Fig. 1: Pressure dependence of \u039bFp25 and \u039bFp75 for polycrystalline oxides at room temperature. Though \u039bFp25 (black symbols) and \u039bFp75 (red symbols) both initially increase with compression, they present different pressure evolution. Presumably due to the spin transition of iron, the \u039bFp25 decreases by ~30% at P\u2009~\u200943\u201354\u2009GPa, while the \u039bFp75 decreases by a larger extent of ~45% at higher pressures of P\u2009>\u2009~\u200953\u2009GPa. In the low-spin state, the \u039bFp75 has a much smaller pressure slope than the \u039bFp25. Each symbol shape represents individual measurement run. The vertical bar at each datum point indicates the data uncertainty of ~10\u201315%. Literature results for single-crystalline Fp8 (orange dashed curve)14<\/a>, Fp10 (blue dashed curve)14<\/a>, and Fp56 (green dashed curve)14<\/a>, as well as sintered polycrystalline Fp20 (bright green dashed line from ambient to 15\u2009GPa)15<\/a> and Fp19 (magenta open stars)25<\/a> are plotted for comparison.<\/p>\n In contrast, \u039bFp75 (red symbols) is systematically lower than \u039bFp25, presumably resulting from the higher FeO content and Fe3+\/\u2211Fe (see Methods), which induce stronger phonon-impurity and phonon-defect scatterings, respectively, with larger resistance to heat transport. At ambient conditions, \u039bFp75 is ~1.9\u2009W\u2009m\u22121 K\u22121, ~27% lower than \u039bFp25. Under compression, \u039bFp75 increases much less with increasing pressure than \u039bFp25, and reaches a much lower peak value of ~8.5\u2009\u00b1\u20091\u2009W\u2009m\u22121 K\u22121 at P\u2009~\u200940\u201353\u2009GPa. Afterwards, the \u039bFp75 decreases by a larger extent (\u2009~\u200945%) and then remains approximately constant, showing a \u201csluggish\u201d variation at a higher-pressure range than Fp25 through the spin transition. Finally, the low-spin \u039bFp75 only increases to ~6.5\u2009W\u2009m\u22121 K\u22121 at 99\u2009GPa, ~4-fold lower than \u039bFp25 at similar pressures.<\/p>\n We note that several factors could affect the \u0245Fp, including FeO content and ferric iron ratio (Fe3+\/\u2211Fe) (representing impurity and defect in the crystal), and grain size. The Fp8, Fp10, and Fp56 samples in Ref. 14<\/a> are single crystals with minor amounts of ferric iron, while the Fp25 and Fp75 samples in the present study are polycrystalline powders with ~6 and ~12% ferric iron, respectively (see Methods). At ambient conditions, the \u0245Fp56 is expected to be lower than \u0245Fp25, e.g., Ref. 25<\/a>. However, our data (Fig.\u00a01<\/a>) show that the present \u0245Fp25 seems to be comparable to the literature \u0245Fp56 up to ~100\u2009GPa, but with slightly lower onset pressure for the \u0245 reduction across the spin transition. Compared to the literature Fp56 sample, the lower FeO content yet with ~6% ferric iron in the present Fp25 sample may have counter-balanced the opposite effects between FeO content (the fewer FeO, the smaller thermal resistance due to less impurity scattering) and ferric iron (the more ferric iron, the larger thermal resistance due to more defects), leading to comparable thermal conductivity. The much lower \u0245Fp75 along with a weaker pressure dependence in high-spin state (its low-spin state even plateaus) is therefore presumably due to the large amounts of FeO and ferric iron. In addition, using electron backscattered diffraction we found that the typical grain size of present polycrystalline Fp25 and Fp75 samples before and after high-pressure measurements were both ~15\u201340\u2009\u03bcm, comparable or larger than our laser spot size in TDTR thermal conductivity measurements. We thus expect that the grain boundary plays minor roles here in affecting their thermal conductivity. Understanding the detailed, fundamental mechanisms for thermal transport in these samples in high- and low-spin states requires further advanced theoretical and computational studies.<\/p>\n \u039bFp25 and \u039bFp75 across the spin transition at elevated temperatures<\/p>\n To assess how elevated temperature influences \u039bFp25 and \u039bFp75 across the spin transition, we first compressed the sample to ~10\u2009GPa and heated it up to 573\u2009K (300\u2009\u00b0C) using an externally-heated DAC (EHDAC). We then measured \u039b at 573\u2009K from ~12 to 72\u2009GPa across the spin transition. As shown in Fig.