¹Cody Schultz,^2,3Brendan A. Anzures,¹Ralph E. Milliken,¹Taki Hiroi,¹Kevin Robertson
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13946]
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, Rhode Island, 02912 USA
²Lunar and Planetary Institute, USRA, Houston, Texas, 77058 USA
³NASA Johnson Space Center, ARES, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

H2O and OH are readily detected in hydrated minerals in CM chondrites via reflectance spectroscopy due to their characteristic vibration absorptions at infrared wavelengths. Previous spectroscopic work on bulk powdered CM chondrites has shown that spectral parameters, like the wavelength position of the “3 μm absorption feature,” vary systematically with the extent to which the samples have been aqueously altered. However, it is yet unclear how these spectral features may vary across an intact meteorite chip when measured at spatial scales smaller than that of the individual components of the meteorite. Here, we explore the spatial variability of this spectral feature and others on intact CM2 chips which, unlike powders, retain their petrologic and textural characteristics. We also model the modal mineralogy of the bulk meteorite powders and correlate this with key spectral features, demonstrating that microscope Fourier transform infrared spectroscopic mapping provides a powerful, rapid, and non-destructive technique for assessing compositional diversity and variations in water–rock interactions in chondritic planetary materials. In all CM2 chondrites studied here, we find that variations in the position, shape, and strength of the 3 μm absorption feature reveal a single chondrite can exhibit as much spectral variation as the entire suite of CM2 chondrites. The observed variations in the position and shape of the 3 μm feature within individual CM2 chondrite chips suggest a range of alteration products (e.g., Mg-rich to Fe-rich phyllosilicates) are present and record sub-mm scale variations in the amount and/or chemistry of the altering fluids. The samples having experienced the most progressive aqueous alteration show the least amount of variability in features like the 3 μm absorption band minimum position, whereas the least altered samples exhibit the most variability. We also find that the bulk spectral signatures in the least altered samples appear to be biased toward the spectral signatures of clasts versus matrix. By extension, asteroid reflectance spectra exhibiting 3 μm absorption features consistent with those measured here may be interpreted in a similar framework in which the spectrum of what may appear to be the least altered asteroids represents an average that belies the true diversity of mineralogy and chemistry of the body.

¹A. P. Jones,¹N. Ysard
Astronomy & Astrophysics 657, A128 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202141793]
¹Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
Reproduced with permission (C)ESO

Context. Nano-diamonds are an enticing and enigmatic dust component yet their origin is still unclear. They have been unequivocally detected in only a few astronomical objects, yet they are the most abundant of the pre-solar grains, both in terms of mass and number.

Aims. Our goal is to derive a viable set of nano-diamond optical constants and optical properties to enable their modelling in any type of astrophysical object where, primarily, the local (inter)stellar radiation field is well-determined.

Methods. The complex indices of refraction, m(n, k), of nano-diamonds, constrained by available laboratory measurements, were calculated as a function of size, surface hydrogenation, and internal (dis)order, using the THEMIS a-C(:H) methodology optEC(s)(a).

Results. To demonstrate the utility of the optical properties (the efficiency factors Qext, Qsca, and Qabs), calculated using the derived m(n, k) data, we show that nano-diamonds could be abundant in the interstellar medium (ISM) and yet remain undetectable there.

Conclusions. The derived optical constants provide a means to explore the existence and viability of nano-diamonds in a wide range of astronomical sources. Here we show that up to a few percent of the available carbon budget could be hidden in the form of nano-diamonds in the diffuse ISM, in abundances comparable to the pre-solar nano-diamond abundances in primitive meteorites.

¹J. Bourdelle de Micas,^1,2S. Fornasier,³C. Avdellidou,³M. Delbo,⁴G. van Belle,^5,6P. Ochner,⁴W. Grundy,⁴N. Moskovitz
Astronomy & Astrophysics 665, A83 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202244099]
¹LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
²Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France
³Université Côte d’Azur, CNRS-Lagrange, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
⁴Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
⁵INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶Dipartimento di Fisica e Astronomia G. Galilei, Universitá di Padova, Vicolo dell’ Osservatorio 3, 35122 Padova, Italy
Reproduced with permission (C)ESO

Aims. We carried out a spectroscopic survey in order to investigate the composition of 64 asteroids of the inner main belt, which are leftovers of the original planetesimals of our Solar System (we call them inner main belt planetesimals or IMBPs). Following published methods, we identified IMBPs in the inverse size versus semimajor axis (α) space, after the removal of all asteroids belonging to collisional families.

