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International Research
Axial volcanic ridges (AVRs) appear to be nearly ubiquitous
but non-steady state features of slow-spreading ridge segments.
They are tens of kilometres long, a few kilometres wide and a
few hundred metres high. They consist of thousands of small,
circular volcanoes that presumably overlie fissures and feeder
dykes (Smith and Cann, 1993; Head et al., 1996; Lawson et
al., 1996) and therefore should reflect the composition and development
of the uppermost part of the effusive layer of the
oceanic crust and its mantle sources. Individual volcanoes
range from single small cones ~100 m in diameter to large,
flat-topped edifices ~2 km in diameter and occur singly, in
rows or piled into larger edifices. Differences in the appearance
of individual AVRs (e.g., freshness of volcanic structures,
degree of tectonism) suggest an evolutionary model (Parson et
al., 1993; Head et al., 1996; Briais et al., 2000), in which early
voluminous sheet flows give way to fissure volcanism that is increasingly
focussed into central volcanoes. This model is as yet
untested. AVRs are thought to contain the youngest seafloor in
any segment (Hussenoeder et al., 1996; Smith et al., 1999; Briais
et al., 2000), though estimates of their duration range from
~25 ka (Ballard and Van Andel, 1977; Barclay et al., 1998) to
~600 ka (Thatcher and Hill, 1995; Searle et al., 1998).
1). The primary aim of these E-W lines was to give a dense
grid of near-bottom magnetic field observations; however, with
TOBI’s 3-km-range, 6-m-resolution sidescan sonar, this survey
plan also provided complete sidescan coverage in all four
orthogonal look-directions. During the course of this survey,
two MAPRs (courtesy of Ed Baker, NOAA) were deployed on
a cable below TOBI, which was towed at about 400-500 m
above the seafloor. These supplemented TOBI’s own CTD and
light scattering sensor to provide a continuous vertical array of
water column measurements.
During the TOBI survey, which was conducted at about 2
knots, we also ran the ship’s EM120 multibeam echosounder.
The objectives of this study are therefore:
1. To date the growth of an AVR and hence infer rates of melt
supply to the crust;
2. To determine the structural, age and compositional relations
between different volcanic units of the AVR;
3. Hence, to test published models for AVR formation;
4. To infer the nature and variation of mantle sources and their
melting histories.
Shipboard operations
To address these issues RRS James Cook (cruise JC24) visited
the Mid-Atlantic Ridge at 45°30’N in May - June 2008, where
the half-spreading rate is 11 km Ma -1 , and a well-developed
AVR exists with strong geochemical diversity (Keeton and
Searle, 1996; www.petdb.org). We carried out a detailed study
of the AVR using the TOBI deep-towed vehicle (Flewellen et
al., 1993) and the ROV Isis. We also used Mini-Autonomous
Plume Recorders (MAPRs; Baker and Milburn, 1997), both
deployed from TOBI and in separate “tow-yo” surveys, to
search for hydrothermal plumes in the area. In addition to the
shipboard scientific party, our team includes shore-based colleagues
Jon Davidson and Yaoling Niu (Durham University),
Ken Sims (WHOI), Louise Thomas (Open University), and
Milène Cormier (University of Missouri).
The cruise began with a 7-day TOBI survey, guided by an earlier
low-resolution bathymetry map (Keeton and Searle, 1996),
in which we first insonified the AVR from each side with N-S
survey lines, followed by an interleaving survey plan to cover
the AVR with 22 E-W lines spaced about 1.4 km apart (Fig.
Depth (m)
Figure 1: Shaded relief bathymetry of the 45°30’N area with JC24
operations superimposed. Black lines, 500m contours; white line,
TOBI track; yellow lines, ROV Isis tracks; red dots, sample positions
(note that Isis tracks are obscured by the sampling symbols). The AVR
lies between the two N-S TOBI tracks and is roughly outlined by the
3000-m contour.
InterRidge News 12 Vol. 17, 2008