Ti site occupancy in zircon   总被引：1，自引：0，他引：1  

N.D. Tailby  A.M. Walker  J. Hermann  J.A. Mavrogenes  I.S. Rodina  D. Rubatto 《Geochimica et cosmochimica acta》2011,75(3):905-921

Ti site occupancy in zircon (ZrSiO₄) is fundamental to thermobarometry because substitution mechanisms control Ti content-temperature relations. Here we describe the results of three independent methods used to demonstrate that Ti substitutes for Si and not Zr in zircon. Zircon grains were synthesized from oxide powders held in a Na₂WO₄ flux at 1 bar and 1300 °C. Zircon grains equilibrated with rutile + cristobalite show Ti contents (1201 ppm) nearly half that of zircon grains equilibrated with srilankite ((Ti,Zr)O₂) + tetragonal zirconia (2640 ppm). The lower Ti content of zircon grains produced at silica-saturated conditions indicates that Ti substitution predominately occurs on the Si site. Moreover, the higher Ti contents of silica-saturated experiments at 1 bar (1201 ppm), relative to those at 1 GPa (457 ppm, Ferry and Watson, 2007), indicates a substantial pressure effect on Ti solubility in zircon. Measured Ti K-α edge X-ray Absorption Near Edge Structure (XANES) spectra of synthetic zircon grains show energies and normalized intensities akin to those seen among tetrahedrally coordinated Ti-bearing standard minerals, strongly suggesting that Ti occupies the Si site. Density functional theory (DFT) calculations confirm that Ti substitution is most likely to occur on the Si site and predict a Ti-O bond length of 1.797 Å (compared to an average of 2.160 Å for substitution on the Zr site), in excellent agreement with X-ray Absorption Fine Structure (EXAFS) spectra of experimentally grown zircon grains which indicate a value of 1.76(1) Å. The software FEFF 8.4 was used to simulate XANES spectra from the defect structures determined by DFT for Ti substituting on both the Si and Zr sites. The predicted spectrum for Ti on the Si site reproduces all the key features of the experimental zircon spectra, whereas Ti on the Zr site is markedly different. All applied methods confirm that Ti substitutes for Si in zircon. Consequently, the Ti content of zircon at a given pressure is not only a function of temperature, but will increase with decreasing silica activity. Because elements that activate or quench cathodoluminescence (CL) in zircon are incorporated into the Zr site, a decoupling of CL from Ti contents - incorporated on the Si site in zircon is expected. This hypothesis has been verified by a systematic CL-trace element study of natural and experimental zircon.  相似文献   

Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust   总被引：1，自引：0，他引：1  

Olivier Namur  Bernard Charlier  Jörg Hermann  Jacqueline Vander Auwera 《Geochimica et cosmochimica acta》2011,75(17):4998-5018

The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100-500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe-Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An₆₈-An₆₀), trace elements (Sr: 1023-1071 ppm; Ba: 132-172 ppm) and Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr_i: 0.70356-0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An₆₀-An₃₈). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm³ during early fractionation of troctolites and then decreased continuously to 2.16 g/cm³ with fractionation of Fe-Ti oxide-bearing gabbros. Plagioclase (An₆₉-An₆₀) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An₆₀) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid.The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust.  相似文献   

The electric field gradient in natural iron-doped chrysoberyl Al<Subscript>2</Subscript>BeO<Subscript>4</Subscript> and sinhalite MgAlBO<Subscript>4</Subscript> single crystals     

Werner Lottermoser  Günther J. Redhammer  Sven-Ulf Weber  Fred Jochen Litterst  Gerold Tippelt  Stephen Dlugosz  Hermann Bank  Georg Amthauer  Michael Grodzicki 《Physics and Chemistry of Minerals》2011,38(10):787-799

This work reports on the evaluation of the electric field gradient (EFG) in natural chrysoberyl Al₂BeO₄ and sinhalite MgAlBO₄ using two different procedures: (1) experimental, with single crystal Mössbauer spectroscopy (SCMBS) on the three principal sections of each sample and (2) a “fully quantitative” method with cluster molecular orbital calculations based on the density functional theory. Whereas the experimental and theoretical results for the EFG tensor are in quantitative agreement, the calculated isomer shifts and optical d–d-transitions exhibit systematic deviations from the measured values. These deviations indicate that the substitution of Al and Mg with iron should be accompanied by considerable local expansion of the coordination octahedra.  相似文献   

