Geological materials used for luminescence dating and associated with the fall of meteorites on the Earth’s surface are extremely rare. The Morasko region has gained fame over the past 100 years because of a cosmic catastrophe which took place there. After thousands of years, the remains of a large metal meteorite which fell in this area have been found. In this article, we would like to state whether it is possible, using luminescence methods, to determine the moment when the iron meteorite fell on the surface of the Earth. The material which was analysed consisted of meteorite crust layers – melt/fusion and “semi melt/fusion”, including sintered ones, along with the sediments surrounding the meteorite. The final results are connected with four objects of different sizes (large ones and small shrapnel – 261 kg, 34 kg, 970 g and 690 g). The obtained results show a large discrepancy, which is most likely associated with the problem of resetting the luminescence signal of the tested materials.
- meteorite crusts
- OSL dating
The Morasko nature-reserve area is a place in Poland where about 1500 elements of iron meteorites with a total mass of about 2000 kg have been excavated in the last 100 years (Stankowski, 2009; Muszyński
The Morasko nature reserve is also famous for several impact craters resulting from the fall of meteorites weighing at least several hundred tonnes. Such extraordinary findings have led to an increased interest in the history of this particular area, and several attempts have been made to determine the time when these meteorites fell. In the past, luminescence methods were applied to the material obtained from cleaned meteorites (Stankowski
Iron meteorites, when falling into the Earth’s atmosphere, fly with an initial speed of approx. 10–20 km/s. Air resistance causes braking at an altitude of 20–40 km, warming up and shining. During braking, due to the increase in temperature, the total annihilation of such meteoroids usually occurs. But sometimes, in the case of larger objects, the remaining heated parts reach the surface of the Earth. The heated surface of the meteorites is varied by ablative niches with a thin layer/film of fusion. Iron meteorites’ falling into the mineral substrate cause either zone variety or thermal effects. On the surface of a meteorite, a non-continuous and very thin fusion layer can remain. The molten meteorite matter, penetrating the encountered meteorite mineral grains, leads to the formation of a spatially limited “semi-fusion unit” – enabling luminescent dating to be conducted due to the existence of heated mineral grains. Apart from the fusion and the “semi-fusion” zone, a commonly continuous sintered layer is created (all structures seen in
Recognition of the layers: the right fusion with “semi-fusion” and sintering was documented in detail with the use of the Scanning Electron Microscopy (SEM) method and an Energy Dispersive Analyser (EDS), performed on a 970 g meteorite (see
The largest iron-meteorite shower in Central Europe occurred in the Morasko meteorite nature reserve near Poznań (Great Poland Lowland/Wielkopolska), (Pilski and Walton 1999). To date, more than 2000 kg of extraterrestrial iron matter have been officially recognised. It is impossible to determine how many unreported meteorites have also been found. This area is mostly wooded, which makes it harder to search for new findings. All of the found meteorites have a very resistant coating of multiple origins, connected with the thermal and pressure-impact events, and later with the weathering processes. Previous palaeo-environmental studies (morphogenetic, palinologic) predicted the location as well as the time of the Morasko event. Older dates (Stankowski
For this investigation, 18 samples were used. Nine of them were collected from the strata/ground in a close proximity to the investigated meteorites, and the other nine samples came directly from the meteorite cover. Samples from the strata were collected using standard equipment, and it was easy to collect enough material for further investigation. Collecting samples from meteorites is always more complicated.
Detailed descriptions of the samples used are GdTL 2476-GdTL 2481 from the crust of the 34 kg meteorite; GdTL 2482 and GdTL 2483 from sediments surrounding the 261 kg meteorite; GdTL 2484 from the crust of the 690 g meteorite; GdTL 2485, GdTL 2487 and GdTL 2490 from the immediate proximity of the 34 kg meteorite; GdTL 2486 and GdTL 2488 from a distance of about 5 cm from the 34 kg meteorite; GdTL 2489 and GdTL 2491 from a distance of about 10–15 cm from the 34 kg meteorite; GdTL 2492 from the crust of the 34 kg meteorite (another/earlier dating); GdTL 2493 from the crust of the 970 g meteorite. All this data is also shown in
The dose rate results of the analysed samples and their age estimation.
