Cathodoluminescence (CL) is a visible light of emissions when a material is irradiated by an electron beam. The luminescence phenomenon mainly occurs due to a presence of specific luminescence center due to lattice defect or impurity. The CL features in minerals are closely related to varieties of emission centers such as the impurity concentration, defect density, crystal fields (e.g., Marfunin, 1979). Therefore, a CL method has a high advantage to reveal internal textures, growth zoning and antigenic minerals, which are difficult to identify by conventional optical examinations such as a polarization microscopy (e.g., Matsunami
Enstatite is one of most important rock-forming minerals in the terrestrial and extraterrestrial materials. The enstatite CL in the meteorites has been investigated to reveal their thermal histories (e.g., Derham
Yamato 86004 (hereafter Y-86004) is one of Antarctic E-chondrite classified as an EH melt rock, which is characterized by constituent minerals having a feature of crystallization from melts with an Abee matrix (Rubin and Scott, 1997). We have found a CL zonation composed the enstatite with various color in Y-86004 for the first time from the meteorites. In this study, we have conducted to clarify the CL zonation in Y-86004 using CL color imaging and spectroscopy for estimating a thermal history of the meteorite.
The sample of Y-86004 was provided as a polished thin section by the National Institute of Polar Research (NIPR), Japan. It is a rounded shape with ~4.5 mm in diameter (
CL color images were obtained using the Luminoscope (ELM-3), attached with a cooled charge-coupled device (CCD) camera, which consists of a cold cathode discharge tube and a vacuum chamber in which the sample is placed. It was operated stably with electron beams generated by an excitation voltage of 15 kV and a beam current of 0.5 mA. CL spectroscopy was carried out by an SEM-CL system, which consists of an SEM (JEOL, JSM-5410LV) combined with a grating monochromater (Oxford, Mono CL2) with operating conditions of 15 kV and 1.0 nA in a scanning mode. The CL emitted from the sample was collected in the range of 300–800 nm with a photomultiplier tube by a photon counting method. All CL spectra were corrected for total instrumental response, which was measured using of a calibrated standard lamp (Eppley Laboratory: Quartz Halogen Lamp). This correction prevents errors in the peak position of emission bands and allows quantitative evaluation of CL intensity. Detailed construction of the equipment and the analytical procedure can be found in Ikenaga
Y-86004 shows a rounded shape of ~4.5 mm diameter in a thin section, which is completely surrounded by a fusion crust. It exhibits no texture with chondrules and chondrule fragments under a polarizing microscope. According to EPMA and Raman spectroscopy analyses, the meteorite contains orthopyroxine of near-end-member enstatite (20–200 μm), opaque minerals of metallic Fe-Ni and troilite (up to 200 μm), plagioclase with albite composition (up to 30 μm) and silica mineral of tridymite (up to 120 μm) and glassy materials mostly in the fusion crust. Orthoenstatite (Oen) is predominant phase, which occurs as euhedral lath-shape grains, which usually protrude into or enclosed in opaque minerals. Plagioclase and silica mineral are commonly found as interstitial materials in the Oen grains. The petrographic texture characterized by enstatite occurrences in Y-86004 is similar to that observed in the EH melt rock, which may have experienced an impact melt near the surface of the parent body (Rubin and Scott, 1997; Lin and Kimura, 1998). However, Y-86004 has no impact-shock textures (e.g., planar fractures, clinoenstatite lamellae, and mosaicism) in the constituent minerals.
Chemical compositions of the enstatite with different CL color are compiled on an average of six analyses per grain by EPMA (WDS) in
Chemical compositions on an average of three analyses of the enstatite with different CL colors and fusion crust in each zone. n.d.: not detected.
