Rapid Accretion and Early Differentiation of Mars Indicated by ¹⁴² Nd/ ¹⁴⁴ Nd in SNC Meteorites

Abstract

Small differences in the ratio of neodymium-142 to neodymium-144 in early formed mantle reservoirs in planetary bodies are the result of in situ decay of the extinct radionuclide samarium-146 and can be used to constrain early planetary differentiation and therefore the time scale of planetary accretion. The martian meteorite Nakhla (approximately 1.3 billion years old), the type sample of the nakhlite subgroup of the Shergottite-Nakhlite-Chassigny (SNC) meteorites, exhibits a 59 +/- 13 parts per million excess in the ratio of neodymium-142 to neodymium-144 relative to normal neodymium. This anomaly records differentiation in the martian mantle before 4539 million years ago and implies that Mars experienced no giant impacts at any time later than 27 million years after the origin of the solar system.

The evolution of a heterogeneous Martian mantle: Clues from K, P, Ti, Cr, and Ni variations in Gusev basalts and shergottite meteorites

“…These estimates agree with thermal models that suggest the Martian mantle convected more vigorously early (e.g., Schubert et al, 1992;Sleep, 2000) and crustal growth slowed over time (Hauck and Phillips, 2002). In addition, early light REE depletion of the shergottitic source (Harper et al, 1995;Borg et al, 2002) indicates early partial melting and crustal growth.…”

“…Most shergottites tend to be depleted in incompatible elements, particularly in alkalis ( Fig. 1D) and light rare earth elements (REE; e.g., Treiman et al, 1986) and are likely derived from a mantle that has been depleted in these elements for a long time based on Sm-Nd isotope systematics (Harper et al, 1995;Borg et al, 1997). A second, more enriched reservoir must also exist because some shergottites have flatter REE profiles (higher La/Lu ratios) and Sm-Nd systematics that indicate prolonged incompatible element enrichment.…”

Constraints on Martian differentiation processes from RbSr and SmNd isotopic analyses of the basaltic shergottite QUE 94201

“…This value is essentially the same as that measured in the Plag-lR, Plag-2, and Plag-3L fractions (Table 1) because of their very low 87Rb/86Sr, and the relatively young crystallization age of the sample. (Harper et al, 1995;Jagoutz, 1996), and suggests that the sources of these samples experienced similar early differentiation events.…”

“…We also present time-integrated melting models that demonstrate that QUE 94201 can be derived from a primitive (chondritic) mantle that has undergone repeated smallvolume partial melting events. et al (1996), Kring et al (1996), McKay et al (1996, McSween and Eisenhour (1996), McSween et al, (1996), and Mikouchi et al (1996) (Harper et al, 1995;Jagoutz, 1996) Thus, whereas the Nd isotope systematics of the Nakhlites are indicative of derivation from source regions with strong time-integrated LREE depletions, the LREE enrichment of the melts suggest derivation from LREE-enfiched sources. To explain this seeming contradiction, the REE systematics of the Nakhlites have generally been attributed to very small degrees of melting of a previously depleted source (e.g., Nakamura et al, 1982;Longhi, 1991 Longhi, 1991;Longhi et al, 1992;Gleason et al, 1996).…”

Core Formation and Mantle Differentiation on Mars

“…Data from Debaille et al (2007), Caro et al (2008), Debaille et al (2009) The 142 Nd record of the nakhlites is even more difficult to interpret than that of the shergottites. The elevated 142 Nd/ 144 Nd ratio of the nakhlites corresponds to a twostage model age of ∼25 Ma after CAI formation (Harper et al 1995;Foley et al 2005). This age is also consistent with that obtained from combined 142 Nd-182 W systematics for Sm/Nd and Hf/W fractionation in a garnet-bearing shallow mantle (Kleine et al 2004b;Foley et al 2005).…”