Coupled dynamics and evolution of primordial and recycled heterogeneity in Earth’s lower mantle

The nature of compositional heterogeneity in Earth’s lower mantle remains a long-standing puzzle that can inform about the long-term thermochemical evolution and dynamics of our planet. Here, we use global-scale 2D models of thermo- chemical mantle convection to investigate the coupled evolution and mixing of (intrinsically-dense) recycled and (intrinsically- strong) primordial heterogeneity in the mantle. We explore the effects of ancient compositional layering of the mantle, as motivated by magma-ocean solidification studies, and of the physical parameters of primordial material. Depending on these physical parameters, our models predict various regimes of mantle evolution and heterogeneity preservation over 4.5 Gyrs. Over a wide parameter range, primordial and recycled heterogeneity are predicted to co-exist with each other in the lower mantle of Earth-like planets. Primordial material usually survives as mid-to-large scale blobs (or streaks) in the mid-mantle, around 1000-2000 km depth, and this preservation is largely independent on the initial primordial-material volume. In turn, recycled oceanic crust (ROC) persists as large piles at the base of the mantle and as small streaks everywhere else. In models with an additional dense FeO-rich layer initially present at the base of the mantle, the ancient dense material partially survives at the top of ROC piles, causing the piles to be compositionally stratified. Moreover, the addition of such an ancient FeO-rich basal layer significantly aids the preservation of the viscous domains in the mid-mantle. Finally, we find that primordial blobs are commonly directly underlain by thick ROC piles, and aid their longevity and stability. Based on our results, we propose an integrated style of mantle heterogeneity for the Earth, involving the preservation of primordial domains along with recycled piles. This style has important implications for early Earth evolution, and has the potential of reconciling geophysical and geochemical discrepancies on present-day lower-mantle heterogeneity.

Integrated geological-geophysical characterization of the zone of the Kherson—Smolensk transregional tectonic suture — deep long-lived magma- and fluid-conducting channel

The transregional Kherson—Smolensk suture has been established to be located between large meridional faults of the crystalline crust of the Ukrainian Shield (USh) in a strip of 50—70 km width and separates two microplates of different composition of the Precambrian basement. It is traced by subcrustal mantle heterogeneity in the lithosphere and a change in the relief of the main geodynamic boundary. The suture controls the USh large multiphase magmatic massifs and manifestation of the basic mafic magmatism in the Dniepr-Donets Depressin (DDD), which age decreases from south to north from the Early Proterozoic in the shield to the Devonian in the depression. On both sides of it, the crystalline crust differs in a set of parameters including a zone of low velocities in the area of the Novokonstantinovsky ore field of the USh to the east of the Kherson—Smolensk suture, where from DSS data its maximum thickness is 10—15 km in the upper crust. It appears to bea source of abiogenic hydrogen manifestations recorded by mining operations on this field. The Kherson—Smolensk suture, being a transregional mantle feature, unites the existing hydrocarbon manifestation in the USh with the promising hydrocarbon areas of the DDD. The inhomogeneities of the crystalline crust and the uppermost mantle give strong evidences to classify reasonably the transregional tectonic suture Kherson—Smolensk as a powerful mantle long-lived magmatic and fluid-conducting channel. Ores hows and modern degassing of methane are related to it, with methane beingmain greenhouse gas.

Low-spin ferric iron in primordial bridgmanite crystallized from a deep magma ocean

The crystallization of the magma ocean resulted in the present layered structure of the Earth’s mantle. An open question is the electronic spin state of iron in bridgmanite (the most abundant mineral on Earth) crystallized from a deep magma ocean, which has been neglected in the crystallization history of the entire magma ocean. Here, we performed energy-domain synchrotron Mössbauer spectroscopy measurements on two bridgmanite samples synthesized at different pressures using the same starting material (Mg0.78Fe0.13Al0.11Si0.94O3). The obtained Mössbauer spectra showed no evidence of low-spin ferric iron (Fe3+) from the bridgmanite sample synthesized at relatively low pressure of 25 gigapascals, while that directly synthesized at a higher pressure of 80 gigapascals contained a relatively large amount. This difference ought to derive from the large kinetic barrier of Fe3+ rearranging from pseudo-dodecahedral to octahedral sites with the high-spin to low-spin transition in experiments. Our results indicate a certain amount of low-spin Fe3+ in the lower mantle bridgmanite crystallized from an ancient magma ocean. We therefore conclude that primordial bridgmanite with low-spin Fe3+ dominated the deeper part of an ancient lower mantle, which would contribute to lower mantle heterogeneity preservation and call for modification of the terrestrial mantle thermal evolution scenarios.

Boron isotope evidence for devolatilized and rehydrated recycled materials in the Icelandic mantle source

