Early diagenetic siderite as an indicator of depositional environment in the Triassic Rewan Group, southern Bowen Basin, eastern Australia

Abstract

Early concretionary and non‐concretionary siderites are common in subsurface Triassic sandstones and mudrocks of the Rewan Group, southern Bowen Basin. A detailed petrological and stable isotopic study was carried out on these siderites in order to provide information on the depositional environment of the host rocks. The siderites are extremely pure, containing 85–97 mol% FeCO3, and are commonly enriched in manganese. δ13C (PDB) values are highly variable, ranging from ‐ 18·4 to +2·9‰, whereas δ18O (PDB) values are very consistent, ranging from ‐ 14·0 to ‐ 10·2‰ (mean= ‐ 11·9 ± 1·0‰). The elemental and oxygen isotopic composition of the siderites indicates that only meteoric porewaters were involved in siderite formation, implying that host rocks accumulated in totally non‐marine environments. The carbon isotopic composition of the siderites is interpreted to reflect mixing of bicarbonate/carbon dioxide generated by methane oxidation and methanogenesis. Very low δ13C values demonstrate that, contrary to current views, highly 13C‐depleted siderite can be produced at shallow burial depths in anoxic non‐marine sediments.

Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction

“…Siderite formation has been proposed to commonly require biotic mediation to provide the source of both Fe(II) and carbonate through oxidation of organic carbon coupled to dissimilatory Fe(III) reduction (Coleman, 1993;Coleman et al, 1993). Many authigenic Fe carbonates typically have much higher Mg and Ca contents than can be accommodated if they formed in thermodynamic equilibrium at low temperatures (e.g., Mozley and Carothers, 1992;Laverne, 1993;Baker et al, 1995;Hendry, 2002). In addition, experimental studies using Geobacter metallireducens to produce siderite from fluids that contained variable Ca to Mg ratios and constant Fe concentration show that Ca incorporation is rate dependent but that Mg incorporation is rate independent (Mortimer et al, 1997), suggesting that a wide range of metastable Fe carbonate compositions may be produced by DMRB.…”

Temperature and isotopic relations in carbonate minerals in the Middle Jurassic sideritic rocks of central and southern Poland

“…The d 18 O val ues range be tween -7.84 and -1.92‰ VPDB. The for mula af ter Carothers et al (1988) has been used to es ti mate the d 18 O of the pore wa ter re spon si ble for (Baker et al, 1995;Rezaee and Schulz -Rojahn, 1998). Such val ues in di cate a mixture of ma rine and me te oric wa ters.…”

Trace and rare-earth element behaviors during alteration and mineralization in the Attepe iron deposits (Feke-Adana, southern Turkey)

“…This interpretation is confirmed by both the stratigraphic sequence and the δ 13 C PDB value (−15.90‰; Küpeli et al, 2007) of the Precambrian graphitic schists in the Attepe region. Additionally, these C isotope values indicate that the investigated siderite and ankerite were not formed by marine-synsedimentary or diagenetic processes under the reducing conditions that generate high δ 13 C values (Irwin et al, 1977;Baker et al, 1995). The siderite and ankerite were precipitated as a result of the interaction between hydrothermal fluids rich in Fe, Mn and Ba and the Lower-Middle Cambrian marine limestone marbles of the Çaltepe Formation.…”