Anthony C. Withers

Carbon-dioxide-rich silicate melt in the Earth's upper mantle.

The onset of melting in the Earth’s upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic melt could be stable 250 km or less beneath mid-oceanic ridges, owing to the small fraction (∼0.03 wt%) its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present at ∼200 km (refs 3–5) but large melt fractions are thought to be restricted to shallower depths. Here we present experiments on carbonated peridotites over 2–5 GPa that constrain the location and the slope of the onset of silicate melting in the mantle. We find that the pressure–temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and that silicate melting of dry peridotite + CO2 beneath ridges commences at ∼180 km. Accounting for the effect of 50–200 p.p.m. H2O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ∼100 p.p.m. CO2 becomes as deep as ∼220–300 km. We suggest that, on a global scale, carbonated silicate melt generation at a redox front ∼250–200 km deep, with destabilization of metal and majorite in the upwelling mantle, explains the oceanic low-velocity zone and the electrical conductivity structure of the mantle. In locally oxidized domains, deeper carbonated silicate melt may contribute to the seismic X-discontinuity. Furthermore, our results, along with the electrical conductivity of molten carbonated peridotite and that of the oceanic upper mantle, suggest that mantle at depth is CO2-rich but H2O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth’s deep upper mantle, and the inventory of carbon, H2O and other highly incompatible elements at ridges becomes controlled by the flux of the former.

Aluminum coordination and the densification of high-pressure aluminosilicate glasses

Abstract To better understand the relationship between atomic-scale structures and densities of aluminosilicate glasses and liquids, we used 27Al MAS NMR to determine the speciation of aluminum ions in K3AlSi3O9, Na3AlSi3O9, and Ca3Al2Si6O18 glasses quenched from melts at 3 to 10 GPa. These data are a first approximation of high-pressure melt structure and illustrate the effects of the type of modifier cation. High field strength modifier cations (e.g., Ca) clearly induce more high-coordinated Al than lower field strength cations (e.g., Na and K). Measured glass densities show that, especially with rapid decompression, a significant portion of the total densification observed in-situ in melts is retained on return to ambient temperature and pressure. Observed increases in Al coordination are well correlated with decreased volume, which suggests that this structural change is a major part of the mechanism for recovered densification of high-pressure melts. Additionally, 23Na MAS NMR, combined with the 27Al MAS spectra and density determinations, reveal that other changes, such as the compression of modifier cation sites and/or decreased network bond angles, must also be significant, especially at low pressure.

The OH content of pyrope at high pressure

The OH contents of pyrope garnets synthesised at 1000°C and pressures between 2 and 13 GPa have been measured by infrared spectroscopy. We find that under the same conditions of pressure, temperature, aH2O and aSiO2 the OH content of pyrope is similar to that of grossular. This shows that observed differences in nature between water contents of pyrope and grossular are related to paragenesis rather than reflecting higher intrinsic solubility in grossular. In the presence of excess SiO2 the H2O content increases with pressure to 5 GPa (1000 ppm) then decreases to below the detection limit at pressures above 7 GPa. Thus garnet becomes more hydrous with pressure to some critical value, beyond which dehydration occurs even under H2O-saturated conditions. This observation is consistent with the measured partial molar volume of water in hydrogarnet which becomes greater than that in fluid water within the pressure range of this study. When corrected to upper mantle conditions we find that pyrope should dehydrate with increasing depth below about 250 km. Pyrope is unlikely therefore to be a major site of water storage in the transition zone and upper mantle.

Intercalibration of FTIR and SIMS for hydrogen measurements in glasses and nominally anhydrous minerals

Abstract We present new Fourier Transform Infrared Spectroscopy (FTIR) and ion microprobe/secondary ion mass spectrometry (SIMS) analyses of 1H in 61 natural and experimental geological samples. These samples include 8 basaltic glasses (0.17 to 7.65 wt% H2O), 11 rhyolitic glasses (0.143 to 6.20 wt% H2O), 17 olivines (~0 to 910 wt. ppm H2O), 9 orthopyroxenes (~0 to 263 wt. ppm H2O), 8 clinopyroxenes (~0 to 490 wt. ppm H2O), and 8 garnets (~0 to 189 wt. ppm H2O). By careful attention to vacuum quality, the use a Cs+ primary beam, and a resin-free mounting technique, we routinely achieve hydrogen backgrounds equivalent to less than 5 ppm by weight H2O in olivine. Compared to previous efforts, the new calibration extends to a wider range of H2O contents for the minerals and is more reliable owing to a larger number of standards and to characterization of anisotropic minerals by polarized FTIR on oriented crystals. When observed, discrepancies between FTIR and SIMS measurements are attributable to inclusions of hydrous minerals or fluid inclusions in the crystals. Inclusions more commonly interfere with FTIR analyses than with SIMS, owing to the much larger volume sampled by the former. Plots of H2O determined by FTIR vs. (1H/30Si) × (SiO2), determined by SIMS and electron microprobe (EMP) yield linear arrays and for each phase appear to be insensitive to bulk composition. For example, basalt and rhyolite calibration slopes cannot be distinguished. On the other hand, calibration slopes of different phases vary by up to a factor of 4. This reflects either phase-specific behavior of 1H/30Si secondary ion ratios excited by Cs+ ion beams or discrepancies between phase-specific FTIR absorption coefficient schemes.

