Richard Wirth

Focused Ion Beam (FIB) A novel technology for advanced application of micro- and nanoanalysis in geosciences and applied mineralogy

The Focused Ion Beam (FIB) tool has been successfully used as both a means to prepare site-specific TEM foils for subsequent analysis by TEM, as well as a stand-alone instrument for micromachining of materials. TEM foil preparation with FIB technique has drastically changed traditional TEM specimen preparation because it allows site-specific foil preparation. FIB consists of cutting electron transparent foils through Ga-ion milling on a bulk sample. Optical microscopy together with a micromanipulator is used to transfer the foil from the specimen to a TEM grid coated with a holey carbon support film. No further carbon coating is required. This novel technology offers significant benefits in precision and speed of preparing site-specific TEM foils from inclusions in minerals, grain boundaries, microfossils, thin films on substrates and various coatings. FIB cut TEM foils can be investigated with modern TEM techniques such as various diffraction techniques, analytical electron microscopy (AEM) including line scan and elemental mapping, electron energy-loss spectroscopy (EELS) and high-resolution electron microscopy (HREM). With the FIB instrument (FEI FIB200) operated since fall 2002 at GFZ Potsdam we have prepared and investigated several hundreds of high quality TEM foils from silicates, carbonates, metals alloys, ceramic materials and diamond.

Incomplete retention of radiation damage in zircon from Sri Lanka

Abstract A suite of 18 zircon gemstones from placers in the Highland/Southwestern Complex, Sri Lanka, were subjected to a comprehensive study of their radiation damages and ages. The investigation included X-ray diffraction, Raman and PL spectroscopy, electron microprobe, PIXE and HRTEM analysis, as well as (U-Th)/He and SHRIMP U-Th-Pb age determinations. Zircon samples described in this study are virtually homogeneous. They cover the range from slightly metamict to nearly amorphous. Generally concordant U-Th-Pb ages averaging 555 ± 11 Ma were obtained. Late Ordovician zircon (U-Th)/He ages scattering around 443 ± 9 Ma correspond reasonably well with previously determined biotite Rb-Sr ages for rocks from the HSWC. Slightly to moderately metamict zircon has retained the radiogenic He whereas only strongly radiation-damaged zircon (calculated total fluences exceeding ~3.5 × 1018 α-events/g) has experienced significant He loss. When compared to unannealed zircon from other localities, Sri Lanka zircon is about half as metamict as would correspond to complete damage accumulation over a ~555 m.y. lasting self-irradiation period, suggesting significant annealing of the structural radiation damage. Insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon. We suggest to estimate “effective α-doses” for Sri Lanka zircon by multiplying total α-fluences, which were calculated using the zircon U-Th-Pb age, by a correction factor of 0.55. This conversion may be applied to literature data as well, because all gem-zircon samples from Sri Lanka (this work and previous studies) seem to reveal the same general trends of property changes depending on the radiation damage. The use of “effective α-doses” for Sri Lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation-damaged zircon.

Nanometre-sized mineral and fluid inclusions in cloudy Siberian diamonds: new insights on diamond formation

Nanometre-sized isolated inclusions have been studied in four cloudy octahedral diamonds from the Internatsionalnaya and one from the Yubileynaya mines (Yakutia). Transmission electron microscopy (TEM) techniques such as electron diffraction, analytical electron microscopy (AEM), electron energy-loss spectroscopy (EELS) and high-resolution electron microscopy (HREM) were applied as well as line scan and elemental mapping of the samples. All crystals exhibit octahedral external habit with opaque central cuboid cores that contain numerous nano-inclusions. All nano-inclusions in the size range between 30 and 800 nm reflect the diamond habit and are considered primary, syngenetic to host diamond. They are composed of multi-phase assemblages, which include solid phases (silicates, oxides, carbonates), brines (halides), and fluid bubbles. These inclusions are relatively homogeneous in composition and contain distinguishable crystalline and fluid phases. Al-bearing high-Mg silicate, dolomite, Ba-Sr carbonate, phlogopite, ilmenite, ferropericlase, apatite, magnetite, K-Fe sulfides (djerfisherite?) and kyanite have been identified as crystalline mineral phases by electron diffraction patterns, except the Ba-Sr carbonate. Several phases, including CaF 2 and clinohumite-like phases, have never been reported as inclusions in diamond. The halide phase was KCl. Bubbles contained high K, Cl, O, P and less S, Ba, Si, Ti components. Carbonates were identified in TEM foils from all studied diamonds. They occur in all assemblages with silicates, oxides, and sulfides and show a general enrichment in incompatible elements such as Sr and Ba. Some elemental variations may be explained by fractional crystallization of fluid/melt or mixing of fluids with different compositions (carbonatitic, hydrous-silicic, brines).

