Metallographic cooling rates of the IIIAB iron meteorites

An improved computer simulation program has been developed and used to re-measure the metallographic cooling rates of the IIIAB irons, the largest iron meteorite chemical group. The formation of this chemical group is attributed to fractional crystallization of a single molten metallic core during solidification. Group IIIAB irons cooling rates vary by a factor of 6 from 56 to 338 °C/My. The cooling rate variation for each meteorite is much smaller than in previous studies and the uncertainty in the measured cooling rate for each meteorite is greatly reduced. The lack of correction for the orientation of the kamacite-taenite interface in the cooling rate measurement of a given meteorite in previous studies not only leads to large cooling rate variations but also to inaccurate and low cooling rates. The cooling rate variation with Ni content in the IIIAB chemical group measured in this study is attributable, in part, to the variation in nucleation temperature of the Widmanstatten pattern with Ni content and nucleation mechanism. However, the factor of 6 variation in cooling rate of the IIIAB irons is hard to explain unless the IIIAB asteroidal core was exposed or partially exposed in the temperature range in which the Widmanstatten pattern formed. Measurements of the size of the island phase in the cloudy zone of the taenite phase and Re-Os data from the IIIAB irons and the pallasites make it hard to reconcile the idea that pallasites are located at the boundary of the IIIAB asteroid core.     

Compositional trends and cooling rates of group IVB iron meteorites

Based on new neutron activation data for group IVB we find that log-element — log-Ni trends are best understood in terms of core formation and fractional crystallization. The limited compositional range found in group IVB seems to reflect the fact that, because of the low concentrations of S, P and C and the high concentration of Ni, $\text{k}{}_{\mathrm{\chi }}$ values are nearer unity than are those in other magmatic groups. Mean volatile abundances in group IVB are much lower than those found in any group of chondritic meteorites, suggesting that these low abundances were not entirely the result of nebular processes, but that planetary outgassing was also involved.We calculated cooling rates on the basis of a computer simulation of the growth of kamacite crystals; these calculations are particularly straightforward for the high-Ni irons since no local bulk Ni enrichment is involved. We estimate a mean IVB cooling rate of 170–230 K/Ma, the lower values based on 20 K undercooling, the higher on no undercooling. There is no dependence of cooling rate on chemical composition. The mean cooling rate of the low-volatile groups IVB and IVA are both much higher than those typical of iron-meteorite groups. This indicates small parent bodies, and reinforces the above suggestion that the low volatile contents resulted from planetary outgassing.There is a small compositional hiatus in group IVB, but since the sets on both sides of the hiatus form continuous trends on log-element — log-Ni diagrams and have the same cooling rates, it appears that both sets originated in a single oxidized, refractory-rich parent body. This sampling hiatus corresponds to 26% of the original core, a value shown to be typical for a random sequence sampled 11 times.     

Fission tracks and cooling rates of meteorites

Recent work on fission track studies of meteorite samples to obtain cooling rates of metetorite parent bodies is reviewed. The cooling rates of chondrites are in excess of 1^oK/10⁶ yr. Fission track studies of phosphate grains in mesosiderites do not support the extremely slow cooling rates of 0°1^oK/10⁸ yr for these meteorites, inferred from metallographic studies. The accumulating evidence from fission track studies indicates a gross underestimation of the cooling rates of meteorites as determined by the metallographic techniques.     

A nonmagmatic origin of group-IIE iron meteorites

New neutron activation data on 10 elements in 12 IIE and IIE-related irons lead to a reclassification of several irons. Seymchan and Lonaconing are removed from IIE, and Leshan added. Four IIE members are designated IIE-An to call attention to some anomalous properties. The eight normal IIE members define element-Ni trends generally similar to those in the nonmagmatic group IAB; the small negative slopes on W-Ni and Ir-Ni diagrams are strongly indicative of a nonmagmatic origin of the IIE irons. We propose that IIE irons like IAB irons originated as individual pools of impact-produced melt in the near-surface region of a chondritic parent body. The positive As-Ni and Au-Ni trends are the only evidence suggesting fractional crystallization, but their slopes are lower than those in magmatic group IIIAB, and only slightly higher than those of Cu and Sb in IAB. We suggest that the S and C contents of the IIE precursor materials were much lower than those of the IAB precursors, thus higher temperatures were required to generate enough metallic melt to segregate into pools. These higher temperatures are also reflected in the nonchondritic compositions of the silicate inclusions.     

