Electron microscopy study of the iron meteorite Santa Catharina

Abstract— Characterization of the microstructural features of the metal of the Santa Catharina meteorite was performed using a variety of electron optical techniques. Sample USNM#6293 is chemically homogeneous on the micron scale and has a Ni content of 28.2 wt.%. Its microstructure is similar to that of the Twin City ataxite and contains clear taenite II, i.e., fcc taenite with domains of tetrataenite, < 10 nm in size. Sample USNM#3043 is a more typical Santa Catharina specimen with dark and light regions as observed with the light optical microscope. The dark regions are inhomogeneous and contain 45–50 wt.% Ni and 7–12 wt.% O. The light regions are homogeneous and contain 35 wt.% Ni and no detectable oxygen. The microstructure is that of cloudy zone, i.e., islands of tetrataenite, ～20 nm in size, in a honeycomb matrix. The honeycomb phase contains Ni rich oxide in the dark regions and contains metal, fcc taenite, in the light regions. The original metal structure of USNM#3043 is cloudy zone which formed during cooling into the low temperature miscibility gap of the Fe-Ni phase diagram. The dark regions were developed from the metal by selective corrosion of the honeycomb structure, transforming it into Ni containing oxides, possibly non-stoichiometric Fe₂NiO₄ while retaining the tetrataenite islands. Using the results of this study, many of the existing discrepancies concerning the microstructure of Santa Catharina can be explained.     

THE BLOOMINGTON (LL6) CHONDRITE AND ITS SHOCK MELT GLASSES

The Bloomington meteorite, a 67.8 gram veined, brecciated chondrite, fell during the summer of 1938 in Bloomington, Illinois. Its olivine, orthopyroxene and metal compositions (fo₆₉, en₇₄ and Fe₅₂ Ni₄₈ respectively) and its texture identify it as a brecciated LL6 chondrite of shock facies d. Shock melt glasses occur in Bloomington as sparse melt pockets and veins in clasts and as isolated masses in the black, clast-rich matrix. The vein glasses chemically resemble bulk LL-group chondrites and thus appear to reflect total melting of the host meteorite. The melt pocket and matrix glasses, like those described previously in L-group chondrites, have more varied compositions and are typically enriched in normative plagioclase. All glasses that we analyzed in Bloomington have FeO/MgO and Na/Al ratios similar to those of LL-group chondrites, indicating that melting of this meteorite involved neither a significant change in the oxidation state of iron nor loss of sodium to a vapor phase. Bloomington is a monomict breccia whose components formed in place as a result of a single episode of shock and attendant melting.     

Deglaciation of the younger dryas or Loch Lomond Stadial ice-field in the northern Highlands,Scotland

When viewed from the air, Scottish ‘hummocky moraine’ can be resolved into a series of linear ridges that resemble those found at the margins of actively retreating glaciers today. Recent work has supported the interpretation of these linear ridges as ice-marginal landforms and the authors believe that the majority of ‘hummocky moraine’ deposits can be interpreted in this way. Consequently the pattern of deglaciation can be established fairly precisely from the pattern of linear ridges. This approach is applied to the landforms of the northern part of the Loch Lomond Stadial ice-field in order to reconstruct the regional pattern of deglaciation. This leads to important inferences about the significance of topographic control during deglaciation and more importantly it provides fresh insight into the environment of the British Isles during the Loch Lomond Stadial.     

Complex replacement patterns in garnets from Bergen Arcs eclogites: A combined EBSD and analytical TEM study

Eclogite-facies rocks within the Bergen Arcs, western Norway, have formed from granulites along shear zones and fluid pathways. Garnets that were inherited from granulite facies protoliths show different types of replacement patterns due to an incomplete eclogitisation process including concentric rim zoning, zoning along vein fillings and inclusion trails, and zoning bands without inclusions. The interfacial part between the granulitic core and the eclogitic rim of garnet as well as the microstructure of other relevant minerals (omphacite, plagioclase) has been analysed using analytical transmission electron microscopy (ATEM). In garnet, the interface is characterised by gradual changes in composition from X_alm=0.31, X_pyr=0.50 to X_alm=0.54, and X_pyr=0.25 within ≈20 μm and exhibits no distinct change in microstructure. Granulitic plagioclase shows exsolution lamellae of the Bøggild intergrowth. In omphacite, anti-phase domains (APDs) which potentially record the temperature of cation ordering after mineral growth have been observed and their size suggest eclogitisation at 600–700 °C. The electron backscatter diffraction (EBSD) analysis revealed that the lattice orientation of the granulitic feldspar is basically unrelated to tectonic axes whereas newly formed eclogitic minerals omphacite and kyanite show a crystallographic relation to the foliation. In garnet, no change in the basic crystallographic orientation between the eclogitic and granulitic garnet composition was confirmed. However, misorientation analysis suggests a cellular microstructure not more than 1° misorientation in the core of the garnets, which is missing in the eclogitic rim indicating textural equilibration of the latter. The heterogeneous replacement patterns are characteristic for dissolution and re-precipitation reactions in an open system limited to fluid availability. The appearance of the compositional profile in garnet is interpreted as a diffusional re-equilibration step after the time-limited, fluid-mediated eclogitisation event that apparently obscured the initially sharp interface within the further retrograde metamorphic history.     

Reaction mechanism of uranyl in the presence of zero-valent iron nanoparticles

The current study provides an investigation of abiotic reduction of an oversaturated uranyl solution driven by iron nanoparticle oxidation. The reactivity of nano-scale zero-valent iron (ZVI) under mildly oxic conditions (1.2% O₂ and 0.0017% CO₂) was studied in 1000 ppm uranyl solution in the pH range 3-7, at reaction times from 10 min to 4 h. Reductive precipitation of UO₂ was observed as the main process responsible for the removal of uranium from solution with the kinetics of reaction becoming increasingly favourable at higher pH. Despite working with an oversaturated uranium solution, the precipitation of UO₂ occurred in preference to precipitation of UO₃·2H₂O (metaschoepite) at reaction times between 1 and 4 h and for uranyl solutions initially set up at pH ?5. Characterisation of both solid and solution phases was performed using X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB) imaging, X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES).     

The volcano-magnetic effect

Summary A geometrically simple volcano is considered, havig a spherical magma chamber of 2.5 km radius centred at 10 km depth. The Curie point isotherm is assumed to be a plane at 20 km depth, except for the spherical volume which is also non-magnetic. The stress pattern in the vicinity of the spherical chamber, due to regional stress of sufficient intensity to cause an eruptions, is used to calculate the change in magnetization which results from the piezomagnetic effect through the volume of solid rock. The consequent magnetic field anomaly at the surface is then obtaied by numerical integration of the dipole law of force over the stressed volume. For rocks of the type found on the volcanic island of St. Vincent (West Indies), this model gives a maximum local volcano-magnet c effect of about 7 gammas.