The parent body controls on cosmic spherule texture: Evidence from the oxygen isotopic compositions of large micrometeorites

M Van Ginneken,J Gattacceca,P Rochette,C Sonzogni,A Alexandre,V Vidal,M.J Genge

doi:10.1016/j.gca.2017.05.008

Abstract

High-precision oxygen isotopic compositions of eighteen large cosmic spherules (>500µm diameter) from the Atacama Desert, Chile, were determined using IR-laser fluorination – Isotope Ratio Mass spectrometry. The four discrete isotopic groups defined in a previous study on cosmic spherules from the Transantarctic Mountains (Suavet et al., 2010) were identified, confirming their global distribution. Approximately 50% of the studied cosmic spherules are related to carbonaceous chondrites, 38% to ordinary chondrites and 12% to unknown parent bodies. Approximately 90% of barred olivine (BO) cosmic spherules show oxygen isotopic compositions suggesting they are related to carbonaceous chondrites. Similarly, ∼90% porphyritic olivine (Po) cosmic spherules are related to ordinary chondrites and none can be unambiguously related to carbonaceous chondrites. Other textures are related to all potential parent bodies. The data suggests that the textures of cosmic spherules are mainly controlled by the nature of the precursor rather than by the atmospheric entry parameters. We propose that the Po texture may essentially be formed from a coarse-grained precursor having an ordinary chondritic mineralogy and chemistry. Coarse-grained precursors related to carbonaceous chondrites (i.e. chondrules) are likely to either survive atmospheric entry heating or form V-type cosmic spherules. Due to the limited number of submicron nucleation sites after total melting, ordinary chondrite-related coarse-grained precursors that suffer higher peak temperatures will preferentially form cryptocrystalline (Cc) textures instead of BO textures. Conversely, the BO textures would be mostly related to the fine-grained matrices of carbonaceous chondrites due to the wide range of melting temperatures of their constituent mineral phases, allowing the preservation of submicron nucleation sites. Independently of the nature of the precursors, increasing peak temperatures form glassy textures.

Highlights

Micrometeorites are extraterrestrial particles 10 lm– 2 mm in size, which represent in terms of mass the most important part of the flux of extraterrestrial material to accrete to the Earth’s surface (Rubin and Grossman, 2010)
The discovery of a 16O-poor cosmic spherules related to the isotopic Group 4 defined by Suavet et al (2010) confirms that unknown parent bodies contribute significantly to the flux of micrometeorites to Earth
The texture of the cosmic spherules is mainly controlled by the grain-size and mineralogy of the precursor material

Summary

Introduction

Micrometeorites are extraterrestrial particles 10 lm– 2 mm in size, which represent in terms of mass the most important part of the flux of extraterrestrial material to accrete to the Earth’s surface (Rubin and Grossman, 2010). Studies based on ion microprobe analyses have shown that the oxygen isotopic bulk compositions of micrometeorites mostly plot below the TFL, suggesting that these particles have carbonaceous chondrite-related precursors (Clayton and Mayeda, 1999; Engrand et al, 2005; Taylor et al, 2005; Yada et al, 2005). Recent studies aiming at determining the oxygen isotopic composition of melted micrometeorites from the Transantarctic Mountains (TAM) > 500 mm in size, have used the IR-laser fluorination – Isotope Ratio Mass spectrometry (IRMS) technique (Cordier et al, 2011a; Suavet et al, 2010, 2011; Cordier and Folco, 2014) An advantage of this technique is its high reproducibility and accuracy which allow a clear identification of micrometeorites having oxygen isotopic compositions plotting close to the TFL (e.g., ordinary chondrite-related precursors). An important conclusion from these studies is that in the size fractions studied, the contribution of micrometeorites having ordinary chondrite-related precursors is significant (i.e., between 30% and 70%)

Objectives

Methods

Results

Discussion

Conclusion