Shock induced planar deformation structures in quartz from the Ries crater,Germany

Crystalline rocks from breccias of the Ries basin, Germany, contain highly deformed quartz. Various planar deformation structures could be observed and classified into five different types: (1) Decorated planar elements, (2) Non-decorated planar elements, (3) Homogeneous lamellae, (4) Filled lamellae, (5) Planar fractures. All these structures are parallel to crystallographic planes: {10¯13}, {10¯12}, {10¯11}, {0001},{11¯21}, {11¯22}, {21¯31}, {51¯61}, {10¯10}. The most typical and most abundant planar structures are decorated and nondecorated planar elements parallel to {10¯13} and {10¯12}. Planar fractures are parallel to {0001} and {10¯11} and form at lower stress levels, probably earlier than the planar elements.Quartz containing planar elements, especially of the non-decorated type, has lower density, index of refraction and birefringence than normal quartz. This quartz is apparently a mixture of an amorphous phase and crystalline quartz, the amount of which can be calculated using average density or refractive index.Comparison of planar quartz structures found in tectonites and those produced artificially under static or dynamic high pressure conditions demonstrates that Ries quartz closely resembles deformed quartz recovered from shock wave experiments. The planar structures found in Ries quartz have been formed by shock wave actions with peak pressures in the 100–400 kbar range.Planar elements are explained to be traces of gliding processes during shock loading visible due to the fact that a high pressure phase (stishovite and/or a stishovite-like glass phase) has been produced along the glide planes. Upon pressure release most of the high pressure phase was transformed into an SiO₂-glass (diaplectic glass).In comparison with experimental data the amount of residual crystalline quartz as well as type and orientation of planar structures in the quartz grains are clues to estimate the peak pressures responsible for these deformations. Shock waves with peak pressures exceeding about 400 kbar completely transform quartz into diaplectic SiO₂-glass.     

Mineral shock signatures in rocks from Dhala (Mohar) impact structure,Shivpuri district,Madhya Pradesh,India

A concrete study combining optical microscopy, Raman spectroscopy and X-ray diffractometry, was carried out on subsurface samples of basement granite and melt breccia from Mohar (Dhala) impact structure, Shivpuri district, Madhya Pradesh, India. Optical microscopy reveals aberrations in the optical properties of quartz and feldspar in the form of planar deformation feature-like structures, lowered birefringence and mosaics in quartz, toasting, planar fractures and ladder texture in alkali feldspar and near-isotropism in bytownite. It also brings to light incidence of parisite, a radioactive rare mineral in shocked granite. Raman spectral pattern, peak positions, peak widths and multiplicity of peak groups of all minerals, suggest subtle structural/crystallographic deviations. XRD data further reveals minute deviations of unit cell parameters of quartz, alkali feldspar and plagioclase, with respect to standard \({\upalpha }\)-quartz, high- and low albite and microcline. Reduced cell volumes in these minerals indicate compression due to pressure. The \(\hbox {c}_{0}/\hbox {a}_{0}\) values indicate an inter-tetrahedral angle roughly between \(120^{\mathrm{o}}\) and \(144^{\mathrm{o}}\), further pointing to a possible pressure maxima of around 12 GPa. The observed unit cell aberration of minerals may indicate an intermediate stage between crystalline and amorphous stages, thereby, signifying possible overprinting of decompression signatures over shock compression effects, from a shock recovery process.     

Variations in shock deformation at the Slate Islands impact structure,Lake Superior,Canada

A progressive change in the level of shock deformation is documented in autochthonous rocks from the central uplift of the Slate Islands impact structure, Lake Superior. Correlation of these observations, which are based mainly on the relative frequency of planar features of specific crystallographic orientation in quartz, with experimental data is used to estimate the average shock pressures recorded in the samples studied. Recorded pressures range from 5.8 to 15.3 GPa and generally increase towards the proposed shock centre. Variations in the shock response of quartz of different grain size and texture are observed within and between samples. It is apparent that large interlocking quartz grains in eyes record approximately 15–20% higher levels of shock deformation than small grains in mosaics or large isolated phenocrysts. These variations in shock deformation are attributed to the effect of shock wave reverberations between grains and length of shock pulse duration within grains.Comparison of the Slate Islands data with similar observations at the larger Charlevoix impact structure indicates that the rate of change of recorded shock pressure with distance is greater at the Slate Islands structure. This is interpreted as due to variations in the strain rates and/or the rate of shock wave attenuation with radial distance between impact structures of different size.Contribution from Earth Physics Branch No. 626     

