GEMAS: Adaptation of weathering indices for European agricultural soil derived from carbonate parent materials

2021 ◽  
pp. geochem2021-059
Author(s):  
Philippe Négrel ◽  
Anna Ladenberger ◽  
Clemens Reimann ◽  
Alecos Demetriades ◽  
Manfred Birke ◽  
...  
Keyword(s):  
Chemical Weathering ◽  
Large Scale ◽  
Parent Material ◽  
Chemical Compositions ◽  
Secondary Phases ◽  
Good Tool ◽  
Continental Scale ◽  
Rock Types ◽  
Special Adaptation ◽  
Weathering Indices

Carbonate rocks are very soluble and export elements in dissolved form, and precipitation of secondary phases can occur at a large scale. They leave a strong chemical signature in soil that can be quantified and classified by geochemical indices, which are useful for evaluating chemical weathering trends (e.g., the Chemical Index of Alteration CIA or the Mafic Index of Alteration MIA). Due to contrasting chemical compositions and high Ca content, a special adaptation of classical weathering indices is necessary to interpret weathering trends in carbonate-derived soil. In fact, this adaptation seems to be a good tool for distinguishing weathering grades of source rock types at the continental-scale, and allows a more robust interpretation of soil parent material weathering grade and its impact on the current chemical composition of soil. Increasing degree of weathering results in Al enrichment and Mg loss in addition to Fe loss and Si enrichment, leaching of mobile cations such as Ca and Na and precipitation of Fe-oxides and hydroxides. Relation between soil weathering status and its spatial distribution in Europe provides important information on the role played by climate and terrain. Geographical distribution of soil chemistry contributes to a better understanding of soil nutritional status, element enrichment, degradation mechanisms, desertification, soil erosion and contamination.

Major element geochemistry of European agricultural soil: weathering processes of silicate parent materials

2021 ◽  
Author(s):  
Philippe Negrel ◽  
Anna Ladenberger ◽  
Clemens Reimann ◽  
Alecos Demetriades ◽  
Manfred Birke ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Agricultural Soil ◽  
Chemical Weathering ◽  
Large Scale ◽  
Secondary Products ◽  
Transfer Coefficients ◽  
Parent Materials ◽  
Weathering Processes ◽  
Continental Scale ◽  
Weathering Indices

<p>Collection of agricultural soil samples in Europe (0–20 cm, 33 countries, 5.6 million km<sup>2</sup>) during the GEMAS (GEochemical Mapping of Agricultural and grazing land Soil) continental-scale project allowed the study of geochemical behaviour of major elements during weathering (SiO<sub>2</sub>, TiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>, Fe<sub>2</sub>O<sub>3</sub>, MgO, CaO, Na<sub>2</sub>O, K<sub>2</sub>O, P<sub>2</sub>O<sub>5</sub>) using their total concentrations (XRF data). The chemical composition of soil represents to a large extent the primary mineralogy of the source bedrock, the effects of pre- and post-depositional weathering and element mobility, either by leaching or mineral sorting with the addition of formation of secondary products such as clays.</p><p>Bulk geochemistry is used to calculate a set of weathering indices such as chemical index of alteration CIA, reductive and oxidative mafic index of alteration MIA, the change in mass balance t (calculation relative to immobile Nb) for soil derived from silicate parent materials defined as granite, gneiss and schist at the European continental-scale. Silicate minerals of soil parent materials can be either very resistant to weathering or very soluble and export of elements in dissolved form and precipitation of secondary phases can occur at a large scale. Either way, they leave a strong chemical signature in derived soil, which can be quantified and classified with help of geochemical indices that are useful tools to evaluate chemical weathering trends. Weathering indices and gain-loss mass transfer coefficients were applied to agricultural soil to provide an insight into the weathering processes affecting three silicate parent rocks and their impact on soil development at the European scale. Distinct chemical composition and weathering patterns has been evidenced in silicate derived soil. The interpretation of geographical distribution of soil types with silicate substrate allows better understanding of soil nutritional status, metal enrichments, degradation mechanisms under various climate conditions.</p>

scholarly journals Thermoelectric Properties of Thin Films from Sorted Single-Walled Carbon Nanotubes

