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.     

Iron meteorites: Crystallization,thermal history,parent bodies,and origin

We review the crystallization of the iron meteorite chemical groups, the thermal history of the irons as revealed by the metallographic cooling rates, the ages of the iron meteorites and their relationships with other meteorite types, and the formation of the iron meteorite parent bodies. Within most iron meteorite groups, chemical trends are broadly consistent with fractional crystallization, implying that each group formed from a single molten metallic pool or core. However, these pools or cores differed considerably in their S concentrations, which affect partition coefficients and crystallization conditions significantly. The silicate-bearing iron meteorite groups, IAB and IIE, have textures and poorly defined elemental trends suggesting that impacts mixed molten metal and silicates and that neither group formed from a single isolated metallic melt. Advances in the understanding of the generation of the Widmanstätten pattern, and especially the importance of P during the nucleation and growth of kamacite, have led to improved measurements of the cooling rates of iron meteorites. Typical cooling rates from fractionally crystallized iron meteorite groups at 500–700 °C are about 100–10,000 °C/Myr, with total cooling times of 10 Myr or less. The measured cooling rates vary from 60 to 300 °C/Myr for the IIIAB group and 100–6600 °C/Myr for the IVA group. The wide range of cooling rates for IVA irons and their inverse correlation with bulk Ni concentration show that they crystallized and cooled not in a mantled core but in a large metallic body of radius 150±50 km with scarcely any silicate insulation. This body may have formed in a grazing protoplanetary impact. The fractionally crystallized groups, according to Hf–W isotopic systematics, are derived originally from bodies that accreted and melted to form cores early in the history of the solar system, <1 Myr after CAI formation. The ungrouped irons likely come from at least 50 distinct parent bodies that formed in analogous ways to the fractionally crystallized groups. Contrary to traditional views about their origin, iron meteorites may have been derived originally from bodies as large as 1000 km or more in size. Most iron meteorites come directly or indirectly from bodies that accreted before the chondrites, possibly at 1–2 AU rather than in the asteroid belt. Many of these bodies may have been disrupted by impacts soon after they formed and their fragments were scattered into the asteroid belt by protoplanets.     

Main-group pallasites: Thermal history, relationship to IIIAB irons, and origin

We have determined metallographic cooling rates below 975 K for eight main group (MG) pallasites from Ni profiles across taenite lamellae of known crystallographic orientation in metallic regions with Widmanstätten patterns. Comparison with profiles generated by modeling kamacite growth gave cooling rates ranging from 2.5 to 18 K/Myr. Relative cooling rates were also inferred from the sizes of cloudy zone particles in 28 MG pallasites (86-170 nm) and tetrataenite bandwidths in 20 MG pallasites (1050-2170 nm), as these parameters are positively correlated with each other and negatively correlated with the metallographic cooling rates. These three different techniques show that MG pallasites cooled below 975 K at significantly diverse rates. Since samples from the core-mantle boundary should have indistinguishable cooling rates, MG pallasites could not have cooled at this location. Group IIIAB irons, which were previously thought to be core samples from the MG pallasite body, have faster cooling rates (∼50-350 K/Myr) and smaller cloudy zone particle sizes and tetrataenite bandwidths. This shows that IIIAB irons cooled faster than MG pallasites and could not plausibly be from the same body. The absence of related iron meteorites and achondrites and our thermal constraints suggest that MG pallasites cooled at diverse depths in a pallasitic body consisting of well-mixed olivine and metallic Fe-Ni. Such a body may have formed during an impact on a differentiated asteroid or protoplanet that mixed olivine mantle fragments with residual Ir-poor molten metal from the outermost part of a core that chemically resembled the IIIAB core and was ∼80% fractionally crystallized. Separation of the solid core and most of the associated mantle may have resulted from a grazing hit-and-run impact with a larger protoplanet or asteroid. Thermal calculations suggest that the radius of the pallasitic body was 400 km but the likely presence of a regolith would reduce this estimate considerably.     

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.     

