Mantle Composition Research Articles

Elemental assay of the fumarolic microecology of the Los Azufres (México) volcanic complex using nuclear analytical techniques

The elemental composition of the mantle is a key factor for the understanding of the science of the earth’s interior. However, the direct sampling of the mantle is a difficult and involved task. Fumarolic vapours carry a certain imprint of the mantle composition but are also relatively difficult to sample directly given their high temperature and acidity. Many terrestrial fu- marolic vents are colonized by extremophilic mosses that absorb the vapours and become repositories of the mantle elements brought up by the vapours. The elemental composition of the niche ecology of the fumarole therefore may serve as a near “proxy” for the deep-earth elemental profile of the region. Our study concerns the elemental composition of fumarolic plants, their rhizospheric soil, hydrothermal sediment and volcanic substrate collected from three geothermal microecological sites of the Los Azufres volcanic complex of Mexico. This work presents the results for the heavy elements (HE) Cd, Hg, Pb, Th and U that have been determined using the nuclear analytical techniques of Polarized Energy Dispersive Xray Fluorescence (PEDXRF) spectrometry, Instrumental Neutron Activation Analysis (INAA) as well as Inductively Coupled Plasma Mass spectrometry (ICPMS). The distributions of these HE in the aforesaid biogeochemical matrices are examined and rationalized with respect to possible mechanisms of their deposition. Comparisons with literature reports of the HE composition of fumarolic sulphur deposits and fumarolic vapours of the volcanoes of the Kuril Islands and geothermal aquifers of Iceland, are made. The HE composition is found to vary considerably by the geographic location of the fumarole. However, the trend of the concentrations of these 5 HE is found to be approximately the same for the fumarolic plants of Los Azufres as with the fumarolic vapours of the Kuril volcanoes the displacement being only in the magnitudes. Consequently, the concentrations of these 5 HE in the fumarolic plants are direct qualitative analogues of the vapours of those particular fumaroles of Los Azufres and thereby of the deep-earth geochemistry of that particular geothermal zone.

Role of Metasomatism in the Development of the East African Rift at the Northern Tanzanian Divergence: Insights From 3D Magnetotelluric Modeling

AbstractThe Northern Tanzanian Divergence in the East Africa Rift is arguably the best place on Earth to study the controls on rifting of thick lithosphere. Here, where the East Africa Rift intersects the Tanzanian Craton and the Mozambique Belt, the relationships between volcanism, faulting, pre‐existing structures and lithospheric thickness and composition can be observed. In this work, we carry out the first lithospheric‐scale 3D magnetotelluric modeling of the Northern Tanzanian Divergence and combine the results with experimental electrical conductivity and petrology models to calculate mantle composition, which is also inferred in the craton from reanalysis of garnet xenocryst data. Our results show that metasomatic materials exist in the cratonic lithospheric mantle and the relatively undeveloped southern part of the rift zone. However, the lithospheric mantle of the Mozambique Belt and the more developed northern section of the rift is more resistive and does not contain metasomatic phases. Combined with geochemical data from erupted lavas, these results suggest that, in zones that have experienced voluminous Cenozoic magmatism, melting events have destroyed the metasomes and dehydrated the mantle. Since the presence of magma is a primary control of lithospheric strength, rifting may become limited as the lithospheric mantle becomes dehydrated and harder to melt.

High-pressure single-crystal elasticity of corundum: Implication for multiple seismic structure of 660-km discontinuity

The complex multi-discontinuity structure at 660‐800 km depth is likely attributable to lateral mantle composition heterogeneities, which are closely related to the mid-ocean ridge basalts (MORB). To decipher the impact of varying composition on the seismic properties of MORB, detailed knowledge of the elasticity of candidate minerals is thus important. Here we employed Brillouin scattering coupled with diamond anvil cells to determine the single-crystal elasticity of corundum up to 14 GPa and 300 K. Using third-order finite strain equation of state, we calculate the pressure derivatives of adiabatic bulk modulus and shear modulus of corundum, which yields KS0’ = 3.8(1), G0’ = 1.8(1) with KS0 = 256(1) GPa and G0 = 163(1) GPa and ρ0 = 3.987(1) g/cm3. Combined with previous high-temperature data, the velocity and anisotropy of corundum have been calculated at 300 K or along normal geotherm. Our results are applied to model the density and velocity profiles of normal and alkali-depleted MORB. Our modeling demonstrates that varying the alkali content and temperature of MORB can significantly impact the discontinuity depth and velocity jump at 660–800 km depth. For normal MORB, reducing the temperature by 300 K from normal mantle geotherm results in a shift of the velocity jump from 670–710 to 650–710 km depth but hardly affects the magnitude of the velocity jump (5.8–6.8(3)% for VP and 10.0–10.8(5)% for VS). By contrast, in alkali-depleted MORB, the discontinuity will occur at a greater depth from 705–730 to 720–745 km depending on temperature with a VP jump of 3.8–4.6(2)% and VS jump of 6.3–7.4(4)%.

