Fluorine and chlorine in mantle minerals and the halogen budget of the Earth’s mantle

The fluorine (F) and chlorine (Cl) contents of arc magmas have been used to track the composition of subducted components, and the F and Cl contents of MORB have been used to estimate the halogen content of depleted MORB mantle (DMM). Yet, the F and Cl budget of the Earth’s upper mantle and their distribution in peridotite minerals remain to be constrained. Here, we developed a method to measure low concentrations of halogens (≥0.4 µg/g F and ≥0.3 µg/g Cl) in minerals by secondary ion mass spectroscopy. We present a comprehensive study of F and Cl in co-existing natural olivine, orthopyroxene, clinopyroxene, and amphibole in seventeen samples from different tectonic settings. We support the hypothesis that F in olivine is controlled by melt polymerization, and that F in pyroxene is controlled by their Na and Al contents, with some effect of melt polymerization. We infer that Cl compatibility ranks as follows: amphibole > clinopyroxene > olivine ~ orthopyroxene, while F compatibility ranks as follows: amphibole > clinopyroxene > orthopyroxene ≥ olivine, depending on the tectonic context. In addition, we show that F, Cl, Be and B are correlated in pyroxenes and amphibole. F and Cl variations suggest that interaction with slab melts and fluids can significantly alter the halogen content of mantle minerals. In particular, F in oceanic peridotites is mostly hosted in pyroxenes, and proportionally increases in olivine in subduction-related peridotites. The mantle wedge is likely enriched in F compared to un-metasomatized mantle, while Cl is always low (<1 µg/g) in all tectonic settings studied here. The bulk anhydrous peridotite mantle contains 1.4–31 µg/g F and 0.14–0.38 µg/g Cl. The bulk F content of oceanic-like peridotites (2.1–9.4 µg/g) is lower than DMM estimates, consistent with F-rich eclogite in the source of MORB. Furthermore, the bulk Cl budget of all anhydrous peridotites studied here is lower than previous DMM estimates. Our results indicate that nearly all MORB may be somewhat contaminated by seawater-rich material and that the Cl content of DMM could be overestimated. With this study, we demonstrate that the halogen contents of natural peridotite minerals are a unique tool to understand the cycling of halogens, from ridge settings to subduction zones.     

Equation of state of magnesite for the conditions of the Earth’s lower mantle

A semiempirical equation of state was derived for magnesite under the thermodynamic conditions of the Earth’s mantle. Within experimental uncertainties, it is consistent with thermochemical, ultrasonic, X-ray, and shock-wave data at temperatures from 15 K to the melting point and pressures of up to 100–130 GPa. The following values were recommended for the isothermal bulk modulus and its pressure derivative: K _T = 111.71 GPa and K′ = 4.08. Thermodynamic analysis showed that magnesite does not decompose to periclase and CO₂ under the thermodynamic conditions of the Earth’s lower mantle and outer core.     

Estimation of the stress state of the Earth’s crust and the upper mantle in the area of the Southern Kuril Islands

The catastrophic Shikotan earthquake of October 4 (5), 1994, occurred in the Pacific Ocean. Its focus was located 80 km eastward of Shikotan Island. The stress state of the Earth’s crust in this area was estimated by the method of the cataclastic analysis of the whole range of the earthquake mechanisms. The performed reconstruction of the parameters of the current stress state of the Earth’s crust and the upper mantle in the area of the Southern Kuril Islands made it possible to establish that this area is characterized, on the one hand, by the presence of extensive areas of steady behavior of the stress tensor parameters and, on the other hand, by the presence of local sections of anomalously fast changes in these parameters.     

Gravitational interactions of the solid core and the Earth’s mantle and variations in the length of the day

A simple mechanical model explaining the long-period (about 100-year) variations in the Earth’s rotational velocity is proposed. This model takes into account the gravitational interaction of the mantle with the solid core of the Earth and the fact that the core rotation leads that of the mantle. Well-known Earth parameters provide estimates of the gravitational torque that support the proposed model. The mathematical problem involved reduces to the classical problem of a nonlinear oscillator exposed to a constant torque. The well-known parameters of the core-mantle system result in a stable equilibrium and a stable limiting cycle on the phase cylinder of this oscillator. This equilibrium corresponds to a single angular velocity for the mantle and solid core, with no long-period oscillations in the length of the day. The limiting cycle corresponds to the core rotation leading the mantle rotation. In this case, the ellipsoidality of the gravitationally interacting bodies provides a periodic interchange of kinetic angular momentum between the mantle and solid core that results in long-period variations in the length of the day. The proposed model does not support the formerly widespread opinion that the core rotates more slowly than the mantle.     

Is the tilt of the Thames valley towards the east caused by a distal lobe of the Icelandic mantle plume?

