The behavior of protons during wet olivine deformation under the conditions of the kimberlite process

This paper reports the results of optical and electron microscopic investigations of mantle olivine samples with H₂O contents of tens-hundreds ppm weight. Samples were obtained from the xenoliths and xenocrysts of the Udachnaya pipe. At the scale of optical microscope magnification, a peculiar banded microstructure was observed in thin sections prepared parallel to the olivine (010) plane. It is formed by cross-hatched bands parallel to four crystallographic directions of the olivine structure: [100], [001], [101], and [−101]. At the scale of electron optical magnifications, the banded microstructure is observed as nanometer-sized heterogeneities of various types which are related to olivine deformation: (a) planar defects parallel to (100) and (001) corresponding to the (100)[010] and (001)[100] dislocation glide systems, respectively; they are occasionally transformed into lamellae or decorated by nanoinclusions; and (b) nanometer-sized heterogeneities formed by nanoinclusion arrays not related to planar defects and oriented along the same directions of the olivine structure as the optically visible bands. The deformation structures are decorated by coupled point OH-bearing defects, which were initially present in the olivine. The crystallographically oriented arrays of nanoinclusions of high-pressure hydrous silicates are considered as a result of olivine deprotonization (elimination of OH-bearing defects from the olivine structure) in the zones of previous deformation compression in the crystal. Light refraction effects on the nanoinclusions make these zones optically visible and produce the banded microstructure, which is a consequence of previous deformation.     

The influence of the low-high quartz transformation on recrystallization and grain growth during contact metamorphism (Traversella intrusion, North Italy)

Recrystallization and grain growth of quartz occur only within 35 m of the contact. This limit of recrystallization and grain growth coincides with an inferred maximum temperature of 620 ± 20°C, which at about 6–7 km is the temperature for the low-high quartz transformation. The remarkable coincidence of the temperature of about 620°C for the beginning of recrystallization and grain growth with the transformation temperature of quartz is explained by the atomic displacement mechanism of the low-high quartz transformation. The already disordered structure of the grain boundary becomes even more distorted by the fluctuations of the SiO₄ tetrahedra during the low-high quartz transformation. This results in an increase of the number of atoms which are transferred from one grain into the grain boundary, and from the grain boundary to the other grain. A lowering of the activation energy for grain boundary migration results such that grain boundary migration becomes the most important process for recrystallization and grain growth.     

Evaluating sedimentary DNA for tracing changes in cyanobacteria dynamics from sediments spanning the last 350 years of Lake Tiefer See,NE Germany

Journal of Paleolimnology - Since the beginning of the Anthropocene, lacustrine biodiversity has been influenced by climate change and human activities. These factors advance the spread of harmful...     

P-T-X-controlled element transport through granulite-facies ternary feldspar from Lofoten,Norway

Fluid transport on the grain-scale controls many rock properties and governs chemical exchange. Charnockites from Lofoten indicate fluid penetration into ternary alkali feldspars controlled by their microtextures. In a process of fluid infiltration at granulite-facies conditions (∼600°C and 8–11 kbar), tiny pyroxenes enclosed in alkali feldspar reacted to amphiboles, which are always spatially connected to perthitic albite. Investigation of these microtextures by TEM imaging of Focused Ion Beam (FIB) prepared foils revealed that pyroxenes in contact with albite lamellae show dissolution features. An amorphous Fe- and Cl-bearing material interpreted to be a residuum of the percolating fluid was found within albite lamellae. Textures and mineral compositions indicate that a Cl-rich aqueous fluid attacked the lamellae, which then provided pathways for further fluid flow. A correspondence was found between feldspar compositions, their microtexture and their degree of alteration as a result of their permeability for fluids at specific temperatures. Hence, in addition to pressure and temperature, small variations of feldspar composition can strongly influence the fluid permeability of feldspathic rocks under lower crustal conditions.     

Component mobility at 900 °C and 18 kbar from experimentally grown coronas in a natural gabbro

Several approximately 100-μm-wide reaction zones were grown under experimental conditions of 900 °C and 18 kbar along former olivine-plagioclase contacts in a natural gabbro. The reaction zone comprises two distinct domains: (i) an irregularly bounded zone with idiomorphic grains of zoisite and minor corundum and kyanite immersed in a melt developed at the plagioclase side and (ii) a well-defined reaction band comprising a succession of mineral layers forming a corona structure around olivine. Between the olivine and the plagioclase reactant phases we observe the following layer sequence: olivine|pyroxene|garnet|partially molten domain|plagioclase. Within the pyroxene layer two micro-structurally distinct layers comprising enstatite and clinopyroxene can be discerned. Chemical potential gradients persisted for the CaO, Al₂O₃, SiO₂, MgO and FeO components, which drove diffusion of Ca, Al and Si bearing species from the garnet-matrix interface to the pyroxene-olivine interface and diffusion of Mg- and Fe-bearing species in the opposite direction. The systematic mineralogical organization and chemical zoning across the corona suggest that the olivine corona was formed by a “diffusion-controlled” reaction. We estimate a set of diffusion coefficients and conclude that L_AlAl < L_CaCa < (L_SiSi, L_FeFe) < L_MgMg during reaction rim growth.     

