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Although, the reduced status of the Earth’s upper mantle is a possible controller of the deep, rock-hosted biosphere, knowledge of the redox state of the mantle is incomplete. Peridotites (mantle rocks) are composed of ultramafic (Fe, Mgrich) minerals such as olivine and pyroxene. During serpentinization, water and ultramafic minerals react, generating a package of secondary minerals dominated by serpentine. This releases hydrogen gas in amounts dependent on system geochemistry and largely controlled by the Fe(II) budget in the protolith, as well as other products. Microbial life can be fueled by the hydrogen produced by serpentinization in environments that are generally not regarded as hospitable to life—cool, dark, low energy, subseafloor settings. Peridotite-hosted vents in the seabed and springs in continental ophiolites reveal active microbial communities at work in these distinctive serpentinization-associated waters. In this study, 16 variably serpentinized peridotite samples from the Coast Range Ophiolite (CRO) (11 core samples and one hand sample) and Zambales Ophiolite (ZO) (four hand samples) were selected for study based on mineralogy. The objective of this study was to understand better the redox status of Fe in these rocks and produce possible H2 generation values for the CRO and ZO. Each sample was analyzed using X-ray diffraction and thin sections (when available) to identify possible Fe bearing minerals (olivine, spinel, serpentine, pyroxene, magnetite, other Fe-oxides). X-ray fluorescence was used to obtain the bulk concentration of Fe in each sample (~28,000 to 51,000 ppm (~3.7 to 6.5 wt% FeO)). Mössbauer spectroscopy was used to determine the percentage of total Fe that is Fe 2+ (~23 to 70%), Fe 3+ (~14 to 65%), and magnetite (~0 to 63%), which is a combination of Fe 2+ and Fe 3+ . The data sets were integrated into a hydrogen generation model. I assumed that each sample was representative of the peridotite units of the corresponding ophiolite. This permitted computation of a range of total hydrogen production possible by the peridotite considered, until serpentinization is complete (~900 to 4800 Tmol H2 or ~2000 to 12,735 Tmol H2 if density is factored into the calculation). The CRO can produce less H2 per rock volume than the ZO because the CRO samples generally have a lower Fe concentration, but the CRO has a greater volume and can produce a larger total amount of H2. Variability in bulk rock Fe concentration and Fe valence states in samples taken in close proximity indicate diverse serpentinization reaction paths even in a single ultramafic unit. Tectonics, emplacement history, age, climate, composition, and hydrology of the ophiolite all influence the redox status in the modern, ophiolitehosted ultramafics.

REDOX STATUS OF FE IN SERPENTINITES OF THE COAST RANGE AND ZAMBALES OPHIOLITES