Iron oxidation state in impact glass from the K/T boundary at Beloc,Haiti, by high‐resolution XANES spectroscopy

Abstract— We examined the local iron environment in nine impact glasses from the Cretaceous‐Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples have been analyzed by Fe K‐edge high‐resolution X‐ray absorption near edge structure (XANES) spectroscopy to obtain data on both the Fe oxidation state and the coordination number. The pre‐edge peak of our high‐resolution XANES spectra display noticeable variations indicative of significant changes in the Fe oxidation state spanning a wide range from about 20 to 75 mol% trivalent Fe. All data plot along the same trend, falling between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of four‐ and five‐coordinated Fe²⁺) to ^[4]Fe³⁺ and ^[5]Fe³⁺, respectively. Thus, the XANES spectra can be interpreted as a mixture of ^[4]Fe²⁺, ^[5]Fe²⁺, ^[4]Fe³⁺, and ^[5]Fe³⁺. There is no evidence for six‐fold coordinated Fe; however, its presence in small amounts cannot be excluded from XANES data alone. Our observations can be explained by two possible scenarios: either these impact glasses formed under very reducing conditions and, because of their small size, were easily oxidized in air while still molten, or they formed under a variety of different oxygen fugacities resulting in different Fe oxidation states. In the first case, the oxidation state and coordination number would imply similar formation conditions as splash‐form tektites, followed by progressive oxidation.     

Vesta's mineralogical composition as revealed by the visible and infrared spectrometer on Dawn

The Dawn spacecraft mission has provided extensive new and detailed data on Vesta that confirm and strengthen the Vesta–howardite–eucrite–diogenite (HED) meteorite link and the concept that Vesta is differentiated, as derived from earlier telescopic observations. Here, we present results derived by newly calibrated spectra of Vesta. The comparison between data from the Dawn imaging spectrometer—VIR—and the different class of HED meteorites shows that average spectrum of Vesta resembles howardite spectra. Nevertheless, the Vesta spectra at high spatial resolution reveal variations in the distribution of HED‐like mineralogies on the asteroid. The data have been used to derive HED distribution on Vesta, reported in Ammannito et al. (2013), and to compute the average Vestan spectra of the different HED lithologies, reported here. The spectra indicate that, not only are all the different HED lithologies present on Vesta, but also carbonaceous chondritic material, which constitutes the most abundant inclusion type found in howardites, is widespread. However, the hydration feature used to identify carbonaceous chondrite material varies significantly on Vesta, revealing different band shapes. The characteristic of these hydration features cannot be explained solely by infalling of carbonaceous chondrite meteorites and other possible origins must be considered. The relative proportion of HEDs on Vesta's surface is computed, and results show that most of the vestan surface is compatible with eucrite‐rich howardites and/or cumulate or polymict eucrites. A very small percentage of surface is covered by diogenite, and basaltic eucrite terrains are relatively few compared with the abundance of basaltic eucrites in the HED suite. The largest abundance of diogenitic material is found in the Rheasilvia region, a deep basin, where it clearly occurs below a basaltic upper crust. However, diogenite is also found elsewhere; although the depth to diogenite is consistent with one magma ocean model, its lateral extent is not well constrained.     

Yellow impact glass from the K/T boundary at Beloc (Haiti): XANES determination of the Fe oxidation state and implications for formation conditions

Abstract— We determined the iron oxidation state and coordination number in five samples of yellow impact glass from the Cretaceous‐Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples were analyzed by Fe K‐edge XANES spectroscopy and the results were compared with published data on eight black impact glasses and one high Si‐K impact spherule from the same impact layer. The pre‐edge peak of our high‐resolution XANES spectra displays evident variations indicative of significant changes in the Fe oxidation state, spanning a wide range from about 75 to 100 mole% Fe³⁺. Yellow K/T glasses have significantly higher Fe³⁺/(Fe²⁺ + Fe³⁺) ratios compared to black K/T impact glasses (from 20 to 75 mole% Fe³⁺) and high Si‐K glass (20 mole% Fe³⁺). In particular, all the pre‐edge peak data on these three types of impact glasses plot between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of ^[4]Fe²⁺ and ^[5]Fe²⁺) to ^[4]Fe³⁺ and ^[5]Fe³⁺, respectively. Thus, the XANES spectra of the yellow K/T glasses can be interpreted as a mixture of ^[4]Fe^{2+, [5]}Fe^{2+, [4]}Fe³⁺, and ^[5]Fe³⁺. Our observations can be explained by a very large range of oxygen fugacity conditions during melt formation. Furthermore, there is a clear positive relationship between the Fe³⁺/(Fe²⁺ + Fe³⁺) ratio and the Ca content of these glasses, suggesting that the Fe oxidation state was influenced by the relative contribution of Ca‐sulfate‐ and Ca‐carbonate‐bearing sedimentary rocks at the impact site.     

