Long-term fluid circulation in extensional faults in the central Catalan Coastal Ranges: P-T constraints from neoformed chlorite and K-white mica

The neoformation of chlorite and K-white mica in fault rocks from two main faults of the central Catalan Coastal Ranges, the Vall{è}s and the Hospital faults, has allowed us to constrain the P–T conditions during fault evolution using thermodynamic modeling. Crystallization of M1 and M2 muscovite and microcline occured as result of deuteric alteration during the exhumation of the pluton (290 {\textdegree}C > T > 370 {\textdegree}C) in the Permian. After that, three tectonic events have been distinguished. The first tectonic event, attributed to the Mesozoic rifting, is characterized by precipitation of M3 and M4 phengite together with chlorite and Calcite C1 at temperatures between 190 and 310 {\textdegree}C. The second tectonic event attributed to the Paleogene compression has only been identified in the Hospital fault with precipitation of low-temperature Calcite C2. The shortcut produced during inversion of the Vall{è}s fault was probably the responsible for the lack of neoformed minerals within this fault. Finally, the third tectonic event, which is related to the Neogene extension, is characterized in the Vall{è}s fault by a new generation of chlorite, associated with Calcite C4 and laumontite, formed at temperatures between 125 and 190 {\textdegree}C in the absence of K-white mica. Differently, the Hospital fault is characterized by the precipitation of Calcite C3 during the syn-rift stage at temperatures around 150 {\textdegree}C and by low-temperature fluids precipitating Calcites C5, C6 and PC1 during the post-rift stage. During the two extensional events (Mesozoic and Neogene), faults acted as conduits for hot fluids producing anomalous high geothermal gradients (50 {\textdegree}C/km minimum).

Long-term fluid circulation in extensional faults in the central Catalan Coastal Ranges: P–T constraints from neoformed chlorite and K-white mica

The neoformation of chlorite and K-white mica in fault rocks from two main faults of the central Catalan Coastal Ranges, the Vallès and the Hospital faults, has allowed us to constrain the P–T conditions during fault evolution using thermodynamic modeling. Crystallization of M1 and M2 muscovite and microcline occured as result of deuteric alteration during the exhumation of the pluton (290 °C >  T  > 370 °C) in the Permian. After that, three tectonic events have been distinguished. The first tectonic event, attributed to the Mesozoic rifting, is characterized by precipitation of M3 and M4 phengite together with chlorite and Calcite C1 at temperatures between 190 and 310 °C. The second tectonic event attributed to the Paleogene compression has only been identified in the Hospital fault with precipitation of low-temperature Calcite C2. The shortcut produced during inversion of the Vallès fault was probably the responsible for the lack of neoformed minerals within this fault. Finally, the third tectonic event, which is related to the Neogene extension, is characterized in the Vallès fault by a new generation of chlorite, associated with Calcite C4 and laumontite, formed at temperatures between 125 and 190 °C in the absence of K-white mica. Differently, the Hospital fault is characterized by the precipitation of Calcite C3 during the syn-rift stage at temperatures around 150 °C and by low-temperature fluids precipitating Calcites C5, C6 and PC1 during the post-rift stage. During the two extensional events (Mesozoic and Neogene), faults acted as conduits for hot fluids producing anomalous high geothermal gradients (50 °C/km minimum).

Variscan veins: record of fluid circulation and Variscan tectonothermal events in Upper Palaeozoic limestones of the Moravian Karst, Czech Republic

Numerous Variscan syntectonic Calcite veins cross-cut Palaeozoic rocks in the Moravian Karst. A structural, petrographic and stable isotopic analysis of the Calcite veins and a microthermometric study of fluid inclusions in these vein cements have been carried out to determine the origin of the Variscan fluids and their migration during burial and deformation. The isotopic parameters of white (older, more deformed) and rose (younger) Calcites are: Sr-87/Sr-86 is between 0.7078 and 0.7082 (white) and 0.7086 (rose), delta O-18 is between +17.7 and +26.1 (white) and between +14.8 and +20.7 parts per thousand SMOW (rose), delta C-13 ranges from +0.1 to +2.5 (white) and from -0.3 to +1.6 parts per thousand V-PDB (rose). The isotopic signatures point to precipitation in an older fluid system buffered by the host rock (white Calcites) and to an open, younger fluid-dominated system (rose Calcites). Parent fluids (H2O-NaCl system) had salinities between 0.35 and 17.25 eq. wt% NaCl. The pressure-corrected and confined homogenization temperatures suggest formation of the Calcite veins from a fluid with a temperature between 120 and 170 degrees C, a pressure of 300-880 bar at a depth between 2.1 and 3.2 km. The fluids were most likely confined to a particular sedimentary bed as a bed-scale fluid migration (white older Calcite veins) or, later, to a pile of Palaeozoic sediments as a stratigraphically restricted fluid flow (rose younger Calcite veins). The low temperatures and pressures during precipitation of Calcites, which took place close to a peak of burial/deformation, confirm the distal position of the Moravian Karst region within the Variscan orogen.

