Main Group Pallasites Research Articles

Geochemistry of pallasite olivine and the origin of pallasites

I have done major element analyses by electron microprobe, in-situ trace element analyses by laser ablation inductively coupled plasma mass spectrometry, and instrumental neutron activation analyses on bulk samples of olivine grains separated from main-group and Eagle-Station pallasites. Most main-group pallasite olivines have homogeneous Fe/Mg yet have varying Fe/Mn. Those few with anomalously ferroan olivine have Fe/Mn within the range of other main-group pallasites. High-temperature redox process coupled with diffusional exchange resulted in the homogeneous compositions of most main-group pallasites; simple diffusional exchange alone is insufficient. The Eagle-Station pallasites have Fe/Mn twice that of main-group pallasites with similar Fe/Mg, a result of having roughly half the Mn content. The Ni/Co ratio of main-group pallasite olivines is relatively constant and was imposed by the same high-temperature redox/diffusion process that established Fe/Mg-Fe/Mn relationships. Variability in trace lithophile element contents within individual pallasites and within individual olivine grains, coupled with very low contents for some that are inconsistent with formation from a magma, indicate that the current mm-sized olivine grains were recrystallized from a fragmental olivine breccia; grain fragments from different portions of an original dunitic mantle were juxtaposed in the breccia. Main-group pallasites are dimict breccias formed of fragmented and mixed monomict dunite breccia and metallic breccia that were formed in the walls and floor of a large basin that penetrated the mantle of their parent asteroid. Limited data indicate that Eagle-Station pallasites may have been formed by a similar process. Given that Eagle-Station and main-group pallasites were formed in distinct regions of the early Solar System, the pallasite forming process likely was common in early Solar System history.

Reconciling fast and slow cooling during planetary formation as recorded in the main group pallasites

Pallasite meteorites contain evidence for vastly different cooling timescales: rapid cooling at high temperatures (K/yrs) and slow cooling at lower temperatures (K/Myrs). Pallasite olivine also shows contrasting textures ranging from well-rounded to angular and fragmental, and some samples record chemical zoning. Previous pallasite formation models have required fortuitous changes to the parent body in order to explain these contrasting timescales and textures, including late addition of a megaregolith layer, impact excavation, or parent body break-up and recombination. We investigate the timescales recorded in Main Group Pallasite meteorites with a coupled multiscale thermal diffusion modelling approach, using a 1D model of the parent body and a 3D model of the metal-olivine intrusion region, to see if these large-scale changes to the parent body are necessary. We test a range of intrusion volumes and aspect ratios, metal-to-olivine ratios, and initial temperatures for both the background mantle and the intruded metal. We find that the contrasting timescales, textural heterogeneity, and preservation of chemical zoning can all occur within one simple ellipsoidal segment of an intrusion complex. These conditions are satisfied in 13% of our randomly generated models (2200 model runs), with small intrusion volumes (with a mean radius ≲100 m) and colder background mantle temperatures (≲1200 K) favourable. Large rounded olivine can be explained by a previous intrusion of metal into a hotter mantle, suggesting possible repeated bombardment of the parent body. We speculate that the formation of pallasitic zones within planetesimals may have been a common occurrence in the early Solar System, as our model shows that favourable pallasite conditions can be accommodated in a wide range of intrusion morphologies, across a wide range of planetesimal mantle temperatures, without the need for large-scale changes to the parent body. We suggest that pallasites represent a late stage of repeated injection of metal into a cooling planetesimal mantle, and that heterogeneity observed in micro-scale rounding or chemical zoning preservation in pallasite olivine can be explained by diverse cooling rates in different regions of a small intrusion.

Mantle fragmentation and incomplete core merging of colliding planetesimals as evidenced by pallasites

