Excess Argon Research Articles

The HRuCCH, RuCCH2, and Ru-η2-C2H2 Molecules: Infrared Spectra and Density Functional Calculations

Laser-ablated ruthenium atoms undergo reaction with acetylene during condensation in excess neon and argon matrices to form a metallacycle complex, insertion into the C-H bond, and rearrangement to the vinylidene complex. The subject molecules were identified by (13)C(2)H(2) and C(2)D(2), isotopic substitutions and density functional theory (DFT) frequency calculations. The HRuCCH molecule is described by Ru-H, CH, and CC stretching modes and CCH deformation modes. A very strong CC double bond stretching, weak CH stretching, and CCH deformation frequencies were observed for the Ru═C═CH(2) complex. The metallacycle Ru-η(2)-(C(2)H(2)) is characterized through CC double bond stretching, CH stretching and CCH deformation modes. The reaction mechanism for formation of the Ru═C═CH(2) complex was investigated by B3LYP internal reaction coordinate calculations, and the hydrido-alkyny complex is the rate-determining step. The delocalized three-center-four-electron π bond using the Ru 4d(xz) electron pair contributes to the C-C π* orbital and provides stabilization energy (ΔE((2)), second-order perturbation) for the vinylidene Ru═C═CH(2) complex.

過剰アルゴン：輸送・捕獲過程

過剰アルゴン：輸送・捕獲過程

Partitioning of excess argon between alkali feldspars and glass in a young volcanic system

Argon system behavior in a young volcanic system has been investigated through analysis of alkali feldspar phenocrysts and glass separates in order to constrain the potential effects of excess Ar upon 40Ar– 39Ar ages on feldspars and the crystal/melt partition coefficient. The importance of syn-and post-emplacement processes was investigated by sampling glass and pumice clasts derived from varying post-eruptive conditions, including welded sections, vapor-phase altered sections and heavily weathered portions of the ignimbrite. Excess 40Ar was detected in all glass samples at 1–10 ppb level, yet despite this, step-heating of co-existing alkali feldspar revealed a total gas age of 0.644 ± 0.038 Ma compared with the generally accepted age of 0.668 ± 0.004 Ma for the timing of emplacement of the Arico Formation. This suggests that despite high concentrations of Ar dissolved in the glass at quenching, the alkali feldspar crystallizing from excess Ar-rich melt contain very little excess Ar. No correlation was observed between excess Ar in the glass and total gas age obtained from the co-existing alkali feldspar and a partition coefficient (K D) between feldspar and glass was constrained to be less than 10 − 3 . A correlation between the concentrations of excess Ar and atmospheric derived 36Ar in the glass (manifested as a restricted range of 40Ar/ 36Ar up to ~ 320) suggests a uniform mechanism of incorporation of Ar from these reservoirs measured in all samples prior to eruption. In an assessment of post-eruptive factors, diffusional loss of Ar was not significant during the elevated temperatures (~ up to 670 °C) associated with welding processes at the relevant timescales over which welding occurs. Since diffusive exchange occurs between crystals and melt over long periods in the magma chamber, ages determined on volcanic alkali feldspars reflect the eruption age as a result of the very low K D, but in systems containing excess Ar the ultimate accuracy of such ages will be limited by the actual value of K D. We observe that biotite, a mineral with a higher K D, yields anomalously old ages in such systems, an observation made previously for other volcanic systems. Diffusive loss of Ar from volcanic glass occurs on timescales approximating that of volcanic eruptions, suggesting that excess Ar measured in the glass separates is representative of a degassed state, and indicates even higher pre-eruptive Ar concentrations present in the magma chamber prior to eruption. Thus our measured K D is likely to be a maximum value. The significance of physical contamination of feldspar by glass is also highlighted and quantified demonstrating that less than 0.01% by weight of adhered phonolite glass was required to elevate a measured feldspar age. This is particularly significant for young samples where any such contamination effect would be magnified proportionally to the small amount of in-grown radiogenic Ar.

