Eagle Station 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.

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.

Karavannoe: Mineralogy, trace element geochemistry, and origin of Eagle Station group pallasites

ABSTRACTKaravannoe is a pallasite found in Russia in 2010. The mineralogy, chemistry, and oxygen isotopic composition indicate that Karavannoe is a member of the Eagle Station Pallasite (ESP) group. Karavannoe contains mostly olivine and subdued interstitial Fe,Ni‐metal. Zoned distribution of FeO in small, rounded grains of olivine and FeO and Al2O3 in chromite shows that the cooling rate of the melt was fast during the crystallization of the round olivine grains. Siderophile element distribution and correlations of Au‐As and Os‐Ir concentrations in Karavannoe and the other ESP metal record its magmatic origin. FeO‐rich composition of olivine, low W and Ga, and high Ni abundances in the Karavannoe metal indicate the formation of the metal from an oxidized chondrite precursor. Model calculations demonstrate that the ESPs’ metal compositions correspond to the solids of the fractional crystallization of CV‐ or CO‐chondrite‐derived metallic liquids. The Karavannoe metal composition corresponds to the solid fraction crystallized after ~40% fractional crystallization. The Mg/(Mg+Fe) atom ratio of complementary silicate liquid corresponds to Fo70, possibly indicating that the olivine is not in equilibrium with the metal and could have been a product of the late evolutionary processes in the Karavannoe parent body mantle. In any ESP genesis Karavannoe was not in equilibrium with its metal and is a product of mantle differentiation processes. Olivine of Karavannoe and ESPs is similar in composition, while the metal is different. We propose a model of ESP formation involving an impact‐induced intrusion of liquid core metal into a basal mantle layer, followed by fractional crystallization of the metal. The metal textures and chemical zoning of Karavannoe minerals point to remelting and rapid cooling due to a later impact event.

Evaluating the O‐Cr‐Mo isotope signatures in various meteorites representing core–mantle–crust fragments: Implications for partially differentiated planetesimal(s) accreted in the early outer solar system

AbstractThe Allende, Eagle Station pallasites (ESP), some ungrouped achondrites (UA), and ungrouped irons (UI) represent different types of meteorites; these are traditionally distinguished as chondrule‐bearing chondrite, chondrule‐free stony irons, stones, and irons, respectively. Oxygen isotopic compositions of meteorites have long been used as a robust tool to identify the parent bodies of these extraterrestrial materials. We revisited the δ18O‐∆17O, along with ε54Cr and ε92Mo, of these meteorites and established a genetic link that possibly suggests that undifferentiated carbonaceous chondrites and their differentiated counterparts belong to a common reservoir. We observed that the differentiated counterparts of Allende CV3 including ESP, ungrouped achondrites, and ungrouped irons collectively provide the oxygen isotope trends comparable to those of the Y&R and the PCM lines, and are likely to represent the primitive isotope reservoirs in the solar nebula. ε54Cr and ε92Mo data of these meteorite types support their oxygen isotope trends. The idea of a common partially differentiated parent body also lends support from natural remnant magnetization in the CV chondrites and Eagle Station olivine. Furthermore, ∆17O‐ε54Cr data suggest that these meteorites originated from carbonaceous planetesimal(s) accreted at different heliocentric distances in the solar nebula in the outer solar system. As proposed earlier, these bodies remained separated from the inner solar system due to formation of Jupiter. Taken together, we propose that the ungrouped irons, ESP, and ungrouped achondrites could possibly represent the differentiated sections, such as core, core–mantle, and mantle of a planetesimal(s), respectively, with the Allende CV3 representing an undifferentiated chondritic crust.