Main group pallasites were likely formed by the collision of their parent body with a smaller impactor that caused a mixing of mantle and core material from the target and impactor, respectively. In order to better constrain the collision, we present particle size distribution (PSD) analyses of olivines in seven main group pallasites and of the Eagle station pallasite. The PSDs of two fragmental pallasites (Admire and Huckitta) contain linear segments in bi-logarithmic particle number versus size diagrams that are similar to terrestrial cataclasites or impact-related rocks. On the other hand, the PSDs of four angular pallasites only display linear segments above their respective average grain sizes. We argue that fragmental pallasites record brittle rock deformation close to the impact site of the collision, while angular pallasites represent deeper-lying mantle rocks of the target body that were disintegrated by the downward percolation of core metal from the impactor. High strain-rate deformation experiments with the system olivine – FeS melt ± Au melt produced microstructures and PSDs that are broadly similar to these two textural groups. The experiments also suggest that de-localized metal melt percolation and concomitant mantle disintegration as evidenced in angular pallasites is facilitated by weak grain boundaries caused by a small fraction of previously present metal melt in the mantle as opposed to localized diking that is dominant in a melt-free mantle. The former mechanism is expected to prevent efficient core-merging and instead causes mantle-impregnation with metal melt, which could be active when planetesimal mantles were still warm due to short-lived radiogenic heating. In addition to the parent body of the PMG, asteroid 16 Psyche may be an example of this inefficient core-merging mechanism.

Iron isotope evidence of an impact origin for main-group pallasites

Iron isotope evidence of an impact origin for main-group pallasites

Olivine aggregates reveal a complex collisional history of the main group pallasite parent body

AbstractOlivine aggregates, bodies found in pallasites that consist of olivines with coherent grain boundaries and minor amounts of Fe‐Ni and troilite, likely represent well‐preserved samples of different mantle regions of pallasite parent bodies (PPBs). We investigated olivine aggregates from the main group pallasites Fukang, Esquel, Imilac, and Seymchan and compare their textures with results from deformation experiments. Our measurements reveal an inverse relationship between the grain size of olivines and the primary metal fraction inside olivine aggregates, which is explained by simultaneous grain growth retarded by Zener pinning in different mantle regions. Textural evidence indicates that the mantle has remained at high temperatures before initial cooling occurred shortly after pallasite formation that was likely caused by an impact. Different degrees of annealing of the deformation textures suggest that the postcollisional cooling occurred in the order Seymchan, Imilac, Esquel, and Fukang. We interpret this observation with an increasing burial depth after the collision. We also demonstrate that the mantle has not been convecting before the impact despite being at high temperature. Using the minimum critical Rayleigh number, we estimate PPB radii assuming different core radii. Our results question the recent ferromagmatism hypothesis for pallasite formation and support a multistage formation process that includes one or several impacts.

Tungsten and molybdenum isotopic evidence for an impact origin of pallasites

The origin of pallasites—stony-iron meteorites mainly composed of olivine and Fe-Ni metal—is debated and proposed formation scenarios broadly range from models that explain pallasite formation by internal processes in the mantle of a differentiated planetesimal to those that involve impact–induced mixing of core and mantle materials. Here, the origin of pallasites is examined by studying the nebular source regions of their precursor material using Mo isotopes and their history of metal-silicate segregation using Hf-W chronometry. We report new Mo and W isotopic data for a large suite of pallasite metal samples, alongside Pt isotope data to quantify superimposed cosmic ray exposure effects. Most main-group pallasites exhibit uniform pre-exposure 182W and Mo isotopic compositions that bear an excellent similarity to those of IIIAB iron meteorites. Four main-group pallasites and the IIIAB iron Thunda have more radiogenic pre-exposure 182W compositions, but display the same Mo isotopic composition as other main-group pallasites and IIIAB irons. This strong chronological and genetic link strongly suggests that main-group pallasite metal originated in the IIIAB parent body core. This, combined with prior Pd-Ag chronometric evidence for an early collisional disruption of the IIIAB parent body, implies that main-group pallasites formed by impact–induced mixing of metal and silicates rather than by an internal process on the IIIAB parent body. This mixing led to elevated 182W compositions in some pallasites, which are best accounted for by partial re-equilibration of IIIAB metal with radiogenic 182W from the colliding body. Collectively, our results support models that explain main-group pallasite formation by injection of pallasite metal into the mantle of another differentiated body, implying that pallasite silicates did not primarily derive from the IIIAB mantle, but instead from that of the colliding body.

Isotopic evidence for pallasite formation by impact mixing of olivine and metal during the first 10 million years of the Solar System.

Pallasites are mixtures of core and mantle material that may have originated from the core-mantle boundary of a differentiated body. However, recent studies have introduced the possibility that they record an impact mix, in which case an isotopic difference between metal and silicates in pallasites may be expected. We report a statistically significant oxygen isotope disequilibrium between olivine and chromite in main group pallasites that implies the silicate and metal portions of these meteorites stem from distinct isotopic reservoirs. This indicates that these meteorites were formed by impact mixing, during which a planetary core was injected into the mantle of another body. The impactor likely differentiated within ∼1-2Myr of the start of the Solar System based on Hf-W chronology of pallasite metal, and we infer the age of the impact based on Mn-Cr systematics and cooling rates at between ∼1.5 and 9.5Myr after Ca-Al-rich inclusions (CAIs). When combined with published slow subsolidus cooling rates for these meteorites and considering that several pallasite groups exist, our results indicate that such impacts may be an important stage in the evolution of planetary bodies.

