IAB Iron Meteorites Research Articles

Precise ReOs determinations and systematics of iron meteorites

The ReOs system for samples of FeNi, sulphide, and phosphide from iron meteorites was investigated. Techniques were developed which yield reproducible analyses for Re/Os at the 2%‰ level and which permit complete isotopic exchange between sample and tracer, as is necessary for concentration measurements of Re and Os by isotope dilution. High precision osmium and rhenium isotope data have been obtained using negative ion thermal ionization, with ionization efficiencies of up to 10% for Os and 20% for Re, both for normals and for Re and Os extracted from the samples. Replicate analyses of Re/Os are in good agreement, within ±2.5%o. The results show a well defined correlation line on a 187Re- 187Os evolution diagram for iron meteorites from groups IAB, IIAB, IIIAB, IVA, and IVB, all taken together. This correlation line yields a slope of 0.07863 ± 0.00031 (2σ) and initial 187Os/ 188Os = 0.09560 ± 0.00018 (2σ). If the individual groups of iron meteorites for which there is sufficient dispersion in Re/Os are considered, data on the IIAB and on the IVA irons appear to indicate a difference in age of 60 ± 45 Ma, with the IVA group being older. This age difference is qualitatively the same as obtained for PdAg data but is larger. Sulphides from two IAB iron meteorites show extremely low concentrations of Re and Os and indicate that Re and Os are not partitioned into this phase during planetary differentiation. There is evidence for recent element remobilization or contamination, corresponding to relative enrichment of Re or loss of Os in the sulphides. Schreibersites contain small but significant amounts of Re and Os, with high Re/Os relative to the metal phases and with 187Os/ 188Os much more radiogenic than in the metal. Model ages for the Schreibersites are relatively young (4.3–3.5 AE) and indicate that the Schreibersites were open-systems for ReOs at least 0.5–1 AE after the original formation of the iron meteorites. It now appears possible to use metal- schreibersite pairs to determine internal isochrons. Based on the schreibersite model ages, the cooling rates for the two IAB meteorites are estimated to be ∼ 1°C/Ma, more than an order of magnitude lower than the most recently determined metallographic cooling rates for IAB irons (Herpfer et al., 1994).

The Lueders, Texas, IAB iron meteorite with silicate inclusions

Abstract The Lueders iron meteorite with silicate inclusions was recovered as a single specimen of ∼35.4 kg in Shackelford County, Texas, in 1973 and recognized as a meteorite in 1993. Siderophile element concentrations indicate chemical classification as a low‐Ni IAB iron meteorite closely related to Landes; like Landes, it has a Cu content ∼4σ above the main IAB‐IIICD trend and therefore we also designate Lueders as an anomalous member of IAB. The metallic host is composed of equigranular kamacite but with a suggestion of octahedral structure and with a bandwidth of 1.4 mm, suggesting structural classification as a coarse octahedrite (Og). The meteorite contains ∼23 wt% of roughly millimeter to centimeter‐sized angular silicate inclusions. Classification as a IAB is confirmed by O isotopic analysis of silicate inclusions. These inclusions contain an assemblage rich in silicates, troilite and graphite; lack certain minor phases (e.g., daubreelite); and have angular shapes. A variety of processes (e.g., fragmentation, partial melting, reduction) appear to have played a significant role in the formation of Lueders and all IAB iron meteorites. Petrologic and chemical differences confirm that Lueders is not paired with the widely distributed Odessa meteorite.