Land-use transition for bioenergy and climate stabilization: model comparison of drivers,impacts and interactions with other land use based mitigation options   总被引：1，自引：0，他引：1  

Alexander Popp  Steven K. Rose  Katherine Calvin  Detlef P. Van Vuuren  Jan Phillip Dietrich  Marshall Wise  Elke Stehfest  Florian Humpenöder  Page Kyle  Jasper Van Vliet  Nico Bauer  Hermann Lotze-Campen  David Klein  Elmar Kriegler 《Climatic change》2014,123(3-4):495-509

In this article, we evaluate and compare results from three integrated assessment models (GCAM, IMAGE, and ReMIND/MAgPIE) regarding the drivers and impacts of bioenergy production on the global land system. The considered model frameworks employ linked energy, economy, climate and land use modules. By the help of these linkages the direct competition of bioenergy with other energy technology options for greenhouse gas (GHG) mitigation, based on economic costs and GHG emissions from bioenergy production, has been taken into account. Our results indicate that dedicated bioenergy crops and biomass residues form a potentially important and cost-effective input into the energy system. At the same time, however, the results differ strongly in terms of deployment rates, feedstock composition and land-use and greenhouse gas implications. The current paper adds to earlier work by specific looking into model differences with respect to the land-use component that could contribute to the noted differences in results, including land cover allocation, land use constraints, energy crop yields, and non-bioenergy land mitigation options modeled. In scenarios without climate change mitigation, bioenergy cropland represents 10–18 % of total cropland by 2100 across the different models, and boosts cropland expansion at the expense of carbon richer ecosystems. Therefore, associated emissions from land-use change and agricultural intensification as a result of bio-energy use range from 14 and 113 Gt CO2-eq cumulatively through 2100. Under climate policy, bioenergy cropland increases to 24–36 % of total cropland by 2100.  相似文献   

The value of bioenergy in low stabilization scenarios: an assessment using REMIND-MAgPIE     

David Klein  Gunnar Luderer  Elmar Kriegler  Jessica Strefler  Nico Bauer  Marian Leimbach  Alexander Popp  Jan Philipp Dietrich  Florian Humpenöder  Hermann Lotze-Campen  Ottmar Edenhofer 《Climatic change》2014,123(3-4):705-718

This study investigates the use of bioenergy for achieving stringent climate stabilization targets and it analyzes the economic drivers behind the choice of bioenergy technologies. We apply the integrated assessment framework REMIND-MAgPIE to show that bioenergy, particularly if combined with carbon capture and storage (CCS) is a crucial mitigation option with high deployment levels and high technology value. If CCS is available, bioenergy is exclusively used with CCS. We find that the ability of bioenergy to provide negative emissions gives rise to a strong nexus between biomass prices and carbon prices. Ambitious climate policy could result in bioenergy prices of 70 $/GJ (or even 430 $/GJ if bioenergy potential is limited to 100 EJ/year), which indicates a strong demand for bioenergy. For low stabilization scenarios with BECCS availability, we find that the carbon value of biomass tends to exceed its pure energy value. Therefore, the driving factor behind investments into bioenergy conversion capacities for electricity and hydrogen production are the revenues generated from negative emissions, rather than from energy production. However, in REMIND modern bioenergy is predominantly used to produce low-carbon fuels, since the transport sector has significantly fewer low-carbon alternatives to biofuels than the power sector. Since negative emissions increase the amount of permissible emissions from fossil fuels, given a climate target, bioenergy acts as a complement to fossils rather than a substitute. This makes the short-term and long-term deployment of fossil fuels dependent on the long-term availability of BECCS.  相似文献   

New findings concerning the Pleistocene Glaciation of the Leh Basin,Ladakh (34°03′ N/77°38′ E)     

Hermann Achenbach 《山地科学学报》2010,7(4):367-374

Like for most parts of High Asia, researches concerning the Pleistocene landscape evolution of the Leh Basin (34°03′ N/77°38′ E) have also left contradictions. To push this topic, three up to now unexplored Ladakh Range tributaries of the Leh Basin (Stagmo-, Arzu- and Nang-Valley) have been investigated. U-shaped profiles, transfluence passes, moraine mantled and glacially rounded peaks and ridges, roches moutonnées, glacial flank polishings and ground moraines document the former glaciation of the study area. The ice fillings of these tributaries reached a minimum thickness up to 540 m. Even at the valley outlets and on the orographic right side of the Leh Basin, the glaciation was more than 350 m thick. Based on these empirically extracted results, theoretical snow line considerations lead to the conclusion that the whole Leh Basin was filled up by a former Indus-Valley glacier. An ice injection limited to the nourishment areas of the Ladakh Range valleys could not have caused the reconstructed ice cover (down to 3236 m a.s.l.), which is proved by extended ground moraine complexes. Only an Indus ice stream network (most likely during the LGP), nourished by inflowing glaciers of the Ladakh- and Stok Range, explains the widespread existence of the glacial sediments at the outlets of the investigated valleys.  相似文献   

