Changing deformation style during natural serpentinite carbonation to talc-magnesite and quartz-magnesite

Strategies of underground carbon sequestration by CO₂ injection into ultramafic rocks at depth, inducing carbonation of Mg-silicates, face challenges to predict and monitor the evolution of reaction progress, fluid flow, and geo-mechanical responses. The fossil geological rock record of naturally carbonated mantle rocks allows to investigate the involved non-trivial coupling of thermal-hydrological-mechanical-chemical feedback processes across the necessarily large spatial and temporal scales.

To explore the interplay between carbonation reactions and deformation, we investigate the field- to micro-scale structures of a sequence of variably carbonated, serpentinized harzburgites from the Advocate complex of the Baie Verte Ophiolite, Newfoundland. The ultramafic rocks were progressively carbonated at 280 – 350 °C to brucite-magnesite bearing serpentinite, magnesite-talc rock and listvenite due to metamorphic fluid infiltration along a nearby fault zone [1].

Serpentinites show the recrystallization of lizardite to antigorite + brucite. This was related to semi-brittle fracturing and brucite-magnetite veins, together with oriented growth of antigorite. Incipient carbonation proceeded along the brucite veins and replacing remnant lizardite domains. Subsequently, reaction of antigorite with CO₂ to magnesite–talc rocks led to talc-rich domains that develop a penetrative foliation. Magnesite shows continued growth of Fe-zoned magnesite, commonly with euhedral facets. In places, talc fringes develop in strain shadows of magnesite grains, indicating that ductile deformation was assisted by dissolution-precipitation.

In contrast, the carbonation reaction talc + CO₂ to quartz–magnesite caused common semi-brittle deformation in listvenite. This is manifested by boudinage and sub-parallel sets of quartz extension veins mostly arranged normal to foliation and in oblique echelon arrays, consistent with syn-reaction shearing. At the outcrop scale, these veins cut listvenite layers and boudins, without continuation into talc-magnesite rock. At the microscale, similar quartz veins transect elongated magnesite porphyroblasts in magnesite-talc-quartz rock and foliated listvenite. Their termination at the porphyroblast rims together with co-precipitated magnesite along the vein-walls indicate that they formed synchronous to carbonation reaction. Strongly foliated transitions from talc-rich lithologies to listvenites further show apparent mylonitic fabrics with crystallographic preferred orientations with maxima of [001]_Mgs normal and [001]_Qtz parallel to foliation. This fabric is inconsistent with low-temperature (< 400°C) dislocation creep, but was likely caused by oriented growth under deviatoric stress. Fuchsite-filled stylolites in quartz-depleted listvenites further attest for prolonged deformation and permeability renewal by pressure solution. Our results indicate that, in line with with experimental evidence [2], carbonation is related to a changing deformation style with increasing reaction extent, from brittle veining in serpentinite, to ductile creep in talc and semi-brittle fracturing in listvenite, although dissolution-precipitation creep mechanisms are relevant during all stages. The studied case example underlines that deformation is a key factor for extensive carbonation. We further show that pressure solution can maintain permeability even in fully carbonated listvenites and may lead to nearly pure magnesite rocks.

 

Funding: RUSTED project PID2022-136471NB-C21 & C22 funded by MCIN/AEI/10.13039/501100011033 and ERDF – a way of making Europe. M.D.M further acknowledges ERC project OZ (grant: 101088573).

 

References:

[1] Menzel et al., 2018, Lithos, doi.org/10.1016/j.lithos.2018.06.001

[2] Eberhard et al., in review, Science Advances