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2015 Annual Science Report

University of Colorado, Boulder Reporting  |  JAN 2015 – DEC 2015

Deciphering the Mineralogy and Geophysical Properties of Serpentinized Rocks

Project Summary

The transformation of mineral phases during water-rock interaction provides critical energy sources for life yet the reaction processes under low temperature conditions are enigmatic. Thus a key goal of the RPL NAI is to decipher the geochemical reaction path history recorded in rocks that have interacted with water at low temperatures. To that end, we are developing and optimizing analytical procedures for quantifying mineralogy, Fe oxidation state, and rock magnetism at the microscale. To date, we are applying an integrated suite of microsopic and spectroscopic techniques to mafic and ultramafic rocks that have undergone low temperature alteration in two ophiolite systems — surface and subsurface serpentinites from Oman and drill core material from the California Coast Range Microbial Observatory. The resulting integration of mineralogical and chemical images is critical in efforts to unravel the sequence of water/rock reactions. In future years, our approaches will be applied to a diversity of rocks to be recovered by drilling at sites undergoing active, low temperature reaction, such as the Atlantis Massif and the Oman ophiolite, as well as samples that will be generated from laboratory-based water/rock reaction experiments.

4 Institutions
3 Teams
0 Publications
3 Field Sites
Field Sites

Project Progress

In Year 1, research focused on surface serpentinite and drill chips from the Oman ophiolite and serpentinite core samples from CROMO that were currently available for study. Mayhew, Miller, Ellison, and Templeton worked to develop and optimize a protocol for complementary and integrated bulk and microscale geochemical data collection. Briefly, subsamples of rocks were powdered for bulk mineralogical identification by x-ray diffraction (XRD) and for assessment of the overall oxidation state of iron by Fe specific synchrotron radiation based x-ray absorption near-edge structure (XANES) spectroscopy. Thin sections were prepared and analyzed by QEMSCAN (combined backscatter electron signal and energy-dispersive x-ray spectrum) for a qualitative, large-scale imaging of mineralogy. Using the distribution and spatial information from QEMSCAN, smaller areas of interest were identified for finer scale Raman spectroscopy to obtain a more quantitative and detailed understanding of the mineralogy and spatial relationships between phases. Hyperspectral Raman mapping revealed the existence of rare, yet potentially key, reactive phases.

In parallel, Cardace has trained students on sample preparation and data collection for XRD and microFTIR. Testing of microFTIR methods have been carried out and will be applied to investigating organic compound distributions in vein filling secondary minerals in serpentinites. The integration of microscale mineralogical and organic analyses will be a powerful approach to understanding water-rock-microbe associations. Currently, rock wafers are being described prior to microbial colonization experiments, to be carried out with CROMO groundwaters. Results are expected to clarify which mineral types and possibly crystallographic parameters control microbial distribution on serpentinites and/or facilitate epilithic biofilm formation.

Synchrotron radiation based x-ray fluorescence mapping and XANES analyses optimized for high resolution around the pre-edge feature of the spectra allow for quantitative assessment of the oxidation state of Fe in specific mineral phases. Mayhew, Miller, Ellison, and Templeton developed a variogram for quantitative assement of the Fe3+/FeTotal in serpentine specific to the samples from Oman and CROMO building on prevous work by Wilke et al. (2001) and Andreani et al., (2013). These analyses are conducted on regions of interest identified by other techniques and are integrated with electron microprobe wave-length dispersive spectral data that measures the Fe content of key mineral phases. These analyses were also integrated with rock magnetic measurements of Oman well chips conducted by Tominaga and Ruchala. One notable outcome from the integration of these data sets was direct evidence for the extensive conversion of brucite to serpentine and magnetite as a function of depth in the Oman aquifer, which forms the basis of a new hypothesis for low-temperature H2 generation that is now presented in Miller et al. (2016).

In 2015, Mayhew submitted an application to be a member of the Science Party for IODP Expedition 357 – Atlantis Massif (AM): Serpentinization and Life. The AM is one of the three serpentinizing systems field sites identified in the RPL NAI proposal and these samples are ideal for deciphering the mineralogy and reaction history of rocks undergoing low temperature alteration in a submarine setting. Mayhew’s application was accepted and she will travel to Bremen, Germany in January 2016 to describe, archive, and sample core.

In December 2015, Mayhew and Templeton prepared and submitted applications to be a member of the Oman Drilling Project Science Party to obtain samples of deep subsurface rocks actively undergoing low temperature alteration in a terrestrial ophiolite sequence. The acquisition of serpentinite cores from Oman will allow for greater comparison of alteration and Fe transformation processes between low temperature serpentinizing systems (the Samail ophiolite, the Coast Range Ophiolite (CRO) in CA, USA, and the Atlantis Massif (AM) marine serpentinites) and will by coupling the core analysis with aqueous geochemistry and dissolved gases, our analyses will provide more in-situ constraints for deciphering low temperature serpentinization processes.