Asteroidal Parent Bodies Research Articles

Mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates

Petrographic, mineralogical and chemical analysis of naturally weathered equilibrated ordinary chondrites collected from ‘ hot’ deserts and Antarctica has revealed striking similarities and also pronounced differences in weathering between the two environments. Terrestrial weathering in all meteorites studied is dominated by oxidation and hydration of Fe,Ni metal, producing Fe-oxides and oxyhydroxides that have partially replaced the metal grains and have also occluded primary intergranular pores to form veins. Troilite weathers readily in ‘ hot’ desert environments but undergoes very little alteration under Antarctic conditions. Most of the primary porosity of ordinary chondrites has been occluded by the time that ∼ 15 to 25% of the initial Fe 0 and Fe 2+ has been oxidised to Fe 3+ in both environments. Results from modelling the volume changes upon alteration of primary minerals to a range of weathering products demonstrates that the primary porosity of most meteorites is sufficient to accommodate weathering products. Dilation of primary pores and brecciation, which has been observed in parts of some meteorites, will only occur if the meteorite is especially metal-rich, or has a low primary porosity. These weathering products are absent from recent falls but have formed in a fall after ∼ 100 yr of museum storage. Cl-bearing akaganéite and hibbingite are common weathering products in Antarctic finds but occur in abundance in only one ‘ hot’ desert meteorite, Daraj 014. The majority of Fe-rich weathering products in meteorites from both environments contain low, but variable concentrations of Si, Mg and Ca. In most meteorites a proportion of these elements are inferred to be present as a very finely crystalline mineral with a ∼ 1.0-nm lattice fringe spacing; where seen within intragranular fractures this mineral has a topotactic relationship with olivine and orthopyroxene. In the heavily-weathered Antarctic finds ALHA 78045 and 77002, Si is concentrated in cronstedtite, a Fe-rich phyllosilicate. An unidentified hydrous Si-Fe-Ni-Mg mineral or gel has also partially replaced taenite in ALHA 78045. In addition to Fe-rich weathering products, ‘ hot’ desert meteorites contain sulphates, Ca-carbonate and silica, whereas such minerals are largely absent from Antarctic finds. The abundance of silicate weathering products in Antarctic meteorites is unexpected and indicates that olivine and pyroxene undergo significant chemical weathering in these environments. As preterrestrial cronstedtite is abundant in CM2 carbonaceous chondrites, the Antarctic environment may be a powerful analog for aqueous alteration in the asteroidal parent bodies of primitive meteorites.

Preparation of TEM samples by focused ion beam (FIB) techniques: applications to the study of clays and phyllosilicates in meteorites

AbstractTransmission electron microscope samples were prepared of ALH 78045 and ALH 88045, two clay-and phyllosilicate-bearing Antarctic meteorites, using argon ion milling and focused ion beam (FIB) techniques. ALH 78045 contains clay- and phyllosilicate-filled veins that have formed by terrestrial weathering of olivine, orthopyroxene and metal. Very narrow (∼10 nm) intragranular clay-filled veins could be observed in the TEM samples prepared by argon ion milling, whereas differential thinning and lack of precision in the location of the electron-transparent areas hindered the study of wider (5 — 15 μm) phyllosilicate-filled intergranular veins. Using the FIB instrument, electron-transparent slices were cut from specific parts of the wider veins and lifted out for TEM study. Results show that these veins are occluded by cronstedtite, a mixed-valence Fe-rich phyllosilicate. This discovery shows that silicates can be both dissolved and precipitated during terrestrial weathering within the Antarctic ice. ALH 88045 is one of a small number of known CM1 carbonaceous chondrites. This meteorite is largely composed of flattened ellipsoidal aggregates of serpentine-group phyllosilicates. To determine the mineralogy and texture of phyllosilicates within specific aggregates, TEM samples were prepared by trenching into the cut edge of a sample using the FIB instrument. Results show that Mg-rich aggregates are composed of lath-shaped serpentine crystals with a ∼0.73 nm basal spacing, which is typical of the products of low temperature aqueous alteration within asteroidal parent bodies. Results of this work demonstrate that the FIB has enormous potential in a number of areas of Earth and planetary science.

