Craig R. Bina

The structure of the core-mantle boundary from diffracted waves

Diffracted P and S waves (Pd, Sd) traveling around the core-mantle boundary (CMB) of the Earth give us information about the velocity structure and therefore the thermochemistry of D″, the base of the Earths mantle. By examining Pd and Sdarrivals we determined the apparent ray parameter for different regions at the base of the mantle. By comparing the data slownesses to those found from reflectivity synthetic seismograms we were able to quantify D″ average velocities. Using these averaged velocities with a thermochemical modeling of lower mantle minerals using a Birch-Mumaghan equation of state, we have been able to make chemical and physical inferences as to the causes of lateral variations at the CMB. Examinations found significant lateral heterogeneity at the base of the mantle, amounting to ≈ 4% for both P and S velocities. These velocities did not always vary in parallel, and the Poisson ratio varied regionally by almost 6%. The most unusual region of the CMB found was under Indonesia, where velocities 3% slower than the preliminary reference Earth models were found adjacent to a region of faster than average velocities. These regions currently correspond to areas of core up welling and down welling (respectively) found by Voorhies (1986), which if mostly held in place by core-mantle coupling might cause a flux of heat and iron into the mantle, making the anomaly both thermally and chemically derived. At the CMB under the northern Pacific rim the fastest shear velocities were found, but the same region yielded slower than average P velocities. While the presence of fast shear velocities here would support the idea that we are seeing the cold dregs of mantle convection, perhaps continuing down from the North Pacific subduction zones, the presence of slow P velocities suggests additional complications. Our thermochemical modeling suggests that the D″ Poisson ratio is very sensitive to variations in the silicate/oxide ratio and that a decrease in the amount of perovskite relative to magnesiowustite may play an important role in this region.

Implications of slab mineralogy for subduction dynamics

Phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs, both under equilibrium and disequilibrium (metastable) conditions. Such perturbations yield anomalies not only in seismic velocities but also in density. The buoyancy forces arising from these density anomalies may exert several important effects. They contribute to the stress field within the slab, in a fashion consistent with observed patterns of seismicity. They may affect subduction rates, both by inducing time-dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions. They may affect slab morphology, possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs. Latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating, which may generate adiabatic shear instabilities capable of triggering deep seismicity.

Phase transition buoyancy contributions to stresses in subducting lithosphere

The sequence of phase transitions undergone by minerals with increasing depth in Earths mantle is perturbed within subducting lithospheric slabs by their thermal structure. Such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab. While other factors contribute to overall slab stresses, thermal and phase transition effects largely control the structure of the stress field within the slab. The resulting maximum in down-dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity. Furthermore, metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity. These observations are independent of the variety of failure mechanisms proposed for deep seismogenesis.

Possible presence of high-pressure ice in cold subducting slabs

During the subduction of oceanic lithosphere, water is liberated from minerals by progressive dehydration reactions and is thought to be critical to several geologically important processes such as island-arc volcanism, intermediate-depth seismicity and chemical exchange between the subducting lithosphere and mantle. Although dehydration reactions would yield supercritical fluid water in most slabs, we report here that the stable phase of H2O should be solid ice VII in portions of the coldest slabs. The formation of ice VII as a dehydration product would affect the generation, storage, transport and release of water in cold subduction zones and equilibrium conditions of dehydration would shift, potentially affecting the depths of seismogenesis and magmagenesis. Large amounts of pure ice VII might accumulate during subduction and, as a sinking slab warms, eventual melting of the ice would release large amounts of water in a small region over a short period of time, with a significant positive volume change. Moreover, the decreasing availability of fluid water, owing to the accumulation of ice VII and its subsequent reaction products in a cooling planetary interior (for example, in Mars or the future Earth), might eventually lead to a decline in tectonic activity or its complete cessation.

Frequency dependence of the visibility and depths of mantle seismic discontinuities

The visibility and apparent position of a seismic gradient zone depends on the seismic wave frequency at which it is observed, which we illustrate by showing how the visibility and depth of the olivine α→β phase transformation (410 km discontinuity) change as temperature varies. Temperature variations change bottomside reflection coefficients by factors of 2–4 in the 0.5–1.0 Hz frequency band, which can explain spatial variability in short period P′ 410P′ observations and the small apparent variability in observed 410 km discontinuity depths. Discontinuity depths inferred from the time lags of these reflections are also frequency dependent, leading to a frequency dependence of estimates of mantle lateral thermal variability based on discontinuity displacements. The frequency dependent seismic Clapeyron slopes for the olivine α→β phase change we compute are 2.04 MPa K−1 at 0.2 Hz and 2.12 MPa K−1 at 0.05 Hz, and differ significantly from thermodynamic slope for the Mg2SiO4 end-member.

