A paradigm change: Lithospheric lid thickness, not mantle potential temperature, controls the extent of mantle melting, depth of melt extraction & basalt compositions in all tectonic settings on Earth

Yaoling Niu

Department of Earth Sciences, Durham University, Durham DH1 3LE, UK; yaoling.niu@durham.ac.uk

This webpage is a summary of: Niu, Y.L. (2021) Lithosphere thickness controls the extent of mantle melting, depth of melt extraction and basalt compositions in all tectonic settings on Earth – A review and new perspectives, Earth-Science Reviews 217, 103614; https://doi.org/10.1016/j.earscirev.2021.103614

Introduction

Basalts are the most abundant igneous rocks on Earth and their petrology and geochemistry have been used to infer the thermal structure and composition of the mantle and to research chemical differentiation in Earth. However, it was unclear until the 1960s that the mantle consists of peridotites whose partial melting produces basaltic magmas. This problem was finally solved by experimental petrology (e.g., Ringwood, 1962; Green & Ringwood, 1967; O’Hara, 1963, 1968; Kushiro, 1968). However, debate continued on what may actually cause partial melting until the 1970s when decompression melting became gradually accepted (Carmichael et al., 1974; Yoder, 1976). This concept had been conceived earlier–in the 1950s (Verhoogen, 1954) and had been developed further in the 1960s by comparing natural basalts with experimental melts (Green & Ringwood, 1967). These early experimental studies have formed a solid foundation for many aspects of our present-day understanding of basalt genesis in terms of peridotite compositions, pressure and temperature conditions, the effect of volatiles, and phase equilibria. This has, however, not developed into a consensus paradigm in the context of plate tectonics because of the ~30 year long debate on the nature of primary magma among experimental petrologists and in the petrology community.

Continued experimental petrology in parallel with worldwide sampling and study of mid-ocean ridge basalts (MORB) brought about new insights. This culminated in a theoretical model in the 1980s that mantle (potential) temperature (T_MP) variations control the extent and pressure (depth) of mantle melting and basalt compositions (e.g., Klein & Langmuir, 1987; McKenzie & Bickle, 1988). This model states that hotter rising mantle begins to melt deeper, has a taller melting column, and thus melts more and produces thicker crust and a shallower ridge. The melt has the signature of a higher extent and pressure of melting than melt from a colder mantle source.

These same ideas have been widely applied to the study of intraplate magmatism in ocean basins and continental interiors. Basalt generation above subduction zones has, on the other hand, been generally accepted since the early 1980s to result from slab-dehydration induced mantle wedge melting. Nevertheless, recent studies also advocate mantle temperature variations as the primary control on the extent of mantle wedge melting. All these views, as a result of popular consensus, have formed a paradigm of mantle melting and basaltic magmatism.

Driven by curiosity, and a need to understand each and every element of this paradigm, I have finally concluded over the past 30 years of my MORB studies that this paradigm has fundamental problems (e.g., Niu, 1997, 2004, 2016, 2021; Niu & Hékinian, 1997a; Niu & O’Hara, 2008). Rigorous analysis of global data in combination with experimental petrology has demonstrated unequivocally that it is lithospheric lid thickness that controls the extent of mantle melting (F), pressure (depth) of melt extraction (P_F) and basalt compositions. This applies not only to MORB but in all tectonic settings on Earth including intra-plate ocean island basalts (OIB), volcanic arc basalts above subduction zones (VAB) and basalts in continental interiors (CIB).

1. Three major problems with the paradigm, and MORB mantle melting controlled by the lithospheric lid effect

Problem [1]: The assumption that decompression melting continues all the way up to the Moho ignores the presence and effect of the conductive thermal boundary layer (CTBL) at the top of the mantle.

