Modeling incompatible trace-element abundances in plagioclase in the Skaergaard intrusion using the trapped liquid shift effect

Incompatible trace-element abundances in minerals and whole rocks from layered intrusions have been used to model the fractionation processes and evolving liquid compositions. Many such models assume that the analyzed concentration in a mineral represents that of the mineral when it first crystallized. However, overgrowth from residual liquid and subsequent diffusive equilibration can result in significant changes to the bulk mineral compositions (the more incompatible the element the more dramatic the subsequent changes). The proportion of that residual liquid relative to the cumulus minerals is the most important parameter in determining the magnitude of this effect (trapped liquid shift effect). Calculations involving Ba and La contents in plagioclase quantitatively demonstrate this effect. For Ba and La (partition coefficients of 0.4 and 0.04), 50% trapped liquid in a sample can result in two and sevenfold increases, respectively, in concentration between original and final bulk mineral compositions. Different cumulus assemblages also have a major effect on final compositions. We use examples of the concentrations of Ba and La in plagioclase from the Skaergaard intrusion from previous publications to illustrate the importance of this effect. Specifically, the La content of bulk plagioclase steadily decreases upward from the Lower Zone to Upper Zone c, and Ba in plagioclase shows no increase from the Lower Zone to the top of the Middle Zone. Such results are not explicable by fractionation processes, but can be modeled by the trapped liquid shift effect, assuming the well-established evidence for upward decrease in trapped liquid proportion through these zones.     

Space- and Time-Dependent Probabilities for Earthquake Fault Systems from Numerical Simulations: Feasibility Study and First Results

In weather forecasting, current and past observational data are routinely assimilated into numerical simulations to produce ensemble forecasts of future events in a process termed “model steering”. Here we describe a similar approach that is motivated by analyses of previous forecasts of the Working Group on California Earthquake Probabilities (WGCEP). Our approach is adapted to the problem of earthquake forecasting using topologically realistic numerical simulations for the strike-slip fault system in California. By systematically comparing simulation data to observed paleoseismic data, a series of spatial probability density functions (PDFs) can be computed that describe the probable locations of future large earthquakes. We develop this approach and show examples of PDFs associated with magnitude M > 6.5 and M > 7.0 earthquakes in California.     

The petrogenesis of Merensky Reef potholes at the Western Platinum Mine, Bushveld Complex: Sr-isotopic evidence for synmagmatic deformation

Potholes represent areas where the normally planar PGE-rich Merensky Reef of the upper Critical Zone of the Bushveld Complex transgresses its footwall, such geometric relationships being unusual in layered intrusions. The recognition of vertical dykes of Merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull-apart sites resulting from tensional deformation due to the loading effects of major new magma additions. In contrast, crosscutting anorthosite veins display physical and isotopic evidence of upward emplacement. The Merensky Reef and its footwall have distinct initial Sr-isotope ratios (R ₀ > 0.7066 and <0.7066, respectively), which may be used to constrain these processes related to pothole formation. Merensky Reef in potholes (R ₀ = 0.7069−0.7078) shows no isotopic evidence of assimilation of, or reaction with, footwall material. Discrete, discordant replacement bodies of anorthosite extend from the footwall lithologies to cross-cut the Merensky Reef and its hanging wall. The initial Sr-isotope ratio in these replaced rocks is totally reset to footwall values (R ₀ = 0.7066), and immediately adjacent stratiform lithologies are slightly modified towards footwall values. In contrast, Neptunian pyroxenitic (Merensky) dykes cross-cutting the footwall lithologies, with a large surface area to volume ratio, and low Sr content, do not display footwall-like Sr-isotope initial-ratios (R ₀ = 0.7077), and thus show no evidence for assimilation of or reaction with footwall material. Furthermore, pegmatoidal replacement pyroxenite (“replacement pegmatoid”), at the base of the Merensky Reef within potholes, has a high initial-ratio (R ₀ > 0.7071), and so models of pervasive metasomatism by footwall material are not applicable. This isotopic evidence indicates that there was no active interaction of footwall material with the overlying magma during, or after, the formation of Merensky Reef potholes, a basic tenet of existing pothole formation hypotheses involving footwall mass-transfer. In contrast, the isotopic data are entirely consistent with an extensional model for pothole formation, with the more radiogenic Merensky magma migrating laterally to fill extensional zones in the footwall layers. Received: 11 October 1997 / Accepted: 21 December 1998     