\u00a02<\/a>, \u039bFp25 at 573\u2009K (black symbols) initially increases with pressure from 12.9 to ~35\u2009GPa and, given the data uncertainty of ~10\u201315%, remains at ~11.5\u2009\u00b1\u20091\u2009W\u2009m\u22121 K\u22121 between 35 and 46\u2009GPa. We then observed a slight decrease (\u2009~\u200917%) until ~56\u2009GPa, after which the \u039bFp25 re-increases with pressure to ~11.8\u2009W\u2009m\u22121 K\u22121 at 71\u2009GPa. Overall, in contrast to the behavior at room temperature through the spin transition (Fig.\u00a01<\/a>), the \u039bFp25 at 573\u2009K is systematically lower, with a plateau and fairly gentle decrease over a wider pressure range (\u2009~\u200935\u201356\u2009GPa). In addition, to explore its pressure dependence in the low-spin state, we have further measured the \u039bFp25 at 873\u2009K (blue symbols) in a separate experimental run, which shows a smaller pressure slope than at room temperature. Finally, \u039bFp75 (red symbols) at 573\u2009K is, again, systematically lower than the \u039bFp25, with a plateau at ~6.5\u2009\u00b1\u20090.5\u2009W\u2009m\u22121 K\u22121 at higher pressures of ~38\u201350\u2009GPa. We also observed a sluggish decrease (\u2009~\u200930%) to ~4.6\u2009W\u2009m\u22121 K\u22121 until ~66\u2009GPa. Though our measurement pressure was only extended to 72\u2009GPa, the low-spin \u039bFp75 seems to have a weak pressure dependence as that at room temperature.<\/p>\n Fig. 2: Pressure dependence of the \u039bFp25 and \u039bFp75 at elevated temperatures. Compared to the pressure evolution at room temperature, the \u039bFp25 (black symbols) and \u039bFp75 (red symbols) at 573\u2009K both soften in a more sluggish and gentle manner across the spin transition, where the pressure interval shifts to slightly higher pressures. The low-spin \u039bFp25 at 873\u2009K (blue symbols) has a weaker pressure slope than at room temperature. Each symbol shape represents individual experimental run. The vertical bar at each datum point shows its uncertainty of ~10\u201315%.<\/p>\n Since the \u039b of a material scales with the square of its sound velocity14<\/a>,26<\/a> (proportional to elastic constants), the pressure evolution of \u039bFp25 and \u039bFp75 in Figs.\u00a01<\/a> and 2<\/a> can be primarily accounted for by the softening of elastic constants across the spin transition14<\/a>,18<\/a>. Moreover, compared to room temperature, the extent of decrease in \u039b through the spin transition at 573\u2009K is more sluggish, and the transition extends to higher pressures. These observations are in good agreement with the qualitative evolution of elastic constants across the spin transition at elevated temperatures as predicted by ab initio calculations18<\/a>,19<\/a>.<\/p>\n Temperature dependence of the low-spin \u039bFp25 and \u039bFp75<\/p>\n Understanding the temperature dependence of \u039bFp25 and \u039bFp75 in the low-spin state is critical to better constrain the deep-mantle thermal state. To this end, we explored how the low-spin \u039bFp25 and \u039bFp75 change with temperature to ~973\u2009K (Fig.\u00a03<\/a>). To quantify their temperature dependence, for simplicity, we assumed that the transition to the low-spin state is complete above ~62\u201364\u2009GPa and that low-spin \u039bFp25 and \u039bFp75 can be modelled as \u039b(T)\u2009=\u2009\u039bLST n, where \u039bLS is a normalization constant. Such a power law dependence for \u039b(T) is based on fundamental heat transport theory under Debye approximation and on an assumption that the three phonon umklapp scattering between acoustic phonons is the predominant mechanism for heat transport27<\/a>,28<\/a>,29<\/a>,30<\/a>. Since phonon relaxation time can be approximated to be inversely proportional to T, heat transport theory predicts that for a pure crystal, \u039b(T) would scale with T \u22121, while for an impurity-bearing crystal, \u039b(T) would scale with T \u22120.5, see Refs. 27<\/a>,28<\/a>,29<\/a>,30<\/a> for detailed derivations of the theory for thermal conductivity at high temperatures. We determined the temperature exponent n by linearly fitting a regression line in the ln\u039b-lnT plot. For low-spin \u039bFp25, we found n\u2009=\u2009\u20130.36(\u2009\u00b1\u20090.07) at P\u2009=\u200962\u201368\u2009GPa and n\u2009=\u2009\u20130.41(\u2009\u00b1\u20090.06) at P\u2009=\u200972\u201378\u2009GPa (Fig.