Basalts record a limited extent of mantle depletion: cause and chemical geodynamic implications
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Keywords: Mantle evolution, isotope ratios, incompatible elements, continental growth, mantle plumes, mantle convection, chemical geodynamics
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Abstract
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 Figure 1 Generation of variably ICE depleted peridotite by partial melting average peridotitic mantle to a maximum extent of melting of 16 %. Lower right panel shows calculated isotopic evolution within 1.5 Myr. Elemental concentrations before depletion: CNd = 0.713 μg/g, CSm = 0.270 μg/g (Salters and Stracke, 2004). Bulk partition coefficients: DNd = 0.024, DSm = 0.037 (Stracke and Bourdon, 2009). Depletion age: 1.5 Ga. Initial 143Nd/144Nd (at 1.5 Ga) = 0.510877. Decay constant: λ147Sm = 6.54 × 10−12. |  Figure 2 (a) Isotope ratio of aggregated melt (basalt) formed by mixing melts from variably ICE depleted peridotite and recycled crust as calculated with Equation 1, (b) Nd concentrations, and (c) Nd isotope ratios in variably ICE depleted peridotite aged for 1.5 Gyr before renewed melting. Recycled oceanic crust is a mixture of 96 % mafic crust (CNd = 5.83 μg/g, CSm = 2.01 μg/g) and 4 % sediments (CNd = 27.6 μg/g, CSm = 6.0 μg/g), also aged 1.5 Gyr with an initial 143Nd/144Nd = 0.509667 for sediments and 0.510879 for mafic crust before melting at present. Present-day melting degrees are 8 % for peridotite and 64 % for recycled oceanic crust, and are constant for simplicity. Bulk partition coefficients used for calculating melt compositions, CperNd and CrcNd, are DrcNd= 0.024, DperNd= 0.148 (Stracke and Bourdon, 2009). Each curve in (a) corresponds to a distinct amount of recycled oceanic crust, mrc, in the mantle source (4–12 %), and each point of a curve corresponds to a distinct degree of peridotite depletion FD (0–16 %). |  Figure 3 Isotope ratio of aggregated melt (basalt) formed by mixing melts from variably ICE depleted peridotite and recycled crust (parameters as in Fig. 2), showing that there are two principal reasons why OIB, on average, have lower Hf and Nd isotope ratios than MORB: 1) they contain more recycled crust (for similar average peridotite compositions), or 2) they contain similar amounts, but, on average, more extensively depleted peridotite (for similar recycled crust compositions). |  Figure 4 (a) Peridotite depletion FD (%) versus temperature T (°C) of a model mantle source with 9 % of recycled oceanic crust (frc = 0.09). (b) Excess temperature, ΔT (°C), required to make a mixture of variably depleted peridotite (FD) and recycled crust neutrally buoyant. δρ is the difference to the reference density ρ0 = 3300 kg m−3. Shown here is δρ for all combinations of FD and Tp, as discussed in the text. |
| Figure 1 | Figure 2 | Figure 3 | Figure 4 |
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Mantle Compositional Evolution
Earth’s mantle evolves continuously by returning peridotite melted under mid-ocean ridges into the deeper mantle via subduction, and remelting these residual, variably incompatible element (ICE) depleted peridotites once they are transported back into the shallow mantle. This basic concept (e.g., Langmuir et al., 1992
Langmuir, C.H., Klein, E.M., Plank, T. (1992) Petrological systematics of mid-ocean ridge basalts: constraints on melt generation beneath ocean ridges. Geophysical Monograph Series 71, 183–280. https://doi.org/10.1029/GM071p0183
; Willig et al., 2020Willig, M., Stracke, A., Beier, C., Salters, V.J.M. (2020) Constraints on mantle evolution from Ce-Nd-Hf isotope systematics. Geochimica et Cosmochimica Acta 272, 36–53. https://doi.org/10.1016/j.gca.2019.12.029
; Stracke, 2021Stracke, A. (2021) A process-oriented approach to mantle geochemistry. Chemical Geology 579, 120350. https://doi.org/10.1016/j.chemgeo.2021.120350
; Fig. 1) explains why ICE depleted basalts erupt at mid-ocean ridges. The Nd-Hf isotope ratios of mid-ocean ridge basalts (MORB) further show that these peridotites have resided in the mantle for some 107–109 years before being remelted, because the Nd-Hf isotope ratios are higher, on average, than those of the Bulk Silicate Earth (BSE; White et al., 2015White, W.M. (2015) Isotopes, DUPAL, LLSVPs, and Anekantavada. Chemical Geology 419, 10–28. https://doi.org/10.1016/j.chemgeo.2015.09.026
; Stracke et al., 2022Stracke, A., Willig, M., Genske, F., Béguelin, P., Todd, E. (2022) Chemical geodynamics insights from a machine learning approach. Geochemistry, Geophysics, Geosystems 23, e2022GC010606. https://doi.org/10.1029/2022GC010606
). This cycle of melting and remelting of Earth’s peridotitic mantle generates oceanic crust, and is the surface expression of the internal engine that drives plate tectonics. The number of melting cycles the peridotites may have undergone, their corresponding extent of ICE depletion, and their residence time in the mantle, however, remain major unknowns in our understanding of mantle compositional evolution, and mantle-crust mass fluxes in general.Figure 1 Generation of variably ICE depleted peridotite by partial melting average peridotitic mantle to a maximum extent of melting of 16 %. Lower right panel shows calculated isotopic evolution within 1.5 Myr. Elemental concentrations before depletion: CNd = 0.713 μg/g, CSm = 0.270 μg/g (Salters and Stracke, 2004
Salters, V.J.M., Stracke, A. (2004) Composition of the depleted mantle. Geochemistry, Geophysics, Geosystems 5, Q05B07, http://doi.org/10.1029/2003GC000597
). Bulk partition coefficients: DNd = 0.024, DSm = 0.037 (Stracke and Bourdon, 2009Stracke, A., Bourdon, B. (2009) The importance of melt extraction for tracing mantle heterogeneity. Geochimica et Cosmochimica Acta 73, 218–238. https://doi.org/10.1016/j.gca.2008.10.015
). Depletion age: 1.5 Ga. Initial 143Nd/144Nd (at 1.5 Ga) = 0.510877. Decay constant: λ147Sm = 6.54 × 10−12.An integral aspect of partial mantle melting, under mid-ocean ridges or the shallow mantle of actively upwelling mantle (“plumes”), is that peridotites that move on different paths through the melting region melt to different extents (Fig. 1). The peridotites that return and become recirculated within the mantle after partial melting are highly heterogeneous, and range from compositions similar to average “depleted mantle” (Salters and Stracke, 2004
Salters, V.J.M., Stracke, A. (2004) Composition of the depleted mantle. Geochemistry, Geophysics, Geosystems 5, Q05B07, http://doi.org/10.1029/2003GC000597
