A depleted, not ideally chondritic bulk Earth: The explosive-volcanic basalt loss hypothesis     

Paul H. Warren 《Geochimica et cosmochimica acta》2008,72(8):2217-2235

It has long been customary to assume that in the bulk composition of the Earth, all refractory-lithophile elements (including major oxides Al₂O₃ and CaO, all of the REE, and the heat-producing elements Th and U) occur in chondritic, bulk solar system, proportion to one another. Recently, however, Nd-isotopic studies (most notably Boyet M. and Carlson R. W. (2006) A new geochemical model for the Earth’s mantle inferred from ¹⁴⁶Sm-¹⁴²Nd systematics. Earth Planet. Sci. Lett.250, 254-268) have suggested that at least the outer portion of the planet features a Nd/Sm ratio depleted to ∼0.93 times the chondritic ratio. The primary reaction to this type of evidence has been to invoke a “hidden” reservoir of enriched matter, sequestered into the deepest mantle as a consequence of primordial differentiation. I propose a hypothesis that potentially explains the evidence for Nd/Sm depletion in a very different way. Among the handful of major types of differentiated asteroidal meteorites, two (ureilites and aubrites) are ultramafic restites so consistently devoid of plagioclase that meteoriticists were once mystified as to how all the complementary plagioclase-rich matter (basalt) was lost. The explanation appears to be basalt loss by graphite-fueled explosive volcanism on roughly 100-km sized planetesimals; with the dispersiveness of the process dramatically enhanced, relative to terrestrial experience, because the pyroclastic gases expand into vacuous space (Wilson L. and Keil K. (1991) Consequences of explosive eruptions on small Solar System bodies: the case of the missing basalts on the aubrite parent body. Earth Planet. Sci. Lett.104, 505-512). By analogy with lunar pyroclastic products, the typical size of pyroclastic melt/glass droplets under these circumstances will be roughly 0.1 mm. Once separated from an asteroidal or planetesimal gravitational field, droplets of this size will generally spiral toward the Sun, rather than reaccrete, because drag forces such the Poynting-Robertson effect quickly modify their orbits (the semimajor axis, in a typical scenario, is reduced by several hundred km during the first trip around the Sun). Assuming a similar process occurred on many of the Earth’s precursor planetesimals while they were still roughly 100 km in diameter, the net effect would be a depleted composition for the final Earth. I have modeled the process of trace-element depletion in the planetesimal mantles, assuming the partial melting was nonmodal and either batch or dynamic in terms of the melt-removal style. Assuming the process is moderately efficient, typical final-Earth Nd/Sm ratios are 0.93-0.96 times chondritic. Depletion is enhanced by a relatively low assumed residual porosity in batch-melting scenarios, but dampened by a relatively high value for “continuous” residue porosity in dynamic melting scenarios. Pigeonite in the source matter has a dampening effect on depletion. There are important side effects to the Nd/Sm depletion. The heat-producing elements, Th, U and K, might be severely depleted. The Eu/Eu^∗ ratio of the planet is unlikely to remain precisely chondritic. One of the most inevitable side effects, depletion of the Al/Ca ratio, is consistent with an otherwise puzzling aspect of the composition of the upper mantle. A perfectly undepleted composition for the bulk Earth is dubious.  相似文献   

Density functional theory predictions of equilibrium isotope fractionation of iron due to redox changes and organic complexation     

Shawn D. Domagal-Goldman  James D. Kubicki 《Geochimica et cosmochimica acta》2008,72(21):5201-5216

We present molecular orbital/density functional theory (MO/DFT) calculations that predict a greater isotopic fractionation in redox reactions than in reactions involving ligand exchange. The predicted fractionation factors, reported as 1000·ln(^56-54α), associated with equilibrium between Fe-organic and Fe-H₂O species were <1.6‰ in vacuo and <1.2‰ in solution when the oxidation state of the system was held constant. These fractionation factors were significantly smaller than those predicted for equilibrium between different oxidation states of Fe, for which 1000·ln(^56-54α) was >2.7‰ in vacuo and >2.2‰ in solution when the bound ligands were unchanged. The predicted ⁵⁶Fe/⁵⁴Fe ratio was greater in complexes containing Fe³⁺ and in complexes with shorter Fe-O bond lengths; both of these trends follow previous theoretical results. Our predictions also agree with previous experimental measurements that suggest that the largest biological fractionations will be associated with processes that change the oxidation state of Fe, and that identification of biologically controlled Fe isotope fractionation may be difficult when abiotic redox fractionations are present in the system. The models studied here also have important implications for future theoretical isotope calculations, because we have discovered the necessity of using vibrational frequencies instead of reduced masses when predicting reduced partition functions in aqueous-phase species.  相似文献   

