A cross-hole, multi-year tracer injection experiment in the volcanic ocean crust

Author(s): Fisher, A. T.; Neira, N. M.; Wheat, C. G.; Clark, J. F.; Becker, K.; Hsieh, C. C.; Rappe, M. S.
Author Affiliation(s): Primary:
University of California Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, CA, United States
University of California Santa Barbara, Santa Barbara, CA, United States
National Undersea Research Program, University of Alaska, United States
University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, FL, United States
University of Hawaii at Manoa, Honolulu, HI, United States
Volume Title: AGU 2014 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2014; American Geophysical Union 2014 fall meeting, San Francisco, CA, Dec. 15-19, 2014. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: We present preliminary results from the first cross-hole tracer injection experiment in the volcanic ocean crust. The test site is on 3.5 to 3.6 M.y. old seafloor on the eastern flank of the Juan de Fuca Ridge. Six borehole subseafloor observatories (CORKs) were installed during three scientific ocean drilling expeditions, five arrayed along a 1 km profile aligned with the strike of underlying abyssal hills (Holes 1026B, 1301A/B, and 1362A/B), and one offset 2.4 km to the east (1027C). Before installing the sixth CORK in Hole 1362B, in 2010, we injected a mixture of tracers (dissolved gas, metal salts, particles) during 24 hours into the upper ocean crust. Seafloor samplers connected CORKs, sampling from different locations in the crust, were recovered during servicing expeditions in 2011 and 2013; downhole samplers that contain records from the full four years following tracer injection will be recovered in Summer 2014. Analyses of dissolved gas tracers collected with wellhead samplers through 2013 suggest that the dominant flow direction in upper basement is south to north, as inferred from regional thermal data and the chemistry of geochemical (pore fluid and borehole) samples. The apparent tracer flow rate in upper basement is on the order of meters/day, but calculations are complicated by an incomplete CORK seal in Hole 1301A, which resulted in discharge from this system that also "pulled" water and tracer to the south. Samples were collected from the tracer injection borehole, Hole 1362B, and a sampling site 200 m to the north, Hole 1362A, beginning one year after tracer injection, after opening a large-diameter ball valve on the wellhead of Hole 1362B to initiate a long-term free flow experiment. Analyses of these samples suggest that much of the tracer injected in 2010 remained close to Hole 1362B rather than being advected and dispersed into the formation. It also appears that much of the tracer transport to Hole 1362A occurred within one or more relatively thin/isolated zones, because tracer concentrations remain relatively high and there is a long tail of gradually decreasing values during the last two years of sampling. This interpretation is consistent with the highly layered and laterally continuous volcanic stratigraphy observed in basement boreholes drilled at Sites 1301 and 1362.
Year of Publication: 2014
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; Crust; East Pacific; Endeavour Ridge; Expedition 327; IODP Site U1301; IODP Site U1362; Integrated Ocean Drilling Program; Juan de Fuca Ridge; Lithosphere; North Pacific; Northeast Pacific; Oceanic crust; Oceanic lithosphere; Pacific Ocean; Tracers
Coordinates: N474530 N474540 W1274540 W1274543
N474500 N474600 W1274600 W1274600
Record ID: 2016017741
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