125
Sub-project 2.4
Dredge sampling of the Walvis Ridge, Meteor Rise – Shona Ridge,
Madagascar Ridge and Discovery Seamounts, South Atlantic
Participants
* Coordinators
Institution
Names
Email addresses
Alfred Wegener Institute
(AWI)
W. Jokat*
wjokat@awi-bremerhaven.de
University of Cape Town
(UCT)
A.P. le Roex*
aleroex@geology.uct.ac.za
Lamont-Doherty Earth
Observatory, Columbia
University (LDEO)
C. Class
class@ldeo.columbia.edu
University of Kiel
J. O’Connor
joconnor@geomar.de
Requested Funding
Total for the 5-year duration project beginning in 2004: Euros 666400
(Excluding costs for ship time)
(costs distributed among German participant institutes)
2004
2005
2006
2007
2008
AWI
61400
221800
308200
RSA
20000
25000
30000
81400
246800
338200
Univ. Kiel
Totals
126
Summary
During the Mesozoic opening of the South Atlantic and the
South-West Indian oceans several large-scale volcanic structure
were formed, most prominently the Walvis and Madagascar
ridges and the Shona-Discovry seamounts. To understand their
evolution in conjunction with the processes before and during the
break-up of Gondwanaland, the current data base is insufficient
by far. Some areas are poorly or not at all sampled. Thus, this
projects aims to close this important gap in knowledge. The
sampling of the ridge and seamounts will allow us to draw
conclusions about the temporal and spatial relationship between
the onshore volcanism of southern Africa and their counterparts.
For the Walvis Ridge, for example, it is of interest whether there
was a continuous volcanic activity between 135 and 80 Ma or if
volcanism ceased after emplacement of the Etendeka basalt
volcanism. Furthermore, the analysis of dredge samples will
provide new information on the problem of a fixed hotspot frame
of reference. Here, high precision age determinations are
required to constrain the absolute movement of the African plate.
The role of mantle plumes in the development of passive
continental margins is poorly understood. Here, the long-term
history of mantle plumes are of interest and how they interact
with the newly formed rifted margins.
127
Rationale and motivation
The South Atlantic is a key area for addressing two important scientific questions: the fixity
of hotspots in the lower mantle, and the origin of extreme geochemical mantle components
postulated to occur in the sub-lithospheric oceanic mantle. With regard to the first, the
reliable measurement of African absolute plate motion is the key to establishing the role
played by mantle plumes in continental rifting and testing the notion that hotspots are fixed in
the lower mantle. However, a recent re-evaluation of the migration rates of volcanism along
the St. Helena and Walvis hotspot trails has revealed - unexpectedly - that current estimates
of this basic parameter are seriously flawed (O'Connor et al., 1998). With regard to the
second, the sub-South Atlantic mantle is unique the world over in hosting a number of
compositionally extreme end-member mantle components (EM-I, DUPAL and LOMU),
whose presence is evident from limited samples suites available from oceanic basalts from
this region.
Fixed hotspots?
It is generally assumed that hypothesized mantle plumes have their origins deep in the earth's
interior and so represent a fundamental form of mantle circulation (e.g., Morgan, 1971,
1972). Seamount chains and aseismic ridges created during the migration of lithospheric
plates over the hotspots fed by these plumes provide therefore opportunities for gaining a
better insight into the dynamics in the deeper mantle. Morgan (1971, 1972) proposed that
mantle plumes are both long-lived and stationary (i.e., with respect to each other and the
lower mantle). Thus, hotspots should represent a 'fixed reference frame' from which the
absolute motion of lithospheric plates can be determined. This hypothesis is very important
because lithospheric plate movements have destroyed all other fixed coordinate systems (with
the exception of that based on the earth's geomagnetic field). The hotspot frame of reference
has therefore been extensively used in a wide range of studies requiring an understanding of
past lithospheric plate positions.
Nonetheless, the notion of fixed hotspots has been very controversial for many years.
