Ferropicrites as evidence for lithological heterogeneity in the mantle source of continental fl ood basalts

The sublithospheric mantle must be chemically heterogeneous as a consequence of billions of years\ud worth of continuous subduction. Evidence for heterogeneity is strong in ocean island settings, where\ud variability in isotopic, major and trace element composition is frequently attributed to variability in\ud the mantle source. In continental fl ood basalt (CFB) provinces, signals of mantle-derived hetero-\ud geneity are obscured by fractionated and contaminated nature of CFB magmas. However, outcrops of\ud primitive magmas that more closely represent primary mantle melts are found in some CFB provinces.\ud This study focuses on picrites and ferropicrites from the Paraná-Etendeka CFB province, and also ex-\ud amines ferropicrites from the Karoo CFB province, which are found as dykes and thin fl ows. Ferropi-\ud crites are Fe-rich, Al-poor magmas equally primitive and Mg-rich as the peridotite-derived picrites,\ud but cannot have formed from peridotite melting.\ud The sample sets are investigated by supplementing published whole-rock and mineral analyses\ud with new mineral chemistry and melt inclusion data. Incompatible trace element compositions of their\ud olivine-hosted melt inclusions are very homogeneous, suggesting that their primary fractional mantle\ud melts underwent extensive mixing prior to the onset of crystallisation. Compared with primitive melt\ud inclusions from other settings, inclusions in these and other CFB provinces are well-mixed. The\ud crystallisation temperatures of olivine phenocrysts in these samples were determined by Al-in-olivine\ud thermometry, with maximum crystallisation temperatures of ∼ 1500 °C identifi ed in some Etendeka\ud picrites. These require a very high mantle potential temperature (TP ); the ferropicrites crystallised at\ud somewhat lower temperatures. XANES analyses confi rmed that the spinel Fe\ud 3+\ud /FeT were within the\ud thermometer’s calibrated range.\ud Ferropicrite has been suggested to originate from high pressure, low fraction melting of mantle\ud pyroxenite. The Etendeka ferropicrite geochemistry is examined with the aid of a new thermodynamic\ud model in order to interrogate its mantle source lithology and melting conditions. Modelling indicates\ud that both the major and trace element composition of ferropicrite is indeed more compatible with\ud garnet clinopyroxenite melting than peridotite melting, and that elevated TP plume conditions are\ud required in its formation. By comparison, picrite major element chemistry is consistent with a high\ud TP depleted peridotite melt. If a depleted peridotite and hybrid pyroxenite source mineralogy are\ud used for picrite and Etendeka ferropicrite, respectively, then they both represent ∼ 10–20% mantle\ud melting

Ferropicrites as evidence for lithological heterogeneity in the mantle source of continental flood basalts

The sublithospheric mantle must be chemically heterogeneous as a consequence of billions of years worth of continuous subduction. Evidence for heterogeneity is strong in ocean island settings, where variability in isotopic, major and trace element composition is frequently attributed to variability in the mantle source. In continental fl ood basalt (CFB) provinces, signals of mantle-derived hetero- geneity are obscured by fractionated and contaminated nature of CFB magmas. However, outcrops of primitive magmas that more closely represent primary mantle melts are found in some CFB provinces. This study focuses on picrites and ferropicrites from the Paraná-Etendeka CFB province, and also ex- amines ferropicrites from the Karoo CFB province, which are found as dykes and thin fl ows. Ferropi- crites are Fe-rich, Al-poor magmas equally primitive and Mg-rich as the peridotite-derived picrites, but cannot have formed from peridotite melting. The sample sets are investigated by supplementing published whole-rock and mineral analyses with new mineral chemistry and melt inclusion data. Incompatible trace element compositions of their olivine-hosted melt inclusions are very homogeneous, suggesting that their primary fractional mantle melts underwent extensive mixing prior to the onset of crystallisation. Compared with primitive melt inclusions from other settings, inclusions in these and other CFB provinces are well-mixed. The crystallisation temperatures of olivine phenocrysts in these samples were determined by Al-in-olivine thermometry, with maximum crystallisation temperatures of ∼ 1500 °C identifi ed in some Etendeka picrites. These require a very high mantle potential temperature (TP ); the ferropicrites crystallised at somewhat lower temperatures. XANES analyses confi rmed that the spinel Fe 3+ /FeT were within the thermometer’s calibrated range. Ferropicrite has been suggested to originate from high pressure, low fraction melting of mantle pyroxenite. The Etendeka ferropicrite geochemistry is examined with the aid of a new thermodynamic model in order to interrogate its mantle source lithology and melting conditions. Modelling indicates that both the major and trace element composition of ferropicrite is indeed more compatible with garnet clinopyroxenite melting than peridotite melting, and that elevated TP plume conditions are required in its formation. By comparison, picrite major element chemistry is consistent with a high TP depleted peridotite melt. If a depleted peridotite and hybrid pyroxenite source mineralogy are used for picrite and Etendeka ferropicrite, respectively, then they both represent ∼ 10–20% mantle melting

