EXAFS Determination of Clay Minerals in Martian Meteorite Allan Hills 84001 and Its Implication for the Noachian Aqueous Environment

Ryoichi Nakada,Tetsuya Yokoyama,Tomohiro Usui,Iori Kajitani,Masashi Shidare,Gaku Tanabe

doi:10.3390/min11020176

Abstract

The aqueous environment of ancient Mars is of significant interest because of evidence suggesting the presence of a large body of liquid water on the surface at ~4 Ga, which differs significantly from the modern dry and oxic Martian environment. In this study, we examined the Fe-bearing minerals in the 4 Ga Martian meteorite, Alan Hills (ALH) 84001, to reveal the ancient aqueous environment present during the formation of this meteorite. Extended X-ray absorption fine structure (EXAFS) analysis was conducted to determine the Fe species in ALH carbonate and silica glass with a high spatial resolution (~1–2 μm). The μ-EXAFS analysis of ALH carbonate showed that the Fe species in the carbonate were dominated by a magnesite-siderite solid solution. Our analysis suggests the presence of smectite group clay in the carbonate, which is consistent with the results of previous thermochemical modeling. We also found serpentine in the silica glass, indicating the decrease of water after the formation of carbonate, at least locally. The possible allochthonous origin of the hematite in the carbonate suggests a patchy redox environment on the ancient Martian surface.

Highlights

Geological and geomorphological evidence suggests the presence of a large amount of liquid water on the surface of early Mars [1,2,3,4,5]
The Extended X-ray absorption fine structure (EXAFS) analysis showed that Fe species in all Allan Hills (ALH) carbonates contained magnesite and siderite, whereas the presence of ankerite or dolomite was not suggested
Magnetite was suggested for all analyzed phases, namely, rosette carbonate, slab carbonate, and silica glass, whereas hematite and pyrite were only suggested for rosette and slab carbonate, respectively

Summary

Introduction

Geological and geomorphological evidence suggests the presence of a large amount of liquid water on the surface of early Mars [1,2,3,4,5] Observations by both the orbiter and rover detected a variety of hydrous minerals and evaporites [6,7,8,9]. Smectites were observed in crater-rim-derived rocks in the Gale crater, possibly formed by diagenesis in the early Hesperian [12,13]. Despite their ubiquity on the Martian surface, clay minerals have only been observed in small-scattered regions in a ~4.1 Ga Martian meteorite, Allan Hills (ALH) 84001 [14]

Results

Discussion

Conclusion