Search

We discuss elemental abundances of noble gases in targets exposed to the solar wind (SW) onboard the “Genesis” mission during the three different SW “regimes”: “Slow” (interstream, IS) wind, “Fast” (coronal hole, CH) wind and solar wind related to coronal mass ejections (CME). To this end we first present new Ar, Kr, and Xe elemental abundance data in Si targets sampling the different regimes. We also discuss He, Ne, and Ar elemental and isotopic abundances obtained on Genesis regime targets partly published previously. Average Kr/Ar ratios for all three regimes are identical to each other within their uncertainties of about 1% with one exception: the Fast SW has a 12% lower Xe/Ar ratio than do the other two regimes. In contrast, the He/Ar and Ne/Ar ratios in the CME targets are higher by more than 20% and 10%, respectively, than the corresponding Fast and Slow SW values, which among themselves vary by no more than 2–4%. Earlier observations on lunar samples and Genesis targets sampling bulk SW wind had shown that Xe, with a first ionisation potential (FIP) of ∼12 eV, is enriched by about a factor of two in the bulk solar wind over Ar and Kr compared to photospheric abundances, similar to many “low FIP” elements with a FIP less than ∼10 eV. This behaviour of the “high FIP” element Xe was not easily explained, also because it has a Coulomb drag factor suggesting a relatively inefficient feeding into the SW acceleration region and hence a depletion relative to other high FIP elements such as Kr and Ar. The about 12% lower enrichment of Xe in Genesis’ Fast SW regime observed here is, however, in line with the hypothesis that the depletion of Xe in the SW due to the Coulomb drag effect is overcompensated as a result of the relatively short ionisation time of Xe in the ion-neutral separation region in the solar chromosphere. We will also discuss the rather surprising fact that He and Ne in CME targets are quite substantially enriched (by 20% and 10%, respectively) relative to the other solar wind regimes, but that this enrichment is not accompanied by an isotopic fractionation. The Ne isotopic data in CMEs are consistent with a previous hypothesis that isotopic fractionation in the solar wind is mass-dependent.

Electron probe microanalyzer measurements of trace elements with high accuracy are challenging. Accurate Al measurements in olivine are required to calibrate SIMS implant reference materials for measurement of Al in the solar wind. We adopt a combined EPMA/SIMS approach that is useful for producing SIMS reference materials as well as for EPMA at the ~100 μg g(−1) level. Even for mounts not polished with alumina photoelectron spectroscopy shows high levels of Al surface contamination. In order to minimize electron beam current density, a rastered 50 × 100 μm electron beam was adequate and minimized sensitivity to small Al-rich contaminants. Reproducible analyses of eleven SIMS-cleaned spots on San Carlos olivine agreed at 69.3 ± 1.0 μg g(−1)• The known Al mass fraction was used to calibrate an Al implant into San Carlos. Accurate measurements of Al were made for olivines in the pallasites: lmilac, Eagle Station and Springwater. Our focus was on Al in olivine, but our technique could be refined to give accurate electron probe measurements for other contamination-sensitive trace elements. For solar wind, it is projected that the Al/Mg abundance ratio can be determined to 6%, a factor of 2 more precise than the solar spectroscopic ratio.

The Genesis mission captured Solar Wind (SW) and delivered it to Earth for laboratory analyses. Due to advanced mass spectrometric techniques developed specifically for analyses of returned Genesis SW-collectors, SW-oxygen, nitrogen and noble gas isotopes have been successfully measured providing new insights for cosmo- and geochemistry. SW-Xe collected by Genesis is the heaviest and the least abundant SW element analyzed. Here we describe in detail the experimental improvements we made over last 5 years and a latest refined SW-Xe isotopic composition. Combined with earlier, already published SW-Xe analyses, our new results provide the best current estimate for SW-Xe collected by Genesis: 136Xe/130Xe = 1.818 ± 0.004; 134Xe/130Xe = 2.242 ± 0.005; 132Xe/130Xe = 6.063 ± 0.010; 131Xe/130Xe = 5.010 ± 0.012; 129Xe/130Xe = 6.314 ± 0.013; 128Xe/130Xe = 0.510 ± 0.001; 126Xe/130Xe = 0.0256 ± 0.0004; 124Xe/130Xe = 0.0292 ± 0.0004 (all errors are 1σ). The achieved precision allows resolving small, but now statistically significant isotopic difference between solar wind Xe and Xe trapped in lunar regolith samples. This emerging difference, not apparent prior to this study, likely points to the composition of indigenous lunar Xe and to the temporal evolution of terrestrial Xe. Combining our Xe fluence with that for other high first ionization potential (FIP) elements, we find that the depletion of elements with the FIP greater than 12 eV is not constant but monotonically decreases as FIP increases.

NASA's Genesis mission was flown to capture samples of the solar wind and return them to the Earth for measurement. The purpose of the mission was to determine the chemical and isotopic composition of the Sun with significantly better precision than known before. Abundance data are now available for noble gases, magnesium, sodium, calcium, potassium, aluminum, chromium, iron, and other elements. Here, we report abundance data for hydrogen in four solar wind regimes collected by the Genesis mission (bulk solar wind, interstream low-energy wind, coronal hole high-energy wind, and coronal mass ejections). The mission was not designed to collect hydrogen, and in order to measure it, we had to overcome a variety of technical problems, as described herein. The relative hydrogen fluences among the four regimes should be accurate to better than ±5-6%, and the absolute fluences should be accurate to ±10%. We use the data to investigate elemental fractionations due to the first ionization potential during acceleration of the solar wind. We also use our data, combined with regime data for neon and argon, to estimate the solar neon and argon abundances, elements that cannot be measured spectroscopically in the solar photosphere.