Crystallization histories of the group IIF iron meteorites and Eagle Station pallasites

AbstractThe group IIF iron meteorites and Eagle Station pallasites (PES) have highly siderophile element abundances (HSE; Re, Os, Ir, Ru, Pt, and Pd) of metal that are consistent with formation in planetesimal cores by fractional crystallization with minor to major solid metal–liquid metal mixing. Modeling of HSE abundances of the IIF irons indicates a complex formation history that included the mixing of primitive and evolved solid and liquid metals. By contrast, modeling of HSE abundances of PES metal suggests these meteorites formed mainly as equilibrium solids from a common liquid. Abundances of some of the siderophile elements in the IIF irons and PES are permissive of a common core origin; however, the abundances of W and Ni indicate the PES ultimately formed on a more oxidized body. The PES most likely formed by the injection of olivine present at the core–mantle boundary into a metallic core liquid as a result of impact. The core then crystallized inward, trapping the olivine.

Study of the Pallasite Radiation History by Track Analysis

This work was performed within the OLYMPIA experiment on the study of tracks of heavy and superheavy cosmic ray nuclei in olivine crystals from Marjalahti and Eagle Station pallasites. Depth distributions of the track formation rate for heavy cosmic ray nuclei in olivine crystals from pallasites of different pre-atmospheric sizes were obtained. The dependences obtained were used to analyze the data on the track density in olivine crystals from the Marjalahti pallasite. In three crystals, the track distribution with a high density gradient was detected, which indicates a complicated radiation history of the meteorite.

Carbonaceous and noncarbonaceous iron meteorites: Differences in chemical, physical, and collective properties

AbstractIron‐meteorite groups that appear from published isotopic data to have been derived from melted carbonaceous‐chondrite‐like precursors (CC irons) (IIC, IID, IIF, IIIF, IVB) tend to have higher median refractory siderophile element (RSE) contents, higher median Ni contents, and higher median Ir/Ni and Ir/Au ratios than magmatic noncarbonaceous (NC) iron‐meteorite groups (IC, IIAB, IIIAB, IIIE, IVA). (Group IIG is also NC.) One potential source of RSEs in magmatic CC irons is the set of refractory metal nuggets from inherited CAIs. Magmatic CC‐iron groups tend to have longer cosmic‐ray exposure (CRE) ages than magmatic NC‐iron groups, indicating long residence times as small bodies in interplanetary space. The lower membership of CC‐iron groups is probably mainly due to the high oxidation state of their precursors. Such oxidation would have produced lesser amounts of free metal; parent body differentiation of such bodies would have produced smaller cores, resulting in fewer samples available to make CC‐iron meteorites in the first place. (Ungrouped magmatic irons, most of which can be considered groups with only one member, also tend to be carbonaceous.) It is possible that a subset of the chondrule‐poor dark inclusions in many carbonaceous chondrites represent unmelted materials related to the precursors of the CC irons. The Eagle Station pallasites (also CC‐related) are analogous to CC irons in being more oxidized, richer in Ni and RSEs, and fewer in number than main‐group pallasites (PMG). However, Eagle Station has a shorter CRE age than most PMG.

Accretion timescales and style of asteroidal differentiation in an 26Al-poor protoplanetary disk