Group IVA irons: New constraints on the crystallization and cooling history of an asteroidal core with a complex history

We report analyses of 14 group IVA iron meteorites, and the ungrouped but possibly related, Elephant Moraine (EET) 83230, for siderophile elements by laser ablation ICP-MS and isotope dilution. EET was also analyzed for oxygen isotopic composition and metallographic structure, and Fuzzy Creek, currently the IVA with the highest Ni concentration, was analyzed for metallographic structure. Highly siderophile elements (HSE) Re, Os and Ir concentrations vary by nearly three orders of magnitude over the entire range of IVA irons, while Ru, Pt and Pd vary by less than factors of five. Chondrite normalized abundances of HSE form nested patterns consistent with progressive crystal-liquid fractionation. Attempts to collectively model the HSE abundances resulting from fractional crystallization achieved best results for 3 wt.% S, compared to 0.5 or 9 wt.% S. Consistent with prior studies, concentrations of HSE and other refractory siderophile elements estimated for the bulk IVA core and its parent body are in generally chondritic proportions. Projected abundances of Pd and Au, relative to more refractory HSE, are slightly elevated and modestly differ from L/LL chondrites, which some have linked with group IVA, based on oxygen isotope similarities.Abundance trends for the moderately volatile and siderophile element Ga cannot be adequately modeled for any S concentration, the cause of which remains enigmatic. Further, concentrations of some moderately volatile and siderophile elements indicate marked, progressive depletions in the IVA system. However, if the IVA core began crystallization with ∼3 wt.% S, depletions of more volatile elements cannot be explained as a result of prior volatilization/condensation processes. The initial IVA core had an approximately chondritic Ni/Co ratio, but a fractionated Fe/Ni ratio of ∼10, indicates an Fe-depleted core. This composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron, consistent with an oxidized parent body. The depletions in Ga may reflect decreased siderophilic behavior in a relatively oxidized body, and more favorable partitioning into the silicate portion of the parent body.Phosphate inclusions in EET show Δ¹⁷O values within the range measured for silicates in IVA iron meteorites. EET has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite. Chemical and isotopic evidence for a genetic relation between EET and group IVA is strong, but the high Ni content and the newly determined, rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped. Previously reported metallographic cooling rates for IVA iron meteorites have been interpreted to indicate an inwardly crystallizing, ∼150 km radius metallic body with little or no silicate mantle. Hence, the IVA group was likely formed as a mass of molten metal separated from a much larger parent body that was broken apart by a large impact. Given the apparent genetic relation with IVA, EET was most likely generated via crystal-liquid fractionation in another, smaller body spawned from the same initial liquid during the impact event that generated the IVA body.     

Silica and pyroxene in IVA irons; possible formation of the IVA magma by impact melting and reduction of L-LL-chondrite materials followed by crystallization and cooling

