ION IRRADIATION EXPERIMENTS ON THE MURCHISON CM2 CARBONACEOUS CHONDRITE

Introduction. Remote sensing observations show that space weathering processes affect all airless bodies in the Solar System to some degree. Sample analyses and lab experiments provide insights into the chemical, spectroscopic and mineralogic effects of space weathering and aid in the interpretation of remote-sensing data. For example, analyses of particles returned from the S-type asteroid Itokawa by the Hayabusa mission revealed that space-weathering on that body was dominated by interactions with the solar wind acting on LL ordinary chondrite-like materials [1, 2]. Understanding and predicting how the surface regoliths of primitive carbonaceous asteroids respond to space weathering processes is important for future sample return missions (Hayabusa 2 and OSIRIS-REx) that are targeting objects of this type. Here, we report the results of our preliminary ion irradiation experiments on a hydrated carbonaceous chondrite with emphasis on microstructural and infrared spectral changes. Samples and Methods. A polished thin section of the Murchison CM2 carbonaceous chondrite was irradiated with 4 kV He + (normal incidence) to a total dose of 1x10 18 He + /cm 2 over an area of ~5x5 mm 2 . The irradiated area included abundant matrix and chondrules. We obtained ex situ Fourier-transform infrared (FTIR) reflectance spectra from multiple areas of matrix, ~150 m 2 in size, using a Hyperion microscope on a Vertex Bruker FTIR bench. A JEOL 7600F field emission scanning electron microscope (SEM) was used to study the morphological effects of the irradiation. Following the SEM analyses, we extracted thin sections from both irradiated and unirradiated regions in matrix using focused ion beam (FIB) techniques. We used electron beam deposition for the protective carbon strap to minimize surface damage artifacts from the FIB milling. The FIB sections were analyzed using the JEOL 2500SE scanning and transmission electon microscope (STEM). Results and Discussion. Optical examination showed that the irradiated area was visibly darker than the un-irradiated parts of the thin section. FTIR reflectance spectra were collected from irradiated and unirradiated regions of fine-grained matrix. The irradiated matrix showed lower reflectance in the near-IR and a red-sloped continuum compared to the un-irradiated matrix spectra. The depth of the 3 m feature is decreased in the irradiated regions relative to un