Dinantian dolomites from East Fife: hydrothermal overprinting of early mixing-zone stable isotopic and Fe/Mn compositions

Abstract

Thin marine limestones from the Dinantian of the St Monans syncline, East Fife, are extensively dolomitized. They occur within a deltaic sequence cut by Upper Carboniferous volcanic rocks. The volcanic rocks have been extensively modified by CO 2 -enriched hydrothermal fluids. The dolomite is ferroan and nonluminescent. Stable isotope compositions for groundmass dolomite range from δ 18 O = −4.76‰ to −0.47‰ and δ 13 C = −3.29‰ to 0.60‰. Later, void-filling dolomite falls into two groups; the first is 18 O depleted, with δ 18 O as low as −10.6‰, while the second is 18 O-enriched, δ 18 O = −2.8‰ to −1.2‰. Despite almost homogeneous staining characteristics, backscattered SEM imaging shows the dolomites to be chemically extremely heterogeneous. Dolomitization was a multistage process in which an early ‘mixed water’ component was variably remobilized to form the ‘matrix dolomite’. Biomolds and other vuggy post-early-dolomite porosity are filled by dolomite precipitated within a volcanically driven hydrothermal system. The chemistry of the later dolomite reflects the mixing of upward moving hydrothermal fluids, modified by interaction with volcanic material, with downward moving seawater feeding the hydrothermal system. In addition, material released during normal processes of siliciclastic diagenesis added a further component to the late stage dolomitization. Only two of the limestones escaped complete dolomitization and appear to have undergone early post-depositional diagenesis within meteoric porefluids. They contain early meteoric calcite cements and later post-compaction calcite cements, dolomite and siderite. One of the implications of this study is that the interpretation of dolomite geochemical data can be vastly enhanced by the use of back scattered electron microscopy to reveal otherwise hidden compositional heterogeneity.