Over the last 30 years, a growing body of research has shown that first-order control of the elevation of Earth's surface is exercised by thermal anomalies in the upper asthenosphere. One line of research is to test models and observations of mantle behaviour against the sedimentary record. A second line of research is to use the sedimentary record to further understanding of mantle behaviour. Here this second line of research is adopted: a particular hypothesis of mantle behaviour is tested against the Quaternary sedimentary record of the Thames valley, England. Schoonman et al. (2017) have proposed that a warm finger of mantle material extending from the Icelandic plume underlies southern England at the present day. That warm finger would represent the distal end of the influence of the Icelandic plume in this area, and would have advanced broadly from west to east, causing a progressive tilt of the surface of the Thames valley towards the east. The warm-finger hypothesis is supported by the evidence reviewed here. That evidence consists of two main sets of observations, both sets established beyond reasonable doubt by many researchers over many years. First, there is the progressive increase in elevation westward from the present-day coast of the North Sea of the 2.5–2 Ma shallow-marine Red and Norwich Crags. Second, there is the subsequent Quaternary record of progressive eastward tilting of the Thames valley shown in the river terraces.     

Lithospheric structure and crust–mantle decoupling in the southeast edge of the Tibetan Plateau

Convergence between the Indian plate and the Eurasian plate has resulted in the uplift of the Tibetan Plateau, and understanding the associated dynamical processes requires investigation of the structures of the crust and the lithosphere of the Tibetan Plateau. Yunnan is located in the southwest edge of the plateau and adjacent to Myanmar to the west. Previous observations have confirmed that there is a sharp transition in mantle anisotropy in this area, as well as clockwise rotations of the surface velocity, surface strain, and fault orientation. We use S receiver functions from 54 permanent broad-band stations to investigate the structures of the crust and the lithosphere beneath Yunnan. The depth of the Moho is found to range from 36 to 40 km beneath southern Yunnan and from 55 to 60 km beneath northwestern Yunnan, with a dramatic variation across latitude 25–26°N. The depth of the lithosphere–asthenosphere boundary (LAB) ranges from 180 km to less than 70 km, also varying abruptly across latitude 25–26°N, which is consistent with the sudden change of the fast S-wave direction (from NW–SE to E–W across 26–28°N). In the north of the transition belt, the lithosphere is driven by asthenospheric flow from Tibet, and the crust and the upper mantle are mechanically coupled and moving southward. Because the northeastward movement of the crust in the Burma micro-plate is absorbed by the right-lateral Sagaing Fault, the crust in Yunnan keeps the original southward movement. However, in the south of the transition belt, the northeastward mantle flow from Myanmar and the southward mantle flow from Tibet interact and evolve into an eastward flow (by momentum conservation) as shown by the structure of the LAB. This resulting mantle flow has a direction different from that of the crustal movement. It is concluded that the Sagaing Fault causes the west boundary condition of the crust to be different from that of the lithospheric mantle, thus leading to crust–mantle decoupling in Yunnan.     

Origin of the LLSVPs at the base of the mantle is a consequence of plate tectonics——A petrological and geochemical perspective

In studying the petrogenesis of intra-plate ocean island basalts(OIB) associated with hotspots or mantle plumes, we hypothesized that the two large-low-shear-wave-velocity provinces(LLSVPs) at the base of the mantle beneath the Pacific(Jason) and Africa(Tuzo) are piles of subducted ocean crust(SOC)accumulated over Earth's history. This hypothesis was formulated using petrology, geochemistry and mineral physics in the context of plate tectonics and mantle circulation. Because the current debate on the origin of the LLSVPs is limited to the geophysical community and modelling discipline and because it is apparent that such debate cannot be resolved without considering relevant petrological and geochemical information, it is my motivation here to objectively discuss such information in a readily accessible manner with new perspectives in light of most recent discoveries. The hypothesis has the following elements:(1) subduction of the ocean crust of basaltic composition to the lower mantle is irreversible because(2) SOC is denser than the ambience of peridotitic composition under lower mantle conditions in both solid state and liquid form;(3) this understanding differs from the widespread view that OIB come from ancient SOC that returns from the lower mantle by mantle plumes, but is fully consistent with the understanding that OIB is not derived from SOC because SOC is chemically and isotopically too depleted to meet the requirement for any known OIB suite on Earth;(4) SOC is thus the best candidate for the LLSVPs, which are, in turn, the permanent graveyard of SOC;(5) the LLSVPs act as thermal insulators, making core-heating induced mantle diapirs or plumes initiated at their edges, which explains why the large igneous provinces(LIPs) are associated with the edges of the LLSVPs;(6) the antipodal positioning of Jason and Tuzo represents the optimal momentum of inertia, which explains why the LLSVPs are stable in the spinning Earth.     

A brief review of isotopically light Li – a feature of the enriched mantle?

