Several methods have been developed to assess the thermal state of the mantle below oceanic ridges, islands, and plateaus, on the basis of the petrology and geochemistry of erupted lavas. One leads to the conclusion that mantle potential temperature (i.e., TP) of ambient mantle below oceanic ridges is 1430°C, the same as Hawaii. Another has ridges with a large range in ambient mantle potential temperature (i.e., TP = 1300–1570°C), comparable in some cases to hot spots (Klein and Langmuir, 1987; Langmuir et al., 1992). A third has uniformly low temperatures for ambient mantle below ridges, ∼1300°C, with localized 250°C anomalies associated with mantle plumes. All methods involve assumptions and uncertainties that we critically evaluate. A new evaluation is made of parental magma compositions that would crystallize olivines with the maximum forsterite contents observed in lava flows. These are generally in good agreement with primary magma compositions calculated using the mass balance method of Herzberg and O'Hara (2002), and differences reflect the well-known effects of fractional crystallization. Results of primary magma compositions we obtain for mid-ocean ridge basalts and various oceanic islands and plateaus generally favor the third type of model but with ambient mantle potential temperatures in the range 1280–1400°C and thermal anomalies that can be 200–300°C above this background. Our results are consistent with the plume model.

/pdf/temperatures-in-ambient-mantle-and-plumes-constraints-from-t2dh8hln70.pdf

Temperatures in Ambient Mantle and Plumes: Constraints from Basalts, Picrites, and Komatiites

This chapter is in part an update of a previous, more abbreviated review ( Hofmann, 1997 ) It covers the subject in greater depth, and it reflects some significant changes in the author's views since the writing of the earlier paper In particular, the spatial range of equilibrium attained during partial melting may be much smaller than previously thought, because of new experimental diffusion data and new results from natural settings Also, the question of ‘layered’ versus ‘whole-mantle’ convection, including the depth of subduction and of the origin of plumes, has to be reassessed in light of the recent breakthroughs achieved by seismic mantle tomography As the spatial resolution of seismic tomography and the pressure range, accuracy, and precision of experimental data on melting relations, phase transformations, and kinetics continue to improve, the interaction between these disciplines and geochemistry sensu stricto will continue to improve our understanding of what is actually going on in the mantle The established views of the mantle being engaged in simple two- or single-layer convection are becoming obsolete In many ways, we are just at the beginning of this new phase of mantle geology, geophysics, and geochemistry

Sampling mantle heterogeneity through oceanic basalts: Isotopes and trace elements

A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle

New finite-frequency tomographic images of S-wave velocity confirm the existence of deep mantle plumes below a large number of known hot spots. We compare S-anomaly images with an updated P-anomaly model. Deep mantle plumes are present beneath Ascension, Azores, Canary, Cape Verde, Cook Island, Crozet, Easter, Kerguelen, Hawaii, Samoa, and Tahiti. Afar, Atlantic Ridge, Bouvet(Shona), Cocos/Keeling, Louisville, and Reunion are shown to originate at least below the upper mantle if not much deeper. Plumes that reach only to midmantle are present beneath Bowie, Hainan, Eastern Australia, and Juan Fernandez; these plumes may have tails too thin to observe in the lowermost mantle, but the images are also consistent with an interpretation as “dying plumes” that have exhausted their source region. In the tomographic images, only the Eifel and Seychelles plumes are unambiguously confined to the upper mantle. Starting plumes are visible in the lowermost mantle beneath South of Java, East of Solomon, and in the Coral Sea. All imaged plumes are wide and fail to show plumeheads, suggesting a very weakly temperature-dependent viscosity for lower mantle minerals, and/or compositional variations. The S-wave velocity images show several minor differences with respect to the earlier P-wave results, including plume conduits that extend down to the core-mantle boundary beneath Cape Verde, Cook Island, and Kerguelen. A more substantial disagreement between P-wave and S-wave images reopens the question on the depth extent of the Iceland plume. We suggest that a pulsating behavior of the plume may explain the shape of the conduit beneath Iceland.

A catalogue of deep mantle plumes: New results from finite‐frequency tomography

New high-precision samarium-neodymium isotopic data for chondritic meteorites show that their 142Nd/144Nd ratio is 20 parts per million lower than that of most terrestrial rocks This difference indicates that most (70 to 95%) of Earth9s mantle is compositionally similar to the incompatible element–depleted source of mid-ocean ridge basalts, possibly as a result of a global differentiation 453 billion years ago (Ga), within 30 million years of Earth9s formation The complementary enriched reservoir has never been sampled and is probably located at the base of the mantle These data influence models of Earth9s compositional structure and require revision of the timing of global differentiation on Earth9s Moon and Mars

https://science.sciencemag.org/content/sci/309/5734/576.full.pdf

142Nd Evidence for Early (>4.53 Ga) Global Differentiation of the Silicate Earth

[1] Geodynamic models of thermo-chemical plumes rising in a mantle wind suggest that we should abandon some paradigms based on the dynamics of purely thermal axisymmetric plumes. The head-tail structure is possible but not unique and the lack of a plume head does not preclude a deep origin. Our results suggest that the surface expression of some thermo-chemical plumes may be a headless, age-progressive volcanic chain. Plume tails are laterally heterogeneous, rather than concentrically zoned, because deep heterogeneities are sheared into distinct and long-lasting filaments that will be successively sampled by different volcanoes, as the oceanic plate moves over the plume tail. Finally, calculated S-wave velocity anomalies are consistent with recent plume tomographic images, showing that compositional heterogeneities in the lowermost mantle favour the coexistence of a great variety of plume shapes and sizes.

/pdf/beyond-the-thermal-plume-paradigm-3614t4k5mv.pdf

Beyond the thermal plume paradigm

http://www.geosci.usyd.edu.au/users/prey/Teaching/Geos-3003/geos3003pict/Farnetani.pdf

Mixing and deformations in mantle plumes

https://www.ipgp.jussieu.fr/%7ecinzia/2003-SamuelFarnetani.pdf

Thermochemical convection and helium concentrations in mantle plumes

https://www.ipgp.fr/~cinzia/2010-FarnetaniHofmann.pdf

Dynamics and internal structure of the Hawaiian plume

https://www.ipgp.jussieu.fr/%7ecinzia/2009-FarnetaniHofmann.pdf

Cinzia G. Farnetani

Papers

Beyond the thermal plume paradigm

Mixing and deformations in mantle plumes

Thermochemical convection and helium concentrations in mantle plumes

Dynamics and internal structure of the Hawaiian plume

Dynamics and internal structure of a lower mantle plume conduit