Environmental responses to Lateglacial climatic fluctuations recorded in the sediments of pre‐Alpine Lake Mondsee (northeastern Alps)

Investigation of the sedimentary record of pre‐Alpine Lake Mondsee (Upper Austria) focused on the environmental reaction to rapid Lateglacial climatic changes. Results of this study reveal complex proxy responses that are variable in time and influenced by the long‐term evolution of the lake and its catchment. A new field sampling approach facilitated continuous and precisely controlled parallel sampling at decadal to sub‐annual resolution for µ‐XRF element scanning, carbon geochemistry, stable isotope measurements on ostracods, pollen analyses and large‐scale thin sections for microfacies analysis. The Holocene chronology is established through microscopic varve counting and supported by accelerator mass spectrometry ¹⁴C dating of terrestrial plant macrofossils, whereas the Lateglacial age model is based on δ¹⁸O wiggle matching with the Greenland NGRIP record, using the GICC05 chronology. Microfacies analysis enables the detection of subtle sedimentological changes, proving that depositional processes even in rather large lake systems are highly sensitive to climate forcing. Comparing periods of major warming at the onset of the Lateglacial and Holocene and of major cooling at the onset of the Younger Dryas reveals differences in proxy responses, reflecting threshold effects and ecosystem inertia. Temperature increase, vegetation recovery, decrease of detrital flux and intensification of biochemical calcite precipitation at the onset of the Holocene took place with only decadal leads and lags over a ca. 100 a period, whereas the spread of woodlands and the reduction of detrital flux lagged the warming at the onset of the Lateglacial Interstadial by ca. 500–750 a. Cooling at the onset of the Younger Dryas is reflected by the simultaneous reaction of δ¹⁸O and vegetation, but sedimentological changes (reduction of endogenic calcite content, increase in detrital flux) were delayed by about 150–300 a. Three short‐term Lateglacial cold intervals, corresponding to Greenland isotope substages GI‐1d, GI‐1c2 and GI‐1b, also show complex proxy responses that vary in time. Copyright © 2011 John Wiley & Sons, Ltd.     

Effects of detrital carbonate on stable oxygen and carbon isotope data from varved sediments of the interglacial Piànico palaeolake (Southern Alps,Italy)

Contamination with detrital matter is a well‐known bias in δ¹⁸O records from lake carbonates but quantitative information of this effect is yet lacking. Therefore, we developed a new methodological approach combining isotope analyses with microfacies, X‐ray diffraction and micro‐X‐ray fluorescence data and applied this in a case study for the Piànico interglacial lake record in order to provide a quantitative estimate of the effect of detrital carbonate on stable δ¹⁸O and δ¹³C data. The Piànico record contains a long series of distinct and well‐preserved calcite varves and is correlated to Marine Isotope Stage 11. Intercalated in the varve sequence are detrital layers triggered by surface erosion events. These detrital layers are mainly composed of dolomite, thus reflecting the mineralogical signature of the catchment. Microfacies analyses of a 9350 varve year interval allows the identification of detrital layers down to sub‐millimetre scale and a precise selection of three different types of samples for isotope analyses: (1) pure endogenic calcite varves (five varves per sample) without detrital contamination; (2) individual detrital layers; and (3) ‘mixed’ samples including five calcite varves and up to four thin detrital layers. Detrital samples show the isotopic signature of the catchment dolomite and are up to 5.7‰ enriched in δ¹⁸O values with respect to endogenic calcite samples. In mixed samples, the degree of isotopic enrichment is directly related to the amount of detrital contamination; δ¹⁸O of bulk carbonates is significantly biased when the detrital component amounts to more than 5% of the sample. It is also shown that samples containing detrital material have an influence on the calculation of the covariance between δ¹³C and δ¹⁸O. Covariance is high (r = 0.76) when the correlation coefficient is calculated on the base of all samples, but absent (r = ?0.43) when samples containing detrital dolomite are excluded. It has been demonstrated that microfacies analysis is a quick tool to avoid or reduce detrital contamination in bulk carbonate samples during sample selection. Copyright © 2009 John Wiley & Sons, Ltd.