Differentiation 2: mantle, crust OUTLINE

Transcription

1 Differentiation 2: mantle, crust OUTLINE Reading this week: Should have been White Ch 10 and 11!! 7- Nov Differentiation of the Earth, Core formation W , Nov Moon, crust, mantle, atmosphere W 11.2, 11.3 evolution Today 1.Core last lecture, now the rest: 2.Mantle, crust 1

2 Core Formation Recent model: early collisional heating => deep magma ocean (persisted?) BUT possible lower mantle not hot enough to melt: could retain many siderophile elements as molten Fe sinks through from above Wood et al. (2006) 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.,

3 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? 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 3

4 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 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 4

5 NEXT: Other layers When did differentiation happen? About 4.5 billion years ago After beginning of Earth s accretion at Ga Before the formation of the Moon s oldest known rocks, 4.47 billion years ago Þ ~100 Ma window Formation of The Moon Giant impact as last major event, aka starting point: impactor s core largely transferred to Earth Moon accretes from debris in orbit (85% impactor, 15% Earth) High temperatures: evaporated the most volatile elements Lunar siderophile element depletion: it formed a core twice: once prior to impact, once after impact 5

6 Formation of our moon Highland anorthosites (white), explained by low density feldspar floating to surface of magma ocean =hot! Crust formed by time of oldest lunar rocks ~4.47 Ga. Heavy impact bombardment continued until ~3.9 Ga Ga: Basalts fill some of the large craters (Mare) => Use this for Earth analog! Earth s Mantle Depth range is 40 km to 2900 km The mantle consists of rocks of intermediate density, mostly compounds of O, Mg, Fe, Si Continents produced from melting mantle 6

7 Evidence for mantle composition: What samples do we have again? Sampled by xenoliths, occasionally exposed by crustal deformation (ophiolites) Peridotite Eclogite ß What is eclogite? Seismic velocities match both rocks Must melt to form basaltic magma Peridotite melting max 40% Eclogite melting nearly 100% Mantle compositional estimates Models on the right are still reasonable today: Pyrolite: a mix of mantle samples Anderson s model adds eclogite, to undo melting to make crust Recycled crust 7

8 Mantle structure & composition Little access = uncertainty: we use seismology, geochemistry (isotopes) and mineral physics experiments for constraints Tackley, 00 Earth s Crust Lighter rocks floated to the surface of the magma ocean. Light materials with low melting temperature, up to 40 km thick. Generally compounds of Si, Al, Fe, Ca, Mg, Na, K, O Ga zircons from western Australia have δ 18 O isotopes characteristic of liquid water: Þ Earth cooled enough for solid crust + liquid water within 100 Ma after the Giant impact (Moon > 4.47Ga) Þ The zircons origin is still debated though 8

9 Bimodal distribution of topography a hypsometric curve: two modes (left) or two plateaus (right) on curve with little transition continental crust: ~1km oceanic crust: ~ -4km from: Topography and isostasy Crust is less dense than the mantle, and basically floats on it. Continental crust = numerous rock types, but its mean density =2.7 g/cm 3. Continent = granodioriteandesite, not really granitic Oceanic crust = largely basaltic, its mean density = 2.8 g/cm 3. Isostasy = equal standing: column of mantle + crust = equal at a reference depth; thick lower density continent floats higher Low density relates to different chemical composition 9

10 Forms at mid-ocean ridges, cools/deepens away from ridge until ~180Ma basalt/gabbro -as expected from (partial) melting of the mantle All other solar system crusts are basaltic Systematic oceanic crust Hot spots are also largely basaltic (e.g. Hawaii). Hotspot melting probably deeper 10

11 How to make continental crust Mantle melting makes basalt (45-55% SiO 2 ), so how to make rocks with SiO 2 > 60% Continental crust age distribution Low density continental crust does not subduct, it just folds. ÞContinents up to 4 Ga, only continental mass recycled is small amounts of sediment on oceanic plates (small flux) ÞLand keeps being added 11

12 Where to add to a continent? At convergent plate margins (volcanic arcs) water added to the mantle from the subducted lithosphere causes melting - flux melting - calc-alkaline basalt (so still not silicic) Adding mass to a continent Step 1: accrete terranes to the continental margin; i.e. blocks of unrelated origin got assembled together Model would be initially to have island arcs collide 12

13 11/8/17 Make the granitoids Within the continental arcs Great example: coastal batholiths What we think happens: Existing low(er) SiO2 rocks get reheated by repeated intrusion and remelt/mix (just the low-temperature melting components) Compositions by Goldschmidt s classes Split primitive mantle to crust, mantle; elements divided: Lithophiles mostly in crust; ionic bonds; large ions. O, Mg, Fe, Si in mantle too Chalcophiles split between mantle, crust, core; covalent Siderophiles mostly in the core (metal) 13