Records of ancient cosmic radiation in extraterrestrial rocks

Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of²⁶A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (J _s,R _o)=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×10⁵ to 2×10⁶ years. Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (⁵³Mn,²⁸Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and²²Ne/²¹Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.²²Ne/²¹Ne ratio <1·06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from⁶⁰Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid. Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid. Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.     

A study on cosmogenic nuclides in the Jilin meteorite and its irradiation history

44 samples of the Jilin meteorite were analyzed by various laboratories for spallogenic, radiogenic, and trapped rare gases. A non-uniform distribution has been found for the rare gases of different origins. There have been found correlations among the spallogenic rare gases, with apparent depth effects. 43 samples were analyzed for their cosmogenic radionuclides⁶⁰Co,²⁶Al,³⁶Cl,⁴⁰K,⁵³Mn,⁵⁴Mn, and²²Na. Correlations have been found between⁶⁰Co and⁵³Mn and between⁶⁰Co and²¹Ne. The Jilin meteorite has a complex history of cosmic-ray irradiation. According to the two-stage model and the other fragments have a burying depth two stages,T ₁=11 m.y. andT ₂=0.3 m.y. The burying depth of all samples in the parent body can be obtained by the content of²¹Ne. Jilin meteorite No. 1 is located 20–142 cm from the surface, No. 4 ranges from 106–134 cm, and the other fragments have a burying depth between 15 and 150 cm. The equation of the reference plane for the surface of the 1-stage Jilin meteoroid is 0.24x+0.81y+0.53z+0.5=0. Use can also be made of⁶⁰Co to determine the burying depth of all samples in the 2-stage Jilin meteoroid (under a geometry of 4π), and further to restore the preatmospheric form and size of the parent body. During its atmospheric passage, the Jilin had an ablation rate of ca. 30%. On the basis of the two-stage model and the concentration of²¹Ne and⁶⁰Co in the samples, we propose a scheme to restore the relative position of all samples in the parent body. 12 sample-distribution regions can be sketched out. Also can be restored the relative position of all the samples in the parent body during the two stages.     

Cosmic-ray production rates of neon isotopes in meteorite minerals

The rates of production of²¹Ne and²²Ne in spallation reactions, both due to solar as well as galactic cosmic rays, in some major meteoritic minerals, e.g. olivines, feldspars and pyroxenes, are calculated using their energy spectra and excitation functions. The production profiles of²¹Ne and²²Ne due to galactic cosmic rays, and the²²Ne/²¹Ne ratio depend upon the size of the meteoroid. The²²Ne/²¹Ne ratio is very sensitive to the abundance of sodium and consequently its depth profile is distinctly different in feldspars, the ratio increasing with depth rather than decreasing as in pyroxenes and olivines. In the near-surface regions, up to a depth of 2cm, production due to solar flare protons dominates, giving rise to a steep gradient in isotopic production as well as in the²²Ne/²¹Ne ratio. Composite production profiles are given and compared with measurements in some meteorites.     

Ionization effects from cosmic sources of x-rays in the lower atmosphere

Detectable ionization effects in the ionosphericD-region from individual, strong and steady x-ray sources such as Sco X-1 and transient x-ray sources such as Cet X-2 have been reported by us and many others previously based on the field strength and phase variations of the VLF data. As a follow up to these investigations, we have examined the integrated effects of many of the known x-ray sources discovered by UHURU, ANS, Ariel V and SAS-3 satellites, in order to understand the totality of their effects. These effects are examined in the present paper for 0° and +38° geographic latitudes corresponding to midnight conditions and for different times of the year. Such effects are compared, in turn, with those of the known steady sources responsible forD-region ionization such as Lyman-alpha and galactic cosmic radiations. The results are presented as profiles of electron production rates as a function of height. Our study leads to the conclusion that there should be detectable annual variations of the electron density which are pronounced around May-August. Further, the results of the computations on electron production rates corresponding to the spectacular x-ray nova A0620-00 are also included in the present paper.     

