OLIVINES IN MARTIAN METEORITE NWA 10416: ALTERATION AND OXYGEN ISOTOPE EVOLUTION

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. 47^th LPSC. [2] Vaci Z. et al., 2016. Abstract #2538. 47^th LPSC.