The case for old basaltic shergottites
Introduction
The SNC meteorites (historically Shergotty–Nakhla–Chassigny) are widely considered to have been derived from Mars. Oxygen isotopes indicate that these rocks are from a single planetary body (Clayton and Mayeda, 1983, Franchi et al., 1999), and the abundance patterns of gases (particularly rare gases) occluded in pockets of shock melt (Bogard and Johnson, 1983, Pepin, 1985) are very similar to the Martian atmosphere as analyzed by the Viking landers (Owen, 1976). Furthermore, the apparently young (< 1 Ga) radiometric ages of these meteorites call for a parent planet with equally young volcanic activity, which at first glance seems to fit the well-preserved morphology of Martian volcanoes. A broad consensus has emerged over the last two decades that nakhlites and chassignites are ∼ 1.3 Ga old, whereas shergottites are much younger, on the order of a few hundred million years. Two distinctly different classes of shergottites have been identified, basaltic and lherzolitic. Only the former will be discussed in detail here. Numerous isotopic data from different long-lived radiogenic systems (87Rb–87Sr, 147Sm–143Nd, 176Lu–176Hf, U–Pb), as well as exposure ages, have been obtained for basaltic shergottites and are summarized by Nyquist et al. (2001). Most mineral (internal) isochron ages cluster around 180 Ma and 330–475 Ma (Nyquist et al., 2001), which, since Jones (1986), generally is taken as the crystallization ages of these rocks. The whole-rock data, however, are inconsistent with this interpretation: scatter makes the 147Sm–143Nd data essentially useless for dating purposes, while the > 4 Ga age indicated by the shergottite 87Rb–87Sr whole-rock errorchron so far has been seen as reflecting the time of reservoir differentiation (Borg et al., 1997, Borg et al., 2005, Shih et al., 1982). Exposure ages are fully consistent from one chronometer to another (Nyquist et al., 2001), thus bestowing strong credibility on this body of data. Importantly, they define multiple groups, which exclude the interpretation that the shergottites could have been extracted from the Martian surface or subsurface by a single impact event.
Because volcanic rocks and sediments are absent from the samples suspected to originate from Mars, it seems likely that shergottites, the most abundant type among the SNC meteorites and equivalent to plutonic rocks, represent a dominant lithology of the planetary subsurface. Blichert-Toft et al. (1999) and more recently Bouvier et al. (2005) have questioned the evidence that basaltic shergottites crystallized as shallow intrusions as recently as 180 Myr ago. Our previous arguments can be combined with additional evidence and summarized as follows:
Regardless of the uncertainties, cratering chronology (Barlow, 1988, Hartmann and Neukum, 2001, Werner et al., 2005) and stratigraphy (Tanaka and Scott, 1987, Tanaka et al., 1992) assign an age greater than 2 Ga to most of the Martian surface. As discussed by Nyquist et al., 1998, Nyquist et al., 2001, the inordinate number of apparently young shergottites contrasts with the relatively small proportion of adequately young terrains. Hartmann and Neukum (2001) reiterate that “only a modest percentage of the known volcanics are younger than 1.3 Gyr [i.e., the age of nakhlites]”. The diversity of exposure ages (Nyquist et al., 2001) thus leaves unanswered the question of how multiple impacts could have extracted nearly exclusively young material from the Martian surface.
Evidence of old Pb–Pb ages in basaltic shergottites, for which the labile component (dominated by apatite) has been removed by acid leaching, is overwhelming: leached maskelynite (the current amorphous state of plagioclase left by impact) and other minerals give ages in excess of 4 Ga. A tentative case for an old component was first made by Chen and Wasserburg (1986a), and later by Jagoutz (1991) for basaltic shergottites, and then again by Misawa et al. (1997) for the lherzolitic shergottite Y-793605. Recently, Bouvier et al. (2005) produced a statistically significant Pb–Pb isochron for Zagami with an age of 4.048 ± 0.017 Ga and observed that Pb from Los Angeles, EETA 79001, and Shergotty also fell on –or very near– the Zagami isochron. Likewise, excellent linear arrays were obtained in Pb–Pb isochron plots by Gaffney et al. (2007) for the most depleted member of the basaltic shergottite suite, QUE 94201, with correlation coefficients > 0.999, but their 4.3 Ga age were dismissed as valid isochrons by the authors who rather consider these alignments as contamination artifacts.
The preservation of extinct radioactivity anomalies in shergottites, notably 142Nd and 182W (Foley et al., 2005, Harper et al., 1995, Kleine et al., 2004, Lee and Halliday, 1997), is perplexing if these meteorites are young. For such anomalies to have been preserved until the recent past in the mantle source of young shergottite magmas, mantle convection must have been either extremely sluggish or virtually non-existent for the entire 4.5 Gyr of the planet's history. In the light of evidence of an active dynamo during the first 500 Myr of Martian history (Connerney et al., 2001, Weiss et al., 2002), such a scenario seems unlikely.
Unleached whole-rock 87Rb–87Sr data for shergottites define an errorchron with an apparent age in excess of 4 Ga (Borg et al., 1997, Borg et al., 2005, Shih et al., 1982).
Other indications of old ages include 4.0 to 2.5 Ga fission tracks in phosphate grains (Rajan et al., 1986) and K–Ar and 39Ar–40Ar ages in excess of emplacement ages (Bogard and Garrison, 1999, Mathew et al., 2003, Walton et al., 2007).
The debate about the true age of shergottites was fueled again recently by the reporting of young Pb–Pb ages on baddeleyite in basaltic shergottites (Herd et al., 2007, Misawa and Yamaguchi, 2007). In order to evaluate this new line of evidence, we undertook a combined Pb–Pb, Sm–Nd, and Lu–Hf isotope study of two fragments of Los Angeles and Shergotty to complement our earlier results for Zagami. Our new data confirm that, although the Sm–Nd and Lu–Hf ages are young, hence in accordance with previous results, the Pb isotope compositions are all consistent with the ∼ 4.1 Ga isochron defined previously by Zagami (Bouvier et al., 2005). In the following discussion we review ages from the shergottite literature and argue that they allow us to maintain the claim (Bouvier et al., 2005) that basaltic shergottites are ancient rocks in full agreement with the old ages estimated by crater size-frequency for most of the Martian surface.
Section snippets
Analytical techniques
We analyzed the Pb–Pb, Sm–Nd, and Lu–Hf isotope compositions of whole-rocks and mineral separates (maskelynite, augite, and pigeonite) from a 2.8 g piece of the basaltic shergottite Shergotty (fall) obtained from the Museum National d'Histoire Naturelle (MNHN, sample #430) and a 2.3 g piece of the basaltic shergottite Los Angeles (find) obtained from the American Museum of National History (AMNH). The analytical techniques employed for this work, carried out at the Ecole Normale Supérieure in
Results
The Sm–Nd and Lu–Hf isotope data are presented in Table 2. Internal isochrons combining results on unleached whole-rock powder and leached maskelynite, pigeonite, and/or augite separates from Shergotty give a Sm–Nd age of 172 ± 40 (MSWD = 2.0) (Fig. 1a) and a Lu–Hf age of 188 ± 91 (MSWD = 3.1) (Fig. 1b). The precision on the Lu–Hf age can be improved to 188 ± 24 Ma (MSWD = 0.01) if the whole-rock (WR2b in Table 2) is removed from the calculation. The three available measurements on Shergotty whole-rock
Discussion
The new young Sm–Nd and Lu–Hf internal isochron ages for Shergotty and Los Angeles are in accordance with previous findings, including our own. The replicate whole-rock Sm–Nd and Lu–Hf analyses of different fragments or powder fractions of a same meteorite, however, do not form alignments with meaningful ages (Fig. 1, Fig. 2). The scatter is larger than any potential inter-laboratory bias and therefore, from the outset, indicates complexities in the Lu–Hf and Sm–Nd isotope systematics, which
Conclusions
Our new Pb isotope data on Shergotty and Los Angeles combined with our previous Pb data on Zagami and Pb data from the literature define an isochron indicating that shergottites crystallized ∼ 4.1 Gyr ago. The conflict between different isotopic systems is only apparent from and boils down to a contrast between mineral and whole-rock isochron ages. The old Pb–Pb age is in agreement with evidence drawn from the whole-rock 87Rb–87Sr systematics of shergottites. In contrast, the young ages obtained
Acknowledgements
We are grateful to the curating committee of the Muséum National d'Histoire Naturelle in Paris and to Denton Ebel at the American Museum of Natural History in New York for donating samples of, respectively, Shergotty and Los Angeles. We thank Alan Brandon, Larry Nyquist, Chi-Yu Shih, and Steven Symes for generously providing some of their unpublished Rb–Sr data in tabulated form. We further are indebted to Philippe Telouk for assistance with the Nu Plasma and to Chantal Douchet for day-to-day
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Present address: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.