EGU22-5656, updated on 28 Mar 2022
https://doi.org/10.5194/egusphere-egu22-5656
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Decoupling of chemical and isotope fractionation processes during atmospheric entry of S-type micrometeorites

Seppe Lampe1,2, Bastien Soens2,3, Stepan Chernonozhkin4,5, Claudia González de Vega4, Matthias van Ginneken6, Flore Van Maldeghem2, Frank Vanhaecke4, Billy Glass7, Ian Franchi8, Herman Terryn9, Vinciane Debaille3, Philippe Claeys2, and Steven Goderis2
Seppe Lampe et al.
  • 1Hydrology and Hydraulic Engineering (HYDR), Vrije Universiteit Brussel, Brussels, Belgium (seppe.lampe@vub.be)
  • 2Analytical-, Environmental-, and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, B-1050 Brussels, Belgium
  • 3Laboratoire G-Time, Université Libre de Bruxelles, B-1050 Brussels, Belgium
  • 4Atomic & Mass Spectrometry – A&MS Research Unit, Ghent University, B-9000 Ghent, Belgium
  • 5Lehrstuhl Allgemeine und Analytische Chemie, Montanuniversität Leoben, 8700 Leoben, Austria
  • 6Centre for Astrophysics and Planetary Science, University of Kent, CT2 Canterbury, United Kingdom
  • 7Department of Geological Sciences, University of Delaware, DE Newark, United States of America
  • 8Planetary and Space Sciences, The Open University, MK7 Milton Keynes, United Kingdom
  • 9Electrochemical and Surface Engineering, Vrije Universiteit Brussel, B-1050 Brussels, Belgium

During atmospheric entry, micrometeorites experience variable degrees of (i) evaporation due to gas drag heating and (ii) mixing with atmospheric oxygen. Evaporation affects the physical properties and chemical and isotopic compositions of fully melted cosmic spherules (CSs). Oxygen isotope ratios of pristine micrometeorites are commonly used to relate these particles to their appropriate parent bodies. However, the degree of mixing with atmospheric oxygen and isotope fractionation by evaporation in CSs generally remains unclear, leading to uncertainties in their initial oxygen isotope ratios, which in turn complicates the precursor body identification. Previously, several studies have estimated the degree of evaporation based on contents of major refractory elements Ca and Al in combination with Fe/Si atomic ratios. This now commonly adopted chemical classification system has not yet been assessed with O and Fe isotope variability. As evaporation leads to both isotope and chemical fractionation, it is imperative to verify whether the predicted amounts of evaporation based on isotopic and chemical proxies converge.

Here, we measure the major and trace element compositions of 57 chondritic (mostly vitreous) CSs, along with their Fe isotope ratios. The δ56Fe isotope and chemical (K, Zn, Na or CaO and Al2O3 concentrations) fractionation in these particles show no correlation. This can be interpreted in two ways: (i) separate processes govern chemical and isotope fractionation or (ii) the selected proxies for isotope and/or chemical fractionation are inadequate. Because the initial Fe isotope ratios of chondrites display limited variation (0.005 ± 0.008‰ δ56Fe), Fe isotope ratios in CSs are assumed to only have changed through evaporation. At the same time, the chemical compositions of CSs show larger variability, so the CSs are thus often not chemically representative of their precursor bodies.

As oxygen isotope ratios are commonly used to identify the precursor bodies of (micro)meteorites, triple oxygen isotope ratios are measured in 37 of the 57 CSs. Based on the relationship between δ57Fe and δ18O, the effect of evaporation on the O isotope ratios can be corrected, which allows for a more precise precursor body reconstruction. Via this method, two 16O-poor spherules with greatly varying degrees of isotope fractionation (~1.0‰ and 29.1‰ δ56Fe, respectively) can be distinguished. Furthermore, it is observed that CSs that likely have an OC-like heritage all underwent the same degree of atmospheric mixing (~8‰ δ18O). These findings highlight the potential of including Fe isotope measurements to the regular methodologies applied to CS studies.

How to cite: Lampe, S., Soens, B., Chernonozhkin, S., González de Vega, C., van Ginneken, M., Van Maldeghem, F., Vanhaecke, F., Glass, B., Franchi, I., Terryn, H., Debaille, V., Claeys, P., and Goderis, S.: Decoupling of chemical and isotope fractionation processes during atmospheric entry of S-type micrometeorites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5656, https://doi.org/10.5194/egusphere-egu22-5656, 2022.

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