Materials ◽  
2020 ◽  
Vol 13 (17) ◽  
pp. 3808 ◽  
Author(s):  
Blazej Podlesny ◽  
Bogumila Kumanek ◽  
Angana Borah ◽  
Ryohei Yamaguchi ◽  
Tomohiro Shiraki ◽  
...  
Keyword(s):  
Carbon Nanotubes ◽  
Thermoelectric Properties ◽  
Large Scale ◽  
Single Walled Carbon Nanotubes ◽  
Parent Material ◽  
Free Standing ◽  
Two Phase ◽  
Single Walled Carbon ◽  
Walled Carbon Nanotubes ◽  
Aqueous Two Phase Extraction

Single-walled carbon nanotubes (SWCNTs) remain one of the most promising materials of our times. One of the goals is to implement semiconducting and metallic SWCNTs in photonics and microelectronics, respectively. In this work, we demonstrated how such materials could be obtained from the parent material by using the aqueous two-phase extraction method (ATPE) at a large scale. We also developed a dedicated process on how to harvest the SWCNTs from the polymer matrices used to form the biphasic system. The technique is beneficial as it isolates SWCNTs with high purity while simultaneously maintaining their surface intact. To validate the utility of the metallic and semiconducting SWCNTs obtained this way, we transformed them into thin free-standing films and characterized their thermoelectric properties.

scholarly journals Analog Downscaling of Seasonal Rainfall Forecasts in the Murray Darling Basin

2013 ◽  
Vol 141 (3) ◽  
pp. 1099-1117 ◽  
Author(s):  
Andrew Charles ◽  
Bertrand Timbal ◽  
Elodie Fernandez ◽  
Harry Hendon
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Seasonal Prediction ◽  
General Circulation Models ◽  
Meridional Wind ◽  
Seasonal Forecasts ◽  
Local Climate ◽  
Continental Scale ◽  
Predictive Skill ◽  
Murray Darling Basin

Abstract Seasonal predictions based on coupled atmosphere–ocean general circulation models (GCMs) provide useful predictions of large-scale circulation but lack the conditioning on topography required for locally relevant prediction. In this study a statistical downscaling model based on meteorological analogs was applied to continental-scale GCM-based seasonal forecasts and high quality historical site observations to generate a set of downscaled precipitation hindcasts at 160 sites in the South Murray Darling Basin region of Australia. Large-scale fields from the Predictive Ocean–Atmosphere Model for Australia (POAMA) 1.5b GCM-based seasonal prediction system are used for analog selection. Correlation analysis indicates modest levels of predictability in the target region for the selected predictor fields. A single best-match analog was found using model sea level pressure, meridional wind, and rainfall fields, with the procedure applied to 3-month-long reforecasts, initialized on the first day of each month from 1980 to 2006, for each model day of 10 ensemble members. Assessment of the total accumulated rainfall and number of rainy days in the 3-month reforecasts shows that the downscaling procedure corrects the local climate variability with no mean effect on predictive skill, resulting in a smaller magnitude error. The amount of total rainfall and number of rain days in the downscaled output is significantly improved over the direct GCM output as measured by the difference in median and tercile thresholds between station observations and downscaled rainfall. Confidence in the downscaled output is enhanced by strong consistency between the large-scale mean of the downscaled and direct GCM precipitation.

scholarly journals Ocean deoxygenation, the global phosphorus cycle and the possibility of human-caused large-scale ocean anoxia

2017 ◽  
Vol 375 (2102) ◽  
pp. 20160318 ◽  
Author(s):  
Andrew J. Watson ◽  
Timothy M. Lenton ◽  
Benjamin J. W. Mills
Keyword(s):  
Chemical Weathering ◽  
Large Scale ◽  
Oxygen Demand ◽  
Subsurface Water ◽  
Phosphorus Cycle ◽  
Low Oxygen ◽  
Positive Feedbacks ◽  
Future Possibility ◽  
Warming World ◽  
Ocean Deoxygenation

