The equation of state of CaSiO3 perovskite to 108 GPa at 300 K

Pressure–volume measurements have been performed for CaSiO₃ perovskite to 108 GPa at 300 K using NaCl and argon pressure-transmitting media, and energy dispersive X-ray diffraction (EDXD) in a diamond-anvil cell (DAC). By determining a parameter that is the product of the elastic anisotropy, S, and the uniaxial stress component, t, for each data point, we define the stress condition of the sample. For different points at the same pressure in a temperature-quenched sample, the St value can differ by as much as a factor of 5, indicating heterogeneity in the stress condition. This may be responsible for the large scatter of earlier P–V measurements in the DAC which in general used a large diameter X-ray beam. Also, the St value provides insight into the elastic anisotropy, S, of CaSiO₃ perovskite and platinum. The sign of S (positive) for CaSiO₃ perovskite agrees with first principles calculations but the magnitude may be inconsistent. A new compression curve at 300 K was obtained for CaSiO₃ perovskite by using those data points which represent the most nearly hydrostatic conditions. It is observed that the data points with high St values yield larger volumes than the points with small St values at a given pressure. By selecting the data points having low St values (St≤0.005), combining with lower pressure large volume press (LVP) measurements and fitting to third order Birch–Murnaghan equation of state (EOS), we find that CaSiO₃ perovskite is more compressible (V₀=45.58±0.05 ų, K_T0=236±4 GPa, and K_T0′=3.9±0.2 GPa) than suggested by previous studies. The density and bulk modulus of CaSiO₃ perovskite at lower mantle pressures and 300 K are 1–3% greater and 5–15% smaller, respectively, than found in previous studies. This study demonstrates that defining the stress state of the sample is crucial to obtain an accurate 300 K compression curve for unquenchable high-pressure phases.     

The energetics of aluminum solubility into MgSiO3 perovskite at lower mantle conditions

Experiments [T. Irifune (1994) Nature 370, 131–133; E. Ito et al. (1998) Geophys. Res. Lett. 25, 821–824; A. Kubo, M. Akaogi (2000) Phys. Earth Planet. Int. 121, 85–102] indicate that (Mg,Fe)SiO₃ perovskite, commonly believed to be the most abundant mineral in the Earth, is the preferred host phase of Al₂O₃ in the Earth’s lower mantle. Aiming to better understand the effects of Al₂O₃ on the thermoelastic properties of the lower mantle, we use atomistic models to examine the chemistry and elasticity of solid solutions within the MgSiO₃(perovskite)–Al₂O₃(corundum)–MgO(periclase) mineral assemblage under conditions pertinent to the lower mantle: low Al cation concentrations, P=25–100 GPa, and T=1000–2000 K. We assess the relative stabilities of two likely substitution mechanisms of Al into MgSiO₃ perovskite in terms of reactions involving MgSiO₃, MgO, and Al₂O₃, in a manner similar to the 0 Kelvin calculations of Brodholt [J.P. Brodholt (2000) Nature 407, 620–622] and Yamamoto et al. [T. Yamamoto et al. (2003) Earth Planet. Sci. Lett. 206, 617–625]. We determine the equilibrium composition of the assemblage by examining the chemical potentials of the Al₂O₃ and MgO components in solid solution with MgSiO₃, as functions of concentration. We find that charge coupled substitution dominates at lower mantle pressures and temperatures. Oxygen vacancy-forming substitution accounts for 3–4% of Al substitution at shallow lower mantle conditions, and less than 1% in the deep mantle. For these two pressure regimes, the corresponding adiabatic bulk moduli of aluminous perovskite are 2% and 1% lower than that of pure MgSiO₃ perovskite.     