Comment on: “Investigating Earth’s Formation History Through Copper & Sulfur Metal–Silicate Partitioning During Core-Mantle Differentiation” by Mahan et al. (2018)

The physical and chemical conditions of terrestrial core formation play a key role in the distribution of elements between the Earth’s silicate mantle and metallic core. To explore this, Mahan et al. (2018a) present experimentally-derived partitioning data, showing how Cu distributes itself between metal and silicate at lower-mantle PT conditions with implications for planetary accretion and core formation. Eight experiments were performed in a diamond anvil cell (DAC) and each sample was welded to a copper grid for analysis. An offset in partitioning behaviour was subsequently noted between the high-P experiments and the lower-P dataset. However, when analysing the DAC experiments by electron probe microanalysis, the authors did not account for the secondary fluorescence of Cu that arises from the sample holder. Using Monte Carlo simulations of X-ray and electron transport, we show that the fluorescence of the Cu grid, originating from high energy continuum X-rays emitted from the sample, makes a significant contribution to the reported measurement of Cu in both the silicate and metallic phases. This is in good agreement with previous measurements made on Cu-free analogues. On average, around 70% of the published Cu concentrations are attributable to X-rays that originate externally to the sample. The reported offset in KDmet-sil at high pressures may reflect the different experimental and analytical protocol used, rather than a true pressure effect. Although adequate post-hoc corrections can be made, uncertainties around the exact sample and detector geometries make it difficult to refine simulations and derive accurate correction factors for each experiment

Karoon suuren magmaprovinssin Mg-rikkaimpien magmojen kiteytymislämpötilat Al-oliviini termometrin perusteella

