• Vesta
• HED
• mesosiderites
• Un-Hadid
• petrography
• oxygen isotopes
• meteorites

Mesosiderites are an enigmatic group of differentiated meteorites consisting of metal and silicate clasts in a matrix made of metal plus silicates with either fragmental or igneous textures. The origin of such a complex mixture of differentiated materials is poorly understood and the structure of the mesosiderite parent body is a matter of debate. In particular, it is not clear whether the mesosiderite metal component is the product of planetary igneous differentiation or a later addition from an iron impactor on an already differentiated parent body, and if the latter was asteroid 4Vesta one of the largest (~500 km diameter) differentiated survivors in the Main Belt, target of the recent NASA mission Dawn. This thesis aims to fill a significant gap in our knowledge of mesosiderites by making a comprehensive petrological study of three mesosiderites (Um-Hadid, Estherville and Mt Padbury), and by providing the oxygen isotopes of nine mesosiderites (Um-Hadid, Donnybrook, Hainholz, Mount Egerton, Simondium, Lamount, Estherville, Dong Ujimqin qi and Mincy). Moreover, to put the studied mesosiderites in a broader context, a total of 84 thin and thick sections of 42 different meteorites (mainly achondrites), and 12 different mesosiderites that were studied using petrographic and stereoscopic microscopes.
The first part of this thesis is to classify Um-Hadid, a mesosiderite found in Saudi Arabia in the 19th century, inside the mesosiderites classes (of the 241 approved mesosiderites only 65 classify into the mesosiderites classes) and to study a gabbroic igneous enclave of this meteorite. As mesosiderites are difficult to study, complex and heterogeneous meteorites, a matrix fragment of Estherville and a basaltic enclave of Mt. Padbury, two of the most studied mesosiderites, were also studied to be used as a point of comparison. The petrological characterization of the meteorites was carried out using a series of different analytical techniques, including optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron microprobe, X-ray fluorescence, and digital image analysis tools. Based on the resulting data Um-Hadid was classified as a 4A mesosiderite having an igneous texture and being plagioclase-rich. In addition, the gabbroic enclave of Um-Hadid provided information about igneous processes in the early solar system, showing similarities and differences with HEDs.
The second part of this thesis is to provide new high precision oxygen isotopes data on mesosiderites (only 7% of mesosiderites have oxygen isotope data). A total of nine mesosiderites were analyzed providing the oxygen isotopes analysis on mesosiderites matrix with the highest precision up to date. The analyses were performed using the laser fluorination system of the Open University (UK). The results are δ17O‰ 1.670 ± 0.178 (2σ), δ18O‰ 3.635 ± 0.360 (2σ) and Δ17O‰ -0.236 ± 0.027 (2σ) and are compatible with the previous works but slightly higher and almost in complete superposition with typical values of HEDs. It was also confirmed the classification of Mount Egerton as an aubrite, as recommended by the Meteoritical Bulletin, otherwise classified as a mesosiderite in various collections. The new oxygen isotopes analyses allow us to furtherly constrain the values of the mesosiderites oxygen isotopes and their link with HEDs.
It is now accepted that HED and mesosiderites have different parent bodies, being 4 Vesta the one of HEDs. However, the striking similar petrography between mesosiderites and HED plus their undistinguishable oxygen isotope composition strongly link them. This defines a unique relationship between these meteorites in which their parent bodies are not the same but they are brothers, developing in the same region of the Early Solar System (ESS). This thesis shows the complexity of the active processes in the Early Solar system in which two planetesimals that were formed in the same region of it ended up having completely different histories: 4 Vesta is almost pristine and is the second largest body of the Main Asteroid Belt; while the mesosiderite parent body was completely transformed by the impact with a metal rich body ending up as a brecciated mixture of metals and silicates.