A New Constraint on the Physicochemical Condition of Mars Surface during the Amazonian Epoch Based on Chemical Speciation for Secondary Minerals in Martian Nakhlites

Hiroki Suga,Akira Yamaguchi,Yasuo Takeichi,Tomohiro Usui,Takuji Ohigashi,Oki Sekizawa,Keika Suzuki,Kiyofumi Nitta,Yoshio Takahashi

doi:10.3390/min11050514

Abstract

Iddingsite in Martian nakhlites contains various secondary minerals that reflect water–rock interaction on Mars. However, the formation processes of secondary Fe minerals in iddingsite are unclear because they include carbonates precipitated under reductive and alkaline conditions and sulfates that are generally precipitated under oxidative and acidic conditions. Mineral types cannot coexist under equilibrium. Herein, we characterize the carbonate phase of meteorite Yamato 000593 as siderite and Mn-bearing siderite via field-emission electron probe microanalyzer (FE-EPMA). Then, we examined the distribution and speciation of trace Cr and S within the carbonates through synchrotron micro-focused X-ray fluorescence-X-ray absorption fine structure and scanning transmission X-ray microscopy (μ-XRF-XAFS/STXM) analysis to estimate the transition history of Eh-pH conditions during siderite formation to explain the coexistence of carbonate and sulfate phases in the nakhlite vein. Specifically, the distribution and speciation of S in the mesostasis and carbonate phases and the heterogeneous distribution of Mn-FeCO3 incorporating Cr(III) in the carbonate constrain the Eh-pH condition. The conditions and transition of the fluid chemistry determined herein based on speciation of various elements provide a new constraint on the physicochemical condition of the water that altered the nakhlite body during the Amazonian epoch.

Highlights

The current Martian surface environment is cold and dry, past Martian climate was generally wetter and warmer, and surface oceans were present with liquid water supporting habitability, as recently assessed in the literature [1]
These occurrences are similar to the observation results of clay minerals and poorly crystalline silicates sandwiched by siderite in the iddingsite [24]
We investigated the chemical compositions of the iddingsite via field-emission electron probe microanalyzer (FE-EPMA) in the plotted carbonate portions shown in Supplementary Figure S1; the carbonate compositions are listed in Supplementary Table S2

Summary

Introduction

The current Martian surface environment is cold and dry, past Martian climate was generally wetter and warmer, and surface oceans were present with liquid water supporting habitability, as recently assessed in the literature [1]. A recent investigation on brecciated Martian meteorites implies that liquid water existed on Mars until about 2100 Ma during the early Amazonian epoch [4]. Nakhlite Martian meteorites contain iddingsite, which is characterized by an aqueous-altered texture. Investigating the vestiges of Martian aqueous events preserved in Martian meteorites as secondary minerals is important. Such analysis can reveal details of past Martian liquid water conditions (e.g., cation composition and pH) to better understand past and present Martian habitability

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Conclusion