While it is conceptually correct that hotter upwelling mantle intersects the solidus (at P_O) and begins to melt deeper than colder upwelling mantle, it is incorrect in concept and unlikely in nature that decompression melting will continue all the way up to Earth's surface or even to Moho depth (Figure 1a,b). It will be arrested by the CTBL (Figure 1c,d; Figure. 2), whose presence is inevitable because of mantle heat loss to the seafloor. The thickness of the CTBL, i.e. the depth to its base, that determines the depth of melting cessation (at P_F) is a consequence of the heat balance between adiabatic upwelling of hot asthenosphere and heat loss to the seafloor. This is ultimately controlled by the rate of mantle upwelling.

Figure 1: Decompression melting of adiabatically rising mantle in P-T space. [a] and [b] decompression melting continues all the way to the Moho (Mh) is assumed in the paradigm. [c] and [d] decompression melting is capped by the conductive thermal boundary layer (CTBL) atop the mantle beneath ocean ridges. Click here or on image to enlarge.

Figure 2: Schematic diagram showing current understanding of ocean ridge mantle melting regime and its thermal structure in P-T space. Click here or on image to enlarge.

Plate separation causes sub-ridge mantle to rise and melt by decompression (Figure 2). Fast upwelling beneath fast-spreading ridges allows the adiabat to extend to shallow level, overcoming the conductive cooling to the seafloor, and resulting in a thin CTBL (shallow P_F), a large decompression melting interval (P_O-P_F) and a high extent of melting (high F ∝ P_O-P_F; Figure 1c). By contrast, slow upwelling beneath slow-spreading ridges allows conductive cooling to penetrate to greater depth against the adiabat, resulting in a thick CTBL (deep P_F), a small decompression interval (P_O-P_F) and a low extent of melting (low F ∝ P_O-P_F; Figure 1d).

This theoretical picture is consistent with MORB (and abyssal peridotites, the MORB melting residue) compositional systematics–the extent of mantle melting increases with increasing spreading rate (Niu & Hékinian, 1997a). Likewise, the compositionally (major elements) fertile mantle constitutes a mineral assemblage of high density, resulting in deep ridges, whereas compositionally depleted mantle is buoyant and gives rise to shallow ridges. Dense, fertile mantle beneath deep ridges upwells only sluggishly in response to plate separation. This limits upwelling and allows conductive cooling to penetrate to great depth (deep P_F). In turn, this shortens the melting interval and reduces the amount of melt produced (low F ∝ P_O-P_F; Figure 1d) relative to the more refractory and buoyant mantle beneath shallow ridges (shallow P_F and high F ∝ P_O-P_F; Figure 1c). This model is in agreement with MORB (and abyssal peridotite) compositional variation as a function of ocean ridge depth (Niu & O’Hara, 2008; Niu, 2016, 2021). The bottom line is that it is the base of the CTBL, P_F, that determines the extent of melting and basaltic compositions–the concept of the lid effect (Niu et al., 2001).

Problem [2]: The petrological parameter Fe₈ used to calculate the initial depth of mantle melting has no significance.

Experimental petrology (Jaques & Green, 1980) shows that both FeO and MgO in mantle melts increase with increasing pressure of melting. However, the Mg# (= 100*Mg/[Mg+Fe]) ≥ 72 has limited variation, being constrained by mantle olivine with Fo (= 100*Mg/[Mg+Fe]) ≥ 90 or Fo₉₀. Klein & Langmuir (1987) proposed the parameter Fe₈ (FeO wt% corrected to a constant MgO = 8%) to calculate P_O and T_O by applying the equations (Langmuir et al., 1992):

P_O = 6.11Fe₈-34.5 (kbar)
T_O = 1150 + 13P_O (°C)

which give:

T_MP = T_O-P_O*1.8 (°C).

These calculable P_O, T_O and T_MP values led to the paradigm that mantle potential temperature variations of 250 K control the extent and pressure of mantle melting beneath global ocean ridges.

Nevertheless, all these are based on a single parameter, Fe₈, that is petrologically invalid. Global Fe₈ varies from 6.5 to 11.5, corresponding to variably evolved MORB melts with Mg# = 68 to 56. The use of Fe₈ values that represent such variably and highly evolved melts to discuss mantle melting conditions violates basic petrological principle!  Primitive MORB melts with Mg# ≥ 72 that are in equilibrium with mantle olivine of Fo ≥ 90 could be used to discuss mantle sources and processes, not melts with Mg# < 72 because of significant crustal-level differentiation (e.g., Mg# = 68 to 56) (Niu & O’Hara, 2008). Therefore, values of calculated P_O, T_O and T_MP obtained using Fe₈ have no significance.

