Masahiro Kayama

Discovery of seifertite in a shocked lunar meteorite

Many craters and thick regoliths of the moon imply that it has experienced heavy meteorite bombardments. Although the existence of a high-pressure polymorph is a stark evidence for a dynamic event, few high-pressure polymorphs are found in a lunar sample. α-PbO₂-type silica (seifertite) is an ultrahigh-pressure polymorph of silica, and is found only in a heavily shocked Martian meteorite. Here we show evidence for seifertite in a shocked lunar meteorite, Northwest Africa 4734. Cristobalite transforms to seifertite by high-pressure and -temperature condition induced by a dynamic event. Considering radio-isotopic ages determined previously, the dynamic event formed seifertite on the moon, accompanying the complete resetting of radio-isotopic ages, is ~2.7 Ga ago. Our finding allows us to infer that such intense planetary collisions occurred on the moon until at least ~2.7 Ga ago.

Chapter 14 c. 1450 Ma regional felsic volcanism at the fringe of the East Indian Craton: constraints from geochronology and geochemistry of tuff beds from detached sedimentary basins

Abstract New geochronological and geochemical data from bedded porcellanitic tuffs present within two sedimentary basins at the eastern fringe of the Archaean Bastar Craton, eastern India (the Ampani and Khariar basins) are presented and compared with data available from tuffaceous beds present within adjoining basins. U–Th–total Pb electron probe microanalysis data of monazite grains from the Ampani tuff revealed several age data clusters: c. 2400, c. 2130, c. 1600, c. 1450 and c. 1000 Ma. An age of 1446±21 Ma is proposed as the depositional/crystallization age for the Ampani tuff, considering its maximum probability. Comparable ages for the tuffaceous units from the Khariar (1455±47 Ma) and Singhora (c. 1500 Ma) basins allow us to infer a major felsic volcanic event during c. 1450 Ma at the eastern margin of the Indian Craton. Detailed geochemical data suggest rhyolite to andesite character for the siliceous tuff units from three geographically separated basins and point towards the presence of an active volcanic arc in a subduction-related setting in the region. The geochronological and geochemical data prompted us to search for other contemporaneous events in the Indian continent and beyond, that is, within its erstwhile neighbours in the Precambrian supercontinent ‘Columbia’.

Cathodoluminescence of alkali feldspars and radiation effects on the luminescent properties

Abstract Cathodoluminescence (CL) spectroscopy provides useful information about the existence of radiation-induced defect centers with a few micrometer resolutions and therefore has great potential to estimate the accumulated dose of natural radiation in micrometer-ordered mineral grains from radioactive decay. Although great scientific interest exists concerning the CL of various types of minerals, very few investigation have been conducted on the luminescence properties of radiation-induced alkali feldspars. This study, therefore, has sought a clarification of radiation effects on emission centers detected by CL analysis of alkali feldspar implanted with He+ ions at 4.0 MeV, which corresponds to the energy of an α particle derived from radioactive decay of 238U and 232Th. Panchromatic CL images of cross sections of sanidine, orthoclase, and microcline show a dark line with ~1 μm width on the bright luminescent background at 12-15 μm beneath the implanted surface, of which behavior may be corresponding to the electronic energy loss process of 4.0 MeV He+ ion. CL and Raman spectroscopy revealed that He+ ion implantation may leads to a partial destruction of the feldspar framework and Na+ migration, resulting in a quenching of CL emission from alkali feldspar, proportional to the radiation dose. CL spectra of unimplanted and He+-ion-implanted sanidine, orthoclase and microcline have emission bands at ~400-410 nm and at ~730 nm. Deconvolution of the CL spectra can successfully separate these emission bands into emission components at 3.05, 2.81, 2.09, 1.73, and 1.68 eV. These components are assigned to the Ti4+ impurity, Al-O--Al/Ti defect, a radiation-induced defect center, and Fe3+ impurities on the T1 and T2 sites, respectively. The intensity at 3.05 eV negatively correlates with radiation dose owing to decreases in the luminescence efficiency. A slight Na+ diffusion and breaking of the linkage between Ti4+ and oxygen as a ligand might reduce the activation energy, which decreases the availability of radiative energy in the luminescence process of Ti4+ impurity centers. Furthermore, He+ ion implantation causes electron holes to be trapped at and released from Löwenstein bridges as a consequence of Na+ migration and leads to a partial destruction of Al-O bonds, which might be responsible for an increase and decrease in the intensity of emission component at 2.81 eV. With an enhanced radiation dose, there is a decrease in intensity at 1.73 eV and an increase in intensity at 1.68 eV. Deconvoluted CL spectra of the alkali feldspars reveal a positive correlation between intensity at 2.09 eV and the radiation dose, which may be due to the formation of a radiation-induced defect center. These correlations can be fitted by an exponential curve, where the gradients differ between the alkali feldspars studied, and are largest for the microcline, followed by the orthoclase and then the sanidine. The intensity at 2.09 eV has the potential to be used in geodosimetry and geochronometry.

