Constraints on the petrogenesis of Martian meteorites from the Rb-Sr and Sm-Nd isotopic systematics of the lherzolitic shergottites ALH77005 and LEW88516

Abstract

Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 ± 11 Ma and a Sm-Nd age of 173 ± 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 ± 10 and 166 ± 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 ± 4, and the initial ε_Nd value is +11.1 ± 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 ± 4 and an initial ε_Nd value of +8.2 ± 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low ¹⁴³Nd/¹⁴⁴Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 ± 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental.

Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ∼4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.

Introduction

The nine Martian basaltic meteorites that have been analyzed have a larger range of initial Sr and Nd isotopic compositions than all terrestrial basalts. This range has been interpreted to reflect mixing of components derived from highly fractionated mantle sources and evolved crustal-like sources (e.g., Shih et al 1982, Jones 1989, Borg et al 1997. Whereas the isotopic composition of the mantle end-member is well defined by meteorites such as QUE94201 (QUE), which are characterized by very low initial ⁸⁷Sr/⁸⁶Sr ratios and high initial ε_Nd¹⁴³ and ε_Nd¹⁴² values, the isotopic composition of the crust-like component remains poorly defined. This in part stems from the fact that these meteorites do not define a single liquid line of descent, as evidenced by a variety of parental magma compositions Stolper and McSween 1979, McSween and Jarosewich 1983, McSween et al 1988, Longhi and Pan 1989, McSween et al 1996 and ages (165 to 575 Ma; Nyquist et al 1979, Borg et al 2001. The goal of this study is to better constrain the composition of the Martian crust-like component present in the shergottites by examining the Sr and Nd isotopic systematics of two closely related lherzolitic shergottites ALH77005 (ALH) and LEW88516 (LEW) in the context of assimilation and fractional crystallization (AFC).

The Rb-Sr and Sm-Nd isotopic systematics of Martian meteorites, particularly the shergottite subgroup, are extremely complicated. This complexity arises for several reasons, including (a) the Rb-Sr and Sm-Nd isotopic systems often do not yield concordant ages; (b) when the two systems do yield concordant ages, each is sometimes defined by different mineral fractions and/or leachates; and (c) all of the mineral fractions, whole rocks, and leachates from a single meteorite usually do not lie on an individual isochron. Many of these features have been attributed to disturbance of the isochron by shock metamorphism or weathering processes occurring on Mars and Earth. Shock metamorphism can produce glasses in which parent/daughter nuclides are fractionated (Jagoutz, 1989) or add extraneous material to the meteorite (Rao et al., 1999). Likewise, weathering can add secondary alteration products Zipfel et al 1999, Crozaz and Wadhwa 2001 that are not in isotopic equilibrium with the igneous components of the meteorite Borg et al 1999a, Borg et al 2000. The second goal of this study, therefore, is to assess the effects of shock metamorphism and secondary alteration by comparing the Rb-Sr and Sm-Nd isotopic systematics of two nearly identical but unpaired lherzolitic shergottites.