IIIAB Iron Meteorites Research Articles

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.

Chemical projectile–target interaction and liquid immiscibility in impact glass from the Wabar craters, Saudi Arabia

Impact glasses are usually strongly affected by secondary alteration and chemical weathering. Thus, in order to understand relevant formation processes, detailed petrographic studies on unweathered impact glasses are necessary as preserved heterogeneities in quenched impact glasses may serve as a tool to better understand their genesis. Here, we report on petrography and microchemistry of impact glasses from the Wabar impact craters (Saudi Arabia) that, with an age of ∼300years, are among the youngest terrestrial impact craters. The fact that parts of the IIIAB iron meteorite have survived impact and subsequent weathering is granting Wabar a special role among the presently 184 confirmed terrestrial impact structures. Electron microprobe analysis (EMPA) and transmission electron microscopy (TEM) obtained on the black impact melt/glass variety at Wabar suggest that meteoritic Fe was selectively mixed with high-silica target melt at high temperatures due to selective oxidation, resulting in high Fe/Ni ratios for the black melt (37 on average, individual values range from 13 to 449) and low Fe/Ni ratios for projectile droplets (“FeNi spheres” with a Fe/Ni ratio of 3 on average; Fe/Ni ratio for the meteorite is ∼12). The black melt shows emulsion textures that are the result of silicate liquid immiscibility. Liquid–liquid phase-separation resulted in the formation of a poorly polymerized, ultrabasic melt (Lfe) rich in divalent cations like Fe2+, Ca2+, or Mg2+, that is dispersed in a highly polymerized, high-silica melt (Lsi) matrix. The typical Wabar black melt emulsion displays a spheres-in-a-matrix texture of ∼10–20% Lfe homogeneously dispersed in the form of two sets of spheres and droplets (10–30nm and 0.1–0.4μm in diameter) in ∼80–90% Lsi matrix, plus occasionally disseminated FeNi spheres. Around large (>10μm) FeNi spheres, however, the typical emulsion texture changes to ∼21% Lsi dispersed in ∼79% Lfe. This change of texture is interpreted as evidence for the transfer of meteoritic Fe from the meteoritic FeNi spheres into the target melt due to selective oxidation of Fe over Ni and Co. Variations in the bulk composition of Wabar black melt largely depend on the volume ratios between immiscible ultrabasic Lfe, felsic Lsi, and remains of meteoritic FeNi spheres. Based on natural occurrences of phase-separated glasses (this work and literature) and quenching experiments (literature), there is growing evidence that liquid immiscibility is a major process in the formation of glassy impactites.

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.

Mineral and chemical composition of the new iron meteorite Javorje from Slovenia

Abstract– The single‐piece iron meteorite Javorje, with a mass of 4920 g, is the heaviest and largest meteorite found in the territory of Slovenia. The meteorite Javorje is a medium octahedrite with kamacite bandwidth of 0.85 ± 0.26 mm. The bulk composition of Ni (7.83 wt%), Co (0.48 wt%) and trace elements Ga (25 μg/g), Ge (47 μg/g), Ir (7.6 μg/g), As (5.8 μg/g), Au (0.47 μg/g), and Pt (13.4 μg/g) indicates that the meteorite Javorje belongs to the chemical group IIIAB. Mineral and bulk chemical compositions are consistent with other reported group IIIAB meteorites. The presence of numerous rhabdites, carlsbergite, sparse troilite, and chromite and abundance of daubréelites are in accordance with low‐Ni and low‐P IIIAB iron meteorites. The severely weathered surface and secondary weathering products in the interior of the meteorite suggest its high terrestrial age.