To defend the parameter Fe₈, Gale et al. (2014) re-corrected the data to show Fe₈ = Fe₉₀ (MORB melt in equilibrium with mantle olivine Fo₉₀). However, Fe₈ = Fe₉₀ is entirely incorrect because it is petrologically impossible as demonstrated simply by Niu (2016; see his Fig. 3). Therefore, the conclusion, on the basis of MORB Fe₈ (or any other form such as Fe₉₀), that global ocean ridge T_MP variations of up to ΔT_MP = 250 K control the extent and pressure of mantle melting is simply the result of artificial data manipulation and has no petrological foundation.

In this context, we should note that when corrected to Mg# = 72, Fe₇₂ of Pacific MORB shows variations that correlate with other major elements, the abundances and ratios of incompatible elements and radiogenic isotopes (Niu et al., 2002), and Fe isotopes (Sun et al., 2020a). This points to mantle compositional heterogeneity.

Problem [3]: The assumption that erupted basalts record the initial depth of mantle melting ignores the fact that effective and efficient melt-solid equilibration occurs in the melting mantle at least for the olivine-making chemical elements Si, Mg and Fe.

Figure 2 illustrates our current understanding that plate separation induced decompression melting begins when the upwelling mantle intersects the solidus at P_O (corresponding to T_O) and continues until the upwelling melting mantle reaches P_F (corresponding to T_F), the final depth of melting or melt-solid equilibration, which is the base of the CTBL (Region 2) beneath the ridge. Because of the buoyancy contrast, the melt (red-arrowed thin dash lines) ascends faster than the solid residue (blue-arrowed thick lines). The melt is extracted to form the ocean crust and the residue contributes to the lithospheric mantle. The latter, when tectonically exposed on the seafloor, is sampled as abyssal peridotites. During near-fractional decompression melting in the melting Region 1 (Figure 2) from P_O to P_F, the 1–2% (or much less) melt in physical contact with 98–99% (or greater) solid matrix ensures effective and efficient melt-solid re-equilibration. This applies at least for the elements Si, Mg and Fe, which are controlled by olivine, the most abundant mantle mineral. The thousands of years that melting takes ensures complete melt-solid re-equilibration that is readily achieved in just tens of hours in peridotite melting experiments.

In a plain language:

Figure 3 illustrates the lid effect on mantle melting.

Figure 3. Illustration of the lid effect on mantle melting beneath intra-plate ocean islands and OIB petrogenesis. Click here or on image to enlarge.

2. The lithospheric lid effect on mantle melting beneath intra-plate ocean islands for OIB

The lithospheric lid thickness beneath ocean ridges is known to be the thinnest with rather small variation in a global context. The resulting small lid effect is well recorded in MORB compositions, demonstrating that basalt composition is indeed sensitive to the lid effect. To test this idea further, we examined global OIB data since they are affected by large lid thickness variations, from thin near ocean ridges to up to ~ 90 km beneath mature oceanic lithosphere older than 70 Ma (Humphrey & Niu, 2009). The significant correlation of OIB compositions with thickness of the oceanic lithosphere at the time of volcanism is a simple manifestation of the lid effect. OIB erupted on thick lithosphere has the petrological signature of a low extent of melting (short decompression interval) and high pressure of melt extraction (deep P_F), whereas the opposite is true for OIB erupted on thin lithosphere (Figure 3; Niu et al., 2011).

