Surface Of Parent Body Research Articles

Geochemical evidence for the origin of the IIE parent body from H chondrite‐like material

AbstractIsotopic compositions of O, Mo, and Cu in the IIE iron meteorites have indicated a close affinity to the H chondrite group. The diversity of trace element compositions and their abundance of silicate inclusions indicate that IIE iron meteorites were formed in multistage processes. To better constrain the formation of the IIE irons, this study analyzed elemental abundances in the metal of five IIE irons (Elga, Miles, Tobychan, Verkhne Dnieprovsk, and Watson) by laser ablation inductively coupled plasma mass spectrometry. The data are interpreted in terms of a new model of IIE crystallization from the metal fraction of completely molten H chondrite‐like material based on the solid/liquid distribution coefficients of siderophile and chalcophile elements changing simultaneously with changes of S concentrations in the remaining liquid during the crystallization of the Fe,Ni phase in the Fe‐Ni‐S system. The model showed that IIE iron compositions could be produced as solid phases at 40–73 wt% of fractional crystallization of the metal component of a bulk H chondrite‐like metallic melt. We propose that IIE iron metal could have originated from the solidified core of a differentiated body of H chondrite‐like composition and sampled different fractions of that core exposed during a catastrophic disruption of the body. The present structure of metal and silicate inclusions of IIE irons was formed by remelting and metal–silicate mixing during late impact event(s) on the parent body surface.

Aubrite Pesyanoe: Clues to composition and evolution of the enstatite achondrite parent body

AbstractThe Pesyanoe aubrite is an essentially polymict regolith breccia comprised by fragments of different highly magnesian pyroxenitic lithologies: albite; anorthoclase and labradorite‐bearing pyroxenites; diopside and magnesian augite pyroxenites; roedderite‐ and forsterite‐bearing pyroxenites; and impact glasses; porphyritic and melt matrix breccia fragments; FeO‐rich chondritic inclusions; and exotic oxidized clasts. The parent magma of Pesyanoe probably was carbon saturated, as suggested by pyroxenite fragments containing igneous‐textured carbon phases, possibly graphite. The composition of feldspar and trapped melt inclusions in enstatite indicates occurrence of at least three metaluminous melt sources with different (K + Na)/Al and K/(K + Na) atomic ratios on the Pesyanoe parent body and has records of K and Na loss from the melt, possibly due to evaporation from the parent body surface. The roedderite‐ and forsterite‐bearing rocks probably crystallized from a peralkaline melt. We propose that peralkaline melt could be formed from a metaluminous melt(‐s) due to gravitational segregation of djerfisherite‐bearing metal‐sulfide liquid in the lower horizon of the magma chamber and following oxidation of the magma. This should lead to enrichment of silicate melt in K2O and Na2O and increasing of (K + Na)/Al > 1, allowing forsterite and roedderite to crystallize. Rocks enriched in K and containing rare K‐bearing minerals were found among both magmatic and melt rocks. This may imply an insignificant role of regolith transport in the process of the breccia’s formation and, therefore, an origin of all of the breccia components from a local region of the Pesyanoe parent body, probably from a single complex igneous massif.

Cathodoluminescence color zonation in the Antarctic meteorite (enstatite chondrite) of Yamato 86004

AbstractEnstatite in Yamato 86004 classified as EH melt rock shows cathodoluminescence (CL) zonation as arranged in a concentric pattern from within outward blue, light blue, red and non-CL areas (fusion crust). The zonation observed in the meteorite results from different distribution ratio of the enstatite with various CL colors. CL spectra of the enstatite have two broad emission bands at around 400 nm in the blue region and at around 670 nm in a red region. The emission components obtained by a spectral deconvolution can be assigned to three defect centers (2.73, 3.13–3.15 and 3.77 eV) in a blue region and to impurity centers of Cr3+ion (1.71 eV) and Mn2+ion (1.86–1.91 eV) in a red region. According to the CL related to structural defects in the enstatite, blue-CL enstatite might be originally formed from the melt by a quenching from the melt on the surface of parent body. The enstatite with light blue and red CL might be thermally altered from blue-CL enstatite with phase transitions during a flash heating when the meteorite passed through the atmosphere. Therefore, the color CL zonation reflects a thermal history recorded in the meteorite.

