Differentiation 1: core formation OUTLINE

Transcription

1 Differentiation 1: core formation Reading this week: White Ch 12 OUTLINE Today 1.Finish some slides 2.Layers 3.Core formation 1

2 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 (S 2- ) Lithophile silicate liking ([SiO 4 ] n, also O loving in practice) Atmophile gas phase liking Where do these groups reside? GG325 L31, F2013 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 L31, F2013 2

3 Major elements in mantle compared to meteorites Primitive Upper Mantle (PUM) composition is determined from intersection of chondritic meteorite array with mantle xenolith array PUM is not equal to any class of meteorites, Mg/Si higher in Earth; Ca/Si, Al/Si show solar system refractory vs. silicate phases Element Relationships: Earth and C1 Chondrites Most important siderophile and lithophile elements: BULK Earth has higher Fe/Si and Mg/Si than the chondrites (Sun) => if bulk earth» CI chondrite: lower mantle / core must host Si, or we got < chondrite 3

4 Volatiles Earth is variably depleted in volatile elements (e.g., K, Rb, Cs, etc.) relative to chondrites The Earth s Interior Earth is radially zoned, with layers of increasing density toward the center. Crust and mantle ~1/2 radius Outer and inner core other ~1/2 Crust Mantle Upper Mantle Transition Zone Lower Mantle Depth (km) ÞHow do we know? Outer Core (liquid) 2898 Core 5145 Inner Core (solid)

5 Seismic Data Density inferred from the seismological data Crust Velocity (km/sec) Lithosphere Asthenosphere Think: higher density = faster Mantle 2000 S waves P waves Mesosphere Outer Core 3000 Depth (km) 4000 Liquid 5000 Inner Core 6000 S waves Solid Main mineral phases Matching seismic velocities to high P-T experiments, starting with peridotite for mantle, Fe-alloy for core we get x-section How do we know where the phase changes happen? Major seismic discontinuities Note: spinel and perovskite (now called bridgmanite) are high P versions of olivine 5

6 Starting from the center: What s in the core? Seismic data suggests Fe alloy like Fe meteorites at high P: We think»85% Fe +»5% Ni Problem: density too high, need 10% light element Primary contenders: O, S, Si, C, P, Mg and H No direct evidence, so this is modeling, experiments, meteorite analogs ÞStill hotly debated Core formation in light of accretion 3 scenarios for accretion: v homogeneous Earth accretes from materials of the same composition AFTER condensation, followed by differentiation v heterogeneous Earth accretes DURING condensation, forming a differentiated planet as it grows v intermediate between these two end-members 6

7 Basic concept of core formation Both homogeneous and heterogeneous accretion models: core segregates by: 1) melting of accreted Fe. 2) molten Fe sinks as droplets to the Earth s center (density) Details differ between models Heterogeneous accretion Accretion during condensation requires very rapid build up of earth, ~10Ka from initiation of condensation Density control time Early core = refractory early condensates (Ca, Al) Silicate and metal phases condense next, with heating molten Fe replaces refractory core Temperature control GG325 L32, F2013 7

8 Pros: Heterogeneous accretion 1. Explains relative proportions of refractory, later elements of inner planets with decreasing nebular temperature outward 2. Some siderophile elements condense later and never equilibrate with molten iron, remain in the silicate mantle. Major problem: ultramafic phases have Fe, but Fe should have segregated to core while still hot: no Fe expected in lower mantle, but seismology: Fe/(Mg+Fe) ~0.1 Homogeneous accretion Condensation first, then Earth builds from cool materials and becomes hotter Aka: Earth accreted mostly homogeneously after condensation was complete Important aspects: a. Heat builds up as the planet accretes. b. Sometime afterwards, the core formed by Fe melting, accompanied by other chemical transformations (see next slide) 8

9 entirely Temperature controlled As things heat up, you lose volatiles, moderately volatiles, etc GG325 L32, F2013 Homogeneous accretion 1) Accretion starts with oxidized materials (some volatiles around, like C1-C3 chondrites) 2) Presence of H 2 reduces Fe to metal 3) Later, Earth heats up: 4) Then some Fe 2+ incorporated in silicates (olivines, pyroxenes) 9

10 Homogeneous accretion Heavy Fe sinks, light rock floats, but volatile loss not 100% ~10% of silicate earth retained volatiles during accretion, other 90% was degassed = primitive mantle Pros: 1. Allows volatiles in the core 2. Explains Fe in core + silicates Homogeneous accretion 3. Provides heat source for early mantle melting/ formation of proto continents Cons: 1. Degassing of all but 10% of the volatile elements doesn t work for all elements 2. Not all siderophile elements agree with this core formation model 3. Heat for melting Fe comes later: Is there enough heat to melt Fe? 10

11 Core Formation Core formation = closely linked to accretion and requires: Immiscible components (iron metal and silicate). Macro-segregation of components: at least one of which was molten or mostly molten. Substantial difference in density of components. Gravitational settling Fe melt layer collects, sinks as diapir (or crack), provides additional gravitational heat for melting Core Formation Recent model: early collisional heating => deep magma ocean (persisted?) Lower mantle not necessarily heated: can retain many siderophile elements as molten Fe sinks through from above Wood et al. (2006) 11

12 When? Core Formation Timing 182 Hf to 182 W (T 1/2 ~9 Ma = max ~50 Ma extinct) Hf is one of the most refractory elements => Earth should be ~chondritic Core formation: W into core, Hf into mantle/crust If core formation happened AFTER 182 Hf- 182 W was extinct, Earth should have chondritic W isotopes IT DOESN T => Models suggest core within Ma Chondrite Yin et al., 2002 Compositional caveat Arguments like W=core, Hf=silicates requires knowledge of core composition so what s in the core? Iron meteorites = 5-10% Ni. Great: Chondrite 6% Ni (to core)» primitive mantle Density arguments (seismology) require 10% of some light element(s) ÞWhat light elements are in the core? 12

13 Light elements in the core Contenders: O, S, Si, C, P, Mg and H. Hotly debated, but many people like S, O: FeS is miscible with Fe liquid at low and high temperatures S more depleted in silicate Earth than similar volatility elements Iron meteorites contain FeS (troilite) FeO miscibility requires high pressures and temperatures Together with S affects how much sidero/chalcophile elements enter core: needed to explain mantle What the chalcophile elements say Chalcophiles (S-loving) depleted in silicate Earth vs chondrites, though siderophiles more depleted. Þcould argue against much S in the core (if there s more S loving elements in the mantle than expected, S probably same) ongoing problem 13

14 What the siderophile elements say Siderophiles not as low in the mantle as expected from pure metal-silicate equilibration times more enriched than expected for complete silicate-fe equilibrium Volatile siderophiles even more enriched than non-volatile ones. Þ3 possible causes: 1)incomplete equilibration 2)an impure Fe phase 3)addition of a volatile rich component after core formation, aka late veneer 14