Partitioning of iron and magnesium between crystals and partial melts in peridotite upper mantle

The iron-magnesium distribution coefficient, $$K'_D = (X_{\Sigma {\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{\Sigma {\text{FeO}}} )^{{\text{liquid}}} ,$$ has frequently been used as a means of testing whether experimental and natural silicate liquids could have been in equilibrium with olivine of mantle composition. It is shown here that this K′ _D decreases with increasing oxygen fugacity (xxx) for a hydrous partial melt in equilibrium with a natural spinel peridotite assemblage under pressure and temperature conditions corresponding to those of the upper mantle (from 0.52 at the xxx of the iron-wüstite buffer to 0.04 at the xxx of the magnetite-hematite buffer). K′ _D also increases with increasing pressure, with decreasing temperature, and probably with increasing Mg/(Mg+∑ Fe) of the parental peridotite, suggesting that $$K_D = (X_{{\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{{\text{FeO}}} )^{{\text{liquid}}}$$ also increases with increasing pressure and decreasing temperature. Thus, unless these four variables (P, T, xxx, silicate composition) are known for a natural magma, K′ _D and probably K _D are variables, and the Mg/(Mg+∑ Fe) of such a magma cannot be correlated to that of the parent. The K _D determined at 1 atm pressure by Roeder and Emslie has frequently been used to test whether the Mg/(Mg+∑ Fe) ratios of experimentally formed liquids at high pressure in equilibrium with olivine of known Fo content represent the equilibrium Mg/(Mg+Fe²⁺) of this liquid, assuming that ∑Fe=Fe²⁺ and that K′ _D does not vary with P, T, and composition of the system. Published data demonstrate that the oxygen fugacities of the experimental designs employed by different laboratories vary between those of the magnetite-hematite and magnetite-wüstite buffers (6 orders of magnitude), resulting in K′ _D between 0.04 and 0.31 at 1050° C and 15 kbar, for example. Thus, published arguments as to whether the quenched liquids represent equilibrium compositions based on iron-magnesium partitioning are inadequate. The effects of P, T, xxx, and the composition of the starting material must also be considered.     

Pressure dependence of nickel partitioning between forsterite and aluminous silicate melts

Nickel partitioning between forsterite and aluminosilicate melt of fixed bulk composition has been determined at 1300°C to 20 kbar pressure. The value of the forsterite-liquid nickel partition coefficient is lowered from >20 at pressures equal to or less than 15 kbar to <10 at pressures above 15 kbar.Published data indicate that melts on the join Na₂O-Al₂O₃-SiO₂ become depolymerized in the pressure range 10–20 kbar as a result of Al shifting from four-coordination at low pressure to higher coordination as the pressure is increased. This coordination shift results in a decreasing number of bridging oxygens in the melt. It is suggested that the activity coefficient of nickel decreases with this decrease in the number of bridging oxygens. As a result, the nickel partition coefficient for olivine and liquid is lowered.Magma genesis in the upper mantle occurs in the pressure range where the suggested change in aluminum coordination occurs in silicate melts. It is suggested, therefore, that data on nickel partitioning obtained at low pressure are not applicable to calculation of the nickel distribution between crystals and melts during partial melting in the upper mantle. Application of high-pressure experimental data determined here for Al-rich melts to the partial melting process indicates that the melts would contain about twice as much nickel as indicated by the data for the low-pressure experiments. If, as suggested here, the polymerization with pressure is related to the Al content of the melt, the difference in the crystal-liquid partition coefficient for nickel at low and high pressure is reduced with decreasing Al content of the melt. Consequently, the change ofD_Ni^{ol-andesite melt} is greater than that ofD_Ni^{ol-basalt melt}, for example.     

Risø 1978: Further investigations into the effects of local terrain irregularities on tower-measured wind profiles

Observations of flow over complex terrain taken at Risø during June–July 1978 and numerical studies confirm earlier findings that small variations in surface elevation have significant effects on mean wind profiles. Measured shear stresses in the nonequilibrium region of the flow are consistent with theory but quite different from those obtained assuming simple flux-profile relationships. These findings imply that flux-profile relationships can be quite complicated over other than simple homogeneous terrain.     

The PT phase relations of a primary oceanite from the Reykjanes peninsula,Iceland

The field relationships and petrographic features of the oceanites of the Reykjanes peninsula suggest that they might have originated as primary magmas. The principal phase relationships of primary liquids formed by the partial melting of lherzolites are defined. The phase relations obtained for oceanite RE78 between 0 and 30 kbar at dry conditions suggest that the oceanite originated as a primary magma at 25 kbar and 1580°C, and erupted at a temperature near 1300°C.     

Heat budget of Flakevatn,a subpolar lake

Zusammenfassung Durch Kenntnis von Sommer- und Wintertemperaturen in dem südnorwegischen Gebirgssee Flakevatn (Meeresh?he 1448 m) konnte die Thermik des subpolaren Seetypus charakterisiert werden. Im Sommer herrscht in Flakevatn wiederholte Isothermie bei relativ hohen Temperaturen, im Winter wird der See ganz ausserordentlich abgekühlt (Fig. 1, Tabelle). Die sommerliche W?rmespeicherung betr?gt +5800, die winterliche −22500 cal/cm².      

