Ultramafic Rock Carbonation: Constraints From Listvenite Core BT1B, Oman Drilling Project

Abstract

The occurrence of the quartz-carbonate alteration assemblage (listvenite) in ophiolites indicates that ultramafic rock represents an effective sink for dissolved CO 2. However, the majority of earlier studies of ultramafic rock carbonation had to rely on the surface exposure of reaction textures and field relationships. Here we present the first observations on ultramafic rock alteration obtained from the 300 m deep BT1B drill hole, ICDP Oman Drilling Project, allowing for a continuous and high-resolution investigation. Hole BT1B recovered continuous drill core intersecting surface alluvium, 200 m of altered ultramafic rock comprising mainly listvenite and minor serpentinite bands at 90 and 180 m depth, and 100 m of the underlying metamorphic sole. Textural evidence suggests that the carbonation of fully serpentinized harzburgite commenced by non-equilibrium growth of spheroidal carbonate characterized by sectorial zoning resulting from radially oriented low-angle boundaries. In the serpentinite, carbonate spheroids are composed of alternating magnesite cores and dolomite rims, whereas texturally similar carbonate in the listvenite is composed of Fe-rich magnesite cores and CaFe rich magnesite rims. The distinct compositions and mineral inclusions indicate that the carbonation extent was controlled by fluid accessibility resulting in the simultaneous formation of limited carbonate in the serpentinite bands and complete carbonation in the listvenite parts of BT1B. The presence of euhedral magnesite overgrowing spheroidal carbonate in the listvenite suggests near-equilibrium conditions during the final stage of carbonation. The carbonate clumped isotope thermometry constrains carbonate crystallization between 50°C and 250°C, implying repeated infiltration of reactive fluids during ophiolite uplift and cooling.

Brittle Deformation of Carbonated Peridotite—Insights From Listvenites of the Samail Ophiolite (Oman Drilling Project Hole BT1B)

“…Clasts are commonly rotated and rounded—in particular where cataclasites have been reactivated several times (Figure 3c). Magnesite mineral clasts locally show a chemical zonation of Fe contents—similar to magnesite in host listvenite (e.g., Beinlich, Plümper, et al, 2020; Lafay et al, 2020). The Fe zonation is truncated sharply at the rims of magnesite clasts (Figure 4b).…”

Brittle Deformation of Carbonated Peridotite – Insights from Listvenites of the Samail Ophiolite (Oman Drilling Project Hole BT1B)

“…The listvenites at Wadi Mansah are an outstanding natural example showing that peridotite carbonation reactions can go to completion on large scale (Falk and Kelemen, 2015). Where listvenite is in contact with serpentinized peridotite, the presence of intergrown antigorite + quartz ± talc suggests temperatures of 80 -130 °C, broadly consistent with clumped-isotope thermometry of carbonates (45 -245 °C; Falk and Kelemen, 2015;Beinlich et al, 2020), and the presence of microcrystalline quartz (recrystallized from opal or chalcedony) (Streit et al, 2012). An internal Rb -Sr isochron gives an age of 97 ± 29 Ma (Falk and Kelemen, 2015), in agreement with the age of emplacement of the ophiolite.…”

“…Listvenite not affected by overprinting (i.e. "primary listvenite") can be massive, containing spheroidal magnesite grains and pseudomorphs of the typical mesh and bastite textures of (Beinlich et al, 2020;Jöns and Bach, 2013;Lafay et al, 2020). In other parts of the core, primary listvenite has a well-developed foliation defined by lithological banding and at the microscale ellipsoidal carbonate particles with an iron oxide core.…”

“…Magnesite mineral clasts locally show a chemical zonation of Fe contentssimilar to magnesite in host listvenite (e.g. Beinlich et al, 2020;Lafay et al, 2020). The Fe zonation is truncated sharply at the rims of magnesite clasts (Fig.…”

Searching for biosignatures in sedimentary rocks from early Earth and Mars

“…When seen at the surface, the light-coloured magnesium carbonate and magnesium hydroxide minerals formed at low temperatures stand out from the ultramafic rocks. These precipitates can be millimetre to decimetre thin, as is the case of hydromagnesite and brucite crusts that cement the clasts on the scree slopes of some serpentinized ultramafic bodies, veins and mine openings 162,211,226 . The net export of Mg 2+ (aq) from ultramafic rocks in highlands to an adjacent basin at Gumigil-Kunwarara precipitated magnesite in two settings: in highlands as vein fills and in sedimentary lowlands as fine-grained cement and concretions 223,227 (Fig.…”