) to peridotites, which are almost devoid of ICEs (Fig. 1). In other words, any ICE depleted mantle sampled today is, by origin, a highly heterogeneous assemblage of compositionally, and isotopically, variable peridotites.During residence in the mantle, these heterogeneous peridotites are stretched, disintegrated, redistributed, stirred and become dispersed by the convective flow (Fig. 1), but they do not homogenise diffusively (Hofmann and Hart, 1978
Hofmann, A.W., Hart, S.R. (1978) An assessment of local and regional isotopic equilibrium in the mantle. Earth and Planetary Science Letters 38, 44–62. https://doi.org/10.1016/0012-821X(78)90125-5
; Hart, 1993Hart, S.R. (1993) Equilibration during mantle melting: a fractal tree model. Proceedings of the National Academy of Sciences 90, 11914–11918. https://doi.org/10.1073/pnas.90.24.11914
). An inevitable, but mostly neglected, consequence is that the radiogenic isotope ratios of variably ICE depleted and aged peridotites develop extreme values during several 107–109 years of residence in the mantle. These values reach far beyond the isotopic spectrum of MORB (Fig. 1), as observed in abyssal peridotites (e.g., Stracke et al., 2011Stracke, A., Snow, J.E., Hellebrand, E., von der Handt, A., Bourdon, B., Birbaum, K., Günther, D. (2011) Abyssal peridotite Hf isotopes identify extreme mantle depletion. Earth and Planetary Science Letters 308, 359–368. https://doi.org/10.1016/j.epsl.2011.06.012
; Sani et al., 2023Sani, C., Sanfilippo, A., Peyve, A.A., Genske, F., Stracke, A. (2023) Earth Mantle’s Isotopic Record of Progressive Chemical Depletion. AGU Advances 4, e2022AV000792. https://doi.org/10.1029/2022AV000792
). A fundamental question, therefore, is: why do oceanic basalts, i.e. MORB and basalts from oceanic islands (OIB), not mirror this enormous range of radiogenic isotope ratios of the peridotitic mantle?top
Melting Earth’s Heterogeneous Mantle
The answer to the question above is simple: partial melting of peridotite produces basaltic oceanic crust, which is returned into the deeper mantle together with the residual, melt-depleted peridotites. The recycled crust and peridotites become stirred and dispersed within the mantle by convection. As a result, Earth’s present mantle is chemically and lithologically heterogeneous, and consists of variably ICE depleted peridotite plus ICE enriched recycled oceanic (± continental) crust (Fig. 1; e.g., Stracke 2012
Stracke, A. (2012) Earth’s heterogeneous mantle: A product of convection-driven interaction between crust and mantle. Chemical Geology 330, 274–299. https://doi.org/10.1016/j.chemgeo.2012.08.007
, 2021Stracke, A. (2021) A process-oriented approach to mantle geochemistry. Chemical Geology 579, 120350. https://doi.org/10.1016/j.chemgeo.2021.120350
and references therein). Hence, oceanic basalts are mixtures of melts from chemically and lithologically heterogeneous mantle. As such, oceanic basalts represent a weighted average of melts from the different mantle components, but their isotope ratios are rarely, if ever, identical to those of a single mantle component.Specifically, isotope ratios in oceanic basalts are mixtures between melts from peridotites and recycled crust (± other components). Peridotites are the volumetrically dominant component (>85–90 %), but are also ICE depleted, whereas recycled crust is a minor (<10–15 %), but ICE enriched, component in Earth’s mantle (e.g., Stracke et al., 2022
Stracke, A., Willig, M., Genske, F., Béguelin, P., Todd, E. (2022) Chemical geodynamics insights from a machine learning approach. Geochemistry, Geophysics, Geosystems 23, e2022GC010606. https://doi.org/10.1029/2022GC010606
and references therein). The isotopic range of the heterogeneous peridotites is, therefore, not conveyed unchanged to oceanic basalts, but rather significantly shifted toward the isotope ratio of melts from the ICE enriched mantle component(s). Because lithophile elements with geologically useful radiogenic isotope ratios (Sr, Ce, Nd, Hf, Pb) are all ICEs, their inventory in oceanic basalts derives mostly from melts of the ICE enriched mantle components (i.e. recycled oceanic ± continental crust). The latter is demonstrated below with the example of melting of a two-component mantle that consists of ICE depleted peridotite (‘per’) and ICE enriched recycled crust (‘rc’, i.e. oceanic ± continental crust). The isotope ratio (IR) of element i in the derivative basalt is the weighted average of melts from the two mantle components:Eq. 1
with f = mass fraction of melt, which relates to the mass fraction, m, in the mantle by f = m × F, F being the degree of melting in %, and Ci being the concentrations of element i in melts from peridotite and recycled crust.
Equation 1 shows that the isotope ratios in basalts (IRibasalt) are a trade-off between the abundance (m) of the two mantle components, the extent to which the peridotite and recycled crust melt (F), the average composition of the melts from peridotite and recycled crust (Ci), and, of course, the corresponding isotope ratios in these melts (IRi). Isotope ratios in peridotites (IRiper) can be extreme, but are coupled to exceedingly low ICE concentrations (Fig. 1). Hence, although extreme IRiper have a large leverage in Equation 1, these isotopic signatures are moderated by correspondingly low concentrations in the peridotites and their derivative melts (Ciper). This is illustrated in Figure 2, which shows calculated IRibasalt that are a mix of a constant proportion of melts from recycled crust and peridotite from which variable amounts of melt have been extracted and aged for 1.5 Ga (Figs. 1, 2, Eq. 1). Although IRiper becomes extremely high with an increasing degree of ancient melt extraction (FD, subscript D for depletion) and, thus, an increasing extent of ICE depletion before renewed melting (Fig. 1), IRibasalt peak at low to intermediate values of FD in Figure 2, when CiperIRiper reaches its maximum value (note that fper and frc are constant in the calculation shown in Fig. 2). With increasing extent of peridotite depletion (FD), Ciper → 0, and thus the term fperCiperIRiper also approaches 0. Effectively then, fbasaltCibasaltIRibasalt → frcCircIRirc (Eq. 1), and IRibasalt → IRirc. Hence, there is a natural limit to which the range of isotope ratios of the peridotitic mantle, and especially the IRiper of the most ICE depleted peridotites, are conveyed to the basalts.