A Study of Ozone Laminae Using Diabatic Trajectories, Contour Advection and Photochemical Trajectory Model Simulations.

In this paper, we show that the rate of ozone loss in both polar and mid-latitudes, derived from ozonesonde and satellite data, has almost the same vertical distribution (although opposite sense) to that of ozone laminae abundance. Ozone laminae appear in the lower stratosphere soon after the polar vortex is established in autumn, increase in number throughout the winter and reach a maximum abundance in late winter or spring. We indicate a possible coupling between mid-winter, sudden stratospheric warmings (when the vortex is weakened or disrupted) and the abundance of ozone laminae using a 23-year record of ozonesonde data from the World Ozone Data Center in Canada combined with monthly-mean January polar temperatures at 30 hPa.Results are presented from an experiment conducted during the winter of 1994/95, in phase II of the Second European Stratospheric And Mid-latitude Experiment (SESAME), in which 93 ozone-enhanced laminae of polar origin observed by ozonesondes at different time and locations are linked by diabatic trajectories, enabling them to be probed twice or more. It is shown that, in general, ozone concentrations inside laminae fall progressively with time, mixing irreversibly with mid-latitude air on time-scales of a few weeks. A particular set of laminae which advected across Europe during mid February 1995 are examined in detail. These laminae were observed almost simultaneously at seven ozonesonde stations, providing information on their spatial scales. The development of these laminae has been modelled using the Contour Advection algorithm of Norton (1994), adding support to the concept that many laminae are extrusions of vortex air. Finally, a photochemical trajectory model is used to show that, if the air in the laminae is chemically activated, it will impact on mid-latitude ozone concentrations. An estimate is made of the potential number of ozone molecules lost each winter via this mechanism.     

Amplitudes of long-period PcP and the core-mantle boundary

The amplitudes of the core reflection PcP are sensitive to the wave velocities and densities in the neighborhood of the core-mantle boundary (CMB). We study the amplitude ratio of the long-period phases PcP and P from two South American deep-focus earthquakes with favorable fault-plane solution, depth and magnitude, as recorded by WWNSS and CSN stations in North America.Comparison is made with long-period PcP/P amplitude ratios, derived from theoretical seismograms for a variety of CMB models. Models from previous studies, which were mainly derived from short-period PcP observations and which are characterized by discrete layers above the CMB, are almost all inconsistent with the long-period data. The data also discriminate against low nonzero S velocities below the CMB. Simple first-order-discontinuity models of the CMB, for instance according to the Jeffreys-Bullen earth model or according to recent models based mainly on free oscillations, explain the data reasonably well.Model improvements are attempted by varying the P-velocity gradient above the CMB. The best amplitude fit is obtained for a rather strong decrease in P velocity with depth in this zone which, however, gives no acceptable traveltime fit for PcP. The scatter in body-wave amplitudes is considerable even for long-period waves and may prevent the correct assessment of that part of the amplitude variation of a phase with distance that is due to the variation of velocities and densities with depth alone.     

Tveitite,a new calcium yttrium fluoride

Tveitite is a new mineral with formula of the type $\text{Ca}{}_{\mathrm{1?x}}\text{(}\text{Y, RE}\text{)}{}_{\mathrm{x}}\text{F}{}_{\mathrm{2+x}}\text{where}\text{x}$ is approximately 0.3. It is found in a cleavelandite pegmatite at Høydalen in Tørdal, Telemark, S. Norway. The sub-cell is thought to be monoclinic, $\text{a}\text{′=3.924,}\text{b}\text{′=3.893,}\text{c}\text{′=5.525, ß=90°26′,}\text{Z}\text{=2}$. The structure is very similar to that of fluorite and can be described in terms of a pseudo-cubic cell $\text{[1}\text{1}\text{0]}{}_0\text{= 5.527, [110]}{}_0\text{= 5.527,}\text{c}\text{= 5.525, α = 90°18′, γ = 89°37′}$ which contains approximately 1[YCa₃F₉]. The mineral shows twinning in at lest three different directions giving a grating structure probably due to high-low inversion. DTA studies show an inversion point at about 670°C. Material heated above this temperature gives X-ray diffraction patterns corresponding to yttrofluorite (a₀ 5.528). The effect of trivalent ions on the stability of fluorite is discussed.     

Post-depositional remanent magnetization in a re-deposited deep-sea sediment

Re-deposition of some deep-sea clay material in the earth's magnetic field is described. At the time of an induced reversal of the ambient field declination, calcium carbonate layers were introduced for identification purposes. NRM and bulk susceptibility measurements along the still moist sediment cores showed the presence of a time lag of around 10 days between a change in the ambient field direction and a corresponding change in the stable magnetization. AF-demagnetization of discs sampled from a dried core suggested that this time lag was caused by a post-depositional remanent magnetization mechanism. Magnetomineralogical investigations show the presence of the thermally unstable magnetic mineral phase, probably titano-maghemite. A positive correlation between NRM-intensities and water temperature during deposition is discussed. It is suggested that consolidation-rate is a major factor influencing the NRM intensity in deep-sea sediments.