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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* /
University of Cape Town
(UCT) / A.P. le Roex* /
Lamont-Doherty Earth Observatory, Columbia University (LDEO) / C. Class /
University of Kiel / J. O’Connor /

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

Univ. Kiel

Totals

/

81400

/

246800

/

338200

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.

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.

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.

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, 209-221.

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.

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;
  • 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 long-lived 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.

Timeframe

Walvis Ridge2006 (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

Duration2 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

Duration2 years€ 14,400

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Total manpower€ 576,000

InvestmentsNONE

Consumables

Walvis Ridge (2006, 2007)

Ar-Ar dating (40 samples at €500 each)€20,000

XRF major elements (80 samples at €20 each)€ 1,600

ICP-MS analyses (80 samples at €40 each)€ 3,200

Thin sections (80 samples at €5 each)€ 400

Sr, Nd and Pb isotopes (40 samples at €200 each)€ 8,000

Subtotal€33,200

Shona Ridge – Meteor Rise and Discovery Seamount chain (2007, 2008)

Ar-Ar dating (40 samples at €500 each)€20,000

XRF major elements (80 samples at €20 each)€ 1,600

ICP-MS analyses (80 samples at €40 each)€ 3,200

Thin sections (80 samples at €5 each)€ 400

Sr, Nd and Pb isotopes (40 samples at €200 each)€ 8,000

Subtotal€33,200

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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