Partial melt composition of enstatite chondritic mantle around the rheological transition at 23 GPa: Implications for the chemical differentiation of the Earth's mantle

Terrestrial planets are thought to be made of chondritic materials. However, previous studies have suggested that the Earth's upper mantle is depleted in some incompatible refractory lithophile elements (RLEs), such as U and Th in comparison with chondritic values. To explain the compositional contradiction between the upper mantle and chondrites, a hidden reservoir for incompatible RLEs in the lower mantle has been proposed. These studies have invoked a possible formation of hidden reservoir for incompatible RLEs during magma ocean solidification and by subsequent mantle overturn due to the gravitational instability of Fe-rich dense cumulates. However, the density of partial melt and its fate in a crystallizing deep magma ocean is still debated. Here we report partial melt compositions of enstatite chondritic mantle at 23 GPa as a function of the extent of melting. The results show that bridgmanite crystallized in a chondritic magma ocean becomes enriched in Fe and Ca with decreasing extent of melting and prevents the crystallization of ferropericlase and CaSiO3 perovskite (Davemaoite). At the rheological transition (the extent of melting of 40% based on mass balance calculation), where the solid-liquid separation efficiently occurs even in the case of equilibrium crystallization, the solid part is mostly composed of bridgmanite with a trace amount of majorite, and the melt becomes enriched in FeO and CaO compared to the initial chondritic composition. The calculated density of the melt at the rheological transition is lighter than bridgmanite, but denser than majorite, suggesting that the melt may have ponded at the bottom of the mantle transition zone. The melt ponded around 660 km depth may have been enriched in incompatible RLEs and formed the hidden reservoir for missing RLEs if this melt-bearing region is stable against subsequent mantle convection.

Contrasting Diamond Potentials of Kimberlites of the V. Grib and TsNIGRI-Arkhangelskaya Pipes (Arkhangelsk Diamondiferous Province) as a Result of the Different Compositions and Evolution of the Lithospheric Mantle: Data on the Contents of Major and Trace Elements in Garnet Xenocrysts

Abstract —The paper presents data on the contents of major and trace elements in garnet xenocrysts from kimberlites of the highly diamondiferous V. Grib pipe (1100 grains) and weakly diamondiferous TsNIGRI-Arkhangelskaya pipe (446 grains). We have established that the high diamond potential of the V. Grib kimberlite pipe is due to several factors related to the composition and structure of the lithospheric mantle represented by kimberlite: (1) a “cold” regime, with a heat flow of 36–38 mW/m2; (2) a thick “diamond window” (70–102 km), with the depth level of the lower boundary of the lithospheric mantle estimated at >200 km; (3) the high degree of preservation of diamondbearing peridotites under the P–T conditions of diamond stability despite the high degree of impregnation of the lithospheric-mantle rocks by high-temperature silicate melts. The low diamond content of the TsNIGRI-Arkhangelskaya kimberlite pipe as compared with the V. Grib pipe is due to the following factors: (1) a more intense heat flow in the lithospheric mantle, 38–42 mW/m2; (2) a thinner “diamond window”, 10–60 km, with the depth level of the lower boundary of the lithospheric mantle estimated at <200 km; (3) weak impregnation of the rocks of the middle and lower lithospheric mantle by CHO fluid/melt, which might have induced diamond formation; (4) minimum preservation of diamond-bearing peridotites in the lower lithospheric mantle, partly because of the possible impregnation of this zone by high-temperature silicate melts.

Heavy Mo isotope enrichment in the Pitcairn plume: Implications for the subduction cycle of anoxic sediments

