Kimberlite petrogenesis: Insights from clinopyroxene-melt partitioning experiments at 6 GPa in the CaO-MgO-Al2O3-SiO2-CO2 system

In this experimental study, we examine the mineral-melt partitioning of major and trace elements between clinopyroxene and CO₂-rich kimberlitic melts at a pressure of 6 GPa and temperatures of 1410°C and 1430°C. The melts produced contain ∼ 28 wt% dissolved CO₂, and are saturated with olivine and clinopyroxene. To assess the effects of temperature, crystal and melt compositions on trace element partitioning, experiments were performed in the model CaO-MgO-Al₂O₃-SiO₂-CO₂ system. Our results reveal that all the elements studied, except Al, Mg, Si, and Ga, are incompatible in clinopyroxene. Partition coefficients show a considerable range in magnitude, from ∼ 10⁻³ for D_U and D_Ba to ∼ 2.5 for D_Si. The two experimental runs show similar overall partitioning patterns with the D values being lower at 1430°C. Rare earth elements display a wide range of partition coefficients, D_La (0.012-0.026) being approximately one order of magnitude lower than D_Lu (0.18-0.23). Partition coefficients for the 2+ and 3+ cations entering the M2-site exhibit a near-parabolic dependence on radius of the incorporated cations as predicted from the lattice strain model. This underlines the contribution made by the crystal structure toward controlling the distribution of trace elements. Using data obtained in this study combined with that in the published literature, we also discuss the effects that other important parameters, namely, melt composition, pressure, and temperature, could have on partitioning.Our partition coefficients have been used to model the generation of the Group I (GI) kimberlites from South Africa. The numerical modeling shows that kimberlitic melts can be produced by ∼0.5% melting of a MORB-type depleted source that has been enriched by small-degree melts originating from a similar depleted source. This result suggests that the source of GI kimberlites may be located at the lithosphere-asthenosphere transition. Percolation of small degree melts from the asthenosphere would essentially create a metasomatic horizon near the bottom of the non-convecting sublithospheric mantle. Accumulation of such small degree melts together with the presence of volatiles and conductive heating would trigger melting of the ambient mantle and subsequently lead to eruption of kimberlitic melts. Additionally, our model shows that the GI source can be generated by metasomatism of a 2 Ga old MORB source ca. 1 Ga ago. Assuming that MORB-type mantle is the most depleted source of magmas on earth, then this is the oldest age at which the GI source could have existed. However, this age most likely reflects the average age of a series of metasomatic events than that of a single event.     

Making ends meet, in the household and on the planet

Policy-making in relation to sustainable development is usually at the national (or, in relation to climate change, the global) level, yet the consumption it seeks to modify takes place at the household level. If households all ‘made ends meet’ in the same way then the much-relied upon notion of per capita consumption would be valid and we could rely on ‘top-down’ modelling to guide policy. Cultural Theory, however, predicts that there are five socially viable ways of making ends meet, and that all of them will be found (in varying proportions) within any nation. This prediction has been tested on a sample of 220 British households and shown to be well supported. Top-down modelling, it is argued, has to give way to a constructive interplay between the reflexive policy-maker and a plurally responsive citizenry. This revised version was published online in August 2006 with corrections to the Cover Date.     

Using Shoreline Video Assessment for coastal planning and restoration in the context of climate change in Kien Giang,Vietnam

Kien Giang, bordering Cambodia in the Mekong River Delta, is one of the two most vulnerable provinces in the region to coastal erosion and flooding. Coastal protection can conflict with current land use and economic development activities. The conditions of the mangrove forest and mainland coastline of the Kien Giang province were assessed using the Shoreline Video Assessment Method (SVAM) backed up with information from satellite images. Half of the 206 km Kien Giang coastline has been eroded or is being eroded. Protective mangrove forests naturally occurred in 74% of the coastline but have been under threat from illegal cutting, erosion and coastal retreat. Accurate information on the state of the coastline and mangrove forest health provided invaluable data for developing a new coastal rehabilitation plan to guard against future sea level rise. In contrast to the current boundary management of land and natural resources, this plan divided the provincial coastline into 19 sections based on the landscape condition and exposure to erosion. Priority strategic actions for erosion management, mangrove restoration and sustainable livelihood development for local communities for each section of coast were developed based on an integrated cross sectoral approach and practical experience in the Conservation and Development of the Kien Giang Biosphere Reserve Project.

     

Extending the Capture Map Concept to Estimate Discrete and Risk-Based Streamflow Depletion Potential

A popular and contemporary use of numerical groundwater models is to estimate the discrete relation between groundwater extraction and surface-water/groundwater exchange. Previously, the concept of a “capture map” has been put forward as a means to effectively summarize this relation for decision-making consumption. While capture maps have enjoyed success in the environmental simulation industry, they are deterministic, ignoring uncertainty in the underlying model. Furthermore, capture maps are not typically calculated in a manner that facilitates analysis of varying combinations of extraction locations and/or reaches. That is, they are typically constructed with focus on a single reach or group of reaches. The former of these limitations is important for conveying risk to decision makers and stakeholders, while the latter is important for decision-making support related to surface-water management, where future foci may include reaches that were not the focus of the original capture analysis. Herein, we use the concept of a response matrix to generalize the theory of the capture-map approach to estimate spatially discrete streamflow depletion potential. We also use first-order, second-moment uncertainty estimation techniques with the concept of “risk shifting” to place capture maps and streamflow depletion potential in a stochastic, risk-based framework. Our approach is demonstrated for an integrated groundwater/surface-water model of the lower San Antonio River, Texas, USA.