IVA Iron Meteorites Research Articles

Trace elements in IVA iron meteorites explained by limited solid–liquid equilibration during inward solidification

Iron meteorites are windows into the formation and evolution of planetesimal cores. The trace element compositions of IVA iron meteorites are enigmatic; specifically, to explain the fractionations of different elements requires various sulfur contents of the parent liquid. Here, we propose a possible solution to this problem. IVA irons are thought to sample an exposed core that underwent inward solidification. In an inward solidifying core, sulfur-rich liquids expelled by crystallization are buoyant and stably stratified in the interstices of the mushy solidification front, until they eventually solidify at the eutectic point. Solidification proceeds through in-situ dendritic crystallization of mushy parcels of identical compositions, with the absence of chemical fractionation. In order for fractionation to take place, “pristine” liquids must flow into the mushy front and react with solids, which would be possible if circulation is driven by external forcing, for example, collisions. In this picture, the fluid exchange (which enables fractionation) is driven by occasional events, and each incremental solid can react with only a limited amount of liquid during solidification. We develop a simple model to describe the fractionation associated with this limited solid–liquid equilibration. With this model, we can explain the concentrations of different elements satisfactorily with a single sulfur content (∼5wt%) of the IVA iron parent liquid. Assuming that the stirring is caused by collisions to the solidifying body, we combine the new model for element fractionation with a model for solidification (as a Stefan problem) to suggest a frequency on the order of once per few thousand years for collisions that are large enough to cause the required stirring.

Characterization of iron meteorites by scanning electron microscopy, X‐ray diffraction, magnetization measurements, and Mössbauer spectroscopy: Gibeon IVA

AbstractGibeon IVA iron meteorite fragment was characterized using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X‐ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. Optical microscopy and SEM made on the polished section of the meteorite, show the presence of α‐Fe(Ni, Co) and γ‐Fe(Ni, Co) phases and plessite structures. There are no troilite inclusions observed in the studied section. EDS studies indicate some variations in the Ni concentrations: (i) within the α‐Fe(Ni, Co) phase in the range ~5.0 ± 0.1 – ~7.5 ± 0.1 at% and (ii) within the γ‐Fe(Ni, Co) phase in the range ~26.0 ± 0.2 – ~36.1 ± 0.2 at%. The latter Ni concentration range indicates the presence of small amount of the paramagnetic γ‐phase in addition to the ferromagnetic γ‐phase. EDS also shows that Ni content in two plessite structures is varying in the range ~16–37 at%, which can indicate the presence of only the α2‐Fe(Ni, Co) and γ‐Fe(Ni, Co) phases in the duplex plessite structure. This may be a result of the γ‐phase decomposition with the incomplete martensitic transformation: γ → α2 + γ due to a faster cooling rate. XRD indicates the presence of ~1.3 wt% of the γ‐Fe(Ni, Co) phase in Gibeon VIA. The saturation magnetization moment of 185(2) emu g−1 obtained also confirms the presence of phases with low and high Ni concentrations. The most appropriate fit of the Gibeon IVA Mössbauer spectrum demonstrates the presence of five magnetic sextets and one paramagnetic singlet which are assigned to the ferromagnetic α2‐Fe(Ni, Co), α‐Fe(Ni, Co), γ‐Fe(Ni, Co), and paramagnetic γ‐Fe(Ni, Co) phases. The relative average Fe contents in these phases are: 13.4% in the α2‐Fe(Ni, Co) phase, 78.3% in the α‐Fe(Ni, Co) phase, and 8.3% in the ferromagnetic and paramagnetic γ‐Fe(Ni, Co) phases.

IVA iron meteorites as late-stage crystallization products affected by multiple collisional events