Subduction redistributes elements between Earth's principal geochemical reservoirs, modifying the chemical composition of Earth's mantle, crust, atmosphere, and hydrosphere, and consequently having an impact on the evolution of life itself. Subduction of surface material that has been geochemically modified by low-temperature processes leads to mineralogical and chemical heterogeneities in mantle reservoirs over time and is recorded in modern ocean island basalts. One of the principal geochemical end members of the heterogeneous deep mantle, the enriched mantle 1 (EM-1) source of Pitcairn Island, has been attributed to the contribution of crustal material with vastly different chemical compositions and ages. The Mo isotope composition of lavas from Pitcairn Island constrains the nature of this recycled crustal component. Pitcairn lavas have elevated δ98/95Mo relative to the depleted mantle. The high δ98/95Mo is associated with high time-integrated 232Th/238U and 87Rb/86Sr, and low time-integrated 147Sm/144Nd and 238U/204Pb. These characteristics can be attributed to the recycling of nearly pristine pelagic sediments that were deposited in a Proterozoic anoxic deep-ocean into the sources of the Pitcairn Island lavas. The isotope composition of these lavas is similar to that of EM-1 hotspots from the South Atlantic, indicating the addition of reduced sediments in both of Earth's large low shear wave velocity provinces (LLSVPs). Consistent data from both locations imply that the subduction cycling of sedimentary redox-sensitive elements such as Mo, S, Se, and U into arc magmas was in these cases inefficient in the Precambrian and the chemical and isotopic signature of reduced sediments is preserved in the source of ocean island basalts bearing the EM-1 characteristics.

Osmium Isotope Heterogeneity of the Upper Mantle: Evidence From the Bay of Islands Ophiolite Complex, Newfoundland

AbstractUnderstanding and determining the composition and evolution of the upper mantle is valuable to unravel Earth's evolutionary geodynamics. The compositions of oceanic basalts are inevitably biased by the preferential melting of fusible mantle components and the pooling of melts from multiple sources. Here we present whole rock and mineral chemistry as well as Re‐Os isotopic systematics of the Table Mountain ophiolitic mantle massif from the Bay of Islands ophiolite complex, complemented by thermodynamic models of melt extraction. The Table Mountain lherzolites were formed by moderate degrees (5%–15%) of shallow polybaric melting. We divide the harzburgites into two types. Type I harzburgites represent residues of high‐pressure melting, and Type II harzburgites appear to have been further modified by second‐stage flux melting above a subduction zone induced by slab sediment‐derived aqueous fluids. The Table Mountain peridotites display highly variable “initial” 187Os/188Os485 Ma isotope compositions, including 0.1252–0.1253 for the Type I harzburgites, 0.1217–0.1236 and 0.1192–0.1198 for both lherzolites and Type II harzburgites. These Os isotopic variations cannot be generated by different degrees of partial melting of a homogenous fertile mantle immediately prior to obduction, and hence must have been inherited from ancient melting events. The Table Mountain lherzolites with inherited ancient melting signals are of particular significance because they demonstrate that the record of earlier melting events is not limited to ultra‐refractory harzburgites but is also present even in relatively fertile lherzolites. Our observations underscore the ubiquity of ancient melting signatures in the entire compositional spectrum of the depleted upper mantle.

Garnet pyroxenites explain high electrical conductivity in the East African deep lithosphere

In Tanzania, the deep lithospheric mantle (> 70 km depth) is characterized by significantly higher electrical conductivity within the cratonic root than in the Mozambique belt. Such contrasts are typically attributed to changes in volatiles and/or melt content, with changes in mineralogy deemed insufficient to impact conductivity. To test this assumption, electrical conductivity measurements were conducted at pressure-temperature conditions relevant to the Tanzanian lithosphere (1.5 and 3 GPa; from 400 to >1500 °C) on dunite (depleted) and pyroxenite (fertile) xenoliths from Engorora, Northern Tanzania. Once garnet becomes stable in the fertile mantle rock (> 60 km, 1.7 GPa), it nucleates at grain boundaries, forming the backbone of a conductive network. At 3 GPa, such garnet-rich networks increase conductivity by a factor of 100 regardless of temperature. Numerical models demonstrate that the observed low (< 10−2 S⋅m−1) and high (> 10−1 S⋅m−1) conductivity values can be explained by low and high degrees of garnet connectivity, respectively. Such high electrical conductivities in cratonic roots can be explained by the presence of connected garnet clusters or garnet pyroxenites, suggesting mantle fertilization. This new source for electromagnetic signal generation at appropriate pressures and temperatures must be factored in where interpreting magnetotelluric signals at relevant depths in the lithosphere. Plain language summaryThe deep root of the Tanzanian craton is characterized by high electrical conductivity. In this study, we reproduce this anomaly in the laboratory. We use powders of Tanzanian xenoliths, which are natural rocks similar to what is expected in the deep Tanzanian lithosphere. Two extremes of mantle composition are tested: dunite and pyroxenite, respectively typical of a depleted mantle (already produced melts) and fertile mantle (high chemical diversity). Experiments at high pressures and temperatures representative of the deep Tanzanian lithosphere reveal that the high electrical conductivity is generated by the establishment of a connected network of garnet pyroxenites. Minerals such as garnet and pyroxenes are known to conduct electricity much better than olivine – the major component of most mantle rocks – especially when they contain limited amounts of hydrogen. This “water” is incorporated in crystals and at their boundaries and promotes high-conductivity pathways. A significant increase in conductivity occurs once the pressure is high enough for garnet to become stable. These results suggest that the composition of the deep cratonic root in Tanzania is closer to a pyroxenite than a peridotite. This difference in composition compared to the average mantle is expected as an impact of the East African superplume.

