doi:10.1016/S0016-7037(02)01273-5 Destabilization of olivine by 30-keV electron irradiation: A possible mechanism of space weathering affecting interplanetary dust particles and planetary surfaces L. LEMELLE, 1, * L. BEAUNIER, 2 S. BORENSZTAJN, 2 M. FIALIN, 3 and F. GUYOT 4 1 Laboratoire de Sciences de la Terre, Ecole Normale Supe ´rieure de Lyon, UMR5570, 46 alle ´e d’Italie, 69364 Lyon Cedex 07, France 2 Laboratoire de Physique des Liquides et Electrochimie, case 133, 4 place Jussieu, 75252 Paris Cedex 05, France 3 Service de Microanalyse CAMPARIS-CNRS, Universite ´ Pierre et Marie Curie, case 110, 4 place Jussieu, 75252 Paris Cedex 5, France 4 Laboratoire de Mine ´ralogie et Cristallographie de Paris, case 115, and IPGP, 4 place Jussieu, 75252 Paris Cedex 05, France (Received February 14, 2002; accepted in revised form September 18, 2002) Abstract—Electron irradiation experiments were performed using a 30-keV electron beam on single crystals of olivine in a scanning electron microscope (SEM) and in an electron microprobe (EMP). We determined that, under certain conditions, structural damage is caused to the irradiated surface of iron-bearing olivines. The irradiated areas comprise spherules with sizes of hundreds of nanometers and micrometer-sized holes. In the immediate vicinities of the irradiated areas, droplets with sizes of tens of nanometers and branching tracks are observed. With increasing total charge, the hundreds of nanometer-sized spherules become larger and more irregular in shape. The size and shape of the nanometer-sized droplets remain almost constant, but their surface density increases (in m -2 ). Chemical fractionations compared to the initial olivine were found: the irradiated areas are slightly enriched in MgO, whereas the deposits are enriched in SiO 2 . Destabilization of olivine is not due to the dissipation of the implanted energy as heat, but results most probably from electrostatic discharges leading to the breakdown of the dielectric lattice. The possibility that such processes could be responsible for significant space weathering of interplanetary dust particles and regoliths of planetary surfaces should be taken into account. In the interplanetary medium, 10-keV range electrons are carried by the solar wind, whereas at 1 AU from the Sun, the lifetime of cometary dust and the exposure time of lunar regolith are, at least, 10 to 100 times greater than the duration required to accumulate the damaging electronic doses applied in this study. Moreover, the comparison of the microstructures of samples irradiated in the present study with features of lunar regolith grains reveals several chemical and structural similarities. Copyright © 2003 Elsevier Science Ltd 1. INTRODUCTION Space weathering is due to a family of processes active in the space environment, such as heating, micrometeorite impacts, stellar winds, or cosmic ray irradiation, which combine to alter the physical and/or compositional properties of space materials (Pieters, 1998). The identification of a prevailing process, if any, is important for interpreting planetary surface properties, as well as the bulk composition of dust particles. The study of space weathering in the interplanetary medium benefits from the constraints obtained from investigations of both space- weathered lunar samples (Keller and McKay, 1997) and of the solar wind (Reames, 1999). Direct implications concern bodies of the solar system devoid of atmosphere, including interplan- etary dust particles and cometary dust (Brownlee, 1985; Jess- berger et al., 1988; Bradley et al., 1989; Hanner et al., 1994; Crovisier et al., 1997; Wooden et al., 1999), as well as aster- oidal and lunar surfaces. Numerous experiments have shown specific structural and chemical effects of irradiation on silicates by one kind of charged species present in the solar wind. Irradiation with MeV protons or helium or krypton cations induces the formation of nuclear tracks and of amorphous surfaces having higher O/Si ratios than in the bulk (Bibring et al., 1972; Wang et al., 1991; Bradley, 1994). It has also been shown that proton irradiation of olivine leads to lower Mg/Si ratios (Bradley, 1994). Mech- anisms previously proposed to describe lattice damage include physical processes that are critically dependent on the cation mass. Irradiation with keV protons, helium and argon cations may modify the chemical composition and the microstructure of the silicate surface (Hochella et al., 1988; Demyk et al., 2001). The reduction of iron and silicon cations in olivine by protons was suggested by Dukes et al. (1999). Irradiation with 100-keV range electrons generates the formation of amorphous rims in silicates (e.g., beryl, cordierite, nepheline, sphene; see Vance et al., 1986; Gong et al., 1998). The breakdown of olivine into associated MgO crystallites and an amorphous SiO 2 -rich phase, with a lower Mg/Si ratio than the initial olivine was reported by Carrez et al. (2001). Among possible irradiation processes, the irradiation by 10- keV range electrons from the solar wind (Lin et al., 1997; Krucker et al., 1999) is considered in this paper. Our aim was to investigate the chemical and structural response of olivine, an ubiquitous phase in the solar system, subjected to 30-keV electrons. Natural San Carlos olivine and synthetic iron-free forsterite single crystals were irradiated with various electron doses in a scanning electron microscope (SEM) and in an electron microprobe (EMP). Microstructures and chemical compositions of the samples were then analyzed by both SEM and EMP. The most extensively studied case of space weath- ering is the one affecting grains of the lunar regolith (Keller and McKay, 1997), subjected both to solar wind and to microme- teoritic impacts. A fruitful approach to discuss the mechanism * Author to whom correspondence should be addressed (llemelle@ens- lyon.fr). Pergamon Geochimica et Cosmochimica Acta, Vol. 67, No. 10, pp. 1901–1910, 2003 Copyright © 2003 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/03 $30.00 + .00 1901