Annealing radiation damage and the recovery of cathodoluminescence

The structural recovery upon heat treatment of a highly metamict, actinide-rich zircon (U~6000 ppm) has been studied in detail using a range of techniques including X-ray powder diffraction, Raman spectroscopy, SHRIMP ion probe, electron microprobe, transmission electron microscopy and cathodoluminescence analysis. The structural regeneration of the amorphous starting material depends on random nucleation. It starts between 800 and 950°C when amorphous ZrSiO4 decomposes to form crystalline ZrO2 and amorphous SiO2. At around 1100°C, well-crystallised ZrSiO4 grows at the expense of the oxides. U has been retained in the newly grown zircon whereas Pb was evaporated during the heat treatment. This process is in marked opposition to the reconstitution of moderately metamict minerals, which experience a gradual recovery controlled by the epitaxial growth at the crystalline–amorphous boundaries. Both of these recovery processes are not the direct inverse of metamictisation. The structural regeneration was found to be connected with a significant increase in the emission of CL. In all cases (annealing heavily damaged zircon and moderately damaged zircon and monazite), we observe that the final, wellcrystallised annealing products emit more intense CL than their radiation-damaged starting minerals, although having almost identical elemental composition. Our observations are taken as evidence that the CL is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres.

Chemical and phase composition of particles produced by laser ablation of silicate glass and zircon—implications for elemental fractionation during ICP-MS analysis

The chemical and phase compositions of particles produced by laser ablation (266 nm Nd:YAG) of silicate NIST glasses and zircon were studied by SIMS and HR-TEM techniques. The data suggest that the formation of phases of different mineralogy and/or chemical composition from the original sample at the ablation site can result in elemental fractionation (non-stoichiometric sampling) in material delivered to the ICP-MS for quantitative analysis. Evidence of the element fractionation is preserved in chemically zoned ejecta deposited around the ablation pit. The chemical composition and mineralogy of particles varies with particle size so that the efficiency of transport of particles also plays a role in elemental fractionation. During the first 250 pulses in a typical ablation experiment using a 266 nm laser, particle sizes are mainly <2.5 μm; thereafter they decrease to <0.3 μm. Pb and U are fractionated significantly during the ablation of both silicate glass and zircon. During the ablation of glass, both micron-sized, melt-derived, spherical particles, and nm-sized, condensate-derived particle clusters, are produced; the very smallest particles (<0.04 μm) have anomalously high Pb/U ratios. For zircon, both larger (0.2–0.5 μm) spherical particles and agglomerates of smaller (∼0.005 μm) particles produced by ablation are mixtures of amorphous and crystalline materials, probably zircon, baddeleyite (ZrO2) and SiO2. Evidence for thermal decomposition of zircon to baddeleyite and SiO2 is preserved in the wall of the ablation pit, and may lead to the commonly observed increase in Pb/U recorded during laser ablation ICP-MS analysis. It follows that a matrix-matched external calibration is essential for achieving highly precise and accurate laser (266 nm wavelength) ablation ICP-MS analysis of Pb and U in silicate samples.

TEM imaging and analysis of microinclusions in diamonds: A close look at diamond-growing fluids

Abstract Fluid-bearing microinclusions in diamonds (<1 μm) provide a unique source of information on the diamond-forming medium. Transmission electron microscopy (TEM) investigation of such microinclusions enables the detailed study of their size, external habit, internal morphology, and mineralogy, and yields information on the chemical composition and crystallography of the included phases. Here we present a detailed TEM examination of microinclusions in four fibrous diamonds from Canada and Siberia, each with a distinctive inferred original fluid composition. Most microinclusions contain multi-phase assemblages that include carbonate, halide, apatite, possible pyroxene, and high-silica mica (6.8-7.7 Si atoms per formula unit) whose composition lies along the phlogopite-Al-celadonite join. The TEM results, together with the tight range of composition detected by electron probe microanalysis (EPMA) and the volatiles detected by infrared (IR) spectroscopy, suggest that the microinclusions trapped a uniform, dense, supercritical fluid and that the crystallized minerals grew as secondary phases during cooling. Carbonates appear in all assemblages, together with either halides or silicates, indicative of the importance of carbonatitic high-density fluid during diamond growth and fluid evolution. The presence of halide-carbonate or silicate-carbonate assemblages is in agreement with the bulk composition of the microinclusions as detected by EPMA. The high K content of some microinclusions detected by EPMA cannot be accounted for by the solid phases analyzed by TEM. This discrepancy suggests that K is concentrated in the residual fluid that is lost during TEM sample preparation. In addition to microinclusions, large cavities containing amorphous phases were found in the inner parts of one Siberian and one Canadian diamond. An Al-rich phase is the most abundant, and it is accompanied by Ca-rich and Si-rich phases. These phases may be explained by amorphization of crystalline phases. A breakdown of a single melt into three immiscible components is less likely.

