Neodymium,strontium and chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the chronology of the angrite parent body

Abstract— Neodymium, strontium, and chromium isotopic studies of the LEW86010 angrite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar nebula. Pyroxene and phosphate were found to contain ～98% of its Sm and Nd inventory. A conventional ¹⁴⁷Sm-¹⁴³Nd isochron yielded an age of 4.53 ± 0.04 Ga (2 σ) and ?¹⁴³ _Nd = 0.45 ± 1.1. An ¹⁴⁶Sm-¹⁴²Nd isochron gives initial ¹⁴⁶Sm/¹⁴⁴Sm = 0.0076 ± 0.0009 and ?¹⁴³ _Nd = ?2.5 ± 0.4. The Rb-Sr analyses give initial ⁸⁷Sr/⁸⁶Sr (I⁸⁷_Sr) = 0.698972 ± 8 and 0.698970 ± 18 for LEW and ADOR, respectively, relative to ⁸⁷Sr/⁸⁶Sr = 0.71025 for NBS987. The difference, ΔI⁸⁷_Sr, between I⁸⁷_Sr for the angrites and literature values for Allende CAIs, corresponds to ～9 Ma of growth in a solar nebula with a CI chondrite value of ⁸⁷Rb/⁸⁶Sr = 0.91, or ～5 Ma in a nebula with solar photospheric ⁸⁷Rb/⁸⁶Sr = 1.51. Excess ⁵³Cr from extinct ⁵³Mn (t1/2 = 3.7 Ma) in LEW86010 corresponds to initial ⁵³Mn/⁵⁵Mn = 1.44 ± 0.07 × 10^?6 and closure to Cr isotopic homogenization 18.2 ± 1.7 Ma after formation of Allende inclusions, assuming initial ⁵³Mn/⁵⁵Mn = 4.4 ± 1.0 × 10^?5 for the inclusions as previously reported by the Paris group (Birck and Allegre, 1988). The ¹⁴⁶Sm/¹⁴⁴Sm value found for LEW86010 corresponds to solar system initial (¹⁴⁶Sm/¹⁴⁴Sm)_o = 0.0080 ± 0.0009 for crystallization 8 Ma after Allende, the difference between Pb-Pb ages of angrites and Allende, or 0.0086 ± 0.0009 for crystallization 18 Ma after Allende, using the Mn-Cr formation interval. The isotopic data are discussed in the context of a model in which an undifferentiated “chondritic” parent body formed from the solar nebula ～2 Ma after Allende CAIs and subsequently underwent differentiation accompanied by loss of volatiles. Parent bodies with Rb/Sr similar to that of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic systematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, ～2.6 Ma after the CAIs, to satisfy the Sr and Cr isotopic systematics.     

Impact histories of angrites,eucrites, and their parent bodies

Abstract– Eucrites, which are probably from 4 Vesta, and angrites are the two largest groups of basaltic meteorites from the asteroid belt. The parent body of the angrites is not known but it may have been comparable in size to Vesta as it retained basalts and had a core dynamo. Both bodies were melted early by ²⁶Al and formed basalts a few Myr after they accreted. Despite these similarities, the impact histories of the angrites and eucrites are very different: angrites are very largely unshocked and none are breccias, whereas most eucrites are breccias and many are shocked. We attribute the lack of shocked and unbrecciated angrites to an impact, possibly at 4558 Myr ago—the radiometric age of the younger angrites—that extracted the angrites from their original parent body into smaller bodies. These bodies, which may have had a diameter of approximately 10 km, suffered much less impact damage than Vesta during the late heavy bombardment because small bodies retain shocked rocks less efficiently than large ones and because large bodies suffer near‐catastrophic impacts that deposit vastly more impact energy per kg of target. Our proposed history for the angrites is comparable to that proposed by Bogard and Garrison (2003) for the unbrecciated eucrites with Ar‐Ar ages of 4.48 Gyr and that for unbrecciated eucrites with anomalous oxygen isotopic compositions that did not come from Vesta. We infer that the original parent bodies of the angrites and the anomalous eucrites were lost from the belt when the giant planets migrated and the total mass of asteroids was severely depleted. Alternatively, their parent bodies may have formed in the terrestrial planet region and fragments of these bodies were scattered out to the primordial Main Belt as a consequence of terrestrial planet formation.     