Plagioclase Lherzolite Melting: Experimental Constraints on a Primary, High‐Alumina MORB From the Southwest Indian Ridge

AbstractWe experimentally investigated the phase relations of a primary basaltic glass erupted on the 10–15°E ultraslow spreading (∼0.4 cm/yr) oblique amagmatic segment of the Southwest Indian Ridge (SWIR). Piston‐cylinder experiments were conducted at 9–13 kbar and 1270–1310°C, in accordance with the predicted pressure and temperature (9.5 ± 0.8 kbar and 1290 ± 12°C) of saturation with a mantle lherzolite assemblage using the recently developed multiphase reverse fractional crystallization model ReversePetrogen (RevPet, Krein et al., 2021, https://doi.org/10.1029/2020JB021292). The experiments were multiply saturated with a plagioclase lherzolite assemblage (olivine + augite + orthopyroxene + plagioclase) at 10 kbar and 1290°C, within the uncertainty of the RevPet prediction. We use the forward model Petrogen (Krein et al., 2020, https://doi.org/10.1029/2020JB019612) to find the best fitting mantle composition and melting conditions that could produce glass KN162‐9 48‐21. This model independently finds a melting pressure and temperature of 9.5 kbar and 1285°C, which agree remarkably well with the RevPet predictions and our experiments. This sample is also significantly more depleted in incompatible trace elements than other glasses from the same ridge segment, suggesting its mantle source experienced prior episodes of depletion. This is consistent with previous work suggesting that ultraslow spreading at the SWIR preserves heterogeneities on a local scale. The variability of erupted compositions largely reflects aggregation and mixing of heterogeneous near‐fractional melts produced in the spinel and plagioclase stability fields along similar melting paths and their fractional crystallization products. More generally, plagioclase‐field mantle melting does occur at mid‐ocean ridges and is preserved in some melting environments.

Petrogenesis of Paleocene−Eocene gabbros in the Gangdese belt: Geochemical tracking of transitioning from oceanic subduction to continental collision related magmatism in southern Tibet

The composition of the sub-arc mantle and the mode and nature of geodynamic processes during the India-Asia collision that controlled the melt evolution beneath the Gangdese belt (southern Tibet) are still unclear. Here, we present new U-Pb ages and Hf isotopes of zircon, and whole-rock geochemical and Sr-Nd-Pb isotopic data of the Paleocene−Eocene Najinla gabbros from the East Gangdese magmatic belt, aiming to track the transitioning magmatism formed from oceanic subduction to continental collision in the region. Zircon U-Pb analyses of these mafic rocks yield emplacement ages of 54 ± 1 Ma and 63 ± 1 Ma. The gabbros are characterized by variable SiO2 (45.87−55.44 wt%), MgO (1.03−8.18 wt%), FeOT (3.74−12.33 wt%), and Al2O3 (13.45−25.45 wt%) contents. Most samples exhibit high Al2O3 (17.15−25.45 wt%) and relatively low MgO (1.03−6.11 wt%), similar to typical high-alumina basalts and high-alumina basaltic andesites. The Najinla gabbros show characteristic subduction-related signatures with enriched large-ion lithophile elements and depleted high field strength elements. They have depleted Sr-Nd isotopic compositions with low and relatively homogeneous initial 87Sr/86Sr isotopic ratios of 0.7045−0.7049 and positive εNd(t) ratios of +2.2 to +3.2. The Najinla gabbroic rocks also have positive zircon εHf(t) values, ranging from +5.6 to +10.9. These results collectively suggest that magmas of the gabbros formed by partial melting of the asthenosphere with negligible crustal contamination during their emplacement. We propose that the mantle source of the Najinla gabbros was strongly influenced and metasomatized by subducted Neotethyan oceanic crust-derived fluids in the mantle wedge. Rollback of the subducted Neotethyan slab in the early Eocene led to partial melting of the subduction-modified mantle and the formation of these gabbros.