The distribution of short-lived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization

We evaluate initial ( 26Al/ 27Al) I, ( 53Mn/ 55Mn) I, and ( 182Hf/ 180Hf) I ratios, together with 207Pb/ 206Pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common, closed isotopic system, i.e., the early solar nebula. The relative initial isotopic abundances of 26Al, 53Mn, and 182Hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values, here denoted by I(Al) SS, I(Mn) SS, and I(Hf) SS, respectively. I(Mn) SS and I(Hf) SS = 9.1 ± 1.7 × 10 −6 and 1.07 ± 0.08 × 10 −4, respectively, correspond to “canonical” I(Al) SS = 5.1 × 10 −5. I(Hf) SS so determined is consistent with I(Hf) SS = 9.72 ± 0.44 × 10 −5 directly determined from an internal Hf–W isochron for CAI minerals. I(Mn) SS is within error of the lowest value directly measured for CAIs. We suggest that erratically higher values measured for CAIs in carbonaceous chondrites may reflect proton irradiation of unaccreted CAIs by the early Sun after other asteroids destined for melting by 26Al decay had already accreted. The 53Mn incorporated within such asteroids would have been shielded from further “local” spallogenic contributions from within the solar system. The relative initial isotopic abundances of the short-lived nuclides are less consistent with the 207Pb/ 206Pb ages of the corresponding materials than with one another. The best consistency of short- and long-lived chronometers is obtained for ( 182Hf/ 180Hf) I and the 207Pb/ 206Pb ages of angrites. ( 182Hf/ 180Hf) I decreases with decreasing 207Pb/ 206Pb ages at the rate expected from the 8.90 ± 0.09 Ma half-life of 182Hf. The model solar system age thus determined is T SS,Hf–W = 4568.3 ± 0.7 Ma. ( 26Al/ 27Al) I and ( 53Mn/ 55Mn) I are less consistent with 207Pb/ 206Pb ages of the corresponding meteorites, but yield T SS,Mn–Cr = 4568.2 ± 0.5 Ma relative to I(Al) SS = 5.1 × 10 −5 and a 207Pb/ 206Pb age of 4558.55 ± 0.15 Ma for the LEW86010 angrite. The Mn–Cr method with I(Mn) SS = 9.1 ± 1.7 × 10 −6 is useful for dating accretion (if identified with chondrule formation), primary igneous events, and secondary mineralization on asteroid parent bodies. All of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies. For I(Mn) SS = 9.1 ± 1.7 × 10 −6, parent body differentiation is found to extend at least to ∼5 Ma post- T SS, i.e., until differentiation of the angrite parent body ∼4563.5 Ma ago, or ∼4564.5 Ma ago using the directly measured 207Pb/ 206Pb ages of the D’Orbigny-clan angrites. The ∼1 Ma difference is characteristic of a remaining inconsistency for the D’Orbigny-clan between the Al–Mg and Mn–Cr chronometers on one hand, and the 207Pb/ 206Pb chronometer on the other. Differentiation of the IIIAB iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body, and at nearly the same time as one another as shown by the Mn–Cr ages of IIIAB irons and ureilites, respectively. The latest recorded episodes of secondary mineralization are for carbonates on the CI carbonaceous chondrite parent body and fayalites on the CV carbonaceous chondrite parent body, both extending to ∼10 Ma post- T SS.

Shişr 043 (IIIAB medium octahedrite): The first iron meteorite from the Oman desert

Abstract— The iron meteorite Shişr 043 is a single mass of 8267 g found in the south Oman desert 42 km NE of the Shişr village. It is the first iron identified among the &gt;1400 individual meteorites reported from Oman. The meteorite is a slightly elongated mass showing only minor rusting, a partially smooth and partially rough surface with octahedral cleavage, and a partially preserved metallic fusion crust typically 0.75 mm thick.The undeformed Widmanstätten pattern with a mean kamacite bandwidth of 1.0 ± 0.1 mm (n = 97) indicates structural classification as a medium octahedrite. From the bulk composition, Ni = 8.06 wt%, Ga = 18.8 ppm, Ge = 37.25 ppm, and Ir = 3.92 ppm, the meteorite is classified as IIIAB, the most common group of iron meteorites. The cosmic‐ray exposure (CRE) age based on 3He, 21Ne, 38Ar concentrations and 10Be‐21Ne, 26Al‐21Ne, and 36Cl‐36Ar ratios is 290 ± 20 Ma. This age falls within the range observed for type IIIAB iron meteorites, but does not coincide with the main cluster. The cosmogenic noble gas and radionuclide data indicate that Shişr 043 had a relatively small preatmospheric mass. The low degree of weathering is consistent with a young terrestrial age of &lt;10,000 years based on the saturated 41Ca concentration. Shişr 043 is not paired with any of the other eight known iron meteorites from the Arabian Peninsula.