Isotopic composition of xenon and krypton in silicate-graphite inclusions of the El Taco, Campo del Cielo, IAB iron meteorite

Noble gases, extracted by temperature-stepped pyrolysis from acid-resistant silicate, schreibersite, and graphite residues of inclusions from the IAB iron meteorite El Taco (Campo del Cielo), have been analyzed for their isotopic composition. The concentrations of spallogenic (light) gases agree with previous data on adjacent specimens. Nominal feldspar and pyroxene fractions have lost up to 80% of their 3He during, or towards the end of, the exposure of El Taco to the cosmic radiation. In olivine the 3 Ne 21 Ne and NePNe ratios are as low as 2.18 ± 0.01 and 1.09 ± 0.03, respectively, presumably because the silicates were irradiated within a FeNi matrix under extremely heavy shielding. Argon, Kr, and Xe in the silicates are dominated by a trapped “planetary” component with 100 ≤ 36 Ar 132 Xe ≤ 700 and 1.7≤ 84 Kr 132 Xe ≤ 2.2 . Xenon released from the silicates at extraction temperatures > 1000°C is isotopically close to ureilite Xe, while the high- T Xe from graphite and schreibersite is unlike any of the presently well-established types of Xe. The isotopic composition of this Xe can be generated as a mixture of mass-fractionated UXe, with light isotopes depleted by (2.2 ± 0.1) % per atomic mass unit, and 244PuXe. The fission component, 10% in the case of 136Xe, is not due to in situ-production; it must have been added prior to incorporation of the Xe into the El Taco parent body and presumably underwent the mass fractionation process together with UXe. From the silicates 127I( n, γ)-produced 128Xe ∗ and radiogenic 129Xe ∗ are released together, with the same 128 Xe ∗ 129 Xe ∗ ratio in nominal olivine, pyroxene, and feldspar although the maximum release from pyroxene occurs about 300°C higher than from feldspar. From this the cooling rate of silicates for the temperature range 1500-1200°C is deduced to be ≥100°C per million years. The 80 Kr 82 Kr ratio of 79,81Br( n, γ)-produced 80,82Kr is 2.86 ± 0.04, in agreement with the value found in inclusions of the IAB Linwood but higher than observed in almost all stone meteorites.

Sulfur and selenium in chondritic meteorites

Abstract— We report here new analyses of S and Se in carbonaceous chondrites (2 CIs, 11 CMs, 6 CO3s, 7 CV3s, 2 C4s, 4 CRs, and 1 CH), 2 rumurutiites, ordinary chondrites (2 Hs, 2 Ls, and 1 LL), 3 anomalous chondrites, 3 acapulcoites, 3 lodranites, and in silicate inclusions of the Landes IAB iron meteorite. To avoid problems from inhomogeneous distribution of sulfides, the same samples that had been analysed for Se by INAA were analysed for S using a Leybold Heraeus Carbon and Sulfur Analyser (CSA 2002).With the measured CI contents of 5.41% S and 21.4 ppm Se a CI S/Se ratio of 2540 is obtained. A nearly identical S/Se ratio of 2560 ± 150 is found for carbonaceous chondrites (average of falls). The average ratio of all meteorite falls analysed in this study was 2500 ± 270. These data suggest that the new S content of Orgueil with 5.41% provides a reliable estimate for the average Solar System. The new solar system abundance of S of 4.62 × 105 (atoms/106 Si) is in good agreement with the solar photospheric abundance of 7.21 (log (a(H)) = E12) (Anders and Grevesse, 1989).Among the 50 analysed meteorites, 24 were finds from hot (Australia, Africa) and cold (Antarctica) deserts. Weathering effects in the carbonaceous chondrites and in one lodranite from the hot deserts resulted in losses of S, Se, Na and occasionally Ni. Sulfur is apparently more affected by weathering than Se. No losses were observed in ordinary chondrite finds and in meteorites collected in the Antarctica, except for the obvious loss of Na in the CM‐chondrite Y 74662.The low S‐content of 0.096% in Gibson, a lodranite, is probably not representative of this group of meteorites. Gibson is a find from the Australian desert and has lost S and also Se by weathering. Two other lodranites, finds from Antarctica, have about 2% S.

