Response to the comment by G. Dreibus and H. Wänke on “Comparative geochemistry of basalts from the Moon, Earth, HED asteroid, and Mars: Implications for the origin of the Moon” (2001) Geochim. Cosmochim. Acta 65, 979–997

The central theses of our work (Ruzicka et al 1998, 2001) are that the composition of the Moon is not unique in the solar system, that it resembles the composition of the parent body of HED meteorites (likely the asteroid 4-Vesta) to a remarkable extent, and that geochemical data do not support an origin of the Moon by rotational fission (Binder, 1986) or small-impact collisional ejection (Ringwood, 1986, 1990). Furthermore, the data are consistent with a giant-impact origin for the Moon (Hartmann and Davis, 1975; Hartmann, 1986; Cameron, 1986, 1997, 2000; Canup and Asphaug, 2001) only if the Moon largely inherited the composition of the impactor. Few researchers dispute the idea that the Moon and HED parent body have similar abundances of many volatile (e.g., Na and K) and siderophile elements (e.g., W). The controversy (Dreibus and Wanke, 2002) centers instead on the interpretation of geochemical data for a few key elements, including Ni, Co, Cr, and Mn. According to Wanke and colleagues, for these elements the composition of the Moon and Earth are similar, but very different for the Moon and HED parent body (Dreibus et al 1977, 1979; Wanke and Dreibus, 1986). Various authors have reached similar conclusions for Cr and Mn (Drake et al 1989; Ringwood, 1989; Ringwood et al 1990; O’Neill 1991) and for Ni and Co (Ringwood and Seifert, 1986; Ringwood, 1989; O’Neill, 1991), using either the modelled abundances of Wanke and colleagues, or approaches similar to these researchers. In contrast, based on correlated (albeit non-linear) abundances for these elements, we argue that Ni, Co and Cr abundances are similar for the Moon and HED asteroid, and different for the Moon and Earth. In contrast to Wanke and Dreibus (1986), we also argue that Mn data do not necessarily imply special conditions for the formation of the Earth and Moon. The different interpretations stem in part from the different methodologies and samples used. We based conclusions on geochemical data for analogous basaltic samples from each planetary body, and inferred the compositions of the source regions, which could correspond to large portions of the mantles of the parent objects. For lunar samples we concentrated on mare basalts, as opposed to KREEP basalts or aluminous basalts. This was because the latter appear to represent unusual differentiates and/or impact-melt rocks, which are not analogous to the basalts from other objects. In contrast, Wanke and colleagues based their conclusions on derivative compositions of the Moon, HED asteroid, Earth, and Mars, estimated from models that differed from one object to the other, that varied depending on the element being considered, and that were based on different types of rocks, including impact breccias, volcanic rocks, and plutonic rocks. The Ni, Co, Cr, and Mn abundances modelled for the HED asteroid (Dreibus et al 1977; Dreibus and Wanke, 1980) depend strongly on the assumed composition and proportion of an olivine-rich mantle component that has not yet been sampled. This olivine-rich component could have a composition similar to lunar dunite (Dreibus et al 1977), pallasitic olivine (Dreibus and Wanke, 1980), or something else altogether. The proportion of this olivine-rich component is also uncertain as it depends on the nature of the protolith. Dreibus et al (1977) and Dreibus and Wanke (1980) assumed a protolith similar to CI-chondrites, but significantly different parent body compositions are obtained for protoliths similar to ordinary chondrites or enstatite chondrites (Ruzicka et al 1997). A different choice for the composition and proportion of the HED mantle component will change the model Ni, Co, Cr, and Mn abundances in the HED asteroid. For the Moon, the bulk composition inferred by Wanke and coworkers was inferred mainly from the composition of highland breccias, which are acknowledged to be contaminated by meteoritic debris. The abundances of these elements in the Moon were derived by making assumptions about the proportion and composition of the components (KREEP, anorthosite, Mg-rich component) out of which the breccias are assumed composed, and depend on the composition of the assumed protolith (e.g., CI-chondrites) (Wanke et al 1977a, 1977b, 1978). Incorrect assumptions about any of these will lead to an incorrect derivative composition for the Moon. For Ni and Co, the bulk composition of the Moon estimated by Wanke et al (1978) was obtained only after additional assumptions were made regarding the amount and type of meteoritic contamination. Whether or not these models lead to a valid bulk lunar composition is debatable. In contrast, we argue that the compositions of eucrite and mare basalts themselves show good evidence for being controlled by igneous fractionation of mafic minerals (mainly olivine and pyroxene) from protoliths with similar Ni, Co, and Cr abundances. Although the Ni and Co abundances of eucrites are lower than that in mare basalts, the Ni-MgO and Co(MgO FeO) systematics for eucrites and all mare basalts fall on the same trend, and this trend is displaced from the compositions shown by terrestrial basalts and komatiites. On plots * Author to whom correspondence should be addressed (ruzickaa@pdx.edu). Pergamon Geochimica et Cosmochimica Acta, Vol. 66, No. 14, pp. 2633–2635, 2002 Copyright © 2002 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/02 $22.00 .00