Elemental and oxygen isotopic analysis of highly vaporized cosmic spherules from Widerøefjellet, Sør Rondane Mountains (East Antarctica)
: Towards a better understanding of cosmic spherule progenitors, chemical modifications during atmospheric entry and subsequent alteration on Earth’s surface

Scriptie/Masterproef: Master's Thesis

Samenvatting

Every year, 40,000 ± 20,000 metric tons of extraterrestrial matter is accreted to Earth. This flux is dominated by micrometeorites, dust particles within the size range of 10 – 2,000 μm. During atmospheric passage, a substantial fraction of these micrometeorites melt almost entirely, and is referred to as cosmic spherules. A small subset among these cosmic spherules indicate anomalously high degrees of vaporization. Here, a set of such highly vaporized cosmic spherules from Widerøefjellet (Sør Rondane Mountains, East Antarctica) is characterized for their petrographic features, major and trace element concentrations, and their oxygen isotopic compositions. Using Scanning Electron Microscope – Energy-Dispersive X-ray Spectrometry (SEM-EDS), an initial screening for petrographic and chemical features was made. From the resulting elemental data, cosmic spherules with high contents of Al, Ca and Ti – refractory elements that can be used as indicators for a high degree of evaporation – were selected for further analysis. Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS) was used to constrain the major and trace element geochemistry. Based on the chemical parameters defined by Cordier et al. (2011) particles were classified into ‘normal chondritic’, ‘CAT-like’ (with ‘CAT’ standing for Ca, Al and Ti) and ‘High Ca-Al’ cosmic spherules. The abundant highly vaporized ‘CAT-like’ spherules (making up ~71 % of the initial selection) were thoroughly studied for their trace element composition and compared to the available data for the other chemical groups of cosmic spherules. As 40 % of the studied ‘CAT-like’ spherules show degrees of evaporation above 90 % (Mg/Al < 1), chemical fractionation during atmospheric entry appears to have affected the chemical composition of these cosmic spherules more strongly than those of ‘normal chondritic’ cosmic spherules from Widerøefjellet (Goderis et al., 2020) and other Antarctic sampling locations (e.g., Victoria Land Transantarctic Mountains) (Cordier et al., 2011). The low Fe contents, which are generally observed for ‘CAT-like’ cosmic spherules, originate from two distinct processes; metal bead formation and subsequent ejection, and evaporative loss of Fe due to its relatively low condensation temperature (50 % TC = 1334 K). To assess the degree of elemental mobilization processes in cosmic spherules, Laser Ablation – Time-Of-Flight – Inductively Coupled Plasma – Mass Spectrometry (LA-TOF-ICP-MS) was used to generate two-dimensional elemental maps of three highly vaporized ‘CAT-like’ cosmic spherules. Through comparison with the chemical composition of the surrounding sediment, enrichments in Sr, Ba, Ce, Th and U were identified in the outer rims of the particles. Despite the cold conditions in Antarctic environment, chemical alteration with mobilization of these elements from the sediment into the cosmic spherules has occurred. To study the nature of micrometeorite parent material, the degree of evaporation experienced, and the extent of exchange with atmospheric O2, oxygen isotope ratios were determined for a subset of the particles using Secondary-Ion Mass Spectrometry (SIMS). These data indicate that the degree of oxygen isotopic fractionation varies considerably amongst the studied particles, showing a large spread in δ18O and δ17O for both ‘CAT-like’ (δ17O = 4 – 30 ‰ and δ18O = 8 – 59 ‰) and ‘normal chondritic’ (δ17O = 4 – 29 ‰ and δ18O = 13 – 52 ‰) particles. The measured data cover and extend the range previously reported in literature. Even among the ‘CAT-like’ particles, cosmic spherules with normal degrees of evaporation are present. Based on the Δ17O data for 10 ‘normal chondritic’ cosmic spherules, 30 % of these particles were found to derive from ordinary chondrites (OC) whereas 70 % have a carbonaceous (CC) chondritic percentage, which is in reasonable agreement with literature data. A similar distribution is observed for 9 analysed ‘CAT-like’ particles, although a significant fraction of the Δ17O data (~33 %) is too close to the terrestrial fractionation line (TFL) to provide a decisive answer on the parent material. While the initial classification scheme of Cordier et al. (2011) based on chemical criteria proves a useful tool, the outline of this classification became inadequate when incorporating oxygen isotope data. In general, a large variety of processes (including fragmentation, differential melting, metal bead extraction, redox shifts and evaporation) affect the final elemental and oxygen isotopic composition of cosmic spherules. Based on the results from this work, the studied ‘CAT-like’ cosmic spherules sample a variety of different primitive and differentiated parent bodies. Hence, the construction of a single unambiguous classification scheme that comprises of all types of highly vaporized cosmic spherules is complex and needs to take into account petrographic features, elemental compositions and oxygen isotopic compositions. With the currently available data, and as most ‘CAT-like’ spherules of this study comprised V-type and white barred olivine spherules, a classification should mainly be based on petrographic features (in the case of barred olivine textures), or follow the chemical classification system of Cordier et al. (2011) for vitreous spherules. As other textural types prove more challenging to classify and a continuum possibly exists, a scheme that combines the respective petrographic with chemical and isotopic information is currently in development, allowing for a more reliable evaluation of the degree of evaporation experienced by each individual particle.
Datum prijs30 jun 2020
Originele taalEnglish
Prijsuitreikende instantie
  • Scheikunde
  • Vrije Universiteit Brussel
BegeleiderSteven Goderis (Promotor)

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