Although IVA irons have O- and Cr-isotopic compositions resembling those of equilibrated LL chondrites, the bulk composition of refractory elements (e.g., Re, Ir, Pt) in the IVA core appears to be significantly lower than LL. These compositional discrepancies suggest known IVA irons may be missing early crystallized samples. We hypothesize the bulk composition of the IVA core is LL-like, but current collections do not include early fractional-crystallization IVA products. Our fractional-crystallization modeling of element vs. Au trends suggests that extant IVA irons are products of > 40% crystallization of the core, assuming an initial 2.9 wt.% S content. The model-derived bulk (Ni-normalized) composition of the IVA core is depleted relative to LL in most moderate volatiles: S (82% depletion), Ge (99.9% depletion), Ga (95% depletion), As (50% depletion); however, Au is enriched by 10%. Because moderate volatiles with depletions > 80% relative to LL have 50%-condensation temperatures < 1,020 K, it seems likely these depletions reflect post-accretion impact-induced volatilization of the IVA asteroid. The mean Ni-normalized compositions of analyzed IVA irons yield a lesser depletion of As (30%) and greater enrichment of Au (48%) relative to LL. The IVA asteroid may have experienced a complex parent-body thermal and collisional history: (1) differentiation, (2) impact-induced mantle stripping, devolatilization, and fractional condensation, (3) rapid crystallization of the core from the outside inwards, (4) shattering of the core after ∼75% crystallization, (5) quenching of thinly insulated samples (e.g., Fuzzy Creek), (6) formation of amorphous free silica in several IVA irons after impact-induced vaporization of portions of the overlying silicate mantle, followed by fractional condensation, (7) loss of portions of the core representing the first 40% of crystallization, (8) reaccretion of some core fragments, facilitating relatively slow cooling of a few IVA irons (e.g., Duchesne, Duel Hill (1854), Chinautla), and (9) collisional resetting of the Re-Os clock 4456 ± 25 Ma ago.

Variations in the Magnetic Properties of Meteoritic Cloudy Zone

AbstractIron and stony‐iron meteorites form the Widmanstätten pattern during slow cooling. This pattern is composed of several microstructures whose length‐scale, composition and magnetic properties are dependent upon cooling rate. Here we focus on the cloudy zone: a region containing nanoscale tetrataenite islands with exceptional paleomagnetic recording properties. We present a systematic review of how cloudy zone properties vary with cooling rate and proximity to the adjacent tetrataenite rim. X‐ray photoemission electron microscopy is used to compare compositional and magnetization maps of the cloudy zone in the mesosiderites (slow cooling rates), the IAB iron meteorites and the pallasites (intermediate cooling rates), and the IVA iron meteorites (fast cooling rates). The proportions of magnetic phases within the cloudy zone are also characterized using Mössbauer spectroscopy. We present the first observations of the magnetic state of the cloudy zone in the mesosiderites, showing that, for such slow cooling rates, tetrataenite islands grow larger than the multidomain threshold, creating large‐scale regions of uniform magnetization across the cloudy zone that render it unsuitable for paleomagnetic analysis. For the most rapidly cooled IVA meteorites, the time available for Fe‐Ni ordering is insufficient to allow tetrataenite formation, again leading to behavior that is unsuitable for paleomagnetic analysis. The most reliable paleomagnetic remanence is recorded by meteorites with intermediate cooling rates ( 2–500 °C Myr ) which produces islands that are “just right” in both size and degree of Fe‐Ni order.

Pb isotope evidence for rapid accretion and differentiation of planetary embryos

Group IVA iron and siliate-iron meteorites record a large range of cooling rates attributed to an impact-related disruption of a molten and differentiated ca. 1000 km diameter planetary embryo of chondritic composition before re-accretion of mainly the metallic core with minor silicates. To better understand the timing of primary accretion, disruption, re-accretion and cooling of the Group IVA parent body, we have determined Pb-Pb and Al-Mg ages for the Group IVA silicate-iron Steinbach meteorite. A Pb-Pb age based on multiple fractions of late-phase, slowly-cooled orthopyroxene from Steinbach yields an absolute age of 4565.47 ± 0.30 Ma corresponding to a relative age of 1.83 ± 0.34 Myr after formation of calcium-aluminium-rich inclusions (CAIs). This is the oldest U-corrected Pb-Pb absolute age for a differentiated meteorite. We use the deficit Al-Mg dating method on one whole rock sample and two mineral separates to produce a model age of 1.3−0.3+0.5 Myr after CAI formation corresponding to the depletion age of Al relative to Mg in the source material for Steinbach. Assuming this fractionation event occurred in the pre-impact parent body, this provides a maximum time after CAI formation for the disruption of the original Group IVA parent body. Together, these ages require that the original parent body accreted very early and differentiated prior to the impact-related break up, re-accretion and cooling between 1.3−0.3+0.5 Myr and 1.83 ± 0.34 Myr after CAI formation. These ages are fully consistent with a growing body of evidence from meteorites and astronomical observation supporting the early and efficient growth of planetary embryos and with numerical models of pebble accretion that predict rapid growth of embryos in the presence of chondrules. This time frame for the efficient formation of planetary embryos by chondrule accretion is inconsistent with a proposed ∼1.5 Myr delay in chondrule formation, a contradiction that is resolved by a non-canonical abundance of 26Al in the inner Solar System during at least the first million years of the protoplanetary disk.