Last Glacial Maximum to Early Holocene Wind Strength in the Mid-latitudes of the Southern Hemisphere from Aeolian Dust in the Tasman Sea

Dust transported by Southern Hemisphere mid-latitude westerly winds from Australia and deposited in the Tasman Sea shows no evidence for stronger winds during the last glacial maximum (LGM), compared to the Holocene. Features of the particle-size distributions of the dust do, however, indicate enhanced dry deposition of dust in the LGM changing to rainfall scavenging during deglaciation and the early Holocene as climate ameliorated. From these results it appears that activation of desert dunefields over 40% of Australia during the LGM was the result of a reduction in stabilizing vegetation and more frequent episodes of sand movement rather than of increased wind strength. The LGM climate of inland Australia must have been considerably more stressful for plants as a result of lower precipitation and/or carbon dioxide stress to achieve the implied levels of surface destabilization. Enhanced atmospheric dust loads in the Southern Hemisphere and deposition over Antarctica were most likely the result of greatly expanded source areas in the mid-latitude southern continents and a weaker hydrological cycle rather than greater entrainment or more efficient transport by stronger winds. During the LGM wind strength appears to have varied regionally, and predominantly in high latitudes, rather than uniformly for all zonal winds.     

Management Strategies to Sequester Carbon in Agricultural Soils and to Mitigate Greenhouse Gas Emissions

Carbon sequestration in agricultural soils is frequently promoted as a practical solution for slowing down the rate of increase of CO₂ in the atmosphere. Consequently, there is a need to improve our understanding of how land management practices may affect the net removal of greenhouse gases (GHG) from the atmosphere. In this paper we examine the role of agriculture in influencing the GHG budget and briefly discuss the potential for carbon mitigation by agriculture. We also examine the opportunities that exist for increasing soil C sequestration using management practices such as reduced tillage, reduced frequency of summer fallowing, introduction of forage crops into crop rotations, conversion of cropland to grassland and nutrient addition via fertilization. In order to provide information on the impact of such management practices on the net GHG budget we ran simulations using CENTURY (a C model) and DNDC (a N model) for five locations across Canada, for a 30-yr time period. These simulations provide information on the potential trade-off between C sequestration and increased N₂O emissions. Our model output suggests that conversion of cropland to grassland will result in the largest reduction in net GHG emissions, while nutrient additions via fertilizers will result in a small increase in GHG emissions. Simulations with the CENTURY model also indicated that favorable growing conditions during the last 15 yr could account for an increase of 6% in the soil C at a site in Lethbridge, Alberta. Presented at the International Workshop on Reducing Vulnerability of Agriculture and Forestry to Climate Variability and Climate Change, Ljubljana, Slovenia, 7–9 October 2002.     

Non-local mixing of momentum in the convective boundary layer

Recently Frech and Mahrt proposed a closure scheme which includes alarge-scale stress term to represent the effects of non-local momentummixing in the convective boundary layer. Here large-eddy simulation (LES)datasets are used to evaluate the performance of this scheme across a rangeof stabilities between neutral and highly convective conditions, and as afunction of baroclinity. Generally the inclusion of the non-local term inthe closure model leads to results in better agreement with LES, althoughsome modifications to the model formulation are suggested.     

Temporal, geomagnetic and related attributes of kimberlite magmatism at Ekati, Northwest Territories, Canada

