DOCSLIB.ORG

Joint Mineral Physics and Seismic Wave Traveltime Analysis of Upper Mantle Temperature

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Joint Mineral Physics and Seismic Wave Traveltime Analysis of Upper Mantle Temperature Joint mineral physics and seismic wave traveltime analysis of upper mantle temperature Jeroen Ritsema1, Paul Cupillard2, Benoit Tauzin3, Wenbo Xu1, Lars Stixrude4, Carolina Lithgow-Bertelloni4 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA 2Seismological Laboratory, University of California–Berkeley, Berkeley, California 94720, USA 3Ecole et Observatoire des Sciences de la Terre, Institut de Physique du Globe de Strasbourg, 67084 Strasbourg, France 4Department of Earth Sciences, University College London, London WC1E 6BT, UK ABSTRACT CONVERTED WAVE TRAVELTIMES We employ a new thermodynamic method for self-consistent Teleseismic P wave conversions, denoted as Pds, propagate as com- computation of compositional and thermal effects on phase transition pressional (P) waves through the deep mantle and convert to shear (S) depths, density, and seismic velocities. Using these profi les, we compare waves at an upper mantle discontinuity at depth d beneath the seismom- theoretical and observed differential traveltimes between P410s and eter. The phases P410s and P660s, for example, are waves that have P (T410) and between P600s and P410s (T660–410) that are affected only by converted from P to S at the 410 km and the 660 km transitions, respec- seismic structure in the upper mantle. The anticorrelation between T410 tively (Fig. 1). and T660–410 suggests that variations in T410 and T660–410 of ~8 s are due to Our data are recorded traveltime differences between the phases lateral temperature variations in the upper mantle transition zone of P410s and P (T410) and between P660s and P410s (T660–410). T410 and ~400 K. If the mantle is a mechanical mixture of basaltic and harzbur- T660–410 are unaffected by imprecise earthquake locations and lower mantle gitic components, our traveltime data suggest that the mantle has an hetero geneity. Although P410s and P660s are weak signals, they have pre- average temperature of 1600 ± 50 K, in agreement with temperature dictable slowness (i.e., angle of incidence at the surface). It is therefore estimates from magma compositions of mid-ocean ridge basalts. We straight forward to enhance P410s and P660s amplitudes by summation infer a 100 K hotter mantle if we assume the mantle to have a homoge- (stacking) of a large (>50–100) number of teleseismic (epicentral dis- neous pyrolitic composition. The transition-zone temperature beneath tances >4000 km) receiver functions from regional (Li et al., 2000; Shen hotspots and within subduction zones is relatively high and low, respec- et al., 1998; Simmons and Gurrola, 2000) or global (Chevrot et al., 1999; tively. However, the largest variability in T410 and T660–410 is recorded by Lawrence and Shearer, 2006; Tauzin et al., 2008) networks. global stations far from subduction zones and hotspots. This indicates We consider two sets of T410 and T660–410 measurements. The fi rst set that the 400 K variation in upper mantle temperature is complicated by is derived from ~7000 shallow (depth <50 km) earthquake recordings at tilted upwellings, slab fl attening and accumulation, ancient subduction, broadband seismic instruments in California. These recordings feature and processes unrelated to present-day subduction and plume ascent. simple fi rst-arriving P waves that have amplitudes at least three times larger than the coda. We measure the variation in T410 and T660–410 using INTRODUCTION multiple stacks with common P410s conversion points within circular Constraints on the variation of temperature in the Earth’s mantle are caps with radii of 1.5° across California and portions of adjacent states critical for our understanding of Earth’s convective behavior. The best (Fig. 2). The second set of T410 and T660–410 measurements is from the study constraints on temperature in the deeper parts of the mantle are obtained of Tauzin et al. (2008). T410 and T660–410 from this data set are measured from seismic studies. In particular, analyses of traveltimes of refl ected using single-station stacks for 167 global network stations. It is the most (Shearer, 1991) and converted (Vinnik, 1977) waves that have interacted with wave speed and density discontinuities due to the olivine to wadsley- ite (the 410 km) and the ringwoodite to perovskite + ferropericlase (the 660 km) phase transitions, yield localized constraints on temperature in Surface the transition zone (the region between the 410 km and the 660 km transi- tions). Since their Clapeyron slopes have opposite signs (Katsura and Ito, 1989), the 410 km transition is raised in a relatively cold upper mantle 410 km while the 660 km transition is lowered, thereby thickening the transition zone. Opposite defl ections of the 410 km and 660 km and, hence, thin- ner