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1.
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]Fe2+, [5]Fe2+, [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.  相似文献   

2.
Abstract— Libyan Desert Glass (LDG) is an enigmatic type of glass that occurs in western Egypt in the Libyan Desert. Fairly convincing evidence exists to show that it formed by impact, although the source crater is currently unknown. Some rare samples present dark‐colored streaks with variable amounts of Fe, and they are supposed to contain a meteoritic component. We have studied the iron local environment in an LDG sample by means of Fe K‐edge highresolution X‐ray absorption near edge structure (XANES) spectroscopy to obtain quantitative data on the Fe oxidation state and coordination number in both the Fe‐poor matrix and Fe‐rich layers. The pre‐edge peak of the high‐resolution XANES spectra of the sample studied displays small but reproducible variations between Fe‐poor matrix and Fe‐rich layers, which is indicative of significant changes in the Fe oxidation state and coordination number. Comparison with previously obtained data for a very low‐Fe sample shows that, while iron is virtually all trivalent and in tetrahedral coordination ([4]Fe3+) in the low‐Fe sample, the sample containing the Fe‐rich layers display a mixture of tetra‐coordinated trivalent iron ([4]Fe3+) and penta‐coordinated divalent iron ([5]Fe2+), with the Fe in the Fe‐rich layer being more reduced than the matrix. From these data, we conclude the following: a) the significant differences in the Fe oxidation state between LDG and tektites, together with the wide intra‐sample variations in the Fe‐oxidation state, confirm that LDG is an impact glass and not a tektite‐like glass; b) the higher Fe content, coupled with the more reduced state of the Fe, in the Fe‐rich layers suggests that some or most of the Fe in these layers may be directly derived from the meteoritic projectile and that it is not of terrestrial origin.  相似文献   

3.
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.  相似文献   

4.
Abstract— In order to elucidate the formation of low‐H kaersutites in Martian meteorites, the Fe3+/ΣFe ratio of Chassigny kaersutites in magmatic inclusions was directly determined by the synchrotron microXANES analysis. XANES analysis for standard kaersutites with known Fe3+/ΣFe ratios shows a linear relationship between centroid energy positions of XANES pre‐edge spectra and the Fe3+/ΣFe ratio. Based on the linear relationship, the Fe3+/ΣFe ratio of Chassigny kaersutites is estimated to be about 0.05. The low Fe3+/ΣFe ratio clearly suggests that low‐H kaersutites in Chassigny are not likely to be formed by the oxidation‐dehydrogenation reactions. The low‐H content of the Chassigny kaersutites is mainly due to the presence of a Ti oxy‐component.  相似文献   

5.
We report measurements of the oxidation state of Fe nanoparticles within lunar soils that experienced varied degrees of space weathering. We measured >100 particles from immature, submature, and mature lunar samples using electron energy‐loss spectroscopy (EELS) coupled to an aberration‐corrected transmission electron microscope. The EELS measurements show that the nanoparticles are composed of a mixture of Fe0, Fe2+, and Fe3+ oxidation states, and exhibit a trend of increasing oxidation state with higher maturity. We hypothesize that the oxidation is driven by the diffusion of O atoms to the surface of the Fe nanoparticles from the oxygen‐rich matrix that surrounds them. The oxidation state of Fe in the nanoparticles has an effect on modeled reflectance properties of lunar soil. These results are relevant to remote sensing data for the Moon and to the remote determination of relative soil maturities for various regions of the lunar surface.  相似文献   

