| Abstract: | Abstract— The Lafayette meteorite, a nakhlite of the SNC (Martian?) group, contains hydrous alteration materials as intergranular films and as veinlets and patches replacing olivine, pyroxenes, and high-Si glass. The alteration materials (“iddingsite”) consist of ferroan smectite clays, magnetite (or maghemite), and ferrihydrite, as shown by SEM and TEM. Three varieties of veinlets are present and formed in the order: coarse phyllosilicate; fine-grained (phyllosilicate-oxide), and porous oxide. Veinlets of fine-grained material cross-cut coarse phyllosilicate veinlets. The alteration materials are preterrestrial, as they are older than Lafayette's fusion crust, which is glassy and not affected by any alterations. Approaching the crust, the veinlets are progressively modified to the point of melting, and progressively depleted in adsorbed volatile constituents (S, Cl, and P). Bulk compositions of the alteration veinlets (SEM and TEM EDX) are consistent with the observed mineralogy, and suggest: that the smectite contains significant adsorbed S and Cl; that the ferrihydrite contains significant adsorbed S, but not Cl; that rare grains of sulfate (Ca?) and chloride (Na or K?) are present; and that the compositions of the alteration materials are approximated by Lafayette's olivine + high-Si glass + water. We estimate that Lafayette's alteration materials formed at less than 100 °C. The oxidation potential of the water was near or slightly below that of the magnetite-hematite buffer. The presence of sulfate and chloride in discrete phases and as adsorbates on ferrihydrite and smectite suggests that the altering solutions were saline. However, relatively little mass was transferred into or out of Lafayette because the bulk composition of the alteration materials is nearly isochemical with a mixture of magmatic silicate phases and water. Chemical transport within Lafayette was also limited, as alteration materials preserve some chemical signature of their host minerals. Presence of alteration materials along only some grain boundaries and some cracks suggests that Lafayette was not soaked in fluid. These last two inferences suggest that the alteration of Lafayette took place during episodic infiltrations of small volumes of saline water. |
