A comparative study of melilite and fassaite in Types B1 and B2 refractory inclusions

Most of the petrologic data available for Type B inclusions comes from Type B1s. Relatively little comes from the B2s, and there has not been a systematic comparison of the properties of their two most abundant minerals. In this work, we document the compositions and zoning patterns of melilite and fassaite in Type B2 inclusions, and compare and contrast them with the features of their counterparts in Type B1 inclusions. We find that melilite compositions in Type B2 inclusions are similar to those of Type B1s, with maximum Åk contents of ∼75 mol % and a positive correlation between Åk and Na₂O contents. Asymmetrically zoned melilite is common in Type B2s as are melilite grains with reversely zoned regions, and the reversely zoned portions of crystals are thicker than in B1s. In B2s, like B1s, fassaite is zoned with decreasing Ti, Sc, and V oxide contents from cores to rims of grains. Approximately half of the Ti is trivalent, but unlike that in B1s, within fassaite grains in B2s the Ti³⁺/(Ti³⁺ + Ti⁴⁺) ratio does not decrease from core to rim, and sharp enrichments (“spikes”) in Ti³⁺ and V are not observed. Sector-zoned fassaite is much more common in B2s than in B1s. The differences we observed can be accounted for by the differences in bulk compositions between B1s and B2s. Type B2 inclusions tend to have higher SiO₂ contents, hence higher An/Ge component ratios, than Type B1s. Phase equilibria show that, compared to B1s, in B2s less melilite should crystallize prior to the appearance of fassaite, so that in B2s a higher proportion of melilite cocrystallizes with fassaite, causing more of the crystals to be reversely zoned; more melilite crystallizes while adjacent to other crystals, leading to asymmetrical zoning; and with more liquid available, transport of components to growing fassaite occurs more readily than in B1s, facilitating crystal growth and giving rise to sector zoning. The lack of zoning with respect to Ti³⁺/Ti^tot and the absence of Ti³⁺-, V-rich spikes suggest that Type B2 melts maintained equilibrium with the nebular gas throughout crystallization, while the interiors of B1s were probably isolated from the gas, perhaps by their melilite mantles. This makes the similarity of Na-Åk relationships in B1 and B2 melilite difficult to understand, but apparently enclosure by melilite mantles was not necessary for the retention of Na₂O during crystallization of Type B refractory inclusions.     

Valence state of titanium in the Wark-Lovering rim of a Leoville CAI as a record of progressive oxidation in the early Solar Nebula

Simon et al. (2005) reported low Ti³⁺/Ti⁴⁺ values in Ti-rich pyroxenes in the Wark-Lovering rim (WL) of a Leoville CAI (144A) as compared to the interior of the inclusion. These electron microprobe analyses were interpreted as evidence that the growth of the WL rim is the manifestation of an evolution to a more oxidizing environment. Further work by Simon et al. (2007) used XANES analyses to argue for higher Ti³⁺ abundances and interpreted the data of Simon et al. (2005) as the result of X-ray contamination by neighboring phases, specifically spinel. Late-stage alteration was also included as a possible explanation.To investigate further the oxidation state of Ti in WL rims, we re-analyzed Leoville 144A to obtain a more complete data set of Ti³⁺/Ti⁴⁺ values in the Wark-Lovering rims. We conducted experiments on spinel-mixing to determine whether this was a plausible explanation for the observed paucity of Ti³⁺ in WL rims. While we found a wider range of Ti³⁺/Ti⁴⁺ in these WL rim data than in our original study, our new data show that the original conclusion that rims are lower in Ti³⁺/Ti⁴⁺ than interiors remains valid. We conclusively rule out spinel-mixing as an explanation for our data, and we see no clear inconsistency between our electron microprobe data and the XANES data. The WL rim of CAI Ef3 was also analyzed by EMPA and compared to the results of Leoville 144A.To predict compositional consequences of this hypothesis, we constructed a reaction space between Ti-rich pyroxene in the WL rim, perovskite, Mg_(g), Ca_(g), O_2(g), and SiO_(g). We find the oxidation of Ti³⁺, coupled with Ti loss via perovskite formation, explains many features of WL rim EMPA analyses. We maintain that the WL rim pyroxenes are compositionally distinct from those in the interior, and are evidence of a more oxidizing environment during WL rim formation.     

Ti K-edge XANES study on the coordination number and oxidation state of Titanium in pyroxene,olivine, armalcolite,ilmenite, and silicate glass during mare basalt petrogenesis