Solubility of FeO in (Mg,Fe)SiO3 perovskite and the post-perovskite phase transition

Phase relations in Mg_0.5Fe_0.5SiO₃ and Mg_0.25Fe_0.75SiO₃ were investigated in a pressure range from 72 to 123 GPa on the basis of synchrotron X-ray diffraction measurements in situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC). Results demonstrate that Mg_0.5Fe_0.5SiO₃ perovskite is formed as a single phase at 85–108 GPa and 1800–2330 K, indicating a high solubility of FeO in (Mg,Fe)SiO₃ perovskite at high pressures. Post-perovskite appears coexisting with perovskite in Mg_0.5Fe_0.5SiO₃ above 106 GPa at 1410 K, the condition very close to the post-perovskite phase transition boundary in pure MgSiO₃. The coexistence of perovskite and post-perovskite was observed to 123 GPa. In addition, post-perovskite was formed coexisting with perovskite also in Mg_0.25Fe_0.75SiO₃ bulk composition at 106–123 GPa. In contrast to earlier experimental and theoretical studies, these results show that incorporation of FeO stabilizes perovskite at higher pressures. This could be due to a larger ionic radius of Fe²⁺ ion, which is incompatible with a small Mg²⁺ site in the post-perovskite phase.     

Silicate perovskite analogue ScAlO3: temperature dependence of elastic moduli

The elastic moduli of ScAlO₃ perovskite, a very close structural analogue for MgSiO₃ perovskite, have been measured between 300 and 600 K using high precision ultrasonic interferometry in an internally heated gas-charged pressure vessel. This new capability for high temperature measurement of elastic wave speeds has been demonstrated on polycrystalline alumina. The temperature derivatives of elastic moduli of Al₂O₃ measured in this study agree within 15% with expectations based on published single-crystal data. For ScAlO₃ perovskite, the value of (∂K_S/∂T)_P is −0.033 GPa K⁻¹ and (∂G/∂T)_P is −0.015 GPa K⁻¹. The relative magnitudes of these derivatives agree with the observation in Duffy and Anderson [Duffy, T.S., Anderson, D.L., 1989. Seismic velocities in mantle minerals and the mineralogy of the upper mantle. J. Geophys. Res. 94, 1895–1912.] that |(∂K_S/∂T)_P| is typically about twice |(∂G/∂T)_P|. The value of (∂K_S/∂T)_P for ScAlO₃ is intermediate between those inferred less directly from V(P,T) studies of Fe-free and Fe- and Al-bearing MgSiO₃ perovskites [Wang, Y., Weidner, D.J., Liebermann, R.C., Zhao, Y., 1994. P–V–T equation of state of (Mg,Fe)SiO₃ perovskite: constraints on composition of the lower mantle. Phys. Earth Planet. Inter. 83, 13–40; Mao, H.K., Hemley, R.J., Shu, J., Chen, L., Jephcoat, A.P., Wu, Y., Bassett, W.A., 1991. Effect of pressure, temperature and composition on the lattice parameters and density of (Mg,Fe) SiO₃ perovskite to 30 GPa. J. Geophys. Res. 91, 8069–8079; Zhang, Weidner, D., 1999. Thermal equation of state of aluminum-enriched silicate perovskite. Science 284, 782–784]. The value of |(∂G/∂T)|_P for ScAlO₃ is similar to those of most other mantle silicate phases but lower than the recent determination for MgSiO₃ perovskite [Sinelnikov, Y., Chen, G., Neuville, D.R., Vaughan, M.T., Liebermann, R.C., 1998. Ultrasonic shear wave velocities of MgSiO₃ perovskite at 8 GPa and 800K and lower mantle composition. Science 281, 677–679].

Combining the results from the previous studies and current measurements on ScAlO₃ perovskite, we extracted the parameters (q and γ₀) needed to fully specify its Mie–Grüneisen–Debye equation-of-state. In this study, we have demonstrated that acoustic measurements of K_S(T), unlike V(P,T) data, tightly constrain the value of q. It is concluded that ScAlO₃ has ‘normal’ γ₀ (1.3) and high q (3.6). The high value of q indicates that ScAlO₃ has very strong intrinsic temperature dependence of the bulk modulus; similar behaviour has been observed in measurements on Fe- and Al-bearing silicate perovskites (Mao et al., 1991; Zhang and Weidner, 1999).     