Datura family: the 2009 update

Context. Research of asteroid families has been recently refreshed by the discovery of very young ones. These families are of great interest because they represent the product of their parent body fragmentation before orbital and physical evolutionary processes can change them. A cluster of seven objects around the largest body (1270) Datura is of particular interest because it has enough known members and resides in the inner part of the main asteroid belt, facilitating observations. Aims. We carried out photometric observations of the two largest members of the Datura family – asteroids (1270) Datura and (90265) 2003 CL5 – with the goal of inferring their physical parameters. We also used numerous astrometric observations of Datura-family members in the past few years to revisit the age of this cluster. Methods. We performed numerous photometric observations of (1270) Datura over several oppositions. We then used the lightcurve inversion method to determine the spin state and shape of this asteroid. In the case of (90265) 2003 CL5, for which only limited lightcurve data have been acquired so far, we used Fourier analysis to determine the synodic rotation period during the 2008 apparition. We also used backward numerical integration of the improved orbits of Datura family members to reduce uncertainty in its age. Results. We determined the rotation state of (1270) Datura, the largest member of its own family. Its major properties are a short rotation period of ~3.36 h and small obliquity, which, however, exhibits ~ ± 15° excursions because of a forced Cassini state of the proper nodal frequency. Any possible initial non-principal rotation state has probably been damped and the asteroid rotates about the shortest axis of the inertia tensor. Its global shape, although convex in our representation, may reflect regions related to the excavation of the family members from the parent body surface. Interestingly, the second largest member of the Datura family – (90265) 2003 CL5 – appears to be very slow rotator with the rotation period ~24 h. The large amplitude of its rotation curve suggests that its shape is extremely elongated, possibly bi-lobed. Improved orbits of the family members allow us to re-determine the possible age of this family. We find an age that is slightly older than previously reported. Using a conservative approach, we obtain an age in the 450 to 600 kyr range. With strengthened, but plausible, conditions, we find that the current data may support an age of 530 ± 20 kyr. Further astrometric and photometric observations of the Datura cluster members are needed to determine its age more accurately.

Iodine-xenon analysis of Chainpur (LL3.4) chondrules

We present I-Xe analyses of ten chondrules from Chainpur LL3.4 by IR laser-stepped heating. Five chondrules provided isochrons of varying quality, giving a range of ages from 0.5 Ma before Shallowater to 17.8 after Shallowater. This confirms the extended range of Chainpur chondrule ages determined by previous data. We discuss evidence for fluid alteration, shock, and thermal events in explaining the chondrule ages and suggest that chondrule remelting events, presumably from bombardment of the parent body surface, are responsible for resetting the I-Xe chronometer. Previous data show a negative correlation between 132Xe/ 129Xe of the trapped Xe component and 127I/ 129I of an initial iodine component. This behaviour that requires the presence of a component with trapped 129Xe/ 132Xe lower than the planetary value has been cited as evidence for closed system evolution of the I-Xe system. We find no evidence of an unambiguous trapped component lower than planetary and no evidence of a negative correlation in our data. Therefore we suggest that open system behaviour more suitably explains the I-Xe systematics of Chainpur chondrules.

