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Open Questions in Geosciences Mapping Elements & Structure in the Earth Geophysics tells us where we are at today Geochemistry tells us how we got there… Collaborators: - Ricardo Arévalo, Mario Luong : UMD - Kevin Wheeler, Dave Walker : Columbia Univ - Corgne, Keshav & Fei : Geophysical Lab, CIW - John Learned : University of Hawaii - Steve Dye: Hawaii Pacific University 5 Big Questions: - What is the Planetary K/U ratio? planetary volatility curve - Radiogenic contribution to heat flow? secular cooling - Distribution of reservoirs in mantle? whole vs layered convection - Radiogenic elements in the core?? Earth energy budget - Nature of the Core-Mantle Boundary? hidden reservoirs How much Th, U and K is there in the Earth? Heat flow measurements Geochemical modeling Neutrino Geophysics Time Line 1st order Structure of Earth Rock surrounding metal 1897 Emil Wiechert 1915 1925 1935 1970 1995 PLATE TECTONICS Earth’s Total Surface Heat Flow • Conductive heat flow measured from bore-hole temperature gradient and conductivity Data sources Total heat flow Conventional view 463 TW Challenged recently 311 TW Urey Ratio and Mantle Convection Models radioactive heat production Urey ratio = heat loss • Mantle convection models typically assume: mantle Urey ratio: 0.4 to 1.0, generally ~0.7 • Geochemical models predict: mantle Urey ratio 0.3 to 0.5 Discrepancy? • Est. total heat flow, 46 or 31TW est. radiogenic heat production 20TW or 31TW give Urey ratio ~0.3 to ~1 • Where are the problems? – Mantle convection models? – Total heat flow estimates? – Estimates of radiogenic heat production rate? • Geoneutrino measurements can constrain the planetary radiogenic heat production. “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 U and Th (and K) Distribution in the Earth • U and Th (K?) are thought to be absent from the core and present in the mantle and crust. – Core: Fe-Ni metal alloy – Crust and mantle: silicates • U and Th (and K) concentrations are the highest in the continental crust. – Continents formed by melting of the mantle. – K, U and Th prefer to enter the melt phase • Continental crust: insignificant in terms of mass but major reservoir for U, Th, K. Th & U Volatility trend @ 1AU from Sun Silicate Earth Normalized concentration REFRACTORY ELEMENTS VOLATILE ELEMENTS Allegre et al (1995), McD & Sun (’95) Palme & O’Neill (2003) ? Lyubetskaya & Korenaga (2007) Potassium in the core Half-mass Condensation Temperature What is the K/U Th/U ratios for the Earth and modern mantle? sub-title: Implications and History -- implications: K, Th and U are the radioactive elements that provide the sum of the internal radiogenic heat for the planet -- history: Urey 1950s Wasserburg 1960s Jochum 1980s First observations -- got it right at the 1-sigma level SCIENCE Accepted as the fundamental reference and set the bar at K/U = 104 Th/U = 3.5 to 4.0 MORB (i.e., the Depleted Mantle ~ Upper Mantle) K/U ~ 104 and slightly sub-chondritic Th/U DM & Continental Crust – complementary reservoirs DM + Cc = BSE ahh, but the assumptions and samples… Two types of crust: Oceanic & Continental Oceanic crust: single stage melting of the mantle Continental crust: multi-stage melting processes Compositionally distinct Continental Crust Not simple to interpret, but K/U ~104 10 Th/U 8 6 4 2 Granites Loess and Shales 0 1000 10000 K/U 100000 Th/U ~ 4 Oeanic Crust K/U ~ 104 Th/U ~ 4 Crust above = basalt (melt, enriched in K, Th U) Mantle beneath = Refractory peridotite (residue, deplete in K, Th U) MORB Sample Locations a compositional spectrum PM Normalized MORBs: 10 Atlantic Ocean 1 Ba Th U Nb K La Ce Pb Pr Sr Nd Zr Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 100 1 Indian Ocean 0.1 PM Normalized 10 10 1 Pacific Ocean 0.1 Ba Th U Nb K La Ce Pb Pr Sr Nd Zr SmEuGd Tb Dy Ho Er TmYb Lu Ba Th U Nb K La Ce Pb Pr Sr Nd Zr SmEu Gd Tb Dy Ho Er Tm Yb Lu 100000 N-MORB E-MORB K/U 10000 MORB Average 19,200 ± 4300 (2Mean ± 590) 1000 100 1000 10000 K (g/g) Avg. K/U 2σ Mean 2σ RSD % n All MORB N-MORB E-MORB OIB BABB 19030 19208 19759 18139 15055 20185 495 586 420 1558 1240 949 4168 4267 2518 6423 3037 3550 22 22 13 35 20 13 71 53 36 17 6 14 *BABB not included in above diagram,nor in the MORB average Urey Ratio and Mantle Convection Models radioactive heat production Urey ratio = heat loss • Mantle convection models typically assume: mantle Urey ratio: 0.4 to 1.0, generally ~0.7 • Geochemical models predict: Urey ratio 0.3 to 0.5. Mantle is depleted in some elements (e.g., Th & U) that are enriched in the continents. -- models of mantle convection and element distribution Th & U poor Th & U rich 5 Big Questions: - What is the Planetary K/U ratio? planetary volatility curve - Radiogenic contribution to heat flow? secular cooling - Distribution of reservoirs in mantle? whole vs layered convection - Radiogenic elements in the core?? Earth energy budget - Nature of the Core-Mantle Boundary? hidden reservoirs Long-lived chronometer: 147Sm 143Nd (T1/2 = 106 Ga) 147Sm abundance decreased by only 3% in 4.56 Ga Short-lived chronometer: 146Sm 142Nd (T1/2= 103 Ma) 146Sm exists only in the first 500 Myr of solar system history 100000 Published ID-MS & ICP-MS Jochum et al. (1983) Our LA-ICP-MS Data K/U 10000 Basalts (MORB, OIB, BABB) 1000 100 1000 K (g/g) 19,000 ± 500 (2Mean) 10000