Lunar mare basalts are a product of partial melting of the lunar mantle under more reducing conditions when compared to those expected for the Earth’s upper mantle. Alongside Fe, Ti can be a major redox sensitive element in lunar magmas, and it can be enriched by up to a factor of ten in lunar basaltic glasses when compared to their terrestrial counterparts. Therefore, to better constrain the oxidation state of Ti and its coordination chemistry during lunar magmatic processes, we report new X-ray absorption near edge structure (XANES) spectroscopy measurements for a wide range of minerals (pyroxene, olivine, Fe–Ti oxides) and basaltic melt compositions involved in partial melting of the lunar mantle. Experiments were conducted in 1 bar gas-mixing furnaces at temperatures between 1100 and 1300 °C and oxygen fugacities (fO₂) that ranged from air to two orders of magnitude below the Fe–FeO redox equilibrium. Run products were analysed via electron microprobe and XANES Ti K-edge. Typical run products had large (>?100 µm) crystals in equilibrium with quenched silicate glass. Ti K-edge XANES spectra show a clear shift in energy of the absorption edge features from oxidizing to reducing conditions and yield an average valence for Fe–Ti oxides (armalcolite and ilmenite) of 3.6, i.e., a 40% of the overall Ti is Ti³⁺ under fO₂ conditions relevant to lunar magmatism (IW ??1.5 to ??1.8). Pyroxenes and olivine have average Ti valence of 3.75 (i.e., 25% of the overall Ti is trivalent), while in silicate glasses Ti is exclusively tetravalent. Pre-edge peak intensities also indicate that the coordination number of Ti varies from an average V-fold in silicate glass to VI-fold in the Fe–Ti oxides and a mixture between IV and VI-fold coordination in the pyroxenes and olivine, with up to 82% ^[IV]Ti⁴⁺ in the pyroxene. In addition, our results can help to better constrain the Ti³⁺/∑Ti of the lunar mantle phases during magmatic processes and are applied to provide first insights into the mechanisms that may control Ti mass-dependent equilibrium isotope fractionation in lunar mare basalts.     

Redox state of iron in peralkaline rhyolitic glass/melt: X-ray absorption micro-spectroscopy experiments at high temperature

Assessing the ferric-ferrous ratio in magmas prior to eruption remains a challenging task. X-ray absorption near-edge structure (μXANES) spectra were collected at the iron K-edge in water-rich peralkaline silicic melt/glass inclusions trapped in quartz. These experiments were carried out between 800 and 20 °C. The chemical environment of iron was also determined in the naturally quenched samples (glass inclusions and matrix glass) and in the peralkaline rhyolitic reference glasses, with variable [Fe³⁺ / ∑Fe] ratios.In the reference glasses, both the intensity of the pre-peaks (Fe²⁺, Fe³⁺) and site geometry of iron change as the oxidation state increases. Fourfold-coordinated Fe³⁺ prevails in highly oxidised peralkaline silicic glasses, using alkalis for charge balance. The position of the pre-edge centroid of the 1s-3d transition correlates with the Fe³⁺ / ΣFe ratios that allowed calibration of the redox state of iron of our natural samples.At high temperatures, Fe²⁺ dominates in the pre-edge structure of melt inclusions. Upon cooling down to 20 °C, the intensity of the Fe³⁺ peak increases, the centroid position of the pre-edge features shifts by nearly 0.5 eV and the main edge moves slightly towards higher energies. The slower the cooling rate, the higher the ferric iron contribution. Iterative μXANES experiments performed on the same samples show that the process is reversible. However, this apparent oxidation of iron upon cooling is an artefact of changes in Fe coordination. It implies that the [Fe³⁺ / ΣFe] ratio of glassy samples, measured at 20 °C, may be overestimated by a factor > 1.7, and that this ratio cannot be reliably retrieved by probing naturally cooled glass inclusions, and most silicate glasses. High temperature μXANES experiments led first to an assessment of the ferric-ferrous ratio in the water-rich peralkaline melt in pre-eruptive magmatic conditions and second to the determination of the corresponding oxygen fugacity at 740 °C.     

The oxidation state of sulfur in synthetic and natural glasses determined by X-ray absorption spectroscopy

Sulfur K-edge X-ray absorption near edge structure (XANES) spectra were recorded for experimental glasses of various compositions prepared at different oxygen fugacities (fO₂) in one-atmosphere gas-mixing experiments at 1400 °C. This sample preparation method only results in measurable S concentrations under either relatively reduced (log fO₂ < −9) or oxidised (log fO₂ > −2) conditions. The XANES spectra of the reduced samples are characterised by an absorption edge crest at 2476.4 eV, typical of S²⁻. In addition, spectra of Fe-bearing compositions exhibit a pronounced absorption edge shoulder. Spectra for all the Fe-free samples are essentially identical, as are the spectra for the Fe-bearing compositions, despite significant compositional variability within each group. The presence of a sulfide phase, such as might exsolve on cooling, can be inferred from a pre-edge feature at 2470.5 eV.The XANES spectra of the oxidised samples are characterised by an intense transition at 2482.1 eV, typical of the sulfate anion SO₄²⁻. Sulfite (SO₃²⁻) has negligible solubility in silicate melts at low pressures. The previous identification of sulfite species in natural glass samples is attributed to an artefact of the analysis (photoreduction of S⁶⁺). S⁴⁺ does, however, occur unambiguously with S⁶⁺ in Fe-free and Fe-poor compositions prepared in equilibrium with CaSO₄ at 4-16 kbar, and when buffered with Re/ReO₂ at 10 kbar. Solubility of S⁴⁺ thus requires partial pressures of SO₂ considerably in excess of 1 bar. A number of experiments were undertaken in an attempt to access intermediate fO₂s more applicable to terrestrial volcanism. Although these were largely unsuccessful, S²⁻ and S⁶⁺ were found to coexist in some samples that were not in equilibrium with the imposed fO₂.The XANES spectra of natural olivine-hosted melt inclusions and submarine glasses representative of basalts at, or close to, sulfide saturation show mainly dissolved S²⁻, but with minor sulfate, and additionally a peak at 2469.5 eV, which, although presumably due to immiscible sulfide, is 1 eV lower than that typical of FeS. These sulfate and sulfide-related peaks disappear with homogenisation of the inclusions by heating to 1200 °C followed by rapid quenching, suggesting that both these features are a result of cooling under natural conditions. The presence of small amounts of sulfate in otherwise reduced basaltic magmas may be explained by the electron exchange reaction: S²⁻ + 8Fe³⁺ = S⁶⁺ + 8Fe²⁺, which is expected to proceed strongly to the right with decreasing temperature. This reaction would explain why S²⁻ and S⁶⁺ are frequently found together despite the very limited fO₂ range over which they are thermodynamically predicted to coexist. The S XANES spectra of water-rich, highly oxidised, basaltic inclusions hosted in olivine from Etna and Stromboli confirm that nearly all S is dissolved as sulfate, explaining their relatively high S contents.     