Attogram dust grains in the solar system

In this paper we conceptually develop the theory of attogram dust grain formation by the cosmic-ray ion-enhanced brittle microerosion of atmosphereless solar system bodies. According to our theory, the multicharged cosmic ray ions penetrating into the solid material of planetary satellites, asteroids, comets and smaller space bodies give rise to microscopic cylindrical overheated zones and generate mechanical impulses and thermoelastic stresses exceeding the material tensile strength. The impulses may cause micro-scale shattering of the surfaces and the ejection of ultrafine (attogram) dust particles off the parent body. The attogram grains formed by this mechanism (the so-called track-breaking mechanism) have an average mass of 10-19–10-17 g, a very flaky shape and ejection velocities of from tens to hundreds . Depending on the ejection velocity and a large array of non-gravitational forces, such grains can deposit and accumulate on the parent body surface, orbit around the body thus forming a bound dust cocoon, or finally, escape the body as ultrafine-dust wind. Quantitative data on dust contributions to these varieties of attogram population depending on the sizes and parameters of parent body material are obtained and discussed. The volumetric track-breaking resulting in attogram grain deposition in the ice-rock mixture of cometary nuclei during the whole period of their formation is analysed. Taking the example of Halley's comet we demonstrate that the mechanism under study makes it possible to quantitatively explain the in situ measured considerable excess of attogram grains within the cometary environment in its circum-solar path sections. The problem of track-breaking disintegration of small-sized bodies in the Kuiper Belt, which are permanently subjected to a cosmic-ray ion-enhanced brittle microerosion heavier than in the Main Belt, is investigated. The track-breaking survival timescale for submicron and over hundred micron-sized grains is found to be considerably smaller than the escape timescale of the Poynting-Robertson and plasma drags, as well as the collisional mechanism. Track-breaking can also be competitive to the collisional escape in the range of grain sizes of 1 to 100 microns. A quantitative analysis of the calculations shows that the present theory may adequately explain both the observable micron/submicron dust grain depletion of the Kuiper Belt and the loss of its bulk mass during evolution. Some possible observational tests for the future revealing the predicted populations of attogram dust in the solar system are also discussed.

Chromite-plagioclase assemblages as a new shock indicator; implications for the shock and thermal histories of ordinary chondrites

Chromite in ordinary chondrites (OC) can be used as a shock indicator. A survey of 76 equilibrated H, L and LL chondrites shows that unshocked chromite grains occur in equant, subhedral and rounded morphologies surrounded by silicate or intergrown with metallic Fe-Ni and/or troilite. Some unmelted chromite grains are fractured or crushed during whole-rock brecciation. Others are transected by opaque veins; the veins form when impacts cause localized heating of metal-troilite intergrowths above the Fe-FeS eutectic (988°C), mobilization of metal-troilite melts, and penetration of the melt into fractures in chromite grains. Chromite-plagioclase assemblages occur in nearly every shock-stage S3-S6 OC; the assemblages range in size from 20–300 μm and consist of 0.2–20-μm-size euhedral, subhedral, anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition. Plagioclase has a low impedance to shock compression. Heat from shock-melted plagioclase caused adjacent chromite grains to melt; chromite grains crystallized from this melt. Those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in Al 2O 3 than unmelted, matrix chromite grains in the same meteorite. Chromite veinlets (typically 0.5–2 μm thick and 10–300 μm long) occur typically in the vicinity of chromite-plagioclase assemblages. The veinlets formed from chromite-plagioclase melts that were injected into fractures in neighboring silicate grains; chromite crystallized in the fractures and the residual plagioclase-rich melt continued to flow, eventually pooling to form plagioclase-rich melt pockets. Chromite-rich “chondrules” (consisting mainly of olivine, plagioclase-normative mesostasis, and 5–15 vol.% chromite) occur in many shocked OC and OC regolith breccias but they are absent from primitive type-3 OC. They may have formed by impact melting chromite, plagioclase and adjacent mafic silicates during higher-energy shock events. The melt was jetted from the impact site and formed droplets due to surface tension. Crystallization of these droplets may have commenced in flight, prior to landing on the parent-body surface. Chromite-plagioclase assemblages and chromite veinlets occur in 25 out of 25 shock-stage S1 OC of petrologic type 5 and 6 that I examined. Although these rocks contain unstrained olivine with sharp optical extinction, most possess other shock indicators such as extensive silicate darkening, numerous occurrences of metallic Cu, polycrystalline troilite, and opaque veins. It seems likely that these rocks were shocked to levels at least as high as shock-stage S3 and then annealed by heat generated during the shock event. During annealing, the olivine crystal lattices healed but other shock indicators survived. Published Ar-Ar age data for some S1 OC indicate that many shock and annealing events occurred very early in the history of the parent asteroids. The common occurrence of shocked and annealed OC is consistent with collisions being a major mechanism responsible for metamorphosing OC.