The major biogeochemical cycles that keep the present-day Earth habitable are linked by a network of feedbacks, which has led to a broadly stable chemical composition of the oceans and atmosphere over hundreds of millions of years. This includes the processes that control both the atmospheric and oceanic concentrations of oxygen. However, one notable exception to the generally well-behaved dynamics of this system is the propensity for episodes of ocean anoxia to occur and to persist for 10 5 –10 6 years, these ocean anoxic events (OAEs) being particularly associated with warm ‘greenhouse’ climates. A powerful mechanism responsible for past OAEs was an increase in phosphorus supply to the oceans, leading to higher ocean productivity and oxygen demand in subsurface water. This can be amplified by positive feedbacks on the nutrient content of the ocean, with low oxygen promoting further release of phosphorus from ocean sediments, leading to a potentially self-sustaining condition of deoxygenation. We use a simple model for phosphorus in the ocean to explore this feedback, and to evaluate the potential for humans to bring on global-scale anoxia by enhancing P supply to the oceans. While this is not an immediate global change concern, it is a future possibility on millennial and longer time scales, when considering both phosphate rock mining and increased chemical weathering due to climate change. Ocean deoxygenation, once begun, may be self-sustaining and eventually could result in long-lasting and unpleasant consequences for the Earth's biosphere. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.

Global scale hydrological modelling at 100 m, 1 h resolution, in Python

2021 ◽  
Author(s):  
Kor de Jong ◽  
Marc van Kreveld ◽  
Debabrata Panja ◽  
Oliver Schmitz ◽  
Derek Karssenberg
Keyword(s):  
Large Scale ◽  
Model Building ◽  
Flow Direction ◽  
Building Blocks ◽  
Global Scale ◽  
Data Availability ◽  
Hydrological Models ◽  
Continental Scale ◽  
Modelling Framework ◽  
Scale Models

<p>Data availability at global scale is increasing exponentially. Although considerable challenges remain regarding the identification of model structure and parameters of continental scale hydrological models, we will soon reach the situation that global scale models could be defined at very high resolutions close to 100 m or less. One of the key challenges is how to make simulations of these ultra-high resolution models tractable ([1]).</p><p>Our research contributes by the development of a model building framework that is specifically designed to distribute calculations over multiple cluster nodes. This framework enables domain experts like hydrologists to develop their own large scale models, using a scripting language like Python, without the need to acquire the skills to develop low-level computer code for parallel and distributed computing.</p><p>We present the design and implementation of this software framework and illustrate its use with a prototype 100 m, 1 h continental scale hydrological model. Our modelling framework ensures that any model built with it is parallelized. This is made possible by providing the model builder with a set of building blocks of models, which are coded in such a manner that parallelization of calculations occurs within and across these building blocks, for any combination of building blocks. There is thus full flexibility on the side of the modeller, without losing performance.</p><p>This breakthrough is made possible by applying a novel approach to the implementation of the model building framework, called asynchronous many-tasks, provided by the HPX C++ software library ([3]). The code in the model building framework expresses spatial operations as large collections of interdependent tasks that can be executed efficiently on individual laptops as well as computer clusters ([2]). Our framework currently includes the most essential operations for building large scale hydrological models, including those for simulating transport of material through a flow direction network. By combining these operations, we rebuilt an existing 100 m, 1 h resolution model, thus far used for simulations of small catchments, requiring limited coding as we only had to replace the computational back end of the existing model. Runs at continental scale on a computer cluster show acceptable strong and weak scaling providing a strong indication that global simulations at this resolution will soon be possible, technically speaking.</p><p>Future work will focus on extending the set of modelling operations and adding scalable I/O, after which existing models that are currently limited in their ability to use the computational resources available to them can be ported to this new environment.</p><p>More information about our modelling framework is at https://lue.computationalgeography.org.</p><p><strong>References</strong></p><p>[1] M. Bierkens. Global hydrology 2015: State, trends, and directions. Water Resources Research, 51(7):4923–4947, 2015.<br>[2] K. de Jong, et al. An environmental modelling framework based on asynchronous many-tasks: scalability and usability. Submitted.<br>[3] H. Kaiser, et al. HPX - The C++ standard library for parallelism and concurrency. Journal of Open Source Software, 5(53):2352, 2020.</p>

scholarly journals Herbarium specimens reveal substantial and unexpected variation in phenological sensitivity across the eastern United States