This paper outlines the development of a multi-disciplinary strategy to focus exploration for economic kimberlites on the Ekati property. High-resolution aeromagnetic data provide an over-arching spatial and magnetostratigraphic framework for exploration and kimberlite discovery at Ekati, and hence also for this investigation. The temporal, geomagnetic, spatial and related attributes of kimberlites with variable diamond content have been constrained by judiciously augmenting the information gathered during routine exploration with detailed, laboratory-based or field-based investigations. The natural remanent magnetisation of 36 Ekati kimberlites has been correlated with their age as determined by isotopic dating techniques, and placed in the context of a well-constrained geomagnetic polarity timescale. Kimberlite magmatism occurred over the period 75 to 45 Ma, in at least five temporally discrete intrusive episodes. Based on current evidence, the older kimberlites (75 to 59 Ma) have low diamond contents and are distributed throughout the property. Younger kimberlites (56 to 45 Ma) have moderate to high diamond contents and occur in three distinct intrusive corridors with NNE to NE orientations. Economic kimberlite pipes erupted at 55.4±0.4 Ma along the A154-Lynx intrusive corridor, which is 7 km wide and oriented at 015°, and at 53.2±0.3 Ma along the Panda intrusive corridor, which is 1 km wide and oriented at 038°. The intrusion ages straddle a paleopole reversal at Chron C24n, consistent with the observation that the older economic kimberlites present as aeromagnetic “low” anomalies while the younger economic pipes are characterised as aeromagnetic “highs”. The aeromagnetic responses for these kimberlites are generally muted because they contain volcaniclastic rock types with low magnetic susceptibility. Kimberlites throughout the Ekati property carry a primary natural magnetic remanence (NRM) vector in Ti-bearing groundmass magnetite, and it dominates over vectors related to induced magnetisation. Magnetostratigraphic correlation of Ekati kimberlites may therefore present a powerful adjunct to existing exploration techniques, mainly because the diamond content of Ekati kimberlites apparently is related more to the age of eruption than to any other parameter investigated in this work.     

Platelet development in cuboid diamonds: insights from micro-FTIR mapping

Fourier transform infrared mapping of diamonds can reveal detailed information on impurities, with a spatial context. We apply this technique, combined with in situ isotopic analysis of carbon, to the study of cuboid diamond growth in a sample that exhibits some mixed-habit growth. While there has been some uncertainty in the literature regarding sectoral differences in nitrogen aggregation and subsequent platelet development, the data from this study appear far more conclusive. We show that despite nitrogen being concentrated in octahedral sectors, there is no detectable difference in the concentration-adjusted rate of nitrogen aggregation within octahedral and cuboid sectors. However, the resultant platelet development is significantly reduced in cuboid sectors compared to contemporaneously formed octahedral sectors. This finding has significant implications for the classification of diamonds using the relationship between their platelet intensity and the absorption caused by B centres. It means that cuboid diamonds naturally fall below the linear relationship that has been termed regular, which would lead to them being incorrectly interpreted as having experienced heating or deformation. The data also support earlier suggestions that large hydrogen concentrations in the diamond-forming fluid may be required for cuboid growth. We further suggest that high nitrogen and hydrogen concentrations are required for mixed-habit diamond growth, which might be the product of specific fluid chemistries that occur in reducing mantle environments.     

Climate forcing by carbonaceous and sulfate aerosols

 An atmospheric general circulation model is coupled to an atmospheric chemistry model to calculate the radiative forcing by anthropogenic sulfate and carbonaceous aerosols. The latter aerosols result from biomass burning as well as fossil fuel burning. The black carbon associated with carbonaceous aerosols is absorbant and can decrease the amount of reflected radiation at the top-of-the-atmosphere. In contrast, sulfate aerosols are reflectant and the amount of reflected radiation depends nonlinearly on the relative humidity. We examine the importance of treating the range of optical properties associated with sulfate aerosol at high relative humidities and find that the direct forcing by anthropogenic sulfate aerosols can decrease from −0.81 W m^-2 to −0.55 Wm^-2 if grid box average relative humidity is not allowed to increase above 90%. The climate forcing associated with fossil fuel emissions of carbonaceous aerosols is calculated to range from +0.16 to +0.20 Wm^-2, depending on how much organic carbon is associated with the black carbon from fossil fuel burning. The direct forcing of carbonaceous aerosols associated with biomass burning is calculated to range from −0.23 to −0.16 Wm^-2. The pattern of forcing by carbonaceous aerosols depends on both the surface albedo and the presence of clouds. Multiple scattering associated with clouds and high surface albedos can change the forcing from negative to positive. Received: 29 September 1997 / Accepted: 10 June 1998     

The Effects of Informal Settlement on Suburban Property Values in Cape Town,South Africa

During the late 1980s some informal settlements were established within or near affluent Cape Town suburbs. Despite objections by suburban residents that these settlements would lead to a rapid escalation in crime and a dramatic fall in the value of properties, the authorities allowed the settlements to remain. Using data on property transfers, which are then mapped using GIS, the paper examines the effects that the informal settlement of Marconi Beam had on house prices and housing turnover within the adjacent suburb of Milnerton. The results show that most of the objections regarding Marconi Beam's effect on property values were groundless as the settlement had only a negligible effect on property values in the adjacent suburb.