“Mantle plumes” are NOT from ancient oceanic crust

Yaoling Niu¹ & Michael J. O'Hara<sup>2

1</sup>Durham University, Durham DH1 3LE, UK, yaoling.niu@durham.ac.uk
²University of Wales, Aberystwyth SY23 3DB, mio@aber.ac.uk

1. Introduction

Following the paper by Niu et al. (2002), we point out that there are many more difficulties than certainties in models invoking ancient recycled oceanic crust as the source material for ocean island basalt (OIB). Our paper Niu & O'Hara (2004) titled “Mantle plumes are not from ancient oceanic crust” in the book Oceanic Hotspots summarises our work in progress. Our perspective contrasts with that advanced in the classic paper, published 20 years earlier by Hofmann & White (1982), titled “Mantle plumes from ancient oceanic crust”. We published a more comprehensive treatment a year later (Niu & O'Hara, 2003) titled “Origin of ocean island basalts: A new perspective from petrology, geochemistry and mineral physics considerations”. It is apparent from reading the papers by Sobolev et al. (2005, 2007) on the same subject, that our views have been overlooked.

In this brief webpage, we summarise our views (Niu & O'Hara, 2003) in an accessible form for readers. We do not ignore the interesting issues raised by Sobolev et al. (2005, 2007). Figures and page numbers mentioned in the text (e.g., ECV 5-2) refer to those in (Niu & O'Hara, 2003).

2. Background and context

If we assume the Earth’s primitive mantle (PM) to be compositionally uniform, we must explain why the mantle source for mid-ocean ridge basalts (MORB) is more depleted both isotopically and in terms of incompatible elements than the mantle source for OIB. By interpreting MORB mantle depletion as resulting from continental crust extraction in Earth’s early history (Armstrong, 1968; Gast, 1968; Hofmann, 1988), we could be satisfied with the OIB source being less depleted than the MORB source. However, OIB mantle source is not just less depleted, but enriched in incompatible elements relative to the PM. It also varies significantly in inferred abundances and ratios of incompatible elements as well as radiogenic isotopes from one island to another and from one group of islands to another group. Therefore, the mantle source for OIB is heterogeneous on all scales.

By accepting the assumption that the entire mantle is in the solid state and that solid-state elemental fractionation is unlikely (Hofmann & Hart, 1978), it is logical to suspect that processes known to occur in the upper mantle and crust (e.g., partial melting and magma evolution, dehydration, alteration/metamorphism, differential weathering, transport and sedimentation) are the likely causes of elemental fractionation. These shallow or near-surface fractionated materials are then introduced into the mantle source regions of oceanic basalts through subduction zones. Mantle compositional heterogeneity is thus a general consequence of plate tectonics because of crust-mantle recycling. Among many contributions endeavoring to understand the origin of mantle compositional heterogeneity in the context of plate tectonics is the classic paper by Hofmann & White (1982). They proposed that "oceanic crust is returned to the mantle during subduction… Eventually, it becomes unstable (at the core-mantle boundary or CMB; see Christensen & Hofmann, 1994) as a consequence of internal heating, and the resulting diapirs become the source plumes of oceanic island basalts (OIB) and hot spot volcanism." They also stressed that this recycled ancient oceanic crust, the plume material which is the source of OIB, is geochemically more ‘‘enriched’’ in K, Rb, U, Th, and light rare-earth elements relative to the more "depleted" source of MORB. While some details are considered conjectural, the principal idea of the model has, since 1982, been widely accepted by the solid Earth community as being correct, except for those who share the view of Niu & O'Hara ( 2003).

3. Niu & O’Hara (2003) demonstrated that there is no obvious association between ancient recycled oceanic crust and OIB source in terms of petrology, geochemistry, and mineral physics