2018 ◽  
Vol 374 (1763) ◽  
pp. 20170394 ◽  
Author(s):  
Daniel S. Park ◽  
Ian Breckheimer ◽  
Alex C. Williams ◽  
Edith Law ◽  
Aaron M. Ellison ◽  
...  
Keyword(s):  
United States ◽  
Climatic Change ◽  
Large Scale ◽  
Critical Role ◽  
Flowering Plant ◽  
Reproductive Phenology ◽  
Herbarium Specimens ◽  
Eastern United States ◽  
Regional Patterns ◽  
Continental Scale

Phenology is a key biological trait that can determine an organism's survival and provides one of the clearest indicators of the effects of recent climatic change. Long time-series observations of plant phenology collected at continental scales could clarify latitudinal and regional patterns of plant responses and illuminate drivers of that variation, but few such datasets exist. Here, we use the web tool CrowdCurio to crowdsource phenological data from over 7000 herbarium specimens representing 30 diverse flowering plant species distributed across the eastern United States. Our results, spanning 120 years and generated from over 2000 crowdsourcers, illustrate numerous aspects of continental-scale plant reproductive phenology. First, they support prior studies that found plant reproductive phenology significantly advances in response to warming, especially for early-flowering species. Second, they reveal that fruiting in populations from warmer, lower latitudes is significantly more phenologically sensitive to temperature than that for populations from colder, higher-latitude regions. Last, we found that variation in phenological sensitivities to climate within species between regions was of similar magnitude to variation between species. Overall, our results suggest that phenological responses to anthropogenic climate change will be heterogeneous within communities and across regions, with large amounts of regional variability driven by local adaptation, phenotypic plasticity and differences in species assemblages. As millions of imaged herbarium specimens become available online, they will play an increasingly critical role in revealing large-scale patterns within assemblages and across continents that ultimately can improve forecasts of the impacts of climatic change on the structure and function of ecosystems. This article is part of the theme issue ‘Biological collections for understanding biodiversity in the Anthropocene’.

Late-summer carbon fluxes from Canadian forests and peatlands along an east–west continental transect

2006 ◽  
Vol 36 (3) ◽  
pp. 783-800 ◽  
Author(s):  
Carole Coursolle ◽  
Hank A Margolis ◽  
Alan G Barr ◽  
T Andrew Black ◽  
Brian D Amiro ◽  
...  
Keyword(s):  
Large Scale ◽  
Ecosystem Respiration ◽  
Late Summer ◽  
Ecosystem Productivity ◽  
Diurnal Courses ◽  
Continental Scale ◽  
Young Forest ◽  
C Cycle ◽  
Use Efficiency ◽  
East West

Net ecosystem productivity (NEP) during August 2003 was measured by using eddy covariance above 17 forest and 3 peatland sites along an east–west continental-scale transect in Canada. Measured sites included recently disturbed stands, young forest stands, intermediate-aged conifer stands, mature deciduous stands, mature conifer stands, fens, and an open shrub bog. Diurnal courses of NEP showed strong coherence within the different ecosystem categories. Recently disturbed sites showed the weakest diurnal cycle; and intermediate-aged conifers, the strongest. The western treed fen had a more pronounced diurnal pattern than the eastern shrub bog or the Saskatchewan patterned fen. All but three sites were clearly afternoon C sinks. Ecosystem respiration was highest for the young fire sites. The intermediate-aged conifer sites had the highest maximum NEP (NEPmax) and gross ecosystem productivity (GEPmax), attaining rates that would be consistent with the presence of a strong terrestrial C sink in regions where these types of forest are common. These results support the idea that large-scale C cycle modeling activities would benefit from information on the age-class distribution and disturbance types within larger grid cells. Light use efficiency followed a pattern similar to that of NEPmax and GEPmax. Four of the five recently disturbed sites and all three of the peatland sites had low water use efficiencies.

