ALBERT

Description: SUMMARY Active high-temperature (〉150 °C) geothermal areas like the Krafla caldera, NE-Iceland, often show distinct magnetic lows in aeromagnetic anomaly maps suggesting a destruction of magnetic minerals by hydrothermal activity. The main alteration processes in such an environment are low-temperature oxidation (〈350 °C, maghemitization) and fluid–rock interactions. We investigated the rock magnetic properties [natural remanent magnetization (NRM) magnetic susceptibility and their temperature and field variation] and the mineralogy, using X-ray diffraction, microscopic methods and electron microprobe analyses, of two drill cores (KH1 and KH3) from the rim of the Krafla caldera. The drill cores have distinctly lower NRM values (average 〈3 A m –1 ) compared to younger surface basalts (average 20 A m –1 ) along with a large variation in magnetic susceptibility (1.3 × 10 −7 – 4.9 × 10 −5 m 3 kg –1 ). The secondary mineral assemblage (sulphides, sphene, rutile and chlorite) indicates an alteration within the chlorite–smectite zone for both cores without depth zoning. Optical miscroscopy in combination with the Bitter technique and backscatter electron microscopy along with the thermomagnetic analyses allow distinguishing two different magnetomineralogical groups of titanomaghemite: (1) titanomaghemite with intermediate titanium concentration and probably high vacancy concentration, and (2) titanomaghemite with low titanium concentration and low vacancy concentration. The mineral assemblages, textures and magnetic properties deduced from the mentioned magnetic measurements indicate two-stage transformation mechanism: (1) Dissolution of titanium at low pH under oxidizing conditions. The ulvöspinel component of titanomagnetite and ilmenite forms rutile or sphene, and Fe 2 + migrates out of the spinel lattice forming titanomaghemite. (2) Formation of pyrite and dissolution of remaining titanomaghemite under reducing and acidic conditions. The latter mechanism produces ghost textures (all titanomaghemite is transformed and only their former grain shapes are preserved), with only paramagnetic minerals left and ferrimagnetic minerals nearly dissolved. This mechanism could explain the significant magnetization loss, which is seen in many local magnetic anomaly lows within the oceanic crust and volcanic islands like Iceland or Hawaii. The production of nanoporous textures in titanomaghemites is suggested as a mechanism for the enhancement of the magnetic susceptibility values related to the hydrothermal alteration of Krafla.

Description: Synthetic polycrystalline samples of Fe-Ti oxides (titanomagnetite, Tmtss; ilmenite-hematitess, Ilmss; pseudobrookitess, Psbss) are very sensitive to changes in the redox conditions at high temperatures, either during synthesis experiments or during thermomagnetic measurements. For instance, exposure to air for a few seconds at the end of a synthesis run at 1300°C of a Tmtss-Ilmss sample produces surficial oxidation down to a depth of some 100 μm. This oxidation zone is well visible on backscattered electron images of polished sections through the sample pellet. It is characterized by so-called trellis “oxyexsolution” textures, i.e., fine lamellae of Ilmss within the Tmtss crystals and lamellae of Psbss within the Ilmss crystals. In this oxidation zone the newly grown Ilmss lamellae and the surrounding Tmtss are more Fe rich than the original crystals. The presence of trellis textures in the crystals of both coexisting phases, Tmtss and Ilmss, show that only short-scaled elemental transport within the crystals was involved and that equilibrium was not attained. Even though the oxidation zone is very narrow, the imprint of the new Tmtss compositions is well recognizable in temperature-dependent magnetic susceptibility curves. In temperature-dependent saturation magnetization (MS-T) curves, however, the contribution of more Fe-rich Tmtss from the oxidation zone can be easily overseen. However, surficial oxidation of Tmtss does occur during MS-T measurements with a variable field translation balance, apparently in relation with insufficient Ar flowing around the sample. Further examples of rapid surficial oxidation of Fe-Ti oxide samples are also discussed.

Description: [1]  Shock experiments with pressures ranging from 3 to 30 GPa have been conducted on a mixed assemblage of hexagonal and monoclinic pyrrhotite. All samples were studied with respect to their particular shock-induced microstructures and magnetic properties at high and low temperatures. Up to 8 GPa, microstructures in shocked pyrrhotite are characterized by mechanical deformation producing a damage of the crystal structure. At pressures of 20 GPa and upward, amorphization and mechanical twinning are the dominant structural features induced by shock. Within the lower-pressure range coercivity, saturation isothermal remanent magnetization and coercivity of remanence increase with shock pressures, in agreement with more single-domain (SD)-like behavior. Simultaneously, the λ-peak of hexagonal pyrrhotite decreases and the 34 K transition of monoclinic pyrrhotite broadens and is depressed. Magnetic hardening is triggered by grain-size reduction, but also by the formation of SD within discrete multidomain grains. Planar deformation features subdivide such multidomain grains into lath-shaped domains with average sizes lying in the SD range. The planar deformation features disappear at 20 GPa and irregular, nanometer-sized “amorphous domains” occur instead. Pressure release from 30 GPa finally triggers partial melting of pyrrhotite. The sharp interfaces between molten and crystalline pyrrhotite document a rapid change of thermal conditions. Within molten pyrrhotite, quenched iron crystals occur. The presence of native iron strongly influences the magnetic properties, depending on the particular amount in the studied sample and likely affects the magnetic properties of impact lithologies on Earth and extraterrestrial material.