Calculating reliable temperatures of Mg-rich magmas is problematic because melt composition and KD(Fe-Mg)ol-liq, the key parameters of many traditional thermometers, are difficult to constrain precisely. The recently developed Al-in-olivine thermometer [Coogan, L.A., Saunders, A.D., Wilson, R.N., 2014. Aluminum-in-olivine thermometry of primitive basalts: Evidence of an anomalously hot mantle source for large igneous provinces. Chemical Geology 368, 1–10] circumvents these problems by relying on the temperature-dependent exchange of Al between olivine and spinel crystallising in equilibrium with each other. This thermometer is used to re-evaluate the crystallisation temperatures of the most Mg-rich magma type identified from the Karoo large igneous province (LIP), known as the Vestfjella depleted ferropicrite suite. Previous temperature estimates for the suite were based on olivine-melt equilibria and indicated anomalously high crystallisation temperatures in excess of 1600 °C. We also present crystallisation temperatures for another Antarctic Karoo magma type, Group 3 dykes from Ahlmannryggen, which are derived from a pyroxene-rich mantle source. Our high-precision analysis of Al in olivine-spinel pairs indicate crystallisation temperatures from 1391±42 °C to 1481±35 °C for the Vestfjella depleted ferropicrite suite (Fo88–92) and from 1253±64 °C to 1303±40 °C for the Group 3 dykes (Fo79–82). Although the maximum temperature estimates for the former are over 100 °C lower than the previously presented estimates, they are still ~200 °C higher than those calculated for mid-ocean ridge basalts using the same method. Although exact mantle potential temperatures are difficult to estimate, the presented results support elevated sub-Gondwanan upper mantle temperatures (generated by a mantle plume or internal mantle heating) during the generation of the Karoo LIP.Calculating reliable temperatures of Mg-rich magmas is problematic because melt composition and KD(Fe-Mg)ol-liq, the key parameters of many traditional thermometers, are difficult to constrain precisely. The recently developed Al-in-olivine thermometer [Coogan, L.A., Saunders, A.D., Wilson, R.N., 2014. Aluminum-in-olivine thermometry of primitive basalts: Evidence of an anomalously hot mantle source for large igneous provinces. Chemical Geology 368, 1–10] circumvents these problems by relying on the temperature-dependent exchange of Al between olivine and spinel crystallising in equilibrium with each other. This thermometer is used to re-evaluate the crystallisation temperatures of the most Mg-rich magma type identified from the Karoo large igneous province (LIP), known as the Vestfjella depleted ferropicrite suite. Previous temperature estimates for the suite were based on olivine-melt equilibria and indicated anomalously high crystallisation temperatures in excess of 1600 °C. We also present crystallisation temperatures for another Antarctic Karoo magma type, Group 3 dykes from Ahlmannryggen, which are derived from a pyroxene-rich mantle source. Our high-precision analysis of Al in olivine-spinel pairs indicate crystallisation temperatures from 1391±42 °C to 1481±35 °C for the Vestfjella depleted ferropicrite suite (Fo88–92) and from 1253±64 °C to 1303±40 °C for the Group 3 dykes (Fo79–82). Although the maximum temperature estimates for the former are over 100 °C lower than the previously presented estimates, they are still ~200 °C higher than those calculated for mid-ocean ridge basalts using the same method. Although exact mantle potential temperatures are difficult to estimate, the presented results support elevated sub-Gondwanan upper mantle temperatures (generated by a mantle plume or internal mantle heating) during the generation of the Karoo LIP.Peer reviewe

Diamond anvil cell partitioning experiments for accretion and core formation: testing the limitations of electron microprobe analysis

Metal-silicate partitioning studies performed in high pressure, laser-heated diamond anvil cells (DAC) are commonly used to explore element distribution during planetary-scale core-mantle differentiation. The small run-products contain suitable areas for analysis commonly less than tens of microns in diameter and a few microns thick. Because high spatial resolution is required, quantitative chemical analyses of the quenched phases is usually performed by electron probe microanalysis (EPMA). Here, EPMA is being used at its spatial limits, and sample thickness and secondary fluorescence effects must be accounted for. By using simulations and synthetic samples, we assess the validity of these measurements, and find that in most studies DAC sample wafers are sufficiently thick to be characterised at 15 kVacc. Fluorescence from metal-hosted elements will, however, contaminate silicate measurements, and this becomes problematic if the concentration contrast between the two phases is in excess of 100. Element partitioning experiments are potentially compromised; we recommend simulating fluorescence and applying a data correction, if required, to such DAC studies. Other spurious analyses may originate from sources external to the sample, as exemplified by 0.5 to > 1 wt.% of Cu arising from continuum fluorescence of the Cu TEM grid the sample is typically mounted on

Vaipan sulien sekoittuminen syvällä laakiobasalttiprovinssien alla: Rajoituksia primitiivisten magmojen oliviinin sulasulkeumista

