Mantle Geochemistry Research Articles

A REE Inverse Model From Bulk Distribution Coefficients and Boundary Conditions: Results for Shield and Rejuvenated Stage Hawaiian Volcanoes

AbstractA major challenge in mantle geochemistry is determining the source composition and melt fraction involved in melting. We provide a new Rare‐Earth Element (REE) inverse model that provides source concentration, source and melt mineral modes, and melt fraction based on the difference between separate determinations of bulk distribution coefficients and constrained by boundary conditions. An analytical inverse of the batch melting equation provides expressions for source, , and bulk distribution coefficient of the mantle, , with two unknowns, the initial concentration of La in the mantle, , and Pi, the bulk distribution coefficient of the melt. We traverse through a range of steps and examine thousands of melt modes, Pi, at each step. Thousands of trial melt modes fail by generating that are inconsistent with partition coefficients. Many surviving trials cannot be inverted to estimate a mantle mode. Other boundary conditions eliminate even more trials. Surviving trials are ordered by the difference between calculated from the REE data of a lava suite and calculated from partition coefficients and mantle mode. We select the solution with the closest fit that passes all the boundary conditions. We tested our new model with lava suites from Hawaii where different lines of evidence suggest that they melted from different mantle sources, Mauna Kea representing shield‐stage lava and submarine Kiekie representing rejuvenated stage lava. Our inverse determination of mantle composition and melting parameters was consistent with earlier models based on assumptions of HREE composition.

Detrital peridotite minerals reveal recycled arc mantle beneath Marion Rise, Southwest Indian Ridge

AbstractThe recycling of subcontinental lithospheric mantle back into the asthenosphere during continental breakup is potentially a major but underappreciated process in plate tectonics. Here, we report major and trace elements for detrital peridotite minerals in carbonate breccias dredged from the Marion Rise, Southwest Indian Ridge. With an emphasis on the trace elements of the relatively abundant orthopyroxene detrital grains, the results show that a portion of pyroxene grains was likely derived from a mantle source that experienced hydrous melting and slab‐derived melt metasomatism above a subduction zone. This finding provides a novel view into the mantle geochemistry at midocean ridges, as well as evidence that Marion Rise is at least partially supported by chemically buoyant recycled refractory mantle wedge material.

Iron isotope evidence in continental intraplate basalts for mantle lithosphere imprint on heterogenous asthenospheric melts

Iron isotope studies on ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) have disclosed the contribution of pyroxenite lithologies in the generation of basaltic magmas. Whether Fe isotopic compositions of continental intraplate basalts can be applied to trace lithological heterogeneity within the mantle source regions remains poorly investigated. To explore the systematics of stable Fe isotopes as a potential probe of lithological heterogeneity in the source of continental intraplate basalts, we present twenty-four Fe isotope data on a suite of well-characterized Cenozoic basalts from SE China. The samples show a large range of Fe isotope values (δ56Fe = +0.09‰ to +0.20‰), which correlate with SiO2, CaO/Al2O3, Ti/Eu, Hf/Hf*, Zr/Nb, Dy/Yb, La/Yb, Nb/Y, K/La, Sr/Ce, δ66Zn and estimated equilibrium pressures. The samples with the highest δ56Fe values represent early-stage low-silica basalts with moderately enriched Sr-Nd isotope ratios and high δ66Zn values, while the late-stage high-silica basalts display a broad δ56Fe decrease with increasing 87Sr/86Sr and with decreasing εNd and δ66Zn. We demonstrate that the heaviest Fe isotope signatures cannot be derived from a pure peridotite source and require a pyroxenite component in the source. Combined with other geochemical proxies, we show that the heaviest basalt compositions are consistent with low-degree melts produced by the adiabatic decompression of a carbonated pyroxenite-bearing asthenospheric mantle. Mixing of these hybrid melts with melt produced from in-situ melting of the subduction-modified sub-continental lithospheric mantle (SCLM) reproduces the Fe-isotope variability of the late-stage basalts. Our results demonstrate the significance of lithological heterogeneity in the mantle source of continental intraplate basalts and highlight the subsequent imprint of mantle lithosphere in the evolution of their Fe isotope compositions. Finally, this study exemplifies the potential of the Fe-Zn stable isotope pair for mantle geochemistry; coupled with traditional radiogenic isotopes and major and trace element concentrations, they formed an efficient tool to trace the nature and contribution of the mantle source components in the formation of intraplate basalts.