Magnesium isotopic constraints on the lithospheric mantle metasomatism beneath the craton margin

The magnesium (Mg) isotopic distinction between surficial carbonate and mantle rock makes the Mg isotopes a potential tracer for recycled crustal materials (especially the carbonate-bearing materials) within the mantle. However, whether or to what extent recycled carbonate-bearing crustal materials could influence the Mg isotopes of the lithospheric mantle is still in debate. Here, we report the Mg isotope data for three well-characterized alkaline complexes from the northern margin of the North China Craton, i.e., the Hekanzi, Yaojiazhuang and Saima alkaline complexes. These alkaline rocks were all derived from enriched lithospheric mantle sources beneath the craton margin that were variably metasomatized by carbonate-bearing recycled crustal materials. However, they have relatively homogeneous and normal mantle-like δ26Mg values, i.e., −0.23 to −0.25‰ for the Hekanzi, −0.20 to −0.30‰ for the Yaojiazhuang and − 0.25 to −0.30‰ for the Saima alkaline rocks, respectively. These Mg isotopic characteristics were unrelated to shallow crustal processes (i.e., surface alteration, fractional crystallization and crustal assimilation) but inherited from the mantle sources. The mantle-like Mg isotopes suggest that the recycled carbonate-bearing crustal materials did not change the Mg isotopic compositions of the lithospheric mantle overlying the subduction zone, which mainly ascribes to the large contrast of MgO contents between the recycled crustal materials and the lithospheric mantle peridotite. The MgO contents of calcite-rich carbonates in the metasomatic agents released from the subducted crustal materials are too low to induce Mg isotopic anomaly in the MgO-rich peridotite. Our work reveals that Mg isotopes might not be a sensitive tracer of carbonate melt/fluid metasomatism in the lithospheric mantle beneath the craton margins.

Bayesian inference on the isotopic building blocks of Mars and Earth

Elements with differing siderophile affinities provide insights into various stages of planetary accretion. The isotopic anomalies they exhibit offer a deeper understanding of the materials responsible for the formation of terrestrial planets. By analyzing new iron isotopic anomaly data from Martian meteorites and drawing insights from published data for O, Ca, Ti, Cr, Fe, Ni, Sr, Zr, Mo, Ru, and Si, we scrutinize potential changes in the isotopic composition of the material accreted by Mars and Earth during their formation. Our methodology employs a Bayesian inference method, executed using a Markov Chain Monte Carlo (MCMC) algorithm.The Bayesian method helps us balance the task of emulating the compositions of the terrestrial planets (reflected in the likelihood) with the flexibility to explore the isotopic compositions of mixing components within permissible intervals (indicated by the priors of the nuisance parameters). A Principal Component Analysis of isotopic anomalies in meteorites identifies three main clusters (forming the three parts of the isotopic trichotomy): CI, CC=CM + CO + CV + CR, and NC = EH + EL + H + L + LL. We adopt CI, COCV, O, and E as endmember compositions in the mixtures. We are concerned here with explaining isotopic anomalies in Mars and Earth, not their chemical compositions. Previous studies have shown that Earth's chemical composition could not be well explained by solely considering undifferentiated meteorites, as it requires incorporation of a refractory component enriched in forsterite. The endmember chondrite components considered here are assumed to be representative of isotopic reservoirs that were present in the solar nebula, but the actual building blocks could have had different chemical compositions, and we use cursive letters to denote those putative building blocks (CI for CI, COCV for COCV, O for O, and E for E).Because Earth's mantle composition is an endmember for some isotopic systems, it cannot be reproduced exactly by considering known chondrite groups only and requires involvement of a component that is missing from meteorite collections but is likely close to enstatite meteorites. With this caveat in mind, our results suggest that Earth is primarily an isotopic mixture of ~92% E, 6% CI, and < 2% COCV and O. Mars, on the other hand, appears to be a mixture of ~65% E, 33% O, and < 2% CI and COCV. We establish that Earth's CI contribution substantially increased during the latter half of its accretion. Mars began accreting a mix of O and E but predominantly accreted E later.Mars' changing isotopic makeup during accretion can be explained if it underwent gas-driven type I migration from its origin near the O - E boundary to a location well within the E region during the first few million years of solar system history. Earth's later increased CI contribution may be attributed to the stochastic impact of an interloper carbonaceous embryo that moved inside the inner solar system region while nebular gas was still present, and subsequently participated in the stage of chaotic growth.The discovery that a significant portion of Earth's building blocks closely resembles enstatite chondrites contrasts with recent findings of Si isotopic anomalies in enstatite chondrites when compared to terrestrial rocks. We suggest that this discrepancy likely stems from insufficient correction for high-temperature equilibrium isotopic fractionation, whether of nebular or planetary origin. With appropriate adjustments for this influence, both the silicate Earth and enstatite chondrites exhibit comparable Si isotopic anomalies, reaffirming a genetic link between them.