Figure 2 (a) Isotope ratio of aggregated melt (basalt) formed by mixing melts from variably ICE depleted peridotite and recycled crust as calculated with Equation 1, (b) Nd concentrations, and (c) Nd isotope ratios in variably ICE depleted peridotite aged for 1.5 Gyr before renewed melting. Recycled oceanic crust is a mixture of 96 % mafic crust (CNd = 5.83 μg/g, CSm = 2.01 μg/g) and 4 % sediments (CNd = 27.6 μg/g, CSm = 6.0 μg/g), also aged 1.5 Gyr with an initial 143Nd/144Nd = 0.509667 for sediments and 0.510879 for mafic crust before melting at present. Present-day melting degrees are 8 % for peridotite and 64 % for recycled oceanic crust, and are constant for simplicity. Bulk partition coefficients used for calculating melt compositions, CperNd and CrcNd, are DrcNd= 0.024, DperNd= 0.148 (Stracke and Bourdon, 2009
Stracke, A., Bourdon, B. (2009) The importance of melt extraction for tracing mantle heterogeneity. Geochimica et Cosmochimica Acta 73, 218–238. https://doi.org/10.1016/j.gca.2008.10.015
). Each curve in (a) corresponds to a distinct amount of recycled oceanic crust, mrc, in the mantle source (4–12 %), and each point of a curve corresponds to a distinct degree of peridotite depletion FD (0–16 %).Likewise, there is also a limit to which the isotope ratios of recycled components, IRirc, are reflected in basalts. For ICEs frcCirc > fperCiper, and thus IRibasalt also approaches IRirc, especially if IRirc > IRiper, e.g., for Sr isotope ratios. For compatible elements, however, frcCirc << fperCiper, i.e. that IRirc only has a significant effect on IRibasalt if IRirc >> IRiper. The latter is critical for evaluating whether recycled components can have significant imprint on the stable isotope ratios of compatible elements in oceanic basalts, e.g., for Cr, Ni, Mg, but also Zn, Fe and other transition elements. Equation 1 allows evaluating if natural and/or partial melting induced stable isotope variation in recycled components suffice to affect IRibasalt, but this is rarely considered (cf., Sodermann et al., 2022
Soderman, C.R., Shorttle, O., Matthews, S., Williams, H.M. (2022) Global trends in novel stable isotopes in basalts: theory and observations. Geochimica et Cosmochimica Acta 318, 388–414. https://doi.org/10.1016/j.gca.2021.12.008
).Hence, the value of IRibasalt is determined by the parameters in Equation 1, i.e. the relative abundance, composition and intrinsic isotopic range of individual mantle components, which are, of course, inherently heterogeneous and not limited to only two components. The style and efficiency of mixing melts from individual components during extraction from the mantle and melt evolution in the crust also plays a crucial role for how the isotopic range of heterogeneous mantle components are reflected in the generated melts (e.g., Stracke and Bourdon, 2009
Stracke, A., Bourdon, B. (2009) The importance of melt extraction for tracing mantle heterogeneity. Geochimica et Cosmochimica Acta 73, 218–238. https://doi.org/10.1016/j.gca.2008.10.015
; Koornneef et al., 2012Koornneef, J.M., Stracke, A., Bourdon, B., Meier, M.A., Jochum, K.P., Stoll, B., Grönvold, K. (2012) Melting of a two-component source beneath Iceland. Journal of Petrology 53, 127–157. https://doi.org/10.1093/petrology/egr059
; Rudge et al., 2013Rudge, J.F., Maclennan, J., Stracke, A. (2013) The geochemical consequences of mixing melts from a heterogeneous mantle. Geochimica et Cosmochimica Acta 114, 112–143. http://dx.doi.org/10.1016/j.gca.2013.03.042
; Shorttle et al., 2014Shorttle, O., Maclennan, J., Lambart, S. (2014) Quantifying lithological variability in the mantle. Earth and Planetary Science Letters 395, 24–40. http://dx.doi.org/10.1016/j.epsl.2014.03.040
, 2016Shorttle, O., Rudge, J.F., Maclennan, J., Rubin, K.H. (2016) A Statistical Description of Concurrent Mixing and Crystallization during MORB Differentiation: Implications for Trace Element Enrichment. Journal of Petrology 57, 2127–2162. http://dx.doi.org/10.1093/petrology/egw056
; Neave et al., 2019Neave, D.A., Namur, O., Shorttle, O., Holtz, F. (2019) Magmatic evolution biases basaltic records of mantle chemistry towards melts from recycled sources. Earth and Planetary Science Letters 520, 199–211. https://doi.org/10.1016/j.epsl.2019.06.003
; Genske et al., 2019Genske, F., Stracke, A., Berndt, J., Klemme, S. (2019) Process-related isotope variability in oceanic basalts revealed by high-precision Sr isotope ratios in olivine-hosted melt inclusions. Chemical Geology 524, 1–10. https://doi.org/10.1016/j.chemgeo.2019.04.031
; Stracke, 2021Stracke, A. (2021) A process-oriented approach to mantle geochemistry. Chemical Geology 579, 120350. https://doi.org/10.1016/j.chemgeo.2021.120350
). Although understanding the mechanisms and effects of melt mixing on the relation between mantle and basalt heterogeneity is still in its infancy (e.g., Rudge et al., 2013Rudge, J.F., Maclennan, J., Stracke, A. (2013) The geochemical consequences of mixing melts from a heterogeneous mantle. Geochimica et Cosmochimica Acta 114, 112–143. http://dx.doi.org/10.1016/j.gca.2013.03.042
), it has become clear that the isotopic range of oceanic basalts is offset from and much smaller than that of the peridotitic mantle, as long as some degree of mixing between melts from heterogeneous peridotites and ICE enriched mantle components occurs (Eq. 1, Fig. 2). Melt mixing is unavoidable, because melts from several 10–100 km wide and deep melting regions are funnelled to the localised eruption sites at the ridge axis or volcano at the surface (e.g., Stracke, 2021Stracke, A. (2021) A process-oriented approach to mantle geochemistry. Chemical Geology 579, 120350. https://doi.org/10.1016/j.chemgeo.2021.120350
).top
Implications for Mantle-Crust Mass Fluxes
Another critical effect of melt mixing is that oceanic basalts do not reflect the average extent of ICE depletion of the underlying peridotitic mantle. A column of peridotitic mantle that melted between 0 and 16 % and is 1.5 Ga old (Fig. 1), for example, has average 143Nd/144Nd = 0.514327 (ɛNd = +33.1), and 176Hf/177Hf = 0.285310 (ɛHf = +89.3). If a mantle source consisting of such heterogeneous peridotite also contains 3 % of recycled crust with 143Nd/144Nd = 0.512762 (ɛNd = +2.6), and 176Hf/177Hf = 0.282756 (ɛHf = −1.0), and the peridotites melt, on average, to 8 % and recycled crust to 64 %, the total aggregated melt has 143Nd/144Nd = 0.512866 (ɛNd = +4.6), and 176Hf/177Hf = 0.283029 (ɛHf = +8.6) (Eq. 1).
Using this average isotope ratio of the total extracted melts from such a heterogeneous mantle source, i.e. the isotope ratios of oceanic basalts, to evaluate the extent of mantle depletion will, therefore, by far underestimate the extent of ICE depletion of the peridotitic mantle, and thus the average rate of processing mantle through melting regions in the shallow mantle, which scales with the mantle’s convective vigor. This also means that mantle-crust mass fluxes may be higher than inferred from MORB-based estimates of mantle depletion (e.g., Jacobsen and Wasserburg, 1979
Jacobsen, S.B., Wasserburg, G.J. (1979) Mean age of mantle and crustal reservoirs. Journal of Geophysical Research 84, 7411–7427. https://doi.org/10.1029/JB084iB13p07411
; Galer et al., 1989Galer, S.J.G., Goldstein, S.L., O’Nions, R.K. (1989) Limits on chemical and convective isolation in the Earth’s interior. Chemical Geology 75, 257–290. https://doi.org/10.1016/0009-2541(89)90001-6
; Kumari et al., 2016Kumari, S., Paul, D., Stracke, A. (2016) Open system models of isotopic evolution in Earth’s silicate reservoirs: Implications for crustal growth and mantle heterogeneity. Geochimica et Cosmochimica Acta 195, 142–157. http://dx.doi.org/10.1016/j.gca.2016.09.011
; Tucker et al., 2020Tucker, J.M., van Keken, P.E., Jones, R.E., Ballentine, C.J. (2020) A role for subducted oceanic crust in generating the depleted mid‐ocean ridge basalt mantle. Geochemistry, Geophysics, Geosystems 21, e2020GC009148. https://doi.org/10.1029/2020GC009148
).top
Implications for Isotopic Differences Between MORB and OIB
Another implication of Equation 1 and Figures 2 and 3 is that isotopic differences between oceanic basalts can be explained in several ways. For example, ɛNd basalt can decrease from +8 to +5 because there is a larger contribution of melts from recycled crust with ɛNd rc = −1, or because the peridotite melts derive from, on average, more ICE depleted peridotite (Fig. 3). The latter may, at first, be counterintuitive, because highly ICE depleted and aged peridotites have extremely high ɛNd per (Figs. 1, 2). However, because they are highly ICE depleted, high ɛNd per are compensated by CperNd → 0, i.e. the contribution of Nd from peridotite melts, fperCperNd ɛNd per → 0, and thus ɛNd basalt → ɛNd rc, irrespective of high ɛNd per (Fig. 3). Hence, the, on average, lower Hf-Nd and higher Ce isotope ratios of OIB compared to MORB (White, 2015
White, W.M. (2015) Isotopes, DUPAL, LLSVPs, and Anekantavada. Chemical Geology 419, 10–28. https://doi.org/10.1016/j.chemgeo.2015.09.026
; Willig et al., 2020Willig, M., Stracke, A., Beier, C., Salters, V.J.M. (2020) Constraints on mantle evolution from Ce-Nd-Hf isotope systematics. Geochimica et Cosmochimica Acta 272, 36–53. https://doi.org/10.1016/j.gca.2019.12.029
; Stracke et al., 2022Stracke, A., Willig, M., Genske, F., Béguelin, P., Todd, E. (2022) Chemical geodynamics insights from a machine learning approach. Geochemistry, Geophysics, Geosystems 23, e2022GC010606. https://doi.org/10.1029/2022GC010606
) could either mean that 1) the mantle sources of OIB contain more recycled crust (for similar average peridotite compositions) than most MORB sources, or that 2) OIB sources contain similar amounts of recycled crust as MORB sources, but, on average, more extensively depleted peridotites (Fig. 3).Figure 3 Isotope ratio of aggregated melt (basalt) formed by mixing melts from variably ICE depleted peridotite and recycled crust (parameters as in Fig. 2), showing that there are two principal reasons why OIB, on average, have lower Hf and Nd isotope ratios than MORB: 1) they contain more recycled crust (for similar average peridotite compositions), or 2) they contain similar amounts, but, on average, more extensively depleted peridotite (for similar recycled crust compositions).
