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Sulfate-Rich Scapolite on Mars?

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Sulfate-Rich Scapolite on Mars? Lunar and Planetary Science XXXVIII (2007) 1152.pdf Sulfate-rich Scapolite on Mars? J.J. Papike, J.M. Karner, and C.K. Shearer Astromaterials Institute, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 INTRODUCTION also shows selected bonds to neighboring Ca atoms. The surface of Mars shows abundant evidence for This configuration is remarkably stable in P-T space sulfur activity over a prolonged period in martian discussed below. history. Evidence goes back to the Viking landers in STABILITY 1976 but more recent orbital and landed in situ Figure 3 illustrates the stability field of SM [3]. The missions, including the two MER rovers, which are diagram shows that SM can be stable over the still active, confirm this. The identification of the pressure interval ~ 10 to 27 Kb at 1200 degrees C. sulfate jarosite with ferric iron confirms highly This corresponds to a depth interval of ~90 – 243 km oxidizing conditions near the martian surface. We depth in Mars (Fig. 4). In this depth range SM may speculate that conditions oxidizing enough to crystallize if the fO2 is high enough for S to exist as + crystallize a SO4 containing scapolite exist in the 6 . lower martian crust or upper mantle. This scapolite TERRESTRIAL OCCURENCES could be a important reservoir for sulfate in Mars and Terrestrial occurrences of igneous scapolite are might form either directly from sulfur-rich, oxidizing discussed in [2,4, and 5]. Boivin and Camus [4] magmas or by metasomatism of previously discuss a scapolite assemblage found as megacrysts crystallized plagioclase in basalt. in tephra in Massif Central, France and in a cinder CRYSTAL CHEMISTRY cone, Sequeika volcano, Algeria. Conditions of the For a brief review of scapolite crystal chemistry see formation of a sulfate rich scapolite are pressure, 5- [1]. Most scapolites are solid solutions between 15 Kb and T ~1100 ºC. Goff et al. [5] describe an marialite, Na4Al3Si9O24Cl and meionite, occurrence of SM as phenocrysts in a latite dome in Ca4Al6Si6O24CO3 . However most terrestrial igneous northwest Arizona. The authors suggest that the scapolites have high sulfate contents and some scapolite crystallized at temperatures of 850-900 ºC approach compositions of Ca4Al6Si6O24SO4 which and 3-6 Kb total pressure. Teertstra et al. [2] describe has the name silvialite [2] or may be more simply SM from upper-mantle xenoliths garnet-granulite referred to as sulfate-Me [3] or in this abstract as SM. xenoliths hosted by olivine nephelinite, from The most likely space group for ordered SM is McBride Province, North Queensland, Australia. tetragonal P42/n with two formula units per unit cell. They estimate the scapolite formed at 900-1000 ºC at The focus in these discussions is on the tetrahedral 8-12 Kb under high fSO2 and fO2. SO4 group which in space group P42/n occupies IMPLICATIONS FOR MARS special position b (Wyckoff notation) and has site Sulfate-rich scapolite may occur in Mars in at least symmetry bar 4 which is consistent with on ordered two types of occurrences. The first would be in primary igneous occurrences were the melt SO4 group. The multiplicity of this site is 2 or there conditions had the appropriate high fO2 and fSO2. are 2 SO4 groups per unit cell. We, together with the essential help of Eric Dowty, prepared two new The second would be in a metosomatic replacement structure drawings to display some of the important assemblage causes by hot sulfate-rich brine structural features of scapolite and the environment interactions with previously formed plagioclase of the sulfate group in a large cage in the scapolite assemblages. Once again the brines have to be at a relatively high fO2. structure which can accommodate, SO4, CO3, or Cl. Figure 1 shows the structure projected down the c- ACKNOWLEDGEMENTS axis. The yellow tetrahedra contain Al and Si with an This research was funded by a NASA/ ordered alternation like in the anorthite end-member Cosmochemistry grant to JJP. We sincerely thank of plagioclase. The purple spheres represent Ca in Eric Dowty for producing the beautiful scapolite the oval-shaped channels that run parallel to the c- crystal structure drawings. axis. The sulfur atoms are shown in orange. Oxygen REFERENCES. [1] Papike (1988) Reviews of Geophysics, 26, 407-444. [2] Teertstra et al. (1999) is illustrated with blue spheres. Figure 2 shows a projection of the structure down the a-axis ( 90 Min. Mag., 63, 321-329. [3] Newton and Goldsmith degrees from Fig. 1) and shows nicely the large (1976) Zeit. Fur Krist, 143, 333-353. [4] Boivin and Camus (1981) Contrib. Mineral Petrol., 77, 365-375. silicate tetrahedral cage that accommodate SO4. It [5] Goff et al. (19820 EPSL, 60, 86-92. Lunar and Planetary Science XXXVIII (2007) 1152.pdf s C z + Q 30 y + 4 K O + Gr S a C z + ] Q r r + y o a K 20 C b + r Ca Al Si O SO k 4 6 6 G 24 4 + [ (Sul q e fate-Me) i r n A L u s s e r 10 P An + CaSO 4 0 800 1000 1200 1400 1600 a1 Temperature [°C] Figure 3. After Newton and Goldsmith (1976). Gr = a2 grossular garnet , Ca3Al2Si3O12; Ky = kyanite, Al2SiO5; Qz = quartz, SiO2; Cs = coesite, SiO2; Cor = corundum, Al2O3. Figure 1. Sulfate -Me structure projected down c. See text for discussion. CRUST (10 - 50 km thick) te scapolite stable sulfa ~90 - 2 43 50 km Olivine - pyroxene - garnet 0 k m M A 1 N 00 0 T km L E Spinel - majorite 1 500 km Metallic iron 200 0 k m C 2 O 500 R k m E 300 0 k c m a1 Figure 4. The interior of Mars with proposed sulfur Figure 2. Sulfate- Me structure projected down a. scapolite stability region. With the help of Dave See text for discussion. Draper. .
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