Multiscale evidence for weathering and the preservation of carbonaceous material in an Antartic micrometeorite

sem images of mm cp94

Micrometeorites (MMs) recovered from the Earth's surface may have undergone significant changes prior to their collection, including weathering while residing in the terrestrial environment.  These alteration processes, such as the precipitation of hydrous phases, may overprint both primary and atmospheric entry features, obscuring pre-existing material properties.  In addition, weathering exerts a prominent control on the geochemical interactions, such as species mobility between extraterrestrial material and the terrestrial environment, particularly in Antarctica.  

In the paper 'Multiscale evidence for weathering and the preservation of carbonaceous material in an Antarctic micrometeorite', a team of researchers from this department, The University of Alabama, the Natural History Museum (London), the University of London and Imperial College, London, characterise the textural and compositional consequences of weathering on an unmelted, fine-grained, Antarctic MM (which includes the mapping of nanometre-scale features using atom probe tomography).

In particular, the team investigated geochemical behaviour across textural boundaries in the MM and observed nanoscale elemental heterogeneity within complex alteration assemblages.  In one sample region, a compositional boundary is highlighted by distinct elemental differences, consistent with a weathering encrustation of mixed mineralogies, while analyses in other targeted regions show evidence for nanoscale elemental networks, as well as a grain boundary adjacent to a carbon-rich region.


The team's findings are presented in the context of terrestrial weathering as a dominant cause for the nanoscale features observed.  Weathering processes responsible for these features including the leaching of extraterrestrial material, precipitation of secondary alteration products with associated layering, and the influence of mechanical stress on pre-existing weaknesses.  From these results, the team derived a weathering sequence to explain the formation of the alteration product assemblage and highlighted the controls on MM geochemistry in the terrestrial environment.  Their observations show that nanoscale carbonaceous material may be preserved in oxy-hydroxides under icy conditions, which can also act as tracers for local environmental changes.