We present major and trace element compositions of 154 re-homogenised olivine-hosted melt inclusions found in primitive rocks (picrites and ferropicrites) from the Mesozoic Paraná–Etendeka and Karoo Continental Flood Basalt (CFB) provinces. The major element compositions of the melt inclusions, especially their Fe/Mg ratios, are variable and erratic, and attributed to the re-homogenisation process during sample preparation. In contrast, the trace element compositions of both the picrite and ferropicrite olivine-hosted melt inclusions are remarkably uniform and closely reflect those of the host whole-rocks, except in a small subset affected by hydrothermal alteration. The Paraná–Etendeka picrites and ferropicrites are petrogenetically related to the more evolved and voluminous flood basalts, and so we propose that compositional homogeneity at the melt inclusion scale implies that the CFB parental mantle melts were well mixed prior to extensive crystallisation. The incompatible trace element homogeneity of olivine-hosted melt inclusions in Paraná–Etendeka and Karoo primitive magmatic rocks has also been identified in other CFB provinces and contrasts with findings from studies of basalts from mid-ocean ridges (e.g. Iceland and FAMOUS on the Mid Atlantic Ridge), where heterogeneity of incompatible trace elements in olivine-hosted melt inclusions is more pronounced. We suggest that the low variability in incompatible trace element contents of olivine-hosted melt inclusions in near-primitive CFB rocks, and also ocean island basalts associated with moderately thick lithosphere (e.g. Hawaii, Galápagos, Samoa), may reflect mixing along their longer transport pathways during ascent and/or a temperature contrast between the liquidus and the liquid when it arrives in the crust. These thermal paths promote mixing of mantle melts prior to their entrapment by growing olivine crystals in crustal magma chambers. Olivine-hosted melt inclusions of ferropicrites from the Paraná–Etendeka and Karoo CFB have the least variable compositions of all global melt inclusion suites, which may be a function of their unusually deep origin and low viscosity.Peer reviewe

The dependence of metal-silicate partitioning of moderately volatile elements on oxygen fugacity and Si contents of Fe metal: Implications for their valence states in silicate liquids

The volatile siderophile elements are important tracers of the delivery of volatile elements to the Earth. Their concentrations in the bulk silicate Earth are a function of the relative timing of their accretion and their sequestration into the core: a comprehensive understanding of their metal-silicate partitioning behaviour is therefore required in order to infer the volatile element accretion history. We present new partitioning data between liquid metal and liquid silicate at 11 GPa for a suite of volatile siderophile elements: Ag, As, Au, Cu, Ge, P, Pb, Sb, Sn. We focus particularly on determining their valence states and the effects of Si on partitioning, which are required in order to extrapolate from experimental conditions to core-formation conditions. It was found that all elements have weak to strong positive interaction parameters with Si. At low fO2, redox equilibria dictate that the siderophile elements should become more siderophile. However, at low fO2, Si also partitions more strongly into the metal. Given the repulsive nature of the interaction between Si and the elements of interest, the increased Si concentration at low fO2 will counteract the expected increase in the partition coefficient, making these elements less siderophile than expected at very reducing conditions. This causes the linear relationship between fO2 and log(D) to become non-linear at low fO2, which we account for by fitting an interaction parameter between Si and the elements of interest. This has implications for the interpretation of experimental results, because the valence cannot be determined from the slope of log(D) vs. logfO2 if low fO2, high Si metal compositions are employed without applying an activity correction. This also has implications for the extrapolation of experimental partitioning data to core-formation conditions: reducing conditions in the early stages of core formation do not necessarily result in complete or even strong depletion of siderophile elements when Si is present as a light element in the core-forming metal phase