The Achievements of the RockStar Group (Perugia) on Astrophysical Modelling and Pallasite Geochemistry

In the present work we summarize the first achievements of the RockStar Group of the Department of Physics and Geology (at the University of Perugia, Italy), which is made of a strict collaboration between Physicists and Geologists on astrophysical and planetological studies. The RockStar Group acts on two research lines: (i) astrophysical modeling and (ii) mineralogical and geochemical studies of meteorites. In the first part of the article we review the recent results concerning the development of theoretical modeling of nucleosynthesis and mixing process in asymptotic giant branch. In the second part we report (1) the catalog of the Meteorite collection of University of Perugia and (2) major and trace elements mapping, performed through EPMA and LA-ICP-MS, of the Mineo pallasite, a unique sample hosted by the collection. The new data constrain the Mineo meteorite among the Main Group Pallasites and support the hypothesis of the “early giant impact” formation.

Keplerite, Ca9(Ca0.5☐0.5)Mg(PO4)7, a new meteoritic and terrestrial phosphate isomorphous with merrillite, Ca9NaMg(PO4)7

Abstract Keplerite is a new mineral, the Ca-dominant counterpart of the most abundant meteoritic phosphate, which is merrillite. The isomorphous series merrillite-keplerite, Ca9NaMg(PO4)7-Ca9(Ca0.5☐0.5) Mg(PO4)7, represents the main reservoir of phosphate phosphorus in the solar system. Both minerals are related by the heterovalent substitution at the B-site of the crystal structure: 2Na+ (merrillite) → Ca2+ + ☐ (keplerite). The near-end-member keplerite of meteoritic origin occurs in the main-group pallasites and angrites. The detailed description of the mineral is made based on the Na-free type material from the Marjalahti meteorite (the main group pallasite). Terrestrial keplerite was discovered in the pyrometamorphic rocks of the Hatrurim Basin in the northern part of Negev desert, Israel. Keplerite grains in Marjalahti have an ovoidal to cloudy shape and reach 50 μm in size. The mineral is colorless, transparent with a vitreous luster. Cleavage was not observed. In transmitted light, keplerite is colorless and non-pleochroic. Uniaxial (−), ω = 1.622(1), ε = 1.619(1). Chemical composition (electron microprobe, wt%): CaO 48.84; MgO 3.90; FeO 1.33; P2O5 46.34, total 100.34. The empirical formula (O = 28 apfu) is Ca9.00(Ca0.33Fe0.202+☐0.47)1.00Mg1.04P6.97O28. The ideal formula is Ca9(Ca0.5☐0.5)Mg(PO4)7. Keplerite is trigonal, space group R3c, unit-cell parameters refined from single-crystal data are: a = 10.3330(4), c = 37.0668(24) Å, V = 3427.4(3) Å3, Z = 6. The calculated density is 3.122 g/cm−3. The crystal structure has been solved and refined to R1 = 0.039 based on 1577 unique observed reflections [I>2σ(I)]. A characteristic structural feature of keplerite is a partial (half-vacant) occupancy of the sixfold-coordinated B-site (denoted as CaIIA in the earlier works). The disorder caused by this cation vacancy is the most likely reason for the visually resolved splitting of the ν1 (symmetric stretching) (PO4) vibration mode in the Raman spectrum of keplerite. The mineral is an indicator of high-temperature environments characterized by extreme depletion of Na. The association of keplerite with “REE-merrillite” and stanfieldite provides evidence for the similarity of temperature conditions that occurred in the Mottled Zone to those expected during the formation of pallasite meteorites and lunar rocks. Because of the cosmochemical significance of the merrillite-keplerite series and by analogy to plagioclases, the Na-number measure, 100×Na/(Na+Ca) (apfu), is herein proposed for the characterization of solid solutions between merrillite and keplerite. The merrillite end-member, Ca9NaMg(PO4)7, has the Na-number = 10, whereas keplerite, Ca9(Ca0.5☐0.5)Mg(PO4)7, has Na-number = 0. Keplerite (IMA 2019-108) is named in honor of Johannes Kepler (1571–1630), a prominent German naturalist, for his contributions to astronomy and crystallography.

