Seafloor Volcanic and Hydrothermal Controls on Early Life Preserved in an Archean Greenstone Belt (Canada)
What: The Abitibi Greenstone Belt (AGB), and specifically the Blake River Group, affords us the relatively rare opportunity to study seafloor volcanic and hydrothermal processes in an area that is both accessible and has a significant thickness of exposed extrusive pile. We plan to study mafic volcanic rocks of the AGB that were erupted as part of an ancient Archean seafloor sequence. Our proposed detailed mapping, physical properties, geochemical, and petrographic studies will contribute to the geologic understanding of seafloor volcanic and hydrothermal processes within the context of modern ocean crustal processes and provide us with a better understanding of how hydrothermal fluid flow patterns may have operated early in Earth’s history.
When and Where: July 17-August 14, 2008. Students will arrive the Williams-Mystic campus in Mystic, CT by July 17, 2008. A few days will be spent in and around Mystic on field and laboratory methods and selecting projects. Students will live cooperatively in Williams-Mystic historic houses while in Mystic, and will have full access to the new Marine Science Center and Mystic Seaport. We will fly as a group to Rouyn, Quebec, and rent vehicles to drive to the nearby University of Quebec in Abitibi-Témiscamingue (UQAT), where we will stay in campus apartments. We will begin the field portion of the project with a two-day field trip to selected sites in the Blake River Group and several days of group reconnaissance, followed by fieldwork on projects. We will depart Rouyn, Quebec, and then spend another few days together in Mystic preparing samples, doing initial laboratory work or sending samples out to other analytic laboratories, and analyzing data. Departure from Mystic will be on/by August 14, 2008.
Who: Six students. Professor Lisa Gilbert (Williams) and Professor Neil Banerjee (U. of Western Ontario)
Project Description and Goals
Greenstone belts are useful for understanding ancient seafloor processes that we cannot access directly. When we study the ocean crust it is generally either just on the very surface of the seafloor, in a one-dimensional hole drilled into the crust, or by some remote method that prohibits detailed mapping. At the AGB, we will map and sample in several dimensions for studies of physical properties (including density, velocity, and porosity) and geochemistry to help constrain original setting, understand fluid flow and porosity of extrusive rocks, and explore the variety of compositions and relationships between volcanic facies preserved. We will put our results into context, using comparisons from other greenstone belts, ophiolites, and modern seafloor rocks.
We plan to use these results to better understand the importance of fluid flow to the distribution of microbial trace fossils that have been recently discovered in samples from the AGB. The microfossils occur in hyaloclastite samples and consist of micron-sized tubular structures mineralized by titanite. These structures are identical to microfossils described from 3.5 Ga greenstone belts from South Africa and Australia (e.g., Furnes et al., 2004; Banerjee et al., 2006; Banerjee et al., 2007). Based on their similarity to textures observed in recent glassy pillow basalts, these structures are interpreted to represent compelling evidence of ancient mineralized traces of microbial activity. Initial results from AGB samples lend evidence for microbial activity, but in order to further substantiate this interpretation we need to do more detailed sample collecting and mapping to place the samples in a volcanological and hydrothermal context, first at the outcrop scale and later in a more regional context. In addition to sample collection we will map out the distribution of originally glassy lithologies and make special note of primary alteration zones that may have acted as fluid flow zones within a hydrothermal context.
Listed below are potential student projects within our overall goal of assessing controls on the geologic and hydrothermal features of the Abitibi Greenstone Belt and the early microbial biosphere that thrived in the presence of hot circulating fluids in the Archean.
- Mapping and image analysis of the inter-pillow zones to estimate the original connected porosity and permeability of upper ocean crust, key controls on fluid flow, and thus on life in a Precambrian subseafloor (Figure 2).
- Characterization of the lava drain-back features and vesicles formed during emplacement.
- Paleoslope and depth of emplacement of the pillow lavas from 2-D and 3-D field measurements of pillow morphology and laboratory estimates of porosity.
- A comparative study of the microfossils preserved in different volcanic facies (i.e., pillow lavas, hyaloclastites, and volcanoclastic breccias; Figure 2). Field mapping of altered glass in these facies will be complimented by fine-scale sampling for possible microprobe work on thin sections.
- Detailed mapping of the glassy fragments in a continuous 1 km hyaloclastite layer and a geochemical comparison (carbon isotopes, trace elements, and oxygen isotopes) of hyaloclastite samples hosting microfossils.
- Spatial characterization of hydrothermal mineral assemblages around an Archean hydrothermal vent.
Students will live cooperatively and will be expected to participate in cooking and general community duties both at UQAT and at Williams-Mystic. No special gear is required: full kitchens and laundry are available at both sites, and students need only supply their own linens, clothes, and personal items.
Your general approach to scientific questions, work ethic, and interest in this project are more important than any specific courses taken. More important than any particular coursework, we are seeking students with a demonstrated ability to think creatively. Prior experience with field mapping (and/or field camp) would be helpful, but not required. Some coursework in one or more of the following fields might also be useful, but not necessary: mineralogy, oceanography, geophysics, structural geology, geochemistry, molecular biology, statistics, or computer science. Students without a valid passport should apply for one as soon as possible, preferably before May 1.