The effect of potassium on aluminous phase stability in the lower mantle

The aluminous calcium-ferrite type phase (CF) and new aluminous phase (NAL) are thought to hold the excess alumina produced by the decomposition of garnet in MORB compositions in the lower mantle. The respective stabilities of CF and NAL in the nepheline-spinel binary (NaAlSiO4 –MgAl2O4 ) are well established. However with the addition of further components the phase relations at lower mantle conditions remain unclear. Here we investigate a range of compositions around the nepheline apex of the nepheline-kalsilite-spinel compositional join (NaAlSiO4 –KAlSiO4–MgAl2O4 ) at 28–78 GPa and 2000 K. Our experiments indicate that even small amounts of a kalsilite (KAlSiO4 ) component dramatically impact phase relations. We find NAL to be stable up to at least 71 GPa in potassium-bearing compositions. This demonstrates the stabilizing effect of potassium on NAL, because NAL is not observed at pressures above 48 GPa on the nepheline-spinel binary. We also observe a broadening of the CF stability field to incorporate larger amounts of potassium with increasing pressure. For pressures below 50 GPa only minor amounts (<0.011(1) K/(K+Na+Mg) ) of potassium are soluble in CF, whereas at 68 GPa, we find a solubility in CF of at least 0.088(3) K/(K+Na+Mg). This indicates that CF and NAL are suitable hosts of the alkali content of MORB compositions at lower mantle conditions. For sedimentary compositions at lower mantle pressures, we expect K-Hollandite to be stable in addition to CF and NAL for pressures of 28–48 GPa, based on our simplified compositions

SiO2 glass density to lower-mantle pressures

The convection or settling of matter in the deep Earth’s interior is mostly constrained by density variations between the different reservoirs. Knowledge of the density contrast between solid and molten silicates is thus of prime importance to understand and model the dynamic behavior of the past and present Earth. SiO2 is the main constituent of Earth’s mantle and is the reference model system for the behavior of silicate melts at high pressure. Here, we apply our recently developed x-ray absorption technique to the density of SiO2 glass up to 110 GPa, doubling the pressure range for such measurements. Our density data validate recent molecular dynamics simulations and are in good agreement with previous experimental studies conducted at lower pressure. Silica glass rapidly densifies up to 40 GPa, but the density trend then flattens to become asymptotic to the density of SiO2 minerals above 60 GPa. The density data present two discontinuities at ∼17 and ∼60  GPa that can be related to a silicon coordination increase from 4 to a mixed 5/6 coordination and from 5/6 to sixfold, respectively. SiO2 glass becomes denser than MgSiO3 glass at ∼40  GPa, and its density becomes identical to that of MgSiO3 glass above 80 GPa. Our results on SiO2 glass may suggest that a variation of SiO2 content in a basaltic or pyrolitic melt with pressure has at most a minor effect on the final melt density, and iron partitioning between the melts and residual solids is the predominant factor that controls melt buoyancy in the lowermost mantle

Kelyphite textures experimentally reproduced through garnet breakdown in the presence of a melt phase

Complex multiphase reaction rims that form during garnet breakdown are known as kelyphite coronae and are common amongst exhumed mantle xenoliths. It has long been established that a reaction of garnet and olivine produces kelyphite corona consisting of spinel and pyroxenes, and that preservation of high-pressure garnet cores requires sufficiently rapid uplift of material through the spinel lherzolite stability field from depths of at least 60 km.We present new high-pressure, high-temperature experiments of garnet breakdown in the spinel-lherzolite stability field demonstrating that a series of cascading reactions can reproduce the multilayer, multiphase kelyphites seen in nature. In all experiments where breakdown occurred, a melt appears to have moderated the reactions towards equilibrium; we believe this to be the first experimental confirmation of the importance of such melts in garnet breakdown reactions. In our experiments at least three distinct zones of concentric kelyphite growth can occur at a single pressure, temperature condition; we suggest, therefore, that such kelyphites seen in natural samples do not have to be caused by a multistage uplift path as is often assumed.Kelyphitic coronae surrounding garnet have previously been used to estimate uplift rates, however, the lack of kinetic data for relevant exhumation reactions has limited their use for PTt pathway estimations and the understanding of emplacement mechanisms. In order to constrain accurate PTt pathways we use reaction rim thickness as a proxy for reaction progress and present preliminary results for the kinetics of garnet breakdown