Estimates of motion between hotspots have varied from as little as 5 mm/yr to most recently
as high as 34 mm/yr (e.g., Morgan, 1981; Duncan, 1981; Molnar and Stock, 1987; Mueller et
al. 1993; Tarduno and Gee, 1995; Tarduno and Cottrell, 1998). If hotspots are not fixed, a
wide body of scientific data is invalidated making the stakes in this longstanding debate very
high. The controversy over hotspot fixity has recently been heightened significantly by the
inference - from new Ocean Drilling Program paleomagnetic data for Pacific seamounts - that
very rapid motion (34 ± 9 mm/yr) has occurred between Pacific and Indo-Atlantic hotspots
(e.g., Tarduno and Gee, 1995, Tarduno and Cottrell, 1998).
Most tests of hotspot fixity incorporate the basic assumption that the history of African plate
motion over its hotspots since the mid-Cretaceous is reliably measured from dated S. Atlantic
hotspot trails. However, we showed recently using new 40Ar/39Ar data for the St. Helena
hotspot trail that this pivotal assumption is seriously flawed (O'Connor et al., 1999),, leading
to the inevitable conclusion that most studies of hotspot fixity on a global scale must now be
called into question.
128
Continental Margins
The role of mantle plumes in the development of passive continental margins is poorly
understood. Vast outpourings of flood basalts on continental margins point to sudden
catastrophic plume events (‘starting plume’ model) (Richards et al., 1989) triggering rifting
in many regions, including the S. Atlantic. However, recent studies of the Iceland plume
(O'Connor et al., 2000) point to a much longer history of plume activity prior to the onset of
continental rifting (‘plume incubation’ model) (Kerr, 1995). Establishing which of these two
fundamental models is correct has significant implications for measuring the activity of
mantle plumes and understanding how they influence passive continental margins
development. We contend that these models can be tested by understanding the temporal and
spatial behavior of a mantle plume since the onset of continental rifting.
The most relevant study to the research proposed here succeeded in inferring long-term
behavior of the Iceland mantle plume from new information about N. Atlantic oceanic
hotspot volcanism (O'Connor et al., 2000). This key information was used, together with the
extensive published geophysical information for N. Atlantic plume-influenced margins, to 1)
extend backward and forward in time the known history of the Iceland mantle plume, 2)
show that the Iceland plume pulsed with a 5 to 10 Myr periodicity since 70 Ma, and 3) show
that ‘incubating’ mantle plumes can provide a mechanism for continental rifting, despite the
rapid creation of flood basalts bounding passive continental margins. We propose a similar
approach to investigating S. Atlantic passive margin development. Our expectation is that by
combining geophysical data for S. Atlantic passive margins (together with subproject 2.1)
and key new information about the long-term behavior of the Discovery, Shona (and Tristan)
mantle plumes we can distinguish, for example, whether S. Atlantic continental rifting
occurred in response to a sudden plume event (i.e., ‘starting plume model’) or after a much
longer period of plume activity (i.e., ‘incubating plume model’).
Mantle geochemical components
A number of geochemical studies have shown that the South Atlantic hosts some of the more
extreme mantle end-member components (EM-I – Walvis Ridge; DUPAL – Tristan/Gough
and Discovery hotspots; LOMU – southern MAR). The origin of these geochemical
components is poorly constrained and proposals range from constructional heterogeneities
established at the time of Earth formation (Hart, 1984), recycled ancient pelagic sediment
(Weaver, 1990), and delaminated sub-continental lithosphere (Hawkesworth et al., 1986;
Milner and le Roex, 1996; Douglass et al., 1999). To resolve these alternatives, detailed
sample suites are required from along the relevant seamount chains/aseismic ridges to
evaluate the degree of temporal heterogeneity of the respective mantle plumes – Tristan,
Discovery and Shona, to determine whether these are comparatively shallow level, passive,
features located today with the convecting oceanic mantle, or deep-seated (lower mantle?)
anomalies brought to the surface by the upwelling plumes. With the exception of EM-I, these
anomalies are unique to the Gondwana ocean basin systems, and the South Atlantic provides
an ideal location for their study.
