Earth's Heat Loss Research Articles

Hydrothermal Models Constrained by Fine‐Scale Seismic Velocities Confirm Hydrothermal Cooling of 7–63 Ma South Atlantic Crust

AbstractAlthough 70% of the Earth's heat loss occurs in the oceans, the nature of hydrothermal heat flow in oceanic crust is controversial. Lithospheric cooling models, heat flow measurements, and seismic experiments provide conflicting accounts on the longevity of hydrothermal systems and their efficiency at removing heat from the crust. Here we present five hydrothermal models along a crustal flowline in the western South Atlantic at ∼31°S to quantify how conductive and advective heat loss change as a function of crustal age and structure. The model sites cover crustal ages of 7–63 Ma and are collocated with planned drill sites of International Ocean Discovery Program Expeditions 390 and 393. We constrained our hydrothermal models with detailed physical property distributions that we estimated from new high‐resolution seismic velocity models. The hydrothermal models yield conductive and advective heat fluxes that closely match lithospheric cooling models and conductive heat flow measurements on the seafloor, supporting a hydrothermal sealing age of ∼65 Ma. Our results also agree with global estimates of fluid volume flux into the oceans and are consistent with a previously published analysis of upper crustal seismic velocities in the study area, indicating ongoing hydrothermal activity at relatively old crustal ages. This study broadly confirms and unifies existing concepts of oceanic heat flow and its modes of transport. Moreover, it provides a regional framework of seismic velocities and modeled hydrothermal fluxes, in which future in‐situ measurements can be integrated.

Subseafloor Cross‐Hole Tracer Experiment Reveals Hydrologic Properties, Heterogeneities, and Reactions in Slow‐Spreading Oceanic Crust

AbstractThe permeability, connectivity, and reactivity of fluid reservoirs in oceanic crust are poorly constrained, yet these reservoirs are pathways for about a quarter of the Earth's heat loss, and seawater‐rock exchange within them impact ocean chemical cycles. We present results from the second ever cross‐hole tracer experiment within oceanic crust and the first conducted during a single expedition and in slow‐spreading crust west of the Mid‐Atlantic Ridge at North Pond. Here we employed boreholes that were drilled by the Integrated Ocean Drilling Program (Sites U1382 and U1383) that were instrumented and sealed. A cesium salt solution and bottom seawater tracer experiment provided a measure of the minimum Darcy fluid velocity (2 to 41 m/day) within the upper volcanic crust, constraining the minimum permeability of 10−11 to 10−9 m2. We also document chemical heterogeneities in crustal fluid compositions, rebound from drilling disturbances, and nitrification within the basaltic crust, based on systematic differences in borehole fluid compositions over a 5‐year period. These results also show heterogeneous fluid compositions with depth in the borehole, indicating that hydrothermal circulation is not vigorous enough to homogenize the fluid composition in the upper permeable basaltic basement, at least not on the time scale of 5 years. Our work verifies the potential for future manipulative experiments to characterize hydrologic, biogeochemical, and microbial process within the upper basaltic crust.

The diversity and evolution of late-Archean granitoids: Evidence for the onset of “modern-style” plate tectonics between 3.0 and 2.5Ga