Classification and origin of IAB and IIICD iron meteorites

We have analyzed, by duplicate neutron-activation analysis, 125 members of iron meteorite groups IAB and IIICD. These data show no hiatus between the groups, and we recommend that the two sets be treated as a single group until data are obtained that require their separation. In cases where there is no ambiguity, we will use IAB to designate the combined group. Our data allow properties of group IAB to be more tightly constrained than heretofore. We examined the properties of ungrouped irons occupying the same region of GaNi and GeNi space as IAB and found that our more precise data did not add any to the group. Based primarily on CoNi, AuNi, and to a lesser extent, CuNi and AsNi trends we find that three irons (Hassi-Jekna, Magnesia and Qarat al Hanash) previously assigned to IIICD are better designated ungrouped, primarily because their Au contents diverge from those of IAB; six irons (EET 84300, Mertzon, Misteca, Persimmon Creek, Yongning, and Zacatecas (1792)) have also been removed from IAB on the basis of the new data. Fractional crystallization models of magmas saturated in troilite yield trends for several elements quite similar to those observed in IAB and, with moderate modification of the distribution coefficients, these can also account for the IIICD trend. However, this model requires metal segregation and convective stirring at very low temperatures, and predicts a much lower abundance of low-Ni irons than observed. It also fails to account for the ubiquitous presence of trapped melt and chondritic (or subchondritic) silicates in IAB irons. The weight of the evidence supports an impact-melt model in which individual IAB irons are interpreted as melt pools produced by impacts into a chondritic megaregolith. The lower the melt temperature of the melt, the larger the fraction of Ga, Ge, and Ir that remained sequestered in unmelted solids. The increasing range in Ga, Ge, and Ir with increasing Ni content can be explained by mixing between different primary melt pools having compositions along the lower (IIICD) envelope of IAB-IIICD. And we suggest that some high-Ni melt pools having low cotectic temperatures experienced crystal settling on a scale of cm to m; specifically, this process may be responsible for producing the most Ni-rich members of the group. We infer that the IAB precursor materials had properties (high porosity, fine grain size) that made them susceptible to impact melting. A high FeS would also favor melt generation. The oxygen isotope composition of IAB silicates is 0.45%o below the terrestrial-fractionation line in a region occupied by CR chondrites and other carbonaceous-chondrite-related meteorites. This suggests that the chondritic precursor was a metal- and FeS-rich carbonaceous chondrite. Cl-normalized element/Ni ratios in low-Ni (65–68 mg/g) TAB are ≥1.5 for Co, Ga, Ge, and Au. Although the high Ga and Ge abundance ratio could indicate fractional crystallization of elements having high solid/melt distribution coefficients, this explanation cannot account for the Co and Au enrichments. We interpret the high Ga, Ge, and Au ratios in terms of a nebular condensation-accretion model; we suggest that these elements condensed into a late-formed, fine nebular component having an enhanced abundance in the chondritic materials parental to group IAB. Because it is more refractory than Fe and Co, the Ni content of this fine metal was appreciably lower than that of the early formed coarse metal.

Mineralogy of new Antarctic achondrites with affinity to Lodran and a model of their evolution in an asteroid

Abstract— We studied five new Antarctic achondrites, MacAlpine Hills (MAC) 88177, Yamato (Y)74357, Y75274, Y791491 and Elephant Moraine (EET)84302 by mineralogical techniques to gain a better understanding of the mineral assemblages of a group of meteorites with an affinity to Lodran (stony‐iron meteorite) and their formation processes. This group is being called lodranites. These meteorites contain major coarse‐grained orthopyroxene (Opx) and olivine as in Lodran and variable amounts of FeNi metal and troilite etc. MAC88177 has more augite and less FeNi than Lodran; Y74357 has more olivine and contains minor augite; Y791491 contains in addition plagioclase. EET84302 has an Acapulco‐like chondritic mineral assembladge and is enriched in FeNi metal and plagioclase, but one part is enriched in Opx and chromite. The EET84302 and MAC88177 Opx crystals have dusty cores as in Acapulco. EET84302 and Y75274 are more Mg‐rich than other members of the lodranite group, and Y74357 is intermediate. Since these meteorites all have coarse‐grained textures, similar major mineral assemblages, variable amounts of augite, plagioclase, FeNi metal, chromite and olivine, we suggest that they are related and are linked to a parent body with modified chondritic compositions. The variability of the abundances of these minerals are in line with a proposed model of the surface mineral assemblages of the S asteroids. The mineral assemblages can best be explained by differing degrees of loss or movements of lower temperature partial melts and recrystallization, and reduction. A portion of EET84302 rich in metal and plagioclase may represent a type of component removed from the lodranite group meteorites. Y791058 and Caddo County, which were studied for comparison, are plagioclase‐rich silicate inclusions in IAB iron meteorites and may have been derived by a similar process but in a different body.

