Fluid circulation in the upper brittle crust: Thickness distribution, hydraulic transmissivity fluid inclusion and isotopic data of veins hosted in the Oligocene sandstones of the Macigno Formation in southern Tuscany, Italy

Abstract

We present structural analysis, fluid inclusion data on calcite and quartz, and isotopic composition of calcite forming veins occurring in the upper crustal level and hosted in Oligocene sandstone in southern Tuscany (Italy). The veins have been analysed in two sites few kilometres apart, along well-exposed coastal cliffs and in an abandoned quarry. These two sites were at a different depths at the time of the vein formation with a Δh ~ 100 m. Structural analysis of veins provided estimations of stress ratio ( Φ = ( σ 2 − σ 3)/( σ 1 − σ 3)), driving stress ratio ( R′ = ( Pf − σ 3)/( σ 1 − σ 3)) and fluid overpressure ( ΔP o = P f − σ 3) at the time of vein formation. The estimated ΔP o is in the range of 42–103 MPa, Φ = 0.24 and R′ = 0.45, indicating that fluid pressure was higher than the intermediate principal stress at the time of veins formation. The veins' thickness ( t) shows a clear power-law distribution ( D = 1.8835 and R 2 = 0.9762) in the lowermost site (coast) and a negative exponential distribution ( a = 0.6943 and R 2 = 0.9921) in the uppermost site (abandoned quarry). The vein thickness distributions have been used to compute the average transmissivity of the veins in the two sites. The computed transmissivity for the vein formation is ~ 10 −4 m 2 s −1, with higher values attained by the veins having negative exponential thickness distribution. Fluid inclusions studies highlighted that in both calcite and quartz, water-rich inclusions, with salinities of 2.2–4.3 wt.% NaCl equiv., and methane-rich inclusions were coevally trapped during fluid un-mixing processes. Thermogenic origin, from thermal maturation of organic matter present in the Macigno Formation, is proposed for methane. Whereas, the similarity between the δ 18 O (from 14.9 to 17.4‰) and δ 13 C (from −0.4 to −2.4‰) data of representative calcite veins and the isotopic composition ( δ 18 O: 16.1‰, δ 13 C: −1.0‰) of host-rock carbonate component, indicates that the fluid which formed calcite was in isotopic equilibrium with the carbonates present in the Oligocene sandstones. The calculated pressure–temperature conditions during the formation of these inclusions are prevalently within the 40–145 MPa and 160–260 °C ranges. The highest pressure values approximate the lithostatic pressure (~ 120 MPa) computed from geological data and are coherent with a geothermal gradient ranges of 35–45 °C/km. Whereas, the lower pressure values are comparable with hydrostatic pressure conditions. The pressure range indicated by fluid inclusion data is also comparable with the fluid pressure estimated from structural analysis. The considerable pressure range can be related to pressure cycling between lithostatic and hydrostatic conditions as a consequence of fault-valve actions and rock fracturing with subsequent pressure recover due to self-sealing process.