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Transcript
Differentiation of the Eath Sujoy Mukhopadhyay Harvard University CIDER 2008 What we will cover in this lecture Short lived nuclides Differentiation of the Earth (Core, crust, and atmospheric formation) Planet Formation in its simplest form The great temperature differences between the hot inner regions and the cool outer regions of the nebula determined what kinds of condensates were available to form planets. Near Mercury’s orbit, metal started to condense. Moving outwards to Venus and Earth, more rock condensed Only beyond the frost line, which lay between the present-day orbits of Mars and Jupiter, were temperatures low enough for hydrogen compounds to condense into ices. 4567.2±0.6 Ma (Amelin et al., 2002) What is the age of the Earth Chondrules -> 4564.7±0.6 Ma (Amelin et al., 2002) Eucrites (achondrites) ~4560 Ma So did the Earth also form within a few (ten) million years after the start of the solar system? Extinct radionuclides and early Earth history (182Hf Æ 182W, 146Sm Æ 142Nd, 129I Æ 129Xe Age equation for a long-lived radioactive element Pr Dr Dr + (e λt − 1) = D s today D s initial D s today Measurable quantities For an extinct radionuclide we have Dr Dr Pr = + D s today D s initial D s initial But Pr does not exist anymore….. Dr Dr Pr Ps = + D s today D s initial Ps initial D s Pr is radioactive parent Dr is daughter produced from radioactive decay of Pr Ds is stable isotope of Dr and not produced by radioactive decay Ps stable isotope of Pr λ is the decay constant Extinct radionuclides: 182Hf Æ 182W Dr Ds 182 W 183 W today 182 Dr Pr = + today D s initial Ps initial W 183 W = today 182 W 183 W + initial t2 W 183 W initial 182 Hf 180 Hf initial 180 Hf 183 W 182 Hf 182 Hf 180 = 180 exp[−λ∆t ] Hf 2 Hf 1 ∆t = t 2 − t1 slope=(Pr/Ps)=182Hf/180Hf t1 182 Ps Ds 180 Hf 183 W Hafnium –Tungsten (182Hf-182W) systematics; t1/2 = 9 M.y Hf is highly lithophile, retained in the silicate portion of the Earth. W is moderately siderophile, most enters core, some retained in the silicate mantle. So ideal for tracking core formation Silicate mantle Core Growth Evolution due to decay of 182Hf t1/2 = 9 m.y 182W/183W 182W* 0 Silicate Mantle z Bulk Earth Core Time (Ma) 60 So what do the data look like? Compared to primitive meteorites, Earth’s mantle has an excess of 182W. Core formation must have happened when 182Hf was still alive. Mean time of core formation was 11 Myr and coreformation was completed within 30 Myr Moon forming impact at ~30 Myr (Yin et al 2002; Jacobsen Kleine et al. 2002, Yin et al. 2002 2005) ε is the deviation of a sample from a reference or standards in parts per 10,000 Yin et al., 2002 Mean time of core formation was 11 Myr and core-formation was completed within 30 Myr Moon forming impact at ~30 Myr (Yin et al 2002; Jacobsen 2005) Was there equilibration between metal and silicate during core formation? Nimmo and Agnor, 2006 Nimmo and Agnor, 2006 Nimmo and Agnor, 2006 Without significant metal-silicate equilibration during accretion silicate Earth would end up too radiogenic in ε182W Has the final word on the moon forming impact been said? Recent Hf-W data from the moon Touboul et al. (2007) Lithophile – silicate loving Siderophile – iron loving Chalcophile – sulfur loving Atmophile – species prefer gas/ fluid phase Ratio of these elements are chondritic Æ used to argue for a late veneer after core formation Elemental abundance versus 50% condensation temperature (Wood et al., 2006) Relative to volatility trend, some elements are grossly depleted in silicate portion of the earth Samarium –Neodymium (146Sm-142Nd) systematics; t1/2 = 103 M.y Both Sm and Nd are lithophile. Both are incompatible. However Nd is more incompatible than Sm. So ideal for tracking crust-mantle differentiation. Crust Silicate mantle The early differentiation of the Earth’s mantle: evidence from 142Nd 146Sm Æ 147Sm Æ 142Nd; t1/2= 103 My. 143Nd; t 1/2 = 109 Gy Early crust-mantle differentiation; ~30 Myr after start of the solar system. Boyet and Carlson (2005) Boyet and Carlson (2005) attributed the 142Nd difference due to formation of an early enriched layer (at ~ 30 Myr) that subsequently sank back into the mantle; this hidden layer is not sampled today at either mid ocean ridge volcanism or ocean island volcanism. The enriched layer would be enriched in heat producing radioactive elements But are there other interpretations? But what we one assumes that the different stellar components were not well mixed in the solar system? Is it possible that because of incomplete mixing of r- and s- process components Earth and meteorites had different starting 142Nd/144Nd values? Solar Nebula Earth Meteorite Parent Bodies (142Nd/144Nd)0,a (142Nd/144Nd)0,b To test this idea one needs to find another heavy element having isotopes produced by both the r- and s-process