A new high-pressure phase of Ca2Al2SiO7 and implications for the earth''s interior

Gehlenite (Ca₂Al₂SiO₇) has been found to transform to a new phase at pressures greater than 100 kbar and at about 1000°C, using a diamond-anvil pressure cell coupled with laser heating. The atoms of the new phase appear to be arranged in a perovskite-related structure similar to that described for Na₂Ti₃O₇. The structure probably consists of layers of (Al₂SiO₇)⁴⁻, which are built up from blocks of edge-sharing (Al, Si)O₆ octahedra and these blocks are joined by common octahedra corners. A small cubic unit cell with a = 3.719 ± 0.004 Å indexes completely the strong lines of the powder diffraction pattern, and a superlattice with a = 14.88 ± 0.02 Å satisfies all the observed weak lines in addition to the strong ones. However, the cell may be pseudocubic. The small cell contains a half of the gehlenite formula while the large cell contains 32 gehlenite formulae. Hence the molar volume for the new phase of Ca₂Al₂SiO₇ is calculated to be 61.96 ± 0.20 cm³ at atmospheric pressure and room temperature. The new sodium titanate-type structure is probably more closely packed than an ordinary perovskite-type structure in which all octahedral corners are shared. This view is strongly supported by the very great density of this new phase, which is about 8% denser than the equivalent mixture of CaAl₂O₄ (calcium ferrite type) plus CaSiO₃ (cubic perovskite type). The new phase is probably the most closely packed silicate known. Mg₂SiO₄ (spinel) was found to transform to an assemblage containing MgSiO₃ (perovskite) plus MgO (periclase) at P-T conditions equivalent to the upper part of the lower mantle. By reacting with MgO, the perovskite modification of both MgSiO₃ and MgSiO₃ · xAl₂O₃ may adopt the sodium titanate structure at the still greater depths of the lower mantle. If the sodium titanate structures of Mg₂(Al₂Si)O₇ and Mg₂(MgSi₂)O₇ are present in the deep part of the lower mantle, MgO does not exist as a separate phase at the mantle-core boundary. This might be an obstacle to the possibility of dissolving these oxides (specifically the FeO component) in the molten Fe in the outer core as suggested by geophysical and geochemical studies of the earth's interior. The mechanism for developing the chemical plumes in the deep mantle proposed by Anderson does not appear to be consistent with studies of phase transformations in Ca-Al-rich compounds as outlined in this paper.     

Elastic and anelastic anomalies in (Ca,Sr)TiO3 perovskites: Analogue behaviour for silicate perovskites

Values of bulk modulus (K), shear modulus (G) and mechanical quality factor (Q) have been determined for polycrystalline samples across the CaTiO₃ (CST0)–SrTiO₃ (CST100) solid solution by resonant ultrasound spectroscopy. Because of similarities with low frequency elastic and anelastic anomalies due to twin wall motion reported in previous studies, a working hypothesis is developed in which dissipation processes are interpreted in terms of twin wall displacements. At high temperatures in CST50 the stability field of the I4/mcm structure is marked by the disappearance of all resonance peaks (superattenuation). This is attributed to anelastic domain wall sliding. At room temperature the I4/mcm phase of CST70 and CST80 has values of G which are lower than those of cubic or orthorhombic phases, and a concomitant drop in Q is interpreted as implying that the domain wall pinning process reported elsewhere to occur below 400–450 K is only partial. A similar drop in G and Q was found in CST95 below the transition at 238 K. The I4/mcm ↔ Pbcm transition in CST70 at 230 K is marked by an abrupt increase in Q, suggesting that mobile twins in crystals with the I4/mcm structure become effectively immobile in antiferroelectric crystals with the Pbcm structure. The I4/mcm ↔ Pnma transition in CST50 is marked by a similarly abrupt increase in Q, consistent with twin walls becoming effectively immobile also in crystals with the Pnma structure. A fall in Q below 800 K in CST0, however, could imply that a degree of twin wall mobility might develop in Pnma crystals if the tetragonal spontaneous strain departs significantly from zero. The remarkable attenuation behaviour of crystals with the I4/mcm structure at the relatively low stress conditions which apply during resonances of a parallelepiped with edge dimensions of 2–4 mm, is consistent with the view that a characteristic signature for tetragonal CaSiO₃ in the Earth's lower mantle should be a marked attenuation of seismic waves.     