The decay of radioactive 26Al to 26Mg (half-life of 730,000years) is postulated to have been the main energy source promoting asteroidal melting and differentiation in the nascent solar system. High-resolution chronological information provided by the 26Al–26Mg decay system is, therefore, intrinsically linked to the thermal evolution of early-formed planetesimals. In this paper, we explore the timing and style of asteroidal differentiation by combining high-precision Mg isotope measurements of meteorites with thermal evolution models for planetesimals. In detail, we report Mg isotope data for a suite of olivine-rich [Al/Mg∼0] achondritic meteorites, as well as a few chondrites. Main Group, pyroxene and the Zinder pallasites as well as the lodranite all record deficits in the mass-independent component of μ26Mg (μ26Mg∗) relative to chondrites and Earth. This isotope signal is expected for the retarded ingrowth of radiogenic 26Mg∗ in olivine-rich residues produced through partial silicate melting during 26Al decay and consistent with their marginally heavy Mg isotope composition relative to ordinary chondrites, which may reflect the early extraction of isotopically light partial melts from the source rock. We propose that their parent planetesimals started forming within ∼250,000years of solar system formation from a hot (>∼500K) inner protoplanetary disk region characterized by a reduced initial (26Al/27Al)0 abundance (∼1–2×10−5) relative to the (26Al/27Al)0 value in CAIs of 5.25×10−5. This effectively reduced the total heat production and allowed for the preservation of solid residues produced through progressive silicate melting with depth within the planetesimals. These ‘non-carbonaceous’ planetesimals acquired their mass throughout an extended period (>3Myr) of continuous accretion, thereby generating onion-shell structures of incompletely differentiated zones, consisting of olivine-rich residues, overlaid by metachondrites and undifferentiated chondritic crusts. In contrast, individual olivine crystals from Eagle Station pallasites record variable μ26Mg∗ excesses, suggesting that these crystals captured the 26Mg∗ evolution of a magmatic reservoir controlled by fractional crystallization processes during the lifespan of 26Al. Similar to previous suggestions based on isotopic evidence, we suggest that Eagle Station pallasites formed from precursor material similar in composition to carbonaceous chondrites from a cool outer protoplanetary disk region characterized by (26Al/27Al)0⩾2.7×10−5. Protracted planetesimal accretion timescales at large orbital distances, with onset of accretion 0.3–1Myr post-CAIs, may have resulted in significant radiative heat loss and thus efficient early interior cooling of slowly accreting ‘carbonaceous’ planetesimals.

Timing of metal–silicate differentiation in the Eagle Station pallasite parent body

The time of the metal–silicate differentiation of the Eagle Station pallasite (ESP) parent body was investigated using the 26Al–26Mg short-lived chronometer (half-life of 0.72Myr). The Mg isotope ratios were measured in ESP olivines by both MC–SIMS and HR-MC–ICPMS, allowing us to check the consistency between the results given by two different analytical protocols and data reduction processes. Results show that the two datasets are consistent, with a (δ26Mg*)av. value of –0.003 (±0.005)‰ (2 s.e., n=89). Such a value, associated with data from the 182Hf–182W short-lived systematics (half-life of 8.9Myr), indicates an ESP parent body metal–silicate differentiation occurring most likely at least at ∼2Ma, but possibly 4Ma, after CAI formation. From the 27Al/24Mg ratios measured in ESP olivines using MC–SIMS, the duration of the olivine crystallization process was inferred to have lasted over ∼275kyr if the core has differentiated as early as 2Ma after CAIs, while in the case of a core differentiation occurring 4Ma after CAIs, the silicate–silicate differentiation should have lasted for another 4Myr.

Magnesium isotopes constraints on the origin of Mg-rich olivines from the Allende chondrite: Nebular versus planetary?

High precision Mg isotope measurements by multi-collector ion microprobe show that refractory olivines from the Allende chondrite, either olivines isolated in the matrix (2 samples studied) or olivines in type I chondrules (6 samples studied), have variable δ26Mg* enrichments and deficits (calculated in permil as the 26Mg deviation from the instrumental mass fractionation line) relative to the Earth. Most average δ26Mg* (noted δ26Mg*av) values (between 10 and 20 analyses per chondrule) are negative but the total range is from −0.029 (±0.010) ‰ (2 sigma errors) to +0.011 (±0.011) ‰ with an exception of one olivine at +0.043 (±0.023) ‰. These variations in δ26Mg*av reflect the formation of the olivines from reservoirs enriched in various amounts of 26Mg by the decay of short-lived 26Al (T1/2=0.73Ma). Similarly, 30 analyses of olivines from the Eagle Station pallasite show a δ26Mg*av value of −0.033±0.008‰, as negative as some olivines from Allende chondrules and the Solar system initial δ26Mg* value of −0.038±0.004‰ (defined at the time of formation of type B Ca–Al-rich inclusions – CAIs – when 26Al/27Al=5.23×10−5, Jacobsen et al., 2008).Because olivines are Al-poor and because their Mg isotopic compositions are not reset during the chondrule forming events, their δ26Mg*av can be used to calculate model crystallization ages relative to various theoretical Mg isotope growth curves. The two end-member scenarios considered are (i) a “nebular” growth in which the Al/Mg ratio remains chondritic and (ii) a “planetary” growth in which a significant increase of the Al/Mg ratio can be due to, for instance, olivine magmatic fractionation. The low δ26Mg*av value of olivines from the Eagle Station pallasite demonstrate that metal-silicate differentiation occurred as early as ~0.15-0.23+0.29 Ma after CAIs in either of the growth scenarios. Similarly the variable δ26Mg*av values of refractory olivines can be understood if they were formed in planetesimals which started to differentiate as early as the Eagle Station parent body. Accretion of these planetesimals must have been coeval to the formation of CAIs and their disruption could explain why their fragments (Mg-rich olivines) were distributed in the chondrule forming regions of the disk.

