K and Cl concentrations on the Martian surface determined by the Mars Odyssey Gamma Ray Spectrometer: Implications for bulk halogen abundances in Mars

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 37,, doi: /2010gl043528, 2010 K and Cl concentrations on the Martian surface determined by the Mars Odyssey Gamma Ray Spectrometer: Implications for bulk halogen abundances in Mars G. Jeffrey Taylor, 1 William V. Boynton, 2 Scott M. McLennan, 3 and Linda M. V. Martel 1 Received 12 April 2010; revised 17 May 2010; accepted 21 May 2010; published 30 June [1] Orbital gamma ray spectrometry shows that the Martian surface has a mean Cl/K ratio of 1.3 ± 0.2, indistinguishable from the ratio in CI chondrites (1.28). Although Cl and K fractionate by magma degassing and aqueous processing, during igneous partial melting both elements are highly incompatible. Thus, if the surface Cl/K reflects the bulk crustal value, then the mantle, hence primitive silicate Mars, also has a roughly CI ratio. Martian meteorite data indicate that Cl/Br is also approximately chondritic, suggesting that elements that condensed in the nebula between 1000 K (K and Cl) to 500 K (Br) are uniformly depleted in Mars at about 0.6 CI chondrite concentrations. Mars clearly does not contain 0.6 CI levels of H 2 O, which would be 6 wt%, indicating that Mars was constructed by planetesimals rich in volatile elements, but not in water. Citation: Taylor, G. J., W. V. Boynton, S. M. McLennan, and L. M. V. Martel (2010), K and Cl concentrations on the Martian surface determined by the Mars Odyssey Gamma Ray Spectrometer: Implications for bulk halogen abundances in Mars, Geophys. Res. Lett., 37,, doi: / 2010GL Introduction [2] The best estimate for the bulk composition of the silicate portion of Mars is that developed by Wänke and Dreibus in a series of papers [Dreibus and Wänke, 1984, 1987; Wänke and Dreibus, 1988, 1994]. This robust estimate is based on reasonable cosmochemical assumptions and the compositions of Martian meteorites. However, there is one curious discrepancy. The estimated abundance of Cl is much lower than other elements with similar condensation temperatures, such as K (50% condensation temperature of 1006 K cf 948 K for Cl). The standard Wänke Dreibus model composition for moderately volatile and volatile lithophile elements are given in Table 1, along with their 50% condensation temperatures [Lodders, 2003], and listed in order of decreasing condensation temperature. The low Cl carries over to the values for the other halogens, except for F. [3] The Mars Odyssey Gamma Ray Spectrometer (GRS) has returned data that allow us to determine the K, Cl, and other elemental concentrations in the upper few decimeters 1 Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii, USA. 2 Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA. 3 Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York, USA. Copyright 2010 by the American Geophysical Union /10/2010GL of the Martian surface. As described below, although K and Cl are not correlated, the average Cl/K is 1.3, close to the CI chondrite value of 1.28 [Palme and Jones, 2004], but very different from the Wänke Dreibus value of 0.08 (Table 1). We argue here that bulk silicate Mars has a CI chondrite Cl/K ratio and that the low Cl value in the Wänke Dreibus model is caused by Martian meteorites being depleted in Cl because of loss of this volatile during eruption or nearsurface intrusion [also see McSween et al., 2001]. A roughly chondritic Cl/Br in Martian meteorites suggests that Cl/Br is also chondritic and the small difference in the condensation temperatures of Br and I suggests that all three halogens are in chondritic proportions. 