39Ar‐40Ar chronology of R chondrites

Abstract— This study presents the first determinations of 39Ar‐40Ar ages of R chondrites for the purpose of understanding the thermal history of the R chondrite parent body. The 39Ar‐40Ar ages were determined on whole‐rock samples of four R chondrites: Carlisle Lakes, Rumuruti, Acfer 217, and Pecora Escarpment #91002 (PCA 91002). All samples are breccias except for Carlisle Lakes. The age spectra are complicated by recoil and diffusive loss to various extents. The peak 39Ar‐40Ar ages of the four chondrites are 4.35, ˜4.47 ± 0.02, 4.30 ± 0.07 Ga, and 4.37 Ga, respectively. These ages are similar to Ar‐Ar ages of relatively unshocked ordinary chondrites (4.52–4.38 Ga) and are older than Ar‐Ar ages of most shocked ordinary chondrites («4.2 Ga).Because the meteorites with the oldest (Rumuruti, ˜4.47 Ga) and the youngest (Acfer 217, ˜4.30 Ga) ages are both breccias, these ages probably do not record slow cooling within an undisrupted asteroidal parent body. Instead, the process of breccia formation may have differentially reset the ages of the constituent material, or the differences in their age spectra may arise from mixtures of material that had different ages. Two end‐member type situations may be envisioned to explain the age range observed in the R chondrites. The first is if the impact(s) that reset the ages of Acfer 217 and Rumuruti was very early. In this case, the ˜170 Ma maximum age difference between these meteorites may have been produced by much deeper burial of Acfer 217 than Rumuruti within an impact‐induced thick regolith layer, or within a rubble pile type parent body following parent body re‐assembly. The second, preferred scenario is if the impact that reset the age of Acfer 217 was much later than that which reset Rumuruti, then Acfer 217 may have cooled more rapidly within a much thinner regolith layer. In either scenario, the oldest age obtained here, from Rumuruti, provides evidence for relatively early (˜4.47 Ga) impact events and breccia formation on the R chondrite parent body.

Organic Compounds in Carbonaceous Meteorites

AbstractFor Abstract see ChemInform Abstract in Full Text.

Possible in situ formation of meteoritic nanodiamonds in the early Solar System.

Grains of dust that pre-date the Sun provide insights into their formation around other stars and into the early evolution of the Solar System. Nanodiamonds recovered from meteorites, which originate in asteroids, have been thought to be the most abundant type of presolar grain. If that is true, then nanodiamonds should be at least as abundant in comets, because they are thought to have formed further out in the early Solar System than the asteroid parent bodies, and because they should be more pristine. Here we report that nanodiamonds are absent or very depleted in fragile, carbon-rich interplanetary dust particles, some of which enter the atmosphere at speeds within the range of cometary meteors. One interpretation of the results is that some (perhaps most) nanodiamonds formed within the inner Solar System and are not presolar at all, consistent with the recent detection of nanodiamonds within the accretion discs of other young stars. An alternative explanation is that all meteoritic nanodiamonds are indeed presolar, but that their abundance decreases with heliocentric distance, in which case our understanding of large-scale transport and circulation within the early Solar System is incomplete.

Organic compounds in carbonaceous meteorites.

The carbonaceous chondrite meteorites are fragments of asteroids that have remained relatively unprocessed since the formation of the solar system 4.6 billion years ago. These carbon-rich objects contain a variety of extraterrestrial organic molecules that constitute a record of chemical evolution prior to the origin of life. Compound classes include aliphatic hydrocarbons, aromatic hydrocarbons, amino acids, carboxylic acids, sulfonic acids, phosphonic acids, alcohols, aldehydes, ketones, sugars, amines, amides, nitrogen heterocycles, sulfur heterocycles and a relatively abundant high molecular weight macromolecular material. Structural and stable isotopic characteristics suggest that a number of environments may have contributed to the organic inventory, including interstellar space, the solar nebula and the asteroidal meteorite parent body. This review covers work published between 1950 and the present day and cites 193 references.