Major and Trace Element Characteristics of Apatites in Granitoids from Central Kazakhstan: Implications for Petrogenesis and Mineralization

This paper presents abundances of major and trace elements of apatites in granitic rocks associated with different types of ore deposits in Central Kazakhstan on the basis of electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. Our results demonstrate that the concentrations and ratios of elements in apatites from different granitoid rocks show distinct features, and are sensitive to magma evolution, petrogenetic and metallogenetic processes. Apatites in the rocks associated with Mo‐W deposits have high content of F and MnO, low content of Cl, which may be indicative of sedimentary sources, while apatites from a Pb‐Zn deposit show relatively high content of Cl and low F content, which possibly suggest a high water content. In these apatites, Sr contents decrease, while Mn and Y contents increase with magma evolution. This relationship reflects that these elements in apatites are related with the degree of magmatic differentiation. Four types of REE patterns in apatites are identified. Type 1 character of highest (La/Yb)_N in apatites of Aktogai porphyry Cu‐Mo deposit, Sayak‐I skarn Cu deposit and Akzhal skarn Pb‐Zn depposit is likely produced by the crystallization of heavy REE‐enriched minerals. Type 2 character of upward‐convex light REE in apatite of Aktogai porphyries likely results from La‐enriched mineral crystallization. Type 3 feature of Nd depletion in apatites of East Kounrad and Zhanet deposits both from Mo‐W deposits primarily inherits the character of host‐rock. Type 4 apatites of Aktogai deposit and Akshatau W‐Mo deposit with wide range of REE contents may suggest that apatites crystallize under a wide temperature range. Three types of apatite with distinct redox states are identified based on Eu anomaly. The Aktogai apatite with slight negative Eu anomaly displays the most oxidized state of the magma, and the apatites of other samples at Aktogai, East Kounrad and Akzhal with moderate negative Eu anomaly show moderate oxidizing condition of these rocks, while the remaining apatites with strong En anomaly indicate a moderate reductive state of these rocks.     

Theoretical analysis of X-ray absorption near-edge structure in forsterite, Mg2SiO4-Pbnm, and fayalite, Fe2SiO4-Pbnm, at room temperature and extreme conditions

We calculated the forsterite Mg K-edge and the fayalite Fe K-edge X-ray absorption spectra both for the M 1 and M 2 sites and for the overall edge by using the one-electron multiple-scattering theory. The validity of the theoretical model is well illustrated by comparison of calculations with experimental data at the Mg K-edge of MgO (periclase) and at the Mg and Fe K-edges spectra of forsterite and fayalite. Starting from these results at room conditions, we calculated the Mg and Fe K-edges X-ray absorption spectra of forsterite and fayalite at low and high temperatures and at high pressures as well. Variations of fine structures occur mostly in the intermediate multiple scattering (IMS) regions and as a result of the applied pressure. In order to demonstrate the capability of XAS to lead to deeper knowledge of structure relevant to Earth's upper mantle we also attempted calcuating the high-P edge for Fe ²⁺ in low-spin using a different occupation of valence electrons. If a change in spin state really occurs in fayalite, our simple model shows that XAS would evidence it easily even with low resolution.     

A probabilistic approach for the classification of earthquakes as ��triggered�� or ��not triggered��

The occurrence time of earthquakes can be anticipated or delayed by external phenomena that induce strain energy changes on the faults. ??Anticipated?? earthquakes are generally called ??triggered??; however, it can be controversial to label a specific earthquake as such, mostly because of the stochastic nature of earthquake occurrence and of the large uncertainties usually associated to stress modelling. Here we introduce a combined statistical and physical approach to quantify the probability that a given earthquake was triggered by a given stress-inducing phenomenon. As an example, we consider an earthquake that was likely triggered by a natural event: the M?=?6.2 13 Jan 1976 Kópasker earthquake on the Grímsey lineament (Tj?rnes Fracture Zone, Iceland), which occurred about 3?weeks after a large dike injection in the nearby Krafla fissure swarm. By using Coulomb stress calculations and the rate-and-state earthquake nucleation theory, we calculate the likelihood of the earthquake in a scenario that contains only the tectonic background and excludes the dike and in a scenario that includes the dike but excludes the background. Applying the Bayes?? theorem, we obtain that the probability that the earthquake was indeed triggered by the dike, rather than purely due to the accumulation of tectonic strain, is about 60 to 90?%. This methodology allows us to assign quantitative probabilities to different scenarios and can help in classifying earthquakes as triggered or not triggered by natural or human-induced changes of stress in the crust.     

Studies on the contribution of some geological factors to seismic hazard

The present study is primarily meant to make a separation of the amplification effects in the intensity distribution within the macroseismic field function of their different causes, i.e. the activation of some regional tectonic lines during the seismic motion and local seismogeological conditions.With this end in view, the methodology used consists of filtering the direct macroseismic observations by means of the weighted mean-values method and outlining the regional and local field anomalies.The regional anomaly map of the macroseismic field of the 4 March 1977 Vrancea earthquake, including the southern and south-eastern part of the Outer Carpathian Zone, has been analyzed. Some dislocations, located in the platform basement, which functioned as seismic energy amplifiers during the event, have been identified.The results obtained can be used in seismotectonic and microzoning studies.Paper presented at the 21st General Assembly of the European Seismological Commission, Sofia, 1988.     