129
Figure 2.4.1 Location of the three areas to be sampled: the Walvis Ridge, the Discovery and Shona hot spot tracks.
Key References
Douglass, J., Schilling, J.-G., Kingsley, R.H., Small, C. (1995) Influence of the Discovery and Shona
mantle plumes on the southern Mid-Atlantic Ridge: Rare earth evidence, Geophys. Res.
Lett., 22, 2893-2896.
Douglass, J, Schilling, J.-G., Fontignie, D. (1999) Plume-ridge interactions of the Discovery and
Shona mantle plumes with the southern Mid-Atlantic Ridge (40°-55°S), J. Geophys. Res.
104, 2941-2962.
Duncan, R.A. (1981) Hotspots in the Southern oceans - an absolute frame of reference for motion of
the Gondwana continents, Tectonophysics, 74, 29-42.
Fontignie, D., Schilling, J.-G. (1996) Mantle heterogeneities beneath the South Atlantic: a Nd-Sr-Pb
isotope study along the Mid-Atlantic Ridge (3°S-46°S), Earth Planet. Sci. Lett., 142, 209221.
Hartnady, C.J.H., le Roex, A.P. (1985) Southern Ocean hotspot tracks and Cenozoic absolute motion
of the African, Antarctic, and South American plates, Earth Planet. Sci. Lett., 75, 245-257.
Hawkesworth C. J., Mantovani M. S. M., Taylor P. N., Palacz Z. (1986). Evidence from the Parana
of south Brazil for a continental contribution to Dupal basalts. Nature 322, 356-359.
Kerr, A. C. (1995) The melting processes and composition of the North Atlantic (Iceland) plume:
geochemical evidence from the Early Tertiary basalts. J. Geol. Soc. Lond. 152, 975-978.
le Roex, A.P., Dick, H.J.B., Gulen, A.M., Erlank, A.J. (1987) Local and regional heterogeneity in
MORB from the Mid-Atlantic Ridge between 54.5°S and 51°S: Evidence for geochemical
entrainment, Geochim. Cosmochim. Acta 51, 541-555.
Milner S. C., le Roex A. P. (1996) Isotope characteristics of the Okenyenya igneous complex,
northwestern Namibia: constraints on the composition of the early Tristan plume and the
origin of the EM 1 mantle component. Earth and Planetary Science Letters 141, 277-291.
Molnar, P., Stock, J. (1987) Relative motions of hotspots in the Pacific, Atlantic and Indian Oceans
since late Cretaceous time, Nature, 327, 587-591.
Morgan, W.J. (1971) Convection plumes in the lower mantle, Nature, 230, 42-43.
130
Morgan, W.J. (1972) Plate motions and deep mantle convection, Geol. Soc. Am. Mem., 132, 7-22.
Morgan, W.J. (1981) Hot spot tracks and the opening of the Atlantic and Indian Oceans, in: Emiliani,
C. (ed.), The Oceanic Lithosphere. The Sea, Vol. 7, 443-487, Wiley, New York.
Müller, R.D., Royer, J.-Y., Lawver, L.A. (1993) Revised plate motions relative to the hotspots from
combined Atlantic and Indian Ocean hotspot tracks, Geology, 21, 275-278.
O'Connor, J..M., Stoffers, P., van den Bogaard, P. & McWilliams, M. First seamount age evidence
for significantly slower African plate motion since 19 to 30 Ma, Earth and Planet. Sci. Lett.,
171, 575–589, 1999.
O'Connor, J.M., Stoffers, P. & Wijbrans, J.R. Evidence from episodic seamount volcanism for
pulsing of the Iceland plume in the past 70 Myr. Nature 408, 954-958, 2000.
Richards, M.A., Duncan, R.A., Courtillot, V.E. (1989) Flood basalts and hot-spot: Plume heads and
tails, Science 246, 103-107).