transition zones, characterize the relatively warm regions of the upper Transition zone 660 km mantle (Bina and Helffrich, 1994). We introduce a new thermometer for the upper mantle based on a P wave combination of mineral physics constraints and the analysis of traveltimes S wave of compressional waves that have converted to shear waves in the tran- P660s P410s P sition zone. Contrary to previous global studies (Chevrot et al., 1999; Lawrence and Shearer, 2006; Tauzin et al., 2008), we do not infer the temperature from estimates of transition zone thickness that rely on upper mantle velocity corrections. Instead, our analysis employs theoretical one- Figure 1. Ray paths of teleseismic P, P410s, and P660s through upper dimensional profi les of seismic velocity based on realistic upper mantle mantle. Phase P410s converts from P to S wave at 410 km discontinu- bulk compositions and geotherms. These profi les account for the full com- ity, while P660s converts from P to S at 660 km discontinuity. P wave plexity of the phase diagram and include all the mineralogical and thermal and S wave segments are indicated by solid and dashed lines, re- spectively. Note that P, P410s, and P660s have nearly parallel P wave effects on phase transition depths, density, and wave speeds. Hence, they propagation paths in the lower mantle and that P410s and P660s differ enable us to constrain the potential temperature of the mantle from travel- only in the transition zone. Differential traveltimes T410 and T660–410 are times directly and self-consistently. affected only by structure in upper 660 km of mantle. © 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, April April 2009; 2009 v. 37; no. 4; p. 363–366; doi: 10.1130/G25428A.1; 4 fi gures. 363 predicted value) in California is due to reduced shear velocities in the rela- A tively warm uppermost mantle beneath western North America. The set of global T410 measurements exhibits a broad variation due to the relatively strong (thermal and compositional) heterogeneity in the 40 uppermost 200–300 km of mantle. T410 peaks near 43.5 s, a value that is nearly identical to the ak135 predicted value. T410 measurements are skewed to low values because the majority of global seismic stations are located on relatively cool continental lithosphere. The global set of T660–410 has a narrower distribution than T410. T660–410 is sensitive only to transition zone thickness and velocity variations. 35 ′ Q Q TRAVELTIME PREDICTIONS We determine theoretical values of T410 and T660–410 from the velocity profi les of Xu et al. (2008), computed using the self-consistent thermo- Latitude (°N) dynamic method of Stixrude and Lithgow-Bertelloni (2005a, 2005b) and the updated parameters in Xu et al. (2008). For a given mantle composition 30 and geotherm, velocity profi les are determined by computing equilibrium phase assemblages, the physical properties of the individual phases via stress and temperature derivatives of the Gibbs free energy, and the elastic T 660–410 properties of the assemblage from the Voigt-Reuss-Hill average. T410 and T660–410 are determined using refl ectivity seismograms (Fuchs and Muller, 22.25 22.65 23.05 23.50 23.95 24.35 24.75 1971) that fully incorporate the effects on P410s and P660s waveforms 25 due to the abrupt 410 km and 660 km and the gradual transformations in 125 120 115 110 the non-olivine components. Longitude (°W) Xu et al. (2008) considered the mantle to be a mixture of basalt (XB) and harzburgite (XH) with a basalt fraction of 18%. They considered two Q Q′ models of identical bulk composition that represent contrasting views of 0 the reequilibration of subducted heterogeneity: (1) The equilibrium assem- B blage (EA) represents perfect re-equilibration of basalt and harzburgite 10 into a homogeneous pyrolite. (2) The mechanical mixture (MM) main- tains perfect disequilibrium. The MM value is motivated by the suggestion 20 that the compositionally stratifi ed oceanic lithosphere is responsible for hetero geneity in the mantle (Allègre and Turcotte, 1986). 30 RESULTS 40 Figure 4 compares T410 and T660–410 for both the California and global data sets to theoretical values for the MM and EA and a range of adia- 50 batic geotherms with potential temperature that vary between 1400 K and 1800 K. Low values for T410 and high values for T660–410 are recorded at Time since P (s) 60 stations in subduction zones. High and low values for T410 and T660–410 are recorded at hotspot stations above putative mantle plumes, as expected if 70 the traveltime variations refl ect temperature heterogeneity in the mantle. However, the total global variation in T410 and T660–410 among the normal 80 stations is as large as the differences in T410 and T660–410 between hotspot 122 120 118 116 114 and subduction zone stations. Longitude (°W) Our computations indicate that the range in T410 and T660–410 values of ~8 s can be explained by MM for geotherms with potential temperatures Figure 2.