On the Relationship between Refractive Index and Density for SiO2-polymorphs

Five different refraction formulas were applied to SiO₂ polymorphs in order to determine the most suitable refractive index-density relation. 13 SiO₂ polymorphs with topological different tetrahedral frameworks are used in this study including eight new low density SiO₂ polymorphs — so called “guest free porosils”. These SiO₂ polymorphs cover a density range from 1.76 to 2.92 g/cm³. The mean refractive indices (ovn) of the porosils have been determined by the immersion method, the densities (ρ) were calculated from the unit cell parameters. Assuming the polarizability (α) of all SiO₂ polymorphs to be constant the general refractivity formula $$\{ 2\overline {11} 0\} \langle 0001\rangle $$ turned out to be the most suitable for SiO₂ polymorphs. Regression analysis yields an electronic overlap parameter b=1.2(1).     

Crystal structure of xinganite

Xinganite is a new REE-Be-rich silicate discovered in China. Its ideal formula is: (Y, Ce)Be SiO₄ OH. The mineral is of monoclinic system. The intensity data were collected with a single-crystal four-circle diffractometer. The lattice parameters are: a=4.7681 (± 0.00263) Å,b=7.7657 (± 0.00686) Å, c=9.9301 (± 0.00639) Å; α =90°, β=90.171° (±0.0053°), γ=90° space group p2¹/c;,Z=4. The crystal structure has been determined by direct methods and electron density synthesis methods. The least squares refinement gave a final discrepancy indexR=0.086. The crystal structural analysis shows that xinganite is of datolite-type structure.     

Moho structure of Central America based on three-dimensional lithospheric density modelling of satellite-derived gravity data

The Central American isthmus hosts a highly variable Moho structure due to the diverse origin and composition of the crustal basement and the influence of large-scale neotectonic processes. Gravity data from the combined geopotential model EGM2008 were interpreted via forward modelling to outline the three-dimensional lithospheric density structure along the Middle American Trench, as well as the segmentation of the oceanic Cocos and Nazca plates and the overriding Caribbean plate. In this work, results for the depth of the Moho obtained from the density model are presented. The Quaternary volcanic arc correlates with a maximum Moho depth of 44 km in western Guatemala. To the south-east of the continental shelf, the Caribbean plate shows Moho depths between 20 and 12 km whereas to the north, values as shallow as 8 km are observed at the Cayman trough. For the oceanic Cocos plate, depths between 16 and 21 km are obtained for the Moho along the Cocos ridge, contrasting with values between 15 and 12 km for the seamount segment and 8 and 11 km for the segments of the crust that are not affected by the Galapagos hot-spot track.     

Ebb-tidal fronts in Charleston Harbor,South Carolina: Physical and biological characteristics

Surface accumulations of foam and flotsam as well as sharp salinity, density, turbidity gradients and regions of acoustic scatter were characteristic of ebb-tidal fronts in Charleston Harbor, South Carolina. Surface convergence velocities at these fronts averaged 0.06 m s^?1 into the front at an angle of 30° to 60° with respect to the frontal axis, indicating along-front transport during the ebb. These fronts are tidally-induced, forming on the late flood and ebb along the interfaces of water masses. Horizontal and vertical measurements of density revealed that the upper harbor fronts form along the margin of a freshwater lens produced by riverine input. The hypothesis that these frontal zones have higher densities of phytoplankton and zooplankton than adjacent water masses was tested using chlorophylla measurements and net collections. The fronts did not demonstrate any significant accumulations of phytoplankton or zooplankton during the ebb tide. The results of this study suggest that the physical characteristics of ebb-tidal estuarine fronts in Charleston Harbor are periodic in nature and may indirectly affect plankton transport in this coastal plain estuary.     