Genetics, crystallization sequence, and age of the South Byron Trio iron meteorites: New insights to carbonaceous chondrite (CC) type parent bodies.

The nucleosynthetic Mo, Ru, and W isotopic compositions of the South Byron Trio iron meteorite grouplet (SBT) are consistent with all three meteorites originating on a single parent body that formed in the carbonaceous chondrite (CC) isotopic domain within the Solar nebula. Consistent with a common origin, the highly siderophile element (HSE) concentrations of the SBT can be related to one another by moderate degrees of fractional crystallization of a parental melt with initially chondritic relative abundances of HSE, and with initial S and P contents of ~7 and ~1 wt. %, respectively. Tungsten-182 isotopic data for the SBT indicate the parent body underwent metal-silicate differentiation 2.1 ± 0.8 Myr after calcium aluminum rich inclusion formation, and thermal modeling suggests the parent body formed 1.1 ± 0.5 Myr after CAI formation. This accretion age is not resolved from the accretion ages of other CC and most noncarbonaceous (NC) type iron meteorite parent bodies. Comparison of the projected parental melt composition of the SBT to those projected for the IVA and IVB iron meteorite groups suggests that at least some portions of the CC nebular domain were more oxidized compared to the NC domain. In addition, comparison of the SBT parental melt S content to estimates for parent bodies of the IIAB, IIIAB, IVA, IID, and IVB "magmatic" iron meteorite groups suggests that CC type iron meteorite parental melts were characterized by a general depletion in S, in addition to depletions in some other moderately volatile elements. Based on chemical and O isotope similarities, prior studies have suggested the possibility of a common parent body for the SBT and the Milton pallasite. Molybdenum and Ru isotopic compositions of Milton also provide permissive evidence for this. The HSE concentrations in the Milton metal, however, cannot be related to the SBT by any known crystal-liquid fractionation or mixing path. Thus, Milton more likely formed on a different, chemically distinct, but genetically identical parent body present in the CC nebular domain.

Cation distribution in orthopyroxenes from São João Nepomuceno iron meteorite inferred from 57Fe Mössbauer spectroscopy: Implications for thermal history and origin of IVA parent body

AbstractSão João Nepomuceno (SJN) is an IVA iron meteorite, found in Minas Gerais state (Brazil) in 1960, that consists of Fe‐Ni metal matrix and coarse‐grained silicate inclusions. In spite of the extensive work performed on the IVA irons, there is still no consensus about their origin and thermal history. Their particular chemistry and range in metallographic cooling rates are difficult to explain using conventional models. Furthermore, metal–silicate mixing of the IVA group remains a complex issue. In this work, the 57Fe Mössbauer spectroscopy was applied for measuring the intracrystalline Fe‐Mg distribution in orthopyroxenes extracted from SJN. The Mössbauer data associated with Ganguly's cooling rate numerical model were used to investigate the thermal history of SJN meteorite. The results are the background for discussions about the IVA formation models, aiming to improve the understanding of the origin of IVA iron meteorite group.

Uranium isotope ratios of Muonionalusta troilite and complications for the absolute age of the IVA iron meteorite core

