Trace elements in the Allende meteorite—IV. Amoeboid olivine aggregates

INAA data for Ca, Sc, Hf, La, Ce, Sm, Eu, Tb, Yb, Lu, Os, Ir, Ru, Na, Cl, Br, Fe, Mn, Cr, Co, Au, As, and Sb are presented for ten amoeboid aggregates from the Allende meteorite. Only one lacks olivine. Seven of the remainder, as a group, have cosmic proportions of refractory lithophile and siderophile elements and appear to have formed when coarse-grained Allende inclusion material underwent partial reaction with a low-temperature nebular gas and mixture with FeO-rich olivine. The other two have highly fractionated abundances of refractory elements relative to one another compared to Cl chondrites, including Group II REE patterns, and probably formed by the mixing of fine-grained Allende inclusion material with FeO-rich olivine. Non-refractory siderophile components are also different in composition in each type of amoeboid olivine aggregate.     

Nickel partitioning between olivine and silicate melt

Partitioning of Ni between olivine and silicate melt has been determined for compositions in the system Fo-Ab-An (1 atm) for temperatures ranging from 1250°C to 1450°C. Nickel concentrations were determined by electron microprobe; concentration levels in the liquids ranged from 0.1% to 0.5%. Platinum capsules or Pt wire loops were used as containers. Equilibrium was evaluated from kinetic considerations and by variation of run parameters; it was documented in one case by a bracketed reversal. No evidence was found for a dependence of the partition coefficient D (Ni in olivine/Ni in liquid) on Ni concentration. D is strongly dependent on melt composition, varying linearly with (1/MgO) at constant temperature. The intrinsic temperature dependence of D is small; the apparent temperature dependence reported in previous studies is largely related to the variation of melt composition with temperature. Our D values determined in the simple system Fo-An-Ab agree well with those reported by Leeman for natural (Fe-bearing) basalt systems. Overall variation of D in our system (and in natural basalts) can be expressed by the regression: D = (124/MgO) ? 0.9Our data are used to evaluate published Ni-MgO relationships in natural basalt series from Kilauea, Crozet, Cape Verde and Baffin Bay. A combination of olivine accumulation and fractional crystallization processes are sufficient to model these series. Using our data, unique “parental” liquids can be specified for each of these series; the MgO content of these liquids varies from 6% to 13%. Basalts with MgO contents greater than these “parental” liquids must be accumulative. The linear Ni-MgO trends, high absolute Ni concentrations, and large spread of Ni contents for the high-MgO basalts argue convincingly against their being “primary” liquids. Models such as those of O'Hara [6,13] and Clarke [24], based on the assertion of primary high-MgO liquids, must therefore be re-evaluated.Because of the high Si/O ratio and low MgO content of island arc andesites, the Ni partition coefficient D may be quite high. Therefore, the relatively low Ni content of such andesites may not be an argument against their derivation as direct partial melts of the mantle.     

A finite element approach to watershed runoff

A numerical finite element based implicit recurrence relationship is developed for the analysis of watershed direct runoff problems. The algorithm is presented in a two dimensional form for the full momentum and continuity equations and unidimensionally for the kinematic wave theory. Since the whole domain is represented and solved as a single set of matrix equations, the advent of shocks for domains where appreciable changes in slope and/or resistance to flow occur, is immediately distinguishable.     

Studies of K-Ar dating and xenon from extinct radioactivities in breccia 14318; implications for early lunar history

The rare gases argon and xenon were studied intensively in lunar breccia 14318, one of a family of three Apollo 14 breccias exhibiting similarities, including substantial amounts of ‘parentless’ xenon from the spontaneous fission of extinct ²⁴⁴Pu. We made stepwise heatings on both unirradiated and pile-irradiated specimens. The isotopic composition of the xenon from fission was determined by a new method which invokes a minimum of assumptions; it is shown to be from ²⁴⁴Pu and almost certainly parentless. For example, the fission component, although not appreciably fractionated with respect to the trapped component during stepwise heating, has a low temperature character so that, relatively speaking, it appears to be more surficial than xenon emanating from uranium sites in the irradiated sample. We demonstrate that this effect is not an artifact of the neutron irradiation. The breccia contains abundant trapped argon with a high ${}^{40}\text{Ar}{}^{36}\text{Ar}$ ratio for lunar material—~14. Otherwise the argon is radiogenic and gives a convincing K-Ar age of 3.69 ± 0.09 b.y. by the stepwise ⁴⁰Ar-³⁹Ar method, nearly in agreement with ages for other Apollo 14 breccias obtained in our laboratory and elsewhere. Rock 14301, another of the family of breccias and one which has been studied in other laboratories, contains similar trapped argon and parentless xenon. Unlike 14318 it also contains a conspicuous excess of ¹²⁹Xe from the radioactive decay of extinct ¹²⁹I. Implications of the parentless xenon from extinct sources, as seen in these different rocks, depend upon the model adopted for its evolution and storage. We present four different models, all of which are unsatisfactory in some respects, so that we are presently unable to narrow the question. We must stress that other Apollo 14 breccias, such as 14321, contain fission xenon from ²⁴⁴Pu which was apparently produced in situ.     

