Carbon Geochemistry of the Active Serpentinization Site at the Wadi Tayin Massif: Insights From the ICDP Oman Drilling Project: Phase II

Abstract

A large part of the hydrated oceanic lithosphere consists of serpentinites exposed in ophiolites. Serpentinites constitute reactive chemical and thermal systems and potentially represent an effective sink for CO2. Understanding carbonation mechanisms within ophiolites are almost exclusively based on studies of outcrops, which can limit the interpretation of fossil hydrothermal systems. We present stable and radiogenic carbon isotope data that provide insights into the isotopic trends and fluid evolution of peridotite carbonation in ICDP Oman Drilling Project drill holes BA1B (400‐m deep) and BA3A (300‐m deep). Geochemical investigations of the carbonates in serpentinites indicate formation in the last 50 kyr, implying a distinctly different phase of alteration than the initial oceanic hydration and serpentinization of the Samail Ophiolite. The oldest carbonates (∼31 to >50 kyr) are localized calcite, dolomite, and aragonite veins, formed between 26°C and 43°C and related to focused fluid flow. Subsequent pervasive small amounts of dispersed carbonate precipitated in the last 1,000 years. Macroscopic brecciation and veining of the peridotite indicate that carbonation is influenced by tectonic features allowing infiltration of fluids over extended periods and at different structural levels such as along fracture planes and micro‐fractures and grain boundaries, causing large‐scale hydration of the ophiolite. The formation of dispersed carbonate is related to percolating fluids with δ18O lower than modern ground and meteoric water. Our study shows that radiocarbon investigations are an essential tool to interpret the carbonation history and that stable oxygen and carbon isotopes alone can result in ambiguous interpretations.

Mineralogical and Geochemical Characterization of the Sta. Cruz Nickel Laterite Deposit, Zambales, Philippines

“…The associated volume expansion can both seal existing fractures in the system and induce new fractures at the same time. These tension cracks or cross-fractures [56] are formed during earlier, moderately high-temperature (>200 • C) serpentinization, although we recognize that this process may continue at lower temperatures (<100 • C) [58][59][60][61][62][63][64]. Relatively high Ni content in the least.…”

An untargeted exometabolomics approach to characterize dissolved organic matter in groundwater of the Samail Ophiolite

“…Similarly, relatively high abundances of features annotated as intermediates of propanoate degradation in the hyperalkaline wells raise the intriguing possibility that the abiotic production of propanoate could serve as a carbon source for the microbial communities found there ( Shock and Schulte, 1998 ). Recent carbon analysis of serpentinized rocks obtained in the Oman Drilling Project shows that non-carbonate-carbon loadings are approximately 140–360 ppm ( Ternieten et al, 2021 ). Thus, there may be multiple primary sources of organic carbon in this system: autotrophic carbon fixation and turnover of biomass, heterotrophic utilization of reduced carbon accumulated in the rock during prior water/rock interaction, or ongoing abiotic organic synthesis.…”

Gas Migration Episodes Observed During Peridotite Alteration in the Samail Ophiolite, Oman

“…A variety of petrological, hydrological, geochemical, and microbiological investigations have been carried out for borehole BA1B (e.g., Cocomazzi et al., 2020 ; Ellison et al., 2021 ; Kelemen et al., 2021 ; Ternieten et al., 2021 ), providing important context for interpreting our results. We cannot determine the composition of the bubbles we detected on the basis of our acoustic data, but peridotite alteration (Nothaft et al., 2021 ), and microbial processes supported by alteration products (Schrenk et al., 2013 ), can both generate H 2 and CH 4 gas.…”

“…Vein filling alteration products in the upper part of the formation include both carbonate and serpentinite minerals. Low‐temperature alteration of previously altered harzburgite in this near‐surface environment has been identified as a means to produce H 2 gas through the replacement of Fe(II)‐rich brucite with Fe(III)‐bearing serpentine (Kelemen et al., 2021 ; Ternieten et al., 2021 ). Additionally, in some of the MBO boreholes, the partial pressure of oxygen (oxygen fugacity, f O 2 ) recorded by both mineral assemblages and borehole waters at depths of 200–400 m below the surface is on the order of 10 −80 bars (Kelemen et al., 2021 ), which is at the limit where H 2 O breaks down to form H 2 .…”