Changes in fluid pathways in a calcite vein mesh (Natih Formation, Oman Mountains): insights from stable isotopes

Abstract

We present a structural, microstructural, and stable isotope study of a calcite vein mesh within the Cretaceous Natih Formation in the Oman Mountains to explore changes in fluid pathways during vein formation. Stage 1 veins form a mesh of steeply dipping crack‐seal extension veins confined to a 3.5‐m‐thick stratigraphic interval. Different strike orientations of Stage 1 veins show mutually crosscutting relationships. Stage 2 veins occur in the dilatant parts of a younger normal fault interpreted to penetrate the stratigraphy below. The δ18O composition of the host rock ranges from 21.8‰ to 23.7‰. The δ13C composition ranges from 1.5‰ to 2.3‰. This range is consistent with regionally developed diagenetic alteration at top of the Natih Formation. The δ18O composition of vein calcite varies from 22.5‰ to 26.2‰, whereas δ13C composition ranges from −0.8‰ to 2.1‰. A first trend observed in Stage 1 veins involves a decrease of δ13C to compositions nearly 1.3‰ lower than the host rock, whereas δ18O remains constant. A second trend observed in Stage 2 calcite has δ18O values up to 3.3‰ higher than the host rock, whereas the δ13C composition is similar. Stable isotope data and microstructures indicate an episodic flow regime for both stages. During Stage 1, formation of a stratabound vein mesh involved bedding‐parallel flow, under near‐lithostatic fluid pressures. The 18O fluid composition was host rock‐buffered, whereas 13C composition was relatively depleted. This may reflect reaction of low 13C CO2 derived by fluid interaction with organic matter in the limestones. Stage 2 vein formation is associated with fault‐controlled fluid flow accessing fluids in equilibrium with limestones about 50 m beneath. We highlight how evolution of effective stress states and the growth of faults influence the hydraulic connectivity in fracture networks and we demonstrate the value of stable isotopes in tracking changes in fluid pathways.

Solid bitumen in calcite veins from the Natih Formation in the Oman Mountains: Multiple phases of petroleum migration in a changing stress field

“…Mechanical twinning is common in veins in the study area; (Gomez-Rivas et al, 2014;Arndt et al, 2014) only the last generations of calcite veins are undeformed.…”

“…They were studied in two outcrops: (1) close to the village Al Raheba west of the road leading to Jebel Shams, located approximately 200 m north east of the vein mesh described by Arndt et al (2014) and (2) in Wadi Ghul between the entrance of Wadi Nakhr and the road to Sint (Tab. 1).…”

Multiphase Structural Evolution of a Continental Margin During Obduction Orogeny: Insights From the Jebel Akhdar Dome, Oman Mountains

“…These studies include reconstructions of the kinematics of the obducted nappes (Béchennec et al, ; Cooper, ; Searle, ) and address timing and kinematic framework of ophiolite obduction, which started with oceanic subduction (e.g., El‐Shazly & Sisson, ; Ghent & Stout, ; Hacker & Mosenfelder, ; Searle & Cox, ). The Jebel Akhdar window exposes lower grade metamorphic rocks, and here the tectonic evolution has been studied less comprehensively (Arndt et al, ; Breton et al, ; Cooper et al, ; Gomez‐Rivas et al, ; Hilgers et al, ; Holland, Urai, et al, ; Le Metour et al, ; Mozafari et al, ; Virgo, Arndt, et al, ). Most studies focused on the southern flank of the Jebel Akhdar Dome (Figure ) and recognized a series of multiple deformation phases based on overprinting relations between veins, faults, and stylolites.…”

“…At least seven consecutive deformation phases have been identified. Stable isotope analysis of different vein generations revealed two calcite sources: early veins (Generation I–III of Virgo, ) show host rock buffered stable isotope signatures suggesting a local calcite source (Arndt et al, ; Hilgers et al, ). This is corroborated by the ubiquitous occurrence of pressure solution features such as stylolites.…”

Chemostratigraphy of the Upper Albian to mid‐Turonian Natih Formation (Oman) – how authigenic carbonate changes a global pattern

“…A plot of all available carbon‐isotope data (Fig. ) from the Natih Formation (Wagner, ; Zhao et al ., ; Vahrenkamp, ; Arndt et al ., and this study) points at differences in absolute values. Large variations in amplitude can also be observed.…”