Experimental constraints on the origins of primitive potassic lavas from the Trans-Mexican Volcanic Belt

Primitive magmas in the Trans-Mexican Volcanic Belt (TMVB) span a wide geochemical range that includes calc-alkaline basalt and basaltic andesite, potassic shoshonites, and intraplate alkaline basalts, indicating that the subarc mantle wedge is chemically heterogeneous. The aim of this study is to experimentally constrain the origins of potassic lavas that have erupted along the volcanic front in the TMVB. We used a piston-cylinder apparatus to determine the P–T–H₂O near-liquidus phase relations for two primitive potassic lavas: a hornblende trachybasalt (shoshonite) from Cerro La Pilita in the central TMVB and a high-K calc-alkaline basalt from Ayutla in the western TMVB. Experiments were conducted at mantle pressures (0.8–2.5 GPa) and temperatures (1,100–1,400 °C) with 1.5–6 wt% H₂O. Results show that both samples were last equilibrated with an olivine + clinopyroxene assemblage at upper mantle pressures. Integrating our results with trace element characteristics, we conclude that the potassic magmas formed by a complex, multistage process in which melts from the hottest part of the mantle wedge either reequilibrated with clinopyroxene-rich veins in the shallow upper mantle or caused melting of such veins by advective heating. We combine our results with previous experiments on TMVB lavas to provide an along-arc perspective of melt equilibration depths in the mantle wedge. The results suggest that although melts may initially form deep in the wedge, they commonly reequilibrate with heterogeneous mantle at shallower depths. Primitive, medium-K basaltic andesites in the TMVB form by reequilibration with harzburgite, which we infer to be a common lithology in the upper mantle, whereas some potassic magmas like the ones studied here form through reequilibration with or melting of veins of olivine + clinopyroxene ± phlogopite. Though somewhat rare at the volcanic front relative to the more abundant medium-K volcanic rocks, the potassic magmas are an important lava type for revealing mantle chemical heterogeneities.     

The organic composition of Comet C/2000 WM1 (LINEAR) revealed through infrared spectroscopy

We investigated the parent volatile composition of the Oort cloud Comet C/2000 WM₁ (LINEAR) on 23-25 November 2001, using the Near Infrared Echelle Spectrograph on the Keck II telescope. Flux-calibrated spectra, absolute production rates, and mixing ratios are presented for H₂O, HCN, CH₄, C₂H₂, C₂H₆, H₂CO, CH₃OH and CO. Compared with “organics-normal” comets, WM₁ is moderately depleted in HCN, CH₄ and CH₃OH, and is even more depleted in C₂H₂ and CO. Its composition is thus intermediate to comets that are severely depleted in their organic volatile composition and those that exhibit “normal” organic volatile abundances. We argue that WM₁ may have formed closer to the young Sun than “organics-normal” comets, but at greater distance than the severely depleted comets, before its ejection to the Oort cloud. The mixing ratios of the above-listed organic volatiles agree day-by-day for 23-25 November 2001. Thus, there is no evidence of macroscopic heterogeneity in chemistry of this comet’s nucleus at the achieved measurement accuracy. As the first comet to show moderate organic depletion in parent volatiles, WM₁ represents an important addition to the emerging taxonomic classification based on chemical composition.     

Habitat suitability curves for benthic macroinvertebrates from a small New Zealand river

We developed habitat suitability curves (HSC) using generalised additive models (GAMs) for nine benthic macroinvertebrate taxa from a small New Zealand river for hydraulic-habitat modelling assessments of instream flow requirements. We included interaction terms between the primary variables (water depth, velocity, substrate) when significant, to address a longstanding criticism of univariate HSC. To date, only large-river univariate HSC have been available and these have been used in hydraulic-habitat applications on small rivers, despite doubt over the transferability of HSC between rivers of different size and type. We tested the outcome on the predicted abundance–flow relationship of applying the small-river habitat suitability GAMs versus large-river GAMs for two taxa on the same small river. We found the effects of flow allocation were overestimated by the large-river GAMs relative to the small-river GAMs. Further research to develop general HSC for categories of river size and type is needed to better inform hydraulic-habitat modelling applications.     

Thermal Infrared Spectra of Comet Hale-Bopp at Heliocentric Distances of 4 and 2.9 AU

We present 10 and 20 μm spectra of comet Hale-Bopp taken at UKIRT on 1996 June 20 and 1996 September 29.The 10 μm spectra clearly show a strong silicate feature with peaks at 10.0 and 11.2 μm. The 20 μm spectrum on September 29 has strong excess flux relative to a blackbody and a peak near 19μm, in good agreement with the ISO SWS spectra obtained a week later. However, the 20 μm spectrum on June 20 has significantly lower flux than would be expected based on a blackbody extrapolation from the flux at 12.5 μm. This revised version was published online in July 2006 with corrections to the Cover Date.     