Low-δ56Fe fluid released from serpentinite as recorded by iron isotopes of Myanmar jadeitites

Melt extraction and subduction zone fluid infiltration can produce light Fe isotope compositions of arc lavas relative to the MORBs. However, the Fe isotope signature of subduction zone fluid is poorly constrained, which has hampered our understanding of Fe cycling in the subduction zone. Jadeitite veins in forearc serpentinite provide a unique record for the characteristics of subduction zone fluids. Here we report Fe isotope data for Myanmar jadeitites and associated rocks to constrain the Fe isotope signature of subduction zone fluids. Two types of jadeitites from Myanmar display distinct δ56Fe much lower than typical igneous rocks. White jadeitites, directly precipitated from fluids, have variable δ56Fe from −0.60‰ to −0.04‰, while green jadeitites, formed by fluid-chromite interaction, have δ56Fe (−0.44‰ to −0.19‰) similar to the associated amphibole-rich blackwalls (−0.47‰ to −0.23‰). The light Fe isotope signature of the jadeitites is not induced by weathering, kinetic fractionation, dissolution of Fe-rich carbonate, or mineral dissolution-reprecipitation process. Instead, it most likely reflects the Fe isotope variation of serpentinite-derived fluids. Correlations of δ56Fe with REEs, Sb, and U in white jadeitites suggest that the jadeitite-forming fluid is a mixture of serpentinite-derived and sediment-derived fluids. In contrast, correlations of δ56Fe with Si, Al, and Cr in green jadeitites record the interaction of low-δ56Fe fluids with chromite. Considering the abundance of the serpentinite-derived fluid, it can effectively influence the Fe isotope compositions of the ambient mantle and arc lavas.

Thermal state of the Siberian craton marginal zone at the time of Mesozoic kimberlitic magmatism within the Kuoika field (Yakutian diamondiferous province)

Research subject. Clinopyroxene xenocrysts from the heavy-mineral concentrates of 14 kimberlite bodies (western part of the field: Obnazhennaya, Rubin, Seraya, Vodorazdelnaya, Vtorogodnitsa, Antoshka; central part of the field: Sliudyanka, Skiff II; western part of the field: Irina, Noyabrskaya, Vechernyaya, Lyusya, Dianga and Zhila 79) of the Kuoika field (Yakutian diamondiferous province, Siberian craton). Aim. To reconstruct the mantle paleogeotherms Under the six kimberlite pipes (Vodorazdelnaya, Obnazhennaya, Vtorogodnitsa, Sliudyanka, Dianga, and Zhila 79) using two independent approaches. Materials and methods. The chemical composition of clinopyroxene xenocrysts was investigated and last equilibrium temperatures and pressures were estimated using single-crystal thermobarometry. The geothermal line fitting to the P-T data set was performed using two methods. The first method is based on the D. Hasterok and D.S. Champan model, and the second - on the D. McKenzie model. Results. The value of lithosphere thickness falls within the margin of error for the two methods, being also comparable with earlier reconstructions for the Vtorogodnitsa, Dyanga, and Obnazhennaya pipes. The obtained results indicate that, during the Mesozoic kimberlitic magmatism, the lithosphere thickness beneath the Kuoika field was about 200 km. Conclusion. The determined peculiarities of the chemical composition of clinopyroxene xenocrysts indicate heterogeneity of the lithospheric mantle composition. Different depths of transported mantle material for kimberlite pipes of the Kuoika field, which are coeval and are located a few km apart, may be related to peculiarities of kimberlite magma ascent to the surface and the presence of intermediate magma chambers. The eastern part of the Kuoika field contains more garnet and garnet-spinel peridotites compared to the central and western parts, which may indirectly indicate a greater diamondiferous potential of the eastern block, where the diamondiferous Dianga pipe is located. The absence of diamonds in other discovered pipes of the Kuoika field may be connected with the metasomatic enrichment of the lithospheric mantle in the area of the “diamond window”, which is confirmed by a large number of high-temperature clinopyroxenes at these depths.

Mesoarchaean peridotite-norite cumulates of SW Greenland – The Miaggoq ultramafic complex