Nazarovite, Ni12P5, a new terrestrial and meteoritic mineral structurally related to nickelphosphide, Ni3P

Abstract Nazarovite, Ni12P5, is a new natural phosphide discovered on Earth and in meteorites. Terrestrial nazarovite originates from phosphide assemblages confined to pyrometamorphic suite of the Hatrurim Formation (the Mottled Zone), the Dead Sea basin, Negev desert, Israel. Meteoritic nazarovite was identified among Ni-rich phosphide precipitates extracted from the Marjalahti meteorite (main group pallasite). Terrestrial mineral occurs as micrometer-sized lamella intergrown with transjordanite (Ni2P). Meteoritic nazarovite forms chisel-like crystals up to 8 μm long. The mineral is tetragonal, space group I4/m. The unit-cell parameters of terrestrial and meteoritic material, respectively: a 8.640(1) and 8.6543(3), c 5.071(3), and 5.0665(2) Å, V 378.5(2), and 379.47(3) Å3, Z = 2. The crystal structure of terrestrial nazarovite was solved and refined on the basis of X-ray single-crystal data (R1 = 0.0516), whereas the structure of meteoritic mineral was refined by the Rietveld method using an X-ray powder diffraction profile (RB = 0.22%). The mineral is structurally similar to phosphides of schreibersite–nickelphosphide join, Fe3P-Ni3P. Chemical composition of nazarovite (terrestrial/meteoritic, electron microprobe, wt%): Ni 81.87/78.59, Fe <0.2/4.10; Co <0.2/0.07, P 18.16/17.91, total 100.03/100.67, leading to the empirical formula Ni11.97P5.03 and (Ni11.43Fe0.63Co0.01)12.07P4.94, based on 17 atoms per formula unit. Nazarovite formation in nature, both on Earth and in meteorites, is related to the processes of Fe/Ni fractionation in solid state, at temperatures below 1100 °C.

A Time‐Resolved Paleomagnetic Record of Main Group Pallasites: Evidence for a Large‐Cored, Thin‐Mantled Parent Body

AbstractSeveral paleomagnetic studies have been conducted on five Main Group pallasites: Brenham, Marjalahti, Springwater, Imilac, and Esquel. These pallasites have distinct cooling histories, meaning that their paleomagnetic records may have been acquired at different times during the thermal evolution of their parent body. Here, we compile new and existing data to present the most complete time‐resolved paleomagnetic record for a planetesimal, which includes a period of quiescence prior to core solidification as well as dynamo activity generated by compositional convection during core solidification. We present new paleomagnetic data for the Springwater pallasite, which constrains the timing of core solidification. Our results suggest that in order to generate the observed strong paleointensities (∼65–95 μT), the pallasites must have been relatively close to the dynamo source. Our thermal and dynamo models predict that the Main Group pallasites originate from a planetesimal with a large core (>200 km) and a thin mantle (<70 km).

New constraints on the formation of main group pallasites derived from in situ trace element analysis and 2D mapping of olivine and phosphate

Pallasites are stony‑iron meteorites consisting mainly of olivine and Fe-Ni metal with a formation history that remains widely debated. Despite their simple mineralogy, relatively limited data are available on their geochemistry and the lateral elemental distribution in individual pallasite olivine crystals. In this work, laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) was used for the elemental analysis of both pallasite olivine and phosphate phases, including 2D trace element mapping of olivine crystals. While the results obtained are in good agreement with literature values, important differences are observed for Al and Ni concentrations compared with bulk analytical methods. The element distribution maps reveal complex zoning in pallasite olivine, which can be explained based on i) diffusion gradients formed during olivine cooling, ii) the crystal chemistry of element substitution due to charge-balancing, and iii) inherited features of olivine before the metal-olivine mixing. Oscillatory zoning in an olivine crystal from Imilac provides strong evidence for the olivine not being a restite of partial melting, but rather having crystallized from a melt. Concentrations of Cr and Al are correlated both within single olivine crystals and between olivine crystals of different pallasites, forming a 1:1 linear trend. This likely results from a spinel-type charge-balancing substitution mechanism in pallasite olivine. Additionally, principle component analysis of laterally resolved multi-element concentration data of pallasite olivine provides an independent measure of the genetic relationships between the different main group pallasites within their parent body (−ies). Rare earth element (REE) contents of pallasite phosphate grains range from concentrations typically measured for chondritic or primitive achondrite-like phosphates to more (light) REE-depleted signatures, revealing the overall primitive nature of the melts from which the phosphate minerals crystallized. Phosphate in pallasites may thus have formed through the consumption of apatites during pro-grade metamorphism, melting, and melt extraction. Combined, the trace element signatures of olivine and phosphate in pallasites of the main group (PMG) suggest that these meteorites are common products of planetary processes on chondritic or primitive achondrite-like precursor bodies, with the different subgroups highlighting distinct formation and evolution histories.