Effect of structural transitions on properties of high-pressure silicate melts: 27Al NMR, glass densities, and melt viscosities

Abstract The densities and viscosities of silicate melts depend strongly on pressure, in part because of potentially measurable structural rearrangements. In an attempt to further understand these changes and how they affect macroscopic properties, we have used 27Al MAS NMR to determine the coordination of the Al cations in a series of aluminosilicate glasses quenched from melts at pressures of 2 to 8 GPa, have measured the glass densities, and have applied an in-situ falling sphere method to measure melt viscosities at high pressure. Spectra from these four- and five-component glasses show increasing Al coordination with increasing pressure and with increasing average field strength of the modifier cation, as was previously reported for simpler compositions. These data also indicate that when multiple modifier cations are present (e.g., Ca and K), the Al coordination is lower than what would be expected from linear combinations of the appropriate aluminosilicate end-members. The viscosity of Ca3Al2Si6O18 melts, measured using a falling sphere method that combines multianvil techniques with synchrotron X-ray radiography, may reach a minimum at a pressure below 6 GPa. A quasi-thermodynamic approach using equilibrium constants for the reactions that generate high-coordinated Al suggests that this pressure may be related to a maximum in the concentration of five-coordinated Al. These results further support the concept that pressure-induced network structural transitions have direct implications for the macroscopic properties of high-pressure melts.

The effect of Fe on olivine H2O storage capacity: Consequences for H2O in the martian mantle

Abstract To investigate the influence of chemical composition on the behavior of H2O in Fe-rich nominally anhydrous minerals, and to determine the difference between H2O behavior in the martian and terrestrial mantles, we conducted high-pressure H2O storage capacity experiments employing a wide range of olivine compositions. Experiments were conducted with bulk compositions in the system FeO-MgO-SiO2-H2O with Mg no. [Mg no. = 100 × molar Mg/(Mg+Fe)] ranging between 50 and 100 at 3 GPa in a piston-cylinder and at 6 GPa in a multi-anvil apparatus. Experiments at 3 GPa were conducted at 1200 °C, with fO₂ buffered by the coexistence of Fe and FeO, and at 1300-1500 °C in unbuffered assemblies. Experiments at 6 GPa were conducted at 1200 °C without buffers. Experiments at 1200 °C produced olivine+orthopyroxene+hydrous liquid (liq), and higher T experiments produced olivine+liq. Additionally, we synthesized a suite of 7 olivine standards (Mg no. = 90) for low blank secondary ion mass spectrometry (SIMS) analysis of H in multi-anvil experiments at 3-10 GPa and 1250 °C, resulting in large (200-400 μm) homogeneous crystals with 0.037 to 0.30 wt% H2O. Polarized Fourier transform infrared (FTIR) measurements on randomly oriented grains from the synthesis experiments were used to determine principal axis spectra through least-squares regression, and H contents were calculated from the total absorbance in the OH stretching region. Using these olivines as calibrants for SIMS analyses, the H contents of olivines and pyroxenes from the variable Mg no. experiments were measured by counting 16OH ions. Ignoring any matrix effects owing to variation in Mg no., H contents of olivine and pyroxene increase linearly with decreasing Mg no. At 6 GPa and 1200 °C, olivine H contents increase from 0.05 to 0.13 wt% H2O (8360 to 23 900 H/106 Si) as olivine Mg no. decreases from 100 to 68, and at 3 GPa and 1200 °C olivine H contents increase from 0.017 to 0.054 wt% (278 to 10 000 H/106 Si) as Mg no. decreases from 100 to 55. The partition coefficient for H between pyroxene and olivine, DHopx/ol, decreases from 1.05 at 3 GPa and 1200 °C to 0.61 at 6 GPa and 1200 °C. The storage capacity of Fe-rich olivines with compositions expected in the martian mantle is -1.5 times greater than those in the terrestrial mantle, suggesting that the geochemical behavior of H2O in the mantles of the two planets are quite similar. If 50% of the K2O on Mars remains in its mantle (Taylor et al. 2006), then a similar or greater proportion of the H2O is also in the mantle. Given accretionary models of the total martian H2O budget (Lunine et al. 2003), this suggests concentrations of 100-500 ppm H2O in the martian mantle and 0.1-1.9 wt% H2O in primary martian basalts.

