Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Alexis S. Templeton

Second Advisor

Hubert Staudigel

Third Advisor

Tom McCollom

Fourth Advisor

G. Lang Farmer

Fifth Advisor

Steve Mojzsis

Abstract

The discoveries of tubular alteration features of potential biological origin in subseafloor basalt glasses, ophiolites, and ancient greenstones has important implications for increasing our understanding of global biogeochemical cycling in hard-rock systems and the evolution of life on Earth, and for exploring other planets for signs of life. Given the technological challenges in accessing the modern subseafloor and the possibility that it could take thousands of years to form these features, at this point it is not possible to observe their formation in-situ, nor have they been successfully modeled in the lab. Thus, we must infer the formation and preservation mechanisms of these putative biosignatures from physical and chemical clues. We have used a number of micro-analytical techniques to geochemically characterize these features at the submicron scale.

Using focused ion beam milling we prepared ultra-thin cross-sections of the tubules, which enabled high-resolution imaging and chemical analyses by transmission electron microscopy and electron-dispersive spectroscopy. These analyses revealed leached rims around the tubule margins, and partially crystalline Fe-bearing phyllosilicates infilling the tubules. In addition, we combined four different high-resolution synchrotron-based techniques, including X-ray fluorescence microprobe mapping, micro-diffraction, and absorption spectroscopy to determine the distribution and speciation of key major and trace elements. These studies revealed consistent patterns of Ca, Mg, Ti, Fe, Mn, S, and P distributions, metal oxidation, and authigenic precipitation of secondary phases, as well as identified possible biominerals and organic material. Put together, these results have provided insight into the processes involved in tubular alteration and mineralization. The formation mechanism involves initial incongruent dissolution of the glass potentially accompanied by the authigenic precipitation of biominerals, specifically Fe-oxides and sulfate. Successive stages of fluid flow then infill the tubules with a variety of Feand Ti-rich minerals, with concomitant partial to complete oxidation of the redox active elements. The consistency of these results across a 100Ma suite of subseafloor samples and one ophiolite sample implies that the alteration and mineralization patterns are consistent spatially and temporally. These results also add significant evidence to the argument for the biological formation of the tubules by revealing potential signs of microbial geochemical processing.

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