Mantle Plumes – the Persistent Myth

Australia: The Land Where Time Began

According to Anderson the fields of seismology, thermodynamics and classical physics show that beneath large tectonic plates that are long-lived the ambient shallow mantle is at temperatures that are hundreds of degrees hotter than the passive upwellings at the global spreading ridge system, and that mantle temperatures below about 200 km generally decrease with depth, and that the deep mantle low shear wave–speed features, instead of being narrow like mantle plumes, are sluggish and dome-like. The surface boundary layer of the mantle is more voluminous, as well as potentially being hotter, than are regions that are usually considered to be sources for intraplate volcanoes. The effect of this is that the ‘mantle plume’ explanation for Hawaii and large igneous provinces is not necessary. Upwellings are positive and large in isolated systems that are heated from within and cooled from above. Which suggests that at least until a small intrinsic buoyancy at shallow depths is induced by melting, tomographic features and upwellings are responses to plate tectonics, upwellings and subduction. According to Anderson melting anomalies, or ‘hotspots’, are side effects of plate tectonics that are primarily fed by processes in the boundary layer (BL) that are shear driven, and so not by deep buoyant upwellings. The lower boundary layer of the mantle is further stabilised by a dense basal mélange (BAM) component. Anderson suggests the transition region is the probable origin of mid-ocean ridges and associated broad passive depleted mantle (DM) upwellings, while deeper upwellings are broad domes that remain in the lower mantle.

Anderson has included a quote from Sir Arthur Stanley Eddington in The Nature of the Physical World (1929) in which Eddington says that the law that entropy always increases is supreme among the laws of nature. If a pet theory disagrees with Maxwell’s equations, then Maxwell’s equations lose out, the same goes for experimental results. But when the theory disagrees with the 2^nd law of thermodynamics it is the theory that must be wrong.

Conclusions

For the development of chemical geodynamic models that are based on fluid injection experiments (Maxwell’s Demons) and inspection and intuitive interpretations of selected saturated colour images obtained from crude forms of seismic imaging (TTT), are regarded as evidence of through going thermal features. It is accepted by geochemical modellers that the depth, temperature, composition, helium content and degassing history of the sources of hotspot magmas are all constrained by isotope geochemistry. The beliefs regarding mantle structure and convection are now intimately intertwined, though Anderson claims that basic physics has not been considered, and this omission rules out many of the bedrock assumptions. Models and ideas that were plume- and paradox-free which were developed in the 1960s and earlier by Holmes, Birch, Gutenberg, Verhoogen, Orowan, Elsasser, Hales, Ringwood and Green, and in the 1970s by Armstrong, Jacoby, Forsyth, Uyeda, Garfunkel, Richter, Tatsumoto, Tozer and Kaula, and from surface wave and mantle anisotropy studies in the 1980s and 1990s, Anderson says nicely account for contemporary and subsequent discoveries.

Anderson says it has now been documented abundantly (see Geological Society of America Special Papers 388, 430, and 470 and www.mantleplumes.org) that:

1. Essentially all assumptions and predictions of the plume paradigm are wrong;

2. The seismological models that are well-constrained, plate tectonics, recycling and surface boundary layer and transition zone sources and processes, can explain the geochemical, petrological and geophysical data, and at the same time are compatible with physics and thermodynamics; and

3. That shallow mantle beneath a plate is hotter than mantle beneath a ridge and that deeper mantle is, on average, subadiabatic.

Anderson says these can all be verified while not straying too far from the confines of S&N (Tozer, 1973; Anderson et al., 1992; Tackley et al., 1993; Ekstrom & Dziewonski, 1998; Hofmeister, 1999; Anderson, 2001; McNutt, 2007; Pilet et al., 2008; Kawakastu et al., 2009; Cao et al., 2011; Conrad et al., 2011; King, 2011; Murakami et al., 2012). Few of these papers mention plumes, and none of them support the assumptions that underlie the plume hypotheses.

Sources & Further reading