Identifying Accessory Mineral Saturation during Differentiation in Granitoid Magmas: an Integrated Approach

Numerical reconstructions of processes that may have operatedduring igneous petrogenesis often model the behaviour of importanttrace elements. The geochemistry of these trace elements maybe controlled by accessory mineral saturation and fractionation.Determination of the saturation point of accessory mineralsin granitoid rocks is ambiguous because assumptions about crystalmorphology and melt compositions do not always hold. An integratedapproach to identifying accessory mineral saturation involvingpetrography, whole-rock geochemical trends, saturation calculationsand mineral chemistry changes is demonstrated here for a compositionallyzoned pluton. Within and between whole-rock samples of the BoggyPlain zoned pluton, eastern Australia, the rare earth element(REE)-enriched accessory minerals zircon, apatite and titaniteexhibit compositional variations that are related to saturationin the bulk magma, localized saturation in intercumulus meltpools and fractionation of other mineral phases. Apatite isidentified as having been an early crystallizing phase overnearly the whole duration of magma cooling, with zircon (andallanite) only saturating in more felsic zones. Titanite andmonazite did not saturate in the bulk magma at any stage ofdifferentiation. Although some trace elements (P, Ca, Sc, Nb,Hf, Ta) in zircon exhibit compositional variation progressingfrom mafic to more felsic whole-rock samples, normalized REEpatterns and abundances (except Ce) do not vary with progressivedifferentiation. This is interpreted to be a result of limitationsto both simple ‘xenotime’ and complex xenotime-typecoupled substitutions. Our data indicate that zircon REE characteristicsare not as useful as those of other REE-rich accessory mineralsas a petrogenetic indicator. KEY WORDS: saturation; zircon; apatite; titanite; magma differentiation; trace elements; REE patterns     

Rural Self-Reliance in Bondolfi, Zimbabwe: the role of beekeeping

Community-based economic self-reliance has become critical in many African countries because of the severity of prevailing soci-economic and environmental constraints. Concerns over food security have encouraged a re-evaluation of the importance of indigenous technical knowledge. This study examines a rural area in Zimbabwe where members of the local community have established a successful beekeeping association to improve their livelihoods. The history of the initiative, its broader impact on the community and the role played by external facilitation feature in the study.     

Mountain Biking in the Manawatu Region: Participants, Perceptions, and Management Dimensions

The increase in the number of mountain bikers world‐wide has raised a number of issues associated with impacts and appropriate management strategies. Studies in the USA, UK and New Zealand have indicated a growing concern with environmental impacts, demonstrated attitudes of other recreationalists to mountain bikers and identified a number of management issues. This article presents the findings from field research conducted during the southern summer of 1997/8. The research was carried out in association with the Palmerston North City Council (PNCC) and the Manawatu Mountain Bike Club. The aims of the research were to investigate the demographics of mountain bikers and related management issues. Key findings of the field research were that: access to mountain biking opportunities was a problem; information provision was poor; there was conflict between user groups; there was a lack of agreement on whether there should be dedicated or multi‐use tracks. The article discusses how the research findings have been used to assist in the creation of an outdoor recreation and tourism strategy for Palmerston North. A model for managing recreation /tourism activities is also suggested.     

Playa‐lake sedimentation and organic matter accumulation in an Andean piggyback basin: the recent record from the Cuenca de Pozuelos,North‐west Argentina

