William O. Hibberson

Subduction of continental crust and terrigenous and pelagic sediments: an experimental study

Abstract An important role for the recycling of terrigenous sediments in the mantle was proposed by Armstrong [1,2]. Aspects of this hypothesis have been tested via an experimental study of phase equilibria in a composition similar to that of average upper continental crust, over the pressure interval 6–24 GPa. This composition is sufficiently close to those of the principal argillaceous and siliceous classes of pelagic sediments to use the results to interpret the behaviour of these latter sediments, in addition to terrigenous sediments, during subduction. The subsolidus phase relationships displayed by lithologies derived from these materials are complex, with major roles being played by K-hollandite, garnet, Na-clinopyroxene and stishovite. Continentally derived lithologies achieve densities similar to, or greater than, surrounding mantle at depths greater than 200 km. The geodynamical implications of these buoyancy relationships are discussed. A reconnaissance study has also been made to assess the melting and element partitioning behaviours in the subducted continental crust lithologies. During partial melting at relatively low pressures (5–10 GPa), orthoclase, wadeite and K-hollandite are eliminated near the solidus, whereas the stability field of Na-clinopyroxene extends to temperatures well above the solidus. Liquids formed by small to moderate degrees of partial melting of subducted, continentally derived lithologies in this pressure interval have high K Na ratios and high SiO2 contents. At higher pressures (16–24 GPa), the stability fields of K-hollandite and stishovite extend towards the liquidus, whereas sodium-bearing phases are eliminated much closer to the solidus. Resultant partial melts consequently possess relatively low K Na ratios and SiO2 contents. Lead possesses a high crystal-liquid partition coefficient in K-hollandite, whereas uranium is excluded. The broad stability field of K-hollandite during partial melting of these lithologies at pressures above 15 GPa therefore has the capacity to cause fractionation of lead from uranium. The petrological and geochemical implications of these results are discussed in the context of the partial melting of terrigenous materials during subduction and the interactions of the partial melts with the surrounding mantle.

Water and its influence on the lithosphere–asthenosphere boundary

The Earth has distinctive convective behaviour, described by the plate tectonics model, in which lateral motion of the oceanic lithosphere of basaltic crust and peridotitic uppermost mantle is decoupled from the underlying mechanically weaker upper mantle (asthenosphere). The reason for differentiation at the lithosphere-asthenosphere boundary is currently being debated with relevant observations from geophysics (including seismology) and geochemistry (including experimental petrology). Water is thought to have an important effect on mantle rheology, either by weakening the crystal structure of olivine and pyroxenes by dilute solid solution, or by causing low-temperature partial melting. Here we present a novel experimental approach to clarify the role of water in the uppermost mantle at pressures up to 6 GPa, equivalent to a depth of 190 km. We found that for lherzolite in which a water-rich vapour is present, the temperature at which a silicate melt first appears (the vapour-saturated solidus) increases from a minimum of 970 °C at 1.5 GPa to 1,350 °C at 6 GPa. We have measured the water content in lherzolite to be approximately 180 parts per million, retained in nominally anhydrous minerals at 2.5 and 4 GPa at temperatures above and below the vapour-saturated solidus. The hydrous mineral pargasite is the main water-storage site in the uppermost mantle, and the instability of pargasite at pressures greater than 3 GPa (equivalent to more than about 90 km depth) causes a sharp drop in both the water-storage capacity and the solidus temperature of fertile upper-mantle lherzolite. The presence of interstitial melt in mantle with more than 180 parts per million of water at pressures greater than 3 GPa alters mantle rheology and defines the lithosphere-asthenosphere boundary. Modern asthenospheric mantle acting as the source for mid-oceanic ridge basalts has a water content of 50-200 parts per million (refs 3-5). We show that this matches the water content of residual nominally anhydrous minerals after incipient melting of lherzolite at the vapour-saturated solidus at high pressure.The Earth has distinctive convective behaviour, described by the plate tectonics model, in which lateral motion of the oceanic lithosphere of basaltic crust and peridotitic uppermost mantle is decoupled from the underlying mechanically weaker upper mantle (asthenosphere). The reason for differentiation at the lithosphere–asthenosphere boundary is currently being debated with relevant observations from geophysics (including seismology) and geochemistry (including experimental petrology). Water is thought to have an important effect on mantle rheology, either by weakening the crystal structure of olivine and pyroxenes by dilute solid solution, or by causing low-temperature partial melting. Here we present a novel experimental approach to clarify the role of water in the uppermost mantle at pressures up to 6 GPa, equivalent to a depth of 190 km. We found that for lherzolite in which a water-rich vapour is present, the temperature at which a silicate melt first appears (the vapour-saturated solidus) increases from a minimum of 970 °C at 1.5 GPa to 1,350 °C at 6 GPa. We have measured the water content in lherzolite to be approximately 180 parts per million, retained in nominally anhydrous minerals at 2.5 and 4 GPa at temperatures above and below the vapour-saturated solidus. The hydrous mineral pargasite is the main water-storage site in the uppermost mantle, and the instability of pargasite at pressures greater than 3 GPa (equivalent to more than about 90 km depth) causes a sharp drop in both the water-storage capacity and the solidus temperature of fertile upper-mantle lherzolite. The presence of interstitial melt in mantle with more than 180 parts per million of water at pressures greater than 3 GPa alters mantle rheology and defines the lithosphere–asthenosphere boundary. Modern asthenospheric mantle acting as the source for mid-oceanic ridge basalts has a water content of 50–200 parts per million (refs 3–5). We show that this matches the water content of residual nominally anhydrous minerals after incipient melting of lherzolite at the vapour-saturated solidus at high pressure.