Evidence for a late thermal event of unequilibrated enstatite chondrites: A Rb-Sr study of Qingzhen and Yamato 6901 (EH3) and Khairpur (EL6)

Abstract— The Rb-Sr whole rock and internal systematics of two EH3 chondrites, Qingzhen and Yamato 6901, and of one EL6 chondrite, Khairpur, were determined. Sulfides were separated using a stepwise dissolution technique. The mineral species in each fraction were estimated based on the chemical analyses of 12 major elements. The internal Rb-Sr systematics of the EH3 chondrites are highly disturbed. Fractions corresponding to sulfide phases show excess ⁸⁷Sr, while other fractions corresponding to silicate phases produce a linear trend on a Rb-Sr evolution diagram. If these linear relations are interpreted as isochrons, the ages of the silicate phases are 2.12 ± 0.23 Ga and 2.05 ± 0.33 Ga with the initial Sr isotopic ratios of 0.7112 ± 0.0018 and 0.7089 ± 0.0032, for Qingzhen and Yamato 6901, respectively. The process of the isotopic disturbance probably involved the breakdown of the major K-bearing sulfide (djerfisherite), and a lack of isotopic exchange between sulfide and silicate phases indicates moderate temperatures of reheating. Although a complete Sr isotopic re-homogenization among silicate phases was not attained, we interpret the Rb-Sr results as indicative of a late thermal event about 2 Ga ago on the parent bodies of these EH3 chondrites. These ages agree well with previously published K-Ar ages. An older isochron age of 4.481 ± 0.036 Ga with a low initial Sr isotopic ratio of 0.69866 ± 0.00038 was obtained for the data from silicate fractions of Khairpur, indicating early petrological equilibration on the parent body of EL6 chondrites.     

147Sm‐143Nd and 176Lu‐176Hf systematics of eucrite and angrite meteorites

Comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present ¹⁴⁷Sm‐¹⁴³Nd and ¹⁷⁶Lu‐¹⁷⁶Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a ¹⁴⁷Sm‐¹⁴³Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous ¹⁴⁷Sm‐¹⁴³Nd and ¹⁷⁶Lu‐¹⁷⁶Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.     

Basaltic volcanism on the angrite parent body: Comparison with 4 Vesta

Carbon and nitrogen data from stepped combustion analysis of eight angrites, seven eucrites, and two diogenites, alongside literature data from a further nine eucrites and two diogenites, have been used to assess carbon and nitrogen incorporation and isotope fractionation processes on the angrite parent body (APB), for comparison with volatile behavior on the HED parent body (4 Vesta). A subset of the angrite data has been reported previously (Abernethy et al. 2013 ). Two separate families of volatile components were observed. They were (1) moderately volatile material (MVM), mostly combusting between ~500 and 750 °C and indistinguishable from terrestrial contamination and (2) refractory material (RM), mainly released above 750 °C and thought to be carbon (as ) and nitrogen (as N₂ or ) dissolved within the silicate lattice, fitting with the slightly oxidized (~IW to IW+2) angrite fO₂ conditions. Isotopic fractionation trends for carbon and nitrogen within the plutonic and basaltic (quenched) angrites suggest that the behavior of the two volatile elements is loosely coupled, but that the fractionation process differs between the two angrite subgroups. Comparison with results from eucrites and diogenites implies similarities between speciation of carbon and nitrogen on 4 Vesta and the APB, with the latter being more enriched in volatiles than the former.     

Samarium-neodymium and rubidium-strontium systematics of nakhlite Governador Valadares

Abstract— The Sm-Nd systematics of whole-rock and mineral separate samples from nakhlite Governador Valadares define a good ¹⁴⁷Sm-¹⁴³Nd mineral isochron age of 1.37 ± 0.02 Ga. This age is in excellent agreement with the ³⁹Ar-⁴⁰Ar and Rb-Sr ages obtained previously for this meteorite. However, the Rb-Sr isotopic data for our sample show that the isotopic system is disturbed. The lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases. The whole-rock and acid-washed mineral data yield a Rb-Sr age of 1.20 ± 0.05 Ga, which probably represents a lower limit to the crystallization age of the rock. The petrographic evidence indicates that this meteorite is a clinopyroxene cumulate that probably crystallized in a subsurface sill (McSween, 1994). Thus, the Sm-Nd isotopic age probably represents the age of such a magmatic event. The initial ε¹⁴³Nd value determined for the rock at 1.37 Ga is +17 ± 1, indicating that the parent magma of the rock came from a light-rare-earth-element-depleted source of ¹⁴⁷Sm/¹⁴⁴Nd = ～0.237 based on a simple two-stage evolution model. Results of the same model calculation for the initial ⁸⁷Sr/⁸⁶Sr ratio of the rock suggest that its source material was depleted in ⁸⁷Rb/⁸⁶Sr by ～50% relative to the estimated martian value at 1.37 Ga. Both the high Sm/Nd and low Rb/Sr values support a clinopyroxene-rich cumulate source for the genesis of the nakhlite Governador Valadares. Furthermore, our Sm-Nd age and ε¹⁴³Nd data and the previously published ε¹⁴²Nd datum for the rock (Harper et al., 1995) are consistent with early differentiation of the parent planet, formation of cumulate sources ～4.56 Ga ago, and late melting of the sources and formation of the rock ～1.37 Ga ago. The good agreement of isotopic ages and petrographic features among Governador Valadares, Nakhla, and Lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories. The nakhlite age data suggest that isotopic heterogeneity in the martian mantle sources existed up to ～1.37 Ga ago and early mantle structures probably have not been disturbed for a significant portion of martian history.     