The crystallization ages of planetary crustal material (given by basaltic meteorites) and planetary cores (given by iron meteorites) provide fiducial marks for the progress of planetary formation, and thus, the absolute ages of these objects fundamentally direct our knowledge and understanding of planet formation and evolution. The lone precise absolute age of planetary core material was previously obtained on troilite inclusions from the IVA iron meteorite Muonionalusta. This previously reported Pb–Pb age of 4565.3 ± 0.1 Ma—assuming a 238U/235U =137.88—only post-dated the start of the Solar System by approximately 2–3 million years, and mandated fast cooling of planetary core material. Since an accurate Pb–Pb age requires a known 238U/235U of the sample, we have measured both 238U/235U and Pb isotopic compositions of troilite inclusions from Muonionalusta. The measured 238U/235U of the samples range from ∼137.84 to as low as ∼137.22, however based on Pb and U systematics, terrestrial contamination appears pervasive and has affected samples to various extents for Pb and U. The cause of the relative 235U excess in one sample does not appear to be from terrestrial contamination or the decay of short-lived 247Cm, but is more likely from fractionation of U isotopes during metal–silicate separation during core formation, exacerbated by the extreme U depletion in the planetary core. Due to limited Pb isotopic variation and terrestrial disturbance, no samples of this study produced useful age information; however the clear divergence from the previously assumed 238U/235U of any troilite in Muonionalusta introduces substantial uncertainty to the previously reported absolute age of the sample without knowledge of the 238U/235U of the sample.Uncertainties associated with U isotope heterogeneity do not allow for definition of a robust age of solidification and cooling for the IVA core. However, one sample of this work—paired with previous work using short-lived radionuclides—suggests that the cooling age of the IVA core may be significantly younger than previously thought. This work indicates the metallic cores of protoplanetary bodies solidified no earlier than the first ∼5–10 million years of the Solar System.

Atom‐probe tomography and transmission electron microscopy of the kamacite–taenite interface in the fast‐cooled Bristol IVA iron meteorite

AbstractWe report the first combined atom‐probe tomography (APT) and transmission electron microscopy (TEM) study of a kamacite–tetrataenite (K–T) interface region within an iron meteorite, Bristol (IVA). Ten APT nanotips were prepared from the K–T interface with focused ion beam scanning electron microscopy (FIB‐SEM) and then studied using TEM followed by APT. Near the K‐T interface, we found 3.8 ± 0.5 wt% Ni in kamacite and 53.4 ± 0.5 wt% Ni in tetrataenite. High‐Ni precipitate regions of the cloudy zone (CZ) have 50.4 ± 0.8 wt% Ni. A region near the CZ and martensite interface has &lt;10 nm sized Ni‐rich precipitates with 38.4 ± 0.7 wt% Ni present within a low‐Ni matrix having 25.5 ± 0.6 wt% Ni. We found that Cu is predominantly concentrated in tetrataenite, whereas Co, P, and Cr are concentrated in kamacite. Phosphorus is preferentially concentrated along the K‐T interface. This study is the first precise measurement of the phase composition at high spatial resolution and in 3‐D of the K‐T interface region in a IVA iron meteorite and furthers our knowledge of the phase composition changes in a fast‐cooled iron meteorite below 400 °C. We demonstrate that APT in conjunction with TEM is a useful approach to study the major, minor, and trace elemental composition of nanoscale features within fast‐cooled iron meteorites.

Pd-Ag chronometry of IVA iron meteorites and the crystallization and cooling of a protoplanetary core

To constrain the timescales and processes involved in the crystallization and cooling of protoplanetary cores, we examined the Pd-Ag isotope systematics of the IVA iron meteorites Muonionalusta and Gibeon. A Pd-Ag isochron for Muonionalusta provides an initial 107Pd/108Pd=(2.57±0.07)×10−5. The three metal samples analyzed from Gibeon plot below the Muonionalusta isochron, but these samples also show significant effects of cosmic ray-induced neutron capture reactions, as is evident from 196Pt excesses in the Gibeon samples. After correction for neutron capture effects on Ag isotopes, the Gibeon samples plot on the Muonionalusta isochron, indicating that these two IVA irons have indistinguishable initial 107Pd/108Pd. Collectively, the Pd-Ag data indicate cooling of the IVA core below Pd-Ag closure between 2.9±0.4Ma and 8.9±0.6Ma after CAI formation, where this age range reflects uncertainties in the initial 107Pd/108Pd ratios of the solar system, which in turn result from uncertainties in the Pb-Pb age of Muonionalusta. The Ag isotopic data indicate that the IVA core initially evolved with a modestly elevated Pd/Ag, but the low Ag concentrations measured for some metal samples indicate derivation from a source with much lower Ag contents and, hence, higher Pd/Ag. These contrasting observations can be reconciled if the IVA irons crystallized from an initially more Ag-rich core, followed by extraction of Fe-S melts during compaction of the nearly solidified core. Owing to its strong tendency to partition into Fe-S melts, Ag was removed from the IVA core during compaction, leading to the very low Ag concentration observed in metal samples of IVA irons. Alternatively, Ag was lost by evaporation from a still molten metallic body just prior to the onset of crystallization. The Pd-Ag isotopic data indicate that Muonionalusta cooled at >500K/Ma through the Pd-Ag closure temperature of ∼900K, consistent with the rapid cooling inferred from metallographic cooling rates for IVA irons. Combined, these observations are consistent with cooling of IVA irons in a metallic body with little or no silicate mantle.

