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

Author

Amy Stander, University of Rhode IslandFollow

Date of Award

2014

Degree Type

Thesis

Degree Name

Master of Science in Biological and Environmental Sciences (MSBES)

Specialization

Environmental and Earth Sciences

Department

Geosciences

First Advisor

Dawn Cardace

Abstract

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, Mg-rich) 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 Fe²⁺ (~23 to 70%), Fe³⁺ (~14 to 65%), and magnetite (~0 to 63%), which is a combination of Fe²⁺ and Fe³⁺. 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, ophiolite-hosted ultramafics.

Recommended Citation

Stander, Amy, "REDOX STATUS OF FE IN SERPENTINITES OF THE COAST RANGE AND ZAMBALES OPHIOLITES" (2014). Open Access Master's Theses. Paper 460.
https://digitalcommons.uri.edu/theses/460