Mn‐Cr isotope systematics of the D'Orbigny angrite

Abstract— We have conducted a detailed study of the Mn‐Cr systematics of the angrite D'Orbigny. Here, we report Cr isotopic abundances and Mn/Cr ratios in olivine, pyroxene, glass, chromite, and bulk rock samples from D'Orbigny. ⁵³Cr excesses in these samples correlate well with their respective Mn/Cr ratios and define an isochron with a slope that corresponds to an initial ⁵³Mn/⁵⁵Mn ratio = (3.24 ± 0.04) × 10^?6 and initial ⁵³Cr/⁵²Cr ratio of ?(53) = 0.30 ± 0.03 at the time of isotopic closure. The ⁵³Mn/⁵⁵Mn ratio of the D'Orbigny bulk rock is more than two‐fold the ⁵³Mn/⁵⁵Mn ratio of the angrites Lewis Cliff 86010 (LEW) and Angra dos Reis (ADOR) and implies an older Mn‐Cr age of 4562.9 ± 0.6 Ma for D'Orbigny relative to a Pb‐Pb age of 4557.8 ± 0.5 Ma for LEW and ADOR. One of the most unusual aspects of D'Orbigny is the presence of glass, a phase that has not been identified in any of the other angrites. The Mn‐Cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite. Since the Mn‐Cr age of D'Orbigny is ?5 Ma years older than the angrites LEW and ADOR, D'Orbigny likely represents an earlier stage in the evolution of the angrite parent body.     

Rb‐Sr and Sm‐Nd isotopic systematics of the lherzolitic shergottite GRV 99027

Abstract– Rb‐Sr and Sm‐Nd isotopic analyses of the lherzolitic shergottite Grove Mountains (GRV) 99027 are reported. GRV 99027 yields a Rb‐Sr mineral isochron age of 177 ± 5 (2σ) Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio (I_Sr) of 0.710364 ± 11 (2σ). Due to larger uncertainties of the Sm‐Nd isotopic data, no Sm‐Nd isochron age was obtained for GRV 99027. The ε¹⁴³Nd value is estimated approximately +12.2, assuming an age of 177 Ma. The I_Sr of GRV 99027 is distinguishable from other lherzolitic shergottites, confirming our previous conclusion that it is not paired with them ( Lin et al. 2005 ). The new data of GRV 99027 support the same age of approximately 180 Ma for most lherzolitic shergottites, and fill the small gap of I_Sr between Allan Hills A77005 and Lewis Cliff 88516 ( Borg et al. 2002 ). All available data are consistent with a single igneous source for the intermediate subgroup of lherzolitic shergottites.     

Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures

Noble gases in the five angrites Northwest Africa (NWA) 1296, 2999, 4590, 4801, and 4931 were analyzed with total melting and stepwise heating methods. The noble gases consist of in situ components: spallogenic, radiogenic, nucleogenic, and fission. Cosmic-ray exposure ages of the angrites (including literature data) spread uniformly from <0.2 to 56 Ma, and coarse-grained angrites have longer exposure ages than fine-grained angrites. It is implied that the parent bodies from which the two subgroups of angrites were ejected are different and have distinct orbital elements. The ²⁴⁴Pu-¹³⁶Xe relative ages of the angrites obtained by using ²⁴⁴Pu/¹⁵⁰Nd ratios are as old as that of Angra dos Reis, reflecting their early formation. On the other hand, another method to obtain ²⁴⁴Pu-¹³⁶Xe relative ages, using fission ¹³⁶Xe, spallogenic ¹²⁶Xe, and Ba/REE ratios, yields systematically older ²⁴⁴Pu-¹³⁶Xe ages than those obtained by using ²⁴⁴Pu/¹⁵⁰Nd ratios, which is explained by apparently high Ba/REE ratios caused by Ba contamination during terrestrial weathering. The ²⁴⁴Pu/²³⁸U ratio at 4.56 Ga of angrites is estimated as 0.0061 ± 0.0028, which is consistent with those for chondrules, chondrites, achondrites, and a terrestrial zircon. It is suggested that initial ²⁴⁴Pu/²³⁸U ratio has been spatially homogeneous at least in the inner part of the early solar system.     