The end of the Archean aeon (3.0–2.5 Ga) was a period of fundamental change in many aspects of the geological record. In Archean cratons, this timespan is marked by a considerable diversification in both the nature and petrogenesis of granitoid rocks. In this article, we review the nature, petrogenesis and global evolution of late-Archean granitoids and discuss their geodynamic significance. Late-Archean granitoids can be classified into four groups: (1) volumetrically-dominant and juvenile tonalites, trondhjemites and granodiorites (TTGs), whose geochemistry is consistent with an origin through partial melting of meta-igneous mafic rocks at various pressures; (2) Mg-, Fe- and K-rich, metaluminous (monzo)diorites and granodiorites, referred to as sanukitoids s.l., which derive primarily from hybridization between mantle peridotite and a component rich in incompatible elements; (3) peraluminous and K-rich biotite- and two-mica granites, formed through melting of older crustal lithologies (TTGs and meta-sediments, respectively); and (4) hybrid high-K granites with mixed characteristics from the first three groups. The chronology of granitoid emplacement in late-Archean times is different from one craton to another but, in general, follows a very specific two-stage sequence: (1) a long period (0.2–0.5 Ga) of TTG emplacement; (2) a shorter period (0.02–0.15 Ga) during which all other granitoid types were generated. We propose that this sequence represents the first global subduction–collision cycle in the Earth's history. Although possibly present in the geological record prior to 3.0 Ga, such mechanisms became progressively prevalent on a planetary scale only between 3.0 and 2.5 Ga, indicating that the late-Archean geodynamic changes resulted from the global initiation of “modern-style” plate tectonics. The Archean–Proterozoic transition thus represents a major change in the mechanisms of the Earth's heat loss: before 3.0–2.5 Ga, it took place by large-scale magmatic differentiation characterized by generation of proto-continents that underwent crustal maturation locally, but without obvious cyclic activity on a planetary scale. After this, heat loss became accommodated by plate tectonics and global Wilson subduction–collision cycles. These changes were the consequence of the Earth's cooling, which in turn controlled a number of different parameters locally (thickness, temperature, volume and rheology of the crust). This explains why the changes took place over a short timespan (~ 0.5 Ga) relative to the Earth's history, but at different times and with different characteristics from one craton to another.

Why Was Kelvin's Estimate of the Earth's Age Wrong?

This is a companion to our previous paper1 in which we give a published example, based primarily on Perry's work,2,3 of a graph of ln y versus t when y is an exponential function of t. This work led us to the idea that Lord Kelvin's (William Thomson's) estimate of the Earth's age was wrong not because he did not account for radioactivity, as is commonly believed,4 but because he used the wrong model for Earth's heat loss. We feel this idea is worth spreading. To this end (following England et al.2,3), we examine two questions, the first about the radioactivity part and the second about Perry's alternate model for Earth's heat loss.

Effects of continental insulation and the partitioning of heat producing elements on the Earth's heat loss

Continental lithosphere influences heat loss by acting as a local insulator to the convecting mantle and by sequestering heat‐producing radioactive elements from the mantle. Continental heat production can have a two‐part effect since it decreases the amount of internal heat driving convection, which lowers mantle temperature, while also increasing the local insulating effect of continental lithosphere, which raises mantle temperature. We explored these competing effects using simulations that incorporated enriched continents within a mixed internal‐ and bottom‐heated convecting mantle. Increasing continental surface area was found to enhance global heat loss for a range of heat production distributions and Rayleigh numbers. The effect of enriched continents was evident as a double peak in the continental surface area values that maximize global heat loss. That the presence of continental lithosphere could increase average mantle temperature despite the mantle being depleted suggests that continents can significantly influence mantle potential temperature.

Introduction to Special Section: Hawaii Scientific Drilling Project

Intraplate or “hot spot” volcanic island chains, exemplified by Hawaii, play an important role in plate tectonic theory as reference points for absolute plate motions, but the origin of these volcanoes is not explained by the plate tectonic paradigm [Engebretson et al., 1985; Molnar and Stock, 1987; Morgan, 1971, 1981, 1983; Wilson, 1963]. The most widely held view is that these chains of volcanoes form from magma generated by decompression melting of localized, buoyant upwellings in the mantle [Ribe and Christensen, 1994; Richards et al., 1988; Sleep, 1990; Watson and McKenzie, 1991]. These upwellings, or “plumes,” are believed to originate at boundary layers in the mantle (e.g., at the core‐mantle boundary or near the boundary at ∼670 km between the upper and lower mantle), and the cause of the buoyancy may be both compositional and thermal [Campbell and Griffiths, 1990; Griffiths, 1986; Richards et al., 1988; Watson and McKenzie, 1991]. Mantle plumes are responsible for about 10% of the Earth's heat loss and constitute an important mechanism for cycling mass from the deep mantle to the Earth's surface.