Sample name Dose rate for age calculation (Gy/ka) Equivalent dose (Gy) Age (ka) CAM GdTL 2476 (meteorite 34 kg) 1.27 ± 0.11 6.9 ± 0.4 5.4 ± 0.6 GdTL 2477 (meteorite 34 kg) 1.36 ± 0.12 9.6 ± 1.0 7.0 ± 0.9 GdTL 2478 (meteorite 34 kg) 1.43 ± 0.13 29.5 ± 2.4 20.5 ± 2.5 GdTL 2479 (meteorite 34 kg) 1.42 ± 0.12 30.5 ± 3.3 21.4 ± 2.9 GdTL 2480 (meteorite 34 kg) 1.14 ± 0.10 5.8 ± 0.4 5.0 ± 0.5 GdTL 2481 (meteorite 34 kg) 1.33 ± 0.18 11.4 ± 1.2 8.5 ± 1.5 GdTL 2482 (surrounding meteorite 261 kg) 1.67 ± 0.17 18.1 ± 1.6 10.8 ± 1.5 GdTL 2483 (surrounding meteorite 261 kg) 1.58 ± 0.15 9.4 ± 1.1 5.9 ± 0.9 GdTL 2484 (meteorite 690 g) 1.12 ± 0.11 24.9 ± 2.0 22.2 ± 2.9 GdTL 2485 (surrounding meteorite 34 kg) 2.40 ± 0.19 8.5 ± 1.0 3.5 ± 0.5 GdTL 2486 (surrounding meteorite 34 kg) 3.21 ± 0.24 17.0 ± 1.2 5.2 ± 0.5 GdTL 2487 (surrounding meteorite 34 kg) 3.79 ± 0.26 14.7 ± 1.6 3.8 ± 0.5 GdTL 2488 (surrounding meteorite 34 kg) 4.37 ± 0.30 11.3 ± 0.5 2.5 ± 0.2 GdTL 2489 (surrounding meteorite 34 kg) 3.84 ± 0.29 59.5 ± 4.4 15.4 ± 1.6 GdTL 2490 (surrounding meteorite 34 kg) 2.22 ± 0.17 6.8 ± 0.7 3.0 ± 0.4 GdTL 2491 (surrounding meteorite 34 kg) 3.17 ± 0.24 48.7 ± 4.1 15.3 ± 1.7 GdTL 2492 (meteorite 34 kg) 1.04 ± 0.10 79.0 ± 8.0 75.9 ± 10.7 GdTL 2493 (meteorite 970 g) 1.34 ± 0.13 37.2 ± 1.9 27.6 ± 3.0
The dose rate results of the analysed samples and their age estimation.
In order to determine the period in which the luminescence signal was reset the previous time, it is necessary to determine the following two independent parameters: the dose rate (Gy/year) and the equivalent dose (Gy). The final age result is always a quotient of these parameters, according to the simple equation presented below.
In the case of meteorite fusion crusts, which are non-standard samples, obtaining the appropriate material for the luminescent analysis is difficult, yet possible. It should also be noted that, from the point of view of achieving the final results, the greatest problem which we have exposed was the correct definition of the annual dose rate. The dose rate is the amount of energy absorbed per year from radiation in the environment surrounding the measured material, and can be derived by directly measuring the amount of radioactivity, or by a chemical analysis of the surrounding material. Usually, in order to determine the dose rate, high-resolution gamma spectrometry or mass spectrometry (ICP-MS) are used.
To derive the dose rate for meteorites as well as for sediment samples, high-resolution gamma spectrometry, equipped with an HPGe gamma ray detector manufactured by CANBERRA, was used. This equipment measured the concentration in the U-238 series, the Th-232 series, and the concentration of K-40. Besides this, the activity of the Cs-137 artificial isotope was measured in samples to recognise modern sediment mixing. The main source of this isotope is provided by nuclear weapon tests and, additionally, by the nuclear power plant accident in Chernobyl. The samples for measurement were dried, homogenised and placed in measurement containers. After being placed in the measurement containers, the samples were stored for about 4 weeks to ensure equilibrium between gaseous 222Rn and 226Ra in the 238U decay chain. Typical counting time was about 80 ks. The resolution of the detector was about 1.8 keV and the relative efficiency was 35%. As a standard, materials manufactured by IAEA (IAEA-375, RGU, RGTh, RGK) were used. Additionally, a reference material of IAEA-385 was used to check the calibration quality. To calculate the U-238 content, the following gamma lines were taken: 295.1 keV (Pb-214), 352.0 keV (Pb-214), 609.3 keV (Bi-214) and 1120.3 keV (Bi-214). In the case of the Th-232 decay chain the following gamma lines were considered: 583.0 keV (TL-208), 911.2 keV (Ac-228) and 2614.4 keV (Tl-208). To calculate the K-40 content the 1460.8 keV gamma line was taken. To calculate the Cs-137 content the 661.7 keV gamma line was taken. The activities of radioisotopes were converted into dose rates by using the conversion factors described by Guerin
For OSL measurements, coarse grains and fine grains of quartz were extracted from the samples. Coarse-grained quartz was extracted from soil samples from meteorite environments, and fine-grained quartz was extracted from the meteorite samples. Both fractions were treated with 20% hydrochloric acid (HCl) and 20% hydrogen peroxide (H2O2). The last step was to add hexafluorosilicic acid (H2SiF6) for fine grains and hydrofluoric acid (HF) for coarse-grained quartz.
All OSL measurements were performed using an automated Daybreak 2200 TL/OSL reader (Bortolot, 2000). This reader uses the Hoya U-340 optical filter with blue diodes (470 ± 4 nm), delivering about 60 mW/cm2 at the sample position.
Laboratory irradiations were performed using a calibrated 90Sr/90Y beta source mounted onto the reader, delivering a dose rate of 2.8 Gy/min. Equivalent doses were determined using the single-aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000). The OSL SAR protocol, which was used in our measurements, contains the following steps.