Geochemistry and neodymium-strontium isotope signature of tektite-like objects from Siberia (urengoites,South-Ural glass)

Abstract— We report Sr-Nd isotope parameters, rare earth element (REE), and major element data for isolated findings of tektite-like objects from western Siberia (urengoites, South-Ural glass), as well as for two indochinites. The latter were recovered in Vietnam and their overall geochemical characteristics equal those of other tektites from the indochinite subgroup of the Australasian strewn field. The three urengoites (～24 Ma) are extremely silica-rich (89 to 96 wt% SiC₂), and their REE abundances vary between 45 and 76 ppm. With La_N/Yb_N ranging from 7.6 to 10.4 and Eu_N/EU* between 0.69 and 0.75, their REE distribution patterns match that of average upper crust. The urengoites have present-day ?_Sr of +155 to +174 and ?_Nd ranging from ?18 to ?23. Their model ages in million years are: T^SruR = 1200 up to 4060 and T^NdcHUR = 1570 up to 2070. Data points for the urengoites plot colinearly in the Rb-Sr evolution diagram. The age corresponding to the slope is 183 ± 30 Ma (2s?), which is indistinguishable from the intercept age of 211 Ma in the T^Sr_UR vs. l/f_Rb diagram. Rubidium-strontium and Sm-Nd systematics of the urengoites indicate a heterogeneous precursor material, derived from Paleoproterozoic continental crust, which underwent Rb/Sr fractionation and partial Sr isotope homogenization in Jurassic times. Any relation between the urengoites and the Haughton impact crater, having within 2s? errors an identical age, can be excluded on the basis of isotope relationships and geochemical data. The only known South-Ural glass (～6.2 Ma) is characterized by intermediate SiO₂ (65 wt%), high Al₂O₃ (14 wt%) and CaO (12 wt%), and low FeO_TOT (0.4 wt%) contents. This unique tektite-like object contains 110 ppm REE displaying a steeply negative C1 normalized distribution with La_N/Yb_N of 17, and Eu_N/Eu¹ of 0.71. The Rb abundance (10 ppm) and Rb/Sr ratio are low, and combined with a “crustal” ⁸⁷Sr/⁸⁶Sr ratio of 0.722, yielding an unrealistic T^SruR age of 2.5 Ga. The Rb-Sr systematics imply a rather recent parent/daughter element decoupling. The T^NdCHUR age of the South-Ural glass is ～1690 Ma. Geochemical data suggest that urengoites and the South-Ural glass belong to two discrete groups of tektites, whose source craters remain to be discovered.     

The abundance and isotopic composition of water in eucrites

Volatile elements play a key role in the dynamics of planetary evolution. Extensive work has been carried out to determine the abundance, distribution, and source(s) of volatiles in planetary bodies such as the Earth, Moon, and Mars. A recent study showed that the water in apatite from eucrites has similar hydrogen isotopic compositions compared to water in terrestrial rocks and carbonaceous chondrites, suggesting that water accreted very early in the inner solar system given the ancient crystallization ages (~4.5 Ga) of eucrites. Here, the measurements of water (reported as equivalent H₂O abundances) and the hydrogen isotopic composition (δD) of apatite from five basaltic eucrites and one cumulate eucrite are reported. Apatite H₂O abundances range from ~30 to ~3500 ppm and are associated with a weighted average δD value of ?34 ± 67‰. No systematic variations or correlations are observed in H₂O abundance or δD value with eucrite geochemical trend or metamorphic grade. These results extend the range of previously published hydrogen isotope data for eucrites and confirm the striking homogeneity in the H‐isotopic composition of water in eucrites, which is consistent with a common source for water in the inner solar system.     