Crystal chemistry of schreibersite, (Fe,Ni)3P

Abstract Schreibersite, (Fe,Ni)3P, the most abundant cosmic phosphide, is a principal carrier of phosphorus in the natural Fe-Ni-P system and a likely precursor for prebiotic organophosphorus compounds at the early stages of Earth’s evolution. The crystal structure of the mineral contains three metal sites allowing for unrestricted substitution of Fe for Ni. The distribution of these elements across the structure could serve as a tracer of crystallization conditions of schreibersite and its parent celestial bodies. However, discrimination between Fe (Z = 26) and Ni (Z = 28) based on the conventional X-ray structural analysis was for a long time hampered due to the proximity of their atomic scattering factors. We herein show that this problem has been overcome with the implementation of area detectors in the practice of X-ray diffraction. We report on previously unknown site-specific substitution trends in schreibersite structure. The composition of the studied mineral encompasses a Ni content ranging between 0.03 and 1.54 Ni atoms per formula unit (apfu): the entire Fe-dominant side of the join Fe3P-Ni3P. Of 23 schreibersite crystals studied, 22 comprise magmatic and non-magmatic iron meteorites and main group pallasites. The near end-member mineral (0.03 Ni apfu) comes from the pyrometamorphic rocks of the Hatrurim Basin, Negev desert, Israel. It was found that Fe/Ni substitution in schreibersite follows the same trends in all studied meteorites. The dependencies are nonlinear and can be described by second-order polynomials. However, the substitution over the M2 and M3 sites within the most common range of compositions (0.6 < Ni <1.5 apfu) is well approximated by a linear regression: Ni(M2) = 0.84 × Ni(M3) – 0.30 apfu (standard error 0.04 Ni apfu). The analysis of the obtained results shows a strong divergence between the variation of unit-cell parameters of natural schreibersite and those of synthetic (Fe,Ni)3P. This indicates that Fe/Ni substitution trends in the mineral and its synthetic surrogates are different. A plausible explanation might be related to the differences in the system equilibration time of meteoritic schreibersite (millions of years) and synthetic (Fe,Ni)3P (~100 days). However, regardless of the reason for the observed difference, synthetic (Fe,Ni)3P cannot be considered a structural analog of natural schreibersite, and this has to be taken into account when using synthetic (Fe,Ni)3P as an imitator of schreibersite in reconstructions of natural processes.

Crystal Chemistry of Stanfieldite, Ca7M2Mg9(PO4)12 (M = Ca, Mg, Fe2+), a Structural Base of Ca3Mg3(PO4)4 Phosphors

Stanfieldite, natural Ca-Mg-phosphate, is a typical constituent of phosphate-phosphide assemblages in pallasite and mesosiderite meteorites. The synthetic analogue of stanfieldite is used as a crystal matrix of luminophores and frequently encountered in phosphate bioceramics. However, the crystal structure of natural stanfieldite has never been reported in detail, and the data available so far relate to its synthetic counterpart. We herein provide the results of a study of stanfieldite from the Brahin meteorite (main group pallasite). The empirical formula of the mineral is Ca8.04Mg9.25Fe0.72Mn0.07P11.97O48. Its crystal structure has been solved and refined to R1 = 0.034. Stanfieldite from Brahin is monoclinic, C2/c, a 22.7973(4), b 9.9833(2), c 17.0522(3) Å, β 99.954(2)°, V 3822.5(1)Å3. The general formula of the mineral can be expressed as Ca7M2Mg7(PO4)12 (Z = 4), where the M = Ca, Mg, Fe2+. Stanfieldite from Brahin and a majority of other meteorites correspond to a composition with an intermediate Ca≈Mg occupancy of the M5A site, leading to the overall formula ~Ca7(CaMg)Mg9(PO4)12 ≡ Ca4Mg5(PO4)6. The mineral from the Lunar sample “rusty rock” 66095 approaches the M = Mg end member, Ca7Mg2Mg9(PO4)12. In lieu of any supporting analytical data, there is no evidence that the phosphor base with the formula Ca3Mg3(PO4)4 does exist.