Paleomagnetic evidence for dynamo activity driven by inward crystallisation of a metallic asteroid

The direction in which a planetary core solidifies has fundamental implications for the feasibility and nature of dynamo generation. Although Earth's core is outwardly solidifying, the cores of certain smaller planetary bodies have been proposed to inwardly solidify due to their lower central pressures. However, there have been no unambiguous observations of inwardly solidified cores or the relationship between this solidification regime and planetary magnetic activity. To address this gap, we present the results of complimentary paleomagnetic techniques applied to the matrix metal and silicate inclusions within the IVA iron meteorites. This family of meteorites has been suggested to originate from a planetary core that had its overlaying silicate mantle removed by collisions during the early solar system. This process is thought to have produced a molten ball of metal that cooled rapidly and has been proposed to have inwardly solidified. Recent thermal evolution models of such a body predict that it should have generated an intense, multipolar and time-varying dynamo field. This field could have been recorded as a remanent magnetisation in the outer, cool layers of a solid crust on the IVA parent core. We find that the different components in the IVA iron meteorites display a range of paleomagnetic fidelities, depending crucially on the cooling rate of the meteorite. In particular, silicate inclusions in the quickly cooled São João Nepomuceno meteorite are poor paleomagnetic recorders. On the other hand, the matrix metal and some silicate subsamples from the relatively slowly cooled Steinbach meteorite are far better paleomagnetic recorders and provide evidence of an intense (≳100 μT) and directionally varying (exhibiting significant changes on a timescale ≲200 kyr) magnetic field. This is the first demonstration that some iron meteorites record ancient planetary magnetic fields. Furthermore, the observed field intensity, temporal variability and dynamo lifetime are consistent with thermal evolution models of the IVA parent core. Because the acquisition of remanent magnetisation by some IVA iron meteorites require that they cooled below their Curie temperature during the period of dynamo activity, the magnetisation carried by Steinbach also provides strong evidence favouring the inward solidification of its parent core.

Joegoldsteinite: A new sulfide mineral (MnCr2S4) from the Social Circle IVA iron meteorite

Joegoldsteinite, a new sulfide mineral of end-member formula MnCr_2S_4, was discovered in the Social Circle IVA iron meteorite. It is a thiospinel, the Mn analog of daubreelite (Fe^(2+)Cr_2S_4), and a new member of the linnaeite group. Tiny grains of joegoldsteinite were also identified in the Indarch EH4 enstatite chondrite. The chemical composition of the Social Circle sample determined by electron microprobe is (wt%) S 44.3, Cr 36.2, Mn 15.8, Fe 4.5, Ni 0.09, Cu 0.08, total 101.0, giving rise to an empirical formula of (Mn_(0.82)Fe_(0.23))Cr_(1.99)S_(3.95). The crystal structure, determined by electron backscattered diffraction, is a Fd3m spinel-type structure with a = 10.11 A, V = 1033.4 A^3, and Z = 8.