Impact ages of meteorites: A synthesis

Abstract— Isotopic ages of meteorites that indicate chronometer resetting due to impact heating are summarized. Most of the ages were obtained by the ³⁹Ar-⁴⁰Ar technique, but several Rb-Sr, Pb-Pb, and Sm-Nd ages also suggest some degree of impact resetting. Considerations of experimental data on element diffusion in silicates suggest that various isotopic chronometers ought to differ in their ease of resetting during shock heating in the order K-Ar (easiest), Rb-Sr, Pb-Pb, and Sm-Nd, which is approximately the order observed in meteorites. Partial rather than total chronometer resetting by impacts appears to be the norm; consequently, interpretation of the event age is not always straightforward. Essentially all ³⁹Ar-⁴⁰Ar ages of eucrites and howardites indicate partial to total resetting in the relatively narrow time interval of 3.4–4.1 Ga ago (1 Ga = 10⁹ years). Several disturbed Rb-Sr ages appear consistent with this age distribution. This grouping of ages and the brecciated nature of many eucrites and all howardites argues for a large-scale impact bombardment of the HED parent body during the same time period that the Moon received its cataclysmic bombardment. Other meteorite parent bodies such as those of mesosiderites, some chondrites, and IIE irons also may have experienced this bombardment. These data suggest that the early bombardment was not lunar specific but involved much of the inner Solar System, and may have been caused by breakup of a larger planetismal. Although a few chondrites show evidence of age resetting ～3.5–3.9 Ga ago, most impact ages of chondrites tend to fall below 1.3 Ga in age. A minimum of ～4 impact events, including events at 0.3, 0.5, 1.2, and possibly 0.9 Ga appear to be required to explain the younger ages of H, L, and LL chondrites, although additional events are possible. Most L chondrites show evidence of shock, and the majority of ³⁹Ar-⁴⁰Ar ages of L chondrites fall near 0.5 Ga. The L chondrite parent body apparently experienced a major impact at this time, which may have disrupted it. The observations (1) that lunar highland rocks experienced major impact resetting of various isotopic chronometers ～3.7–4.1 Ga ago; (2) that the HED parent body experienced widespread impact resetting of the K-Ar chronometer but only modest disturbance of other isotopic systems, during a similar time period; (3) that ordinary chondrite parent bodies show much more recent and less extensive impact resetting; and (4) that impacts, which initiated cosmic-ray exposure of most stone meteorites almost never reset isotopic chronometers, may all be a consequence of relative parent body size. Greater degrees of isotopic chronometer resetting occur in larger and warmer impact ejecta deposits that cool slowly. The relatively greater size of bodies like the Moon and Vesta (assumed to be the parent asteroid of HED meteorites) both permit such favorable ejecta deposits to occur more easily compared to smaller parent bodies (generally assumed for chondrites) and also protect parent objects from collisional disruption. Thus, impacts on larger bodies would tend to more easily reset chronometers, consistent with the observed relative ease of resetting of Moon (easiest), HED, chondrites and of K-Ar (easiest), Rb-Sr, other chronometers. In contrast, the more recent impact ages of chondrites are postulated to represent collisional disruption of smaller parent objects whose fragments are more readily removed from the meteorite source reservoirs. Impacts that initiate cosmic-ray exposure are mostly small in scale and produce little heating.     

Ibitira: A basaltic achondrite from a distinct parent asteroid and implications for the Dawn mission

Abstract— I have done a detailed petrologic study of Ibitira, a meteorite that has been classified as a basaltic eucrite since 1957. The mean Fe/Mn ratio of pyroxenes in Ibitira with <10 mole% wollastonite component is 36.4 ± 0.4; this value is well resolved from those of similar pyroxenes in five basaltic eucrites studied for comparison, which range from 31.2 to 32.2. Data for the latter five eucrites completely overlap. Ibitira pyroxenes have lower Fe/Mg than the basaltic eucrite pyroxenes; thus, the higher Fe/Mn ratio does not reflect a simple difference in oxidation state. Ibitira also has an oxygen isotopic composition, alkali element contents, and a Ti/Hf ratio that distinguish it from basaltic eucrites. These differences support derivation from a distinct parent asteroid. Thus, Ibitira is the first recognized representative of the fifth known asteroidal basaltic crust, the others being the HED, mesosiderite, angrite, and NWA 011 parent asteroids. 4 Vesta is generally assumed to be the HED parent asteroid. The Dawn mission will orbit 4 Vesta and will perform detailed mapping and mineralogical, compositional, and geophysical studies of the asteroid. Ibitira is only subtly different from eucritic basalts. A challenge for the Dawn mission will be to distinguish different basalt types on the surface and to attempt to determine whether 4 Vesta is indeed the HED parent asteroid.     