Charge spectrum of galactic cosmic ray nuclei as measured in meteorite olivines

This paper presents experimental results on galactic cosmic ray nuclei in olivine crystals from the Marjalahti and Eagle Station pallasites. The charge spectrum of the nuclei is measured to be in good agreement with the experimental data from the HEAO-3 and ARIEL-6satellite missions.

Trace element chemistry of Cumulus Ridge 04071 pallasite with implications for main group pallasites

Abstract— Pallasites have long been thought to represent samples from the metallic core‐silicate mantle boundary of a small asteroid‐sized body, with as many as ten different parent bodies recognized recently. This report focuses on the description, classification, and petrogenetic history of pallasite Cumulus Ridge (CMS) 04071 using electron microscopy and laser ablation ICP‐MS. Most olivines are angular in CMS 04071, but there are some minor occurrences of small rounded olivines, such as in the Eagle Station pallasite. Olivine, chromite, and metal compositions indicate that CMS 04071 can be classified as a Main Group pallasite. The kamacite/taenite partition coefficients (D) for highly siderophile elements (HSE) are all close to 1, but comparison with previous studies on iron meteorites and pallasites shows that variation of some D values is controlled by the Ni content of taenite. D(HSE)metal/sulfide for Re, Cu, and Cr all are < 1, indicating chalcophile behavior for these three elements, in agreement with experimental Dmetal/sulfide. D(HSE)metal/olivine are variable, which is perhaps due to small metallic inclusions in the olivine that are present to variable extents in different pallasites. All of these data, together with results from previous studies, indicate that the CMS pallasites were likely formed at the core‐mantle boundary of a small asteroid, but not necessarily related to the core that produced the IIIAB irons. In addition, they share a similar volatile element depletion to HEDs that is distinct from other bodies such as Earth, Mars, Angrite Parent Body, and the parent body of the brachinites.

