Shyh-Lung Hwang

Genesis of microdiamonds from melt and associated multiphase inclusions in garnet of ultrahigh-pressure gneiss from Erzgebirge, Germany

Abstract The common association of microdiamonds with the phase assemblage: phlogopite, apatite, paragonite and α-quartz (containing amorphous Na–Al silicate inclusions), as inclusions in garnets of quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany, was studied by analytical electron microscopy. The assemblage implies a precedent melt, which coexisted with the microdiamonds before and after entrapment in the garnet host, and subsequently crystallized. The formation of microdiamonds in this metamorphic rock could be explained by cotectic-induced partial melting of a subducted continental slab at about 4–6 GPa and 1000°C, as constrained by the occurrence of TiO 2 with an α-PbO 2 -type structure at the peak metamorphic conditions, and by the catalytic effect of siderophile and chalcophile elements.

Metal-sulfur-COH-silicate fluid mediated diamond nucleation in Kokchetav ultrahigh-pressure gneiss

Submicron metal sulfides, randomly oriented microdiamonds and phlogopite have been identified as multiphase inclusions in garnet of a garnet-clinopyroxene-quartz crustal rock from the Kokchetav Massif in Kazakhstan. Analytical electron microscopy shows that metal sulfides are entrapped among diamond aggregates away from infiltration cracks and do not exhibit any specific crystallographic relationship with associated microdiamonds, thus implying a possible syngenetic nucleation of microdiamonds from a precursive fluid containing dissolved metal sulfides. The diamonds increased in size via a spiral growth mechanism, as manifested by dislocations radiating from nuclei toward faceted surface outcrops.

Hematite and magnetite precipitates in olivine from the Sulu peridotite: A result of dehydrogenation-oxidation reaction of mantle olivine?

Abstract Analytical electron microscopic observations have been carried out on a garnet peridotite from the Maobei area, Sulu ultrahigh-pressure terrane. The results showed that olivine in this garnet peridotite (5.3-6.6 GPa; 853-957 °C), contains precipitates of chromian magnetite and chromian-titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and the olivine host, viz. [101]Mt//[001]Ol, [110]Mt//[01̄1]Ol, and [01̄1]Mt//[011]Ol; and [0001]Hm//[100]Ol and [101̄0]Hm//[001]Ol. These oriented oxides are not associated with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation. It is postulated that these magnetite and hematite precipitates most likely have resulted from dehydrogenation-oxidation of nominally anhydrous mantle olivine during rock exhumation. In view of the contrasting diffusion rates of H and Fe in the olivine lattice, it is suggested that the formation process might actually take place in steps. Hydrogen diffusion with concomitant quantitative oxidation of Fe2+ to Fe3+ in olivine occurred early during initial rock exhumation and was followed by slow Fe diffusion forming magnetite/hematite at stacking faults and dislocations within the olivine lattice. Two requirements are essential under such a scenario: an ample amount of H content of the olivine, and an appropriate exhumation rate, probably in the range of 6-11 mm/year, of the host rock. It is also noted that such dehydrogenation-oxidation processes may hamper a correct estimate of the actual P-T conditions and mantle oxidation state based on mineral chemistries present in mantle eclogite/peridotite. The present study demonstrates that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through a series of more complicated reaction mechanisms.

Kumdykolite, an orthorhombic polymorph of albite, from the Kokchetav ultrahigh-pressure massif, Kazakhstan

Kumdykolite, an orthorhombic polymorph of albite, has been identified for the first time by analytical electron microscopy. It occurs in association with diopside, quartz/cristobalite, phengite/phlogopite, an unidentified aluminosilicate, calcic amphibole, dolomite, calcite, or talc, as micrometer-scale mineral inclusions in omphacite of eclogite from the Kumdy Kol, Kokchetav ultrahigh-pressure massif in northern Kazakhstan. The unit-cell parameters of kumdykolite were determined to be a = 8.24(1) A, b = 8.68(1) A, and c = 4.84(1) A, V = 346.17 A3, and with Z = 2. The space group could be either P 2 nn or Pmnn, but is probably Pmnn . Kumdykolite is presumed to be a metastable phase formed at high temperatures followed by rapid cooling in the absence of water. It is further postulated that kumdykolite may have resulted from the interaction between infiltrated melt and omphacite when the Kokchetav massif was exhumed from mantle depths to the base of the crust.