2. GRS Methods [4] The GRS determines elemental concentrations from the intensities of gamma rays produced by neutron capture and/or scattering reactions (Si, Fe, Ca, Al, Cl, H) or by radioactive decay (K, Th). Details of data acquisition and reduction are given by Boynton et al. [2007, 2008]. The data we use here for Cl and K were obtained between June 8, 2002 and April 2, The GRS spatial resolution is defined as the nadir centered region within which 50% of the signal originates, which is 3.7 arc radius ( 220 km), although this varies somewhat with the gamma ray energy. The data were binned initially at , smoothed using a boxcar filter over a much larger radius (5 for K, 10 for Cl) and then rebinned to 5 5 grid points. The measured gamma rays are produced in the upper few tens of centimeters of the Martian surface, much deeper than sensed by visible to thermal infrared spectral instruments ( mm). We use only points in regions where H contents are low enough not to interfere in the determination of Cl concentrations. Hydrogen has a high cross section for capturing thermal neutrons, significantly affecting the neutron flux in the upper 30 cm of the Martian surface. The GRS data are corrected for this effect, but the correction is uncertain at polar latitudes where H dominates elemental signatures. Accordingly, we constrained our results using a mask based on H concentration, corresponding to roughly ±45 of latitude from the equator. (The concentration of H does not affect K data because its g rays result from radioactive decay, but we use data only within the H mask to compare with Cl concentrations.) 3. Overview of K and Halogen Geochemical Behavior [5] Potassium, Cl, Br, and I are all large ion lithophile elements that behave incompatibly during partial melting, 1of5

2 Table 1. Standard Wänke Dreibus Model Mars Bulk Composition Compared to Model Described in This Paper 50% Condensation Temperature a (K) Wänke Dreibus Wänke Dreibus b (ppm) (Mars/CI) This Work This Work (ppm) (Mars/CI) K Na Cl Rb Cs F Br I a Lodders [2003]. b Dreibus and Wänke [1984, 1987] and Wänke and Dreibus [1988, 1994]. CI chondrite values from Palme and Jones [2004]. hence preserving the mantle source rock Cl/K in the magma produced [Schilling et al., 1980; Deruelle et al., 1992; Pyle and Mather, 2009]. The fluorine ion has a much smaller ionic radius than the other halides and tends to behave somewhat less incompatibly during partial melting [Pyle and Mather, 2009]. Thus, Cl/K (and Br/K and I/K) in surface rocks would give a good approximation to the mantle values if the halogens were not lost by degassing during magma ascent and eruption. Note that K is not lost via magma degassing. [6] Loss of halogens during magma degassing depends on magma composition and H 2 O concentration (see summary by Aiuppa et al. [2009]), but fractionation relative to each other is probably less affected and is related to their ionic radii. Experiments by Bureau et al. [2000] show that fluidmagma partition coefficients for the halogens increase with ionic radius: F (D f/m = 0.1, ionic radius = 1.33 Å), Cl (4.5, 1.81 Å), Br (14, 1.96 Å), I (78, 2.2 Å). (Ionic radii are for 6 fold coordination and taken from Shannon [1976]; data for F are from Webster [1992].) On the other hand, F is retained by lavas, so its concentration in volcanic rocks more accurately reflects its abundance in the mantle. Thus, the behavior of F in magmatic systems leads to a more accurate estimate of its concentration in bulk silicate Mars (Table 1). [7] Once on the surface, K and the halogens behave quite differently. K is retained in feldspars and residual igneous glass until sufficient weathering takes place to begin to dissolve those phases, whereas the Cl, Br, and I are present in highly soluble salts. Thus, surface aqueous processes continue to fractionate K from Cl and the other large ion halogens. The geochemistry of halogens on the Martian surface is complicated [e.g., Marion et al., 2009] and although Cl and Br behave similarly, they can still fractionate significantly from each other [e.g., Clark et al., 2005; Haskin et al., 2005]. Extracting the bulk crustal Cl, Br, and I from surface analyses is not yet possible with existing data from the Mars Exploration Rovers, so below we use Cl/Br and Cl/I ratios in Martian meteorites to evaluate Br and I in the mantle. This approach assumes that we can ignore the differences in fluid/magma partition coefficients. 