The origin of iron silicides in ureilite meteorites

Ureilite meteorites contain iron silicide minerals including suessite (Fe,Ni)₃Si, hapkeite (Fe₂Si) and xifengite (Fe₅Si₃). Despite occurring mostly in brecciated varieties presumed to be derived from the regolith of the ureilite parent asteroid, suessite has also been confirmed in one lithology of a dimict ureilite (NWA 1241). In contrast, Si-bearing Fe-metals occur in both brecciated and unbrecciated ureilites, implying that they were formed throughout the ureilite parent asteroid. We examined major, minor and trace element data of Fe-metals in seven brecciated ureilites (DaG 319, DaG 999, DaG 1000, DaG 1023, DaG 1047, EET 83309, and EET 87720) in addition to the dimict ureilite NWA 1241.In this study we show that the silicides and Si-bearing metals in ureilites have similar siderophile trace element patterns; therefore, the precursors to the silicides were indigenous to the ureilite parent body. Si-free kamacite grains in brecciated ureilites show flatter, more chondritic siderophile element patterns. They may also be derived from the interior of the ureilite parent body, but some may be of exogenous origin (impactor debris), as are rare taenite grains.On Earth, iron silicides are often formed under high-temperature and strongly reducing conditions (e.g. blast furnaces, lightning strikes). On the Moon, hapkeite (Fe₂Si) and other silicides have been found in the regolith where they were formed by impact-induced space weathering. In the Stardust aerogel, iron silicides derived from comet Wild2 were also formed by an impact-related reduction process. Silicides in ureilite regolith breccias may have formed by similar processes but ureilites additionally contain abundant elemental carbon which probably acted as a reducing agent, thus larger and more abundant silicide grains were formed than in the lunar regolith or cometary material. The origin of suessite in NWA 1241 may be analogous to that of reduced lithologies in the terrestrial mantle, although a regolith origin may also be possible since this sample is shown here to be a dimict breccia.     

Thermal history and origin of the IVB iron meteorites and their parent body

We have determined metallographic cooling rates of 9 IVB irons by measuring Ni gradients 3 μm or less in length at kamacite-taenite boundaries with the analytical transmission electron microscope and by comparing these Ni gradients with those derived by modeling kamacite growth. Cooling rates at 600-400 °C vary from 475 K/Myr at the low-Ni end of group IVB to 5000 K/Myr at the high-Ni end. Sizes of high-Ni particles in the cloudy zone microstructure in taenite and the widths of the tetrataenite rims, which both increase with decreasing cooling rate, are inversely correlated with the bulk Ni concentrations of the IVB irons confirming the correlation between cooling rate and bulk Ni. Since samples of a core that cooled inside a thermally insulating silicate mantle should have uniform cooling rates, the IVB core must have cooled through 500 °C without a silicate mantle. The correlation between cooling rate and bulk Ni suggests that the core crystallized concentrically outwards. Our thermal and fractional crystallization models suggest that in this case the radius of the core was 65 ± 15 km when it cooled without a mantle. The mantle was probably removed when the IVB body was torn apart in a glancing impact with a larger body. Clean separation of the mantle from the solid core during this impact could have been aided by a thin layer of residual metallic melt at the core-mantle boundary. Thus the IVB irons may have crystallized in a well-mantled core that was 70 ± 15 km in radius while it was inside a body of radius 140 ± 30 km.     