Irradiation with the regenerative beta dose Preheating at a temperature of 260°C for 10 s Blue-light stimulation at a temperature of 125°C for 100 s Irradiation with the test dose Cut-heat at a temperature of 220°C Blue-light stimulation at a temperature of 125°C for 100 s.
Irradiation with the regenerative beta dose
Preheating at a temperature of 260°C for 10 s
Blue-light stimulation at a temperature of 125°C for 100 s
Irradiation with the test dose
Cut-heat at a temperature of 220°C
Blue-light stimulation at a temperature of 125°C for 100 s.
For an equivalent dose calculation, the first second of the signal was used and the background was estimated from the last 10 seconds. The SAR dose response curves were best represented by a single saturating exponential. A dose recovery test was carried out for both samples, performed under the standard SAR conditions. For each sample, 5 different aliquots were used. Subsequently, all aliquots were bleached with blue light for 100 s (at room temperature) and after a pause of 10000 s were bleached for another 100 s. After the bleaching, a laboratory dose with a value similar to the equivalent dose of each sample was administered and measured using the SAR protocol. The results of the obtained dose recovery ratio were close to unity.
Equivalent doses were determined using the single-aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000), and OSL age results were obtained using the Central Age Model (CAM) (Galbraith
In the research conducted for over a dozen years by Stankowski, associated with the reliable determination of the impact time of the Morasko meteorites, significant progress has taken place in terms of the ability to employ luminescence methods. The most important change was the use of materials from one and the same meteorite, but collected from different places at the surface. All samples from the meteorite weighing 34 kg contain a thin layer of sediment which was adhered to the metal surface, along with the material which comes from the molten or semi-molten zone. This material, according to our knowledge, was heated to high temperatures in the past, during the fall of the heated iron meteorite in this place. In the ideal case, all the investigated samples would be bleached to a residual level at the same time in the past, so the obtained results should be very similar. These results are presented in
It is inevitable to refer to the previous luminescence analyses conducted more than 10 years ago by Stankowski and Bluszcz (Stankowski
The reflections on the possibility of determining the time of the meteorite fall in Morasko, as presented here, indicate that this problem is complex and difficult to be interpreted using luminescence methods. Despite the fact that 7 independent samples were collected from the same meteorite, the obtained results were far from those expected. Most probably, this might have been related to the large temperature differences of individual pieces of the fragmented meteorite (heated surface and cold interior). Of course, this is associated with a significant discrepancy in the obtained results. However, a reference can be made to
The dose rate results of the analysed samples and their age estimation.
|Sample name||Dose rate for age calculation (Gy/ka)||Equivalent dose (Gy)||Age (ka) CAM|
|GdTL 2476 (meteorite 34 kg)||1.27 ± 0.11||6.9 ± 0.4||5.4 ± 0.6|
|GdTL 2477 (meteorite 34 kg)||1.36 ± 0.12||9.6 ± 1.0||7.0 ± 0.9|
|GdTL 2478 (meteorite 34 kg)||1.43 ± 0.13||29.5 ± 2.4||20.5 ± 2.5|
|GdTL 2479 (meteorite 34 kg)||1.42 ± 0.12||30.5 ± 3.3||21.4 ± 2.9|
|GdTL 2480 (meteorite 34 kg)||1.14 ± 0.10||5.8 ± 0.4||5.0 ± 0.5|
|GdTL 2481 (meteorite 34 kg)||1.33 ± 0.18||11.4 ± 1.2||8.5 ± 1.5|
|GdTL 2482 (surrounding meteorite 261 kg)||1.67 ± 0.17||18.1 ± 1.6||10.8 ± 1.5|
|GdTL 2483 (surrounding meteorite 261 kg)||1.58 ± 0.15||9.4 ± 1.1||5.9 ± 0.9|
|GdTL 2484 (meteorite 690 g)||1.12 ± 0.11||24.9 ± 2.0||22.2 ± 2.9|
|GdTL 2485 (surrounding meteorite 34 kg)||2.40 ± 0.19||8.5 ± 1.0||3.5 ± 0.5|
|GdTL 2486 (surrounding meteorite 34 kg)||3.21 ± 0.24||17.0 ± 1.2||5.2 ± 0.5|
|GdTL 2487 (surrounding meteorite 34 kg)||3.79 ± 0.26||14.7 ± 1.6||3.8 ± 0.5|
|GdTL 2488 (surrounding meteorite 34 kg)||4.37 ± 0.30||11.3 ± 0.5||2.5 ± 0.2|
|GdTL 2489 (surrounding meteorite 34 kg)||3.84 ± 0.29||59.5 ± 4.4||15.4 ± 1.6|
|GdTL 2490 (surrounding meteorite 34 kg)||2.22 ± 0.17||6.8 ± 0.7||3.0 ± 0.4|
|GdTL 2491 (surrounding meteorite 34 kg)||3.17 ± 0.24||48.7 ± 4.1||15.3 ± 1.7|
|GdTL 2492 (meteorite 34 kg)||1.04 ± 0.10||79.0 ± 8.0||75.9 ± 10.7|
|GdTL 2493 (meteorite 970 g)||1.34 ± 0.13||37.2 ± 1.9||27.6 ± 3.0|