39Ar‐40Ar ages of eucrites and thermal history of asteroid 4 Vesta

Abstract— Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period.To further understand the complex HED parent body thermal history, we determined new 39Ar‐40Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic‐brecciated eucrites. Precise Ar‐Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar‐Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb‐Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4–4.515 Gyr, and 147Sm‐143Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41–4.55 Gyr. Similar Ar‐Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K‐Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar‐Ar ages at ˜4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ˜4.48 Gyr ago, which quickly cooled the samples and started the K‐Ar chronometer. A large (˜460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb‐Pb and Sm‐Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar‐Ar age of 4.48 Gyr, and the few older Pb‐Pb and Sm‐Nd ages may reflect an isotopic closure before the large cratering event.One cumulate eucrite gives an Ar‐Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar‐Ar ages of 3.4–3.7 Gyr; and 2 unbrecciated eucrites give Ar‐Ar ages of ˜3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar‐Ar impact‐reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5–4.1 Gyr. Among these, Piplia Kalan, the first eucrite to show evidence for extinct 26Al, was strongly impact heated ˜3.5 Gyr ago. When these data are combined with eucrite Ar‐Ar ages in the literature, they confirm that several large impact heating events occurred on Vesta between ˜4.1–3.4 Gyr ago. The onset of major impact heating may have occurred at similar times for both Vesta and the moon, but impact heating appears to have persisted for a somewhat later time on Vesta.

Classification of mafic clasts from mesosiderites: Implications for endogenous igneous processes

We have analyzed thirteen igneous pebbles from the Vaca Muerta, EET87500 and Bondoc mesosiderites by electron microprobe and instrumental neutron activation and combined these data with literature data for forty-three analyzed mesosiderite clasts. We classify these well-characterized clasts into the following five principal groups: 1. 1) Polygenic and monogenic cumulates (39%) are coarse-grained gabbros that are highly depleted in incompatible elements (relative to H chondrites); they formed at moderate depth either as residues of low-degree partial melting of pre-existing cumulate eucrites or as cumulates from parent melts similar to cumulate eucrites. 2. 2) Polygenic basalts (30%) are finer-grained rocks with positive europium anomalies, La/Lu ratios < 1, and lower rare earth element abundances than basaltic eucrites. It seems likely that these rocks were formed near their parent body surface by remelting mixtures of major amounts of basaltic eucrites and lesser amounts of cumulate eucrites. 3. 3) Quench-textured rocks comprise two compositional groups, (a) those which resemble basaltic eucrites (5%) and (b) those which resemble cumulate eucrites (2%). The quench-textured rocks are probably monogenic; they formed most likely when small-scale impacts at their parent body surface totally melted small amounts of basaltic or cumulate eucrite material. 4. 4) Monogenic basalts (11%) resemble basaltic eucrites and formed by endogenous igneous processes on the mesosiderite parent body (MPB). The monogenic basalts and those quench-textured rocks that resemble basaltic eucrites in composition plot subparallel to the Nuevo Laredo trend eucrites on diagrams of incompatible element concentration vs. mg ∗. Previous work has shown that the Nuevo-Laredo trend eucrites were probably produced by fractional crystallization; it seems likely that the monogenic basalts from the MPB had a similar origin. 5. 5) Ultramafic rocks are cumulates consisting mainly of large crystals of orthopyroxene (9%) or olivine (4%). Orthopyroxenite clasts closely resemble diogenites and were formed most likely by endogenous igneous processes. Olivine crystals vary significantly in major element composition; some may be endogenous, but others may have been derived from the disrupted mantle of a different parent body.

Petrogenesis of mesosiderites: I. origin of mafic lithologies and comparison with basaltic achondrites

New petrologic and trace element data on basaltic and gabbroic clasts in mesosiderites and basaltic achondrites, combined with existing petrologic and trace element data, have served as a basis for interpretation of the petrogenesis of mesosiderite clasts. Compared with the basaltic achondrites, the mesosiderite basaltic and gabbroic clasts contain more abundant modal tridymite and merrillite, and more commonly contain augite as a late magmatic phase. Their pyroxenes tend to be more MgO-rich, and have lower Fe/Mn ratios which are positively correlated with the Fe/Mg ratio. Some of the mesosiderite basaltic and gabbroic clasts contain xenocrystic plagioclase. The basaltic clasts commonly show super-chondritic Eu/Sm ratios and slight LREE depletions. The cumulate gabbro clasts are extremely depleted in LIL, have very high Eu/Sm ratios and high Lu/Sm ratios. All of these features can be explained by a model in which the mafic lithologies of mesosiderites were formed by remelting of a mixed basalt-cumulate gabbro-metal source region near the parent body surface. Reducing agents (P, C?, S?) from the metallic fraction reduce FeO out of the silicate melt and result in the positive correlation of Fe/Mn with Fe/Mg observed in pyroxene analyses and the modal abundances of tridymite, merrillite, and augite. Xenocrystic plagioclase is a natural consequence of remelting a cumulate gabbro-rich source, as are the unique REE patterns observed.