The isotopic composition and concentration of Ag in iron meteorites and the origin of exotic silver

The isotopic composition of Ag and the concentration of Ag and Pd have been determined in Canyon Diablo (IA), Grant (IIIB), Hoba, Santa Clara, Tlacotepec and Warburton Range (IVB), Piñon and Deep Springs (anom.). Troilite from Grant and Santa Clara have also been analyzed. All of these meteorites, with the exception of Canyon Diablo, give ${}^{107}\text{Ag}{}^{109}\text{Ag}$ in the metal phase that is greater than the terrestrial value with the enrichments of ¹⁰⁷Ag ranging from ~2% to 212%. These data show that Ag of anomalous isotopic composition is common to all IVB and anomalous meteorites. The results on Grant suggest that the anomalies may be widespread including more common meteorite groups. There is a general correlation of ${}^{107}\text{Ag}{}^{109}\text{Ag}$ with $\text{Pd}\text{Ag}$ except for the data from FeS of Santa Clara. It is concluded that the excess ¹⁰⁷Ag is the result of decay of ¹⁰⁷Pd, a nuclide that is extinct at present with an abundance of ${}^{107}\text{Pd}{}^{108}\text{Pd}$ of about 3 × 10^?5. The troilite in Grant exhibits normal ${}^{107}\text{Ag}{}^{109}\text{Ag}$ to within errors, a high Ag concentration and a low ratio of ${}^{108}\text{Pd}{}^{109}\text{Ag}$ ~0.17. Grant metal has ${}^{107}\text{Ag}{}^{109}\text{Ag}$ that is ~2% greater than normal and a high ratio of ${}^{108}\text{Pd}{}^{109}\text{Ag}$ ~ 10³. The data from Grant appear to represent a ¹⁰⁷Pd-¹⁰⁷Ag isochron and indicate that the cooling rate at elevated temperatures was sufficiently rapid to preserve substantial isotopic differences between metal and troilite. Troilite in Santa Clara was found to contain Ag with a very high ${}^{107}\text{Ag}{}^{109}\text{Ag}$ ratio (108% above normal), an Ag concentration only a factor of three above the metal and a high value of ${}^{108}\text{Pd}{}^{109}\text{Ag}$ ~1.3 × 10⁴. The troilite has a higher ${}^{107}\text{Ag}{}^{109}\text{Ag}$ than the metal. These data are not compatible with a simple model of in situ decay and subsequent local Ag redistribution between metal and troilite during cooling. These data suggest that Ag in Santa Clara and possibly other IVB meteorites is made up of almost pure ¹⁰⁷Ag produced from ¹⁰⁷Pd decay and ¹⁰⁹Ag produced by nuclear reactions with only a small amount of “normal” Ag. This indicates an intense energetic particle bombardment history in the early solar system (~10²⁰ p/m²) which occurred after the formation of small planetary bodies. We infer that a T-Tauri activity by the early sun contributed to some late stage “nucleosynthesis” and the heating of a dust cloud. In addition, implications on the early thermal evolution of iron meteorites are presented based on ¹⁰⁷Pd decay and models of the cooling history.     

Petrology and origin of the shergottite meteorites

Shergottites contain cumulus pigeonite and augite, probably without cumulus plagioclase and crystallized under relatively oxidizing conditions. Shergotty and Zagami may differ in the relative proportions of cumulus pyroxenes and crystallized intercumulus liquid, but the compositions of pyroxenes and liquid are similar in both meteorites. Absence of olivine in melting experiments suggests that the shergottites crystallized from fractionated derivatives of primary liquids. Low-Ca pyroxene and augite apparently began to crystallize from these primary liquids prior to plagioclase. Shergottites can be readily distinguished from other achondrite groups by their mineralogies, crystallization sequences and inferred source region compositions. However, the source regions of the shergottites may be related to those of other achondrite types by addition or loss of volatile components.The bulk composition of the Earth's upper mantle overlaps that of permissible shergottite source regions. Shergottites and terrestrial basalts display similarities in oxidation state and concentrations of trace and minor elements with a wide range of cosmochemical and geochemical affinities. Accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body or accretion of the Earth's upper mantle from planetesimals similar to the shergottite parent body may account for many of their similarities. Models of the origin of the Earth's upper mantle which attribute its oxidation state, its siderophile element abundances and its volatile element abundances to uniquely terrestrial processes or conditions, or to factors unique to the origin and differentiation of large bodies, are unattractive in light of the similarities between shergottites and terrestrial basalts.     