Main-group pallasites

Abstract We used neutron activation to characterize the metal of 33 main-group pallasites (PMG), widely held to be samples of a core–mantle interface. Most PMG cluster in a narrow range of metal and silicate compositions, but 6 are assigned to an anomalous subset (PMG-am) because of their deviant metal compositions, and 4 others to another anomalous subset (PMG-as) because of their appreciably higher olivine Fa contents. Metal compositions in all PMG are closely related to those in evolved IIIAB irons, and are generally consistent with their formation in the IIIAB parent asteroid. On element-Au diagrams for incompatible elements the normal PMG plot near an extrapolation of IIIAB trends to higher Au concentrations. On element-Au plots of compatible elements such as Ir or Pt the loci of PMG spread out over a broader region explainable by mixing of evolved IIIAB magma with early-crystallized core or mantle-residue solids. Two features of PMG require special models: (1) Ga and Ge contents are generally high (≈1.5×) compared to the IIIAB-based mixing model: and (2) the FeO/(FeO + MgO) ratios span a surprisingly wide range, from 0.11–0.14 in normal PMG to 0.16 to 0.18 in PMG-as This range is larger than expected in a cumulate layer at the base of a mantle. We suggest that both features may be related to the interaction of PMG precursors with a highly evolved magmatic gas phase, and that some or all of these anomalies may have resulted from vapor deposits in voids near the core–mantle interface. An important boundary condition for understanding the detailed PMG origin at the core–mantle interface is the large difference between the solidus temperature of Fa11 olivine (≈2000 K) and the liquidus temperature of an evolved IIIAB melt containing >100 mg/g S and some P (≈1600 K). Following the mixing event that formed the PMG it is therefore reasonable that there would have been olivine rubble floating on top of the IIIAB-like magma, but with appreciable void space present just above the upper level reached by the magma. These voids would have contained gases released from the magma during its flow into the PMG region. We suggest that Ga and Ge, the two most volatile siderophiles in our element set, were added to PMG metal from the magmatic gas. We also suggest that the magmatic gas was oxidizing and that the PMG having high olivine fayalite contents formed in regions where the ratio of void to olivine was high, and that some metallic Fe was oxidized and entered the olivine (or the phosphoran olivine). In support of the latter idea is the observation that both Ni and Co are elevated in the PMG-as (Fa≥16) compared to values predicted by IIIAB trends. We analyzed two Eagle-Station pallasites (PES); after correction for weathering effects in Cold Bay, its composition is found to closely resemble that of Eagle Station but to represent a more evolved composition (i.e., lower Ir, higher Au). Vermillion and Yamato 8451 have been called pyroxene pallasites but have metal compositions (unrelated to those of the PMG or PES) that are too different from each other to even allow assignment to the same grouplet.

On Search and Identification of Fossil Tracks dne to Superheavy Cosmic Ray Nuclei (Z ≥ 110) in Meteoritic Crystals

A new method for investigation of the ultra-heavy component of Galactic cosmic-ray nuclei (Z = 50 – 92) has been developed since 1980. It depends on the ability of extraterrestrial silicate crystals (olivine) to register and store during many million years the tracks due to cosmic-ray nuclei with Z ≥ 22. Our approach bases on the partial annealing of both “fossil” tracks and the tracks due to accelerated Kr, Xe, Au, Pb and U ions, and on the chemical etching of total volume track length of the cosmic-ray nuclei in the olivine crystals. The crystals taken from Marjalahti and Eagle Station pallasites (the radiation ages are 180 and 45 million years respectively) were annealed at 703 K during 32 h and etched. The volume tracks due to Z ≥ 50 cosmic-ray nuclei were then measured. The intercomparison of track length spectra of the “fossil” tracks and the tracks due to the accelerated 238U and 208Pb nuclei proves unambiguously that the last two abundant peaks of “fossil” track length spectra (120 – 140 μm and 180 μm, first observed in 1980 in Dubna) are products of the recent nucleosynthesis in our Galaxy due to Pt-Pb and Th-U groups of cosmic-ray nuclei. During 1980 – 1996 more than 1600 Th-U “fossil” tracks were measured and 11 anomalously long tracks (L = 340 – 370 μm) were found. Concerning the origin of the anomalously long tracks we have determined the orientation of these tracks in olivine crystal with the Laue-Roentgen method. We estabilished that five anomalously long “fossil” tracks have an angle ≥ 15° to the main crystal plane (010) and subsequently can not be attributed to any actinide nuclei in meteoritic olivine crystals. Moreover, in the fossil track study of olivines annealed at 703 K during 32 h we found the 250 μm – long track which could not be attributed to Th-U nuclei at any orientation in the crystal lattice. Now we can set an upper limit of Z ≥ 110 (superheavy) nuclei abundance at the level NZ μ 110 / NTh, U ≤ (1÷3) · 10−3. Our goal is to obtain the final, necessary and sufficient proofs of the existence of SHE nuclei in Galactic matter.

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 (<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.

