Fabio Cammarano

Insights into the nature of the transition zone from physically constrained inversion of long-period seismic data

Imposing a thermal and compositional significance to the outcome of the inversion of seismic data facilitates their interpretation. Using long-period seismic waveforms and an inversion approach that includes constraints from mineral physics, we find that lateral variations of temperature can explain a large part of the data in the upper mantle. The additional compositional signature of cratons emerges in the global model as well. Above 300 km, we obtain seismic geotherms that span the range of expected temperatures in various tectonic regions. Absolute velocities and gradients with depth are well constrained by the seismic data throughout the upper mantle, except near discontinuities. The seismic data are consistent with a slower transition zone and an overall faster shallow upper mantle, which is not compatible with a homogenous dry pyrolite composition. A gradual enrichment with depth in a garnet-rich component helps to reduce the observed discrepancies. A hydrated transition zone would help to lower the velocities in the transition zone, but it does not explain the seismic structure above it.

Synthetic seismic signature of thermal mantle plumes

Abstract The first seismic images of mantle plumes have been a source of significant debate. To interpret these images, it is useful to have an idea of a plume’s expected seismic signature. We determined a set of dynamic thermal whole-mantle plumes, with parameters appropriate for the Earth’s mantle and shallow-mantle temperature contrasts compatible with surface observations. We explore the sensitivity of amplitude and width of thermal plume anomalies to model parameters. The conversion of thermal to seismic structure accounts for effects of temperature, pressure, an average mantle composition including phase transitions, and anelasticity. With depth-dependent expansivity and temperature- and depth-dependent viscosity, these relatively weak plumes have lower-mantle diameters of 300–600 km at one half of the maximum temperature anomaly. To attain the narrow upper-mantle plumes inferred from surface observations and tomography, viscosity reduction by a factor 30–100 is necessary, either as a jump or as a strong gradient. All model plumes had buoyancy fluxes ≥4 Mg/s and it seems difficult to generate whole-mantle thermal plumes with fluxes much lower. Due to changing seismic sensitivity to temperature with depth and mineralogy, variations in the plumes’ seismic amplitude and width do not coincide with those in their thermal structure. Velocity anomalies of 2–4% are predicted in the uppermost mantle. Reduced sensitivity in the transition zone as well as complex velocity anomalies due to phase boundary topography may hamper imaging continuous whole-mantle plumes. In the lower mantle, our plumes have seismic amplitudes of only 0.5–1%. Unlike seismic velocities, anelasticity reflects thermal structure closely, and yields plume anomalies of 50–100% in dln(1/QS).

Quantitative integration of geophysical methods for archaeological prospection

Multi-method surveys have been used with the aim of detecting either sharp discontinuities (boundary of the cavity, fractures in the medium, etc.) or volumetric variations (bodies with different physical properties), at three different archaeological test sites. For the survey a combination of passive and active methods (magnetic, GPR and dipole–dipole geoelectric method) has been used. With all methods a high-resolution data acquisition method has been adopted with the aim of reconstructing a global vision of the area investigated. The enhancement of the processing technique towards the integration of different geophysical methods, enabling one to better define the location, depth and geometry of any archaeological body, has been adopted. Copyright

Effects of chemical composition, water and temperature on physical properties of continental crust

We explore the influence of major elements chemistry and H2O-content on the density and seismic velocity of crustal rocks by computing stable and metastable crustal mineralogy and elastic properties as a function of pressure and temperature (P-T). Proposed average compositions of continental crust result in significantly different properties, for example a difference in computed density of ∼ 4 % is obtained at a given P-T. Phase transformations affect crustal properties at the point that crustal seismic discontinuities can be explained with mineral reactions rather than chemical stratification. H2O, even if introduced in small amount in the chemical system, has an effect on physical properties comparable to that attributed to variations in major elements composition. Thermodynamical relationships between physical properties differ significantly from commonly used empirical relationships. Density models obtained by inverting CRUST 1.0 compressional wave velocity are different from CRUST 1.0 density and translate into variations in isostatic topography and gravitational field that ranges ±600 m and ±150 mGal respectively. Inferred temperatures are higher than reference geotherms in the upper crust and in the deeper portions of thick orogenic crust, consistently with presence of metastable rocks. Our results highlight interconnections/dependencies among chemistry, pressure, temperature, seismic velocities and density that need to be addressed to better understand the crustal thermo-chemical state.