Formation of IIAB iron meteorites

Group IIAB is the third largest group of iron meteorites and the second largest group that formed by fractional crystallization; many of these irons formed from the P-rich portion of a magma consisting of two-immiscible liquids. We report neutron-activation data for 78 IIAB irons. These confirm earlier studies showing that the group has the largest known range in Ir concentrations (a factor of 4000) and that slopes are steeply negative on plots of Ir vs. Au or As (or Ni). High negative slopes imply relatively high distribution coefficients for Ir, Au, and As (but, with rare exceptions, remaining less than unity for the latter). IIAB appears to have had the highest S contents of any magmatic group of iron meteorites, consistent with its high contents of other volatile siderophiles, particularly Ga and Ge. Large fractions of trapped melt were present in the IIAB irons with the highest Au and As and lowest Ir contents. As a result, when these irons crystallized, the D_Au and D_As values can, with moderate accuracy, be estimated to have been roughly 0.53 and 0.46, respectively. These low values imply that the initial nonmetal (S + P) content of the magma was much lower than 170 mg/g, as estimated in earlier studies; our estimate is 75 mg/g. Our results are consistent with an initial P/S ratio of 0.25, similar to the ratio estimated for other magmatic groups. There is little doubt that incompatible S-rich and P-rich metallic liquids were involved during the formation of group IIAB. After 20% crystallization of our assumed starting composition the two-liquid boundary is encountered (at 72 mg/g S and 18 mg/g P). Initially the volume of S-rich liquid is very small, but continued crystallization increased the volume of this phase and decreased its P/S ratio while increasing this ratio in the P-rich liquid. Most crystallization of the IIAB magma would have occurred in the lower, P-rich portion of the core. However, metal was still a liquidus phase at the top of the core and, because both the immiscible liquids would have convected, they may have approached equilibrium throughout the very limited crystallization of the magma recorded in group IIAB. All IIAB irons contain trapped melt, and this melt will have had very different compositions depending on whether the liquid is S-rich (at the outer solid/liquid interface) or P-rich (at the inner interface). The P/S ratio in the melt trapped in the Santa Luzia iron is about 0.6 g/g, consistent with our modeling of Ir-Au and Ir-As trends implying that Santa Luzia formed in the lower, P-rich portion of the core after about 48% crystallization of the magma. Because the liquids were in equilibrium, the point at which immiscibility first occurred is not recorded by a dramatic change in the trends on element-Au diagrams; the main compositional effect is recorded in the P/S ratio of the trapped melt. The high-Au (>0.8 μg/g) irons for which large sections are available all contain skeletal schreibersite implying a relatively high (>0.3 g/g) P/S ratio; none of these irons could have crystallized from the S-rich upper layer of the core.     

Primary fractionation of elements among iron meteorites

The primary fractionation process in iron meteorites is that responsible for the distribution of elements between the groups, most notably Ga and Ge, which show concentration ranges of 10³ and 10⁴ respectively. To investigate the cause of the primary fractionation, concentrations of 16 elements were converted to relative abundances by dividing the element/Ni ratio by the CI chondrite ratio. These abundances were plotted on logarithmic graphs with data for each group (except IB and IIICD) and each cluster of closely related anomalous irons averaged.Co, P, Au, As, Cu, Sb, Ge and Zn are positively correlated with Ga. For most groups (except IA, IC and IIAB) relative abundances of these elements tend to decrease from about 1 in approximately the order listed above. This is the expected order in which these elements will condense into Fe, Ni during equilibrium nebular condensation. Mean relative abundances of refractory elements in groups generally lie within a narrow range of 0.5–2, and are uncorrelated with Ga. Although the equilibrium model may be only a gross approximation, it suggests that most primary fractionation did occur during nebular condensation.The anomalous irons are essential for defining many of the primary fractionation trends. On several element-Ga graphs the displacements of the anomalous irons from the primary curves indicate that these irons experienced the same secondary fractionation process (probably fractional crystallization) that produced the trends within most groups. The anomalous irons appear to be samples from over 50 minor groups, which have similar histories to the 12 major groups.     

