DI24A-01
Broadband SPdKS waveforms reveal ULVZ ridge in the central Pacific
Over the past decade, the seismic phase SPdKS has been used to examine anomalous boundary layer structure at the core-mantle boundary (CMB), and revealed evidence for the presence of ultralow-velocity zones (ULVZ) right at the CMB. Studies have mapped ULVZs with thicknesses up to 40 km, S-wave velocity reductions from 5 to 45%, and P-wave velocity reductions from 5 to 15%. Some significant modeling uncertainties are present for ULVZ mapping. The nature of ULVZs may be intimately linked to overall mantle processes (e.g., the birth place of plumes), thus we desire better structural and geographical constraints. Here we investigate ULVZ structure in the central Pacific utilizing 37 southwest Pacific deep focus events with impulsive source mechanisms recorded in the Americas, focusing on the broadband (BB) SKS and SPdKS wavefields. SPdKS is an SKS wave that intersects the CMB at the critical angle for ScP, thus initiating short segments of diffracted P-waves (Pd) along the CMB at the core entry and exit locations. Analysis of these data show a cluster of highly anomalous SPdKS waveforms: SPdKS initiates at an epicentral distance of around 103°, roughly 7° earlier than in models lacking ULVZ structure (e.g., PREM). Also, the SPdKS amplitudes are larger than SKS. The anomalous waveforms are observed for multiple events recorded at stations across N. America, with the commonality that the source-side Pd CMB inception points are grouped together forming a ridge-like pattern (~200 km in length trending NW). SPdKS source-side inception points to the NE of this ridge display milder ULVZ characteristics, whereas inception points to the NW or SE of the ridge display PREM-like characteristics. We model these P/SV- waveforms using the 2.5D axi-symmetric finite difference algorithm PSVaxi. Although PSVaxi does not incorporate full 3D geometry, it is useful here as the geometry of the observed ridge is perpendicular to the station-receiver great circle path, and we are capable of computing synthetics with 6 sec dominant periods. We show that our observations are compatible with PSVaxi predictions for a ULVZ ridge shaped structure, trending to the NW, and sloping off to thinner ULVZ to the NE. The location of this ULVZ ridge is located in the center of the Pacific Large Low Shear Velocity Province (LLSVP). The nature of the LLSVP is uncertain, but appears consistent with a chemically distinct origin, e.g., a thermochemical pile. Geodynamic modeling suggests the existence of internal upwellings and downwellings within piles, resulting in hot ridges around the pile margins, as well as within the pile. For this possibility, if ULVZ material is significantly more-dense than pile material, ULVZs may accumulate beneath the upwelling regions within the piles. The ridge-like ULVZ feature we observe is consistent with hot linear basal temperature anomalies in regional upwelling zones in geodynamic calculations. This ULVZ modeled here may be associated with upwellings giving rise to the Samoan hot spot.
DI24A-02 INVITED
The base of the mantle beneath the Pacific: noLVZ, LVZ and ULVZ
A large, high quality P-wave data set comprising short-period and broadband signals sampling four regions in the lowermost mantle beneath the Cocos plate, Mexico, the north Pacific, and the central Pacific is analyzed using regional one-dimensional double-array stacking and synthetic modeling. A data-screening criterion retains only events with stable PcP energy in the final data stacks used for modeling and interpretation. This significantly improves the signal in the stacks compared to stacks that uses all observations and allows tight bounds to be placed on P-wave velocity structure above the core-mantle boundary (CMB). The PcP reflections under the Cocos plate are well-modeled without any ultra-low velocity zone from 5 - 20°N. The data stacks for paths with PcP reflection points below the eastern equatorial Pacific, Mexico and the Gulf of Mexico have PcP images that are well-matched with the simple IASP91 structure. Beneath the north Pacific near the Aleutian Islands, PcP arrivals are compatible with LVZ (dlnVp of about 0 to -3%) only a few km thick. Data sampling a 200 X 200 km patch of the lowermost mantle beneath the central Pacific confirm the presence of a low velocity zone just above the CMB about 15 km thick, with very little topography, dlnVp of around -3 to -4% and dlnVs that varies from -4 to -8%. The data are insensitive to any density contrasts. The velocity drops observed in this region are small compared to some ULVZ observations beneath other parts of the Pacific, which have dlnVp and dlnVs of -10 and -30%, respectively. P- and S-wave data in this region also indicate laterally varying discontinuities shallower in D". The data sample the margin of the central Pacific's large low shear velocity province and our results may reflect a gradual change in Fe or Al content. Our results indicate localized occurrence of detectable LVZ structures rather than ubiquitous ULVZ structure and show structural differences between circum-Pacific regions and the large low shear velocity province under the central Pacific.