Shock-induced mechanical deformations in biotites from crystalline rocks of the ries crater (Southern Germany)

Mechanical deformation features in shocked biotites from crystalline rocks of the Ries crater are: kink bands, planar elements, and plastic lattice deformations as determined by X-ray investigations.Kink bands can be observed in micas of various pressure histories (stages 0, I, II and less frequently stage III of shock metamorphism). Kink bands in shocked micas are less symmetrical than kinks of static origin. Asymmetry increases with increasing dynamic pressures. Moreover, kink band width is sensitive against changing peak pressures. Distribution of kinked and undistorted micas within a rock permits to fix the shock front direction. Shock-induced kinks in micas are produced by various gliding processes in the cleavage plane (001).Planar elements seldom occur in biotites of shock stages II and III and have never been described in endogenic rocks. Up to now orientations of planar elements parallel to (111), (1¯11), (112) and (11¯2) have been determined. Planar elements are interpreted as planes of plastic lattice gliding. {[110]} is supposed to be the main gliding direction. In the same pressure region other plastic lattice deformations have been determined. They are orientated parallel to (001), (100) and (¯132) or (201) which results from single crystal X-ray investigations and may represent planes of plastic lattice gliding. The dependency of formation of gliding planes and gliding directions on increasing dynamic pressures will be discussed.     

CO2-rich fluid inclusions along planar elements of quartz in basement rocks of the Vredefort Dome,South Africa

Along a NW-SE profile through the basement core, starting below the sedimentary unconformity and ending in the center of the nearly circular structure, the constituent quartz grains and their fluid inclusions exhibit the following characteristics:In the NW, fluid inclusions composed of CO₂ and occasionally up to 50 Vol.% H₂O occur along shock-induced planar elements following predominently {0 0 0 1} of coarse, largely unrecrystallized quartz grains. The planar elements are partly still open microcracks, partly they are healed, the fluid inclusions decorating the former sites of the cracks. Along these planar elements recrystallization into fine grained new quartz fabrics starts, this process increasing decidedly towards the southeast; nevertheless fluid inclusions are still retained. — Near and within the center of the dome the formerly coarse quartz grains are completely recrystallized to medium grained annealing fabrics, in which — surprizingly — the fluid inclusions have often retained their original positions relative to the old grains, so that their planar alignment now traverses the new grain boundaries. Here the enclosed fluid is pure CO₂ as far as can be determined.On the basis of the homogenization temperatures of the fluid inclusions measured, and of independent petrologic geothermometry of the basement rocks near the center, the fluids trapped after the shock event had exhibited partial pressures of CO₂ as high as 3 kbars at temperatures around 850° C. The derivation of these CO₂-rich, post-shock fluids is either through release of older fluid inclusions from the lower crustal granulites affected by the catastrophic shattering event, or it is from a direct mantle source that might be genetically connected with the Vredefort event itself.     

Shocked monazite chronometry: integrating microstructural and in situ isotopic age data for determining precise impact ages