Load more

Recommended publications
  • Effects of Water on Seismic Wave Velocities in the Upper Mantle Terior. Therefore, If One Understands the Relation Between Water
    No. 2] Proc. Japan Acad., 71, Ser. B (1995) 61 Effects of Water on Seismic Wave Velocities in the Upper Mantle By Shun-ichiro KARATO Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455 (Communicated by Yoshibumi ToMODA,M. J. A., Feb. 13, 1995) Abstract : Possible roles of water to affect seismic wave velocities in the upper mantle of the Earth are examined based on mineral physics observations. Three mechanisms are considered: (i) direct effects through the change in bond strength due to the presence of water, (ii) effects due to the enhanced anelastic relaxation and (iii) effects due to the change in preferred orientation of minerals. It is concluded that the first direct mechanism yields negligibly small effects for a reasonable range of water content in the upper mantle, but the latter two indirect effects involving the motion of crystalline defects can be significant. The enhanced anelastic relaxation will significantly (several %) reduce the seismic wave velocities. Based on the laboratory observations on dislocation mobility in olivine, a possible change in the dominant slip direction in olivine from [100] to [001] and a resultant change in seismic anisotropy is suggested at high water fugacities. Changes in seismic wave velocities due to water will be important, particularly in the wedge mantle above subducting oceanic lithosphere where water content is likely to be large. Key words : Water; seismic wave velocity; upper mantle; anelasticity; seismic anisotropy. Introduction. Water plays important roles in the terior. Therefore, if one understands the relation melting processes and hence resultant chemical evolu- between water content and seismic wave velocities, tion of the solid Earth.
    [Show full text]
  • Geomagnetism and Paleomagnetism Membership Survey
    Membership Survey: Geomagnetism and Paleomagnetism (GP) Section What is your primary section or focus group affiliation? Response Answer Options Response Count Percent Geomagnetism and Paleomagnetism (GP) Section 71.0% 363 Study of Earths Deep Interior (SEDI) Focus Group 4.5% 23 Planetary Sciences (P) Section 2.0% 10 Tectonophysics (T) Section 7.0% 36 Near-Surface Geophysics (NS) Focus Group 5.1% 26 Other (please specify) 10.4% 53 answered question 511 skipped question 0 What is your primary section or focus group affiliation? Geomagnetism and Paleomagnetism (GP) Section Study of Earths Deep Interior (SEDI) Focus Group Planetary Sciences (P) Section Tectonophysics (T) Section Near-Surface Geophysics (NS) Focus Group Other (please specify) Membership Survey: Geomagnetism and Paleomagnetism (GP) Section What (if any) is your secondary affiliation? Response Answer Options Response Count Percent Geomagnetism and Paleomagntism Section 24.0% 121 Study of Earth's Deep Interior Focus Group 15.0% 76 Planetary Sciences Section 13.7% 69 Tetonophysics Section 22.8% 115 Near-Surface Geophysics Focus Group 10.7% 54 None 24.2% 122 answered question 505 skipped question 6 What (if any) is your secondary affiliation? 30.0% 25.0% 20.0% 15.0% 10.0% 5.0% 0.0% None and Section Section Geophysics FocusGroup Near-Surface Planetary Focus Group DeepInterior Tetonophysics Geomagnetism Paleomagntism Study of Earth's Study of SciencesSection Membership Survey: Geomagnetism and Paleomagnetism (GP) Section List additional secondary affiliations (if any): Answer Options
    [Show full text]
  • Earth, Environmental and Planetary Sciences 1
