Early differentiation of the Earth and the Moon

Abstract

We examine the implications of new 182 W and 142 Nd data for Mars and the Moon for the early evolution of the Earth. The similarity of 182 W in the terrestrial and lunar mantles and their apparently differing Hf/W ratios indicate that the Moon-forming giant impact most probably took place more than 60 Ma after the formation of calcium-aluminium-rich inclusions (4.568 Gyr). This is not inconsistent with the apparent U-Pb age of the Earth. The new 142 Nd data for Martian meteorites show that Mars probably has a superchondritic Sm/Nd that could coincide with that of the Earth and the Moon. If this is interpreted by an early mantle differentiation event, this requires a buried enriched reservoir for the three objects. This is highly unlikely. For the Earth, we show, based on new mass-balance calculations for Nd isotopes, that the presence of a hidden reservoir is difficult to reconcile with the combined 142 Nd-143 Nd systematics of the Earth's mantle. We argue that a likely possibility is that the missing component was lost during or prior to accretion. Furthermore, the 142 Nd data for the Moon that were used to argue for the solidification of the magma ocean at ca 200 Myr are reinterpreted. Cumulate overturn, magma mixing and melting following lunar magma ocean crystallization at 50-100 Myr could have yielded the 200 Myr model age.

Non-chondritic Sm/Nd ratio in the terrestrial planets: Consequences for the geochemical evolution of the mantle–crust system

“…Therefore, if the hidden reservoir is very old, it must be produced by (and therefore postdate) crystallization of the terrestrial magma ocean. This observation is seemingly difficult to reconcile with the non-chondritic 142 Nd composition of the Bulk Silicate Moon (See Section 2), as this would require isolation of the hidden reservoir before the giant impact Bourdon et al, 2008). In the following section, we investigate mass balance relationships in the mantle-crust system using the nonchondritic BSE parameters defined in Section 2.…”

“…Because the presentday 143 Nd signature of the MORB source puts limits on the magnitude of this early Sm/Nd fractionation, the hidden reservoir would have had to form less than 30 Myr after formation of the solar system (Boyet and Carlson, 2005), and probably less than 10 Myr if the effect of continental crust extraction on the 143 Nd signature of the depleted mantle is taken into account Korenaga, 2009). A major difficulty arises from the fact that this event needs to have taken place prior to the giant impact (Touboul et al, 2007;Bourdon et al, 2008;Halliday, 2008), a constraint confirmed by recent assessments of lunar composition Touboul et al, 2009). However, it is difficult to envision how a differentiated reservoir could have survived mixing in an entirely molten mantle (Canup, 2004), and in which the characteristic timescale of homogenization is expected to be no longer than a week (Pahlevan and Stevenson, 2007).…”

Re-evaluating 142Nd/144Nd in lunar mare basalts with implications for the early evolution and bulk Sm/Nd of the Moon

“…First, the most recent W isotope data for the Moon now shows it to have identical e 182 W for the Earth (Touboul et al, 2007(Touboul et al, , 2009. This would give a mean accretion age for the Moon at no earlier than 4.517 Ga (52 Ma ONC ) to no later than 4.417 Ga (152 Ma ONC , Touboul et al, 2007Touboul et al, , 2009Bourdon et al, 2008;Halliday, 2008). Solidification of a global-scale magma ocean as early as 4.44 Ga ($129 Ma ONC ) as constrained by KREEP source formation at this time , would reduce the crystallization time to a maximum of $60-80 Ma after accretion of the Moon and possibly much less (Taylor et al, 2009).…”

“…This difference may have resulted from 2 processes. One possibility is nebular materials that accreted to make the bulk Earth may have had a slightly higher Sm/Nd ratio than those materials that accreted to make average chondrites Bourdon et al, 2008). An alternative possibility is that the bulk Earth has an identical 142 Nd/ 144 Nd ratio to that of the average solar system but there is an early-formed missing or hidden reservoir with a subchondritic Sm/Nd ratio that offsets the $20 ppm higher 142 Nd/ 144 Nd ratio observed in all measured terrestrial materials that are accessible relative to the average solar system.…”

“…Comparing 142 Nd-143 Nd systematics for samples from the two different studies indicates that the discrepancy reflects differences in measured 142 Nd/ 144 Nd ratios and may, in part, be related to analytical artifacts. The issue of whether the well-defined 142 Nd-143 Nd apparent isochrons in the lunar data have age implications for processes of the Moon is a matter of debate (Rankenburg et al, 2006;Boyet and Carlson, 2007;Caro et al, 2008;Bourdon et al, 2008). Rankenburg et al (2006) suggested that the 215 Ma age represented closure of the mare basalt source regions upon cooling of a lunar magma ocean.…”