Pb/Pb geochronology, petrography and chemistry of Zr-rich accessory minerals (zirconolite, tranquillityite and baddeleyite) in mare basalt 10047

In situ U-Pb isotopic measurements were carried out by ion microprobe on the Zr-rich accessory minerals zirconolite [CaZrTi₂O₇], tranquillityite [Fe₈²⁺(ZrY)₂Ti₃Si₃O₂₄] and baddeleyite [ZrO₂] in low-K, high-Ti mare basalt 10047 collected during the Apollo 11 mission. The analysed minerals are concentrated in pockets of late-stage mesostasis that comprises an intergrowth of silica, barian K-feldspar and Si-Al-K glass, from a phaneritic, subophitic, basalt comprising mainly pyroxene, plagioclase, ilmenite, cristobalite and troilite. Most Zr-rich minerals are unaltered, however, some tranquillityite is replaced by a complex intergrowth of zirconolite, baddeleyite, ilmenite and fayalite, suggesting that the mineral became unstable during crystallization. Several baddeleyite crystals have also undergone alteration to secondary zircon. Zirconolite was analysed in thin section 10047,11 and tranquillityite and baddeleyite in 10047,227, using a ∼6 μm primary ion beam. Both zirconolite and tranquillityite have significant U and low initial Pb contents, and are highly suitable for Pb/Pb dating. Fifteen analyses of zirconolite give a ²⁰⁷Pb/²⁰⁶Pb age of 3708 ± 7 Ma (²⁰⁷Pb/²⁰⁶Pb:²⁰⁴Pb/²⁰⁶Pb isochron; 95% confidence, including renormalisation of ratios) and twenty-five analyses of tranquillityite give 3710 ± 6 Ma. The ²⁰⁷Pb/²⁰⁶Pb dates are consistent with each other and refine results from an earlier study. Baddeleyite data were less precise, mainly due to lower secondary ionisation efficiency. Our results show that zirconolite and tranquillityite can provide precise isotopic dates and, given their presence in other samples, they represent important U-Pb chronometers for refining lunar geology.     

Sulfur K-edge XANES analysis of natural and synthetic basaltic glasses: Implications for S speciation and S content as function of oxygen fugacity

XANES analyses at the sulfur K-edge were used to determine the oxidation state of S species in natural and synthetic basaltic glasses and to constrain the fO₂ conditions for the transition from sulfide (S²⁻) to sulfate (S⁶⁺) in silicate melts. XANES spectra of basaltic samples from the Galapagos spreading center, the Juan de Fuca ridge and the Lau Basin showed a dominant broad peak at 2476.8 eV, similar to the spectra obtained from synthetic sulfide-saturated basalts and pyrrhotite. An additional sharp peak at 2469.8 eV, similar to that of crystalline sulfides, was present in synthetic glasses quenched from hydrous melts but absent in anhydrous glasses and may indicate differences in sulfide species with hydration or presence of minute sulfide inclusions exsolved during quenching. The XANES spectra of a basalt from the 1991 eruption of Mount Pinatubo, Philippines, and absarokitic basalts from the Cascades Range, Oregon, USA, showed a sharp peak at 2482.8 eV, characteristic of synthetic sulfate-saturated basaltic glasses and crystalline sulfate-bearing minerals such as hauyne. Basaltic samples from the Lamont Seamount, the early submarine phase of Kilauea volcano and the Loihi Seamount showed unequivocal evidence of the coexistence of S²⁻ and S⁶⁺ species, emphasizing the relevance of S⁶⁺ to these systems. XANES spectra of basaltic glasses synthesized in internally-heated pressure vessels and equilibrated at fO₂ ranging from FMQ − 1.4 to FMQ + 2.7 showed systematic changes in the features related to S²⁻ and S⁶⁺ with changes in fO₂. No significant features related to sulfite (S⁴⁺) species were observed. These results were used to construct a function that allows estimates of S⁶⁺/ΣS from XANES data. Comparison of S⁶⁺/ΣS data obtained by S Kα shifts measured with electron probe microanalysis (EPMA), S⁶⁺/ΣS obtained from XANES spectra, and theoretical considerations show that data obtained from EPMA measurements underestimate S⁶⁺/ΣS in samples that are sulfate-dominated (most likely because of photo-reduction effects during analysis) whereas S⁶⁺/ΣS from XANES provide a close match to the expected theoretical values. The XANES-derived relationship for S⁶⁺/ΣS as a function of fO₂ indicates that the transition from S²⁻ to S⁶⁻ with increasing fO₂ occurs over a narrower interval than what is predicted by the EPMA-derived relationship. The implications for natural systems is that small variation of fO₂ above FMQ + 1 will have a large effect on S behavior in basaltic systems, in particular regarding the amount of S that can be transported by basaltic melts before sulfide saturation can occur.     