The Trace Element Chemistry of the Metal in IAB Iron Meteorites

Abstract Metal grains from a silicate inclusion in Landes, a group IAB iron-meteorite, were separated and analyzed via IN A A for 14 siderophile elements. The same 14 elements were determined in matrix metal from Landes, 5 other IAB meteorites and three anomalous iron meteorites. Compared to the other samples and to chondritic metal, the inclusion metal is enriched in siderophile elements. In chondritic metal, the siderophile element contents are higher when Fe0 is oxidized to FeO. Silicates in IAB inclusions contain little FeO, yet the metal is as rich in siderophile elements as the most oxidized chondrites. Evidently the IAB body contained its full cosmic complement of S resulting in an FeS content ~5 x that of ordinary chondrites. This explains the composition of the metal since sulfurization, like oxidation, reduces the available Fe0. IAB meteorites contain their full cosmic complement of other moderately volatile elements, which implies an equally high S content. A high S content significantly affects the melting and freezing of FeNi-metal. About 85% of the metal melts at the Fe-FeS eutectic (~960 °C), and all of the metal is molten at ~1050°C. During cooling, ~85% of the metal crystallizes with a Ni content of 6±1% between 1050°c and 960°C. Besides a small depletion in elements with low melting points or chalcophile tendencies this metal is little fractionated. Bellow 960 °C, the Ni content of the metal increases until the melt is exhausted. This Ni-rich metal also becomes progressively depleted in refractory elements and enriched in volatile elements. The crystallization process thus yields products that mimic the abundance and composition of group IAB meteorites.

I-Xe and 40Ar- 39Ar analyses of silicate from the Eagle Station pallasite and the anomalous iron meteorite Enon

Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and 40Ar- 39Ar techniques, Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic 40Ar at low temperature, and gives a plateau age of 4.59 ± 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred Pu U ratio is more than 2σ above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 6 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The Eagle Station pallasite, the most 16O-rich meteorite known, gives a complex 40Ar- 39Ar age pattern which suggests a recent (≲0.85 Ga) severe thermal disturbance. The absence of excess 129Xe, and the low trapped Ar and Xe contents, are consistent with this interpretation. The similarity between 40Ar- 39Ar data for Eagle Station and for the olivine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pallasites, in the sense that similar processes operating at similar times on different parent bodies may have been involved in the formation of olivine in both types of meteorites.