Populations of nanodiamond grains in meteorites from the data on isotopic composition and content of nitrogen

The present study has shown that the dependence of the isotopic composition of nitrogen on the N/C ratio, revealed from the data for bulk samples of meteoritic nanodiamond, can be obtained within the framework of the following model of the composition of populations of nanodiamond grains: (a) initial nanodiamond, i.e., the nanodiamond in the protoplanetary cloud before the accretion of the meteorite parent bodies, was composed mainly of grains of two populations (denoted as C_N and C_F), the ratio of which changed in meteorites depending on the degree of hydrothermal metamorphism; (b) only the grains of one of these populations (C_N) contain volume-bound nitrogen with δ¹⁵N = ?350‰; (c) the grains of both populations contain surface-bound nitrogen (δ¹⁵N ≡ 0). The calculations revealed the following properties of population grains in this model. (1) The grains of the C_N and C_F populations are most likely the same in isotopic composition of carbon and heterogeneous in distribution of its isotopes: the central part of grains is enriched with the δ¹²C isotope relative to the remainder of the grain. While the value of δ¹³C is ?37.3 ± 1.1‰ for carbon in the central part, it is ?32.8 ± 1.5‰ for the whole volume of the grains. (2) The noble gases of the HL component, specifically Xe-HL, are anomalous in isotopic composition and are most likely contained in the third population of nanodiamond grains (denoted as C_HL), the mass fraction of which is negligible relative to that for other grain populations. Only the grains of the C_HL population have an undoubtedly presolar origin, while the grains of the other nanodiamond populations could have formed at the early stages of the evolution of the protoplanetary cloud material before the accretion of the meteoritic parent bodies.     

Cosmogenic and fissiogenic noble gases and 81Kr-Kr exposure age clusters of eucrites

Abstract— The isotopic composition and concentrations of noble gases were measured in the eucrites Bereba, Cachari, Caldera, Camel Donga, Chervony Kut, Ibitira, Jonzac, Juvinas, Millbillillie, Moore County, Padvarninkai, Pasamonte, Pomozdino, Serra de Magé, Sioux County, and Vetluga. The distribution of ⁸¹Kr-Kr exposure ages shows “clusters” at (7 ± 1) Ma, (10 ± 1) Ma, (14 ± 1) Ma, (22 ± 2) Ma, and (37 ± 1) Ma that agree with those for howardites, eucrites, and diogenites (HED) at (6 ± 1) Ma, (12 ± 2) Ma, (21 ± 4) Ma, and (38 ± 8) Ma. This most likely indicates a common origin of HED meteorites. Correlation equations for the shielding-sensitive cosmogenic ratios ⁷⁸Kr/⁸³Kr, ⁸⁰Kr/⁸³Kr, ⁸²Kr/⁸³Kr, and ¹²⁴Xe/¹³¹Xe were obtained. Comparison with data from simulation experiments suggests that most eucrites were exposed to the cosmic radiation as somewhat large meteoroids with diameters of ～1 m or more. The shielding-dependence of the ⁷⁸Kr and ¹²⁶Xe production rates was found to be small, with a few exceptions the variations aren <10%–15%. Concentrations of spallogenic ³He indicate diffusive losses of up to 70% that can be, in first approximation, described by a model of quasi-continuous losses during the exposure to the cosmic radiation with a loss rate of the order of ～3 × 10^?8 a^?1. Radiogenic ⁴He shows additional substantial losses that occurred at the time of, or prior to, the separation of the meteoroids from their parent body. Typical ⁴⁰Ar retention in eucrites is 50%–60% which corresponds to a ⁴⁰Ar-K retention age of 3.4–3.6 Ga. In all analyzed unbrecciated eucrites, the retention is distinctly larger (70%–100%). The ²⁴⁴Pu fission ratio (⁸⁶Kr/¹³⁶Xe)_Pu, was evaluated from the data on Pomozdino samples to be 0.039 ± 0.014.     

Highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body

Abstract– Aubrites exhibit a wide range of highly siderophile element (HSE—Re, Os, Ir, Ru, Rh, Pt, Pd, Au) concentrations and ¹⁸⁷Os/¹⁸⁸Os compositions. Their HSE concentrations are one to three orders of magnitude less than chondrites, with the exception of the Shallowater and Mt. Egerton samples. While most aubrites show chondritic HSE abundance ratios, significant enrichments of Pd and Re relative to Os, Ir, and Ru are observed in 12 of 16 samples. Present‐day ¹⁸⁷Os/¹⁸⁸Os ratios range from subchondritic values of 0.1174 to superchondritic values of up to 0.2263. Half of the samples have ¹⁸⁷Os/¹⁸⁸Os ratios of 0.127 to 0.130, which is in the range of enstatite chondrites. Along with the brecciated nature of aubrites, the HSE and Re‐Os isotope systematics support a history of extensive postaccretion processing, including core formation, late addition of chondritic material and/or core material and potential breakup and reassembly. Highly siderophile element signatures for some aubrites are consistent with a mixing of HSE‐rich chondritic fragments with a HSE‐free aubrite matrix. The enrichments in incompatible HSE such as Pd and Re observed in some aubrites, reminiscent of terrestrial basalts, suggest an extensive magmatic and impact history, which is supported by both the ¹⁸⁷Re‐¹⁸⁷Os isotope system and silicate‐hosted isotope systems (Rb‐Sr, K‐Ar) yielding young formation ages of 1.3–3.9 Ga for a subset of samples. Compared with other differentiated achondrites derived from small planetary bodies, aubrites show a wide range in HSE concentrations and ¹⁸⁷Os/¹⁸⁸Os, most similar to angrites. While similarities exist between the diverse groups of achondrites formed early in solar system history, the aubrite parent body(ies) clearly underwent a distinct evolution, different from angrites, brachinites, ureilites, howardites, eucrites, and diogenites.     