Enriched mantle heterogeneities are widely considered to be generated through subduction, but the connections between specific subducted materials and the chemical signatures of mantle heterogeneities are not clearly defined. Boron is strongly isotopically fractionated at the surface and traces slab devolatilization, making it a potent tracer of previously subducted and recycled materials. Here, we present high-precision SIMS boron concentrations and isotope ratios on a comprehensive suite of quenched basaltic glasses from all neovolcanic zones in Iceland, two rhyolite glasses, and a set of primitive melt inclusions from central Iceland. Boron isotope ratios (δ11B) in Icelandic basalts and melt inclusions range from -11.6‰ to -1.0‰, averaging -4.9‰, which is higher than mid-ocean ridge basalt (MORB; δ11B = -7.1‰). Because the δ11B value of the Icelandic crust is low, the high δ11B compositions of the Icelandic lavas are not easily explained through crustal assimilation processes. Icelandic basalt glass and melt inclusion B/Ce and δ11B values correlate with trace element ratio indicators of the degree of mantle partial melting and mantle heterogeneity (e.g. Nb/Zr, La/Yb, Sm/Yb), which indicate that the boron systematics of basalts are controlled by mantle heterogeneity. Additionally, basalts with low B/Ce have high 206Pb/204Pb, further indicating mantle source control. These correlations can be used to deduce the boron systematics of the individual Icelandic mantle components. The enriched endmember within the Iceland mantle source has a high δ11B value and low B/Ce, consistent with the composition of “rehydrated” recycled oceanic crust. The depleted endmember comprises multiple distinct components with variable B/Ce, likely consisting of depleted MORB mantle and/or high 3He/4He mantle and two more minor depleted components that are consistent with recycled metasomatized mantle wedge and recycled slab gabbro.The compositions of these components place constraints on the devolatilization history of recycled oceanic crust. The high δ11B value and low B/Ce composition of the enriched component within the Iceland mantle source is inconsistent with a simple devolatilization process and suggests that the recycled oceanic crust component may have been isotopically overprinted by B-rich fluids derived from the underlying hydrated slab lithospheric mantle (i.e. “rehydration”). Further, the B/Ce and δ11B systematics of other OIBs can be used to constrain the devolatilization histories of recycled components on a global scale. Globally, most OIB B/Ce compositions suggest that recycled components have lost >99% of their boron, and their δ11B values suggest that rehydration may be a sporadic process, and not ubiquitous.

Coupled dynamics and evolution of primordial and recycled heterogeneity in Earth's lower mantle

The nature of compositional heterogeneity in Earth's lower mantle remains a long-standing puzzle that can inform about the long-term thermochemical evolution and dynamics of our planet. Here, we use global-scale 2D models of thermochemical mantle convection to investigate the coupled evolution and mixing of (intrinsically dense) recycled and (intrinsically strong) primordial heterogeneity in the mantle. We explore the effects of ancient compositional layering of the mantle, as motivated by magma ocean solidification studies, and of the physical parameters of primordial material. Depending on these physical parameters, our models predict various regimes of mantle evolution and heterogeneity preservation over 4.5 Gyr. Over a wide parameter range, primordial and recycled heterogeneity are predicted to co-exist with each other in the lower mantle of Earth-like planets. Primordial material usually survives as medium- to large-scale blobs (or streaks) in the mid-mantle, around 1000–2000 km depth, and this preservation is largely independent of the initial primordial-material volume. In turn, recycled oceanic crust (ROC) persists as large piles at the base of the mantle and as small streaks everywhere else. In models with an additional dense FeO-rich layer initially present at the base of the mantle, the ancient dense material partially survives at the top of ROC piles, causing the piles to be compositionally stratified. Moreover, the addition of such an ancient FeO-rich basal layer significantly aids the preservation of the viscous domains in the mid-mantle. Finally, we find that primordial blobs are commonly directly underlain by thick ROC piles and aid their longevity and stability. Based on our results, we propose an integrated style of mantle heterogeneity for the Earth involving the preservation of primordial domains along with recycled piles. This style has important implications for early Earth evolution and has the potential to reconcile geophysical and geochemical discrepancies on present-day lower-mantle heterogeneity.

A subduction influence on ocean ridge basalts outside the Pacific subduction shield

The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow. Hence it is puzzling why melts of subduction-modified mantle have rarely been recognized in mid-ocean ridge basalts. Here we report the first mid-ocean ridge basalt samples with distinct arc signatures, akin to back-arc basin basalts, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence in this region. This influence can also be identified in Atlantic and Indian mid-ocean ridge basalts but is nearly absent in Pacific mid-ocean ridge basalts. Such a hemispheric-scale upper mantle heterogeneity reflects subduction modification of the asthenospheric mantle which is incorporated into mantle flow, and whose geographical distribution is controlled dominantly by a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr. Simple modeling suggests that a slab flux equivalent to ~13% of the output at arcs is incorporated into the convecting upper mantle.

Mantle source of tephritic porphyry in the Tarim Large Igneous Province constrained from Mg, Zn, Sr, and Nd isotope systematics: Implications for deep carbon cycling

The nature and source of magmatism associated with large igneous provinces (LIPs) remain disputed. Here we investigate the role of recycled materials that contributed to mantle heterogeneity in the Tarim Large Igneous Province (TLIP) in China through integrated Zn−Mg−Sr−Nd isotopes of a rare tephritic rock suite. The Sr−Nd isotopes [(87Sr/86Sr)i = 0.70368−0.70629; εNd(t) = −0.25−4.64] and δ26Mg values (−0.23‰ to −0.34‰) of the tephritic porphyries are consistent with a normal mantle origin. In contrast, the whole rock and pyroxene phenocrysts yield δ66Zn values of +0.28‰ to +0.46‰ and +0.30‰ to +0.39‰, which are slightly heavier than those of the terrestrial mantle (+0.16 ± 0.06‰) and mid-oceanic-ridge basalts (MORBs) (+0.27 ± 0.05‰). We exclude the possibility that the heavy Zn isotopes of the Wajilitag tephritic porphyries are caused by magmatic processes such as fractional crystallization and partial melting and correlate the isotopic features to the role of altered oceanic crust along with magnesite in the mantle source. The Wajilitag tephritic porphyry displays trace-element patterns similar to those of the melts from natural hornblendite, especially those of hornblendite + peridotite. Additionally, the geochemical characteristics such as enrichment in Nb and Ta, depletion in K, high TiO2, and constant Na2O/K2O ratios also suggest that the tephritic porphyries were derived from an amphibole-bearing source contributed by altered oceanic crust along with sedimentary carbonates. Our study provides insight into the contribution of subducted materials to the mantle heterogeneity beneath the TLIP and highlights the role of subduction in the deep carbon cycle and subducted slab-lithosphere-plume interaction in the generation of LIPs.