The ancient rock record for Mars has long been at odds with climate modelling. The presence of valley networks, dendritic channels and deltas on ancient terrains points towards running water and fluvial erosion on early Mars
 1
 , but climate modelling indicates that long-term warm conditions were not sustainable
 2
 . Widespread phyllosilicates and other aqueous minerals on the Martian surface3–6 provide additional evidence that an early wet Martian climate resulted in surface weathering. Some of these phyllosilicates formed in subsurface crustal environments
 5
 , with no association with the Martian climate, while other phyllosilicate-rich outcrops exhibit layered morphologies and broad stratigraphies
 7
 consistent with surface formation. Here, we develop a new geochemical model for early Mars to explain the formation of these clay-bearing rocks in warm and wet surface locations. We propose that sporadic, short-term warm and wet environments during a generally cold early Mars enabled phyllosilicate formation without requiring long-term warm and wet conditions. We conclude that Mg-rich clay-bearing rocks with lateral variations in mixed Fe/Mg smectite, chlorite, talc, serpentine and zeolite occurrences formed in subsurface hydrothermal environments, whereas dioctahedral (Al/Fe3+-rich) smectite and widespread vertical horizonation of Fe/Mg smectites, clay assemblages and sulphates formed in variable aqueous environments on the surface of Mars. Our model for aluminosilicate formation on Mars is consistent with the observed geological features, diversity of aqueous mineralogies in ancient surface rocks and state-of-the-art palaeoclimate scenarios. A geochemical model for the state of early Mars suggests that short-term warm events grafted onto a generally cold climate could form Al-rich phyllosilicates in short-lived surface water bodies and Mg-rich ones in hydrothermal subsurface environments.

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Surface clay formation during short-term warmer and wetter conditions on a largely cold ancient Mars

The equilibrium diagrams of zeolites and the different possibilities of synthesizing zeolites starting from chemicals, minerals, and natural glasses are reviewed so to have a general picture of the conditions of crystallization of these minerals. Subsequently, a description and interpretation is given of the geological environments where zeolites crystallize in nature. Key-words: zeolite, diagenesis, very-low-grade metamorphism, hydrothermalism, volcanic glass.

The genesis of zeolites

The role of basalt weathering in the Sr isotope budget of the oceans

Secondary minerals near and within fractures in Columbia River basalts contain objects the size and shape of bacteria. These bacteriomorphs are most commonly rods or ellipses but also include cocci and diplococci forms, vibrioids and club-shaped rods, and associated pairs of objects that suggest cellular division by binary fission. Secondary minerals associated with, enclosing, and making up bacteriomorphs include iron oxyhydroxides, sulfides, and smectites containing ferrous iron. The secondary minerals are intimately intermixed with kerogen. Moreover, bacteriomorphs in the pyrite consist of kerogen. Careful consideration of mineral associations, the occurrence of organic carbon, and the spatial context of bacteriomorphs indicate that they are microfossils. The association of microfossils with minerals formed in reducing environments suggests an ancient ecosystem dominated at least in part by sulfate-reducing bacteria, similar to communities within these basalts today.

https://es.ucsc.edu/~rcoe/eart290C_2006/McKinley_CRBbacteria_Geomicrobio00.pdf

Microfossils and Paleoenvironments in Deep Subsurface Basalt Samples

Middle Jurassic sandstones contain a variety of authigenic clay materials. The origin and distribution of these are related to the influence of several factors, principally depositional pore-fluid chemistry, sandbody geometry and the migration of aggressive fluids. Pore-lining illite, pore-lining chlorite and pore-filling vermiform kaolinite in the Ravenscar Group mutually exclude each other because of depositional pore-water chemistry; seawater in the case of the illite, anoxic freshwater in the case of chlorite and oxygenated freshwater in the case of the kaolinite. Blocky pore-filling dickite occurs ubiquitously within the large connected sandbodies. Its origin may be related to the migration of aggressive fluids and its distribution to depositional sandbody geometry. Mixed-layer chlorite-vermiculite also occurs, and is believed to have formed from chlorite during Recent weathering. Similar patterns occur in Ninian Field Brent Group sandstones, although the situation is more complicated (in these sandstones the kandite subgroup minerals are undifferentiated in this paper). Here too, pore-lining illite occurs in marine sandstones and pore-filling vermiform kandites in non-marine sandstones. However, vermiform kandites also occur in the marine sandstones, perhaps due to freshwater-table development following progradation. The more blocky kandites occur in large connected sandbodies. Finally, a second phase of illitization occurs, postdating blocky kandites, perhaps caused by alkaline formation waters. The occurrence of chlorite in the Broom Formation is anomalous and its possible origin is discussed.

The Nature, Origin and Distribution of Authigenic Clay Minerals from Middle Jurassic Ravenscar and Brent Group Sandstones

The principal components of secondary mineral assemblages found in Pasco Basin basalts are iron-rich smectite (nontronite), clinoptilolite, and silica. Silica occurs as quartz, cristobalite, tridymite, and opal-CT. Extractable iron within the nontronite suggests the presence of an iron-bearing oxyhydroxide phase intercalated with the nontronite. Other components present in minor or trace amounts are mordenite, celadonite, apatite, pyrite, phillipsite, gypsum, erionite, and chabazite. The generalized precipitation sequence with time and/or depth was found to be clay (usually nontronite) clinoptilolite silica and/ or clay. Nontronite, the first phase to form, is present at nearly al sampled depths. Clinoptilolite is apparently restricted to depths below about 350 m. Quartz is ubiquitous whereas opal and cristobalite appear to be abundant only below 600 m. Mordenite occurs only at depths below about 900 m, which correlates roughly with the first occurrence of dissolution-etched clinoptilolite. These observations as well as comparisons with data on secondary mineral assemblages from other basaltic and felsic systems suggest that the geochemical evolution of Pasco Basin basalts probably occurred under conditions similar to those existing today.

L. V. Benson

Papers

Diagenesis of Basalts from the Pasco Basin, Washington--I. Distribution and Composition of Secondary Mineral Phases