Smith, W.H.F., Sandwell, D.T. (1994) Bathymetric prediction from dense satellite altimetry and
sparse shipboard bathymetry, J. Geophys. Res., 99, 21803-21824.
Tarduno, J.A., Gee, J. (1995) Large-scale motion between Pacific and Atlantic hotspots, Nature, 378,
477-480.
Tarduno, J.A., Cottrell, R.D. (1998) Large scale motion between hotspot groups and its geodynamic
implications (abstract), EOS, Trans. AGU, 79, 241.
Weaver B. L. (1990) Geochemistry of highly-undersaturated ocean island basalt suites from the
South Atlantic Ocean: Fernando de Noronha and Trindade islands. Contributions to
Mineralogy and Petrology 105, 502-515.
White, W.M. (1985) Sources of oceanic basalts; radiogenic isotope evidence, Geology, 13, 115-118.
Zindler, A., Hart, S. (1986) Chemical geodynamics, Ann. Rev. Earth Planet. Sci., 14, 493-571.
Key questions



Is the 'incubation' or 'starting' plume model applicable in the context of S. Atlantic
opening and margin development?
How do mantle plumes behave, develop and interact with the lithosphere?
Are (all) hotspots initiated/maintained by deep (plume?) or shallow sources in the
mantle?
Objectives
We propose a systematic dredge sampling program of the Discovery and Shona hotspot
systems, and more detailed sampling of the Walvis Ridge (Fig. 2.4.1). The resulting age and
compositional data determined for these dredge samples will reveal the long-term temporal,
spatial and geochemical development of Walvis, Discovery and Shona hotspot magmatism
on the African plate. It is proposed to also extend the work to sampling the Madagascar
Ridge.
Such information will allow us to address the following objectives and questions:
 Establish the migration rates of volcanism along the Discovery and Shona hotspot chains;
 Use these rates - in combination with other African hotspot trails - to establish a new,
more robust measurement of African absolute plate since the onset of S. Atlantic
opening/continental rifting;
 Use new absolute motion Euler poles to test for fixed S. Atlantic hotspots and the more
controversial hypothesis of fixed Pacific versus Indo-Atlantic hotspot groups;
 Use new estimate of African absolute motion to determine if the onset of S. Atlantic
plume activity was synchronous with the development of the Parana-Etendeka continental
flood basalts and/or the early rifting of the South Atlantic;
131



Reconstruct the physical and geochemical transition from on-ridge to intraplate Discovery
and Shona hotspot volcanism;
Mapping the composition of synchronous volcanism on the Discovery, Shona (and
Tristan) hotspot trails will provide ~2000 km long NW-SE trending profiles crossing three
hotspot systems - Shona, Discovery and Tristan. This should provide an important window
into the composition of reservoirs in the deep S. Atlantic mantle. Determining the
composition of such deep mantle reservoirs, e.g., EM-I, HIMU, DUPAL, FOZO (White,
1985; Zindler and Hart, 1986, Hart et al., 1992), and placing constraints on their possible
origin, is one of the most important challenges in establishing the scale and origin of
mantle heterogeneity;
Detailed age and compositional mapping of important hotspot trails, such as Walvis,
Discovery and Shona should shed new insights on long-held, largely untested ideas about
plume-lithospheric interaction and mantle plume dynamics.
Methodological Approach and techniques







Comprehensive along and cross chain dredge-sampling of the Walvis,, Madagascar,
Shona and Discovery hotspot lineaments
Initial description and selection of rocks recovered at dredge stations
Detailed post-cruise petrological description of selected rock samples
Major element analyses (XRF) of selected samples
Geochemical analyses (ICP-MS) of selected samples
High precision whole rock and mineral 40Ar/39Ar ages
Isotope analyses (Sr, Nd, Pb etc)
Expected outcomes/deliverables






Determine when volcanism began on the Walvis and Shona ridges in order to establish
whether it was synchronous with the Parana-Etendeka continental flood basalts and/or
the early rifting of the South Atlantic (‘starting’ versus ‘incubation’ plume model).
