Elemental Composition of the Martian Crust REVIEW
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REVIEW critically evaluate geochemical data from all these sources to constrain the composition of the crust and consider how martian magmatism may have Elemental Composition differed from that on Earth. of the Martian Crust Geochemical Data Sets The Gamma-Ray Spectrometer (GRS) on the Mars Odyssey orbiter has provided elemental 1 2 3 Harry Y. McSween Jr., * G. Jeffrey Taylor, Michael B. Wyatt abundance data and global distribution maps forH,Si,Ca,K,Cl,Fe,andTh(7). A global The composition of Mars’ crust records the planet’s integrated geologic history and provides mapwillbeavailableforAl,butatpresentwe clues to its differentiation. Spacecraft and meteorite data now provide a global view of the use only the global Al mean value. We used chemistry of the igneous crust that can be used to assess this history. Surface rocks on data collected from June 2002 to January 2006. Mars are dominantly tholeiitic basalts formed by extensive partial melting and are not highly Reduced elemental concentrations were origi- weathered. Siliceous or calc-alkaline rocks produced by melting and/or fractional crystallization nally binned at 0.5° by 0.5° and smoothed using of hydrated, recycled mantle sources, and silica-poor rocks produced by limited melting of mean filters over radii of 5° (K), 10° (H, Fe, and alkali-rich mantle sources, are uncommon or absent. Spacecraft data suggest that martian Th), or 15° (Si and Ca). For our analysis, we meteorites are not representative of older, more voluminous crust and prompt questions about rebinned data to 5° by 5° grid points, resulting their use in defining diagnostic geochemical characteristics and in constraining mantle in large spatial resolution. We used only points compositional models for Mars. in regions where H contents are low enough not ver the past decade, instruments on orbiting spacecraft, landers, and rovers A 9 GRS assumes have measured the abundances of ele- Gusev average Na/K Trachyte O Rhyolite ments present in martian rocks and soils. Some Trachy- on May 7, 2009 analyses are incomplete, and the scales of ana- Basaltic andesite trachy- Gusev lyzed areas range from centimeters to hundreds Tephrite andesite Rock RAT of kilometers in diameter, complicating compar- Trachy- Rock brush 6 Soil isons. Martian meteorites [shergottite, nakhlite, basalt and chassignite (SNC)] constitute another im- Foidite Basalt Pathfinder rock portant source of geochemical data. Although O (wt. %) 2 GRS Martian meteorites the meteorites come from as-yet undetermined Meridiani Basaltic shergottite O+K locations on Mars, laboratory analyses permit 2 soils Ol-phyric shergottite 3 Na complete chemical characterizations that cannot TES Lherzolitic shergottite www.sciencemag.org Dacite be obtained by remote sensing techniques. Picrobasalt Nakhlite Andesite Based on these data, Mars has been viewed Pathfinder Basaltic soils as a basalt-covered world (1). Although basalts andesite TES point density Bounce rock are ubiquitous on rocky planets, the apparent lack 1 10 20 30-50 of other rock compositions suggests that the geo- 0 35 45 55 65 75 logic evolution of Mars has been distinct from SiO (wt. %) Earth. Rocks at the Mars Pathfinder landing site 2 previously identified as andesite (2)maybe Downloaded from B Nepheline Plagioclase Quartz coated with alteration rinds, and martian spectral Normative Pathfinder rock signatures formerly interpreted as andesitic (3) Bounce rock are now attributed to the effects of chemical compositions weathering (4, 5). Only a few occurrences of evolved siliceous rocks have been discovered in Martian meteorites Gusev global spectral surveys (6), supporting the view Basaltic shergottite Rock RAT Ol-phyric shergottite that magmatic differentiation has been very Rock brush Lherzolitic shergottite limited. Although much of the surface is cov- Nakhlite ered by sediments, these materials largely re- tain the chemical compositions of their basaltic Olivine Hypersthene precursors. Fig. 1. (A) Total alkalis-silica diagram used for classification of volcanic rocks. Gusev RAT-ground Sufficient geochemical data now exist to bet- and RAT-brushed compositions for the same rocks are connected by tie-lines. Analyses of Gusev ter characterize the crust and the igneous pro- rocks and soils, martian meteorites, and global GRS data (calculated on a volatile-free basis) cesses that produced it. Here, we compare and indicate a crust dominated by basalts. TES-derived data and possibly the Mars Pathfinder rock composition may reflect alteration. Data sources in this and other figures are discussed in the text. 1 Planetary Geosciences Institute and Department of Earth (B) Calculated normative minerals in the martian crust. The three triangles correspond (from left to and Planetary Sciences, University of Tennessee, Knoxville, right) to alkaline basalts, olivine tholeiites, and quartz tholeiites. The critical plane of silica TN 37996–1410, USA. 2Hawai’i Institute for Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, undersaturation separates alkaline basalts and olivine tholeiites; fractionating liquids to the left of HI, 96822, USA. 3Department of Geological Sciences, Brown this plane form silica-deficient compositions, whereas those to the right evolve to silica-enriched University, Providence, RI 02912–1846, USA. compositions. Contours indicate the relative abundances of terrestrial basaltic rocks (37). Martian *To whom correspondence should be addressed. E-mail: meteorites and Gusev rocks plot mostly in the fields of olivine tholeiites and quartz tholeiites; [email protected] nepheline-normative rocks have not been encountered. 