Group IVA is a large magmatic group of iron meteorites. The mean Δ¹⁷O (=δ¹⁷O − 0.52·δ¹⁸O) of the silicates is ∼+1.2‰, similar to the highest values in L chondrites and the lowest values in LL chondrites; δ¹⁸O values are also in the L/LL range. This strongly suggests that IVA irons formed by melting L-LL parental material, but the mean Ni content of IVA irons (83 mg/g) is much lower than that of a presumed L-LL parent (∼170 mg/g) and the low-Ca pyroxene present in two IVA meteorites is Fs13, much lower than the Fs20-29 values in L and LL chondrites. Thus, formation from L-LL precursors requires extensive addition of metallic Fe, probably produced by reduction of FeS and FeO. Group IVA also has S/Ni, Ga/Ni, and Ge/Ni ratios that are much lower than those in L-LL chondrites or any chondrite group that preserves nebular compositions, implying loss of these volatile elements during asteroidal processing. We suggest that these reduction and loss processes occurred near the surface of the asteroid during impact heating, and resulted partly from reduction by C, and partly from the thermal dissociation of FeS and FeO with loss of O and S. The hot (∼1770 K) low-viscosity melt quickly moved through channels in the porous asteroid to form a core. Two members of the IVA group, São João Nepomuceno (hereafter, SJN) and Steinbach, contain moderate amounts of orthopyroxene and silica, and minor amounts of low-Ca clinopyroxene. Even though SJN formed after ∼26% crystallization and Steinbach formed after ∼77% crystallization of the IVA core, both could have originated within several tens of meters of the core-mantle interface if 99% of the crystallization occurred from the center outwards. Two other members of the group (Gibeon and Bishop Canyon) contain tabular tridymite, which we infer to have initially formed as veins deposited from a cooling SiO-rich vapor. The silicates were clearly introduced into IVA irons after the initial magma crystallized. Because the γ-iron crystals in SJN are typically about 5 cm across, an order of magnitude smaller than in IVA irons that do not contain massive silicates, we infer that the metal was in the γ-iron field when the silicates were injected. The SJN and Steinbach silicate compositions are near the low-Ca-pyroxene/silica eutectic compositions. We suggest that a tectonic event produced a eutectic-like liquid and injected it together with unmelted pyroxene grains into fissures in the solid metal core. Published estimates of IVA metallographic cooling rates range from 20 to 3000 K/Ma, leading to a hypothesized breakup of the core during a major impact followed by scrambling of the core and mantle debris [Haack, H., Scott, E.R.D., Love, S.G., Brearley, A. 1996. Thermal histories of IVA stony-iron and iron meteorites: evidence for asteroid fragmentation and reaccretion. Geochim. Cosmochim. Acta60, 3103-3113]. This scrambling model is physically implausible and cannot explain the strong correlation of estimated cooling rates with metal composition. Previous workers concluded that the low-Ca clinopyroxene in SJN and Steinbach formed from protopyroxene by quenching at a cooling rate of 10¹² K/Ma, and suggested that this also supported an impact-scrambling model. This implausible spike in cooling rate by a factor of 10¹⁰ can be avoided if the low-Ca clinopyroxene were formed by a late shock event that converted orthopyroxene to clinopyroxene followed by minimal growth in the clinopyroxene field, probably because melt was also produced. We suggest that metallographic cooling-rate estimates (e.g., based on island taenite) giving similar values throughout the metal compositional range are more plausible, and that the IVA parent asteroid can be modeled by monotonic cooling followed by a high-temperature impact event that introduced silicates into the metal and a low-temperature impact event that partially converted orthopyroxene into low-Ca clinopyroxene.     

Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies

Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (∼50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by ²⁶Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.     

Oxygen isotopic compositions of IVA iron meteorites: implications for the thermal evolution derived from in situ ultraviolet laser microprobe analyses

Oxygen isotopic compositions of silicate inclusions in IVA iron meteorites have been measured with an in situ UV laser microprobe technique. The homogeneity of oxygen isotopic compositions within and among individual mineral grains has also been examined. Oxygen isotope fractionations between coexisting mineral pairs were utilized in oxygen isotope thermometry. Our measured Δ¹⁷O values, ranging from 0.97 to 1.25‰, are characteristic of a single reservoir and fully confirm the oxygen isotopic similarity between IVA irons and L/LL chondrites. Steinbach and São João Nepomuceno, containing inclusions of two silicate minerals in mutual contact, exhibit a mass-dependent fractionation of ¹⁸O/¹⁶O between tridymite and bronzite with apparent oxygen isotopic heterogeneity. The SiO₂-bearing member, Gibeon, gives homogeneous oxygen isotopic compositions without detectable fractionation of ¹⁸O/¹⁶O between tridymite and quartz. Oxygen isotope equilibrium temperatures are estimated for coexisting tridymite and bronzite in the same sample slabs or clusters in Steinbach and São João Nepomuceno. The fractionations of ¹⁸O/¹⁶O between bronzite and tridymite range from 1.6 to 2.3‰ in different sample slabs or clusters. On the basis of the closure temperature concept, cooling rates are estimated at approximately 20 to 1000°C/Myr between 800 and 1000°C, a range of temperatures not accessible to other cooling rate methods. Using the Fast Grain Boundary diffusion model, we have demonstrated that significant oxygen heterogeneity both in tridymite and bronzite is probably due to isotope exchange during cooling between minerals with various grain sizes and mineral abundances in different regions of the samples. The new estimates of cooling rate by oxygen isotope thermometry refine previous cooling curves of IVA irons and support the breakup-reassembly model for the IVA parent body.     