Uranophane dissolution and growth in CaCl₂-SiO₂(aq) test solutions     

James D. Prikryl 《Geochimica et cosmochimica acta》2008,72(18):4508-4520

The dissolution and growth of uranophane [Ca(UO₂)₂(SiO₃OH)₂·5H₂O] have been examined in Ca- and Si-rich test solutions at low temperatures (20.5 ± 2.0 °C) and near-neutral pH (∼6.0). Uranium-bearing experimental solutions undersaturated and supersaturated with uranophane were prepared in matrices of ∼10⁻² M CaCl₂ and ∼10⁻³ M SiO₂(aq). The experimental solutions were reacted with synthetic uranophane and analyzed periodically over 10 weeks. Interpretation of the aqueous solution data permitted extraction of a solubility constant for the uranophane dissolution reaction and standard state Gibbs free energy of formation for uranophane ( kJ mol⁻¹).  相似文献   

Crystal-Melt Separation and the Development of Isotopic Heterogeneities in Hybrid Magmas     

Beard  James S. 《Journal of Petrology》2008,49(5):1027-1041

If a magma is a hybrid of two (or more) isotopically distinctend-members, at least one of which is partially crystalline,separation of melt and crystals after hybridization will leadto the development of isotopic heterogeneities in the magmaas long as some of the pre-existing crystalline material (antecrysts)retains any of its original isotopic composition. This holdstrue whether the hybridization event is magma mixing as traditionallyconstrued, bulk assimilation, or melt assimilation. Once a magma-scaleisotopic heterogeneity is formed by crystal–melt separation,it is essentially permanent, persisting regardless of subsequentcrystallization, mixing, or equilibration events. The magnitudeof the isotopic variability resulting from crystal–meltseparation can be as large as that resulting from differentialcontamination, multiple isotopically distinct sources, or insitu isotopic evolution. In one model, a redistribution of one-thirdof the antecryst cargo yielded a crystal-enriched sample with⁸⁷Sr/⁸⁶Sr of 0·7058, whereas the complementary crystal-poorsample has ⁸⁷Sr/⁸⁶Sr of 0·7068. In other models, crystal-richsamples are enriched in radiogenic Sr. Isotopic heterogeneitiescan be either continuous (controlled by the modal distributionof crystals and melt) or discontinuous (when there is completeseparation of crystals and liquid). The first case may be exemplifiedby some isotopically zoned large-volume rhyolites, formed bythe eruptive inversion of a modally zoned magma chamber. Inthe latter case, the isotopic composition of any (for example)interstitial liquid will be distinct from the isotopic compositionof the bulk crystal fraction. The separation of such an interstitialliquid may explain the presence of isotopically distinct late-stageaplites in plutons. Crystal–melt separation provides anadditional option for the interpretation of isotopically zonedor heterogeneous magmas. This option is particularly attractivefor systems whose chemical variation is otherwise explicableby fractionation-dominated processes. Non-isotopic chemicalheterogeneities can also develop in this fashion. KEY WORDS: isotopic heterogeneity; zoning; hybrid magma; crystal separation; Sr isotopes; aplite; rhyolite  相似文献   

The hydrochemistry of a semi-arid pan basin case study: Sua Pan,Makgadikgadi, Botswana     

Frank D. Eckardt  Robert G. Bryant  Graham McCulloch  Baruch Spiro  Warren W. Wood 《Applied Geochemistry》2008

