Carbon and mineral transformations in seafloor serpentinization systems

Abstract

This thesis examines abiotic processes controlling the transformation and distribution of carbon compounds in seafloor hydrothermal systems hosted in ultramafic rock. These processes have a direct impact on carbon budgets in the oceanic lithosphere and on the sustenance of microorganisms inhabiting hydrothermal vent ecosystems. Where mantle peridotite interacts with carbon-bearing aqueous fluids in the subseafloor, dissolved inorganic carbon can precipitate as carbonate minerals or undergo reduction by H 2(aq) to form reduced carbon species. In Chapters 2 and 3, I conduct laboratory experiments to assess the relative extents of carbonate formation and CO 2 reduction during alteration of peridotite by CO 2(aq)-rich fluids. Results from these experiments reveal that formation of carbonate minerals is favorable on laboratory timescales, even at high H 2(aq) concentrations generated by serpentinization reactions. Although CO 2(aq) attains rapid metastable equilibrium with formate, formation of thermodynamically stable CH 4(aq) is kinetically limited on timescales relevant for active fluid circulation in the subseafloor. It has been proposed that CH 4 and potentially longer-chain hydrocarbons may be sourced, instead, from fluid inclusions hosted in plutonic and mantle rocks. Chapter 4 analyzes CH 4-rich fluid inclusions in olivine-rich basement rocks from the Von Damm hydrothermal field and the Zambales ophiolite to better understand the origin of abiotic hydrocarbons in ultramaficinfluenced hydrothermal systems. Comparisons of hydrocarbon abundances and stable isotopic compositions in fluid inclusions and associated vent fluids suggest that fluid inclusions may provide a significant contribution of abiotic hydrocarbons to both submarine and continental serpentinization systems.

Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions

“…As is the case for submarine hydrothermal systems, derivation of abiotic CH 4 by leaching from fluid inclusions is consistent with available constraints in continental serpentinization sites. Helium isotopes and stable carbon isotopic compositions of CH 4 in fluid inclusions overlap with those measured at continental CH 4 seeps (1,6). Moreover, fluid inclusions were formed in the distant geological past (>50 ka) and therefore CH 4 is likely radiocarbon-free, as is observed in CH 4 from continental seeps (1).…”

“…Leaching of olivinehosted fluid inclusions represents a mechanism for the addition of abiotic CH 4 to circulating hydrothermal fluids in submarine serpentinization systems that is not at odds with available geological and geochemical constraints (2, 3, 6). Methane trapped in fluid inclusions has carbon isotopic compositions consistent with those of abiotic CH 4 in submarine serpentinization systems (6). Because CH 4 is formed in isolation from circulating vent fluids, the carbon required for CH 4 formation is not derived from circulating fluids, in keeping with measured vent fluid compositions (3).…”

“…A rock containing 10 5 inclusions per cm 3 would contain 2.5 to 363 nmol CH 4(g) per gram of olivine. For comparison, recent whole rock analyses of partially serpentinized peridotite from the Mid-Cayman Rise and the Zambales ophiolite indicate minimum CH 4(g) contents of 2 to 37 nanomoles (nmol) per gram of rock (6). If it is assumed that peridotite constitutes at least 5% of the seafloor in the Atlantic (12), Arctic, and Indian Oceans, and that 77% of the peridotite contains 75 wt.…”

“…Gabbro is the dominant rock type in the lower oceanic crust (12) and, as the present study demonstrates, olivine in gabbro can contain abundant CH 4(g) -rich secondary fluid inclusions. Grozeva et al (6) reported minimum concentrations of 72 to 310 nmol CH 4(g) per gram of gabbro in fluid inclusions hosted in olivine, plagioclase, and clinopyroxene. If we assume that gabbro occurs in least 50% of the area in the Atlantic, Arctic, and Indian Oceans, that gabbro contains at least 72 nmol CH 4(g) g −1 (6), and that the thickness of the gabbro layer at slow-spreading ridges, although highly variable, is close to 4 km on average (13,14), the total amount of CH 4(g) contained in the lower oceanic crust is on the order of 4.8 Pg.…”

“…Recently, McDermott et al (3) used carbon isotopic and mass balance constraints to demonstrate that P CO 2 reduction by H 2 does not yield CH 4 during convection of hydrothermal fluids at the Von Damm hydrothermal field, suggesting that abiotic CH 4 formation and convective seawater circulation are decoupled. This challenged the paradigm of significant abiotic CH 4 formation during active fluid circulation and led to the suggestion that abiotic CH 4 observed in deep-sea hydrothermal fluids associated with ultramafic rocks may be leached from fluid inclusions (3,(6)(7)(8). Many important questions remain regarding fluid inclusion prevalence, formation, internal fluid-mineral interaction, and their contributions of CH 4 to venting fluids and global reservoirs.…”