Infrared Spectra and Quantum Chemical Calculations of the Bridge-Bonded HC(F)LnF2 (Ln = La–Lu) Complexes

Lanthanide metal atoms, produced by laser ablation, were condensed with CHF3 (CDF3) in excess argon or neon at 4 K, and new infrared absorptions are assigned to the oxidative addition product fluoromethylene lanthanide difluoride complex on the basis of deuterium substitution and density functional theory frequency calculations. Two dominant bands in the 500 cm–1 region are identified as metal–fluorine stretching modes. A band in the mid-600 cm–1 region is diagnostic for the unusual fluorine bridge bond C–(F)–Ln. Our calculations show that most of the bridged HC(F)LnF2 structures are 3–6 kcal/mol lower in energy than the open CHF-LnF2 structures, which is in contrast to the open structures observed for the corresponding CH2-LnF2 methylene lanthanide difluorides. Argon-to-neon matrix shifts are 15–16 cm–1 to the blue for stretching of the almost purely ionic Ln–F bonds, as expected, but 10 cm–1 to the red for the bridge C–(F)–Ln stretching mode, which arises because Ar binds more strongly to the electropositive Ln center, decreasing the bridge bonding, and thus allowing a higher C–F stretching frequency.

A Carboniferous high-pressure klippe in the western Acatlán Complex of southern México: implications for the tectonothermal development and palaeogeography of Pangea

High-pressure (HP) rocks are critical for palinspastic restorations because they mark inferred subducted/extruded oceanic crust; knowledge of their geometric, geodynamic, and age relationships provide essential constraints on palaeogeographic reconstructions. The westernmost HP belt (Ixcamilpa) in the Acatlán Complex of southern Mexico has been inferred to be a mid-Late Ordovician backarc basin on the southern Iapetan margin that was subducted beneath eastern Laurentia and extruded up the subduction zone during the Early Silurian. Re-examination of Ixcamilpa HP rocks has revealed that they comprise lower Palaeozoic rift-passive margin protoliths and occur in a W-vergent klippe (not a suture) formed during polyphase deformation. Peak metamorphic mineral assemblages of blueschist-amphibole eclogite facies underwent retrogression through epidote amphibolite to greenschist facies. 40Ar/39Ar dating of various rocks yielded plateau ages of 344–339 Ma for calcic amphibole, 318 ± 4 Ma for glaucophane, and 329–325 ± 2 Ma for muscovite (excess argon), which clearly indicate a Carboniferous tectonothermal event. We interpret the 20 million years range in amphibole ages as reflecting progressive unroofing. The terminal stage of progressive thrusting placed the HP rocks above the middle Mississippian Zumpango Unit, during which a single penetrative sub-greenschist fabric was produced. Subsequent Permian or Laramide deformation refolded all the rocks about NE-trending upright folds. We postulate that the root zone of the HP nappe lies to the east in the median HP belt, which has a structure consistent with an extrusion zone. Inasmuch as similar units of the Acatlán Complex bound this HP root zone on either side, it is inferred to have been extruded into the upper plate above the subduction zone, and thus is not an oceanic suture. Our new data provide constraints for a Carboniferous palaeogeographic reconstruction, whereby subduction erosion of passive margin rocks took place along the western margin of Pangea and were subsequently extruded into the upper (Acatlán) plate.

Infrared Spectrum of the CH3OCH2 Radical in Solid Argon

The methoxymethyl radical, CH(3)OCH(2), is prepared via hydrogen photodissociation from dimethyl ether during codeposition of CH(3)OCH(3) in excess argon at 4 K with laser-excited metal plume radiation. The spectrum of this radical is characterized by four infrared absorptions at 1468.1, 1253.9, 1226.6, and 944.4 cm(-1), which are assigned by deuterium substitution as well as frequency and intensity calculations using density functional theory. The O-CH(2) bond length is calculated to be 0.07 Å shorter than the CH(3)-O bond due to additional π bonding interactions. In the matrix near-UV irradiation destroys the CH(3)OCH(2) radical with the formation of HCO radical and CH(4), which is different from the decomposition mechanism of CH(3)OCH(2) radical to H(2)CO and CH(3) radical proposed for the gas phase process.

