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NASA's Genesis Mission returned solar wind (SW) to the Earth for analysis to derive the composition of the solar photosphere from solar material. SW analyses control the precision of the derived solar compositions, but their ultimate accuracy is limited by the theoretical or empirical models of fractionation due to SW formation. Mg isotopes are “ground truth” for these models since, except for CAIs, planetary materials have a uniform Mg isotopic composition (within ≤1‰) so any significant isotopic fractionation of SW Mg is primarily that of SW formation and subsequent acceleration through the corona. This study analyzed Mg isotopes in a bulk SW diamond‐like carbon (DLC) film on silicon collector returned by the Genesis Mission. A novel data reduction technique was required to account for variable ion yield and instrumental mass fractionation (IMF) in the DLC. The resulting SW Mg fractionation relative to the DSM‐3 laboratory standard was (−14.4‰, −30.2‰) ± (4.1‰, 5.5‰), where the uncertainty is 2ơ SE of the data combined with a 2.5‰ (total) error in the IMF determination. Two of the SW fractionation models considered generally agreed with our data. Their possible ramifications are discussed for O isotopes based on the CAI nebular composition of McKeegan et al. (2011).

Solar abundances are important to planetary science since the prevalent model assumes that the composition of the solar photosphere is that of the solar nebula from which planetary materials formed. Thus, solar abundances are a baseline for planetary science. Previously, solar abundances have only been available through spectroscopy or by proxy (CI). The Genesis spacecraft collected and returned samples of the solar wind for laboratory analyses. Elemental and isotopic abundances in solar wind from Genesis samples have been successfully measured despite the crash of the re‐entry capsule. Here we present science rationales for a set of 12 important (and feasible postcrash) Science and Measurement Objectives as goals for the future (Table 1). We also review progress in Genesis sample analyses since the last major review (Burnett 2013). Considerable progress has been made toward understanding elemental fractionation during the extraction of the solar wind from the photosphere, a necessary step in determining true solar abundances from solar wind composition. The suitability of Genesis collectors for specific analyses is also assessed. Thus far, the prevalent model remains viable despite large isotopic variations in a number of volatile elements, but its validity and limitations can be further checked by several Objectives.

We compare element and isotopic fractionations measured in solar wind samples collected by NASA's Genesis mission with those predicted from models incorporating both the ponderomotive force in the chromosphere and conservation of the first adiabatic invariant in the low corona. Generally good agreement is found, suggesting that these factors are consistent with the process of solar wind fractionation. Based on bulk wind measurements, we also consider in more detail the isotopic and elemental abundances of O. We find mild support for an O abundance in the range 8.75 - 8.83, with a value as low as 8.69 disfavored. A stronger conclusion must await solar wind regime specific measurements from the Genesis samples., Comment: 6 pages, accepted by Astrophysical Journal Letters

An amorphous diamond-like carbon film deposited on silicon made at Sandia National Laboratory by pulsed laser deposition was one of several solar wind (SW) collectors used by the Genesis Mission (NASA Discovery Class Mission #5). The film was ~1 μm thick, amorphous, anhydrous, and had a high ratio of sp 3–sp 2 bonds (>50%). For 27 months of exposure to space at the first Lagrange point, the collectors were passively irradiated with SW (H fluence ~2 × 1016 ions cm−2; He fluence ~8 × 1014 ions cm−2). The radiation damage caused by the implanted H ions peaked at 12–14 nm below the surface of the film and that of He about 20–23 nm. To enable quantitative measurement of the SW fluences by secondary ion mass spectroscopy, minor isotopes of Mg (25Mg and 26Mg) were commercially implanted into flight-spare collectors at 75 keV and a fluence of 1 × 1014 ions cm−2. The shapes of analytical depth profiles, the rate at which the profiles were sputtered by a given beam current, and the intensity of ion yields are used to characterize the structure of the material in small areas (~200 × 200 ± 50 μm). Data were consistent with the hypothesis that minor structural changes in the film were induced by SW exposure. Electronic supplementary material The online version of this article (doi:10.1007/s10853-017-1267-3) contains supplementary material, which is available to authorized users.

Ti valence measurements in MgAl_2O_4 spinel from calcium-aluminum-rich inclusions (CAIs) by X-ray absorption near-edge structure (XANES) spectroscopy show that many spinels have predominantly tetravalent Ti, regardless of host phases. The average spinel in Allende type B1 inclusion TS34 has 87% Ti^(+4). Most spinels in fluffy type A (FTA) inclusions also have high Ti valence. In contrast, the rims of some spinels in TS34 and spinel grain cores in two Vigarano type B inclusions have larger amounts of trivalent titanium. Spinels from TS34 have approximately equal amounts of divalent and trivalent vanadium. Based on experiments conducted on CAI-like compositions over a range of redox conditions, both clinopyroxene and spinel should be Ti^(+3)-rich if they equilibrated with CAI liquids under near-solar oxygen fugacities. In igneous inclusions, the seeming paradox of high-valence spinels coexisting with low-valence clinopyroxene can be explained either by transient oxidizing conditions accompanying low-pressure evaporation or by equilibration of spinel with relict Ti^(+4)-rich phases (e.g., perovskite) prior to or during melting. Ion probe analyses of large spinel grains in TS34 show that they are enriched in heavy Mg, with an average Δ^(25)Mg of 4.25 ± 0.028‰, consistent with formation of the spinel from an evaporating liquid. Δ^(25)Mg shows small, but significant, variation, both within individual spinels and between spinel and adjacent melilite hosts. The Δ^(25)Mg data are most simply explained by the low-pressure evaporation model, but this model has difficulty explaining the high Ti^(+4) concentrations in spinel.

