Origin and Early Evolution of the Earth: a volatile elements perspective Cider 2010 Bill McDonough Geology, University of Maryland Support from:
Time Line 1st order Structure of Earth Rock surrounding metal 1897 Emil Wiechert 1915 CORE-MANTLE 1925 UPPER-LOWER MANTLE INNER-OUTER CORE 1935 PLATE TECTONICS 1970 1995
5 Big Questions: • What is the Planetary K/U ratio? • Is the mantle in-gassing or de-gassing? • Distribution of volatiles in mantle? • Volatiles in the core? • Volatiles at Core-Mantle Boundary? planetary volatility curve secular changes whole vs layered convection Light element in the core hidden reservoirs
Role of Giant Impacts: volatiles • Earth’s volatile budget was likely shaped by Mars-sized impacted events. • Did the late veneer introduce HSE and volatiles? • Differences in the volatile budget of the Earth and Moon?
More volatile questions • What are the volatile elements? • What are their abundances in the Earth? • When did we inherit them? • How did we inherited? • Is there a secular variation in the volatile elements abundances of the Earth?
Volatiles: defined • - H2O, CO2, N2, CH4, (i.e., H, C, N, O) • Noble gases (group 18 elements) • - elements with half-mass condensation T <1250 K • - elements readily degassed (e.g., Re, Cd, Pb…) • - chalcogens (group 16: i.e., O, S, Se and Te) • - halides (group 17: i.e., F, Cl, Br, I)? • alkali metals (group 1: Cs, Rb, K…)?
What are the moderately volatile elements? Refractory >1400 K “Si, Mg, Fe, Ni… 1350 to 1250 K Moderately volatile 1250 to 650 K Volatile <650 K classified affinity where Siderophile iron core Lithophile oxide mantle Chalcophile sulfur mostly core Redox conditions in the Solar System…..
What is the composition of the Earth? and where did this stuff come from? Nebula Meteorite Heterogeneous mixtures of components with different formation temperatures and conditions Planet: mix of metal, silicate, volatiles
Volatiles: distribution • Atmosphere (N2 78%, O2 21%, Ar 1%, other) • Mantle volatiles: H2O, C(C, CO2, CO, CH4), • sulfides, etc • Core volatiles: FeC, FeN, FeO, FeS, FeH
Rings aroundbPictoris Okamoto et al (2004, Nature) 63 light yrs away Astro-mineralogy -- determine size, crystal structure and chemistry of dust grains in space, often around protostars (observations usually at mid-infrared wavelengths (2–30 mm)). 0.1 μm- and 1.5 μm-sized olivine, pyroxene and quartz.
What’s in the ISM (interstellar medium) Mid-infrared spectroscopy (IRS) Spitzer X-ray absorption fine structure (XAFS) Chandra <2.2% crystallinity in silicate exist indiffuse ISM. In the Galactic ISM Si exists in the form of silicates, whereas a significant fraction of S exists in the gas phase. ISM/solar O/Si 0:63 ± 0:17 Mg/Si 1:14 ± 0:13 S/Si 1:03 ± 0:12 Fe/Si 0:97 ± 0:31 The ratio of Mg to Fe in olivine is >1.2 and 15%–37% of the total O atoms in the ISM must be contained in silicate grains.
