ABSTRACTThe carbonate platforms of the Wetterstein Formation of the Eastern Alps (Drau Range and Northern Calcareous Alps) show a distinct facies zonation of reefs and lagoons. While some lagoonal areas were episodically emerged and formed lagoonal islands, others remained permanently flooded. The scale of near surface, meteoric or marine diagenesis was related to this lagoonal topography. At shallow burial depth, cementation was dominated by altered marine solutions, which additionally caused recrystallization of metastable constituents of the sediment and earlier marine cements (high magnesian calcite, aragonite) connected with a carbon and oxygen isotopic change to more negative values.Deeper burial cementation shows a succession with two types of saddle dolomite and three types of blocky calcite. Carbon and oxygen isotopic values of these cements show a trend towards more negative values from the first to the last generation, in the following succession: clear saddle dolomite—zoned blocky calcite—cloudy saddle dolomite—post‐corrosion blocky calcite—replacive blocky calcite. Fluid inclusion studies of the carbonate cements are interpreted to indicate a deeper burial temperature development that first increases from 175 to 317°C, followed by a temperature decrease to 163–260°C, and subsequent increase up to 316°C, whereby the samples of the Drau Range always show the lowest values. Calculations of the isotopic composition of the water, from which the carbonate cements were precipitated, yielded positive δ18O values from 6.66 to 17.81%o (SMOW), which are characteristic for formation and/or metamorphic waters. Also, the isotopic compositions of the palaeofluids probably changed during deeper burial diagenesis, following the temperature development.

ABSTRACTThe Late Proterozoic Pedro Leopoldo facies (Bambuí Group) in the vicinity of Belo Horizonte, Brazil, comprises alternating laminated microsparitic limestones (10–35 mm thick beds) and fibrous limestones (10–55 mm thick). The latter are composed of a mosaic of sparry calcite crystals. These irregularly crosscut rays and fans are composed of feathery precursor crystal bundles with squared‐off growth zones. Ghosts of an original fibrous mineral, hexagonal in cross‐section, are visible. The petrographic characteristics, very high strontium content and low magnesium content of the fibrous beds, as well as microspar beds, strongly argue for an original aragonitic mineralogy. The rays are interpreted as having formed by precipitation at the sediment‐water interface, whereas the micrite was precipitated from the water column prior to deposition on the sea floor. The lack of emergence features suggests widespread aragonite precipitation under persistently subtidal conditions.

ABSTRACTThe replacement by ferroan calcite with preservation of the original structures can be used as a new criterion for identifying skeletons originally composed of high‐magnesian calcite. This applies to bryozoa, rugose corals, echinoderms, many foraminifera, most ostracods, red algae, and serpulids. On the other hand, skeletons originally composed of low‐magnesian calcite were never replaced by ferroan calcite, as shown by belemnites, brachiopods, and most of the pelecypods.Using this criterion, an original low‐magnesian calcite composition is inferred for Tentaculites and some ostracods and foraminifera, whereas a previous high‐magnesian calcite composition is inferred for trilobites, oligostegina and certain ooids. Chemical instability of high‐magnesian calcite is suggested to be the driving force of the replacement by ferroan calcite. In most of the thirty‐seven samples investigated, of Oligocene to Devonian age, the ferrous iron concentration of the interstitial fluid increased during diagenesis, as shown by well established sequences of cement A and B and fissure fill. This offers a relative time scale for diagenetic processes.Ferroan calcites contain up to 6 mol % FeCO3 and up to 5 mol % MgCO3. In this range of concentration, the distribution coefficients for Fe and Mg between calcite and solution at about 25°C are about 1 to 0‐03, respectively, according to experiments. Possible sources of iron are iron oxides and hydroxides as well as clay minerals including glauconite.Though a submarine origin below the sediment surface is conceivable for ferroan calcite, there are serious limiting conditions such as low Eh and, at the same time, lack in sulphate‐reducing bacteria. On the other hand, ferroan ‘dedolomite’, compositional zonality in individual ferroan calcite overgrowths, low δ18C and δ18O values, and low Mg concentrations point more to a meteoric‐phreatic origin of many ferroan calcite occurrences.

42 Individuen von Ophiuroideen (Schlangensterne) und 48 Individuen von Asteroideen (Seesterne) rezenter Mee- resbereiche wurden hinsichtlich ihrer Skelettzusammen- setzung vor allem röntgendiffraktometrisch untersucht (Methodik s. RICHTER, 1984). Neben drei Lokalitäten der Ägäis (Lesbos, Palea Epidavros und Ag. Theodori; mittlere jährliche Temperatur des Oberflächenwassers: \9°C) konnten Exemplare aus dem Übergangsbereich Är- melkanal/Atlantik (Roscoff/Breta gne; Yl.S'Kl) und aus der Nordsee (Helgoland; \\°C) berücksichtigt werden. Bei den aus 1-3 m Wassertiefe im Sommer beprobten Stel- len wich die zeitgleich gemessene Wassertemperatur na- türlich vom Jahresmittel ab, während bei den Lokalitäten in 10-30 m Wassertiefe die im Sommer (Ägäis) bzw. im Winter (Roscof f) gemessenen Daten mit den Jahresmitteln übereingestimmt haben. Der mögliche Einfluß der Wachstumszeit bzw. -geschwindigkeit (vgl. WEBER, 1973) auf die Zusam- mensetzung der Skelette wird nachfolgend nicht disku- tiert. Generell erfolgt der Skelettaufbau im gesamten Jahr, wobei jedoch die Wachstumsraten im Sommer gegenüber dem Winter meist höher sind (SWAN, 1966).