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Following bandwagons is easy, but insights matter!  (Yaoling Niu, October 15, 2017, Durham, UK)

I admire Arthur Holmes (that’s why I am now settled in Durham after travelling around the globe) and Harry Hess – the two most creative and inventive Earth Scientists of the 20th Century. Both were renowned geochemists and petrologists, and both creatively put forward hypotheses on how the Earth works – mantle convection, driving forces for continental drift, seafloor spreading etc., that laid the foundation of the powerful and elegant theory of plate tectonics. Thus, as a field geologist by training, I use petrology and geochemistry to study Earth processes on all scales. I emphasize observations (field, laboratory and global) and integrate apparently unrelated observations into coherent and logical lines of thought to gain insights to identify problems and solve them. In my research, I choose not to follow bandwagons, but enjoy thinking independently, and make effective contributions at the forefront of scientific debates on models of Earth’s evolution and processes. Below are brief summaries of my research in two sections, showing my twin commitments to discovery and to scientific debates (numerals in brackets, e.g., [9], refer to papers in my publication list: Niu (or He orHis) refers to the work by myself or the work led by myself with co-authors.

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Clarivate Analytics (WoS): 2017, 2018, 2019 Highly Cited Researcher (HCR):

1. Niu’s leading role in the well-known debates

(1) Global ocean ridge processes and mid-ocean ridge basalts (MORB) genesis:

(a) Niu argues that the global correlation of MORB chemistry with ridge depth is largely controlled by fertile mantle compositional (hence, mineralogy, density, solidus etc.) variation [39,77]. This conclusion is further verified by the updated global data [183], arguing against the inferred large mantle temperature control in the widely referenced model by C. H. Langmuir’s group.  (bNiu correlates global MORB (also abyssal peridotite) chemistry with plate spreading rate variation and demonstrates that this correlation is a simple consequence of the extent of sub-ridge mantle melting that increases with increasing plate spreading rate [20,22], which verifies the earlier (early 1980’s) thermal model prediction, and is further confirmed by the updated global data [183], arguing against the subjective statement by Langmuir’s group “There is no correlation between the chemical parameters and spreading rate.”  Niu [183] concludes “The meaning of the global MORB major element compositions may continue to be debated, but the conclusions offered here are the most objective and logical, and are most consistent with petrological, geochemical, geological and geophysical principles and observations.” I am delighted that this global ocean ridge problem is perfectly understood and I am confident that those who disagree will happily agree after having read Niu (2016) carefully with objective mind.

(2) Petrogenesis of global intra-plate ocean island basalts (OIB):

Mantle plumes from ancient oceanic crust” (standard model) advocated by A.W. Hofmann and W.W. White (1982) has convinced many and has thus been widely accepted. Niu demonstrated, however, that the major aspect of this interpretation is inconsistent with observations in terms of basic petrology, geochemistry and mineral physics. The observations are consistent with melting of low-degree-melt metasomatized mantle peridotites [42,48,52,80,90,101]. The OIB olivine trace/minor element abundances (Ni, Cr, Ca, Mn) variation by Hofmann’s group (Sobolev et al, 2005, 2007) is not evidence for recycled ocean crust for OIB, but pressure effect determined by lithosphere thickness variation (i.e., the “lid-effect”) [92,117,125]. Please note, Niu recognizes the significance of subducted ocean crust in basaltic magmatism as a consequence of plate tectonics [25,28,117,125,143,169,185,216], but objects the standard model in that (a) recycled ocean crust is chemically and isotopically too depleted to meet the requirement for any OIB suite on Earth and (b) ocean crust (basaltic vs. peridotitic) subducted into the lower mantle is too dense (> 2% denser than the ambience) to rise to the upper mantle feeding OIB. I am delighted that this first order global OIB problem is perfectly understood and I am confident that those who disagree will happily agree after having read Niu (2018) carefully with objective mind.

