Glenn A. Gaetani

Partitioning of moderately siderophile elements among olivine, silicate melt, and sulfide melt: Constraints on core formation in the Earth and Mars

Abstract This study investigates the effects of variations in the fugacities of oxygen and sulfur on the partitioning of first series transition metals (V, Cr, Mn, Fe, Co, Ni, and Cu) and W among coexisting sulfide melt, silicate melt, and olivine. Experiments were performed at 1 atm pressure, 1350°C, with the fugacities of oxygen and sulfur controlled by mixing CO2, CO, and SO2 gases. Starting compositions consisted of a CaOMgOAl2O3SiO2FeONa2O analog for a barred olivine chondrule from an ordinary chondrite and a synthetic komatiite. The f o 2/f s 2 conditions ranged from log f o 2 = −7.9 to −10.6, wi log f s 2 values ranging from −1.0 to −2.5. Our experimental results demonstrate that the f o 2/f s 2 dependencies of sulfide melt/silicate melt partition coefficients for the first series transition metals are proportional to their valence states. The f o 2/f s 2, dependencies for the partitioning of Fe, Co, Ni, and Cu are weaker than predicted on the basis of their valence states. Variations in f o 2/f s 2 conditions have no significant effect on olivine/melt partitioning other than those resulting from f o 2-induced changes in the valence state of a given element. The strong f o 2/f s 2, dependence for the olivine/silicate melt partitioning of V is attributable to a change of valence state, from 4+ to 3+, with decreasing f o 2. Our experimentally determined partition coefficients are used to develop models for the segregation of sulfide and metal from the silicate portion of the early Earth and the Shergottite parent body (Mars). We find that the influence of S is not sufficient to explain the overabundance of siderophile and chalcophile elements that remained in the mantle of the Earth following core formation. Important constraints on core formation in Mars are provided by our experimental determination of the partitioning of Cu between silicate and sulfide melts. When combined with existing estimates for siderophile element abundances in the Martian mantle and a mass balance constraint from Fe, the experiments allow a determination of the mass of the Martian core (∼17 to 22 wt% of the planet) and its S content (∼0.4 wt%). These modeling results indicate that Mars is depleted in S, and that its core is solid.

Open system behavior of olivine-hosted melt inclusions

Abstract It is commonly assumed that an olivine-hosted melt inclusion is chemically isolated from the melt surrounding its host crystal. This assumption is probably valid for slow-diffusing, incompatible trace elements, but for readily exchanged major elements such as Fe and Mg, it is much more tenuous. We used numerical simulations constrained by experimental data on olivine–liquid equilibrium and interdiffusion rates of Fe and Mg in olivine to assess the extent to which post-entrapment processes alter the major element compositions of olivine-hosted melt inclusions. The results indicate that extensive diffusive communication between a melt inclusion and the melt surrounding its host olivine occurs at cooling rates as rapid as 1–2°C/yr. An included melt undergoes significant and irreversible compositional changes in order to maintain Fe–Mg exchange equilibrium with the fractionating external melt. When a magnesian olivine containing an exotic liquid, such as an ultra-depleted melt, becomes entrained in a basalt and equilibrates with the new external melt by diffusive exchange, the ensuing compositional changes to the included melt cannot be removed simply by adding olivine back into the final (measured) melt composition. Observable disequilibrium between an inclusion and its host olivine is likely to have been produced during cooling of the erupted lava. Simulations, supplemented by experiments carried out on inclusion-bearing olivines, demonstrate that laboratory heating can reverse the effects of syn-eruptive crystallization on melt inclusion composition, and the presence or absence of compositional zoning in the immediately adjacent olivine can provide an indication of how closely the experiment reproduced the pre-eruptive temperature.

Rapid reequilibration of H2O and oxygen fugacity in olivine-hosted melt inclusions

The solubility of H 2 O in silicate melt drops substantially with decreasing pressure, so that a magma initially containing several weight percent H 2 O in a crustal magma reservoir is left with only a few thousand parts per million following ascent and eruption at the Earth’s surface. This rapid release of volatiles makes determining the pre-eruptive H 2 O contents of magmas very difficult. Olivine-hosted melt inclusions are thought to retain their H 2 O because they are protected from decompression by the strength of the host crystal, and pre-eruptive concentrations obtained from melt inclusions have been used to both estimate the amount of H 2 O in the upper mantle and investigate its role in the melt generation process. The greatest uncertainty involved in constraining upper mantle conditions from melt inclusions is the potential for rapid diffusive loss or gain of H + (protons) through the host olivine. Here we present results from hydration and dehydration experiments that demonstrate that, contrary to the widely held view, H 2 O loss or gain in melt inclusions is not limited by redox reactions and significant fluxes of H + through the host olivine are possible on very short time scales. We also show that the Fe 3+ /ΣFe of an olivine-hosted melt inclusion maintains equilibrium with the external environment via diffusion of point defects through the host olivine. Our results demonstrate that, while pre-eruptive H 2 O and Fe 3+ /ΣFe can be reliably estimated, olivine-hosted melt inclusions do not necessarily retain a record of the H 2 O and O 2 fugacity conditions at which they formed. High-H 2 O melt inclusions are particularly susceptible to diffusive dehydration, and therefore are not reliable proxies for the state of the upper mantle.

