Thermomagnetic analysis of meteorites, 2. C2 chondrites

Samples of all eighteen of the known C2 chondrites have been analyzed thermomagnetically. For eleven of these, initial Fe₃O₄ content is low (generally <1%) and theJ_s-T curves are irreversible. The heating curves show variable greater (up to 10 times) than it is initially. This behavior is attributed to the production of magnetite from a thermally unstable phase — apparently FeS. Four of the remaining seven C2 chondrites contain Fe₃O₄ as the only significant magnetic phase: initial magnetite contents range from 4 to 13%. The remaining three C2 chondrites contain iron or nickel-iron in addition to Fe₃O₄. These seven C2 chondrites show little evidence of the breakdown of a thermally unstable phase.     

Diel vertical migration of zooplankton in contrasting habitats of the Chesapeake Bay

Two different habitats in the Chesapeake Bay were tested for the occurrence of diel vertical migration (DVM) by members of the zooplankton community. During July of 1997 diel samples were collected from Jones Creek, a small, protected, 3-m deep tidal gut, and a station in Hampton Roads, a 20-m deep, expansive section of the bay. Temperature, salinity, light penetration, water clarity, dissolved oxygen, total particulate nitrogen and carbon, chlorophyll concentration, and algal density were also determined for depth increments at these stations, ca. midday and midnight. Significant differences in zooplankton depth distribution occurred over the day at both stations. The calanoid copepod,Acartia tonsa, dominated the zooplankton community and the larger stages were strong migrators. Mysid shrimp, the largest sized crustaceans, displayed significant DVM at both stations. Although diel differences were also detected in chlorophylla concentrations, with 25% reduction at night in Jones Creek and 18% in Hampton Roads, counts of algal cells revealed no differences between night and day numbers. The nightly losses of chlorophyll were not driven by mesozooplankton grazers. Using grazing rates from the literature, we estimated that mesozooplankton harvested less than 5% of the standing crop of phytoplankton each day in both locations. Food quantity and quality (as determined by chlorophyll concentrations, cell counts, and C:N ratio) were uniform in the Jones Creek water column, but higher near the surface in Hampton Roads. While it was clearly advantageous for zooplankton to migrate to surface waters at night for feeding in Hampton Roads, it is paradoxical for DVM to persist in Jones Creek when ample food was found in deeper water.     

Loss of patch-scale heterogeneity on secondary productivity in the arid shrubland of Western Australia

General models of degradation suggest soil and nutrients are lost and conversion of rainfall into primary productivity is diminished when rangeland is degraded. These models are supported by studies on non-resilient landscapes, where loss of primary productivity also translated into loss of secondary productivity, but have not been tested on resilient landscapes. Elsewhere we showed that loss of chenopod shrubs from a landscape characterized as resilient was associated with declines in plant productivity and efficiency of conversion of rainfall into plant mass. To explore whether these differences in primary productivity translated into differences in secondary productivity, we grazed sheep at five rates of stocking for 10 years on 2000 ha of this landscape. The experiment was necessarily replicated in time not space (i.e. pseudo-replicated), which limits confident extrapolation of results to other landscapes.Productivity of sheep at all except highest rates of stocking varied little between sites where shrubs were abundant or scarce. From an animal production point of view, greater rates of stocking were unsustainable through dry years on the degraded site, but animal performance was generally unaffected on the non-degraded site, where shrubs were abundant. While these results provide evidence of economic penalties associated with degrading a resilient landscape, important ecological penalties were only partially explored.     

'What a dolt one is': language learning and fieldwork in geography

This paper discusses aspects of the theory and practice of foreign language learning for fieldwork in geography, a rarely discussed set of issues despite the personal investment that language learning requires from the researcher. In some cases of cross-cultural research, it may be particularly desirable to learn a vernacular language for which there are few (or no) study aids. However, there is little guidance about how learning such a language should be approached, or discussion of its significance for the subsequent research. Through a discussion of personal experience in Ethiopia, the paper describes one way of learning a vernacular language, and explores the advantages that this brought to the fieldwork and to the resulting research understandings.     

The extrapolation of elastic moduli to high pressure and temperature

The elastic constants of a crystal under stress, defined as the second derivative of the crystal free energy with respect to strain, require a correction related to the static pressure at non-zero pressures. The corrections required for the elastic constants calculated by the free energy minimisation code PARAPOCS are described and tested by comparison with the elastic constants calculated numerically by applying small stresses in the appropriate orientations to simulated crystals of fluorite, forsterite, α-quartz and albite. The corrected elastic constants are then used to investigate the extrapolation of the bulk and shear moduli (and hence also the seismic wave velocities V _p and V _s) of β-spinel and forsterite to upper mantle pressures. A Murnaghan equation, thirdorder Eulerian finite strain equation, second order polynomial equation and a logistic equation were all fitted to the simulated bulk and shear moduli between 0 and 3 GPa pressure. The parameters derived for these equations are used to extrapolate the bulk and shear moduli to 14 GPa and the results are compared to the simulated high pressure moduli. Over this pressure range, the second order polynomial provides the best extrapolation of the bulk modulus, but the use of the logistic equation results in the best extrapolation of the shear modulus.     