    Earth, Environmental and Planetary Sciences 1 MATH 0090 Introductory Calculus, Part I (or higher) Earth, Environmental PHYS 0050 Foundations of Mechanics (or higher) Nine Concentration courses and Planetary Sciences EEPS 0220 Earth Processes 1 EEPS 0230 Geochemistry: Earth and Planetary 1 Materials and Processes Chair EEPS 0240 Earth: Evolution of a Habitable Planet 1 Greg Hirth Select three of the following: 3 Students in Earth, Environmental, and Planetary Sciences develop a EEPS 1240 Stratigraphy and Sedimentation comprehensive grasp of principles as well as an ability to think critically EEPS 1410 Mineralogy and creatively. Formal instruction places an emphasis on fundamental EEPS 1420 Petrology principles, processes, and recent developments, using lecture, seminar, EEPS 1450 Structural Geology laboratory, colloquium, and field trip formats. Undergraduates as well as graduate students have opportunities to carry out research in current fields Two additional upper level EEPS courses or an approved 2 of interest. substitute such as a field course One additional upper level science or math course with approval 1 The principal research fields of the department are geochemistry, from the concentration advisor. mineral physics, igneous petrology; geophysics, structural geology, tectonophysics; environmental science, hydrology; paleoceanography, Total Credits 13 paleoclimatology, sedimentology; and planetary geosciences. Emphasis in these different areas varies, but includes experimental, theoretical, and Standard program for the Sc.B. degree observational approaches as well as applications to field problems. Field This program is recommended for students interested in graduate study studies of specific problems are encouraged rather than field mapping for and careers in the geosciences and related fields. its own sake. Interdisciplinary study with other departments and divisions is encouraged.
    [Show full text]
  • The Upper Mantle and Transition Zone
    The Upper Mantle and Transition Zone Daniel J. Frost* DOI: 10.2113/GSELEMENTS.4.3.171 he upper mantle is the source of almost all magmas. It contains major body wave velocity structure, such as PREM (preliminary reference transitions in rheological and thermal behaviour that control the character Earth model) (e.g. Dziewonski and Tof plate tectonics and the style of mantle dynamics. Essential parameters Anderson 1981). in any model to describe these phenomena are the mantle’s compositional The transition zone, between 410 and thermal structure. Most samples of the mantle come from the lithosphere. and 660 km, is an excellent region Although the composition of the underlying asthenospheric mantle can be to perform such a comparison estimated, this is made difficult by the fact that this part of the mantle partially because it is free of the complex thermal and chemical structure melts and differentiates before samples ever reach the surface. The composition imparted on the shallow mantle by and conditions in the mantle at depths significantly below the lithosphere must the lithosphere and melting be interpreted from geophysical observations combined with experimental processes. It contains a number of seismic discontinuities—sharp jumps data on mineral and rock properties. Fortunately, the transition zone, which in seismic velocity, that are gener- extends from approximately 410 to 660 km, has a number of characteristic ally accepted to arise from mineral globally observed seismic properties that should ultimately place essential phase transformations (Agee 1998). These discontinuities have certain constraints on the compositional and thermal state of the mantle. features that correlate directly with characteristics of the mineral trans- KEYWORDS: seismic discontinuity, phase transformation, pyrolite, wadsleyite, ringwoodite formations, such as the proportions of the transforming minerals and the temperature at the discontinu- INTRODUCTION ity.