DI24A-03 INVITED
Melting at the core-mantle boundary
The ULVZ at the core-mantle boundary is characterized by a complex, three dimensional structure. ScP wave data from events near the Tonga-Fiji region displays a laterally variable topography of the ULVZ corresponding to shear and P wave velocity reductions of 10-30% and and 0-10%, respectively. A laterally varying density structure, leading up to an increase of 50% is also observed. We use a multiparticle, Boundary Integral Method to evaluate the melt fraction corresponding to this velocity structure. The average variation in the velocity can be explained by the presence of approximately 10-15% melt by volume, indicating that the ULVZ is not disaggregated. This extent of melting is likely to reduce the viscosity of the aggregate by a factor of 0.08 to 0.02. Laboratory experiments on the stability of a dense, low viscosity fluid layer at the base of the mantle also predicts such a viscosity contrast between the ULVZ and the surrounding mantle. However, both experimental and theoretical measurements of density of silicate melts under core-mantle boundary conditions fail to explain the observed density contrast for 10-15% melting, indicating the likely presence of a compositional change.
DI24A-04 INVITED
Dynamics of the Ultra Low Velocity Zone
The Ultra Low Velocity zone (ULVZ) is characterized by a sharp drop in seismic velocities in certain regions of the lowermost mantle. Recent seismic observations indicate that the ULVZ may have a morphology of small, kilometer-scale packets of anomalously high-density material. Potential explanations for the cause of the ULVZ include the presence of partial melt and/or fine-scale chemical heterogeneity possibly due to iron enrichment. A fundamental question regarding both hypotheses pertains to whether mantle convection is dynamically capable of supporting such small-scale, high-density structures. We perform high-resolution, whole-mantle thermochemical convection calculations to test the feasibility of this hypothesis. We find that convection acts to segregate and support very thin layers of ultra-dense material into geometries that resemble those inferred from seismic models. Furthermore, we find that ULVZ material accumulates at the base of upwelling regions, providing a link between observations and dynamics. We show that continued high resolution mapping of ULVZ geography promises to provide constraints upon the style of larger scale mantle convection.
DI24A-05 INVITED
Elastic properties of the post-perovskite phase of Fe2O3 and implications for ultra-low velocity zones.
Seismological studies indicate the presence of ultra-low velocity zones at the base of the lower mantle. In these 5-20 km thick layers, compressional-wave velocities are depressed by between 5 to 10 percent and shear-wave velocities by between 10 to 30 percent, indicative of a change in phase or chemical composition. The most common explanation for these observations is that the discontinuities mark the onset of partial melting, but a more recent interpretation is that they could comprise localized zones of iron enriched post- perovskite. Theoretical calculations indicate that, in the lowermost mantle, ferrous iron in post-perovskite will undergo a valence disproportionation reaction yielding ferric iron and iron metal. In view of this, we performed ab initio calculations to calculate the elastic properties of the post-perovskite phase of Fe2O3 at 136 GPa and 0 K. These were then used, in combination with data for the post-perovskite phase of MgSiO3, to estimate high-temperature values for a range of lower mantle minerals assemblages. Comparison of these with seismic constraints for ultra-low velocity zones allowed us to ascertain if iron-enriched post-perovskite could explain the anomalous regions.
DI24A-06 INVITED
Where is mantle's carbon?