Oral histories in meteoritics and planetary science: I. Edward Anders

Abstract— In this interview, taped in 2000 August, during the meeting of the Meteoritical Society in Chicago, Edward Anders ascribes his interest in meteorites to a single moment of inspiration while he was in graduate school. He then reviews those advances that he has found most exciting. Some were contributed by his own research group, but many by others. Among the topics are the following: origin of meteorites (lunar or asteroidal parent bodies, diamonds, Widmanstätten pattern, nebular condensation, meteorite orbits, asteroid spectra); primitive meteoritic matter (Cl chondrites, calcium‐aluminum‐rich inclusions (CAIs), solar‐system abundances, organic matter); meteorite ages (radiogenic ages, cosmic‐ray ages, charged particle tracks); classification of meteorites (chondrites, iron meteorites, unequilibrated chondrites); isotopic anomalies (oxygen, heavy elements, noble gases, interstellar grains); and meteorite impacts (impact craters, K/T mass extinction). Anders spent most of his career (1955–1991) at the University of Chicago where he was the Horace B. Horton Professor of Chemistry from 1973–1991. The Meteoritical Society honored Anders with its Leonard Medal in 1974, and he served as its president in 1991–1992. He then dropped out of science. After residing in Switzerland for several years, Anders returned to the USA where he maintains residences in Burlingame, California, and Chicago, Illinois. Today, Anders can be reached at: eanders1@concentric.net.

Modeling aqueous alteration of CM carbonaceous chondrites

Abstract— Results from an inorganic geochemical modeling study support a scenario in which low‐temperature aqueous alteration of an anhydrous CM asteroidal parent body and melt water from H2O and CO2 ices produces the altered assemblage observed in CM carbonaceous chondrites (chrysotile, greenalite, tochilinite, cronstedtite and minor calcite and magnetite). We consider a range of possible precursor mineral assemblages, varying with respect to the Fe‐oxidation state of the initial anhydrous phases. The aqueous solutions produced by this alteration are generally strongly basic and reducing and a large quantity of H2, and possible CH4, gas can be released during aqueous alteration.

Efficient delivery of meteorites to the Earth from a wide range of asteroid parent bodies

Almost all meteorites come from asteroids, but identifying their specific parent bodies, and modelling their transport to the Earth, has proved to be difficult. The usual model of delivery through orbital resonances with the major planets has recently been shown to deplete the supply of meteorites much too rapidly to explain either the observed flux at the Earth, or the length of time the meteorites have spent in space (as measured by cosmic-ray exposure ages). Independently, it has been found that a force arising from anisotropically emitted thermal radiation from asteroidal fragments (the 'Yarkovsky effect') influences the fragments' orbits in important ways. Here we report the results of a detailed model for the transport of meteorites to the Earth, which includes the Yarkovsky effect and collisional evolution of the asteroidal fragments. We find that the Yarkovsky effect significantly increases the efficiency of the delivery of meteorites to the Earth, while at the same time allowing a much wider range of asteroids to contribute to the flux of meteorites. Our model also reproduces the observed distribution of cosmic-ray exposure ages of stony meteorites.

Thermal alteration of asteroids: evidence from meteorites

The world's meteorite collections contain meteorites from at least ∼27 primitive, chondritic and ∼108 partially or totally melted asteroids. Detailed studies of these meteorites have shown that all of their asteroidal parent bodies have been thermally altered by internal heating to some degree. These alterations range from low temperature, aqueous processes (∼0 to <∼300°C), to thermal metamorphism (∼400 to ∼950°C), to partial melting and formation of unfractionated (∼980–1050°C) and fractionated (∼1000 to >1250°C) residues, to partial and complete melting, differentiation and fractional crystallization of asteroids (∼1150 to ⪢1250°C). The most likely heat source was the decay of short-lived radionuclides, notably 26 Al . These thermal alterations took place penecontemporaneously on all asteroids of which we have samples, and in the first few Ma of solar system history. The asteroid 4 Vesta, the likely parent body of the HED meteorites, is a highly differentiated object, may have a metal core, and can be viewed as the smallest of the terrestrial planets. It accreted, was heated, was partially to completely melted, and formed an extrusive basaltic crust, all within a few Ma of formation of CAIs and the dawn of the solar system.