    [Show full text]
  • Magnetic Properties of Single and Multi-Domain Magnetite Under Pressures from 0 to 6
    GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L10612, doi:10.1029/2004GL019844, 2004 Magnetic properties of single and multi--domain magnetite under pressures from 0 to 6 GPa Stuart A. Gilder,1 Maxime LeGoff,1 Jean-Claude Chervin,2 and Jean Peyronneau1 Received 1 April 2004; revised 13 April 2004; accepted 21 April 2004; published 26 May 2004. [1] Using novel experimental methods, we measured the [3] How stress influences magnetic remanence also acquisition of isothermal remanent magnetization, direct depends on the size and shape of the substance. The field demagnetization, and alternating field demagnetization magnetizations of single domain (SD) and multi-domain of multi-domain (MD) and single domain (SD) magnetite (MD) magnetite grains react differently to imposed stresses. under hydrostatic pressures to 6 GPa. We find that Stress may cause the net spontaneous moment in a SD grain the saturation remanence of MD magnetite increases to reorient to a new position depending on the angle 2.8 times over initial, non-compressed values by 6 GPa, between the stress and magnetic easy axis direction, the while its remanent coercivity remains relatively constant. grain’s shape, etc. [Hodych, 1977]. The individual domains For SD magnetite, remanent coercivity and saturation in a multi-domain grain grow or shrink to compensate for remanence vary little from 0 to 1 GPa, increase markedly the stress, which modify the grain’s magnetic intensity and from 1 to 3 GPa, then plateau above 3 GPa. These new direction [Bogdanov and Vlasov, 1966]. In sum, two pro-
    [Show full text]
  • The Time-Averaged Geomagnetic Field: Global and Regional Biases for 0-5 Ma
    Geophys. J. Int. (1997) 131, 643--666 The time-averaged geomagnetic field: global and regional biases for 0-5 Ma Catherine L. Johnson’ and Catherine G. Constable2 Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W. Washington, DC 20015, USA Institute,forGeophysics and Planetary Physics, Scripps Institution of Oceanography, La Jollu, CA 92093-0225, USA Accepted 1997 June 27. Received 1997 June 12; in original form 1997 January 21 SUMMARY Palaeodirectional data from lava flows and marine sediments provide information about the long-term structure and variability in the geomagnetic field. We present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava-flow data. Time-averaged field models are constructed for normal and reverse polarity periods for the past 5 Ma, using the combined data sets. Non- zonal models are required to satisfy the lava-flow data, but not those from sediments alone. This is in part because the sediment data are much noisier than those from lavas, but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the Earth’s core. Different average field configurations for normal and reverse polarity periods are consistent with the palaeo- magnetic directions; however, the differences are insignificant relative to the uncertainty in the average field models. Thus previous inferences of non-antipodal normal and reverse polarity field geometries will need to be re-examined using recently collected high-quality palaeomagnetic data. Our new models indicate that current global sediment and lava-flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0-5 Ma time-averaged field, highlighting the need for the collection of additional high-latitude palaeomagnetic data.
    [Show full text]
  • Towards a Mineral-Physics Reference Model for the Moon's Core
    Towards a mineral-physics reference model for the Moon’s core Daniele Antonangeli Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC) UMR CNRS 7590, Sorbonne Universités – UPMC Univ. Paris 6, Muséum National d’Histoire Naturelle, IRD The Moon Moon seismology: the Apppgollo Lunar Surface Experiments Package after Wieczorek et al., Elements 2009 Moon’s seismic models Garcia et al., PEPI 2011 Weber et al., Science 2011 seismic investigation of the deepp(p)est lunar structures (> 900 km depth) remains very challenging mineral physics constraints density and sound velocities: the link between seismic observations and models Iron phase diagram vs. planetary cores P-T conditions hcp (or ε) structure stable at Earth’s core P-T conditions 1 fcc (or γ) structure stable at P-T conditions of cores of small telluric planets and satellites Tateno et al., Science 2010 Tsujino et al., EPSL 2013 fcc-iron EOS XRD in combination with multi-anvil press Tsujino et al., EPSL 2013 + thermodynamic constraints (metallurgy, phase relations) P-V-T relations at Moon’s conditions First checks on recent seismic models P-V-T relations ditdensity 767.6-787.8 g/cm3 We ber et al., SiScience 2011 What about sound velocities? Inelastic X-ray Scattering (IXS) Ein, kin, in E, Q • Energy transfer E = Eout –Ein (E<< Ein) • Momentum ttfransfer Q = kout – kin = 2k sin (/2) (koutkink) Directional analysis of the scattered photons Q Energy analysis of the scattered photons E Phonon dispersion curves and elasticity Ex. bcc‐V at HP Sound velocity from