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.

Accurate measurement of silver isotopic compositions in geological materials including low Pd/Ag meteorites

Very precise silver (Ag) isotopic compositions have been determined for a number of terrestrial rocks, and high and low Pd/Ag meteorites by utilizing multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The meteorites include primitive chondrites, the Group IAB iron meteorites Canyon Diablo and Toluca, and the Group IIIAB iron meteorite Grant. Silver isotopic measurements are primarily of interest because 107Ag was produced by decay of the short-lived radionuclide 107Pd during the formation of the solar system and hence the Pd-Ag chronometer has set constraints on the timing of early planetesimal formation. A 2σ precision of ±0.05‰ can be obtained for analyses of standard solutions when Ag isotopic ratios are normalized to Pd, to correct for instrumental mass discrimination, and to bracketing standards. Caution must be exercised when making Ag isotopic measurements because isotopic artifacts can be generated in the laboratory and during mass spectrometry. The external reproducibility for geological samples based on replicate analyses of rocks is ±0.2‰ (2σ). All chondrites analyzed have similar Ag isotopic compositions that do not differ significantly (>0.3‰) from the ‘terrestrial’ value of the NIST SRM 978a Ag isotope standard. Hence, they show no evidence of excess 107Ag derived from 107Pd decay or, of stable Ag isotope fractionation associated with volatile element depletion within the accretion disk or from parent body metamorphism. The Group IAB iron meteorite samples analyzed show evidence of complex behavior and disturbance of Ag isotope systematics. Therefore, care must be taken when using this group of iron meteorites to obtain chronological information based on the Pd-Ag decay scheme.

Sulfur contents of the parental metallic cores of magmatic iron meteorites

Magmaticiron meteorites are thought to be samples of the central metallic coresof asteroid-sized parent bodies. Sulfur is believed to have been animportant constituent of these parental cores, but due to the lowsolubility of S in solid metal, initial S-contents for the magmaticgroups cannot be determined through direct measurements of the ironmeteorites. However, experimental solid metal-liquid metal partitioncoefficients show a strong dependence on the S-content of the metallicliquid. Thus, by using the experimental partition coefficients to modelthe fractional crystallization trends within magmatic iron meteoritegroups, the S-contents of the parental cores can be indirectlyestimated. Modeling the Au, Ga, Ge, and Ir fractionations in four ofthe largest magmatic iron meteorite groups leads to best estimates forthe S-contents of the parental cores of 12 ± 1.5 wt% forthe IIIAB group, 17 ± 1.5 wt% for the IIAB group, and 1± 1 wt% for the IVB group. The IVA elementalfractionations are not adequately fit by a simple fractionalcrystallization model with a unique initial S-content. These S-contentestimates are much higher than those recently inferred fromcrystallization models involving trapped melt. The discrepancy is duelargely to the different partition coefficients that are used by thetwo models. When only partition coefficients that are consistent withthe experimental data are used, the trapped melt model, and the lowS-contents it advocates, cannot match the Ge and Ir fractionations thatare observed in IIIAB ironmeteorites.

