Transmission Electron Microscopy of CONCORDIA UltraCarbonaceous Antarctic MicroMeteorites (UCAMMs): Mineralogical properties

Abstract

We performed mineralogical and petrographic studies of three UltraCarbonaceous Antarctic Micrometeorites (UCAMMs) by analytical transmission electron microscopy (TEM). The UCAMMs were identified in the CONCORDIA micrometeorite collection (2002 and 2006) recovered from central Antarctic snow, and are of probable cometary origin. UCAMMs are dominated by disordered carbonaceous matter that extends over surfaces of up to ∼90% of the particle. Embedded in this carbonaceous matter, we observed small and complex assemblages of fine-grained mineral phases, isolated minerals, glassy phases that resemble Glass with Embedded Metal and Sulfides (GEMS) that were first found in Interplanetary Dust Particles (IDPs), and rounded objects containing both glass and crystalline materials. The mineral assemblages are chondritic in composition, within a factor of 2. Crystalline materials represent at least 25% of mineral phases. This value is much larger than the upper limit of crystallinity measured in the diffuse interstellar medium (<2.2 wt%). Crystalline phases are dominated by low-Ca, Mg-rich pyroxenes, Mg-rich olivine and low-Ni Fe-sulfides. Exotic phases such as Mn-, Zn-rich sulfide and perryite have also been found as accessory minerals. The variety of high temperature mineral phases observed in UCAMMs is similar to that reported in chondritic porous IDPs and 81P/Wild 2 samples. The close association of high temperature crystalline phases with the low temperature carbonaceous matter in UCAMMs supports the hypothesis of a large-scale radial mixing in the early solar nebula. This new type of carbon-rich micrometeorites containing crystalline material provides the opportunity to analyze in situ, without any chemical processing, the association of materials formed at both low and high temperatures in the protoplanetary disk. A better knowledge of these samples is also important to prepare for future cometary missions, like ROSETTA in 2014.