Several studies focused on the ultramafic bodies of the Archaean continental crust in southern Greenland in order to gain information on early Earth petrogenetic, metamorphic and metasomatic processes. This research provides the first petrological dataset of the Miaggoq Ultramafic Complex (∼1 km2) in the Akia terrane, with a minimum age of 2997 ± 15 Ma. It comprises ultramafic (dunite, peridotite) and mafic (orthopyroxenite, norite) rocks along with chromitites and provides a window into Mesoarchaean mantle compositions. Field observations, such as chromitite bands, mineral layering, and orthopyroxenite oikocrysts in peridotites coupled with chemical analysis displaying high abundance of chromites in the dunitic rocks and high forsterite contents (Mg# 91 to 92.5) of the olivines, all point to a layered cumulate origin for the Miaggoq body. Pseudosection calculations along with geothermometry estimations reveal peak metamorphic conditions of 850–1100 °C at pressures of 0.7–1.25 GPa under anhydrous conditions followed by a possible metamorphic overprint at 650–800 °C and 0.7 GPa with relatively dry melting (0.025–0.125 wt% H2O). MELTS fractional crystallization coupled with cumulate modelling approximated the compositional trends with conditions on ∼3 kbar with 1 wt% H2O. This research concludes that the Miaggoq body represents a layered cumulate complex that was derived by large degrees of partial melting of the mantle with possible assimilation (synonymous with contamination) of basalts in the crust. Overall, this study provides complementary data for the Mesoarchaean cumulate bodies of the Akia Terrane and their petrological processes.

Lunar mantle structure and composition inferred from Apollo 12 – Explorer 35 electromagnetic sounding

Constraints on the interior structure of the Moon have been derived from its inductive response, principally as measured by the magnetic transfer function (TF) between the distantly orbiting Explorer 35 satellite and the Apollo 12 surface station. The most successful prior studies used a dataset spanning 0.01–1 mHz, so the lunar response could be modeled as a simple dipole. However, earlier efforts also produced transfer functions up to 40 mHz. The smaller electromagnetic skin depth at higher frequency would better resolve the uppermost mantle—where key information about primitive lunar evolution may still be preserved—but requires a multipole treatment.I compute new profiles of electrical conductivity vs depth using both low- and high-frequency ranges of published Apollo-Explorer TFs. Using the low-frequency data, I derive temperature profiles at depths >400 km (<1 mHz) consistent with conductive heat loss and expectations of the iron (and possibly water) content of the mantle. The near-constant iron fraction (Mg# 81 ± 10) could imply efficient mixing due to now-defunct convection. Alternatively, incomplete overturn of gravitationally unstable magma-ocean cumulates could have left a heterogeneous distribution of minerals at hundred-km scales that are not resolved by electromagnetic sounding. A third explanation is that the electromagnetically probed region may be the initial equilibrium crystallization in a mantle that did not buoyantly overturn.In contrast, the high-frequency data produced higher conductivities than expected, requiring unrealistically low Mg# or high water content at depths 200–400 km. Either the published transfer functions > > 1 mHz are incorrect, or the TF multipole method at the Moon is unreliable. Future electromagnetic sounding using the magnetotelluric method can operate up to 100 s Hz and is largely insensitive to multipole effects, resolving structure to 100 km or less.

Syn‐Rift Magmatism and Sequential Melting of Fertile Lithologies in the Lithosphere and Asthenosphere

AbstractThe timing and rate of decompression melting of a compositionally heterogeneous mantle during continental rifting are assessed with a new one‐dimensional geodynamic code, MELT1D. MELT1D computes pressure and temperature in the extending lithosphere and rising asthenosphere and calculates the resulting melt fraction and eruption rates for different lithologies. A series of models simulate syn‐rift melt production from (a) dry and wet depleted lherzolite similar to the mantle that underlies most mid‐ocean ridges, (b) dry and wet relatively fertile ultramafic compositions representing plume or primitive mantle material, (c) pyroxenite representing recycled ultramafic oceanic crust or magmatic metasomes, and (d) basalt representing recycled mafic crust or metasomes. The models predict sequential melting of the different compositions that is broadly consistent with basalt eruption histories in many Phanerozoic rifts. Results show a progressive transition in magma sources as the lithosphere thins, beginning with melting of wet mantle and compositionally fertile mafic components near the lithosphere‐asthenosphere boundary during the earliest stages of extension. This transitions to magmatism dominated by melting of relatively fertile ultramafic components (pyrolitic and pyroxenitic compositions) as extension progresses, and finally to melting of ambient lherzolite asthenosphere as lithosphere thinning approaches breakup. Mantle composition, pre‐rift lithosphere thickness, and mantle temperature exert the greatest controls on the timing and volumes of magmas produced from each lithology. In general, a cool or thick lithosphere has a greater capacity to sequester fertile lithologies than thin or warm lithosphere, and thus has a greater capacity to produce early syn‐rift magmas without requiring a hot mantle plume.