Isotopic composition of carbonate in EETA 79001 and its relation to parent body volatiles

Calcite, a probable extraterrestrial weathering product in the shergottite EETA 79001, has δ 13 C = +9.7%. and δ 18 O = +21.0%.. In order to account for the large excess of 18O relative to the silicate substrate ( δ 18 O = +4.2%.), a parent body surface reservoir containing both water and carbon dioxide is required. The inferred H 2O CO 2 ratio is consistent with estimates for the degassed reservoir on Mars.

A STUDY ON COSMOGENIC NUCLIDES IN JILIN METEORITE AND ITS TWO-STAGE IRRADIATION HISTORY

Forty-four burk samples of the H chondrite Jilin have been analysed for their contents ofspallogenic, trapped and radiogenic noble gases, and thirty-five samples have been determinedfor their radiocosmogenic nuclides. The Jilin meteorite has an extreme complicated history ofcosmic-ray irradiation. In the 1st-stage, the Jilin meteorite might be located between 0.2--2mbelow the surface of the parent body travelling about 8 m. y. in space and under a geometryof 2π subjected to the radiation of cosmic-ray. According to the data of ~(60)Co dating the burialdepth of all samples in the 2nd-stage can be determined and the preatmospheric form andsize of the parent body can be restored (under a geometry of 4π). The result of this recon-struction is that, the 2nd-stage has a nearly spheric form, with a radius of 85±5 cm, weighing9.7±1.7 Ts. The position of the center of the preatmospheric body is: X=55, Y=55 andZ=39. Jilin meteorite No.1 is located at the center of the parent body and is surroundedwith the fragments. The ablation rate is estimated at about 30%. We have proposed a new modelof the two-stage irradiation, and based on the contents of ~(21)Ne-~(60)Co in the samples, a classificationscheme can be worked out for the restoration of relative position of the samples within theparent body.

Impact melt‐rock clasts in the Hvittis Enstatite chondrite breccia: Implications for a genetic relationship between El chondrites and aubrites

The Hvittis enstatite chondrite breccia contains 5±2 vol% dark clasts which appear to be impact melt‐rocks. The largest clast (Clast A) is 2.5 cm in diameter; it has an igneous texture, no chondrules, and is depleted in metallic Fe, Ni and sulfide. Its fine grain size, low‐Ni kamacite and Schreibersite, and rare martensite resulted from rapid cooling. The abundance of melt‐rock clasts in Hvittis suggests that the breccia resided at its parent body surface. However, the apparent absence of EL chondrites with solar wind‐implanted rare gases suggests that the EL parent body was too small to develop a substantial regolith. The bulk composition of Clast A is very similar to that of average aubrites, except for a sevenfold enrichment in S, Cr, and Mn. These elements are concentrated in the sulfides. It thus is possible that the clast was derived from one of the sulfide‐rich regions of the Hvittis matrix. If so, then aubrites may be genetically related to EL chondrites.

Metamorphic reactions in mesosiderites: origin of abundant phosphate and silica

The high modal abundances of merrillite [Ca 3(PO 4) 2] and tridymite in most mesosiderites are not the result of igneous fractionation but are attributed to redox reactions between silicates and P-bearing Fe-Ni metal within a limited T-fO 2 range at low pressure. The Emery mesosiderite is the most tridymite- and merrillite-rich mesosiderite so it is used as the model for this study. Examination of reactions in the system CaO-SiO 2-MgSiO 3-Fe-P-O indicate that essentially all of the present phosphorus in Emery should have been dissolved in the metallic portion (calculated to have contained 0.65 wt% P originally), and that it largely reacted to form phosphate. The thermodynamic calculations predict that the reactions would have occurred between 970°C, log fO 2 = −16.5 and 1030°C, log fO 2 = −15.0 for the range of phase compositions in Emery. A narrower range of conditions is expected for other mesosiderites. Phosphide (schreibersite) formed only later at temperatures < 600°C during slow cooling. The recalculated amounts of dissolved P and S in the metallic portion of Emery reduce the temperature of the metal liquidus to < 1350°C and the solidus to < 800°C. Thus mixing of liquid metal with cold silicates near the parent body's surface would not have resulted in substantial melting of the silicates but would have resulted in their metamorphism, which is consistent with the textural relationships observed in Emery. This scenario of redox reactions and redistributions of components between metal and silicates represents a new insight into the complexities of mesosiderite processing that helps unravel the mesosiderite history and recalculate their original components.