Disproportionation of Iron in Almandine‐Pyrope‐Grossular Garnet From 25 to 65 GPa

AbstractThe production of metal via the iron disproportionation reaction in the deep Earth has been a long debated topic with important implications for the geochemistry of the lower mantle. To explore the occurrence of the iron disproportionation reaction from 25 to 65 GPa, a natural almandine‐pyrope‐grossular garnet was studied with in situ X‐ray diffraction (XRD) in the laser‐heated diamond anvil cell and ex situ scanning electron microscopy (SEM) techniques. Upon heating the natural almandine‐pyrope‐grossular garnet up to 3000 K up to 65 GPa, the formation of phase assemblage consisting of bridgmanite, stishovite, and davemaoite was confirmed by XRD, but because of the low abundance of Fe metal and small grain size, XRD was determined not to be effective in detecting the disproportionation reaction. Examination of the samples recovered between 39 and 64 GPa by SEM analysis revealed the presence of nm‐scale disproportionated iron metal grains as an additional product of this reaction that was not detectable in the XRD patterns. Volume compression data of bridgmanite synthesized in the experiments were fit to the Birch‐Murnaghan equation of state and compared to similar compositions. Bridgmanite was found to decompress to the LiNbO3‐type structure, indicating a high FeAlO3 content, in accordance with the occurrence of a disproportionation reaction. The experimental confirmation of disproportionated metallic Fe has significant implications for the distribution of siderophile and volatile elements in the lower mantle.

Mapping the Structure and Metasomatic Enrichment of the Lithospheric Mantle Beneath the Kimberley Craton, Western Australia

AbstractThe lithology, geochemistry, and architecture of the continental lithospheric mantle (CLM) underlying the Kimberley Craton of north‐western Australia has been constrained using pressure‐temperature estimates and mineral compositions for >5,000 newly analyzed and published garnet and chrome (Cr) diopside mantle xenocrysts from 25 kimberlites and lamproites of Mesoproterozoic to Miocene age. Single‐grain Cr diopside paleogeotherms define lithospheric thicknesses of 200–250 km and fall along conductive geotherms corresponding to a surface heat flow of 37–40 mW/m2. Similar geotherms derived from Miocene and Mesoproterozoic intrusions indicate that the lithospheric architecture and thermal state of the CLM has remained stable since at least 1,000 Ma. The chemistry of xenocrysts defines a layered lithosphere with lithological and geochemical domains in the shallow (<100 km) and deep (>150 km) CLM, separated by a diopside‐depleted and seismically slow mid‐lithosphere discontinuity (100–150 km). The shallow CLM is comprised of Cr diopsides derived from depleted garnet‐poor and spinel‐bearing lherzolite that has been weakly metasomatized. This layer may represent an early (Meso to Neoarchean?) nucleus of the craton. The deep CLM is comprised of high Cr2O3garnet lherzolite with lesser harzburgite, and eclogite. The peridotite components are inferred to have formed as residues of polybaric partial mantle melting in the Archean, whereas eclogite likely represents former oceanic crust accreted during Paleoproterozoic subduction. This deep CLM was metasomatized by H2O‐rich melts derived from subducted sediments and high‐temperature FeO‐TiO2melts from the asthenosphere.

Activity model for 36 elements in Fe-Ni-Si-S-C liquids with application to terrestrial planet accretion and mantle geochemistry: New data for Ru, Re, Pt, Os, Ti, Nb, and Ta

