Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

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.     

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

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.     

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

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.     

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

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.     

Plasma density undulations in the nighttime mid-latitude F-region as observed by CHAMP,KOMPSAT-1, and DMSP F15

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.     

Field test of sub‐basalt hydrocarbon exploration with marine controlled source electromagnetic and magnetotelluric data

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.     

Fault imaging in sparsely sampled 3D seismic data using common‐reflection‐surface processing and attribute analysis – a study in the Upper Rhine Graben

Cost reduction in seismic reconnaissance is an issue in geothermal exploration and can principally be achieved by sparse acquisition. To address the adherent decrease in signal/noise ratio, the common‐reflection‐surface method has been proposed. We reduced the data density of an existing 3D dataset and evaluated the results of common‐reflection‐surface processing using seismic attributes. The application of the common‐reflection‐surface method leads in all cases to an improvement of the signal/noise ratio. The most distinct improvement can be seen in the low fold regions. The improvement depends strongly on the midpoint aperture, and there is a tradeoff between reflector continuity and horizontal resolution. If small scale targets are to be imaged, a small aperture size is necessary, which may be far below the Fresnel zone for a specific reflector. The substantial reduction of the data density leads in our case to an irrecoverable information loss.     

Protracted fluid-induced melting during Barrovian metamorphism in the Central Alps

The timing and dynamics of fluid-induced melting in the typical Barrovian sequence of the Central Alps has been investigated using zircon chronology and trace element composition. Multiple zircon domains in leucosomes and country rocks yield U–Pb ages spanning from ~32 to 22 Ma. The zircon formed during Alpine melting can be distinguished from the inherited and detrital cores on the basis of their age, Th/U (<0.1) and trace element composition. Ti-in-zircon thermometry indicates crystallization temperatures around 620–700°C. Their composition allows discriminating between (1) zircon formation in the presence of early garnet, (2) zircon in equilibrium with abundant L-MREE-rich accessory phases (allanite, titanite and apatite) typical of metatonalites, and (3) zircon formed during melting of metasediments in feldspar-dominated assemblages. The distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single Barrovian metamorphic cycle at roughly constant temperature; that in the country rocks zircon formation was limited to the initial stages of melting, whereas further melting concentrated in the segregated leucosomes; that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids; and that leucosomes were repeatedly melted when fluids became available. The geochronological data force a revision of the temperature–time path of the migmatite belt in the Central Alps. Protracted melting over 10 My followed the fast exhumation of Alpine eclogites contained within the same region and preceded fast cooling in the order of 100°C/Ma to upper crustal levels.