X-ray standing wave fluorescence yield depth profiling was used to determine the solar wind implanted Fe and Ni fluences in a silicon-on-sapphire (SoS) Genesis collector (60326). An internal reference standardization method was developed based on fluorescence from Si and Al in the collector materials. Measured Fe fluence agreed well with that measured previously by us on a sapphire collector (50722) as well as SIMS results by Jurewicz et al. Measured Ni fluence was higher than expected by a factor of two; neither instrumental errors nor solar wind fractionation effects are considered significant perturbations to this value. Impurity Ni within the epitaxial Si layer, if present, could explain the high Ni fluences and therefore needs further investigation. As they stand, these results are consistent with minor temporally-variable Fe and Ni fractionation on the timescale of a year.

The distribution of solar-wind ions in Genesis mission collectors, as determined by depth profiling analysis, constrains the physics of ion-solid interactions involving the solar wind. Thus, they provide an experimental basis for revealing ancient solar activities represented by solar-wind implants in natural samples. We measured the first depth profile of ^4He in a Genesis collector; the shallow implantation (peaking at

We have measured the isotopic composition and fluence of solar-wind nitrogen in a diamond-like-carbon collector from the Genesis B/C array. The B and C collector arrays on the Genesis spacecraft passively collected bulk solar wind for the entire collection period, and there is no need to correct data for instrumental fractionation during collection, unlike data from the Genesis “Concentrator.” This work validates isotopic measurements from the concentrator by Marty et al. (2010, 2011); nitrogen in the solar wind is depleted in ^(15)N relative to nitrogen in the Earth’s atmosphere. Specifically, our array data yield values for ^(15)N/^(14)N of (2.17 ± 0.37) × 10^(−3) and (2.12 ± 0.34) × 10^(−3), depending on data-reduction technique. This result contradicts preliminary results reported for previous measurements on B/C array materials by Pepin et al. (2009), so the discrepancy between Marty et al. (2010, 2011) and Pepin et al. (2009) was not due to fractionation of solar wind by the concentrator. Our measured value of ^(15)N/^(14)N in the solar wind shows that the Sun, and by extension the solar nebula, lie at the low-^(15)N/^(14)N end of the range of nitrogen isotopic compositions observed in the solar system. A global process (or combination of processes) must have operated in interstellar space and/or during the earliest stages of solar system formation to increase the ^(15)N/^(14)N ratio of the solar system solids. We also report a preliminary Genesis solar-wind nitrogen fluence of (2.57 ± 0.42) × 10^(12) cm^(−2). This value is higher than that derived by backside profiling of a Genesis silicon collector (Heber et al. 2011a).

We present bulk solar wind isotopic and elemental ratios for Ar, Kr, and Xe averaged from up to 14 individual analyses on silicon targets exposed to the solar wind for ~2.3 years during NASA’s Genesis mission. All averages are given with 1σ standard errors of the means and include the uncertainties of our absolute calibrations. The isotopic ratios ^(86)Kr/^(84)Kr and ^(129)Xe/^(132)Xe are 0.303 ± 0.001 and 1.06 ± 0.01, respectively. The elemental ratios ^(36)Ar/^(84)Kr and ^(84)Kr/^(132)Xe are 2390 ± 120 and 9.9 ± 0.3, respectively. Average fluxes of ^(84)Kr and ^(132)Xe in the bulk solar wind in atoms/(cm^2 s) are 0.166 ± 0.009 and 0.017 ± 0.001, respectively. The flux uncertainties also include a 2% uncertainty for the determination of the extracted areas. The bulk solar wind ^(36)Ar/^(38)Ar ratio of 5.50 ± 0.01 and the ^(36)Ar flux of 397 ± 11 atoms/(cm^2s) determined from silicon targets agree well with the ^(36)Ar/^(38)Ar ratio and the ^(36)Ar flux determined earlier on a different type of target by Heber et al. (2009). A comparison of the solar wind noble gas/oxygen abundance ratios with those in the solar photosphere revealed a slight enrichment of Xe and, within uncertainties a roughly uniform depletion of Kr–He in the solar wind, possibly related to the first ionization potentials of the studied elements. Thus, the solar wind elemental abundances He–Kr display within uncertainties roughly photospheric compositions relative to each other. A comparison of the Genesis data with solar wind heavy noble gas data deduced from lunar regolith samples irradiated with solar wind at different times in the past reveals uniform ^(36)Ar/^(84)Kr ratios over the last 1–2 Ga but an increase of the ^(84)Kr/^(132)Xe ratio of about a factor of 2 during the same time span. The reason for this change in the solar wind composition remains unknown.

