Chromites in ordinary chondrite fusion crusts

Meteorites are several orders of magnitude older than homo sapiens – or any other homo genus, for that matter. Their origin dates back to the formation of the solar system (4.56 billion years ago), whereas our species traces its roots back to two hundred thousand years in the past. Even though anthropologists are expected to push this estimate further back in the past, humankind is bound to remain a latecomer in the history of Universe. Considering that the beginning of our Universe is currently placed at 13.8 billion years ago, it is correct to say that the solar system itself is very young. The time when humankind began to guess the true nature of the space rocks is far more recent than 200,000 years – we should drop three zeros and write 200 years. The term ‘origin’ thus applies to two very different contexts: the epoch when meteorites formed and the time when people started to regard them as celestial remnants of a very remote past, when our Sun and the planets came to life. Meteorites are the falling debris of a vast class of solar system objects – the minor bodies, which include asteroids and comets as well as a host of objects discovered in the last decades: trans-Neptunian objects (TNOs), plutinos, Centaurs, Kuiper Belt and scattered disk objects (KBOs), Oort Cloud members etc.. Prior to the Apollo missions, meteorites were the only extraterrestrial material available on Earth and thus an extremely valuable source of information about the origin of the solar system. Minor bodies, planets and satellites all share a common origin. However, the building blocks of the larger bodies have been heavily altered by gravity, pressure and heat – a process which has erased previous traces of their history. On the other hand, small solar system objects preserve a record of their birth; this fact, only recently recognized by researchers, confers on them the very special status of time capsules from a very remote past. But these capsules have had their own evolution, though less destructive with respect to their larger siblings. Evidence of aqueous alteration and/or thermal metamorphism is very common among meteorites; they have a complex history, of which the formation of a fusion crust on entering Earth’s atmosphere is only the final chapter. Crusts have always received less attention than the inner part (bulk) of meteorites: however, interest has been constantly increasing, mainly sparked by the desire to build a detailed theoretical model of the atmospheric flight. Due to the scarcity of reliable data, current knowledge of physical mechanisms leading to the formation of a fusion crust is very crude and inadequate. Alternative approaches aiming at collecting more information have been tried, ranging from plasmatron experiments (in an attempt to create synthetic analogs of real crusts) to chemical analysis of the minerals formed inside the crust. This study focuses upon some minerals forming within the crust during atmospheric flight – chromites, a class of spinels. Although secondary in meteorites (because not present from the start), once they are formed spinels are remarkably resistant to alteration and can be found in the sample even after a very long time. In search of quantitative results, various authors have tried to take advantage of this stability. We have studied ten samples of meteorites – H, L and LL ordinary chondrites – searching for bulk as well as crust chromites. Almost 300 chromites were found, from bulk and crust in equal proportions; SEM analyses confirm significant differences in size, shape and chemical composition between crust and bulk. Results may help shed light upon the processes operating during atmospheric flight, as well as make a comparison with chromites found in layers of terrestrial remnants of impact craters. Such chromites have often been cited as samples of extraterrestrial matter and considered reliable markers of asteroid (or comet) impacts. The issue is very complex and still actively debated: we hope to give a little, but valuable contribution with the present work.