An integrated mineralogical, petrologic and isotopic study of ureilites

Smith, Caroline Louise (2003). An integrated mineralogical, petrologic and isotopic study of ureilites. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f6f6

Abstract

The ureilites are essentially carbon-bearing, ultramafic meteorites that comprise the second largest of the achondrite meteorite groups. The mineral assemblage, petrography and certain aspects of mineral chemistry that the ureilites display suggest a differentiated and igneous petrogenetic history. However, in terms of oxygen isotope compositions and high carbon contents they appear similar to the primitive and unprocessed carbonaceous chondrites; evidence which has been taken to suggest a genetic link between the ureilites and the carbonaceous chondrites. Ureilites may be divided into four distinct groups on the basis of constant Δ¹⁷O compositions, suggesting formation on four parent bodies or four separate regions on a single parent body. Samples analysed herein have been taken from two oxygen groups to gain a better understanding of inter-/intra-group relationships and the petrogenetic history of the parent body (ies).

The first detailed mineralogical and petrological examination of 17 ureilite samples has been carried out in combination with high-resolution stepped combustion analyses of 11 of the samples to determine carbon form, abundance and isotopic composition. High- resolution stepped combustion analyses have also been carried out for the first time on 10 samples from Disko Island, Greenland and 3 terrestrial graphites.

Results from Disko Island samples (potential terrestrial analogues to the ureilites) suggest that carbon isotopic compositions of lithologies that have undergone reduction reactions and thermal metamorphism will be little changed compared with the original composition. This poses a major problem to models of ureilite petrogenesis that suggest the carbonaceous chondrites as the ureilite progenitor material. The carbon abundance and isotopic composition of ureilites may be affected by fluid flow on the parent body; perhaps as a secondary process resulting from the (re)mobilisation of carbon-bearing fluids (formed though reduction reactions) during a catastrophic shock event. Indication of disruption and subsequent re-aggregation of the parent-body is provided by Frontier Mountain ureilite samples, which display mineralogically and isotopically heterogeneous lithologies in intimate contact with each other. This also suggests that the ureilite parent body was heterogeneous on a much smaller scale than previously believed.

Results from stepped combustion of terrestrial graphites indicate a correlation between degree of graphite crystallinity and stepped-combustion behaviour. Combined with detailed petrographic analyses and stepped combustion results, shock metamorphism appears to be the dominant process affecting the crystallinity of ureilite graphite.

Strong similarities in carbon form, abundance and isotopic composition and mineralogy and petrography between ureilites from different oxygen groups indicates all suffered similar petrogenetic processes to similar degrees. This is more consistent with derivation from a single ureilite parent body, which retained oxygen isotope heterogeneity.