Plasma density undulations in the nighttime mid-latitude F-region as observed by CHAMP,KOMPSAT-1, and DMSP F15   总被引：1，自引：0，他引：1  

Jaeheung Park  Hermann Lühr  Kyoung Wook Min  Jae-Jin Lee 《Journal of Atmospheric and Solar》2010,72(2-3):183-192

We investigate plasma density undulations in the nighttime mid-latitude topside F-region. During solar maximum years the undulations are found at CHAMP, KOMPSAT-1, and DMSP F15 altitudes. The occurrence rate is higher at KOMPSAT-1 than at DMSP F15 altitude. The undulations occur infrequently during equinoxes, and the occurrence peaks are in the Asian/Oceanian (eastern Pacific/American) region during June (December) solstice. At CHAMP altitude the undulations are observed all through the night, and the occurrence rate is anti-correlated with the solar cycle. As all these results are in general agreement with known climatology of MSTIDs, we suggest that the undulations are a topside signature of MSTIDs. The undulations are often but not always accompanied by magnetic signatures indicating the presence of field-aligned current (FAC). The partial lack in correspondence might be due to the ionospheric conductivity variation. The similar distribution is, however, in support of a connection between density undulations and FACs.  相似文献   

Introduction to “Global Tsunami Science: Past and Future,Volume I”     

Eric L. Geist  Hermann M. Fritz  Alexander B. Rabinovich  Yuichiro Tanioka 《Pure and Applied Geophysics》2016,173(12):3663-3669

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Field test of sub‐basalt hydrocarbon exploration with marine controlled source electromagnetic and magnetotelluric data          下载免费PDF全文

David Myer  David Alumbaugh  Oliver Hermann  Randall Hobbet 《Geophysical Prospecting》2015,63(5):1284-1310

The recent use of marine electromagnetic technology for exploration geophysics has primarily focused on applying the controlled source electromagnetic method for hydrocarbon mapping. However, this technology also has potential for structural mapping applications, particularly when the relative higher frequency controlled source electromagnetic data are combined with the lower frequencies of naturally occurring magnetotelluric data. This paper reports on an extensive test using data from 84 marine controlled source electromagnetic and magnetotelluric stations for imaging volcanic sections and underlying sediments on a 128‐km‐long profile. The profile extends across the trough between the Faroe and Shetland Islands in the North Sea. Here, we focus on how 2.5D inversion can best recover the volcanic and sedimentary sections. A synthetic test carried out with 3D anisotropic model responses shows that vertically transverse isotropy 2.5D inversion using controlled source electromagnetic and magnetotelluric data provides the most accurate prediction of the resistivity in both volcanic and sedimentary sections. We find the 2.5D inversion works well despite moderate 3D structure in the synthetic model. Triaxial inversion using the combination of controlled source electromagnetic and magnetotelluric data provided a constant resistivity contour that most closely matched the true base of the volcanic flows. For the field survey data, triaxial inversion of controlled source electromagnetic and magnetotelluric data provides the best overall tie to well logs with vertically transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data a close second. Vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data provided the best interpreted base of the volcanic horizon when compared with our best seismic interpretation. The structural boundaries estimated by the 20‐Ω·m contour of the vertical resistivity obtained by vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data gives a maximum geometric location error of 11% with a mean error of 1.2% compared with the interpreted base of the volcanic horizon. Both the model study and field data interpretation indicate that marine electromagnetic technology has the potential to discriminate between low‐resistivity prospective siliciclastic sediments and higher resistivity non‐prospective volcaniclastic sediments beneath the volcanic section.  相似文献   

Source Models and Near-Field Impact of the 1 April 2007 Solomon Islands Tsunami     

Yong Wei  Hermann M. Fritz  Vasily V. Titov  Burak Uslu  Chris Chamberlin  Nikos Kalligeris 《Pure and Applied Geophysics》2015,172(3-4):657-682

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