Energy gap and density in SiO2 polymorphs

A relationship between the energy gap (E _G) and density (ρ) for pure SiO₂ polymorphs is derived from atomic weights and first ionization potentials of free silicon and oxygen atoms. Theoretical considerations are based on the Lorentz electron theory of solids. The eigenfrequency v₀ of elementary electron oscillators, in energy units h v₀, is identified with the energy gap of a solid. The numerical relation is expressed as \(E_G = \sqrt {139.24 - 13.8327\rho } \) is in eV. For low-quartz with a density of 2.65 g/cm³ and also for stishovite with a density of 4.28 g/cm³, the energy gap E _G=10.1 eV and 8.9 eV, respectively. From laboratory measurements for low-quartz E _G=10.2 eV. The energy gap-density relation suggests a critical density value of ρ_x ≈ 10.1 g/cm³ for an SiO₂ phase when the energy gap vanishes (E _G=0), which is consistent with estimated densities for a high pressure silica polymorph with the fluorite structure.     

The cosmic ray record in the San Juan Capistrano meteorite

An integrated study of the cosmic ray exposure history of the San Juan Capistrano meteorite was carried out using measurements of rare gas isotopic abundances, particle track densities and radioisotope concentrations. Spallation systematics determined for Kr isotopes in lunar samples are shown to be valid also for the San Juan Capistrano and St. Severin meteorites, thus allowing us to determine a reliable ⁸¹Kr/⁸³Kr production ratio as needed for applying the ⁸¹Kr-Kr dating method. The ⁸¹Kr-Kr age of SJC is 28.7 ± 2.0 Myr, about 35% longer than ages determined by spallation He or Ne. The minimum observed track production rate (2.6 × 10⁵ tracks/cm² · Myr) sets a minimum of 8 cm for the preatmospheric radius of an assumed spherical body. Track density gradients and the low ⁶⁰Co activity (<2.9 dpm/g Co) both set an upper limit of 10 cm to the radius. Track results show that ablation losses have averaged 6cm. The relative spallation yields of ⁷⁸Kr and ⁸³Kr, and the ratios ³He/²¹Ne and ²²Ne/²¹Ne are all compatible with a hard irradiation as would be experienced by a sample depth of about 6 cm in a body of 8–10 cm. The low activities of ⁵⁴Mn, ²²Na and ²⁶Al are also consistent with these irradiation conditions.     

Long-term increase in diffuse groundwater recharge following expansion of rainfed cultivation in the Sahel,West Africa

Rapid population growth in sub-Saharan West Africa and related cropland expansion were shown in some places to have increased focused recharge through ponds, raising the water table. To estimate changes in diffuse recharge, the water content and matric potential were monitored during 2009 and 2010, and modeling was performed using the Hydrus-1D code for two field sites in southwest Niger: (1) fallow land and (2) rainfed millet cropland. Monitoring results of the upper 10 m showed increased water content and matric potential to greater depth under rainfed cropland (>2.5 m) than under fallow land (≤1.0 m). Model simulations indicate that conversion from fallow land to rainfed cropland (1) increases vadose-zone water storage and (2) should increase drainage flux (～25 mm year^?1) at 10-m depth after a 30–60 year lag. Therefore, observed regional increases in groundwater storage may increasingly result from diffuse recharge, which could compensate, at least in part, groundwater withdrawal due to observed expansion in irrigated surfaces; and hence, contribute to mitigate food crises in the Sahel.     

Processes controlling the heavy metal distribution in pacific ferromanganese nodules and crusts

The ferromanganese precipitates existing in deep-sea waters of the Pacific consist of two types of deposits: (1) nodules mainly are distributed in pelagic basins beneath the CCD (Calcite Compensation Depth) where the rate of sedimentation is low; (2) polymetallic encrustations are formed on exposed seamount rocks where currents prevent normal sediment accumulation. Nodules, being formed in areas bordering the equatorial zone of high biological productivity, grow by two different processes: (A) early diagenetic growth by supply of metals and metal compounds from pore water and (B) hydrogenetic growth by supply of colloidal particles from near-bottom seawater. These processes lead to different kinds of oxide and different metal contents. The diagenetic growth process takes place under oxidizing to suboxidizing conditions and is supplied by an ionic solution of Mn²⁺ and other divalent metal ions. The mobilization of Mn is caused by the decomposition of organic matter. The growth features of the early diagenetic nodules show alternating laminae of crystalline and amorphous material. These rhythmic sequences of different microlayers are explained by physico-chemical changes (variation of pH) in the microenvironment of the accreting nodule surface. The hydrogenetic crust growth on seamounts leads to ferromanganese precipitates which are in particular rich in Co. The Co concentration is inversely related to the water depth. Co is positively correlated to Mn which can be derived from the oxygen minimum zone. Contrary to the diagenetic nodule growth, the crust accretion is also a colloidal precipitation process. In the water column below the oxygen minimum zone, a mixture of particles of Mn-Fe-oxyhydroxide and silicate accrete together on the surface of substratum rocks. Surface chemical mechanisms control the enrichment of Ni, Co, Pb, and other metals from the seawater; for Pt, a coprecipitation with MnO₂ caused by a redox reaction is proposed. Distinct oceanographical and geological conditions enable or promote, respectively, the ferromanganese crust formation on seamounts.     