Heat flow from the Earth's interior: Analysis of the global data set

We present a new estimate of the Earth's heat loss based on a new global compilation of heat flow measurements comprising 24,774 observations at 20,201 sites. On a 5° × 5° grid, the observations cover 62% of the Earth's surface. Empirical estimators, referenced to geological map units and derived from the observations, enable heat flow to be estimated in areas without measurements. Corrections for the effects of hydrothermal circulation in the oceanic crust compensate for the advected heat undetected in measurements of the conductive heat flux. The mean heat flows of continents and oceans are 65 and 101 mW m−2, respectively, which when areally weighted yield a global mean of 87 mW m−2 and a global heat loss of 44.2 × 1012 W, an increase of some 4–8% over earlier estimates. More than half of the Earth's heat loss comes from Cenozoic oceanic lithosphere. A spherical harmonic analysis of the global heat flow field reveals strong sectoral components and lesser zonal strength. The spectrum principally reflects the geographic distribution of the ocean ridge system. The rate at which the heat flow spectrum loses strength with increasing harmonic degree is similar to the decline in spectral strength exhibited by the Earth's topography. The spectra of the gravitational and magnetic fields fall off much more steeply, consistent with field sources in the lower mantle and core, respectively. Families of continental and oceanic conductive geotherms indicate the range of temperatures existing in the lithosphere under various surface heat flow conditions. The heat flow field is very well correlated with the seismic shear wave velocity distribution near the top of the upper mantle.

Parameterized thermal convection in a layered region and the thermal history of the Earth

Parameterized convective schemes for constant viscosity fluids are used to investigate the influence of layering on the relationship between the rate of heat loss from and heat supply to a number of convecting layers of different viscosity and thicknesses. Simple analytic expressions are obtained which accurately predict the behavior to a step decrease in the heat supply, and show that layering considerably increases the response time. The same arguments are applied to a planet consisting of spherical shells. They show that the response of the earth's heat loss to the decay of radioactive elements depends strongly on whether or not mantle convection extends from the core to the surface. If it does so, the surface heat flux is only about 50% greater than the heat generation rate. However if upper and lower mantle convection occurs in two separate shells, as seismological and geochemical arguments suggest, the response time is increased. The present surface heat flux is then principally controlled by the thermal structure of the earth at the time it was formed. The best estimate of the present ratio of the heat loss to heat generation is about two, and this value is consistent with the behavior of an earth model consisting of a two‐layered mantle overlying a core.

Submarine geothermal resources

Approximately 20% of the earth's heat loss (or 2 × 10 12 cal/s) is released through 1% of the earth's surface area and takes the form of hydrothermal discharge from young (Pleistocene or younger) rocks adjacent to active seafloor-spreading centers and submarine volcanic areas. This amount is roughly equivalent to man's present gross energy consumption rate. A sub-seafloor geothermal reservoir, to be exploitable under future economic conditions, will have to be hot, porous, permeable, large, shallow, and near an energy-deficient, populated land mass. Furthermore, the energy must be recoverable using technology achievable at a competitive cost and numerous environmental, legal and institutional problems will have to be overcome. The highest-temperature reservoirs should be found adjacent to the zones of the seafloor extension or volcanism that are subject to high sedimentation rates. The relatively impermeable sediments reduce hydrothermal-discharge flow rates, forcing the heat to be either conducted away or released by high-temperature fluids, both of which lead to reservoir temperatures that can exceed 300°C. There is evidence that the oceanic crust is quite permeable and porous and that it was amenable to deep (3–5 km) penetration by seawater at least some time in the early stages of its evolution. Most of the heat escapes far from land, but there are notable exceptions. For example, in parts of the Gulf of California, thermal gradients in the bottom sediments exceed 1°C/m. In the coastal areas of the Gulf of California, where electricity and fresh water are at a premium, this potential resource lies in shallow water (< 200 m) and within sight of land. Other interesting areas include the Sea of Japan, the Sea of Okhotsk and the Andaman Sea along the margins of the western Pacific, the Tyrrhenian Sea west of Italy, and the southern California borderland and west flank of the Juan de Fuca Ridge off the west coast of the United States. Many questions remain to be answered about the physical and other characteristics of these systems before they can be considered a viable resource. Until several of the most promising areas are carefully defined and drilled, the problem will remain unresolved.

The driving mechanism of plate tectonics

With the general acceptance of plate tectonics there is the question of its cause. An energy source is required which can account for the earth's heat loss, seismicity, volcanism, and tectonics. The probable energy source is the heat production due to radioactive decay of uranium, thorium, and potassium [Lliboutry, 1972; Turcotte and Oxburgh, 1972]. Alternative energy sources are differentiation processes in the core and mantle and solid earth tides.