Due to the strongly reducing conditions (the presence of metallic iron was suggested both by experiments [1] and theory [2]), diamond was believed to be the main host of carbon through most of the lower mantle [3]. We showed [4] that cementite Fe3C is another good candidate to be the main host of "reduced" carbon in the mantle, reinforcing an earlier hypothesis [5]. The fate of "oxidised" carbon (in subducted slabs) is of particular importance – if carbonates decompose producing fluid CO2, this would have important implications for the chemistry and rheology of the mantle. Knowledge of crystal structures and phase diagrams of carbonates is crucial here. The high-pressure structures of CaCO3 were predicted [6] and subsequently verified by experiments. For MgCO3, Isshiki et al. [7] found a new phase above 110 GPa, and several attempts were made to solve it [8,9]. Here [4], using an evolutionary algorithm for crystal structure prediction [10], we show that there are two post-magnesite phases at mantle-relevant pressure range, one stable at 82-138 GPa, and the other from 138 GPa to ~160 GPa. Both are based on threefold rings of CO4-tetrahedra and are more favourable than all previously proposed structures. We show that through most of the P-T conditions of the mantle, MgCO3 is the major host of oxidized carbon in the Earth. We predict the possibility of CO2 release at the very bottom of the mantle (in SiO2-rich basaltic part of subducted slabs), which could enhance partial melting of rocks and be related to the geodynamical differences between the Earth and Venus. 1.Frost D.J., Liebske C., Langenhorst F., McCammon C.A., Tronnes R.G., Rubie D.C. (2004). Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle. Nature 428, 409-412. 2.Zhang F., Oganov A.R. (2006). Valence and spin states of iron impurities in mantle-forming silicates. Earth Planet. Sci. Lett. 249, 436-443. 3.Luth R.W. (1999). Carbon and carbonates in the mantle. In: Mantle Petrology: Field Observations and High Pressure Experimentation: A Tribute to Francis R. (Joe) Boyd. Geochemical Soc., Special Publication No. 6. Eds: Y. Fei, C.M. Bertka, B.O. Mysen. 4.Oganov A.R., Ono S., Ma Y., Glass C.W., Garcia A. (2008). Novel high-pressure structures of MgCO3, CaCO3 and CO2 and their role in the Earth's lower mantle. Earth Planet. Sci. Lett. 273, 38-47 5.Scott H.P.,, Williams Q., Knittle E. (2001). Stability and equation of state of Fe3C to 73 GPa: Implications for carbon in the Earth's core. Geoph. Res. Lett. 28, 1875-1878. 6.Oganov A.R., Glass C.W., Ono S. (2006). High-pressure phases of CaCO3: crystal structure prediction and experiment. Earth Planet. Sci. Lett. 241, 95-103. 7.Isshiki M., Irifune T., Hirose K., Ono S., Ohishi Y., Watanuki T., Nishibori E., Takadda M., and Sakata M. (2004). Stability of Magnesite and its high-pressure form in the lowermost mantle. Nature 427, 60-63. 8.Skorodumova N.V., Belonoshko A.B., Huang L., Ahuja R., Johansson B. (2005) Stability of the MgCO3 structures under lower mantle conditions. Am. Mineral. 90, 1008-1011. 9.Panero W.R., Kabbes J.E. (2008). Mantle-wide sequestration of carbon in silicates and the structure of magnesite II. Geophys. Res. Lett. 35, L14307. 10.Oganov A.R., Glass C.W. (2006). Crystal structure prediction using ab initio evolutionary algorithms: principles and applications. J. Chem. Phys. 124, art. 244704.
DI24A-07 INVITED
Helium and Other Geochemical Evidence on the Mantle Composition Above the Core- Mantle Boundary
Helium is a unique tracer for the geodynamic evolution of the Earth. It does not get recycled back into the mantle like other incompatible elements but rather gets degassed to the atmosphere during crust formation and subduction and then is lost to space. Thus, the presence of primordial 3He in the Earth poses fundamental questions: How does Earth retain primordial helium during accretion and catastrophic moon formation? Where is primordial helium preserved during subsequent whole mantle convection, and continuous degassing through magma generation. How are 3He/4He up to 50Ra in plume-related basalts explained when continuous production of 4He from Th+U decay reduces the 3He/4He ratio of the Earth? The last few years have seen an explosion of new ideas to address these questions. Three are highlighted here. (1) Through whole mantle convection, the mantle that has been degassed through ocean and continental crust formation remixes with yet undegassed portions of the mantle, resulting in preservation of high 3He/4He in the depleted mantle (Class & Goldstein 2005, Nature 436, 1107-1112). (2) Helium diffused early in Earth history into refractory harzburgite melt residues, where it is preserved and later sampled during melting (Albarède 2008, Science 319, 943-945). (3) Excesses of Ti correlate positively with 3He/4He, possibly suggesting association of ancient slabs composed of refractory eclogite with high 3He/4He peridotite (Jackson et al. 2008, G3 9, Q04027). Here we show that these seemingly contradictory interpretations merge into a hybrid helium evolution model. Including early Iceland plume and Samoa data, the global correlation between helium isotope ratios and Th persists, pointing towards a depleted composition for the high 3He/4He endmember, where a correlation with Ti excess is superimposed. For a given Th content ocean island basalts vary in helium isotope ratios from mid-ocean ridge like helium towards higher or lower values, but not both, emphasizing the fundamental significance of mid-ocean ridge-type helium. This requires stratification of the helium isotope ratios in the mantle with depth, caused either by age or variable mixing efficiency. The relationship to Ti excess can be ascribed to the slab graveyard at the core-mantle boundary forming the hot core of plumes, which are most likely to sample the high 3He/4He but incompatible element depleted lithologies of the deep mantle.
