Shergottite Mixing Relations, Partial Melting Models, and the Nd Evolution of the Martian Mantle

Details Shergottite Mixing Relations, Partial Melting Models, and the Nd Evolution of the Martian Mantle Shergottite Mixing Relations, Partial Melting Models, and the Nd Evolution of the Martian Mantle

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.511g&link_type=abstract

Meteoritics, vol. 30, no. 5, page 511

Computer Science

Basalt, Isotopes, Mars, Meteorites, Differentiated, Snc, Nd, Shergottites

Scientific paper

The SNC class of meteorites includes basalts and cumulate ultramafic rocks with isotopic and chemical characteristics indicating derivation from the same parent body, most likely Mars [1]. The nakhlites (N) and Chassigny (C) have igneous crystallization ages of 1.26 Ga and appear to be cogenetic, while a recently discovered SNC orthopyroxenite (ALH84001) appears to be 4.5Ga [2]. A third age group, the shergottites (S), all have recently confirmed igneous crystallization ages of 180 Ma [2] but are less clearly related by simple igneous processes [3]. Several workers have suggested that their complex isotopic and rare earth element (REE) systematics can be explained by mixing with an additional component [e.g., 3,4,5], based partly on the observation that the shergottites fall on a 1.22 Ga Sm-Nd "pseudochron" or mixing line [2]. We have modeled these complexities quantitatively using mixing and assimilation-fractionation (AFC) equations and the experimentally determined shergottite parent melt compositions of [3,6]. These calculations successfully relate all the shergottites to two distinct parent magmas, which were derived from similar depleted mantle sources. These two magmas mixed to produce a hybrid magma, but only one underwent significant assimilation of an enriched basaltic component as suggested by [5]. If the shergottites were produced from parent magmas derived from long-term light rare earth element (LREE) depleted mantle (epsilon(sub)Nd = +15 to +20 at 180 Ma), then Nd isotopes suggest that the martian mantle evolved down a fairly uniform Sm/Nd trajectory (147Sm/144Nd = 0.22 to 0.23) starting with earliest planetary differentiation [7]. However, partial melting models for the REE indicate that 147Sm/144Nd ratios of the model shergottite source (0.40 to 0.55) are much higher than epsilon(sub)Nd values allow, unless the source was depleted only a few hundred million years before shergottite magmatism at 180 Ma. Also, attempts to produce a match for the model shergottite source by partially melting a primitive mantle with a chondritic REE pattern are unsuccessful. No combination of incremental batch melting models appears to be capable of reproducing the model shergottite source pattern. Both garnet and clinopyroxene, which are assumed to be the major mantle phases exerting influence on the REE pattern [8], yield similar LREE depleted residues during melting which lack the high Dy/Yb and La/Ce ratios characteristic of the shergottite model source and magmas. The apparent simple Nd isotopic evolution of martian mantle thus appears to be decoupled from REE systematics, which suggest a much more complex chemical evolution for magma sources. The unusual S-shaped shergottite REE patterns bear some resemblance to REE patterns in clinopyroxenes from terrestrial mantle thought to have been metasomatized by CO2-rich fluids or small-volume enriched melts [9]. Mantle metasomatism has previously been suggested to explain the enriched trace-element signatures in the nakhlites and Chassigny [5], which were derived from long-term LREE depleted (high Sm/Nd) sources as required by Nd isotopes [7]. Such processes may have acted to "suppress" Sm/Nd ratios in martian mantle source regions over time, with periodic removal of small melt fractions (e.g., Nakhla-type melts) reflecting these enrichment processes. SNC mantle sources thus appear to have been "buffered" at relatively constant time-averaged Sm-Nd ratios by complex interactions involving melt migration (enrichment) and removal (depletion) at various times throughout their evolution. References: [1] McSween H. Y. (1994) Meteoritics, 29, 757-779. [2] Nyquist L. E. (1995) LPS XXVI, 1065-1066. [3] Wadhwa M. et al. (1994) GCA, 58, 4213-4229. [4] Jones J. H. (1989) Proc. LPSC 19th, 465-474. [5] Longhi J. (1991) Proc. LPS, Vol. 21, 695-709. [6] McKay G. A. et al. (1986) GCA, 50, 927-937. [7] Harper C. L. et al. (1995) Science, 267, 213-217. [8] Longhi J. et al. (1992) in Mars, 184-208, Univ. of Arizona, Tucson. [9] Johnson K. T. M. et al. (1990) JGR, 95, 2661-2678.

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