Siderophile elements in brecciated HED meteorites and the nature of projectile materials in HED meteorites

Petrological, mineralogical and geochemical studies were performed on five brecciated HED meteorites (ALH 76005, EET 92003, LEW 85300, LEW 87026 and GRO 95633) in order to elucidate the nature of impactors on the HED parent body. Some brecciated HED meteorites contain exotic materials such as FeNi-metal grains with low Co/Ni ratios (ALH 76005, EET 92003 and GRO 95633) and carbonaceous chondrite clasts (LEW 85300) in a clastic and/or impact melt matrix. Such exotic materials were incorporated during brecciation. Platinum group element (PGE) abundances vary significantly (CI × 0.002–0.05), but are higher than those of pristine rocks from the HED parent body. The PGE ratios for the five HED meteorites are inconsistent with each other, implying that the impactor components of each HED meteorites are different from each other. The various PGE ratios are consistent with those for metals from chondrites and iron meteorites, and carbonaceous chondrites. This study provides the evidence that IAB and IVA iron meteorites, and carbonaceous chondrites (CM, CO, CV, CK, CB and CR), ordinary chondrites (L and H) and enstatite chondrite (EL) are candidates of the impactor materials on the HED parent body. It is highly probable that significant amounts of siderophile elements were incorporated into the inner solar system objects like the HED parent body from both chondritic materials and differentiated materials like iron meteorites during heavy bombardment. The HED meteorites in this study and metals from mesosiderite have different Pd/Ir ratios, probably implying that HED meteorites and mesosiderites formed either at distinct settings on one common parent body or on similar parent bodies.

Determining cooling rates of iron and stony-iron meteorites from measurements of Ni and Co at kamacite–taenite interfaces

Analyses and modeling of Ni zoning in taenite in differentiated meteorites provide metallographic cooling rates at ∼500°C that are inconsistent with conventional formation models. Group IVA iron meteorites have very diverse cooling rates of 100–6600°C/Myr indicating that they cooled inside a large metallic body with little or no silicate mantle (Yang et al., 2007). Wasson and Hoppe (2012) have questioned these diverse cooling rates on the basis of their ion probe measurements of Ni/Co ratios at the kamacite–taenite interface in two group IVA and in two group IIIAB iron meteorites. To investigate their claims and to assess methods for determining relative cooling rates from kamacite–taenite interface compositions, we have analyzed 38 meteorites—13 IVA, 14 IIIAB irons, 4 IAB complex irons, 6 pallasites and a mesosiderite—using the electron probe microanalyzer (EPMA). Ni concentrations in taenite (Niγ) and kamacite (Niα) at kamacite–taenite interfaces are well correlated with metallographic cooling rates: Niγ values increase from 30 to 52wt.% while Niα decreases from 7 to 4wt.% as cooling rates decrease. EPMA measurements of Niγ, Niα, and Niα/Niγ, can therefore be used to provide order-of-magnitude estimates of relative cooling rates. Concentrations of Co in kamacite and taenite at their interface (Coα, Coγ) are controlled by bulk Ni and Co composition, as well as cooling rate. The ratios Coα/Coγ and (Co/Ni)α/(Co/Ni)γ are correlated with cooling rate, but because of significant scatter, these parameters should not be used to estimate cooling rates. Our analyses of 13 group IVA irons provide robust support for diverse cooling rates that decrease with increasing bulk Ni, consistent with measurements of cloudy zone size and tetrataenite width. Apparent equilibration temperatures, which are inferred from Niγ values and the Fe–Ni–P phase diagram and Ni diffusion rates in taenite, show that cooling rates of IVA irons vary by a factor of ≈100, in excellent agreement with the metallographic cooling rates. Similar calculations using Niγ/Niα and Coα/Coγ ratios and phase diagram data give factors that are an order of magnitude lower but have larger uncertainties. Thus we strongly disagree with the conclusion of Wasson and Hoppe (2012) that interface concentrations of Ni and Co are in any way in conflict with the cooling rates of Yang et al. (2008). Our measurements confirm that the IVA irons could not have cooled in an asteroidal core surrounded by a silicate mantle, and also that main-group pallasites cooled slower than IIIAB irons and did not cool at the boundary between the mantle and core from which the IIIAB irons originated. Our data provide additional evidence that mesosiderites, which formed by impact mixing of Fe–Ni melt and crustal rocks, cooled at uniquely slow rates.

Fe2+-Mg order–disorder study in orthopyroxenes from São João Nepomuceno (IVA) iron meteorite

Sao Joao Nepomuceno (SJN) is an iron meteorite belonging to IVA chemical group. It consists of Fe-Ni metal matrix and silicate inclusions made of a coarse-grained mixture of tridymite, orthopyroxene and clinopyroxene. In spite of the extensive work performed on the IVA group there is still no consensus about their origin and its thermal history is subject of ongoing debates. In this work, we report preliminary results on Fe2+ distribution in the non-equivalent octahedral sites in orthopyroxene crystals extracted from Sao Joao Nepomuceno in order to infer on the thermal history of this meteorite and therefore acquire more information related to the origin of the IVA iron meteorite group.

