Evaporite mineral assemblages in the nakhlite (martian) meteorites

A mineralogical study of the three nakhlite (martian) meteorites has revealed that they contain evaporite mineral assemblages. Lafayette has Ca-siderite and clay minerals (smectite/illite) along fractures within olivine; Governador Valadares contains clay mineral veins in olivine, with siderite, gypsum and anhydrite in interstitial areas; Nakhla has clay and gypsum veins in olivine, with Mg-, Mn-rich siderite, anhydrite and halite in interstitial sites. Minor goethite is also present in the three meteorites. Lafayette siderite 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 for clay, siderite and gypsum are all similar with LREE, Y > HREE, Zr, Nb and La 0.9-95 x CI; Y 0.2-2.4 x CI. This pattern of abundances reflects the trace element contents of the parent fluid, which in turn were derived through dissolution of LREE-enriched feldspathic mesostasis. The close similarities in silicate petrography and radiometric ages determined by other workers for these olivine clinopyroxenites suggests that the parent rocks were close to one another on Mars and therefore the same fluid may have been responsible for the precipitation of the evaporite mineral assemblages. Lafayette contains the mineral assemblage and siderite composition which are least soluble in water and Nakhla contains the most soluble minerals and carbonate composition. On the basis of our new data we consider a new model of progressive 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. Partial dissolution of near-surface rocks by the acidic brine released Fe, Mg and trace elements from mesostasis and olivine into the fluid. The Lafayette assemblage was formed where > 25% volume of the water remained following evaporation, Governador Valadares 20% and Nakhla < 10%. This model is consistent with martian ground temperatures at >0 degrees C and a thicker atmosphere (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.