Wetting of mantle olivine by sulfide melt: implications for Re/Os ratios in mantle peridotite and late-stage core formation

This study investigates the effects of variations in the relative fugacities of oxygen and sulfur on the wetting of mantle olivine by molten sulfide. Experiments were performed on mixtures of San Carlos olivine and synthetic FeS at 1 bar and 1350°C. Crucibles were fabricated from San Carlos olivine, and the fugacities of oxygen and sulfur were controlled by mixing CO2, CO, and SO2 gases. Experimental conditions ranged from logfO2=−7.9 to −10.3 and from logfS2=−1.5 to −2.5. Our experimental results demonstrate that, at a given temperature and pressure, the olivine–sulfide melt dihedral angle is controlled by the concentration of O dissolved in an anion-rich melt. Trace amounts of O dissolve in sulfide melt at fO2 conditions near the iron–wustite oxygen buffer and the dihedral angle is 90°. At fO2 conditions near the fayalite–magnetite–quartz oxygen buffer the concentration of dissolved O is near 9 wt% and the dihedral angle is 52°, allowing small amounts of sulfide melt to form an interconnected network in olivine-rich rocks and to migrate via porous flow. These results indicate that sulfide melt is likely to be mobile at current upper mantle fO2 and fS2 conditions. In mantle peridotite, the addition or removal of sulfide melt by porous flow will variably fractionate Re/Os, U/Pb, and Th/Pb ratios because Os and Pb are more chalcophile than Re, U, and Th. The Re/Os ratio of the peridotite is especially sensitive to this process. The mobility of sulfide melt at oxidizing conditions implies that the addition of oxidized chondritic material during the later stages of the accretion of the Earth may have facilitated the segregation of core-forming material by porous flow if temperatures were in excess of the sulfide solidus.

Partitioning of rare earth elements between clinopyroxene and silicate melt: Crystal-chemical controls

Abstract-This study explores some of the effects of major element compositional variations on the partitioning of trivalent rare earth elements between high-Ca clinopyroxene and silicate melt. Experiments performed in the system CaO-MgO-Al@-Si02 at 1 atm pressure, over a small temperature range, differentiate crystal-chemical controls on mineral/melt partitioning from the effects of pressure and tem- perature. The experimental results demonstrate that the Ca-Tschermakite content of high-Ca clinopyroxene exercises an important control on rare earth element partitioning for pyroxene coexisting with basaltic melt. A comparison of our experimental results with those from two-liquid partitioning and thermal diffusion studies demonstrates that melt structure has only a minor influence on clinopyroxene/melt partitioning for basaltic compositions, but becomes progressively more important as polymerization of the melt increases. Melt structure exercises the dominant control on partitioning for highly polymerized melts such as high- silica rhyolites. Semiempirical expressions developed using equilibrium constants for pyroxene/melt exchange reactions successfully predict the partitioning of Ce3+ and Yb’+ for a broad range of synthetic and natural coexisting melt and clinopyroxene compositions. The ability of our model to predict the par- titioning behavior of trivalent rare earth elements over a wide range of experimental conditions (O.OOOl- 3.0 GPa; 1234- 1430°C) indicates that consideration of the compositions of coexisting clinopyroxene and melt is adequate to account for the effects of varying pressure and temperature.

Microtomography of Partially Molten Rocks: Three-Dimensional Melt Distribution in Mantle Peridotite

As mantle rocks melt, an interconnected network of liquid drives the ascent of magma to the sea floor. The permeability of the upper mantle controls melt segregation beneath spreading centers. Reconciling contradictory geochemical and geophysical observations at ocean ridges requires a better understanding of transport properties in partially molten rocks. Using x-ray synchrotron microtomography, we obtained three-dimensional data on melt distribution for mantle peridotite with various melt fractions. At melt fractions as low as 0.02, triple junctions along grain edges dominated the melt network; there was no evidence of an abrupt change in the fundamental character of melt extraction as melt fraction increased to 0.2. The porosity of the partially molten region beneath ocean ridges is therefore controlled by a balance between viscous compaction and melting rate, not by a change in melt topology.