Metal and Bi/Pb microdistribution studies of an L3 chondrite: their implications for a meteorite parent body

We find strong localizations (relative to bulk) of Bi and to a lesser extent Pb. in some of the kamacite grains in Khohar. Other kamacite grains show no such enrichments. There are distinctive and correlated differences in the Ni contents of the two kamacite populations, with the Bi/Pb-rich kamacite grains having consistently lower Ni levels (sometimes unusually low. ~ 2% Ni) than the Bi/Pb-poor kamacite, which typically have ~ 6–7% Ni. The Bi/Pb-rich kamacite grains are also distinguished on the basis of their etching behavior, exhibiting a highly reactive attack, which has not been observed previously and which we believe may be due to the fact that the Bi/Pb-rich kamacite is finely polycrystalline.We conclude that the trace element microdistributions were not established in the nebula. Nor is it likely that the enrichments occurred with slow cooling in the presence of a vapor phase during the kamacite-taenite phase transition. Rather, the Bi/Pb-rich kamacite most likely reflect the occurrence of a brief reheating episode (or episodes), which may have been shock-induced and which was followed by rapid cooling. We find fine-grained metal-sulfide intergrowths which testify to such a reheating event, and one likely candidate for the site of this event is a hot ejecta blanket at the parent body surface. Iron oxides are found in our Khohar sections. We believe that they are not due to terrestrial alteration, that they are magnetite and that the magnetite probably originated in the same dynamic event in which the Bi/Pb distributions were established. The present data do not allow us to confidently determine whether the event occurred prior to, during, or after the compaction of this meteorite, although the simplest interpretation of the data would indicate the first alternative. Bulk Bi data for Khohar has been used for inferring accretion temperatures and this now appears inappropriate.

History of the Pasamonte achondrite: Relative susceptibility of the Sm [sbnd]Nd, Rb [sbnd]Sr, and U [sbnd]Pb systems to metamorphic events

The Rb Sr, Sm Nd, and U Pb systematics of the eucrite Pasamonte have been studied in order to investigate the relative susceptibility of the different systems to post-crystallization events and to determine the age and history of the meteorite. The Rb Sr and 238U- 206Pb data of mineral separates do not define an isochron but the Sm Nd data define an internal isochron which corresponds to the formation age of 4.58 ± 0.12b.y. (10 9 years). The 207Pb- 206Pb data of mineral separates define an apparent age of 4.53 ± 0.03b.y., however we conclude that this age, while in agreement with the Sm Nd age, is not strictly valid since the U Pb data indicate at least three stages of evolution. The U Pb data indicate that the parent body of the meteorite experienced brecciation shortly after the formation of the parent body surface ( ∼4.2–4.45b.y. ago) and a recent disturbance (collision?) 6 ± 30m.y. ago. The latter age is within the range of cosmic ray exposure ages for achondrites.

The Tucson meteorite

Tucson is an unusual iron meteorite which contains highly reduced silicate inclusions and elemental silicon and chromium in solution. The metal matrix of Tucson was found to be chemically uniform, suggesting that homogenization has occurred at elevated temperatures. The microstructure of the metal consists of plessite and thin ribbons of kamacite. Nickel and phosphorus concentrations indicate that kamacite nucleated along prior taenite grain boundaries at ~650°C, and grew upon cooling to 500°C. Kamacite growth calculations show that Tucson cooled at ~1°C/1000 yr, a rate which corresponds to a depth of burial at the center of a 15 km radius parent body or closer to the surface of parent bodies of larger sizes. The shapes of the Tucson irons, and the presence and distribution of silicate inclusions in the Fe-Ni matrix appear to be a result of a solidification process.