Frank E. Brenker

The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks

Abstract In a scanning electron microscope (SEM) an electron beam sets up an omni-directional source of scattered electrons within a specimen. Diffraction of these electrons will occur simultaneously on all lattice planes in the sample and the backscattered electrons (BSE), which escape from the specimen, will form a diffraction pattern that can be imaged on a phosphor screen. This is the basis of electron backscatter diffraction (EBSD). Similar diffraction effects cause individual grains of different orientations to give different total BSE. SEM images that exploit this effect will show orientation contrast (OC). EBSD and OC imaging are SEM-based crystallographic tools. EBSD enables measurement of the crystallographic orientation of individual rock-forming minerals as small as 1 μm, and the calculation of misorientation axes and angles between any two data points. OC images enable mapping of all misorientation boundaries in a specimen and thus provide a location map for EBSD analyses. EBSD coupled to OC imaging in the SEM enables complete specimen microtextures and mesotextures to be determined. EBSD and OC imaging can be applied to any mineral at a range of scales and enable us to expand the microstructural approach, so successful in studies of quartz rocks, for example, to the full range of rock-forming minerals. Automated EBSD analysis of rocks remains problematic, although continuing technical developments are enabling progress in this area. EBSD and OC are important new tools for petrologists and petrographers. Present and future applications of EBSD and OC imaging include phase identification, studying deformation mechanisms, constraining dislocation slip systems, empirical quantification of microstructures, studying metamorphic processes, studying magmatic processes, and constraining geochemical microsampling. In all these cases, quantitative crystallographic orientation data enable more rigorous testing of models to explain observed microstructures.

Crystal plasticity of natural garnet: New microstructural evidence

Scanning electron microscope (SEM) orientation contrast images of mantle nodules show that garnets contain cellular domains of different crystallographic orientation. Electron backscatter diffraction shows small crystallographic mismatches (

Cation ordering in omphacite and effect on deformation mechanism and lattice preferred orientation (LPO)

We present microstructural data and lattice preferred orientations (LPOs) of omphacites from a suite of eclogites, from the Adula/Cima Lunga nappe (Central Alps). Our work shows a surprisingly strong correlation between the measured LPO and the ordering state of cations in omphacite. Estimates of deformation temperature from metamorphic petrology, together with measured omphacite compositions and LPOs, determine the field (ordering state), on the omphacite phase diagram, into which each sample falls. LPOs dominated by L-type and S-type signatures are developed in samples that fall in the P2/n field (ordered structure) and C2/c field (disordered structure), respectively. Dislocations with b=1/2〈−110〉 or b=[001] are observed in the transmission electron microscope (TEM) in all samples. The former change from a perfect dislocation in the C2/c structure to a partial in P2/n. Any movement of a partial dislocation requires the formation or growth of a stacking fault. Furthermore, in order to pass an obstacle a partial dislocation has to constrict to a unit dislocation. The energy to form a constriction is high in omphacite due to the large separation width. Thus, the activity of the b=1/2〈−110〉 dislocation is hindered in the P2/n structure relative to the C2/c structure, which change the balance between the two and might give rise to the different LPOs.

Spectroscopic 2D-tomography Residual pressure and strain around mineral inclusions in diamonds

We have studied high-pressure inclusions (Ca-silicates, coesite, graphite) in three large diamonds, one from the Kankan district, Guinea, and the other two from the Panda kimberlite, Ekati diamond mines, Canada. Using the in situ point-by-point mapping technique with a confocal Raman system, the mineralogy of the inclusions, as well as their area distribution pattern ( e.g. , of different Ca-silicate phases) and their order-disorder distribution pattern (shown for graphite/disordered carbon), were determined. Raman mapping of the host diamonds yielded 2D-tomographic pressure and strain distribution patterns and provided information on the residual pressure of the inclusions (∼ 2.3 GPa for a coesite inclusion and ∼ 2.6 GPa for a graphite inclusion). The inclusions are surrounded by haloes of significantly enhanced pressure, several hundred σm across. These haloes exhibit complex pressure relaxation patterns that consist of micro-areas affected by both compressive and dilative strain, with the latter being intensive enough to result in apparent “negative pressures”.