Combined Triple and Double Bonds to Uranium: The N≡U═N−H Uranimine Nitride Molecule Prepared in Solid Argon

Reactions of laser-ablated U atoms with N(2) and H(2) mixtures upon codeposition in excess argon at 5 K gave strong NUN and weak UN infrared absorptions and new bands at 3349.7, 966.9, 752.4, and 433.0 cm(-1) for the unusual new U(V) molecule N≡U=N-H, uranimine nitride, containing both triple and double bonds. This identification is based on D and (15)N isotopic substitution and comparison with frequencies computed by density functional theory for the (2)Δ ground state NUNH. Calculated bond lengths are compared to those of the (1)Σ(g)(+) ground state of U(VI) uranium dinitride N≡U≡N, the (2)Φ ground state of the isoelectronic nitride oxide N≡U=O, and the (3)A ground state of the U(IV) uranimine dihydride HN=UH(2) molecule, which have all been prepared in solid argon matrices. Mulliken bond orders based on the CASSCF orbitals for N≡U=N-H are 2.91, 2.19, and 1.05, respectively. Here, the terminal nitride is effectively a triple bond, just as found for N≡U≡N. The solid argon matrix is a convenient medium to isolate reactive terminal uranium nitrides for examination of their spectroscopic properties.

Infrared Spectra of Rh Atom Reaction Products with C2H2: The HRhCCH, RhCCH, RhCCH2, and Rh-η2-C2H2 Molecules

Laser-ablated Rh atoms react with C(2)H(2) upon co-condensation in excess argon and neon to form the insertion product HRhCCH, the alkyne RhCCH, the vinylidene RhCCH(2), and the metallacycle complex Rh-η(2)-(C(2)H(2)). These species are identified through (13)C(2)H(2), C(2)D(2), and mixed C(2)HD isotopic substitutions and density functional theory isotopic frequency calculations. The HRhCCH molecule is characterized by the CH stretching mode at 3306.2 cm(-1) (Ar) and 3310.9 cm(-1) (Ne), the Rh-H stretching mode at 2090.8 cm(-1) (Ar) and 2111.0 cm(-1) (Ne), and two CCH deformation modes at 584.3 and 573.3 cm(-1) (Ar) and 587.1 and 580.3 cm(-1) (Ne). The absorptions for the vinylidene RhCCH(2) complex are observed at 3150.9 (Ar), 3147.2 (Ne) (CH stretching), 1690.1 (Ar), 1694.3 (Ne) (CC stretching), and 804.9 (Ar), 810.5 cm(-1) (Ne) (CCH deformation). The metallacycle Rh-η(2)-(C(2)H(2)) complex is also identified through CC stretching and CCH deformation modes. The insertion reaction of ground Rh atom to the C-H bond is spontaneous on the basis of the growth of HRhCCH absorptions upon annealing in both solid neon and argon. Here, we show that atomic Rh can convert acetylene to the simple Rh vinylidene complex, analogous to that found for ligand-supported Rh complexes.

K-Ar ages of the Quaternary basalts in the Jeongok area, the central part of Korean Peninsula

The Quaternary basalts along the Chugaryeong Fault zone in central part of the Korean Peninsula are composed of two stratigraphic units in the Jeongok area, calling the Chatan and Jeongok basalts. K-Ar analyses of the mixture of the fine-grained plagioclase and anorthoclase from the 18 basalt samples were carried out to reveal the volcanic activity of the area. The results show that the Chatan and Jeongok basalts give a uniform age, giving the weighted average ages of 0.15 ± 0.01 and 0.51 ± 0.01 Ma, respectively. This reveals the age gap between the Chatan and Jeongok basalts was 360,000 years, providing a unique volcanic history for the Quaternary volcanism. The volcanism formed the two large amounts of alkali olivine basalt flows that are similar to each other in primitive chemical composition, phenocryst composition and viscosity with an interval of 360,000 years. The results also make a time constraint on the Jeongok Paleolithic Site where the Jeongok basalt occurs below the sediments containing the Paleolithic artifacts. The age (0.51 Ma) of the Jeongok basalt provides the lower limit for the age of Paleolithic artifacts. Olivine phenocrysts from both the Chatan and Jeongok basalts have significant amount of excess argon, giving apparent older ages on the whole rock K-Ar dating by previous researchers.