Cr diffusion in orthopyroxene: Experimental determination, Mn-Cr thermochronology, and planetary applications

We have determined Cr diffusion coefficients (D) in orthopyroxene parallel to the a-, b-, and c-axial directions as a function temperature at f(O₂) corresponding to those of the wüstite-iron (WI) buffer. Diffusion is found to be significantly anisotropic with D(//c) > D(//b) > D(//a), conforming to an earlier theoretical prediction. Increase of f(O₂) from WI buffer conditions to 4.5 log unit above the buffer at 950 and 1050 °C leads to decrease of D(Cr) by a factor of two to three, possibly suggesting significant contribution from an interstitial diffusion mechanism. We have used the diffusion data to calculate the closure temperatures (T_c) of the Mn-Cr decay system in orthopyroxene as a function of initial temperature (T₀), grain size (a) and cooling rate for spherical and plane sheet geometries. We also present graphical relations that permit retrieval of cooling rates from knowledge of the resetting of Mn-Cr ages in orthopyroxene during cooling, T₀ and a. Application of these relations to the Mn-Cr age data of the cumulate eucrite Serra de Magé yields a T_c of 830-980 °C, and cooling rates of 2-27 °C/Myr at T_c and ∼1-13 °C/Myr at 500 °C. It is shown that the cooling of Serra de Magé to the closure temperature of the Mn-Cr system took place at its original site in the parent body, and thus implies a thickness for the eucrite crust in the commonly accepted HED parent body, Vesta, of greater than 30 km. This thickness of the eucrite crust is compatible only with a model of relatively olivine-poor bulk mineralogy in which olivine constitutes 19.7% of the total asteroidal mass.     

The thermal history of equilibrated ordinary chondrites and the relationship between textural maturity and temperature

The compositions and textures of phases in eleven equilibrated ordinary chondrites from the H, L, and LL groups spanning petrographic types 4-6 were studied and used to constrain the thermal histories of their parent bodies. Based on Fe-Mg exchange between olivine and spinel, average equilibration temperatures for type 4-6 chondrites encompass a small range, 586-777 °C, relative to what is commonly assumed for peak temperatures (600-950 °C). The maximum temperatures recorded by individual chondrites, which are minima relative to peak metamorphic temperatures, increase subtly but systematically with metamorphic type and are tightly clustered for H4-6 (733-754 °C) and LL4-6 (670-777 °C). For the Ls, Ausson (L5) records a higher maximum olivine-spinel temperature (761 °C) than does the L4 chondrite Saratov (673 °C) or the L6 chondrite Glatton (712 °C). Our data combined with olivine-spinel equilibration temperatures calculated for other equilibrated ordinary chondrites using mineral compositions from the literature demonstrate that, in general, type 4 chondrites within each chemical group record temperatures lower than or equal to those of types 5-6 chondrites.For H chondrites, the olivine-spinel closure temperature is a function of spinel grain size, such that larger grains, abundant in types 5-6 chondrites, record temperatures of ∼740 °C or more while smaller grains, rare in types 5-6 but abundant in type 4 chondrites, record lower temperatures. Olivine-spinel temperatures in the type 6 chondrites Guareña and Glatton are consistent with rapid (50-100 °C/Myr) cooling from high temperatures in the ordinary chondrite parent bodies. With one exception (∼500 °C/Myr), olivine-spinel data for St.-Séverin (LL6) are consistent with similar cooling rates. Cooling rates of order 100 °C/Myr at ∼750 °C for type 6 chondrites are considerably higher than previously determined cooling rates for lower temperatures (?550 °C) based on metallography, fission tracks, and geochronology. For H chondrites, current thermal models of an “onion shell” parent body are inconsistent with a small range of peak temperatures based on olivine-spinel and two pyroxene thermometry combined with a wide dispersion of cooling rates at low temperatures. Equilibrated chondrites may have sampled regions near a major transition in physical properties such as near the base of a regolith pile.     

