Irradiation history and atmospheric ablation of meteorites: Results from cosmic ray track studies

The dominant component of nuclear tracks observed in meteoritic minerals poor in uranium is produced by cosmic ray very heavy (vh:Z>20) nuclei. Studies of cosmic ray tracks and other cosmogenic effects in meteorites give us information on the irradiation history of these meteorites and enable us to estimate the extent of ablation during their atmospheric transit, and hence their pre-atmospheric masses. In a specific type of meteorite, known asgas-rich meteorite, one finds individual grains and xenoliths that have received solar flare and galactic cosmic ray irradiation prior to the formation of these meteorites. Detailed studies of these exotic components give insight into the accretionary processes occurring in the early history of the solar system. Some of the important results obtained from such studies and their implications to meteoritics are summarized.     

Cosmogenic isotope and track records in meteorites and their implications to cosmic ray fluxes

Study of several cosmic ray effects, such as VH track density, spallogenic²⁶Al and⁵³Mn activity,²¹Ne and²²Ne/²¹Ne ratio, made in the same sample or in cores taken from different meteorites can identify parameters related to the exposure history of meteorites and cosmic ray flux variations. Meteorites with single or multiple exposure can be distinguished from a track production rate —²²Ne/²¹Ne correlation diagram and cosmic ray flux variations over 10⁶–10⁷ years can be deduced from a three-isotope correlation diagram of²⁶Al,⁵³Mn and²¹Ne. Isotopic data based on chondrites with simple, one-stage exposure are consistent with the same average galactic cosmic ray intensity over the past 2 million years as that during the past 10⁷ years.     

吉林陨石的热释光剖面与母体宇宙暴露历史示踪

对３个已知取样浓度和３个未知取样部位的吉林陨石样品的热释光研究表明,其自然热释光值具有宇宙成因核素含量相似的性质即浓度分布效应,随着样品距离表面位置的增大而降低,这表征了母体暴露期间宇宙射线的分布状况。诱导热释光数据表明,母体表面部分的样品的峰温和峰宽高于较深部位样品,说明这些样品中发光体矿物由有序态向无序态转化趋势明显。表面位置样品明显高的热释光数据说明在母体暴露过程中,除了银河宇宙射线外,太阳     

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.     

Long-term galactic cosmic ray variations over the last billion years based on the cosmic-ray exposure ages of iron meteorites

The distribution of the cosmic-ray exposure ages (T) of iron meteorites was analyzed to establish the possible variations in the intensity of the galactic cosmic ray (GCR) over the last billion years. The analysis was made for the entire data set containing ~80 age values from the literature (Voshage et al., 1983) and the corrected set after the exclusions of paired meteorites (using the Akaike information criterion). The dependence of the criterion χ² in the distribution of the phase values Ph = T/t–int(T/t) on the values of the assumed period (t) of GCR variations was analyzed for both sets of meteorites. The significant deviations of these parameters from the respective average values were found for t ~ 400–500 Myr and, in part, for t ~ 150 Myr. These deviations were interpreted by numerical modeling using the values of ages randomly distributed in the range of 0–1000 Ma. It was found that for variations with a period of 450 Myr, the distribution of the phase values and cosmic-ray exposure ages in the model data set is similar to that of iron meteorites. These results testify to the existence of the GCR variations with a period of ~400–500 Myr during the last 1 Gyr. The variations in the GCR flux can be explained by periodic galactic spiral arm crossings of the solar system. The GCR variations with a period of ~150 Myr discussed in the previous studies (Shaviv, 2002; 2003; Scherer et al., 2006) appears to be less certain.     

Glacial retreat history of Nanhuta Shan (north-east Taiwan) from preserved glacial features: the cosmic ray exposure perspective.

Taiwan, located at the junction of the Pacific Ocean, the Eurasian continent, and the South China marginal Sea, is of particular interest for reconstructing paleoclimate periods in Eastern Asia. This study reports the first cosmic ray exposure dating (CRE) of glacial features in Taiwan. Among the areas where glacial relicts have been described in Taiwan, the Nanhuta Shan range is probably the place where glacial landforms are best preserved. We consequently focused on this area combining glacial geomorphology observations together with CRE dating using in situ produced ¹⁰Be of erratic boulders and ice-sculpted surfaces. When combined with the geomorphic characteristics of the sampled areas, the obtained minimum CRE ages suggest that the glacial retreat in the Nanhuta Shan commenced about 10±3 ka ago and retreat was complete by 7±1 ka ago. This is consistent with the Holocene warming trend deduced from other biological and physico-chemical paleoclimatic records for the region. Estimates of local bedrock surface denudation rates either directly from in situ produced ¹⁰Be measurements or from geomorphic considerations are employed to determine the preservation of such glacial features within the highly dynamic setting of Taiwan.     