Expansive playa‐lake systems situated in high‐altitude piggyback basins are important and conspicuous components of both modern and ancient cordilleran orogenic systems. Extant playa lakes provide vital habitat for numerous endemic species, whereas sediments from these deposystems may record signals of climate change or develop natural resources over geological time. Laguna de los Pozuelos (North‐west Argentina) provides the opportunity for an actualistic sedimentological and geochemical assessment of a piggyback basin playa lake in an area of critical interest for understanding Quaternary palaeoclimate dynamics. Silty clays and diatom ooze are the dominant playa‐lake centre microfacies, with concentrations of total organic carbon and biogenic silica commonly exceeding 1·5 wt% in this sub‐environment. Elemental and stable isotopic analyses point to a mixed organic matter composition in the playa‐lake centre, with substantial contributions from algae and transported aquatic macrophytes. Bulk sediment and organic mass accumulation rates in the southern playa‐lake centre approach 0·22 g cm^?2 year^?1 and 2·89 mg cm^?2 year^?1, respectively, indicating moderately rapid deposition with negligible deflation over historic time. Playa margin facies contain higher percentages of fragmented biogenic carbonate (ostracods and charophytes) and inorganically precipitated aragonite crusts due to seasonal pumping and evaporation of ground water. Organic matter accumulation is limited along these heavily bioturbated wet and dry mud flats. Fluvial–lacustrine transitional environments, which are key waterbird habitats, are either silty terminal splay (northern axis) or sandy deltas (southern axis) containing highly oxidized and partially allochthonous organic matter. Modern analogue data from Laguna de los Pozuelos provide key insights for: (i) environmental reconstructions of ancient lake sequences; and (ii) improving facies models for piggyback basins.     

REE-Depleted Leucogranites, Black Hills, South Dakota: a Consequence of Disequilibrium Melting of Monazite-Bearing Schists

Two isotopically distinct but otherwise chemically similar leucogranitesuites in the Proterozoic Horney Peak Granite, Black Hills,South Dakota, have contrasting light rare earth element (LREE)concentrations. Most samples of a relatively ¹⁸O-depleted suitehave LREE- enriched, chondrite-normalized patterns, typicalof melts derived from metasedimentary protoliths, whereas allsamples of the regionally significant, relatively ¹⁸O-enrichedsuite have LREE-depleted patterns. The latter patterns are interpretedto have resulted from disequilibrium melting of schists. Monaziteand perhaps other accessory minerals remained armored by biotiteand garnet which did not partake in the muscovite dehydration-meltingreaction that produced LREE-depleted melts. The REE concentrationsin the LREE-depleted samples are below saturation levels formonazite at reasonable melting temperatures and melt water contents,whereas the REE concentrations in the LREE-enriched samplesyield 700–800C monazite saturation temperatures, reasonablefor biotite dehydration-melting reactions. LREE depletions,analogous to those in the LREE-depleted granites, are also foundin leucosomes of partially molten schists, thought to be theprotolith for the granite. In contrast, the melanosomes holdthe accessory minerals and bulk of the LREEs. KEY WORDS: accessory minerals; leucogranites; Black Hills; monazite; partial melting ^*Corresponding author at Department of Geological Sciences, University of Missouri. Telephone: 314-884-6463. Fax: 314-882-5458. e-mail: geolpin{at}showme.missouri.edu.     

Recent sedimentary processes in Scoresby Sund, East Greenland

The fjord system of Scoresby Sund on the east coast of Greenland has been the subject of two research cruises by RV Polarstern in 1988 and 1990. Most of the year, the fjord is covered by sea-ice. Sediment input takes place mostly via drifting icebergs during the short summer period. Depending on the distance to glaciers, surface sediments carry varying proportions of coarse ice-rafted debris (IRD). The degree of sediment reworking by scouring icebergs is controlled by the depth of the fjord, with the most intense reworking in areas shallower than about 450 m depth. Both IRD contribution and intensity of sediment scouring clearly control the distribution pattern of benthic organisms.     

Radiocarbon-dating of transitions between marine and lacustrine sediments and their relation to the development of lakes

In three different areas in western Norway, large errors are obtained in the radiocarbon dates from lacustrine sediments close to marine/lacustrine sediment boundaries. Differences occur between radiocar-bon and pollenanalytic dates and between dates at isolation/ingression contacts for lakes of the same altitude above the present sea level. Younger radiocarbon dates are also obtained below older ones in undisturbed sediments. When divergent dates occur, the radiocarbon dates always seem to be the youngest. Large differences are also found between NaOH soluble and insoluble fractions of the same sediment samples. Insoluble fractions generally yield younger dates than the soluble. Differences are not found, however, for dates younger than about 8,000 B.P. The dating errors are connected to periods with more oligotrophic conditions with isoetides. Their roots penetrate older sediments. Due to contamination of the organic part of the sediment from partly decomposed roots, some radiocarbon dates will be too young. The isoetide vegetation and the dating errors disappear when the lakes become dystrophic.