We report concentrations and isotopic compositions of He, Ne, and Ar measured with high spatial resolution along a radial traverse of a silicon carbide (SiC) quadrant of the Genesis mission concentrator target. The Ne isotopic composition maps instrumental fractionation as a function of radial position in the target: the maximum observed isotopic fractionation is approximately 33‰ per mass unit between the center and periphery. The Ne fluence is enhanced by a factor of 43 at the target center and decreases to 5.5 times at the periphery relative to the bulk solar wind fluence. Neon isotopic profiles measured along all four arms of the "gold cross" mount which held the quadrants in the concentrator target demonstrate that the concentrator target was symmetrically irradiated during operation as designed. We used implantation experiments of Ne into SiC and gold to quantify backscatter loss and isotopic fractionation and compared measurements with numerical simulations from the code "stopping and range of ions in matter." The ^(20)Ne fluence curve as a function of radial distance on the target may be used to construct concentration factors relative to bulk solar wind for accurate corrections for solar wind fluences of other light elements to be measured in the concentrator target. The Ne isotopic composition as a function of the radial distance in the SiC quadrant provides a correction for the instrumental mass-dependent isotopic fractionation by the concentrator and can be used to correct measured solar wind oxygen and nitrogen isotopic compositions to obtain bulk solar wind isotopic compositions.

Ion microprobe elemental and isotopic determinations can be precise but difficult to quantify. Error is introduced when the reference material and the sample to be analysed have different compositions. Mitigation of such “matrix effects” is possible using ion implants. If a compositionally homogeneous reference material is available which is “matrix-appropriate,” i.e., close in major element composition to the sample to be analysed, but having an unknown concentration of the element, E, to be determined, ion implantation can be used to introduce a known amount of an E isotope, calibrating the E concentration and producing a matrix-appropriate calibrator. Nominal implant fluences (ions cm^(−2)) are inaccurate by amounts up to approximately 30%. However, ion implantation gives uniform fluences over large areas, thus it is possible to “co-implant” an additional reference material of any bulk composition having known amounts of E, independently calibrating the implant fluence. Isotope-ratio measurement standards can be produced by implanting two different isotopes, but permil level precision requires post-implant calibration of the implant isotopic ratio. Examples discussed include: (1) standardising Li in melilite; (2) calibrating a ^(25)Mg implant fluence using NIST SRM 617 glass; and (3) using Si co-implanted with ^(25)Mg alongside NIST SRM 617 to produce a calibrated measurement of Mg in Si.

We propose a theory for crystal-melt trace element partitioning that considers the energetic consequences of crystal-lattice strain, of multi-component major-element silicate liquid mixing, and of trace-element activity coefficients in melts. We demonstrate application of the theory using newly determined partition coefficients for Ca, Mg, Sr, and Ba between pure anorthite and seven CMAS liquid compositions at 1330 °C and 1 atm. By selecting a range of melt compositions in equilibrium with a common crystal composition at equal liquidus temperature and pressure, we have isolated the contribution of melt composition to divalent trace element partitioning in this simple system. The partitioning data are fit to Onuma curves with parameterizations that can be thermodynamically rationalized in terms of the melt major element activity product (a_(Al_2O_3))(a_(SiO_2_)^2 and lattice strain theory modeling. Residuals between observed partition coefficients and the lattice strain plus major oxide melt activity model are then attributed to non-ideality of trace constituents in the liquids. The activity coefficients of the trace species in the melt are found to vary systematically with composition. Accounting for the major and trace element thermodynamics in the melt allows a good fit in which the parameters of the crystal-lattice strain model are independent of melt composition.

International audience; A method to quantitatively determine the fluences of shallowly-implanted solar wind ions returned to Earth by the Genesis Discovery mission is described. Through backside depth-profiling, we recover nearly complete depth profiles of implanted solar wind for several nonvolatile elements, including Mg, Al, Ca, Cr, and, to a lesser extent, Na, in silicon targets that collected bulk solar wind and solar wind from specific velocity regimes. We also determine the fluences of the volatile elements C, N, and O in silicon targets that collected bulk solar wind. By the use of appropriately calibrated ion implanted standards, fluences as low as 2 x 10(10) atoms cm(-2) can be determined with precision and accuracy typically in the few percent range. Specific approaches to sample preparation, sputtering artifacts during depth profiling by secondary ion mass spectrometry, and quantification including the production of ion implant standards are discussed. (C) 2014 Elsevier B.V. All rights reserved.

A new generation of time-of-flight mass spectrometers that implement ion sputtering and laser desorption for probing solid samples and can operate in regimes of laser post-ionization secondary neutral mass spectrometry and secondary ion mass spectrometry is being developed at Argonne National Laboratory. These new instruments feature novel ion optical systems for efficient extraction of ions from large laser post-ionization volumes and for lossless transport of these ions to detectors. Another feature of this design is a new in-vacuum all-reflecting optical microscope with 0.5-μm resolution. Advanced ion and light optics and three ion sources, including a liquid metal ion gun (focusable to 50 nm) and a low energy ion gun, give rise to an instrument capable of quantitative analyses of samples for the most challenging applications, such as determining elemental concentrations in shallow implants at ultra-trace levels (for example, solar wind samples delivered by NASA Genesis mission) and analyzing individual sub-micrometer particles on a sample stage (such as, interstellar dust delivered by NASA Stardust mission). Construction of a prototype instrument has been completed and testing is underway. A more advanced instrument of similar design is under construction. The overall design of the new instrument and the innovations that make it unique are outlined. Results of the first tests to characterize its analytical capabilities are presented also.