This study presents results on the fluid and salt chemistry for the Makgadikgadi, a substantial continental basin in the semi-arid Kalahari. The aims of the study are to improve understanding of the hydrology of such a system and to identify the sources of the solutes and the controls on their cycling within pans. Sampling took place against the backdrop of unusually severe flooding as well as significant anthropogenic extraction of subsurface brines. This paper examines in particular the relationship between the chemistry of soil leachates, fresh stream water, salty lake water, surface salts and subsurface brines at Sua Pan, Botswana with the aim of improving the understanding of the system’s hydrology. Occasionally during the short wet season (December–March) surface water enters the saline environment and precipitates mostly calcite and halite, as well as dolomite and traces of other salts associated with the desiccation of the lake. The hypersaline subsurface brine (up to TDS 190,000 mg/L) is homogenous with minor variations due to pumping by BotAsh mine (Botswana Ash (Pty) Ltd.), which extracts 2400 m³ of brine/h from a depth of 38 m. Notable is the decrease in TDS as the pumping rate increases which may be indicative of subsurface recharge by less saline water. Isotope chemistry for Sr (⁸⁷Sr/⁸⁶Sr average 0.722087) and S (δ³⁴S average 34.35) suggests subsurface brines have been subject to a lithological contribution of undetermined origin. Recharge of the subsurface brine from surface water including the Nata River appears to be negligible.  相似文献   

Quantitative sinkhole hazard assessment. A case study from the Ebro Valley evaporite alluvial karst (NE Spain)     

Francisco Gutiérrez  Jesús Guerrero  Pedro Lucha 《Natural Hazards》2008,45(2):211-233

Quantitative sinkhole hazard assessments in karst areas allow calculation of the potential sinkhole risk and the performance of cost-benefit analyses. These estimations are of practical interest for planning, engineering, and insurance purposes. The sinkhole hazard assessments should include two components: the probability of occurrence of sinkholes (sinkholes/km² year) and the severity of the sinkholes, which mainly refers to the subsidence mechanisms (progressive passive bending or catastrophic collapse) and the size of the sinkholes at the time of formation; a critical engineering design parameter. This requires the compilation of an exhaustive database on recent sinkholes, including information on the: (1) location, (2) chronology (precise date or age range), (3) size, and (4) subsidence mechanisms and rate. This work presents a hazard assessment from an alluvial evaporite karst area (0.81 km²) located in the periphery of the city of Zaragoza (Ebro River valley, NE Spain). Five sinkholes and four locations with features attributable to karstic subsidence where identified in an initial investigation phase providing a preliminary probability of occurrence of 0.14 sinkholes/km² year (11.34% in annual probability). A trenching program conducted in a subsequent investigation phase allowed us to rule out the four probable sinkholes, reducing the probability of occurrence to 0.079 sinkholes/km² year (6.4% in annual probability). The information on the severity indicates that collapse sinkholes 10–15 m in diameter may occur in the area. A detailed study of the deposits and deformational structures exposed by trenching in one of the sinkholes allowed us to infer a modern collapse sinkhole approximately 12 m in diameter and with a vertical throw of 8 m. This collapse structure is superimposed on a subsidence sinkhole around 80 m across that records at least 1.7 m of synsedimentary subsidence. Trenching, in combination with dating techniques, is proposed as a useful methodology to elucidate the origin of depressions with uncertain diagnosis and to gather practical information with predictive utility about particular sinkholes in alluvial karst settings: precise location, subsidence mechanisms and magnitude, and timing and rate of the subsidence episodes.  相似文献   

Drought and institutional adaptation in the Great Plains of Alberta and Saskatchewan, 1914–1939     

Gregory P. Marchildon  Suren Kulshreshtha  Elaine Wheaton  Dave Sauchyn 《Natural Hazards》2008,45(3):391-411

Agriculture in the southern Great Plains of Canada has been particularly vulnerable to prolonged episodes of drought. Using climate data and a precipitation minus potential evapotranspiration index, the extent of the region’s exposure to drought is examined. Between 1914 and 1917, the Dry Belt was particularly vulnerable to drought, whereas after 1928, a much larger region known as the Palliser Triangle covering most of southern Alberta and Saskatchewan was much more exposed to drought. These droughts provoked major institutional adaptation, in particular the establishment of the Special Areas Board by the Government of Alberta, and the creation of the Prairie Farm Rehabilitation Administration by the Government of Canada. Both organizations have proved to be relatively permanent public adaptations to the natural hazard of drought in the region. Moreover, these earlier experiences with prolonged drought as well as institution-building may be of value in helping the residents of the Palliser Triangle adapt to predicted climate changes in the future as well as anticipate some of the barriers to effective institutional adaptation.  相似文献   