Majoritic garnet: A new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamond

A means for estimating pressures in natural samples based on both the coupled substitution (Na+)^[1+] (Ti + ^[VI]Si)^[4+] = (M)^[2+] (Al + Cr)^[3+], and the classic pyroxene-stoichiometry majorite-substitution into garnet at high-pressure, is derived for garnets with majoritic chemistry. The technique is based on a compilation of experimental data for different bulk compositions. It is compositionally and thermally robust and can be used to estimate pressures experienced by natural materials during formation of majoritic garnet. In addition, it can be used either retrospectively, or in new experimental studies to establish the pressures of crystallization of reaction products, and determine if disequilibrium is recorded by the chemistries of majoritic garnets. Pressures are calculated based on majoritic chemistries in chondritic meteorites and diamond inclusions. Majoritic garnets associated with Mg perovskite in shocked L chondrites (n = 4) yield uniform pressures of 23.8 ± 0.2 GPa that are slightly higher than pressures recorded by majoritic garnet in shock-derived melt veins in L chondrites (22.4 ± 0.6 GPa; n = 5). Similar pressures are also exhibited by shock-derived majoritic garnets in H chondrites (22.2 ± 1.1 GPa; n = 3). Diamond inclusions with eclogitic and peridotitic majoritic garnet chemistries exhibit mean pressures of 10.7 ± 2.7 GPa (n = 30) and 8.3 ± 1.6 GPa (n = 15) respectively, consistent with a sub-lithospheric origin. However, pressures defined by majoritic diamond inclusions from Jagersfontein (22.3 ± 0.8 GPa and 16.9 ± 1 GPa), Monastery (15.7 ± 7 GPa) and Kankan (15.5 ± 0.2 GPa) show that these inclusions originated from the mantle transition zone. Thus, this new single-phase method for pressure estimation has unmatched potential to map the depth of formation of garnets with majoritic chemistries that occur as diamond inclusions in all parageneses except those that include Ca silicate perovskite. The derived pressures confirm the sub-lithospheric origin of eclogitic majoritic diamond inclusions, and thus provide a more comprehensive picture of the important role of storage of oceanic lithosphere in the transition zone.     

The U-Pb systematics of angrite Sahara 99555

Angrite Sahara 99555 (hereafter SAH), precisely dated by Baker et al. (Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature436, 1127-1131), has been proposed as a new reference point for the early Solar System timescale and for calculation of the revised minimum age of our Solar System. The Pb-Pb age of SAH of 4566.18 ± 0.14 Ma, reported by Baker et al., differs from the Pb-Pb age of D’Orbigny, another basaltic angrite, of 4564.42 ± 0.12 Ma (Amelin Y. (2008) U-Pb ages of angrites. Geochim. Cosmochim. Acta72, 221-232), despite the fact that the relative ⁵³Mn-⁵³Cr and ¹⁸²Hf-¹⁸²W ages of these meteorites are identical. Here I report U-Pb data for 21 whole rock and pyroxene fractions from SAH, analyzed using the same approach as D’Orbigny (Amelin, 2008). These fractions contain between 1.3 and 8.9 pg of total common Pb, slightly more than analytical blank. Measured ²⁰⁶Pb/²⁰⁴Pb ratios are between 625 and 2817 for D’Orbigny, blank-corrected ²⁰⁶Pb/²⁰⁴Pb ratios are between 1173 and 6675. Eight acid-washed whole rock fractions yielded an isochron age of 4564.86 ± 0.38 Ma, MSWD = 1.5. Data for pyroxene fractions plot mostly above the whole rock isochron, and do not form a linear array in ²⁰⁷Pb/²⁰⁶Pb vs. ²⁰⁴Pb/²⁰⁶Pb isochron coordinates. The ²⁰⁷Pb^∗/²⁰⁶Pb^∗ model dates of the pyroxene fractions vary from 4563.8 ± 0.3 to 4567.1 ± 0.5 Ma. The difference between whole rock and pyroxene U-Pb systematics may be a result of re-distribution of radiogenic Pb at a mineral grain scale several million years after crystallization. Complexities of Sm-Nd, Lu-Hf, and possibly ²⁶Al-²⁶Mg mineral systematics of SAH, described previously, may be related to the same process that caused the re-distribution of radiogenic Pb. Disturbance of isotopic chronometers renders SAH an imperfect anchor for the early Solar System timescale. The problems with age determination revealed by the studies of SAH call for greater attention in Pb-isotopic dating of angrites and other achondrites.     

