Precambrian Potassic Ultramafic Rocks: South Greenland

Small hypabyssal intrusives of biotite pyroxenite and biotiteperidotite are described from part of theGardar alkaline province.The intrusives, composed essentially of diopside, olivine, phlogopiteand Fe-Ti oxides (and numerous accessory minerals) are inferredto have crystallized from silica-undersaturated ultramafic magmasof similar bulk composition. Crystallization occurred at anestimated depth of 3–4 km. Despite relatively rapid consolidation(probably attendant on a partial devolatilization) producingtypically fine-grained and sometimes porphyritic rocks, small-scaledifferentiation nevertheless occurred. The differentiates, asveins, pegmatoidal patches and roofing-facies tend to be olivine-poorphlogopite+ferriandiopside+ Fe-Ti oxide rocks, in which perovskite,andradite, apatite and sphene are significant components. Pyroxenecompositions in the ultramafites are consistent with the thesisthat the rocks formed from soda-deficient magmas in stronglyoxidizing conditions. Relative potassium enrichment (averageK₂O: Na₂O wt. % = 1.6) is expressed modally in the ubiquitouspresence of phlogopite and normatively by an average of 6.9%or and/or Ic. Despite compositional affinity with certain olivinemelilitites, melilite is absent, a fact that may be attributableto the comparatively high hydrostatic pressure (c. 1 kb) pertainingduring crystallization. Petrographic and geochemical evidencesuggests that the ultramafic magmas were residua after fractionationof forsteritic olivine from more magnesian magmas of kimberlitetype. It is proposed that the latter may in turn have been residuafrom eclogite fractionation of Fe-Ti rich primary magmas inthe upper mantle.     

Could G-class asteroids be the parent bodies of the CM chondrites?

Abstract— I review the dynamical and compositional evidence for possibly linking CM chondrites and asteroids having G-class taxonomic designations. Three G asteroids have been identified through previous theoretical studies as being likely meteorite source bodies due to their locations near resonances. Two of these objects, 19 Fortuna and 13 Egeria, have spectral properties that are consistent with such a linkage with CM chondrites. Fortuna has a similar strength 0.7 μm absorption feature and near-infrared spectral slope to CM chondrites but a weaker ultraviolet feature. Egeria also has the characteristic 0.7 μm feature of CM chondrite spectra but does not match as well in the near-infrared. However, since the 0.7 μm feature is apparent in the spectra of approximately one-half of measured C-type asteroids, no definitive statement about any linkages can be made. Ceres is spectrally different from known meteorites in the 3 μm wavelength region and cannot be convincingly linked with any meteorite group.     

Chemical compositions of Kilauea east-rift lava, 1968-1971

The major element chemical compositions of lava from four eruptionson the east rift zone of Kilauea between August 1968 and October1971 reflect three petrologic processes:

1. Production of chemically distinct batches of magma in the mantle.
2. Separation of olivine, augite, and plagioclase from liquidduringflow in the rift-zone conduits.
3. Mixing of differentmagmas during ascent to the surface.

Chemically none of the four Kilauea east-rift eruptions matchesthe preceding summit eruption in Halemaumau that ended in July1968. The Mauna Ulu eruption, May 1969 to October 1971 (thelast of flie east-rift eruptions), can be divided into fiveolivine-controlled and chemically distinct variants. Three ofthese characterize the first seven months of the eruption andare closest in composition to the 1967–8 Halemaumau eruption.Variants 4 and 5 were erupted later and have compositions thatare distinctly different from that of the 1967–8 eruption.Major differences are higher Al₂O₃ (0?15–0?23 per cent),and lower K₂O (0?07–0?10 per cent) and TiO₂ (0?12–0?23per cent) in variants 4 and 5 at the same MgO content. Somelavas from eruptions in August and October 1968 and February1969, have olivine-controlled magma compositions that are identicalto mixtures of Mauna Ulu variants 1–3 and the 1967–8composition. This observation fits an hypothesis advanced earlierby T. L. Wright and R. S. Fiske that magmas in the central magmachamber become mixed with magmas in the rift zone and can beidentified as mixing components of rift eruption magmas beforethey appear as distinctive magmas in summit eruptions. Lavas representing mixing of olivine-controlled magma with differentiatedmagma were erupted in October 1968, February 1969, and in Mayand December 1969. The changes in amount of K₂O and TiO₂ during the latter partof the 1969–71 Mauna Ulu eruption are the reverse of theoverall secular change in composition of Kilauea summit lavasfrom pre-1750 through 1967–8. The K₂O and TiO₂ contentsof the latest overflows during the 1969–71 Mauna Ulu eruption(April 1971) are comparable to that of lava erupted at Kilaueasummit prior to 1750. The changing chemistry of Kilauea magma is found to be of useas a ‘tracer’ in the complex Kilauea conduit system.Application of these data to older lava sequences is difficulbecause of the complexity of the processes controlling lavacomposition and the absence of detailed information about thetime-space chemical variation during individual eruptions.     