Thermoluminescence and the shock and reheating history of meteorites—II: Annealing studies of the Kernouve meteorite

Samples of the unshocked, equilibrated chondrite, Kernouve (H6), have been annealed for 1–100 hours at 500–1200°C, their thermoluminescence sensitivity measured and Na, K, Mn, Ca and Sc determined by instrumental neutron activation analysis. The TL sensitivity decreased with temperature until by 1000°C it had fallen by 40%. The process responsible has an activation energy of ~8 kcal/mole and probably involves diffusion. Samples annealed 1000–1200°C had TL sensitivities 10^?2 times the unannealed values, most of the decrease occurring ~1100°C. This process has an activation energy of ~100 kcal/mole and is probably related to the melting of the TL phosphor, feldspar, with some decomposition and loss of Cs, Na and K. Meteorites whose petrography indicates healing > 1100°C by natural shock heating events (shock facies d-f). have TL sensitivities similar to samples annealed > 1100°C. Our own and literature compositional data indicate that TL is more stable to annealing than Ag, In, Tl, Bi, Zn and Te and less stable than Na, K, Mn, Ca, Se and Co, while the TL decrease resembles very closely the pattern of Cs loss on annealing.     

Differentiation and magmatic history of Vesta: Constraints from HED meteorites and Dawn spacecraft data

Quantifying the amounts of various igneous lithologies in Vesta’s crust allows the estimation of petrologic ratios that describe the asteroid’s global differentiation and subsequent magmatic history. The eucrite:diogenite (Euc:Diog) ratio measures the relative proportions of mafic and ultramafic components. The intrusive:extrusive (I:E) ratio assesses the effectiveness of magma ascent and eruption. We estimate these ratios by counting numbers and masses of eucrites, cumulate eucrites, and diogenites in the world’s meteorite collections, and by calculating their proportions as components of crustal polymict breccias (howardites) using chemical mixing diagrams and petrologic mapping of multiple thin sections. The latter two methods yield a Euc:Diog ratio of ∼2:1, although meteorite numbers and masses give slightly higher ratios. Surface lithologic maps compiled from spectra of Dawn spacecraft instruments (VIR and GRaND) yield Euc:Diog ratios that bracket estimates of Euc:Diog from the meteorites. The I:E ratios from HEDs lie between 0.5–2.1:1, due to uncertainties in identifying cumulate eucrite. Gravity mapping of Vesta by the Dawn spacecraft supports the existence of diogenite plutons in the crust. Quantifying the proportion of high-density diogenitic crust in the gravity map yields I:E ratios of 0.8-1:2:1, values which are bracketed by calculations based on HEDs. The I:E ratio for Vesta is lower than for Earth and Mars, consistent with physical modeling of asteroid-size bodies. Nevertheless, it indicates a significant role for pluton emplacement during the formation of Vesta’s crust. These results are inconsistent with simple differentiation models that produce the crust by crystallization of a global magma ocean, unless residual melts are extracted into crustal magma chambers.     

Chemical and physical studies of type 3 chondrites—IV: Annealing studies of a type 3.4 ordinary chondrite and the metamorphic history of meteorites

Samples of a type 3.4 chondrite have been annealed at 400–1000°C for 1–200 hours, their thermoluminescence properties determined and analyzed for K, Na, Mn, Sc and Ca by instrumental neutron activation analysis. After annealing at ?900°C, the samples showed a 50% decrease in TL sensitivity, while after annealing at 1000°C it fell to 0.1-0.01 times its unannealed value and loss of Na and K occurred. The TL and compositional changes resemble those observed for the equilibrated Kernouve chondrite after similar annealing treatments, except that the sharp TL decrease, and element loss, occurred at ~ 1100°C; this difference is presumably due to petrographic differences in the feldspar of the two meteorites. The temperature and the width of the TL peak showed a discontinuous increase after annealing at 800°C; peak temperature jumped from 130 to 200°C and peak width increased from 90 to 150°C. The activation energies for these TL changes are 7–10 kcal/mole. Similar increases in the TL peak temperature have been reported in TL studies of Amelia, VA, albite, where they were associated with the low to high-temperature transformation. However, the activation energy for the transformation is ~80 kcal/mole. These changes in TL emission characteristics resemble trends observed in type 3 ordinary chondrites and it is suggested that type 3.3–3.5 chondrites have a low-feldspar as TL phosphor and > 3.5 have high-feldspar as the phosphor. Thermoluminescence therefore provides a means of palaeothermometry for type 3 ordinary chondrites.     