Monazite is a robust geochronometer and occurs in a wide range of rock types. Monazite also records shock deformation from meteorite impact but the effects of impact-related microstructures on the U–Th–Pb systematics remain poorly constrained. We have, therefore, analyzed shock-deformed monazite grains from the central uplift of the Vredefort impact structure, South Africa, and impact melt from the Araguainha impact structure, Brazil, using electron backscatter diffraction, electron microprobe elemental mapping, and secondary ion mass spectrometry (SIMS). Crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal-plastic deformation features, including shock twins, planar deformation bands and neoblasts. Shock twins were documented in up to four different orientations within individual monazite grains, occurring as compound and/or type one twins in (001), (100), \(\left( 10\bar{1} \right)\), \(~\{110\}\), \(\left\{ 212 \right\},\) and type two (irrational) twin planes with rational shear directions in \([0\bar{1}\bar{1}]\) and \([\bar{1}\bar{1}0]\). SIMS U–Th–Pb analyses of the plastically deformed parent domains reveal discordant age arrays, where discordance scales with increasing plastic strain. The correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event. Neoblasts in granular monazite domains are strain-free, having grown during the impact events via consumption of strained parent grains. Neoblastic monazite from the Inlandsee leucogranofels at Vredefort records a ²⁰⁷Pb/²⁰⁶Pb age of 2010?±?15 Ma (2σ, n?=?9), consistent with previous impact age estimates of 2020 Ma. Neoblastic monazite from Araguainha impact melt yield a Concordia age of 259?±?5 Ma (2σ, n?=?7), which is consistent with previous impact age estimates of 255?±?3 Ma. Our results demonstrate that targeting discrete microstructural domains in shocked monazite, as identified through orientation mapping, for in situ U–Th–Pb analysis can date impact-related deformation. Monazite is, therefore, one of the few high-temperature geochronometers that can be used for accurate and precise dating of meteorite impacts.     

Shock-induced growth and metastability of stishovite and coesite in lithic clasts from suevite of the Ries impact crater (Germany)

The microtextures of stishovite and coesite in shocked non-porous lithic clasts from suevite of the Ries impact structure were studied in transmitted light and under the scanning electron microscope. Both high-pressure silica phases were identified in situ by laser-Raman spectroscopy. They formed from silica melt as well as by solid-state transformation. In weakly shocked rocks (stage I), fine-grained stishovite (≤1.8 μm) occurs in thin pseudotachylite veins of quartz-rich rocks, where it obviously nucleated from high-pressure frictional melts. Generally no stishovite was found in planar deformation features (PDFs) within grains of rock-forming quartz. The single exception is a highly shocked quartz grain, trapped between a pseudotachylite vein and a large ilmenite grain, in which stishovite occurs within two sets of lamellae. It is assumed that in this case the small stishovite grains formed by the interplay of conductive heating and shock reverberation. In strongly shocked rocks (stages Ib–III, above ∼30 GPa), grains of former quartz typically contain abundant and variably sized stishovite (<6 μm) embedded within a dense amorphous silica phase in the interstices between PDFs. The formation of transparent diaplectic glass in adjacent domains results from the breakdown of stishovite and the transformation of the dense amorphous phase and PDFs to diaplectic glass in the solid state. Coesite formed during unloading occurs in two textural varieties. Granular micrometre-sized coesite occurs embedded in silica melt glass along former fractures and grain boundaries. These former high-pressure melt pockets are surrounded by diaplectic glass or by domains consisting of microcrystalline coesite and earlier formed stishovite. The latter is mostly replaced by amorphous silica.     

Studies on the lattice distortion and substructures of shocked lamellae in naturally shocked quartz

One unshocked and 9 naturally shocked single quartz crystal grains with 1–6 sets of shock lamellae from the Ries, West Germany, and the Lake Lappajärvi, Finland, covering a range from unshocked quartz withNo = 1.544 to nearly completely glassy quartz withNo = 1.461 have been used for X-ray precession and Laue investigations. Four of the shocked grains have preliminarily been studied under a transmission electron microscope. It is found that quartz havingNo less than 1.539 shows intensive anisotropic cell expansion and lattice disordering which gradually increase asNo decreases. Shock-induced lattice distortion of quartz is clearly shown on both precession and Laue photographs. For the weakly shocked quartz (p < 200 kb) slight to pronounced spreading of spots is observed. When the pressure reaches 200 kb, both concentric spreading of spots having long ‘tails’ and concentric rings (powder pattern) are revealed on the same photograph, which means that besides a part of single crystal there also exist randomly oriented tiny ‘fragments’ of quartz in this shocked quartz grain. As pressure increases from 230 to 315 kb, more and more crystalline puases in the quartz grains have transformed from solid state into silica glass, and the concentric rings and the long ‘tails’ disappear and the spot spreading becomes slight again, but reflection intensities become much lower in comparison with those of weakly shocked quartz. TEM investigations show three kinds of substructures of shock lamellae. The glass contents of two of the four grains (73% and 84% respectively) were measured on TEM photographs with the help of an image analysis system. On the basis of above investigations a six-terminal-state model for the mechanism of deformation in shock metamorphosed quartz is presented.     