Neutron capture on Pt isotopes in iron meteorites and the Hf–W chronology of core formation in planetesimals

The short-lived 182Hf–182W isotope system can provide powerful constraints on the timescales of planetary core formation, but its application to iron meteorites is hampered by neutron capture reactions on W isotopes resulting from exposure to galactic cosmic rays. Here we show that Pt isotopes in magmatic iron meteorites are also affected by capture of (epi)thermal neutrons and that the Pt isotope variations are correlated with variations in 182W/184W. This makes Pt isotopes a sensitive neutron dosimeter for correcting cosmic ray-induced W isotope shifts. The pre-exposure 182W/184W derived from the Pt–W isotope correlations of the IID, IVA and IVB iron meteorites are higher than most previous estimates and are more radiogenic than the initial 182W/184W of Ca–Al-rich inclusions (CAI). The Hf–W model ages for core formation range from +1.6±1.0 million years (Ma; for the IVA irons) to +2.7±1.3Ma after CAI formation (for the IID irons), indicating that there was a time gap of at least ∼1Ma between CAI formation and metal segregation in the parent bodies of some iron meteorites. From the Hf–W ages a time limit of <1.5–2Ma after CAI formation can be inferred for the accretion of the IID, IVA and IVB iron meteorite parent bodies, consistent with earlier conclusions that the accretion of differentiated planetesimals predated that of most chondrite parent bodies.

Hf–W chronometry of core formation in planetesimals inferred from weakly irradiated iron meteorites

The application of Hf–W chronometry to determine the timescales of core formation in the parent bodies of magmatic iron meteorites is severely hampered by 182W burnout during cosmic ray exposure of the parent meteoroids. Currently, no direct method exists to correct for the effects of 182W burnout, making the Hf–W ages for iron meteorites uncertain. Here we present noble gas and Hf–W isotope systematics of iron meteorite samples whose W isotopic compositions remained essentially unaffected by cosmic ray interactions. Most selected samples have concentrations of cosmogenic noble gases at or near the lowermost level observed in iron meteorites and, for iron meteorite standards, have very low noble gas and radionuclide based cosmic ray exposure ages (<60Ma). In contrast to previous studies, no corrections of measured W isotope compositions are required for these iron meteorite samples. Their ε182W values (parts per 104 deviations from the terrestrial value) are higher than those measured for most other iron meteorites and range from −3.42 to −3.31, slightly elevated compared to the initial 182W/184W of Ca–Al-rich Inclusions (CAI; ε182W=−3.51±0.10). The new W isotopic data indicate that core formation in the parent bodies of the IIAB, IIIAB, and IVA iron meteorites occurred ∼1–1.5Myr after CAI formation (with an uncertainty of ∼1Myr), consistent with earlier conclusions that the accretion and differentiation of iron meteorite parent bodies predated the accretion of most chondrite parent bodies. One ungrouped iron meteorite (Chinga) exhibits small nucleosynthetic W isotope anomalies, but after correction for these anomalies its ε182W value agrees with those of the other samples. Another ungrouped iron (Mbosi), however, has elevated ε182W relative to the other investigated irons, indicating metal–silicate separation ∼2–3Myr later than in the parent bodies of the three major iron meteorite groups studied here.

Pd–Ag chronology of volatile depletion, crystallization and shock in the Muonionalusta IVA iron meteorite and implications for its parent body

