| Popis: |
Research investigating the terrestrial weathering experienced by ordinary and CM chondrites at high spatial resolution is sparse, with most published work using either bulk mineralogical or chemical analysis techniques. With asteroid sample return missions, Hayabusa 2 and OSIRIS-REx expected to return samples in near future, understanding the effects of terrestrial contamination and weathering in fine detail is of paramount importance. The research presented here sets out to investigate terrestrial weathering by analysing the alteration products found around Fe,Ni metal grains by coupled chemical and mineralogical analysis for the first time. Analysis is conducted using a suite of high spatial resolution imaging and spectroscopic techniques including EDS spectroscopy and Raman spectroscopy, an approach with very limited previous use in this field. Initially characterisation of the effects of terrestrial weathering on ordinary chondrites, which should have only experienced significant aqueous alteration in the terrestrial environment, is conducted. The effects of weathering in three climates (the Antarctic, Sahara and Western Australia) are investigated to compare the intensity of alteration between typical find environments. The results of ordinary chondrite analysis reveal Cl-bearing akaganeite is the first terrestrial alteration product to form, followed by goethite and a poorly crystallised hydrated iron III oxide or oxyhydroxide. Analysis also reveals that terrestrial weathering was least intense in cold desert environments (Antarctica) and most intense in semi-arid environments (Western Australia). Having characterised the terrestrial weathering products which form around Fe,Ni metal the effects of terrestrial weathering on CM chondrites, which have experienced significant aqueous alteration both terrestrially and on their parent bodies, is investigated. Analysis of the weathering products in CM chondrites revealed the terrestrial alteration products akaganeite and goethite eroding/replacing parent body alteration products tochilinite and magnetite. The work presented here highlights the need for high resolution, chemical and mineralogical analysis of the alteration products found in ordinary and CM chondrites in order to prevent misinterpretation of terrestrially derived alteration products as evidence of parent body alteration. |
