Water contents of Roberts Victor xenolithic eclogites: primary and metasomatic controls

A suite of eclogites from the Roberts Victor kimberlite has been extensively characterized in terms of petrology and geochemical compositions (Gréau et al. in Geochim Cosmochim Acta 75(22):6927–6954, 2011; Huang et al. in Lithos 142–143:161–181, 2012a). In the present study, the water contents of eclogitic garnet and omphacite were analyzed by Fourier transform infrared spectrometry. Garnet does not contain measureable OH in any sample. The water content of omphacite in the studied eclogites ranges from 211 to 1,496 ppm. Mantle metasomatism has modified the water content of some of the eclogites, while others retain water contents characteristic of their original environment. The OH contents of the metasomatized eclogites may be mainly controlled by the H₂O fugacity and mineral compositions. The OH contents of the non-metasomatized samples are interpreted to be more sensitive to their mantle equilibration temperature, pressure, and the local fugacities of H₂O and O₂. The calculated water content of the metasomatic medium is similar to that of carbonatitic–kimberlitic melts/fluids. Eclogites contain more water than peridotites recorded in the literature (341 ± 161 vs 122 ± 54 ppm) and represent an important water reservoir in the lithospheric mantle wherever they occur. This is an important parameter to be considered in the interpretation of mantle processes and geophysical data such as seismic wave speeds and electrical conductivity, and in geodynamic modeling.     

Petrogenesis of base metal sulphide assemblages of some peridotites from the Kaapvaal craton (South Africa)

Thirty-two peridotite xenoliths from kimberlitic pipes of the Kaapvaal craton were analysed for S and studied in reflected light microscopy and electron microprobe. Correlation between whole-rock S concentrations and sulphide modal abundances has been obscured by kimberlite-related sulphur within the mantle and by low-temperature contamination processes during emplacement. Mantle-derived base metal sulphides (BMS) occur as solitary inclusions (SI) and intergranular blebs. Unfractured SI encloses intergrowths of Ni-poor and Ni-rich monosulphide solid solution (Mss) phases, coexisting with pentlandite (Pn) and Cu-rich sulphides. Textural relationships between Mss phases and Cu-sulphides are consistent with fractional crystallization of Mss from a Cu–Fe–Ni sulphide melt. Pn-rich euhedral SI may have crystallized from a more metal-rich sulphide melt. However, the opaque mineral assemblages of both fractured sulphide inclusions and intergranular BMS point to a progressive desulphurization of Mss, yielding Pn-rich grains, often replaced by Fe-poor heazlewoodite and abundant magnetite, while Cu-sulphides are replaced by native copper. This trend is consistent with reducing conditions generated by low-temperature serpentinization. A residual origin cannot be ruled out for the Mss enclosed in the most refractory peridotites, although their Ni/Fe ratios are too low to be consistent with an equilibration with olivine at magmatic temperatures. Modal abundances of mantle-derived BMS increase in the Fe-enriched metasomatized peridotites. At least two BMS precipitation processes can be recognized: (1) precipitation of Fe–Cu-rich immiscible sulphide melts in phlogopite-(ilmenite–rutile) peridotites and (2) sulphidation reactions from an H₂S-rich fluid phase in phlogopite-K richterite peridotites.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Quality analysis applied on eddy covariance measurements at complex forest sites using footprint modelling

Summary Measuring turbulent fluxes with the eddy covariance method has become a widely accepted and powerful tool for the determination of long term data sets for the exchange of momentum, sensible and latent heat, and trace gases such as CO₂ between the atmosphere and the underlying surface. Several flux networks developed continuous measurements above complex terrain, e.g. AmeriFlux and EUROFLUX, with a strong focus on the net exchange of CO₂ between the atmosphere and the underlying surface. Under many conditions basic assumptions for the eddy covariance method in its simplified form, such as stationarity of the flow, homogeneity of the surface and fully developed turbulence of the flow field, are not fulfilled. To deal with non-ideal conditions which are common at many FLUXNET sites, quality tests have been developed to check if these basic theoretical assumptions are valid.In the framework of the CARBOEUROFLUX project, we combined quality tests described by Foken and Wichura (1996) with the analytical footprint model of Schmid (1997). The aim was to identify suitable wind sectors and meteorological conditions for flux measurements. These tools were used on data of 18 participating sites. Quality tests were applied on the fluxes of momentum, sensible and latent heat, and on the CO₂-flux, respectively. The influence of the topography on the vertical wind component was also checked. At many sites the land use around the flux towers is not homogeneous or the fetch may not be large enough. So the relative contribution of the land use type intended to be measured was also investigated. Thus the developed tool allows comparative investigations of the measured turbulent fluxes at different sites if using the same technique and algorithms for the determination of the fluxes as well as analyses of potential problems caused by influences of the surrounding land use patterns.     