The Acapulco meteorite: Chemistry, mineralogy and irradiation effects

The Acapulco meteorite fell in August, 1976, at El Quemado, near Acapulco, Mexico. It is a unique object with chondritic composition but achondritic texture. High degree of recrystallisation and mineral chemical data indicate formation of the meteorite under redox conditions intermediate between those of H- and E-chondrites at ~ 1100°C, from which it cooled at a rate > 10°C/Myr. The major element composition is within the range of H-chondrites. Troilite and metal, and associated trace elements, are inhomogeneously distributed. Chromium is a factor of two higher than in H-chondrites. Enrichments of P and U indicate high phosphate content. Limited extent of partial melting may explain the light REE enrichment. However other incompatible elements have normal H-chondritic abundances or are even depleted like K and Rb. Moderately volatile or volatile elements (e.g. Mn, Ga, Ge, Zn) are enriched nearly to the level of C1-chondrites. Planetary noble gases are also significantly higher than in equilibrated ordinary chondrites. High temperature recrystallisation has not affected volatile element abundances. Compared to H-chondrites Acapulco is enriched in refractory siderophile elements. The distribution of W and other siderophile elements between metal and silicate phases are indicative of the higher temperature and lower oxygen fugacity of the assemblage. However, contrary to previous claims, the distribution of W cannot be used to calculate the equilibration temperature. Low K and high U contents are also reflected in the anomalous amounts of 40Ar and 4He. The old K-Ar age (4.7 ± 0.3 Gyr) and high 244Pu track densities indicate mobilisation of U and Pu-rich phases shortly after formation of the parent material. This and other evidence suggests that Acapulco may represent a rock formed in the early stages of incipient melting of a chondritic parent body. However, since compositional differences between Acapulco and H-chondrites cannot be explained by fractionation processes on the Acapulco parent body. Acapulco must have originated from a different parent body. Lack of depletion of volatile elements, absence of chondrules and reduced mineral composition indicate some relationship of Acapulco to silicate inclusions in iron meteorites and to other unusual meteorites. Oxygen isotopes and chemical data suggest that there are at least three different groups of reduced chondritic meteorites: 1. (a) Acapulco, Lodran, and probably Allan Hills A 77081; 2. (b) Pontlyfni, Mount Morris, Winona and silicate inclusions in IAB iron meteorites, and 3. (c) Kakangari. An exposure age of 5 × 10 6 yr is deduced from spallogenic rare gas data.

40Ar- 39Ar dating of inclusions from IAB iron meteorites

Silicate and troilite from IAB iron meteorites were dated by the 40Ar- 39Ar technique. Silicate from four IAB meteorites gave well-defined apparent-age plateaus which accounted for 71–99% of the released 39Ar. At low temperatures, only Copiapo showed appreciable loss of 40Ar, while Mundrabilla and Woodbine released excess 40Ar. The plateau ages are: 4.50 Byr for Copiapo, 4.57 Byr for Mundrabilla, 4.57 Byr for Woodbine, 4.54 Byr for unetched Pitts, and 4.57 Byr for etched Pitts; the 1σ error in each case is ± 0.03 Byr. A poorly-defined age plateau for Landes gives an age of 4.48 Byr, while the total K-Ar age (4.55 Byr) is significantly higher. The average ( 40Ar/ 36Ar) trapped ratio for all silicate samples is 0.4 ± 0.4. Simple and undisturbed K-Ar systems are rare for meteorites, yet it appears to be a common feature for IAB silicates. In addition, plateau ages of IAB silicates are as old or older than the mean age of unshocked chondrites (4.47 Byr). Troilite samples yielded complex patterns which were evaluated via 40Ar/ 36Ar vs 39Ar/ 36Ar plots. Data for Pitts troilite are consistent with silicate and troilite retaining Ar at about the same time initially, but then 4.25 Byr ago nearly all the Ar in troilite was redistributed. The 700–1000°C points for Mundrabilla troilite define a line which gives an age of 6.2 Byr and ( 40Ar/ 36Ar) trapped = 42 . This line may be an artifact, perhaps produced by homogenization of Ar and K. Approximate estimates of cosmic-ray exposure ages are 240 Myr for Landes, 130 Myr for Copiapo, 190 Myr for Woodbine, 170 Myr for Mundrabilla troilite, and 60 Myr for Pitts troilite. The I-Xe study of these same samples revealed a good correlation between well-defined I-Xe ages of silicates and Ni contents of metal ( Niemeyer, 1979). The poorer resolution of the 40Ar- 39Ar technique hampers a similar evaluation; nevertheless, plateau ages of the silicates suggest a systematic trend with Ni contents.