Compression of single-crystal magnesium oxide to 118 GPa and a ruby pressure gauge for helium pressure media

Abstract The pressure-volume equation of state (EoS) of single-crystal MgO has been studied in diamondanvil cells loaded with helium to 118 GPa and in a non-hydrostatic KCl pressure medium to 87 GPa using monochromatic synchrotron X-ray diffraction. A third-order Birch-Murnaghan fit to the nonhydrostatic P-V data (KCl medium) yields typical results for the initial volume, V0 = 74.698(7) Å3, bulk modulus, KT0 = 164(1) GPa, and pressure derivative, K′ = 4.05(4), using the non-hydrostatic ruby pressure gauge of Mao et al. (1978). However, compression of MgO in helium yields V0 = 74.697(6) Å3, KT0 = 159.6(6) GPa, and K′ = 3.74(3) using the quasi-hydrostatic ruby gauge of Mao et al. (1986). In helium, the fitted equation of state of MgO underdetermines the pressure by 8% at 100 GPa when compared with the primary MgO pressure scale of Zha et al. (2000), with KT0 = 160.2 GPa and K′ = 4.03. The results suggest that either the compression mechanism of MgO changes above 40 GPa (in helium), or the ruby pressure gauge requires adjustment for the softer helium pressure medium. We propose a ruby pressure gauge for helium based on shift of the ruby-R1 fluorescence line (Δλ/λ0) and the primary MgO pressure scale, with P (GPa) = A/B{[1 + (Δλ/λ0)]B - 1}, where A is fixed to 1904 GPa and B = 10.32(7).

Patterns of deep seismicity reflect buoyancy stresses due to phase transitions

Thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies. Finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near ∼400 km depth, down-dip compression over ∼400–690 km (peaking at ∼550 km), and transition to rapidly fading tension below ∼690 km. Such features, even when complicated by olivine metastability, are consistent with observed patterns of deep seismicity. That such a simple model, neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations, successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs. It also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release.

Effects of slab mineralogy on subduction rates

Although velocities of subducting slabs should be controlled primarily by their negative thermal buoyan- cies, their mineralogy can also have signicant eects. We explore this by using thermo-kinetic modeling to predict mineralogy and compare the resultant buoyant (driving) force to the opposing viscous drag. Phase transitions of (Mg; Fe)2SiO4 in subducting slabs depend on thermal struc- ture in two ways. First, equilibrium phase boundaries should be deflected, causing local buoyancy anomalies whose sign depends upon that of the Clapeyron slope. As slabs rst enter the transition zone, negative anomalies should acceler- ate them, but positive anomalies that arise when they fully penetrate the transition zone should slow them. Such ef- fects may induce geologically abrupt changes in plate mo- tions. Second, olivine that persists metastably in slabs will form regions of positive buoyancy which should reduce slab velocities. The coldest and fastest slabs should be slowed more greatly, thus narrowing the range of feasible subduc- tion rates. Decreased descent rates, however, allow slabs to warm and metastable wedges to thermally erode. Such neg- ative feedback mechanisms may serve to regulate subduction rates.

Diversity and species abundance patterns in late Cenomanian black shale biofacies, Western Interior, U.S.

Questions concerning the application of established biofacies models to mid-Cretaceous black shales prompted a study of diversity characteristics in a fauna from the Late Cenomanian Hartland Shale Member, Western Interior basin. Numerical faunal data are used to assess species abundance patterns, and a new method of analyzing diversity is introduced that incorporates species richness, Shannon index, and equitability into a single plot. In addition, numerical simulations designed to emulate the sampling of species-abundance distributions are used to improve data interpretation. The study illustrates how measured diversity results from the combination of primary ecological controls and sampling effects. Proximal offshore assemblages are characterized by high diversity and log series species-abundance patterns, interpreted as truncated (incompletely sampled) log-normal distributions. Primary ecological controls include variable physical/chemical parameters, biological factors such as predation and competition, and intermediate disturbance frequency. Distal offshore assemblages are characterized by low diversity with patterns of species abundance resembling geometric series. These are interpreted as truncated log-series distributions (sampling effect) that reflect dominance of multiple opportunists, abundant resources in a dysoxic environment, and high disturbance frequency. The data are used to develop an ecological model for diversity levels in basinal black shale facies based on the interplay of recruitment, growth rate, tolerance to low oxygen and sulphide, and disturbance frequency (due to fluctuations of the redox boundary). Although certain taxa (chiefly Inoceramidae) evolved highly opportunistic life strategies to exploit basinal paleoenvironments, it was the unpredictable interaction of these four factors that determined diversity patterns. Analysis of Hartland Shale biofacies illustrates the difficulties in applying a strictly linear relationship between paleo-oxygen levels and diversity.

Confidence limits for silicate perovskite equations of state

We examine thermoelastic equations of state for silicate perovskite based on data from recent static compression studies. We analyze trade-offs among the fitting parameters and examine data sets for possible effects of metastability and of Mg-Fe solid solution. Significant differences are found between equations of state based on low pressure measurements obtained for perovskite outside of its stability field. Increasingly consistent results are obtained when higher pressure data are used, despite differences in the zero-pressure parameters used to describe the equations of state. The results highlight the importance of measuring the thermoelastic properties of perovskite at high pressure, specifically within its stability field, and the potential problems associated with large extrapolations of equations of state in the analysis of seismic data.