A New Occurrence and New Data on Akdalaite, a Retrograde Mineral from UHP Whiteschist, Kokchetav Massif, Northern Kazakhstan

Akdalaite (5Al2O3·H2O), as well as hogbomite, kyanite, Zn-bearing staurolite, Mg-chlorite, gahnite, hercynite, quartz, apatite, and zircon, are present as submicrometer-size minerals associated with rutile aggregates occurring as inclusions in garnet porphyroblasts of whiteschist from the Kulet Kol region, Kokchetav Massif, northern Kazakhstan. The akdalaite crystals, ∼0.5 to 1 μm in size, are subhedral to euhedral, and are bounded by (0001), (0001), {1100}, {1101} facets. EDX analyses show minor Si, Ti, Cr, Fe, Mg, Zn, and Ga, in addition to the major component Al. Electron diffraction data yield the following crystal structural information: space group P63mc, and a = 5.58(1) Å, c = 8.86(2) Å, which are similar to those of synthetic tohdite, but are distinctly differentfrom those reported for the type specimen of akdalaite (P612 or P61 , a = 14.97 Å, c = 2.87 Å). The akdalaite type specimen, stored in the museum of the All-Russian Institute of Mineral Resources (VIMS) in Moscow, had also been re-examined; it has crystallographic parameters similar to tohdite. It is therefore concluded that crystallographic data of the akdalaite type specimen derived from powder X-ray experiments are in error, and that akdalaite is the natural counterpart of tohdite.

Defect microstructures of minerals as a potential indicator of extremely rapid and episodic exhumation of ultrahigh-pressure metamorphic rock: implication to continental collision orogens

Exhumation of subducted continental crust to the Earth’s surface was presumably rapid, though at an uncertain rate, to retain coesite, diamond and α-PbO2-type TiO2. Here we report unique defect microstructures of minerals as a potential indicator of a rapid and episodic exhumation process in a fossil fracture zone of coesite eclogite from the Sulu terrain, eastern China having the most negative δ18O value ever reported for eclogite-facies metamorphic rock. Analytical electron microscopy indicates that semi-brittle deformation occurred in kyanite/omphacite/spinel with extensive and unusual fine-scale twin lamellae and that brittle deformation occurred in garnet with hardly healed {110} microcleavages. These unique defect microstructures can be rationalized by a high strain rate at local weakening and deep faulting of continental collision orogens.

On the transformation pathways of α-PbO2-type TiO2 at the twin boundary of rutile bicrystals and the origin of rutile bicrystals

Extraction and electron irradiation (under transmission electron microscopy) of an epitaxial nanometer-thick α-PbO 2 -type TiO 2 slab between twinned rutile bicrystals in ultra-high pressure metamorphic rock caused phase changes into a modified fluorite-type and then an amorphous phase. This martensitic-type transition process accounts for the dislocations and stacking faults of the slab and disordering of Ti in the adjoined rutile bicrystals. Additional hydrothermal experiments of sol-gel TiO 2 -Al 2 O 3 performed at 8.5–9 kbar and 675–800°C in the piston-cylinder apparatus indicated that twinned rutile bicrystals were shaped in mirror image without the formation of α-PbO 2 -type TiO 2 slab at the twin boundary and with no other planar defects for the bicrystals. The twinned bicrystals can be rationalized by growth and/or coalescence processes. Accordingly, it is not justified to assume a precursor phase of α-PbO 2 -type structure for twinned rutile bicrystals when there is no such relic. Rutile, unless exsolved epitaxially from a host mineral such as garnet, does not constitute evidence for unusually deep burial for ultra-high pressure terranes.

