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Philip T Staudigel

and 7 more

The geochemical and ultrastructural properties of thermally altered skeletal carbonate are expected to be compromised to varying degrees by disequilibrium processes between solids and the ambient aqueous fluids. When assessing the alteration history of carbonates, it is important to apply models that quantitatively describe these diagenetic processes on multiple geochemical systems, such that they can be identified in natural samples. Carbonate clumped isotope analysis provides a unique tool for validating such models and can be combined with other geochemical tools/proxies to more comprehensively describe the processes and products. Here, we have analyzed bivalve shells that have undergone hydrothermal alteration (experimental diagenesis) in high water/rock ratio experiments at 130 and 160{degree sign}C, demonstrating that non-linear changes in ∆47 and ∆48 values can be attributed to heterogeneous replacement of precursor carbonates. Importantly, this model predicts decoupled ∆47 and ∆48 values, despite all reactions occurring at clumped isotope equilibrium with respect to the experimental temperature. We demonstrate that the rapid, thermally induced re-equilibration occurs in a “closed system” with minimal exchange with the ambient fluid, similar to the results of heating experiments conducted without an extraneous fluid. Later stages of alteration occur in an “open system” wherein internal fluid is exchanged with the external fluid at a similar rate to recrystallization and neomorphism. In these experiments, some oxygen from the original inorganic-organic composite-biomineral is inherited, indicating restrictions on the availability of fluid oxygen. Our experiments and models validate a novel application for dual-clumped isotopes for reconstructing hydrothermal temperatures and fluid δ18O compositions.
Carbonate platform background fracture networks are rarely utilized in subsurface models, and it is unclear how they relate to regional stress (other than faults and folds) and burial. We combine structural analysis and drone imagery to investigate the geometry, kinematics, and topological characteristics of (background) fractures at the Latemar platform (both limestones and dolostones; Northern Italy). Deformation was accommodated by a dense network of mode I and conjugate hybrid fractures/veins and conjugate reverse faults, all associated with sub-vertical stylolites. Conjugate fractures and associated sub-vertical stylolites are organized in two systems, constraining the major stress fields. Differences lie in the permutation of the position in the space of the principal stress with depth. Specific burial depth windows are significant in distinguishing the different spatial positions of the principal stresses. Changes in overburden provide the major driving factor in determining the position of background structures that develop during the burial trajectory. Topologically, background fractures in lime- and dolostone pavements show distinct characteristics. In limestone pavements, fractures form a network with a high proportion of I-node and I-C to C-C branches, resulting in a low to moderate connectivity (i.e., CB = ~ 1.5). In dolostones, a complex network with abundant Y-to X- nodes and I-C to C-C branches is found (moderate to a high degree of connectivity CB = ~ 1.7). Topological pathways provide important insights into how background fractures are connected and shed light on the significance of these features in the context of subsurface fluid flow.