All planetary materials sampled thus far vary in their relative abundance of the major isotope of oxygen, ^(16)O, such that it has not been possible to define a primordial solar system composition. We measured the oxygen isotopic composition of solar wind captured and returned to Earth by NASA’s Genesis mission. Our results demonstrate that the Sun is highly enriched in ^(16)O relative to the Earth, Moon, Mars, and bulk meteorites. Because the solar photosphere preserves the average isotopic composition of the solar system for elements heavier than lithium, we conclude that essentially all rocky materials in the inner solar system were enriched in ^(17)O and ^(18)O, relative to ^(16)O, by ~7%, probably via non–mass-dependent chemistry before accretion of the first planetesimals.

We used focused ion beam thin section preparation and scanning transmission electron microscopy (FIB/STEM) to examine the interfacial region between spinel and host melilite for spinel grains in type B1 inclusions from the Allende and Leoville carbonaceous chondrites. Boundary clinopyroxenes decorating spinel surfaces have compositions similar to those of coarser clinopyroxenes from the same region of the inclusion, suggesting little movement after formation. Host melilite displays no anomalous compositions near the interface and late-stage minerals are not observed, suggesting that boundary pyroxenes did not form by crystallization of residual liquid. Allende spinels display either direct spinel-melilite contact or an intervening boundary clinopyroxene between the two phases. Spinel-melilite interfacial regions in a Leoville B1 are more complex, with boundary clinopyroxene, as observed in Allende, but also variable amounts of glass, secondary calcite, perovskite, and an Mg-, Al-, OH-rich and Ca-, Si-poor crystalline phase that may be a layered double hydrate. One possible scenario of formation for the glass veins is that open system alteration of melilite produced a porous, hydrated aggregate of Mg-carpholite or sudoite + aluminous diopside that was shock melted and quenched to a glass. The hydrated crystalline phase we observed may have been a shocked remnant of the precursor phase assemblage, but is more likely to have formed later by alteration of the glass. In the mantle, boundary clinopyroxenes may have been crystallized from Ti-rich liquids formed by the direct dissolution of perovskite and an associated Sc-Zr-rich phase or as a reaction product between dissolving perovskite and liquid. In the core, any perovskite and associated Ti-enriched liquids that may have originally been present disappeared before the growth of boundary clinopyroxene, and the observed boundary clinopyroxene may have nucleated and grown from the liquid, along with the larger core clinopyroxene.

The Genesis mission returned samples of solar wind to Earth in September 2004 for ground-based analyses of solar-wind composition, particularly for isotope ratios. Substrates, consisting mostly of high-purity semiconductor materials, were exposed to the solar wind at L1 from December 2001 to April 2004. In addition to a bulk sample of the solar wind, separate samples of coronal hole (CH), interstream (IS), and coronal mass ejection material were obtained. Although many substrates were broken upon landing due to the failure to deploy the parachute, a number of results have been obtained, and most of the primary science objectives will likely be met. These objectives include He, Ne, Ar, Kr, and Xe isotope ratios in the bulk solar wind and in different solar-wind regimes, and 15N/14N and 18O/17O/16O to high precision. The greatest successes to date have been with the noble gases. Light noble gases from bulk solar wind and separate solar-wind regime samples have now been analyzed. Helium results show clear evidence of isotopic fractionation between CH and IS samples, consistent with simplistic Coulomb drag theory predictions of fractionation between the photosphere and different solar-wind regimes, though fractionation by wave heating is also a possible explanation. Neon results from closed system stepped etching of bulk metallic glass have revealed the nature of isotopic fractionation as a function of depth, which in lunar samples have for years deceptively suggested the presence of an additional, energetic component in solar wind trapped in lunar grains and meteorites. Isotope ratios of the heavy noble gases, nitrogen, and oxygen are in the process of being measured.

Analysis of the Genesis samples is underway. Preliminary elemental abundances based on Genesis sample analyses are in good agreement with in situ-measured elemental abundances made by ACE/SWICS during the Genesis collection period. Comparison of these abundances with those of earlier solar cycles indicates that the solar wind composition is relatively stable between cycles for a given type of flow. ACE/SWICS measurements for the Genesis collection period also show a continuum in compositional variation as a function of velocity for the quasi-stationary flow that defies the simple binning of samples into their sources of coronal hole (CH) and interstream (IS).

We obtained two-dimensional concentration maps for the minor elements Fe and V in 21 spinel crystals in the Allende type B1 inclusion TS-34 with a 4–5 μm resolution. Locally high concentrations of Fe occur along at least one edge of the spinels and decrease toward the center of the grains. Enrichment in V can also occur along edges or at corners. In general, there is no overall correlation of the Fe and V distributions, but in local regions of two grains, the V and Fe distributions are correlated, strongly suggesting a local source for both elements. In these two grains, opaque assemblages are present that appear to locally control the V distributions. This, coupled with previous work, suggests that prior to alteration, TS-34 contained V-rich metal. Oxidation of this metal during alteration can account for the edge/corner V enrichments, but provide only minor FeO contributions, explaining the overall lack of correlation between Fe and V. Most of the FeO appears to have been externally introduced along spinel boundaries during alteration. These alteration phases served as sources for diffusion of FeO into spinel. FeO distributions in spinel lead to a mean attenuation length of ~8 μm and, using literature diffusion coefficients in isothermal and exponential cooling approximations for peak temperatures in the range 600–700°C, this leads to a time scale for calcium-aluminum-rich inclusion (CAI) alteration in the range of decades to centuries.