Methods. We conducted several ground-based observational campaigns of these IMBPs in the visible range at the 1.82 m Asiago telescope, and in the near-infrared range at the Telescopio Nationale Galileo, the Lowell Discovery Telescope, and the NASA InfraRed Telescope Facility telescopes. As several of the identified planetesimals already have spectra published in the literature, we collected all the available data and focused the telescope time to investigate those never observed before, or to complete the 0.45–2.5 μm range spectrum for those for which there is only partial spectral coverage or data with poor signal-to-noise ratio. In this way, we obtained new spectra for 24 IMBPs. Combining new and literature observations, we present spectra for 60 IMBPs in both the visible and near-infrared range, and 4 IMBPs in the visible only. All spectra were classified following well-established taxonomies. We also characterized their spectral absorption bands – when present –, their spectral slopes, and their mineralogy. In addition, we performed curve matching between astronomical and laboratory spectra in order to identify the closest meteorite analog using the RELAB database.

Results. The majority of the IMBPs belong to the S-complex; the latter are best matched with ordinary chondrite meteorites, and their olivine/(olivine and pyroxene) abundance ratio is not correlated with the semi-major axis. This result does not support the hypothesis that this ratio increases with heliocentric distance. Furthermore, ~27% of the IMBPs belong to the C-complex, where Ch/Cgh types dominate, meaning that most of the carbonaceous-rich planetesimals were aqueously altered. These are best fitted by CM2 carbonaceous chondrite meteorites. Finally, the remaining IMBPs (~20%) belong to the X-complex, and have various mineralogies and meteorite matches, while a few are end-member classes, including L-, K-, V-, and D- or T-types.

Conclusions. Our spectroscopic investigation of IMBPs confirms that silicate-rich bodies dominated the inner main belt where temperature has permitted the condensation of silicate rocks. However, almost all the spectral types are found, with the notable exception of olivine-rich A-types and Q-type asteroids. Their absence, as well as the absence of the R- and O-types among planetesimals, might be due to the rarity of these types among large asteroids. However, the absence of Q-types among primordial planetesimals is expected, as they have undergone surface rejuvenating processes. Therefore, Q-types have relatively young and less weathered surfaces compared to other types. Our results support the hypothesis of compositional mixing in the early Solar System. In particular, the fact that most of the C-complex planetesimals are aqueous altered, and the presence of three D- or T-type asteroids among them indicate that these bodies migrated from beyond 3 au to their current position.

^1,2J. Mah,³R. Brasser,^4,5J. M. Y. Woo,^3,7,8A. Bouvier,¹S. J. Mojzsis
Astronomy & Astrophysics 660, A36 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202142926]
¹Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²Earth Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan
³Origins Research Institute, Research Centre for Astronomy and Earth Sciences, 15–17 Konkoly Miklós Thege utca, 1121 Budapest, Hungary
⁴Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
⁵Laboratoire Lagrange, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France
⁶Bayerisches Geoinstitut, Universität Bayreuth, 95447 Bayreuth, Germany
⁷Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria
⁸Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309-0399, USA
Reproduced with permisson (C)ESO

Not only do the sampled terrestrial worlds (Earth, Mars, and asteroid 4 Vesta) differ in their mass-independent (nucleosynthetic) isotopic compositions of many elements (e.g. ε48Ca, ε50Ti, ε54Cr, ε92Mo), the magnitudes of some of these isotopic anomalies also appear to correlate with heliocentric distance. While the isotopic differences between the Earth and Mars may be readily accounted for by the accretion of mostly local materials in distinct regions of the protoplanetary disc, it is unclear whether this also applies to asteroid Vesta. Here we analysed the available data from our numerical simulation database to determine the formation location of Vesta in the framework of three planet-formation models: classical, Grand Tack, and Depleted Disc. We find that Vesta has a high probability of forming locally in the asteroid belt in models where material mixing in the inner disc is limited; this limited mixing is implied by the isotopic differences between the Earth and Mars. Based on our results, we propose several criteria to explain the apparent correlation between the different nucleosynthetic isotopic compositions of the Earth, Mars, and Vesta: (1) these planetary bodies accreted their building blocks in different regions of the disc, (2) the inner disc is characterised by an isotopic gradient, and (3) the isotopic gradient was preserved during the formation of these planetary bodies and was not diluted by material mixing in the disc (e.g. via giant planet migration).