6.
The valence of iron has been used in terrestrial studies to trace the hydrolysis of primary silicate rocks. Here, we use a similar approach to characterize the secondary processes, namely thermal metamorphism and aqueous alteration, that have affected carbonaceous chondrites. X‐ray absorption near‐edge structure spectroscopy at the Fe‐K‐edge was performed on a series of 36 CM, 9 CR, 10 CV, and 2 CI chondrites. While previous studies have focused on the relative distribution of Fe0 with respect to oxidized iron (Feox = Fe2+ + Fe3+) or the iron distribution in some specific phases (e.g., Urey–Craig diagram; Urey and Craig 1953), our measurements enable us to assess the fractions of iron in each of its three oxidation states: Fe0, Fe2+, and Fe3+. Among the four carbonaceous chondrites groups studied, a correlation between the iron oxidation index (IOI = [2(Fe2+) + 3(Fe3+)]/[FeTOT]) and the hydrogen content is observed. However, within the CM group, for which a progressive alteration sequence has been defined, a conversion of Fe3+ to Fe2+ is observed with increasing degree of aqueous alteration. This reduction of iron can be explained by an evolution in the mineralogy of the secondary phases. In the case of the few CM chondrites that experienced some thermal metamorphism, in addition to aqueous alteration, a redox memory of the aqueous alteration is present: a significant fraction of Fe3+ is present, together with Fe2+ and sometimes Fe0. From our data set, the CR chondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR chondrites, the three oxidation states of iron coexist. Even in the least‐altered CR chondrites, the fraction of Fe3+ can be high (30% for MET 00426). This observation confirms that oxidized iron has been integrated during formation of fine‐grained amorphous material in the matrix (Le Guillou and Brearley 2014; Le Guillou et al. 2015; Hopp and Vollmer 2018). Last, the IOI of CV chondrites does not reflect the reduced/oxidized classification based on metal and magnetite proportions, but is strongly correlated with petrographic types. The valence of iron in CV chondrites therefore appears to be most closely related to thermal history, rather than aqueous alteration, even if these processes can occur together (Krot et al. 2004; Brearley and Krot 2013).  相似文献   

7.
Abstract— The Elephant Moraine (EET) 96001 ureilite contains a remarkable diversity of feldspars, which occur as tiny (no more than 60 μm maximum dimension) grains within a few Fe,S‐rich (now weathered to mostly Fe oxide) veins. Molar S: Fe ratio in the veins averages 0.08 ± 0.02. The veins meander and feature large fluctuations in apparent width; they appear to have entered this monomict breccia by a gentle, percolative process, not by violent impact injection. The feldspars are accompanied by a diverse suite of K‐rich (and generally also Ti‐rich) feldspathic glasses, and also major proportions of silica and pyroxene, which is largely fassaitic. A rhönite‐like phase is also found, and, as inclusions in one of the fassaites, a Cr‐poor spinel‐like phase. The feldspars mostly feature remarkably high K/Na compared to feldspars of comparable An from polymict ureilites. The EET 96001 feldspathic component was probably once part of a thin basaltic crust on a ureilite asteroid. The spinel included in one of the fassaites formed at remarkably high f02 (apparent oxidation state of iron: ～41 atom% Fe3+), suggesting that the parent magma possibly assimilated near‐surface water (however, the Fe3+ was not directly measured, and has conceivably been affected by terrestrial weathering; also, there is no assurance that this fassaite originated together with the typical feldspar). We speculate that the feldspathic component was mixed into the dense, Fe,S‐rich vein material, and very soon thereafter the Fe,S‐rich vein material was emplaced adjacent to the EET 96001 host ureilite, at an advanced stage in a chaotic catastrophic disruption and partial reassembly process that affected all ureilites. The high‐K nature of the EET 96001 feldspathic component, including the feldspathic glasses, suggests that fractional fusion may not have been as common during ureilite anatexis as has been inferred from recent studies of clast assemblages in polymict ureilites.  相似文献   

8.
Maohokite, a post‐spinel polymorph of MgFe2O4, was found in shocked gneiss from the Xiuyan crater in China. Maohokite in shocked gneiss coexists with diamond, reidite, TiO2‐II, as well as diaplectic glasses of quartz and feldspar. Maohokite occurs as nano‐sized crystallites. The empirical formula is (Mg0.62Fe0.35Mn0.03)2+Fe3+2O4. In situ synchrotron X‐ray microdiffraction established maohokite to be orthorhombic with the CaFe2O4‐type structure. The cell parameters are a = 8.907 (1) Å, b = 9.937(8) Å, c = 2.981(1) Å; V = 263.8 (3) Å3; space group Pnma. The calculated density of maohokite is 5.33 g cm?3. Maohokite was formed from subsolidus decomposition of ankerite Ca(Fe2+,Mg)(CO3)2 via a self‐oxidation‐reduction reaction at impact pressure and temperature of 25–45 GPa and 800–900 °C. The formation of maohokite provides a unique example for decomposition of Fe‐Mg carbonate under shock‐induced high pressure and high temperature. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2017‐047). The mineral was named maohokite after Hokwang Mao, a staff scientist at the Geophysical Laboratory, Carnegie Institution of Washington, for his great contribution to high pressure research.  相似文献   

