The weathering of micrometeorites from the Transantarctic Mountains

Matthias Van Ginneken,Matthew J Genge,Luigi Folco,Ralph P Harvey

doi:10.1016/j.gca.2015.11.045

Abstract

Micrometeorites are cosmic dust particles recovered from the Earth’s surface that dominate the influx of extraterrestrial material accreting to our planet. This paper provides the first in-depth study of the weathering of micrometeorites within the Antarctic environment that will allow primary and secondary features to be distinguished. It is based on the analysis of 366 particles from Larkman Nunatak and 25 from the Transantarctic Mountain collection. Several important morphological categories of weathering effects were identified: (1) irregular and faceted cavities, (2) surface etch pits, (3) infilled cavities, (4) replaced silicate phases, and (5) hydrated and replaced metal. These features indicate that congruent dissolution of silicate phases, in particular olivine, is important in generating new pore space within particles. Comparison of the preservation of glass and olivine also indicates preferential dissolution of olivine by acidic solutions during low temperature aqueous alteration. Precipitation of new hydrous phases within cavities, in particular ferrihydrite and jarosite, results in pseudomorph textures within heavily altered particles. Glass, in contrast, is altered to palagonite gels and shows a sequential replacement indicative of varying water to rock ratios. Metal is variably replaced by Fe-oxyhydroxides and results in decreases in Ni/Fe ratio. In contrast, sulphides within metal are largely preserved. Magnetite, an essential component of micrometeorites formed during atmospheric entry, is least altered by interaction with the terrestrial environment. The extent of weathering in the studied micrometeorites is sensitive to differences in their primary mineralogy and varies significantly with particle type. Despite these differences, we propose a weathering scale for micrometeorites based on both their degree of terrestrial alteration and the level of encrustation by secondary phases. The compositions and textures of weathering products, however, suggest open system behaviour and variable water to rock ratios that imply climatic variation over the lifetime of the micrometeorite deposits.

Highlights

Micrometeorites are cosmic dust particles recovered from the Earth’s surface that dominate the influx of extraterrestrial material accreting to our planet
We propose a weathering scale for micrometeorites based on both their degree of terrestrial alteration and the level of encrustation by secondary phases
The overabundance of magnetite dominated particles (I-type and G-type cosmic spherules) from the deep-sea, which are more resistant to weathering than silicate-dominated particles, shows that terrestrial weathering can introduce an important bias in micrometeorite collections

Summary

Introduction

Micrometeorites are cosmic dust particles recovered from the Earth’s surface that dominate the influx of extraterrestrial material accreting to our planet. Knowing how terrestrial weathering will affect the preservation of micrometeorites and evaluating possible biases introduced in large micrometeorite collections can have a significant impact for studies focusing on the estimation of the flux of extraterrestrial matter to the Earth’s surface. This is especially the case for old collections, which can give important information on the variability of this flux over the recent geological past (e.g., the TAM collection; Rochette et al, 2008)

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