Primordial and other noble gases in micrometeorites

• Main Author: Baecker, Bastian
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2014
• Subjects: 550; 550 Earth sciences; Dome Fuji; Miller Butte; St. Louis; Transantarctic Mountains; Antarc*; Antarctica
• Online Access: https://archiv.ub.uni-heidelberg.de/volltextserver/16747/; https://doi.org/10.11588/heidok.00016747; https://nbn-resolving.org/urn:nbn:de:bsz:16-heidok-167474

The present study, realized within the framework of the DFG (Deutsche Forschungsgemeinschaft) SPP (Special Priority Program) 1385 "The First 10 Million Years of the Solar System - a Planetary Materials Approach", addresses the proposition of analyzing micrometeorites (MMs) (sizes of 50μm - 1mm) on their noble gas content along with their isotopic composition. The experimental studies and the dissertation was performed at the Max-Planck-Institute for Chemistry in Mainz in collaboration with the Heidelberg University since 2010. In particular, the similarities and differences of MMs to larger meteorites were studied. Also of interest was the thermal history of MMs along with changes in their noble gas inventory while passing Earth's atmosphere. In our study we focused on xenon especially in unmelted MMs. This required the operation of a special version of a noble gas mass spectrometer (MS) which in this case was the "Noblesse" from Nu Instruments. Continuous care and optimization of the measurement methods were essential. The specialty of this device is the ion-counting multicollector system for especially high sensitivity and detection of small amounts of noble gases. Worldwide there is only one more device of this type, at Washington University, St. Louis (USA), where, among other things, the solar wind noble gases of the Genesis mission have been determined. Two different groups of micrometeorites were examined. Firstly MMs collected from a geological trap on top of the Miller Butte ridge, in the area of the Transantarctic Mountains. These micrometeorites were spotted and collected by Luigi Folco, Pierre Rochette and colleagues during a PNRA mission (see Rochette et al. (2008)). The MMs are relatively large (250 - 1000 μm), they sometimes show weathering and were deposited for probably about 1 Ma. In October 2010, with kind cooperation of Luigi Folco, Carole Cordier and Matthias van Ginneken, 103 particles of 51 micrometeorites (MMs) were selected for the noble gas measurements. These MMs consist of the following three main groups - "Cosmic spherules - CS " (more or less glass spherules caused by complete melting of unmelted MMs), "scoriaceous MMs - ScMMs" (partially molten MMs ) and "unmelted MMs - UnMMs. The melting processes are caused by the entry and passage of these small extraterrestrial particles through the Earth's atmosphere. The second group of MMs originated from the snow of the central region of Antarctica (Dome C (DC) - CONCORDIA Collection (see Duprat et al. (2007)) and from Cap Prudhomme (CP) (Maurette et al. (1991)). These particles were extracted and collected by melting snow of the Dome C area and are particularly small. The MMs are deposited only for a short time period (a few decades), thus are "fresh" and relatively unweathered. In November 2010, samples of these MMs were provided with kind support by Jean Duprat, Cécile Engrand and Michel Maurette (University of South-Paris, on the campus in Orsay, France). We were able to acquire a total of 50 MM particles from 28 different MMs. Among them were crystalline (Xtal), fine-grained unmelted and carbonaceous (FgC) and partially molten (Sc) micrometeorites. Before the first MM measurements were carried out, the measuring facility was tested on reliability and accuracy by using small particles of larger meteorites. We gradually degassed the MMs by using a 30W CO2-Laser system. For a complete degassing a power of ~1W was sufficient in most cases. After extraction, the inert gases were partitioned in He + Ne, Ar, Kr and Xe fractions. Furthermore, Ar, Kr and Xe were adsorbed at a cold trap filled with active charcoal. Then the noble gases successively were analyzed using the "Noblesse". The sample measurements were supplemented by "blank" and calibration measurements. A total of 29 particles of 25 different TAM MMs and 11 particles of a total of 10 different DC and CP MMs were measured. For most of the particles we were able release sufficient noble gas quantities. However, in particular the very strongly melted "Cosmic spherules " and some of the less melted "scoriaceous MMs" showed clear signs of pre-degassing, probably through the interaction with the Earth's atmosphere. The MMs of Dome C and Cap Prudhomme are very small, but, however, the vast majority of these samples shows high noble gas concentrations compared to the TAM MMs. Measured 3He/4He ratios were between 0.9 x 10-4 and 149 x 10-4 for the TAM MMs and between 3 x 10-4 and 50 x 10-4 for the MMs of DC and CP, although sometimes with accompanied large uncertainties. Part of the TAM UnMMs and the DC MMs showed a composition in the range between solar wind (SW) and fractionated solar wind ("FSW"). The other (slightly more abundant) part showed - beside SW - significant signs of cosmogenic 3He contributions. Also in