Cumberland Falls chondritic inclusions: Mineralogy/petrology of a forsterite chondrite suite

As previously found for a chondritic inclusion of unknown affinity, mineralogic and petrologic properties of 9 inclusions in the Cumberland Falls enstatitc achondrite are primitive members of the forsterite (F) chondrite group, hitherto defined by 4 meteorites of similar redox state. The inclusions define a primitive suite with properties indicating 8 as F3 and one of even lower petrologic type. The abundant minerals include: low-Ca pyroxene, olivine, plagioclase, kamacite, taenite, schreibersite, troilite, ferroan alabandite and daubreelite. Diopside, oldhamite and a Ti-rich sulfide are present in one or two inclusions. Petrologic textures and jadeitic pyroxene, hitherto unidentified in meteorites, indicate substantial degree of shock. The inclusions acquired their chemical characteristics during nebular condensation and accretion over a broad redox range (metal-silicate trends in them verify Prior's Rules): their parent body later impacted the enstatite meteorite parent body. During impact, the inclusions were shocked and incorporated with enstatite achondrite host as a breccia that would become Cumberland Falls.     

Noble gas and oxygen isotope studies of aubrites: A clue to origin and histories

Noble gas measurements were performed for nine aubrites: Bishopville, Cumberland Falls, Mayo Belwa, Mount Egerton, Norton County, Peña Blanca Spring, Shallowater, ALHA 78113 and LAP 02233. These data clarify the origins and histories, particularly cosmic-ray exposure and regolith histories, of the aubrites and their parent body(ies). Accurate cosmic-ray exposure ages were obtained using the ⁸¹Kr-Kr method for three meteorites: 52 ± 3, 49 ± 10 and 117 ± 14 Ma for Bishopville, Cumberland Falls and Mayo Belwa, respectively. Mayo Belwa shows the longest cosmic-ray exposure age determined by the ⁸¹Kr-Kr method so far, close to the age of 121 Ma for Norton County. These are the longest ages among stony meteorites. Distribution of cosmic-ray exposure ages of aubrites implies 4-9 break-up events (except anomalous aubrites) on the parent body. Six aubrites show “exposure at the surface” on their parent body(ies): (i) neutron capture ³⁶Ar, ⁸⁰Kr, ⁸²Kr and/or ¹²⁸Xe probably produced on the respective parent body (Bishopville, Cumberland Falls, Mayo Belwa, Peña Blanca Spring, Shallowater and ALHA 78113); and/or (ii) chondritic trapped noble gases, which were likely released from chondritic inclusions preserved in the aubrite hosts (Cumberland Falls, Peña Blanca Spring and ALHA 78113). The concentrations of ¹²⁸Xe from neutron capture on ¹²⁷I vary among four measured specimens of Cumberland Falls (0.5-76 × 10⁻¹⁴ cm³STP/g), but are correlated with those of radiogenic ¹²⁹Xe, implying that the concentrations of (¹²⁸Xe)_n and (¹²⁹Xe)_rad reflect variable abundances of iodine among specimens. The ratios of (¹²⁸Xe)_n/(¹²⁹Xe)_rad obtained in this work are different for Mayo Belwa (0.045), Cumberland Falls (0.015) and Shallowater (0.001), meaning that neutron fluences, radiogenic ¹²⁹Xe retention ages, or both, are different among these aubrites. Shallowater contains abundant trapped Ar, Kr and Xe (2.2 × 10⁻⁷, 9.4 × 10⁻¹⁰ and 2.8 × 10⁻¹⁰ cm³STP/g, respectively) as reported previously (Busemann and Eugster, 2002). Isotopic compositions of Kr and Xe in Shallowater are consistent with those of Q (a primordial noble gas component trapped in chondrites). The Ar/Kr/Xe compositions are somewhat fractionated from Q, favoring lighter elements. Because of the unbrecciated nature of Shallowater, Q-like noble gases are considered to be primordial in origin. Fission Xe is found in Cumberland Falls, Mayo Belwa, Peña Blanca Spring, ALHA 78113 and LAP 02233. The majority of fission Xe is most likely ²⁴⁴Pu-derived, and about 10-20% seems to be ²³⁸U-derived at ¹³⁶Xe. The observed (¹³⁶Xe)_Pu corresponds to 0.019-0.16 ppb of ²⁴⁴Pu, from which the ²⁴⁴Pu/U ratios are calculated as 0.002-0.009. These ratios resemble those of chondrites and other achondrites like eucrites, suggesting that no thermal resetting of the Pu-Xe system occurred after ∼4.5 Ga ago. We also determined oxygen isotopic compositions for four aubrites with chondritic noble gases and a new aubrite LAP 02233. In spite of their chondritic noble gas signatures, oxygen with chondritic isotopic compositions was found only in a specimen of Cumberland Falls (Δ¹⁷O of ∼0.3‰). The other four aubrites and the other two measured specimens of Cumberland Falls are concurrent with the typical range for aubrites.     