With only a few exceptions, the solar photosphere is thought to have retained the mean isotopic composition of the original solar nebula, so that, with some corrections, the photosphere provides a baseline for comparison of all other planetary materials. There are two sources of information on the photospheric isotopic composition: optical observations, which have succeeded in determining a few isotopic ratios with large uncertainties, and the solar wind, measured either in situ by spacecraft instruments or as implanted ions into lunar or asteroidal soils or collection substrates. Gravitational settling from the outer convective zone (OCZ) into the radiative core is viewed as the only solar modification of solar-nebula isotopic compositions to affect all elements. Evidence for gravitational settling is indirect, as observations are presently less precise than the predictions of

Apatite, titanite and olivine samples were bombarded with a ~ 150 MeV proton beam to produce ~ 10^8 atoms/mg of spallation ^3He. High-precision stepped-heating experiments were then performed in which the artificial ^3He and, for apatite and titanite, the natural radiogenic ^4He were measured to characterize the diffusive behavior of each isotope. Helium-3 diffusion coefficients are in excellent agreement with concurrently and/or previously determined He diffusion coefficients for each mineral. Our results indicate that proton-induced ^3He is uniformly distributed and that radiation damage associated with a proton fluence of ~ 5 x 10^(14) protons/cm^2 does not cause noticeable changes in ^4He diffusion behavior in at least apatite and titanite. Proton-induced ^3He can therefore be used to establish He diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural ^4He distribution is inhomogeneous. In addition,step-heating ^4He/^3He analysis of a mineral with a uniform synthetic ^3He concentration provides a means by which a natural ^4He distribution can be determined.

The design and operation of the Genesis Solar-Wind Concentrator relies heavily on computer simulations. The computer model is described here, as well as the solar wind conditions used as simulation inputs, including oxygen charge state, velocity, thermal, and angular distributions. The simulation included effects such as ion backscattering losses, which also affect the mass fractionation of the instrument. Calculations were performed for oxygen, the principal element of interest, as well as for H and He. Ion fluences and oxygen mass fractionation are determined as a function of radius on the target. The results were used to verify that the instrument was indeed meeting its requirements, and will help prepare for distribution of the target samples upon return of the instrument to earth. The actual instrumental fractionation will be determined at that time by comparing solar-wind neon isotope ratios measured in passive collectors with neon in the Concentrator target, and by using a model similar to the one described here to extrapolate the instrumental fractionation to oxygen isotopes.

The primary goal of the Genesis Mission is to collect solar wind ions and, from their analysis, establish key isotopic ratios that will help constrain models of solar nebula formation and evolution. The ratios of primary interest include 17O/16O and 18O/16O to ±0.1%, 15N/14N to ±1%, and the Li, Be, and B elemental and isotopic abundances. The required accuracies in N and O ratios cannot be achieved without concentrating the solar wind and implanting it into low-background target materials that are returned to Earth for analysis. The Genesis Concentrator is designed to concentrate the heavy ion flux from the solar wind by an average factor of at least 20 and implant it into a target of ultra-pure, well-characterized materials. High-transparency grids held at high voltages are used near the aperture to reject >90% of the protons, avoiding damage to the target. Another set of grids and applied voltages are used to accelerate and focus the remaining ions to implant into the target. The design uses an energy-independent parabolic ion mirror to focus ions onto a 6.2 cm diameter target of materials selected to contain levels of O and other elements of interest established and documented to be below 10% of the levels expected from the concentrated solar wind. To optimize the concentration of the ions, voltages are constantly adjusted based on real-time solar wind speed and temperature measurements from the Genesis ion monitor. Construction of the Concentrator required new developments in ion optics; materials; and instrument testing and handling.

One of the major goals of the Genesis Mission was to refine our knowledge of the isotopic composition of the heavy noble gases in solar wind and, by inference, the Sun, which represents the initial composition of the solar system. This has now been achieved with permil precision: ^(36)Ar/^(38)Ar = 5.5005 ± 0.0040, ^(86)Kr/^(84)Kr = .3012 ± .0004, ^(83)Kr/^(84)Kr = .2034 ± .0002, ^(82)Kr/^(84)Kr = .2054 ± .0002, ^(80)Kr/^(84)Kr = .0412 ± .0002, ^(78)Kr/^(84)Kr = .00642 ± .00005, ^(136)Xe/^(132)Xe = .3001 ± .0006, ^(134)Xe/^(132)Xe = .3691 ± .0007, ^(131)Xe/^(132)Xe = .8256 ± .0012, ^(130)Xe/^(132)Xe = .1650 ± .0004, ^(129)Xe/^(132)Xe = 1.0405 ± .0010, ^(128)Xe/^(132)Xe = .0842 ± .0003, ^(126)Xe/^(132)Xe = .00416 ± .00009, and ^(124)Xe/^(132)Xe = .00491 ± .00007 (error-weighted averages of all published data). The Kr and Xe ratios measured in the Genesis solar wind collectors generally agree with the less precise values obtained from lunar soils and breccias, which have accumulated solar wind over hundreds of millions of years, suggesting little if any temporal variability of the isotopic composition of solar wind krypton and xenon. The higher precision for the initial composition of the heavy noble gases in the solar system allows (1) to confirm that, exept ^(136)Xe and ^(134)Xe, the mathematically derived U–Xe is equivalent to Solar Wind Xe and (2) to provide an opportunity for better understanding the relationship between the starting composition and Xe-Q (and Q-Kr), the dominant current “planetary” component, and its host, the mysterious phase-Q.

RIMS depth profiles have been measured for Cr and Ca in Genesis solar wind collector made from Si and compared to such measurements for ion-implanted Si reference material. The presence of surface contamination has been shown to be a significant factor influencing the total Ca and Cr fluence measured in the Genesis collectors. A procedure to remove the contaminant signal from these depth profiles using the reference material implanted with a minor isotope demonstrated that 36% of the measured Ca fluence in our Genesis sample comes from terrestrial contamination.