On the coherency-controlled growth habit of precipitates in minerals

Minimization of interfacial energy is the dominant factor governing the equilibrium habit, i.e. orientation and morphology, of a precipitate in the parent crystal. Existing models of optimum interphase boundaries have established criteria that minimize the lattice misfit and/or the strain energy by the comparison of the interplanar spacings of the two interpenetrating lattices. Based on a detailed analytical transmission electron microscopy (TEM) study of two precipitate–matrix systems with different degrees of structural similarity in oxygen sublattices, i.e. hematite in rutile and magnetite in clinopyroxene from metamorphic rocks, it is reported that the optimum interphase boundary and the equilibrium habit of a precipitate in these structurally complex systems can be defined from the special geometry characteristics of diffraction patterns along specific zone axes. Analysis of the TEM results shows that a precipitate in a parent crystal, either with or without a similar oxygen framework, always poses a specific crystallographic relationship with the differences in the g vector pairs (Δg), i.e. misfits of diffraction-spot pairs, of two structures aligned in the same direction, and that the precipitate exhibits a growth habit plane normal to Δg. This new insight into the interfacial energetics, rather than kinetically or topologically controlled mineral growth, can be understood by the coherency of lattice planes in the interphase boundaries and rationalized by the normal-to-Δg principle originally developed for structural transformations in alloys with simple close-packed structures. The normal-to-Δg principle is formulated to calculate the optimum interphase boundaries in the present systems, feldspars and clinopyroxenes of broad geological/mineralogical interest, and gives results comparable to other models yet in a more straightforward manner. This alternative approach can be readily applied in interphase boundary modelling of minerals and sheds light on the growth habit as a potential geothermobarometer of metamorphic rocks.

α-PbO2–Type TiO2: From Mineral Physics to Natural Occurrence

α-PbO2–type TiO2 (space group Pbcn, a thermodynamically predicted high-pressure polymorph of rutile) was synthesized by a number of investigators using shock and static compression experiments. Recently, in situ high-pressure and high-temperature studies employing the multi-anvil device and white-beam (using a synchrotron radiation source) energy-dispersive method indicated that the transformation pressure is lower for nanophase material (∼4 GPa and 900°C) than for the bulk (∼6 GPa and 850°C). In addition, the phase boundary of rutile/α-PbO2-type TiO2 changes from a negative to a positive slope with increasing temperature. This timely knowledge provides indicative pressure-temperature (P-T) constraints on the natural occurrence of α-PbO2-type TiO2, recently identified by analytical electron microscopy as an epitaxial nanometer-thick slab between twinned rutile bicrystals in almandine-rich garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany. The stability field of “bulk” α-PbO2-structured TiO2 shows that the minimum stabilization pressure of transition is ∼6 GPa and could have been up to 2 GPa lower as a result of the nanophase effect. This suggests burial of continental crustal rocks to depths of at least 130-200 kilometers. Thus, α-PbO2-type TiO2 inclusions in garnet may be a useful P-T indicator in the diamond stability field. Furthermore, the possibility of finding α-PbO2-type TiO2 or even a higher-P polymorph (e.g., baddeleyite-structured TiO2) at impact sites of meteorite craters is increased, in view of the recent identification of post-stishovite (isostructure of rutile) SiO2 polymorphs in the meteorite Shergotty, and the alleged identification of α-PbO2-type TiO2 by Raman spectroscopy in shocked gneisses from the Ries Meteorite Crater, Germany.

Inferring the Effects of Compositional Boundary Layers on Crystal Nucleation, Growth Textures, and Mineral Chemistry in Natural Volcanic Tephras through Submicron-Resolution Imaging

Crystal nucleation and growth are first order processes captured in volcanic rocks and record important information about the rates of magmatic processes and chemical evolution of magmas during their ascent and eruption. We have studied glass-rich andesitic tephras from the Central Plateau of the Southern Taupo Volcanic Zone by electron- and ion-microbeam imaging techniques to investigate down to sub-micrometre scale the potential effects of compositional boundary layers (CBLs) of melt around crystals on the nucleation and growth of mineral phases and the chemistry of crystal growth zones. We find that CBLs may influence the types of mineral phases nucleating and growing, and growth textures such as the development of swallowtails. The chemistry of the CBLs also has the capacity to trigger intermittent overgrowths of nanometre-scale bands of different phases in rapidly growing crystals, resulting in what we refer to as cryptic phase zoning. The existence of cryptic phase zoning has implications for the interpretation of microprobe compositional data, and the resulting inferences made on the conditions of magmatic evolution. Identification of cryptic phase zoning may in future lead to more accurate thermobarometric estimates and thus geospeedometric constraints. In future, a more quantitative characterization of CBL formation and its effects on crystal nucleation and growth may contribute to a better understanding of melt rheology and magma ascent processes at the onset of explosive volcanic eruptions, and will likely be of benefit to hazard mitigation efforts.