I-Xe dating of silicate and troilite from IAB iron meteorites

The IAB iron meteorites may be related to the chondrites: siderophile elements in the metal matrix have chondritic abundances, and the abundant silicate inclusions are chondritic both in mineralogy and in chemical composition. Silicate and troilite (FeS) from IAB irons were analyzed by the I-Xe technique. Four IAB silicate samples gave well-defined I-Xe ages [in millions of years relative to Bjurböle; the monitor error (± 2.5 Myr) is not included]: −3.7 ± 0.3 for Woodbine, −0.7 ± 0. 6 for Mundrabilla, +1.4 ± 0.7 for Copiapo, and +2.6 ± 0.6 for Landes. The ( 129Xe/ 132Xe) trapped ratios are consistent with previous values for chondrites, with the exception of Landes which has an extraordinary trapped ratio of 3.5 ± 0.2. Both analyses of silicate from Pitts gave anomalous I-Xe patterns. Troilite samples were also analyzed: Pitts troilite gave a complex I-Xe pattern, which suggests an age of +17 Myr; Mundrabilla troilite defined a good I-Xe correlation, which after correction for neutron capture on 128Te gave an age of −10.8 ± 0.7 Myr. Thus, surprisingly, low-melting troilite substantially predates high-melting silicate in Mundrabilla. Abundances of Ga, Ge, and Ni in metal from these meteorites are correlated with I-Xe ages of the silicate; meteorites with older silicates have greater Ni contents. No model easily accounts for this result as well as other properties of IAB irons; nevertheless, these results, taken at face value, overall favor a nebular formation model (e.g. Wasson, 1970, Icarus 12, 407–423). The great age of troilite from Mundrabilla suggests that this troilite formed in a different nebular region from the silicate and metal, and was later mechanically mixed with these other phases. The correlation between the trace elements in the metal and the I-Xe ages of the silicate provides one of the first known instances in which another well-defined meteoritic property correlates with I-Xe ages. In addition, almost all the 129Xe in Mundrabilla silicate (etched in acid) was correlated with 128Xe. These two results further support the validity of the I-Xe dating method.

Genetic relations between iron and stony meteorites

The main group pallasites and the mesosiderites fall within the oxygen isotope group previously determined for the calcium-rich achondrites (eucrites, howardites and diogenites), consistent with derivation from a common source material, and perhaps a common parent body. The group IIE iron meteorites were derived from the same source material as H-group ordinary chondrites. The chondrite-like silicate inclusions in group IAB iron meteorites are not related to the ordinary chondrites, but may be related to the enstatite chondrites. Several meteorites previously considered “anomalous” fall into these groups: Pontlyfni and Winona with the IAB irons, and Netschaëvo possibly with the H chondrites and IIE irons. The unusual pallasites Eagle Station and Itzawisis have remarkable oxygen isotopic compositions, and have more of the 16O-rich component than any other meteorite. Bencubbin and Weatherford are also unusual in their isotopic compositions, and may bear some relationship to the C2 carbonaceous chondrites. Lodran and Enon are isotopically similar to one another and are close to the achondrite-mesosiderite-pallasite group.

9. Unusual Primitive Meteorites: Chondritic Inclusions from Group IAB Iron Meteorites

Several meteorite types, distinct from the large chondrite groups (H, L, LL, E and carbonaceous), are known to also have chondritic compositions. These meteorites preserve information on conditions at additional formation locations in the early solar nebula. Silicate inclusions from group IAB iron meteorites are one such type. Evidence for their chondritic nature is given and their formation discussed.

The Shaw chondrite, I. The case of the missing metal

The mineralogy, elemental and isotopic composition of the Shaw meteorite indicate that it is a highly metamorphosed L-group chondrite which has lost a portion of its metal and sulfide. The metal which remains has an unusual composition relative to that in other L-group chondrites. It is enriched in Ga, Ge, Ir, Mo, Os, Pt, Re and Ru and depleted in As, Au, Cu and Sb. A comparison of the relative enrichments and depletions in Shaw to those observed in San Cristobal, the extreme end-member of group IAB iron meteorites, shows that the metal phases in these two meteorites have complementary compositions. This implies that the metal in Shaw represents the residual solid of a partial melting process while the missing metal, which drained away, may have gone to form an iron meteorite, like San Cristobal.