Pt-Re-Os systematics of group IIAB and IIIAB iron meteorites

The Pt-Re-Os isotopic and elemental systematics of 13 group IIAB and 23 group IIIAB iron meteorites are examined. As has been noted previously for iron meteorite groups and experimental systems, solid metal-liquid metal bulk distribution coefficients (D values) for both IIAB and IIIAB systems show DOs>DRe>>DPt>1 during the initial stages of core crystallization. Assuming closed-system crystallization, the latter stages of crystallization for each core are generally characterized by DPt>DRe>DOs. The processes governing the concentrations of these elements are much more complex in the IIIAB core relative to the IIAB core. Several crystallization models utilizing different starting parameters and bulk distribution coefficients are considered for the Re-Os pair. Each model has flaws, but in general, the results suggest that the concentrations of these elements were dominated by equilibrium crystallization and subsequent interactions between solid metal and both equilibrium and evolved melts. Late additions of primitive metal to either core were likely minor or nonexistent.The 187Re-187Os systematics of the IIAB and IIIAB groups are consistent with generally closed-system behavior for both elements since the first several tens of Ma of the formation of the solar system, consistent with short-lived chronometers. The Re-Os isochron ages for the complete suites of IIAB and IIIAB irons are 4530 ± 50 Ma and 4517 ± 32 Ma, respectively, and are similar to previously reported Re-Os ages for the lower-Ni endmembers of these two groups. Both isochrons are consistent with, but do not require crystallization of the entire groups within 10–30 Ma of the initiation of crystallization.The first high-precision 190Pt-186Os isochrons for IIAB and IIIAB irons are presented. The Pt-Os isochron ages for the IIAB and IIIAB irons, calculated using the current best estimate of the λ for 190Pt, are 4323 ± 80 Ma and 4325 ± 26 Ma respectively. The Re-Os and Pt-Os ages do not overlap within the uncertainties. The younger apparent ages recorded by the Pt-Os system likely reflect error in the 190Pt decay constant. The slope from the Pt-Os isochron is combined with the age from the Re-Os isochron for the IIIAB irons to calculate a revised λ of 1.415 × 10−12 a−1 for 190Pt, although additional study of this decay constant is still needed.

In search of extant Tc in the early solar system: 98Ru and 99Ru abundances in iron meteorites and chondrites

The short-lived 98Tc– 98Ru (Half-life between 4 and ∼10 Ma) and 99Tc– 99Ru (Half-life 0.21 Ma) decay systems have the potential to provide important information on the relative chronology of processes that affected metal phases in the early solar system. The proof of extant 99Tc in the solar system would also tightly constrain the time interval between production of the nuclide and injection of s-process isotopes into the protosolar cloud. High-precision Ru isotopic data for group IIAB (Negrillos, Bennett County, Coahuila, Filomena, Old Woman) and IIIAB (Casas Grandes, Costilla Peak) iron meteorites, and the chondrites Allende (CV3) and Allegan (H5) indicate that the isotopic compositions of 98Ru and 99Ru in these meteorites overlap with the terrestrial values at the ±0.8 and ±0.3 ε levels (parts in 10,000), respectively. Previous reports of positive deviations of 98Ru in Negrillos and Casas Grandes likely reflect inaccurate measurements. Evaluation of processes that may fractionate Tc/Ru in solar system objects suggests that this ratio likely varied by only a factor of 2 or less. Using this constraint and solar system initial abundances of 99Tc derived from astrophysical models, evolution models for the 99Tc– 99Ru system predict deviations in 99Ru of >0.3 ε only, if these fractionation events occurred during collapse of the molecular cloud (<10 ka). Thus, positive identification of enriched 99Ru may require better than 0.1 ε unit resolution. Absence of resolvable 98Ru deviations indicates 98Tc/ 96Ru ⊙i<2×10 −5.