Star (~1 Myr) with a clearing disk low-mass pre–main-sequence star Spitzer Space Telescope Infrared Spectrograph Glassy olivine Cleared out D’Alessio et al (2005) ApJ
Astro-Mineralogy Von Boekel et al (2004; Nature) HD142527 inner disk Olivine Pyroxene hydrosilicate ISM
“Standard” Planetary Model • Chondrites, primitive meteorites, are key • So too, the composition of the solar photosphere • Refractory elements (RE) in chondritic proportions • Absolute abundances of RE – model dependent • Mg, Fe & Si are non-refractory elements • Chemical gradient in solar system • Non-refractory elements: model dependent • U & Th are RE, whereas K is moderately volatile
H O C N Solar photosphere (atoms Si = 1E6) B Li C1 carbonaceous chondrite (atoms Si = 1E6)
Inner nebular regions of dust to be highly crystallized, Outer region of one star has - equal amounts of pyroxene and olivine - while the inner regions are dominated by olivine. Boekel et al (2004; Nature) Olivine-rich Ol & Pyx
Mg/Si variation in the SS Forsterite -high temperature -early crystallization -high Mg/Si -fewer volatile elements Enstatite -lower temperature -later crystallization -low Mg/Si -more volatile elements
Olivine Potential temperature gradient Pyroxene
EARTH CO CV CI CM H LL L EL EH
Earth @ 1 AU Mars @ 2.5 AU Olivine-rich EARTH CO CV CI CM H LL L MARS SS Gradients EL -thermal -compositional -redox EH Pyroxene-rich
Planetary Compositional Models - Earth Mg/Si -- unknown needs to be fixed Hidden reservoirs -- maybe? 142Nd Early Earth Reservoir -- unlikely “Chondritic Earth” -- yes, (RLE)! but… 5) Future research -- geoneutrinos -KamLAND, Borexnio, SNO+, etc
Earth is “like” an Enstatite Chondrite! Mg/Si -- is very different shared isotopic Xi -- O, Cr, Mo,Ru, Nd, shared origins -- unlikely core composition -- no K, U in core.. S+ “Chondritic Earth” -- lost meaning… 6) Javoy’s model? -- needs to be modified
Ca, Al, REE, K, Th & U Lithophile elements Atmophilie elements Core Mantle Fe, Ni, P, Os Siderophile elements
Si Fe Mg weight % elements
Th & U Volatility trend @ 1AU from Sun
Silicate Earth REFRACTORY ELEMENTS VOLATILE ELEMENTS Allegre et al (1995), McD & Sun (’95) Palme & O’Neill (2003) ? Lyubetskaya & Korenaga (2007) Normalized concentration Potassium in the core Half-mass Condensation Temperature
Core elements remaining in the Silicate Earth Siderophile*and Chalcophile* *dominant chemical characteristic, but not an exclusive definition
4 most abundant elements in the Earth: Fe, O, Si and Mg 6 most abundance elements in the Primitive Mantle: - O, Si, Mg, and – Fe, Al, Ca This result and 1st order physical data for the core yield a precise estimate for the planet’s Fe/Al ratio : 20 ± 2
What’s in the core? What would you like? Constraints: density profile, magnetic field, abundances of the elements, Insights from: cosmochemistry, geochemistry, thermodynamics, mineral physics, petrology, Hf-W isotopes (formation age) How well do we know some elements?
Core compositional models Model 1 Model 2 others
Earth’s D/H ratio • Do we really know comets • D/H ratio of the oceans • What do chondrites tell us? • Source of water and other volatiles vs the sources of noble gases? Ref: Owen and Bar-Nun, in R. M. Canup and K. Righter, eds., Origin of the Earth and Moon (2000), p. 463
Last CIDER report on volatiles in the Earth - Saal et al 2009 Progress Report Conclusions: Approximate concentrations Depleted Mantle H2O 50 ppm; CO2 20 ppm; Cl 1 ppm; F 7 ppm Enriched Mantle H2O 500 ppm; CO2 420 ppm; Cl 10 ppm; F 18 ppm Total Mantle H2O 366 ppm; CO2 301 ppm; Cl 7 ppm; F 15 ppm • Earth: 61024 kg Oceans: 1.41021 kg • Ordinary chondritic planet -- 4 oceans • Carbonaceous chondritic planet -- 600 oceans • Enstatite chondritic planet -- ~2-4 oceans
Volatile Budget! H/C ratio of the bulk silicate Earth is superchondritic, owing chiefly to the high H/C ratio of the exosphere. H/C ratio of the mantle is lower than that of the exosphere, requiring significant H/C fractionation during ingassing or outgassing at some point in Earth history. Hirschmann and Dasgupta (2009)
Earth’s volatiles from chondrites? Let’s hear from what Sujoy has to say!…
When it comes to volatiles…. remember, always, the words of Francis Birch (1952) Unwary “readers” should take warning that ordinary language undergoes modification to a high-pressure form when applied to the interior of the Earth. A few examples of equivalents follow: High-pressure form Ordinary meaning certain dubious undoubtedly perhaps positive proof vague suggestion unanswerable argument trivial objection pure iron uncertain mixture of all the elements