57Fe NCR of Fe phases in “magnetic cassiterites”

A pyralspite garnet from an anomalously magnetic concentrate of a pegmatitic cassiterite ore has been investigated using ⁵⁷Fe nuclear gamma-ray resonance spectroscopy. The quadrupole splitting and isomer shift values of 3.6 mm/s and 1.4 mm/s, respectively, are among the largest observed for Fe²⁺ ions and indicate a very low covalency of the dodecahedral Fe²⁺ — O²-bonds. These data support the more recent and lower value (10.2–10.1 kcal/ mole) of White and Moore (1972) for the CFSE of the dodecahedral Fe²⁺ ion and suggest that the CFSE should be a useful approximation to the site preference energy of Fe²⁺ for this site.     

Dislocation strain energy in the Al2SiO5 polymorphs

Molar elastic strain energy arising from dislocations in andalusite and sillimanite were calculated using equations derived from a non-core, linear elasticity model. For perfect (unit) c screw dislocations in these polymorphs, minimum dislocation densities of about 10¹⁰/cm² are necessary to significantly perturb the andalusite=sillimanite equilibrium boundary in P-T space. Compared to unit c dislocations, smaller energy perturbations arise from dissociated c screw dislocations, which are commonly observed in kyanite and sillimanite. A low computed value of stacking fault energy (～30 ergs/cm²) in these polymorphs is compatible with the large separations of dissociated dislocations in these phases. Dislocation densities in naturally occurring Al₂SiO₅ polymorphs are typically <10⁸/cm². Assuming that these densities are representative of those existing during metamorphism, as is supported by the lack of microtextures indicative of strong recovery, it is concluded that molar strain energies corresponding to observed dislocation densities (<10⁸/cm²) result in insignificant perturbation of P-T phase equilibrium boundaries of the Al₂SiO₅ polymorphs.     

Strong site preference of Co2+ in olivine,Co1.10Mg0.90SiO4

The crystal structure and site preference of Co²⁺ in a synthetic Co_1.10Mg_0.90SiO₄ olivine have been determined from single crystal X-ray diffraction data collected on an automatic diffractometer. The R factor is 0.044 for 612 reflections. The site occupancies are: Ml site: Co 0.730±0.006; Mg 0.270; M2 site: Co 0.370, Mg 0.630. The Gibbs free energy change, ΔG° for the ion-exchange reaction between M1 and M2 sites is ?4.06 kcals/mole, assuming ideal mixing at each set of sites. This energy may be called ‘site preference energy’ of Co²⁺ in olivine. The strong preference of Co²⁺ for the M1 site can be quantitatively explained by two competing forces: preference of ions larger than Mg²⁺ for the M2 site and stronger covalent bonding of transition metal ions at the M1 site. For Fe²⁺, Mg²⁺, these two effects nearly neutralize each other, explaining the lack of considerable cation-ordering in Fe-Mg olivines.     