Australasian microtektites from Antarctica: XAS determination of the Fe oxidation state

The Fe oxidation state and coordination number of 29 impact glass spherules recently recovered from the Transantarctic Mountains (Antarctica) have been determined by X‐ray absorption near edge structure (XANES) spectroscopy. Based on geochemical, isotopic, and fission track data, these spherules are considered as microtektites from the Australasian tektite/microtektite strewn field. Their find location is the farthest so far discovered from the possible source crater region, and their alkali content is the lowest compared with other published data on Australasian microtektite glasses. The Fe³⁺/(Fe²⁺+Fe³⁺) ratio, determined from the analysis of the pre‐edge peak energy position and integrated intensity, is below 0.1 (±0.04) for all the samples, and is comparable to that of most tektites and microtektites from the Australasian strewn field. Also, the pre‐edge peak integrated intensity, which is sensitive to the average Fe coordination geometry, is comparable to that of other Australasian microtektites reported in the literature. The agreement of the Fe oxidation state and coordination number, between the Transantarctic Mountain microtektites (TAM) and the Australasian tektites and microtektites, further confirms the impact origin of these glass spherules and provides an independent suggestion that they represent a major extension southeastward of the Australasian strewn field. The fact that similar redox conditions are observed in tektites and microtektites within the Australasian strewn field regardless of the distance from the source crater area (up to approximately 11000 km) could be an important constraint for better understanding the different processes affecting microtektite formation and transport. The fact that the Fe oxidation state of microtektites does not increase with distance, as in the case of North American microtektites, means that thermal and redox histories of Australasian and TAM microtektites could differ significantly from those of North American microtektites.     

Unraveling the hidden origin and migration of plagioclase phenocrysts by in situ Sr isotopes: the case of final dome activity at Nisyros volcano,Greece

This contribution reports a detailed study on in situ Sr isotope analyses, along with textural and compositional characteristics, of plagioclase phenocrysts occurring in the rhyodacitic dome-lavas and associated mafic enclaves, erupted during the last magmatic activity at Nisyros volcano (Greece). Dome-lavas and enclaves have a paragenesis dominated by plagioclase. We recognize five different types of plagioclase based on their specific textures and composition. Dome-lava plagioclases (Type-1) are mainly large (1–5 mm), subhedral, clear, and poorly zoned crystals with low An content (An_25–35). The plagioclase phenocrysts (Type-4 and Type-5) and groundmass microlites crystallizing in the enclaves, and found in dome-lavas as xenocrysts, have high An content (An_75–95). In both dome-lavas and enclaves, two other types of plagioclase do also occur: (1) plagioclase phenocrysts with size and core composition similar to those of Type-1 having a dusty sieve zone (DSZ) at the rims (Type-2); (2) plagioclases with a DSZ affecting the entire crystal but a thin rim (Type-3). The drilled plagioclases have ⁸⁷Sr/⁸⁶Sr negatively correlated with their An content. Low An cores of Type-1 and Type-2 have quite homogeneous ⁸⁷Sr/⁸⁶Sr (0.7044–0.7046), whose values are more radiogenic than their host magmas (0.70403–0.70408) and similar to those of the previous Upper Pumice (UP) rhyolite magma (0.70438–0.70456). The DSZs of Type-2 and Type-3 show lower and scattered ⁸⁷Sr/⁸⁶Sr (0.70397–0.70426) with intermediate and variable An content. High An cores of Type-4 and Type-5 have the least radiogenic Sr isotope composition (0.70379) in equilibrium with that measured in the enclaves (0.70384–0.70389). We demonstrate that Type-1 plagioclase crystallizes in the previous UP rhyolitic magmas representing the silica-rich magma from which the dome-lava melts derived by open system evolutionary processes (e.g., mixing, mingling, and crystal migration), caused by successive refilling of mafic enclave-forming magma. The Type-2 plagioclase derives from entrainment of Type-1 into the still molten enclave magma. The DSZs originated in response to the interaction between the low An plagioclase and the enclave mafic melt in which dissolution and re-crystallization acted together as function of the interaction time. Type-1 and Type-2 plagioclases record, therefore, a long-lived timescale of events starting from their crystallization in the UP rhyolite. Instead, the different width of DSZs (Type-2 and Type-3) seems to indicate short different interaction timescales between the single crystals and the enclave melt (from few hours to some 40 days). These microanalytical data contribute to the understanding of the origin of the rhyodacitic dome-lavas at Nisyros volcano and to set robust constraints on the dynamics of mingling/mixing processes in terms of crystal exchange pathways and enclave disaggregation.