The variation range of optical constants and distribution characteristics of shock lamellae in shock-metamorphosed quartz

Two moderately shocked rock samples collected from the Ries Crater, West Germany (granite—gneiss sample RC-647-29 and biotite-granite sample RP-627-55) and two weakly shocked pegmatite samples (Lj-711-12 and Lj-711-5) taken from Lake Lappajarvi, Finland, have been optically studied to establish the variation range of optical constants and distribution characteristics of shock lamellae in shocked quartz. It has been found that sample RC-647-29 contains shocked quartz grains with the average refractive index ranging from 1.4612 to 1.5331, and sample RP-627-55 from 1.5002 to 1.4669, i.e., they cover a wide range of shock pressures. As for the larger quartz grains in samples Lj-711-12 and Lj-711-5, the variation range of the average refractive indices are smaller than those of samples from the Ries Crater. Hence the estimation of degree of shock must est with the investigation of a set of representative shocked quartz crystals from a single shocked rock sample. The optical data on shocked quartz indicate that the degree of shock is highly independent of the number of shock lamellae sets and their orientations; the most sensitive optical indicator is the index of refraction. On the basis of TEM investigations of single crystal grains of shocked quartz differing in refractive index, three mechanisms of formation of shock lamellae have been established: host quartz crystals with lamellae having closely spaced dislocations; host quartz crystals with lamellae of randomly oriented fine grains of quartz; and host quartz crystals or their residual fragments with lamellae of silica glass.     

Microstructural constraints on the mechanisms of the transformation to reidite in naturally shocked zircon

Zircon (ZrSiO₄) is used to study impact structures because it responds to shock loading and unloading in unique, crystallographically controlled manners. One such phenomenon is the transformation of zircon to the high-pressure polymorph, reidite. This study quantifies the geometric and crystallographic orientation relationships between these two phases using naturally shocked zircon grains. Reidite has been characterized in 32 shocked zircon grains (shocked to stages II and III) using a combination of electron backscatter diffraction (EBSD) and focused ion beam cross-sectional imaging techniques. The zircon-bearing clasts were obtained from within suevite breccia from the Nördlingen 1973 borehole, close to the center of the 14.4 Ma Ries impact crater, in Bavaria, Germany. We have determined that multiple sets (up to 4) of reidite lamellae can form in a variety of non-rational habit planes within the parent zircon. However, EBSD mapping demonstrates that all occurrences of lamellar reidite have a consistent interphase misorientation relationship with the host zircon that is characterized by an approximate alignment of a {100}_zircon with a {112}_reidite and alignment of a {112}_zircon with a conjugate {112}_reidite. Given the tetragonal symmetry of zircon and reidite, we predict that there are eight possible variants of this interphase relationship for reidite transformation within a single zircon grain. Furthermore, laser Raman mapping of one reidite-bearing grain shows that moderate metamictization can inhibit reidite formation, thereby highlighting that the transformation is controlled by zircon crystallinity. In addition to lamellar reidite, submicrometer-scale granules of reidite were observed in one zircon. The majority of reidite granules have a topotaxial alignment that is similar to the lamellar reidite, with some additional orientation dispersion. We confirm that lamellar reidite likely forms via a deviatoric transformation mechanism in highly crystalline zircon, whereas granular reidite forms via a reconstructive transformation from low-crystallinity ZrSiO₄ within the reidite stability field. The results of this study further refine the formation mechanisms and conditions of reidite transformation in naturally shocked zircon.     

Analysis of dislocations in some naturally deformed plagioclase feldspars

Dislocations in intermediate plagioclase feldspars, which were deformed under granulite facies conditions, have been analysed. The study reveals extensive ductile deformation by intracrystalline slip and by twinning. Six out of the seven possible Burgers vectors were identified: \(b = \left[ {001} \right],\tfrac{1}{2}\left[ {110} \right],\tfrac{1}{2}\left[ {1\bar 10} \right],\left[ {101} \right],\tfrac{1}{2}\left[ {112} \right]and\tfrac{1}{2}\left[ {1\bar 12} \right]\) . Most, perhaps all, dislocations are dissociated by up to 200 Å. The microstructure is dominated by [001] screw dislocations, most of which appear to be dissociated in (010). The dominant slip system appears to be (010) [001]. Large grain-to-grain variations in the density of free dislocations indicate that the plastic strain in individual grains depended upon the Schmid factor for (010) [001]. The microstructure suggests that the rate-controlling step for high-temperature creep of plagioclase is cross-slip of extended [001] screw dislocations. The rheological contrast between feldspar and quartz is partly due to a difference in stacking fault energy.     