Description: Clay minerals as products of hydrothermal alteration significantly influence the hydraulic and mechanical properties of crystalline rock. Therefore, the localization and characterization of alteration zones by downhole measurements is a great challenge for the development of geothermal reservoirs. The magnetite bearing granite of the geothermal site in Soultz-sous-Forêts (France) experienced hydrothermal alteration during several tectonic events and clay mineral formation is especially observed in alteration halos around fracture zones. During the formation of clay minerals, magnetite was oxidized into hematite, which significantly reduces the magnetic susceptibility of the granite from ferrimagnetic to mostly paramagnetic values. The aim of this study was to find out if there exists a correlation between synthetic clay content logs (SCCLs) and measurements of magnetic susceptibility on cuttings in the granite in order to characterize their alteration mineralogy. Such a correlation has been proven for core samples of the EPS1 reference well. SCCLs were created from gamma ray and fracture density logs using a neural network. These logs can localize altered fracture zones in the GPK1-4 wells, where no core material is available. Mass susceptibility from 261 cutting samples of the wells GPK1–GPK4 was compared with the neural network derived synthetic logs. We applied a combination of temperature dependent magnetic susceptibility measurements with optical and electron microscopy, and energy dispersive X-ray spectroscopy to discriminate different stages of alteration. We found, that also in the granite cuttings an increasing alteration grade is characterized by an advancing oxidation of magnetite into hematite and a reduction of magnetic susceptibility. A challenge to face for the interpretation of magnetic susceptibility data from cuttings material is that extreme alteration grades can also display increased susceptibilities due to the formation of secondary magnetite. Low magnetic susceptibility can also be attributed to primary low magnetite content, if the granite facies changes. In order to interpret magnetic susceptibility from cuttings, contaminations with iron from wear debris of the drilling tools must be eliminated. Provided that the magnetic mineralogy of the granite is known in detail, this method in combination with petrographic investigations is suited to indicate and characterize hydrothermal alteration and the appearance of clay.

Description: [1]  This study presents low-temperature SIRM, susceptibility and hysteresis data of magnetite that have been combined with mineralogic observations. It aims to unravel the origin of two different Verwey transition temperatures observed in different rocks from drill cores of the Chesapeake Bay impact structure (CBIS). We distinguish three types of magnetite in these rocks. (1) “ primary magnetite ” (mt prim) in granites from allochthonous and autochthonous crystalline basement. It has average grain sizes up to some hundreds µm, shows typical multidomain magnetic behaviour and a regular Verwey transition at ~120 K. (2) “Shocked magnetite” occurs in fragments of crystalline rocks and as single grains in the suevite and impact breccia. These rocks show two Verwey transitions - a regular transition at ~120 and a “low–temperature Verwey transition” (LT V ) at around 95 K. The shocked grains are strongly deformed and partially molten. These grains have been strongly alterated during a post-impact phase. The LT V originates from an oxidized magnetite fraction with small grain sizes (some tens of µm). Compared to the larger grain size fraction (some hundreds of µm), the high surface/volume ratio of the small grains allows a more pervasive development of the resulting non-stoichiometry that finally controls T V . (3) “Secondary” magnetite, which formed after the impact from hydrothermal fluids, also shows two Verwey transitions. Besides the regular one, an additional LT V appears between 90 and 100 K. “Secondary” magnetite forms clusters consisting of numerous needle-shaped magnetite crystals, which range from few nm to about 10 µm. This grain size variation is in agreement with hysteresis properties and first-order reversal curves, which indicate superparamagnetic to multidomain magnetic behaviour. The LT V in this population is also the of result of a thin oxidized surface layers that make up an effectual percentage of the bulk volume of the small grains. The Verwey transition is often strongly broadened because this type of magnetite holds a broad range of grain sizes with varying ratios between oxidized surface and bulk volume. This study shows that especially the Verwey transition of small magnetite grains reacts very sensitive to surface oxidation. As the Verwey transition temperature of small grains is very sensitive to post-impact alteration it cannot be used as a reliable shock pressure indicator.

Description: Neonatal thrombocytopenia occurs in about 1% of all newborns. Inherited forms like 11q- or Jacobsen syndrome are rare. However, they may remain undetected with karyotyping because the deleted regions in 11q often involve small subtelomeric regions. Here we report on the detection of deletions in 11q in two newborns with normal routine karyotypes who were shown to carry subtelomeric deletions in 11q by means of fluorescence in situ hybridization (FISH) using a subtelomeric 11q probe (Abbott, Diagnostics, Wiesbaden, Germany). Both children showed thrombocytopenia (18.000/μl and 26.000/μl, respectively) and dysmegakaryopoiesis (absence of normal megakaryocytes and presence of micromegakaryocytes) associated with facial dysmorphism, cardiac defects and psychomotoric retardation. In the second case, the mother and the grandmother also showed mild thrombocytopenia. In both patients, FISH analyses on peripheral blood and bone marrow showed the loss of the telomere-associated region of 11q distal of the MLL gene. In the first patient, the deletion of 11q resulted from an unbalanced complex rearrangement with duplication of 11p. As the source of this chromosomal aberration, a paternal pericentric inversion of chromosome 11 was identified. The partial monosomy 11q and the partial trisomy 11p in the first patient were confirmed by comparative genomic hybridization (CGH) analysis. Array/matrix CGH assisted in determining the breakpoints at 11p15.1 and 11q24.1. No structural aberrations of 11q were found in the mother of the second patient, but further investigations are under way. These findings give further evidence that small subtelomeric deletions of 11q and probably mutations of genes located therein cause thrombocytopenia. Since it can be very difficult to detect these deletions by karyotyping, FISH using a subtelomeric 11q probe seems to be an extremely useful new diagnostic tool. This new method should be applied in children with congenital thrombocytopenia, in particular if they have additional complex dysmorphic features.