Element partitioning between magnesium silicate perovskite and ferropericlase: New insights into bulk lower-mantle geochemistry

In this study, we investigated iron–magnesium exchange and transition-metal trace-element partitioning between magnesium silicate perovskite (Mg,Fe)SiO₃ and ferropericlase (Mg,Fe)O synthetised under lower-mantle conditions (up to 115 GPa and 2200 K) in a laser-heated diamond anvil cell. Recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy (ATEM) and nanometer-scale secondary ion mass spectroscopy (nanoSIMS). Iron concentrations in both phases were obtained from X-ray energy dispersive spectroscopy measurements and nanoSIMS. Our results are the first to show that recently reported spin-state and phase transitions in the lower mantle directly affect the evolution of Fe–Mg exchange between both phases. Mg-perovskite becomes increasingly iron-depleted above 70–80 GPa possibly due to the high spin–low spin transition of iron in ferropericlase. Conversely, the perovskite to post-perovskite transition is accompanied by a strong iron enrichment of the silicate phase, ferropericlase remaining in the Fe-rich phase though. Nanoparticles of metallic iron were observed in the perovskite-bearing runs, suggesting the disproportionation of ferrous iron oxide, but were not observed when the post-perovskite phase was present. Implications on the oxidation state of the Earth and core segregation will be discussed. Transition trace-element (Ni, Mn) concentrations (determined with the nanoSIMS) show similar trends and could thus be used to trace the origin of diamonds generated at depth. This study provides new results likely to improve the geochemical and geophysical models of the Earth's deep interiors.     

Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis

Benthic foraminiferal magnesium/calcium ratios were determined on one hundred and forty core-top samples from the Atlantic Ocean, the Norwegian Sea, the Indian Ocean, the Arabian Sea and the Pacific Ocean, mostly at sites with bottom water temperatures below 5 °C. Mg/Ca ratios are consistently lower, by  0.2 mmol/mol, in samples cleaned using oxidative and reductive steps than using oxidative cleaning. Differences between Cibicidoides species have been identified: Mg/Ca of Cibicidoides robertsonianus > Cibicidoides kullenbergi > Cibicidoides wuellerstorfi. Comparison with bottom water temperatures support observations of lowered Mg/Ca of C. wuellerstorfi at temperature below  3 °C compared with values predicted by published calibrations and from other Cibicidoides species. Hydrographic data shows that carbonate ion saturation (Δ[CO₃²⁻]) decreases rapidly below this temperature. An empirical sensitivity of Δ[CO₃²⁻] on Mg/Ca has been established for C. wuellerstorfi of 0.0086 ± 0.0006 mmol/mol/μmol/kg. A novel application using modern temperatures and Last Glacial Maximum temperatures derived via pore fluid modelling supports a carbonate ion saturation state effect on Mg incorporation. This may significantly affect calculated δ¹⁸O_seawater obtained from foraminiferal δ¹⁸O and Mg/Ca temperature.     

Low temperature magnetic properties of titanomagnetites

New measurements of high field magnetisation (I_s), remanence (I_s), and coercive force (H_c) are presented between 4 and 300 K for x = 0.4, 0.5, 0.6 and 0.8 (Fe_3−xTi_xO₄). For x = 0.4 a pronounced minimum is found at T100 K and for X = 0.5 and 0.6 broad minima occur around T200 K, apparently coinciding with the temperature for K₁0. The magnetic properties below T60 K ar complex and were found to be significantly changed by cooling in the presence of a high magnetic field. With no applied field during cooling, a distinct decrease in I_s is observed for T60 K, at which temperature there is a peak in the value of I_rs. The effe cooling is to eliminate the sharp decrease in I_s, reduce H_c and to increase I_rs below 60 K to a value the peak value, giving essentially a square hysteresis loop. The results are interpreted in terms of a form of crystallographic phase transition coupled to the magnetisation direction, possibly by the magnetostriction. Square hysteresis loops in ferrites have been explained by the presence of Jahn-Teller ions and, in the present case, the low temperature of the observed effect may be a consequence of the weak Fe²⁺ Jahn-Teller ion coupled to other effects such as spin-lattice coupling.

Details of this work can be found in Schmidbauer, E. and Readman, P.W., 1982. Low temperature magnetic properties of Ti-rich Fe---Ti spinels. J. Magn. Magn. Mat., 27: 114–118. A paper reporting further work on Fe_2.4Ti0.6O0.4 is in preparation.     