    [Show full text]
  • DON ANDERSON Themes Developed Across Several Decades
    Don L. Anderson 1933–2014 A Biographical Memoir by Thorne Lay ©2016 National Academy of Sciences. Any opinions expressed in this memoir are those of the author and do not necessarily reflect the views of the National Academy of Sciences. DON LYNN ANDERSON March 5, 1933–December 2, 2014 Elected to the NAS, 1982 Don L. Anderson, the Eleanor and John R. McMillan Professor of Geophysics at the California Institute of Tech- nology, was a geophysicist who made numerous seminal contributions to our understanding of Earth’s origin, composition, structure and evolution. He pioneered applications of seismic anisotropy for global surface waves, contributed to discovering the seismic velocity discontinuities in the mantle’s transition zone and initi- ated their mineralogical interpretation, established deep insights into seismic attenuation, co-authored the most widely used reference Earth structure model, helped to establish the field of global tomography, and re-opened inquiry into the nature of hotspot volcanism and mantle stratification. He contributed to initiatives that upgraded By Thorne Lay global and regional seismic instrumentation. He served as Director of the Caltech Seismological Laboratory for 22 years, overseeing a prolific scientific environment, distilling many advances into his masterpiece book Theory of the Earth. His enthusiasm for debate and his phenomenal editorial efforts raised the scientific standard of students and colleagues alike. Don was a geophysicist of tremendous breadth, publishing about 325 peer-reviewed research papers between 1958 and 2014 in seismology, mineral physics, planetary science, tectonophysics, petrology and geochemistry. He had a remarkable familiarity with the literature across all these disciplines, keeping reprints sorted into a multitude of three-ring binders that he could access in an instant, uniquely positioning him to undertake the writing of his ambitious book, The Theory of the Earth, published in 1989.
    [Show full text]
  • ERTH 401: Introduction to Mineral Physics
    ERTH 401: Introduction to Mineral Physics Course Instructors: Bin Chen ([email protected]), Przemyslaw Dera ([email protected]) Office: POST 819B/E Phone: 956-6908 / 956-6347 Office Hours: by appointment Class location and time: POST 733 (M: 1:30-3:20pm; W: 12:30-1:20 pm) ERTH 401 Introduction to Mineral Physics (3) Scientific study of the materials that make up the Earth and other planets or moons. Properties and behaviors of minerals under high-pressure and temperature conditions found in Earth and planetary interiors. Pre: ERTH 301 and PHYS 272, or consent. (Alt. years) (Cross-listed as ERTH 701) DP ERTH 701 Physics of the Earth's Interior (3) Interpretation of geophysical and laboratory data to understand elastic and anelastic properties, composition, phase relationships, temperature distribution in the Earth. Pre: consent. (Alt. years) Pre: ERTH 301, Mineralogy (4), PHYS 272, Physics II, or consent DiVersification courses DP = Physical Science Course Description The great majority of the Earth and other Earth-like planets or moons is inaccessible to direct sampling. Our knowledge on the physics and chemistry of Earth’s and planetary interiors has relied on multidisciplinary inVestigations from seismology, geodynamics, and geochemistry, as well as mineral physics. Mineral physics plays a central role in Earth and planetary sciences by linking materials properties of planetary constituents with large-scale geological and planetary processes and seismic and astronomical obserVations. It proVides essential information on materials properties for the understanding of the nature and dynamics of planetary interiors. Beyond geology, it has many releVant connections with deVelopment of new adVanced materials for technological applications, such as superconductors, ferroelectrics, abrasiVes, etc.