Density and Porosity of Stone Meteorites: Implications for the Density, Porosity, Cratering, and Collisional Disruption of Asteroids

The grain density, bulk density, and porosity were measured for 19 ordinary chondrite and 5 carbonaceous chondrite meteorites, and the grain density was measured for an additional 30 ordinary chondrite and 7 carbonaceous chondrite meteorites. The unweathered ordinary chondrites generally have porosities ranging from 5 to 25%, while the carbonaceous chondrites have porosities in the range 10 to 30%. These high porosities indicate that the asteroidal parent bodies from which these chondritic meteorites are derived are porous, or contain porous regions. The grain densities of all 49 ordinary chondrites are greater than 3.1 g/cc, the grain densities of all 9 anhydrous carbonaceous chondrites are greater than 3.0 g/cc, and the grain densities of the hydrated carbonaceous chondrites range from 2.5 to 3.0 g/cc. Comparison of these grain densities with spacecraft determinations of the densitiy of the S-type asteroid Ida, 2.6 g/cc, and the anhydrous C-type asteroid Mathilde, 1.3 g/cc, suggests that if these asteroids are composed of material similar in mineralogy to that of the stone meteorites, then Ida and Mathilde must exhibit significant porosity. Cratering and collisional disruption are sensitive to the porosity of the target. The parent bodies of the ordinary and carbonaceous chondrite meteorites appear to contain regions of significant porosity, which may affect their compression and compaction during impact events.

Formation and processing of organics in the early solar system.

Until pristine samples can be returned from cometary nuclei, primitive meteorites represent our best source of information about organic chemistry in the early solar system. However, this material has been affected by secondary processing on asteroidal parent bodies which probably did not affect the material now present in cometary nuclei. Production of meteoritic organic matter apparently involved the following sequence of events: Molecule formation by a variety of reaction pathways in dense interstellar clouds; Condensation of those molecules onto refractory interstellar grains; Irradiation of organic-rich interstellar-grain mantles producing a range of molecular fragments and free radicals; Inclusion of those interstellar grains into the protosolar nebula with probable heating of at least some grain mantles during passage through the shock wave bounding the solar accretion disc; Agglomeration of residual interstellar grains and locally produced nebular condensates into asteroid-sized planetesimals; Heating of planetesimals by decay of extinct radionuclides; Melting of ice to produce liquid water within asteroidal bodies; Reaction of interstellar molecules, fragments and radicals with each other and with the aqueous environment, possibly catalysed by mineral grains; Loss of water and other volatiles to space yielding a partially hydrated lithology containing a complex suite of organic molecules; Heating of some of this organic matter to generate a kerogen-like complex; Mixing of heated and unheated material to yield the meteoritic material now observed. Properties of meteoritic organic matter believed to be consistent with this scenario include: Systematic decrease of abundance with increasing C number in homologous series of characterisable molecules; Complete structural diversity within homologous series; Predominance of branched-chain isomers; Considerable isotopic variability among characterisable molecules and within kerogen-like material; Substantial deuterium enrichment in all organic fractions; Some fractions significantly enriched in nitrogen-15; Modest excesses of L-enantiomers in some racemisation-resistant molecules but no general enantiomeric preference. Despite much speculation about the possible role of Fischer-Tropsch catalytic hydrogenation of CO in production of organic molecules in the solar nebula, no convincing evidence for such material has been found in meteorites. A similarity between some meteoritic organics and those produced by Miller-Urey discharge synthesis may reflect involvement of common intermediates rather than the operation of electric discharges in the early solar system. Meteoritic organic matter constitutes a useful, but not exact, guide to what we shall find with in situ analytical and sample-return missions to cometary nuclei.

Interaction of Resonances and Yarkovsky Non-Gravitational Effects in The Asteroid Belt

The interaction between resonances and dissipative effects has long been investigated in relationship with the dynamics of dust particles or the evolution of planetesimals in the primordial solar nebula. More recently, it has been shown that another important application of this kind of dynamics is the delivery of asteroid fragments (precursors of meteorites and NEAs) to Earth-crossing orbits. The relevant dissipative mechanism, the so-called Yarkovsky effect (Öpik 1951; Peterson 1976; Burns et al. 1979) has been known for a long time, but its “seasonal” and “diurnal” variants, as well as its interaction with the collisional process taking place in the asteroid belt, have been studied quantitatively only in the last few years (Rubincam 1995, 1998; Hartmann et al. 1997, 1998; Farinella et al. 1998; Vokrouhlický and Farinella 1998a,b; Vokrouhlický 1998a,b; Farinella and Vokrouhlický 1998a,b).In the Farinella et al. (1998) paper we have provided a unified discussion of the Yarkovsky effect in both its variants. After computing the rate of the corresponding semimajor axis drift as a function of size and spin rate, and comparing the relevant timescales with those for collisional disruption and spin axis reorientation, we have rediscussed some issues in meteorite science which are put in a new light by the relevance of the Yarkovsky effect. In particular, this mechanism provides a good explanation for the fact that meteorite cosmic ray exposure ages (in particular for irons) are much longer than the dynamical lifetimes of objects delivered to the Earth-crossing region through resonances. Thanks to the Yarkovsky effect, small asteroid fragments in the belt undergo a slow drift in semimajor axis (with a random-walk component related to their rotational state) and therefore have enough mobility to reach the resonances after comparatively long times spent in nonresonant main-belt orbits. Metal-rich fragments have slower Yarkovsky drift rates than stones, but their much longer collisional lifetimes can explain why iron meteorites appear to sample a larger number of asteroid parent bodies compared to ordinary chondrites.

