Olivine enrichment in dehydration veins in serpentinites by reactive fluid flow

<p>Serpentinite dehydration in subduction zones plays an important role in Earth’s deep water cycle. In order to keep this water cycle in balance, an efficient rock dehydration mechanism at depth is needed to keep pace with loss of ocean water due to subduction of hydrated oceanic lithosphere. Field observations in non-deformed meta-serpentinites in Erro Tobbio, Ligurian Alps, show that serpentinite dehydration at depth occurs by a channelized vein network rather than pervasive flow. The mineral assemblage in the veins is characterized by a high abundance of metamorphic olivine. Plümper et al. (2017) showed that on small scales (μm-mm) the formation of these veins is controlled by intrinsic chemical heterogeneities in the rock. Field observations suggest that on larger scales the fluid escape is governed by mechanical processes such as hydraulic fracturing. On small scales, where dehydration is chemically controlled, reactive fluid flow is an important process because changes in the fluid chemistry may trigger or hinder further dehydration reactions in the rock. Because of its high solubility and high abundance as a rock forming component, Si might be a key metasomatic agent for first-order effects on the dehydration process.</p><p>Following the approach of Beinlich et al. (2020) we extended the model of Plümper et al. (2017) to a reactive fluid flow model for serpentinite dehydration that accounts for the Si content of the fluid. As input for our model we use mineral chemical data of non-dehydrated serpentinites from the Mirdita ophiolite in Albania that are representative for serpentinized oceanic lithosphere that enters a subduction zone, hence has not experienced any subduction-related metamorphic processes. The results of our model suggest that the high abundance of metamorphic olivine observed in the Erro Tobbio meta-serpentinites hence the purification towards a olivine-dominated assemblage is the result of interaction with an external fluid in the veins after they have been formed from the intrinsic chemical heterogeneities.</p><p><strong>References</strong></p><ul><li>Beinlich, A. et al. (2020). “Instantaneous rock transformations in the deep crust driven by<br>reactive fluid flow”. In: Nature Geoscience 13.4, pp. 307–311. doi: 10.1038/s41561-<br>020-0554-9.</li> <li>Plümper, O. et al. (2017). “Fluid escape from subduction zones controlled by channel-<br>forming reactive porosity”. In: Nature Geoscience 10.2, pp. 150–156. doi: 10.1038/<br>NGEO2865.</li> </ul>

Reactive flow and homogenization in anisotropic media

<p>Dissolution by reactive fluid flow is a fundamental process in geological systems. It controls diagenesis and karst evolution and has broad implications for groundwater hydrology. Specifically, reactive flow controls the evolution of the void-space structure via the feedback between the reaction and transport. In some instances, advective transport rate is high compared to that of geochemical reactions (low Damkӧhler number, Da), such that the reactive fluid penetrates the system before its reactivity is exhausted, resulting in a relatively spatially-uniform dissolution. Despite the importance of low Da conditions, the emerging transformations in the medium structure, flow field, and its bulk properties are not well understood. Likewise, our ability to decipher diagenetic history and preexisting structure is lacking.</p><p>Here, using a network model, we investigate the evolution of heterogeneous and anisotropic medium during dissolution at low Da conditions. The numerical simulations show that the medium progressively becomes more homogeneous as well as isotropic, which consequently makes the flow field more uniform. Homogenization is particularly notable for anisotropic media, in which the transverse channels are wide relative to the channels parallel to the main flow direction. In this case, flow is initially focused within a few highly tortuous pathways, hence emphasizing the effect of dissolution on flow heterogeneity and tortuosity. The homogenization process is further enhanced when the surface reaction is transport-controlled—that is, when diffusion of dissolved ions away from the mineral surface to the bulk fluid is slow, reducing the reactivity adjacent to the surface: At first, since diffusive transport is more effective in narrow channels, they undergo faster dissolution, which selectively enlarges them leading to an initial steep rise in permeability. Later, however, as dissolution proceeds and the channels broaden, the overall dissolution rate drops, diminishing the growth rate of permeability. Our findings provide fundamental insights into reactive transport and hydrogeological processes in fractured and porous media.</p>