Discussion on ‘Geological offsets and age constraints along the northern Dead Sea fault, Syria'

Searle et al. (2010) present significant new evidence regarding the eruption chronology of the Homs Basalt, which adjoins the Misyaf Fault, part of the northern Dead Sea Fault Zone in western Syria. Their Ar–Ar date of 3.7 ± 0.1 Ma for the youngest basalt east of this fault, at Jebel Barshin, significantly post-dates any previously published; hitherto, the youngest numerical date for any part of the Homs Basalt has been 4.8 ± 0.2 Ma (Sharkov et al. 1994). Searle et al. (2010) also report a 4.0 ± 0.1 Ma Ar–Ar date for a basalt outlier west of the Misyaf Fault, near the village of Kafroun. This is mapped (Ponikarov et al. 1963 a ) as a proximal part of one of the most westerly Homs Basalt flow-units, which cascaded southwestward to the Mediterranean palaeo-coastline around the town of Safsafeh, where it overlies Early Pliocene marine sediments (e.g. Ponikarov et al. 1963 a , b , 1966, 1967). Notwithstanding the vagaries and inconsistencies in the site location information provided by Mouty et al. (1992), the new 4.0 ± 0.1 Ma Ar–Ar date is evidently from the same flow-unit and possibly even from the same outlier as yielded their 8.1 ± 0.2 Ma whole-rock K–Ar date. The revision now evident to the numerical age of this flow-unit brings it into line with the age-constraint from the underlying sediments. Moreover, this revision calls into question all extant dates for the Homs Basalt, from Mouty et al. (1992), Sharkov et al. (1994) and Butler & Spencer (1999); all are whole-rock K–Ar dates, and so may well be systematically old, as a result of the effect of excess argon in phenocrysts (see Kelley 2002). This conclusion is supported by other new Ar–Ar dates of 6.4 ± 0.1 Ma and 5.8 ± …

Matrix Infrared Spectroscopic and Computational Investigations of the Lanthanide−Methylene Complexes CH2LnF2 with Single Ln−C Bonds

Laser-ablated lanthanide metal atoms were condensed with CH(2)F(2) in excess argon at 6 K or neon at 4 K. New infrared absorption bands are assigned to the oxidative addition product methylene lanthanide difluorides on the basis of deuterium substitution and vibrational frequency calculations with density functional theory (DFT). Two dominant absorptions in the 500 cm(-1) region are identified as lanthanide-fluoride stretching modes for this very strong infrared absorption. The predominantly lanthanide-carbon stretching modes follow a similar trend of increasing with metal size and have characteristic 30 cm(-1) deuterium and 14 cm(-1) (13)C isotopic shifts. The electronic structure calculations show that these CH(2)LnF(2) complexes are not analogous to the simple transition and actinide metal methylidenes with metal-carbon double bonds that have been investigated previously, because the lanthanide metals (in the +2 or +3 oxidation state) do not appear to form a π-type bond with the CH(2) group. The DFT and ab initio correlated molecular orbital theory calculations predict that these complexes exist as multiradicals, with a Ln-C σ bond and a single electron on C-2p weakly coupled with f(x) (x = 1 (Ce), 2 (Pr), 3(Nd), etc.) electrons in the adjacent Ln-4f orbitals. The Ln-C σ bond is composed of about 15% Ln-5d,6s and 85% C-sp(2) hybrid orbital. The Ln orbital has predominantly 6s and 5d character with more d-character for early lanthanides and increasing amounts of s-character across the row. The Ln-F bonds are almost purely ionic. Accordingly, the argon-neon matrix shifts are large (13-16 cm(-1)) for the ionic Ln-F bond stretching modes and small (∼1 cm(-1)) for the more covalent Ln-C bond stretching modes.

Lifetime of an ocean island volcano feeder zone: constraints from U–Pb dating on coexisting zircon and baddeleyite, and 40Ar/39Ar age determinations, Fuerteventura, Canary IslandsThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

High-precision isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40Ar/39Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U–Pb ages, despite the expected 1% theoretical bias between 40Ar/39Ar and U–Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (Tc) of zircon (699–988 °C) to amphibole (500–600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40Ar/39Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole Tc is 0.8 Ma, suggesting PX1 lifetime of 520 000 to 800 000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ18O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.

Intermediates of CO oxidation on iron oxides: An experimental and theoretical study

Reactions of laser-ablated iron oxides with CO in excess argon are investigated by infrared adsorption spectroscopy and density functional theoretical calculations. The carbonyl iron oxides OFe(CO)(n) (n = 1-3) and O(2)Fe(CO)(m) (m = 1, 2) are generated during sample deposition or annealing, whereas CO(2) is greatly produced at the expense of these carbonyl iron oxides upon UV irradiation, showing the formation of intermediate carbonyl iron oxides in the oxidation of carbon monoxide to carbon dioxide. These intermediate carbonyl iron oxides are characterized on the basis of isotopic substitution, stepwise annealing, change of CO concentration and laser energy, and comparison with theoretical calculations. The overall agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these complexes from the matrix infrared spectra. The reaction pathways for the formation of the products are proposed based on the experimental and theoretical results presented.