Cathodoluminescence Microscopy and Spectroscopy of Micro- and Nanodiamonds: An Implication for Laboratory Astrophysics

Color centers in selected micro- and nanodiamond samples were investigated by cathodoluminescence (CL) microscopy and spectroscopy at 298 K [room temperature (RT)] and 77 K [liquid-nitrogen temperature (LNT)] to assess the value of the technique for astrophysics. Nanodiamonds from meteorites were compared with synthetic diamonds made with different processes involving distinct synthesis mechanisms (chemical vapor deposition, static high pressure high temperature, detonation). A CL emission peak centered at around 540 nm at 77 K was observed in almost all of the selected diamond samples and is assigned to the dislocation defect with nitrogen atoms. Additional peaks were identified at 387 and 452 nm, which are related to the vacancy defect. In general, peak intensity at LNT at the samples was increased in comparison to RT. The results indicate a clear temperature-dependence of the spectroscopic properties of diamond. This suggests the method is a useful tool in laboratory astrophysics.

Thermal effects on cathodoluminescence in forsterite

Cathodoluminescence (CL) spectral analysis has been conducted for luminescent forsterite (olivine) of terrestrial and meteoritic origins. Two emission bands at 3.15 and 2.99 eV in blue region can be assigned to structural defect centres and two emission bands at 1.91 and 1.74 eV in red region to impurity centres of Mn2+ and Cr3+, respectively. These emissions reduce their intensities at higher temperature, suggesting a temperature quenching phenomenon. The activation energy in the quenching process was estimated by a least-square fitting of the Arrhenius plots using integrated intensity of each component as follows; blue emissions at 3.15 eV: 0.08–0.10 eV and at 2.99 eV: 0.09–0.11 eV, red emissions at 1.91 eV: ∼0.01 eV and at 1.74 eV: ∼0.02 eV. The quenching process can be construed by the non-radiative transition by assuming the Mott-Seitz model. The values of activation energies for blue emissions caused by structural defects correspond to the vibration energy of Si-O stretching mode in the lattice, and the values for red emissions caused by Mn and Cr impurity centres to Mg-O vibration energy. It implies that the temperature quenching energy might be transferred as a phonon to the specific lattice vibration.

Discovery of moganite in a lunar meteorite as a trace of H2O ice in the Moon’s regolith

Discovery of moganite in a lunar meteorite indicates the possibility of H2O ice accumulated in the Moon’s regolith. Moganite, a monoclinic SiO2 phase, has been discovered in a lunar meteorite. Silica micrograins occur as nanocrystalline aggregates of mostly moganite and occasionally coesite and stishovite in the KREEP (high potassium, rare-earth element, and phosphorus)–like gabbroic-basaltic breccia NWA 2727, although these grains are seemingly absent in other lunar meteorites. We interpret the origin of these grains as follows: alkaline water delivery to the Moon via carbonaceous chondrite collisions, fluid capture during impact-induced brecciation, moganite precipitation from the captured H2O at pH 9.5 to 10.5 and 363 to 399 K on the sunlit surface, and meteorite launch from the Moon caused by an impact at 8 to 22 GPa and >673 K. On the subsurface, this captured H2O may still remain as ice at estimated bulk content of >0.6 weight %. This indicates the possibility of the presence of abundant available water resources underneath local sites of the host bodies within the Procellarum KREEP and South Pole Aitken terranes.