Understanding siderophile element partitioning between metal and silicate melts under diverse conditions can be used to place important constraints on the materials and conditions of planetary accretion and core formation, as well as post core formation processes. However, the effects of Si on the partitioning and activity coefficients for these elements are not well known, despite Si likely being one of the dominant light elements in Earth’s core. To address this gap in understanding, we have undertaken a systematic study of the highly siderophile elements Re, Pt, Os, and Ru, and the refractory lithophile elements Nb, Ta and Ti at 1600 °C and 1 GPa, to derive epsilon interaction parameters for these elements in FeSi metallic liquids. Positive epsilon interaction parameters were measured for Nb, Ta, Ti, Ru, Re, Pt, and Os, indicating that dissolved Si in Fe liquids causes a decrease in their metal/silicate partition coefficients (or ‘silicophobic’ behavior). Furthermore, εRe,Os,orRuSi > εRe,Os,orRuS which means Si causes a larger decrease in D(metal/silicate) than S, and the chalcophile behavior expected from some elements will be completely masked by the presence of Si in a metallic liquid. The new parameters are used to update an activity model that now includes 36 siderophile elements in Fe-Ni-Si-S-C liquids (27 trace elements considered here). Systematic assessment of these 27 elements shows which have the strongest affinity for Si, C, and S, and also how activity coefficients for these elements would vary during accretion and core formation in Earth, Mars, and Mercury of widely differing fO2 and core compositional conditions. The activity model is combined with new partitioning expressions for Mo, W, Cr, Re Ru, Pt, and Os and applied to aspects of post core formation mantle geochemistry of Earth, Mars, and Mercury. Our updated expressions show that the BSE Mo/W ratio can easily be achieved with metal/silicate partitioning during growth of the Earth, whereas Re, Os and Ru become lower than and highly fractionated compared with BSE values during core formation and accretion, and thus nearly 99% of their BSE abundances are likely contributed by late accretion. Ru isotopes should be a very good indicator of the source material for the late accretion. The high Pt/Os and Re/Os developed in a deepening magma ocean during the growth of the Earth, indicates 186Os and 187Os isotopes could be coupled if this ancient material remained isolated and subsequently became entrained in mantle plumes and measured in surficial lavas. The extent to which this occurred will be limited by the low Os content of this ancient material, thus requiring mixing as a major component in plume sources. Martian mantle Hf/W ratio stays low during accretion and core formation modelling, suggesting that W isotope anomalies are more likely due to solid/liquid silicate fractionation than to core formation. Finally, Ti contents measured by MESSENGER at Mercury’s surface can be explained by segregation of either a metallic core (IW-6 to -8) or metallic core + sulfide (IW-4 to -7.5) followed by mantle melting.

Geochemical characterization, U–Pb apatite geochronology, and geodynamic significance of olivine minette dykes from the Julian Alps, NE Italy

AbstractWe investigated olivine minette dykes from the Rio Colan Valley of the Julian Alps (NE Italy) to provide new constraints on the mantle geochemistry underneath this extreme sector of the Italian Southern Alps. Petrographic observations, high Mg#s, high Cr and Ni contents, low Dy/YbCN ratios and flat heavy rare earth element (REE) profiles imply these are primary magmas derived from a depleted peridotite with olivine and garnet as possible residual phases. However, high K2O and incompatible trace element contents, coupled with superchondritic Nb/Ta ratios, suggest that the source was modified into a rutile–phlogopite-bearing carbonated peridotite by multiple metasomatic events such as recycling of crustal material and carbonatitic metasomatism related to an old orogenic event and rutile-rich metasomatism linked to the Pangaea break-up. Laser ablation – multi-collector – inductively coupled plasma – mass spectrometer (LA-MC-ICP-MS) U–Pb geochronology of apatites from two dykes yielded differences between unforced and forced discordia ages up to ∼45 Ma, likely due to the initial 207Pb/206Pb correction adopted. Nonetheless, because forced discordia and 206Pb/238U weighted mean ages are overall similar within the uncertainty, our preferred interpretation is that olivine minettes intruded and crystallized at ∼67 Ma. The age and anorogenic signature of these magmas are consistent with the regional extensional tectonics in the Julian Alps due to the advance of the external Dinaric front following Adria plate eastward subduction. Eventually, we show also that Rio Colan magmas are geochemically hybrid products between lamprophyres and lamproites compared with similar Italian and Western Mediterranean volcanics.