The thermal diffusion of an Mg implant in Si has been measured with SIMS and compared to RIMS (resonant ionisation mass spectrometry) measurements of Mg implantation and diffusion in Si wafers exposed to solar wind irradiation in the NASA Genesis mission. The Genesis samples show much more surface segregation that the samples annealed in the laboratory, due to diffusion and segregation of the implanted Mg to the heavily damaged near surface regions of the Genesis wafers. This Mg transport has been modeled by solving a set of stiff differential equations and found to agree with RIMS measurements for a Mg interstitial migration energy of 0.7 eV.

To evaluate the isotopic composition of the solar nebula from which the planets formed, the relation between isotopes measured in the solar wind and on the Sun's surface needs to be known. The Genesis Discovery mission returned independent samples of three types of solar wind produced by different solar processes that provide a check on possible isotopic variations, or fractionation, between the solar-wind and solar-surface material. At a high level of precision, we observed no significant inter-regime differences in 20 Ne/ 22 Ne or 36 Ar/ 38 Ar values. For 20 Ne/ 22 Ne, the difference between low- and high-speed wind components is 0.24 ± 0.37%; for 36 Ar/ 38 Ar, it is 0.11 ± 0.26%. Our measured 36 Ar/ 38 Ar ratio in the solar wind of 5.501 ± 0.005 is 3.42 ± 0.09% higher than that of the terrestrial atmosphere, which may reflect atmospheric losses early in Earth's history.

Regular, geometric, apparently crystallographically-controlled, sector zoning has been produced in synthetic anorthite crystallized from a liquid of Type B Ca,Al-rich inclusion (CAI) composition. Cathodoluminescence (CL) provides a rapid method for observing the sectors. Sharp discontinuities in Mg concentration always accompany sharp changes in CL intensity at sector boundaries. Although CL intensity anti-correlates with Mg concentration, there is not a well-defined quantitative relationship, and Mg concentration steps range from as little as 5% to almost a factor of 2 for different CL boundaries. When measurable, Na also tends to anti-correlate with CL brightness in a given crystal and thus correlates with Mg. The partition coefficients for Mg and Na can vary by up to a factor of 2 depending on the particular sector and thus are not constant for CAI anorthite which formed by fractional crystallization. In contrast, Ti (either Ti^+3 or Ti^+4) concentration is not a function of sector zoning and thus does not affect the CL intensity. The sector zoning and correlated chemical variations for synthetic anorthite are similar to those in natural anorthite from Type B CAIs. The CL intensity, boundary sharpness, and minor element zoning in synthetic anorthite are not affected by annealing at 850°C for three weeks. Overall, our observations suggest that the observed CL patterns and minor element zoning in Type B anorthite are igneous in origin and have not been modified by subsolidus reequilibration or alteration. This leads to the conclusion that the ^(26)Mg-^(26)Al systematics in CAIs (based almost entirely on anorthite) have not been modified by subsolidus reequilibration or alteration.

The Genesis mission Solar Wind Concentrator was built to enhance fluences of solar wind by an average of 20x over the 2.3 years that the mission exposed substrates to the solar wind. The Concentrator targets survived the hard landing upon return to Earth and were used to determine the isotopic composition of solar-wind—and hence solar—oxygen and nitrogen. Here we report on the flight operation of the instrument and on simulations of its performance. Concentration and fractionation patterns obtained from simulations are given for He, Li, N, O, Ne, Mg, Si, S, and Ar in SiC targets, and are compared with measured concentrations and isotope ratios for the noble gases. Carbon is also modeled for a Si target. Predicted differences in instrumental fractionation between elements are discussed. Additionally, as the Concentrator was designed only for ions ≤22 AMU, implications of analyzing elements as heavy as argon are discussed. Post-flight simulations of instrumental fractionation as a function of radial position on the targets incorporate solar-wind velocity and angular distributions measured in flight, and predict fractionation patterns for various elements and isotopes of interest. A tighter angular distribution, mostly due to better spacecraft spin stability than assumed in pre-flight modeling, results in a steeper isotopic fractionation gradient between the center and the perimeter of the targets. Using the distribution of solar-wind velocities encountered during flight, which are higher than those used in pre-flight modeling, results in elemental abundance patterns slightly less peaked at the center. Mean fractionations trend with atomic mass, with differences relative to the measured isotopes of neon of +4.1±0.9 ‰/amu for Li, between -0.4 and +2.8 ‰/amu for C, +1.9±0.7‰/amu for N, +1.3±0.4 ‰/amu for O, -7.5±0.4 ‰/amu for Mg, -8.9±0.6 ‰/amu for Si, and -22.0±0.7 ‰/amu for S (uncertainties reflect Monte Carlo statistics). The slopes of the fractionation trends depend to first order only on the relative differential mass ratio, Δ m/ m. This article and a companion paper (Reisenfeld et al. 2012, this issue) provide post-flight information necessary for the analysis of the Genesis solar wind samples, and thus serve to complement the Space Science Review volume, The Genesis Mission (v. 105, 2003).

The Suess-Urey (S-U) mission has been proposed as a NASA Discovery mission to return samples of matter from the Sun to the Earth for isotopic and chemical analyses in terrestrial laboratories to provide a major improvement in our knowledge of the average chemical and isotopic composition of the solar system. The S-U spacecraft and sample return capsule will be placed in a halo orbit around the L1 Sun-Earth libration point for two years to collect solar wind ions which implant into large passive collectors made of ultra-pure materials. Constant Spacecraft-Sun-Earth geometries enable simple spin stabilized attitude control, simple passive thermal control, and a fixed medium gain antenna. Low data requirements and the safety of a Sun-pointed spinner, result in extremely low mission operations costs.

