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Lunar and Planetary Science XXXI
THE COMPOSITION OF THE EARTHS LOWER MANTLE AND THE BULK
COMPOSITION OF THE MOON?
P. Jakes1 and A. Jambon2 , 1 Institute of Geochemistry, Faculty of Sciences, Charles University,
Albertov 6, 128 43 Praha 2, Czech Republic ([email protected]), 2 Department of
Petrology, University of Pierre et Marie Curies Paris VI, 4 place Jussieu, 75 005 Paris, France
([email protected])
Although the lunar science community accepted the idea of origin of the Moon through the
collision of the Earth and Mars size body, the enigma of fertile Earth and refractory Moon has
remained. Wood suggested that the present day estimates of the terrestrial mantle composition
may not be representative of the composition of mantle at the time of the hypothetical fission
event. Jakeš and Jambon (1998) have argued that bulk Moon composition and mantle
composition of the Earth could be compared only if the lower mantle of the Earth has refractory
nature. On the other hand the Earth science community has accepted the idea of uniform
composition of the mantle, particularly if one cell convecting mantle has been advocated.
The geochemical features of the Moon, i.e., the volatile element depletion, excess of refractories,
depletion of iron and siderophile elements and highly reduced state of the iron as compared to
chondrites contrast to the geochemical features of the recent upper mantle of the Earth. The
abundance of refractory oxyphile elements in the upper mantle of the Earth compared to that of
the Moon are lower, whereas the abundance of siderophile elements and volatile elements are
higher in the upper mantle of the Earth than in the Moon.
The low abundance of siderophile elements in lunar rocks (Newsom, 1984) as compared to the
Earth's upper mantle abundance, strong depletion of highly siderophile elements (Re, or Mo) and
similar abundance of less siderophile elements (e.g., P) suggest different degree of "reduction" of
the Moon than of the recent Earth’s upper mantle.
In order to explain the differences in the compositions of both bodies, e.g., higher contents of
refractory elements or lower contents of siderophile element in the Moon as compared to the
Earth explanation was sought in a high proportion of projectile material, which was strongly
fractionated.
Ringwood, Wanke & Dreibus and others have shown that the Moon has been formed after the
formation of the Earth’s core. The additional siderophile element separation must have taken
place after the separation of the lunar matter from the protoEarth such as the formation of the
lunar core, a natural consequence of large scale melting of the Moon that was advocated since
early work of Wood, Smith and Taylor and Jakeš.
Suggestion on strong compositional differences between lower and upper mantle that has been
recently advocated by Albarede and van der Hilst should appeal to lunar community and to many
petrologists, providing an explanation to the observation, that the Earth’s upper mantle was not
in equilibrium with a metallic core. Such scenario would also appeal to those who advocate a
two-cell mantle. A late cometary veneer, that has accreted to the Earth after the Moon forming
material has been removed, would explain the higher abundance of siderophile and volatile
elements in the upper mantle of the Earth and could account for the abundance of refractory
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Lunar and Planetary Science XXXI
COMPOSITION OF THE EARTH'S LOWER MANTLE: P. Jakes and A. Jambon
elements in the Moon and, consequently, in the lower mantle of the Earth. Superheat during early
formation of the Moon may provide some means of changing the chemical composition (by
volatilization) and through the heat induced reduction i.e., formation of metallic species of
siderophile and transitional elements through the decomposition of oxide phases. Such process
(i.e. superheat) may contribute together with the existing (i.e., chondritic and at this stage already
separated metal) to the core formation, and to the refractory - siderophile poor nature of the
residual silicate phase.
Only later often the Moon forming event the upper mantle of the Earth is settled, formed and
mixed. The temperatures of this event are much lower - no evaporation of silicates (e.g., Si) in
order to obtain the upper mantle Mg/Si ratio for the Earth. These could well be explained through
the formation of oceanic and continental crust. "Missing Si" from upper mantle implied by
chondrite analogies may reside in the transitional zone in the form of eclogite. And missing Si in
lower mantle may reside in the core. Such considerations together with the "geophysical"
controversies concerning the FeO/MgO ratio of lower mantle show how poorly constrained the
knowledge of lower mantle composition is.
There has been little chemical exchange between the upper mantle and core (siderophiles in
upper mantle) and there has not been chemical equilibrium between the "softly plastered"
accreting material and lower mantle. The model of two-cell mantle suggests compositional
differences between the upper and lower mantle. In such model the lower mantle is strongly
siderophile depleted, refractory rich and upper mantle less depleted in siderophiles, moderately
volatile elements, etc. The upper mantle did not suffer such intense heating as the lower mantle.
There may have been a considerable time interval between the formation of lower and upper
mantle.
The possibility that the Moon has been formed in the early history of the Earth, that had (at that
stage) a different composition than the recent upper mantle, offers an alternative explanation .
All the characteristic features (i.e., volatilization, refractory composition, and reduced nature) are
mutually related through a extremely high temperature of the forming protoEarth that has been
followed by the impact induced fission of lunar material. This early composition is possibly
preserved in the lower mantle of the Earth that retained its refractory chemical identity since this
stage.
1228.pdf