About this title - The Geochemistry and Geophysics of the Antarctic Mantle

This Memoir is the first dedicated to the Antarctic mantle. It is a cross-disciplinary reference work combining geochemistry and geophysics to characterize Antarctic mantle properties. Through observations and modelling the mantle structures, compositions and dynamics are characterized at regional and continental scales by subject experts. The Memoir reviews all known occurrences of sub-continental mantle xenoliths in igneous rocks. These studies are presented by region as southern or northern Victoria Land, Marie Byrd Land, the Antarctic Peninsula, East Antarctica and the sub-Antarctic Islands. Sub-oceanic mantle in tectonically emplaced and abyssal settings is also considered where known. This is complemented by a continental-scale mantle xenolith overview, mantle characteristics from igneous rocks and a quantitative mantle fabric study. State-of-the-art, continental-scale geophysical overviews of the Antarctic mantle are presented by discipline as seismology, gravity and magnetics, magnetotellurics, rheology, glacial isostatic adjustment, mantle convection and palaeotopography. This Memoir will be the reference for all researchers interested in the Antarctic mantle and its role in dynamics that shape the Antarctic surface and ice sheets.

An introduction to the geochemistry and geophysics of the Antarctic mantle

Abstract The Antarctic mantle, bounded between the core and the Mohorovičić discontinuity, is one of the most difficult targets of study on Earth because of ice cover and rare outcrops. A multidisciplinary approach is adopted in this volume, using petrology, geochemistry, remote-sensed data and geodesy to characterize the Antarctic mantle. This characterization has application to rates of glacial isostatic adjustment, heat flow, sea-level rise and tectonics. It places the Antarctic mantle domain in a global framework on a scale not attempted before. In this chapter we review the historical development of mantle studies in Antarctica, outline current research directions, introduce the volume chapters and provide a summary and outlook.

Multi‐Stage Evolution of the South Australian Craton: Petrological Constraints on the Architecture, Lithology, and Geochemistry of the Lithospheric Mantle

AbstractTo improve the understanding of the formation and evolution of the sub‐continental lithospheric mantle (SCLM) underlying the South Australian Craton we have conducted a detailed petrological study on >3,000 mantle xenocrysts from 13 kimberlites emplaced across the craton. Pressure (P) and temperature (T) estimates on Cr diopside and garnet have been coupled with their chemical concentrations to constrain lithospheric thickness and chemo‐lithostratigraphy. We show that lithospheric thickness is greatest beneath the Gawler Craton, whereas thinner lithosphere occurs beneath the Adelaide Fold Belt. Mineral compositions highlight two litho‐chemical domains within the shallow and deep SCLM that are separated by a mid‐lithosphere discontinuity (MLD). The shallow SCLM (60–130 km) comprises low Cr2O3 lherzolite and wehrlite. Shallow SCLM xenocrysts record depleted and refertilized compositions enriched in light rare earth elements related to metasomatism by kimberlite or related melts. The mid‐lithosphere (130–160 km) is depleted in garnet and Cr diopside which may relate to a layer of pargasite lherzolite. The deep SCLM (>160 km) comprises high Cr2O3 lherzolite with elevated TiO2 and FeO. We interpret the litho‐chemical stratification of the SCLM to reflect a multi‐stage top‐down growth. The shallow SCLM reflects an amalgamation of Precambrian cratonic nuclei characterized by heterogeneity in geochemical enrichment and depletion. Interaction of the shallow SCLM with mantle plumes accreted melts along the paleo‐lithosphere‐asthenosphere boundary, which now occurs as a MLD. The deep SCLM represents depleted mantle residue formed during mantle plume impingement and thickened during orogenesis. This domain has been metasomatized and refertilized by high‐T melts from the asthenosphere.

Sulfur in Archean komatiite implies early subduction of oceanic lithosphere

Komatiite is a magnesium-rich volcanic rock that erupted almost exclusively during the Archean era, requiring high partial melting of Earth's upper mantle to form. Archean komatiites, therefore, provide the most reliable information on the mantle geochemistry of the early Earth. However, it remains unclear whether komatiite compositions reflect only remnants of an early deep mantle or may provide evidence of recycling of subducted components. Sulfur mass-independent fractionation (S-MIF) could be useful to distinguish the two scenarios because S-MIF is produced only by photochemistry in anoxic atmospheres. We report here multiple sulfur isotope analyses of the 2.7 Ga Belingwe komatiites and related volcanics. Some Belingwe komatiites show clear S-MIF signatures (Δ33S from −0.20‰ to −0.12‰). Petrology, trace element chemistry, and Sr-Nd-Pb isotope compositions demonstrate that the samples contain Ni-poor pristine igneous sulfides and crustal contamination is minimal. The observed S and initial Nd isotope covariation can be reconciled if the komatiite source involved an early depleted shallow mantle which acquired S-MIF from seawater (Δ33S < 0‰) via seafloor alteration. We suggest that the Belingwe komatiites were derived at least partly from a past subducted oceanic lithosphere that stagnated in the mantle from the Archean era to the present. Therefore, the sulfur isotopes in Archean komatiite provide new constraints on the history of subduction and the recycling of crustal material.