Total reflection X-ray fluorescence spectrometry (TXRF) was used to analyze residual surface contamination on Genesis solar wind samples and to evaluate different cleaning methods. To gauge the suitability of a cleaning method, two samples were analyzed following cleaning by lab-based TXRF. The analysis comprised an overview and a crude manual mapping of the samples by orienting them with respect to the incident X-ray beam in such a way that different regions were covered. The results show that cleaning with concentrated hydrochloric acid and a combination of hydrochloric acid and hydrofluoric acid decreased persistent inorganic contaminants substantially on one sample. The application of CO2 snow for surface cleaning tested on the other sample appears to be effective in removing one persistent Genesis contaminant, namely germanium. Unfortunately, the TXRF analysis results of the second sample were impacted by relatively high background contamination. This was mostly due to the relatively small sample size and that the solar wind collector was already mounted with silver glue for resonance ion mass spectrometry (RIMS) on an aluminium stub. Further studies are planned to eliminate this problem. In an effort to identify the location of very persistent contaminants, selected samples were also subjected to environmental scanning electron microscopy. The results showed excellent agreement with TXRF analysis.

It is generally accepted that the primitive Sun, which contains the vast majority of the mass of the solar system, has the same composition as the primitive solar nebula, and that the contemporary Sun has a similar composition except perhaps for light elements modified in main sequence hydrogen burning. The diversity of isotopic and elemental compositions now observed in various solar system reservoirs is most likely the result of subsequent modification and noble gases can provide us with valuable tools to understand the evolutionary paths leading to these different compositions. However, to do this we need to know the composition of the Sun with sufficient precision to delineate the different paths and processes leading to the variations observed and how the present solar wind noble gases may differ from that composition.

We present the elemental and isotopic composition of noble gases in the bulk solar wind collected by the NASA Genesis sample return mission. He, Ne, and Ar were analyzed in diamond-like carbon on a silicon substrate (DOS) and ^(84),^(86)Kr and ^(129),^(132)Xe in silicon targets by UV laser ablation noble gas mass spectrometry. Solar wind noble gases are quantitatively retained in DOS and with exception of He also in Si as shown by a stepwise heating experiment on a flown DOS target and analyses on other bulk solar wind collector materials. Solar wind data presented here are absolutely calibrated and the error of the standard gas composition is included in stated uncertainties. The isotopic composition of the light noble gases in the bulk solar wind is as follows: ^3He/^4He: (4.64 ± 0.09) × 10^(−4), ^(20)Ne/^(22)Ne: 13.78 ± 0.03, ^(21)Ne/^(22)Ne: 0.0329 ± 0.0001, ^(36)Ar/^(38)Ar 5.47 ± 0.01. The elemental composition is:^4He/^(20)Ne: 656 ± 5, and ^(20)Ne/^(36)Ar 42.1 ± 0.3. Genesis provided the first Kr and Xe data on the contemporary bulk solar wind. The preliminary isotope and elemental composition is: ^(86)Kr/^(84)Kr: 0.302 ± 0.003, ^(129)Xe/^(132)Xe: 1.05 ± 0.02, ^(36)Ar/^(84)Kr 2390 ± 150, and ^(84)Kr/^(132)Xe 9.5 ± 1.0. The ^3He/^4He and the ^4He/^(20)Ne ratios in the Genesis DOS target are the highest solar wind values measured in exposed natural and artificial targets. The isotopic composition of the other noble gases and the Kr/Xe ratio obtained in this work agree with data from lunar samples containing “young” (~100 Ma) solar wind, indicating that solar wind composition has not changed within at least the last 100 Ma. Genesis could provide in many cases more precise data on solar wind composition than any previous experiment. Because of the controlled exposure conditions, Genesis data are also less prone to unrecognized systematic errors than, e.g., lunar sample analyses. The solar wind is the most authentic sample of the solar composition of noble gases, however, the derivation of solar noble gas abundances and isotopic composition using solar wind data requires a better understanding of fractionation processes acting upon solar wind formation.

One of the primary objectives of the Genesis sample return mission is to measure the oxygen isotopic composition of the Sun to permil-level precision. The returned samples pose a unique analytical challenge, and around the world, there has been parallel development of several competing techniques to make the oxygen measurement on the Genesis concentrator samples. At UCLA, we have developed a hybrid instrument: a secondary ion mass spectrometer (SIMS) front-end combined with an accelerator mass spectrometer (AMS), whose purpose is to eliminate molecular interferences via dissociation. We describe here the current status of the development of the instrument and the remaining issues to address before we request the Genesis samples for analysis.

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

In this study we use microsampling techniques to explore diagenetic processes in carbonates. These processes are important as they can affect the accuracy of U series chronometry. Fission track maps of deep-sea scleractinian corals show a threefold difference between the minimum and maximum [U] in modern corals, which is reduced to a factor of 2 in fossil corals. We use micromilling and MC-ICP-MS to make detailed analyses of the [U] and δ234Uinitial distributions in corals from 218 ka to modern. Within each fossil coral we observe a large range of δ234Uinitial values, with high δ234Uinitial values typically associated with low [U]. A simple model shows that this observation is best explained by preferential movement of alpha-decay produced 234U atoms (alpha-recoil diffusion). Open-system addition of 234U may occur when alpha-recoil diffusion is coupled with a high [U] surface layer, such as organic material. This process can result in large, whole-coral δ234Uinitial elevations with little effect on the final age. The diagenetic pathways that we model are relevant to both shallow-water and deep-sea scleractinian corals since both exhibit primary [U] heterogeneity and may be subject to U addition.