Muonionalusta, a Group IVA iron meteorite, was analyzed for its 107Pd–107Ag isotope systematics by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in order to better constrain the initial Solar System abundance of 107Pd and to provide high resolution chronology of the evolution of its parent body. Six metal samples from Muonionalusta yield Ag abundances between 0.1012 and 1.461ng/g, 107Ag/109Ag between 1.131 and 1.805, with 108Pd/109Ag ratios of 2201 to 52,300. The metal Pd/Ag and Ag isotopic data are correlated with a slope corresponding to a 107Pd/108Pd of (2.15±0.30)×10−5. If the Pd–Ag and Pb–Pb isotope systems closed at the same time in Muonionalusta, i.e., 2–3Ma after CAI formation, then an initial Solar System ratio of 107Pd/108Pd=(2.8±0.5)×10−5 can be inferred. One troilite sample contains 8.478ng/g Ag and has a 107Ag/109Ag ratio of 1.0833; its Pd concentration is 205.2ng/g corresponding to a low 108Pd/109Ag of 13.56. The Pd–Ag results for this troilite plot near the extrapolation of the line passing through the metal points and define an initial 107Ag/109Ag that is substantially higher than chondritic, indicating that Muonionalusta formed from a precursor with high Pd/Ag. Pd and Ag concentrations in Muonionalusta metal suggest fractional crystallization from a source having Pd/Ag>4500, but the initial Ag isotopic composition for Muonionalusta troilite limits the duration of the high Pd/Ag to an interval of ≤0.6Ma before cooling to closure of the Pd–Ag system. This result suggests that depletion of Ag and other volatile elements occurred shortly before accretion and cooling of the IVA parent body, and may have been associated with violent disruption of a progenitor to the IVA parent. Another troilite sample, its chromite inclusions and adjacent metal were isotopically homogenized locally after 107Pd had decayed, possibly by a later episode of shock >50Ma after Solar System formation.

Osmium isotope and highly siderophile element constraints on ages and nature of meteoritic components in ancient lunar impact rocks

The concentrations of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au) and 187Os/188Os isotope compositions have been determined for 67 subsamples of six lunar impact rocks from the Apollo 14, 16 and 17 landing sites, and the lunar meteorite Dar al Gani (DaG) 400 using inductively coupled plasma mass spectrometry (ICP-MS) and negative thermal ionization mass spectrometry (N-TIMS).We report the first Re–Os isochron age on a lunar impact melt rock. 187Re–187Os isotope systematics for Apollo 16 sample 67935 define an isochron age of 4.21±0.13Ga (MSWD=1.5), which is interpreted to reflect localized partitioning processes between solid metal–liquid metal as this rock melted. The new age adds further constraints on the significance of pre-4.0Ga basin forming impacts on the Moon and possible mixing of ancient impactor compositions in lunar impact rocks.Linear correlations displayed by subsamples of a given impact rock in plots of HSE versus Ir concentrations are explained by dilution processes through essentially HSE-free anorthositic lunar crustal target rocks or binary mixing between a high HSE meteoritic end-member and a low HSE end-member composition. Slope-derived HSE ratios and 187Os/188Os of the meteoritic component in granulitic impactites 67915, 67955 and 79215 are similar to slightly volatile element depleted carbonaceous chondrites. Suprachondritic ratios of Ru/Ir, Pt/Ir, Rh/Ir, and Pd/Ir for Apollo 14 impact melt rock 14310 are similar to ratios observed for other Apollo 14 samples and Apollo 17 poikilitic impact melt rocks. Apollo 16 poikilitic and subophitic impact melt rocks 60315 and 67935 show slightly subchondritic Os/Ir and suprachondritic ratios of 187Os/188Os, Ru/Ir, Pt/Ir, Rh/Ir, Pd/Ir and Au/Ir. Their strongly fractionated HSE compositions are similar to some members of the IVA iron meteorite group and provide further evidence for an iron meteorite impactor component in Apollo 16 impact melt rocks.The range of chondritic to suprachondritic ratios of 187Os/188Os, Ru/Ir, Pt/Ir, Pd/Ir, and chondritic to slightly subchondritic Os/Ir of Apollo 17 and 14 impact melt rocks may be explained by variable mixing of a carbonaceous chondrite-like HSE composition (as in granulitic impactites) with a suprachondritic iron meteorite-like impactor composition as inferred from 67935 and 60315, and subsequent homogenization by younger impact events. The inferred dispersal of these two components across the lunar nearside and their antiquity (4.2Ga) suggest at least two basin-forming impacts that predate the 3.9–3.8Ga basin forming era. The HSE composition of the silicate Earth can be matched by a late veneer comprised of ∼80% volatile depleted carbonaceous chondrite-like material and ∼20% differentiated meteoritic metal, similar to what has been identified in ancient lunar impact rocks.

THE ELUSIVE60Fe 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.