oxygen isotopic fractionation during inorganic Calcite precipitation effects of temperature precipitation rate and ph

Abstract Stable oxygen isotopic fractionation during inorganic Calcite precipitation was experimentally studied by spontaneous precipitation at various pH (8.3  R − 2  h − 1 ) and temperatures (5, 25, and 40 °C) using the CO 2 diffusion technique. The results show that the apparent stable oxygen isotopic fractionation factor between Calcite and water ( α Calcite–water ) is affected by temperature, the pH of the solution, and the precipitation rate of Calcite. Isotopic equilibrium is not maintained during spontaneous precipitation from the solution. Under isotopic non-equilibrium conditions, at a constant temperature and precipitation rate, apparent 1000ln α Calcite–water decreases with increasing pH of the solution. If the temperature and pH are held constant, apparent 1000ln α Calcite–water values decrease with elevated precipitation rates of Calcite. At pH = 8.3, oxygen isotopic fractionation between inorganically precipitated Calcite and water as a function of the precipitation rate ( R ) can be described by the expressions 1000 ln α Calcite – water = – 1.102 log R + 34.56 1000 ln α Calcite – water = – 1.094 log R + 30.87 1000 ln α Calcite – water = – 0.534 log R + 26.80 at 5, 25, and 40 °C, respectively. The impact of precipitation rate on 1000ln α Calcite–water value in our experiments clearly indicates a kinetic effect on oxygen isotopic fractionation during Calcite precipitation from aqueous solution, even if Calcite precipitated slowly from aqueous solution at the given temperature range. Our results support Coplen’s work [Coplen T. B. (2007) Calibration of the Calcite–water oxygen isotope geothermometer at Devils Hole, Nevada, a natural laboratory. Geochim. Cosmochim. Acta 71, 3948–3957], which indicates that the equilibrium oxygen isotopic fractionation factor might be greater than the commonly accepted value.

sr2 ca2 and 44ca 40ca fractionation during inorganic Calcite formation ii ca isotopes

Ca isotope fractionation during inorganic Calcite formation was experimentally studied by spontaneous precipitation at various precipitation rates (1.8 < log R < 4.4 μmol/m2/h) and temperatures (5, 25, and 40 °C) with traces of Sr using the CO2 diffusion technique. Results show that in analogy to Sr/Ca [see Tang J., Kohler S. J. and Dietzel M. (2008) Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic Calcite formation: I. Sr incorporation. Geochim. Cosmochim. Acta] the 44Ca/40Ca fractionation during Calcite formation can be followed by the Surface Entrapment Model (SEMO). According to the SEMO calculations at isotopic equilibrium no fractionation occurs (i.e., the fractionation coefficient αCalcite-aq = (44Ca/40Ca)s/(44Ca/40Ca)aq = 1 and Δ44/40CaCalcite-aq = 0‰), whereas at disequilibrium 44Ca is fractionated in a primary surface layer (i.e., the surface entrapment factor of 44Ca, F44Ca < 1). As a crystal grows at disequilibrium, the surface-depleted 44Ca is entrapped into the newly formed crystal lattice. 44Ca depletion in Calcite can be counteracted by ion diffusion within the surface region. Our experimental results show elevated 44Ca fractionation in Calcite grown at high precipitation rates due to limited time for Ca isotope re-equilibration by ion diffusion. Elevated temperature results in an increase of 44Ca ion diffusion and less 44Ca fractionation in the surface region. Thus, it is predicted from the SEMO that an increase in temperature results in less 44Ca fractionation and the impact of precipitation rate on 44Ca fractionation is reduced.
A highly significant positive linear relationship between absolute 44Ca/40Ca fractionation and the apparent Sr distribution coefficient during Calcite formation according to the equation Δ44/40CaCalcite-aq=(−1.90±0.26)·logDSr−2.83±0.28is obtained from the experimental results at 5, 25, and 40 °C. Thus, Sr partitioning during Calcite formation directly reflects Ca isotopic fractionation, independent of temperature, precipitation rate, and molar (Sr/Ca)aq ratio of the aqueous solution. If the (Sr/Ca)aq ratio is constant, Δ44/40CaCalcite-aq values can be directly followed by the Sr content of the precipitated Calcite. A (Sr/Ca)aq ratio close to that of modern seawater yields the equation
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Our experimental results indicate that neither precipitation rate nor temperature dominantly controls Ca isotope fractionation. However, Ca isotopes and Sr content of inorganic Calcite comprise an excellent environmental multi-proxy in natural and applied systems.