GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 49-5
Presentation Time: 2:40 PM


ZIEGLER, Karen, VACI, Zoltan and AGEE, Carl B., Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131,

NWA 10416, an olivine-phyric basalt, contains large olivine phenocrysts that display hydrous iddingsite alterations. Iddingsite alteration is attributed to pre-terrestrial processes, predating groundmass crystallization. We identified several zones of alteration: orange staining in core center (‘orange iddingsite’), intermingled with dull areas (‘weathered orange iddingsite’), and brown core margins (‘brown iddingsite’). Clear olivine overgrows the altered cores.

Material from the interior of the stone has typical Martian isotope values (δ18O=4.310-4.740, Δ17O=0.168-0.327‰). Material from the exterior, more weathered surface has a wider range of δ18O and smaller Δ17O values (4.592-8.214 ‰, 0.071-0.205 ‰).

Clear overgrowth olivine, brown iddingsite, and the mixed orange and weathered orange iddingsite were handpicked from gently crushed rock. The clear olivine oxygen isotope values (δ18O=4.937‰, Δ17O=0.253‰) coincide with those from the bulk interior analyses and are typical Martian. The brown iddingsite margins of the altered olivine cores have δ18O=8.553, 9.596‰ and Δ17O=0.102, 0.113‰ values. Similar to the bulk material from the exterior surface, these results have a larger range of δ18O, however, Δ17O values are still distinctly non-terrestrial. The orange, heavily altered iddingsite from the centers of the olivine cores yield δ18O=13.417, 12.623‰, and Δ17O=0.026, -0.019‰, suggesting a terrestrial origin.

The fact that iddingsite is overgrown by clear olivine with typical Martian δ18O and Δ17O values dictates that the initial alteration of the olivine cores is Martian. Gradually increasing δ18O values and decreasing Δ17O values of the increasingly more altered (brown, then orange) iddingsite towards the core centers indicate subsequent overprinting by terrestrial alteration. Any secondary alteration would attack and affect the most altered material first; this is demonstrated by the terrestrial δ18O and Δ17O values of the orange iddingsite.

These data support earlier hypothesis [1] that the olivine cores were liberated from their cognate basaltic matrix, altered at or near the surface of Mars, and subsequently incorporated into the basalt that now is NWA 10416.

[1] Herd C. D. K. et al. 2016. Abstract #2527. 47th LPSC. [2] Vaci Z. et al., 2016. Abstract #2538. 47th LPSC.