Origin of kimberlites and related magmas

Abstract Rare earth fractionation in kimberlites implies that they were produced by partial melting in the presence of residual garnet, in accordance with the widely held belief that kimberlites were formed by small degrees of partial melting of a garnet lherzolite lithology in the upper mantle. However, recent discoveries in some kimberlites of diamond xenocrysts containing syngenetic inclusions of majorite, and of xenoliths which originally contained majoritic garnet are suggestive of a deeper, transition zone origin for kimberlites. Experiments on a synthetic Group I kimberlite were carried out using an MA-8 apparatus to evaluate this possibility. At 16 GPa and 1650°C, majorite garnet (13% Al2O3) and β-M2SiO4 crystallize together on the liquidus, showing that this kimberlite magma could have been produced by a small degree of partial melting of a majorite +β-M2SiO4 (or γ-M2SiO4) assemblage in the transition zone (400–650 km). However, the first appearance of garnet well below the liquidus at 10 GPa implies that this typical kimberlite composition could not have been produced by a small degree of partial melting of garnet peridotite at depths shallower than 300 km, and casts doubt on conventional models of kimberlite petrogenesis. Isotopic, trace element and geochemical similarities imply a genetic relationship between kimberlites and ocean island basalts (OIBs). However, kimberlites were derived from a source possessing a higher Mg-number, and lower Na2O, Al2O3 and CaO contents than the OIB source. It is proposed that the ultimate source regions both of kimberlites and OIBs lie in the transition zone, in a boundary layer comprised of mixed domains of subducted former harzburgite and aesthenospheric pyrolite. The boundary layer was refertilized by partial melts derived from garnetite (former subducted oceanic crust) trapped on the 650 km discontinuity.

The eclogite-garnetite transformation at high pressure and some geophysical implications

Mineral assemblages displayed by MORB and alkali-poor olivine tholeiites have been investigated over the pressure interval 4.6–18 GPa at 1200°C. Both compositions crystallize to form normal eclogites between 4.6 and 10 GPa and there is little change in the relative proportions of garnet and pyroxene over this range. However, the proportion of garnet increases rapidly above 10 GPa as pyroxene dissolves in the garnet structure and pyroxene-free garnetites (±stishovite) are produced by 14–15 GPa, dependent upon composition. The garnetite facies for both compositions possess zero-pressure densities of 3.75 g/cm3, implying that subducted oceanic crust remains appreciably denser than surrounding mantle to depths exceeding 600 km. It is demonstrated that the seismic velocity distributions in the mantle between 400 and 650 km are inconsistent with Andersons hypothesis that this region is of eclogitic composition.