The porosities of ordinary chondrites: Models and interpretation

Abstract— Densities and porosities for 285 ordinary chondrites have been assembled and analyzed. Measured chondrite porosities are bimodal; finds have an average porosity of &lt;3%, whereas fall porosities average 7% but range from zero to &gt;30%. We conclude that mild degrees of weathering fill pore spaces, lowering grain densities and porosities without significantly changing the bulk size or mass of the sample. By assuming an original pristine grain density (as a function of the meteorite's mineralogy—determined by its class), we can derive model pristine porosities. These model porosities cluster around an average value of 10% for all classes of ordinary chondrites.Ordinary chondrites do not show any correlation of porosity (model or measured) with petrographic grade or sample size (over a range from 0.2 g to 2 kg). However, we do see a correlation between shock state and porosity. Shock‐blackened meteorites are less porous than other meteorites. Furthermore, less severely shocked meteorites show a much broader range of porosities, with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class. This is consistent with the idea that shock compresses and closes pore space.Analysis of meteorite porosity provides a lower bound to the fine‐scale porosity of asteroids. Our densities, even with 10% primordial porosity, are significantly higher than inferred densities of possible asteroid parent bodies. These asteroids are probably loose piles of rubble.

Meteorite Delivery via Yarkovsky Orbital Drift

We provide a unified discussion of the Yarkovsky effect in both the original, “diurnal” variant and also for the “seasonal” variant which has been recently shown by Rubincam (1995) to be important for meteorite-sized, regolith-free asteroid fragments. After computing the rate of the corresponding semimajor axis drift as a function of size and spin rate, and comparing the relevant time scales with those for collisional disruption and spin reorientation, we discuss some issues in meteorite science which are put in a new light by the relevance of the Yarkovsky effect. In particular, this mechanism provides a good explanation for the fact that meteorite cosmic ray exposure ages (in particular for irons) are much longer than the dynamical lifetimes of objects delivered to the Earth-crossing region through resonances. Thanks to the Yarkovsky effect, small asteroid fragments in the belt undergo a slow drift in semimajor axis (with a random-walk component related to their rotational state) and therefore have enough mobility to reach the resonances after comparatively long times spent in nonresonant main-belt orbits. Metal-rich fragments have slower Yarkovsky drift rates than stones, but their much longer collisional lifetimes may explain why iron meteorites appear to sample a larger number of asteroid parent bodies compared to ordinary chondrites.

A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites

Abstract—Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED mantle indicate that the parent body may have been completely molten during its early history. During cooling of a chondritic composition magma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded liquids are similar in composition to Main Group and Stannern trend eucrites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Subsequent fractional crystallization of a Main Group eucrite liquid, which has been isolated as a body of magma, produces the Nuevo Laredo trend and the cumulate eucrites. The predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta.

Shape and Albedo Variations of Asteroid 15 Eunomia

Near-infrared reflected and thermal emitted lightcurves were measured concurrently for Asteroid 15 Eunomia and were used to calculate relative projected area and relative albedo variations over its surface. The technique for calculating relative variations in projected area and albedo from simultaneously measured reflected and emitted lightcurves can be used to detect changes in the shape and surface brightness of asteroids for which such data exist.Rotational spectral reflectance data obtained in 1981 were used to correlate variations in shape and relative surface albedo with changes in surface composition. Using these data combined with this relative projected area/relative albedo technique, it is confirmed that 15 Eunomia is an elongate asteroid with a generally ovoid (“egg”) shape. Its surface composition varies from metalliferous with major olivine and lesser pyroxene in subequal metal/silicate ratios (much like stony–iron meteorites) for the more “pointed” end to a higher concentration of high-Fe pyroxene and a higher general concentration of silicate (a basaltic composition) for the “blunt” end (Gaffey and Ostro, 1987,Lunar Planet Sci. XVIII,310–311).Asteroid 15 Eunomia may be a cross section through a completely or partially differentiated S-type asteroid parent body. The basaltic blunt end of 15 Eunomia may show a sample of the surface of the parent body with the metalliferous end showing a portion of the core/mantle boundary. If true, this would indicate a mantle thickness on the parent body of approximately 300 km.

