Implications for the meteoritic component added to the Earth-Moon 
system.
Our mineralogical study of carbonaceous chondritic microclasts in HED 
meteorites show that C2 material rather than CI1 one was the dominant dust material 
present in the inner solar system at the solar system’s dawn. Furthemore the relative 
abundance of the CM2- and the CR2-like material was comparable. The dominant 
dust material accreting on Earth now is also a C2 material most probably dominated 
by a CM2-like material  rather than a CR2 (see Engrand and Maurette 1998 for a 
review). Cosmic dust in the 50-300 µm size range is known to be the most abundant 
extraterrestrial matter falling onto Earth at the present time (Love and Brownlee 
1993) but this may not have been true through the whole solar system’s history 
(Hughes ?): the modification of the nature of the inner solar system cosmic dust with 
time may  not be representative of the evolution of the composition of the global flux 
of extraterrestrial matter onto the Earth. To get an idea of the evolution of the 
composition of the meteoritic material accreting on Earth  as a whole we reexamined 
the geochemical data pertaining to Earth and Moon because geochemical data take 
into account the totality of meteoritic material accreting on the Earth-Moon system 
whatever the size of the dominant impactor is. One reason for this reexamination is 
that when most of these geochemical works have been done (during the 70s’ and the 
80s’) the CR2 group had not been yet recognized as a chondrite group and Al Rais as 
one of its anomalous member (Weisberg et al (1993), Kalleymen and Wasson 1994) 
and, consequently, the chemical features of the Earth and the Moon known which we 
know now to bear some similarities with hydrated carbonaceous chondrites (ref) 
were compared to all chondrites groups but the yet unborn CR2 one.
Wasson et al (1975) showed on the basis of the lunar samples Ge/Ir-Au/Ir 
mixing line that CM2-like material was the dominant material  accreting on the 
Moon during the last 3.7 billion years. In figure X we have reported the soil mixing 
line they use (Figure 2of Wasson et al (1975)) and plotted the corresponding values 
of the carbonaceous chondrites groups. The CR2-anomalous meteorite Al Rais and 
the CM2 carbonaceous chondrites group are the only groups to fall on the vicinity of 
the solar soil mixing line. Furthemore Hertogen et al (1977) have shown that the 
composition of the bombarding population on the Moon has changed systematically 
and that the Ir /Au value of the impacting population has been in average decreasing 
through time. This observation is compatible with the population of the impactors on 
the Moon evolving from a mixture of C2 material CR-rich to a mixture of C2 
material CM-rich (table 13).
The situation for the Earth is a little more complicated than for the Moon due 
to the early core-mantle differenciation that happened on our planet (ref). In fact, 
siderophile elements such as Ir, Au or Ge present in the very early Earth (before the 
differenciation) have been accompanying iron into the core and the actual abundance 
of siderophiles elements seen in the Earth’s mantle is due to a late (meaning 
postdifferenciation) veneering of meteoritic material (see Morgan (1985) for a 
discussion). On the other hand, the actual lithophile non-volatile element ratios of 
the Earth primitive mantle are averaging over the time of all the successive 
meteoritic components added to Earth and probably are dominated by the earliest 
ones which actually built up the Earth because such elements were not involved in 
the core formation. The Ir/Au content of the pristine upper mantle as determined by 
Morgan (1985) is 3.18 (see table 13), closer to the CM2 ratio (3.70) than to the CR2 
ratio (4.73) suggesting that the late contribution to the Earth of meteoritical 
component was dominated rather by CM2 material than CR2. This is indeed 
confirmed by the low average Ir/Au of AMMs (1.96) which are the dominant 
contributor to the Earth’s accretion rate now (Love and Brownlee 1993). Allègre et 
al (1995) studied the chemical composition of the Earth and proposed that for non-
volatile lithophiles element ratios the Earth is closer to CM2 carbonaceous 
chondrites than to CI1 carbonaceous chondrites or any other chondrite group. 
Reporting data of the CR2 carbonaceous chondrites group and for Al Rais on their 
element ratios’ correlation diagrams we observed that Al Rais composition matches 
better the Earth composition than CM2 carbonaceous chondrites and that CR2 
carbonaceous chondrites composition matches as well the Earth’s composition as 
CM2 carbonaceous chondrites. The dominant impactor population on the Earth was 
and is chemically related to C2 carbonaceous chondrites and has evolved from a 
population related to the CR2-anomalous Al Rais meteorite to a population more 
enriched in a CM2 component. 
From these data we can conclude that indeed the dominant impactor 
population onto the Earth-Moon system was a C2 material, that it has evolved 
through time from a CR-rich material to a more CM-rich material and finally that 
both CR and CM components may have been anomalous if compared to the well 
identified contemporary CM2 and CR2 carbonaceous chondrites groups. Anomalous 
C2 material such as Antarctic micrometeorites and microclasts in HED achondrites 
may have been the building bricks of the inner solar system. It remains to understand 
why this very peculiar kind of hydrated material has been so abundant since the 
beginning of our solar system and where the extensive hydration has taken place. If 
this happened on something like a parent-body it means that water as the one 
reported by Zolensky et al (1999) in the ordinary chondrite Monahans was 
overwhingly abundant in the solar system.