The fission track records of the Estherville,Nakhla and Odessa meteorites

Investigation of fossil charged-particle track densities in various mineral phases of three meteorites—Estherville, Nakhla and Odessa—coupled with U content determination, has led to the evaluation of various contributions to the total fossil track density, including those due to the spontaneous fission of ²³⁸U and ²⁴⁴Pu. A fission-track age for Estherville of around 4 × 10⁹ yr is found, which is thought to reflect slow cooling of the parent body. A Pu track excess of (106 ± 9): 1 over the spontaneous fission of ²³⁸U is found in Odessa diopside, which is larger than may be allówed on a simple ‘continuous synthesis’ model for the production of heavy elements prior to solar system formation. Possible explanations for this value are discussed, including fractionation of Pu relative to U. No detectable U was found in Nakhla diopside, and a measurable contribution of track densities from the fission of superheavy elements is ruled out on the basis of track-length measurements and laboratory calibration with Fe ions.     

On the irradiation history and origin of gas-rich meteorites

With the transmission electron microscope, we have made detailed studies of the track density gradients and irradiation geometries of track-rich grains and chondrules in sections of Fayetteville and Kapoeta. We have made the same type of studies in sections of lunar breccias and grains from lunar soil for comparison.A substantial fraction (50–90 per cent) of the meteoritic track-rich grains and chondrules show evidence of having been irradiated anisotropically in their different faces, as would be expected for irradiation on the surface of a parent body. Our observations thus support the hypothesis that the irradiation of these grains and chondrules took place on the regoliths of asteroidal-sized bodies.Measurement of steepest track density gradients indicate that while there are finite differences between spectra exhibited by individual gas-rich meteorites, the average solar flare spectral shapes have been similar over the last ~4 b.y. or so.     

Denudation history of the Snowy Mountains: Constraints from apatite fission track thermochronology

Apatite fission track thermochronology from Early Palaeozoic granitoids centred around the Kosciuszko massif of the Snowy Mountains, records a denudation history that was episodic and highly variable. The form of the apatite fission track age profile assembled from vertical sections and hydroelectric tunnels traversing the mountains, together with numerical forward modelling, provide strong evidence for two episodes of accelerated denudation, commencing in Late Permian—Early Triassic (ca 270–250 Ma) and mid‐Cretaceous (ca 110–100 Ma) times, and a possible third episode in the Cenozoic. Denudation commencing in the Late Permian—Early Triassic was widespread in the eastern and central Snowy Mountains area, continued through much of the Triassic, and amounted to at least ～2.0–2.4 km. This episode was probably the geomorphic response to the Hunter‐Bowen Orogeny. Post‐Triassic denudation to the present in these areas amounted to ～2.0–2.2 km. Unambiguous evidence for mid‐Cretaceous cooling and possible later cooling is confined to a north‐south‐trending sinuous belt, up to ～15 km wide by at least 35 km long, of major reactivated Palaeozoic faults on the western side of the mountains. This zone is the most deeply exposed area of the Kosciuszko block. Denudation accompanying these later events totalled up to ～1.8–2.0 km and ～2.0–2.25 km respectively. Mid‐Cretaceous denudation marks the onset of renewed tectonic activity in the southeastern highlands following a period of relative quiescence since the Late Triassic, and establishes a temporal link with the onset of extension related to the opening of the Tasman Sea. Much of the present day relief of the mountains resulted from surface uplift which disrupted the post‐mid‐Cretaceous apatite fission track profile by variable offsets on faults.     

Chemical fractionations in meteorites—VIII. Iron meteorites and the cosmochemical history of the metal phase

