Leibniz University of Hanover

About: Leibniz University of Hanover is a education organization based out in Hanover, Niedersachsen, Germany. It is known for research contribution in the topics: Finite element method & Population. The organization has 14283 authors who have published 29845 publications receiving 682152 citations.

Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle

Abstract: The Earth’s uppermost asthenosphere is generally associated with low seismic wave velocity and high electrical conductivity. The electrical conductivity anomalies observed from magnetotelluric studies have been attributed to the hydration of mantle minerals, traces of carbonatite melt, or silicate melts. We report the electrical conductivity of both H2O-bearing (0–6 wt% H2O) and CO2-bearing (0.5 wt% CO2) basaltic melts at 2 GPa and 1,473–1,923 K measured using impedance spectroscopy in a piston-cylinder apparatus. CO2 hardly affects conductivity at such a concentration level. The effect of water on the conductivity of basaltic melt is markedly larger than inferred from previous measurements on silicate melts of different composition. The conductivity of basaltic melts with more than 6 wt% of water approaches the values for carbonatites. Our data are reproduced within a factor of 1.1 by the equation log σ = 2.172 − (860.82 − 204.46 w 0.5)/(T − 1146.8), where σ is the electrical conductivity in S/m, T is the temperature in K, and w is the H2O content in wt%. We show that in a mantle with 125 ppm water and for a bulk water partition coefficient of 0.006 between minerals and melt, 2 vol% of melt will account for the observed electrical conductivity in the seismic low-velocity zone. However, for plausible higher water contents, stronger water partitioning into the melt or melt segregation in tube-like structures, even less than 1 vol% of hydrous melt, may be sufficient to produce the observed conductivity. We also show that ~1 vol% of hydrous melts are likely to be stable in the low-velocity zone, if the uncertainties in mantle water contents, in water partition coefficients, and in the effect of water on the melting point of peridotite are properly considered.

Degradation behaviour and mechanical properties of magnesium implants in rabbit tibiae

Abstract: To investigate the initial mechanical strength and the degradation behaviour with the associated changes in mechanical properties of magnesium-based osteosynthesis implants, 30 rabbits were implanted with cylindrical pins of the alloys MgCa08 (magnesium with 08 wt% calcium), LAE442 (magnesium with 4 wt% lithium, 4 wt% aluminium and 2 wt% rare earths) and WE43 (magnesium with 4 wt% yttrium and 3 wt% rare earths) The implants were inserted into the medullary cavity of both tibiae After 3 and 6 months, each half of the animals was euthanized, respectively, and the implants were taken out A determination of volume, three-point bending tests, scanning electron microscopy (SEM) and energy dispersive X-ray analyses as well as metallographic and μ-computed tomography examinations were accomplished All implants were clinically well tolerated MgCa-implants showed the least initial strength and the highest loss in volume after 6 months SEM- and μ-computed tomography examinations revealed a pronounced pitting corrosion Therefore, their use as degradable implant material seems to be limited LAE442 has the best initial strength which seems to be sufficient for an application in weight-bearing bones The degradation behaviour is very constant However, possible unknown side effects of the rare earths have to be excluded in further investigations on biocompatibility Considering all results of WE43, its application as osteosynthesis material for fracture repair is ineligible due to its heterogeneous and unpredictable degradation behaviour

Mobilization of Soil and Fertilizer Phosphate by Cover Crops

Abstract: Nine tropical cover crops, which had produced positive residual effects on following year maize yield in field experiments in Northern Nigeria on a luvisol low in available P, and maize were grown in a pot experiment using the same soil. The effect of plant growth on pH, organic acids, soil P fractions and phosphatase activity in bulk and rhizosphere soil was studied. All plant species raised the pH in the rhizosphere, differed widely in acid phosphatase activity, and derived most of their P from the resin and bicarbonate-extractable inorganic P (Pi). Organic P (Po) accumulated especially in the rhizosphere in all plant species. There was a negative correlation between the species-specific rhizosphere phosphatase activity and Po accumulation. Pigeonpea (Cajanus cajan) appeared to be more P-efficient than other plant species because it was less reduced in biomass production at low compared to adequate P supply and the highest rhizosphere acid phosphatase activity. The results of this ongoing research indicate that inclusion of cover crops into cropping systems can contribute to more efficient use of soil and fertilizer P by less P-efficient crops such as maize. However, further work is necessary to get a better quantitative understanding of the mechanisms involved.

Effects of f O2 and H2O on andesite phase relations between 2 and 4 kbar

Abstract: Experimental phase equilibria have been investigated on three medium-K silicic andesite (60–61 wt % SiO2) samples from Mount Pelee at 2–4 kbar, 850–1040°C, under both vapor-saturated CO2-free and vapor-saturated CO2-bearing conditions. Most experiments were crystallization experiments using dry glasses prepared from the natural rocks. Both normal- and rapid quench experiments were performed. Two ranges of oxygen fugacity (fO2) were investigated: NNO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderately oxidizing conditions, plagioclase (pl) and magnetite (mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature (T)-H2O range (875–950°C and 5–7 wt % H2O in melt). Amphibole (am) is stable under near vapor-saturated CO2-free conditions at 876°C. At 900°C, ilmenite (ilm) is found only in experiments less than or equal to NNO. Upon increasing pressure (P) under vapor-saturated CO2-free conditions, pl + mt is replaced by am + mt on the liquidus above 3.5 kbar. For highly oxidizing conditions, mt is the sole liquidus phase at 2 kbar, followed by pl and opx, except in the most H2O-rich part of the diagram at 930°C, where opx is replaced by Ca-rich pyroxene (cpx) and am. Compositions of ferromagnesian phases systematically correlate with changingfO2 Experimental glasses range from andesitic through dacitic to rhyolitic, showing systematic compositional variations with pl + opx + mt fractionation (increase of SiO2 and K2O, decrease of Al2O3, CaO, FeOt, and MgO). FeO*/MgO moderately increases with increasing SiO2. For fO2 conditions typical of calk-alkaline magmatism (approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andesite magmas, and this should stimulate reexamination for the mechanisms of generation of andesites by fractionation from basaltic parents.