The most frequent interfaces in olivine aggregates: the GBCD and its importance for grain boundary related processes

Rocks consist of crystal grains separated by grain boundaries that impact the bulk rock properties. Recent studies on metals and ceramics showed that the grain boundary plane orientation is more significant for grain boundary properties than other characteristics such as the sigma value or disorientation (in the Earth’s science community more frequently termed misorientation). We determined the grain boundary character distribution (GBCD) of synthetic and natural polycrystalline olivine, the most abundant mineral of Earth’s upper mantle. We show that grain boundaries of olivine preferentially contain low index planes, in agreement with recent findings on other oxides (e.g. MgO, TiO₂, Al₂O₃ etc.). Furthermore, we find evidence for a preferred orientation relationship of 90° disorientations about the [001] direction forming tilt and twist grain boundaries, as well as a preference for the 60° disorientation about the [100] axis. Our data indicate that the GBCD, which is an intrinsic property of any mineral aggregate, is fundamental for understanding and predicting grain boundary related processes.     

Zircon geochronology and geochemistry to constrain the youngest eruption events and magma evolution of the Mid-Miocene ignimbrite flare-up in the Pannonian Basin,eastern central Europe

A silicic ignimbrite flare-up episode occurred in the Pannonian Basin during the Miocene, coeval with the syn-extensional period in the region. It produced important correlation horizons in the regional stratigraphy; however, they lacked precise and accurate geochronology. Here, we used U–Pb (LA-ICP-MS and ID-TIMS) and (U–Th)/He dating of zircons to determine the eruption ages of the youngest stage of this volcanic activity and constrain the longevity of the magma storage in crustal reservoirs. Reliability of the U–Pb data is supported by (U–Th)/He zircon dating and magnetostratigraphic constraints. We distinguish four eruptive phases from 15.9 ± 0.3 to 14.1 ± 0.3 Ma, each of which possibly includes multiple eruptive events. Among these, at least two large volume eruptions (>10 km³) occurred at 14.8 ± 0.3 Ma (Demjén ignimbrite) and 14.1 ± 0.3 Ma (Harsány ignimbrite). The in situ U–Pb zircon dating shows wide age ranges (up to 700 kyr) in most of the crystal-poor pyroclastic units, containing few to no xenocrysts, which implies efficient recycling of antecrysts. We propose that long-lived silicic magma reservoirs, mostly kept as high-crystallinity mushes, have existed in the Pannonian Basin during the 16–14 Ma period. Small but significant differences in zircon, bulk rock and glass shard composition among units suggest the presence of spatially separated reservoirs, sometimes existing contemporaneously. Our results also better constrain the time frame of the main tectonic events that occurred in the Northern Pannonian Basin: We refined the upper temporal boundary (15 Ma) of the youngest counterclockwise block rotation and the beginning of a new deformation phase, which structurally characterized the onset of the youngest volcanic and sedimentary phase.     

Experimental petrology constraints on the recycling of mafic cumulate: a focus on Cr-spinel from the Rum Eastern Layered Intrusion,Scotland

The ¹⁷⁶Lu–¹⁷⁶Hf and ¹⁴⁷Sm–¹⁴³Nd decay systems are routinely used to determine garnet (Grt)–whole-rock (WR) ages; however, the ¹⁷⁶Lu–¹⁷⁶Hf age of garnet is typically older than the ¹⁴⁷Sm–¹⁴³Nd age determined from the same aliquots. Here we present experimental data for Lu³⁺ and Hf⁴⁺ diffusion in garnet as functions of temperature, pressure and oxygen fugacity and show that the diffusivity of Hf⁴⁺ in almandine/spessartine garnet is significantly slower than that of Lu³⁺. The diffusive closure temperature (T _C) of Hf⁴⁺ is significantly higher than that of Nd³⁺, and although this property is partly responsible for the observed ¹⁷⁶Lu–¹⁷⁶Hf and ¹⁴⁷Sm–¹⁴³Nd Grt–WR age discrepancies, the difference between the T _C-s of Lu³⁺ and Hf⁴⁺ could lead to apparent Grt–WR ¹⁷⁶Lu–¹⁷⁶Hf ages that are skewed from the age of Hf⁴⁺ closure in garnet. In addition, the slow diffusivity of Hf⁴⁺ indicates that the bulk of metamorphic garnets retain a substantial fraction of prograde radiogenic ¹⁷⁶Hf throughout peak metamorphic conditions, a phenomenon that further complicates the interpretation of ¹⁷⁶Lu–¹⁷⁶Hf garnet ages and invalidates the use of analytical T _C expressions. We argue that the diffusion of trivalent rare earth elements in garnet becomes much faster when their concentration level falls below a few hundred ppm, as in the experiments of Tirone et al. (Geochim Cosmochim Acta 69: 2385–2398, 2005), and further argue that this low-concentration mechanism is appropriate for modeling the susceptibility of ¹⁴⁷Sm–¹⁴³Nd garnet ages to diffusive resetting.     

