The Origin of Iddingsite Veins in Olivine from the Nakhlite Meteorites: New Insights from Analogy with CM Carbonaceous Chondrites and Terrestrial Basalts

The nakhlite meteorites are samples of a ~1300 million year old martian clinopyroxenite lava flow or sill [1, 2]. These rocks contain secondary minerals including hydrous silicates, carbonates, sulphates and Fe-(hydr)oxides that formed by watermediated alteration of the igneous body [3, 4]. A prerequisite for understanding the nature of the aqueous system from which these minerals formed, including water/rock ratio, the provenance of solutes and its longevity, is knowing whether the secondary minerals formed by replacement of primary igneous components (minerals and glasses), or by cementation of pores that were opened by fracturing. A replacive origin would suggest low water/rock ratios with solutions being close to saturation with respect to secondary minerals, and does not require a pre-existing network of pores for fluids to gain access to mineral grain interiors. An origin by cementation would suggest that solutes had been sourced by dissolution of other parts of the nakhlite parent rock or the martian crust and were introduced by fluid flow under relatively high water/rock ratio conditions; a means of fracturing the rock is also required. Here we have sought to answer the question of whether olivine-hosted veins in the nakhlites formed by cementation or replacement by comparing the microstructures of veins in the nakhlite Lafayette with veins in olivine grains from type I chondrules in Murchison (CM2 carbonaceous chondrite). We also draw on previously published work on ‘iddingsite’ veins in olivine from terrestrial basalts.