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The Bulk Composition of Mars

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The Bulk Composition of Mars Chemie der Erde 73 (2013) 401–420 Contents lists available at ScienceDirect Chemie der Erde jou rnal homepage: www.elsevier.de/chemer Invited review The bulk composition of Mars ∗ G. Jeffrey Taylor Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, United States a r t i c l e i n f o a b s t r a c t Article history: An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding plan- Received 16 July 2013 etary accretion, differentiation, mantle evolution, the nature of the igneous parent rocks that were altered Accepted 11 September 2013 to produce sediments on Mars, and the initial concentrations of volatiles such as H, Cl and S, important constituents of the Martian surface. This paper reviews the three main approaches that have been used Keywords: to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophys- Mars ical. The standard model is one developed by Wänke and Dreibus in a series of papers, which is based on Composition compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have Planet formation Cosmochemistry become available to allow a reassessment of the composition of Mars from elemental data, including Geochemistry tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations Terrestrial planets in the Wänke–Dreibus model, but in general confirm its accuracy. Bulk silicate Mars has roughly uniform depletion of moderately volatile elements such as K (0.6 × CI), and strong depletion of highly volatile ele- ments (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with more scatter, with normalized abundances of 0.03 CI. Bulk planetary H2O is much higher than estimated previously: it appears to be slightly less than in Earth, but D/H is similar in Earth and Mars, indicating a common source of water-bearing material in the inner solar system. K/Th ranges from ∼3000 to ∼5000 among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout the inner solar system: ∼3 wt% in Mercury, 8 wt% in Earth and Venus, and 18 wt% in Mars. These dif- ferences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets were formed from fundamentally different materials. The broad chemical similarities among the terres- trial planets indicate substantial mixing throughout the inner solar system during planet formation, as suggested by dynamical models. © 2013 Elsevier GmbH. All rights reserved. Contents 1. Introduction . 402 2. Approaches to estimating bulk composition . 402 2.1. Models based on geochemistry and nebular components . 402 2.1.1. Wänke and Dreibus model . 402 2.1.2. Morgan and Anders model . 404 2.2. Estimates based on isotopic composition of Martian meteorites . 404 2.3. Estimates based on geophysical properties of Mars . 405 2.4. Summary of the models . 405 3. Datasets . 405 4. Complications . 406 4.1. Is the Martian surface composition representative of the entire crust?. 406 4.1.1. Highlands megaregolith . 406 4.1.2. Lava flows . 406 4.1.3. Element ratios . 406 4.2. Heterogeneity of the Martian mantle . 406 ∗ Tel.: +1 808 956 3899; fax: +1 808 956 6322. E-mail address: [email protected] 0009-2819/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.chemer.2013.09.006 402 G.J. Taylor / Chemie der Erde 73 (2013) 401–420 5. A reassessment of Martian bulk composition . 407 5.1. Estimating uncertainties . 407 5.2. Refractory element abundances. 408 5.3. FeO and MnO . 409 5.4. Phosphorous . 411 5.5. Moderately volatile elements . 411 5.6. Highly volatile elements, including halogens . 412 5.7. Ni and Co . 412 5.8. Strongly siderophile elements . 412 5.9. H2O and D/H . 414 6. Discussion . 415 6.1. Mars is rich in FeO . ..
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