but not having any significant contribution from radioactive decay. Barium Æ isotopes are not affected by radioactive decay Chondrites have an excess of r-process produced Ba-isotopes Chondrite Ba Normalized to Earth Ba 0.7 0.6 0.5 εBa 0.4 Allende Murchison Bruderheim Guarena Grady 0.3 Normalizing isotopes 0.2 0.1 0.0 -0.1 133 s r,s s r,s r,s 134 135 136 137 138 Ba Mass Ratio (i/136) Ranen and Jacobsen, 2006 139 The Ba isotopic differences can only be formed by incomplete mixing of stellar components between the Earth and Meteorite parent bodies (i.e. decay of radioisotopes cannot contribute to these variations) So is the Earth chondritic? (Ranen 2008) The results for Ba predict a negative 142Nd effect in chondrites compared to Earth as observed Æ so did the Earth really undergo an early episode of crustal differentiation? How constant is 146Sm/144Sm? Ranen (2008) The effect of in-homogeneous 146Sm in the solar system (Ranen 2008) Evidence of Early Crust {3.7-3.8 Ga Isua metasediments Derived from a mantle source that has already been depleted in rare earth elements (Caro et al.,2003) MORB source as the complement to crust formation Continental crust Mid-ocean ridge basalt The trace element pattern of mid-ocean ridge basalts can be modeled as resulting from melting a source previously depleted of incompatible elements to form the continental crust. The isotopic composition of MORBs can be modeled to generate growth curves for continental crust Using 143Nd/144Nd to constrain continental crust growth Rapid crustal growth in the first 1 Gy of Earth history Areal extent of Archean continental crust is ~14% Jacobsen 1988 The formation of the atmosphere Pu-I-Xe systematics 129I Æ 129Xe; t =16My 1/2 244Pu Æ 136Xe; t 136Xe also produced from U fission = 80My; 1/2 Kunz et al., 1998 I/(Pu+U) ratio determines the slope of the line Fundamental observation: MORB data differ from atmosphere, and show mixing between air and a component with excesses of both 129Xe (from 129I decay) and fissiogenic Xe isotopes (from Pu and/or U decay) Noble gases in the atmosphere of terrestrial planets 1. Massive depletion of volatiles from Earth 2. Abundance pattern looks like carbonaceous meteorites 3. No evidence that present atmosphere is a remnant of a primary atmosphere Solar N/Ne ~1; terrestrial N/Ne ~86,000 =>most of the nitrogen delivered in condensed form. Secondary Atmosphere: Degassing of the Earth’s mantle A reducing atmosphere (H2, CO, CH4, NH3 small amount of CO2) Or a neutral atmosphere (CO2, H2O, N2) - depends if degassing happens before core formation is over So when did the mantle degass? Pu-I-Xe systematics 129I Æ 129Xe; t =16 My 1/2 244Pu Æ 136Xe; t 136Xe also from U fission = 80 My; 1/2 Excess 129Xe in the mantle has been used to calculate mantle degassing age. (e.g., Staudacher and Allegre, 1982) Kunz et al., 1998 Mean age of degassing 15-30 Myr; prior to the end of core formation Æ Early atmosphere was likely to have been reducing But the isotopes do not add up…… Atmosphere and mantle have different composition OIBs MORBs Earth started off with a solar composition; maybe mixture of nebular and planetary gases Primitive meteorites Xenon in the atmosphere Atmospheres on Earth and Martian have preferentially lost the lighter isotopes. An example of what we have done so far: Xe in the mantle Pu-I-Xe systematics 129I Æ 129Xe; t =16 My 1/2 244Pu Æ 136Xe; t 136Xe also from U fission 1/2 = 80 My; Our knowledge of xenon to a large extent is based on a single rock (known as popping rock) from a mid-ocean ridge. Difficult measurements and we need to be able to look through the veil of atmospheric contamination Kunz et al., 1998 Mid Ocean Ridge (MORB) Xenon Atlantic Indian Pacific Air Air-MORB Popping rockmixing line (Kunz et al., 1998) MORBs The upper mantle seems to be homogeneous in it’s composition. Xenon in Ocean Island Basalts Source Iceland (Trieloff et al, 2000) 2.6 2.5 136Xe/130Xe 136Xe/130Xe 2.7 Iceland slope is about 10% steeper than MORBs. 2.4 2.3 2.2 129Xe/136Xe 6.4 6.8 7.2 7.6 8.0 129Xe/130Xe Data from our Lab Iceland sample So….? Air-MORB Air-MORB mixing line 2.6 136Xe/130Xe OIBS and MORBs have the same trend. 2.7 2.5 2.4 136Xe/130Xe 2.1 2.3 2.2 2.1 6.4 129Xe/136Xe 6.8 7.2 129Xe/130Xe 7.6 8.0 What we have learnt have so far We do not have a clear and coherent story on the early history of the atmosphere Æ inventories established early but uncertainties beyond this. Mantle degassing by itself does not generate the secondary atmosphere; if the noble gas signature is not related to a late veneer massive amounts of H2 loss is required to explain the isotopic signature in the atmosphere Æ very early change in the redox state of our planet. Have to be open to the idea that atmosphere and mantle could have evolved with different I/Xe ratios and unfortunately, mantle 129Xe* excess may not yield meaningful degassing age.