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Temperature (°C)

Fig. 1.3 A similar style of plot to that of Fig. 1.2 again showing the A17O values for samples relative to the TF line and the MF line. (Note different scale to that of Fig. 1.2.) Data from

It is assumed that minerals breaking down at the lowest temperatures are also those formed at relatively low temperatures, either on Mars or on Earth and as such should display isotopic compositions reflecting either terrestrial water or that taking part in the Martian hydrological cycle. However, in such anhydrous mineral dominated samples any indigenous signature present in water released below ~150°C is likely to be overwhelmed by adsorbed terrestrial water. From these studies it is impossible to determine whether or not the water extracted at low temperatures originates from minerals formed in the terrestrial environment, or whether in fact the minerals are originally Martian, their water having been completely replaced on Earth. In the case of DaG 476 the former option is probably the case as the sample appeared weathered and rusty prior to analysis, yet even this sample seemed to retain an indigenous component in higher temperature water. The sample extracted at intermediate temperatures can be understood in terms of a mixture - some Martian and some terrestrial - but with the majority of that adsorbed to the sample already removed, most should originate from within minerals. As such the dominant source will be either interlayer water from expanding clays or from the water of hydration from minerals such as sulphates. Water released at higher temperatures will largely originate from the structural OH- groups present in phyllosilicates and also primary hydrated phases such as amphiboles and micas. If, as suggested by D/H measurements, the water in primary minerals has been altered to reflect

hydrothermal fluids, then all indigenous water will isotopically reflect surface reservoirs in contact with the atmosphere. However, if primary water retains its original signature, assumed to be the same as silicates (i.e. + 0.32 %o, A17O), then this will only serve to dilute that originating from hydrothermal fluids possessing a greater 17O enrichment. The lack of equilibrium between oxygen in hydrated phases and that held in the structure of silicates is one of the most important pieces of information to emerge from studies of water in Martian meteorites. The now well established A17O value of the oxygen isotopic composition of silicates (+ 0.32 %o) appears distinct from that found in indigenous water which extends up to + 0.8 % or even higher. The easiest explanation of this discrepancy is that while water has interacted with the silicates to produce hydrated minerals, the reactions have not been of sufficient duration or at a sufficiently high temperature to allow isotopic equilibrium to be achieved and the water signature erased. However, this still leaves us with the question of where the distinct isotopic composition of water originated. Fractionation during atmospheric loss of oxygen, as a result of Jeans escape, can produce a small 17O anomaly that might be capable of generating the general enrichment observed in most meteorites [56]. A possible alternative solution to the problem [18] was suggested after identification of a non-mass dependent isotopic fractionation processes in upper parts of the Earth's atmosphere. One reaction suggested was the photolysis of ozone that ultimately created a 17O enrichment in CO2 that could subsequently be transferred to water. In a later study [57] it was suggested that there may be many such reactions involving UV radiation capable of fractionating oxygen to produce gaseous reservoirs enriched in 17O. On Mars the thin atmosphere allows energetic particles much nearer the surface, therefore any resulting reservoir of isotopically enriched gas would be in more intimate contact with the hydrosphere. Mixing and exchange between water in the hydrological cycle and oxygen in the atmosphere then enabled transfer of the anomaly to water and subsequently to hydrated minerals in the crust. An alternative source for isotopically distinct water is that of a veneer of cometary water as discussed below.

ALH84001

While the general excess of 17O observed over a range of temperatures in Martian meteorites can be explained by internal and atmospheric processes, the anomolous peak observed at about 300°C in ALH 84001 may be best explained by a more fundamental process. In numerous studies of primitive meteorites it has been established that the oxygen isotopic composition of the initial water inventory present on their parent bodies possessed a significant excess of 17O [e.g. 58, 59]. It seems highly likely that this water reflected the majority of that present in the early Solar System, which in turn possessed a composition that may have been inherited from material in the original solar nebula. With a formation age of 4 -4.5 Ga, ALH 84001 is recognised as being much older than other Martian meteorites. It may, therefore, record isotopic evidence of fluids present at or shortly after this time, e.g. carbonates from ALH 84001 have been dated at about

3.9 Ga old [30]. Assuming that the hydrated minerals were formed on Mars prior to the homogenising effect of the proposed warm/wet phase, hydrated minerals may retain the composition of water arriving as a late veneer. Two possible sources of late veneer volatiles have been suggested based upon D/H evidence, either asteroidal or cometary. Had the source of this water been from asteroidal bodies such as those with a primitive carbonaceous chondrite composition, then water would have reflected the composition found after parent body processes (i.e. after reaction between water and solids whilst part of the asteroid), and as such would have had A17O values of close to or below 0 % [60]. However, if the source of water had been from impacting cometary bodies containing large amounts of primitive, relatively unaltered water, then hydrated minerals formed may reflect a water composition possessing a A17O of + 4 % or greater. As discussed previously, this does not constrain the composition of the D/H ratio which can alter independently of the oxygen in different Solar System water bodies. The warm wet phase that was believed to follow the heavy bombardment would, very likely, have resulted in mixing and loss of the majority of any extreme isotopic signature. But had the cometary contribution been sufficiently large, it, rather than atmospheric effects, could have been the source of the moderate A17O excesses seen in most other indigenous phases. Unfortunately our lack of knowledge as to the depth to which any water-crust interaction took place prohibits calculation of the amount of water that may have been involved and consequently oxygen isotopes have little to say about the ultimate quantities of the present Martian water inventory.

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