A Comparison of “identical” Antarctic Micrometeorites from Glacial Ice & Aeolian Sediments

Introduction: Micrometeorites exhibit a broad range of compositions, textures and mineralogy that may be the result of a larger range of sampled extraterrestrial sources. However, the selection effects due to collection methods compound this wide variation. Micrometeorites and cosmic spherules are collected mainly from Antarctica by either melting large volumes of glacial ice [1,2], or collecting sediments from aeolian traps such as moraines near meteorite stranding grounds or glacially eroded mountaintops [3,4]. Aeolian particles are collected dry from moraine sediments or other aeolian traps and therefore do not have contact with hot liquid water during collection like the samples from glacial ice. However, the post collection sorting for micrometeorites is time consuming and laborious due to the high terrestrial background. The terrestrial history of the aeolian micrometeorites is also impossible to know. Particles within the moraine may have fallen millions of years ago or just the day before collection, and may or may not have been incorporated into glacial ice. Samples collected from glacial ice have the advantage of a relatively rapid collection yielding tens of thousands of particles within a single collection [1]. Minimal post collection sorting is needed due to a very low terrestrial lithologic contamination, and their terrestrial history is better known. However, these particles spend many hours in warm liquid water during the melting of the ice. This study was undertaken in order to determine if there are any differences between micrometeorites collected from glacial ice and aeolian traps due to collection methods. Samples and analytical techniques: The glacial ice sample was from the 2004 expedition to Cap Prudhomme (CP) and collected as described in Maurette et al. (2004) [5]. The sample of aeolian sediment was collected from Station D (SD) located at the tip of the Lewis Cliff Ice Tongue, a meteorite stranding ground. The sediment was then wet sieved in cool filtered water, and magnetically separated to remove the bulk of the terrestrial lithologic contamination. Both the CP and SD samples were sorted in 200 proof ethanol using an optical stereomicroscope. Sorting parameters were dark, irregularly shaped magnetic particles, features corresponding to unmelted finegrained, micrometeorites (fgMMs). Approximately 40 potential fgMMs from each of the CP and SD samples were then mounted in EPOTEK 301 and made into polished sections. Each particle was imaged using backscattered electron (BSE) imaging and EDS spectra to confirm features indicative of extraterrestrial origin such as a magnetite rim, the presence of Ni in metal or sulfides, or a “chondritic” composition. They were also examined on the size and shape of vesicles and compositional heterogeneity in order to select those that appeared least melted. Two particles were selected, one each from CP and SD, based upon their similarities in BSE imaging and their relatively low degree of melting (Figure 1).