Fluid inclusions as petrogenetic indicators in granulite xenoliths,Pali-Aike volcanic field,Chile

Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.Examination of fluid inclusions in the granulites indicates the ubiquitous presence of an essentially pure CO₂ fluid phase. Inclusions of three different parageneses have been recognized: Type I inclusions occur along exsolution lamellae in clinopyroxene and are thought to represent precipitation of structurally-bound C or CO₂ during cooling of the gabbro. These are considered the most primary inclusions present. Type II inclusions occur as evenly distributed clusters not associated with any fractures. These inclusions probably represent entrapment of a free fluid phase during recrystallization of the host grains. IIa inclusions are found in granoblastic grains and have densities of 0.68–0.88 g/cm³. Higher density (=0.90–1.02 g/cm³) IIb inclusions occur only in symplectite phases. Secondary Type III CO₂+glass inclusions with =0.47–0.78 g/cm³ occur along healed fractures where basalt has penetrated the xenoliths. Type III inclusions appear related to exsolution of CO₂ from the host basalt during its ascent to the surface. These data suggest that CO₂ is an important constituent of the lower crust under conditions of granulite facies metamorphism, indicated by Type I and II fluid inclusions, and of the mantle, as indicated by Type III inclusions.Correlation of fluid inclusion densities with P-T conditions calculated from both two-pyroxene geothermometry and reation (1) indicate emplacement of a gabbroic pluton at 1,200–1,300° C, 4–6 kb; cooling was accompanied by a slight increase in pressure due to crustal thickening, and symplectite formation occurred at 850±35° C, 5–7 kb. Capture of the xenoliths by the basalt resulted in heating of the granulites, and CO₂ from the basalt was continuously entrapped by the xenoliths over the range 1,000–1,200° C, 4–6 kb. Examination of fluid inclusions of different generations can thus be used in conjunction with other petrologic data to place tight constraints on the specific P-T path followed by the granulite suite, in addition to indicating the nature of the fluid phase present at depth.     

The LaPaz Icefield 04840 meteorite: Mineralogy, metamorphism, and origin of an amphibole- and biotite-bearing R chondrite

The R chondrite meteorite LaPaz Icefield (LAP) 04840 is unique among metamorphosed, non-carbonaceous chondrites in containing abundant OH-bearing silicate minerals: ∼13% ferri-magnesiohornblende and ∼0.4% phlogopite by volume. Other minerals include olivine (Fo₆₂), orthopyroxene (En₆₉Fs₃₀Wo₁), albite (An₈Ab₉₀Or₂), magnetite, pyrrhotite, pentlandite, and apatite. Ferromagnesian minerals are rich in Fe³⁺, as determined by Mössbauer spectrometry and electron microprobe chemical analyses. Fe³⁺/Fe_tot values are olivine ?5%, amphibole 80%, phlogopite 65%, and magnetite 42%. Mineral compositions are nearly constant across grains and the section, except for a small variability in amphibole compositions reflecting the edenite exchange couple (^ANa + ^IVAl ↔ ^A□ + Si). These mineral compositions, the absence of Fe-Ni metal, and the oxygen isotope data support its classification as an R (Rumuruti) chondrite. LAP 04840 is classified as petrologic grade 5, based on the chemical homogeneity of its minerals, and the presence of distinctly marked chondrules and chondrule fragments in a fine-grained crystalline matrix. The mineral assemblage of LAP 04840 allows calculation of physical and chemical conditions at the peak of its metamorphism: T = 670 ± 60 °C from a amphibole-plagioclase thermometer; P_H2O between 250 and 500 bars as constrained by the assemblage phlogopite + orthopyroxene + olivine + feldspar and the absence of diopside; P_CO2 unconstrained; f_O2 at QFM + 0.5 log units; . The hydrogen in LAP 04840 is very heavy, an average δD value of +3660 ± 75‰ in the magnesiohornblende. Only a few known sources of hydrogen have such high δD and are suitable sources for LAP 04840: ordinary chondrite phyllosilicates (as in the Semarkona chondrite), and insoluble organic matter (IOM) in ordinary chondrites and CR chondrites. Hydrogen from the IOM could have been released by oxidation, and then reacted with an anhydrous R chondrite (at high temperature), but it is not clear whether this scenario is correct.     

Vanadium K edge XANES of synthetic and natural basaltic glasses and application to microscale oxygen barometry

A new microscale oxybarometer for solar system basaltic glasses, based on vanadium K-edge X-ray absorption near edge structure (XANES) spectroscopy, is described. Vanadium is unique among abundant elements in siliceous materials in that it can potentially occur in nature in four valence states: V²⁺, V³⁺, V⁴⁺ and V⁵⁺. Consequently, the vanadium redox system is a robust oxybarometer covering at least six orders of magnitude in buffer-relative oxygen fugacity. The method was calibrated using synthetic glass standards produced under known fO₂ and temperature conditions. Correction for temperature differences among standards and unknowns was quantified using microXANES data for isobaric synthetic glass couples. Application of the method to lunar, martian, and terrestrial glasses yielded fO₂ estimates from 1.6 log units more reduced than the iron-wüstite (IW) buffer (IW-1.6) for lunar glasses, to IW + 4.0 for terrestrial glass inclusions. The martian and terrestrial results are in good agreement with previous estimates by other methods. The inferred fO₂ values for lunar pyroclastic glasses are ∼0.5 log unit more reduced than previous estimates, but the differences are comparable to analytical uncertainties. Micro-extended X-ray absorption fine structure spectra were consistent with the valence states determined by microXANES and provided additional constraints on vanadium site geometry. These results demonstrate the value of this new oxybarometer, which can be applied nondestructively to individual grains in conventional thin sections with ∼ micrometer resolution and ∼100 ppm elemental sensitivity.     