Lithium isotope geochemistry is increasingly being used to trace deep-earth processes, reflecting the observed large variation of Li isotope ratios in mantle-derived rocks, including peridotite xenoliths associated with ancient continents. We briefly review the Li isotopic compositions of major geochemical reservoirs, the assumed mechanisms of Li isotopic fractionation, and, in particular, the origins of isotopically light Li in mantle-derived rocks based on the latest developments in Li isotope geochemistry. Comparison of Li isotope data with existing Sr-Nd isotope ratios reflects the subduction-recycling of ancient oceanic crust and the reappearance of Li in volcanic rocks. This circulation may play an important role in generating the isotopically light-Li component in the mantle – perhaps the enriched mantle end member defined by the Sr-Nd isotopic compositions of oceanic basalts.     

Possible physicochemical facies of wehrlitization of ultramafic rocks in the mantle wedge under volcanoes of the Kuril–Kamchatka frontal zone

A quantitative model describing the dynamics of the process of metasomatic wehrlitization of ultramafics is put forward. It is elaborated for the process taking place in permeable fault zones over a time span of 50 kyr with fluid source depths in the range of 150–50 km at initial temperatures of 1000–1200°C. The possibility of existence of two physical–chemical facies of this process has been demonstrated: one occurs at the level of garnet and the other is at the level of spinel depth facies. Their realization is related to the dependence of the activity of Mg–Ca–Si metasomatism against variation in the composition of low–molecular hydrocarbons in a fluid under conditions of changing T and P in a system.     

Hydration and dehydration in the lower margin of a cold mantle wedge: implications for crust–mantle interactions and petrogeneses of arc magmas

Garnet orthopyroxenites from Maowu (Dabieshan orogen, eastern China) were formed from a refractory harzburgite/dunite protolith. They preserve mineralogical and geochemical evidence of hydration/metasomatism and dehydration at the lower edge of a cold mantle wedge. Abundant polyphase inclusions in the cores of garnet porphyroblasts record the earliest metamorphism and metasomatism in garnet orthopyroxenites. They are mainly composed of pargasitic amphibole, gedrite, chlorite, talc, phlogopite, and Cl-apatite, with minor anhydrous minerals such as orthopyroxene, sapphirine, spinel, and rutile. Most of these phases have high X_Mg, NiO, and Ni/Mg values, implying that they probably inherited the chemistry of pre-existing olivine. Trace element analyses indicate that polyphase inclusions are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), and high field strength elements (HFSE), with spikes of Ba, Pb, U, and high U/Th. Based on the P–T conditions of formation for the polyphase inclusions (?1.4 GPa, 720–850°C), we suggest that the protolith likely underwent significant hydration/metasomatism by slab-derived fluid under shallow–wet–cold mantle wedge corner conditions beneath the forearc. When the hydrated rocks were subducted into a deep–cold mantle wedge zone and underwent high-pressure–ultrahigh-pressure (HP–UHP) metamorphism, amphibole, talc, and chlorite dehydrated and garnet, orthopyroxene, Ti-chondrodite, and Ti-clinohumite formed during prograde metamorphism. The majority of LILE (e.g. Ba, U, Pb, Sr, and Th) and LREE were released into the fluid formed by dehydration reactions, whereas HFSE (e.g. Ti, Nb, and Ta) remained in the cold mantle wedge lower margin. Such fluid resembling the trace element characteristics of arc magmas evidently migrates into the overlying, internal, hotter part of the mantle wedge, thus resulting in a high degree of partial melting and the formation of arc magmas.     

Trace element modeling of aqueous fluid – peridotite interaction in the mantle wedge of subduction zones

