Low Temperature Serpentinite Replacement by Carbonates during Seawater Influx in the Newfoundland Margin

Abstract

Serpentinite replacement by carbonates in the seafloor is one of the main carbonation processes in nature providing insights into the mechanisms of CO2 sequestration; however, the onset of this process and the conditions for the reaction to occur are not yet fully understood. Preserved serpentine rim with pseudomorphs of carbonate after serpentine and lobate-shaped carbonate grains are key structural features for replacement of serpentinite by carbonates. Cathodoluminescence microscopy reveals that Ca-rich carbonate precipitation in serpentinite is associated with a sequential assimilation of Mn. Homogeneous δ18O values at the µm-scale within grains and host sample indicate low formation temperature (<20 °C) from carbonation initiation, with a high fluid to rock ratio. δ13C (1–3 ± 1‰) sit within the measured values for hydrothermal systems (−3–3‰), with no systematic correlation with the Mn content. δ13C values reflect the inorganic carbon dominance and the seawater source of CO2 for carbonate. Thermodynamic modeling of fluid/rock interaction during seawater transport in serpentine predicts Ca-rich carbonate production, at the expense of serpentine, only at temperatures below 50 °C during seawater influx. Mg-rich carbonates can also be produced when using a model of fluid discharge, but at significantly higher temperatures (150 °C). This has major implications for the setting of carbonation in present-day and in fossil margins.

Initial Results From the Oman Drilling Project Multi‐Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon

“…Second, it has been proposed that formation of carbonate minerals via reaction of surface waters with peridotite can produce a significant carbon reservoir on this and other planets, and could potentially be emulated in accelerated systems for engineered carbon capture and storage (e.g., Kelemen & Matter, 2008). There have been numerous, related studies of carbon mineralization during natural weathering of peridotite in the Samail ophiolite Falk et al, 2016;Giampouras et al, 2020;Kelemen et al, 2011;Lacinska et al, 2014;Lacinska & Styles, 2013;Leleu et al, 2016;Mervine et al, 2014Mervine et al, , 2015Noël et al, 2018;Paukert Vankeuren et al, 2012, 2019Rajendran et al, 2014;Rajendran & Nasir, 2019;Streit et al, 2012), elsewhere on Earth (e.g., Alt et al, 2012Alt et al, , 2013Andrews et al, 2018;Arcilla et al, 2011;Beinlich et al, 2010;del Real et al, 2016;Gahlan et al, 2020;Manuella et al, 2019;Morrissey & Morrill, 2017;Oskierski, Bailey, et al, 2013;Oskierski et al, 2013aOskierski et al, , 2013bPicazo et al, 2020;Quesnel et al, 2013Quesnel et al, , 2016Sánchez-Murillo et al, 2014;Schwarzenbach et al, 2013Schwarzenbach et al, , 2016Ulrich et al, 2014), andon Mars (e.g., Edwards &Ehlmann, 2015;…”

Carbonate Mineralogy in Mantle Peridotites of the Atlantis Massif (IODP Expedition 357)

“…Previous studies of calcite cements formed in shallow fresh‐water aquifers provide evidence that the uptake of Mn 2+ in carbonates is mainly dependent on Eh, which controls Mn solubility (Barnaby & Rimstidt, 1989; Dromgoole & Walter, 1990). Most marine low‐temperature carbonates have MnO contents of ∼1 wt% (e.g., Eickmann, Bach, Rosner, & Peckmann, 2009; Picazo et al., 2020). At the Atlantis Massif, carbonates exhibit remarkably high and variable MnO concentrations, and we argue that this is caused by a change in the redox state of the hydrothermal fluid.…”

Oxygen isotopes in ophicalcites: an ever-lasting controversy?

“…Data from Weissert and Bernoulli (1984), Früh-Green et al (1990), Pozzorini and Früh-Green (1996. Compilation of values from the Mid Atlantic Ridge (MAR) and from the Iberian and Newfoundlad Margins (IB, NFL) after Picazo et al (2020) Morgan 1996), and oxygen isotope values of calcites from Cretaceous ophicalcites of the Newfoundland Margin cluster in a relatively restricted range of 30.8 to 32.6‰ δ 18 O V-SMOW for replacive calcite grains and at 28 ‰ δ 18 O V-SMOW in calcite veins (Picazo et al 2020). Indeed, all cited examples from the Atlantic Ocean formed near or at the sea floor and at low temperatures (< 20 °C), even where syn-and postrift magmatism occurs as in the Platta unit.…”

“…Data from Weissert and Bernoulli ( 1984 ), Früh-Green et al ( 1990 ), Pozzorini and Früh-Green ( 1996 ). Compilation of values from the Mid Atlantic Ridge (MAR) and from the Iberian and Newfoundlad Margins (IB, NFL) after Picazo et al ( 2020 ) …”

“…The same trend in O-isotope values has been recognized long ago in ophicalcites and overlying oceanic sediments in the Apennines where the O-isotope values become more negative from the more metamorphic Ligurian units in the north to their less metamorphic counterparts in the south (Bonatti et al 1974 ; Barberi et al 1979 ). The ultramafic carbonate breccias from the Mid-Atlantic Ridge vary from 2.5 to 5 ‰ δ 18 O V-PDB or 33.5–36 ‰ δ 18 O V-SMOW (Bonatti et al 1974 ); fracture-filling calcites from the Iberian distal margin from − 0.8 to 1 ‰ δ 18 O V-PDB or around 30 ‰ δ 18 O V-SMOW (Millikan and Morgan 1996 ), and oxygen isotope values of calcites from Cretaceous ophicalcites of the Newfoundland Margin cluster in a relatively restricted range of 30.8 to 32.6 ‰ δ 18 O V-SMOW for replacive calcite grains and at 28 ‰ δ 18 O V-SMOW in calcite veins (Picazo et al 2020 ). Indeed, all cited examples from the Atlantic Ocean formed near or at the sea floor and at low temperatures (< 20 °C), even where syn- and post-rift magmatism occurs as in the Platta unit.…”