We report data from a series of dynamic crystallization experiments that focus on determining the partition coefficients (D’s) for V and Ti in the spinel + liquid system of an average type B1 CAI bulk composition for three different fO_2 conditions. Partitioning data for Ca and Si are also obtained. We show that the D’s for V and Ti are fO_2 dependent with D_(Ti) decreasing at low oxygen fugacity due to the presence of Ti^(3+). D_V is essentially 0 in air, rises to 2.2 at the Fe-FeO buffer and drops to 1.4 at the C-CO buffer. This indicates that V^(3+) is highly compatible in spinel and that higher and lower valence states are much less compatible. We also report data from isothermal experiments that determine diffusion times for V and Ti in same system at a temperature close to the T_(max) for type B1 CAIs. Diffusion of these elements between spinel and liquid is surprisingly rapid, with essentially total equilibration of Ti and V between spinel and liquid in 90 h run duration. Lack of equilibration of Cr, Si, and Ca shows that the Ti and V equilibration mechanism was diffusion and not dissolution and reprecipitation. Our experimental run durations set an upper limit of a few tens of hours on the time that type B1 CAIs were at their maximum temperature. Based on our data we argue that subsolidus reequilibration between spinel inclusion and host-silicate phases within type B CAIs likely explains the observed range of V and Ti concentrations in spinels which are inclusions in clinopyroxene.

Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’), with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection.

We present detailed simulations of the optical spectra emitted from an Argon plasma whose conditions correspond to those thought to prevail within sonoluminescing bubbles. The model incorporates detailed atomic physics based on atomic data from the Opacity Project database, and includes bound-bound, bound-free, and free-free transitions. Line broadening is treated by use of the modified semi-empirical method. The spectral model is used as a postprocessor of hydrodynamic simulations. We find good agreement with experimental spectra and accurately reproduce experimental pulse widths. We also predict that whilst the majority of the optical emission corresponds to bound-free transitions, there remains the possibility of observing line emission in both the UV and IR regions of the spectrum. © 2001 Elsevier Science Ltd. All rights reserved.

The increase in the XUV mass absorption coefficient of liquid aluminium, produced by high-power-laser shock-compression, is measured using XUV laser radiography. At a photon energy of 63 eV a change in the mass absorption coefficient by up to a factor of ~2.2 is determined at densities close to twice that of solid and electron temperatures of the order of 1 eV. Comparison with hydrodynamic simulations indicate that the absorption coefficient scales with density as ρ1.3±0.2.

The Genesis Discovery mission returned solar matter in the form of the solar wind with the goal of obtaining precise solar isotopic abundances (for the first time) and greatly improved elemental abundances. Measurements of the light noble gases in regime samples demonstrate that isotopes are fractionated in the solar wind relative to the solar photosphere. Theory is required for correction. Measurement of the solar wind O and N isotopes shows that these are very different from any inner solar system materials. The solar O isotopic composition is consistent with photochemical self-shielding. For unknown reasons, the solar N isotopic composition is much lighter than essentially all other known solar system materials, except the atmosphere of Jupiter. Ne depth profiling on Genesis materials has demonstrated that Ne isotopic variations in lunar samples are due to isotopic fractionation during implantation without appealing to higher energy solar particles. Genesis provides a precise measurement of the isotopic differences of Ar between the solar wind and the terrestrial atmosphere. The Genesis isotopic compositions of Kr and Xe agree with data from lunar ilmenite separates, showing that lunar processes have not affected the ilmenite data and that solar wind composition has not changed on 100 Ma time scales. Relative to Genesis solar wind, ArKrXe in Q (the chondrite noble gas carrier) and the terrestrial atmosphere show relatively large light isotope depletions.

The outer layers of the Sun are thought to preserve the average isotopic and chemical composition of the solar system. The solar O-isotopic composition is essentially unmeasured, though models based on variations in meteoritic materials yield several predictions. These predictions are reviewed and possible variations on these predictions are explored. In particular, the two-component mixing model of Clayton and Mayeda (1984) (slightly revised here) predicts solar compositions to lie along an extension of the calcium-aluminum-rich inclusion (CAI) ^(16)O line between (δ^(18)O, δ^(17)O) = (16.4, 11.4)%0 and (12.3, 7.5)%0. Consideration of data from ordinary chondrites suggests that the range of predicted solar composition should extend to slightly lower δ^(18)O values. The predicted solar composition is critically sensitive to the solid/gas ratio in the meteorite-forming region, which is often considered to be significantly enriched over solar composition. A factor of two solid/gas enrichment raises the predicted solar (δ^(18)O, δ^(17)O) values along an extension of the CAI ^(16)O line to (33, 28)%0. The model is also sensitive to the nebular O gas phase. If conversion of most of the gaseous O from CO to H_2O occurred at relatively low temperatures and was incomplete at the time of CM aqueous alteration, the predicted nebular gas composition (and hence the solar composition) would be isotopically heavier along a slope 1/2 line. The likelihood of having a single solid nebular O component is discussed. A distribution of initial solid compositions along the CAI ^(16)O line (rather than simply as an end-member) would not significantly change the predictions above in at least one scenario. Even considering these variations within the mixing model, the predicted range of solar compositions is distinct from that expected if the meteoritic variations are due to non-mass-dependent fractionation. Thus, a measurement of the solar O composition to a precision of several permil would clearly distinguish between these theories and should clarify a number of other important issues regarding solar system formation.