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

The deepest rocks known from within Earth are fragments of normal mantle (≈400 km) and metamorphosed sediments (≈350 km), both found exhumed in continental collision terranes. Here, we report fragments of a highly reduced deep mantle environment from at least 300 km, perhaps very much more, extracted from chromite of a Tibetan ophiolite. The sample consists, in part, of diamond, coesite-after-stishovite, the high-pressure form of TiO2, native iron, high-pressure nitrides with a deep mantle isotopic signature, and associated SiC. This appears to be a natural example of the recently discovered disproportionation of Fe2+ at very high pressure and consequent low oxygen fugacity (fO2) in deep Earth. Encapsulation within chromitite enclosed within upwelling solid mantle rock appears to be the only vehicle capable of transporting these phases and preserving their low-fO2 environment at the very high temperatures of oceanic spreading centers.

Nyerereite and nahcolite inclusions in diamond: evidence for lower-mantle carbonatitic magmas

Abstract Nyerereite and nahcolite have been identified as micro- and nano-inclusions in diamond from the Juina area, Brazil. Alongside them are Sr- and Ba-bearing calcite minerals from the periclase-wüstite series, wollastonite II (high), Ca-rich garnet, spinels, olivine, phlogopite and apatite. Minerals of the periclase-wüstite series belong to two separate groups: wüstite and Mg-wüstite with Mg# = 1.9-15.3, and Fepericlase and periclase with Mg# = 84.9-92.1. Wollastonite-II (high, with Ca:Si = 0.992) has a triclinic structure. Two types of spinel were distinguished among mineral inclusions in diamond: zoned magnesioferrite (with Mg# varying from 13.5-90.8, core to rim) and Fe spinel (magnetite). Olivine (Mg# = 93.6), intergrown with nyerereite, forms an elongate, lath-shaped crystal and most likely represents a retrograde transformation of ringwoodite or wadsleyite. All inclusions are composed of poly-mineralic solid mineral phases. Together with previously found halides, sulphates and other mineral inclusions in diamond from Juina, they form a carbonatitic-type mineral paragenesis in diamond which may have originated in the lower mantle and/or transition zone. Wüstite inclusions with Mg# = 1.9-3.4, according to experimental data, may have formed in the lowermost mantle. The source for the observed carbonatitic-type mineral association in diamond is lower-mantle natrocarbonatitic magma. This magma may represent a juvenile mantle melt, or be the result of low-degree partial melting of deeply-subducted carbonated oceanic crust. This magma was rich in volatiles, such as Cl, F and H, which played an important role in the formation of diamond.

Microstructures, petrofabrics and seismic properties of ultra high-pressure eclogites from Sulu region, China: implications for rheology of subducted continental crust and origin of mantle reflections

Ultra high-pressure (UHP) eclogites from Sulu region (China) represent mafic components of the continental crust, which were first subducted to mantle depths greater than 100 km and then exhumed to the earth’s surface. Detailed investigation of microstructures, chemical compositions, petrofabrics and seismic properties of the UHP eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle. We present here results from field, optical and TEM observations, electron back-scattered diffraction (EBSD) measurements and numerical computations of the seismic properties of UHP eclogites collected from fresh surface outcrops at the drill site (Maobei, Donghai County, Jiangsu Province) of the Chinese Continental Scientific Drilling Program (CCSD). Two types of eclogites have been distinguished: Type-1 (coarse-grained) eclogites deformed by recovery-accommodated dislocation creep at the peak metamorphic conditions, and Type-2 (fine-grained) eclogites which are composed of reworked Type-1 materials during recrystallizationaccommodated dislocation creep in shear zones which were active during the exhumation of the UHP metamorphic rocks. Both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the UHP metamorphic conditions. Plasticity of eclogites under UHP conditions can effectively facilitate channeled flow along the interplate shear zone. The

Transmission electron microscope study of polyphase and discordant monazites: Site-specific specimen preparation using the focused ion beam technique

Electron-microprobe (EMP) U-Th-Pb dating on polyphase and discordant monazites from polymetamorphic granulites of the Andriamena unit (north-central Madagascar) reveals inconsistent chemical ages. To explain these drastic variations, transmission electron microscopy (TEM) foils were prepared directly from thin sections by using the focused ion beam technique. The most important result of the TEM study is the demonstration of the presence of small (~50 nm) Pb-rich domains where large variations in EMP ages occur. We suggest that radiogenic Pb was partially reincorporated in monazite during the recrystallization at 790 Ma. Because the excited volume of EMP is ~4 µm3, U-Th-Pb dating yielded various apparent older ages without geological significance. In addition, TEM analysis of the foils revealed the presence of an ~150-nm-wide amorphous zone along the grain boundary of monazite and its host quartz. This Fe-Si-Al–rich phase may have formed as a result of fluid activity at 500 Ma, and the phases amorphous state may be due to the irradiation from U and Th decay in the monazite. This demonstrates for the first time the enormous potential of the TEM investigations on site-specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data.