REE patterns versus the origin of the basaltic achondrites

The spectral uniqueness of asteroid 4 Vesta has led to suggestions that it is the eucrite parent body. However, there exist other basaltic achondrite types besides eucrites; either they also came from Vesta or else there exist other achondrite parent bodies. Howardites appear to be mixtures of eucrites and diogenites, and mesosiderites mixtures of eucrites or howardites and iron; thus one may infer that all four classes come from the same parent body. The REE patterns of eucrites and diogenites are modeled in order to test this hypothesis; eucrites can be made easily, but the patterns of diogenites are more difficult to match. The other basaltic achondrites are so rare that one cannot argue from statistics of abundances against a disrupted parent body for their origin. Pallasites and most irons likely had an origin separate from eucrites, again in parent bodies since disrupted.     

Geochemistry and chronology of the Bunburra Rockhole ungrouped achondrite

Bunburra Rockhole is a unique basaltic achondrite that has many mineralogical and petrographic characteristics in common with the noncumulate eucrites, but differs in its oxygen isotope composition. Here, we report a study of the mineralogy, petrology, geochemistry, and chronology of Bunburra Rockhole to better understand the petrogenesis of this meteorite and compare it to the eucrites. The geochemistry of bulk samples and of pyroxene, plagioclase, and Ca‐phosphate in Bunburra Rockhole is similar to that of typical noncumulate eucrites. Chronological data for Bunburra Rockhole indicate early formation, followed by slow cooling and perhaps multiple subsequent heating events, which is also similar to some noncumulate eucrites. The ²⁶Al‐²⁶Mg extinct radionuclide chronometer was reset in Bunburra Rockhole after the complete decay of ²⁶Al, but a slight excess in the radiogenic ²⁶Mg in a bulk sample allows the determination of a model ²⁶Al‐²⁶Mg age that suggests formation of the parent melt for this meteorite from its source magma within the first ~3 Ma of the beginning of the solar system. The ²⁰⁷Pb‐²⁰⁶Pb absolute chronometer is also disturbed in Bunburra Rockhole minerals, but a whole‐rock isochron provides a re‐equilibration age of ~4.1 Ga, most likely caused by impact heating. The mineralogy, geochemistry, and chronology of Bunburra Rockhole demonstrate the similarities of this achondrite to the eucrites, and suggest that it formed from a parent melt with a composition similar to that for noncumulate eucrites and subsequently experienced a thermal history and evolution comparable to that of eucritic basalts. This implies the formation of multiple differentiated parent bodies in the early solar system that had nearly identical bulk elemental compositions and petrogenetic histories, but different oxygen isotope compositions inherited from the solar nebula.     

