T21C-1977
Is there a signal from geofission neutrinos in reactor experiments?
Antineutrinos are the radiation of choice for probing the radioactivity in the Earth's interior because of their penetrating power. Geoneutrinos are copiously emitted in the radioactive decay of the heat producing elements (HPE) in the earth. Neutrinos emitted in the spontaneous fission of uranium are not generally considered as part of the geoneutrino studies because of their lower relative intensity compared to antineutrinos emitted in the decay of HPE. Antineutrino experiments with fission reactors are continuously in use for studying neutrino properties for over fifty years. A residual fission neutrino background was reported in these experiments. How much of this background could be from the geofission neutrino signal will be examined.
T21C-1978
Considerations for a Dedicated Geoneutrino Detector
A combination of several sources including: radiogenic heating, processes of mantle and core formation and differentiation, delayed radiogenic heating, earthquakes, and tidal friction account for the surface heat flux. Radiogenic heating is of much interest in various fields of geosciences. Inferences from recent experiments with reactor antineutrinos and solar neutrinos showed that the age of geoneutrinos is at hand. Because of the deep penetrating properties of the neutrinos this type of radiation in the decay of the heat producing elements (HPE) is ideally suited for the investigation of the deep interiors of the Earth compared to conventional radiometric methods for HPE employing alpha-, beta- and gamma rays. This presentation will address the considerations for a dedicated geoneutrino detector to be set up for investigating the interior regions all the way to the center of the Earth.
T21C-1979
Heat Loss of the Earth: a new estimate
Determination of the rate of Earth's energy loss requires a very large number of heat flux measurements in a
variety of geological settings. Difficulties in integrating the flux over the Earth surface stem from two facts.
One is that heat flux varies on a wide range of spatial scales and, in continents, is not a function of a single
variable such as geological age, for example. The other difficulty is that the data exhibit large scatter. Notable
advances in the interpretation of heat flux data have been made over the past decades. In the oceans,
these are due to a thorough understanding of hydrothermal circulation through oceanic crust and sediments.
In continents, sampling of old cratons is now adequate and systematic studies of heat flux and heat
production allow strong constraints on the crustal contribution to the surface heat flux.
Heat loss through the ocean floor cannot be determined using the raw data affected by hydrothermal
circulation and irregularities of sediment cover. Predictions of the "half-space" model for the conductive
cooling of oceanic lithosphere are consistent with heat flux measurements in selected "noise-free"
environments as well as with the bathymetry of the sea floor. They are also consistent with values of the
mantle temperature beneath oceanic ridges derived from petrology. This cooling model is also consistent with
numerical calculations of mantle convection with plates. Using an accurate determination of the area extent
of oceanic sea floor including marginal basins and accounting for enhanced heat flux over hot spots, we
estimated the rate of heat loss through the oceans to be 32±2 TW (1012 Watts). This result is valid
only for the present-day age distribution of sea floor and may have changed in the past due to the plates
configuration being different from the present.
For continents, bias due to the very uneven sampling of the surface heat flux is removed by area-weighting
the average. The average value is independent of the size of the sampling window. Estimates of Moho heat
flux beneath stable cratons obtained by several independent methods are within a range of 12-18
mW~m-2. We estimate the continental heat loss to be 14±1 TW of which continental heat
production accounts for about half. Therefore, we obtain that the total heat loss of the Earth is 46±3
TW.