Local environment and valence state of iron in microinclusions in fibrous diamonds: X-ray absorption and Mössbauer data

Iron valence state and local environment in a set of fibrous diamonds from Brazilian and Congolese placers were investigated using X-ray absorption and Mössbauer spectroscopies. It is shown that the diamonds could be divided into two main groups differing in the type of dominant Fe-bearing inclusions. In the first group Fe is mostly trivalent and is present in octahedral coordination; diamonds from the second group contain a mixture of Fe²⁺ and Fe³⁺, most likely with Fe²⁺ in dodecahedral coordination. A few other diamonds contain iron in a more reduced state: The presence of metallic Fe and Fe₃O₄ is inferred from XAS measurements. Spatially resolved XANES and Mössbauer measurements on polished diamond plates show that in some cases the Fe valence state may change considerably between the core and rim, whereas in other cases Fe speciation and valence remain constant. It is shown that Fe valence does not correlate with water and/or carbonate content or ratio, suggesting that iron is a minor element in the growth medium of fibrous diamonds and plays a passive role. This study suggests that, when present, evolution of the C isotopic composition with diamond growth is largely due to changes in chemistry of the growth medium and not due to variations of f_O2.     

Oxygen isotope systematics of chondrules in the Allende CV3 chondrite: High precision ion microprobe studies

The oxygen three-isotope systematics of 36 chondrules from the Allende CV3 chondrite are reported using high precision secondary ion mass spectrometer (CAMECA IMS-1280). Twenty-six chondrules have shown internally homogenous Δ¹⁷O values among olivine, pyroxene, and spinel within a single chondrule. The average Δ¹⁷O values of 19 FeO-poor chondrules (13 porphyritic chondrules, 2 barred olivine chondrules, and 4 chondrule fragments) show a peak at −5.3 ± 0.6‰ (2SD). Another 5 porphyritic chondrules including both FeO-poor and FeO-rich ones show average Δ¹⁷O values between −3‰ and −2‰, and 2 other FeO-poor barred olivine chondrules show average Δ¹⁷O values of −3.6‰ and 0‰. These results are similar to those for Acfer 094 chondrules, showing bimodal Δ¹⁷O values at −5‰ and −2‰. Nine porphyritic chondrules contain olivine grains with heterogeneous Δ¹⁷O values as low as −18‰, indicating that they are relict olivine grains and some of them were derived from precursors related to refractory inclusions. However, most relict olivine grains show oxygen isotope ratios that overlap with those in homogeneous chondrules. The Δ¹⁷O values of four barred olivine chondrules range from −5‰ to 0‰, indicating that not all BO chondrules plot near the terrestrial fractionation line as suggested by previous bulk chondrule analyses. Based on these data, we suggest the presence of multiple oxygen isotope reservoirs in local dust-rich protoplanetary disk, from which the CV3 parent asteroid formed.A compilation of 225 olivine and low-Ca pyroxene isotopic data from 36 chondrules analyzed in the present study lie between carbonaceous chondrite anhydrous mineral (CCAM) and Young and Russell lines. These data define a correlation line of δ¹⁷O = (0.982 ± 0.019) × δ¹⁸O − (2.91 ± 0.10), which is similar to those defined by chondrules in CV3 chondrites and Acfer 094 in previous studies. Plagioclase analyses in two chondrules plot slightly below the CCAM line with Δ¹⁷O values of −2.6‰, which might be the result of oxygen isotope exchange between chondrule mesostasis and aqueous fluid in the CV parent body.     

Seasonal δS variations in two high elevation snow pits measured by S-S double spike thermal ionization mass spectrometry

δ³⁴S and sulfate concentrations were determined in snow pit samples using a thermal ionization mass spectrometric technique capable of 0.2‰ accuracy and requires ≈5 μg (0.16 μmol) natural S. The technique utilizes a ³³S-³⁶S double spike for instrumental mass fractionation correction, and has been applied to snow pit samples collected from the Inilchek Glacier, Kyrgyzstan and from Summit, Greenland. These δ³⁴S determinations provide the first high-resolution seasonal data for these sites, and are used to estimate seasonal sulfate sources. Deuterium (δD) and oxygen (δ¹⁸O) isotope data show that the Inilchek and Summit snow pit samples represent precipitation over ≈20 months.The δ³⁴S values for the Inilchek ranged from +2.6 ± 0.4‰ to +7.6 ± 0.4‰ on sample sizes ranging from 0.3 to 1.8 μmol S. δ³⁴S values for Greenland ranged from +3.6 ± 0.7‰ to +13.3 ± 5‰ for sample sizes ranging from 0.05 to 0.29 μmol S. The concentration ranged from 92.6 ± 0.4 to 1049 ± 4 ng/g for the Inilchek and 18 ± 9 to 93 ± 6 ng/g for the Greenland snow pit. Anthropogenic sulfate dominates throughout the sampled time interval for both sites based on mass balance considerations. Additionally, both sites exhibit a seasonal signature in both δ³⁴S and concentration. The thermal ionization mass spectrometric technique has three advantages compared to gas source isotopic methods: (1) sample size requirements of this technique are 10-fold less permitting access to the higher resolution S isotope record of low concentration snow and ice, (2) the double spike technique permits δ³⁴S and S concentration to be determined simultaneously, and (3) the double spike is an internal standard.     

