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 εNd 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 εNd 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.