Earth’s mantle composition revealed by mantle plumes

Earth’s mantle composition revealed by mantle plumes

Evidence of South American lithosphere mantle beneath the Chile mid-ocean ridge

Numerous geochemical studies on mid-ocean ridge basalts have established the presence of both compositional and lithological heterogeneities in the upper mantle. These studies have successfully constrained the composition and age of the chemical heterogeneities within the Earth's upper mantle. However, the origin of those heterogeneities remains highly debated. The Chile Mid-Ocean Ridge in the southeast Pacific Ocean, located far away from any hotspot, is colliding and subducting under the South America plate promoting the formation of a slab window. Comprehensive geochemical data (major, trace, volatile element contents and Sr, Nd, Hf and Pb isotope ratios) on Chile Ridge submarine glasses collected over the 1000 km ridge length demonstrate significant mantle compositional variability. Four main mantle components have been recognized: the typical Pacific MORB mantle, an enriched mantle (e.g., EM-1), a Subduction Modified mantle, and an anciently depleted mantle with unusually high Hf isotope ratios. Surprisingly, despite the large compositional variability, all glasses – including those with a subduction signature – have volatile-refractory element ratios within the range of the Pacific normal MORB. We propose that the Patagonia sub-continental lithospheric mantle, variably metasomatized since the Early Paleozoic, is eroded and incorporated into an asthenosphere with a south Atlantic MORB mantle composition that is flowing westward across the slab window from South America to the Chile Ridge. This scenario contrasts with a well-accepted geodynamic model that predicts the opposite direction of mantle flow. We speculate that the westward mantle flow across the slab window might be a counter-flow response to the opening of the Drake passage and the eastward movement of Pacific mantle since ∼30 Ma ago.

Chromium isotope variations in garnet-facies mantle rocks and their minerals: Implications for Cr isotope behavior in high-temperature processes

Previous work on Cr isotope composition of the lithospheric mantle focused on shallow spinel-facies peridotites and invoked the effects of partial melting and metasomatism. However, little is known on the Cr isotope composition and behavior in the deeper and more voluminous garnet-facies mantle, notably during generation and passage of magmas and fluids. Here, we present Cr isotope data for 26 garnet and ten spinel whole-rock peridotite xenoliths (mainly harzburgites) from the Udachnaya kimberlite on the Siberian craton and minerals from five of them, as well as for minerals of 14 eclogites and four garnet clinopyroxenites from the Dabie orogen in eastern China. Garnet (Gt) and clinopyroxene (Cpx) are major Cr hosts in the mantle. Our ionic modeling suggests the following order of 53Cr/52Cr values in coexisting minerals: garnet ≈ spinel > Cpx > olivine. Cr isotopes are equally partitioned between garnet and Cpx in four peridotites, while garnet is enriched in heavy Cr in one peridotite and all the eclogites (Δ53CrGt-Cpx 0.01‰ to 0.27‰, av. 0.10‰), but depleted in heavy Cr in the pyroxenites (Δ53CrGt-Cpx −0.03‰ to −0.16‰, av. −0.10‰). The Δ53CrGt-Cpx variations are attributed to effects of chemical composition, equilibration temperature and oxygen fugacity on equilibrium Cr isotope fractionation. Further, the anomalous Δ53CrGt-Cpx values in the Dabie pyroxenites and one peridotite may indicate disequilibrium inter-mineral isotope fractionation via kinetic diffusion during phase transitions as a result of pressure-temperature (P-T) changes and metasomatism.The whole-rock δ53Cr range in the garnet peridotites (−0.27 to 0.13‰; average −0.08 ± 0.18‰, 2 SD, n = 25, excluding an outlier with −0.86‰) overlaps bulk silicate Earth estimates whereas the spinel peridotites tend to have higher δ53Cr (−0.12 to 0.16‰; av. 0.02 ± 0.20‰, 2 SD, n = 10). While these rocks are residues of high degrees (30–40%) of melt extraction, their range contrasts with limited equilibrium Cr isotope fractionation we calculated for melting residues of both spinel and garnet peridotites (Δ53Crresidue-source < 0.03‰). The δ53Cr values in the spinel peridotites show correlations with modal abundances of Cpx, garnet and spinel, as well as major and trace element composition. We posit that the Cr isotope variations in these peridotites are somehow linked to solid–melt and inter-mineral diffusive exchanges during P-T changes, deformation and ingress of metasomatic media previously documented in the Udachnaya peridotites. The average δ53Cr value (−0.09 ± 0.16‰, 2 SD, n = 31) for garnet peridotites (excluding U9) in this and previous studies is similar to those previously obtained for (mainly off-craton) spinel peridotites and komatiites, and appears to be representative of the Cr isotope composition for bulk mantle. Numerical modeling for mantle magmas derived at different depths and by different melting degrees of garnet and spinel peridotites and eclogites yields a narrow δ53Cr range, possibly affected by oxygen fugacity.