Enstatite in zone 1 | Enstatite in zone 2 | Enstatite in zone 3 | Fusion crust in zone 4 | ||
---|---|---|---|---|---|
SiO2 | 59.80 | 59.96 | 59.20 | 53.90 | wt.% |
TiO2 | n.d. | 0.01 | n.d. | n.d. | |
Al2O3 | 0.11 | 0.10 | 0.07 | 4.24 | |
FeO | 0.43 | 0.11 | 0.60 | 30.60 | |
Cr2O3 | 0.02 | n.d. | 0.02 | 0.39 | |
MnO | n.d. | n.d. | 0.04 | 0.94 | |
MgO | 39.81 | 39.74 | 39.48 | 5.05 | |
CaO | 0.26 | 0.28 | 0.24 | 3.55 | |
NiO | 0.01 | 0.01 | 0.06 | n.d. | |
Na2O | 0.01 | n.d. | n.d. | 1.41 | |
K2O | 0.01 | 0.01 | 0.01 | n.d. | |
Total | 100.46 | 100.22 | 99.73 | 100.00 |
Y-86004 shows a rounded shape with blue luminescence as the whole pattern in a color CL image (
The enstatite with low FeO contents (<5 wt.%) in E-chondrites shows visible CL due to a negligibly small concentration of quencher (e.g., Keil, 1968; Weisberg
Color CL imaging in the thin section reveals a concentric zonation mainly attributable to the distribution of the enstatite different in color as arranged from within outward blue CL (zone 1), light blue CL (zone 2), red CL (zone 3) and non-CL zone (zone 4) corresponding to a fusion crust (
Average width of each band in the zonation is ~1.50 mm for zone 1, ~700 μm for zone 2, ~100 μm for zone 3 and ~80 μm for zone 4. Zone 1 and 2 consist of blue and light blue enstatite, dark blue tridymite, reddish-brown plagioclase, and non-CL grains of opaque minerals. These zones have similar petrographic texture, however, the distribution ratio of light-blue-CL enstatite in zone 2 is higher than that of zone 1. Zone 3 is characterized by red-CL enstatite with opaque minerals, but no tridymite. Zone 4 composed of glassy materials including blebs and opaque minerals exhibits no emission, suggesting heating above the melting temperature of the enstatite. Therefore, color CL zonation consisted of the enstatite with various colors should record a thermal history of the meteorite in the process of rapid heating and quenching during a flash heating when it entered into the atmosphere.
The CL spectra of the enstatite in each zone are shown in
According to Ohgo
Color CL image (
The result of polarization microscopy and Raman spectroscopy show two types of enstatite with blue and light-blue CL in zone 2, former of which is virtually identical to Oen with a blue emission in zone 1. The light-blue enstatite has two defect centers of Defect I and Defect II, whereas the enstatite is identified as Oen. It is characterized by the Defect II not detected in the blue enstatite. Inasmuch as the meteorite experienced a flash heating above the melting point of the enstatite, the heating and cooling process during a falling through the atmosphere might affect a creation of defect center (Defect II) and alteration of existing defect (Defect I). Therefore, according to Gasparik (1990), some portion of the enstatite in zone 2 probably might have experienced a phase transition from Oen to protoenstatite (Pen) during a flash heating, which occurs at near 1273 K. In this case, the enstatite with blue CL considerably survived from the original enstatite the same to one in zone 1, suggesting the temperature at around 1273 K. When Pen is rapidly cooled, the phase transition involves physical stress, which might create defect center (Defect II) in the structure.
The enstatite in zone 3 is identified as LT-Cen by a Raman spectroscopy. This result is also supported by the peak energy of the emission component related to Mn2+ impurity. According to Ohgo
Y-86004 suffered a heavy ablation on its surface, which took away melting materials from the meteorite body in a short period. Zone 4 was formed during such ablation with amorphousization of enstatite-rich materials at the surface of the meteorite. Therefore, the surface of the meteorite has been exposed at and above the temperature of a melting point (>1831 K) of the enstatite.
Y-86004 was abruptly heated for a short time when it entered the earth's atmosphere, and rapidly quenched in the Antarctic ice immediately after its falling. In this study, the enstatite with a background of previous phases corresponding to the elevated temperatures can be characterized by CL imaging and spectroscopy, which reveals a mechanism of CL color zonation found in Y-86004.
We have conducted to evaluate the temperate history of Y-86004 during heating when the meteorite entered into the atmosphere. In general, typical falling object collides with the Earth’s atmosphere at velocities of 12–20 km·s–1 (Bottke
where
The result of CL examination suggests that zone 2 in the range of the depth from surface between ~200 and ~900 μm might be heated at around 1273 K, which is the transition temperature from Oen to Pen. Zone 1 was not heated up to 1273 K due to the presence of only Oen with a blue CL. Zone 3 between ~100 and ~200 μm might be expected to be heated up to the melting point of the enstatite at 1831 K due to the presence of HT-Cen supposed by the CL spectroscopy. The elevated temperatures in the zone estimated from the result of CL analysis might be consistent with the temperatures in corresponding depth from the surface calculated from the duration of 20 second (