An Observation Related to the Pressure Dependence of Ionic Radii

Here it is shown that the crystal radii of ions are represented by a simple relation rcryst = rB3√(10 m)/N, where m and N are small integer numbers determined by the principal and orbital quantum numbers and valence, and rB is the Bohr radius. The relation holds to within 5%. This finding elucidates that despite their original definition crystal- and ionic radii are not classical but represent the limiting case of spherically symmetric spatial averages of the valence electron states and, therefore, are able to reflect changes in the valence electron configuration with pressure and temperature. The relation is used to show general pressure-effects on the radii, in particular the increase of bond coordination with pressure and metallization as limiting state. The pressure-effect is exemplified for the elements Mg and Si as major constituent cations in the Earth’s mantle, and for Ba as a large ionic lithophile element. It is found that at least to about 140 GPa the radii depend linearly on pressure. Further, if a generalization is permitted for just three elements, the pressure-dependence is lesser the higher the charge of the ion. The three elements exhibit a much weaker pressure-dependence than previously calculated non-bonding radii. For mantle geochemistry this finding implies that elements incompatible in the upper mantle remain so for the main lower mantle minerals bridgmanite and periclase and are hosted by davemaoite.

Mantle geochemistry in China with a view from modern Earth system science

本文在现代地球系统科学视野下，简要回顾了20世纪国际地幔地球化学学科形成历史；综述了自20世纪70年代以来，中国地幔地球化学发端与发展的历程；强调了在国际岩石圈计划及大陆动力学等计划推进下，以中国大陆岩石圈组成与演化特征研究为重点，先后开展了东部新生代玄武岩及其幔源捕虏体研究、地幔源区地球化学特征及端元厘定、地幔地球化学填图所揭示的地幔源区块体特征以及大陆深俯冲导致的壳幔相互作用和华北克拉通破坏计划与相应研究所取得的进展；其中特别总结了新世纪以来在幔源岩类研究方面的巨大进展，以及在应用非传统稳定同位素、微区原位分析领域以及深地幔超高压矿物研究方面所取得的突破性成果。最后本文强调了现代地球系统科学应回归到它的本来含义，即涵盖深空、深海、深地，从外层空间直至地核的多圈层相互作用及其动力学机制的全方位全过程地球科学，其中，包含核幔相互作用的深部地幔过程，无疑起着主导作用，而地幔地球化学在现代地球系统科学中将依旧作为生力军而不可或缺；同时强调今后应重视拓展微观粒子尺度探索及地球化学基础理论研究的重要性。

A process-oriented approach to mantle geochemistry

The numerous chemical and isotopic studies of oceanic basalts have shaped our perception of mantle geochemistry over the last six decades. As partial melts of Earth’s mantle, basalts are indirect tracers of mantle composition. Because the scale of isotopic heterogeneity is smaller than the scale of melt production, melts from isotopically heterogeneous mantle ingredients mix into variable blends on their way to eruption. Basalts are therefore isotopically less variable than their mantle sources. Decrypting “the message from oceanic volcanism” thus necessitates developing strategies to see through this ubiquitous sampling bias, but also acknowledging the inherent limitations imposed by investigating mantle composition through basalts. Understanding how large the bias between melts and mantle actually is requires decoding process versus source-related causes for the isotopic variability of basalts. Ultimately, deciphering the effective range of isotopic variability in Earth’s mantle is crucial for connecting isotopic signals in basalts to different materials and thus, the geologic processes that govern silicate earth evolution. Invariably, however, basalts are weighted averages of melts from isotopically different mantle constituents. As such, their incompatible element and isotopic composition is inherently biased towards the incompatible element enriched source components. The incompatible element depleted components of Earth’s mantle must therefore range to more extreme compositions than the basalts. But although isotope data from peridotites and olivine-hosted melt inclusions have extended the bounds of mantle heterogeneity, the overall extent of incompatible element depletion and mass fraction of incompatible element depleted mantle is still elusive. Mantle depletion is driven by the rate of melt extraction, or mantle processing, and thus interconnects the geochemical and geodynamical evolution of Earth’s mantle. Better constraining mantle depletion is therefore at the root of understanding our planet’s principal mode of operation.