NASA's Genesis capsule, carrying the first samples ever returned from beyond the Moon, took a hard landing in the western Utah desert on 8 September after its parachutes failed to deploy Despite the impact, estimated at 310 km per hour, some valuable solar wind collector materials have been recovered. With these samples, the Genesis team members are hopeful that nearly all of the primary science goals may be met. The Genesis spacecraft was launched in August 2001 to collect and return samples of solar wind for precise isotopic and elemental analysis. The spacecraft orbited the Earth-Sun Lagrangian point (LI), ˜1.5 million km sunward of Earth, for 2.3 years. It exposed ultrapure materials—including wafers of silicon, silicon carbide, germanium, chemically deposited diamond, gold, aluminum, and metallic glass— to solar wind ions, which become embedded within the substrates' top 100 nm of these materials.

Results of the Genesis mission to sample the solar wind are summarized.

The Genesis spacecraft, launched on 8 August 2001 from Cape Canaveral, Florida, will be the first spacecraft ever to return from interplanetary space. The fifth in NASAs line of low-cost, Discovery-class missions, its goal is to collect samples of solar wind and return them to Earth for detailed isotopic and elemental analysis. The spacecraft is to collect solar wind for over 2 years, while circling the L1 point 1.5 million km Sunward of the Earth, before heading back for a capsule-style re-entry in September 2004. After parachute deployments mid-air helicopter recovery will be used to avoid a hard landing. The mission has been in development over 10 years, and its cost, including development, mission operations, and initial sample analysis, is approximately $209 million.

The Genesis Discovery mission, to be launched in January 2001, will expose ultrapure materials to the solar wind for about 2 years and then return this sample to Earth for isotopic and chemical analysis in terrestrial laboratories. Sample return missions use the best available instrumentation to achieve mission science goals. To complete the Genesis science objectives, advanced instrumentation that surpasses present laboratory sample analysis capabilities is required. Advanced Analytical Instrumentation Facilities (AAIFs) will be created for the mission to ensure that the best analytical instruments are used. This approach also enables broad participation by NASA scientists in solar wind sample return analysis.

The partitioning of Pu and Sm between diopside/liquid and whitlockite/liquid has been investigated experimentally to evaluate the geochemical coherence of Pu and the light REEs. ^(Pu)D/^(Sm)D is 2 ~ for both diopsidic pyroxene and whitlockite. This small amount of fractionation would be decreased further if Pu were compared to Ce or Nd. Our experimental results thus validate the suggestion that Pu behaves as a LREE during igneous processes in reducing environments. Our data and the data of Ray et al. (1983) indicate that temperature rather than melt composition is the most important control on elemental partitioning. This is true even though we demonstrate that additions of only 1–2 wt.% of P2_O_5 to the diopside-anorthite-albite system change ^(Pu)D_(cpx) by a factor of two. Our data suggest that P_2O_5 in aluminosilicate melts serves as a complexing agent for the actinides and lanthanides.

Elemental abundances from Cl carbonaceous chondrite meteorites are thought to represent the average solar system (“cosmic”) composition, based on the agreement between Cl and solar photospheric abundances and on the smoothness of heavy element abundances of odd mass nuclei when plotted as a function of mass number. To test Cl elemental smoothness, we have analyzed Cl meteorites, using conventional PIXE (proton probe) techniques, where we have good sensitivity for the range of elements: Ni to Mo. Preliminary analyses indicate that elemental smoothness is only approximate, with possible deviations of 30% to perhaps even 50%. This may be due to chemical fractionation. Alternatively, explanations for nonsmooth behavior may be understandable with the aid of general ideas of n-capture nucleosynthesis. As Cl abundances are refined, it could be that the lack of elemental smoothness may provide the strongest argument for the identification of Cl with primordial solar system abundances.

We have used an electron microprobe or scanning electron microscope to analyze the Ag produced by exposure of Ilford nuclear emulsions to ^(151)Sm beta particles. The low beam currents of the SEM yielded satisfactory results while the higher current density of the microprobe produced variable amounts of volatilization of the organic constituents of the emulsion. Nonlinearities in Ag counting rate vs. exposure time curves were found at both high and low Ag concentrations. Two different types (mechanisms?) of emulsion fading were observed, one operating on an hour, the other on a day, time scale. The low-Ag nonlinearities are perhaps explained by either dose-dependent short time scale fading or by the requirement of multiple hits to render some AgBr grains developable.

Kodaikanal iron meteorite measurements for cosmogenic rare gases and K-Ar isotopes ages for glass inclusions

A new method involving the detection of fission fragments in mica has been applied to the measurement of the fission cross section of the compound nucleus Tl201 produced by bombardments of Au197 with helium ions. These data have been interpreted in terms of an expression for the ratio of fission to neutron-emission probabilities similar to those used conventionally, but modified to include the effect of quantum-mechanical barrier penetrability. In this way a height of 22.5±1.5 MeV was found for the fission barrier of Tl201 and a lower limit on the width could be established. The above value of the barrier, when interpreted on the basis of the liquid-drop theory, leads to an accurate determination of the ratio of the electrostatic to the surface energy of nuclei. This serves to establish the constant of proportionality between the "fissionability parameter" x and the value of Z2/A as follows: x=(Z2/A)/(48.4±0.5). This measured barrier height, when added to the ground-state mass of Tl201, gives a saddle-point mass of this nucleus equal to 200.9949±0.0015 mass units (carbon scale).

