Geochemistry of Martian Meteorites and the Petrologic Evolution of Mars

Introduction: Mafic igneous rocks serve as probes of the interiors of their parent bodies – the compositions of the magmas contain an imprint of the source region composition and mineralogy, the melting and crystallization processes, and mixing and assimilation. Although complicated by their multifarious history, it is possible to constrain the petrologic evolution of an igneous province through compositional study of the rocks. Incompatible trace elements provide one means of doing this. I will use incompatible element ratios of martian meteorites [1] to constrain the early petrologic evolution of Mars. Incompatible elements are strongly partitioned into the melt phase during igneous processes. The degree of incompatibility will differ depending on the mineral phases in equilibrium with the melt. Most martian meteorites contain some cumulus grains, but nevertheless, incompatible element ratios of bulk meteorites will be close to those of their parent magmas. ALH 84001 is an exception, and it will not be discussed. The martian meteorites will be considered in two groups; a 1.3 Ga group composed of the clinopyroxenites and dunite, and a younger group composed of all others. Planetary Comparisons: On Earth, P and La are highly correlated in igneous rock suites, resulting in a small range in P/La ratios for terrestrial mafic magmas (Fig. 1). Magmas with higher incompatible element contents have lower P/La, indicating La is more incompatible than P. This is also supported by various estimates of the composition of the bulk continental crust [2], which is more highly enriched in La than P compared to an estimated bulk silicate Earth [2]. Mafic rocks from the Moon follow the terrestrial example – P/La ratios exhibit a narrow range (Fig. 1). In contrast, martian meteorites show a wide range in P/La (Fig. 1). The planetary distinctions are also clearly shown on a P vs. Yb diagram (Fig. 1). The terrestrial mafic rocks show no correlation between P and Yb, lunar rocks occupy a band of increasing P and Yb, while in martian meteorites these elements are strongly correlated. This suggests a fundamental difference in the petrologic evolution of Mars as compared to the Earth or Moon. The constancy of Yb/P implies that the partitioning of these elements is governed by a single phase, or two phases either in constant proportions or with similar P/Yb partition coefficient ratios. The phase or phases would have a much lower partition coefficient for La, such that P and La are decoupled. Figure 1. P-La and P-Yb plots for mafic igneous rocks from Mars, the Earth and Moon. Crustal Assimilation: Recently, a number of geochemical characteristics of a subset of martian meteorites have been ascribed to assimilation of crustal material by mafic magmas [e.g. 3]. The P/La ratios for martian meteorites <1 Ga in age are positively correlated with their εNd [1] (Fig. 2), a presumed measure of crustal assimilation/contamination, indicating that this process may have contributed to the scatter in P/La. This is in general in accord with terrestrial geochemistry – the P/La ratio for the bulk continental crust is lower than that estimated for the bulk silicate Earth [2]. However, P and La are enriched in the continental crust by roughly 10× and 30×, respectively [2]. QUE 94201 has the highest εNd (Fig. 2) and the highest P content (Fig. 1). This rock is considered to have suffered minimal crustal contamination [see 3]. Hence, if basalts like Shergotty were formed by contamination of primary or evolved magmas, then these magmas, and possibly their source regions, must have had incompatible element contents different in detail from those of QUE 94201. Thus, for example, their Sm/Nd ratios may have been lower, and the source region would then have had lower εNd. Regardless of complications from possible assimilation of crust, the P/La ratios of martian mafic igneous rocks show wide ranges, while 0.01 0.1 1 10