Evidence for a Distinctive Rare Earth Element-enriched Mantle Reservoir on Mars

Introduction: At least three major geochemical reservoirs have been identified in the crust-mantle system of Mars, including the primitive mantle, an ancient highly depleted mantle, best sampled by the most LREE-depleted basaltic shergottites, and an ancient enriched crust [e.g., Refs. 1-4]. Some workers, evaluating both isotopic and geochemical data, have suggested additional reservoirs, for example, representing the sources of Nakhlites and Chassigny [e.g., 5-7]. In light of the recent dramatic increase in the number of basaltic through ultramafic meteorites derived from Mars, and better understanding of the composition and size of the Martian crust, from Pathfinder chemical data, GRS measurements, the chemistry of shergottites, and geophysical data from Pathfinder and Mars Global Surveyor, there is a need for re-evaluating the character of major geochemical reservoirs in the crust-mantle system on Mars. Primitive Mantle of Mars: The chemical composition of SNC meteorites has long been used to estimate the composition of the Martian primitive mantle. However, it is increasingly clear that the magmatically young shergottites represent a mixture between an ancient, LIL-depleted mantle and an ancient LILenriched crust [e.g., 2,3]. McLennan [8] suggested that certain key element ratios used to estimate the moderately volatile element content of the Martian primitive mantle, notably K/Th, K/U and K/La, are strongly fractionated between depleted mantle and enriched crust. Accordingly, while the composition of the Martian primitive mantle is probably known to a first order, it is likely not well constrained for the critical moderately volatile lithophile elements, such as K, Rb and Cs. On the other hand, essentially all workers agree that refractory lithophile elements should be present in chondritic proportions in the primitive mantles of all the terrestrial planets, including Mars. For example, apart from a few samples affected by U-adddition during terrestrial weathering processes [e.g., 9], SNC meteorites all have Th/U ratios close to the chondritic value of about 3.5 [8]. Both Th and U have similar degrees of incompatibility during most partial melting processes. In other cases, significant differences in solid-liquid partition coefficients can result in major changes in ratios among the refractory lithophile elements during magmatic processes (e.g., La/Sm, Th/Sc, Ba/Y, etc.), but “enriched” and “depleted” reservoirs should be complementary and the average composition of all of the major geochemical reservoirs will result in chondritic ratios among the refractory lithophile elements. La/Th Ratios in SNC Meteorites. Figure 1 plots La/Th ratios against La content for SNC meteorites and these are compared to the terrestrial crust and depleted mantle (as sampled by mid-ocean ridge basalts). Although La and Th typically are both highly incompatible elements, there is enough difference to result in significant fractionation of the La/Th ratio during igneous differentiation processes leading to relatively low La/Th in most felsic rocks and high La/Th in most mafic rocks [12]. This is clearly seen on Earth, where there is a four-fold difference in the average La/Th ratio of continental crust and depleted mantle as sampled by MORB. Nevertheless, terrestrial MORB and continental crust have complementary La/Th ratios relative to the CI value of 8.6 and reasonable mixtures can lead to a chondritic La/Th ratio.