\u00a03a<\/a>), slightly weaker (less sensitive to temperature change) than the typical T \u20130.5 dependence for several Fe-bearing mantle minerals14<\/a>,15<\/a>,31<\/a>,32<\/a>,33<\/a>,34<\/a>. Note that despite the spin state, \u039b(Mg,Fe)O(T) has been conventionally approximated to T \u20130.5 throughout the whole mantle14<\/a>,15<\/a>,31<\/a>,33<\/a>, although a T \u20130.24 dependence for high-spin Fp5 and Fp20 only up to 14\u2009GPa and ~1150\u2009K had once been reported30<\/a>. Furthermore, the \u039bFp75 has a much weaker temperature dependence with n ranging from \u20130.15(\u2009\u00b1\u20090.04) to \u20130.3(\u2009\u00b1\u20090.07), see Fig.\u00a03b<\/a>. We emphasize that our findings represent critical experimental results indicating a weaker temperature dependence for the low-spin \u039bFp25 and \u039bFp75 than conventionally thought. We use this result to update modelled values for the deep-mantle conditions (see discussion below).<\/p>\n Fig. 3: Temperature dependence of \u039bFp25 and \u039bFp75 in the low-spin state. a \u039bFp25 has a temperature dependence T n with n\u2009=\u2009\u20130.36 to \u20130.41, which is slightly weaker than the typical T \u20130.5 for Fe-bearing mantle minerals14<\/a>,15<\/a>,31<\/a>,32<\/a>,33<\/a>,34<\/a>. b \u039bFp75 shows an even smaller temperature exponent of \u20130.15 to \u20130.3 at deep-mantle pressures. The vertical bar at each datum point represents its uncertainty of ~15%. The thermal pressure has minor effects on the temperature dependence, see Supplementary Note\u00a0S2<\/a> for details.<\/p>\n Modelling the thermal conductivity of lower mantle minerals along a mantle geotherm<\/p>\n Our results for \u039bFp25 and \u039bFp75 with different P, T, and FeO content provide better constraints on the thermal conductivity profiles of (Mg,Fe)O and pyrolitic mantle, which in turn play a key role in the thermochemical evolution and dynamics in the deep mantle, including the impacts on LLSVPs and ULVZs. Here we first modelled \u039bFp25 and \u039bFp75 along a representative mantle geotherm35<\/a>, to which we added a steep temperature rise from 2587\u2009K at 2800\u2009km depth (\u2009~\u2009130\u2009GPa)35<\/a> to 4000\u2009K at the CMB (\u2009~\u2009136\u2009GPa). Furthermore, we assumed that the \u039b(T) of high-spin \u039bFp25 and \u039bFp75 follow the typical T \u20130.5, while in the low-spin state they follow the trend we found in Fig.\u00a03\u2014i<\/a>.e., \u0245Fp25 scales with T \u20130.39 and \u0245Fp75 scales with T \u20130.23. In the mixed-spin state, \u039b(T) is approximated as the average of the high- and low-spin states. We find that \u039bFp25 (black solid line in Fig.\u00a04a<\/a>) starts from ~4\u2009W\u2009m\u22121 K\u22121 at 660\u2009km and increases with depth, followed by a slight decrease across the spin transition at ~1000\u20131400\u2009km depth. The low-spin \u039bFp25 then reaches ~11.5\u2009W\u2009m\u22121 K\u22121 at 2800\u2009km depth, and slightly reduces to 10\u2009W\u2009m\u22121 K\u22121 at the CMB due to the steep temperature rise. Compared to the conventionally assumed T \u20130.5 for both high- and low-spin states (black dashed line), the weaker temperature-dependence in the low-spin state (T \u20130.39) we found yields an ~20% higher \u039bFp25 at the lowermost mantle. Moreover, we highlight that the \u039bFp75 (red solid line) remains as low as ~3.6\u20133.8\u2009W\u2009m\u22121 K\u22121 from ~1600\u20132800\u2009km depth (\u2009~\u200950% higher than that with the conventional T \u20130.5 dependence, red dashed line). The CMB\u2019s high temperature further decreases the \u039bFp75 to an exceptionally low value of ~3.4\u2009W\u2009m\u22121 K\u22121.<\/p>\n Fig. 4: Modelled thermal conductivity profiles of lower mantle minerals along a representative mantle geotherm. a The red solid line represents the \u039bFp75 with a conventional T \u20130.5 dependence for high-spin state and a weaker T \u20130.23 dependence as we experimentally found for low-spin state, while, for comparison, the red dashed line indicates the \u039bFp75 with the conventional T \u20130.5 dependence for both high- and low-spin states. The black solid line shows the \u039bFp25 with the conventional T \u20130.5 dependence for high-spin state and a weaker T \u20130.39 dependence for low-spin state; the black dashed line is the \u039bFp25 with the conventional T \u20130.5 dependence for both high- and low-spin states. Overall, both \u039bFp25 and \u039bFp75 increase with depth, except for the range where the spin transition occurs. The low-spin \u039bFp75 remains nearly a constant of ~3.6\u20133.8\u2009W\u2009m\u22121 K\u22121 due to its weak temperature dependence. The small artifact discontinuities at the start and end of the spin transition result from the assumed temperature dependence in the mixed-spin state. Profiles of \u0245FeAl-Bm (FeAl-Bm, green dashed line) and \u0245davemaoite (Dvm, blue dashed line) are plotted for comparison. b From the top to the bottom of the lower mantle, the \u0245LM (blue solid line) increases two fold and converges to the previously modelled one (green dash-dotted line)14<\/a> at 8\u2009W\u2009m\u22121 K\u22121 at the lowermost mantle. The sudden decrease in \u0245 profile above the CMB is due to the steep temperature rise.<\/p>\n Figure\u00a04a<\/a> also shows our modelled \u0245 profile of (Fe,Al)-bearing bridgmanite (FeAl-Bm) with an assumed T \u20130.5 dependence33<\/a> (FeAl-Bm, green dashed line) and CaSiO3 davemaoite with a T \u20131.1 dependence36<\/a> (Dvm, blue dashed line). Combining these two profiles with our \u0245Fp25 and using a Voigt-Reuss-Hill (VRH) volumetric average scheme then provide an estimate of the thermal conductivity of pyrolitic lower-mantle aggregate, \u0245LM. Based on the pyrolite model, the lower mantle is composed of FeAl-Bm (\u2009~\u200975\u2009vol%), (Mg,Fe)O (\u2009~\u200918\u2009vol%), and CaSiO3 davemaoite (\u2009~\u20097\u2009vol%), and for calculations we assume \u0245LM\u2009=\u20090.75\u0245FeAl-Bm\u2009+\u20090.18\u0245Fp25\u2009+\u20090.07\u0245davemaoite. At ~1000\u2009km depth, our present modelled \u0245LM (blue solid line in Fig.\u00a04b<\/a>) is ~22% higher than the previously modelled profile based on room-temperature data (green dash-dotted line in Fig.\u00a04b<\/a> with a potential temperature of 2500\u2009K14<\/a>). Nevertheless, these two lines gradually converge in the deep mantle. Our modelled profile reaffirms that with the davemaoite, \u0245LM is ~8\u2009W\u2009m\u22121 K\u2212114<\/a> at ~2800\u2009km depth, but decreases to ~6.5\u2009W\u2009m\u22121 K\u22121 at the CMB, consistent with previous results, e.g., Refs. 37<\/a>,38<\/a>. For comparison, we also modelled a \u0245LM, where the \u0245Fp instead follows the conventional T \u20130.5, despite its spin state (orange dashed line in Fig.\u00a04b<\/a>). Because the (Mg,Fe)O only contributes ~18\u2009vol%, the \u0245LM with different \u039b(Mg,Fe)O(T) in the low-spin state only differs by <4% throughout the deep mantle. For similar reasons, the variable FeO content in the (Mg,Fe)O with depth also has a minor effect on the modelled \u0245LM14<\/a>. Finally, the \u0245LM is insensitive to the average scheme we adopted: when using Hashin\u2013Shtrikman scheme14<\/a>, the modelled \u0245LM is comparable (\u2009<\u2009~10\u201315% difference) to that using the VRH scheme throughout the lower mantle.<\/p>\n Impacts of thermally-insulating ULVZs on the lower-mantle thermal structure and dynamics and geodynamo evolutions<\/p>\n The anomalously low velocities and higher density of ULVZs relative to ambient mantle imply that they are likely thermochemical heterogeneities1<\/a>,2<\/a>,5<\/a>, e.g., Fe-rich mantle minerals, partial melts, or subducted slab materials. If made of Fe-rich oxides10<\/a>,11<\/a>, e.g., Fp75, ULVZs would have an exceptionally low thermal conductivity, \u0245ULVZ, ~3.4\u2009W\u2009m\u22121 K\u22121 (red short stripe above 2890\u2009km depth in Fig.\u00a04b<\/a>). This conductivity would further decrease with higher FeO content and thus, ULVZs may locally act as a thermal insulator at the CMB, meaning first that the amount of heat flowing from the core to the mantle may be limited at ULVZs locations, and second that ULVZs may have remained hot compared to their surroundings. Estimating temperature excess within ULVZs requires numerical simulations of mantle dynamics. Such excess potentially reaches several tens to a few hundreds of degrees, as suggested by temperature excess within low conductivity LLSVPs39<\/a>. Such low conductivity is comparable to the previously modelled Fe-rich mantle aggregate14<\/a>. Based on the results of Ref. 40<\/a>, we further modelled thermal conductivity of basaltic melt at CMB-relevant high P-T conditions to be ~1.9\u2009W\u2009m\u22121 K\u22121 (black short stripe above 2890\u2009km depth in Fig.