Experimental and theoretical study of the reactions of iron and manganese oxides with dinitrogen in a cryogenic matrix

Reactions of laser-ablated iron and manganese oxides with dinitrogen in excess argon have been investigated by matrix-isolation infrared spectroscopy. The NNMO (M=Fe, Mn), (NN)2FeO, and NNFeO2 complexes are formed as reaction products during sample deposition and/or further annealing. These complexes are characterized on the basis of the results of the isotopic substitution and comparison with theoretical calculations. The agreement between the experimental and calculated vibrational frequencies and isotopic shifts supports the identification of these complexes from the matrix infrared spectra. Reaction mechanisms have been proposed for the formation of these products.

Timing of Magmatic Activity and Mineralization and Evidence of a Long-Lived Hydrothermal System in the Fresnillo Silver District, Mexico: Constraints from 40ar/39Ar Geochronology

The ore deposits of Fresnillo have produced silver for more than 450 years and contain resources of approximately 1 Goz. Mineralization in Fresnillo occurs in epithermal veins hosted by a Middle Jurassic to Late Cretaceous volcanosedimentary sequence (Guerrero terrane). Late Paleocene conglomerates and Eocene to Oligocene tuffs and flows of trachyte, rhyolite, and rhyodacite lie unconformably on top of the Guerrero terrane. Steam-heated acid sulfate alteration containing alunite veins occurs in Eocene tuffs above some of the mineralized veins. Oligocene rhyolitic domes and a quartz-monzonite stock also occur in the area, whereas sporadic Miocene basaltic flows and dikes are the youngest igneous rocks in the district. Spatial association of Tertiary volcanic rocks, intrusions, acid sulfate alteration, and epithermal veins makes the district ideal for evaluating the temporal relations between magmatism and hydrothermal activity using precise 40Ar/39Ar geochronology. Sanidine single crystal laser fusion analyses reveal discrete volcanic eruptions at 44.73 ± 0.04, 40.92 ± 0.04, 40.83 ± 0.04, 31.55 ± 0.03, and 31.04 ± 0.05 Ma. Dating of orthoclase from the Fortuna quartz-monzonite was problematic due to excess argon contamination, but a preferred age of 32.65 ± 0.05 Ma is suggested. Two adularia samples from the Jarillas vein give statistically identical ages of 29.75 ± 0.12 and 29.68 ± 0.10 Ma. Adularia from Valdecanas vein shows a climbing age spectrum related to hydrothermal alteration; however, a terminal step with an age of 29.74 ± 0.07 Ma suggests that the two veins may be coeval. Alunite recorded two distinct ages at 30.56 ± 0.04 and 31.03 ± 0.05 Ma. Alunite and adularia ages suggest that acid sulfate alteration and mineralization occurred separately well after emplacement of the dated phase of the Fortuna stock and, consequently, it cannot be the source of heat for the hydrothermal system that formed those. Finally, the ~3 m.y. difference in age between epithermal mineralization and the Fortuna stock is indicative of a long-lived but episodic hydrothermal system which could have been driven by multiple pulses of small and shallow intrusions.

Interpreting high-pressure phengite 40Ar/39Ar laserprobe ages: an example from Saih Hatat, NE Oman

New single grain fusion and core-rim 40Ar/39Ar laserprobe phengite data from the Saih Hatat high-pressure terrane in NE Oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane. The majority of the determined ages are older than the previously reported U-Pb zircon peak metamorphic age. Core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores, or, rarely, older rims; some samples manifest all three patterns. Numerical diffusion modelling shows that due to the peak temperature of ca. 550°C, the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system. The age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm-cm scale coupled with pervasive and heterogeneously distributed excess argon. Anomalously old eclogite phengite 40Ar/39Ar ages are due either to internally derived 40Ar inherited from a K-bearing precursor, or externally derived 40Ar distributed by grain boundary fluids. Mica-rich schists within the eclogite boudins yield younger phengite ages, suggesting excess argon was absent or diluted. Pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist-facies overprint and yield very variable ages commonly with apparently older rims on younger cores. The grain- and sample-scale age variations measured in Saih Hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport, whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle.