The instability of plagioclase in peridotite at high pressure

Experimental crystallization of forsterite + anorthite (2.2: 1 molar ratio) in a piston-cylinder solid-media apparatus shows that forsterite and anorthite are stable at less than 8.5 ± 0.5 kb at 1250°C, but forsterite + orthopyroxene + clinopyroxene + spinel is the stable assemblage at higher pressures. The fayalite + anorthite assemblage is stable to 7–7.5 kb at 1050°C and to 6–6.5 kb at 900°C, and at higher pressure is replaced by almandine-grossular garnet. Reactions between magnesian olivine (Fo92) and labradorite (An59), and between olivine and plagioclase in a complex peridotite composition (pyrolite), show that there is a 5-phase field of olivine + orthopyroxene + clinopyroxene + plagioclase + aluminous spinel between 9 kb and 11–14 kb (depending on composition) at 1200°C. At higher pressures plagioclase is absent and at lower pressures pyroxenes are absent or present in smaller amounts, and alumina-rich spinel is absent. The experimental data are applied in discussions of corona formation and of the stability of plagioclase in peridorite compositions.

Solubilities of mantle oxides in molten iron at high pressures and temperatures: implications for the composition and formation of Earth's core

The solubilities of several oxide components of the mantle in molten iron have been measured at 16 GPa and 1700–2000°C. Their relative solubilities are as follows: FeO > Cr2O3 > MnO, V2O3, TiO2 > SiO2 > MgO > Al2O3. Oxide contents reach significant levels, varying from ∼ 10 wt% FeO to ∼ 1 wt% SiO2 at the respective metal-oxide eutectics, but are very small for MgO and Al2O3. The solubilities of these oxides are expected to increase substantially at temperatures above 2000°C and at pressures above 16 GPa and are relevant to the core-formation process in the Earth. Individual oxide species dissolve quasi-congruently under conditions which maintain oxygen fugacities near the iron-wustite buffer. However, the dissolution of mantle mineral phases is highly incongruent. If a mixture of metallic iron and mantle silicates were subjected to increasing pressures and temperatures (above 16 GPa and 2000°C) the most soluble species, FeO, would first be extracted into the metallic liquid. When the FeO activity had been lowered sufficiently, significant amounts of SiO2 would then enter this melt. At extremely high temperatures and pressures, appreciable amounts of MgO could even dissolve. These results provide a background for interpreting recent diamond-anvil experiments in which molten iron was observed to react with mantle silicates at temperatures of 2700–3700°C and pressures of 20–70 GPa. They also elucidate the nature of chemical reactions between the core and mantle which may occur in the “D” layer of the lower mantle. The differing solubilities of mantle oxides in molten iron place constraints on models of accretion of the Earth and of accompanying core-formation processes. The observation that TiO2 is undepleted in the Earths mantle (relative to CI chondrites) makes it improbable that the core contains a substantial amount of dissolved Si or SiO2. It therefore seems likely that oxygen (as dissolved FeO) is the principal light element component of the core. This conclusion is reconciled more readily with a modified version of homogeneous accretion of the Earth than with some current models of heterogeneous accretion. Accretion of the Earth from a population of giant planetesimals would have led to complete melting of the mantle and core at very high temperatures. Under these conditions, nearly all FeO and other transition metal oxides would have been extracted from the mantle into the core, together with a significant amount of SiO2. It is difficult to reconcile the geochemical consequences of this accretion scenario with the present composition of the mantle.

Experimental petrology of Apollo 12 basalts: part 1, sample 12009

Abstract The lunar sample, 12009, is a rapidly quenched basalt with microphenocrysts of olivine (∼7%) and spinel in a cryptocrystalline matrix with many small microlites. The rock is olivine-normative (11%) and comparison of the olivine microphenocryst compositions with the experimentally determined liquidus olivine compositions shows that the rock was originally entirely liquid and that none of the observed olivine results from crystal accumulation. The magma (12009) began crystallizing olivine at ∼1230°C, spinel joined the olivine at ∼1210°C, and pigeonitic clinopyroxene would have appeared at ∼1190°C but sudden quenching of the magma occurred before this temperature was reached. Experimental studies at high pressure on 12009 magma show that olivine ceases to be a liquidus phase at pressures above 8kb and the liquidus clinopyroxene becomes more Ca and Al rich with increasing pressure. Although 12009 is not saturated with orthopyroxene at any pressure, a composition of 12009 + 10% olivine (Fo 75 ) has olivine and orthopyroxene as liquidus phases at 15kb. The data are used to infer partial melting of olivine pyroxenite [orthopyroxene + clinopyroxene + olivine, 100 Mg/Mg + Fe = 75–80] at depths > 200 km within the lunar interior, as the primary source of the maria-filling magmas.