1. Melting of oceanic crust with basaltic/picritic compositions cannot produce high-magnesian lavas parental to most OIB. Primitive OIB melts (> 15% MgO) are probably more magnesian than bulk ocean crust (< 13% MgO) (p. ECV 5-2).
2. "One may argue that OIB are derived from melts of recycled oceanic crusts mixed with predominantly peridotite melts. This is possible and will apparently relax the OIB MgO requirement, but then OIB are no longer derived from recycled oceanic crust alone" (p. ECV 5-2). Note that the original theory by Hofmann & White (1982) and Christensen & Hofmann (1994) is incompatible with this possible scenario, but by using it Sobolev et al. ( 2005, 2007) discovered new evidence in support of Hofmann & White (1982) and Christensen & Hofmann (1994) without addressing points 3.1 and 3.2 in this list.
3. Oceanic crust passing through subduction zone dehydration reactions should be depleted in water-soluble incompatible elements such as Ba, Rb, Cs, Th, U, K, Sr, Pb relative to water-insoluble incompatible elements such as Nb, Ta, Zr, Hf, Ti, etc. Residual crust with such trace-element systematics is unsuitable as a fertile source for OIB (Figures 2-3;  p. ECV 5-3,4,5).  Melting or partially melting such residual crust will never produce OIB or any volcanic rocks ever sampled on the Earth’s surface.
4. Ancient oceanic crust is too depleted to produce the Sr-Nd-Hf isotopic signatures of most OIB (Figure 1; p. ECV 5-2,3).
5. OIB Sr-Nd-Hf isotopes preserve no signals that indicate previous subduction-zone dehydration histories (Figures 4-5; p. ECV 5-5,7,8,9).
6. Subducted oceanic crust at shallow lower-mantle conditions forms mineral assemblages that are much (>2%) denser than the ambient peridotitic mantle (Figure 6; p. ECV 5-9,10).
7. If subducted crust melts in the deep lower mantle, this melt, depending on its composition, may have still greater (up to 15%) density than solid peridotitic mantle. Therefore ancient oceanic crust that has subducted into the deep lower mantle will not return in bulk to the upper mantle in either the solid (see 3.6 in this list) or molten state (Figure 7; p. ECV 5-10,11,12).
8. Small fragments of subducted oceanic crust could be returned to the upper mantle source regions of oceanic basalts provided they are carried along with streams of ascending buoyant material. However, there is no convincing evidence for the presence of bulk subducted crust in the source regions of oceanic basalts, noting points 3.3, 3.4 and 3.5 in this list (p. ECV 5-16).
9. It is probable that subducting oceanic crust does not penetrate the 660-km discontinuity, but may be stripped, at least partly, off the slab and remain in the transition zone. This may be a result of its lower density compared with the peridotite mantle around 660 km (Figure 6; p. ECV 5-16).  This scenario would apparently make recycled oceanic crust a less problematic potential source material for OIB, but this model differs from the CMB-origin model (Hofmann & White, 1982; Christensen & Hofmann, 1994). Furthermore, physical difficulties remain. Subducted crust is eclogitic in most of the upper mantle and is probably far too dense to ascend to OIB source regions (p. ECV 5-14).

4. Summary

Models invoking recycled oceanic crust to explain the geochemistry of OIB must be able to:

1. demonstrate how such crust can, by melting, produce high-magnesian lavas in many OIB suites, and
2. explain the lack of subduction-zone dehydration (metamorphic) signatures in OIB. 

The latter includes:

1. why OIB are enriched not only in water-insoluble incompatible elements, but also water-soluble incompatible elements,
2. why OIB are enriched in the progressively more incompatible elements, and
3. why OIB show significant correlated Sr-Nd-Hf isotopic variations, all of which are magmatic (vs. metamorphic) fingerprints.

Models that require ancient subducted crust as plume sources reactivated from the lower mantle also require physical mechanisms to overcome the huge negative buoyancy of subducted crust in both the lower and the upper mantle (p. ECV 5-16)

5. Our perspective of the origin of OIB sources (Niu & O'Hara, 2003)

The recycled deep portions of oceanic peridotitic lithosphere are the best candidate for the source feeding hot spot volcanism and OIB. These deep parts of oceanic lithosphere are likely to have been enriched in water-soluble incompatible elements such as Ba, Rb, Cs, Th, U, K, Sr, Pb as well as all other incompatible elements as a result of low-degree melt metasomatism at the interface between the LVZ and the cooling and thickening oceanic lithosphere. These metasomatized lithospheric materials are peridotitic in bulk composition. They can, by partial or locally total melting, produce high-magnesian melts for primitive OIB. Such materials (vs. crustal compositions) will develop positive thermal buoyancy upon heating, if subducted into the deep mantle, especially in the presence of a peridotitic melt phase, making it possible for the bulk material to ascend and feed OIB volcanism.

Concerning the papers by Sobolev et al. (2005, 2007), we comment that variations, of the type reported, in Ni, Ca, Mn and Cr content in olivine phenocrysts is a straightforward petrologic consequence of peridotite melting processes and mineral facies changes in the upper mantle. We are investigating the extent to which these effects account for the observations quantitatively.

4. Acknowledgements

We thank Gillian Foulger for inviting this contribution.

References

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