¹C. Avdellidou (Χ. Αβδελλίδου),¹M. Delbo,¹A. Morbidelli,²K. J. Walsh,^1,3E. Munaibari,⁴J. Bourdelle de Micas,⁵M. Devogèle,^4,6S. Fornasier,⁷M. Gounelle,⁸G. van Belle
Astronomy & Astrophysics 665, L9 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202244590]
¹Université Côte d’Azur, CNRS – Lagrange, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
²Southwest Research Institute, 1050 Walnut St. Suite 300, Boulder, CO 80302, USA
³Université Côte d’Azur, CNRS – Géoazur, Observatoire de la Côte d’Azur, 250 rue Albert Einstein, Sophia Antipolis, 06560 Valbonne, France
⁴LESIA, Université Paris Cité, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 92190 Meudon, France
⁵Arecibo Observatory, University of Central Florida, HC-3 Box 53995 Arecibo, PR 00612, USA
⁶Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France
⁷Muséum National d’Histoire Naturelle, Sorbonne Universités, CNRS, IMPMC – UMR CNRS 7590, 57 rue Cuvier, 75005 Paris, France
⁸Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
Reproduced with permission (C)ESO

The identification of meteorite parent bodies provides the context for understanding planetesimal formation and evolution as well as the key Solar System events they have witnessed. However, identifying such links has proven challenging and some appear ambiguous. Here, we identify that the family of asteroid fragments whose largest member is (161) Athor is the unique source of the rare EL enstatite chondrite meteorites, the closest meteorites to Earth in terms of their isotopic ratios. The Athor family was created by the collisional fragmentation of a parent body 3 Gyr ago in the inner main belt. We calculate that the diameter of the Athor family progenitor was 64 km in diameter, much smaller than the putative size of the EL original planetesimal. Therefore, we deduce that the EL planetesimal that accreted in the terrestrial planet region underwent a first catastrophic collision in that region, and one of its fragments suffered a more recent catastrophic collision in the main belt, generating the current source of the EL meteorites.

¹Stefan Schröder et al. (>10)
Astronomy & Astrophysics 666, A164 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202244059]
¹Luleå University of Technology, 98128 Kiruna, Sweden
Reproduced with permission (C) ESO

Context. After landing on C-type asteroid Ryugu, MASCOT imaged brightly colored, submillimeter-sized inclusions in a small rock. Hayabusa2 successfully returned a sample of small particles from the surface of Ryugu, but none of these appear to harbor such inclusions. The samples are considered representative of Ryugu.

Aims. To understand the apparent discrepancy between MASCOT observations and Ryugu samples, we assess whether the MASCOT landing site, and the rock by implication, is perhaps atypical for Ryugu.

Methods. We analyzed observations of the MASCOT landing area acquired by three instruments on board Hayabusa2: a camera (ONC), a near-infrared spectrometer (NIRS3), and a thermal infrared imager. We compared the landing area properties thus retrieved with those of the average Ryugu surface.

Results. We selected several areas and landforms in the landing area for analysis: a small crater, a collection of smooth rocks, and the landing site itself. The crater is relatively blue and the rocks are relatively red. The spectral and thermophysical properties of the landing site are very close to those of the average Ryugu surface. The spectral properties of the MASCOT rock are probably close to average, but its thermal inertia may be somewhat higher.

Conclusions. The MASCOT rock can also be considered representative of Ryugu. Some of the submillimeter-sized particles in the returned samples stand out because of their atypical spectral properties. Such particles may be present as inclusions in the MASCOT rock.

¹Maxime Piralla,¹Johan Villeneuve,¹Nicolas Schnuriger,¹David V.Bekaert,¹Yves Marrocchi
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115427]
¹Université de Lorraine, CNRS, CRPG, UMR 7358, Nancy, France
Copyright Elsevier

The chronology of dust formation in the early solar system remains controversial. Chondrules are the most abundant high-temperature objects formed during the evolution of the circumsolar disk. Considering chondrule formation, absolute lead‑lead (Pbsingle bondPb) ages and aluminum‑magnesium (26Alsingle bond26Mg) ages relative to calcium‑aluminum-rich inclusions (CAIs) provide inconsistent chronologies, with Pbsingle bondPb ages showing early and protracted chondrule formation episodes whereas 26Alsingle bond26Mg ages suggest that chondrule production was delayed by >1.5 Ma. Here, we develop a new method to precisely determine in situ 26Alsingle bond26Mg ages of spinel-bearing chondrules without being affected by secondary asteroidal processes. Our data demonstrate that 26Alsingle bond26Mg chondrule formation ages are actually 1 Ma older than previously thought and extend over the entire lifetime of the disk. This shift in chondrule formation ages relative to CAIs, however, is not sufficient to reconcile the Pbsingle bondPb and 26Alsingle bond26Mg chronologies. Thus, either chondrules Pbsingle bondPb ages and volcanic achondrites 26Alsingle bond26Mg ages are incorrect or the age of the solar system age should be reevaluated at 4568.7 Ma to ensure consistency between chronometers. We favor the second hypothesis, given that (i) the canonical age of CAIs was determined using only 4 specimens and (ii) older ages of 4568.2 Ma have also been measured. We show that the adoption of 4568.7 Ma as the new canonical age of CAIs and the use of our new spinel-derived 26Alsingle bond26Mg ages enable reconciling the Pbsingle bondPb and 26Alsingle bond26Mg ages of chondrules and achondrites. This new chronology implies the existence of a 0.7–1 Ma gap between the formation of refractory inclusions and chondrules, and supports the homogeneous distribution of 26Al in the circumsolar disk.