Evidence for a shock-metamorphic breccia within a buried impact crater,Lake Raeside,Yilgarn Craton,Western Australia

Mineral exploration drilling 60 km west of Leonora in 2008 intersected >95 m of poorly consolidated granitoid-dominated breccia at the base of a Cenozoic paleochannel beneath Lake Raeside. The breccia, initially interpreted as a kimberlite, is composed of poorly consolidated fragments of granitic gneiss, felsite and metamorphosed mafic rock within a matrix of fine to medium-grained breccia. Microscopic examination revealed quartz grains displaying well-developed planar deformation features (PDFs) dominated by the ω? {1013} planar set, diaplectic silica glass and diaplectic plagioclase glass. These features constitute the diagnostic hallmarks of shock metamorphism owing to high-velocity impact of a large meteorite or asteroid. The PDFs in quartz grains of the breccia are distinctly different from metamorphic deformation lamellae produced tectonically or in diatremes. Airborne total magnetic intensity data suggest an outline of an 11 km-diameter crater, consistent with the significant thickness of the shock-metamorphosed breccia at >95 m, suggestive of the existence of a large impact structure.     

Microstructures of AIPO4 subjected to high shock pressures

Berlinite single crystal specimens were shocked to peak pressures 12 and 24 GPa. Specimens were placed in an Al capsule to minimize shock-wave reflections at interfaces between specimen and capsule. Shock pressures were achieved with a 6.5-m-long two-stage gun. The shock-induced microstructures in recovered specimens were then investigated by Transmission Electron Microscopy. In the sample shocked at 12 GPa, the prominent shock-induced defects are dislocations and basal a glide appears to be the only glide system activated. In contrast, the sample shocked at 24 GPa exhibits no dislocations. The material is partially converted into an amorphous phase occurring under the form of thin amorphous lamellae parallel to the }10 $\bar 1$ n{ planes (n=0, 2, 3, 4). This microstructure is very similar to the one observed in experimentally shocked quartz.     

Mylonitic deformation of gabbro in the lower crust: A case study from the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan

The mylonitization of the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan, occurred along its western margin at ≈600 MPa and 660–700 °C through dynamic recrystallization of plagioclase and a retrograde reaction from granulite facies to amphibolite facies (orthopyroxene + clinopyroxene + plagioclase + H₂O = hornblende + quartz). The reaction produced a fine-grained (≤100 μm) polymineralic aggregate composed of orthopyroxene, clinopyroxene, quartz, hornblende, biotite and ilmenite, into which strain is localized. The dynamic recrystallization of plagioclase occurred by grain boundary migration, and produced a monomineralic aggregate of grains whose crystallographic orientations are mostly unrelated to those of porphyroclasts. The monomineralic plagioclase aggregates and the fine-grained polymineralic aggregates are interlayered and define the mylonitic foliation, while the latter is also mixed into the former by grain boundary sliding to form a rather homogeneous polymineralic matrix in ultramylonites. However in both mylonite and ultramylonite, plagioclase aggregates form a stress-supporting framework, and therefore controlled the rock rheology. Crystal plastic deformation of pyroxenes and plagioclase with dominant (100)[001] and (001)1/2 slip systems, respectively, produced distinct shape- and crystallographic-preferred orientations of pyroxene porphyroclasts and dynamically recrystallized plagioclase grains in both mylonite and ultramylonite. Euhedral to subhedral growth of hornblende in pyroxene porphyroclast tails during the reaction and its subsequent rigid rotation in the fine-grained polymineralic aggregate or matrix produced clear shape- and crystallographic-preferred orientations of hornblende grains in both mylonite and ultramylonite. In contrast, the dominant grain boundary sliding of pyroxene and quartz grains in the fine-grained polymineralic aggregate of the mylonite resulted in their very weak shape- and crystallographic-preferred orientations. In the fine-grained polymineralic matrix of the ultramylonite, however, pyroxene and quartz grains became scattered and isolated in the plagioclase aggregate so that they were crystal-plastically deformed leading to stronger shape- and crystallographic-preferred orientations than those seen in the mylonite.     