Constraints on the role of impact heating and melting in asteroids

Abstract— Imaging of asteroids Gaspra and Ida and laboratory studies of asteroidal meteorites show that impacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks. However, in recent years, impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids, such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events. The latter were proposed to explain the origin of ureilites, aubrites, mesosiderites, the Eagle Station pallasites, acapulcoites, lodranites, and the IAB, IIICD, and HE irons. We considered fundamental observations from terrestrial impact craters, combined with results from laboratory shock experiments and theoretical considerations, to evaluate the efficiency of impact heating and melting of asteroids.Studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of asteroids will produce two types of impact melts: (1) large‐scale whole rock melts (total melts, not partial melts) at high shock pressure and (2) localized melts formed at the scale of the mineral constituents (mineral specific or grain boundary melting) at intermediate shock pressures. The localized melts form minuscule amounts of melt that quench and solidify in situ, thus preventing them from pooling into larger melt bodies. Partial melting as defined in petrology has not been observed in natural and experimental shock metamorphism and is thermodynamically impossible in a shock wave‐induced transient compression of rocks. The total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater. Internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids, and impact melt rocks are usually clast‐laden. Thermal metamorphism of country rocks by impact is extremely minor. Experimental and theoretical considerations suggest that (1) single disruptive impacts cannot raise the average global temperature of strength‐ or gravity‐dominated asteroids by more than a few degrees; (2) cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter; (3) small crater size, low gravity, and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small (0.01–0.1%) fraction of the total displaced crater volume; (4) total impact melt volume formed during the typical lifetime of an asteroid is a small fraction (<0.001) of the volume of impact‐generated debris; and (5) much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and, thus, not retained on the asteroid.The inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above, and we must turn to processes other than impact, such as decay of short‐lived radionuclides or electromagnetic induction during an early T‐tauri phase of the Sun to explain heating and melting of the parent bodies of these meteorites.

Mbosi: An anomalous iron with unique silicate inclusions

Abstract— The Mbosi iron meteorite contains millimeter size silicate inclusions. Mbosi is an ungrouped iron meteorite with a Ge/Ga ratio >10, which is an anomalous property shared with the five‐member IIF iron group, the Eagle Station pallasites and four other ungrouped irons. Neither the IIF group nor the four other ungrouped irons are known to have silicate inclusions. Chips from three Mbosi inclusions were studied, but most of the work concentrated on a whole 3.1 mm circular inclusion. This inclusion consists of a mantle and a central core of different mineralogies. The mantle is partially devitrified quartz‐normative glass, consisting of microscopic crystallites of two pyroxenes and plagioclase, which are crystalline enough to give an x‐ray powder diffraction pattern but not coarse enough to permit analyses of individual minerals. The core consists of silica. The bulk composition does not match any known meteorite type, although there is a similarity in mode of occurrence to quartz‐normative silicate inclusions in some HE irons. Mbosi silicate appears to be unique. The bulk rare earth element (REE) pattern of the mantle is flat at ≅ 7×C1; the core is depleted in REE but shows a small positive Eu anomaly. The O‐isotope composition of bulk silicate lies on a unit slope mixing line (parallel and close to the C3 mixing line) that includes the Eagle Station pallasites and the iron Bocaiuva (related to the IIF irons); all of these share the property of having Ge/Ga ratios >10. It is concluded that Mbosi silicate represents a silica‐bearing source rock that was melted and injected into metal. Melting occurred early in the history of the parent body because the metal now shows a normal Widmanstätten structure with only minor distortion that was caused when the parent body broke up and released meteorites into interplanetary space. The cause of Ge/Ga ratios being >10 in these irons is unknown. The fact that silicates in Mbosi, Bocaiuva (related to IIF irons) and the Eagle Station trio of pallasites, all characterized by a Ge/Ga ratio >10, lie on a unit slope mixing line in the O‐isotope diagram suggests that their origins are closely related. The C3 chondrites appear to be likely precursors for silicates in Mbosi, Bocaiuva and the Eagle Station pallasites.