The crust and upper mantle discontinuity structure beneath Alaska inferred from receiver functions

In this study, three receiver function stacking methods are used to study the detailed crust and upper mantle structure beneath south-central Alaska. We used teleseismic waveform data recorded by 36 stations in the Broadband Experiment Across the Alaska Range (BEAAR) and 4 permanent stations in Alaska. H − κ stacking method using P-to-S converted wave and its multiply reflected waves between the Earth's surface and the Moho discontinuity is adopted to estimate the crustal thickness (H) and average crustal V_P/V_S ratio (κ) in this region. The receiver function results for 24 stations show that the crustal thickness under Alaska ranges from 26.0 to 42.6 km with an average value of 33.8 km, and the V_P/V_S ratio varies from 1.66 to 1.94 with an average value of 1.81 which corresponds to an average Poisson's ratio of 0.277 with a range from 0.216 to 0.320. High Poisson's ratios under some stations are possibly caused by partial melting in the crust and the uppermost mantle. Common converted point (CCP) stacking results of receiver functions along three lines show clear Moho and slab images under this subduction zone. The depths of the slab from our CCP stacking images are consistent with those estimated from the Wadati–Benioff Zone (WBZ). In the area between two stations DH2 (147.8°W, 63.3°N) and DH3 (147.1°W, 63.0°N), a Moho depth offset of about 10 km is found by both the H − κ and CCP stacking techniques. Common depth point (CDP) stacking of receiver functions shows not only the 410-, 520- and 660-km discontinuities, but also significant variations (−30 to 15 km) in the transition zone thickness under the southwest and southeast parts of the study region. The transition zone becomes thinner by 20–30 km, indicating that the temperature there is 150–200 K higher than that of the normal mantle.     

Temporal–compositional trends over short and long time-scales in basalts of the Big Pine Volcanic Field, California

Primitive basaltic single eruptions in the Big Pine Volcanic Field (BPVF) of Owens Valley, California show systematic temporal–compositional variation that cannot be described by simple models of fractional crystallization, partial melting of a single source, or crustal contamination. We targeted five monogenetic eruption sequences in the BPVF for detailed chemical and isotopic measurements and ⁴⁰Ar/³⁹Ar dating, focusing primarily on the Papoose Canyon sequence. The vent of the primitive (Mg# = 69) Papoose Canyon sequence (760.8 ± 22.8 ka) produced magmas with systematically decreasing (up to a factor of two) incompatible element concentrations, at roughly constant MgO (9.8 ± 0.3 (1σ) wt.%) and Na₂O. SiO₂ and compatible elements (Cr and Ni) show systematic increases, while ⁸⁷Sr/⁸⁶Sr systematically decreases (0.7063–0.7055) and ε_Nd increases (− 3.4 to − 1.1). ¹⁸⁷Os/¹⁸⁸Os is highly radiogenic (0.20–0.31), but variations among four samples do not correlate with other chemical or isotopic indices, are not systematic with respect to eruption order, and thus the Os system appears to be decoupled from the dominant trends. The single eruption trends likely result from coupled melting and mixing of two isotopically distinct sources, either through melt-rock interaction or melting of a lithologically heterogeneous source. The other four sequences, Jalopy Cone (469.4 ± 9.2 ka), Quarry Cone (90.5 ±17.6 ka), Volcanic Bomb Cone (61.6 ± 23.4 ka), and Goodale Bee Cone (31.8 ± 12.1 ka) show similar systematic temporal decreases in incompatible elements. Monogenetic volcanic fields are often used to decipher tectonic changes on the order of 10⁵–10⁶ yr through long-term changes in lava chemistry. However, the systematic variation found in Papoose Canyon (10⁰–10² yr) nearly spans that of the entire volcanic field, and straddles cutoffs for models of changing tectonic regime over much longer time-scales. Moreover, ten new ⁴⁰Ar/³⁹Ar ages combined with chemistry from all BPVF single eruption sequences show the long-term trend of BPVF evolution comprises the overlapping, temporal–compositional trends of the monogenetic vents. This suggests that the single eruption sequences contain the bulk of the systematic chemical variation, whereas their aggregate compositions define the long-term trend of volcanic field evolution.     

