The Magmatism of Mars Inferred from Chemical Composition of Shergottites

Introduction: It is well known that shergottites have a large variation of rare earth elements (REEs) abundance patterns and isotopic compositions. Jones [1] and Borg and Draper [2] suggest that light REEs (LREEs)-enriched shergottites resulted from mixing of a LREEs-enriched component with their parental magma. The LREEsenriched component is thought to be either Martian crust [1] or a KREEP-like component, which is analogous to lunar KREEP formed during the final stage of crystallization of a magma ocean [2], but, it is poorly constrained. Shirai and Ebihara [3] estimated REEs abundances for the Martian crust assuming that the LREEenriched component is Martian crust. In this paper, we present additional Zr and Hf abundances for the Martian crust and discuss the magmatism of Mars. Analytical procedures: We analyzed four olivinephyric shergottites (Y 980459, DaG 476, SaU 005 and EETA 79001A). Bulk major, minor and trace element compositions were measured by using prompt gamma ray analysis (PGA), instrument neutron activation analysis (INAA), instrument photon activation analysis (IPAA) and inductively coupled plasma mass spectrometry (ICP-MS). Result: Our Zr abundances and Zr/Hf ratios for shergottites (solid symbols) are shown in Fig. 1, where literature data also are shown. Although having larger errors than those of [4], our Zr/Hf ratios for olivine-phyric shergottites are in good agreement with [4]. Our Zr/Hf ratios for four olivine-phyric shergottites are consistent with each other and subchondritic. Zr/Hf ratios for these olivine-phyric shergottites are similar to those of lherzolitic shergottites. Zr/Hf ratio of another olivine-phyric shergottite (NWA 1068) is chondritic [5] and similar to those of basaltic shergottites. As shown in Fig. 1, olivinephyric shergottites (Y 980459, DaG 476 and SaU 005) have the lowest Zr/Hf ratios and Zr abundances among shergottites. Discussion: As shown in Fig. 1, olivine-phyric shergottites display a positive correlation between Zr/Hf ratios and Zr abundances. Such a trend can be confirmed even in shergottites including basaltic shergottites. Experimental partitioning studies indicate that Zr is more incompatible than Hf in each system of clinopyroxene, garnet, majorite, Mg-perovskite and ilmenite [6, 7, 8, 9]. Apparently, the correlation trend confirmed in Fig. 1. is consistent with experimental partitioning studies [6, 7, 8, 9]. We examine which mineral controls the fractionation of Zr from Hf in shergottites. First, we have assembled experimental data of partition coefficients for Zr and Hf (denoted as DZr and DHf, respectively) for each phase of clinopyroxene, garnet, Mg-perovskite, majorite and ilmenite. DZr/DHf ratios and their standard deviations for clinopyroxene, garnet, majorite, Mg-perovskite and ilmenite are 0.587 0.118 (n = 11), 0.960 0.265 (n = 16), 0.731 0.099 (n = 5), 0.975 0.184 (n = 8) and 0.851 0.077 (n = 3) (n: number of data). From these values, it can be concluded that Zr and Hf can be fractionated during partial melting and crystallization in the presence of each of clinopyroxene, majorite and ilmenite. Thus, each of clinopyroxene, majorite and ilmenite can be responsible for the fractionation of Zr/Hf in shergottites.