Mn‐Cr chronology of five IIIAB iron meteorites

Abstract— Mn‐Cr systematics in phosphates (sarcopside, graftonite, beusite, galileiite, and johnsomervilleite) in IIIAB iron meteorites were investigated by secondary ion mass spectrometry (SIMS). In most cases, excesses in 53Cr are found and δ53Cr is well correlated with Mn/Cr ratios, suggesting that 53Mn was alive at the time of IIIAB iron formation. The inferred Mn‐Cr “ages” are different for different phosphate minerals. This is presumably due to a combined effect of the slow cooling rates of IIIAB iron meteorites and the difference in the diffusion properties of Cr and Mn in the phosphates. The ages of sarcopside are the same for the IIIAB iron meteorites. Johnsomervilleite shows apparent old ages, probably because of a gain of Cr enriched in 53Cr during the closure process. Apparently, old Mn‐Cr ages reported in previous studies can also be explained in a similar way. Therefore, the IIIAB iron meteorites probably experienced identical thermal histories and thus derived from the core of a parent body. Thermal histories of the parent body of IIIAB iron meteorites that satisfy the Mn‐Cr chronology and metallographic cooling rates were constructed by computer simulation. The thermal history at an early stage (&lt;10 Ma after CAI formation) is well determined, though later history may be more model‐dependent. It is suggested that relative timing of various events in the IIIAB parent body may be estimated with the aid of the thermal history. There is a systematic difference in Mn and Cr concentrations in various minerals (phosphates, sulfide, etc.) among the IIIAB iron meteorites, which seems to be mainly controlled by redox conditions.

Partitioning of siderophile elements (W, Mo, As, Ag, Ge, Ga, and Sn) and Si in the Fe-S system and their fractionation in iron meteorites

The partitioning of some moderately siderophile elements and Si between solid metal and liquid metal-sulfide in the Fe-S system at 1000 to 1400°C and low pressure has been investigated at minor abundances using EPMA, and the effect of S content of the liquid on molar partition coefficient [D(x)] quantified by linear regression. The parameters (C) and (β) for the Jones and Malvin (1990) equation ln D(x) = −β ln(1–2αX S) + C are: W 0.14(26), 2.07(20); Mo −0.52(53), 1.25(44); As −1.30(14), 1.19(15); Ag −2.05(12), −1.77(9); Ge −0.57(8), 1.79(5); Ga −0.18(6), 1.40(6); Sn −1.82(8), 0.40(5), and Si −0.78(41), 2.57(22), respectively, where α = 1.05 and 1.09 for Fe-S and Fe-Ni-S systems, respectively, and X S is the mole fraction of S in the liquid; the parameters for W, Ge and Ga are based on this study and literature results. The fractionation patterns for Ir, W, Au, As, Ge, and Ga in group IIIAB iron meteorites are reproduced quantitatively using a non-equilibrium batch crystallization model with 4 mol% crystallization steps. The Ge fit required correction for the effect of P on D(Ge). The modeled batch size is interpreted to relate to the molar proportion of the crystallized IIIAB asteroidal core that maintained communication with the metal-sulfide liquid. This chemical model is consistent with core evolution by either inward or outward concentric crystallization.

Nitrogen‐isotopic compositions of IIIE iron meteorites

Abstract— The (compositionally) closely related iron meteorite groups IIIE and IIIAB were originally separated based on differences in kamacite bandwidth, the presence of carbides only in the IIIE group, and marginally resolvable differences on the Ga‐Ni and Ge‐Ni diagrams. A total of six IIIE iron meteorites have been analyzed for C and N using secondary ion mass spectrometry, and three of these have also been analyzed for N, Ne, and Ar by stepped combustion.We show that these groups cannot be resolved on the basis of N abundances or isotopic compositions but that they are marginally different in C‐isotopic composition and nitride occurrence. Cosmic‐ray exposure age distributions of the IIIE and IIIAB iron meteorites seem to be significantly different.There is a significant N‐isotopic range among the IIIE iron meteorites. A negative correlation between δ15N and N concentration suggests that the increase in s̀15N resulted from diffusional loss of N.