The composition of the sodium-bearing molecular ion cloud in the vicinity of Io may yield clues to Io's geochemistry. Likely Na-bearing source minerals are Na-sulfides and Na_2SO_4, with the relative composition depending on the distribution of thermal environments in the upper crust. These materials will be sputtered by co-rotating torus ions either from the surface or from atmospheric aerosols. Using laser postionization, we investigated the ion-sputtered neutral products of Na_2SO_4 to determine whether NaO in the Io torus might be a diagnostic indicator for sodium sulfate on Io's surface or in aerosols. With an ArF excimer laser (λ = 6.4 eV), single-photon saturation of the ionization step was achieved for NaS, Na_2O, and the combination Na_2S + Na_2O_2 at mass 78, while other species, including NaO, were close to saturation. Photofragmentation during the ionization step was minimal by all indications. The results predict sputtering ratios NaO/NaS > 8, Na_2O/NaS ∼8, Na_2/NaS ≥ 6, and Na/NaS ≥ 100. Multiphoton ionization was also attempted using a frequency-doubled (3.2-eV) Ti:sapphire femtosecond laser at high intensities, but saturation was not achieved for the major species, and there was evidence of photofragmentation. Velocity distributions of the sputter products show for the first time that significant fractions of sodium-bearing molecules can be sputtered into unbound trajectories from Io's surface, with escape fractions > 50% in the case of NaO. Combining relative sputter yields and velocity distributions gives escape ratios of NaO/NaS > 10, Na_2/NaS ∼ 6, and Na_2O/NaS ∼2.8 from Na_2SO_4. While absolute sputtering and escape ratios need to be compared for Na-sulfides versus Na_2SO_4, this work shows that, in the absence of significant modification by atmospheric chemical reactions, measurement of the neutral NaO/NaS ratio in the vicinity of Io could identify the major Na-bearing minerals.

A Na-S mineral on the surface of Io is required to be the source of the famous atomic cloud. SO_2 is a confirmed atmospheric and surface constituent, and because of the rapid volcanic resurfacing rate, the SO_2 is buried within the crust, where at least occasionally, over many cycles of burial and eruption, it must contact silicate materials at midlevel crustal temperatures. Surface interaction experiments were performed for a wide variety of silicate compositions showing that interaction products of these with SO_2 could be observed at 1123 K on laboratory timescales, even in the absence of external redox agents. Not all experiments produced deposits that could be studied by scanning electron microscopy; some required the greater sensitivity of photoelectron spectroscopy (XPS). Characterization of the alteration products by XPS showed that both oxidized and reduced sulfur species were formed, indicating that a disproportionation mechanism producing a sulfate and a reduced S species although smaller amounts of interaction leading to Na_2SO_3 formation cannot be ruled out. The reduced sulfur species is best explained as elemental S which was independently documented for two compositions. Scanning electron microscopy studies for those compositions where reaction was extensive enough to be observed showed (1) Na_2SO_4 for a soda-lime composition, (2) a mixed Na-Ca-sulfate liquid and CaSO_4 for AbAnDi and a chondrule glass composition, and (3) Fe-sulfate for a natural obsidian. Infrared spectroscopy for the soda-lime glass composition showed peaks best explained by Na_2SO_4. We conclude that SO_2 disproportionation as well as direct formation from SO_3 under oxidizing conditions can produce Na_2SO_4 by interaction of SO_2 with silicates on Io, but Ca and Fe sulfates may form preferentially in more basaltic compositions. As highly oxidizing conditions may be unlikely for Io, the disproportionation mechanism may be more competitive on Io than it is in laboratory experiments. Very low rates of Na_2SO_4 production are required to supply the Io atomic cloud, so the interaction processes can be very inefficient.

The Genesis mission sampled solar wind ions to document the elemental and isotopic compositions of the Sun and, by inference, of the protosolar nebula. Nitrogen was a key target element because the extent and origin of its isotopic variations in solar system materials remain unknown. Isotopic analysis of a Genesis Solar Wind Concentrator target material shows that implanted solar wind nitrogen has a ^(15)N/^(14)N ratio of 2.18 ± 0.02 × 10^(−3) (that is, ≈40% poorer in ^(15)N relative to terrestrial atmosphere). The ^(15)N/^(14)N ratio of the protosolar nebula was 2.27 ± 0.03 × 10^(−3), which is the lowest ^(15)N/^(14)N ratio known for solar system objects. This result demonstrates the extreme nitrogen isotopic heterogeneity of the nascent solar system and accounts for the ^(15)N-depleted components observed in solar system reservoirs.

Laboratory simulations have been carried out to test the possibility that interactions of SO_2 and silicates can produce Na-S compounds which can account for the observed surface enrichment of Na relative to Si on Io. A wide variety of silicate compositions were heated under oxidizing conditions with a SO_2/O_2 = 1 gas phase at a mid-level crustal temperature for Io (1123K). All experiments produced sulfate deposits on the silicate surfaces. The nature of the sulfate changed systematically with the silicate Ca/Na ratio, with mixtures of CaSO_4 and Na-rich sulfate formed by basaltic compositions having higher Ca/Na but only alkali-rich sulfates formed for more granitic (low Ca/Na) compositions. For crystalline albite and an albite-orthoclase eutectic glass composition, K and Al-rich sulfates were formed. Assuming that burial of SO_2 to the temperatures of our experiments is plausible on Io and that somewhat less oxidizing conditions do not qualitatively change the results, SO_2-rock interactions producing Na-rich sulfates could account for the Io surface enrichment in Na. Observations on minor elements, such as K or Ca, in the atomic cloud or in magnetospheric ion spectra could be used to identify sulfates as Io surface phases and, conceivably, even define Ionian crustal magma types. Many analyses show significant S excesses which are best interpreted as due to the presence of bisulfate (HSO_4.) components, and NaHSO_4 deserves consideration as an Io surface mineral. The possibility of elemental S on Io can be regarded as a totally separate issue from the problem of the surface Na enrichment.