Noble gases and strontium isotopes in the unique meteorite Divnoe

Abstract— We present an isotope study of noble gases in Divnoe, an anomalous meteorite, and also Rb-Sr and K-Ar dating of this meteorite. The relatively young Rb-Sr age obtained (3.39 Ga) seems doubtful and, most probably, results from weathering or contamination. The ancient K-Ar age (4.67^+0.20_–0.40), together with clear excess of ¹²⁹Xe, allows the suggestion of very early formation of the Divnoe meteorite. Concentrations and isotope ratios of noble gases in Divnoe are: 17.9 ≤ ³He ≤ 29.0 × 10^?8; ²⁰Ne = 6.22 × 10^?8; 2.44 ≤ ³⁶Ar ≤ 5.10 × 10^?8; ¹³⁰Xe = 41.3 × 10^?12 cm³/g; 0.079 ≤ ³He/⁴He ≤ 0.193; ²⁰Ne/²²Ne = 0.860; ²¹Ne/²²Ne = 0.927; 3.47 ≤ ⁴⁰Ar/³⁶Ar ≤ 9.47; 2.22 ≤ 36Ar/³⁸Ar ≤ 3.27; ¹²⁹Xe/¹³²Xe = 1.09. The exposure age calculated from cosmogenic ³He, ²¹Ne, and ³⁸Ar is 17.9 ± 0.9 Ma. On the basis of the isotope data for the noble gases and O, and abundances of K, Rb, and Sr, an attempt was made to estimate the relationship of Divnoe to other meteorite types. The O-isotope characteristics of Divnoe are clearly distinct from those of ordinary chondrites, acapulcoites/lodranites, and SNC meteorites (Petaev et al., 1994, Clayton, 1993). In plots of ¹³⁶Xe vs. ¹²⁹Xe/¹³⁰Xe, the Divnoe data fall outside of the data fields for carbonaceous and enstatite chondrites. The light noble gas data, especially the ⁴⁰Ar/³⁸Ar ratio, and the ⁴⁰Ar, ³⁸Ar, ³He, and ⁴He contents of Divnoe differ significantly from those of all meteorite types except diogenites. The K, Rb, and Sr abundances in Divnoe are substantially lower than in most other meteorites. In the concentrations of these elements, as well as in the REE pattern, the Divnoe meteorite is similar only to diogenites. Divnoe probably should be treated as a restite remaining after partial melting of the chondritic mantle of a parent asteroid body.     

Earliest high-Ti volcanism on the Moon: 40Ar-39Ar,Sm-Nd,and Rb-Sr isotopic studies of Group D basalts from the Apollo 11 landing site

Abstract High-Ti basalts from the Apollo collections span a range in age from 3.87 Ga to 3.55 Ga. The oldest of these are the common Apollo 11 Group B2 basalts which yield evidence of some of the earliest melting of the lunar mantle beneath Mare Tranquillitatis. Rare Group D high-Ti basalts from Mare Tranquillitatis have been studied in an attempt to confirm a postulated link with Group B2 basalts (Jerde et al., 1994). The initial Sr isotopic ratio of a known Group D basalt (0.69916 ± 3 at 3.85 Ga) lies at the lower end of the tight range for Group B2 basalts (⁸⁷Sr/⁸⁶Sr = 0.69920 to 0.69921). One known Group D basalt and a second postulated Group D basalt yield indistinguishable initial ?_Nd (1.2 ± 0.6 and 1.2 ± 0.3) and again lie at the lower end of the range for the Group B2 basalts from Apollo 11 (+2.0 ± 0.4 to +3.9 ± 0.6, at 3.85 Ga). A third sample has isotopic (⁸⁷Sr/⁸⁶Sr = 0.69932 ± 2; ?_Nd = 2.5 ± 0.4; at 3.59 Ga; as per Snyder et al., 1994b) and elemental characteristics similar to the Group A high-Ti basalts returned from the Apollo 11 landing site. Ages of ⁴⁰Ar-³⁹Ar have been determined for one known Group D basalt and a second postulated Group D basalt using step-heating with a continuous-wave laser. Suspected Group D basalt, 10002, 1006, yielded disturbed age spectra on two separate runs, which was probably due to ³⁹Ar recoil effects. Using the “reduced plateau age” method of Turner et al. (1978), the ages derived from this sample were 3898 ± 19 and 3894 ± 19 Ma. Three separate runs of known Group D basalt 10002, 116 yielded ⁴⁰Ar/³⁹Ar plateau ages of 3798 ± 9 Ma, 3781 ± 8 Ma, and 3805 ± 7 Ma (all errors 2σ). Furthermore, this sample has apparently suffered significant ⁴⁰Ar loss either due to solar heating or due to meteorite impact. The loss of a significant proportion of ⁴⁰Ar at such a time means that the plateau ages underestimate the “true” crystallization age of the sample. Modelling of this Ar loss yields older, “true” ages of 3837 ± 18, 3826 ± 16, and 3836 ± 14 Ma. These ages overlap the ages of Group B2 high-Ti basalts (weighted average age = 3850 ± 20 Ma; range in ages = 3.80 to 3.90 Ga). The combined evidence indicates that the Group D and B2 high-Ti basalts could be coeval and may be genetically related, possibly through increasing degrees of melting of a similar source region in the upper mantle of the Moon that formed >4.2 Ga ago. The Group D basalts were melted from the source first and contained 3–5×more trapped KREEP-like liquid than the later (by possibly only a few million years) Group B2 basalts. Furthermore, the relatively LREE- and Rb-enriched nature of these early magmas may lend credence to the idea that the decay of heat-producing elements enriched in the KREEP-like trapped liquid of upper mantle cumulates, such as K, U, and Th, could have initiated widespread lunar volcanism.