Modeling the major-element evolution of olivine-hosted melt inclusions

Abstract This paper presents a detailed description of an approach for modeling the major element evolution of olivine-hosted melt inclusions. Numerical simulations constrained by experimental data on olivine/melt equilibrium and interdiffusion rates of Fe and Mg in olivine quantify the post-entrapment processes (crystallization, dissolution, Fe–Mg exchange with the host) that influence the major element compositions of included melts. Equilibrium at the olivine/melt interface is described by expressions for FeO and MgO partitioning calibrated using olivine–liquid pairs from the literature spanning temperatures of 1064 to 1950 °C and pressures of 1 bar to 70 kbar. The liquidus temperature and equilibrium olivine composition are calculated simultaneously for melt of a given composition using the partitioning expressions and olivine stoichiometry. Diffusion in the host olivine is modeled by the explicit finite-difference method. The main inputs for a simulation are the initial composition and size of the melt inclusion, the size of the host olivine, and the desired cooling path expressed as a constant cooling rate. The rate of olivine crystallization (d F /d T ) within an inclusion is dynamically adjusted with each time step so that the observed Fe–Mg exchange coefficient ( K D Fe–Mg ) at the inclusion/host interface matches the equilibrium value predicted by the partitioning equations. Application of the approach to model the major element evolution of an inclusion formed within the melting regime beneath an oceanic spreading center indicates that the composition of the included melt is significantly modified during transport to the surface along the mantle geotherm. Further, the compositional path followed by the included melt is dependent upon the capacity of the host olivine to maintain the inclusion at the pressure of entrapment.

Compositional variability in a cold‐water scleractinian, Lophelia pertusa: New insights into “vital effects”

We analyzed Sr/Ca and Mg/Ca ratios in the thecal wall of Lophelia pertusa, a cold-water coral, using SIMS ion microprobe techniques. The wall grows by simultaneous upward extension and outward thickening. Compositional variability displays similar trends along the upward and outward growth axes. Sr/Ca and Mg/Ca ratios oscillate systematically and inversely. The sensitivity of Lophelia Sr/Ca ratios to the annual temperature cycle (−0.18 mmol · mol−1/°C) is twice as strong as that exhibited by tropical reef corals, and four times as strong as the temperature dependence of Sr/Ca ratios of abiogenic aragonites precipitated experimentally from seawater. A comparison of the skeletal composition of Lophelia with results from precipitation calculations carried out using experimentally determined partition coefficients suggests that both temperature-dependent element partitioning and seasonal changes in the mass fraction of aragonite precipitated from the calcifying fluid influence the composition of Lophelia skeleton. Results from calculations that combine these effects reproduce both the exaggerated amplitude of the Sr/Ca and Mg/Ca oscillations and the inverse relationship between Sr/Ca and Mg/Ca ratios.

Experimental Constraints on Melt Generation in the Mantle Wedge

Experimental studies show that H 2 O affects most aspects of melt generation in the sub-arc mantle wedge. For example, dissolved H 2 O modifies the major element composition of peridotite partial melt by increasing the ratio of SiO 2 to MgO + FeO, mimicking the effect of decreasing pressure during anhydrous partial melting. Comparison of the normalized (anhydrous) compositions of experimentally produced hydrous and anhydrous melts shows that SiO 2 increases by ∼1 wt% with addition of 3 to 6 wt% dissolved H 2 O, while FeO + MgO decreases by ∼2 wt%. Furthermore, mobility of partial melt in mantle peridotite may increase due to the influence of H 2 O. Orthopyroxene-melt dihedral angles are ∼70° under anhydrous conditions, trapping small amounts of melt at 4 grain junctions, but they decrease to ∼52° under hydrous conditions, allowing connectivity down to very low melt fractions. Dissolved H 2 O also decreases melt density and viscosity that, combined with enhanced connectivity, allows hydrous melt to segregate very efficiently from residual peridotite. Less melt is produced by hydrous peridotite, relative to anhydrous peridotite, for a given temperature increase or pressure decrease, because of the monotonic decrease of dissolved H 2 O with increasing extent of melting. Primitive arc magmas with high pre-eruptive H 2 O contents may form when a peridotite partial melt that is initially near fluid saturation percolates upward through the mantle wedge, maintaining equilibrium with hotter, overlying peridotite by dissolving the surrounding rock (reactive porous flow). Adiabatic decompression melting may occur in regions where hot mantle flows from the back arc into the wedge corner, generating nearly anhydrous partial melt.

The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment

The NoMelt experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2-D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (>103 Ωm) lithosphere with a less resistive (∼50 Ωm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (≤10 Ωm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 to 0.5°C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3°C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H2O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H2O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory data set of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H2O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid-ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result.