U and P4 Reaction Products: A Quantum Chemical and Matrix Isolation Spectroscopic Investigation

Reactions of laser-ablated U atoms with P(4) molecules upon codeposition in excess argon gave weak new infrared absorptions at 504, 483, and 426 cm(-1), which are best identified as binary uranium phosphide UP(4) species based on extensive B3LYP, BPW91, and PBE density functional and CASSCF/CASPT2 wave function based calculations. These UP(4) adducts may be considered as simple models for P(4) activation products by ligand-supported transition and main group metal complexes. The sought-after PUP molecule in the (3)Φ(u) ground state probably absorbs under the intense P(4) precursor band near 465 cm(-1). The triplet U(IV) molecule PUP is 7 kcal/mol lower in energy than the U(VI) analog, singlet PUP. The (3)Φ(u) ground state of PUP, with 2.54 effective bond order computed by CASPT2, may be compared to the (1)Σ(g)(+) ground state of N≡U≡N with 2.87 effective bond order, where third row elements are less effective for π bond formation than second row elements.

Reactions of Yttrium and Scandium Atoms with Acetylene: A Matrix Isolation Infrared Spectroscopic and Theoretical Study

Laser-ablated yttrium and scandium metal atoms have been codeposited at 4 K with acetylene in excess argon. Products, Y(C(2)H(2)), HYCCH, HScCCH(-), and HScScCCH(-), have been formed in the present experiments and characterized using infrared spectroscopy on the basis of the results of the isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, the change of reagent concentration and laser energy, and the comparison with theoretical predictions. Density functional theory calculations have been performed on these molecules. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these molecules from the matrix infrared spectra. Plausible reaction mechanisms have been proposed to account for the formation of these molecules.

40Ar/ 39Ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations

Artificial mixtures of two age populations of muscovite and of biotite were prepared at relative weight percents of 3:1, 1:1, and 1:3 and analyzed by the 40Ar/ 39Ar vacuum furnace step-heating method. The original micas are Late Jurassic and Late Cretaceous samples that yield flat age spectra, with plateau ages defined by 97–100% of the gas release and no evidence for excess argon. The age spectra from mica mixtures yield patterns that systematically decrease with decreasing Jurassic mica content and increasing Cretaceous component. However, the mixed muscovite spectra are relatively flat, consistent with the two original samples showing similar degassing patterns during vacuum furnace heating. In contrast, the mixed biotite spectra are highly discordant with an overall decrease in age during the step-heat analysis. This is consistent with the degassing patterns of the original mica samples, which indicate the Jurassic biotite (high Fe/Mg) outgases at lower temperatures and the Cretaceous biotite (low Fe/Mg) remains retentive until the higher temperature steps. The individual degassing patterns and compositions show consistencies with known compositional controls on argon diffusivity indicating that micas may maintain argon retention characteristics rooted in crystal chemistry during vacuum furnace heating. All but one of the mixed samples failed to yield ages reflecting the age of the original micas. These results imply that complex age spectra obtained for multiply-deformed rocks with multiple mica populations may define geologically meaningless ages as a result of simultaneous degassing (i.e. mixing of sample reservoirs) during heating. Comparison of laboratory degassing rates and crystal chemistry indicates compositional controls on argon retention are likely preserved during vacuum furnace step heating and micas may degas/retain argon in a predictable manner. These conclusions support assertions that recovery of fossil age gradients in slowly cooled samples may be possible using furnace step heating.

Reactions of molybdenum and tungsten atoms with nitrous oxide in excess argon: A combined matrix infrared spectroscopic and theoretical study

Reactions of laser-ablated Mo and W atoms with the N(2)O molecules in excess argon have been investigated using matrix-isolation infrared spectroscopy. In the reaction of the N(2)O molecule with the Mo atom, the absorptions at 1960.3 and 934.4 cm(-1) are assigned to the N-N and Mo-O stretching vibrations of the OMoNN complex, respectively. An analogous OWNN complex has also been observed in the W + N(2)O reaction. Infrared spectroscopy also provides evidence for the formation of the OW(NN)(2) complexes. Density functional theory calculations have been performed on the products. Overall agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these species from the matrix infrared spectra. Furthermore, a plausible reaction mechanism for the formation of these products has been proposed.