(3) Models on continental crust accretion:

The “island-arc” model by S. R. Taylor (1969, 1977) on continental crust growth has good geochemical footing, and has thus been generally accepted. However, with abundant new observations, Niu argues that this model has many difficulties, e.g., (a) bulk-continental crust is andesitic yet primary arc crust is basaltic, and (b) arc crust production is mass-balanced by subduction erosion and sediment recycling [79,90,133]. Based on his study in southern Tibet, Niu proposes “continental collision zones are primary sites for net continental crust growth” [133], which overcomes all the difficulties of the Island-arc model, and which has been tested to produce, preserve and thus maintain net crustal growth by extensive sampling and studying syncollisional granitoids along several Phanerozoic orogens in continental China (> 10000 km in total length) [142,157,161,167,168,171,173,175,176,184,186,191,194,202212].

(4) Subduction initiation:

There would be no plate tectonics if there were no subduction zones” [49]. Studies of subduction initiation have been many and continued to this day, culminating with three IODP expeditions (Legs 350-352 in 2014) in the western Pacific to test the ideas of spontaneous and induced subduction initiation. Niu considers all the existing models and efforts to be useful but inconclusive, and hypothesizes with insights “Subduction initiation is a consequence of lateral compositional buoyancy contrast within the lithosphere” [49,147] on the basis of petrological observations, physical analysis, and geological evolution. Niu offers a cost-effective method for “Testing the geologically testable hypothesis on subduction initiation” [178] by dredging forearc slopes of the western Pacific trenches because they expose  (through serpentine diapirs) basement rocks of island arcs, which are, prior to subduction initiation, continental (oceanic plateau, to a lesser extent) basement rocks off the prior continental (to a lesser extent oceanic plateau) margins because “large within-lithosphere compositional buoyancy contrast exists at edges of oceanic plateaus in ocean basins and at passive continental margins globally and these sites are loci of future subduction initiation” [178].

2. Niu’s major findings/contributions (which are continuing … …)

Niu uses petrology and geochemistry effectively to tackle global Earth problems, including discovering previously unrecognized efficacies of plate tectonics theory.  His major findings and original contributions, thus far, include the following:  

001:   Niu developed one of the first KD-based models for calculating major element compositions of primary mantle melts produced beneath ocean ridges, which is a conceptual advance, and forms foundation for many subsequent studies to this day on basalt petrogenesis [9].

002:  Niu shows that MORB chemical trends differ between slow- and fast-spreading ridges [12], and in particular the extent of mantle melting beneath ocean ridges, hence the production of ocean crust, increases with increasing spreading rate [22].

003:  Niu shows that the global correlation of MORB chemistry with ridge depth is a result of fertile mantle compositional variation (vs. mantle temperature variation) [77,183]. He also shows that at slow-spreading ridge segments, mantle compositional variation controls not only MORB compositions, but also the extent and depth of mantle melting, mantle flows, ridge morphology and ridge segmentation, elucidating the dynamic control of mantle composition on physical processes contrary to the traditional perception [39].

004:  Niu shows that abyssal peridotites are not simple MORB melting residues, but modified by olivine addition, accompanied by incompatible element refertilization, as a result of ascent and cooling of mantle melts through advanced residues atop the mantle right beneath ocean ridges [20,21,24,58].

005:  Niu developed, from abyssal peridotites, the quantitative description of polybaric decompression melting processes in the spinel peridotite stability field (a cpx + b opx + c spl = d ol + 1.000 melt with b > a) (vs. a > b in isobaric experiments) [20].

006:  Niu shows that intra-plate ocean island basalts are largely sourced from low-degree melt metasomatized mantle peridotites (e.g., deep portions of recycled oceanic mantle lithosphere), rather than from recycled oceanic crust as advocated in a widely referenced model [48,101,125].

007:  Niu hypothesizes that the oceanic crust subducted to the lower mantle is too dense to rise up and will irreversibly sink and contribute to the development of the two large-low-shear-wave-velocity provinces (LLSVPs) at the base of the mantle beneath the Pacific and Africa [207125,48].