The pyroxene pallasites, Vermillion and Yamato 8451: Not quite a couple

Abstract— Two pallasites, Vermillion and Yamato (Y)‐8451, have been studied to obtain petrologic, trace element, and O‐isotopic data. Both meteorites contain low‐Ca and high‐Ca pyroxenes (&lt;2% by volume) and have been dubbed “pyroxene pallasites.” Pyroxene occurs as large individual grains, as inclusions in olivine and in other pyroxene, and as grains along the edges of olivine. Symplectic overgrowths, sometimes found in Main Group and Eagle Station pallasites, are not seen in the pyroxene pallasites. Olivine compositions are Fa10–12, similar to those of Main Group pallasites. Siderophile trace element data show that metal in the two meteorites have significantly differing compositions that are, for many elements, outside the range of the Main Group and Eagle Station pallasites. These compositions also differ from those of IAB and IIIAB iron meteorites. Rare earth element (REE) patterns in merrillite are similar to those seen in other pallasites, indicating formation by subsolidus reaction between metal and silicate, with the merrillite inheriting its pattern from the surrounding silicates. The O‐isotopic compositions of Vermillion and Y‐8451 are similar but differ from Main Group or Eagle Station pallasites, as well as other achondrite and primitive achondrite groups. Although Vermillion and Y‐8451 have similar mineralogy, pyroxene compositions, REE patterns, and O‐isotopic compositions, there is sufficient evidence to resist formally grouping these two meteorites. This evidence includes the texture of Vermillion, siderophile trace element data, and the presence of cohenite in Vermillion.

Massive chromite in the Brenham pallasite and the fractionation of Cr during the crystallization of asteroidal cores

Large (≥2 mm) chromite grains are present in IIIAB iron meteorites and in the main-group pallasites ( pmg), closely related to high-Au IIIAB irons. Pallasites seem to have formed by the intrusion of a highly evolved metallic magma from a IIIAB-like core into fragmented olivine of the overlying dunite mantle. High Cr contents are commonly encountered during the analyses of metallic samples of high-Au IIIAB irons and main-group pallasites, an indication that Cr contents were high in the intruding liquid and that Cr behaved as an incompatible element during the crystallization of the IIIAB magma, contrary to expectations based on the negative IIIAB Cr-Ni and Cr-Au trends among low-Au IIIAB irons. In a region about 10 cm across in the Brenham main-group pallasite massive chromite fills the interstices between olivine grains, the site normally occupied by metal in Brenham and other pallasites. The massive chromite may have formed as a late cumulus phase; because Fe-Ni was also crystallizing, its absence in the chromite-rich region suggests a separation associated with differences in liquid buoyancy. The coexisting chromite and olivine are zoned; in the olivine FeO is highest in pallasitic (olivine-metal) regions, lowest in rims adjacent to chromite, and intermediate in the cores of these olivines. Chromite shows the opposite zoning, with the highest FeO contents at grain edges adjacent to olivine. The observed gradients are those expected to form by Fe-Mg exchange between olivine and chromite during slow cooling at subsolidus temperatures. Compared to normal Brenham, contents of phosphoran olivine and phosphates are higher in the chromitic pallasitic region. We also report data for large-to-massive chromites present in pmg Molong and in high-Au IIIAB Bear Creek that, like Brenham, formed from a highly evolved magma. The Bear Creek chromite has a much lower Mg content than that in the pallasites, implying that, in the pmg, the Mg was extracted from the olivine during high-temperature reaction with the precipitating chromite. There are other circumstantial arguments indicating that Cr was incompatible in the metal during the crystallization of the IIIAB magma, with the concentration in the residual magma rising from an initial value of about 300 μg/g to a value around 700 μg/g when Bear Creek and Brenham were formed. We consider possible explanations for these negative Cr-Au and Cr-Ni trends and find the most probable one to be that they reflect sampling artefacts resulting from analysts avoiding visible chromite (and the commonly associated phase FeS) when choosing metal samples.