This, together with experimental petrological results, has also led to the understanding that the lithosphere-asthenosphere boundary (LAB = P_F) beneath ocean basins is the amphibole dehydration solidus and is characterized by the isotherm (dT/dP = 0) ~ 1100°C at P ≤ ~ 3 GPa (< ~ 90 km depth) and an isobar (dP/dT = 0) of ~ 3 GPa (~ 90 km depth) at T = ~ 1050-1150°C (Niu & Green, 2018). This presents a unifying solution that not only explains why LAB depth increases with increasing seafloor age from beneath ocean ridges (~ 10 km) to beneath seafloor of up to ~ 70 Ma (the square root of age relationship) on a global scale, but also explains the intrinsic control on the globally constant LAB depth (~ 90 km) beneath seafloor older than ~ 70 Ma (isobaric solidus at ~ 90 km).

OIB are widely regarded as the products of deep-rooted mantle plumes. Different mantle plumes are expected to have different T_MP and thus different initial melting depths P_O. However, as explained, OIB only record P_F and have no memory of P_O. If the olivine-making elements Si, Mg and Fe only record P_F and not P_O due to effective melt-olivine equilibrium in the melting mantle, then the extent of mantle melting (F ∝ [P_O-P_F]) recorded by incompatible elements should show varying P_O. And yet OIB still have no record of P_O but only P_F. This indicates that all mantle plumes have similar T_MP and P_O. That is, globally, T_MP of mantle plumes is rather similar! Alternatively, T_MP between mantle plumes may vary very little and be negligible–far smaller than the lid effect (Niu et al., 2011). This issue may still be debated, but the rigorous analysis based on observations described here should not be ignored.

3. The lithospheric lid effect on mantle melting beneath continents producing Continental Interior Basalts (CIB)

The lid effect on MORB and OIB is further confirmed by the prediction that it exerts primary control on mantle melting producing basalts in the continental interiors (CIB). The present-day best test-ground is eastern continental China, where the cratonic lithosphere was thinned in the Mesozoic as a result of Paleo-Pacific subduction and basal hydration weakening (Niu, 2014). This is the same process that developed the seismic low velocity zone (LVZ) with varying LAB depths in the region. Indeed, we have shown that the Cenozoic CIB in eastern China have varying compositions that correlate with LAB depths in a way that is fully consistent with the lid effect as illustrated in Figure 3 (Guo et al., 2020; Sun et al., 2020b).

4. The lithospheric lid effect on mantle wedge melting above subduction zones for Volcanic Arc Basalts (VAB)

Volcanic arc basalts (VAB) above subduction zones are well-understood as resulting from mantle wedge melting induced by the subducting-slab dehydration effect (flux melting). My analysis of global VAB compositions compiled in the recent literature (Turner & Langmuir, 2025a,b) also indicates a significant lid effect. That is, the petrogenesis of VAB is a combination of flux-melting developing mantle diapirs and continued diapiric decompression melting with the extent of melting controlled by the arc lithospheric lid thickness above subduction zones (Niu, 2021).

5. Summary

In addition to the effects of magma evolution at varying depths, the compositional variability of basalts in each and every tectonic setting is also expected to preserve signatures of mantle source compositional variation, initial depth (P_O) of melting due to possible T_MP variation and source fertility variation. We do not ignore these factors, but they are secondary and overshadowed by the lid effect. It is lithosphere lid thickness, not mantle potential temperature, that controls the extent of mantle melting, depth of melt extraction and basalt compositions in all tectonic settings on Earth.  

It may be an exaggeration to claim a paradigm shift but regardless, the “temperature control” paradigm on basalt petrogenesis is inconsistent with all observations and needs change if the science is to progress. Since a change from “temperature control” to “lid effect” is fundamental and may be unfamiliar to many, it will take time for this new thinking to be accepted by the Earth Science community through further debate. To facilitate such debate, I offer two statements:

[1] Global MORB, OIB, VAB, and CIB compositions all show the lid effect (i.e., P_F control), but do not show the effect of “temperature control” (i.e., P_O or T_MP). Temperature may not be important at all in reality or its effects on surface observables must have been obliterated because of effective and efficient melt-solid equilibration in the melting mantle.

[2] Objectiveness and open-mindedness, in contrast to confirmation bias, are requisite twins for insights and discoveries.

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