High-resolution 17O MAS NMR spectroscopy of forsterite (α-Mg2SiO4), wadsleyite (β-Mg2SiO4), and ringwoodite (γ-Mg2SiO4)

Abstract The high sensitivity of the satellite-transition (ST) MAS NMR technique was exploited to obtain high-resolution 17O MAS NMR spectra of the three polymorphs of Mg2SiO4: forsterite (α-Mg2SiO4), wadsleyite (β-Mg2SiO4), and ringwoodite (γ-Mg2SiO4). High NMR sensitivity was important in this application because 17O-enriched, Fe-free materials are required for 17O NMR and high-pressure syntheses of the dense β and γ polymorphs result in a only a few milligrams of these solids. In all, eight distinct O species were identified and assigned: three in forsterite, four in wadsleyite, and one in ringwoodite, in agreement with the number of O sites in their crystal structures. The isotropic chemical shifts extracted are in excellent agreement with a previously published correlation with Si-O bond length. However, unexpectedly large quadrupolar coupling constants were found for the non-bridging O species in the dense polymorphs wadsleyite and ringwoodite.

Partitioning of Cr, V, and Mn between mantles and cores of differentiated planetesimals: Implications for giant impact hypothesis of lunar origin

Abstract Currently preferred versions of the “giant impact” hypothesis of lunar origin imply that the Moon was derived mainly from the mantle of a giant (Martian-sized) planetesimal which struck the Earth. This hypothesis also implies that the depletions of Cr, V, and Mn which are observed in the Moon were inherited from the mantle of the impactor. Experiments have been undertaken to determine whether the formation of an iron core within a differentiated giant planetesimal could have caused depletions of Cr, V, and Mn in the planetesimal mantle, owing to siderophile behavior of these elements during core formation. Partition coefficients of Cr, V, and Mn between metallic iron and the principal mineral phases present in the mantle of a giant planetesimal have been determined at 1500–2000°C and at 3–25 GPa using an MA-8 apparatus. Cr, V, and Mn were found to remain lithophile (D silicate/metal >1) under these conditions. It follows that the formation of an iron core within a giant planetesimal would not have caused any depletion of Cr, V, and Mn in its mantle. Depletions of Cr and V (relative to Mg) cannot be attributed to selective volatilization in the solar nebula, prior to planetesimal and planet formation because V is less volatile than Mg, whilst the condensation temperature of Cr is similar to that of Mg and higher than that of Si. Accordingly, it is concluded that the depletions of Cr and V (and probably Mn) in the Moon were not inherited from the mantle of a giant planetesimal. Cr, V, and Mn are depleted (relative to Mg) in the Earths mantle and the terrestrial depletions for these elements are similar to the lunar depletion factor. This similarity suggests that protolunar material was derived mainly from the Earths mantle. Depletions of Cr and V (and perhaps Mn) in the terrestrial mantle are believed to have been connected with core-formation processes within the Earth which occurred at much higher pressures and temperatures than those prevailing during core formation in giant planetesimals.

Reaction between magnesiowusite of lower mantle composition and core-forming Fe-Ni alloy at 1-40 GPa

Abstract One class of models for the early history of the Earth requires the present-day inventory of siderophile elements in the mantle to have been established by equilibrium partitioning between core-forming metal and mantle minerals at high pressures and temperatures deep inside the Earth. We have accordingly carried out reconnaissance experiments on the partitioning of nickel between model lower mantle magnesiowüstite (Mg′ = 85 and 1.3 wt% NiO) and a model core-forming alloy, Fe94Ni6 (~7 wt% Ni) at pressures between 1-40 GPa and temperatures ranging from 1200 °C to 2000 °C. Reversal experiments were also attempted. Our results highlight the difficulty of attaining equilibrium partitioning in this system and imply that partition coefficients derived from unreversed experiments should accordingly be viewed with reservation. Our data nevertheless imply that the concentration of NiO in lower mantle magnesiowüstite in equilibrium with core-forming metal with ~7 wt% Ni would be extremely low, e.g., about 0.2 wt% NiO. Moreover, equilibrium seems to be fairly insensitive to the effects of either pressure or temperature, and so it is unlikely that magnesiowüstite could acquire 1.3 wt% NiO simply by equilibrating with core-forming metal under special high P-T conditions early in Earth history. Alternative hypotheses for the present-day siderophile element inventory of the mantle are accordingly preferred.