The composition of the metal phase is traced through an idealized, traditional history from condensation, oxidation and accretion in the nebula to melting, segregation and freezing in a parent body. Fifteen elements are considered: Au, Co, Cu, Fe, Ga, Ge, Ir, Mo, Ni, Os, Pd, Pt, Re, Rh and Ru. All are strongly siderophile but differ in volatility and melting-freezing behavior. This simplifies the problem yet provides a means to resolve chemical trends which evolve at different stages in the metal's history.The parent bodies of 5 (IC, IIAB, IIC, IID and IIIAB) of the 12 iron meteorite groups resolved by Scott and Wasson (1975) seem to have had a traditional history. That is condensation, oxidation to various levels, accretion, melting, segregation and fractional crystallization during freezing, presumably in cores. The others seem to have had more unusual histories. The composition of the metal in group IVB matches that predicted for the metal condensate at 1270°K (at$\text{P}{}_{\mathrm{T}}\text{= 10}{}^{\mathrm{?5}}\text{atm}$). This implies accretion at high temperatures; no other combination of the processes can produce this composition. It does not rule out secondary processing, however. The metal in group IVA, whose members have different cooling rates (7–200°/myr), has a composition indicative of aggregates in a body undergoing progressive stages of partial melting. This is consistent with a model in which molten metal collects into pods or raisins at various depths. The composition of the metal in group IAB is indicative of a partial melt which refroze during the initial stages of segregation, before it had managed to aggregate into a single mass. The physical setting implied is consistent with observed inhomogeneities in the metal and abundant inclusions.Three of the 12 groups are deficient in volatiles (IIIF, IVA and IVB) implying a high accretion temperature. In all three cases, cooling rates are comparatively rapid, indicating small bodies or low radioactive element contents. Conceivably all three were deficient in K.     

The early differentiation history of Mars from W-Nd isotope systematics in the SNC meteorites

We report here the results of an investigation of W and Nd isotopes in the SNC (Shergottite-Nakhlite-Chassignite (martian)) meteorites. We have determined that ε¹⁸²W values in the nakhlites are uniform within analytical uncertainties and have an average value of ∼3. Also, while ε¹⁸²W values in the shergottites have a limited range (from 0.3-0.7), their ε¹⁴²Nd values vary considerably (from −0.2-0.9). There appears to be no correlation between ε¹⁸²W and ε¹⁴²Nd in the nakhlites and shergottites. These results shed new light on early differentiation processes on Mars, particularly on the timing and nature of fractionation in silicate reservoirs. Assuming a two-stage model, the metallic core is estimated to have formed at ∼12 Myr after the beginning of the solar system. Major silicate differentiation established the nakhlite source reservoir before ∼4542 Ma and the shergottite source reservoirs at 4525 Ma. These ages imply that, within the uncertainties afforded by the ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd chronometers, the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contemporaneously, possibly during crystallization of a global magma ocean. The distinct ¹⁸²W-¹⁴²Nd isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on Mars, two of which are depleted in incompatible lithophile elements relative to chondrites, and the third is enriched. The two depleted silicate reservoirs most likely reside in the Martian mantle, while the enriched reservoir could be either in the crust or the mantle. Therefore, the ¹⁸²W-¹⁴²Nd isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related. A further implication is that the source reservoirs of the nakhlites and shergottites on Mars have been isolated since their establishment before ∼4.5 Ga. Therefore, there has been no giant impact or efficient global mantle convection to thoroughly homogenize the Martian mantle following the establishment of the SNC source reservoirs.     

Iron meteorites: Crystallization,thermal history,parent bodies,and origin

We review the crystallization of the iron meteorite chemical groups, the thermal history of the irons as revealed by the metallographic cooling rates, the ages of the iron meteorites and their relationships with other meteorite types, and the formation of the iron meteorite parent bodies. Within most iron meteorite groups, chemical trends are broadly consistent with fractional crystallization, implying that each group formed from a single molten metallic pool or core. However, these pools or cores differed considerably in their S concentrations, which affect partition coefficients and crystallization conditions significantly. The silicate-bearing iron meteorite groups, IAB and IIE, have textures and poorly defined elemental trends suggesting that impacts mixed molten metal and silicates and that neither group formed from a single isolated metallic melt. Advances in the understanding of the generation of the Widmanstätten pattern, and especially the importance of P during the nucleation and growth of kamacite, have led to improved measurements of the cooling rates of iron meteorites. Typical cooling rates from fractionally crystallized iron meteorite groups at 500–700 °C are about 100–10,000 °C/Myr, with total cooling times of 10 Myr or less. The measured cooling rates vary from 60 to 300 °C/Myr for the IIIAB group and 100–6600 °C/Myr for the IVA group. The wide range of cooling rates for IVA irons and their inverse correlation with bulk Ni concentration show that they crystallized and cooled not in a mantled core but in a large metallic body of radius 150±50 km with scarcely any silicate insulation. This body may have formed in a grazing protoplanetary impact. The fractionally crystallized groups, according to Hf–W isotopic systematics, are derived originally from bodies that accreted and melted to form cores early in the history of the solar system, <1 Myr after CAI formation. The ungrouped irons likely come from at least 50 distinct parent bodies that formed in analogous ways to the fractionally crystallized groups. Contrary to traditional views about their origin, iron meteorites may have been derived originally from bodies as large as 1000 km or more in size. Most iron meteorites come directly or indirectly from bodies that accreted before the chondrites, possibly at 1–2 AU rather than in the asteroid belt. Many of these bodies may have been disrupted by impacts soon after they formed and their fragments were scattered into the asteroid belt by protoplanets.     