陨石分类研究进展及其地学意义

近40年来陨石分类学经历了3个发展阶段，60－70年代，由根据陨石的矿物结构的分类方法发展为球粒陨石的化学一岩石学分类法和铁陨石的化学群分类法；70－80年代，提出了分异型陨石和未分异型陨石的概念，球粒陨石被认为是未分异型陨石，而其它陨石（铁陨石，石铁陨石和无球粒陨石）大多被划入分异型陨石，80－90年代以来，陨石氧同位素组成成为了陨石成因分类的一个主要依据，使陨石分类学进入了一个新的成因分类阶段，作者对80－90年代以来新确立的R群，K小群，CR群和CK群球粒陨石，以及根据氧同位素划分出的原始型无球粒陨石系列：A－L无球粒陨石，Winonaites无球粒陨石和Brachinites无球粒陨石进行了介绍，笔者对陨石研究和陨石分类学的发展在估算地球整体成分，探讨地球成因和早期演化历史方面的重要意义进行了说明，并建议地球科学家应对陨石学和陨石分类的发展现状给以关注。     

Cumberland Falls chondritic inclusions—II. Trace element contents of forsterite chondrites and meteorites of similar redox state

Mineralogic study of black inclusions in the Cumberland Falls enstatite achondrite revealed that they constitute a highly unequilibrated chondritic suite distinct from other chondrite groups. This highly shocked suite, the forsterite (F) chondrites, exhibits mineralogic trends apparently produced during primary nebular condensation and accretion over a broad redox range. We analyzed these samples and possibly related meteorites for Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn, trace elements known to yield important genetic information. The results demonstrate the compositional coherence and distinctiveness of the F chondrite suite relative to other chondrites. The Antarctic aubrite, ALH A78113, may include more F chondrite material. Trace element contents do not vary with mineral compositions hence do not reflect redox variations during formation of F chondrite parental matter. Trace element mobilization—during secondary heating episodes in the F chondrite parent or during its disruptive collision with the enstatite meteorite parent body—is not detectable. Chemical trends in F chondrites apparently reflect primary nebular processes. Cosmochemical fractionation of lithophiles from siderophiles and chalcophiles occurred at moderately high temperatures, certainly higher than those existing during formation of primitive carbonaceous, enstatite and ordinary chondrites of petrologic type ≤3.     