Recently measured partition coefficients for Rb, Th, U, Nb, La (Ce), Pb, Sr, Sm, Zr, and Y between lherzolite assemblage minerals and H₂O-rich fluid (Ayers et al. 1997; Brenan et al. 1995a,b) are used in a two-component local equilibrium model to assess the effects of interaction between slab-derived aqueous fluids and wedge lherzolite on the trace element and isotopic composition of island arc basalts (IAB). The model includes four steps representing chemical processes, with each process represented by one equation with one adjustable parameter, in which aqueous fluid: (1) separates from eclogite in the subducted slab (Rayleigh distillation, mass fraction of fluid released F  ^fluid); (2) ascends through the mantle wedge in isolated packets, exchanging elements and isotopes with depleted lherzolite (zone refining, the rock/fluid mass ratio n); (3) mixes with depleted lherzolite (physical mixing, the mass fraction of fluid in the mixture X  ^fluid); (4) induces melting to form primitive IAB (batch melting, mass fraction of melt F ^melt). The amount of mantle lherzolite processed by the fluid in step (2) determines its isotopic and trace element signature and the relative contributions of slab and wedge to primitive IAB. Assuming an average depleted lherzolite composition and mineralogy (70% olivine, 26% orthopyroxene, 3% clinopyroxene and 1% ilmenite) and using nonlinear regression to adjust parameter values to obtain an optimal fit to the average composition of IAB (McCulloch and Gamble 1991) yields values of F  ^fluid= 0.20, n= 26, X  ^fluid= 0.17, and F  ^melt= 0.15, with r  ²= 0.995 and the average relative error in trace element concentration = 6%. The average composition of IAB can also effectively be modeled with no contribution from the slab other than H₂O (i.e., skip model step 1): n= 27, X  ^fluid= 0.21, F  ^melt= 0.17, with r  ²= 0.992. By the time the fluid reaches the IAB source, exchange with depleted wedge lherzolite reduces the ⁸⁷Sr/⁸⁶Sr ratio isotopic composition to near-mantle values and the slab contribution to <50% for all but the most incompatible elements (e.g., Pb). The IAB may retain the slab signature for elements such as B and Be that are highly incompatible and that have very low concentrations in the depleted mantle wedge. The relatively high equilibrium D  ^{mineral /   fluid} values measured by Ayers et al. (1997), Brenan et al. (1995a) and Stalder et al. (1998) suggest that large amounts of fluid (>5 wt%) must be added to lherzolite in the IAB source. Decreasing X  ^fluid below 0.05 causes model results to have unacceptably high levels of error and petrologically unreasonable values of F  ^melt. That H₂O contents of IAB are generally <6 wt% suggests that not all of the H₂O that metasomatizes the IAB source remains in the source to dissolve in the subsequently formed melt. Modeling of the compositions of specific primitive IAB from oceanic settings with low sediment input and depleted mantle wedges (Tonga, Marianas) shows a generally lower level of fluid-wedge interaction (low n), and therefore a larger slab component in primitive IAB. Received: 6 October 1997 / Accepted: 8 May 1998     

Persistence of mantle lithospheric Re–Os signature during asthenospherization of the subcontinental lithospheric mantle: insights from in situ isotopic analysis of sulfides from the Ronda peridotite (Southern Spain)

The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling. The Re–Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that of grains in the spinel tectonites. This indicates that the Re–Os radiometric system in sulfides was highly resistant to partial melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re–Os isotopic systematics of sulfides in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2–1.4 Ga, respectively. The emplacement of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent.     

Equation of state of silicate melts with densified intermediate-range order at the pressure condition of the Earth’s deep upper mantle

The knowledge from the compression behavior of densified SiO₂ glass suggests that SiO₂ melt may behave as a single phase having a densified network structure (intermediate-range order) at the pressure condition of the Earth’s deep upper mantle, including the transition zone. A simple and easy-to-use equation of state of silicate melts which is applicable to a wide range of chemical composition at the pressure condition of the deep upper mantle is proposed based on the assumption that SiO₂ component is in its densified state (or phase). The equation of state proposed in this study is consistent with all the available density data of silicate melts with an SiO₂ content of about 35–55 mol% measured with large-volume presses at pressures between 8 and 22 GPa. The equations of state in previous studies differ considerably from each other. The main reason for the discrepancies seems to be that the compression behavior of multiple states (or phases) of silicate melts has been described in most cases with a single equation of state. It is necessary to consider that silicate melts are in their densified states (or phases) in the deep upper mantle.     

Mineralogy of the lower mantle: A review of ‘super-deep’ mineral inclusions in diamond

Starting from the late 1980s, several groups of lower-mantle mineral inclusions in diamond have been found. Three associations were established among them: juvenile ultramafic, analogous to eclogitic, and carbonatitic. The juvenile ultramafic association strongly predominates, and it is composed of ferropericlase, MgSi-, CaSi- and CaTi-perovskites, stishovite, tetragonal almandine-pyrope phase (TAPP), and some others. The association analogous to the upper-mantle eclogitic association, formed from subducting lithosphere, comprises: majorite, CaSi-perovskite bearing compositional Eu anomalies, phase ‘Egg’ with a tetragonal structure, and stishovite. The carbonatitic association is represented by various carbonates, halides, and associated minerals. Some mineral associations (wüstite + periclase and native iron + iron carbides) are, possibly, related to the D″ layer at the core/mantle boundary. The mineralogical composition of the lower mantle is now understood to be more complex than had been suggested in theoretic and experimental works. The proportion of ferropericlase in the lower mantle is higher than it was suggested before, and its composition is more iron-rich (mg = 0.36–0.90) as compared to experimental and theoretical data. Free silica (stishovite) is always present in lower-mantle associations, and a separate aluminous phase (TAPP) has been identified in several areas. These discrepancies suggest that the composition of the lower mantle differs to that of the upper-mantle, and experiments based solely on ‘pyrolitic’ compositions are not, therefore, applicable to the lower mantle. These data indicate a probability of an alternative to the CI-chondrite model of the Earth's formation, for example, an enstatite-chondrite model.