东天山地区中—新生代隆升-剥露过程:来自磷灰石裂变径迹的证据

前人已经对西天山及邻区以及阿尔金断裂带进行了大量中—新生代隆升-剥露的研究工作,但对东天山地区的研究工作很少。天山造山带中—新生代期间的隆升-剥露过程是否具有均一性,目前仍没有确切的认识。为了获得东天山地区中生代以来的隆升-剥露信息,对吐哈盆地东南缘雅满苏地区磷灰石裂变径迹进行了研究。研究表明,在不同构造位置采集的花岗岩、砂岩、火山岩样品年龄集中分布在81~53Ma,样品年龄记录了东天山地区晚白垩世—古新世发生的冷却事件。磷灰石裂变径迹平均长度为13.60~14.36μm,接近于磷灰石初始径迹长度约14.5μm,表明径迹形成后没有发生过明显的退火作用。根据地温梯度计算得到东天山晚白垩世以来的平均隆升速率约为4.31×10-2 mm/a。进一步的热史模拟表明,晚白垩世—古新世(80~50Ma)期间东天山地区经历了一次隆升-剥露事件;始新世以后(50 Ma),东天山地区地壳处于稳定状态,东天山隆起带现在的构造面貌基本继承了中生代的特征。     

Noble gases,nitrogen and cosmic ray exposure age of the Sulagiri chondrite

The Sulagiri meteorite fell in India on 12 September 2008,LL6 chondrite class is the largest among all the Indian meteorites.Isotopic compositions of noble gases(He,Ne,Ar,Kr and Xe) and nitrogen in the Sulagiri meteorite and cosmic ray exposure history are discussed.Low cosmogenic(~(22)Ne/~(21)Ne)_c ratio is consistent with irradiation in a large body.Cosmogenic noble gases indicate that Sulagiri has a 4πcosmic-ray exposure(CRE) age of 27.9 ± 3.4 Ma and is a member of the peak of CRE age distribution of IX chondrites.Radiogenic ~4He and ~(40)Ar concentrations in Sulagiri yields the radiogenic ages as 2.29 and4.56 Ca,indicating the loss of He from the meteorite.Xenon and krypton are mixture of Q and spallogenic components.     

Noble gas concentrations and cosmic ray exposure ages of eight recently fallen chondrites

Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced ³He, ²¹Ne, ²²Ne and ³⁸Ar indicate that all eight samples experienced less than average cosmic ray shielding. ³He and ²¹Ne exposure ages were calculated using shielding corrected chondritic production rates and the measured ${}^{22}\text{Ne}{}^{21}\text{Ne}$. Exposure ages calculated from ${}^{22}\text{Na}{}^{22}\text{Ne}$ and ${}^{26}\text{Al}{}^{21}\text{Ne}$ ratios and constant relative production rates show a bias between the two ages due to variations in ${}^{22}\text{Na}{}^{26}\text{Al}$. Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the ${}^{22}\text{Na}{}^{22}\text{Ne}$ and ${}^{26}\text{Al}{}^{21}\text{Ne}$ production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe.     

中吉天山隆升时代对比——裂变径迹年代学证据

中吉天山成矿带境内外天山在成矿时代、矿产种类、矿床规模等多方面存在重大差异。它们的成矿条件基本类似,是否因为保存条件的不同而产生这种差异值得关注。文章对采自吉尔吉斯斯坦北天山(境外西天山)的磷灰石样品进行了裂变径迹测试分析和温度-时间反演模拟研究,表明吉尔吉斯斯坦北天山在中新生代发生了四期抬升剥露作用,分别为晚侏罗世、晚白垩世、始新世和渐新世,且不同区域其抬升剥露史也不相同:晚侏罗世的抬升局限于伊塞克湖南岸的泰尔斯山脉,始新世的抬升主要发生在伊塞克湖南北两侧的泰尔斯山脉和昆格山脉,晚白垩世和渐新世的抬升为吉尔吉斯斯坦北天山整体抬升。与东部境内西天山对比表明,境内西天山整体隆升时间较早,历时较长,有可能隆升剥蚀程度超过境外西天山,从而造成了成矿方面的重大差异。