Acapulcoite-lodranite meteorites: Ultramafic asteroidal partial melt residues

Acapulcoites (most ancient Hf-W ages are 4,563.1?±?0.8 Ma), lodranites (most ancient Hf-W ages are 4,562.6?±?0.9 Ma) and rocks transitional between them are ancient residues of different degrees of partial melting of a chondritic source lithology (e.g., as indicated by the occurrence of relict chondrules in 9 acapulcoites), although the precise chondrite type is unknown. Acapulcoites are relatively fine- grained (～150–230?μm) rocks with equigranular, achondritic textures and consist of olivine, orthopyroxene, Ca-rich clinopyroxene, plagioclase, metallic Fe,Ni, troilite, chromite and phosphates. Lodranites are coarser grained (540–700?μm), with similar equigranular, recrystallized textures, mineral compositions and contents, although some are significantly depleted in eutectic Fe,Ni-FeS and plagioclase- clinopyroxene partial melts. The acapulcoite-lodranite clan is most readily distinguished from other groups of primitive achondrites (e.g., winoanites/IAB irons) by oxygen isotopic compositions, although more than 50% of meteorites classified as acapulcoites currently lack supporting oxygen isotopic data. The heat source for melting of acapulcoites-lodranites was internal to the parent body, most likely ²⁶Al, although some authors suggest it was shock melting. Acapulcoites experienced lower temperatures of ～980–1170?°C and lower degrees of partial melting (～1–4?vol.%) and lodranites higher temperatures of ～1150–1200?°C and higher degrees (～5?≥?10?vol.%) of partial melting. Hand-specimen and thin section observations indicate movement of Fe,Ni-FeS, basaltic, and phosphate melts in veins over micrometer to centimeter distances. Mineralogical, chemical and isotopic properties, Cosmic Ray Exposure (CRE) ages which cluster around 4–6 Ma and the occurrence of some meteorites consisting of both acapulcoite and lodranite material, indicate that these meteorites come from one parent body and were most likely ejected in one impact event. Whereas the precise parent asteroid of these meteorites is unknown, there is general agreement that it was an S-type object. There is nearly total agreement that the acapulcoite-lodranite parent body was <～100?km in radius and, based on the precise Pb–Pb age for Acapulco of 4555.9?±?0.6 Ma, combined with the Hf/W and U/Pb records and cooling rates deduced from mineralogical and other investigations, that the parent body was fragmented during its cooling which the U/Pb system dates at precisely 4556?±?1 Ma. Hf-W chronometry suggests that the parent body of the acapulcoites-lodranites and, in fact, the parent bodies of all “primitive achondrites” accreted slightly later than those of the differentiated achondrites and, thus, had lower contents of ²⁶Al, the heat producing radionuclide largely responsible for heating of both primitive and differentiated achondrites. Thus, the acapulcoite-lodranite parent body never experienced the high degrees of melting responsible for the formation of the differentiated meteorites, but arrested its melting history at relatively low degrees of ～15?vol.%.     

Genetic relations between meteorites and terrestrial and lunar rocks

According to their genesis, meteorites are classified into heliocentric (which originate from the asteroid belt) and planetocentric (which are fragments of the satellites of giant planets, including the Proto-Earth). Heliocentric meteorites (chondrites and primitive meteorites genetically related to them) used in this study as a characteristic of initial phases of the origin of the terrestrial planets. Synthesis of information on planetocentric meteorites (achondrites and iron meteorites) provides the basis for a model for the genesis of the satellites of giant planets and the Moon. The origin and primary layering of the Earth was initially analogously to that of planets of the HH chondritic type, as follows from similarities between the Earth’s primary crust and mantle and the chondrules of Fe-richest chondrites. The development of the Earth’s mantle and crust precluded its explosive breakup during the transition from its protoplanetary to planetary evolutionary stage, whereas chondritic planets underwent explosive breakup into asteroids. Lunar silicate rocks are poorer in Fe than achondrites, and this is explained in the model for the genesis of the Moon by the separation of a small metallic core, which sometime (at 3–4 Ga) induced the planet’s magnetic field. Iron from this core was involved into the generation of lunar depressions (lunar maria) filled with Fe- and Ti-rich rocks. In contrast to the parent planets of achondrites, the Moon has a olivine mantle, and this fact predetermined the isotopically heavier oxygen isotopic composition of lunar rocks. This effect also predetermined the specifics of the Earth’s rocks, whose oxygen became systematically isotopically heavier from the Precambrian to Paleozoic and Mesozoic in the course of olivinization of the peridotite mantle, a processes that formed the so-called roots of continents.     

Formation of the Bencubbin polymict meteoritic breccia

The Bencubbin meteorite is a polymict breccia consisting of a host fraction of ~60% metal and ~40% ferromagnesian silicates and a selection of carbonaceous, ordinary and ‘enstatite’ chondritic clasts. Concentrations of 27 elements were determined by neutron activation in replicate samples of the host silicates and the ordinary and carbonaceous chondritic clasts; 12 elements were determined in the host metal. Compositional data for the ordinary chondrite clast indicate a classification of LL4 ± 1. Refractory element data for the carbonaceous chondrite clast indicate that it belongs to the CI-CM-CO clan; its volatile element abundances are intermediate between those of CM and CO chondrites. Abundances of nonvolatile elements in the silicate host are similar to those in the carbonaceous chondrite clast and in CM chondrites; the rare earths are unfractionated. We conclude that it is not achondritic as previously designated, but chondritic and that it is probably related to the CI-CM-CO clan; its volatile abundances are lower than those in CO chondrites. Oxygen isotope data are consistent with these classifications. Host metal in Bencubbin and in the closely related Weatherford meteorite has low abundances of moderately volatile siderophiles; among iron meteorite groups its nearest relative is group IIIF.We suggest that Bencubbin and Weatherford formed as a result of an impact event on a carbonaceous chondrite regolith. The impact generated an ‘instant magma’ that trapped and surrounded regolithic clasts to form the polymict breccia. The parent of this ‘magma’ was probably the regolith itself, perhaps mainly consisting of the so-called ‘enstatite’ chondrite materials. Accretion of such a variety of materials to a small parent body was probably only possible in the asteroid belt.     