    [Show full text]
  • EGU2010-1362, 2010 EGU General Assembly 2010 © Author(S) 2009
    Geophysical Research Abstracts Vol. 12, EGU2010-1362, 2010 EGU General Assembly 2010 © Author(s) 2009 Reconciling Seismology and Mineral Physics in the Mantle Brian L. N. Kennett Australian National University, Research School of Earth Sciences, Canberra ACT, Australia ([email protected], 61 2 62572737) The Earth is a complex 3-D mineral assemblage whose gradients are dominantly with depth. As a result much effort has been expended on trying to tie estimates of mineral properties at depth to indirect inferences about radial structure, e.g., from Seismology. At the scale of sampling of seismic waves most of the mantle appears isotropic, but the constituent minerals are not. There are therefore two issues to be resolved when making comparisons between information derived from equations of state and seismology: firstly, the quality of the estimators for individual minerals and secondly, the way that these mineral specific estimates are combined. The standard procedures work quite well, but if we are to be able to pin down effects such as those due to spin transition in iron atoms we need to improve the representations. In particular it is highly desirable that determinations of equations of state include information at the highest pressures (e.g., shock wave or ab initio) so that any estimate of properties is based on interpolants, rather than extrapolation from the pressure range of regular experiment. It is likely that most current estimates of seismic wavespeed variability in the mantle represent underestimates, yet such variability lies well outside the range of applicability of derivatives from a single adiabat, if indeed this is a suitable reference.
    [Show full text]
  • Deep Earth and Recent Developments in Mineral Physics
    Deep Earth and Recent Developments in Mineral Physics Jay D. Bass1 and John B. Parise2 DOI: 10.2113/GSELEMENTS.4.3.157 ery few rocks on the Earth’s surface come from below the crust. In are they made of? What are their thermal structures? What is the fact, most of Earth’s interior is unsampled, at least in the sense that nature of heat and mass transfer Vwe do not have rock samples from it. So how do we know what is down within and between them? there? Part of the answer comes from laboratory and computer experiments These are obviously difficult ques- that try to recreate the enormous pressure–temperature conditions in the tions, because most of the Earth is deep Earth and to measure the properties of minerals under these conditions. inaccessible by drilling or other This is the realm of high-pressure mineral physics and chemistry. By comparing direct means. However, it turns out that a great deal of relevant mineral properties at high pressures and temperatures with geophysical information has been accumulating observations of seismic velocities and density at depth, we get insight into over the past several hundred years. the mineralogy, composition, temperature, and deformation within Earth’s By 1800, the density of the Earth was known to be about 5.5 g/cm3, interior, from the top of the mantle to the center of the planet. very close to the currently accepted value of 5.52 g/cm3. Because this KEYWORDS: Earth’s interior, mineral physics, mantle, core, high pressure density is greater than the value for surface rocks (~2.6 g/cm3), we INTRODUCTION can immediately conclude that the bulk of Earth’s interior As far as we can tell from recorded history, there has always is much denser than rocks at the surface and, therefore, dif- been interest and curiosity about the interior of the Earth.
    [Show full text]
  • Paleomagnetism Indicates That Primary Magnetite in Zircon Records
    Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo John A. Tardunoa,b,1 , Rory D. Cottrella , Richard K. Bonoa,2 , Hirokuni Odac , William J. Davisd , Mostafa Fayeke , Olaf van ’t Ervef , Francis Nimmog , Wentao Huanga , Eric R. Thernh,i , Sebastian Fearnj , Gautam Mitraa, Aleksey V. Smirnovk , and Eric G. Blackmanb aDepartment of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627; bDepartment of Physics and Astronomy, University of Rochester, Rochester, NY 14627; cResearch Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8567, Japan; dGeological Survey of Canada, GSC-Central Canada, Geochronology, Ottawa, ON K1A 0E8, Canada; eDepartment of Geological Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada; fMaterials Science and Technology Division, Naval Research Laboratory, Washington, DC 20375; gDepartment of Earth and Planetary Sciences, University of California, Santa Cruz, CA 95064; hGeochron Research Group, Bentley, WA 6982, Australia; iDepartment of Applied Physics, Curtin University of Technology, Perth, WA 6001, Australia; jDepartment of Geoscience, University of Wisconsin–Milwaukee, Milwaukee, WI 53201-0413; and kDepartment of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI 49931 Edited by Lisa Tauxe, University of California San Diego, La Jolla, CA, and approved December 12, 2019 (received for review September 24, 2019) Determining the age of the geomagnetic field is of paramount tion on the billion-year timescales needed to explore the most importance for understanding the evolution of the planet because ancient geodynamo. Here we use “single-domain–like” in a the field shields the atmosphere from erosion by the solar wind.
    [Show full text]