The early Cambrian Chahmir shale-hosted Zn–Pb deposit,Central Iran: an example of vent-proximal SEDEX mineralization

The Chahmir zinc–lead deposit (1.5 Mt @ 6 % Zn + 2 % Pb) in Central Iran is one among several sedimentary-exhalative Zn–Pb deposits in the Early Cambrian Zarigan–Chahmir basin (e.g., Koushk, Darreh-Dehu, and Zarigan). The deposit is hosted by carbonaceous, fine-grained black siltstones, and shales interlayered with volcaniclastic sandstone beds. It corresponds to the upper part of the Early Cambrian volcano-sedimentary sequence (ECVSS), which was deposited on the Posht-e-Badam Block during back-arc rifting of the continental margin of Central Iran. Based on crosscutting relationships, mineralogy, and texture of sulfide mineralization, four different facies can be distinguished: stockwork (feeder zone), massive ore, bedded ore, and distal facies (exhalites with barite). Silicification, carbonatization, sericitization, and chloritization are the main wall-rock alteration styles; alteration intensity increases toward the proximal feeder zone. Fluid inclusion microthermometry was carried out on quartz associated with sulfides of the massive ore. Homogenization temperatures are in the range of 170–226 °C, and salinity is around 9 wt% NaCl eq. The size distribution of pyrite framboids of the bedded ore facies suggests anoxic to locally suboxic event for the host basin. δ³⁴S_(V-CDT) values of pyrite, sphalerite, and galena range from +10.9 to +29.8?‰. The highest δ³⁴S values correspond to the bedded ore (+28.6 to +29.8?‰), and the lowest to the massive ore (+10.9 to +14.7?‰) and the feeder zone (+11.3 and +12.1?‰). The overall range of δ³⁴S is consistent with a sedimentary environment where sulfide sulfur was derived from two sources. One of them was corresponding to early ore-stage sulfides in bedded ore and distal facies, consistent with bacterial reduction from coeval seawater sulfate in a closed or semiclosed basin. However, the δ³⁴S values of late ore-stage sulfides, observed mainly in massive ore, interpreted as a hydrothermal sulfur component, leached from the lower part of the ECVSS. Sulfur isotopes, along with the sedimentological, textural, mineralogical, fluid inclusion, and geochemical characteristics of the Chahmir deposit are in agreement with a vent-proximal (Selwyn type) SEDEX ore deposit model.     

Calcite Dissolution Kinetics

We compare the canonical treatment of calcite’s dissolution rate from the literature in a closed system, particle batch reactor, with the alternative approach suggested by Truesdale (Aquat Geochem, 2015). We show that the decay of rate over time can be understood in terms of the evolution and distribution of reactive sites on the surface of these particles. We also emphasize that interpretation of observed rates must not exclude the fundamental role of crystal defects, whose importance is already implicitly reflected in the common form of rate laws in geochemistry. The empirical behavior of overall rate in closed systems, such as those described by Truesdale, may thus reflect relationships between defect centers and the generation of steps over the calcite surface (previously documented for silicates), such that below a critical free energy limit, there is insufficient driving force to open hollow cores and thus a loss of reaction mechanism. Dissolution in this very-near-equilibrium regime will be dependent on the distribution of extant steps and the energetics of new kink site nucleation. However, these sensitivities are complicated in the case of particle systems by grain boundaries, edges, corners, and other terminations. Such discontinuities constitute a defect class whose overall kinetic importance will be strongly tied to particle diameter and which can act independently of the internal strain field imposed by screw and edge dislocations.     

Turning points in black holes astrophysics

Black hole astrophysics is expected to have a major breakthrough any day. Theoretical understanding is well advanced, so the breakthrough has to come from observers. Conclusive and direct evidence of event horizons, signatures of tidal deformation of gas clouds (and eventual disappearance of matter behind horizons) could be obtained in matter of months. At this juncture, we wish to summarize how the subject came to this stage and what is the state-of-the-art flow solution around a black hole. We touch upon the most crucial issue of (un)predictability of this subject. We believe that if viscosity is roughly what MRI simulations are pointing to us, we still require very detailed knowledge of properties of the companion, especially time dependent mass transfer from the companion to the black hole.     

Progressive failure leading to the 3 December 2013 rockfall at Puigcercós scarp (Catalonia,Spain)

A detailed analysis of the pre-failure behavior of the 3 December 2013 rockfall (1,012 m³) occurred on Puigcercós pilot study area (Catalonia, Spain) is presented. The exact date of failure was obtained based on a photographic monitoring performed every 4 h. The long-term monitoring (2,217 days) of the rock slope carried out by a Terrestrial LiDAR allowed the early detection of both pre-failure deformation and precursory rockfalls preceding the final failure. By means of the analysis of the pre-failure deformation, four different deformed areas were detected and the tertiary creep phase was observed in three of them. An attempt to predict the time to failure was performed using the Fukuzono’s (1985) method. Furthermore, the temporal evolution of the precursory rockfalls occurred in those four areas during the progressive failure showed a close resemblance with the exponential pattern of the cumulated displacements at tertiary creep stage. Finally, the study of the meteorological conditions did not show any single triggering factor associated with the final failure. Reversely, the increase in the occurrence of precursory rockfalls on several areas of the slope together with the observed acceleration on the deformation pattern reinforce the role of a progressive degradation of the stability conditions, which ultimately leaded to the 3 December rockfall event.