Fluid and melt inclusions in meteorites: clues to the petrology of asteroids and planets in the solar system

摘要

Meteorites are pieces of other bodies of the solar system falling to the ground when a meteor or shooting star flashes through the atmosphere at high speed. The majority originates from the asteroid belt and a few from the Moon and presumably comets and Mars. Stony meteorites consist of chondrites, containing chondrules and having remained unchanged since their formation (4.5 billion years), and achondrites, pieces of asteroid (Vesta) and planets including Mars. As they are mineralised phases, their interest for fluid inclusions studies is established since 1978 at the Mineralogical Department of the Museum National d'Histoire Naturelle. The presence of water-rich inclusions, even in primitives chondrites, is not difficult to accept as water is known to be one component of solar nebulae. But more than 90 % of meteorites are not "falls", but "finds", and may have acquired terrestrial minerals including quartz, sulphate and possibly salt The first fluid inclusions discovered unambiguously were two-phase aqueous within a moving bubble in feldspar and whitlockite from the Peetz chondrite (Fieni et al., 1978) where fission tracks are revealed; that was said to be fascinating by E. Roedder (pers. comm.). Liquid vapour inclusions are to be considered with care as they could be mostly artefacts like those reported in Diogenites (ALHA 77256, Antarctica) by Warner et al., (1983) and discussed in (Rudnick et al., 1985). Aqueous two-phase FI having moving bubbles were reported in the SNC Martian meteorite ALH 84001 and Nakla (Bodnar, 1999). Since such liquid vapour fluid inclusions were never found in other preparations of Nakla and other Naklites (Governador Valadores) their real signification should be discussed. More recently, Zolensky et al., 1999) reported primary and secondary liquid vapour fluid inclusions in purple halite of the meteorite from Monahans and in halite of the Zag meteorite. One conclusion is that, firstly, the preparation for fluid inclusion studies in extraterrestrial samples should be made with a specific care avoiding any contamination, and secondly the unmodified extraterrestrial content is always questionable. In the future in situ isotopic measurements will be the unique means to solve the problem.