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Bulk Composition and Early Differentiation of Mars G JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, E03S10, doi:10.1029/2005JE002645, 2006 [printed 112(E3), 2007] Click Here for Full Article Bulk composition and early differentiation of Mars G. Jeffrey Taylor,1 W. Boynton,2 J. Bru¨ckner,3 H. Wa¨nke,3 G. Dreibus,3 K. Kerry,2 J. Keller,2 R. Reedy,4 L. Evans,5 R. Starr,6 S. Squyres,7 S. Karunatillake,7 O. Gasnault,8 S. Maurice,8 C. d’Uston,8 P. Englert,1 J. Dohm,2,9 V. Baker,2,9 D. Hamara,2 D. Janes,2 A. Sprague,2 K. Kim,2 and D. Drake10 Received 23 November 2005; revised 12 April 2006; accepted 20 April 2006; published 19 December 2006. [1] We report the concentrations of K, Th, and Fe on the Martian surface, as determined by the gamma ray spectrometer onboard the 2001 Mars Odyssey spacecraft. K and Th are not uniformly distributed on Mars. K ranges from 2000 to 6000 ppm; Th ranges from 0.2 to 1 ppm. The K/Th ratio varies from 3000 to 9000, but over 95% of the surface has K/Th between 4000 and 7000. Concentrations of K and Th are generally higher than those in basaltic Martian meteorites (K = 200–2600 ppm; Th = 0.1–0.7 ppm), indicating that Martian meteorites are not representative of the bulk crust. The average K/Th in the crust is 5300, consistent with the Wa¨nke-Dreibus model composition for bulk silicate Mars. Fe concentrations support the idea that bulk Mars is enriched in FeO compared to Earth. The differences in K/Th and FeO between Earth and Mars are consistent with the planets accreting from narrow feeding zones. The concentration of Th on Mars does not vary as much as it does on the Moon (where it ranges from 0.1 to 12 ppm), suggesting that the primary differentiation of Mars differed from that of the Moon. If the average Th concentration (0.6 ppm) of the surface is equal to the average of the entire crust, the crust cannot be thicker than about 118 km. If the crust is about 57 km thick, as suggested by geophysical studies, then about half the Th is concentrated in the crust. Citation: Taylor, G. J., et al. (2006), Bulk composition and early differentiation of Mars, J. Geophys. Res., 111, E03S10, doi:10.1029/ 2005JE002645. [printed 112(E3), 2007] 1. Introduction lution of Mars. In this paper we focus on using the concentrations of K, Th, and Fe to test models for the bulk [2] The Mars Odyssey gamma ray spectrometer (GRS) composition of Mars, which has implications for planetary provides the first direct determination of elemental concen- accretion, and to investigate the formation and subsequent trations of the entire Martian surface [Boynton et al., 2004], differentiation of the planet. including materials at a depth of about one third meter from the surface. Although the spatial resolution is on the order of 500 km, the data allow us to address significant global 2. Methods problems concerning the geochemical and geological evo- [3] The Odyssey GRS and data reduction methods are described by W. V. Boynton et al. (Concentration of H, Si, Cl, K, Fe, and Th in the low and mid latitude regions of 1 Hawaii Institute of Geophysics and Planetology, Honolulu, Hawaii, Mars, submitted to Journal of Geophysical Research, 2006, USA. 2Lunar and Planetary Laboratory, University of Arizona, Tucson, hereinafter referred to as Boynton et al., submitted manu- Arizona, USA. script, 2006). In this paper we present three types of data 3Max-Planck-Institu¨t fu¨r Chemie, Mainz, Germany. collected through March 2005. First, we provide maps of 4Institute of Meteoritics, University of New Mexico, Albuquerque, New the concentrations of K, Th, K/Th, and Fe. These maps are Mexico, USA. 5 derived from a 2 base map and smoothed with a 10 boxcar Computer Sciences Corporation, Lanham, Maryland, USA. ° ° 6Department of Physics, Catholic University of America, Washington, filter. They are useful for showing global variations in DC, USA. concentration. The maps are essentially global for K, Th, 7Center for Radiophysics and Space Research, Cornell University, and K/Th, but are restricted to regions of relatively low Ithaca, New York, USA. hydrogen for Fe (see below). 8Centre d’Etude Spatiale des Rayonnements, Centre National de la Recherche Scientifique/Universite´ Paul Sabatier, Toulouse, France. [4] Second, we use 5° Â 5° binned data for x-y plots. The 9Department of Hydrology and Water Resources, University of Arizona, 5° Â 5° data have been smoothed with a 10° filter around Tucson, Arizona, USA. each point. (Filtering removes significant levels of noise in 10 TechSource, Santa Fe, New Mexico, USA. the initial data.) For Fe data, we report only those points in regions where H contents are low enough to not interfere Copyright 2006 by the American Geophysical Union. 