4. Results: K and Cl Concentrations [8] GRS data show that Cl and K are not correlated (Figure 1), nor should they be considering their vastly different behavior after partial melting, as summarized above. Nevertheless, the mean Cl/K value, 1.27 ± 0.46 (2 sigma uncertainty), is within the uncertainties of the CI chondritic value, 1.28, and substantially greater than the mean value in Martian meteorites, 0.08, determined from data assembled by Meyer [2009]. Figure 2 shows the Cl/K ratio divided into six groups. One group (largest area in Figure 2) is defined by the chondritic ratio plus and minus one standard deviation of the global dataset (0.2). The other ranges are one standard deviation wide. Figure 2 shows that the entire surface of Mars has a substantially higher Cl/K ratio than reported for bulk Martian meteorites. The ratio is everywhere greater than 0.6 and a substantial portion of the surface is within one standard deviation of the chondritic ratio. Cl/K is significantly higher than the chondritic range over Tharsis, west of Tharsis, and in Elysium, and lower in Utopia Planitia. Cl enrichment in volcanic regions could be caused by fairly recent volcanic deposits, such as the high Cl materials in Medusae Fossae [Keller et al., 2006]. 5. Cl Br Relationships [9] Cl/Br in Martian meteorites (180) is close to the CI chondrite ratio (199) (meteorite data cited by Brückner et al. [2008], Banin et al. [1992], and Dreibus et al. [1992, 1994, 1996, 2003a, 2003b]. If reflective of the interior, it implies that Br, like Cl, is much higher in Mars than the estimate made by Wänke and Dreibus. This assumes, of course, Cl and Br are lost from magmas by the same amount during degassing. (Rocks analyzed in Gusev crater do not appear to provide additional data on Cl and Br in igneous rocks. According to Gellert et al. [2006], even the abraded surfaces of these rocks have high S and Cl, suggesting either contamination by debris generated during grinding or that the rocks have experienced aqueous alteration. A compilation of all abraded rocks in Gusev shows Cl up to almost 2 wt%.) 6. Revised Halogens in Bulk Silicate Mars [10] A central issue is whether the mean Cl/K value determined for the upper few decimeters reflects the bulk composition of the planet. Taylor et al. [2006] addressed Figure 1. Mars Odyssey Cl and K concentrations for 5 5 degree grid points. Filled square represents the mean of the GRS data. Solid line is the CI chondritic ratio. Martian meteorites plot along the bottom of the diagram and have substantially lower Cl/K (mean of 0.08), reflecting loss by degassing of magmas at low pressure. 2of5

3 Figure 2. Map showing the distribution of increments in the Cl/K ratio on Mars, inside the H mask (see text). The largest region represents the CI chondritic ratio ± one standard deviation of the population. (The standard deviation is 0.2.) Other increments are one standard deviation wide. The large area occupied by the Cl/K values within one standard deviation of the CI chondritic value suggests that Cl/K at the Martian surface is close to chondritic. this issue with respect to K and Th. Two factors argue that the surface reflects the mean crustal composition. First, the Martian crust was bombarded continuously during most of its formation. This not only mixed previously formed surface and subsurface rocks, but also formed a regolith that could be reworked continuously as the crust grew. Second, isotopic data for Martian meteorites indicate that the crust has not been recycled into the mantle [Borg et al., 2003]. Thus, what went into the crust as it was constructed by magmatism stayed in the crust. Of course, aqueous processes redistributed materials and fractionated K from Cl, but these processes did not necessarily transport most of the crustal inventory of Cl into the uppermost surface layers. Thus, we believe it is reasonable to conclude that bulk Mars has close to a chondritic Cl/K ratio. Using the Wänke Dreibus value for K (305 ppm (Table 1)), we estimate a bulk Cl abundance of 390 ppm (Table 1). [11] We can check the conclusion that Mars has chondritic Cl/K. Assuming that the bulk crust has a chondritic Cl/K ratio, we estimated the K and Cl abundances in bulk silicate Mars. If the crust is 57 km thick [Wieczorek and Zuber, 2004] and the mantle extends to a depth of 1760 km, the crust occupies 4.6 wt% of silicate Mars. If that mass of crust represents the average amount of partial melting of the mantle over time and the distribution coefficients for K and Cl during mantle partial melting are 0.001, use of the standard equations for equilibrium partial melting indicates that the current mean mantle concentration is 170 ppm K and 220 ppm Cl. With 4.6% partial melting and the crustal mass assumed, half the total inventory of K and Cl is in the crust and half is still in the