Oxygen, hydrogen, and strontium isotope constraints on the origin of granites

1988 ◽  
Vol 79 (2-3) ◽  
pp. 317-338 ◽  
Author(s):  
Hugh P. Taylor
Keyword(s):  
Large Scale ◽  
Wide Spectrum ◽  
Volcanic Field ◽  
Parent Material ◽  
Rock Interaction ◽  
Metasedimentary Rocks ◽  
Clear Cut ◽  
End Members ◽  
Type 3

ABSTRACTOxygen isotope data are very useful in determining the source rocks of granitic magmas, particularly when used in combination with Sr, Pb, and Nd isotope studies. For example, unusually high δ18O values in magmas (δ18O> +8) require the involvement of some precursor parent material that at some time in the past resided on or near the Earth's surface, either as sedimentary rocks or as weathered or hydrothermally altered rocks. The isotopic systematics which are preserved in the Mesozoic and Cenozoic batholiths of western North America can be explained by grand-scale mixing of three broadly defined end-members: (1) oceanic island-arc magmas derived from a “depleted” (MORB-type?) source in the upper mantle (δ18O c. +6 and 87Sr/86Sr c. 0·703); (2) a high-18O (c. +13 to +17) source with a very uniform 87Sr/86Sr (c. 0·708 to 0·712), derived mainly from eugeosynclinal volcanogenic sediments and (or) hydrothermally altered basalts; and (3) a much more heterogeneous source (87Sr/86Sr c. 0·706 to 0·750, or higher) with a high δ18O (c. +9 to +15) where derived from supracrustal metasedimentary rocks and a much lower δ18O (c. +7 to +9) where derived from the lower continental crust of the craton. These end-members were successively dominant from W to E, respectively, within three elongate N–S geographic zones that can be mapped from Mexico all the way N to Idaho.18O/16O studies (together with D/H analyses) can, however, play a more important and certainly a unique role in determining the origins of the aqueous fluids involved in the formation of granitic and rhyolitic magmas. Fluid-rock interaction effects are most clear-cut when low-18O, low-D meteoric waters are involved in the isotopic exchange and melting processes, but the effects of other waters such as seawater (with a relatively high δD c. 0) can also be recognised. Because of these hydrothermal processes, rocks that ultimately undergo partial melting may exhibit isotopic signatures considerably different from those that they started with. We discuss three broad classes of potential source materials of such “hydrothermal-anatectic” granitic magmas, based mainly on water/rock (w/r), temperature (T), and the length of time (t) that fluid-rock interaction proceeds: (Type 1) epizonal systems with a wide variation in whole-rock δ18O and extreme 18O/16O disequilibrium among coexisting minerals (e.g. quartz and feldspar); (Type 2) deeper-seated and (or) longer-lived systems, also with a wide spectrum of whole-rock δ18O, but with equilibrated 18O/16O ratios among coexisting minerals; (Type 3) thoroughly homogenised and equilibrated systems with relatively uniform δ18O in all lithologies. Low-18O magmas formed by melting of rocks altered in a Type 2 or a Type 3 meteoric-hydrothermal system are the only kinds of “hydrothermal-anatectic” granitic magmas that are readily recognisable in the geological record. Analogous effects produced by other kinds of aqueous fluids may, however, be quite common, particularly in areas of extensional tectonics and large-scale rifting. The greatly enhanced permeabilities in such fractured terranes make possible the deep convective circulation of ground waters and sedimentary pore fluids. The nature and origin of low-18O magmas in the Yellowstone volcanic field and the Seychelles Islands are briefly reviewed in light of these concepts, as is the development of high-D, peraluminous magmas in the Hercynian of the Pyrenees.

scholarly journals Drought Relief and Reversal over North America from 1500 to 2016

