Romain Lafay's seminar, Géosciences Montpellier

An intimate association between serpentinization and carbonation during the weathering of mantle rocks

Romain Lafay's seminar, 2019

Abstract

The study of ophiolites and the "in-situ" sampling of oceanic systems make it possible to understand the complexity of the processes of alteration of the oceanic lithosphere, particularly mantle rocks. Many questions remain about the reactions involved, the intensity of the alteration of the lithospheric mantle, or the role of metasomatic reactions on rock fracturing, for example.
Since fluids are a driving force in the development of mineralogical reactions operating in the lithosphere, it is important to understand their distribution and the role of their composition in the processes of alteration of oceanic rocks in divergent and convergent systems. One way to do this is to characterize the mineralogical reactions affecting ultramafic rocks.
Carbonation and serpentinization are two major alteration reactions often considered independently. The temperature and the composition of the reaction fluids are two major parameters that explain the distribution of these two alteration reactions in the lithosphere. However, in different contexts, these two reactions can be expressed jointly or in the same alteration sequence. It appears that the fluid / rock interaction processes and the reactions that characterize them are still difficult to define, even in relatively simple systems.
Here we will focus on the alteration of the base of ophiolite from Samail in Oman. This alteration is the expression of a large-scale metasomatism associated with the infiltration of carbonate-rich fluids during the obduction of oceanic rocks. The complexity of the carbonate rocks (listvenites and ophicarbonates) and hydrated rocks (serpentinites) in the basal portion of the samal ophiolite was revealed by drilling the Oman Drilling Project through the ophiolite and its metamorphic base. The preliminary description of the rocks on board the Chikyu and the petrographic and geochemical study presented here illustrate 1) the importance of mantle heritage (heterogeneity / fertility) on development
metamorphic assemblages, 2) contamination related to infiltration of fluids from the metamorphic base, and 3) continuity between serpentinization and carbonation reactions.
The observations made here question the positive feedback between carbonation and serpentinization and the development of serpentinization fronts during the carbonation of ultramafic rocks. This has great consequences for estimating the evolution of the properties (porosity, permeability, reactivity, rheology) of mantle rocks subject to fluid / rock interactions.
The ophiolite of Samail is no exception, the association between hydration and carbonation reaction being recognized in other oceanic and ophiolitic systems. A future challenge is to establish precisely the mass balances characterizing its metamorphic reactions. This is essential for understanding fluid transfers between large terrestrial reservoirs.