Gregory F. Herzog

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (∼180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

Cosmic-Ray Exposure Ages of Meteorites

The concept of a cosmic-ray exposure (CRE) age for a meteorite is based on a simple but useful picture of meteorite evolution, the one-stage irradiation model. CRE ages have implications for several interrelated questions. From how many different parent bodies do meteorites come? How well do meteorites represent the population of their source region? How many distinct collisions on each parent body have created the known meteorites of each type? How often do parent bodies collide? How big and how energetic were the collisions that produced meteoroids? What factors control the CRE age of a meteorite and how do meteoroid orbits evolve through time? This chapter touches on these questions as it examines the data. In recent years, CRE ages also have become important for interpreting small variations in the abundances of stable isotopes.

Mass-dependent fractionation of Mg, Si, and Fe isotopes in five stony cosmic spherules

We have measured with an electron microprobe the Mg, Al, Si, Ca, Ti, Mn, and Fe contents of five strongly heated stony cosmic spherules (sCS) from the South Pole water well. We have also measured the isotopic compositions of Si, and when possible of Mg and of Fe in these objects by ion microprobe. Except for iron, the measured elemental compositions are chondritic within a factor of 2. In four samples, the ratio of 57Fe/56Fe exceeds the terrestrial value by 3.5‰ to 48‰. Mass-dependent fractionation of the isotopes of Si ranges from ∼2 to ∼8 ‰/AMU in three samples. Mg is clearly fractionated in only one sample, for which δ25Mg = ∼8 ‰. The extent of mass-dependent fractionation of the isotopes and, by implication, of evaporative loss generally follows a trend Mg < Si < Fe. The trend is similar to that found in laboratory heating experiments of charges with solar composition. Although the observed isotopic inhomogeneities within some samples call into question the strict validity of the Rayleigh equation for the sCS, its approximate application to our new and to previously published results for Mg suggests that evaporative losses of greater than 40 wt.% occur rarely from sCS, and that the precursor grains of the sCS had a CM-carbonaceous-chondrite-like complement of Mg, Si, Ca, and Al. Low Fe contents relative to CM abundances could reflect an unusual precursor composition, or, more probably, losses by processes that did not fractionate isotopes, i.e., ejection of immiscible FeS and FeNi beads from the melt or rapid, complete separation and decomposition of FeS at the surface.

Isotopic and elemental composition of iron, nickel, and chromium in type I deep-sea spherules: implications for origin and composition of the parent micrometeoroids

