Kay AchenbachMars Seminar Fall 2002Stable Isotopes of Oxygen on MarsIntroductionThe Oxygen Isotope SystemSNC meteoritesO isotope evidence for sedimentary rocksKay AchenbachMars Seminar Fall 2002Stable Isotopes of Oxygen on MarsIntroductionStable isotope systems are extremely useful as a geochemical tool for understanding the origin and history of a rock. Oxygen isotopes have been studied extensively, and as a whole, this system is fairly well understood and well constrained. Stable oxygen isotopes are particularly useful in the study of Mars because oxygen is present in abundance in both the Martian atmosphere (in the form of CO2) and the lithosphere (in various minerals and also as minor interstitial H2O), as extrapolated from SNC meteorites. Furthermore, it is likely that, if the liquid that is responsible for the fluvial structures observed on the surface of Mars is still frozen in the Martian crust, it also contains oxygen.The stable isotope record contained within the oxygen on Mars represents a vast amount of knowledge regarding the origin of Mars and its relation to that of Earth, and the history and current conditions of the Martian lithosphere, atmosphere, and hydrosphere.The Oxygen Isotope SystemThere are three stable isotopes of oxygen: 16O, 17O, and 18O. 16O is by far the mostabundant, followed by 18O and 17O, respectively. The bulk composition of a given body isoften plotted as 17O/16O vs. 18O/16O. Because isotope fractionation is almost always mass-dependent, and the relative masses of these isotopes are constant no matter what the system, a plot of these values from any homogenized, isolated system yields a straightline with a slope of  0.52. (Franchi et al., 1997a) Since the relative abundance of the three isotopes depends on the system being analyzed, different closed systems plot along parallel lines. (Lewis, 1999)Different bodies in the solar system plot along different lines on this three-isotope plot, implying that the solar system was not homogenized with respect to O isotopes. (Lewis, 1999) Materials from the Earth (and the Moon) plot along one line, and many meteorites plot along different lines (Figure 1). These mass-fractionation lines are a goodindicator of origin, such that any object that falls along the mass-fractionation line of Earth, for example, most likely formed within the closed system of Earth. The bulk O isotope abundances of the SNC meteorites clearly define their own mass-fractionation line, which has been termed the Mars Silicate Fractionation Line (MSFL). (Figure 2)Figure 1: Mass fractionation lines of achondrites. (From McSween, 1999.)SNC meteoritesBecause the meteorites plot along a single mass-fractionation line, the SNC meteorites are, without reasonable doubt, from the same parent body. That parent body ismost likely Mars, as demonstrated by the relative abundances of gas trapped inside shock-melted glass created in the meteorites by the impact that knocked them off of their parent body. Those gases are identical in composition to the Martian atmosphere. (McKeegan & Leshin, 2001)The difference between the MSFL and the Terrestrial Fractionation Line is very small—less than 0.3‰--but it is reproducible (1s standard deviation of 0.07‰). The MSFL differs from the TFL by a mean 17O value of +0.321+/- 0.013‰. The meteorites are very tightly clustered along the MSFL, indicating that their source is isotopically homogenized. (Figure 2) Therefore, the 17O of Mars is most likely +0.321+/- 0.013‰ as well. (Franchi et al., 1997a)Figure 2:SNC meteoritesare tightly clustered along the MFL. (From Eileret al., 2000.)The ages of the meteorites cover a very broad range of time, and the homogeneity of the meteorites indicates that the Martian lithosphere must have been isotopically homogenized very early on in its history. (McKeegan & Leshin, 2001)Bulk Composition of MarsSome scientists have designed compositional models for Mars by matching the O isotope composition of Mars (as extrapolated from the SNC meteorites) to mixtures of different classes of chondrites. (McSween, 2002) This has met with only marginal success, as many of the models are in strong disagreement with each other. Most attempts at compositional models suggest that Mars is more Fe-rich than the Earth. However, there is very little agreement on the estimates these models provide of the amount of volatiles present in the original makeup of Mars, and the size, density, and sulfur content of the Martian core. (McSween, 2002) Secondary minerals in SNC meteoritesThe MSFL is derived from the bulk compositions of the SNC meteorites, and, by definition, the bulk compositions lie along it. (McKeegan & Leshin, 2001) However, water obtained from secondary minerals in the meteorites deviate from the MSFL—they lie between the MSFL and the TFL. Therefore, they are clearly out of equilibrium with the silicates in the meteorite, which implies that the Martian hydrosphere and the Martianlithosphere are out of equilibrium. (McKeegan & Leshin, 2001) Some of these secondary minerals are carbonates formed by the evaporations of saline solutions at surface or near-surface conditions. (Eiler et al., 2000) As on Earth, itmay be that 18O-depleted meteoric water on Mars influenced the 18O of precipitates formed from that water. (Eiler et al., 2000) This would occur as secondary minerals incorporated fluids from the hydrosphere that were affected by fractionation in the Martian atmosphere, a hypothesis that is supported by data from the H and C isotopic systems. (McKeegan & Leshin, 2001)These secondary carbonates (along with some secondary silicates) have a higher 18O than their host rock, indicating that they formed at low temperatures (McKeegan & Leshin, 2001). Interestingly, there is some variation in O isotopes with mineral composition (specifically, between earlier-forming Ca-rich carbonates and later-forming Mg-rich carbonates). It is possible that a) the temperature varied during the formation of these minerals, or b) the fluid composition varied. (McKeegan & Leshin, 2001) The fluid hypothesis is more likely, as there is no observable alteration of the basalt host rock as one would expect to see if the temperature had been so high as to cause the discrepancy. (Eiler et al., 2000) Interestingly, these secondary minerals are depleted in 18O compared to terrestrial marine carbonates by 15-35‰, while they are enriched compared to