Planetary Accretion: Understanding the Formation of Earth's Composition, Slides of Geochemistry

Download Planetary Accretion: Understanding the Formation of Earth's Composition and more Slides Geochemistry in PDF only on Docsity! 1 GG325 L33, F2012 Lecture 33 Planetary Accretion – the raw materials and the final compositions Today – Guest Lecturer, Greg Ravizza. 1. Boundary conditions for Planetary Accretion GG325 L33, F2012 Growth and Differentiation of Planet Earth It is difficult to separate the planetary processes of a. accretion and b. differentiation (into core, mantle, crust, atmosphere/hydrosphere) because a number of features suggest they overlapped in time. Thus, we will work at this problem from both ends over the next few lectures, using as many of the Boundary Conditions for earth's early history as we can to infer a likely sequence of events. Today we examine constraints on planetary compositions at the “beginning” and “end” of the accretion sequence. Docsity.com 2 GG325 L33, F2012 Growth and Differentiation of Planet Earth compositional:
relative proportions of refractory, silicate, metal and volatile components on earth compared to the Sun and C1-3 chondrites
how and when these components were segregated into our present core, mantle, crust and atmosphere.
redox conditions during accretion.
density of earth and its neighboring planets. physical:
rates of accretion and thermal history of planetesimals GG325 L33, F2012 evidence from planetary density As per last lecture, the solar nebula probably originally had a fairly regular radial density gradient. (Density anomalies at Mercury, Mars and the Asteroids have been related to peculiarities early in the accretion process.) Compare that to modern planetary densities: Planet density (g/cm3) Mercury 5.42 -----------------------------| Venus 5.25 | stony Earth 5.52 | (terrestrial) Mars 3.94 -----------------------------| planets Asteroids 2.2 (Ceres) Jupiter 1.3 -----------------------------| Saturn 0.7 | gaseous Uranus 1.3 | planets Neptune 1.7 | Pluto 1.1 -----------------------------| Note: Silicate rocks at the earth's surface have mean density ~2.7 g/cm3. Thus, much denser material must exist inside the Earth for the planet to have a bulk density of 5.52. This material is in the metallic core, where the density is closer to 8 g/cm3. The core is ~32% of Earth’s mass. Docsity.com 5 GG325 L33, F2012 Chemical Evidence Goldschmidt’s Classification and the Geochemical Periodic Chart This classification was proposed in the 1920s by geochemist Victor Goldschmidt. It is qualitatively useful for describing the origin of the Earth from materials present in the early solar system—particularly once liquids (iron and silicate) had become important. Goldschmidt compared the distributions of elements in silicate, metal-rich and gas phases in 1. metal-ore smelter materials 2. meteorites, 3. the modern Earth. He recognized a pattern in how elements were distributed with other elements. GG325 L33, F2012 Goldschmidt Classification/Geochemical Periodic Chart Elements can be assigned to more than one group depending on the situation, so this scheme provides only generalities. Siderophile – iron liking (zero-valent Fe) Chalcophile – sulfide liking (S2-) lithophile – silicate liking ([SiO4]n, also O loving in practice) Atmophile – gas phase liking Docsity.com 6 GG325 L33, F2012 Goldschmidt Classification/Geochemical Periodic Chart The groups have a general relationship to the periodic chart, reflecting an underlying relationship to the electronic configurations of the elements in their common forms. GG325 L33, F2012 Element Relationships: Earth and C1 Chondrites Among three of the most important siderophile and lithophile elements, compositional data for the sun, chondrites and bulk Earth indicate that the Earth has higher Fe/Si and Mg/Si than the chondrites. Docsity.com 7 GG325 L33, F2012 …And the Earth is variably depleted in volatile elements (e.g., K, Rb, Cs, etc.) relative to chondrites. GG325 L33, F2012 0.001 0.01 0.1 1 10 Element Al Ca Sc Ti Sr Y Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Mg Si Li B Na Cl F K Rb Cs V Cr Fe W Co Ni P Mo Pd Re Rh OsIr Pt Ru Au Mn Ga Cu Zn In Sn As Cd Ag Sb Ge S Te Se Pb Tl Hg I Bi Br C Zr Refractory M od er at el y S id er op hi le & V ol at ile H ig hl y V ol at ile P M /C I Figure 11.10. Abundances of the elements in the Primitive Mantle compared to CI chondrites. The two primary processes that resulted in relative elemental loss were incorporation into the core and volatilization modified from White, Geochemistry S id er op hi le H ig hl y S id er op hi le Absolute value of close to 3 greater than C1 Chondrites reflects the loss of core components and volatiles from Earth’s primitive mantle Partially lost to the core Mostly lost to the core Mostly evap- orated Partialtly lost to the core and to evap- oration M o d e ra te ly v o la ti le More broadly, relative to C1 chondrite abundances, the Earth’s estimated primitive mantle (i.e., after removal of Fe to the core but assuming no extraction of crust from the mantle) has (see also Lecture 31): • refractory elements (Al, Ca, U, Th, Si, Ba, rare earths,…) ≈ 3 x C1 values. • depletions in other elements relative to C1 values (including Mg and Si). Docsity.com 10 GG325 L33, F2012 What does the differentiated Earth indicate about its composition during accretion? Seismological studies of the interior demonstrate that today the Earth is radially zoned, with layers of increasing density toward the center. GG325 L33, F2012 Density of the solid substrate can be inferred from the seismological data Docsity.com 11 GG325 L33, F2012 β phase From a combination of high-pressure mineralogy, modeling, and seismic studies of high-pressure minerals, we know the main phases associated with each of the major transitions in density in the mantle. GG325 L33, F2012 For the core, the proportions are: Fe (≈85%) Ni (≈5%) light elements (≈10%) The light elements are believe to mostly be O, S, Si and/or C. The light elements can not be directly determined; instead. They are inferred from the presumed conditions of core formation and remain a topic of debate. Docsity.com 12 GG325 L33, F2012 We can approximate the bulk major-element composition of the primitive upper mantle pretty closely by taking a C1 chondrite, removing most of its volatile elements, and removing most of the Fe and Ni (i.e., segregating them to the core). pyrolite is a slight modification of this major element composition. It is commonly used as a model for the starting composition of the mantle after core formation. GG325 L33, F2012 Continued differentiation of the Earth has caused the upper mantle to segregate into compositional domains (we’ll talk more about this soon). Differences in seismic anisotropy between two commonly inferred rock compositions (peridotite and pyroxenite or eclogite, both occasionally brought to the surface as xenoliths in volcanic rocks) are useful in modeling their distribution in the upper mantle. Peridotite must be the most abundant rock type in the upper mantle. Docsity.com