Laboratory measurement of hydrodynamic saline dispersion within a micro-fracture network induced in granite

We report the first measurements of hydrodynamic dispersion in a microfractured granite using a combination of novel techniques. A fracture network was induced in a cylindrical plug of Ailsa Craig micro-granite by thermal stressing, to produce an isotropic network of fractures with an average aperture of  0.3 μm, a density of approximately 4 × 10⁴ fractures/mm³ and a permeability of 5.5 × 10^− 17 m². After saturating the cores with 0.01 M NaCl solution a step in the concentration profile to 1 M was advected into the plug at flow rates of 0.07 to 2.13 cm³ h^− 1. The longitudinal electrical impedance of the plug was measured continuously as the solute front advected through its length until the plug was saturated with the concentrated electrolyte. Analysis of the impedance versus time relationships allows the derivation of the longitudinal dispersion coefficient, D_L, and hydrodynamic retardation, R_H. The Peclet number–dispersion relationship for the micro-fracture network is very similar to that predicted for other, radically different, fracture networks. Thus dispersion may be more dependent on fracture connectivity and length than fracture density and display a relationship similar to that shown by particle beds and clastic sandstones. The high retardation values observed (2.2–4.9) reflect flow behaviour within a fracture network with a proportion of ‘blind’ sections, and demonstrates how such networks can slow the advance of conservative solute components.     

Melting of β-quartz up to 2.0 GPa and thermodynamic optimization of the silica liquidus up to 6.0 GPa

Melting relations of β-quartz were experimentally determined at 1.0 GPa (1900±20 °C), 1.5 GPa (2033±20 °C), and 2.0 GPa (2145±20 °C) using a new high-pressure assembly in a piston–cylinder apparatus and substantial differences were found with data previously reported. The new melting data of β-quartz were combined and optimized with all available thermodynamic, volumetric, and phase equilibria data for β-cristobalite, β-quartz and coesite to produce a P–T liquidus diagram for silica valid up to 6.0 GPa. Using the new optimized thermodynamic parameters, the invariant point β-cristobalite+β-quartz+liquid and β-quartz+coesite+liquid were determined to lie at 1687±17 °C and 0.457 GPa, and 2425±25 °C and 5.00 GPa, respectively.     

Low-degree partial melting trends recorded in upper mantle minerals

The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Minerals in a spinel lherzolite nodule from Grande Comore island contain glass inclusions which, after homogenization by heating, exhibit a continuous suite of chemical compositions clearly distinct from that of the host basanitic lava. The compositions range from silicic, with nepheline–olivine normative, 64 wt% SiO₂ and 11 wt% alkali oxides, to almost basaltic, with quartz normative, 50 wt% SiO₂ and 1–2 wt% alkali oxides. Within a single mineral phase, olivine, the inferred primary melt composition varies from 54 to 64 wt% SiO₂ for MgO content ranging from 8 to 0.8 wt%. An experimental study of the glass and fluid inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at moderate temperature and pressure (T≈1230°C and P≈1.0 Gpa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr₂O₃ and poorer in SiO₂, K₂O, Na₂O, Al₂O₃ and Cl with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts, and indicate that primary melts with SiO₂ >60 wt%, alkali oxides >11%, FeO <1 wt% and MgO <1 wt% are generated during incipient melting of spinel peridotite. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO₂ and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO₂ and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated.     

Andesitic magma degassing investigated through H2O vapour–melt partitioning of halogens at Soufrière Hills Volcano, Montserrat (Lesser Antilles)