mantle [also see Taylor et al., 2006]. Thus, the primitive mantle (same as bulk silicate Mars) has 340 ppm K and 440 ppm Cl. These values are in accord with our revised estimate for bulk silicate Mars (Table 1). [12] Martian meteorites provide evidence for magmas being much richer in Cl before eruption. Filiberto and Treiman [2009] succinctly summarize the evidence. The key evidence is the presence of Cl bearing amphibole inside melt inclusions in some nakhlites and chassinites. Most notable is MIL 03346, which contains amphiboles with up to 8 wt% Cl [Sautter et al., 2006]. In addition, melt inclusions in the Chassigny dunite contain Ti rich biotite with 0.4 wt% Cl [Johnson et al., 1991]. Using a biotite/melt distribution coefficient of 1.5 for Cl [Icenhower and London, 1997], Filiberto and Treiman [2009] calculate a Cl concentration of 0.3 wt% in the magma from which the biotite crystallized. Other investigators have reported evidence for Cl rich fluids exsolving from magmas giving rise to the MIL nakhlite [McCubbin et al., 2009] and Chassigny [McCubbin and Nekvasil, 2008]. While we cannot pin down the total magmatic Cl or the K/Cl from these data, the meteorite evidence supports the idea that Martian magmas contained much higher levels of Cl than bulk meteorite analyses suggest. [13] The correlation between Cl and Br in Martian meteorites [Brückner et al., 2008] suggests that they are in chondritic proportions, though the mean Cl/Br (180) is slightly less than chondritic (199). As discussed in section 3, Br is lost more readily from degassing magmas than is Cl. Thus, the roughly chondritic Cl/Br might have been lower than observed (because Br was higher), so use of the chondritic ratio or the mean for the Martian meteorites is conservative. If anything, the Br concentration might be higher compared to Cl and other moderately volatile elements. [14] We estimate iodine in bulk silicate Mars by assuming it has the same relationship to Cl as does Br. That is, if Cl/Br is roughly chondritic, Cl/I probably is as well, simply because the condensation temperatures of Br and I do not differ significantly (Table 1). However, meteorite data are insufficient to establish that firmly. 7. Discussion [15] Our revised estimate of the abundances of moderately volatile and volatile lithophile elements in bulk silicate Mars is about 0.6 CI chondrites (Table 1) and indicates that 3of5

4 these elements are not fractionated from each other during planetary accretion. This also applies to the abundances of volatile chalcophile elements, significant amounts of which are in the core. Thus, assuming a roughly 0.6 CI initial concentration of volatile chalcophile and siderophile elements could be a useful starting point for geochemical models of core formation. [16] The uniform 0.6 CI fractionation pattern suggests that Mars accreted substantially from planetesimals formed on average at relatively low temperature, perhaps at greater distances from the Sun than the present orbit of Mars, consistent with accretion models [e.g., Chambers, 2004; O Brien et al., 2006]. What does this imply about H 2 O abundance in bulk Mars? CI chondrites have about 10 wt% H 2 O contained in hydrous silicates, salts, and organic compounds [Kaplan, 1971]. It is clearly unrealistic for the Martian bulk water content to be 0.6 of this level (6 wt%). On the other hand, studies of Martian meteorites indicate that the water contents of Martian magmas [e.g., McSween et al., 2001; McCubbin et al., 2010] are too high to be consistent with Wänke and Dreibus s [1988] estimate of only 39 ppm in bulk silicate Mars. Their estimate is tied to their Cl abundance. Using the same approach but with our higher Cl concentration, we estimate 390 ppm of H 2 Oin bulk Mars, still substantially less than 6 wt%, indicating decoupling of volatile lithophile elements and water. McCubbin et al. [2010] estimate from hydrous amphibole in melt inclusions in Chassigny that the mantle source region for the Chassigny magma contained between 130 and 250 ppm H 2 O, also substantially higher than estimated by Wänke and Dreibus [1988]. It appears that except for a small percentage of planetesimals, the raw materials for Mars may have accreted before temperatures dropped to the water condensation T, about 180 K. 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