DI24A-08
Shock-Induced Transformation and Melting of Lower Mantle Minerals: Implications for Earth Evolution
Shock wave techniques can define high-pressure melting relations for deep Earth minerals by several methods. Pressure-volume-energy equations of state and spectral radiation shock temperature measurements are sensitive to the conditions where Hugoniots of lower mantle mineral compositions cross phase boundaries including both polymorphic phase transitions and partial to complete melting. Method 1 uses the velocity of isentropic rarefaction waves to observe the loss of shear modulus upon melting. Method 2 use radiative temperature along the Hugoniot to seek a deflected in temperature upon intersection of Hugoniot and melting curve. Method 3 applies in the case when the liquid phase is denser than the coexisting solid phase(s). In the absence of any other known polymorphic phase change a sudden density increase is attributed to melting. For SiO2, all three of these methods define the shock pressure and temperature where the Hugoniot of fused silica passes from stishovite to partial melt (73 GPa, 4600 K) and where the Hugoniot of crystal quartz passes from the CaCl2 structure to partial melt (116 GPa, 4900 K). In the case of Mg2SiO4, the forsterite Hugoniot passes from periclase+perovskite phases to melt before 152 GPa and 4300 K, whereas initial wadsleyite material follows a colder path, transforming from periclase+post–perovskite to melt before 151 GPa and 4160 K. Recently, we extended the range of the MgSiO3 glass Hugoniot and demonstrated that this glass transforms into the perovskite structure from 80 to 100 GPa. Above 100 GPa and extending to over 160 GPa, the shock state is molten. Since shock states derived from crystal enstatite are also molten above 160 GPa, precise determination of the high pressure Grüneisen parameter,γ , can be obtained from the finite difference V[dP/dE]V. As previously seen in Mg2SiO4 liquid, γ for molten MgSiO3 increases markedly with compression, going from 0.5 to 1.6 over the 0 to 135 GPa range. This property gives rise to larger than expected isentropic rises in temperature with depth in model magma oceans encompassing the entire mantle. Taking into account our interpretation of the shock melting data and a critically evaluated subset of the published database of equation of state determinations from static (multi-anvil and diamond anvil) methods, we construct a proposed deep mantle and core geotherm. We note the similarity of our resulting magma ocean model [Asimow, 2008] and that of Labrosse et al. [2007]. The notion put forward by Labrosse et al. that the present ULVZ is a very thin remnant of the ancient magma ocean, which initially started to crystallize at mid-lower mantle depths, deserves further study. Evidently, if the ULVZ is indeed a dynamically stable, partially molten remnant of the primordial magma ocean, it is a candidate for hosting the deep Earth's hidden reservoir of incompatible elements,including both a substantial portion of the global inventory of heat–producing elements and the missing primordial noble gas isotopes. The density of such melts contributes to this reservoir being effectively unsampled by either solid-state mantle convection or magmatic fluids derived from the very deep Earth. Asimow, P.D., 2007. Magmatism and the evolution of the Earth's interior. Geochimica Et Cosmochimica Acta, 71(15): A40. Labrosse, S., Hernlund, J.W. and Coltice, N., 2007. A crystallizing dense magma ocean at the base of the Earth's mantle. Nature, 450(7171): 866-869.