Use new estimates for the migration rates of volcanism along the Walvis, Discovery and
Shona hotspot chains to provide a more robust measurement of African absolute plate
motion.
Produce detailed maps of the temporal-spatial distribution and geochemical variability of
volcanism along three sub-parallel South Atlantic volcanic lineaments. This will provide
contemporaneous age and compositional information about hotspot volcanism in a series
of ~2000 km long NW-SE profiles between the African continental margin to the South
Atlantic spreading axis.
Use our new ~2000 x ~3000 km window on the deep (assuming mantle plumes) or
shallow (assuming shallow hotspot sources) – in conjunction with relevant information
from margin and onshore studies – in order to address the following:
Distinguish localized (shallow?) lithospheric control of hotspot volcanism from longlived plume related behavior (deep plume?). This is possibly a unique approach to
inferring from hotspot volcanism how mantle plumes behave, develop and interact with
the lithosphere.
Test alternative models for origin of South Atlantic geochemical components (EM-I,
HIMU, DUPAL, FOZO) – e.g., shallow level, passive features located in the convecting
mantle, or deep-seated (lower mantle?) anomalies brought to the surface by the upwelling
plumes.
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Timeframe
Walvis Ridge
2006 (Proposal submitted for FS Meteor)
Discovery/Shona Seamounts
>2007
(no proposal submitted)
Madagascar Ridge
>2007 (no proposal submitted)
Logistic/Equipment requirements
Ship capable of dredging, winch and 10,000 m cable
Estimated Funding Requirements
Manpower:
All salaries based on the German public service scale (BAT)
1 post-doc for isotopic and geochemical studies, Walvis Ridge
Salary group: BAT IIa (€ 4800/month)
Duration:
3 years
€ 172,800
1 PhD student for geochemical and geochronological studies, Walvis Ridge
Salary group: BAT IIa/2 (€ 2400/month)
Duration:
3 years
€ 86,400
1 Student assistant (half-time) for lab preparatory work, Walvis Ridge
Salary group: € 600/month
Duration
2 years
€ 14,400
1 post-doc for isotopic and geochemical studies, Discovery/Shona
Salary group: BAT IIa (€ 4800/month)
Duration:
3 years
€ 172,800
1 PhD for geochemical and geochronological studies, Discovery/Shona
Salary group: BAT IIa/2 (€ 2400/month)
Duration:
3 years
€ 86,400
1 MSc student for geochemical and petrographic analyses, Discovery/Shona
Salary group: € 1200/month
Duration:
2 years
€ 28,800
1 Student assistant (half-time) for preparatory work, Discovery/Shona
Salary group: € 600/month
Duration
2 years
Total manpower
€ 14,400
=========
€ 576,000
133
Investments
NONE
Consumables
Walvis Ridge (2006, 2007)
Ar-Ar dating (40 samples at €500 each)
XRF major elements (80 samples at €20 each)
ICP-MS analyses (80 samples at €40 each)
Thin sections (80 samples at €5 each)
Sr, Nd and Pb isotopes (40 samples at €200 each)
Subtotal
Shona Ridge – Meteor Rise and Discovery Seamount chain (2007, 2008)
Ar-Ar dating (40 samples at €500 each)
XRF major elements (80 samples at €20 each)
ICP-MS analyses (80 samples at €40 each)
Thin sections (80 samples at €5 each)
Sr, Nd and Pb isotopes (40 samples at €200 each)
Subtotal
€20,000
€ 1,600
€ 3,200
€ 400
€ 8,000
€33,200
€20,000
€ 1,600
€ 3,200
€ 400
€ 8,000
€33,200
=======
Total consumables
€66,400
Travel expenses:
Participation of project scientists at scientific congresses
Basis: 4 international (2000 €), 4 national (1000 €) per year
Time frame:
2007-2008
€ 24,000