Magma degassing at Soufrière Hills Volcano (SHV) is characterised by an almost permanent SO₂ flux and a HCl production rate which mainly depends on dome growth rate. Degassing processes have been studied through textural, H₂O and halogen analyses of clasts collected between 1995 and 2006 on the dome and in pyroclastic flows and vulcanian eruption deposits. Cl, Br and I are strongly depleted in melts during H₂O degassing with no significant Cl–Br–I fractionation, whereas F is almost unaffected. All magmas erupted at SHV have followed a multi-step degassing path from the magma chamber up to a shallow depth ( 1 km, P  20 MPa). From that depth, however, effusive and explosive paths are distinct; vulcanian eruptions are the result of closed system degassing (CSD), while effusive dome growth is the result of CSD up to a very shallow depth (≤ 200 m, P  5–2 MPa) followed by open system degassing (OSD). CSD is modelled using the H₂O solubility law, the perfect gas law and partition coefficients of halogens between a rhyolitic melt and H₂O vapour (d_v − lⁱ). Gas loss characteristic of OSD is modelled using a Rayleigh law. Degassing induced crystallisation is introduced through the ratio of crystallisation and degassing rates, which ranges from 150–500. d_v − l^Cl for OSD ranges between 50–300, increasing with melt Cl content. For CSD, the lower effective d_v − l^Cl ( 20) is attributed to kinetic effects.

Dome forming activity has a greater impact on atmospheric chemistry than vulcanian eruptions because OSD is much more efficient at extracting halogens. The model shows that HCl flux is a good proxy for the dome forming eruption rate. Comparison between model and measured gas compositions suggests a high HBr–BrO conversion rate (BrO/Total Br  1/3) in the SHV gas plume.

The degassing behaviour of Cl, Br and I implies similar Cl/Br ( 160) and Br/I ( 90) in initial melts, volcanic clasts and high temperature gases. The low Cl/Br at SHV compared to other island arcs ( 250–300) is attributed to a shallow, pre-eruptive Br enrichment. The almost permanent dome extrusion at SHV since 1995 has likely had a significant regional atmospheric impact because of the very efficient effusive degassing and the high conversion rate of halogens into reactive species within the gas plume.     

Anelastic structure of the upper mantle beneath the northern Philippine Sea

We investigated the upper mantle anelastic structure beneath the northern Philippine Sea region, including the Izu-Bonin subduction zone and the Shikoku Basin. We used regional waveform data from 69 events in the Pacific and the Philippine Sea slabs, recorded on F-net and J-array network broadband stations in western Japan. Using the S–P phase pair method, we obtained differential attenuation factors, δt^*, which represent the relative whole path Q. We conducted a tomographic inversion using 978 δt^* values to invert for a fine-scale (50–100 km) three-dimensional anelastic structure.

The results shows two high-Q regions (Q_P>1000) which are consistent with the locations of the Pacific and the Philippine Sea slabs. Also there is a low-Q (Q_P110) area extending to the deeper parts (350–400 km) of the model just beneath the old spreading center and the Kinan Seamount Chain in the Shikoku Basin. A small depth dependence of the laterally averaged Q_P was found, with values of 266 (0–250 km), 301 (250–400 km), and 413 (400–500 km).     

Dating of prograde metamorphic events deciphered from episodic zircon growth in rocks of the Dabie–Sulu UHP complex, China

The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie–Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1 ± 0.4 Ma, 227.2 ± 0.8 Ma and 219.8 ± 0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7 ± 1.2 and 227.5 ± 1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0 ± 2.6 Ma was obtained from two Sulu zircon samples which contain low pressure–temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below  2.5 Gpa) mineral inclusions. In terms of Fe–Mg exchange of trapped garnet–clinopyroxene pairs within zircon domains, we are able to determine the Pressure–Temperature (PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism ( 2.7 Gpa) began at 242.2 ± 0.4 Ma (n = 74, pooling the Dabie–Sulu data), followed by peak UHP metamorphism (>  4 Gpa) at 227.3 ± 0.7 Ma (n = 72), before exhumation (<  220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie–Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust.     

Calcium isotope fractionation in coccoliths of cultured Calcidiscus leptoporus, Helicosphaera carteri, Syracosphaera pulchra and Umbilicosphaera foliosa

