Japan Geoscience Union Meeting 2016

Presentation information


Symbol P (Space and Planetary Sciences) » P-PS Planetary Sciences

[P-PS12] Formation and evolution of planetary materials in the solar system

Tue. May 24, 2016 3:30 PM - 5:00 PM 104 (1F)

Convener:*Masaaki Miyahara(Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University), Akira Yamaguchi(National Institute of Polar Research), Tomohiro Usui(Department of Earth and Planetary Sciences,Tokyo Institute of Technology), Yoko Kebukawa(Faculty of Engineering, Yokohama National University), Wataru Fujiya(Ibaraki University, College of Science), Yusuke Seto(Graduate School of Science, Kobe University), Shoichi Itoh(Graduate school of Science, Kyoto University), Chair:Akira Yamaguchi(National Institute of Polar Research)

3:45 PM - 4:00 PM

[PPS12-20] A New Interpretation of Pb Isotopic Variation of Shergottites: Evidence for Heterogeneous Mantle and Crustal Assimilation on Mars

*Minato Tobita1, Tomohiro Usui1, Tetsuya Yokoyama1 (1.Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology)

Keywords:Mars, Shergottite, Pb isotope

Geochemical studies of shergottites (Martian basalts) based on the Rb-Sr, Sm-Nd, and Lu-Hf isotopic systems have provided clues to understanding of the geochemical evolution of Martian mantle and identification of the source reservoirs. On the other hand, the U-Pb isotopic systematics has been used to a limited extend for the shergottite petrogenesis, because it is generally difficult to discriminate the indigenous magmatic Pb component from secondary Martian near-surface components and terrestrial contamination. This study compiles and reassesses all the available Pb isotopic compositions of shergottites; the datasets include whole-rocks and mineral separates with acid leaching experiments.
Three geochemical groups of shergottites (enriched, intermediate, and depleted) have different Pb isotopic compositions, reflecting different µ (238U-204Pb) values of their sources. The enriched and depleted shergottites individually exhibit distinct linear arrays in the 206Pb/204Pb-207Pb/204Pb diagram, providing apparent isochron ages of ~4.1 Ga and ~4.3 Ga, respectively. These linear arrays in the Pb isotopic diagram are interpreted as reflecting either (1) a Pb-Pb isochron or (2) a mixing of two components with distinct Pb isotopic compositions. Four possibilities have been suggested to explain the linear variations in the Pb isotopic compositions of shergottites: (1a) Pb-Pb isochron representing a shergottite crystallization age, (1b) Pb-Pb isochron for the formation ages of the shergottite source reservoirs, (2a) mixing of a terrestrial Pb component, and (2b) mixing of Martian surficial Pb by alternation. However, these interpretations of the Pb isotopic variations are inconsistent with other isotopic systematics such as Rb-Sr, Sm-Nd, and Lu-Hf for shergottites.
We propose a new model for the shergottite Pb isotopic variations along with other geochemical evidence, in which the two linear arrays defined by the enriched and depleted shergottites represent assimilation of an ancient high-μ crustal component that has high 207Pb/204Pb and 206Pb/204Pb ratios. Differences in the other ends of the linear arrays are interpreted as reflecting the geochemical heterogeneity of the Martian mantle (i.e. enriched and depleted mantle sources). These interpretations of the Pb isotopic variations are consistent with two models proposed by other geochemical signatures of shergottites, “crustal assimilation” and “mantle heterogeneity”.