008:  Niu shows cratonic lithosphere thinning results from basal hydration-weakening that effectively reduces the viscosity and coverts the basal “lithosphere” into convective “asthenosphere” (e.g., eastern China since the Mesozoic) with the water coming from dehydration of the paleo-Pacific slab stagnant in the mantle transition zone. This is the very process that develops seismic low velocity zone (LVZ) beneath continents [59,143,147,162,169,185], which also explains the lithosphere thinning in eastern Australia and the western US.

009:  Niu argues that initiation of subduction zones is a consequence of lateral compositional buoyancy contrast within the lithosphere [49,147,178, 218]. The large compositional buoyancy contrast in the lithosphere occurs at edges of oceanic plateaus in ocean basins and at passive continental margins. These are the loci of future subduction zones.  Therefore, subduction cannot initiate in normal ocean basins, but only does at edges of oceanic plateaus and at passive continental margins. This leads to the hypothesis that the basement of all the island arcs must have continental (or oceanic plateau) lithosphere affinityNiu proposes a cost-effective approach to test this hypothesis [178].

010:  Niu shows that element ratios Zr/Hf and Nb/Ta are not constant in earth rocks (vs. classic concept), but show up to over two orders of magnitude of correlated fractionations in seafloor basalts, gabbros and abyssal peridotites [21,25,58]. He hypothesizes mass-dependent fractionation to be important under mantle conditions and offers effective ways to test this hypothesis by determining possible 46,47,48,49,50Ti, 90,91,92,94,96Zr, 174,176,177,178,179,180Hf isotopic fractionations in these rocks [120,192].

011:  Niu shows that younger oceanic lithosphere must be decoupled from the subjacent asthenosphere, and that the degree of decoupling increases with increasing plate spreading rate as a result of ridge suction, which drives asthenospheric flows towards ocean ridges to form the new crust and for lithosphere growth [28,53], which explains petrological effects of the “plume-ridge interactions”, and offers insights for understanding upper mantle flow in the context of plate tectonics.

012:  Niu hypothesizes that continental collision zones, rather than active zones of seafloor subduction, are primary sites for net continental crust growth, and the episodic crustal growth in the geological record results from preservations of juvenile (mantle-derived) crustal materials during continental collisions and super-continent amalgamation [79,90,133]. Niu has effectively tested the hypothesis with success [142,157,161,167,168,171,173,175,176,181,184,186,191,194,202212] (see above), and anticipates general appreciation of this hypothesis in the near future.

013:  Niu proposes that the geochemically depleted MORB mantle results from upper mantle asthenosphere stratification processes [90], and may not be the intrinsic property of the oceanic upper mantle associated with continental crust extraction, which is a testable hypothesis underway.

014:  Niu discovered that primitive MORB, prior to plagioclase on the liquidus, have excess Eu (also Sr, Nb, Ta and Ti), leading to the hypothesis that MORB mantle hosts the missing Eu (Sr, Nb, Ta and Ti) in the continental crust [90]. The process or processes that have led to such crust-mantle complementarity is being further investigated beyond common perceptions.

015:  Niu shows that globally oceanic lithosphere thickness variation exerts the primary control on the chemistry of the intra-plate ocean island volcanism (i.e., the “lid effect”) despite the effect of mantle source compositional heterogeneity and possible mantle potential temperature variation [92,117, 209]; the “lid effect” has been shown to be important also for global MORB genesis [22,77,183]. Niu and his team have demonstrated recently that “Lithosphere thickness controls continental basalt compositions” using Cenozoic alkali basalts and contained pyroxene megacrysts in eastern China with a north-south spatial coverage in excess of 2500 km. [231,237].

