ACTIVE IGNEOUS AND HYDROTHERMAL ACTIVITY DURING THE EARLY-MIDDLE AMAZONIAN: INFERENCES FROM THE CHASSIGNITE AND NAKHLITE METEORITES AND

Introduction: The current picture of the present-day martian surface presents a challenge to life [1]. Environmentally-informative mineralogy identified from orbital and in-situ exploration indicates that where water was present during recent times, condi-tions were largely saline, acidic and oxidizing [2]. The martian surface may only have been habitable during the Noachian to early Hesparian, where water was available before it was erased by the emergence of a cold, dry climate that persisted for ~3.5Ga [3]. Below the surface, however, aqueous environments on Mars may be elucidated from the detailed analysis of SNC meteorites; several of which record a magmatic source of water and the potential for young (early-mid Ama-zonian) subsurface hydrothermal activity that could stretch the envelope of martian habitability over both space and time [4-7]. The SNC meteorites represent a direct sampling of igneous processes on Mars, and they are some of the only detailed windows into the martian subsurface that scientists currently have. In recent years, the mineral-ogy of the SNC meteorites has expanded this insight to include subsurface hydrothermal activity on Mars. In particular, the volatile-bearing mineralogy of the Nakhlite and Chassignite meteorites, which includes apatite, amphibole, mica, and jarosite, have recorded both high-temperature and low-temperature interaction with a variety of fluid compositions, including those rich in water, chlorine, sulfur, carbon, iron and alkalis [4-6, 8, 9]. Moreover, many of these fluids were de-rived by magmatic degassing, indicating that the mag-matic source regions were still contributing to the addi-tion of these volatile constituents to the martian surface and subsurface as late as the early-mid Amazonian (the Chassignites and Nakhlites are dated at ~1.3 Ga [10]) [6]. In fact, the recent discovery of methane sources from volcanic provinces on Mars indicates that the same types of processes could even continue today [11]. Areas of young (Amazonian) volcanism have also been identified on Mars from orbital exploration, and several of these locations have been suggested to repre-sent potential source regions for the SNC meteorites [12]. The Chassignites and Nakhlites have been sug-gested to originate from either thick lava flows or shal-low layered intrusions [13], which would likely be as-sociated with Amazonian volcanic provinces. The most prominent young volcanism on Mars is associated with the regions of Tharsis and Elysium. Provinces of Cen-tral Elysium Planitia (southeast of Elysium Mons and Noctic Labyrinthus) and Echus Chasma (east of the Tharsis region) were active until approximately one hundred million years ago [14, 15]. Hydrated light-toned deposits (probably hydrated sulfates or chloride salts) have also been identified in association with these young volcanic features [16]. These deposits are consistent with the presence of aqueous alterations under conditions similar to the cur-rent climate [16]; however, hydrothermal activity can-not be ruled out as a potential source for the deposits. In this contribution, we attempt to synthesize the new findings of evidence for magmatically-derived hydrothermal activity from the Nakhlite and Chassig-nite meteorites with new observations from remote sensing on young Amazonian volcanism on Mars. From this compilation, we are able to make inferences about potential habitable zones at the martian surface and subsurface even after the onset of the cold, dry climate that has existed for much of the Hesparian and Amazonian epochs. Evidence for Hydrothermal Activity from The Nakhlites and Chassignites: At least two types of magmatically-derived hydrothermal fluids have been identified from recent studies of SNC meteorites [4-6, 17]. Both of these fluids were inferred based on the mineral assemblages present within various textural regimes in the meteorites. The water-rich nature of amphibole and mica within olivine-hosted melt inclusions from the Chassigny me-teorite are consistent with magmatically derived fluids that are water-rich. The apatite from these melt inclu-sions indicate that chlorine was also present in the fluid, but it was not the dominant volatile species [5, 6]. These fluids are produced from the magma after reaching fluid-saturation during ascent and crystalliza-tion. At elevated temperatures, these fluids would con-tain primarily silica and un-ionized chlorides of so-dium, potassium, iron, and hydrogen [e.g. 17-21]. Con-tinued crystallization would produce more fluid that would become progressively more dilute (water-rich). At low temperatures, these fluids would be character-ized by neutral to alkaline pH, with moderate-low sa-linity, although some variability could arise from wall-rock interactions. Astrobiology Science Conference 2010 (2010) 5604.pdf