0148-0227/06/2005JE002645$09.00 in the determination of the Fe concentration. Hydrogen has E03S10 1of16 E03S10 TAYLOR ET AL.: MARS COMPOSITION AND DIFFERENTIATION E03S10 a high cross section for capturing thermal neutrons, so it can uncertainty is over 3000. On the other hand, other areas significantly affect the flux of thermal neutrons in the upper appear to be distinctly higher or lower, such as the high 30 cm of the Martian surface. To account for the effect of region in and south of Valles Marinaris, at least at the 1s H, we use a correction procedure that involves both the level. Details of the variations in K/Th are discussed by measured fluxes of gamma rays from H, Fe, and Si, and the Taylor et al. [2006]. fluxes calculated from a neutron transport–gamma ray [7] Fe concentrations (Figure 3) in the northern plains are production model (Boynton et al., submitted manuscript, higher than in the southern highlands. Nevertheless, con- 2006). We accomplished that by using the H mask described centrations are almost everywhere higher than in typical by Boynton et al. (submitted manuscript, 2006). This terrestrial basalts and generally consistent with the high Fe approach has resulted in reasonable values at equatorial contents of Martian meteorites (discussed further below) latitudes. Because the approach results in uncertain values at and with the inferred high FeO in bulk silicate Mars. higher polar latitudes where the influence of hydrogen Additional data are presented in the following sections, in dominates elemental signatures, the results presented here which we compare our GRS data to the compositions are constrained using a mask based upon both H concen- of Martian meteorites, test models for the bulk composition tration values and described by Boynton et al. (submitted of Mars, and explore the record of the planet’s early manuscript, 2006). The H mask corresponds to roughly plus differentiation. or minus 45° of latitude from the equator. We restrict our analysis of binned data for K and Th to between 75° south 4. Relation of Surface Compositions to the Entire and 75° north latitude; at higher latitudes the concentrations Crust of K and Th are diluted by very high water contents, increasing the uncertainty of those measurements. The [8] The GRS measures the composition of the upper few typical uncertainties (relative percent) stemming from tens of centimeters of the dusty Martian surface, yet we are counting statistics for an average point are 5% for K, 10% trying to understand the formation of the entire crust and the for Th, and 5% for Fe. bulk composition of the entire planet. Several factors allow [5] Third, we use summed spectra to compute the con- us to extrapolate our surface measurements to great depth. centrations in specific geologic regions (described below). One is that K and Th have very similar geochemical These spectra involve large counting times (>3 Â 106 s), behavior in igneous systems, as shown by their similar, and because statistical uncertainties vary as the square root and very low (1), crystal-melt distribution coefficients of the counting time, they have correspondingly low statis- [Beattie, 1993; Borg and Draper, 2003; Hauri et al., 1994]. tical uncertainties. The uncertainty varies with the size of Both elements are incompatible and their concentrations in the region, hence with the total counting times in the magmas are not greatly affected by source rock composition summed spectra. Typical uncertainties are 1% for K, 3– or crystallizing phase, even when garnet is involved. There 5% for Th, and 2–4% for Fe. These are the uncertainties in are interesting exceptions, however. Th is highly compatible the measurements and define the confidence to which we in phosphate minerals [Jones, 1995]. Phosphates form late know the means. It does not reflect the variation in in the crystallization of a magma and are unlikely to be concentrations across the Martian surface. retained in a mantle source region, so probably do not play a role in fractioning K from Th during igneous processes. 3. Results However, in principle, it could be significant if mantle regions were matasomatized by fluids that contained [6] Maps of the distribution of K, Th, K/Th, and Fe are phosphate components. K is compatible in phlogopite presented in Figures 1–3. K and Th are not uniformly [Halliday et al., 1995] and somewhat compatible in amphi- distributed on Mars. The northern plains from about À60° bole [Halliday et al., 1995], so if these phases were present to +180°E are rich in both, though the higher-than-average in the Martian mantle, it could lead to fractionation of K Th region extends much further south into the highlands. from Th. Nevertheless, in general, K/Th in a lava flow Both are generally medium to low over Tharsis.
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