We report elemental and isotopic analyses of Fe, Ni, and Cr in type I deep-sea spherules with masses ranging from 43 to 256 μg. We measured (1) the isotopic compositions of Fe and Cr by thermal ionization mass spectrometry; and (2) the elemental concentrations of Fe, Ni, and Cr and the isotopic compositions of Ni by inductively-coupled plasma mass spectrometry. Evaporation of Fe, Ni, and Cr during atmospheric entry led to large and similar average degrees of mass-dependent fractionation, Φ, in most spherules. The average value, ∼16‰/AMU, corresponds to mass losses of 80–85%, assuming open-system evaporation of the atoms. We find ΦCr ∼ ΦFe, in seven spherules. This observation implies similar evaporation rates for Cr and Fe and that the measured Cr/Fe ratios (mass/mass) are close to those of the progenitors. Four spherules have Cr/Fe ∼0.003; two others with low Cr/Fe, ∼8 × 10−4, high Fe/Ni, ∼2000, and ΦCr ∼ ΦFe ∼0, may belong to a different, possibly terrestrial, population. A seventh spherule with “chondritic” Cr/Fe, ∼17 × 10−3 and subaverage ΦCr and ΦFe, 8–10‰/AMU, may represent still another source of particles. Because the higher vapor pressure of pure Cr should lead to ΦCr > ΦFe we infer either that Cr has a low activity coefficient in liquid Fe or that it forms a relatively involatile species there. A best fit correlation between ΦCr and ΦFe can be expressed in the form ΦCr = 0.31 × ΦFe1.47, although the data also are adequately fit by a linear regression. Correlated variation of ΦNi and ΦFe can be fit by the empirical relationship ΦNi = 0.016 × ΦFe2.58. For low ΦFe, we find ΦNi ΦFe, which probably reflects the increase with temperature of the vapor pressure of pure Ni, changes in activity coefficients of Fe and Ni, and the formation of relatively involatile wustite and magnetite. Differences between ΦNi and ΦFe in many samples mean that measured Fe/Ni ratios may differ appreciably from pre-atmospheric values. After compensating for evaporation by using the Rayleigh law, we estimate an average pre-atmospheric Fe/Ni ratio (by mass) in type I spherules of 19 ± 4 (σmean). Similarly, by assuming Ir is involatile, we obtain a preatmospheric ratio of Ir/Ni = 3 × 10−5, which is about 10 times smaller than the average measured value, but similar to the cosmic (CI) abundance ratio of 4 × 10−5. Cosmogenic nuclides have been detected in some Type I spherules at levels indicating irradiation as metal in space. Among conventional meteorites, the best matches to both the Cr/Fe and Fe/Ni ratios inferred for type I progenitors are metal from CO, CV, and CR chondrites and from unequilibrated ordinary chondrites. The match with metal from CM chondrites is acceptable but somewhat poorer. Iron meteorites, because of their low Cr/Fe ratios and low flux to Earth, make unlikely progenitors for type I spherules. We propose that most type I spherules derive from metal grains in carbonaceous-chondrite-like objects that were freed by comminution in space, or, less likely, that collisions of large objects formed droplets rich in metal.

Beryllium-10 contents of core samples from the St. Severin meteorite

Abstract Eleven samples taken from along the 35-cm core AIII of the LL-chondrite St. Severin have 10 Be contents ranging from 20 to 28 dpm/kg and averaging 24.5 ± 0.9 dpm/kg. The 10 Be contents increase with depth in the outermost 8 cm while at depths greater than 8 cm the 10 Be contents change little. Calculations based on cross sections for proton-induced reactions that make 10 Be disagree seriously with the measured values. Relatively large cross sections were constructed for neutron-induced reactions below 1 GeV. They give calculated 10 Be production rates that agree with the measured values to within 10%. Cosmogenic 10 Be in stony meteorites is better described as a medium-energy product than as a high-energy one.

26Al and10Be production in iron meteorites

Abstract We have measured by accelerator mass spectrometry the 26 Al contents of 20 and the 10 Be contents of 14 iron meteorites. The 26 Al contents are typically 30% or more lower than values obtained by counting techniques; the 10 Be contents are 10–15% lower. The production rates ( P ) of these nuclides decrease by more than a factor of two as the 4 He/ 21 Ne ratio increases with increasing shielding from 200 to 400. For the lighter shielding conditions expected in stony meteorites we estimate P 26 (Fe) as 3–4 dpm/kg and P 10 (Fe) as 4–5 dpm/kg. The average P/ 10 P 26 activity ratio is close to 1.5. Exposure ages calculated from 21 Ne/ 26 Al ratios cannot be calibrated so as to agree with both 40 KK/ ages and ages based on the shorter-lived nuclides 39 Ar and 36 Cl. If agreement with the latter is forced, then the disagreement with 40 KK/ ages may signal a 35% increase in the cosmic-ray intensity during the last 10 7 a.

