GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L12204, doi:10.1029/2010GL043528, 2010 Click Here for Full Article 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. Martel1 Received 12 April 2010; revised 17 May 2010; accepted 21 May 2010; published 30 June 2010. [1] Orbital gamma ray spectrometry shows that the Martian of the Martian surface. As described below, although K and surface has a mean Cl/K ratio of 1.3 ± 0.2, indistinguishable Cl are not correlated, the average Cl/K is 1.3, close to the CI from the ratio in CI chondrites (1.28). Although Cl and K chondrite value of 1.28 [Palme and Jones, 2004], but very fractionate by magma degassing and aqueous processing, different from the Wänke‐Dreibus value of 0.08 (Table 1). during igneous partial melting both elements are highly We argue here that bulk silicate Mars has a CI chondrite Cl/K incompatible. Thus, if the surface Cl/K reflects the bulk ratio and that the low Cl value in the Wänke‐Dreibus model crustal value, then the mantle, hence primitive silicate is caused by Martian meteorites being depleted in Cl Mars, also has a roughly CI ratio. Martian meteorite data because of loss of this volatile during eruption or near‐ indicate that Cl/Br is also approximately chondritic, surface intrusion [also see McSween et al., 2001]. A roughly suggesting that elements that condensed in the nebula chondritic Cl/Br in Martian meteorites suggests that Cl/Br is between ∼1000 K (K and Cl) to ∼500 K (Br) are also chondritic and the small difference in the condensation uniformly depleted in Mars at about 0.6 × CI chondrite temperatures of Br and I suggests that all three halogens are concentrations. Mars clearly does not contain 0.6 × CI in chondritic proportions. levels of H2O, which would be ∼6 wt%, indicating that Mars was constructed by planetesimals rich in volatile 2. GRS Methods elements, but not in water. Citation: Taylor, G. J., W. V. Boynton, S. M. McLennan, and L. M. V. Martel (2010), K and Cl concentra- [4] The GRS determines elemental concentrations from tions on the Martian surface determined by the Mars Odyssey the intensities of gamma rays produced by neutron capture Gamma Ray Spectrometer: Implications for bulk halogen abun- and/or scattering reactions (Si, Fe, Ca, Al, Cl, H) or by dances in Mars, Geophys. Res. Lett., 37, L12204, doi:10.1029/ radioactive decay (K, Th). Details of data acquisition and 2010GL043528. 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, 2005. The GRS spatial resolution is 1. Introduction defined as the nadir‐centered region within which ∼50% of ∼ ∼ [2] The best estimate for the bulk composition of the the signal originates, which is 3.7° arc radius ( 220 km), silicate portion of Mars is that developed by Wänke and although this varies somewhat with the gamma ray energy. Dreibus in a series of papers [Dreibus and Wänke, 1984, The data were binned initially at 0.5° × 0.5°, smoothed 1987; Wänke and Dreibus, 1988, 1994]. This robust esti- using a boxcar filter over a much larger radius (5° for K, 10° mate is based on reasonable cosmochemical assumptions for Cl) and then rebinned to 5° × 5° grid points. The mea- and the compositions of Martian meteorites. However, there sured gamma rays are produced in the upper few tens of is one curious discrepancy. The estimated abundance of Cl centimeters of the Martian surface, much deeper than is much lower than other elements with similar condensation sensed by visible to thermal infrared spectral instruments temperatures, such as K (50% condensation temperature of (10–100 mm). We use only points in regions where H contents 1006 K cf 948 K for Cl). The standard Wänke‐Dreibus are low enough not to interfere in the determination of Cl model composition for moderately volatile and volatile concentrations. Hydrogen has a high cross‐section for cap- lithophile elements are given in Table 1, along with their 50% turing thermal neutrons, significantly affecting the neutron condensation temperatures [Lodders, 2003], and listed in flux in the upper ∼30 cm of the Martian surface. The GRS order of decreasing condensation temperature. The low Cl data are corrected for this effect, but the correction is carries over to the values for the other halogens, except for F. uncertain at polar latitudes where H dominates elemental [3] The Mars Odyssey Gamma Ray Spectrometer (GRS) signatures. Accordingly, we constrained our results using a has returned data that allow us to determine the K, Cl, and mask based on H concentration, corresponding to roughly other elemental concentrations in the upper few decimeters ±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 1Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii, USA. compare with Cl concentrations.) 2Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA. 3. Overview of K and Halogen Geochemical 3Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York, USA. Behavior [5] Potassium, Cl, Br, and I are all large‐ion lithophile Copyright 2010 by the American Geophysical Union. elements that behave incompatibly during partial melting, 0094‐8276/10/2010GL043528 L12204 1of5 L12204 TAYLOR ET AL.: MARS BULK COMPOSITION L12204 Table 1. Standard Wänke‐Dreibus Model Mars Bulk Composi- ferent behavior after partial melting, as summarized above. tion Compared to Model Described in This Paper Nevertheless, the mean Cl/K value, 1.27 ± 0.46 (2‐sigma uncertainty), is within the uncertainties of the CI‐chondritic 50% Condensation value, 1.28, and substantially greater than the mean value in Temperaturea Wänke‐Dreibus Wänke‐Dreibusb This Work This Work Martian meteorites, 0.08, determined from data assembled (K) (ppm) (Mars/CI) (ppm) (Mars/CI) by Meyer [2009]. Figure 2 shows the Cl/K ratio divided into K 1006 305 0.56 305 0.56 six groups. One group (largest area in Figure 2) is defined Na 958 3700 0.74 3700 0.74 by the chondritic ratio plus and minus one standard devia- Cl 948 38 0.054 390 0.56 tion of the global dataset (0.2). The other ranges are one Rb 800 1.06 0.46 1.06 0.46 standard deviation wide. Figure 2 shows that the entire Cs 799 0.07 0.37 0.07 0.37 F 734 32 0.55 32 0.55 surface of Mars has a substantially higher Cl/K ratio than Br 546 0.145 0.041 2.2 0.60 reported for bulk Martian meteorites. The ratio is every- I 535 0.032 0.074 0.26 0.60 where greater than 0.6 and a substantial portion of the surface a is within one standard deviation of the chondritic ratio. Cl/K Lodders [2003]. bDreibus and Wänke [1984, 1987] and Wänke and Dreibus [1988, 1994]. is significantly higher than the chondritic range over Tharsis, CI chondrite values from Palme and Jones [2004]. west of Tharsis, and in Elysium, and lower in Utopia Planitia. Cl enrichment in volcanic regions could be caused by fairly hence preserving the mantle source rock Cl/K in the magma recent volcanic deposits, such as the high‐Cl materials in produced [Schilling et al., 1980; Deruelle et al., 1992; Pyle Medusae Fossae [Keller et al., 2006]. and Mather, 2009]. The fluorine ion has a much smaller ionic radius than the other halides and tends to behave 5. Cl‐Br Relationships somewhat less incompatibly during partial melting [Pyle and Mather, 2009]. Thus, Cl/K (and Br/K and I/K) in sur- [9] Cl/Br in Martian meteorites (180) is close to the CI face rocks would give a good approximation to the mantle chondrite ratio (199) (meteorite data cited by Brückner et al. values if the halogens were not lost by degassing during [2008], Banin et al. [1992], and Dreibus et al. [1992, 1994, magma ascent and eruption. Note that K is not lost via 1996, 2003a, 2003b]. If reflective of the interior, it implies magma degassing. that Br, like Cl, is much higher in Mars than the estimate [6] Loss of halogens during magma degassing depends on made by Wänke and Dreibus. This assumes, of course, Cl magma composition and H2O concentration (see summary and Br are lost from magmas by the same amount during by Aiuppa et al. [2009]), but fractionation relative to each degassing. (Rocks analyzed in Gusev crater do not appear to other is probably less affected and is related to their ionic provide additional data on Cl and Br in igneous rocks. radii. Experiments by Bureau et al. [2000] show that fluid‐ According to Gellert et al. [2006], even the abraded surfaces magma partition coefficients for the halogens increase with of these rocks have high S and Cl, suggesting either con- ionic radius: F (Df/m = 0.1, ionic radius = 1.33 Å), Cl (4.5, tamination by debris generated during grinding or that the 1.81 Å), Br (14, 1.96 Å), I (78, 2.2 Å). (Ionic radii are for rocks have experienced aqueous alteration. A compilation of 6‐fold coordination and taken from Shannon [1976]; data all abraded rocks in Gusev shows Cl up to almost 2 wt%.) for F are from Webster [1992].) On the other hand, F is retained by lavas, so its concentration in volcanic rocks 6.