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291.
Albert Günter Herrmann Mark J. Potts Doris Knake 《Contributions to Mineralogy and Petrology》1974,44(1):1-16
Analytical data including major elements, the Rare Earths, Ni, Cu, Zn, Rb, and Sr are presented for twenty-three spilites from the Mid-Atlantic Ridge, the Hercynian part of the Variscan geosyncline in Germany, and several localities in Switzerland. Low grade metamorphism (up to approximately 400° C) and spilitization of basaltic rocks apparently do not alter the original Rare Earth element (REE) distributions. This fact permits comparison of the spilites and unaltered tholeiitic basalts from the Mid-Atlantic Ridge. The relative REE distributions thus appear suitable for delineating the original basalt types of spilites formed by metamorphism. The spilites from the Hercynian part of the Variscan geosyncline have a REE distribution pattern which is characteristic of continental tholeiites. It is thus probable that during the formation of this geosyncline the principal magma extruded was of tholeiitic composition and that these rocks were later converted to spilites by metamorphism. 相似文献
292.
Traceability in Geochemical Analysis 总被引:2,自引:0,他引:2
A review is presented of "traceability" and the way this concept can be demonstrated in the analysis of geochemical samples. It is suggested that the two practical methods are (i) by the analysis of appropriate certified reference materials with an evaluation of the agreement between analysed and certified values, and (ii) by the appropriate use of definitive, reference and standard methods of analysis. The differences between these two methods of demonstrating traceability are considered and the difficulties in applying them are discussed. 相似文献
293.
294.
Deborah S. Kelley Marvin D. Lilley John E. Lupton Eric J. Olson 《Deep Sea Research Part II: Topical Studies in Oceanography》1998,45(12):2665-2682
The rapid field response in March and April 1996 to diking-eruptive events on the Northern Gorda Ridge allowed the first detailed co-measurement of 3He, H2, and CH4 in plumes associated with a submarine eruption, and examination of the chemical transformation of these volatile species during aging of the plumes. The diking-eruptive event generated extensive event plumes with a low 3He/heat ratio of ∼0.4×10-12 cm3 STP gm-1 °C-1, while chronic plumes had 3He/heat ratios of 2.1–6.2×10-12 cm3 STP gm-1 °C-1. Hydrogen concentrations in the initial event plume (EP96A) reached a maximum value of 47 nM, 235 times that of ocean background concentration (0.2 nM). In contrast, roughly one month later H2 concentrations, although still elevated, had drastically dropped to 40 times background values. Methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments, reaching a maximum value of 7 nM. The intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high, localized concentrations of H2. The data indicate, however, that maximum rates of H2 production during event plume formation could have lasted a few days at most. 相似文献
295.
Late Quaternary terrestrial climate records from the semi-arid zone of the Western Cape of South Africa are rare. However, palaeoenvironmental information may be inferred from ancient termite mounds of the region. Calcrete lenses in these mounds have δ13C and δ18O values that show systematic changes with radiocarbon dates, which range from 33,629–36,709 to 21,676–23,256 cal yr BP. These dates confirm that these heuweltjies had been present in the landscape since the last glacial period. The decrease in δ13C and δ18O from 33,629–36,709 to 21,676–23,256 cal yr BP indicates that climate information is recorded by the calcretes. It is suggested that a progressive decline in air temperature and an increase in moisture availability, and a decline in abundance of C4 or CAM plants, occurred in the region during the time heuweltjie calcite precipitated. 相似文献
296.
297.
Peter C. Webb Philip J. Potts Michael Thompson Stephen A. Wilson Charles J. B. Gowing 《Geostandards and Geoanalytical Research》2019,43(3):397-408
Re‐issuing the same test material, its true identity unknown to participants, in two rounds of the GeoPT proficiency testing programme 18 years apart has demonstrated remarkable similarity, and therefore stability, of consensus values independently estimated for over fifty measurands in each round. A comparison of the two data sets, GeoPT5 (AMH‐1) from 1999 and GeoPT41 (ORA‐1) from 2017, shows that corresponding consensus values, rigorously derived by current procedures for complete compatibility, are for the most part statistically indistinguishable when account is taken of their associated uncertainties. Inferences that may be drawn from this exceptional agreement include: (a) the consensus estimation procedure was robust and gave consistent results; (b) overall, the balance of the contributed data was unaffected by significant changes in: (i) the populations of laboratories participating, (ii) the proportions of laboratories producing results by different techniques, (iii) the manner in which systems were calibrated, or (iv) the aspirations of laboratories to attain higher quality results over time; and (c) unsurprisingly perhaps, that the test material itself had remained stable and unchanged during that time. Additionally, a statistical comparison of the original values for AMH‐1 with consensus values derived by current procedures demonstrates that they are also effectively indistinguishable. 相似文献
298.