Clinopyroxene-silicate liquid partition coefficients for U and Th have been determined by particle track radiography from 1 atm crystallization experiments at controlled fO_2. Two natural basaltic and one synthetic composition were studied at fO_2 values from the Ni—NiO oxygen buffer to 1 log unit more oxidizing than Fe—FeO (IW+ 1). Over the range of fO_2 values and compositions studied, D_U^(cpx/liq) = 0.0034–0.015,D_(Th)^(cpx/liq) = 0.008–0.036, and D_(Th)/D_U= 3.4–1.1. With decreasing fO_2, D_(Th)/D_U can decrease by up to a factor of 3 for a given composition, primarily from an increase in D_U^(cpx/liq), which we interpret as resulting from an increase in the proportion of tetravalent U in the system with decreasing fO_2. This demonstrates that crystal-liquid U—Th fractionation is fO_2 dependent and that U in terrestrial magmas is not entirely tetravalent. D_(Th)^(cpx/liq) appears to decrease in the two basalts at the lowest fO_2, possibly as a result of changes in composition with fO_2. Our data show the sense of U—Th fractionation by clinopyroxene-liquid partitioning is consistent with previous experimental determinations, in that D_(Th)^(cpx)/D_U^(cpx)> 1 in all cases. This indicates that, during partial melting, the liquid will have a Th/U ratio less than the clinopyroxene in the source. The observed ^(238)—U^(230)Th disequilibrium in MORB requires that the partial melt should have a Th/U ratio greater than the bulk source, and, therefore, cannot result from clinopyroxene-liquid partitioning. Further, the magnitudes of the measured partition coefficients are too small to generate significant U—Th fractionation in either direction. Assuming that clinopyroxene contains the bulk of the U and Th in MORB source, our results indicate that ^(238)U—^(230)Th disequilibrium in MORB may not be caused by partial melting at all.

One aspect of our recent paper (HAGEE et al., 1990) on actinide abundances in ordinary chondrites is estimation of the amount of ^(244)Pu present in these meteorites at the time of their formation. This estimate is based on the quantity of ^(244)Pu spontaneous fission Xe, a component whose resolution from other Xe components present must be based on observed isotopic compositions in the meteorite and assumed compositions for the contributing components. In their comments, MYERS and KURODA (1991) advocate a different approach to component analysis of Xe than the one we used and arrive at correspondingly different estimates of fission Xe and ^(244)Pu abundances.

Crystal-liquid partitioning in Fe-Ti oxides and zircon was studied in partially melted granodiorite blocks ejected during the climactic eruption of Mt. Mazama (Crater Lake), Oregon. The blocks, which contain up to 33% rhyolite glass (75 wt% SiO_2), are interpreted to be portions of the magma chamber walls that were torn off during eruption. The glass is clear and well homogenized for all measured elements except Zr. Results for Fe-Ti oxides give DU^(oxide/liq) ≈ 0.1. Partitioning of Mg, Mn, Al, Si, V, and Cr in Fe-Ti oxides indicates that grains surrounded by glass are moderately well equilibrated with the melt for many of the minor elements, while those that are inclusions in relict plagioclase are not. Uranium and ytterbium inhomogeneities in zircons indicate that the zircons have only partially equilibrated with the melt and that uranium appears to have been diffusing out of the zircons faster than the zircons were dissolving. Minimum U, Y, and P concentrations in zircons give maximum DU^(zrc/liq) = 13,DY^(zrc/liq) = 23, and DP^(zrc/liq) = 1, but these are considerably lower than reported by other workers for U and Y. Based on our measurements and given their low abundances in most rocks, Fe-Ti oxides probably do not play a major role in U-Th fractionation during partial melting. The partial melts were undersaturated with zircon and apatite, but both phases are present in our samples. This demonstrates an actual case of non-equilibrium source retention of accessory phases, which in general could be an important trace-element fractionation mechanism. Our results do not support the hypothesis that liquid structure is the dominant factor controlling trace-element partitioning in high-silica rhyolites. Rough calculations based on Zr gradients in the glass indicate that the samples could have been partially molten for 800 to 8000 years.

Krypton is the best candidate for determining limits on solid/gas fractionation in the early sun because of the smoothness of the odd-mass abundance curve in its mass region, which permits relatively precise interpolations of its abundance assuming no fractionation. Here we calculate the solar-system Kr abundance from solar-wind noble-gas ratios, determined previously by low-temperature oxidations of lunar ilmenite grains, normalized to Si by spacecraft solar-wind measurements. The estimated ^(83)Kr abundance of 4.1 ± 1.5 per 10^6 Si atoms is within uncertainty of estimates assuming no fractionation, determined from CI-chondrite abundances of surrounding elements. This is significant because it is the first such constraint on solid/gas fractionation, though the large uncertainty only confines it to somewhat less than a factor of two.

Measurements of ^(244)Pu fission Xe, U, Th, and light REE (LREE) abundances, along with modal petrographic determinations of phosphate abundances, were carried out on equilibrated ordinary chondrites in order to define better the solar system Pu abundance and to determine the degree of variation of actinide and LREE abundances. Our data permit comparison of the directly measured Pu/U ratio with that determined indirectly as (Pu/Nd) × (Nd/U) assuming that Pu behaves chemically as a LREE. Except for Guarena, and perhaps H chondrites in general, Pu concentrations are similar to that determined previously for St. Severin, although less precise because of higher trapped Xe contents. Trapped ^(130Xe)/_(136Xe) ratios appear to vary from meteorite to meteorite, but, relative to AVCC, all are similar in the sense of having less of the interstellar heavy Xe found in carbonaceous chondrite acid residues. The Pu/U and Pu/Nd ratios are consistent with previous data for St. Severin, but both tend to be slightly higher than those inferred from previous data on Angra dos Reis. Although significant variations exist, the distribution of our Th/U ratios, along with other precise isotope dilution data for ordinary chondrites, is rather symmetric about the CI chondrite value; however, actinide/(LREE) ratios are systematically lower than the CI value. Variations in actinide or LREE absolute and relative abundances are interpreted as reflecting differences in the proportions and/or compositions of more primitive components (chondrules and CAI materials?) incorporated into different regions of the ordinary chondrite parent bodies. The observed variations of Th/U, Nd/U, or Ce/U suggest that measurements of Pu/U on any single equilibrated ordinary chondrite specimen, such as St. Severin, should statistically be within ±20–30% of the average solar system value, although it is also clear that anomalous samples exist.

