Title: Conditions of meteoric calcite formation along a Variscan fault and their possible relation to climatic evolution during the Jurassic-Cretaceous
Authors: Muchez, Philippe ×
Nielsen, P
Sintubin, Manuel
Lagrou, D #
Issue Date: 1998
Publisher: Blackwell science ltd
Series Title: Sedimentology vol:45 issue:5 pages:845-854
Abstract: Two calcite cements, filling karst cavities and replacing Lower Carboniferous limestones at the Variscan Front Thrust, were precipitated after mid-Jurassic Cimmerian uplift and subsequent erosion but before late Cretaceous strike-slip movement. The first calcite (stage A) is nonferroan and crystals are coated by hematite and/or goethite. These minerals also occur as inclusions along growth zones. The calcite lattice contains < 0.07 mol.% Fe, but Mn concentrations can be as high as 0.72 mol.% in bright yellow luminescent zones. Primary, originally one-phase, all-liquid, aqueous inclusions have a final melting temperature between -0.2 degrees and +0.2 degrees C, indicating a meteoric origin of the ambient water. The delta(13)C and delta(18)O values of the calcites are between -7.3 parts per thousand, and -6.3 parts per thousand, -7.8 parts per thousand, and -5.5 parts per thousand on the Vienna PeeDee Belemnite (VPDB) scale, respectively. The second calcite (stage B) consists of ferroan (0.13-0.84 mol.% Fe) blocky crystals with Mn concentrations between 0.34 and 0.87 mol.%. Primary, single-phase aqueous fluid inclusions indicate precipitation from a meteoric fluid below 50 degrees C. The delta(13)C values of stage B calcites vary between -7.3 parts per thousand and -2.1 parts per thousand VPDB and the delta(18)O values between -7.9 parts per thousand and -7.2 parts per thousand VPDB. A precipitation temperature below 50 degrees C for the stage A calcites and the presence of iron oxide/hydroxide inclusions in the crystals indicate near-surface precipitation conditions. Within this setting, the geochemistry of the nonferroan stage A calcites reflects precipitation under oxic to suboxic conditions. The ferroan stage B calcites precipitated in a reducing environment. The evolution from the stage A to stage B calcites and the associated geochemical changes are interpreted to be related to the change from semiarid to humid conditions in western Europe during late Jurassic-Cretaceous times. A change in humidity can explain the evolution of groundwater from oxic/suboxic to reducing conditions during calcite precipitation. The typically higher delta(13)C values of the stage B compared to the stage A calcites can be explained by a smaller contribution of carbon derived from soil-zone processes than from carbonate dissolution in the groundwater under humid conditions. The small shift to lower delta(18)O between stage A and B calcites may be caused by a higher precipitation temperature or a decrease in the delta(18)O value of the meteoric water. This decrease could have been caused by a change in the source of the air masses or by an increase in the amount of rainfall during the early mid-Cretaceous. Although the latter interpretation is preferred, it cannot be proven.
ISSN: 0037-0746
Publication status: published
KU Leuven publication type: IT
Appears in Collections:Division of Geology
× corresponding author
# (joint) last author

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