RESPONSE OF CATHODOLUMINESCENCE OF ALKALI FELDSPAR TO He + ION IMPLANTATION AND ELECTRON IRRADIATION

Cathodoluminescence (CL) of minerals such as quartz and zircon has been extensively studied to be used as an indicator for geodosimetry and geochronometry. There are, however, very few investigations on CL of other rock-forming minerals such as feldspars, regardless of their great scientific interest. This study has sought to clarify the effect of He+ ion implantation and electron irradiation on luminescent emissions by acquiring CL spectra from various types of feldspars including anorthoclase, amazonite and adularia. CL intensities of UV and blue emissions, assigned to Pb2+ and Ti4+ impurity centers respectively, decrease with an increase in radiation dose of He+ ion implantation and electron irradiation time. This may be due to decrease in the luminescence efficiencies by a change of the activation energy or a conversion of the emission center to a non-luminescent center due to an alteration of the energy state. Also, CL spectroscopy of the alkali feldspar revealed an in-crease in the blue and yellow emission intensity assigned to Al-O−-Al/Ti defect and radiation-induced defect centers with the radiation dose and the electron irradiation time. Taken together these results indicate that CL signal should be used for estimation of the α and β radiation doses from natural radionuclides that alkali feldspars have experienced.

Luminescence spectroscopical properties of plagioclase particles from the Hayabusa Sample Return Mission: An implication for study of space weathering processes in the asteroid Itokawa

We report a systematic spectroscopical investigation of three plagioclase particles (RB-QD04-0022, RA-QD02-0025-01, and RA-QD02-0025-02) returned by the Hayabusa spacecraft from the asteroid Itokawa, by means of scanning electron microscopy, cathodoluminescence microscopy/spectroscopy, and micro-Raman spectroscopy. The cathodoluminescence properties are used to evaluate the crystallization effects and the degree of space weathering processes, especially the shock-wave history of Itokawa. They provide new insights regarding spectral changes of asteroidal bodies due to space weathering processes. The cathodoluminescence spectra of the plagioclase particles from Itokawa show a defect-related broad band centered at around 450 nm, with a shoulder peak at 425 nm in the blue region, but there are no Mn- or Fe-related emission peaks. The absence of these crystal field-related activators indicates that the plagioclase was formed during thermal metamorphism at subsolidus temperature and extreme low oxygen fugacity. Luminescence characteristics of the selected samples do not show any signatures of the shock-induced microstructures or amorphization, indicating that these plagioclase samples suffered no (or low-shock pressure regime) shock metamorphism. Cathodoluminescence can play a key role as a powerful tool to determine mineralogy of fine-grained astromaterials.

Cathodoluminescence of synthetic zircon implanted by He+ ion

Abstract He+ ion implantation at 4.0 MeV, equivalent to energy of α particles from natural radioactive nuclei 238U and 232Th, has been conducted for undoped synthetic zircon. The cathodoluminescence (CL) of implanted samples was measured to clarify the radiation-induced effects. Unimplanted synthetic zircon shows pronounced and multiple blue emission bands between 310 nm and 380 nm, whereas the implanted samples have an intense yellow band at ~550 nm. The blue emission bands can be assigned to intrinsic defect centers formed during crystal growth. The yellow band should be derived from induced-defect centers by He+ ion implantation, which might be related to the metamicitization originated from a self-induced radiation in natural zircon. The yellow band may be separated into two emission components at 1.96 eV and 2.16 eV. The emission component at 2.16 eV is recognized in both unimplanted and implanted samples, and its intensity increases with an increase in the implantation dose. The CL of zircon can be used as the geodosimeter.

Cathodoluminescence and Raman Spectromicroscopy of Forsterite in Tagish Lake Meteorite: Implications for Astromineralogy

The Tagish Lake meteorite is CI/CM2 chondrite, which fell by a fireball event in January 2000. This study emphasizes the cathodoluminescence (CL) and Raman spectroscopical properties of the Tagish Lake meteorite in order to classify the meteoritic forsterite and its relation to the crystallization processes in a parent body. The CL-zoning of Tagish Lake meteorite records the thermal history of chondrules and terrestrial weathering. Only the unweathered olivine is forsterite, which is CL-active. The variation of luminescence in chondrules of Tagish Lake meteorite implies chemical inhomogeneity due to low-grade thermal metamorphism. The blue emission center in forsterite due to crystal lattice defect is proposed as being caused by rapid cooling during the primary crystallization and relatively low-temperature thermal metamorphism on the parent body of Tagish Lake meteorite. This is in a good agreement with the micro-Raman spectroscopical data. A combination of cathodoluminescence and micro-Raman spectroscopies shows some potentials in study of the asteroidal processes of parent bodies in solar system.