Space Science Reviews, 130 (1-4), ISSN:1572-9672, ISSN:0038-6308

Cometary particles returned by the Stardust Discovery Mission are primarily silicate materials of solar system origin. Some of the grains were formed at high temperatures close to the Sun, but then transported far out to the Kuiper belt region of the solar system before being incorporated in the comet.

Minor element variations in MgAl_2O_4 spinel from the type B1 calcium-aluminum-rich inclusion (CAI) Allende TS-34 confirm earlier studies in showing correlations between the minor element chemistry of spinels with their location within the inclusion and with the chemistry of host silicate phases. These correlations result from a combination of crystallization of a liquid produced by re-melting event(s) and local re-equilibration during subsolidus reheating. The correlation of the Ti and V in spinel inclusions with the Ti and V in the adjacent host clinopyroxene can be qualitatively explained by spinel and clinopyroxene crystallization prior to melilite, following a partial melting event. There are, however, difficulties in quantitative modeling of the observed trends, and it is easier to explain the Ti correlation in terms of complete re-equilibration. The correlation of V in spinel inclusions with that in the adjacent host clinopyroxene also cannot be quantitatively modeled by fractional crystallization of the liquid produced by re-melting, but it can be explained by partial re-equilibration. The distinct V and Ti concentrations in spinel inclusions in melilite from the edge regions of the CAI are best explained as being affected by only a minor degree of re-equilibration. The center melilites and included spinels formed during crystallization of the liquid produced by re-melting, while the edge melilites and included spinels are primary. The oxygen isotope compositions of TS-34 spinels are uniformly ^(16)O-rich, regardless of the host silicate phase or its location within the inclusion. Similar to other type B1 CAIs, clinopyroxene is ^(16)O-rich, but melilite is relatively ^(16)O-poor. These data require that the oxygen isotope exchange in TS-34 melilite occurred subsequent to the last re-melting event.

The Genesis Discovery mission will return samples of solar matter for analysis of isotopic and elemental compositions in terrestrial laboratories. This is accomplished by exposing ultra-pure materials to the solar wind at the LI Lagrangian point and returning the materials to Earth. Solar wind collection will continue until April 2004 with Earth return in Sept. 2004. The general science objectives of Genesis are to (1) to obtain solar isotopic abundances to the level of precision required for the interpretation of planetary science data, (2) to significantly improve knowledge of solar elemental abundances, (3) to measure the composition of the different solar wind regimes, and (4) to provide a reservoir of solar matter to serve the needs of planetary science in the 21st century. The Genesis flight system is a sun-pointed spinner, consisting of a spacecraft deck and a sample return capsule (SRC). The SRC houses a canister which contains the collector materials. The lid of the SRC and a cover to the canister were opened to begin solar wind collection on November 30, 2001. To obtain samples of O and N ions of higher fluence relative to background levels in the target materials, an electrostatic mirror (‘concentrator’) is used which focuses the incoming ions over a diameter of about 20 cm onto a 6 cm diameter set of target materials. Solar wind electron and ion monitors (electrostatic analyzers) determine the solar wind regime present at the spacecraft and control the deployment of separate arrays of collector materials to provide the independent regime samples.

We present detailed simulations of the optical spectra emitted from an argon plasma whose conditions correspond to those thought to prevail within sonoluminescing bubbles. The model incorporates detailed atomic physics based on atomic data from the Opacity Project database, and includes bound-bound, bound-free and free-free transitions. Line broadening is treated using the modified semi-empirical method. The spectral model is used as a postprocessor of hydrodynamic simulations. While finding excellent agreement with the shape of experimental spectra, we calculate an intensity that is a factor of 100 greater than that in experiment. We also predict that whilst the majority of the optical emission corresponds to bound-free transitions, there remains the possibility of observing broad line emission in both the UV and IR regions of the spectrum.

Isotope dilution thorium and uranium analyses by inductively-coupled plasma mass spectrometry of 12 samples of Harleton (L6) show a much larger scatter than was previously observed in equilibrated ordinary chondrites. Th/U linearly correlates with 1/U in Harleton and in the total equilibrated ordinary chondrite data set as well. Such a correlation suggests a two component mixture and this trend can be quantitatively modeled as reflecting variations in the mixing ratio between two phosphate phases: chlorapatite and merrillite. The major effect is due to apatite variations, which strongly control the whole rock U concentrations. Phosphorous variations will tend to destroy the Th/U vs. 1/U correlation, and measured P concentrations on exactly the same samples as U and Th show a factor of 3 range. It appears that the P variations are compensated by inverse variations in U (a dilution effect) to preserve the Th/U vs. 1/U correlation. Because variations in whole rock Th/U are consequences of phosphate sampling, a weighted average of high accuracy Th/U measurements in equilibrated ordinary chondrites should converge to a significantly improved average solar system Th/U. Our best estimate of this ratio is 3.53 with σ_(mean) = 0.10.

