Evaporite mineral assemblages in the nakhlite (martian) meteorites

A mineralogical study of the three nakhlite (martian) meteorites has reveal ed that they contain evaporite mineral assemblages. Lafayette has Ca-sideri te and clay minerals (smectite/illite) along fractures within olivine; Gove rnador Valadares contains clay mineral veins in olivine, with siderite, gyp sum and anhydrite in interstitial areas; Nakhla has clay and gypsum veins i n olivine, with Mg-, Mn-rich siderite, anhydrite and halite in interstitial sites. Minor goethite is also present in the three meteorites. Lafayette s iderite has the range of compositions (mol%) CaCO3 21.6-36.8, MnCO3 4.2-35. 3, MgCO3 0.1-1.6, FeCO3 27.5-67.0; Governador Valadares has CaCO3 3.6-11.1, MnCO3 1.1-2.1, MgCO3 9.0-29.2, FeCO3 64.3-77.8; Nakhla has CaCO3 0.1-5.7. MnCO3 1.0-39.9, MgCO3 2.0-40.9, FeCO3 23.2-87.0. Trace element abundances f or clay, siderite and gypsum are all similar with LREE, Y > HREE, Zr, Nb an d La 0.9-95 x CI; Y 0.2-2.4 x CI. This pattern of abundances reflects the t race element contents of the parent fluid, which in turn were derived throu gh dissolution of LREE-enriched feldspathic mesostasis. The close similarit ies in silicate petrography and radiometric ages determined by other worker s for these olivine clinopyroxenites suggests that the parent rocks were cl ose to one another on Mars and therefore the same fluid may have been respo nsible for the precipitation of the evaporite mineral assemblages. Lafayett e contains the mineral assemblage and siderite composition which are least soluble in water and Nakhla contains the most soluble minerals and carbonat e composition. On the basis of our new data we consider a new model of prog ressive evaporation from a Na-Mg-Fe-Ca-SO4-Cl-H2O-HCO3- acidic brine in an area of enclosed drainage (e.g, crater or low-lying flood plain) on Mars. P artial dissolution of near-surface rocks by the acidic brine released Fe, M g and trace elements from mesostasis and olivine into the fluid. The Lafaye tte assemblage was formed where > 25% volume of the water remained followin g evaporation, Governador Valadares 20% and Nakhla < 10%. This model is con sistent with martian ground temperatures at >0 degrees C and a thicker atmo sphere (pCO(2) = 30-100 mbar) at least over brief periods of time less than or equal to 1.3 Ga. (C) 2000 Elsevier Science B.V. All rights reserved.