石英ESR测年信号衰退特征研究进展

电子自旋共振(ESR)测年法是第四纪地质年代学的重要测试方法之一,其测年范围广,特别是对于20万年至百万年尺度的第四纪样品有着重要意义.准确测量古剂量是影响ESR测年法精度和可信度的重要因素之一,而掌握ESR信号衰退特征及机制是获得准确、可信古剂量的重要前提.ESR测年法适用的地质样品种类较多,如热液石英脉、地质断层泥、水系沉积物等,不同样品的ESR信号衰退特征和机制有较大差异.近年来,国内外学者针对不同地质样品中石英ESR信号的衰退特征和机制深入开展了一系列的实验室模拟和野外观测研究,取得了诸多新认识.回顾了近年来在石英ESR信号衰退特征领域取得的进展,特别是水系沉积物石英ESR信号的衰退特征,并展望了这一领域的未来工作重点.     

Rare earth element mobility in the Roffna Gneiss,Switzerland

The Roffna Gneiss, a deformed Hercynian granite porphyry within the Penninic nappes of eastern Switzerland, underwent extreme cataclasis with the progressive development of phengite towards the margins of the nappe under conditions of the glaucophane schist to greenschist facies. This resulted in the selective mobilization of major and trace elements over distances of 10's to 100's of meters and the resetting of the Rb — Sr whole rock isotopic systems some 100 my ago. The component ratios and compositionvolume relationships of progressively deformed gneiss samples studied here suggest that this process was essentially isovolumetric. The mineralogy of the deformation sequence appears to have been controlled by a reaction involving the breakdown of microcline, albite and biotite and the formation of phengite and quartz. The fluids introduced Mg and H₂O, promoting the development of phengite, and removed the Na being released by the breakdown of albite. The fluids were most probably derived from the surrounding Triassic carbonates and quartzites. These relatively high fO₂ and carbonate rich fluids also introduced rare earth elements (REE) into the gneiss. The gneiss was progressively enriched in Eu up to 60%, Y up to 40%, and Yb up to 100%. These enrichments are associated with the development of epitaxial xenotime around zircon in the most phengite-rich sample. While the REE were mobile, uranium and thorium were essentially immobile. The formation of xenotime was suggested to explain the observed heavy REE enrichment when large differences in the REE contents were found for replicate analyses using HF and then lithium metaborate for dissolution. These differences arose because xenotime, like monazite, can be difficult (if not impossible) to dissolve in hydrofluoric acid. Due to the possibility of incomplete sample dissolution, we now recommend fusion with lithium metaborate for all REE, Lu — Hf or Sm — Nd studies.     

The lithospheric mantle beneath the Kerguelen Islands (Indian Ocean): petrological and petrophysical characteristics of mantle mafic rock types and correlation with seismic profiles

Deep-seated meta-igneous xenoliths brought to the surface by alkali basaltic magmas from the Kerguelen Islands reveal that basaltic magmas have intruded the upper mantle throughout their geological evolution. These xenoliths record volcanic activity associated with their early South East Indian Ridge location and subsequent translation to an intraplate setting over the Kerguelen Plume. The meta-igneous xenoliths sample two distinctive geochemical episodes: one is tholeiitic transitional and one is alkali basaltic. Geothermobarometry calculations provide a spatial context for the rock type sequence sampled and for interpreting petrophysical data. The garnet granulites equilibrated over a pressure range of 1.15 to 1.35 GPa and the garnet pyroxenite at 1.8 GPa. Ultrasonic measurements of compressional wave speed V_P have been carried out at pressures up to 1 GPa, and densities measured for representative samples of meta-igneous xenoliths and for a harzburgite that represents the peridotitic mantle. V_P and density have also been calculated using modal proportions of minerals and appropriate elastic properties for the constituent minerals. Calculated and measured V_P agree well for rock types with microstructures not complicated by kelyphitic breakdown of garnet and/or pervasive grain-boundary cracking. Pyroxene granulites have measured and calculated V_P within the range 7.37-7.52 km/s; calculated velocities for the garnet granulites and pyroxenites range from 7.69 to 7.99 km/s, whereas measured and calculated V_P for a mantle harzburgite are 8.45 and 8.29 km/s respectively. The seismic structure observed beneath the Kerguelen Islands can be explained by (1) a mixture of underplated pyroxene granulites and ultramafic rocks responsible for the 2-3 km low velocity transitional zone below the oceanic layer 3, (2) varying proportions of granulites and pyroxenites in different regions within the upper mantle producing the lateral heterogeneities, and (3) intercalation of the granulites and pyroxenites throughout the entire upper mantle column, along with elevated temperatures, accounting for the relatively low mantle velocities (7.70-7.95 km/s).