Origin of arc-like intraplate volcanism by melting of lithospheric mantle pyroxenite of the South China continental margin

Volcanic arcs usually produce silica-rich magmas that may modify the peridotitic sub-arc mantle, but direct observations on the lithology and geochemistry of the sub-arc lithospheric mantle are rare. An Andean-type arc system is inferred on the continental margin of southeastern China during the Mesozoic time; however, direct petrologic evidence for such an ancient arc is lacking. In this study, we have analyzed whole-rock major and trace elements, Sr–Nd–Pb–Hf isotope, and high-precision olivine geochemistry of the basaltic rocks of the Leizhou Peninsula and Hainan Island, on the South China continental margin. Unlike Hainan Island with only typical ocean island basalt (OIB)-type basalts, the Leizhou Peninsula has volcanic rocks ranging from typical OIB-type basalts to island arc basalt (IAB)-type with enrichment in large-ion lithophile elements and depletion in high-field-strength elements. The IAB-like rocks of the Leizhou Peninsula have less radiogenic Nd and Hf isotopes than the Hainan OIB-type basalts and the sub-continental lithospheric mantle. The IAB-like rocks can be explained by interaction of the Hainan plume with a subduction-derived component based on the plots of Nb/Nb* vs. Ce/Pb, Ba/Nb, Pb/Pb⁎, and SiO2. Olivine phenocrysts with anomalously high Ni and low Ca and Mn contents are consistent with a pyroxenite-rich mantle source for the IAB-like rocks. We suggest that the pyroxenite-rich mantle component of the IAB-like rocks formed by metasomatic reaction between silica-rich arc melts and the sub-continental lithospheric mantle during an ancient plate subduction event. We suggest that these arc-like volcanic rocks record melting of lithospheric mantle pyroxenite triggered by the Hainan plume. Our results are consistent with an Andean-type active continental margin related to subduction of the paleo-Pacific Plate beneath the southeastern China during the Mesozoic.

Systematic LREE enrichment of mantle harzburgites: The petrogenesis of San Carlos xenoliths revisited

The dichotomy between partial melting and metasomatism is a paradigm of mantle geochemistry since the pioneering work of Frey and Prinz (1978) on the occurrence of LREE-enriched harzburgites. However, the thermo-chemical implications of such two-stage scenarios are often poorly considered, and the latter fail to explain why trace-element enrichment and major-element depletion are often proportional. We here re-envisage the petrogenesis of the famous San Carlos peridotites based on new petrographic observations and detailed modal, major- and trace-element compositions. The lherzolites (and pyroxenites) are characterized by homogeneously fertile mineral chemistry and LREE-depleted patterns consistent with low degrees of partial melting of the lherzolitic protolith. Bulk compositions and mineral zoning suggest that opx-rich pyroxenites formed by pressure-solution creep during melt-present deformation, locally accompanied by magmatic segregations of cpx. The harzburgites are characterized by stronger mineral zoning with low-Mg# and Na-, Al- and Cr-rich cpx rims, and can be discriminated in a low-Jd and high-Jd cpx groups. The high-Jd group is interpreted as the result of local elemental redistribution in the presence of a low-degree hydrous melt, in good agreement with their wide range of LREE enrichment. In contrast, the MREE-to-HREE fractionation and increasing Cr# in spinel of the low-Jd group indicate that they experienced higher degrees of melting. Open-system melting simulations of trace-element fractionation during hydrous flux melting suggests that the high-Jd harzburgites are the result of low fluid influx producing poorly extracted melt, while higher influx led to higher melting degrees and efficient melt extraction in the low-Jd harzburgites; the lherzolites mostly remained below or near solidus during that process. The lithological and chemical heterogeneity of San Carlos mantle is thus compatible with a single-stage evolution, which is also supported by the striking consistency between Fe-Mg exchange and REE thermometric estimates (1057 and 1074 °C on average, respectively), indicating that harzburgites and lherzolites probably followed a similar P-T path and relatively little sub-solidus re-equilibration. These interpretations suggest that the development of melt extraction pathways promoted by reactive channeling instability is able to produce complex lithological heterogeneities during hydrous flux melting. This process provides a self-consistent explanation for the systematic enrichment of harzburgites observed in many mantle terranes and xenoliths worldwide. We argue that San Carlos is one of such examples where a ca 1.5-Ga continental lithosphere experienced localized flux melting and deformation during the tectonic reactivation of a Proterozoic subduction zone, providing new constraints on the mantle sources of volcanic activity in the Jemez Lineament.