There are more aspects to consider. For example, it is clear that OIB range to different Sr-Nd-Hf-Pb isotope ratios than MORB (Zindler and Hart, 1986
Zindler, A., Hart, S. (1986) Chemical geodynamics. Annual Review of Earth and Planetary Sciences 14, 493–571. https://doi.org/10.1146/annurev.ea.14.050186.002425
; White, 2015White, W.M. (2015) Isotopes, DUPAL, LLSVPs, and Anekantavada. Chemical Geology 419, 10–28. https://doi.org/10.1016/j.chemgeo.2015.09.026
; Stracke, 2021Stracke, A. (2021) A process-oriented approach to mantle geochemistry. Chemical Geology 579, 120350. https://doi.org/10.1016/j.chemgeo.2021.120350
; Stracke et al., 2022Stracke, A., Willig, M., Genske, F., Béguelin, P., Todd, E. (2022) Chemical geodynamics insights from a machine learning approach. Geochemistry, Geophysics, Geosystems 23, e2022GC010606. https://doi.org/10.1029/2022GC010606
and references therein), and their mantle sources may, thus, not only contain different proportions, but also different types of recycled components. Broad linear trends in various combinations of Ce-Nd-Hf isotope ratio diagrams (e.g., Willig et al., 2020Willig, M., Stracke, A., Beier, C., Salters, V.J.M. (2020) Constraints on mantle evolution from Ce-Nd-Hf isotope systematics. Geochimica et Cosmochimica Acta 272, 36–53. https://doi.org/10.1016/j.gca.2019.12.029
) make it difficult to distinguish between different types of recycled crust, but Sr and Pb isotope ratios, in combination with Ce-Nd-Hf isotope ratios, may distinguish between different types of oceanic ± continental crust (e.g., Stracke, 2012Stracke, A. (2012) Earth’s heterogeneous mantle: A product of convection-driven interaction between crust and mantle. Chemical Geology 330, 274–299. https://doi.org/10.1016/j.chemgeo.2012.08.007
and references therein). On the other hand, extremely high Nd-Hf and low Ce isotope ratios in ICE depleted and aged peridotites are a sensitive tracer of variably ICE depleted peridotite (Willig et al., 2020Willig, M., Stracke, A., Beier, C., Salters, V.J.M. (2020) Constraints on mantle evolution from Ce-Nd-Hf isotope systematics. Geochimica et Cosmochimica Acta 272, 36–53. https://doi.org/10.1016/j.gca.2019.12.029
). This is because mixing melts from heterogeneous peridotites and ICE enriched components forms linear trends subparallel to the global trends in Ce-Nd-Hf isotope diagrams, which are variably displaced toward high Hf-Nd and low Ce isotope ratios (Salters et al., 2011Salters, V.J.M., Mallick, S., Hart, S.R., Langmuir, C.E., Stracke, A. (2011) Domains of depleted mantle: New evidence from hafnium and neodymium isotopes. Geochemistry, Geophysics, Geosystems 12, Q10017, https://doi.org/10.1029/2011GC003617
; Béguelin et al., 2019Béguelin, P., Bizimis, M., Mcintosh, E.C., Cousens, B., Clague, D.A. (2019) Sources vs processes: Unraveling the compositional heterogeneity of rejuvenated-type Hawaiian magmas. Earth and Planetary Science Letters 514, 119–129. https://doi.org/10.1016/j.epsl.2019.03.011
; Willig et al., 2020Willig, M., Stracke, A., Beier, C., Salters, V.J.M. (2020) Constraints on mantle evolution from Ce-Nd-Hf isotope systematics. Geochimica et Cosmochimica Acta 272, 36–53. https://doi.org/10.1016/j.gca.2019.12.029
; Sanfilippo et al., 2021Sanfilippo, A., Salters, V.J.M., Sokolov, S.Y., Peyve, A.A., Stracke, A. (2021) Ancient refractory asthenosphere revealed by mantle re-melting at the Arctic Mid Atlantic Ridge. Earth and Planetary Science Letters 566, 116981. https://doi.org/10.1016/j.epsl.2021.116981
; Stracke, 2021Stracke, A. (2021) A process-oriented approach to mantle geochemistry. Chemical Geology 579, 120350. https://doi.org/10.1016/j.chemgeo.2021.120350
).top
Implications for Plume Buoyancy
Many OIB form above mantle upwellings, so called mantle “plumes”, whose buoyancy is driven by thermochemical density deficits relative to the ambient mantle. Higher amounts of recycled crust make plumes denser, meaning that high temperatures are required to make them less dense than the ambient mantle (i.e. positively buoyant). Peridotites, however, also become less dense with increasing extent of melt depletion. Hence, lower temperature excesses are required to make melt-depleted peridotite positively buoyant.
For example, the compositional density of peridotite, Δρc, scales with the extent of melt depletion, FD (Δρc ≈ 214.5 kg m−3 × FD; Afonso and Schutt, 2012
Afonso, J.C., Schutt, D.L. (2012) The effects of polybaric partial melting on density and seismic velocities of mantle restites. Lithos 134, 289–303. https://doi.org/10.1016/j.lithos.2012.01.009
). That is, for ca. 8 % of melt extraction, peridotite density decreases by ∼0.5 % (reference mantle density, ρ0 = 3300 kg m−3). Density deficits of similar magnitude due to temperature alone require temperature excesses >150 °C (Fig. 4, T at FD = 0), assuming that the thermal density deficit, ΔρT, is due to thermal expansion of peridotite by ΔρT = −ρ0 × α × ΔT, with α being the thermal expansivity of 3 × 10−5 (K−1), and ΔT being the excess temperature relative to ambient mantle potential temperature Tp (ΔT = Tp−T0; with T0 = 1350 °C, Ballmer et al., 2013Ballmer, M.D., Ito, G., Wolfe, C.J., Solomon, S.C. (2013) Double layering of a thermochemical plume in the upper mantle beneath Hawaii. Earth and Planetary Science Letters 376, 155–164. https://doi.org/10.1016/j.epsl.2013.06.022
). For an excess density of the recycled oceanic crust of 180 kg m−3 (Ishii et al., 2019Ishii, T., Kojitani, H., Akaogi, M. (2019) Phase relations of harzburgite and MORB up to the uppermost lower mantle conditions: precise comparison with pyrolite by multisample cell high‐pressure experiments with implication to dynamics of subducted slabs. Journal of Geophysical Research: Solid Earth 124, 3491–3507. https://doi.org/10.1029/2018JB016749
), adding ∼9 % of recycled oceanic crust to a peridotitic mantle source (ρ0 = 3300 kg m−3) increases the density by ∼0.5 %. A temperature excess of ∼150 °C, or assuming a more melt-depleted peridotite from which ∼8 % of melt has been extracted, would make this heterogeneous peridotite-recycled crust assemblage neutrally buoyant (Fig. 4).Figure 4 (a) Peridotite depletion FD (%) versus temperature T (°C) of a model mantle source with 9 % of recycled oceanic crust (frc = 0.09). (b) Excess temperature, ΔT (°C), required to make a mixture of variably depleted peridotite (FD) and recycled crust neutrally buoyant. δρ is the difference to the reference density ρ0 = 3300 kg m−3. Shown here is δρ for all combinations of FD and Tp, as discussed in the text.