9.
Lunar regolith breccias are temporal archives of magmatic and impact bombardment processes on the Moon. Apollo 16 sample 60016 is an “ancient” feldspathic regolith breccia that was converted from a soil to a rock at ~3.8 Ga. The breccia contains a small (70 × 50 μm) rock fragment composed dominantly of an Fe‐oxide phase with disseminated domains of troilite. Fragments of plagioclase (An95‐97), pyroxene (En74‐75, Fs21‐22,Wo3‐4), and olivine (Fo66‐67) are distributed in and adjacent to the Fe‐oxide. The silicate minerals have lunar compositions that are similar to anorthosites. Mineral chemistry, synchrotron X‐ray absorption near edge spectroscopy (XANES) and X‐ray diffraction (XRD) studies demonstrate that the oxide phase is magnetite with an estimated Fe3+/ΣFe ratio of ~0.45. The presence of magnetite in 60016 indicates that oxygen fugacity during formation was equilibrated at, or above, the Fe‐magnetite or wüstite–magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidization‐driven mineral replacement of Fe‐metal or desulphurisation from Fe‐sulfides (troilite) at low temperatures (<570 °C) in equilibrium with H2O steam/liquid or CO2 gas. Oxidizing conditions may have arisen from vapor transport during degassing of a magmatic source region, or from a hybrid endogenic–exogenic process when gases were released during an impacting asteroid or comet impact.  相似文献   

10.
This thermal annealing experiment at 1000 K for up to 167 h used a physical mixture of vapor phase‐condensed magnesiosilica grains and metallic iron nanograins to test the hypothesis that a mixture of magnesiosilica grains and an Fe‐source would lead to the formation of ferromagnesiosilica grains. This exploratory study found that coagulation and thermal annealing of amorphous magnesiosilica and metallic grains yielded ferromagnesiosilica grains with the Fe/(Fe + Mg) ratios in interplanetary dust particles. Furthermore, decomposition of brucite present in the condensed magnesiosilica grains was the source for water and the cause of different iron oxidation states, and the formation of amorphous Fe3+‐ferrosilica, amorphous Fe3+‐Mg, Fe‐silicates, and magnesioferrite during thermal annealing. Fayalite and ferrosilite that formed from silica/FeO melts reacted with forsterite and enstatite to form Mg, Fe‐silicates. The presence of iron in different oxidation states in extraterrestrial materials almost certainly requires active asteroid‐like parent bodies. If so, the possible presence of trivalent Fe compounds in comet P/Halley suggests that Halley‐type comets are a mixture of preserved presolar and processed solar nebula dust. The results from this thermal annealing experiment further suggest that the Fe‐silicates detected in the impact‐induced ejecta from comet 9P/Temple 1 might be of secondary origin and related to the impact experiment or to processing in a regolith.  相似文献   