^1,2,3Queenie H. S. Chan,⁴Jonathan S. Watson,⁴Mark A. Sephton,^3,5,6Áine C. O’Brien,⁵Lydia J. Hallis
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13936]
¹Royal Holloway University of London, Surrey, TW20 0EX UK
²The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
³UK Fireball Network (UKFN), UK
⁴Department of Earth Science and Engineering, Imperial College London, London, SW7 2BX UK
⁵School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ UK
⁶UK Fireball Alliance (UKFAll), UK
Published by arrangement with John Wiley & Sons

The rapid recovery of the Winchcombe meteorite offers a valuable opportunity to study the soluble organic matter (SOM) profile in pristine carbonaceous astromaterials. Our interests in the biologically relevant molecules, amino acids—monomers of protein, and the most prevalent meteoritic organics—polycyclic aromatic hydrocarbons (PAHs) are addressed by analyzing the solvent extracts of a Winchcombe meteorite stone using gas chromatography mass spectrometry. The Winchcombe sample contains an amino acid abundance of ~1132 parts-per-billion that is about 10 times lower than other CM2 meteorites. The detection of terrestrially rare amino acids, including α-aminoisobutyric acid (AIB); isovaline; β-alanine; α-, β-, and γ-amino-n-butyric acids; and 5-aminopentanoic acid, and the racemic enantiomeric ratios (D/L = 1) observed for alanine and isovaline indicate that these amino acids are indigenous to the meteorite and not terrestrial contaminants. The presence of predominantly α-AIB and isovaline is consistent with their formation via the Strecker-cyanohydrin synthetic pathway. The L-enantiomeric excesses in isovaline previously observed for aqueously altered meteorites were viewed as an indicator of parent body aqueous processing; thus, the racemic ratio of isovaline observed for Winchcombe, alongside the overall high free:total amino acid ratio, and the low amino acid concentration suggest that the analyzed stone is derived from a lithology that has experienced brief episode(s) of aqueous alteration. Winchcombe also contains 2- to 6-ring alkylated and nonalkylated PAHs. The low total PAHs abundance (6177 ppb) and high nonalkylated:alkylated ratio are distinct from that observed for heavily aqueously altered CMs. The weak petrographic properties of Winchcombe, as well as the discrepancies observed for the Winchcombe SOM content—a low total amino acid abundance comparable to heavily altered CMs, and yet the high free:total amino acid and nonalkylated:alkylated PAH ratios are on par with the less altered CMs—suggest that Winchcombe could represent a class of weak, poorly lithified meteorite not been previously studied.

¹Jesse T.Gu,¹Rebecca, A.Fischer,¹Matthew C.Brennan,²Matthew S.Clement,³Seth A.Jacobson,⁴Nathan A.Kaib,⁵David P.O’Brien,⁶Sean N.Raymond
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115425]
¹Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
²Earth and Planets Laboratory, Carnegie Institution for Science, Washington D.C., CO, USA
³Department of Earth and Environmental Sciences, Michigan State University, East Lansing, MI, USA
⁴HL Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK, USA
⁵Planetary Science Institute, Tucson, AZ, USA
⁶Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, Bordeaux, France
Copyright Elsevier