Whether different proportions of recycled crust or more extensively depleted mantle is responsible for observed isotopic differences between MORB and OIB is, therefore, also a central question for understanding the thermochemical driving forces of active mantle upwellings (e.g., Shorttle et al., 2014
Shorttle, O., Maclennan, J., Lambart, S. (2014) Quantifying lithological variability in the mantle. Earth and Planetary Science Letters 395, 24–40. http://dx.doi.org/10.1016/j.epsl.2014.03.040
; Stracke et al., 2019Stracke, A., Genske, F., Berndt, J., Koornneef, J.M. (2019). Ubiquitous ultra-depleted domains in Earth’s mantle. Nature Geoscience 12, 851–855. https://doi.org/10.1038/s41561-019-0446-z
, 2022Stracke, A., Willig, M., Genske, F., Béguelin, P., Todd, E. (2022) Chemical geodynamics insights from a machine learning approach. Geochemistry, Geophysics, Geosystems 23, e2022GC010606. https://doi.org/10.1029/2022GC010606
), and mantle convective motion in general, and key for relating geochemical observations and geodynamic constraints.top
Summary and Perspectives
Highly heterogeneous peridotites with isotope ratios far beyond the MORB spectrum are a natural consequence of oceanic crust generation and mantle residence, and characterise the majority (>85–90 %) of Earth’s mantle (Fig. 1). Heterogeneous peridotite-derived melts, therefore, always influence oceanic basalt isotope ratios, even if subtly so (Eq. 1, Figs. 2, 3). Equation 1 is a basis for evaluating how basalt compositions reflect individual components of heterogeneous mantle sources, and for better constraining basalt-forming processes (Figs. 2, 3). A key question to address further is whether peridotites in plume sources of OIB are more melt-depleted, and thus lighter than the peridotites of MORB sources. If composition is, indeed, a large factor for thermochemical plume buoyancy, plumes may not necessarily have to originate from thermal boundary layers and peridotite composition may also influence evolving plume structures. Variable peridotite depletion, therefore, connects geochemical and geophysical observables, and is a critical parameter for advancing our understanding of basalt generation, plume formation, and chemical geodynamic models of mantle convection.
top
Acknowledgements
The authors thank Julie Prytulak and an anonymous reviewer for their constructive and thoughtful comments, and editor Horst Marshall for his conscientious and effective editorial handling.
Editor: Horst Marschall
top
References
Afonso, J.C., Schutt, D.L. (2012) The effects of polybaric partial melting on density and seismic velocities of mantle restites. Lithos 134, 289–303. https://doi.org/10.1016/j.lithos.2012.01.009
Show in context For example, the compositional density of peridotite, Δρc, scales with the extent of melt depletion, FD (Δρc ≈ 214.5 kg m−3 × FD; Afonso and Schutt, 2012).
View in article
Ballmer, M.D., Ito, G., Wolfe, C.J., Solomon, S.C. (2013) Double layering of a thermochemical plume in the upper mantle beneath Hawaii. Earth and Planetary Science Letters 376, 155–164. https://doi.org/10.1016/j.epsl.2013.06.022
Show in context Density deficits of similar magnitude due to temperature alone require temperature excesses >150 °C (Fig. 4, T at FD = 0), assuming that the thermal density deficit, ΔρT, is due to thermal expansion of peridotite by ΔρT = −ρ0 × α × ΔT, with α being the thermal expansivity of 3 × 10−5 (K−1), and ΔT being the excess temperature relative to ambient mantle potential temperature Tp (ΔT = Tp−T0; with T0 = 1350 °C, Ballmer et al., 2013).
View in article
Béguelin, P., Bizimis, M., Mcintosh, E.C., Cousens, B., Clague, D.A. (2019) Sources vs processes: Unraveling the compositional heterogeneity of rejuvenated-type Hawaiian magmas. Earth and Planetary Science Letters 514, 119–129. https://doi.org/10.1016/j.epsl.2019.03.011
Show in context This is because mixing melts from heterogeneous peridotites and ICE enriched components forms linear trends subparallel to the global trends in Ce-Nd-Hf isotope diagrams, which are variably displaced toward high Hf-Nd and low Ce isotope ratios (Salters et al., 2011; Béguelin et al., 2019; Willig et al., 2020; Sanfilippo et al., 2021; Stracke, 2021).
View in article
Galer, S.J.G., Goldstein, S.L., O’Nions, R.K. (1989) Limits on chemical and convective isolation in the Earth’s interior. Chemical Geology 75, 257–290. https://doi.org/10.1016/0009-2541(89)90001-6
Show in context This also means that mantle-crust mass fluxes may be higher than inferred from MORB-based estimates of mantle depletion (e.g., Jacobsen and Wasserburg, 1979; Galer et al., 1989; Kumari et al., 2016; Tucker et al., 2020).
View in article
Genske, F., Stracke, A., Berndt, J., Klemme, S. (2019) Process-related isotope variability in oceanic basalts revealed by high-precision Sr isotope ratios in olivine-hosted melt inclusions. Chemical Geology 524, 1–10. https://doi.org/10.1016/j.chemgeo.2019.04.031
Show in context The style and efficiency of mixing melts from individual components during extraction from the mantle and melt evolution in the crust also plays a crucial role for how the isotopic range of heterogeneous mantle components are reflected in the generated melts (e.g., Stracke and Bourdon, 2009; Koornneef et al., 2012; Rudge et al., 2013; Shorttle et al., 2014, 2016; Neave et al., 2019; Genske et al., 2019; Stracke, 2021).
View in article
Hart, S.R. (1993) Equilibration during mantle melting: a fractal tree model. Proceedings of the National Academy of Sciences 90, 11914–11918. https://doi.org/10.1073/pnas.90.24.11914
Show in context During residence in the mantle, these heterogeneous peridotites are stretched, disintegrated, redistributed, stirred and become dispersed by the convective flow (Fig. 1), but they do not homogenise diffusively (Hofmann and Hart, 1978; Hart, 1993).
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Hofmann, A.W., Hart, S.R. (1978) An assessment of local and regional isotopic equilibrium in the mantle. Earth and Planetary Science Letters 38, 44–62. https://doi.org/10.1016/0012-821X(78)90125-5
Show in context During residence in the mantle, these heterogeneous peridotites are stretched, disintegrated, redistributed, stirred and become dispersed by the convective flow (Fig. 1), but they do not homogenise diffusively (Hofmann and Hart, 1978; Hart, 1993).
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Ishii, T., Kojitani, H., Akaogi, M. (2019) Phase relations of harzburgite and MORB up to the uppermost lower mantle conditions: precise comparison with pyrolite by multisample cell high‐pressure experiments with implication to dynamics of subducted slabs. Journal of Geophysical Research: Solid Earth 124, 3491–3507. https://doi.org/10.1029/2018JB016749
Show in context For an excess density of the recycled oceanic crust of 180 kg m−3 (Ishii et al., 2019), adding ∼9 % of recycled oceanic crust to a peridotitic mantle source (ρ0 = 3300 kg m−3) increases the density by ∼0.5 %.