太阳星云凝聚过程的岩石学模型：（Ⅱ）非球陨石、C1陨石及类C1陨石的凝聚成因和小行星区星云凝聚作用

综述了非球陨石（铁陨石，石铁陨石和无球粒陨石）在成分结构方面的非分异成因证据，推断其成因是：星云盘中心层中的星云发生气－液凝聚作用形成的熔滴，在较高温度下彼此合并形成了较大熔体，熔体固化后形成该类陨石母体。根据Ｃ１陨石不含球粒和其它成分特征，推断它们是星云只发生气－固凝聚作用的产物。对近年来新发现的一些特殊成分的碳质球粒陨石进行了综合分析，暂定名为类Ｃ１陨石。通过类Ｃ１陨石与其它球粒陨石及Ｃ１陨石成分结构特征的对比，推断它们是星云盘边缘层星云发生气－液－固和气－固联合凝聚作用，同时发生水化作用的产物。最后，在对所有陨石凝聚成因进行解释的基础上，建立了小行星区星云凝聚模型。     

26Al as a heat source for early melting of planetesimals: Results from isotopic studies of meteorites

Ion microprobe studies of magnesium isotopic composition in igneous components from several chondritic meteorites have been carried out to look for²⁶Mg excess that may be attributed to the presence of the now-extinct radionuclide²⁶Al(τ ∼ 1 Ma) at the time of formation of these objects. A positive evidence for the presence of²⁶Al in the analysed objects will strengthen its case as the primary heat source for the early thermal metamorphism/melting of meteorite parent bodies. Based on calculated temperature profiles inside chondritic objects of different sizes and initial²⁶Al/²⁷Al ratios, we have estimated the initial abundances of²⁶Al needed to provide the heat necessary for the wide range of thermal processing seen in various types of meteorites. The magnesium isotopic data obtained by us do not provide definitive evidence for the presence of²⁶Al at the time of formation of the analysed igneous phases in different chondritic meteorites. Experimental evidence for a planetary scale distribution of²⁶Al in the early solar system to serve as a significant heat source for the thermal metamorphism and melting of meteorite parent bodies (planetesimals) remains elusive.     

Parentage Identification of Differentiated Achondritic Meteorites by Hand‐held Energy Dispersive X‐Ray Fluorescence Spectrometry

We evaluate the potential of a hand‐held energy dispersive XRF spectrometer for the preliminary classification of non‐chondritic differentiated meteorites. The studied achondrites include nine lunar meteorites, seventeen Martian meteorites, five angrites and eighteen meteorites from asteroid 4 Vesta. Analytical precision and accuracy was tested on thirty‐nine terrestrial igneous rock slabs with a wide range of composition. Replicate analyses, performed on the studied meteorites, show that Fe/Mn values together with Si and Ca/K ratio can be used in the discrimination of different achondrite groups. Fusion crust's Fe/Mn values of meteorites from Vesta and Mars are indistinguishable from those of the interior implying that even measurements on the fusion‐crusted external surface could be sufficient to pigeonhole non‐chondritic meteorites. Hand‐held energy dispersive XRF spectrometer is a non‐destructive but very effective technique for preliminary classification of achondrites in the field and in laboratory and for the identification of mislabelled meteorites in museum collections.     

