The origin of the moon and the early history of the earth-A chemical model. Part 1: The moon

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Abstract

The chemical implications of a "giant impact" model for the origin of the Moon are examined, both for the Moon and for the Earth. The Impactor is taken to be an approximately Mars-sized body. It is argued that the likeliest bulk chemical composition of the Moon (including a small Ni-rich metallic core) is quite similar to that of the Earth's mantle, and that this composition may be explained in detail if about 80% of the Moon came from the primitive Earth's mantle after segregation of the Earth's core. The other 20% of the Moon is modelled as coming from (a) the Impactor, which is constrained to be an oxidized, probably undifferentiated body of roughly CI chondritic composition (on a volatile free basis) and (b) a late stage veneer, with a composition and oxidation state similar to that of the H-group ordinary chondrites. This latter component is the source of all the volatile elements in the Moon (e.g., Na, K, Rb, Cs, and the volatile siderophile elements such as Cu, Ge, etc.), which failed to condense from the Earthand Impactor-derived materials; this component constitutes about 4% of the Moon. It is argued that Mo may behave as a volatile element under the relatively oxidising conditions necessary for the condensation of the proto-Moon. The late stage veneer may also be the reducing agent responsible for forming a small lunar core, which depletes the primitive silicate mantle of the Moon in the more siderophile elements. This core is presumed to have formed in equilibrium with the lunar mantle and, therefore, has Ni (Ni + Fe) = 0.45 ± 0.1; a mass balance of Ni in the Moon constrains the lunar core to ∼ 1% of the Moon's mass. Metal separation to the core at an assumed temperature of 1300°C takes place at an oxygen fugacity 0.8 log-bar units below the iron-wüstite (IW) oxygen buffer. These conditions may then be used to calculate the siderophile trace element concentrations in the core-forming metal and thus the siderophile element abundances of the bulk Moon (core plus primitive mantle). The model accounts satisfactorily for most of the siderophile elements, including Fe, Ni, Co, W, P, and Cu. The relatively well-constrained lunar abundances of V, Cr, and Mn are also accounted for; their depletion in the Moon is inherited from the Earth's mantle. The consequences of the model for the composition of the Earth's mantle are addressed in O'Neill (1991).

Original languageEnglish
Pages (from-to)1135-1157
Number of pages23
JournalGeochimica et Cosmochimica Acta
Volume55
Issue number4
DOIs
Publication statusPublished - Apr 1991
Externally publishedYes

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