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Ancient lithosphere blobs beneath the oceans: spelling out the geochemists’ alphabet and understanding ocean basin lithosphere

 

Suzanne O’Reillya, Ming Zhanga, William Griffina & Graham Begga,b

 

aGEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquare University, NSW 2109, Australia

sue.oreilly@mq.edu.au, mzhang1951@live.com.au, bill.griffin@mq.edu.au

bMinerals Targeting International PL, 17 Prowse Street, West Perth, WA 6005, Australia

graham@mineralstargeting.com

 

Also see: www.gemoc.mq.edu.au/Annualreport/annrep2008/Reshigh08.html

 


This webpage is a summary of: O'Reilly, S.Y., Zhang, M., Griffin, W.L., Begg, G. and Hronsky, J. 2009. Ultradeep continental roots and their oceanic remnants: a solution to the geochemical "mantle reservoir" problem? Lithos, 112, S2, 1043-1054.


 

The nature of continental Archean lithospheric mantle

Archean subcontinental lithospheric mantle (SCLM) is distinctive in its highly depleted composition, commonly strong stratification, and the presence of rock types (e.g., depleted, low-Fe harzburgite) absent in younger SCLM (Griffin & O’Reilly, 2007; Griffin et al., 2009). Primary (unmetasomatised) Archean lithosphere is very low in basaltic components such as Al, Ca, Fe, consists dominantly of dunite and Ca-poor harzburgite and has high seismic wave velocities, mainly due to the high proportion of Mg-rich olivine (Fo92-94). Archean lithosphere is significantly less dense than asthenosphere at any depth, and this buoyancy means that it cannot be gravitationally delaminated; it needs mechanical disaggregation (e.g., rifting) and/or metasomatic reworking to be disrupted.

Oceanic Archean mantle revealed in tomographic models

Figure 1 shows tomographic slices through the oceanic lithosphere and upper mantle of the Atlantic Ocean Basin at 0-100 km, 100-175 km and 175-250 km. We use a high-resolution global tomography model derived from SH body-wave travel times based on the approach of Grand (2002) and fully described by Begg et al. (2009).

 

Figure 1. Tomographic model images at three depth slices for the Atlantic Ocean Basin (see text, Begg et al., 2009 and O’Reilly et al, 2009 for details of the model and colour scales). Note that “hot” (red-white) colours indicate higher velocities and cool colours lower velocities. Relevant OIB Provinces are: 1, Azores; 2, Madeira; 3, Canary Islands; 4, Cape Verde; 5, Fernando de Noronha; 6, Ascension Islands; 7, St. Helena; 8, Trindade; 9, Tristan da Cunha (Walvis Ridge at ~130 Ma; Richardson et al., 1984); 10, Bouvet; 11, Crozet Archipelago (Afanasy – Nikitin Rise in the Indian Ocean at Late Cretaceous, ~115-80 Ma; Mahoney et al., 1996); 12, Cameroon Line.

 

In the 0-100 km section, high-velocity regions are obvious. Some are apparently continuous with continental regions (especially off southwestern Africa and southeastern South America) and some occur as discrete “blobs” within the ocean basin, from the continental margins to the mid-ocean ridge. In the layer from 100-175 km, these fast domains persist, and some also show velocity contrasts in the 175-250 km layer.

A traditional interpretation for high-velocity regions at the margins of ocean basins is the effect of cooling of the oceanic lithosphere with time and distance from the ridge. However, this cannot be the explanation for the discrete blobs that lie within the ocean basin, both away from the original rift margins and near the present-day ridge, with some extending to depths of 250 km.

We suggest that these high-velocity volumes represent remnants of depleted (buoyant), ancient continental lithosphere, fragmented and stranded during the rifting process at the opening of the ocean basin. The high-velocity domains extending out from the coastlines are not uniformly distributed along the basin edge. The most marked high-velocity regions, off SE South America and northwest and southwest Africa, appear to be continuous with their respective continental deep structure as seen in the tomographic models. The global magnetic-anomaly map (Korhonen et al., 2007; Figure 2) shows that these regions have a complex magnetic signature that is consistent with extended continental crust, and distinct from that of oceanic lithosphere, which is characterised by the regular magnetic striping produced at spreading centres.

 

Figure 2. Modified extract from the global magnetic anomaly map (Korhonen et al., 2007, Magnetic Anomaly Map of the World Scale: 1:50,000,000, 1st edition, Commission for the Geological Map of the World, Paris, France) showing the Atlantic Ocean Basin and Atlantic coasts of South Africa and South America. Black lines outline the regions with crustal rather than oceanic magnetic characteristics. Click here or on Figure for enlargement.

 

Old Re-Os ages for mantle sulfides in some depleted mantle rock types beneath rift zones and oceanic areas (see references in O’Reilly et al., 2009 and Coltorti et al., 2010) suggest that these high-velocity blobs (inferred to have high Mg# and low density) represent relict Archean to Proterozoic SCLM (now refertilised to varying degrees, during episodes of mantle fluid inflitration reflecting larger-scale tectonic events) that was mechanically disrupted and thinned during the formation of the oceanic lithosphere. This interpretation implies that ocean basins do not form by clean breaks at now-observed continental boundaries, but that significant volumes of buoyant old mantle are embedded within the newly generated oceanic lithosphere. The opening of ocean basins may be largely by listric faulting mechanisms, leaving significant wedges of continental lithosphere at rifted margins, and stranding domains of ancient lithosphere in the upper part of the new oceanic crust-mantle system, where they would remain as buoyant blobs.

If the higher-velocity coherent blobs observed at depths up to >150 km in the upper mantle of the Atlantic Ocean do represent remnant Archean mantle roots, this has important implications for the nature of global convection. Models involving large-scale horizontal movement would be difficult to reconcile with these observations. Instead, convection may be dominantly in the form of upwelling vertical conduits with shallow horizontal flow (Figure 3). The locus of these conduits may be controlled by the geometry of the margins and the coherence of the buoyant lithospheric blobs.

 

Figure 3. Cartoon indicating how high-Vs (low-density), vertically coherent regions extending to up to > 250 km could control convection pathways (modified from O’Reilly et al., 2009).

 

Ocean island basalt goechemical signatures

The localised persistence of ancient SCLM beneath oceans also provides a logical explanation for the “alphabet soup” of mantle sources created by geochemists to describe the isotopic signatures of basalts (EM1, EM2, HIMU, DMM; Hofmann, 1997 and references therein; Figure 4). These components are generally attributed to different geochemical reservoirs within the convecting mantle. However, all of these geochemical fingerprints are found in lithospheric material and have been well characterised in mantle xenolith studies (e.g., Leeman, 1982; Wilson & Downes, 1991; Zhang et al., 2001; Thompson et al., 2005). If lithospheric volumes persist to deep mantle levels (>150 km) in ocean basins, then interaction with upwelling mantle plumes can “contaminate” magmas and fluids (Figure 3), imposing a range of isotopic and trace-element signatures. A detailed examination of the ocean-island database from the Atlantic shows a strong correlation between “continental” signatures (EM1, EM2, etc.) and the presence of high-velocity blobs in the seismic tomography.

 

Figure 4. Isotopic components commonly observed in basaltic magmas and their fields in Nd and Sr isotopic space (Hofmann, 1997, and references therein).

 

This model removes the requirement for hidden source regions embedded within the convecting mantle. Magma interaction with deep ancient SCLM roots also provides a simple explanation for observations such as Archean Re-depletion model ages in oceanic basalts.

 

References

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