We have conducted an electron microprobe study of minor element distributions among spinels from two type B1 calcium-aluminum-rich inclusions (CAIs): Allende TS-23 and Leoville 3537–2. We show that by maintaining the petrologic context (edge, middle, and center of the inclusion plus their host silicate phase), four populations of spinels are resolvable based on their minor element contents. One population resides within the edge area (mainly mantle melilite) and is characterized by the highest V contents. Unlike Leoville 3537–2, many edge grains from Allende TS-23 also have high-Fe contents (up to 4.0 wt%) and low-Cr values. Based on their V and Ti concentrations (which is positively correlated), middle and center grains define a trend that is divided into three populations: spinels enclosed by melilite, fassaite, and anorthite. The overall range in Ti concentration based on fractional crystallization should be much less than a factor of 2; however, the observed range is considerably larger. The minor element contents of these grains are interpreted as recording alteration, primary fractional crystallization, and a complex igneous history that may involve remelting and recrystallization. From our data, Allende TS-23 has experienced more alteration than Leoville 3537–2, which is consistent with previous petrologic studies of silicates within these objects; yet both objects have likely been remelted (at least one additional melting event, possibly two, postdating the initial formation of these CAIs). By invoking a remelting history, the large range ir Ti concentrations and the different populations of spinels can be explained. Although our data suggest that more than one generation of spinels exist within these objects, we are unable to establish any population of relic spinel grains that predate the initial melting event.

We present the results of time-resolved interferometric measurements of the electron-density profile in a discharge-ablated capillary waveguide. We observe the development of a pronounced axial minimum in the electron-density profile with a relative depth of as much as 60% of the axial electron density. Such a profile is suitable for channeling high-intensity laser pulses.

It has been proposed that the composition of the solar wind could be measured directly by transporting ultrapure collectors into space, exposing them to the solar wind, and returning them to earth for analysis. In a study to help assess the applicability of present and future postionization secondary neutral mass spectrometers for measuring solar wind implanted samples, measurements of Zr in Si were performed. A three-color resonant ionization scheme proved to be efficient while producing a background count rate limited by secondary ion signal (5×10^(−4) counts/laser pulse). This lowered the detection limit for these measurements to below 500 ppt for 450,000 averages. Unexpectedly, the Zr concentration in the Si was measured to be over 4 ppb, well above the detection limit of the analysis. This high concentration is thought to result from contamination during sample preparation, since a series of tests were performed that rule out memory effects during the analysis.

The combination of secondary neutral mass spectrometry (SNMS) and resonance ionization spectroscopy (RIS) has been shown to be a powerful tool for the detection of low levels of elemental impurities in solids. Drawbacks of the technique have been the laser-repetition-rate-limited, low duty cycle of the analysis and the fact that RIS schemes are limited to determinations of a single element. These problems have been addressed as part of an ongoing program to explore the usefulness of RIS/SNMS instruments for the analysis of naturally occurring samples. Efficient two-color, two-photon (1+1) resonance ionization schemes were identified for Mo and for four platinum-group elements (Ru, Os, Ir, and Re). Careful selection of the ionization schemes allowed Mo or Ru to be measured simultaneously with Re, Os, or Ir, using two tunable dye lasers and an XeCl excimer laser. Resonance frequencies could be switched easily under computer control, so that all five elements can be rapidly analyzed. In situ measurements of these elements in metal grains from five meteorites were conducted. From the analyses, estimates of the precision and the detection limit of the instrument were made. The trade-off between lower detection limits and rapid multielement RIS analyses is discussed.

We heated mixtures of sulfur and Na-bearing silicates in evacuated silica glass capsules to temperatures between 600°C and 950°C. At or above 850°C, Na-silicate glass reacts with elemental S to form a (Na,K) sulfide. Mobilization of this phase may account for the presence of Na and K on the surface of Io, and hence in the material sputtered into the Jovian magnetosphere.

The Na atmosphere of Io requires a Na-S-O phase in the outer surface layers. Considering the various mechanisms for extraction of Na to the surface, the possible primary phases are Na_2O, Na_2S_x and Na_2SO_4. However, regardless of the primary phases brought to the surface, the shallow crustal recycling of material implied by the ongoing volcanism will tend to produce thermochemical equilibrium and cause all Na to end up as Na_2SO_4 or Na_2S_x. This hypothesis is investigated by relatively model-independent thermodynamic calculations. The major assumption is that material is statistically circulated to sufficiently high temperatures by burial that thermochemical equilibrium can be attained. For a wide range of assumed crustal (PT) conditions, Na_2O will be converted to Na_2SO_4. During residence in the shallowest crustal regions dominated by liquid SO_2, e.g., SO_2 geysers or fumaroles, or for any crustal regimes where SO_2 and S are in comparable abundances, Na-sulfides will be converted to Na_2SO_4. However, in high-temperature, low-pressure regimes with a low relative abundance of SO_2 relative to S (e.g., due to outgasing of SO_2), Na_2SO_4 is converted to Na sulfides. Such regimes could be relatively common on Io, e.g., associated with flows, lava lakes, or shallow intrusions. Consequently, because of thermochemical equilibration in different crustal environments, both Na_2SO_4 and Na sulfides will coexist on the Io surface.