Interpretation of magnetic fabrics in the Dalma volcanic rocks and associated meta-sediments of the Singhbhum Mobile Belt

The present work deals with the generations of Fe–Ti oxides and the variation in magnetic fabrics of the Dalma lavas and associated meta-sediments of the Singhbhum Mobile Belt (SMB) in relation to tectonics. Generations of the Fe–Ti oxides are different in meta-sediments and volcanics, the former preserving upliftment related oxidised grains, whereas the latter contains fresh grains prompting towards their upliftment due to plume upwelling before the volcanic eruption. In the meta-sediments, the magnetic fabric has close accordance with \(\hbox {D}_{2}/\hbox {F}_{2}\) event revealing synchronous development with \(\hbox {D}_{2}\). The Dalma thrust developed a sudden break in the homogeneity of the magnetic fabrics of the rocks where the magnetic foliations are all parallel to the Dalma thrust. This also causes \(P_{j}\) to be highest in this sector. The magnetic fabrics of volcanic rocks are different from the meta-sediments and record no signature of deformation. The pattern of distribution of susceptibility axes are in accordance with the subaerial lava flows. However, their \(\hbox {K}_{1}\) and \(\hbox {K}_{2}\) dispersed throughout the periphery with \(\hbox {K}_{3}\) clustering at the centre. This infers towards the fact that although the volcanism took place in a subaerial environment, calm aqueous environment was locally present where the oblate grains settled on the eruption surface with their \(\hbox {K}_{3}\) vertical.     

Fractal Modelling of the Microstructure Property of Quartz Mylonite During Deformation Process

Mylonite is the result of the dynamic metamorphism and minerals in mylonite are deformed gradually with an increase in the degree of metamorphism. Quantifying the degree of deformation including the irregularities of shapes and the frequency distribution of the minerals becomes one of the most challenging efforts in mylonite analysis. Fractal modelling has been demonstrated in this paper to be an effective mean to achieve the above goal. Perimeter-Area fractal model was used to quantify the irregularities in the geometries and Cumulative Number-Area model is used to characterize the irregularities of distribution of quartzs in mylonites, respectively. Examples of quartz from five types of mylonites with different degree of deformation within the foreland of the Moine Thrust Zone in NW Scotland are chosen to study the evolution processes of deformation. As the main mineral component of quartzite mylonite, patterns are extracted from digital photomicrographics of the multiscale-grey image grid data to show quartz grains with different degree of deformation, The areas and perimeters of the quartz grains were calculated by GIS-based image processing technologies. From type one to type five, with an increase in degree of deformation, the corresponding Perimeter-Area exponent increases from 1.20, 1.28, 1.38, 1.46, to 1.60, respectively, the fractal dimension of the perimeter from 1.07, 1.08, 1.17, 1.23, to 1.44, as well as the exponent of Cumulative Number- Area from 0.50, 0.51, 0.58, 0.82, to 0.85, respectively. The result has shown that as increase of the intensity of deformation, the shape of quartz grains tends to be more irregular, grain size tends to be smaller, and the number of grains increases. The results obtained using GSI model has indicated that as an increase in the intensity of deformation, the patterns of quartz grains tends to be more stratified and randomness increases.     

Discontinuous grain growth of quartz in metacherts: the influence of mica on a microstructural transition

Abstract The microstructure of quartz in metacherts of the Ryoke metamorphic belt in central Japan develops from polygonal, through duplex to irregular with increasing metamorphic grade. The polygonal microstructure is composed of small (mostly 90–160 μm), equant, equigranular, polygonal quartz grains, whereas the irregular microstructure is characterized by large (>300 μm) grains with irregular grain boundaries. The duplex microstructure is a mixture of small polygonal and large irregular grains. The development of these microstructures is interpreted as being due to secondary recrystallization. The size of polygonal grains is greatly influenced by the presence of second-phase minerals, such as mica, whereas that of large irregular grains is unaffected by second-phase minerals. There seems to be a critical grain size for quartz to occur as polygonal aggregates: no polygonal aggregates occur in rocks with larger than the critical grain size. The size (about 140 μm) decreases slightly with increasing volume fraction of mica. The mean grain sizes of polygonal quartz ( D ) and coexisting mica ( d ) in the duplex microstructure are systematically related to the volume fraction of mica ( f ) by D = 0.728 d (1/ f )^0.629.     