Four species of marine calcifying algae, the coccolithophores Calcidiscus leptoporus, Helicosphaera carteri, Syracosphaera pulchra and Umbilicosphaera foliosa were grown in laboratory cultures under temperatures varying between 14 and 23 °C, and one species, C. leptoporus, under varying [CO₃²⁻], ranging from 105 to 219 μmol/kg. Calcium isotope compositions of the coccoliths resemble in both absolute fractionation and temperature sensitivity previous calibrations of marine calcifying species e.g. Emiliania huxleyi (coccolithophores) and Orbulina universa (planktonic foraminifera) as well as inorganically precipitated CaCO₃, but also reveal small species specific differences. In contrast to inorganically precipitated calcite, but similar to E. huxleyi and O. universa, the carbonate ion concentration of the medium has no statistically significant influence on the Ca isotope fractionation of C. leptoporus coccoliths; however, combined data of E. huxleyi and C. leptoporus indicate that the observed trends might be related to changes of the calcite saturation state of the medium. Since coccoliths constitute a significant portion of the global oceanic CaCO₃ export production, the Ca isotope fractionation in these biogenic structures is important for defining the isotopic composition of the Ca sink of the ocean, one of the key parameters for modelling changes to the marine Ca budget over time. For the present ocean our results are in general agreement with the previously postulated and applied mean value of the oceanic Ca sink (Δ_sed) of about − 1.3‰, but the observed inter- and intra-species differences point to possible changes in Δ_sed through earth history, due to changing physico-chemical conditions of the ocean and shifts in floral and faunal assemblages.     

Prediction of siderophile element metal-silicate partition coefficients to 20 GPa and 2800°C: the effects of pressure, temperature, oxygen fugacity, and silicate and metallic melt compositions

We report new metal-silicate partition coefficients for Ni, Co and P at 7.0 GPa (1650–1750°C), and Ni, Co, Mo, W and P at 0.8, 1.0 and 1.5 GPa (1300–1400°C). Guided by thermodynamics, all available metal-silicate partition coefficients, D(i), where i is Ni, Co, P, Mo and W, are regressed against 1/T, P/T, lnf(O₂), ln(1 − X_s) (X_S is mole fraction of S in metallic liquid) and nbo/t (non-bridging oxygen/tetrahedral cation ratio, a silicate melt compositional-structural parameter) to derive equations of the following form: ln D(i) = aln f(O₂) + (b/T) + (cP/T) + d(nbo/t) + eln(1 − X_S) + f. Expressions for solid metal-liquid silicate and liquid metal-liquid silicate partition coefficients are derived for S-free and S-bearing systems.

We investigate whether Earth's upper-mantle siderophile element abundances can be reconciled with simple metal-silicate equilibrium. Sulfur-free metallic compositions do not allow a good fit. However, Ni, Co, Mo, W and P abundances in the upper mantle are consistent with simple metal-silicate equilibrium at mantle pressures and temperatures (27 GPa, 2200 K, ΔIW(iron-wüstite) = −0.15, nbo/t = 2.7; X_S = 0.15). Although these conditions are near the anhydrous peridotite solidus, they are well above the hydrous solidus and probably closer to the liquidus. A hydrous magma ocean and early mantle are consistent with predicted planetary accretion models. These results suggest that siderophile element abundances in Earth's upper mantle were established by liquid metal-liquid silicate equilibrium near the upper-mantle-lower-mantle boundary.     

Can biomass burning produce a globally significant carbon-isotope excursion in the sedimentary record?

Negative carbon-isotope excursions have been comprehensively studied in the stratigraphic record but the discussion of causal mechanisms has largely overlooked the potential role of biomass burning. The carbon-isotopic ratios (δ¹³C) of vegetation, soil organic matter and peat are significantly lower than atmospheric carbon dioxide (CO₂), and thereby provide a source of low ¹³C CO₂ when combusted. In this study, the potential role of biomass burning to generate negative carbon isotope excursions associated with greenhouse climates is modeled. Results indicate that major peat combustion sustained for 1000 yr increases atmospheric CO₂ from 2.5× present atmospheric levels (PAL) to 4.6× PAL, and yields a pronounced negative δ¹³C excursion in the atmosphere ( 2.4‰), vegetation ( 2.4‰) and the surface ocean ( 1.2‰), but not for the deep ocean ( 0.9‰). Release of CO₂ initiates a short-term warming of the atmosphere (up to 14.4 °C, with a duration of 1628 yr), which is consistent with the magnitude and length of an observed Toarcian excursion event. These results indicate that peat combustion is a plausible mechanism for driving negative δ¹³C excursions in the rock record, even during times of elevated pCO₂.