View in article
Jacobsen, S.B., Wasserburg, G.J. (1979) Mean age of mantle and crustal reservoirs. Journal of Geophysical Research 84, 7411–7427. https://doi.org/10.1029/JB084iB13p07411
Show in context This also means that mantle-crust mass fluxes may be higher than inferred from MORB-based estimates of mantle depletion (e.g., Jacobsen and Wasserburg, 1979; Galer et al., 1989; Kumari et al., 2016; Tucker et al., 2020).
View in article
Koornneef, J.M., Stracke, A., Bourdon, B., Meier, M.A., Jochum, K.P., Stoll, B., Grönvold, K. (2012) Melting of a two-component source beneath Iceland. Journal of Petrology 53, 127–157. https://doi.org/10.1093/petrology/egr059
Show in context The style and efficiency of mixing melts from individual components during extraction from the mantle and melt evolution in the crust also plays a crucial role for how the isotopic range of heterogeneous mantle components are reflected in the generated melts (e.g., Stracke and Bourdon, 2009; Koornneef et al., 2012; Rudge et al., 2013; Shorttle et al., 2014, 2016; Neave et al., 2019; Genske et al., 2019; Stracke, 2021).
View in article
Kumari, S., Paul, D., Stracke, A. (2016) Open system models of isotopic evolution in Earth’s silicate reservoirs: Implications for crustal growth and mantle heterogeneity. Geochimica et Cosmochimica Acta 195, 142–157. https://dx.doi.org/10.1016/j.gca.2016.09.011
Show in context This also means that mantle-crust mass fluxes may be higher than inferred from MORB-based estimates of mantle depletion (e.g., Jacobsen and Wasserburg, 1979; Galer et al., 1989; Kumari et al., 2016; Tucker et al., 2020).
View in article
Langmuir, C.H., Klein, E.M., Plank, T. (1992) Petrological systematics of mid-ocean ridge basalts: constraints on melt generation beneath ocean ridges. Geophysical Monograph Series 71, 183–280. https://doi.org/10.1029/GM071p0183
Show in context This basic concept (e.g., Langmuir et al., 1992; Willig et al., 2020; Stracke, 2021; Fig. 1) explains why ICE depleted basalts erupt at mid-ocean ridges.
View in article
Neave, D.A., Namur, O., Shorttle, O., Holtz, F. (2019) Magmatic evolution biases basaltic records of mantle chemistry towards melts from recycled sources. Earth and Planetary Science Letters 520, 199–211. https://doi.org/10.1016/j.epsl.2019.06.003
Show in context The style and efficiency of mixing melts from individual components during extraction from the mantle and melt evolution in the crust also plays a crucial role for how the isotopic range of heterogeneous mantle components are reflected in the generated melts (e.g., Stracke and Bourdon, 2009; Koornneef et al., 2012; Rudge et al., 2013; Shorttle et al., 2014, 2016; Neave et al., 2019; Genske et al., 2019; Stracke, 2021).
View in article
Rudge, J.F., Maclennan, J., Stracke, A. (2013) The geochemical consequences of mixing melts from a heterogeneous mantle. Geochimica et Cosmochimica Acta 114, 112–143. https://dx.doi.org/10.1016/j.gca.2013.03.042
Show in context Although understanding the mechanisms and effects of melt mixing on the relation between mantle and basalt heterogeneity is still in its infancy (e.g., Rudge et al., 2013), it has become clear that the isotopic range of oceanic basalts is offset from and much smaller than that of the peridotitic mantle, as long as some degree of mixing between melts from heterogeneous peridotites and ICE enriched mantle components occurs (Eq. 1, Fig. 2).
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The style and efficiency of mixing melts from individual components during extraction from the mantle and melt evolution in the crust also plays a crucial role for how the isotopic range of heterogeneous mantle components are reflected in the generated melts (e.g., Stracke and Bourdon, 2009; Koornneef et al., 2012; Rudge et al., 2013; Shorttle et al., 2014, 2016; Neave et al., 2019; Genske et al., 2019; Stracke, 2021).
View in article
Salters, V.J.M., Stracke, A. (2004) Composition of the depleted mantle. Geochemistry, Geophysics, Geosystems 5, Q05B07, http://doi.org/10.1029/2003GC000597
Show in context Lower right panel shows calculated isotopic evolution within 1.5 Myr. Elemental concentrations before depletion: CNd = 0.713 μg/g, CSm = 0.270 μg/g (Salters and Stracke, 2004).
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The peridotites that return and become recirculated within the mantle after partial melting are highly heterogeneous, and range from compositions similar to average “depleted mantle” (Salters and Stracke, 2004) to peridotites, which are almost devoid of ICEs (Fig. 1).
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Salters, V.J.M., Mallick, S., Hart, S.R., Langmuir, C.E., Stracke, A. (2011) Domains of depleted mantle: New evidence from hafnium and neodymium isotopes. Geochemistry, Geophysics, Geosystems 12, Q10017, https://doi.org/10.1029/2011GC003617
Show in context This is because mixing melts from heterogeneous peridotites and ICE enriched components forms linear trends subparallel to the global trends in Ce-Nd-Hf isotope diagrams, which are variably displaced toward high Hf-Nd and low Ce isotope ratios (Salters et al., 2011; Béguelin et al., 2019; Willig et al., 2020; Sanfilippo et al., 2021; Stracke, 2021).
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Sanfilippo, A., Salters, V.J.M., Sokolov, S.Y., Peyve, A.A., Stracke, A. (2021) Ancient refractory asthenosphere revealed by mantle re-melting at the Arctic Mid Atlantic Ridge. Earth and Planetary Science Letters 566, 116981. https://doi.org/10.1016/j.epsl.2021.116981
Show in context This is because mixing melts from heterogeneous peridotites and ICE enriched components forms linear trends subparallel to the global trends in Ce-Nd-Hf isotope diagrams, which are variably displaced toward high Hf-Nd and low Ce isotope ratios (Salters et al., 2011; Béguelin et al., 2019; Willig et al., 2020; Sanfilippo et al., 2021; Stracke, 2021).
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Sani, C., Sanfilippo, A., Peyve, A.A., Genske, F., Stracke, A. (2023) Earth Mantle’s Isotopic Record of Progressive Chemical Depletion. AGU Advances 4, e2022AV000792. https://doi.org/10.1029/2022AV000792
Show in context These values reach far beyond the isotopic spectrum of MORB (Fig. 1), as observed in abyssal peridotites (e.g., Stracke et al., 2011; Sani et al., 2023).
View in article
Shorttle, O., Maclennan, J., Lambart, S. (2014) Quantifying lithological variability in the mantle. Earth and Planetary Science Letters 395, 24–40. https://dx.doi.org/10.1016/j.epsl.2014.03.040
Show in context The style and efficiency of mixing melts from individual components during extraction from the mantle and melt evolution in the crust also plays a crucial role for how the isotopic range of heterogeneous mantle components are reflected in the generated melts (e.g., Stracke and Bourdon, 2009; Koornneef et al., 2012; Rudge et al., 2013; Shorttle et al., 2014, 2016; Neave et al., 2019; Genske et al., 2019; Stracke, 2021).
View in article
Whether different proportions of recycled crust or more extensively depleted mantle is responsible for observed isotopic differences between MORB and OIB is, therefore, also a central question for understanding the thermochemical driving forces of active mantle upwellings (e.g., Shorttle et al., 2014; Stracke et al., 2019, 2022), and mantle convective motion in general, and key for relating geochemical observations and geodynamic constraints.