The original certification protocol, published by the International Association of Geoanalysts in 2003, specified that the competence of laboratories selected as competent to contribute certification measurements should be evaluated from their performance in the GeoPT proficiency testing programme. Round 39 of the IAG GeoPT proficiency testing programme provided an opportunity to examine four methods of evaluating laboratory competence based largely on the use of proficiency testing z‐scores as performance indicators. This opportunity arose because two test materials were co‐analysed by participating laboratories in this round: a syenite, SyMP‐1, supplied by the USGS, and an established CRM, the nepheline syenite, CGL 006. The performance of laboratories was assessed in four ways; in each case, consensus values and their uncertainties as derived from selective data sets of competent laboratories were compared with results derived from the routine GeoPT data assessment, involving all submitted measurements. An overall comparison of results showed no significant statistical differences in either consensus values or uncertainties between these data sets. This conclusion was unexpected and calls into question the widely held assumption that ‘better’ consensus data would be obtained from a subset of laboratories judged to be competent on the basis of proficiency testing performance indicators. 相似文献
299.
Jonathan A. Sandor Jay B. Norton Jeffrey A. Homburg Deborah A. Muenchrath Carleton S. White Stephen E. Williams Celeste I. Havener Peter D. Stahl 《Geoarchaeology》2007,22(3):359-386
Research on soil fertility is presented in the context of runoff agriculture, a venerable farming system that has been used for millennia in arid to semiarid regions, where water is a major limiting resource for crop production. The agroecology of runoff farming was studied with the Zuni to evaluate nutrient and hydrologic processes, management, maize productivity, and soil quality in some of the oldest recognized fields in the United States. This ancient Southwest agriculture has functioned without conventional irrigation or fertilization by tapping into biogeochemical processes in natural watersheds connected to fields. Carefully placed fields are managed on alluvial fans and other valley margin landforms to intercept runoff and associated sediment and organic debris transported from adjoining forested uplands. We report on research to evaluate and link nitrogen and phosphorus, two key nutrients for crop production, in watershed, soil, and crop components of this agroecosystem. Nutrient data have been collected by observational and experimental methods for each component and the transport of nutrients from watershed to field to maize. The condition of Zuni agricultural soils suggests that their knowledge and management of soils contributed to effective conservation. This study and others indicate the need for further long‐term monitoring and experimental research on watersheds, runoff processes, field soils, and crops across a range of arid to semiarid ecosystems. © 2007 Wiley Periodicals, Inc. 相似文献
300.
Experimental hydrothermal alteration of a martian analog basalt: Implications for martian meteorites
Leslie L. BAKER Deborah J. AGENBROAD Scott A. WOOD 《Meteoritics & planetary science》2000,35(1):31-38
Abstract— A number of martian meteorite samples contain secondary alteration minerals such as Ca‐Mg‐Fe carbonates, Fe oxides, and clay minerals. These mineral assemblages hint at hydrothermal processes occurring in the martian crust, but the alteration conditions are poorly constrained. This study presents the results of experiments that examined the alteration of a high‐Fe basalt by CO2‐saturated aqueous fluids at 23 and 75 °C and by mixed H2O‐CO2 vapors at 200 and 400 °C and water‐rock ratios of 1:1 and 1:10. Results indicate that observable alteration of the basalt takes place after runs of only seven days. This alteration includes mobilization of silica into phases such as opal‐CT and quartz, as well as the formation of carbonates, oxides, and at some conditions, zeolites and hydrous silicates. The degree of alteration increases with run temperature and, in high‐temperature vapor experiments, with increasing water content of the vapor. The degree of alteration and the mineralogy observed in the martian meteorites suggests that none of these samples were exposed to aqueous fluids for long periods of time. Nakhla and Lafayette probably interacted with water for relatively brief periods of time; if so, silica may have been leached from the parent rocks by the altering fluids. Allan Hills 84001 shows possible evidence for very limited interaction with an aqueous fluid, but the overall slight degree of alteration described for this meteorite strongly suggests that it never interacted extensively or at high temperature with any water‐bearing fluid. Elephant Moraine A79001 may not have been altered by aqueous fluids at all. The results of this study best support models wherein the meteorite parent rocks were wetted intermittently or for brief periods of time rather than models that invoke long‐term reaction with large volumes of water. Our experiments studied alteration of a high‐Fe basalt by dilute, CO2‐saturated, aqueous solutions at 23 and 75 °C and by mixed H2O‐CO2 vapors at 200 and 400 °C. The results suggest that alteration of the parent rock takes place even after very short reaction times of seven days. All experiments produced carbonate minerals, including calcite, and in some cases, magnesite, siderite, and ankerite. A free silica phase, either opal, quartz, or hydrated silica, formed in most experiments. More altered experiments also contained minerals such as zeolites and hydrous phyllosilicates. Clay minerals were not observed to form in any experiments. In aqueous fluids, higher temperature corresponded with a higher degree of alteration, whereas changing fluid composition had no observable effect. In high‐temperature vapors, the degree of alteration was controlled by temperature and the proportion of H2O to CO2, with water‐rock ratio also playing a role in transport of silica. Application of these results to martian meteorites that contain secondary alteration minerals suggests that none of the martian rocks underwent extensive interaction with aqueous fluids. Nakhla and Lafayette contain clay minerals, which suggests that they interacted with water to some extent, possibly at elevated temperatures. Although ALH84001 shows possible evidence of very limited interaction with aqueous fluids, EETA79001 does not. These results support models for the alteration of these meteorites that do not invoke long‐term interaction with water or reaction with large volumes of water. Except for some models for alteration of ALH84001, this conclusion agrees with most of the literature on alteration of martian meteorites. 相似文献