Ejection of Martian meteorites

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.

Section: Validity Of the Ar-ar Agementioning

confidence: 99%

Section: Validity Of the Ar-ar Agementioning

confidence: 99%

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

Nyquist¹,

Bogard²,

Shih³

et al. 2009

“…These estimates are based on the conversion of plagioclase to either a diaplectic 'maskelynite' ($14-45 GPa) or melt glass (>45 GPa) with a resulting increase in the refractive index and/or Raman band broadening, compared to unshocked plagioclase (Fritz et al, 2004). Specifically, for ALH4001, estimated values for peak shock pressure have evolved over time -$50-60 GPa (Langenhorst et al, 2000); <40 GPa (van der Bogert et al, 1999); $35-40 GPa (Stö ffler, 2000); 35.7 ± 4.5 GPa (Fritz et al, 2003); >35-40 GPa (Fritz et al, 2004) -with the best current estimate at 32 ± 1 GPa (Fritz et al, 2005).…”

Section: Introductionmentioning

confidence: 99%

“…Results from simulations of shockwave propagation through compositionally and texturally complex matrices are computationally difficult and sensitive to initial starting conditions, but do indicate that shock heating would have been highly heterogeneous at all size scales. Using a Hugoniot equation-ofstate experimentally derived from Stillwater pyroxenite, Twin Sister dunite (Stö ffler, 1982), and a gabbroic rock (Trunin et al, 2001), the post-shock temperature elevation for ALH84001 associated with a 32 GPa shock event lies in the range of $373-383 K (Fritz et al, 2005). If we assume ALH4001 prior to ejection was at the average Martian ground-surface temperature of $230 K and that the peak shock pressure experienced corresponded to this final ejection event (note, paleomagnetic data of Weiss et al (2000) suggest the interior of ALH84001 was not heated above $313 K), this would imply an average maximum heating, post-ejection, to a temperature of $610 K. This is in broad agreement with recent (U-Th)/He analyses of ALH84001 phosphate grains that suggest a peak ejection temperature of $673 K or less (Min and Reiners, 2006).…”

Section: Introductionmentioning

confidence: 99%

Origins of magnetite nanocrystals in Martian meteorite ALH84001

Thomas‐Keprta

Clemett

McKay

et al. 2009

108

The Martian meteorite ALH84001 preserves evidence of interaction with aqueous fluids while on Mars in the form of microscopic carbonate disks. These carbonate disks are believed to have precipitated 3.9 Ga ago at beginning of the Noachian epoch on Mars during which both the oldest extant Martian surfaces were formed, and perhaps the earliest global oceans. Intimately associated within and throughout these carbonate disks are nanocrystal magnetites (Fe 3 O 4 ) with unusual chemical and physical properties, whose origins have become the source of considerable debate. One group of hypotheses argues that these magnetites are the product of partial thermal decomposition of the host carbonate. Alternatively, the origins of magnetite and carbonate may be unrelated; that is, from the perspective of the carbonate the magnetite is allochthonous. For example, the magnetites might have already been present in the aqueous fluids from which the carbonates were believed to have been deposited. We have sought to resolve between these hypotheses through the detailed characterization of the compositional and structural relationships of the carbonate disks and associated magnetites with the orthopyroxene matrix in which they are embedded. Extensive use of focused ion beam milling techniques has been utilized for sample preparation. We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonates under a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites present in the carbonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead, we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to any shock or thermal processing of the carbonates.

“…Nakhlites are thought to be cumulates that formed subsurface (Reid and Bunch, 1975;Berkley et al, 1980;Harvey and McSween, 1992;McSween, 1994), and they appear to have cooled relatively slowly under oxidizing conditions Miyamoto, 1998, 2002). Nakhlites experienced lower shock levels than any other martian meteorites, and their plagioclases have not been converted to maskelynite (Fritz et al, 2005). A detailed review of nakhlites was presented by Treiman (2005).…”

Section: Introductionmentioning

confidence: 99%

39Ar–40Ar ages of martian nakhlites

Park

Garrison

Bogard

2009

We report 39 Ar- 40 Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of 39 Ar recoil and variable amounts of trapped 40 Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2r errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial 40 Ar on weathered grain surfaces that was degassed along with the first several percent of 39 Ar. A total K-40 Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2r). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of $12 Ma is also similar to CRE ages for other nakhlites. Published by Elsevier Ltd.