Variability of the Al26 production rate in ordinary chondrites

Abstract 15 ordinary chondrites for which unusually high spallogenic Ne 22 Ne 21 or He 3 Ne 21 ratios had been reported and one meteorite with marked shock characteristics were selected in order to investigate the relations between Ne 22 Ne 21 ratios, Al26 contents and depth. We report Al26 and K contents of 13 samples from 11 of these and-noble gas contents of 30 samples from all of these stones. A decrease in the Al26 production rate accompanies the increase of Ne 22 Ne 21 towards the pre-atmospheric surface: Al obs 26 Al calc 26 = 3.2−2.0 Ne 22 Ne 21 for 1.08 ≤ Ne 22 Ne 21 ≤ 1.2 . Large deviations from this relationship may indicate that a meteorite experienced an abnormal flux of cosmic rays. For Ne 2 Ne 21 > 1.2 this trend continues but the data scatter more, probably because of the steadily increasing influence of pre-atmospheric size. Ne 22 Ne 21 ratios increase most rapidly in the outermost few centimeters according both to a plot of Ne 22 Ne 21 vs (recovered mass) 1 3 and to track studies. The increase seems to derive from the enhanced importance of nuclear reactions on Si. Ne 22 Ne 21 defines a region where the Al26 production rates are less sensitive to depth and vanish in the limit of large shielding; the weak correlation between Ne 22 Ne 21 and Al26 in this region rules out the use of the Ne 22 Ne 21 ratio as a basis for a shielding correction to Al26.

Beryllium-10 in Australasian Tektites: Evidence for a Sedimentary Precursor

Each of seven Australasian tektites contains about 1 x l08 atoms of beryllium-10 (half-life, 1.53 x 106 years) per gram. Cosmic-ray bombardment of the australites cannot have produced the measured amounts of beryllium-10 either at the earths surface or in space. The beryllium-10 contents of these australites are consistent with a sedimentary precursor that adsorbed from precipitation beryllium-10 produced in the atmosphere. The sediments must have spent several thousand years at the earths surface within a few million years of the tektite-producing event.

THE ELUSIVE 60Fe IN THE SOLAR NEBULA

No 60Ni or 61Ni anomalies have been detected in troilite inclusions from Muonionalusta, a 4565.3 ± 0.1 Ma old IVA iron meteorite, to the level of the analytical precision of 10 ppm. Because Muonionalusta troilite is very old, has a high Fe/Ni ratio, and is free of 61Ni anomalies, it is the ideal material in which to search for potential excesses of 60Ni produced by the decay of 60Fe (t 1/2 = 2.62 Ma). The 60Ni/58Ni and Fe/Ni ratios (up to 1680) measured here imply an upper limit for the initial 60Fe/56Fe for the Muonionalusta troilite of 3 × 10–9. Assuming that 60Fe was homogeneously distributed across the nebula, this result suggests that the solar system initial 60Fe/56Fe ratio was less than 5 × 10–9, which is lower than previously measured by at least a factor of 50.

Exposure histories of the lunar meteorites: MAC88104, MAC88105, Y791197, and Y86032

Four lunar meteorites, MacAlpine Hills (MAC) 88104, MacAlpine Hills 88105, Yamato (Y) 791197, and Yamato 86032 were analyzed for the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca. From these and published data, histories of exposure to cosmic rays were modelled in terms of two-stage irradiations each with a long first stage on the Moon lasting a time T2π > 5 Ma at a burial depth d2π[gcm2] followed by a second stage in space, i.e., the transit time between the Moon and the Earth, lasting a time T4π [Ma] in a body of typical meteoroidal size. The terrestrial age Tt [Ma]gives the time elapsed between meteorite fall and recovery in Antarctica. The following sets of parameters were obtained: MAC88104/5, 390 ≤ d2π ≤ 500, 0.04 ≤ T4π ≤ 0.11, 0.10 ≤ Tt ≤ 0.19; Y791197, d2π 1000, T4π = 10 ± 2, 0.08 < Tt < 0.12. From the number and exposure histories of lunar meteorites we infer a production rate on the order of 5 Ma−1 and an arrival rate worldwide of about 3 × 106 meteorites Ma−1. These results suggest that each impact event large enough to produce lunar meteorites sends a large number of them to the Earth.