Formation conditions of aluminum-rich chondrules

We have studied the formation conditions of Al-rich chondrules by doing isothermal and dynamic crystallization experiments at one atmosphere on four different chondrule analogue compositions within the pure CaO-MgO-Al₂O₃-SiO₂ system. For the dynamic crystallization experiments, we cooled from both liquidus and subliquidus peak temperatures (T_max), at cooling rates from 5-1000 °C/h. The starting compositions include two with anorthite and two with forsterite as the dominant liquidus phases, all at or near spinel-saturation. One of each pair evolves towards diopside crystallization, and the others cordierite or enstatite crystallization, giving a total of four completely different crystallization sequences analogous to the four basic varieties of Al-rich chondrule recently proposed. Bulk composition is the main controlling factor, both in terms of mineralogy and texture. The textures of the anorthite-rich compositions are more sensitive to T_max than they are to cooling rate, whereas the textures of the forsterite-rich compositions are more sensitive to cooling rate. Comparisons of natural Al-rich chondrules having similar compositions to our synthetic analogues indicate that the natural objects reflect a range of peak heating temperatures, ∼1400-1500 °C, and cooling rates of 10-500 °C/h for porphyritic chondrules and possibly higher (1000 °C/h) for barred chondrules. These conditions are consistent with the conditions inferred for ferromagnesian chondrules but differ from those inferred for some calcium-aluminum-rich inclusions.     

Iron meteorites with low Ga and Ge concentrations—composition,structure and genetic relationships

Twenty-one iron meteorites with Ge contents below 1 μg/g, including nine belonging to groups IIIF and IVB, have been analyzed by instrumental neutron activation analysis (INAA) for the elements Co, Cr, As, Au, Re, Ir and W. Groups IIIF and IVB show positive correlations of Au, As and Co (IIIF only) with published Ni analyses, and negative correlations of Ir, Re, Cr (IVB only) and W (IIIF only) with Ni. On element-Ni plots, the gradients of the least squares lines are similar to those of many other groups, excluding IAB and IIICD. With the inclusion of a new member, Klamath Falls, group IIIF has the widest range of Au, As and Co contents of any group and the steepest gradients on plots of these elements against Ni. It is likely that these trends in groups IIIF and IVB were produced by fractionation of elements between solid and liquid metal, probably during fractional crystallization.It has been suggested that some of the 15 irons with <l μg/g Ge which lie outside the groups might be related. However, the INAA data indicate that no two are as strongly related as two group members. These low-Ge irons and the members of groups IIIF, IVA and IVB tend to have low concentrations of As, Au and P, low $\text{Co}\text{Ni}$ ratios and high Cr contents. The depletion of the more volatile elements probably results from incomplete condensation into the metal from the solar nebula.The structures of low-Ge irons generally reflect fast cooling rates (20–2000 K Myr^?1). When data for all iron meteorites are plotted on a logarithmic graph of cooling rate against Ge concentration and results for related irons are averaged, there is a significant negative correlation. This suggests that metal grains which inefficiently condensed Ge and other volatile elements tended to accrete into small parent bodies.     

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.     

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.     

LA-MC-ICPMS Pb-Pb dating of rutile from slowly cooled granulites: Confirmation of the high closure temperature for Pb diffusion in rutile

Rapid Pb-Pb dating of natural rutile crystals by laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) is investigated as a tool for constraining geological temperature-time histories. LA-MC-ICPMS was used to analyse Pb isotopes in rutile from granulite-facies rocks from the Reynolds Range, Northern Territory, Australia. The resultant ages were compared with previous U-Pb zircon and monazite age determinations and new mica (muscovite, phlogopite, and biotite) Rb-Sr ages from the same metamorphic terrane. Rutile crystals ranging in size from 3.5 to 0.05 mm with ?20 ppm Pb were ablated with a 300-25 μm diameter laser beam. Crystals larger than 0.5 mm yielded sufficiently precise ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios to correct for the presence of common Pb, and individual rutile crystals often exhibited sufficient Pb isotopic heterogeneity to allow isochron calculations to be performed on replicate analyses of a single crystal. The mean of 12 isochron ages is 1544 ± 8 Ma (2 SD), with isochron ages for single crystals having uncertainties as low as ±1.3 Myr (2 SD). The ²⁰⁷Pb-²⁰⁶Pb ages calculated without correction for common Pb are typically <0.5% higher than the common-Pb-corrected isochron ages reflecting the very minor amounts of common Pb present in the rutile. The LA-MC-ICPMS method described samples only the outer 0.1-0.2 mm of the rutile crystals, resulting in a grain size-independent apparent closure temperature (T_c) for Pb diffusion in rutile that is less than the T_c of monazite ?0.1 mm in diameter, but significantly higher than the Rb-Sr system in muscovite (550 °C), phlogopite (435 °C) and biotite (400 °C). Even small rutile crystals are extremely resistant to isotopic resetting. For the established slow cooling rate of ca. 3 °C/Myr, the T_c for Pb diffusion in the analysed rutile is ca. 630 °C. This is in excellent agreement with recent experimental results that indicate that rutile has a higher T_c than previously thought (ca. 600-640 °C for rutile 0.1-0.2 mm diameter cooled at 3 °C/Myr; near 600 °C [Cherniak D.J., 2000. Pb diffusion in rutile. Contrib. Mineral. Petrol. 139, 198-207], versus 400 °C [Mezger, K., Hanson G.N., Bohlen S.R., 1989a. High precision U-Pb ages of metamorphic rutile: applications to the cooling history of high-grade terranes. Earth Planet. Sci. Lett. 96, 106-118.] for 1 °C/Myr), and with current T_c estimates for monazite and other high temperature geochronometers, which have been revised upwards in recent years. The new rutile ages, together with the other geochronological data from the region, support the interpretation that the Reynolds Range underwent prolonged slow cooling on a conductive geotherm, under nearly steady-state conditions. Slow cooling at ca. 3 °C/Myr persisted for at least 40 Myr followed the peak of high-T/low-P metamorphism to granulite-facies conditions, and probably continued at ca. 2-3 °C/Myr for ca. 200 Myr overall.     