The phosphates of IIIAB iron meteorites

Abstract— Thirteen phosphate minerals are found in IIIAB iron meteorites. Four of these (sarcopside, graftonite, johnsomervilleite, and galileiite) constitute the majority of occurrences. The IIIB iron meteorites are confined to occurrences of only these four phosphates. The IIIA iron meteorites may contain one or more of these four phases; they may also contain other rarer phosphates, and silica (in two instances) and a silicate rock (in one instance). Thus, the IIIA lithophile chemistry is more varied than that of the IIIB meteorites. Based on petrographic relations, sarcopside appears to be the first phosphate to form. Graftonite is probably formed by recrystallization of sarcopside. Johnsomervilleite and galileiite exsolved as enclaves in sarcopside or graftonite at lower temperatures, although some of these also nucleated as separate crystals. The IIIAB phosphates are carriers of a group of incompatible lithophile elements: Fe, Mn, Na, Ca, and K, and, rarely, Mg as well as Pb. These elements (and O) were concentrated in a residual, S‐rich liquid during igneous fractional crystallization of the IIIAB core mass. The phosphates formed by oxidation of P as the core solidified and excluded O, which increased its partial pressure in the residual liquid. The trace siderophile trends in bulk IIIAB metal are paralleled by a mineralogical trend of the phosphate minerals that formed. For IIIAB meteorites with low‐Ir contents in the metal, the phosphates are mainly Fe‐Mn phases; at intermediate Ir values, more Na‐bearing phosphates appear; at the highest Ir values, the rarer Na‐, K‐, Mg‐, Cr‐, and Pb‐bearing phosphates appear. The absence of significant amounts of Mg, Si, Al, and Ti suggest depletion of these elements in the core by the overlying mantle.

Re-Os systematics in pallasite and mesosiderite metal

Re-Os data on pallasite metal yield a sufficient range in Re/Os to permit the determination of a whole rock isochron, if pallasites from different groups are considered together. The results are indistinguishable from the data obtained on Groups IAB, IIAB, IID, IIIAB, IVA, and IVB iron meteorites. Pallasite data were obtained on samples of the Main Group, which have been considered related to Group IIIAB irons, and on samples of the distinctive pallasites Eagle Station, Finmarken, and Marjalahti. It appears that iron meteorites and pallasites crystallized within a time interval of 20 Ma. This result is consistent with the evidence from the short-lived chronometers 107Pd- 107Ag and 53Mn- 53Cr. It is also consistent with the evidence from 182Hf- 182W that the time interval over which molten FeNi metal from different iron meteorites equilibrated with and became segregated from silicates was ∼10 Ma. The well-defined 187Re- 187Os whole-rock isochron for pallasites and iron meteorites requires that they were formed in parent bodies that had been partially molten and segregated FeNi metal and FeS and then all crystallized within a 20 Ma time interval. This conclusion appears to apply to members of distinct groups of iron meteorites and pallasites which are viewed as not being cogenetic. The short time-scale requires that the irons and pallasites were formed in small parent planets of ≲10 km radius or near the surface of larger bodies. In contrast to the pallasites, we find that FeNi samples from mesosiderites have a narrow range in Re/Os so that it is not possible to determine a whole-rock isochron for these samples. The mesosiderite data also lie somewhat displaced from the iron meteorite and pallasite isochron indicating a more complex multi-stage evolution. Based on Sm-Nd data on mesosiderite silicate clasts, the Re-Os results on mesosiderite metal are compatible with the model of melted or partially molten FeNi cores or pods still preserved in protoplanetary bodies when these bodies were disrupted up to 150 Ma after early formation and the FeNi was splashed onto the surfaces of other small, differentiated planetesimals.