Late Eocene post-collisional magmatic rocks from the southern Qiangtang terrane record the melting of pre-collisional enriched lithospheric mantle

Abstract Diverse rock types and contrasting geochemical compositions of post-collisional mafic rocks across the Tibetan Plateau indicate that the underlying enriched lithospheric mantle is heterogeneous; however, how these enriched mantle sources were formed is still debated. The accreted terranes within the Tibetan Plateau experienced multiple stages of evolution. To track the geochemical characteristics of their associated lithospheric mantle through time, we can use mantle-derived magmas to constrain the mechanism of mantle enrichment. We report zircon U-Pb ages, major and trace element contents, and Sr-Nd isotopic compositions for Early Cretaceous and late Eocene mafic rocks in the southern Qiangtang terrane. The Early Cretaceous Baishagang basalts (107.3 Ma) are characterized by low K2O/Na2O (&amp;lt;1.0) ratios, arc-like trace element patterns, and uniform Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7067–0.7073, εNd(t) = −0.4 to −0.2]. We suggest that the Baishagang basalts were derived from partial melting of enriched lithospheric mantle that was metasomatized by subducted Bangong–Nujiang oceanic material. We establish the geochemistry of the pre-collisional enriched lithospheric mantle under the southern Qiangtang terrane by combining our data with those from other Early Cretaceous mafic rocks in the region. The late Eocene (ca. 35 Ma) post-collisional rocks in the southern Qiangtang terrane have low K2O/Na2O (&amp;lt;1.0) ratios, and their major element, trace element, and Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7042–0.7072, εNd(t) = −4.5 to +1.5] are similar to those of the Early Cretaceous mafic rocks. Based on the distribution, melting depths, and whole-rock geochemical compositions of the Early Cretaceous and late Eocene mafic rocks, we argue that the primitive late Eocene post-collisional rocks were derived from pre-collisional enriched lithospheric mantle, and the evolved samples were produced by assimilation and fractional crystallization of primary basaltic magma. Asthenosphere upwelling in response to the removal of lithospheric mantle induced the partial melting of enriched lithospheric mantle at ca. 35 Ma.

AFRP20: New P‐Wavespeed Model for the African Mantle Reveals Two Whole‐Mantle Plumes Below East Africa and Neoproterozoic Modification of the Tanzania Craton

AbstractAfrica's Cenozoic tectonism is often attributed to mantle plumes, particularly below East Africa, but their morphology, number, location, and impact on the African lithosphere are debated. The broad slow wavespeed African Superplume, ubiquitous in large‐scale tomographic models, originates below South Africa, reaching the surface somewhere below East Africa. However, whether the diverse East African mantle geochemistry is best reconciled with one heterogeneous upwelling, or current tomographic models lack the resolution to image multiple distinct plumes, remains enigmatic. S‐wavespeed tomographic images of Africa are legion, but higher frequency P‐wavespeed whole‐mantle models possessing complementary diagnostic capabilities are comparatively lacking. This hinders attempts to disentangle the effects of Cenozoic hotspot tectonism and Pan African (and older) tectonic events on the East African lithosphere. Here we develop a continental‐scale P‐wave tomographic model capable of resolving structure from upper‐to‐lower mantle depths using a recently developed technique to extract absolute arrival‐times from noisy, temporary African seismograph deployments. Shallow‐mantle wavespeeds are δVP ≈ −4% below Ethiopia, but less anomalous (δVP ≥–2%) below other volcanic provinces. The heterogeneous African Superplume reaches the upper mantle below the Kenyan plateau. Below Ethiopia/Afar we image a second sub‐vertical slow wavespeed anomaly rooted near the core‐mantle boundary outside the African LLVP, meaning multiple disparately sourced whole‐mantle plumes may influence East African magmatism. In contrast to other African cratons, wavespeeds below Tanzania are only fast to 90–135 km depth. When interpreted alongside Lower Eocene on‐craton kimberlites, our results support pervasive metasomatic lithospheric modification caused by subduction during the Neoproterozoic Pan‐African orogeny.

