Description of Manganese events in hydrothermal hypogene speleogenesis

The phreatic speleothems of hydrothermal speleogenesis are mainly mammillaries and spar calcite. In this study, seven samples from four caves in three areas of Greece are investigated. Morphological evidence from these caves along with the presence of the phreatic calcite indicate their hydrothermal origin. The analyzed calcite deposits contain layers of metallic oxides that were investigated with scanning electron microscopy aided by energy dispersive spectrometer semi-quantitative measurements. Their chemical composition is dominated by manganese. Rarely, double-terminated phantom calcite crystals display two repetitions of these manganese oxide layers along the growth surfaces, but most commonly mammillaries exhibit one to four manganese oxide layers. In one case, the manganese layer was formed along with partial dissolution of the calcite that resulted in small pits on the growth surfaces. We interpret these layers in the phantom spar crystals and cave mammillaries to changes in the fluid conditions and fluctuations that are common in phantom crystals result in the deposition of mangese oxides along with or without contemporary dissolution. In order to mark the changes in conditions of their formation, these layers are described as “manganese events.” Because of the low deposition rate of the mammillaries, the repetitions of manganese events are plausibly independent of exogenous factors, such as seeping water. This event can be found in the secondary minerals of both shallow and deep phreatic settings of hydrothermal speleogenesis. The available data set shows that the manganese event is part of a repetitive depositional pattern that appears in at least 17 % of the carbonate speleothems of hydrothermal speleogenesis.