View in article
Shorttle, O., Rudge, J.F., Maclennan, J., Rubin, K.H. (2016) A Statistical Description of Concurrent Mixing and Crystallization during MORB Differentiation: Implications for Trace Element Enrichment. Journal of Petrology 57, 2127–2162. https://dx.doi.org/10.1093/petrology/egw056
Show in context The style and efficiency of mixing melts from individual components during extraction from the mantle and melt evolution in the crust also plays a crucial role for how the isotopic range of heterogeneous mantle components are reflected in the generated melts (e.g., Stracke and Bourdon, 2009; Koornneef et al., 2012; Rudge et al., 2013; Shorttle et al., 2014, 2016; Neave et al., 2019; Genske et al., 2019; Stracke, 2021).
View in article
Soderman, C.R., Shorttle, O., Matthews, S., Williams, H.M. (2022) Global trends in novel stable isotopes in basalts: theory and observations. Geochimica et Cosmochimica Acta 318, 388–414. https://doi.org/10.1016/j.gca.2021.12.008
Show in context Equation 1 allows evaluating if natural and/or partial melting induced stable isotope variation in recycled components suffice to affect IRibasalt, but this is rarely considered (cf., Sodermann et al., 2022).
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Stracke, A. (2012) Earth’s heterogeneous mantle: A product of convection-driven interaction between crust and mantle. Chemical Geology 330, 274–299. https://doi.org/10.1016/j.chemgeo.2012.08.007
Show in context As a result, Earth’s present mantle is chemically and lithologically heterogeneous, and consists of variably ICE depleted peridotite plus ICE enriched recycled oceanic (± continental) crust (Fig. 1; e.g., Stracke 2012, 2021 and references therein).
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Broad linear trends in various combinations of Ce-Nd-Hf isotope ratio diagrams (e.g., Willig et al., 2020) make it difficult to distinguish between different types of recycled crust, but Sr and Pb isotope ratios, in combination with Ce-Nd-Hf isotope ratios, may distinguish between different types of oceanic ± continental crust (e.g., Stracke, 2012 and references therein).
View in article
Stracke, A. (2021) A process-oriented approach to mantle geochemistry. Chemical Geology 579, 120350. https://doi.org/10.1016/j.chemgeo.2021.120350
Show in context This basic concept (e.g., Langmuir et al., 1992; Willig et al., 2020; Stracke, 2021; Fig. 1) explains why ICE depleted basalts erupt at mid-ocean ridges.
View in article
As a result, Earth’s present mantle is chemically and lithologically heterogeneous, and consists of variably ICE depleted peridotite plus ICE enriched recycled oceanic (± continental) crust (Fig. 1; e.g., Stracke 2012, 2021 and references therein).
View in article
The style and efficiency of mixing melts from individual components during extraction from the mantle and melt evolution in the crust also plays a crucial role for how the isotopic range of heterogeneous mantle components are reflected in the generated melts (e.g., Stracke and Bourdon, 2009; Koornneef et al., 2012; Rudge et al., 2013; Shorttle et al., 2014, 2016; Neave et al., 2019; Genske et al., 2019; Stracke, 2021).
View in article
Melt mixing is unavoidable, because melts from several 10–100 km wide and deep melting regions are funnelled to the localised eruption sites at the ridge axis or volcano at the surface (e.g., Stracke, 2021).
View in article
For example, it is clear that OIB range to different Sr-Nd-Hf-Pb isotope ratios than MORB (Zindler and Hart, 1986; White, 2015; Stracke, 2021; Stracke et al., 2022 and references therein), and their mantle sources may, thus, not only contain different proportions, but also different types of recycled components.
View in article
This is because mixing melts from heterogeneous peridotites and ICE enriched components forms linear trends subparallel to the global trends in Ce-Nd-Hf isotope diagrams, which are variably displaced toward high Hf-Nd and low Ce isotope ratios (Salters et al., 2011; Béguelin et al., 2019; Willig et al., 2020; Sanfilippo et al., 2021; Stracke, 2021).
View in article
Stracke, A., Bourdon, B. (2009) The importance of melt extraction for tracing mantle heterogeneity. Geochimica et Cosmochimica Acta 73, 218–238. https://doi.org/10.1016/j.gca.2008.10.015
Show in context Bulk partition coefficients: DNd = 0.024, DSm = 0.037 (Stracke and Bourdon, 2009).
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Present-day melting degrees are 8 % for peridotite and 64 % for recycled oceanic crust, and are constant for simplicity. Bulk partition coefficients used for calculating melt compositions, CperNd and CrcNd, are DrcNd= 0.024, DperNd= 0.148 (Stracke and Bourdon, 2009).
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The style and efficiency of mixing melts from individual components during extraction from the mantle and melt evolution in the crust also plays a crucial role for how the isotopic range of heterogeneous mantle components are reflected in the generated melts (e.g., Stracke and Bourdon, 2009; Koornneef et al., 2012; Rudge et al., 2013; Shorttle et al., 2014, 2016; Neave et al., 2019; Genske et al., 2019; Stracke, 2021).
View in article
Stracke, A., Snow, J.E., Hellebrand, E., von der Handt, A., Bourdon, B., Birbaum, K., Günther, D. (2011) Abyssal peridotite Hf isotopes identify extreme mantle depletion. Earth and Planetary Science Letters 308, 359–368. https://doi.org/10.1016/j.epsl.2011.06.012
Show in context These values reach far beyond the isotopic spectrum of MORB (Fig. 1), as observed in abyssal peridotites (e.g., Stracke et al., 2011; Sani et al., 2023).
View in article
Stracke, A., Genske, F., Berndt, J., Koornneef, J.M. (2019). Ubiquitous ultra-depleted domains in Earth’s mantle. Nature Geoscience 12, 851–855. https://doi.org/10.1038/s41561-019-0446-z
Show in context Whether different proportions of recycled crust or more extensively depleted mantle is responsible for observed isotopic differences between MORB and OIB is, therefore, also a central question for understanding the thermochemical driving forces of active mantle upwellings (e.g., Shorttle et al., 2014; Stracke et al., 2019, 2022), and mantle convective motion in general, and key for relating geochemical observations and geodynamic constraints.
View in article
Stracke, A., Willig, M., Genske, F., Béguelin, P., Todd, E. (2022) Chemical geodynamics insights from a machine learning approach. Geochemistry, Geophysics, Geosystems 23, e2022GC010606. https://doi.org/10.1029/2022GC010606
Show in context
The Nd-Hf isotope ratios of mid-ocean ridge basalts (MORB) further show that these peridotites have resided in the mantle for some 107–109 years before being remelted, because the Nd-Hf isotope ratios are higher, on average, than those of the Bulk Silicate Earth (BSE; White et al., 2015; Stracke et al., 2022).
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Peridotites are the volumetrically dominant component (>85–90 %), but are also ICE depleted, whereas recycled crust is a minor (<10–15 %), but ICE enriched, component in Earth’s mantle (e.g., Stracke et al., 2022 and references therein).
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Hence, the, on average, lower Hf-Nd and higher Ce isotope ratios of OIB compared to MORB (White, 2015; Willig et al., 2020; Stracke et al., 2022) could either mean that 1) the mantle sources of OIB contain more recycled crust (for similar average peridotite compositions) than most MORB sources, or that 2) OIB sources contain similar amounts of recycled crust as MORB sources, but, on average, more extensively depleted peridotites (Fig. 3).
View in article
For example, it is clear that OIB range to different Sr-Nd-Hf-Pb isotope ratios than MORB (Zindler and Hart, 1986; White, 2015; Stracke, 2021; Stracke et al., 2022 and references therein), and their mantle sources may, thus, not only contain different proportions, but also different types of recycled components.