B-polarization induction in two generalized thin sheets at the surface of a conducting half-space

Summary. A new closed-form solution is obtained analytically for a B- polarization induction problem of geophysical interest, in which a local region of the Earth is represented by a generalized thin sheet at the surface of and in electrical contact with a uniformly conducting half-space. The generalized sheet, first introduced by Ranganayaki & Madden, is a mathematical idealization of a double layer which consists, in this problem, of two adjacent half-planes with distinct conductances representing a surface conductivity discontinuity such as an ocean—coast boundary, underlain by a uniform sheet of finite integrated resistivity representing the lower crust. The resistive sheet exerts a considerable mathematical influence on the solution causing, under certain conditions, an additional pole to appear in one of the forms of contour integral by which the solution can be expressed; it also weakens or eliminates field singularities that would otherwise occur at the conductance discontinuity. A numerical calculation is made for model parameters typifying an ocean—coast boundary underlain by a highly resistive crust. It is found that the residue of the pole associated with the resistive sheet dominates the solution for this example, the main consequence of which is a huge increase in the horizontal range over which the induced currents adjust themselves between the different 'skin-effect' distributions at infinity on either side of the model. Moreover the solution shows that this 'adjustment distance' has a more complicated dependence on the conductance and integrated resistivity of the sheet than that given simply by the square root of their product which was the length parameter proposed by Ranganayaki & Madden.     

H-polarization induction over an ocean edge coupled to the mantle by a conducting crust

Summary. The Wiener—Hopf technique is used to obtain an exact analytical solution for the problem of H -polarization induction over the edge of a perfectly conducting thin sheet, representing an ocean, electrically connected to a perfectly conducting mantle through a slab of finite conductivity and thickness, which represents the Earths crust. It is shown that the induced currents resulting from this type of induction process are drawn up into the sea from the cust and mantle with the greatest concentration of current near the ocean edge. The surface impedance over the land surface, is calculated for various mantle depths and is shown to increase sharply as the coastline is approached. The magnetic field along the ocean floor is also plotted as a function of distance from the coastline, and the results are found to agree very well with those calculated previously by approximate and numerical methods.     

The ultraviolet spectrum of periodic comet encke (1980 XI)

A comparison of the ultraviolet spectrum of periodic Comet Encke (1980 XI), recorded by the IUE between 24 October and 5 November 1980 with similar spectra of short- and long-period comets shows the gaseous composition of P/Encke to be nearly identical to that of the other comets observed by the IUE. If P/Encke is indeed the remains of a once giant comet, this similarity implies a homogeneous radial structure for the cometary ice nucleus. The OH brightness distribution shows a spatial variation similar to the visible fan-shaped image of the comet, suggestive of a nonuniform distribution of volatile ices on the surface of the nucleus. The total derived water production rate appears to be a factor of 5 higher than that derived from HI Lyman-α observations made during the 1970 apparition and shows a variation with heliocentric distance (r) as r^?3.3 over the range 0.81 to 1.02 AU.     

Late Quaternary evolution of the Madeira Abyssal Plain,Canary Basin,NE Atlantic

The deepest part of the Canary Basin, the Madeira Abyssal Plain, receives allochthonous sediments derived from a large drainage basin which, if its subaerial continuation is included, covers an area of 3.36 times 10⁶ km². An international research effort over the last 10 years has recovered over 160 sediment cores from the plain, and the development of a high-resolution stratigraphy has enabled individual turbidites to be correlated layer by layer. Sedimentation on the Madeira Abyssal Plain during the late Quaternary is dominated by thick turbidite muds separated by thin pelagic intervals. The core density has allowed the mapping of each sedimentary unit throughout the abyssal plain, thus building up a layer by layer picture of sediment accumulation. Over the last 300 kyr, 600 km³ of turbidites compared to 60 km³ of pelagic sediments have been deposited on the plain. Sedimentary structures developed in the coarse basal facies of the larger turbidites are more complex than simple models predict due to surging flows, fluctuating flow velocities and reflection from adjacent high ground. Over the last 300 kyr, there has been a switching of entry points for turbidity currents entering the abyssal plain. From 300 ka to 200 ka, organic-rich turbidites were emplaced predominantly from the south but around 200 ka this source switched off and subsequent organic- and volcanic-rich turbidites, which included units deposited by giant, possibly hyperconcentrated flows, were emplaced from northern or eastern sources. Although restricted to the late Quaternary, the data presented provides a detailed case study of the evolution of an oceanic basin fill.     

Regional theory and regionalism: Towards rethinking the regional question

This article presents an historical review of the evolution of regional planning ideas under the growth pole paradigm, from its beginnings in the early 1950s to the current deluge of criticism. Then the outlines of an alternative approach are suggested, emphasizing what is called territorial development, a concept which focuses on meeting the cultural, political and economic needs of regional population groups — as opposed to urging their functional integration into the broader national and world economy. It is argued that territorial development can only be achieved by arousing the regional communities themselves into seeking selective regional closure and strategic regional advantage through willful community action.