the IAB iron-meteorite complex: A group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts

We present new data for iron meteorites that are members of group IAB or are closely related to this large group, and we have also reevaluated some of our earlier data for these irons. In the past it was not possible to distinguish IAB and IIICD irons on the basis of their positions on element-Ni diagrams, but we now show that plotting the new and revised data yields six sets of compact fields on element-Au diagrams, each set corresponding to a compositional group. The largest set includes the majority (≈70) of irons previously designated IA; we christened this set the IAB main group. The remaining five sets we designate “subgroups” within the IAB complex. Three of these subgroups have Au contents similar to the main group, and form parallel trends on most element-Ni diagrams. The groups originally designated IIIC and IIID are two of these subgroups; they are now well resolved from each other and from the main group. The other low-Au subgroup has Ni contents just above the main group. Two other IAB subgroups have appreciably higher Au contents than the main group and show weaker compositional links to it. We have named these five subgroups on the basis of their Au and Ni contents. The three subgroups having Au contents similar to the main group are the low-Au (L) subgroups, the two others the high-Au (H) subgroups. The Ni contents are designated high (H), medium (M), or low (L). Thus the old group IIID is now the sLH subgroup, the old group IIIC is the sLM subgroup. In addition, eight irons assigned to two grouplets plot between sLL and sLM on most element-Au diagrams. A large number (27) of related irons plot outside these compact fields but nonetheless appear to be sufficiently related to also be included in the IAB complex.Many of these irons contain coarse silicates having similar properties. Most are roughly chondritic in composition; the mafic silicates show evidence of reduction during metamorphism. In each case the silicate O-isotopic composition is within the carbonaceous chondrite range (Δ¹⁷O ≤ −0.3‰). In all but four cases these are within the so-called IAB range, −0.30 ≥ Δ¹⁷O ≥ −0.68‰. Fine silicates appear to be ubiquitous in the main group and low-Au subgroups; this requires that viscosities in the parental melt reached high values before buoyancy could separate these.The well-defined main-group trends on element-Au diagrams provide constraints for evaluating possible models; we find the evidence to be most consistent with a crystal segregation model in which solid and melt are essentially at equilibrium. The main arguments against the main group having formed by fractional crystallization are: a) the small range in Ir, and b) the evidence for rapid crystallization and a high cooling rate through the γ-iron stability field. The evidence for the latter are the small sizes of the γ-iron crystals parental to the Widmanstätten pattern and the limited thermal effects recorded in the silicates (including retention of albitic plagioclase and abundant primordial rare gases). In contrast, crystal segregation in a cooling metallic melt (and related processes such as incomplete melting and melt migration) can produce the observed trends in the main group. We infer that this melt was formed by impact heating on a porous chondritic body, and that the melt was initially hotter than the combined mix of silicates and metal in the local region; the melt cooled rapidly by heat conduction into the cooler surroundings (mainly silicates). We suggest that the close compositional relationships between the main group and the low-Au subgroups are the result of similar processes instigated by independent impact events that occurred either at separate locations on the same asteroid or on separate but compositionally similar asteroids.     