2021 ◽  
Vol 25 (1) ◽  
pp. 94-107
Author(s):  
M. C. A. Torbenson ◽  
D. W. Stahle ◽  
I. M. Howard ◽  
D. J. Burnette ◽  
D. Griffin ◽  
...  
Keyword(s):  
Soil Moisture ◽  
North America ◽  
Land Surface ◽  
Large Scale ◽  
Temporal Stability ◽  
Warm Season ◽  
Cool Season ◽  
Continental Scale ◽  
The North ◽  
Drought Relief

Abstract Season-to-season persistence of soil moisture drought varies across North America. Such interseasonal autocorrelation can have modest skill in forecasting future conditions several months in advance. Because robust instrumental observations of precipitation span less than 100 years, the temporal stability of the relationship between seasonal moisture anomalies is uncertain. The North American Seasonal Precipitation Atlas (NASPA) is a gridded network of separately reconstructed cool-season (December–April) and warm-season (May–July) precipitation series and offers new insights on the intra-annual changes in drought for up to 2000 years. Here, the NASPA precipitation reconstructions are rescaled to represent the long-term soil moisture balance during the cool season and 3-month-long atmospheric moisture during the warm season. These rescaled seasonal reconstructions are then used to quantify the frequency, magnitude, and spatial extent of cool-season drought that was relieved or reversed during the following summer months. The adjusted seasonal reconstructions reproduce the general patterns of large-scale drought amelioration and termination in the instrumental record during the twentieth century and are used to estimate relief and reversals for the most skillfully reconstructed past 500 years. Subcontinental-to-continental-scale reversals of cool-season drought in the following warm season have been rare, but the reconstructions display periods prior to the instrumental data of increased reversal probabilities for the mid-Atlantic region and the U.S. Southwest. Drought relief at the continental scale may arise in part from macroscale ocean–atmosphere processes, whereas the smaller-scale regional reversals may reflect land surface feedbacks and stochastic variability.

scholarly journals P–T history of kimberlite-hosted garnet lherzolites from South-West Greenland

2007 ◽  
Vol 13 ◽  
pp. 45-48 ◽  
Author(s):  
Mark T. Hutchison ◽  
Louise Josefine Nielsen ◽  
Stefan Bernstein
Keyword(s):  
The Body ◽  
Mantle Xenoliths ◽  
Chemical Compositions ◽  
Equilibrium Pressure ◽  
West Greenland ◽  
Rock Types ◽  
South West ◽  
Three Phase ◽  
Two Samples ◽  
Granite Complex