\u00a04b<\/a>). This value should be considered an upper bound relative to thermal conductivity of Fe-enriched melt40<\/a>. If formed by heavy melt patches originated from the remnants of basal magma ocean41<\/a>,42<\/a> or subducted slabs4<\/a>,7<\/a>, ULVZs have not only ultralow seismic velocities, but also ultralow thermal conductivity, typically about 3-fold lower than the ambient mantle.<\/p>\n The different thickness and amplitude of velocity and density anomalies among ULVZs observed at different locations2<\/a>,4<\/a> may imply that they result from distinct formation mechanisms or are composed of different combinations of candidate materials. Recent seismic studies suggest that the internal structure of ULVZs may be complex with chemical heterogeneity and\/or multilayer structures41<\/a>,43<\/a>, making it difficult to resolve the spatial changes in the thermal conductivity within individual ULVZ. Nevertheless, since ULVZs are small-sized and share common features of Fe-enrichment and low-velocity, these properties coherently reduce their ability to conduct heat. We thus expect that the ultralow \u0245ULVZ we modelled based on that of Fp75 and basaltic melt (Fig.\u00a04b<\/a>) should be reasonably representative of \u0245ULVZ, although, again, different ULVZs may have different origin and nature.<\/p>\n Our findings have important consequences on the evolution of the CMB region. The ultralow \u0245ULVZ with strong thermal insulating effect would promote accumulation of heat (and temperature) at their base, thus delaying cooling, which in turn further reduces \u0245, as the \u0245 decreases with higher temperature (Fig.\u00a03<\/a>). This positive feedback mechanism results in thermal runaway that raises the local temperature, enhancing the buoyancy of ULVZs, their interactions with the surrounding LLSVPs and mantle, as well as the formation of thermal plumes rooted in ULVZs. Meanwhile, the large radial temperature gradient across the thin ULVZ patches caused by the efficient thermal insulation could produce a local, relatively cooler condition above them. Such reduced temperature may facilitate the growth of pPv above the ULVZs, and induce small-scale lateral thermal instability that could alter the regional mantle dynamics. Moreover, the thermal conductivity of the outer core, depending on its composition and investigation methods, has been suggested to a wide range of ~25\u2013250\u2009W\u2009m\u22121 K\u22121, e.g., Refs. 24<\/a>,44<\/a>,45<\/a> and references therein. Thus, within ULVZs, thermal conductivity on the mantle side of the CMB would be much lower than that on the outermost core side (Fig.\u00a05<\/a>) by at least one-order-of-magnitude. Combined with a high ULVZ temperature, TULVZ, this may significantly depress the local QCMB. Furthermore, if TULVZ is higher than the CMB temperature, local patches of negative QCMB, where heat flows from the mantle to the core, may be triggered. Such patches could in turn influence core flow, at least locally. Patches of very low or negative QCMB would further strongly increase the amplitude of lateral variations in QCMB, increasing the Q* parameter33<\/a> (defined as the ratio between the amplitude in QCMB lateral variations and twice the difference between the horizontally average QCMB and the core adiabatic heat flux). These effects would then impact the spatial and temporal evolution of the geodynamo, including the timing between polarity reversals of geomagnetic field46<\/a>. Future direct experimental measurements on the thermal conductivity of (Mg,Fe)O and other candidate ULVZ materials under deep-mantle\u2019s high P-T conditions will enable assessment of the validity of our modelled conductivity profiles and offer crucial insights to the heat transport at the lowermost mantle. Coupled with advanced numerical modelling that considers the effects of deep-mantle heterogeneities (e.g., LLSVPs, ULVZs, and slabs), these studies should bring better constraints on the complex evolution of lower-mantle thermochemical structures, the formation and growth of plumes, changes in the CMB heat flux pattern, and the stability of geodynamo.<\/p>\n Fig. 5: Illustration for the modelled thermal conductivity in the lower mantle.![\"figure](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2025\/11\/41467_2025_65430_Fig1_HTML.png\")
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