New Chilean iron meteorites: Medium octahedrites from Northern Chile are unique

Abstract— We report data on three new iron meteorites from Northern Chile and propose names. All are unnamed iron meteorites from the meteorite collection at the Universidad de La Serena. For two, the provenance is unknown; for the third, the presumed discovery site is in the countryside east of Iquique. The three meteorites have been analyzed by instrumental neutron activation analysis (INAA) and their structures examined with a binocular microscope. La Serena is a complete 663 g iron, a new member of group IIICD; it is not paired with any other iron. Elqui has a mass of 260 g; two faces are fractures, possibly produced by human actions, but fusion crust appears to be present on some of the remaining surface. It is a hexahedrite and a member of group IIAB, but its composition differs from that of all other Chilean hexahedrites. The third iron, Pozo Almonte, is a medium octahedrite member of group IIIAB, one of the most common meteorite groups. To find out whether it is paired, we assembled a full set of IIIAB iron meteorites from Northern Chile. Our compositional data show that Pozo Almonte is not paired with any other IIIAB iron, and that there are no pairings within the full set with the possible exception of Joel's Iron and Sierra Sandon, which differ only in their contents of Ir, 0.39 and 0.34 μg/g, respectively. However, Buchwald's (1975) structural observations rule out this possible pairing. We find appreciable differences in Cu, As and Au between the previously paired IIIAB irons Chañaral and Ilimaës and conclude that these should not be paired.

Phosphorus Behaviour in Pallasites: Experimental Constraints

Pallasites are stony-iron meteorites mainly composed of olivine (Fa11-20) and iron-nickel metal in approximately equal weight proportions. They are believed to have formed at the core-mantle boundary of planetary bodies. An alternative model is that these meteorites represent pieces of metal pools or 'raisins' isolated within the silicate mantle of their parent body. The minor-element content of the pallasites metal suggest that pallasites are linked to the IIIAB iron-meteorite group (Scott, 1977). Trace and minor element variations within the IIIAB group has been successfully modelled considering that these iron meteorites are pieces of a single asteroidal core material which solidified according to a fractional crystallisation process (Haack and Scott, 1993). Following this model, the high Ni-content of the pallasites metal relative to IIIAB iron-meteorites and the presence of olivine crystals is consistent with a late crystallizing core-liquid, enriched in incompatible elements, which has intruded the asteroid olivine-mantle as already proposed by Scott (1977). A remarkable feature in pallasites is the occurrence of phosphates coexisting with schreibersite, (Fe,Ni)3P. Whitlockite, Ca3(PO4)2, farringtonite, ( M g , F e ) 3 ( P O 4 ) 2 , a n d s t a n f i e l d i t e , Ca4(Mg,Fe)5(PO4)6, are the most widespread species. Phosphoran olivine, a forsterite-rich olivine containing up to 4 -5 wt.% P205 , has also been reported in some pallasites. Phase equilibria involving both phosphates and phosphides have been used to infer oxygen fugacity in pallasites. If we assume that pallasites are composed of metal (either metal pools or core) and olivine of mantle origin and since phosphorus is a siderophile element, accordingly schreibersite phosphorus was dissolved in the metal. The origin of the phosphorus in phosphates, however, is not as obvious. Phosphates could be of mantle origin where they would coexist with olivine. This would rule out the core-mantle origin for pallasites since in terrestrial magmatic systems, phosphates are known to be late crystallizing phases unlikely to occur in magmatic cumulates. However, on the basis of R E E pattern analyses, Davis and Olsen (1991) pointed out that some calcium-rich phosphates (whitlockite and stanfieldite) in Springwater and Santa Rosalia have crystallised from a chondritic melt. The R E E pattern and content of some other phosphate crystals occurring in phosphate-pyroxenetroilite intergrowths between olivine and metal, is different and seem inherited from olivine. In that case, phosphates are supposed to have formed by subsolidus reaction between metal and olivine as suggested by the textural observations. Buseck and Holdsworth (1977) have described striking phosphate textures which rather suggest that phosphates where present as molten droplets attached onto the olivines surface. Phosphates appear to be key minerals in order to decipher the pallasites origin. To our knowledge, no experimental data on the silicate-metal-phosphate system relevant to pallasite formation. Only thermodynamic calculations have been performed in that system in order to derive oxygen fugacity in pallasites (e.g. Righter et al., 1990).