SIMULATING A MARTIAN FUMAROLE: UNDERSTANDING THE EFFECTS OF A DEGASSING MARTIAN MAGMA ON SURROUNDING ROCK.

Introduction: The Martian crust has been studied extensively through measurements from orbit and the surface, indicating that most of the crust is composed of mafic igneous rock[1]. Much of the surface also shows some extent of low temperature alteration. While most of this alteration can be explained by aqueous processes[2,3], there has been abundant orbital evidence for local hydrothermal metamorphism operating in some areas [4,5]. It has also been demonstrated that impacts into Mars’ surface could release sufficient energy to produce long-lived hydrothermal systems in and around craters[6,7]. MER rover Spirit encountered likely hydrothermal terrain at Home Plate, in Gusev Crater, with elevated levels of halogens, sulfur, and other volatile elements [8,9]. While it seems apparent that some form of hydrothermal alteration has taken place on the Martian surface, constraining the specific process remains difficult[9]. Phyllosilicates, unequivocal products of alteration, are generally produced in water-rich environments; however, much of the chemical alteration that has taken place on Mars throughout its most recent history has likely occurred at very low water-rock ratios[10] and in local environments where magmatic gases may have played an important role, such as in fumarolic systems. Understanding the mineralogy and chemistry of such systems is crucial for identifying other similar locales on the Martian surface. Furthermore, being able to recognize signatures of potential fumaroles from orbital observations could prove valuable when searching for prospective landing sites for future survey or sample return missions. These areas of magmatic activity would also be prime areas to continue to search for evidence of past or present life on the Martian surface[11]. To understand this process on Mars, we have initiated the design and implementation of a simulated fumarole. Experimental design: There are many considerations that go into designing an experiment that can accurately mimic a fumarolic environment on the Martian surface. There must be a source material or “magma” that generates a vapor phase. There also must be a country rock (the “target rock”) representing the material that is being altered. The source rock must be at higher temperature than the target. An oxygen fugacity consistent with the Martian surface must be maintained, and all should be isolated as much as possible from the terrestrial environment. Ideally, the source magma for the vapor phase should have a composition that is similar to rock found on Mars. The MER rovers analyzed several finegrained igneous rocks that may well have risen close to the surface while mostly, if not entirely liquid[12]. The rock Irvine is an example of one such basalt and chosen here as a source rock. Using this composition as a starting point, we are able to produce vaporsaturated “magmas” capable of releasing volatile gases. A target “rock” with plagioclase, pyroxene, olivine and glass can be synthesized using the same Irvine composition, but allowing it to partially crystallize. This is an approximate representation of an “average” Martian crust. We sought varied target rocks in order to better understand the effect of protolith composition on alteration products, so we will use synthesized Irvine glass as target rock because it should be more readily altered. Natural minerals such as albite, augite, and olivine were also used in this preliminary study. Experimental details: We have synthesized two “source magma” mixtures of Irvine composition using oxide and silicate powders. One anhydrous mix contained halogens, added as MgCl2 and MgF2. A second hydrous mixture, but free of halogens contained water, added in the form of brucite (Mg[OH]2). Powders were separately loaded into individual graphite-lined platinum capsules, each pressurized to 1 GPa in a piston cylinder press, melted at 1400oC, then quenched to a glass. Equal parts of each glass were weighed and crushed together with added Zinc and Rubidium (as ZnO and RbN2). Separate mixes were employed in order to maximize the dissolved volatiles in each melt; chemical analyses are in Table 1. Note the low totals for the hydrous liquid, this is likely due to an added 4.65% water; however detailed FTIR to quantify dissolved water has not been completed yet. For the alteration experiments, this glass was loaded into a Au80Pd20 capsule that was lightly crimped at the top to allow gases to escape. This capsule is placed inside the silica glass tube, on top of a small spacer (Fig. 1).