Different degrees of aqueous alteration in sulphides within the CI1 chondrites

Introduction: It has been well documented that all of the CI1 chondrites have undergone extensive aqueous alteration, [1] which has modified the texture and composition of the minerals present. In this study, we look at the effect of aqueous alteration on Fe-Ni sulphides in four different CI1 chondrites, with a view to identifying variable degrees of aqueous alteration within CI1 chondrites. Method: Images were obtained using a Jeol 5900LV SEM operating at 20kV and 1nA, with a working distance of 10mm. Quantitative analyses were performed using a Cameca SX50 EMP operating at 20kV and 20nA. Elemental mapping was done both on the SEM and the EMP. Two thin sections of Alais, two of Tonk, one of Ivuna and three of Orgueil were examined. The sections of Orgueil were prepared without using water, in order to try and preserve any water-soluble phases present. Looking at thin sections allowed us to study the sulphides in situ, and observe their spatial relationships to other minerals. This contrasts with the study of CI1 pyrrhotite and pentlandite performed by [2], in which they analysed hand-picked or magnetically separated grains. Observations: The main type of Fe-Ni sulphide found in the CI1 chondrites is pyrrhotite. Occasional small (~10µm) irregular inclusions of pentlandite have been found within pyrrhotite grains in Alais and Tonk. These have not been seen in either Orgueil or Ivuna. The pyrrhotites are hexagonal or lath-shaped, and range in size from approximately 10µm in diameter to around 80µm. Those in Alais and Tonk are, on average, larger than those seen in Orgueil and Ivuna. Some of the pyrrhotite grains are extensively corroded (fig. 1), particularly those in Orgueil and Ivuna. Corrosion occurs at the edge of grains, as well as the interior, with material similar in composition to the matrix filling the spaces, although it is difficult to obtain accurate analyses as the corroded areas are so small. Both Orgueil and Ivuna contain more magnetite than Alais or Tonk, with magnetite occurring as framboids, spherules and plaquettes.