Experimental investigations of trace element fractionation in iron meteorites,II: The influence of sulfur

We have investigated the partitioning of Ir. Ge, Ga, W, Cr, Au, P, and Ni between solid metal and metallic liquid as a function of temperature and S-concentration of the metallic liquid. Partition coefficients for siderophile elements such as Ir, W, Ga and Ge increase by factors of 10–100 as the Sconcentration of the metallic liquid increases from 0–30 wt%. Partition coefficients for other siderophile elements such as Ni, Au and P increase by only factors of 2–3. In contrast, partition coefficients for the more chalcophile element Cr decrease. These experimentally-determined partition coefficients have been used in conjunction with a fractional crystallization model to reproduce the geochemical behavior of Ni, P, Au and Ir during the magmatic evolution of groups IIAB, IIIAB, IVA and IVB iron meteorites. The mean S-concentration for each group increases in the order IVB, IVA, IIIAB, IIAB, in accord with cosmochemical prediction. However, we are unable to reproduce the geochemical behavior of Ge, Ga, W and Cr in an internally consistent way. We conclude that the magmatic histories of these iron meteorite groups are more complex than has been generally assumed.     

Determining accurate temperature-time paths from U-Pb thermochronology: An example from the Kaapvaal craton, southern Africa

Thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure. However, many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited. This study uses apatite (T_c = 450-550 °C) and titanite (T_c = 550-650 °C) U-Pb ID-TIMS thermochronology at the single- to sub-grain scale to separate the variable effects of volume diffusion of Pb from metamorphic (over)growth above and below the T_c of a mineral. Data are presented from two ca. 3227 Ma tonalite samples from north and south of the Barberton Greenstone Belt (BGB), southern Africa. Two distinct populations of apatite from a sample north of the BGB record fast cooling followed by metamorphic growth ∼10 Myr later. Both apatite and titanite dates from south of the BGB show a strong correlation with the grain size and record 100 Myr of post-emplacement cooling. Complex core-rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the T_c. The age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite-difference numerical model to show that slow, non-linear, cooling and not thermal resetting is responsible for the observed distribution. The thermal histories from either side of the BGB are very different and provide unique insight into the BGB’s tectonic evolution: a ∼70 Myr period of apparent stability after ca. 3.2 Ga terrane assembly was followed by fast exhumation south of the BGB that led to lower-crustal melting and intrusion of granitic batholiths ca. 3.14-3.10 Ga.     

Metallic minerals,thermal histories and parent bodies of some xenolithic,ordinary chondrite meteorites

We have studied metal grains in the hosts and lithic fragments of widely differing petrologic types in four xenolithic chondrftes, using reflected-light microscopy and electron-probe analysis. In Weston and Fayetteville, which both contain solar-flare tracks and solar-wind gases, kamacite, taenite and tetrataenite (ordered FeNi) and troilite show a variety of textures. On a Wood plot of central Ni content vs dimension, taenite analyses scatter as if metal grains cooled at rates of 10–1000 and 1–100 K/Myr respectively through 700 K, although metal in an H6 clast in Fayetteville plots coherently with a cooling rate of 50 K/Myr. We propose that metal grains cooled at these rates in chondritic clasts at different locations before host and clasts were compacted, and were not subsequently heated above 650 K. We predict a similar history for all gas-rich ordinary chondrites.By contrast, metallic minerals throughout Bhola and Mezö-Madaras show more uniform textures and plot coherently giving cooling rates in the range 750 to ~600 K of 0.1 and 1 K/Myr, respectively. We conclude that host and xenoliths in both chondrites were slowly cooled after compaction. Thus clasts in these chondrites experienced peak metamorphic temperatures and slow cooling through 700 K in different environments.According to the conventional onion-shell model for H, L or LL chondrite parent bodies, material of petrologic types 3–5 was arranged in successive shells around a type 6 core prior to catastrophic collisions which mixed all types intimately. But if peak metamorphic temperatures were reached during, not after accretion, as seems plausible, maximum metamorphism may have occurred in planetesimals <10 km in radius. Cooling through 700 K may then have occurred in larger bodies that accreted from these planetesimals. Iron meteorites, mesosiderites and some achondrites may also have experienced melting in planetesimals and slow cooling in larger bodies.     

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.