T21C-1980
Conserving mass and energy in cooling models of oceanic lithosphere requires upper mantle origins for trends in subsidence and heat flux and indicates global power of 30 TW
One-dimensional conductive cooling models of ocean lithosphere fail to predict the lateral variation in oceanic heat flux and provide problematic calculations of subsidence, for reasons enumerated below. Our new model follows conservation laws and shows that bathymetric trends are tied to upper mantle temperature variations, given realistic values for thermal expansivity. Heat flux increases towards mid-ocean ridges due to (1) flux varying across upper mantle convection cells and (2) redistribution of mantle heat (Qmtl) by moving magma, and also by (3) hydrothermal circulation. Foremost, widespread, lateral, uptake of Qmtl as latent heat occurs during deep lithospheric melting but this energy is released near ridges through dike emplacement during seafloor spreading. Redistribution and energy conservation account for the local heat flux maximum near x=1200 km, heretofore unexplained. We show that the trend Qmtl(x) far from the ridge is consistent with behavior near the ridge and measured global power of <30 TW , which is compatible with quasi-steady-state conditions and an enstatite chondrite model for the Earth. Observables, such as the pattern of mid-ocean ridges on the globe, point to layered convection and lack of vigor, and gross characteristics of the Earth are supported by an enstatite chondrite model. Our analysis circumvents problems associated with 1-d conductive cooling models of the lithosphere: (1) Existing models replaced conservation of rock-mass with isostatic balance, which unwittingly created subsidence by converting lithosphere to ocean. (2) Half-space models incorrectly cancelled infinities. (3) Plate models omitted latent heat which is immense. (4) 1-d models only permit vertical contraction. These faulty constructs fitted seafloor depths through erroneous use of volumetric (αV=3αL) thermal expansivity coupled with great leeway in cross-multiplied parameters. The underlying premise that thermal aspects of lithosphere can be separately evaluated from those of the mantle is incorrect because the lithosphere is volumetrically significant (16% of the upper mantle), but not the main part of mantle circulation.
T21C-1981
Estimating Earth's Heat Flux
The fundamental difference between recent estimates of Earth's surface heat flux is whether heat conduction models of spreading ridges or strict acceptance of oceanic heat flow observations accurately represents oceanic heat flow. From a theoretical perspective, there are also questions about values of the sources of Earth's internal heat, particularly the radiogenic component. We address these issues using numerical models of heat flow at the ocean ridges and analyses of the relation between heat flow and radioactive heat production. The numerical model parameters include thermal conductivity variation with temperature, a fixed T-z profile at the ridge that follows the mantle liquidus, constant spreading rate, and constant heat flow into the base of the lithosphere. The output of the model is a 2-D temperature-depth grid that provides a comparison with various analytical models of oceanic heat flow. The relations between heat flow, radioactive heat production, tectonic evolution, and physical properties of the lithosphere show that conductive heat flow is predictable. Our results do not contradict previous work that places Earth's heat flux at 44 TW.
T21C-1982 INVITED
Where are U and He?
There is no plausible mechanism for creating a uniform distribution of U, Th and K in the mantle, or for having the mantle cool uniformly or for retaining undegassed reservoirs. Accretion of planets is an energetic affair that results in melting, degassing and differentiation rather than homogenization. A planet is stratified by volatility, composition, radioactivity and density by radial zone refining. Volatiles, including noble gases and heat producing elements, were sweated out of the materials that are now in the deep interior and concentrated into the outer shells. Delaminated and tectonically eroded crust and carbonatites from the slab are overlooked mantle components that account for missing U and Th and heat-flow; these oversights have promoted models of deep enriched, hidden and accessible, reservoirs. About half the continental crust is recirculating in the mantle at any given time and carbonatites contain upwards of a thousand times the concentrations of incompatible elements of primitive mantle. These components are also required to balance the concentrations in MORB and the surface crust. Most of the radioactivity that is not in the crust is in these enriched components in the upper mantle. There is little U, Th and K in the deep mantle. The concentration of heat production in the shallow mantle has not been allowed for in any model investigated by isotope geochemists or convection modelers although it was widely discussed by Birch, Armstrong and others prior to the widespread use of over- simplified geochemical box and reservoir, and whole mantle convection models. Stratification of the mantle is largely irreversible but the eclogite and carbonatite cycles are reversible on a short time scale because of the drastic lowering of the melting point. The upper mantle is mainly depleted and infertile harzburgite, populated by fertile blobs that contain essentially all of the heat producing elements. These blobs rise and fall, driven by phase changes (basalt-eclogite, eclogite-melt) and act as heat exchangers. Every part of the chain that starts out with K, U and Th and ends up with Ar, He, Ne, Pb, continental crust, heat and short-lived isotopes is plagued with paradoxes and conundrums. This indicates that the underlying assumptions, models and dogmas are wrong.