016:  Niu proposes, based on many years’ studies in both oceanic (observational) petrology and experimental  petrology, that the nature of lithosphere-asthenosphere boundary (LAB) is a petrological boundary (amphibole dehydration solidus)[28,42,48,53,80,90,117], which is a unifying concept that not only explains the globally constant LAB depth (~ 90 km) beneath seafloor older than ~ 70 Ma, but also explains why the LAB depth increases with increasing seafloor ages from beneath ocean ridges to beneath seafloor of ~ 70 Ma. See [209]

017:  Niu revealsefficacies of plate tectonics theory beyond popular perception, e.g., (a) slab-pull directly drives the Pacific-type seafloor spreading, and the Atlantic-type seafloor spreading and continental drift are passive movement in response to trench retreat in the Pacific and Indian [147,162]; (b) continued India-Asia convergence since the collision at ~ 55 Ma results from the slab pull of the same giant India-Australia plate into the Sumatra–Indonesia trench (until its total disintegration in the future) [147,193,218]; (c) the “intra-plate” basaltic and granitoid magmatism since the Mesozoic in eastern continental China results from plate tectonics [59,147,162,215,220]; (d) the eastern Eurasia continent has migrated (continental drift) eastward for ~ 2500 km since ~ 50 Ma in response to western Pacific trench retreat [147].

018:  Niu hypothesizes [162] that the Chinese continental shelf (beneath East and South China Seas) is of exotic origin (micro-continent), transported along with the paleo-Pacific plate and jammed the trench at ~ 100 Ma. The locus (“suture”) of the jammed trench is marked by the arc-shaped southeast coastline of continental China [162,177]. This work explains many aspects of the geological evolution in the region, and offers solid constraints on plate tectonics reconstruction in the vast western Pacific and Eurasia since the Mesozoic.

019:  Niu hypothesizes “continental lithospheric thinning results from hydration weakening, not “delamination”, and is a special consequence of plate tectonics” [261], demonstrated using the example of eastern continental China [59,147,162,]. The summary is given here [Hydration weakening;doc1,doc2,doc3,doc4].

020:  Niu advocates that an effective approach to understanding mantle compositional heterogeneity is to carry out petrological and geochemical studies of subduction-zone metamorphism [101], which has led to a series of studies on “geochemical consequences of subduction-zone metamorpgism” [121,131,139,172,188,195], as well as collaborative studies on high- and ultra-high pressure metamorphic rocks [54,55,60,61,64,72,74,86,102,114,158,204].

021:  Niu participates in the great mantle plume debate [62] and emphasizes that the current debate has been ineffective and will have no way out! The mantle plume hypothesis can be effectively tested by drilling into the Kamchatka-Okhotsk Sea basement [198], which is the best candidate for the Hawaiian mantle plume head product [49,51]. This is because the original mantle plume hypothesis requires mantle plumes to be derived from the core-mantle thermal boundary layer and volumetrically significant lower-to-upper mantle material transport can only be accomplished through mantle plume heads. If the Hawaiian volcanism is indeed the very “classic” mantle plume, it must have the plume head product kept in the Kamchatka-Okhotsk Sea basement. If there is no such plume head product, then the claim for the “Hawaiian mantle plume” would be questionable or the mantle plume hypothesis would need revision [198].

022:  Niu argues with observations that “The continued India-Asia convergence since the collision ~55 million years ago has been driven by the subducting slab pull of the giant Indo-Australia plate at the Sumatra-Java trench. The convergence will cease to continue once the Indo-Australia plate disintegrates into several smaller plates in the future.” [147,229].

023:  Niu argues with petrological, geochemical and geophysical illustrations that “the LLSVPs at the base of the mantle are a consequence of plate tectonics.” [147,229].

024:  Niu argues with illustrations that “continental breakup is a straightforward consequence of plate tectonics without requiring mantle plumes. Mantle plumes, if needed, may be of help at the early rifting stage, but cannot lead to complete breakup, let alone to drive long distance dispersal of broken continents. Superplumes invoked by many do not exist.” [235].

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