Exhumation of lower mantle inclusions in diamond: ATEM investigation of retrograde phase transitions, reactions and exsolution

A multiphase inclusion (KK-83) in a diamond from the Kankan deposits in Guinea, resulting from complex reactions between primary lower mantle phases, was examined in detail using analytical transmission electron microscopy. The inclusion consists of a large (300 μm) diopside crystal with a small symplectitic intergrowth in one corner, comprising olivine and tetragonal almandine pyrope phase (TAPP). The olivine part of the symplectite contains small exsolutions of Mg-Al-chromite and rare Ca-carbonate. A second inclusion of ferropericlase observed in the same diamond indicates a primary origin within the lower mantle. The clinopyroxene formed through a series of reactions at the expense of touching inclusions of CaSi- and MgSi-perovskite during convective ascent of the diamond through the transition zone. The proposed reaction sequence is: MgSiPvk+CaSiPvk→MgSiIlm+CaSiPvk→ringwoodite+stishovite+CaSiPvk→clinopyroxene+relict ringwoodite. The high Al-content of the bulk symplectite indicates ringwoodite as precursor phase coexisting with clinopyroxene. A slight deficiency of SiO2 during the diopside forming reaction leads to a small amount of relict ringwoodite (<0.1 vol% of the total inclusion is occupied by ringwoodite) coexisting with diopside. The ringwoodite captures the observed high amount of Al. Subsequent breakdown of ringwoodite to wadsleyite+TAPP produces the observed symplectitic intergrowth. During the phase transformation of wadsleyite to olivine, exsolution of Mg-Al-chromite occurs further decreasing the original solubility of Al, Cr and Ti, in accordance with experimental data on the element partitioning between wadsleyite and olivine. Based on these observations, TAPP can form as a retrograde phase within the transition zone of the Earth’s mantle and is not restricted to the upper part of the lower mantle. High Fe3+-contents may favour its formation.

On the formation of peridotite-derived Os-rich PGE alloys

Abstract Osmium-rich Pt group element (PGE) alloys occur worldwide in association with chromite in ultramafic (peridotite) complexes. It has been suggested that these Os-rich alloys formed under extreme P-T conditions in the lowermost mantle, before the metallic core of the Earth formed, or later, in the outer core, and have been transported to the upper mantle as xenoliths in deep-rooted mantle plumes. Our investigation of syn- and pregenetic inclusions (including silicate and chromite) found in Os-rich alloys from peridotites in northern California and southwest Oregon yield no evidence that these alloys formed under extreme P-T conditions. Instead these inclusions point to a hydrous magmatic origin in the shallow upper mantle, most likely in an arc-environment. Indeed, the common occurrence of Os-rich PGE alloys as primary inclusions in massive (commonly podiform), chromite deposits and, conversely, the occurrence of chromite, olivine, pyroxene, laurite, and siliceous (boninitic) melt inclusions in Os-rich PGE alloys suggest a common origin for all these minerals. Integrating our observations with recent experimental work and with observed field relations, we find support for a model in which massive chromite deposits, olivine, laurite, and Os-rich PGE alloys form in a single magmatic process. In an arc-environment, H2O-rich fluids and siliceous melts (e.g., boninites) are produced in the mantle wedge above the descending and dehydrating plate. Large differences in interfacial energy between the precipitated chromite and PGE alloys, and the hydrous fluid(s) and siliceous melt(s), cause a strong concentration of chromite and PGE alloys in the hydrous fluid(s). This general scenario is capable of simultaneously explaining all key observations, including: (1) the formation of massive chromite deposits; (2) nodular chromite textures; (3) Os-rich PGE alloys, laurite, olivine, and pyroxene as common inclusions in massive chromite; (4) inclusions of chromite, olivine, pyroxene, and hydrated siliceous inclusions (the current study) in the Os-rich PGE alloys; and (5) a similar range of variation in 187Os/188Os ratios among Os-rich PGE alloys and massive chromite deposits from individual ultramafic bodies world-wide.