T21C-1983 INVITED
How to estimate the heat production of a 'hidden' reservoir in Earth's mantle
The possibility of a hidden geochemical reservoir in the deep mantle has long been debated in geophysics and geochemistry, because of its bearings on the structure of the core-mantle boundary region, the origin of hotspots, the style of mantle convection, the history of the geomagnetic field, and the thermal evolution of Earth. The geochemical nature of a hidden reservoir, however, has been estimated based on composition models for the bulk silicate Earth, although these models preclude, in principle, the presence of such reservoir. Here we present a new self-consistent framework to estimate the neodymium and samarium concentration of a hidden reservoir and also constrain the heat production of the bulk silicate Earth, based on the notion of early global differentiation. Our geochemical inference is formulated as a nonlinear inverse problem, and the permissible solution space, delineated by Markov chain Monte Carlo simulations, indicates that an early enriched reservoir may occupy ~13% of the mantle with internal heat production of ~6~TW. If a hidden reservoir corresponds to the D" layer instead, its heat production would be only ~4~TW. The heat production of the bulk silicate Earth is estimated to be 18.9±3.8~TW, which is virtually independent of the likely reservoir size.
T21C-1984 INVITED
Earth's Global Heat Loss: a New Look at the Oceans
The Earth is losing heat at a rate of 44.2 × 1012 W with about 70% occurring through the oceanic lithosphere. This estimate is based on measurements of heat flow on continents and at sea, filtered for extraneous skin effects that perturb some measurements rendering them not representative of heat loss for the deeper crust and mantle. This filter is particularly useful in young sea floor where shallow hydrothermal convection is ubiquitous. The advective heat flow for the entire oceanic ridge system is about 1013 W, with about two-thirds of the heat loss occurring in sea floor younger than 8 Ma. The mass flow necessary to produce this heat loss is probably greater than 1015 kg/yr, equivalent to cycling the entire mass of the oceans through the upper oceanic crust in less than a million years. Recent, misguided efforts to reduce substantially the estimate of Earth's heat loss are based on a fundamental misunderstanding of oceanic heat flow measurements and the role of hydrothermal convection in causing a systematic bias towards low heat flow in some regions of young sea floor. We review advances in understanding the hydrology of the oceanic lithosphere (Davis and Elderfield, 2004) and show how targeted experiments have confirmed the details of shallow hydrothermal convection. Building on recent discoveries as well as the earlier work of Sclater and Crowe (1979) that identified sediment cover and distance from exposed igneous basement as critical parameters affecting hydrothermal convection, we have revisited the current marine heat flow data set. We filter the data to eliminate heat flow sites that have less than 325 m of sediment and are within 90 km of an exposed seamount and find an exceptional fit (heat flow versus age) to simple cooling curves for oceanic lithosphere that are consistent with global heat loss of 44 TW.
T21C-1985
Global Heat Flow: A New Database and a New Approach
Armed with an updated and revised global heat flow database (GHFD 09), we estimate global heat loss resulting from secular cooling and radioactive heat generation. Since the last GHFD update in 1993, nearly 20,000 new data points have been incorporated, nearly doubling the size of the existing dataset. The prior estimate of the global heat loss rate is 44 TW, which is based on data from the previous version of the GHFD. We attempt both to refine and improve upon previous estimates by: (1) refining heat flow estimates for the oceanic lithosphere using bathymetry constrained cooling models; (2) improving estimates in continental data-poor regions using new radioactivity models based on geochemical and geophysical studies; and (1) correcting shallow boreholes for paleoclimate effects resulting from Pleistocene glaciation. Our new estimate of global heat loss is not expected to change more than ~10% as additional data are acquired. A revised global heat flow map will help constrain global patterns in mantle heat flow and models of global heat loss.