Majoritic garnet grains within shock-induced melt veins in amphibolites from the Ries impact crater suggest ultrahigh crystallization pressures between 18 and 9 GPa

Shock-induced melt veins in amphibolites from the Nördlinger Ries often have chemical compositions that are similar to bulk rock (i.e., basaltic), but there are other veins that are confined to chlorite-rich cracks that formed before the impact and these are poor in Ca and Na. Majoritic garnets within the shock veins show a broad chemical variation between three endmembers: (1) \({}^{\text{VIII}}{{\text{M}^{2+}}_3} {}^{\text{VI}}{\text{Al}}_{2} ({}^{\text{IV}}{\text{SiO}}_{4} )_{3}\) (normal garnet, Grt), (2) \({}^{\text{VIII}}{{\text{M}^{2+}}_3} {}^{\text{VI}}[{\text{M}}^{2 + } ({\text{Si,Ti}})]({}^{\text{IV}}{\text{SiO}}_{4} )_{3}\)  (majorite, Maj), and (3) \({}^{\text{VIII}}({{\text {Na} {\text M}^{2+}}_2}) {}^{\text{VI}}[ ({\text{Si,Ti}}){\text {Al}}]({}^{\text{IV}}{\text{SiO}}_{4} )_{3}\) (Na-majorite₅₀Grt₅₀), whereby M²⁺ = Mg²⁺, Fe²⁺, Mn²⁺, Ca²⁺. In particular, we observed a broad variation in ^VI(Si,Ti) which ranges from 0.12 to 0.58 cations per formula unit (cpfu). All these majoritic garnets crystallized during shock pressure release at different ultrahigh pressures. Those with high ^VI(Si,Ti) (0.36–0.58 cpfu) formed at high pressures and temperatures from amphibole-rich melts, while majoritic garnets with lower ^VI(Si,Ti) of 0.12–0.27 cpfu formed at lower pressures and temperatures from chlorite-rich melts. Furthermore, majoritic garnets with intermediate values of ^VI(Si,Ti) (0.24–0.39) crystallized from melts with intermediate contents of Ca and Na. To the best of our knowledge the ‘MORB-type’ Ca–Na-rich majoritic garnets with maximum contents of 2.99 wt% Na₂O and calculated crystallisation pressures of 16–18 GPa are the most extreme representatives ever found in terrestrial shocked materials. At the Ries, the duration of the initial contact and compression stage at the central location of impact is estimated to only ~ 0.1 s. We used a ~ 200-µm-thick shock-induced vein in a moderately shocked amphibolite to model its pressure–temperature–time (P–T–t) path. The graphic model manifests a peak temperature of ~ 2600 °C for the vein, continuum pressure lasting for ~ 0.02 s, a quench duration of ~ 0.02 s and a shock pulse of ~ 0.038 s. The small difference between the continuum pressure and the pressure of majoritic garnet crystallization underlines the usefulness of applying crystallisation pressures of majoritic garnets from metabasites for calculation of dynamic shock pressures of host rocks. Majoritic garnets of chlorite provenance, however, are not suitable for the determination of continuum pressure since they crystallized relatively late during shock release. An extraordinary glass- and majorite-bearing amphibole fragment in a shock-vein of one amphibolite documents the whole unloading path.     

TitaniQ: a titanium-in-quartz geothermometer

Titanium is one of many trace elements to substitute for silicon in the mineral quartz. Here, we describe the temperature dependence of that substitution, in the form of a new geothermometer. To calibrate the “TitaniQ” thermometer, we synthesized quartz in the presence of rutile and either aqueous fluid or hydrous silicate melt, at temperatures ranging from 600 to 1,000°C, at 1.0 GPa. The Ti contents of quartz (in ppm by weight) from 13 experiments increase exponentially with reciprocal T as described by: