Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346Ma old basaltic shergottite related to “lherzolitic” shergottites

Abstract

Multiple lines of evidence show that the Rb-Sr, Sm-Nd, and Ar-Ar isotopic systems individually give robust crystallization ages for basaltic (or diabasic) shergottite Northwest Africa (NWA) 1460. In contrast to other shergottites, NWA 1460 exhibits minimal evidence of excess 40 Ar, thus allowing an unambiguous determination of its Ar-Ar age. The concordant RbSr, Sm-Nd, and Ar-Ar results for NWA 1460 define its crystallization age to be 346 ± 17 Ma (2r). In combination with petrographic and trace element data for this specimen and paired meteorite NWA 480, these results strongly refute the suggestion by others that the shergottites are $4.1 Ga old. Current crystallization and cosmic-ray exposure (CRE) age data permit identification of a maximum of nine ejection events for Martian meteorites (numbering more than 50 unpaired specimens as of 2008) and plausibly as few as five such events. Although recent high resolution imaging of the Martian surface has identified limited areas of sparsely cratered terrains, the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come, or that the cratering chronology needs recalibration. Time-averaged 87 Rb/ 86 Sr = 0.16 for the mantle source of the parent magma of NWA 1460/480 over the $4.56 Ga age of the planet is consistent with previously estimated values for bulk silicate Mars in the range 0.13-0.16, and similar to values of $0.18 for the ''lherzolitic" shergottites. Initial e Nd for NWA 1460/480 at 350 ± 16 Ma ago was +10.6 ± 0.5, which implies a time-averaged 147 Sm/ 144 Nd of 0.217 in the Martian mantle prior to mafic melt extraction, similar to values of 0.211-0.216 for the ''lherzolitic" shergottites. These time-averaged values do not imply a simple two-stage mantle/melt evolution, but must result from multiple episodes of melt extractions from the source regions. Much higher ''late-stage" e Nd values for the depleted shergottites imply similar processes carried to a greater degree. Thus, NWA 1460/480, the ''lherzolitic" shergottites and perhaps EET 79001 give the best (albeit imperfect) estimate of the Sr-and Nd-isotopic characteristics of bulk silicate Mars. Published by Elsevier Ltd.

Lithium isotope constraints on crust–mantle interactions and surface processes on Mars

“…This is evident from the comparison of published data for Zagami (cf. this study; Magna et al, 2006;Reynolds et al, 2006;Seitz et al, 2006) that appear to potentially reflect the lithological diversity of Zagami (McCoy et al, 1999), but also because enriched shergottites are probably not representative of the pristine martian mantle as they record contamination by either martian crust or mixing of heterogeneous mantle components (e.g., Herd et al, 2002;Borg and Draper, 2003;Jones, 2003;Warren et al, 2004;Debaille et al, 2007Debaille et al, , 2008Symes et al, 2008;Basu Sarbadhikari et al, 2009;Nyquist et al, 2009;Lapen et al, 2010).…”

“…(2) only high-MgO (>15 wt.%) samples are taken into account because they are considered to best reflect the composition of bulk silicate Mars (Nyquist et al, 2009) and because they either reflect primitive melt compositions or are cumulates from high-temperature crystallization of olivine; (3) it is assumed that, at high temperatures, the d 7 Li of the melts directly reflects the d 7 Li value of their martian mantle source with insignificant 7 Li/ 6 Li fractionation by other processes (Tomascak et al, 1999b;Jeffcoate et al, 2007). Using these criteria, new and published data (Table 1) for MgO-rich cold desert/fall meteorites ALH 77005, EETA 79001A, LAR 06319, RBT 04262, Tissint, Y-000097 and Y-980459 are used in the initial estimate, yielding an average d 7 Li of 4.0 ± 1.6& (2r) for the martian mantle.…”

“…Examples of "non-SNC" martian meteorites are ALH 84001, a $4.1 Ga orthopyroxenite, thought to represent ancient martian crust (e.g., Lapen et al, 2010), and the $4.4 Ga polymict breccia NWA 7034 (Agee et al, 2013;Humayun et al, 2013). Previous studies have demonstrated the origin of these meteorites from Mars (e.g., Bogard and Johnson, 1983;Franchi et al, 1999;Nyquist et al, 2001), and also revealed evidence for prolonged geologic activity on their parent body (e.g., Borg et al, 2008;Misawa et al, 2008;Nyquist et al, 2009).…”

Isotopic and petrographic evidence for young Martian basalts

“…Young ages are thus interpreted to represent the last re-equilibration of phosphate with Martian groundwater. Although the interpretation of shergottites as ancient rocks reconciles the age of the Martian surface with that of the current inventory of Martian meteorites, as well as allowing for active convection of the Martian mantle, strong arguments against this concept have been made (Jones, 1986;Borg et al, 2005;Nyquist et al, 2006;Fritz et al, 2007;Gaffney et al, 2007;Jones, 2007). The proposal that shergottites are 4.0 Ga has received considerable attention, because if proven correct, it would have the important implication that the Martian surface is older than previously thought.…”

39Ar‐40Ar dating of the Zagami Martian shergottite and implications for magma origin of excess 40Ar

“…Bouvier et al (2005Bouvier et al ( , 2007, using Pb-Pb isotopic data from Zagami and a few other shergottites, suggested that these shergottites actually formed 4.0 Gyr ago and that the Sm-Nd and Rb-Sr chronometers of some minerals, particular phosphates, were reset by acidic aqueous solutions percolating through the Martian surface. However, other chronologists have strongly argued for young ages for the shergottites (Nyquist 2006;Nyquist et al 2001Nyquist et al , 2006bMisawa et al 2006;Herd et al 2007;Shih et al 2007;Jones 2007;Bogard and Park 2007b). Experimental studies suggest that pyroxene would be more strongly weathered than plagioclase in aqueous solutions ranging from moderately acidic to moderately basic (McAdam et al 2006).…”