T21C-1986
Thermal Evolution, Contraction, and Isostasy of a Viscoelastic Ocean Lithosphere
The formation, evolution, and large-scale structure of oceanic lithosphere is the result of dominantly conductive cooling of upper mantle over a large temperature range (1300 K) and pressures up to ~6 GPa. Heat transport properties exhibit strong dependence on phase mineralogy and over this temperature and pressure range. In contrast, the semi-infinite and boundary layer (plate) models assume constant properties of dependent parameters. While the models appear to predict observations fairly well, faulty mathematics and a limited model phenomenology paint an incomplete and often unsatisfactory picture of reality which impedes exploration of finer structure. I introduce a numeric finite-difference model of oceanic lithosphere consisting of 3 major parts: 1) Thermal evolution is modeled with T-dependent thermal properties for composite mantle material phases, 2) A viscoelastic lithosphere model is used to calculate thermal stresses from cooling by linear contraction and 'absorption' into the vertical by viscous relaxation, and 3) an isostasy model that conserves mass is combined with the surface contraction of the viscoelastic model to predict net seafloor surface subsidence. Results show that direct application of recent measurements of mantle mineral parameters results in a thicker lithosphere that is significantly cooler near the surface and slightly warmer at depth. Prediction of surface heat flux depends on interdependence of specific heat and thermal conductivity. If specific heat is assumed constant at 1100 J/g K, then surface heat flux is close to predictions of the classical models. The viscoelastic model clearly shows that the effective expansivity is lower than has been assumed since a large amount of thermal stress cannot relax in low-T mantle, but is also much higher than the linear expansivity since horizontal (thermal) stresses are relaxed in the vertical direction. Lastly, the subsidence model conserves mass by accounting for displaced mantle which results in adjacent uplift (including ridges) and independently models integrated sources of surface contraction and isostatic adjustment.
T21C-1987
Heat Flow and Thermal Modeling of the Gyeongsang Basin and the Ulleung Basin in Korea
The Gyeongsang Basin located at the southeastern part of Korea has relatively high surface heat flow
(~72 mW/m2), and surface heat flow in the Gyeongsang Basin gradually increases from the
western Gyeongsang Basin (~55 mW/m2) toward the Ulleung Basin located in East Sea (~100
mW/m2).
In this study, to investigate the causes of high surface heat flow in the Gyeongsang Basin, we evaluate four
hypotheses that could cause surface heat flow variation in the Gyeongsang Basin. Four hypotheses include:
1) regional groundwater flow in the Gyeongsang Basin, 2) heat refraction caused by differences of crust
thickness related to East Sea opening (25~15 Ma), 3) volcanic activities occurred at ~5 Ma, and 4)
mantle heat flow variation in the transition zone between Korean continental margin and the Ulleung Basin.
Calculated Peclet Number for the Gyeongsang Basin is too low to affect surface heat flow variation in
Gyeongsang Basin. Therefore, groundwater flow could not be a cause of surface heat flow variation in the
Gyeongsang Basin. Additionally, heat refraction could not explain surface heat flow variation in the
Gyeongsang Basin, because a steady-state thermal model indicates heat refraction effect on surface heat
flow is negligible. A transient thermal model shows that the volcanic activities older than ~8 Ma could
not affect surface heat flow variation in the Gyeongsang Basin. Consequently, considering that the age of
the youngest volcanic rock is ~15 Ma in the study area, the volcanic activity also could not explain
surface heat flow variation in the Gyeongsang Basin. Finally, a thermal model for the mantle heat flow
variation effect shows that the calculated surface heat flow from the thermal model agrees with observed
surface heat flow.
A mechanism for describing mantle heat flow variation in the deep subsurface could relate to back-arc basin
(East Sea). The asthenosphere rises near the transition zone related to East Sea extension due to thermal
buoyancy, and sinks near the mantle wedge due to ocean plate (subducting slab) friction. Consequently, the
asthenosphere circulation (mantle convection) in back-arc basin could gradually increase the mantle heat
flow in the transition zone.
Therefore, high surface heat flow of the Gyeongsang Basin is likely to be caused by gradually increasing
mantle heat flow in the transition zone.