CELLULAR TOWER PROLIFERATION IN THE UNITED STATES

ABSTRACT. Since the early 1980s the growing popularity of cellular communication has wrought dramatic landscape changes on the American scene through an invasion of thousands of cellular telephone towers. Objections raised to new tower construction by local residents, interest groups, and regulatory boards range from visual impacts to perceived health risks. This essay traces the origins of wireless telephony, its proliferation across the United States, and the visual impacts associated with tower construction. Three stages in the geographical expansion of wireless networks are identified.     

Impact of the Arctic Fox Alopex lagopus on nesting success of geese in southeast Svalbard, 1989

Goose populations, with special emphasis on Light-bellied Brent Geese Branta bernicla hrota , were censured in Tusenøyane and Tjuvfjorden, southeast Svalbard, July-August 1989. A total of 425 non-breeding Brent Geese, 210 Barnacle Geese Branta leucopsis and 421 Pink-footed Geese Anser bra-chyrhynchus and 11, 2 and 3 families of the three species, respectively, were counted. Brent Geese attempted to nest on 6 of the 20 islands surveyed, and were successful on four. Barnacle Geese attempted to nest on three islands and were successful on two. Many islands known to be traditional nesting sites were entirely void of geese. Pink-footed Geese were only seen in Tjuvfjorden. The breeding failure on the islands was linked to the presence of the Arctic Fox Alopex lagopus which probably caused the geese to completely give up the attempt to nest. On one island the fox had apparently arrived after nest initiation and ravaged 45 Brent Goose and Barnacle Goose nests. It is likely that the foxes stranded on the islands during ice break-up. As a consequence of the presence of foxes on most of Tusenøyane, more than half of the potential breeding segment of the Svalbard Brent Goose population failed to nest in the summer of 1989.     

Optically stimulated luminescence dating as a tool for calculating sedimentation rates in Chinese loess: comparisons with grain-size records

Understanding loess sedimentation rates is crucial for constraining past atmospheric dust dynamics, regional climatic change and local depositional environments. However, the derivation of loess sedimentation rates is complicated by the lack of available methods for independent calculation; this limits interpretation of the environmental changes revealed by the loess record. In particular, while the Quaternary/Neogene Chinese loess and Red Clay sequences have the potential to provide detailed records of past sedimentation and climate change, there is great uncertainty concerning: (i) the influences on sediment grain‐size and accumulation; and (ii) their relationship through time and across the depositional region. This uncertainty has led to the widespread use of assumptions concerning the relationship between sedimentation rate and grain‐size in order to derive age models and climate reconstructions. To address this uncertainty, detailed independent age models, based on optically stimulated luminescence dating, undertaken at 10 to 40 cm intervals at five sections across the Loess Plateau in China, have been used to calculate sedimentation rates and make comparisons with grain‐size changes over the late Pleistocene and Holocene. The results demonstrate that sedimentation rates are site specific, extremely variable over millennial timescales and that this variation is often not reflected in grain‐size changes. In the central part of the Loess Plateau, the relationship between grain‐size and sedimentation rate appears most complex, suggesting an interplay between local conditions at source and sink and a changing emplacement mechanism. This observation further undermines the common use of loess sedimentation age models that rely on a derived relationship between grain‐size and sedimentation rate from a type section. The results also highlight the difficulty in assigning specific environmental causes to sedimentation rate changes and, to a lesser extent, grain‐size shifts.     

Time Scales of Crystal Fractionation in Magma Chambers--Integrating Physical, Isotopic and Geochemical Perspectives

A simple heat balance model for an evolving magma chamber isused to make predictions of the time scales for magma differentiation,which are compared with geological and isotopic constraintson the rates of crystallization and differentiation. In a 10km³ magma chamber releasing thermal energy at a rate of 100MW, basalt and rhyolite magmas should reach 50% crystallizationafter     

Lateglacial varve chronology and biostratigraphy of lakes Holzmaar and Meerfelder Maar, Germany

In this paper we briefly compare previously published data on four lateglacial sediment profiles from lakes Holzmaar and Meerfelder Maar in the Eifel region. Two of these profiles (one from each lake) are varvedated, whereas the other two pollen profiles are on depth scales. In general, the palynological signal in all profiles compares well, proving the regional validity of the individual data sets. There are some discrepancies between Holzmaar and Meerfelder Maar, however, mainly concerning varve dating of major lateglacial features, and these are explained after detailed correlation using a combination of varve dating and palynological signals. The result is a consistent varve-dated biostratigraphy for the Lateglacial in the Eifel region. Moreover, it is demonstrated that a combination of varve chronology and high resolution palynostratigraphy in several lake profiles enables even gaps of a few decades up to a few centuries to be detected, thus providing a valuable tool for precise regional palaeoenvironmental study. This multi-core study on two lakes from the same region demonstrates the likelihood of undetected errors in single-core studies on non-varved sediments.