An experimental study of the solubility and speciation of neodymium (III) fluoride in F-bearing aqueous solutions

The solubility of neodymium (III) fluoride was investigated at temperatures of 150, 200 and 250 °C, saturated water vapor pressure, and a total fluoride concentration (HF°_aq + F⁻) ranging from 2.0 × 10⁻³ to 0.23 mol/l. The results of the experiments show that Nd³⁺ and NdF²⁺ are the dominant species in solution at the temperatures investigated and were used to derive formation constants for NdF²⁺ and a solubility product for NdF₃. The solubility product of NdF₃(logK_sp=loga_Nd³⁺+3loga_F^-) is −24.4 ± 0.2, −22.8 ± 0.1, and −21.5 ± 0.2 at 250, 200 and 150 °C, respectively, and the formation constant of NdF²⁺(logβ=loga_NdF²⁺-loga_Nd³⁺-loga_F^-) is 6.8 ± 0.1, 6.2 ± 0.1, and 5.5 ± 0.2 at 250, 200 and 150 °C, respectively. The results of this study show that published theoretical predictions significantly overestimate the stability of NdF²⁺ and the solubility of NdF₃.The potential impact of the results on natural systems was evaluated for a hypothetical fluid with a composition similar to that responsible for REE mineralization in the Capitan pluton, New Mexico. In contrast to results obtained using the theoretical predictions of Haas [Haas J. R., Shock E. L., and Sassani D. C. (1995) Rare earth elements in hydrothermal systems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta59, 4329-4350.], which indicate that NdF²⁺ is the dominant species in solution, calculations employing the data presented in this paper and previously published experimental data for chloride and sulfate species [Migdisov A. A., and Williams-Jones A. E. (2002) A spectrophotometric study of neodymium(III) complexation in chloride solutions. Geochim. Cosmochim. Acta66, 4311-4323; Migdisov A. A., Reukov V. V., and Williams-Jones A. E. (2006) A spectrophotometric study of neodymium(III) complexation in sulfate solutions at elevated temperatures. Geochim. Cosmochim. Acta70, 983-992.] show that neodymium chloride species predominate and that neodymium fluoride species are relatively unimportant. This suggests that accepted models for REE deposits that invoke fluoride complexation as the method of hydrothermal REE transport may need to be re-evaluated.     

The preparation and characterization of vanadium-substituted goethite: The importance of temperature

Despite a close geo-chemical association between vanadium (V) and iron (Fe) in natural environments, there is little research on the substitution of V in goethite. To assess the effect of temperature on V-substitution in goethite, a series of V-substituted goethite were prepared under varying synthesis temperatures, and analysed using wet chemical and multi-spectroscopic techniques. Vanadium substitution was inversely related to synthesis temperatures and was hindered by the oxidation of V³⁺ to V^4+/5+ as indicated by X-ray absorption near-edge spectroscopy. The presence of V (V⁵⁺ > V⁴⁺ > V³⁺) at high temperature hindered the nucleation of goethite and crystal growth along particular faces resulting in large-sized and twinned crystals as shown by transmission electron microscopy. The large-sized goethite crystals released more Fe (mmoles) per unit surface area during proton-promoted dissolution than the smaller-sized crystals, which could be due to distorted V^4+/5+ local coordination environments in the mineral structure. The dissolution studies showed a heterogeneous distribution of V and/or crystal defects in goethite crystals. The results show that low synthesis temperatures preserved the oxidation state of V³⁺, which has ionic radius and hydrolytic properties similar to Fe³⁺, and hence resulted in as much as 13.3 mol per cent substitution. The structural stability of the goethite decreased upon V-substitution in order; V³⁺ > V⁴⁺ > V⁵⁺. This research provides important information about the interaction between temperature, V incorporation, and crystal structure properties of goethite for V sequestration and other potentially toxic metal cations.     

