Stable isotope and trace element studies in carbonate diagenesis

The thesis is presented as a series of papers. 1. Middle Jurassic hardgrounds from the Lincolnshire Limestone demonstrate synsedimentary submarine cementation. Intensity of early cementation reflects sea-water turbulence. Composite fringing cements have isotopic and magnesium content (δ13C = 3, δ18O = 0 and [backsimeq] 2wt%MgC03) interpreted as relics of high-magnesium calcite cements which stabilised in early diagenesis. Marine isotopic values are preserved despite subsequent precipitation of ferroan calcite (δ18O = -9). 2. The isotopic composition of Mesozoic, Tethyan limestones is related to their lithologies. Micritic, pelagic limestones have δ18O from -0.9. to 0.38, they were cemented on the sea-floor. Heavy δ180 in widespread nodular limestones reflects their former (high-magnesium calcite) mineralogy. Shelf carbonates (δ18O from -2.5 to -6.5) were fully cemented, after burial and modification of pore-water isotopic composition. 3. Sequences of zones in calcites filling phragmacone chambers of three Jurassic ammonites result from neomorphism and cementation of acicular carbonates during burial. Isotopic trends (δ18O from 0 to -20%., δ13C from -26 to 0%. ) demonstrate[s] shallow-burial cementation with input of organogenic carbon followed by cementation and neomorphism after temperature increase, δ18O depletion and introduction of marine bicarbonate. Isotopic, trace element and inclusion density variation between zones reflects neomorphic rate and degree of exchange with burial pore-waters. 4. Fibrous calcite veins in shales grew by successive antitaxial addition at the vein-margins. Isotopic data suggest a late diagenetic origin. Discontinuities are reflected in reversals in isotopic and trace element composition trends. Changes reflect renewed ground-water flow tapping different sources of bicarbonate. 5. The isotopic composition of 'normal' epicontinental marine limestones reflects the modal proportions of allochems and cements. Allochems retain marine isotopic composition (δ18O = O δ13C =+l to +3) by mineralogical stabilisation with minimal external pore-water exchange. Late cements (with δ18O -5 to +13 and marine δ13C) precipitated during normal sediment burial but, depth of burial and cause of the ubiquitous δ18O depletion remain problematical. Limestones and diagenetic calcites are clearly inhomogenous; individual components preserve isotopic and trace element compositions, which reflect the environment of precipitation or stabilisation, throughout later phases of diagenesis.