Petrogenesis of main group pallasite meteorites based on relationships among texture, mineralogy, and geochemistry

AbstractMain group pallasite meteorites are samples of a single early magmatic planetesimal, dominated by metal and olivine but containing accessory chromite, sulfide, phosphide, phosphates, and rare phosphoran olivine. They represent mixtures of core and mantle materials, but the environment of formation is poorly understood, with a quiescent core–mantle boundary, violent core–mantle mixture, or surface mixture all recently suggested. Here, we review main group pallasite data sets and petrologic characteristics, and present new observations on the low‐MnO pallasite Brahin that contains abundant fragmental olivine, but also rounded and angular olivine and potential evidence of sulfide–phosphide liquid immiscibility. A reassessment of the literature shows that low‐MnO and high‐FeO subgroups preferentially host rounded olivine and low‐temperature P2O5‐rich phases such as the Mg‐phosphate farringtonite and phosphoran olivine. These phases form after metal and silicate reservoirs back‐react during decreasing temperature after initial separation, resulting in oxidation of phosphorus and chromium. Farringtonite and phosphoran olivine have not been found in the common subgroup PMG, which are mechanical mixtures of olivine, chromite with moderate Al2O3 contents, primitive solid metal, and evolved liquid metal. Lower concentrations of Mn in olivine of the low‐MnO PMG subgroup, and high concentrations of Mn in low‐Al2O3 chromites, trace the development and escape of sulfide‐rich melt in pallasites and the partially chalcophile behavior for Mn in this environment. Pallasites with rounded olivine indicate that the core–mantle boundary of their planetesimal may not be a simple interface but rather a volume in which interactions between metal, silicate, and other components occur.

Meteorite cloudy zone formation as a quantitative indicator of paleomagnetic field intensities and cooling rates on planetesimals

Metallic microstructures in slowly-cooled iron-rich meteorites reflect the thermal and magnetic histories of their parent planetesimals. Of particular interest is the cloudy zone, a nanoscale intergrowth of Ni-rich islands within a Ni-poor matrix that forms below ∼350°C by spinodal decomposition. The sizes of the islands have long been recognized as reflecting the low-temperature cooling rates of meteorite parent bodies. However, a model capable of providing quantitative cooling rate estimates from island sizes has been lacking. Moreover, these islands are also capable of preserving a record of the ambient magnetic field as they grew, but some of the key physical parameters required for recovering reliable paleointensity estimates from magnetic measurements of these islands have been poorly constrained. To address both of these issues, we present a numerical model of the structural and compositional evolution of the cloudy zone as a function of cooling rate and local composition. Our model produces island sizes that are consistent with present-day measured sizes. This model enables a substantial improvement in the calibration of paleointensity estimates and associated uncertainties. In particular, we can now accurately quantify the statistical uncertainty associated with the finite number of islands acquiring the magnetization and the uncertainty on their size at the time of the record. We use this new understanding to revisit paleointensities from previous pioneering paleomagnetic studies of cloudy zones. We show that these could have been overestimated by up to one order of magnitude but nevertheless still require substantial magnetic fields to have been present on their parent bodies. Our model also allows us to estimate absolute cooling rates for meteorites that cooled slower than <10,000°CMy−1. We demonstrate how these cooling rate estimates can uniquely constrain the low-temperature thermal history of meteorite parent bodies. Using the main-group pallasites as an example, we show that our results are consistent with the previously-proposed unperturbed, conductive cooling at low temperature of a ∼200-km radius main-group pallasite parent body.

Study of metallic Fe-Ni-Co alloy and stony part isolated from Seymchan meteorite using X-ray diffraction, magnetization measurement and Mössbauer spectroscopy

A detailed study of Fe-Ni-Co alloy and stony part extracted from Seymchan (main group pallasite) meteorite fragment using X-ray diffraction, magnetization measurements, and Mössbauer spectroscopy with a high velocity resolution was carried out. A complex phase composition in Fe-Ni-Co alloy was observed. The Mössbauer spectrum of this alloy was fitted using eight magnetic sextets which were related to α2-Fe(Ni, Co), α-Fe(Ni, Co) and γ-Fe(Ni, Co) phases. The stony part consists of the following iron-bearing phases: olivine, troilite, Ca-rich clinopyroxene, chromite and probably iron-magnesium chromite. Estimations of the Fe2+ partitioning between the M1 and M2 sites in olivine and Ca-rich clinopyroxene were based on X-ray diffraction and Mössbauer data. The distribution coefficients KD and the temperatures of Fe2+ and Mg2+ cations equilibrium distribution Teq in olivine were evaluated using X-ray diffraction data and Mössbauer parameters.