11.
Optical absorption and diffuse reflectance spectra were obtained for simulated lunar glasses of four different compositions, both in their as-quenched (reduced) states and following mild subsolidus oxidation. The transmission spectra, when normalized by the FeO content of the glasses, differed from one another only in the relative intensity of an unresolved band in the UV. For fixed melting conditions the strength of this band in the as-quenched glasses increased with increasing FeO, or with increasing TiO2 for a fixed FeO content. Electron spin resonance (ESR) experiments have demonstrated the absence of Fe3+ or Ti3+ and the presence of metallic iron in these materials; all other transition-group elements were excluded in preparation. The unresolved UV absorption edge in the as-quenched reduced glasses is therefore tentatively ascribed to Fe2+Ti4+ intervalency charge transfer transitions. A similar UV edge was also produced by oxidation, leading to the conclusion that the assignment of this band would be ambiguous in the absence of an independent determination of the valence states of Fe and Ti. The relationship between the transmission spectra of polished samples and the reflectance spectra of sieved powders of the same materials is shown to be well described by the Kubelka-Munk approximation. Using this insight, it is possible to understand the spectral characteristics both of oxidation darkening of synthetic glass powders and of maturation darkening of lunar soils in terms of (1) the growth of the aforementioned charge transfer band(s) and (2) the development of opaque surface phases. It is shown that mechanism (1) is of primary importance in lunar highland materials and that mechanism (2) dominates in mare materials. The present results, coupled with previous findings, suggest that lunar soil maturation darkening may result from vitrification only if accompanied by (a) enrichment in the elements Fe and Ti, (b) changes in valence states of these elements, (c) partial crystallization of opaque phases such as iron, ilmenite or magnetite, or (d) a combination of (a), (b), and (c).  相似文献   

12.
Abstract— The crystal structure of druse clinopyroxene from the D'Orbigny angrite, (Ca0.944 Fe2+0.042 Mg0.010Mn0.004) (Mg0.469Fe2+0.317Fe3+0.035Al0.125Cr0.010Ti0.044) (Si1.742Al0.258) O6, a = 9.7684(2), b = 8.9124(2), c = 5.2859(1) Å, β = 105.903(1)°, V = 442.58 Å3, space group C2/c, Z = 2, has been refined to an R1 index of 1.92% using single‐crystal X‐ray diffraction data. The unit formula, calculated from electron microprobe analysis, and the refined site scattering values were used to assign site populations. The distribution of Fe2+and Mg over the M1 and M2 sites suggests a closure temperature of 1000 °C. Mössbauer spectroscopy measurements were done at room temperature on a single crystal and a powdered sample. The spectra are adequately fit by a Voigt‐based quadrupole‐splitting distribution model having two generalized sites, one for Fe2+with two Gaussian components and one for Fe3+with one Gaussian component. The two ferrous components are assigned to Fe2+at the M1 site, and arise from two different next‐nearest‐neighbor configurations of Ca and Fe cations at the M2 site: (3Ca,0Fe) and (2Ca,1Fe). The Fe3+/Fetot ratio determined by Mössbauer spectroscopy is in agreement with that calculated from the electron microprobe analysis. The results are discussed in connection with the redox and thermal history of D'Orbigny.  相似文献   

13.
Abstract— Electron microprobe and laser ablation, inductively‐coupled plasma mass spectrometer analyses of 24 georgiaites show that these tektites are all Si‐rich (79–83 wt% SiO2) glasses with variable major and trace element abundances (e.g., FeO varies from 2.1 to 3.7 wt%). Glass compositions are similar to but not identical with average upper continental crust. For example, georgiaites are light rare earth element enriched with small negative Eu anomalies (Eu/Eu*=0.73‐0.86) and La‐Th‐Sc systematics are intermediate between that of Archean and post‐Archean continental crust. When the georgiaite data are placed in the context of data for all North American tektites, triangular arrays appear on some oxide‐oxide plots (e.g., FeO‐MgO). Large variations in refractory element abundances and ratios compared to the variation in SiO2 favors mixing over volatilization as a cause of the compositional variation. If all the tektites formed as a result of a single impact, then triangular arrays in oxide‐oxide variation diagrams require at least three source components. These components include a Si‐rich material, probably a quartz‐rich sand that was predominant in the formation of georgiaites. Two relatively silica‐poor and Fe‐rich components have compositional characteristics similar to shales and greywackes. The La‐Th‐Sc systematics of the georgiaites and most other North American tektites are distinctive and could potentially be used to link the tektites to Eocene sediments at the Chesapeake Bay impact structure.  相似文献   