Solute carbon and carbon segregation in magnesium oxide single crystals — a secondary ion mass spectrometry study

If carbon is to be analyzed by secondary ion mass spectroscopy (SIMS) in an oxide such as MgO, one has to know how the carbon is incorporated in the oxide host structure, before a successful experiment can be planned. If the carbon impurities derive from dissolved CO₂ component which form a solid solution while the crystal grew from a melt in equilibrium with CO₂, upon cooling, the solid solution becomes supersaturated with respect to the volatile CO₂ component. This creates a thermodynamic driving force for exsolution leading to carbon segregation towards the surface. At the surface rapid degassing occurs in vacuum, enhanced by ion bombardment and electron irradiation. Using freshly cleaved synthetic MgO single crystals it can be shown by SIMS (i) that contamination during short exposure to air and during evacuation remains slight, (ii) that rapid surface/subsurface segregation of solute carbon seems to compete with rapid degassing so that, while no extended segregation profile builds up, the carbon concentration in the bulk beneath the surface decreases to a constant level, (iii) that electron irradiation speeds up degassing, (iv) that heating speeds up carbon diffusion, hence its segregation from the bulk, and (v) that Ar⁺ ion sputtering for the purpose of removing possible contaminants reduces the driving force for carbon surface segregation to the point that no segregation profile can be observed. By placing freshly cleaved MgO crystals under isotopically 99 percent pure ¹³CO₂ for various periods of time subsequent SIMS analysis reveals extended ¹²C profiles, probably about 1 μm wide, which can only have formed by ¹²C segregation from the bulk. These results confirm earlier reports that solute carbon exists as mobile impurity in synthetic MgO and natural olivine, probably due to dissolved CO₂ component.     

Crystal chemistry of a natural schorlomite and Ti-andradites synthesized at different oxygen fugacities

Ti-andradites were synthesized at a pressure of P(H₂O)=3 kbar and temperatures of 700–800° C. Oxygen fugacities were controlled by solid state buffers (Ni/NiO; SiO₂ + Fe/Fe₂SiO₄). The Fe²⁺-and Fe³⁺-distribution was determined by low temperature Mössbauer spectroscopy. The water content was measured by a solid's moisture analyzer. The chemical composition of the synthetic and the natural sample has been determined by electron microprobe. Ti-andradites from runs at high oxygen fugacities have Fe³⁺ on octahedral and tetrahedral sites; Ti-andradites from runs at low oxygen fugacities have tetrahedrally and octahedrally coordinated Fe²⁺ as well. These “reduced” garnets must also contain Ti³⁺ on octahedral sites. Charge balance is maintained due to substitution of O^2? by (OH)^? by two mechanisms: (SiO₄)^4? ? (O₄H₄)^4? and (Fe³⁺O₆)^9? ? (Fe²⁺O₅OH)^9?. FTIR spectra of the synthetic samples do show the presence of structurally bound (OH)^?. In a natural sample tetrahedrally and octahedrally coordinated Fe³⁺ are observed together with Fe²⁺ on all three cation sites of the garnet structure.     

The lattice energy of forsterite. Charge distribution and formation enthalpy of the SiO 4 4− ion

In the lattice energy expression of forsterite, based on a Born-Mayer (electrostatic+repulsive+dispersive) potential, the oxygen charge z _o, the hardness parameter ρ and the repulsive radii r _Mg and r _Si appear as unknown parameters. These were determined by calculating the first and second partial derivatives of the energy with respect to the cell edges, and equalizing them to quantities related to the crystal elastic constants; the overdetermined system of equations was solved numerically, minimizing the root-mean-square deviation. To test the results obtained, the SiO ₄ ^4? ion was assumed to move in the unit-cell, and the least-energy configuration was sought and compared with the experimental one. By combining the two methods, the optimum set of parameters was: z _o=?1.34, ρ=0.27 Å, r _Mg=0.72 Å, r _Si=0.64 Å. The values ?8565.12 and ?8927.28 kJ mol^?1 were obtained, respectively, for the lattice energy E _Land for its ionic component E _L ⁰ ,which accounts for interactions between Mg²⁺ and SiO ₄ ^4? ions only. The charge distribution calculated on the SiO ₄ ^4? ion was discussed and compared with other results. Using appropriate thermochemical cycles, the formation enthalpy and the binding energy of SiO ₄ ^4? were estimated to be: ΔH _f(SiO ₄ ^4? )=2117.6 and E(SiO ₄ ^4? )=708.6 kJ mol^?1, respectively.