Evidence for solar nebula signatures in the matrix of the Allende meteorite

Carbonaceous chondrites of type 3 (e.g. Allende) are among the most primitive meteorites. They contain components that formed prior to the accretion of a parent asteroid and thus record conditions of the ambient nebular gas, the source of the material from which the solid bodies of our solar system formed. Identification of nebular signatures is often difficult as thermal metamorphism and/or aqueous alteration on the meteorite parent body may have erased mineralogical evidence of nebular processes. The major fraction of Ca in the Allende matrix is contained in relatively large, 20–50 μm, Ca,Fe-rich aggregates (CFA) commonly assumed to have formed by parent body processes. To better constrain their origin a transmission electron microscopic study of these CFA was performed. They consist mainly of hedenbergitic pyroxenes with minor andradite and sulfide. We found that pyroxenes with low Mg content belong to the space group P2/n, whereas the expected C2/c structure is restricted to pyroxenes with higher Mg content. A hedenbergitic pyroxene with space group P2/n has never been reported in the literature and is considered metastable. The relationship between composition and space group can be explained best assuming ferrobustamite (with wollastonite structure) as a precursor phase for the P2/n pyroxenes. Above 970°C a two phase field exists between ferrobustamite and augite. The miscibility gap widens towards higher temperatures. In one case intergrown P2/n with C2/c pyroxenes were found. Their compositions fit well into the ferrobustamite–augite two phase field above 1050°C. Very fast cooling (>10°C/h) controls the incomplete transformation from ferrobustamite to hedenbergite resulting in the observed P2/n space group of Mg-poor pyroxenes. Thus, the Ca,Fe-pyroxenes provide strong evidence for a high temperature origin (>1050°C) followed by rapid cooling (>10°/h), implying that the CFA in the Allende matrix formed in the solar nebula as the Allende parent asteroid has never seen such temperatures and possible cooling rates on a kilometer sized body are orders of magnitude lower. The conditions required for the formation of the CFA suggest either transport from a high temperature to a low temperature environment or very localized heating events in the solar nebula. In addition strongly oxidized conditions (log fO2 (bar)=−15 to −10) are required to stabilize andradite against hedenbergite.

Chain multiplicity faults in deformed omphacite from eclogite

Deformed and recovered omphacite from an eclogite sample of the Lower Schist Cover of the Tauern Window was studied by conventional and high resolution transmission electron microscopy. The rock sample, which was used before for petrological investigations (Zimmermann & Franz, 1988) and radiometric age dating (Von Quadt et al. , 1997), shows chain multiplicity faults (CMFs) in omphacite which have not yet been described. The CMFs are usually intercalations of one or two double chains parallel to (010) in the single chain omphacite. They are mostly terminated by partial dislocations. Contrast analysis showed that the displacement vector R and the Burgers vector b of the partial dislocations are probably 1/2[011], less likely interpreted as 1/2[101]. It is assumed that the CMFs were formed by stress-induced dissociation of [011] dislocations. A direct proof of CMFs acting as slip planes is given. The frequent occurrence of CMFs in omphacite suggests that 1/2[011](010) is an important slip system which is probably responsible for lattice preferred orientation. The interaction observed between CMFs and antiphase domain boundaries provides a recovery mechanism hitherto not reported from omphacite.

Late-stage, high-temperature processesing in the Allende meteorite: Record from Ca,Fe-rich silicate rims around dark inclusions

Abstract Secondary Ca,Fe-rich minerals (CFM; diopside-hedenbergite, andradite, wollastonite, kirschteinite) are widespread in the Allende carbonaceous chondrite. About 90 vol% of the total CaO content of the Allende matrix is concentrated in CFM. The conditions and environment (solar nebular or asteroidal) of this alteration are still matters of controversial scientific discussion. Here we present evidence for late-stage, high-temperature processes recorded in Ca,Fe-rich rims around Allende dark inclusions 3529 and IV-1 studied using scanning (SEM) and transmission electron microscopy (TEM), and electron probe microanalysis. The rims show a multilayered structure, with the outermost layer intergrown with the matrix olivines and chondrule fragments of the Allende host, indicative of in situ formation. The central portion of the rim around IV-1 contains several wollastonite polytypes (a polysynthetically twinned polytype of pseudowollastonite, wollastonite-2M, and wollastonite-1T) and an intergrowth of hedenbergite-PM (primitive monoclinic) and augite-hedenbergitess. These findings require temperatures above 1000 °C and fast cooling rates after formation of the central part of the rim. The close occurrences of three different polymorphs of wollastonite suggest that this process was highly localized and may have resulted from shock metamorphism.

Variation of antiphase domain size in omphacite: A tool to determine the temperature–time history of eclogites revisited

Abstract A systematic transmission electron microscopy (TEM) study was performed on the size distribution of antiphase domains in omphacite from eclogites from the Adula/Cima Lunga nappe. The measured mean antiphase domain size is shown to depend on peak temperature, duration of peak metamorphism, cooling rate, and composition. The systematics of the size distribution are modified by dislocation interaction, recrystallization, and the time of growth during the temperature-timedeformation history of the rock. Based on a new model for the size distribution of antiphase domains, we are able to show the potential and limits to estimating time and peak temperature of metamorphism and the subsequent cooling history of this important rock type