On the irradiation history and origin of gas-rich meteorites

With the transmission electron microscope, we have made detailed studies of the track density gradients and irradiation geometries of track-rich grains and chondrules in sections of Fayetteville and Kapoeta. We have made the same type of studies in sections of lunar breccias and grains from lunar soil for comparison.A substantial fraction (50–90 per cent) of the meteoritic track-rich grains and chondrules show evidence of having been irradiated anisotropically in their different faces, as would be expected for irradiation on the surface of a parent body. Our observations thus support the hypothesis that the irradiation of these grains and chondrules took place on the regoliths of asteroidal-sized bodies.Measurement of steepest track density gradients indicate that while there are finite differences between spectra exhibited by individual gas-rich meteorites, the average solar flare spectral shapes have been similar over the last ~4 b.y. or so.     

Chemical fractionation in iron meteorites and its interpretation

Published analyses of trace and minor elements in iron meteorites have been compiled and the distributions interpreted with the chemical groups defined by Wasson. When each element is plotted against Ni on log scales, groups are often clearly resolved with all the members of a group falling within the limits of sampling and analytical error on a straight line. The lines for groups IIIa,b and IVa are generally parallel with IIa,b plotting on a steeper gradient. In contrast to Ga and Qe, many elements show variations within a group which may approach that shown by all the iron meteorites. Group I members have a fairly uniform concentration of elements which are severely fractionated in the other major groups. There are also fewer correlations of elements in group I.     

Mineralogy and petrology of silicate inclusions in iron meteorites

Silicate inclusions in 17 iron meteorites have been analyzed by the electron microprobe and classified, according to their phase assemblages, compositions, and textures, into three major types: Odessa, Copiapo, and Weekeroo Station, and three miscellaneous types: Enon, Kendall County, and Netschaëvo. Phase compositions in both Odessa- and Copiapo-type inclusions are very similar, but the two types are different in texture and constituent phases. Weekeroo Station-type inclusions are very different in every respect from other inclusions.For Odessa- and Copiapo-type inclusions, the distribution coefficients of Fe²⁺ and Mg in coexisting orthopyroxene and clinopyroxene indicate equilibration temperatures of 1,000° C, and the Ca/(Ca+Mg) ratios indicate temperatures of 900° C to 1,000° C. Equilibration temperatures determined for chromite-olivine pairs have a higher range of 1,154° C to 1,335° C. Minor element distributions among coexisting ferromagnesian silicates in these inclusions follow consistent patterns and are constant for any given sample, suggesting equilibrium assemblages. Major and minor element distributions for Weekeroo Station inclusions are anomalous, indicating nonequilibrium.Compositional data, the fragmentary shapes of many inclusions, the highly differentiated characteristic of two types of inclusions, the apparent disequilibrium between kamacite in inclusions and kamacite of the iron host, and the relict chondrules found in Netschaëvo suggest that many of the inclusions did not form cogenetically with the iron host, but represent pre-existing stony material that was taken up by an iron melt, probably not in the core of the parent body (or bodies).     

Nitrogen and xenon in acid residues of iron meteorites

Total nitrogen, measured by neutron activation analysis, is highly enriched in residues from iron meteorites obtained by dissolution of the metal in dilute H₂SO₄, relative to the bulk value. On the average, the residues, representing 3% mass, contain 22% of total N. Group IA has more dissolved N than IIIA. Lithium and Ir show a distribution pattern parallel to N. Total Xe has been measured in several residues and its isotopic composition is, similar to atmospheric Xe for mass numbers 131 to 136 but not for ¹²⁴Xe and ¹²⁶Xe which are strongly depleted in the non-magnetic residues. It is suggested that iron meteorites have trapped in their micro-inclusions, some pre-solar nebular matter which is isotopically heterogeneous.     