We define in situ trace element microanalysis as the measurement of concentrations of trace elements (< 1000 ppm) in individual mineral phases in polished sections, analogous to electron microprobe analysis for major elements. It is our opinion that such measurements are important and can provide a new dimension to petrology and geochemistry. This article is both a review of work already done and a summary of potential advances in the future. The advantages of in situ analyses, as opposed to methods involving physical and/or chemical mineral separations, are that the trace element data can be interpreted in a petrographic context and that ambiguities associated with the purity of the mineral separates can be avoided. With data on individual grains, comparisons of intergrain and intragrain (zoning) variations between major and trace elements can be made, and the importance of inclusions can be assessed, within the spatial resolution of the microanalysis technique.

The use of the ion microprobe for quantitative analysis of Sr, Y, Zr, La, Sm, and Yb in melilite and pyroxene is evaluated. Three trace element-doped synthetic glasses of composition Ak_40, Ak_80, and Di_2AbAn were analyzed by ion microprobe (IMP) using ion yields determined from Corning glass standards. IMP-determined oxide concentrations in the Di_2AbAn glass agree well with electron microprobe (EMP) analyses (to within 6%), but IMP analyses of the melilite glasses deviate from EMP averages by up to 19%. The deviations are due to erroneous SiO_2 estimates caused by suppression of Si ion intensities by the enhanced concentrations of Ca and Al in the melilite glasses compared to the standards. Thus, in order to determine compositions of melilite, diopside, and glass from subliquidus experiments on each of the three starting compositions, we adopted a new set of ion yields such that IMP analyses of the three starting glasses reproduce the EMP average compositions. Further IMP and EMP comparisons of the subliquidus assemblages show that quantitative analyses of melilite, diopside, and glass can be obtained by IMP that are within 10% of the concentrations obtained by EMP, when ion yields determined from glass starting compositions are used. EMP-IMP comparison of crystal and glass analyses also suggests that a structural matrix effect may result in overestimation of SrO (10–12%) in melilite by IMP. Comparison of our data for Ak_12 and Ak_90 melilite compositions with literature results shows that melilite/liquid for REE^3+ determined by IMP decrease with increasing X_Ak (Ak_90: D_La = 0.038, D_Sm = 0.032, D_Yb = 0.0086; Ak: 0.67, 0.75, 0.25, respectively) while that for Sr (=Eu^2+) changes only slightly (0.99 to 0.78, respectively). Since X_Ak increases with decreasing temperature for all melilite with X_Ak

The depth profile of the neutron-induced fission rate of U-235 was directly measured to a depth of 350 g/sq cm by the Apollo 17 Lunar Neutron Probe Experiment. The fission rate rises sharply from the surface to a broad maximum from 110 to 160 g/sq cm and drops off at greater depths. The shape of the theoretical depth profile of Lingenfelter et al (1972) fits the measured capture rates well at all depths. The absolute magnitude of the experimental fission rates are (11 plus or minus 17)% lower than those calculated theoretically. The excellent agreement between theory and experiment implies that conclusions drawn previously by interpreting lunar sample data with the theoretical capture rates will not require revision. In particular, lunar surface processes, rather than uncertainties in the capture rates, are required to explain the relatively low neutron fluences observed for surface soil samples compared to the fluences expected for a uniformly mixed regolith.

Unusual silica-bearing chondrules and clasts have been observed in several unequilibrated ordinary chondrites. There appear to be two distinct types: (I) silica, low-Ca pyroxene assemblages, and (II) silica, fayalite intergrowths coexisting with low-Ca pyroxene. Only the former types are chondrules. The bulk compositions of most of these objects can be expressed in terms of three components—MgO, FeO, and SiO_2—since Ca, Al, and Na are essentially trace elements. Secondary alteration has probably perturbed the Fe/Mg ratios in most objects and produced Ca-rich pyroxenes in some type II clasts, but the silica, low-Ca pyroxene and fayalite are hard to understand as other than primary phases. Such silica-rich bulk compositions can be understood as an intermediate temperature condensate of a solar gas, formed by the loss of refractory solid material by gas-solid separation before the complete condensation of silicon. Alternatively, the silicarich bulk compositions could have been formed by extreme reduction of normal chondritic olivine and orthopyroxene; however, the observed Ca, Al depletion and the lack of Ni-poor metals are not explained by this model. The fayalite-bearing clasts can be explained by a relatively complex two-stage process involving reduction followed by oxidation. Partial reduction of olivine in the presence of S produces Mg-rich pyroxene, silica, and FeFeS; oxidation of metal and sulfide followed by reaction with silica forms fayalite. An alternative is that the fayalite-bearing clasts result from the decomposition of ferrosilite, but this requires the production of metastable ferrosilite during shock or some other high-pressure event. The simplest unflawed alternative is that the fayalite-bearing clasts result from the mixing of a partially molten silica, fayalite liquid with Mg-rich pyroxene.

New data on the U, Pu, and P distributions in less metamorphosed H-chondrites (type 3–5), coupled with literature results, permit a provisional picture to be assembled of the chemistry of these elements and for the rare earth elements in ordinary chondrites and the changes brought about by chondritic metamorphism. Preferential associations of phosphates with metals and/or sulndes in all chondrites strongly indicate an “initially” siderophile or conceivably chalcophile character for P in ordinary chondrite precursor materials with phosphate subsequently formed by oxidation. This oxidation occurred prior to or during chondritic metal-silicate fractionation. Uranium is initially concentrated in chondrule glass at ~ 100 ppb levels with phosphates (primarily merrillite) in H-3 chondrites being essentially U-free (

The diffusion of Xe in olivine, a major mineral in both meteorites and lunar samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500 C. The fraction of implanted Xe retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100% occurred for an implantion dose of 3 x 10 to the 15th power Xe ions/sq cm. Retention was less at lower doses, with (approximately more than or = 50% loss at one hundred trillion Xe ions/sq cm. Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 micrometer layer for ten million years was calculated as a function of metamorphic temperature.