Electrochemical measurements and thermodynamic calculations of redox equilibria in pallasite meteorites: Implications for the eucrite parent body

The intrinsic oxygen fugacity (IOF) of olivine separates from the Salta, Springwater, and Eagle Station pallasites was measured between 850 and 1150°C using oxygen-specific solid zirconia electrolytes at 10 5 Pa. The experimental apparatus consisted of a double-opposed electrolyte configuration with a CO-CO 2 gas-mix bridging atmosphere. Four initial IOF measurements on Salta olivines revealed the effects of grain size and cell memory on experimental results; experiments with an IW cell memory and fine grain size (<45 μm) gave results most consistent with thermodynamic calculations. The IOF experimental results for olivine separates from Salta, Springwater, and Eagle Station fall within one log unit of the iron-wüstite oxygen buffer (IW). While these measurements may provide only an upper limit to the IOF of the pallasites, they correlate with the compositions of the olivine and metal in these samples: Salta, the most reduced pallasite studied (IW − 0.5 log fO 2), contains Fa 12.5 and Fe 90.5Ni 9.0; Springwater, of intermediate redox state (~IW), contains Fa 18.0 and Fe 87.5Ni 12.1; Eagle Station, the most oxidized sample ( IW + 0.5 logfO 2), contains Fa 20.5 and Fe 85.0Ni 14.6. Electron microbeam characterization of the starting materials and run products from these experiments have shown that olivine is the only phase present. Thermodynamic calculations of redox equilibria involving equilibrium pallasite assemblages are in good agreement with our experimental results and provide a lower limit to pallasite redox stability; others involving disequilibrium assemblages, suggest that pallasites experienced localized, late-stage oxidation and reduction effects. Consideration of the redox buffer metal-olivine-orthopyroxene utilizing calculated Eucrite Parent Body (EPB) mantle phase compositions indicates that small redox gradients may have existed in the EPB. Such gradients may have produced strong compositional variation within the EPB. In addition, there is apparently significant redox heterogeneity in the source area of Eagle Station Trio (EST) pallasites and Bocaiuva iron meteorites.

The meteoritic chromium isotopic composition and limits for radioactive 53Mn in the early solar system

We have studied the Cr isotopic composition of early solar system samples to search for anomalies caused by either the decay of extinct radio-nuclide 53Mn or the nucleosynthetic heterogeneities in Cr itself. The samples are spinels from five Allende inclusions, one “chromite carbon” Allende residue, and olivines from both Eagle Station pallasite and Angra dos Reis pyroxenite. We have also analyzed silicates from five Allende inclusions which are known to have a variety of anomalies. Our detection limits are 0.6%. and 1.4%. for fractionation corrected 53Cr 52Cr and 54Cr 52Cr , respectively. No isotopic effects beyond these limits were found among the samples studied. The limits for 53Mn 55Mn for Allende Cr-rich phases, Eagle Station, and Angra dos Reis are inferred to be 85, 11, and 2.8 × 10 −6 at their respective formation times. The lack of detectable 53Mn effects could be due to late formation of the high Mn Cr phases and heterogeneity in the 53Mn distribution for the low Mn Cr samples. It may also be caused by unusually large dilution factor for Mn. The lack of 53Cr effect does not support models in which the pre-solar source for the 26Al was a massive star. Neither does it support a proton irradiation production for 26Al. Our relatively poor detection limit on 54Cr does not permit verification of small (≲.8%.) effects reported recently by Birck and Allègre (1984).

I-Xe and 40Ar- 39Ar analyses of silicate from the Eagle Station pallasite and the anomalous iron meteorite Enon

Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and 40Ar- 39Ar techniques, Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic 40Ar at low temperature, and gives a plateau age of 4.59 ± 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred Pu U ratio is more than 2σ above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 6 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The Eagle Station pallasite, the most 16O-rich meteorite known, gives a complex 40Ar- 39Ar age pattern which suggests a recent (≲0.85 Ga) severe thermal disturbance. The absence of excess 129Xe, and the low trapped Ar and Xe contents, are consistent with this interpretation. The similarity between 40Ar- 39Ar data for Eagle Station and for the olivine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pallasites, in the sense that similar processes operating at similar times on different parent bodies may have been involved in the formation of olivine in both types of meteorites.