The compositional classification of some Chinese iron meteorites

Based on structural observations and the concentrations of Cr, Co, Ni, Cu, Ga, Ge, As, Sb, Re, Ir, and Au by neutron-activation analysis we have classified 14 Chinese iron meteorites. Thirteen are members of the large groups IAB, IIICD, IIIAB and IVA. Leshan is an ungrouped iron meteorite that falls within the IIE field on some element-Ni diagrams, but is distinctly outside this field on plots of Cu, W, and Ir vs. Ni; it is very similar in composition to Techado, another ungrouped iron. The high Cu content of Leshan in consistent with other evidence indicating that Cu is a valuable parameter for classifying iron meteorites. IIICD Dongling appears not to be a new meteorite, but to be paired with Nantan; Dongling was recovered about 50 km from the location of the Nantan shower. In view of the fact that Yongning is highly oxidized, we assign it to group IAB but cannot rule out IIICD. IVA-An Longchang has many characteristics of IVA irons, but has been remelted, probably in a terrestrial setting. Five irons belong to group IVA, a remarkably large number. Three are identical in composition, and we suspect that the two from Hubei, Guanghua and Huangling, are paired. Thus this set of 14 irons includes 12 independent falls.     

The distribution of total nitrogen in iron meteorites

Total nitrogen abundances in 123 iron meteorites have been determined by inert carrier-gas fusion extraction-gas chromatography. The median value for the iron meteorites was found to be 18 ppm N. The N contents of Sulfide inclusions are greater, in nine cases out of ten, than the corresponding metallic phase. The N content of the iron meteorites is positively correlated with germanium content. The effects of terrestrial weathering and heat treatment by man are discussed in relation to the N contents measured for certain specimens. A correlation between N and cooling rates was found, with lower cooling rates associated with greater N abundances.     

Modeling fractional crystallization of group IVB iron meteorites

A ¹⁸⁷Re-¹⁸⁷Os isochron including data for all twelve IVB irons gives an age of 4579 ± 34 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os of 0.09531 ± 0.00022, consistent with early solar system crystallization. This result, along with the chemical systematics of the highly siderophile elements (HSE) are indicative of closed-system behavior for all of the HSE in the IVB system since crystallization.Abundances of HSE measured in different chunks of individual bulk samples, and in spot analyses of different portions of individual chunks, are homogeneous at the ±10% level or better. Modeling of HSE in the IVB system, therefore, is not impacted by sample heterogeneities. Concentrations of some other elements determined by spot analysis, such as P, Cr and Mn, however, vary by as much as two orders of magnitude and reflect the presence of trace phases.Assuming initial S in the range of 0 to 2 wt.%, the abundances of the HSE Re, Os, Ir, Ru, Pt, Rh, Pd and Au in bulk IVB irons are successfully accounted for via a fractional crystallization model. For these elements, all IVB irons can be interpreted as being representative of equilibrium solids, liquids, or mixtures of equilibrium solids and liquids.Our model includes changes in bulk D values (ratio of concentration in the solid to liquid) for each element in response to expected increases in S and P in the evolving liquid. For this system, the relative D values are as follow: Os > Re > Ir > Ru > Pt > Rh > Pd > Au. Osmium, Re, Ir and Ru were compatible elements (favor the solid) throughout the IVB crystallization sequence; Rh, Pd and Au were incompatible (favor the liquid). Extremely limited variation in Pt concentrations throughout the IVB crystallization sequence requires that D(Pt) remained at unity.In general, D values derived from the slopes of logarithmic plots, compared with those calculated from recent parameterizations of D values for metal systems are similar, but not identical. Application of D values obtained by the parameterization method is problematic for comparisons of the compatible elements with similar partitioning characteristics. The slope-based approach works well for these elements. In contrast, the slope-based approach does not provide viable D values for the incompatible elements Pd and Au, whereas the parameterization method appears to work well. Modeling results suggest that initial S for this system may have been closer to 2% than 0, but the elements modeled do not tightly constrain initial S.Consistent with previous studies, our calculated initial concentrations of HSE in the IVB parent body indicate assembly from materials that were fractionated via high temperature condensation processes. As with some previous studies, depletions in redox sensitive elements and corresponding high concentrations of Re, Os and Ir present in all IVB irons are interpreted as meaning that the IVB core formed in an oxidized parent body. The projected initial composition of the IVB system was characterized by sub-chondritic Re/Os and Pt/Os ratios. The cause of this fractionation remains a mystery. Because of the refractory nature of these elements, it is difficult to envision fractionation of these elements (especially Re-Os) resulting from the volatility effects that evidently affected other elements.