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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

New constraints on shergottite petrogenesis from analysis of  Pb isotopic compositional space: Implications for mantle heterogeneity and crustal assimilation on Mars

Minato Tobita, Tomohiro Usui, Tetsuya Yokoyama
Geochemical Journal, Vol. 51, No. , P. 81-94, 2017

ABSTRACT

Geochemical studies of shergottites (Martian basalts) based on Rb-Sr, Sm-Nd, and Lu-Hf isotopic systematics have provided clues to understanding the geochemical evolution of the Martian mantle and identification of the source reservoirs. However, U-Pb isotopic systematics has been used to a limited extent for shergottite petrogenesis, because it is generally difficult to discriminate indigenous magmatic Pb components from secondary Martian near-surface components and terrestrial contamination. This study compiles and reassesses all the available Pb isotopic data of shergottites, as well as their Rb-Sr, Sm-Nd, and Lu-Hf isotope systematics. The Sr-Nd-Hf isotopic systematics suggests that the geochemical variability of the shergottite suite (i.e., enriched, intermediate, and depleted shergottites) reflects a mixture of two distinct source reservoirs. In contrast, the Pb isotopic systematics does not support the two-component mixing model for shergottites, because the geochemically enriched, intermediate, and depleted shergottites do not participate in a two-component mixing array in Pb isotopic space. To reconcile the isotopic signatures of the Sr-Nd-Hf and Pb systems, we propose a new mixing model in which the geochemically enriched, intermediate, and depleted shergottites were derived from compositionally distinct mantle sources that had different μ (238U/204Pb) values. Moreover, a linear mixing trend defined by the enriched shergottites in Pb isotopic space is interpreted as the incorporation of a high-μ Martian crustal component into a parental magma derived from a fertilized Martian mantle source. Our model implies that the geochemical diversity of shergottites reflects heterogeneous mantle sources and an assimilated high-μ crustal component on Mars.

KEYWORDS

shergottite, Pb isotopes, Martian geochemistry, crustal assimilation, mantle heterogeneity

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