TRACE ELEMENT AND εNd SYSTEMATICS IN THE NAKHLITE MIL 03346 AND THE ORTHOPYROXENITE ALH 84001: IMPLICATIONS FOR THE MARTIAN MANTLE

Introduction: MIL 03346 is a newly discovered clinopyroxenite belonging to the nakhlite class of SNC (martian) meteorites [1]. Although the petrographic and geochemical characteristics of MIL 03346 are broadly similar to those of previously known nakhlites, there are also some differences. In particular, it has the highest abundance of glassy mesostasis among the nakhlites, possibly indicative of rapid cooling close to the top of a cumulate pile (e.g., [2,3]). Preliminary data suggest that its crystallization age is similar to that of the other nakhlites (i.e., ~1.3 Ga) [4]. In contrast, ALH 84001 is an orthopyroxenite that is unique among the martian meteorites in having the oldest crystallization age of ~4.5 Ga [5,6]. Here we report εNd values for MIL 03346 and ALH 84001. We also report rare earth element (REE) abundances in minerals of MIL 03346, with particular focus on obtaining the precise abundances of Eu and Gd in the augites to constrain the magmatic redox conditions. The goal of these investigations was to constrain the isotopic and geochemical characteristics of mantle sources on Mars. Analytical Techniques: Chemical separation of Nd was performed at the University of New Mexico using procedures similar to those described earlier [7,8]. The neodymium isotopic compositions of whole rock aliquots of MIL 03346, ALH 84001, and two terrestrial basalt standards (BCR-1 and BHVO1), as well as a +0.50 εNd standard, were measured by multicollector inductively coupled plasma mass spectrometry at the Field Museum using analytical protocol described in [8]. Abundances of the REE were measured in various phases in MIL 03346 with the Washington University modified Cameca ims 3f ion microprobe using the techniques described by [9]. Results: The three standards yielded εNd values of +0.01 ± 0.10 (BCR-1), –0.03 ± 0.09 (BHVO-1), and +0.49 ± 0.08 (+0.50 εNd standard). Both MIL 03346 and ALH 84001 have resolvable excesses in the Nd/Nd ratio relative to the terrestrial standard. While MIL 03346 yields a εNd value of 0.75 ± 0.06, ALH 84001 has a value of 0.19 ± 0.13 (errors are 2σmean). Figure 1 shows the εNd values for MIL 03346 and ALH 84001 reported here along with the εNd and εW values reported earlier for the various martian meteorites ([8] and references therein). -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2