The compositional classification of chondrites: IV. Ungrouped chondritic meteorites and clasts

Six specimens of unusual chondritic materials were analyzed by neutron activation for 30 elements in order to assess their degree of chondritic compositional pristinity and to search for evidence of genetic links to other chondrites. Five have highly recrystallized textures; the other, the Cumberland Falls chondrite, has suffered minor metamorphic recrystallization. Acapulco and Allan Hills A77081, are closely related and have subpristine compositions; they are more distantly related to Enon which has an altered composition. Udei Station appears to be a IAB meteorite even though its $\text{FeO}\text{(FeO + MgO)}$ ratio is slightly above the IAB field. The highly weathered meteorite Tierra Bianca is closely related to IAB but has a δ¹⁸O value 5 standard deviations higher than the IAB mean and is designated ungrouped. Udei Station and Tierra Bianca have altered compositions; rare earth element patterns indicate loss of a phosphate phase. The elemental composition of the Cumberland Falls chondrite is virtually identical to that of LL chondrites and its O-isotope composition is closely similar to those of some unequilibrated ordinary chondrites including LL Semarkona. The $\text{FeO}\text{(FeO + MgO)}$ ratios in its olivine are generally much lower than those in pyroxene, a relationship we show to be indicative of in situ reduction resulting from exchange with the aubritic host. The names winonaites and forsterite chondrites have no taxonomic utility.     

Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies

A number of distinct methodologies are available for determining the oxygen isotope composition of minerals and rocks, these include laser-assisted fluorination, secondary ion mass spectrometry (SIMS) and UV laser ablation. In this review we focus on laser-assisted fluorination, which currently achieves the highest levels of precision available for oxygen isotope analysis. In particular, we examine how results using this method have furthered our understanding of early-formed differentiated meteorites. Due to its rapid reaction times and low blank levels, laser-assisted fluorination has now largely superseded the conventional externally-heated Ni “bomb” technique for bulk analysis. Unlike UV laser ablation and SIMS analysis, laser-assisted fluorination is not capable of focused spot analysis. While laser fluorination is now a mature technology, further analytical improvements are possible via refinements to the construction of sample chambers, clean-up lines and the use of ultra-high resolution mass spectrometers.High-precision oxygen isotope analysis has proved to be a particularly powerful technique for investigating the formation and evolution of early-formed differentiated asteroids and has provided unique insights into the interrelationships between various groups of achondrites. A clear example of this is seen in samples that lie close to the terrestrial fractionation line (TFL). Based on the data from conventional oxygen isotope analysis, it was suggested that the main-group pallasites, the howardite eucrite diogenite suite (HEDs) and mesosiderites could all be derived from a single common parent body. However, high precision analysis demonstrates that main-group pallasites have a Δ¹⁷O composition that is fully resolvable from that of the HEDs and mesosiderites, indicating the involvement of at least two parent bodies. The range of Δ¹⁷O values exhibited by an achondrite group provides a useful means of assessing the extent to which their parent body underwent melting and isotopic homogenization. Oxygen isotope analysis can also highlight relationships between ungrouped achondrites and the more well-populated groups. A clear example of this is the proposed link between the evolved GRA 06128/9 meteorites and the brachinites.The evidence from oxygen isotopes, in conjunction with that from other techniques, indicates that we have samples from approximately 110 asteroidal parent bodies (∼60 irons, ∼35 achondrites and stony-iron, and ∼15 chondrites) in our global meteorite collection. However, compared to the likely size of the original protoplanetary asteroid population, this is an extremely low value. In addition, almost all of the differentiated samples (achondrites, stony-iron and irons) are derived from parent bodies that were highly disrupted early in their evolution.High-precision oxygen isotope analysis of achondrites provides some important insights into the origin of mass-independent variation in the early Solar System. In particular, the evidence from various primitive achondrite groups indicates that both the slope 1 (Y&R) and CCAM lines are of primordial significance. Δ¹⁷O differences between water ice and silicate-rich solids were probably the initial source of the slope 1 anomaly. These phases most likely acquired their isotopic composition as a result of UV photo-dissociation of CO that took place either in the early solar nebula or precursor giant molecular cloud. Such small-scale isotopic heterogeneities were propagated into larger-sized bodies, such as asteroids and planets, as a result of early Solar System processes, including dehydration, aqueous alteration, melting and collisional interactions.There is increasing evidence that chondritic parent bodies accreted relatively late compared to achondritic asteroids. This may account for the fact that apart from a few notable exceptions’ such as the aubrite-enstatite chondrite association, known chondrite groups could not have been the parents to the main achondrite groups.     