Tidal inundation mapping under enhanced land subsidence in Semarang,Central Java Indonesia   总被引：3，自引：0，他引：3  

Muh Aris Marfai  Lorenz King 《Natural Hazards》2008,44(1):93-109

Tidal inundation by high tide under enhanced land subsidence is a damaging phenomenon and a major threat to the Semarang urban area in Indonesia. It impacts on economic activities, as well as the cost of an emergency program and causes interruption of pubic services, danger of infectious diseases and injury to human lives. This study examines a spatial analysis tool on the GIS-raster system for the tidal inundation mapping based on the subsidence-benchmark data and modified detail digital elevation model. Neighborhood operation and iteration model as a spatial analysis tool have been applied in order to calculate the encroachment of the tidal inundation on the coastal area. The resulting map shows that the tidal flood spreads to the lowland area and causes the inundation of coastal settlement, infrastructure, as well as productive agricultural land, i.e., the fish-pond area. The monitoring of the vulnerable area due to the tidal inundation under the scenario of extended land subsidence plays an important role in long-term coastal zone management in Semarang.  相似文献   

Deep-seated gravitational slope deformations and their influence on consequent mass movements (case studies from the highest part of the Czech Carpathians)   总被引：3，自引：0，他引：3  

Jan Hradecký  Tomáš Pánek 《Natural Hazards》2008,45(2):235-253

Gravitational spreading of mountain ridges displays primary disequilibrium of flysch mountain areas of the Czech Carpathians. The progression of various types of mass movements is a product of long-term ridge disintegration and is predisposed by the geological structure of the area and the upper Tertiary-Quaternary morphogenesis of the mountain area. Deep-seated slope deformations are spatially interconnected by the occurrence of some other types of slope deformations (e.g. debris flows, debris slides, slumps, rock avalanches, etc.), which pose a considerable risk for the existence of human society. An important causative factor in these dynamically developing hazardous processes is, among other factors, the way in which land has been used in the last three centuries. Therefore, the occurrence of various types of slope deformations is studied in terms of their relation to deep-seated gravitational deformations and in terms of other limiting factors (structural geological, morphological and climatic factors, manmade impacts, etc.). The paper presents several case studies of slope deformations (Velká Čantoryje Mt, Lysá hora Mt, Ropice Mt and Smrk Mt) in the area of the Outer Carpathians within the territory of the Czech Republic and also adverts to some consequences in terms of the socioeconomic structure of the landscape.  相似文献   

The state of land subsidence and prediction approaches due to groundwater withdrawal in China   总被引：13，自引：6，他引：7  

Ye-Shuang Xu  Shui-Long Shen  Zheng-Yin Cai  Guo-Yun Zhou 《Natural Hazards》2008,45(1):123-135

This article gives a general introduction to land subsidence with the prediction approaches due to withdrawal of groundwater in three subsided/subsiding regions in China: the deltaic plain of Yangtse River (YRDP), North China Plain (NCP), and Fenwei Plain (FP). On YRDP, Shanghai is the typical subsided/subsiding city; on NCP Tianjin is the typical subsided/subsiding city, and on FP Taiyuan is the typical subsided/subsiding city. The subsided area with subsidence over 200 mm on YRDP is about 10,000 km² and the maximum subsided value reached 2.9 m at Shanghai; on NCP the subsided area reached 60,000 km² with the maximum subsidence of 3.9 m at Tianjing; on FP the subsided area is relatively smaller than that on the other two plains and is about 1,135 km² with maximum subsidence of 3.7 m at Taiyuan city. In order to protect the civil and industrial facilities, it is necessary to predict the future development of land subsidence based on present state. Many researchers proposed several approaches to predict the land subsidence due to groundwater withdrawal according to different geological conditions and groundwater withdrawal practice. This article classifies these approaches into five categories: (i) statistical methods; (ii) 1D numerical method; (iii) quasi-3D seepage model; (iv) 3D seepage model; (v) fully coupled 3D model. In China, the former four categories are presently employed in the prediction practice and their merits and demerits are discussed. According to the prediction practice, 3D seepage model is the best method presently.  相似文献