The chemical compositions of Earth’s core and mantle provide insight into the processes that led to their formation. N-body simulations, on the other hand, generally do not contain chemical information, and seek to only reproduce the masses and orbits of the terrestrial planets. These simulations can be grouped into four potentially viable scenarios of Solar System formation (Classical, Annulus, Grand Tack, and Early Instability) for which we compile a total of 433 N-body simulations. We relate the outputs of these simulations to the chemistry of Earth’s core and mantle using a melt-scaling law combined with a multi-stage model of core formation. We find the compositions of Earth analogs to be largely governed by the fraction of equilibrating embryo cores (kcore_emb) and the initial embryo masses in N-body simulations, rather than the simulation type, where higher values of kcore_emb and larger initial embryo masses correspond to higher concentrations of Ni, Co, Mo, and W in Earth analog mantles and higher concentrations of Si and O in Earth analog cores. As a result, larger initial embryo masses require smaller values of kcore_emb to match Earth’s mantle composition. On the other hand, compositions of Earth analog cores are sensitive to the temperatures of equilibration and fO2 of accreting material. Simulation type may be important when considering magma ocean lifetimes, where Grand Tack simulations have the largest amounts of material accreted after the last giant impact. However, we cannot rule out any accretion scenarios or initial embryo masses due to the sensitivity of Earth’s mantle composition to different parameters and the stochastic nature of N-body simulations. We use our compiled simulations to explore the relationship between initial embryo masses and the melting history of Earth analogs, where the complex interplay between the timing between impacts, magma ocean lifetimes, and volatile delivery could affect the compositions of Earth analogs formed from different simulation types. Comparing the last embryo impacts experienced by Earth analogs to specific Moon-forming scenarios, we find the characteristics of the Moon-forming impact are dependent on the initial conditions in N-body simulations where larger initial embryo masses promote larger and slower Moon-forming impactors. Mars-sized initial embryos are most consistent with the canonical hit-and-run scenario onto a solid mantle. Our results suggest that constraining the fraction of equilibrating impactor core (kcore) and the initial embryo masses in N-body simulations could be significant for understanding both Earth’s accretion history and characteristics of the Moon-forming impact.

^1,2Nicole X.Nie,^3,4Xin-Yang Chen,¹Zhe J.Zhang,⁵Justin Y.Hua,²Weiyi Liu,²Francois L. H. Tissot,³Fang-Zhen Teng,⁶Anat Shahar,¹Nicolas Dauphas
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.01.004]
¹Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
²The Isotoparium, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
³Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA
⁴Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu, China
⁵Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
⁶Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA
Copyright Elsevier

As moderately volatile elements, isotopes of Rb and K can trace volatilization processes in planetary bodies. Rubidium isotopic data are however very scarce, especially for non-carbonaceous meteorites. Here, we report combined Rb and K isotopic data (δ87/85Rb and δ41/39Κ) for 7 ordinary, 6 enstatite and 4 Martian meteorite falls to understand the causes for the variations in volatile abundances and isotopic compositions. Bulk Rb and K isotopic compositions of planetary bodies are estimated to be (Table 1): Mars +0.10±0.03 ‰ for Rb and −0.26±0.05 ‰ for K, bulk OCs ‰ for Rb and ‰ for K, bulk ECs + ‰ for Rb and ‰ for K. The bulk K isotopic compositions of subgroup OCs are estimated to be ‰ for H chondrites, ‰ for L chondrites, and ‰ for LL chondrites. A broad correlation between the Rb and K isotopic compositions of planetary bodies is observed. The correlation follows a slope that is consistent with kinetic evaporation and condensation processes, suggesting volatility-controlled mass-dependent isotope fractionation (as opposed to nucleosynthetic anomalies).

Individual ordinary and enstatite chondrites show large Rb and K isotopic variations (−1.02 to +0.29 ‰ for Rb and −0.91 to −0.15 ‰ for K). Samples of lower metamorphic grades display correlated elemental and isotopic fractionation between Rb and K, while samples of higher metamorphic grades show great scatter, suggesting that chondrite parent-body processes have decoupled the two elements and their isotopes at the sample scale. Several processes could have contributed to the observed isotopic variations of Rb and K, including (i) chondrule “nugget effect”, (ii) volatilization during parent-body thermal metamorphism (heat-induced vaporization and gas transport within parent bodies), (iii) thermal diffusion during parent-body metamorphism, and (iv) impact/shock heating. Quantitative modeling of the first two processes suggests that neither of them could produce isotopic variations large enough to explain the observed isotopic variations. Volatilization during parent-body thermal metamorphism [the scenario (ii)], which has been commonly invoked to explain the isotopic variations of volatile elements, is gas transport-limited and its effect on isotopic fractionations of moderately volatile elements should be negligible. Modeling of diffusion processes suggests that (iii) could produce K isotopic variation comparable to the observed variation. The large isotopic variations in non-carbonaceous meteorites are thus most likely due to diffusive redistribution of K and Rb during metamorphism and/or shock-induced heating and vaporization.