736 8 MAY 2009 VOL 324 SCIENCE www.sciencemag.org REVIEW to interfere in the determination of Si, Fe, and Ca concentrations. Hydrogen has a high cross- A 0.5 Martian meteorites section for capturing thermal neutrons, substan- Basaltic shergottite tially affecting the neutron flux in the upper 0.45 Bounce rock Ol-phyric shergottite Lherzolitic shergottite ~30 cm of the martian surface. We corrected 0.4 Nakhlite/chassignite the data for this effect (7) by a process that Nakhlite/chassignite uses measured fluxes of g-rays from H, Fe, Si, 0.35 and Ca, and the fluxes calculated from a neu- Meridiani soils tron transport g-ray production model. This ap- 0.3 Pathfinder soils GRS Gusev proach produces reasonable values at equatorial 0.25 Rock RAT latitudes but uncertain values at higher polar Rock brush latitudes where H dominates elemental signa- 0.2 Soil tures. Accordingly, we constrained our results Ca/Si (wt. ratio) 0.15 TES using a mask based on H concentration, corre- Viking soils sponding to roughly T45° of latitude from the 0.1 1 5 10 15-30 Pathfinder rock TES point density equator. The concentration of H does not affect 0.05 Ca/Si = -0.31 Mg/Si + 0.38 K and Th data because their g-rays result from 2 GRS R =0.85 radioactive decay. To compare igneous rock 0 compositions, we further adjusted the data to a 0 0.2 0.4 0.6 0.8 1.0 1.2 volatile-free basis by removing H2O, Cl, and SNC Mg/Si (wt. ratio) Gusev SO2, the quantities of which were calculated from the S/Cl ratio of ~5 found at rover landing sites. We represent GRS element abundances as Martian meteorites Basaltic shergottite boxes defined by global averages and standard 1 5 10 15-35 B Ol-phyric shergottite Gusev deviations (1s). Lherzolitic shergottite TES point density Rock RAT 4 Nakhlite Rock brush Measurements of the Thermal Emission Spec- on May 7, 2009 Pathfinder rock Soil trometer (TES) onboard the Mars Global Survey- or orbiter are sensitive to the chemistry and Meridiani soils Viking soils Anhydrous TH structure of silicates (8). Complex mixtures can 3 fractionation CA be deconvolved into mineral abundances using Pathfinder soils GRS a spectral library of known minerals (9). The major Bounce rock oxide concentrations can be estimated from TES data to within T5 weight percent (wt %) using 2 Hydrous known mineral chemistries and deconvolved min- fractionatio n TES eral abundances from thermal emission studies www.sciencemag.org (10, 11). We modeled the spectra (12) over 233 FeO*/MgO (wt. ratio) 1 to 508 cm−1 and 825 to 1301 cm−1 usinganend- member set consisting of primary (e.g., plagio- Hydrous melting clase, pyroxene, and olivine) and secondary (e.g., 0 phyllosilicates, sulfates, and oxides) phases with 40 45 50 55 60 known chemistries. Finally, we calculated major SiO2 (wt. %) oxide concentrations on a H2O-free and CO2- free basis. The data clouds in our graphs repre- Fig. 2. (A) Ca/Si-Mg/Si diagram used for classification of martian meteorites. The GRS-measured Ca/Si Downloaded from sent derived chemical compositions from global ratio and standard deviation is represented by a horizontal band. Global Mg/Si can be estimated from TES data binned at 4 pixels per degree. the intersection of that band with the regression line for shergottites or from the average Mg/Si value In comparing GRS analyses and TES- for Gusev rocks and soils (red arrows). (B) FeO*/MgO-silica diagram used for distinguishing dry tholeiitic derived compositions, it is necessary to realize (TH) and wet calc-alkaline (CA) rocks. All martian samples are tholeiitic. TES-derived compositions result that g-rays can penetrate to depths of 20 to 30 cm from alteration. Arrows represent melting and fractionation trends in terrestrial magmas. and thus analyze a much greater volume of material, relative to thermal emission spectra compositions measured by Spirit, but Viking, The MER have performed analyses of more that sample only the outermost 10 to 100 mm. Pathfinder, and Opportunity soils are illustrated than 250 rocks with their APXS (15, 16). Rock Thus, surface alteration processes may have a by ovoids enclosing the data, to minimize com- Abrasion Tools (RAT) brushed away surface profound effect on geochemical classifications plexity in our diagrams. dust and ground into rock interiors. Detailed based on TES data. Two different calibrations of five rock analy- observations of RAT holes and comparison of Sediments potentially sample broad areas of ses by the Mars Pathfinder APXS have been brushed and abraded rock compositions reveal the crust, although fractionation of heavy min- published (17, 18).