Noble gases in the unique Kaidun meteorite

Two examined fragments of the Kaidun meteorite principally differ in the concentrations of isotopes of noble gases and are very heterogeneous in terms of the isotopic composition of the gases. Because these fragments belong to two basically different types of meteoritic material (EL and CR chondrites), these characteristics of noble gases could be caused by differences in the cosmochemical histories of the fragments before their incorporation into the parent asteroid. As follows from the escape kinetics of all gases, atoms of trapped and cosmogenic noble gases are contained mostly in the structures of two carrier minerals in the samples. The concentrations and proportions of the concentrations of various primary noble gases in the examined fragments of Kaidun are obviously unusual compared to data on most currently known EL and CR meteorites. In contrast to EL and CR meteorites, which contain the primary component of mostly solar provenance, the elemental ratios and isotopic composition of Ne and He in the fragments of Kaidun correspond to those typical of the primary components of A and Q planetary gases. This testifies to the unique conditions under which the bulk of the noble gases were trapped from the early protoplanetary nebula. The apparent cosmic-ray age of both of the Kaidun fragments calculated based on cosmogenic isotopes from ³He to ¹²⁶Xe varies from 0.027 to 246 Ma as a result of the escape of much cosmogenic isotopes at relatively low temperatures. The extrapolated cosmic-ray age of the Kaidun meteorite, calculated from the concentrations of cosmogenic isotopes of noble gases, is as old as a few billion years, which suggests that the material of the Kaidun meteorite could be irradiated for billions of years when residing in an unusual parent body.     

Chondritic lithic clasts in the CB/CH-like meteorite Isheyevo: Fragments of previously unsampled parent bodies

The CB/CH-like chondrite Isheyevo is characterized by the absence of fine-grained interchondrule matrix material; the only present fine-grained material is found as chondritic lithic clasts. In contrast to the pristine high-temperature components of Isheyevo, these clasts experienced extensive aqueous alteration in an asteroidal setting. Hence, the clasts are foreign objects that either accreted together with the high-temperature components or were added later to the final Isheyevo parent body during regolith gardening. In order to constrain the origin and secondary alteration of the clasts in Isheyevo, we studied their mineralogy, petrography, structural order of the polyaromatic carbonaceous matter, and oxygen isotopic compositions of carbonates. Three main groups of clasts were defined based on mineralogy and petrology. Group I clasts consist of phyllosilicates, carbonates, magnetite, and lath-shaped Fe,Ni-sulfides. Group II clasts contain different abundances of anhydrous silicates embedded in a hydrated matrix; sulfides, magnetite, and carbonates are rare. With only a few exceptions, groups I and II clasts did not experienced significant thermal metamorphism. Group III clasts are characterized by the absence of magnetite and the presence of Fe,Ni-metal. In addition to aqueous alteration, they experienced thermal metamorphism as reflected by the structure of their polyaromatic carbonaceous matter. While there are some similarities between the Isheyevo clasts, CI chondrites, and the matrices of CM and CR chondrites, on the whole, the characteristics of the clasts do not match those of any of these aqueously altered meteorite classes. Nor do they match those of similar material in various types of chondritic clasts present in other meteorite groups. We conclude that the Isheyevo clasts represent fragments of previously unsampled parent bodies.     

Comments on the paper ‘Subduction of a fossil slow–ultraslow spreading ocean: a petrology-constrained geodynamic model based on the Voltri Massif,Ligurian Alps,NW Italy’ by G. B. Piccardo

Analyses were made of samples of the several classes of iron meteorites: (hexahedrites, octahedrites, ataxites, and troilite inclusions) in further study of the isotopic composition of primordial lead and toward establishing correlation between the distribution of lead among the mineral inclusions and the nickel-iron mass of the meteorite. Two groups of iron meteorites can be distinguished on the basis of isotopic composition lead suggesting two ages for the parent bodies of common iron meteorites. The distribution of lead in iron meteorites ranges markedly but no relation could be found between isotopic composition of lead and the several structures and compositions. The content of lead in troilites are one or two orders of magnitude higher than in the nickel-iron phase.-- M. Russell.     

Cosmic ray exposure age and time of formation of a new meteorite,Dhofar 007 achondrite from Oman: A rock fragment from the asteroid Vesta

The isotopic composition of noble gases was investigated in the Dhofar 007 meteorite. Petrographic and mineralogical observations suggested that it is a brecciated cumulate eucrite with high contents of siderophile elements. The concentrations of noble gases in Dhofar 007 are identical to those of other eucrites. Its cosmic ray exposure age was estimated as 11.8 ± 0.8 Ma, which coincides with a maximum on the histogram of comic ray exposure ages of eucrite meteorites. It can be supposed that, similar to other eucrites, Dhofar 007 was ejected from the surface of their parent body (presumably, asteroid Vesta) about 12.0 Ma ago. The crystallization age of the Dhofar 007 eucrite was estimated from the ratio of plutonogenic Xe to Nd as 4476 ± 22 Ma. The potassium-argon age is much younger, 3.7–4.1 Ga, which indicates partial loss of radiogenic argon during the history of the meteorite, most likely related to impact metamorphic events.     

