No abstract

Some minerals are considered to be representative of specific natural environments. Among them glauconite is considered to be formed in marine deep platform conditions. However, the term glauconitic is misused to designate green minerals formed in marine outer‐shelf environments. X‐ray diffraction, scanning electron microscopy and energy‐dispersive X‐ray analyses of individual green materials were carried out leading to the identification of glauconite in the Purbeckian facies. Green glauconite grains predominantly occur as burrow fills and occasionally as faecal pellet replacements. Two depositional environments have been identified from bottom to top of the succession: (1) an argillaceous dolomitic lagoonal sediment formed in saline, shallow water; (2) a marl‐limestone alternation deposited in a brackish water estuary. The crystallochemical properties of the glauconites change abruptly. These findings show that glauconite may form in coastal environments and not only in mid‐shelf to upper deep water platform environments as classically assumed. Moreover, the glauconite composition changes with the chemical conditions imposed by the local environment.

The chemical index of alteration (CIA) is a tool to calculate the weathering intensity (Nesbitt and Young, 1982). It has been largely used to reconstitute the past climates on Earth at different epochs and to determine the sediment source rocks from shale and graywacke stratigraphical series. However, because it induces some uncertainties, we propose a new approach based on the M ؉ -4Si-R 2؉ system (M ؉ ‫؍‬ Na ؉ ؉ K ؉ ؉ 2Ca 2؉ ; 4Si ‫؍‬ Si/4; R 2؉ ‫؍‬ Fe 2؉ ؉ Mg 2؉ ) that takes silica into account. In these coordinates, the chemical compositions of the weathered granitic, mafic and ultramafic rocks determine clearly separated trends which all converge toward the 4Si pole, namely the kaolinite composition (chlorites plot near the R 2؉ pole). Consequently, the alteration intensity for a given parent rock can be measured by the migration of its chemical composition toward the kaolinite pole: ⌬4Si% ‫؍‬ [(4Si altered sample ؊ 4Si unaltered parent rock ) ؋ 100]/(100 ؊ 4Si unaltered parent rock ). The ultimate stage of weathering leads to the progressive accumulation of the insoluble R 3؉ components (R 3؉ ‫؍‬ Al 3؉ ؉ Fe 3؉ ). It is attained in bauxite deposits where kaolinite is replaced by gibbsite. This ratio varies from 0 to 1 with increasing weathering degree, namely the leaching of soluble components (M ؉ and R 2؉ ), the oxidation of Fe 2؉ and the concentration of the residual ones (Al 3؉ , Fe 3؉ ). Because hydroxides cannot be represented in the M ؉ -4Si-R 2؉ system, the Weathering Intensity Scale (WIS) including the ultimate bauxite stage must be based on the co-variation of the ⌬4Si% parameter and the R 3؉ /(R 3؉ ؉ R 2؉ ؉ M ؉ ) ratio.The effects of the diagenetic illitization on the composition of sediments (socalled K-metasomatism) are difficult to measure since the K 2 O amount of sediments may be highly variable due to the inheritance of detrital potassic minerals. In spite of this variability, if the compositions of the shales in a given series are aligned toward the diagenetic illite pole ([Si 3.30 Al 0.70 ]O 10 (Al 1.78 Fe 3؉ 0.05 Mg 0.17 )(OH) 2 K 0.87 ) in the M ؉ -4Si-R 2؉ coordinates, a correction is required. The illite amount has been calculated for the Gulf Coast shale series assuming that the K 2 O percent of the original sediment (K 2 O sediment ) is equal to that of the shale sample having the lowest K 2 O%: (K 2 O shale ؊ K 2 O sediment ) ؋ 100/9.9. The correction modifies the amounts of all the elements and not only that of potassium. Consequently, the normalized values of the M ؉ , 4Si, and R 2؉ parameters for the corrected composition are changed and the ⌬4Si% parameter modified.The WIS has been tested on a Neoproterozoic shale-graywacke series (Mirbat Group, Oman) in which the sediments have been deposited in contrasted climatic conditions (glacial and interglacial). The variation of the ⌬4Si% parameter along the stratigraphic pile exhibits large and low magnitude oscillations. The former are coherent with that given by the CIA and could correspond to changes in global conditions (temperatur...

During the Hadean to early Archean period (4.5-3.5 Ga), the surface of the Earth's crust was predominantly composed of basalt and komatiite lavas. The conditions imposed by the chemical composition of these rocks favoured the crystallization of Fe-Mg clays rather than that of Al-rich ones (montmorillonite). Fe-Mg clays were formed inside chemical microsystems through sea weathering or hydrothermal alteration, and for the most part, through post-magmatic processes. Indeed, at the end of the cooling stage, Fe-Mg clays precipitated directly from the residual liquid which concentrated in the voids remaining in the crystal framework of the mafic-ultramafic lavas. Nontronite-celadonite and chlorite-saponite covered all the solid surfaces (crystals, glass) and are associated with tiny pyroxene and apatite crystals forming the so-called "mesostasis". The mesostasis was scattered in the lava body as micro-settings tens of micrometres wide. Thus, every square metre of basalt or komatiite rocks was punctuated by myriads of clay-rich patches, each of them potentially behaving as a single chemical reactor which could concentrate the organics diluted in the ocean water. Considering the high catalytic potentiality of clays, and particularly those of the Fe-rich ones (electron exchangers), it is probable that large parts of the surface of the young Earth participated in the synthesis of prebiotic molecules during the Hadean to early Archean period through innumerable clay-rich micro-settings in the massive parts and the altered surfaces of komatiite and basaltic lavas. This leads us to suggest that Fe,Mg-clays should be preferred to Al-rich ones (montmorillonite) to conduct experiments for the synthesis and the polymerisation of prebiotic molecules.