Age of the eucrite “Caldera” from convergence of long-lived and short-lived chronometers

The long-lived 147Sm143Nd chronometer has been used in conjunction with the short-lived146Sm142Nd and 53Mn53Cr chronometers to constrain the time of formation for the eucrite Caldera. The combination of these three chronometers has produced results that are consistent with one another and converge on a precise, high resolution age of 4537 ± 12 Ma (within 95% confidence limits). So far this is the youngest crystallization age that has been confirmed for a noncumulate eucrite. Thus, this basaltic meteorite formed relatively late as compared to other noncumulate eucrites on a differentiated asteroidal parent body, most likely between 17 and 41 million years after the formation of the first condensates in the early Solar System. From these age constraints it is evident that Caldera either formed within a magma body on a large (>500 km) asteroid or in an extensive impact melt on the crust of its parent body.

Dark clasts in the Khor Temiki aubrite: Not basalts

Abstract— The enstatite achondrite meteorites (aubrites) are ultramafic assemblages with highly variable bulk rare earth element (REE) compositions. An enrichment of REE in a dark clast from the Khor Temiki aubrite led Wolf et al. (1983) to suggest that such dark clasts could be the basaltic (i.e., enstatite‐plagioclase) complements to the ultramafic aubrites, with the relatively high REE contents resulting from the presence of plagioclase, which is a common carrier of the REEs. We have studied several dark clasts from the Khor Temiki aubrite and find no evidence for a basaltic character for such material. The microscopic character of the dark clasts is not significantly different from the main portions of Khor Temiki and consists either of highly brecciated material, containing a fine‐grained matrix, or of enstatite grains with abundant inclusions. We suggest that the dark clasts are shock‐darkened, heterogeneous Khor Temiki material that, by chance, contained variable trace contents of oldhamite (CaS), which has been shown to be a major carrier of REE in aubrites. We find that the REE contents of the clasts range from 0.1 to ∼20× CI. Most have negative Eu anomalies, but one has a small positive anomaly. Extensive searches have failed to identify basaltic material in Khor Temiki and other aubrites. The absence of basaltic material is consistent with, but does not prove, the model of Wilson and Keil (1991). They calculate that, on an asteroidal parent body &lt; ∼100 km in radius, a volatile‐rich basaltic partial melt erupted with a velocity greater than the escape velocity of the asteroid and, thus, was lost into space ∼ 4.55 Ga ago.

Volatiles in unequilibrated ordinary chondrites: Abundances, sources and implications for explosive volcanism on differentiated asteroids

Abstract— Keil and Wilson (1993) proposed that, during partial melting of some asteroidal meteorite parent bodies, explosive pyroclastic volcanism accelerated S‐rich Fe, Ni‐FeS cotectic partial melts into space. These authors argued that this process was responsible for the S‐depletion of many of the magmas from which the magmatic iron meteorites formed. This process only requires the presence of a few hundred to thousand ppm of volatiles in asteroids &lt; ∼100 km in radius. If the precursor materials of these magmatic iron meteorite groups were similar in composition to unequilibrated ordinary chondrites, then the volatile contents of the latter may be a measure of the potential effectiveness of the process. Analysis of volatile contents of seven unequilibrated ordinary chondrite falls by dynamic high‐temperature mass spectrometry revealed that thousands of ppm of indigeneous volatiles, mostly CO, Cl, Na and S, are released at temperatures near the Fe, Ni‐FeS cotectic melting temperature of ∼980 °C. If these volatiles are largely retained in the asteroidal parent bodies until onset of partial melting, S depletion of the residual melt might have been achieved by ejection of S‐rich partial Fe, Ni‐FeS melts by pyroclastic volcanism.