The role of S in the evolution of the parental cores of the iron meteorites

Most iron meteorites presumably formed from the cores of parent bodies having more or less chondritic bulk compositions. Consideration of the behavior of S during condensation and core formation indicates that these cores, at least in the case of groups having high or moderate volatile contents (IIAB, IIIAB), contained a substantial amount of S. When elemental fractionations observed in these iron meteorite groups are compared to model calculations of fractional crystallization it becomes evident that at least the IIAB parent melt, and very likely the IIIAB parent melt as well, did not contain the full S complement of the parent body. We consider three possible scenarios to account for the S depletion: (1) Outgassing of S during parent body differentiation; this was probably only possible if the parent body contained organic material, which is improbable for IIIAB. (2) Liquid immiscibility. Our fractional crystallization model would predict curved log Xvs. log Ni relationships in this case, which for many elements are not observed. (3) Formation of metastable liquid layers by episodic melting during core formation. This is based on the fact that the difference in melting temperature between a FeFeS eutectic and FeNi metals is ～500 K. Two melting episodes would tend to form distinct liquid layers that maintain their identities over the crystallization lifetime of the core.Solidification of the cores parental to the main iron meteorite groups should also produce a significant number of sulfide meteorites. The scarcity of sulfide-rich meteorites can be attributed to their lower mechanical resistance to space attrition, higher ablation during atmospheric passage, and faster weathering on earth.     

The stable carbon isotopes in enstatite chondrites and Cumberland Falls

The carbon isotopic composition of the total carbon in the enstatite chondrites Indarch, Abee, St. Marks, Pillistfer, Hvittis and Daniel's Kuil and the enstatite achondrite Cumberland Falls has been measured. The empirical relationhip between carbon isotopic composition and total carbon content is distinct from that of carbonaceous and ordinary chondrites. Within the enstatite chondrite group the average ¹³C content increases with petrographic type: E4 < E5 < E6. Daniel's Kuil shows the largest ¹³C enrichment in the bulk carbon of any meteorite. The carbon isotopic composition is most clearly correlated with the abundance of the elements Zn, Cd and In. Insofar as these elements may hold the key to the understanding of enstatite chondrites, more detailed combined carbon isotope and trace element studies of these meteorites will play an important role in the deciphering of their history.     

玄武质陨石NWA8545的岩石地球化学特征以及对其母体岩浆过程的启示

NWA 8545 是一块玄武质无球粒陨石,它与碳质陨石(CC)NWA 011 成对.CC 被认为是来自于外太阳系的一类物质,由于同位素异常,它们区别于来自内太阳系的非碳质陨石(NC).NWA 011 及其成对陨石作为CC中稀有的玄武质无球粒陨石,其记录的岩浆过程可以被用来研究外太阳系早期行星母体的岩浆活动.本文利用扫描电镜、电子探针和激光剥蚀电感耦合等离子质谱仪(LA-ICP-MS)对NWA 8545 中的辉石、斜长石和陨磷钙钠石进行岩相学以及原位主微量元素的分析,并根据矿物模式丰度计算全岩稀土元素含量.电子探针结果显示NWA 8545 与 HED族陨石Eucrite(钙长辉长无球粒陨石)具有相似的主量元素特征,同时其岩相学与5 型Eucrite类似.激光微量数据表明辉石、斜长石和陨磷钙钠石的稀土元素配分都表现出略微的Ce异常,但其辉石的Ba、Sr等元素并未出现明显的富集现象,即该陨石受地球风化作用影响不明显.利用辉石和斜长石的稀土元素含量,计算平衡熔体的成分,显示其平衡熔体的成分都与全岩的成分比较接近,可以认为两者是在封闭的体系下接近同时结晶.结合变质过程和母体岩浆的成分,本文认为NWA 8545 是由其母体岩浆在经历分离结晶过程后喷发到母体行星表面冷却形成的.     

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 $\text{Pu}\text{U}$ 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.