14.
To shed light on the mechanism of formation of nanophase iron particles (npFe) in space-weathered materials from airless bodies, we analyzed exsolved and unexsolved space-weathered lunar pyroxenes from Apollo 17 sample 71501. The exsolved pyroxene allowed for the observation of the effects of space weathering on similar mineral phases with variable composition. Using coordinated scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS), we determined that two coexisting pyroxenes in the exsolved grain showed systematic variations in response to space weathering, despite equivalent exposure conditions. The npFe in the space-weathered rim of augite lamellae were smaller and fewer than the npFe in the rim of pigeonite lamellae. EELS spectrum imaging revealed the presence and heterogeneous distribution of Fe0, Fe2+, and Fe3+ in the exsolved pyroxene. Metallic iron occurred in the npFe, a mixture of Fe2+ and Fe3+ occurred in the pigeonite lamellae, and the augite lamellae contained virtually all Fe3+. Approximately 50% of the total Fe measured in the exsolved pyroxene grain was ferric. Partitioning of Fe2+ and Fe3+ among the lamellae is invoked to explain the difference in npFe development in pigeonite and augite. The results of this study, the first to identify Fe3+ in a crystalline lunar ferromagnesian silicate, have implications for our understanding of how space weathering might proceed in oxidized phases. Furthermore, the discovery of an Fe3+-rich pyroxene also supports attribution of the 0.7 μm absorption feature observed in Galileo Solid State Imager data to oxidized Fe in clinopyroxenes.  相似文献   

15.
Abstract— We have analyzed fluorine and boron in nine tektites from all four strewn fields, and in a suite of impact glasses and target rocks from the Zhamanshin and Darwin impact craters, as well as Libyan Desert Glass and Aouelloul impact glass samples. Fluorine and boron are useful indicators for the volatilization and temperature history of tektites and impact glasses. Tektites from different strewn fields show a limited range of F and B contents and have F/B ratios near unity. Most splash-form tektites have lower average F and B contents than Muong Nong type tektites, which is similar to the relation between irghizites and zhamanshinites. The F and B contents in target rocks from the Zhamanshin and Darwin impact craters are similar to normal terrestrial sediments. Fluorine in impact glasses and tektites is more depleted compared to their (known or inferred) target rocks than is boron, which is caused by the higher volatility of F. The F/B ratios therefore decrease with increasing temperature of formation (suggesting that irghizites were formed at a higher temperature than zhamanshinites, and Muong Nong type tektites at a lower temperature than splash-form tektites). Mixing of local country rocks together with partial loss of the volatiles F and B can reproduce the F and B contents of impact glasses.  相似文献   

16.
Abstract— We have the elemental abundances and isotopic compositions of noble gases in Muong Nong‐type tektites from the Australasian strewn field by crushing and by total fusion of the samples. We found that the abundances of the heavy noble gases are significantly enriched in Muong Nong‐type tektites compared to those in normal splash‐form tektites from the same strewn field. Neon enrichments were also observed in the Muong Nong‐type tektites, but the Ne/Ar ratios were lower than those in splash‐form tektites because of the higher Ar contents in the former. The absolute concentrations of the heavy noble gases in Muong Nong‐type tektites are similar to those in impact glasses. The isotopic ratios of the noble gases in Muong Nong‐type tektites are mostly identical to those in air, except for the presence of radiogenic 40Ar. The obtained K‐Ar ages for Muong Nong‐type tektites were about 0.7 Myr, similar to ages of other Australasian tektites. The crushing experiments suggest that the noble gases in the Muong Nong‐type tektites reside mostly in vesicles, although Xe was largely affected by adsorbed atmosphere after crushing. We used the partial pressure of the heavy noble gases in vesicles to estimate the barometric pressure in the vesicles of the Muong Nong‐type tektites. Likely, Muong Nong‐type tektites solidified at the altitude (between the surface and a maximum height of 8–30 km) lower than that for splash‐form tektites.  相似文献   