neon, most of the Un- and ScMMs clearly showed signs of solar wind and spallogenic contributions, the latter especially within the TAM MMs 45c.29, 45b.17 and 45b.08. Here, not surprisingly, in particular the CS and ScMMs show clear traces of Earth's atmosphere records. The highest measured concentrations for solar 20Ne in the TAM MMs were measured in UnMM X1 (~3.5 x 10-6 cc/g), whereas in the case of MMs from DC and CP this was DC 06_09_149 with ~8.0 x 10-5 cc/g. The DC values are in the range of what has been found for Dome Fuji MMs measured by Osawa and Nagao (2002). The values for the TAM MMs are significantly lower. The fact that for most TAM and DC/CP MMs the 40Ar/36Ar ratios are considerable lower than in air (40Ar/36Ar = 298.56; Lee et al. (2006)) suggests that these samples include extraterrestrial primordial Ar. Air contamination was indeed detectable, but except for 40Ar usually negligible. The lowest 40Ar/36Ar of 2.0 ± 1.8 was found for TAM UnMM 45c.33(1). The next lowest value, with a much smaller uncertainty is that of TAM MM 45c.35(3), with 4.1 ± 0.1. In the same context the low 38Ar/36Ar ratio of 0.185 ± 0.002 indicates a high proportion of the planetary component "Q(P1)". This component is detected in higher concentrations in carbonaceous chondrites. The same is true for the crystalline Dome C MM DC 06_09_149 which shows 40Ar/36Ar = 5.62 and 38Ar/36Ar = 0.186. Krypton in extraterrestrial samples typically shows only small variations in its isotopic composition and pure Kr evaluations are challenging. Nevertheless, for most of the MMs sufficient Kr-amounts were detected and particularly in association with the results for Ar and Xe interpretations are conceivable. It seems that especially isotopic fractionation processes are explainable by using combined elemental and isotopic plots of Ar, Kr and Xe. Here, isotopic fractionation processes are clearly present and often occur due to Earth atmosphere transitions of MMs, extraterrestrial radiation as well as terrestrial weathering. An example is 36Ar/132Xe versus 84Kr/132Xe, which shows for most of the ScMMs and CS a clear indication of fractionated air. The unmelted UnMMs, however, show noble gas compositions similar to Q (P1). A few MMs indicate a solar influence not only in He and Ne, but also in Ar - especially in the case of TAM UnMM X1 as well as a number of DC MMs . The main focus of the measurements was on xenon. On the one hand, the MS "Noblesse" gave us the opportunity to measure Xe in small quantities along with high sensitivities. On the other hand, so far, Xe has not been adequately measured within MMs. The results in this work represent a significant improvement compared to previous measurements of Osawa and Nagao (2002). Of the overall 29 TAM MMs we were able to analyze Xe for 24 particles, however, partially and in particular for the light Xe isotopes with large uncertainties. Here, UnMM 45c.35(3) shows the highest concentration of 132Xe with ~1.1 x 10-8 cc STP/g. The highest concentration for DC and CP MMs was found in DC 06_09_189, with ~1.7 x 10-7 cc STP/g. Overall, however, the Xe concentration was for both, the TAM MMs as well as the DC and CP MMs, at a similar level. Two distinct groupings are present. One group shows Xe with an isotopic composition similar to that of the Q(P1) component. This is usually true for the UnMMs. The other group shows Xe ratios similar to isotopically fractionated air Xe. A single MM sample is particularly and distinctly different of all others - 45c.29. The two measured particles of this MM show within all noble gas ratios increased, however, variable spallogenic contributions. Also, this MM contains 244Pu fission Xe. The probable origin (parent body) of this MM has so far not been revealed by both the noble gas and by petrological and mineralogical studies and requires further research. Using the detected spallogenic 21Ne concentrations for the two MMs of 45c.29 and for 45b.17, we were able to calculate rather large CRE-ages of ~9-71 Ma. Cosmogenic 38Ar, on the other hand, revealed a range of ~5-186 Ma. However, the majority of TAM MMs show CRE-ages of less than 1 Ma. The cosmogenic contents in MMs of DC and CP were negligible due to the high levels of solar neon and solar / primordial Ar. In addition, one must consider that the specimens may have lost a large proportion of their original trapped inventory during passage through the Earth's atmosphere, especially concerning the lighter noble gases He and Ne. On Earth, micrometeorites are explored in large numbers, located in a variety of mineralogical compositions as well as sizes and in different climatic regions - certainly the best region would be those of ice and snow. Many MMs show alteration and weathering effects along with signs for interaction with the Earth's atmosphere. However, there exist some specimens, which are virtually unchanged and show the complete spectrum of extraterrestrial components. The results obtained in this work show that the investigation of these particles may contribute to the understanding of the origin of Earth's atmosphere - and thus also of other planetary atmospheres. Furthermore micrometeorites may also help to understand how tiny particles behave and evolve in the solar system.