Water in Martian Meteorites

The notion of extracting sufficient water from any of the Martian meteorites for isotopic analysis appears on first appraisal rather unlikely. Yet all members contain some hydrated minerals, either primary or secondary and by carefully gauging the water content and the sample size available, useful measurements can be achieved. Two distinct types of hydrated minerals are present in Martian meteorites: 1. those formed as primary magmatic phases such as biotite, amphibole and apatite that crystallised at high temperatures in the presence of water vapour and 2. those that were formed at low temperature at, or close to, the Martian surface in the presence of liquid water, e.g. aluminosilicate clays. Magmatic minerals are uncommon but have been positively identified in a number of Martian meteorites, including Chassigny, Shergotty and Zagami. Their relative paucity is believed to reflect the low water content of Martian magmas [27]. Processes of secondary alteration are reflected by minerals such as illite, calcium sulphate and magnesium sulphate along with phases such as iddingsite and various phyllosilicates. These entities require the presence of liquid water over an extended period of time and at temperatures sufficiently high (>0°C) to promote alteration of the silicates. Small quantities of various of these alteration products have been observed in many of the Martian meteorites [28].

The timing of the formation of hydrated phases could be an important factor in identifying the source of water but is generally poorly defined. Radiometric isotopes can provide some clues in that crystallisation ages indicate the age of original magmatic phases. The only other constraint we have is from shock ages, usually provided by Ar/Ar dating techniques. However, Martian meteorites possess complex shock histories including large shock events early in their history, as well as that resulting from the final ejection impact. The isotopic evidence of such a sequence of shock events can be difficult to unravel, and as each shock has the potential to mobilise components within the rock, particularly important when studying D/H ratios [29], this makes understanding the age and origin of any water-related components very difficult. Consequently while we have a good idea of the age of any original magmatic water, the timing of any subsequent alteration that this may have undergone or the timing of the formation of any new hydrous phases is less clear. The exception to this rule is ALH 84001 for which the carbonates have been dated at about 4 billion years old, close to the crystallisation age [30].

The overall water abundance in different Martian meteorites varies not only with the prevalence of indigenous hydrated minerals, but crucially also on the amount of terrestrial alteration and contamination that has taken place. In those meteorites that are observed entering the Earth's atmosphere and collected soon after (known as "falls") the amount of alteration is usually small. While in those that are not observed to fall and merely collected ("finds"), a protracted period of terrestrial residence may mean that the majority of water released during analysis originates from terrestrial alteration products. The contribution of terrestrial contamination will also depend upon the surface conditions at the site of collection and has been found to be particularly severe for those meteorites recovered from desert locations. For example DaG 476, a meteorite collected from the Libyan desert, was found to have a water content of an order of magnitude greater than most other Martian samples so far analysed [31]. This probably results from aggressive physical weathering, which allows access to water vapour and liquid water from sporadic rainfall. The quantities of indigenous water (after a pre-combustion stage to remove terrestrial contamination) were found to range between 130 and 350 ppm [32]. Those meteorites possessing the greatest indigenous water contents were those observed during mineralogical studies to possess aqueous alteration products assumed to have been formed during hydrothermal processes [33]. The Nakhlites in particular are recognised to have interacted with fluids to create iddingsite, a composite of a range of smectite clays, sulphates and carbonates [34], while others, notably ALH 84001, EETA 79001 and Chassigny have all been found to contain carbonates, again suggesting interaction with Martian crustal fluids [35, 36, 37].

0 0

Post a comment