View in article
Whether different proportions of recycled crust or more extensively depleted mantle is responsible for observed isotopic differences between MORB and OIB is, therefore, also a central question for understanding the thermochemical driving forces of active mantle upwellings (e.g., Shorttle et al., 2014; Stracke et al., 2019, 2022), and mantle convective motion in general, and key for relating geochemical observations and geodynamic constraints.
View in article
Tucker, J.M., van Keken, P.E., Jones, R.E., Ballentine, C.J. (2020) A role for subducted oceanic crust in generating the depleted mid‐ocean ridge basalt mantle. Geochemistry, Geophysics, Geosystems 21, e2020GC009148. https://doi.org/10.1029/2020GC009148
Show in context This also means that mantle-crust mass fluxes may be higher than inferred from MORB-based estimates of mantle depletion (e.g., Jacobsen and Wasserburg, 1979; Galer et al., 1989; Kumari et al., 2016; Tucker et al., 2020).
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White, W.M. (2015) Isotopes, DUPAL, LLSVPs, and Anekantavada. Chemical Geology 419, 10–28. https://doi.org/10.1016/j.chemgeo.2015.09.026
Show in context The Nd-Hf isotope ratios of mid-ocean ridge basalts (MORB) further show that these peridotites have resided in the mantle for some 107–109 years before being remelted, because the Nd-Hf isotope ratios are higher, on average, than those of the Bulk Silicate Earth (BSE; White et al., 2015; Stracke et al., 2022).
View in article
Hence, the, on average, lower Hf-Nd and higher Ce isotope ratios of OIB compared to MORB (White, 2015; Willig et al., 2020; Stracke et al., 2022) could either mean that 1) the mantle sources of OIB contain more recycled crust (for similar average peridotite compositions) than most MORB sources, or that 2) OIB sources contain similar amounts of recycled crust as MORB sources, but, on average, more extensively depleted peridotites (Fig. 3).
View in article
For example, it is clear that OIB range to different Sr-Nd-Hf-Pb isotope ratios than MORB (Zindler and Hart, 1986; White, 2015; Stracke, 2021; Stracke et al., 2022 and references therein), and their mantle sources may, thus, not only contain different proportions, but also different types of recycled components.
View in article
Willig, M., Stracke, A., Beier, C., Salters, V.J.M. (2020) Constraints on mantle evolution from Ce-Nd-Hf isotope systematics. Geochimica et Cosmochimica Acta 272, 36–53. https://doi.org/10.1016/j.gca.2019.12.029
Show in context This basic concept (e.g., Langmuir et al., 1992; Willig et al., 2020; Stracke, 2021; Fig. 1) explains why ICE depleted basalts erupt at mid-ocean ridges.
View in article
Hence, the, on average, lower Hf-Nd and higher Ce isotope ratios of OIB compared to MORB (White, 2015; Willig et al., 2020; Stracke et al., 2022) could either mean that 1) the mantle sources of OIB contain more recycled crust (for similar average peridotite compositions) than most MORB sources, or that 2) OIB sources contain similar amounts of recycled crust as MORB sources, but, on average, more extensively depleted peridotites (Fig. 3).
View in article
Broad linear trends in various combinations of Ce-Nd-Hf isotope ratio diagrams (e.g., Willig et al., 2020) make it difficult to distinguish between different types of recycled crust, but Sr and Pb isotope ratios, in combination with Ce-Nd-Hf isotope ratios, may distinguish between different types of oceanic ± continental crust (e.g., Stracke, 2012 and references therein).
View in article
On the other hand, extremely high Nd-Hf and low Ce isotope ratios in ICE depleted and aged peridotites are a sensitive tracer of variably ICE depleted peridotite (Willig et al., 2020).
View in article
This is because mixing melts from heterogeneous peridotites and ICE enriched components forms linear trends subparallel to the global trends in Ce-Nd-Hf isotope diagrams, which are variably displaced toward high Hf-Nd and low Ce isotope ratios (Salters et al., 2011; Béguelin et al., 2019; Willig et al., 2020; Sanfilippo et al., 2021; Stracke, 2021).
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Zindler, A., Hart, S. (1986) Chemical geodynamics. Annual Review of Earth and Planetary Sciences 14, 493–571. https://doi.org/10.1146/annurev.ea.14.050186.002425
Show in context For example, it is clear that OIB range to different Sr-Nd-Hf-Pb isotope ratios than MORB (Zindler and Hart, 1986; White, 2015; Stracke, 2021; Stracke et al., 2022 and references therein), and their mantle sources may, thus, not only contain different proportions, but also different types of recycled components.
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Figures
Figure 1 Generation of variably ICE depleted peridotite by partial melting average peridotitic mantle to a maximum extent of melting of 16 %. Lower right panel shows calculated isotopic evolution within 1.5 Myr. Elemental concentrations before depletion: CNd = 0.713 μg/g, CSm = 0.270 μg/g (Salters and Stracke, 2004
Salters, V.J.M., Stracke, A. (2004) Composition of the depleted mantle. Geochemistry, Geophysics, Geosystems 5, Q05B07, http://doi.org/10.1029/2003GC000597
). Bulk partition coefficients: DNd = 0.024, DSm = 0.037 (Stracke and Bourdon, 2009Stracke, A., Bourdon, B. (2009) The importance of melt extraction for tracing mantle heterogeneity. Geochimica et Cosmochimica Acta 73, 218–238. https://doi.org/10.1016/j.gca.2008.10.015
). Depletion age: 1.5 Ga. Initial 143Nd/144Nd (at 1.5 Ga) = 0.510877. Decay constant: λ147Sm = 6.54 × 10−12.
Figure 2 (a) Isotope ratio of aggregated melt (basalt) formed by mixing melts from variably ICE depleted peridotite and recycled crust as calculated with Equation 1, (b) Nd concentrations, and (c) Nd isotope ratios in variably ICE depleted peridotite aged for 1.5 Gyr before renewed melting. Recycled oceanic crust is a mixture of 96 % mafic crust (CNd = 5.83 μg/g, CSm = 2.01 μg/g) and 4 % sediments (CNd = 27.6 μg/g, CSm = 6.0 μg/g), also aged 1.5 Gyr with an initial 143Nd/144Nd = 0.509667 for sediments and 0.510879 for mafic crust before melting at present. Present-day melting degrees are 8 % for peridotite and 64 % for recycled oceanic crust, and are constant for simplicity. Bulk partition coefficients used for calculating melt compositions, CperNd and CrcNd, are DrcNd= 0.024, DperNd= 0.148 (Stracke and Bourdon, 2009
Stracke, A., Bourdon, B. (2009) The importance of melt extraction for tracing mantle heterogeneity. Geochimica et Cosmochimica Acta 73, 218–238. https://doi.org/10.1016/j.gca.2008.10.015
). Each curve in (a) corresponds to a distinct amount of recycled oceanic crust, mrc, in the mantle source (4–12 %), and each point of a curve corresponds to a distinct degree of peridotite depletion FD (0–16 %).
Figure 3 Isotope ratio of aggregated melt (basalt) formed by mixing melts from variably ICE depleted peridotite and recycled crust (parameters as in Fig. 2), showing that there are two principal reasons why OIB, on average, have lower Hf and Nd isotope ratios than MORB: 1) they contain more recycled crust (for similar average peridotite compositions), or 2) they contain similar amounts, but, on average, more extensively depleted peridotite (for similar recycled crust compositions).
Figure 4 (a) Peridotite depletion FD (%) versus temperature T (°C) of a model mantle source with 9 % of recycled oceanic crust (frc = 0.09). (b) Excess temperature, ΔT (°C), required to make a mixture of variably depleted peridotite (FD) and recycled crust neutrally buoyant. δρ is the difference to the reference density ρ0 = 3300 kg m−3. Shown here is δρ for all combinations of FD and Tp, as discussed in the text.