POC and particulate Th export fluxes estimated using Th/U disequilibrium in an enclosed Eastern Mediterranean region (Saronikos Gulf and Elefsis Bay, Greece) in seasonal scale

Activity concentrations of the naturally occurring, short-lived and highly particle-reactive radionuclide tracer ²³⁴Th in the dissolved and particulate phase were determined at 7 shallow-water stations (maximum depths: 30 (S.1 and S.2), 65 (S.3), 97 (S.5), 105 (S.6) and 220 m (S.4 and S.7) in Saronikos Gulf and Elefsis Bay (central Aegean Sea, Greece) during 3 seasonal cruises (summer 2008, autumn 2008 and winter 2009) to assess the time scales of the dynamics and the depositional fate of particulate matter (POC, particulate ²³⁴Th). For that reason, in situ filtrating systems were deployed in several depths of the water column consisting of GF/A disc prefilters to scavenge particulate fraction of ²³⁴Th and organic carbon and impregnated cartridges to adsorb dissolved ²³⁴Th.The obtained data showed average particulate ²³⁴Th activity concentrations of 3.7 ± 0.4 Bq m⁻³ in summer, 2.1 ± 0.2 Bq m⁻³ in autumn and 2.4 ± 0.2 Bq m⁻³ in winter. The respective average dissolved ²³⁴Th activity concentrations were 30.1 ± 2.8 Bq m⁻³ in summer, 30.2 ± 2.9 Bq m⁻³ in autumn and 27.4 ± 3.0 Bq m⁻³ in winter. The activity ratios of total ²³⁴Th and its long-lived conservative parent ²³⁸U were below unity in most of the stations indicating radioactive disequilibrium throughout the water column, thus very dynamic trace-metal scavenging and particle export from the water column. These profiles (²³⁴Th and ²³⁸U) were used to estimate the export fluxes and scavenging rates of ²³⁴Th, as well as their residence times in the water column. The average cumulative export fluxes of particulate ²³⁴Th were estimated to be 33 ± 4 Bq m⁻² d⁻¹ in summer, 35 ± 5 Bq m⁻² d⁻¹ in autumn and 45 ± 6 Bq m⁻² d⁻¹ in winter, whereas the respective average cumulative scavenging rates of dissolved ²³⁴Th were 39 ± 5, 33 ± 5 and 50 ± 7 Bq m⁻² d⁻¹. Moreover, the cumulative average residence times of ²³⁴Th were 25 ± 4 d in summer, 45 ± 6 d in autumn and 64 ± 7 d in winter 2009 for the dissolved fraction and 4 ± 1, 3 ± 1 and 4 ± 1 d for the particulate one, respectively.POC/ ratio profiles decreased versus depth showing a variety of marine processes, such as loss of POC due to dissolution after biological activity, impact of minerals in particle sinking and microbial remineralization. Average cumulative export fluxes of POC were 162 ± 18 mmol m⁻² d⁻¹ in summer, 107 ± 19 mmol m⁻² d⁻¹ in autumn and 157 ± 25 mmol m⁻² d⁻¹ in winter 2009. The seasonal data of POC fluxes certified the existence of phytoplankton bloom in winter for Saronikos Gulf. In addition, after evaluating the maxima of POC fluxes in Elefsis Bay (a small embayment in northern Saronikos Gulf) during summer, potential bloom of phytoplankton also concluded; this approach is in agreement with previous data of the same area. Finally, the elevated POC concentrations and fluxes in the region certify that the Gulf is still one of the most organic polluted in the Mediterranean Sea.     

Adsorption of Rb and Sr at the orthoclase (0 0 1)-solution interface

Adsorption of Rb⁺ and Sr²⁺ at the orthoclase (0 0 1)-solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb⁺ adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K⁺ site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å²). These results are consistent with an ion-exchange reaction in which Rb⁺ occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO₃)₂ solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr²⁺ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr²⁺ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb⁺, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr²⁺ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr²⁺ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr²⁺ on orthoclase. Differences in the partitioning of Sr²⁺ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson-Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge.     

The morphology and chemistry of K-feldspar megacrysts from Ikizdere Pluton: evidence for acid and basic magma interactions in granitoid rocks, NE-Turkey

Ikizdere Pluton consists of granite, granodiorite, tonalite, monzonite, quartz monzonite containing pinkish colored K-feldspar megacrysts (KFMs). The crystal sizes of the KFMs range from 1 to 4 cm. The lath-shaped megacrysts are uniformly (i.e., randomly) distributed in the host plutonic rocks and have mafic and felsic inclusions whose crystal sizes are smaller than 1 mm. The crystal inclusions are biotite, slightly annitic in composition with X_Mg[=Fe_tot/(Fe_tot+Mg)]=0.50-0.58, amphibole (magnesio-hornblende, X_Mg[=Mg/(Mg+Fe_tot)]=0.70-0.79), iron-titanium oxide (low titanium magnetit and ilmenite), plagioclase (Ab₇₅₋₂₅An₆₅₋₃₅) and as minor quartz. The compositions of the KFMs range from Or₉₅Ab₅An₀ to Or₈₂Ab₁₇An₁. BaO contents of the megacrysts increase from core to rim. The mafic and felsic inclusions are compositionally similar those of the host rocks.The chemical and textural features of K-feldspar are typical for megacrysts that grew as phenocrysts in dynamic granitoidic magma systems. The overgrowth of KFMs and mafic magma injections (magma mixing) may be related to temperature, pressure and compositional fluctuations in the magma chamber. Remnant of earlier formed K-feldspar crystals remain in the felsic magma system, while the mafic injection can decompose some earlier precipitated KFMs. The remnant of K-feldspar remaining after mafic injection are overgrown by rapid diffusion of Ba, K and Na elements in liquid phase, during the later stages of crystallization of the host magma.