17.
Abstract— To improve the scarce data base of H2O content in tektites and impact glasses, we analyzed 26 tektites from all four strewn fields and 25 impact glass samples for their H2O content. We used the fourier-transformed infrared (FTIR) spectrometry method, which permits measurement of areas of ～40 μm in diameter. Our results show that the tektites have H2O contents ranging from 0.002 to 0.030 wt% (average 0.014 ± 0.008 wt%). Ivory Coast tektites have the lowest H2O abundances (0.002–0.003 wt%), and Muong Nong-type indochinites and some North American tektites having the highest contents (up to ～0.03 wt%). Impact glass samples (from the Zhamanshin, Aouelloul, and Rio Cuarto craters) yielded H2O contents of 0.008 to 0.13 wt% H2O. Typical impact glasses from the Aouelloul and Zhamanshin craters have low H2O contents (0.008 to 0.063 wt%). Libyan Desert Glasses and Rio Cuarto glasses have higher H2O contents (～0.11 wt%). We also analyzed glasses of unknown origin (e.g., urengoites; glass fragments from Tikal), which showed very low H2O contents, in agreement with an origin by impact. Our data confirm that all tektites found on land have very low H2O contents (<0.03 wt% H2O), while impact glasses have slightly higher H2O contents. Both glass types are very dry compared to volcanic glasses. This study confirms that the low H2O contents (<0.05 wt%) of such glasses can be considered good evidence for an origin by impact.  相似文献   

18.
Abstract— Three samples of Darwin Glass, an impact glass found in Tasmania, Australia at the edge of the Australasian tektite strewn field were dated using the 40Ar/39Ar single‐grain laser fusion technique, yielding isochron ages of 796–815 ka with an overall weighted mean of 816 ± 7 ka. These data are statistically indistinguishable from those recently reported for the Australasian tektites from Southeast Asia and Australia (761–816 ka; with a mean weighted age of 803 ± 3 ka). However, considering the compositional and textural differences and the disparity from the presumed impact crater area for Australasian tektites, Darwin Glass is more likely to have resulted from a distinct impact during the same period of time.  相似文献   

19.
Oxidation is observed in Ca‐pyroxene subjected to a range of shock pressures (21–59 GPa). Changes in the pyroxene redox ratio as measured by the changes in %Fe3+ ranged from 2–6 times the starting composition. Mössbauer and reflectance spectroscopy record the changing Fe3+ concentration as a preferential oxidation of Fe2+ in the M2 crystallographic site. The oxidation is also accompanied by mechanical changes in the pyroxene crystals including fracturing, linear defects, and twinning. As oxygen fugacity is often calculated using mineral redox ratios and thought to represent the prevailing fO2 during crystallization, it is imperative to recognize that the fO2 values measured in impact‐derived materials may represent that of the impact and not the magma source region.  相似文献   

20.
Abstract— The Chesapeake Bay impact structure, which is about 35 Ma old, has previously been proposed as the possible source crater of the North American tektites (NAT). Here we report major and trace element data as well as the first Sr‐Nd isotope data for drill core and outcrop samples of target lithologies, crater fill breccias, and post‐impact sediments of the Chesapeake Bay impact structure. The unconsolidated sediments, Cretaceous to middle Eocene in age, have ?Srt = 35.7 Ma of +54 to +272, and ?Ndt = 35.7 Ma ranging from ?6.5 to ?10.8; one sample from the granitic basement with a TNdCHUR model age of 1.36 Ga yielded an ?Srt = 35.7 Ma of +188 and an ?Ndt = 35.7 Ma of ?5.7. The Exmore breccia (crater fill) can be explained as a mix of the measured target sediments and the granite, plus an as‐yet undetermined component. The post‐impact sediments of the Chickahominy formation have slightly higher TNdCHUR model ages of about 1.55 Ga, indicating a contribution of some older materials. Newly analyzed bediasites have the following isotope parameters: +104 to +119 (?Srt = 35.7 Ma), ?5.7 (?Ndt = 35.7 Ma), 0.47 Ga (TSrUR), and 1.15 Ga (TNdCHUR), which is in excellent agreement with previously published data for samples of the NAT strewn field. Target rocks with highly radiogenic Sr isotopic composition, as required for explaining the isotopic characteristics of Deep Sea Drilling Project (DSDP) site 612 tektites, were not among the analyzed sample suite. Based on the new isotope data, we exclude any relation between the NA tektites and the Popigai impact crater, although they have identical ages within 2s? errors. The Chesapeake Bay structure, however, is now clearly constrained as the source crater for the North American tektites, although the present data set obviously does not include all target lithologies that have contributed to the composition of the tektites.  相似文献   

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