of different models. 相似文献
In this paper, a method of assessing water quality from satellite data is introduced. The composite pollution index (CPI) was calculated from measured chemical oxygen demand (COD) and nutrient concentration. The relationships between CPI and 240 band combinations of SeaWiFS water-leaving radiance were analyzed and the optimal band combination for estimating CPI was chosen from the 240 band combinations. An algorithm for retrieval of CPI was developed using the optimal band combination, (L443 × L510)/(L412 + L490). The CPI was estimated from atmospherically corrected SeaWiFS data by employing the algorithm. Furthermore, the CPI value range for each water quality level was determined based on data obtained from 850 samples taken in the Pearl River Estuary. The remotely sensed CPIs were then transferred to water quality levels and appropriate maps were derived. The remotely sensed water quality level maps displayed a similar distribution of levels based on in situ investigation issued by the State Ocean Administration, China. This study demonstrates that remote sensing can play an important role in water quality assessment. 相似文献