ROLE OF FLUIDS DURING METAMORPHISM ON ORDINARY CHONDRITE PARENT BODIES

The first samples to be returned from an asteroid were obtained by the JAXA Hyabusa mission, which visited asteroid Itokawa in 2005 and returned to Earth in 2010. Itokawa is an S-type asteroid, 500 m in length. Although the mass of material that was captured and returned is very small (e.g. Nakamura et al., 2011: Science 333, 1113), studies of individual particles have shown unequivocally that Itokawa consists of material identical to a major group of ordinary chondrite meteorites, the LL chondrites. This is the first confirmed link between a known group of meteorites and an asteroid. It is well known that ordinary chondrites (OCs) underwent metamorphism on their parent asteroids. Metamorphism is commonly assumed to have taken place under dry, fluid-free conditions. However, our recent work on feldspar and phosphate minerals in ordinary chondrites shows that fluids played a more significant role during heating of the parent body than has hitherto been recognized. Feldspar shows the effects of metasomatic alteration reactions, including albitization and infiltration of fluids, resulting in replacement of feldspar along crystallographically controlled planes. In some cases, fluids appear to have been chlorine-bearing. Phosphate minerals, merrillite and apatite, show evidence for fluid-controlled replacement reactions: they show strong similarities to terrestrial examples of dissolution / precipitation reaction products. However, although activity of fluids seems inescapable, determining the nature and origin of the fluids is more elusive. Ion microprobe analyses of chlorapatite show little to no water in the structure. Our interpretation is that dry, halogen-rich fluids were released during partial melting of the interior of OC parent asteroids, and interacted with chondritic materials during late stages of metamorphism. Small asteroids such as Itokawa represent fragments of larger bodies on which metasomatism, metamorphism and partial melting took place in the first few million years of solar system history.