Updated map of the mafic dike swarms of Brazil based on airborne geophysical data

Identification of mafic dike swarms and LIPs (Large Igneous Provinces) are of vital importance in geologic history because they provide information on geodynamics, mantle geochemistry, and paleomagnetism. These data provide key information for paleogeographic reconstructions with the aid of barcode matches and precise radiometric ages. Considering such issues, the Brazilian Precambrian shield can be used as a case for refining the cartography of the relevant intraplate activity (e.g., dikes, sills, flood basalts) in space and time.This work presents an updated map of Brazilian mafic dike swarms produced from airborne geophysical maps (Series 1000 – Geological Survey of Brazil). Linear and strong anomalies found on aeromagnetic maps using First Vertical Derivative of the Magnetic Field and Amplitude of the Analytic Signal were mapped on a GIS platform. The obtained data were compared to ternary radiometric maps and geological maps in order to exclude those that do not correspond to mafic dikes. The remaining structures - those believed to represent mafic dikes - were classified based on data compiled from the literature.The updated map exhibits more than 5000 elements, including dikes and magmatic suites, in which about 75% were geologically identified and divided into 60 dike swarms and 10 igneous suites and/or units. The dikes were grouped into sixteen extensional episodes from the Archean to the Cenozoic, although some are related to extension/transtension domains within regional compressive zones akin to orogenic settings. The most frequent records refer to the Proterozoic, representing intraplate episodes, some of them consistent with LIPs. The dataset also includes a large record of the Mesozoic age, which corresponds to major LIP events related to the opening of the Atlantic Ocean and the fragmentation of Gondwana.

Geochemistry of a lateritic mantle developed on basalt in the Cameroon Western Highlands (Cameroon Volcanic Line)

Although lateritic mantles have been strongly documented, there is need to continuously characterise and update information at site-scale so as to optimize exploration and exploitation operations in various industrial domains. In this paper, four soil profiles developed on basalt are described in detail to appreciate their morphology, mineralogy, geochemistry, pedogenesis and ore potential. The basalt is composed of olivine, plagioclase feldspars and pyroxenes and accessory magnetite, sphene and apatite. The basalt shows high levels of Ba, Cr, V, Sr, Ni, Zn and Zr; meanwhile total REEs range from 185.03 to 199.45 mg kg−1. The weathering indices reveal an extreme weathering intensity of the parent rock. Newly formed minerals are kaolinite (15–48%), haematite (13–40%), Goethite (10.5–19%) and gibbsite (10–12%), magnetite (4–9.56%), feldspars (4.67–6.10), apatite (2.8–3.9) and sphene (1.77–2.46%). The weathered materials show high levels of Cr (40–850 mg kg−1), V (145–582 mg kg−1), Zr (300–583 mg kg−1) and Ce (24.2–510 mg kg−1). The total REEs of the weathered materials range from 126.52 to 1267.69 mg kg−1, with the predominance of LREE. The REE basalt-normalized patterns revealed slight LREE depletion, more pronounced for the B horizons relative to the underlying C horizons as well as HREE enrichment that increase with depth and altitude. In these soils, mass balance calculations reveal depletion of Si, Ca, Mg, Ba, K, Na, Mn, P, Rb, Sr, La and Lu, enrichment of Al, Fe, Ti, Ga, U, Zr, Dy, Hf, U, Tm and Yb meanwhile Cr, Cs, Sn, Ta, Th, Tl, V, W, Y, Nb, Ce, Pr, Nd, Sm, Eu, Gd, Tb and Ho are either depleted or enriched from one profile to the other. Apart from Cr, V, Zr and Ce whose concentrations respectively attain 850 mg kg−1, 582 mg kg−1, 487 and 510 mg kg−1, the mining potential of the studied lateritic mantle is low.