Reconciliation of experimental results on H2O speciation in rhyolitic glass using in-situ and quenching techniques

In order to resolve the controversy over how to interpret experimental H2O speciation results using in-situ and quenching techniques, we have carried out an infrared spectroscopic study to determine whether the molecular H2O (5230 cm 1 ) and OH (4520 cm 1 ) band intensity variation with measurement temperature below glass transition is owing to species interconversion or to the temperature dependence of molar absorptivities. By comparing rhyolitic glasses with different total H2O content from 0.18 to 0.76 wt.%, we show that the peak height of the 4520 cm 1 band increases by a similar relative amount (about 2% if the baseline is fit with a flexicurve and 10% if the baseline is fit by a straight line) from 25 to 400oC, independent of the total H2O content. The results show that (1) the molar absorptivities do indeed change with temperature, and (2) in our experiments below the glass transition temperature, species concentrations do not change noticeably with temperature, and the band intensity variations are caused mainly by changes in the shape of the absorbance bands with temperature. The absence of unquenchable species reaction in the glass state (on our experimental time scale) confirms that speciation data can be obtained using the quench technique from 400 to 600oC. On the other hand, the temperature dependence of the molar absorptivities must be quantified for the full potential of the in-situ technique to be realised.

A first-principles investigation of hydrous defects and IR frequencies in forsterite: The case for Si vacancies

Abstract We investigate charge-balanced hydrous magnesium and silicon defects [(2H)XMg, (4H)XSi] by first principles. Two new lowest-energy hydrogen configurations are proposed for (4H)XSi. With these new configurations, the distribution of O-H stretching phonon frequencies in Group I (>3450 cm-1) are better reproduced. Substitution of silicon with four hydrogen atoms gives rise to significant elongation of distances between O atoms at the tetrahedron of the silicon vacancy. Our calculations indicate that the correlation between O-O distances and O-H stretching phonon frequencies, which has been well established for hydrous minerals, does not apply directly to nominally anhydrous minerals and should not be used to determine the identity of hydrous defects responsible for infrared absorption peaks.

On the use of unpolarized infrared spectroscopy for quantitative analysis of absorbing species in birefringent crystals

Abstract There is an understandable desire to use simple unpolarized infrared analysis of unoriented anisotropic samples to extract quantitative information, rather than using more demanding polarized techniques. Owing to the fact that unpolarized infrared absorbance in birefringent media deviates from the Beer-Lambert law, previous studies have either warned against using unpolarized spectroscopy for quantitative purposes, or have used flawed error analysis to justify using simple averages of integrated absorbance of multiple absorbance bands as a proxy for total integrated polarized absorbance in the principal spectra. It is shown here that unpolarized infrared absorbance is correctly calculated by averaging in the transmission domain. The errors in estimates of principal absorbance by averaging of unpolarized absorbance spectra are evaluated using correct theory of unpolarized infrared transmission. Correction schemes for integrated absorbance based on linear-absorbance error calculations are shown to be inappropriate. A theory is developed that allows the sum of the polarized principal absorbance spectra to be estimated from multiple unpolarized measurements of randomly oriented samples. The systematic errors that arise when averaging in the absorbance domain are avoided by use of exact theory rather than an approximation. Numerical simulation shows that applying the new procedure to 10 unpolarized measurements of OH stretching bands in olivine results in convergence of the estimated total integrated principal polarized absorbance to within 10% of the true value for a sample size of 10 measurements, but the technique is limited to spectral regions that do not contain absorption bands that are simultaneously intensely absorbing and strongly anisotropic.

A new method for determining the P-V-T properties of high-density H2O using NMR: results at 1.4–4.0 gpa and 700–1100°c

A new method for determining the density of H2O at high pressures and temperatures has been developed using 1H magic angle spinning NMR. Powdered γ-Al2O3 was recrystallised in the piston–cylinder apparatus under hydrothermal conditions to form corundum containing submicroscopic fluid inclusions. After quenching, 1H NMR spectra of the included water were measured at room temperature with the result that the shift relative to H2O at 25°C and 1 bar was found to be dependent on the predicted density of the trapped fluid. The shifts appear to be related to changes in the bulk magnetic susceptibility of water that increases linearly with density. The linear relationship between density and measured shift was calibrated using two measurements of the depression of freezing point in high-density water and enabled us to determine densities in the range 1.0–1.25 g cm−3. Therefore, the technique is complementary to those based on homogenisation of two phase (liquid–vapour) fluid inclusions; the latter techniques are only applicable where water density is <1 g cm−3. Because of the importance of high-density fluids under subduction zone conditions the density of water was determined using the new methodology at 1.4–4.0 GPa and 700–1100°C to test the available equations of state. We find that for the equations of state of Brodholt and Wood, Saul and Wagner, and Pitzer and Sterner the average difference between the calculated and measured values of density is about 1%, less than our estimate of the experimental error. Other widely used equations of state predict densities that are not consistent with our data. For example, those of Saxena and Fei and Holland and Powell predict densities, which are too high by an average of 7.6% and 3.8%, respectively, over the pressure and temperature range studied here.