The oxygen isotope compositions of olivine in main group (MG) pallasites: New measurements by adopting an improved laser fluorination approach

AbstractOxygen isotope measurements of olivine in main group (MG) pallasites by traditional laser fluorination method are associated with some uncertainties including terrestrial weathering, incomplete olivine reaction, and sample state. We improved our laser fluorination approach by pretreating olivine grains with acid to remove terrestrial weathering products and by modifying the sample holder for an efficient and complete laser reaction. Our experiments on Brahin olivine demonstrate that acid‐washing successfully removes the terrestrial weathering with &lt;0.1‰ variation in δ18O value and, at the same time, improving the ∆17O value significantly. We also achieved a complete olivine fluorination by employing a custom‐designed sample holder with “V”‐shaped profile having rounded bottom because incomplete/partial reaction of olivine gives comparatively lighter δ18O values. Using these new techniques, we present precise triple oxygen isotope data (N = 72) of 25 olivine samples separated from main group pallasites. The data are, on average, ~0.5‰ heavier in δ18O relative to the values published in the literature for the same samples. Critically, the ∆17O values of MG pallasites and to some extent their Fo‐contents suggest that there are at least two populations of olivine. Based on our improved data set, we propose that MG pallasites potentially have high‐∆17O‐ and low‐∆17O‐bearing subgroups that are statistically distinct. The subgroups present average ∆17O values of −0.166 ± 0.003 (2SE; N = 16) and −0.220 ± 0.003 (2SE; N = 9), respectively. Furthermore, the high‐∆17O‐bearing subgroup samples trend toward lower Fo‐contents compared to the other subgroup. Taken together, our data provide evidence that argues against a single parent body origin for MG pallasites.

Multiple sulfur isotopic composition of main group pallasites support genetic links to IIIAB iron meteorites

This study reports the quadruple sulfur isotope composition of troilite nodules from Main Group Pallasites. Values range from −0.23‰ to 0.34‰ (average = 0.03 ± 0.17‰ S.D.) in δ34S and 0.008‰ to 0.025‰ (average = 0.018 ± 0.006‰ S.D.) in Δ33S and −0.38 to −0.01 (average = −0.17 ± 0.11‰ S.D.) in Δ36S. The variance of these analyses is comparable to estimates of analytical uncertainty (±0.3‰, ±0.008‰, and ±0.3‰, for δ34S, Δ33S, and Δ36S, respectively) and the average of these values is taken as a constraint on the composition of sulfur in the MG Pallasite parent body. The different Δ33S value compared to CDT and IAB iron meteorites at a similar δ34S value is interpreted as a mass-independent signature. This signature is similar in magnitude and direction to previously published values observed in IIIAB iron meteorites, further supporting a genetic relationship between the two groups of meteorites.

Chemical and mineralogical characterization of the Mineo (Sicily, Italy) pallasite: A unique sample

AbstractThe Mineo pallasite is characterized here for the first time. The only 42 g still available worldwide is part of the collection of the Department of Physics and Geology, University of Perugia. A multianalytical approach was used, joining field‐emission scanning electron microscopy, Raman analysis, X‐ray powder diffraction, electron‐probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry. Results highlighted that (1) the Mineo pallasite belongs to the Main Group pallasites; (2) the silicate component is essentially olivine, with no pyroxene component; (3) the olivine chemical composition varies in terms of both iron and trace elements; (4) the metal phase is essentially kamacite with the taenite mainly found in the plessite structure; (5) phosphide phases are present as schreibersite and barringerite. The observed compositional variability in olivines as well as their occurrence as both angular and rounded crystals suggest that the Mineo pallasite could have been derived from a large impact of a differentiated parent body with a larger solid body. The resulting pallasite conglomerate consists of the compositionally different olivines, likely coming from different areas of the same differentiated parent body, and the residual molten Fe‐Ni.