Exploration for diamonds in West Greenland has experienced a major boost within the last decade following the establishment of world-class diamond mines within the nearby Slave Province of the Canadian Arctic. Numerous companies have active programmes of diamond exploration and increasingly larger diamonds have been discovered, notably a 2.392 carat dodecahedral stone recovered by the Canadian exploration company Hudson Resources Inc. in January 2007. The Geological Survey of Denmark and Greenland (GEUS) is currently carrying out several studies aimed at understanding the petrogenesis of diamondiferous kimberlites in Greenland and the physical and chemical properties of their associated mantle source regions (e.g. Hutchison 2005; Nielsen & Jensen 2005). Constraint of the mantle geotherm, i.e. the variation of temperature with depth for a particular mantle volume, is an important initial step in assessing the likelihood of such a volume to grow diamonds and hence the diamond potential of associated deep-sourced magmatic rocks occurring at surface. Cool geotherms are often present within old cratonic blocks such as West Greenland (Garde et al. 2000) and provide a good environment for the formation of diamonds (Haggerty 1986). This study aims to constrain the mantle geotherm for the southern extent of the North Atlantic Craton in Greenland by applying three-phase geothermobarometry calculations using chemical compositions of clinopyroxene, orthopyroxene and garnet from four-phase kimberlite-hosted lherzolite xenoliths. Xenoliths have been sampled from kimberlites from two areas in South-West Greenland: Midternæs and Pyramide- fjeld (Fig. 1). Kimberlites in the Pyramidefjeld area principally occur as sheeted sills hosted in the Pyramidefjeld granite complex of Palaeoproterozoic Ketilidian age. In contrast, Midternæs kimberlites occur as outcrops within a single, extensive and undulating sill hosted within pre-Ketilidian granodioritic gneiss and Ketilidian supracrustal rocks. Pyramidefjeld kimberlites have been shown to be Mesozoic (Andrews & Emeleus 1971), and work is currently being carried out to further constrain the ages of these and the Midternæs kimberlites and also xenoliths using modern methods. No attempt is made herein to provide a correct petrological classification of the rocks hosting the xenoliths; however, the abundance of clinopyroxene reported by Andrews & Emeleus (1971) suggests that further work may more correctly conclude a classification as ‘orangeite’ after Mitchell (1995). Notwithstanding this, the term ‘kimberlite’ is employed throughout in order to be consistent with that adopted by previous authors. The Precambrian Pyramide fjeld granite complex and adjacent Archaean granod ioritic gneisses are host to several kimberlite sheets located at various levels between 400 and 900 m elevation (Fig. 1A; Andrews & Emeleus 1971, 1975). Kimberlites are mainly found as loose blocks in scree; however, these are almost always sourced locally from in situ bodies. Sheets can often be found deep within overhanging clefts, particularly in granitic walls. The kimberlite bodies are gently dipping, typically 20 degrees, and with a range of strikes. The maximum thickness of sills is approximately 2 m but thickness varies significantly over short distances. In many instances, the occurrence of kimberlite is seen to be controlled locally by structures in the country rocks. Field observations of the range of orientations of intrusive bodies do not appear to suggest a particular focal point which could be a likely location for an intrusive centre such as a pipe. This observation is in line with what is seen throughout West Greenland where kimberlite emplacement appears as dykes and sills (Larsen & Rex 1992) rather than the pipes and blows which are common in other world-wide settings. The occurrence of xenoliths amongst Pyramidefjeld kimberlites is highly variable with the most xenolith-rich localities being in the vicinity of Safirsø (Fig. 1A). The majority of xenoliths are dunites with occasional wehrlites and lherzolites (Emeleus & Andrews 1975). Of particular interest from the point of view of thermobarometry is the occurrence of garnet. This is rarely found, even in clinopyroxene-bearing samples, and the two samples chosen for thermobarometry (Fig. 1A) represent the majority of the garnet-bearing xenoliths identified within an estimated total population of 75 xenoliths collected. The Midternæs kimberlites are hosted in Archaean gneisses and Proterozoic supracrustal rocks (Fig. 1B; Andrews & Emeleus 1971, 1975). The style of kimberlite emplacement and occurrence of garnet-bearing xenoliths are closely similar to those of Pyramidefjeld. Contours of elevation between outcrops suggest that the kimberlites form parts of a largely contiguous single body dipping at approximately 30 degrees to the west-south-west. Individual outcrops as in Pyramidefjeld indicate that the body varies in thickness and undulates in response to local structure. The south-western portion of the body which outcrops near the glacier Sioralik Bræ, is considerably thicker than elsewhere (Fig. 2) and in some places is seen to have a true thickness in excess of 4 m. Xenoliths are less abundant on average than in Pyramidefjeld kimberlites, but a similar variety and proportion of rock types and infrequent occurrence of garnet is observed. The kimberlites from both areas were intruded along zones of platy jointing which likely were caused by degassing of the magma and formed just prior to the kimberlite intrusion. In contrast to some kimberlites in other cratons, very few xenoliths of local, lower crustal rock types have been recognised in the kimberlites from Pyramidefjeld and Mid ternæs. The intrusions are therefore believed to have been of a non-explosive nature, perhaps because of host-rock rheol - ogy or due to emplacement at relatively deep crustal levels. Here we report on calculations of equilibrium pressure and temperature using compositions of three-phase assemblages of garnet, orthopyroxene and clinopyroxene from Midternæs and Pyramidefjeld mantle xenoliths.

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