Adcumulus dunite growth in a laboratory thermal gradient

Laboratory experiments near 1450° C at 1 bar (QFM) on komatiite bulk composition show olivine and liquid in cumulus textures which evolve with experiment duration. Orthocumulus texture with settled olivine crystals separated by liquid matrix is developed within a day. Experiments quenched after a few days to a week show a progression of textures which include development of columns of olivine crystals separated by channels of liquid. Olivine grain sizes increase with the cube root of time suggesting that dissolution and reprecipitation of olivine may be involved in the organization into columns and channels. Experiments quenched after two weeks have well developed adcumulus texture. The basal polycrystalline granular olivine aggregate forms from the decay of the olivine columns. Melt expulsion from the aggregate can be virtually complete, leaving 1% or less of the melt originally present.Buoyancy-driven compaction of olivine is not the mechanism responsible for this textural evolution because the final basal aggregate sometimes contains vesicles. An addition proof of the inadequacy of buoyancy is provided by raising the crucible slightly above the thermal symmetry point of the furnace. The aggregate then compacts on top of a crystal-free liquid. The thermal gradients above and below the furnace hot spot are thought to be primarily responsible for the olivine redistributions observed. Diffusion of olivine components in the liquid is driven along a saturation gradient resulting from the temperature gradient. The process, called thermal migration in geological literature, is essentially the same as traveling solvent zone refining in metallurgy. Differential solubility and Soret fractionation both contribute to olivine redistribution to the cold region of the crystal-liquid aggregate. There may be some applications of these results to natural cumulate rock petrogenesis.     

Monazite solubility and dissolution kinetics: implications for the thorium and light rare earth chemistry of felsic magmas

A series of monazite dissolution experiments was conducted in a hydrous (1–6 wt.%) granitic melt at 8 kbar over the temperature range 1,000–1,400° C. A polished cube of monazite was immersed in a natural obsidian melt and allowed to partially dissolve. Electron microprobe traverses perpendicular to the crystal-melt interface revealed concentration gradients in the LREEs and P. Diffusivities of the LREEs and P were calculated from these profiles, yielding the following Arrhenius relations for the LREEs: D=0.23 exp(–60.1 kcal mol^–1/RT) at 6% water D=2.30×10⁷ exp(–122.1 kcal mol^–1/RT) at 1% water These results demonstrate the importance of dissolved water on REE diffusion. Phosphorus diffusivities are nearly identical to those of the rare-earths, suggesting that P diffusion charge-compensates REE diffusion. The concentration of LREEs required for monazite saturation in these melts is given by the level of dissolved LREEs at the crystal-melt interface. These values also show a dependence on dissolved water, with LREE_sat=60 ppm at 6% H₂O when extrapolated down to 700° C, and LREE_sat=30 ppm at 1% H₂O. Calculated dissolution rates based on the above parameters indicate that minute (<30 m diameter) monazite crystals will be readily digested by an enclosing anatectic magma under reasonable geologic conditions (i.e., T=700–800° C and >2% H₂O), whereas larger (> 50 m) crystals will likely be residual over the duration of an anatectic event. The low solubility of monazite in this melt suggests that the LREE depletion observed in some felsic differentiation suites may be the result of monazite crystallization. Limited experimental and geochemical/petrologic evidence indicates that compositional changes in the melt accompanying differentiation decrease the solubility of monazite drastically. Kinetic and chemical constraints may also lead to localized monazite saturation and inclusion in major phases or even other accessories. Variations in the REE composition of monazite from different parageneses probably reflects the REE pattern of the parent melt, and may be due to gradational differences in the stability of individual or subgroup REE-complexes as a function of melt composition. Particularly important in this regard seems to be the lime+alkali/alumina balance of the melt and its volatile content.     

The Minnesota long-term temperature record

A remarkable climatological record exists in the heart of the North American interior that deserves recognition and exploitation. The objective of this paper is to bring this unique record that commenced in October, 1819, at Ft. Snelling, Minn., to the attention of the climate change community.Minnesota Agricultural Experiment Station, University of Minnesota, Scientific Journal Series Paper No. 13975.