High-resolution geophysical prospecting with integrated methods. The ancient Acropolis of Veio (Rome, Italy)

The fundamental geophysical goal in archaeological prospection, in near-surface investigations and generally in environmental applications is to construct as complete as possible maps of subsurface targets. The main effort in archaeology is also the integration of different, absolutely non-invasive techniques, especially if they are used in the high-resolution three-dimensional tomographic mode. In the present work a combination of fluxgate differential magnetic, ground penetrating radar and dipole–dipole geoelectric methods has been used with the aim of detecting superficial structures (wall remains and traces of an ancient road) contained in an archaeological test area (ancient Acropolis of Veio, Rome). With all geophysical techniques a high-resolution data acquisition method has been adopted with the aim of reconstruction of a global vision of the area investigated (20 m×20 m). Some signal processing and tomographic representation techniques have been used for data elaboration and interpretation. The results of the geophysical surveys have been confirmed by direct archaeological excavations carried out only at anomalous zones.

S−to−P heterogeneity ratio in the lower mantle and thermo-chemical implications

We evaluate the thermo-chemical state of the lower mantle by analysing the differences in the pattern of heterogeneity between shear and compressional velocity variations and the S−to−P heterogeneity ratio ( RS/P=δlnVS/δlnVP) as mapped in our model SPani and in alternative joint models. Robust structural differences between VP and VS evidence the presence of compositional heterogeneity within the two Large Low Shear Velocity Provinces (LLSVPs). We find also an increasing decorrelation with depth that can be associated with compositional layering of the LLSVPs. In addition, our model shows heterogeneity in the transition zone and mid mantle by complex morphology of subducting slabs and further differences between VP and VS that point to an unexpected heterogeneous lower mantle. Precise estimates of compositional heterogeneities are not yet affordable because of the difficulty to provide quantitative measure of RS/P, making it difficult to use this ratio to evaluate chemical heterogeneity. For instance, RS/P global median value ( ) drops from ∼2.8 to ∼1.9, at 2500 km depth when the VP component of SPani is replaced by a VP model resulting from a differently regularized inversion and obtaining an equally good data fit. An increase of 20% of the SPani VP anomalies also drastically reduces without significantly degrading the data fit. Noise in model parameters also leads to overestimate RS/P in the two LLSVPs as we show with synthetic tests. Additional mineral physics uncertainties for compositional effects on RS/P and for the conversion of δlnVS and δlnVP into density further complicates a precise chemical interpretation.

Hydration of marginal basins and compositional variations within the continental lithospheric mantle inferred from a new global model of shear and compressional velocity

We present a new global model of shear and compressional wave speeds for the entire mantle, partly based on the data set employed for the shear velocity model savani. We invert Rayleigh and Love surface waves up to the sixth overtone in combination with major P and S body wave phases. Mineral physics data on the isotropic δlnVS/δlnVP ratio are taken into account in the form of a regularization constraint. The relationship between VP and VS that we observe in the top 300 km of the mantle has important thermochemical implications. Back-arc basins in the Western Pacific are characterized by large VP/VS and not extremely low VS at ∼150 km depth, consistently with presence of water. Most pronounced anomalies are located in the Sea of Japan, in the back-arc region of the Philippine Sea, and in the South China Sea. Our results indicate the effectiveness of slab-related processes to hydrate the mantle and suggest an important role of Pacific plate subduction also for the evolution of the South China Sea. We detect lateral variations in composition within the continental lithospheric mantle. Regions that have been subjected to rifting, collisions, and flood basalt events are underlain by relatively large VP/VS ratio compared to undeformed Precambrian regions, consistently with a lower degree of chemical depletion. Compositional variations are also observed in deep lithosphere. At ∼200 km depth, mantle beneath Australia and African cratons has comparable positive VS anomalies with other continental regions, but VP is ∼1% higher.

Geophysical Investigations of Habitability in Ice‐Covered Ocean Worlds

Geophysical measurements can reveal the structure of icy ocean worlds and cycling of volatiles. The associated density, temperature, sound speed, and electrical conductivity of such worlds thus characterizes their habitability. To explore the variability and correlation of these parameters, and to provide tools for planning and data analyses, we develop 1-D calculations of internal structure, which use available constraints on the thermodynamics of aqueous MgSO

Inferring upper-mantle temperatures from seismic velocities

_4

Caveats on tomographic images

, NaCl (as seawater), and NH