Statistics – Computation
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...2..521l&link_type=abstract
Treatise on Geochemistry, Volume 2. Editor: Richard W. Carlson. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp
Statistics
Computation
11
Scientific paper
The Earth's core was discovered in 1906, when Oldham inferred the existence of a low-velocity region inside the Earth from changes in the amplitude of compressional waves traveling through the Earth's interior (Oldham, 1906). Over the last century, a wealth of knowledge has been obtained on the nature and dynamics of the core ( Figure 1; Dziewonski and Anderson, 1981; Stacey, 1992). Residing in the center of the planet, the core has a radius of 3,480 km, more than half of the Earth's radius. It occupies roughly one-eighth of the Earth's volume, and accounts for nearly one-third of its mass. The mass fraction in the core is much higher than its volume fraction, because the density in the core is much higher than that of the mantle ( Figure 2). With the density jumping from 5.5 g cm-3 to 9.9 g cm-3, the density contrast at the core-mantle boundary (CMB) is the largest in the whole planet. Based on analyses of the velocity and attenuation of seismic waves, the core has been established to have a layered structure. The central part comprising less than 5% of the core's mass or volume is solid, while the rest is largely molten. The pressure in the core ranges from 136 GPa (1,380 kbar) at the CMB to 360 GPa at the very center. In order to stay in the liquid state under such high pressures, the temperature in the core must be high as well. The temperature of the liquid-solid interface at the inner-outer core boundary (ICB) is estimated at 5,400±400 K (e.g., Brown and McQueen, 1986; Boehler, 2000; Hemley and Mao, 2001). (5K)Figure 1. Cross-section of the Earth showing its layered structure (source Dziewonski and Anderson, 1981). (6K)Figure 2. Preliminary reference Earth model (PREM) (source Dziewonski and Anderson, 1981). Combining geochemical and seismological observations on the Earth with laboratory measurements on relevant materials, more than 80 wt.% of the core has been deduced to consist of iron. Other elements with significant concentrations in the core include nickel (˜5 wt.%) and one or more elements that are lighter than iron (e.g., Stevenson, 1981; Jacobs, 1987; Jeanloz, 1990). The age of the core has been determined using isotope geochronometers. According to the most recent measurements based on tungsten-hafnium systematics, most of the core-mantle segregation took place in less than 30Myr ( Yin et al., 2002; Kleine et al., 2002). In other words, the core is almost as old as the Earth itself. Core formation occurred as soon as the Earth accreted, or simultaneously with accretion.Despite its old age, the core is dynamically active. The geomagnetic field observed on the Earth's surface is believed to originate in the outer core through the convection of the liquid conductive metal (Jacobs, 1987). A number of observations have been made recently that are indicative of chemical reaction and dynamical coupling between the core and the mantle (e.g., Lay et al., 1998). Moreover, the core is widely accepted to be a major energy source for our planet. Not only did it acquire a large amount of energy early on during core-mantle segregation from radioactive decay of short-life isotopes, but it is also capable of producing more heat through solidification of the liquid outer core. Speculation suggests that the core contains a significant amount of potassium, which could have been generating heat over the history of the Earth (e.g., Jacobs, 1987; see Chapter 2.15).The surface of the core is only 2,900 km in depth. However, drilling into the Earth's interior turns out to be much more challenging than flying into outer space. Spacecrafts have reached Jupiter, more than 600 million km away from the Earth, but the deepest hole we have successfully drilled has reached less than 14 km below the Earth's surface. Volcanic eruptions are unlikely to bring up pristine samples of the core to the surface of the Earth. To date, the most direct observations of the core have come from seismological studies using remote-sensing techniques. Due to the complex structure of the Earth's interior, seismic investigations require extensive data coverage, sophisticated modeling and efficient data analytical methods. Deciphering geochemical core signatures carried by mantle plumes faces similar challenges. In addition, experimental and computational simulations have been hindered by the necessity to approach extreme pressure and temperature conditions prevalent in the core. For these reasons, many fundamental issues concerning the Earth's core remain controversial or poorly understood.Along with steady improvement in observational, experimental, and computational techniques, research interest in the Earth's core has been growing over the last few decades. Issues of current interest include the timing, duration, and mechanism of core formation, the identity and abundance of light elements in the core, the thermal and chemical evolution of the core, the possibility of ongoing radioactive decay in the core, evidence for continuing core-mantle interaction, structure and dynamics of the outer and inner core, and the origin, structure, and evolution of the geomagnetic field. Progress in the study of the Earth's core has been reviewed by a number of researchers with different perspectives (e.g., Stevenson, 1981; Jacobs, 1987; Jeanloz, 1990; Poirier, 1994; Hillgren et al., 2000). The aim of this chapter is to provide an updated summary of our understanding of the composition of the Earth's core, with an emphasis on experimental constraints. Another chapter focusing on cosmochemical and geochemical observations can be found in Chapter 2.15.We will start with a description of experimental and analytical techniques used in the studies of the Earth's core. Following a review of geophysical and geochemical evidence for iron being the most abundant element in the core, we will provide a summary of experimental data on the phase diagram, equation-of-state (EOS), and physical properties of iron and discuss their implications for the core composition. The discussion of the role of nickel will be brief, as the number of experimental studies of nickel is limited. A large portion of the chapter will be devoted to constraints on the light element composition of the core, a highly controversial subject, with direct implications for diverse topics concerned with the origin, evolution, and current state of the Earth. Finally, we summarize the latest experimental results on potassium, niobium, rhenium, and osmium, some of the interesting minor and trace elements in the core.
Fei Yingwei
Li Jiying
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