Markov chain simulations of vertical dispersion from elevated sources into the neutral planetary boundary layer

A Lagrangian statistical-trajectory model based on a Markov chain relation is used to investigate vertical dispersion from elevated sources into the neutral planetary boundary layer. The model is fully two-dimensional, in that both vertical and longitudinal velocity fluctuations, and their correlation, are simulated explicitly. The best observational information currently available is used to characterize the mean and turbulent structure of the neutral boundary layer. In particular, a realistic vertical profile of the Lagrangian integral time scale is proposed, based partly on a review of direct measurements and partly on a comparison of the model predictions with published diffusion data. The model predictions are shown to agree well with a variety of dispersion observations. The model is used to study vertical diffusion as a function of release height H, friction velocity u* and surface roughness z ₀ for downwind distances up to 10 km from the source. The equivalent Gaussian dispersion parameter Σ _z is shown to decrease slightly with an increase in H, and to increase with increases in z ₀ or u*. It is demonstrated that relationships valid in a field of homogeneous turbulence can be applied to vertical dispersion in the atmosphere if the release occurs above the region of strongest gradients in the mean and turbulent parameters. Scaling in terms of the standard deviation in elevation angle of the wind at the release point leads to a universal curve which provides accurate estimates of Σ _z over a wide range of values of H, z ₀ and the meteorological parameters.     

Site-specific multipath characteristics of global IGS and CORS GPS sites

The site-specific multipath characteristics of 217 Global Positioning System (GPS) sites worldwide were analyzed using the variability of the post-fit phase residuals. Because the GPS satellite constellation returns to the same configuration in a sidereal day (23 h 56 min 4 s), the multipath repeats on that period. However, daily GPS position estimates are usually based on the solar day. When several days of GPS data are processed, this steady change in the orientation of the satellite constellation with respect to the station manifests itself in the form of patterns in the post-fit phase residuals which shift by 3 min 56 s per day. It was found that the mean root mean square of the time-shifted post-fit phase residuals is highly dependent on the GPS antenna type. The conclusions derived from the analysis of the time-shifted post-fit residuals were verified by performing a cross-correlation of the post-fit residuals across many days for selected sites.     

Trace element signature and U–Pb geochronology of eclogite-facies zircon,Bergen Arcs,Caledonides of W Norway

Secondary-ion mass spectrometry (SIMS) U–Pb and trace element data are reported for zircon to address the controversial geochronology of eclogite-facies metamorphism in the Lindås nappe, Bergen Arcs, Caledonides of W Norway. Caledonian eclogite-facies overprint in the nappe was controlled by fracturing and introduction of fluid in the Proterozoic—Sveconorwegian—granulite-facies meta-anorthosite-norite protolith. Zircon grains in one massive eclogite display a core–rim structure. Sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith, i.e. 0.31Th/U0.89, heavy rare earth element (HREE) enrichment, and negative Eu anomaly. Weakly-zoned to euhedral oscillatory-zoned Caledonian rims are characterized by Th/U0.13, low LREE content (minimum normalized abundance for Pr or Nd), variable enrichment in HREE, and no Eu anomaly. A decrease of REE towards the outermost rim, especially HREE, is documented. This signature reflects co-precipitation of zircon with garnet and clinozoisite in a feldspar-absent assemblage, and consequently links zircon to the eclogite-facies overprint. The rims provide a mean ²⁰⁶Pb/²³⁸U crystallization age of 423±4 Ma. This age reflects eclogite-forming reactions and fluid–rock interaction. This age indicates that eclogite-facies overprint in the Lindås nappe took place at the onset of the Scandian (Silurian) collision between Laurentia and Baltica.