Rb Sr isotopic analyses were made on silicate inclusions from Kodaikanal, a brecciated fine octahedrite. Twelve analyses of different mineral fractions and separate inclusions were made. The silicates in this iron meteorite are highly enriched in alkalis and show ^(87)Sr/^(86)Sr ratios ranging up to 8.8. The samples lie on a well defined isochron on the Sr-Rb evolution diagram and indicate an age of 3.8 ± 0.1 × 10^9 y and an initial ^(87)Sr/^(86)Sr ratio between 0.69 and 0.73. These data provide unambiguous evidence for the ‘formation’ of younger solid objects in the solar system in a process which demands extensive chemical differentiation. This is evidence for an intermediate history for some meteoritic objects of a different sort than has been previously observed.

Analysis of seven different portions of Norton County yielded Rb/Sr ratios ranging from 0.15 to 2.3 and permitted the determination of an isochron with high precision. The Rb and Sr concentrations were found to be as low as ∼0.16 ppm. All of the data lie to within their experimental errors on a line in the Sr evolution diagram of slope 0.0654 ± 0.0014, and intercept 0.700 ± 0.002. This determines an age of 4.7 ± 0.1 × 10^9 yr for λ = 1.39 × 10^(−11) yr^(−1). ^(40)K^(40)Ar ages on samples containing 42, 53 and 107 ppm K gave ages of 4.2 – 4.5 × 10^9 yr which are compatible with the Rb-Sr age. The K appears to be concentrated in alkali feldspar grains of about 5–50 microns. The ^(87)Rb^(87)Sr age and primary ^(87)Sr/^(86)Sr value is of sufficient precision to permit the age resolution of meteoritic objects which were formed 0.3 × 10^9 yr apart. No evidence of element redistribution was found for Norton Country as indicated by the consistency of our Rb-Sr and K-Ar results.

Chemical and mineralogical study of Kodaikanal meteorite silicate and metal phases, using optical, X ray and electron microprobe techniques

Rb-Sr measurements were made on silicate inclusions extracted from the iron meteorites Toluca, Odessa, Four Corners, Linwood, Pine River, Colomera and Weekeroo Station. Strontium isotopic analyses were made on samples as small as 24 ng. The typical Sr and Rb blanks were 2 × 10^(−9) g and 2 × 10^(−10) g, respectively. In certain cases it was possible to obtain relatively precise isochrons. With the exception of Colomera, all of these samples gave ages compatible with 4.4 to 4.8 × 10^9 y. The Colomera data scatter widely and do not meet the requirements for defining a reliable age. None of the samples are compatible with an age of 6 × 10^9 y. The primary strontium observed or estimated for Toluca and Pine River agrees with that obtained from achondrites. It is evident that extensive Rb-Sr isotopic studies may be made on a large number of rather typical iron meteorites.

Analyses of the argon and potassium in silicate inclusions were made on Weekerro Station, Toluca, Four Corners and Kodaikanal. The 40Ar40K ratios of all samples except Kodaikanal are compatible with an age of 4.5±0.15 × 109yr. These results are in agreement with the SrRb results obtained previously but not with the so-called 40Ar40K ages determined on iron meteorites by other workers. The 40Ar40K age of 3.5±0.1 × 109yr on Kodaikanal is consistent with the 87Sr87Rb age of 3.8±0.1 × 109yr. Estimates of the U + Th content of the silicate inclusions were made based on the 4He contents. Kodaikanal was found to have a very young exposure age for an iron meteorite (⪅40 m.y) based on upper limits to the cosmogenic He, Ne and Ar in the metal phase.

The distributions in masses and total kinetic energies of fission fragments from a number of elements ranging from erbium to bismuth have been measured. The nuclei undergoing fission were produced by bombarding a variety of targets with projectiles ranging from He4 to O16. The energies of coincident fission fragments were measured using solid-state counters. The energy data were transformed to give mass-total kinetic energy density-of-events distributions. These distributions were compared with those calculated from an approximate version of the liquid-drop model which applies to this region of elements. General agreement in the shapes and widths of the distributions was found, particularly in the cases which involved small angular momenta and small nuclear temperatures. The dependence of the widths of the experimental distributions on the nuclear temperature was found to differ from that predicted by the theory, but uncertainties in the evaluation of nuclear temperatures for the reactions investigated may be large. Analysis of these uncertainties indicated that they were not large enough to alone account for the discrepancy, although this analysis was subject to some error. Angular momentum effects were studied by using certain combinations of targets and projectiles to give the compound nucleus Os186 at the same excitation energy but with different angular momenta. The effect of increasing angular momentum was to broaden and change the shape of the experimental distributions.

This article summarizes research since the last detailed reviews of meteorite ages by Anders [1963] and Reynolds [1967]. Only crystallization ages based on parent-daughter isotopic relationships resulting from the decay of naturally occurring radioactive nuclei will be discussed. The basic principles and techniques for age determinations are discussed in many of the papers cited and, along with summaries of scientific results, in several recent books [Dalrymple and Lanphere, 1969; Doe 1970; Hamilton, 1965; Schaeffer and Zahringer, 1966; Faul, 1966]. However, developments in the field have made some of the material in the books obsolete.