Micro‐scale chemical and physical patterns in an interface of hydrothermal dolomitization reveals the governing transport mechanisms in nature: Case of the Layens anticline, Pyrenees, France

Abstract

Hydrothermal dolomitization is an important diagenetic process that occurs in tectonic environments worldwide and forms conventional reservoirs associated with ore deposits and hydrocarbon accumulation, while forming efficient reservoirs for carbon sequestration. However, the current state of knowledge about the availability and reaction rate of Mg in dolomitizing fluids fails to explain the large volumes of hydrothermal dolomites geobodies observed in extended margins or in fold-and-thrust belts. To better understand this widespread phenomenon, it is essential to recognize the governing and limiting transport mode of the dolomitizing fluid. This contribution investigates the chemical and physical patterns developed between the original calcite and the newly formed dolomite. An extensive analytical study of well-preserved dolomitization interfaces observed at outcrop scale in Callovian-Oxfordian limestones in the Layens anticline (north-western Pyrenees, France) is presented. Through the use of scanning electron microscopy, electron backscattered diffraction, X-ray microtomography, laser ablation inductively coupled plasma mass spectrometry (and mapping), the replacement related variations in elementary content, rock density, crystallographic properties and phase volumes and distribution were constrained. The results indicate a sequence of replacement, beginning with a fluid which starts to infiltrate the host rock by advection in the grain boundary network causing at the same time the replacement of calcite by diffusion-limited dissolution and associated dolomite precipitation. The progressive replacement of calcite grains by dolomite is led by dissolution inside the grain enhanced by replacement related porosity creation, leading to a progressive decrease of local calcite grain size isolated as islands until the replacement is complete. The replacement of calcite by dolomite led to a mass loss without volume change, through generation of ca 11 vol.% porosity. Based on analytical observations of a natural sample, a conceptual model that accounts for the transport mode governing the different steps of hydrothermal dolomitization at crystal-scale is proposed.

Volcanic events‐related hydrothermal dolomitisation and silicification controlled by intra‐cratonic strike‐slip fault systems: Insights from the northern slope of the Tazhong Uplift, Tarim Basin, China

“…In a deep‐burial setting, dolomitisation and silicification related to tectonic‐hydrothermal activities are the most common diagenetic processes responsible for the enhancement of carbonate reservoir quality and thus have gained increasing attention (e.g., Centrella et al., 2020; Chen et al., 2015; Davies & Smith, 2006; Machel & Lonnee, 2002; Packard et al., 2001). The Tarim Basin is the largest superimposed basin in China (Kang & Kang, 1994), where significant oil and gas is produced from the Ordovician (e.g., Jia et al., 2008).…”

How the geochemistry of syn-kinematic calcite cement depicts past fluid flow and assists structural interpretations: a review of concepts and applications in orogenic forelands

“…Though beyond the scope of this review, it is worth mentioning that the fluid chemistry may itself impact fluid flow, for instance through dissolution or fluid-mediated replacement, that will affect the fluid dynamics down to the crystal scale. Classic examples of the latter in carbonates are dolomitization or apatitization processes, and recent experimental works (Jonas et al 2014;Pedrosa et al 2017;Weber et al 2021) and natural observations (Centrella et al 2021) highlighted the prominent role of grain boundaries and mineralogical defects like cleavage planes on the fluid flow at that scale (Fig. 3).…”

Isotope imaging of ultra-traces by LA-fs HR-ICP-MS for U-series dating (U/Th) of archaeological biominerals: how far can we go?

“…This femtosecond laser is coupled to an ICP-MS Element Xr Jet Interface (Thermo Fisher Scientific, Bremen, Germany). This single-sector field mass spectrometer has the specificity of a double focusing of the ions: this offers a better sensitivity and a background noise reduced by a factor of 10 compared to conventional ICP-MS systems, but it also leads to better detection limits and more accurate and reproducible measurements of the isotopic ratios [5,6,7]. This particular combination allows to produce maps of the isotopes relevant for U/Th dating ( 238 U, 235 U, 234 U, 230 Th and 232 Th), while necessitating less than 1 mg of material [8].…”