Differentiation 2: mantle, crust OUTLINE

Transcription

1 Differentiation 2: mantle, crust Reading this week: White Ch 12 OUTLINE Today 1. Core last lecture, now the rest: 2. Mantle, crust 1

2 QoD? 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) What light elements are in the core? 2

3 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 are depleted in the silicate Earth relative to chondrites, but not as depleted as many of the siderophiles are. could argue against much S in the core (if there s more S loving elements in the mantle than expected, S probably same) ongoing problem 3

4 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. 3 possible causes: 1) incomplete equilibration 2) an impure Fe phase 3) addition of a volatile rich component after core formation, aka late veneer 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 ~100 Ma window 4

5 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 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! 5

6 Earth s Mantle Lies between the crust and the core. Depth range is 40 km to 2900 km. The mantle consists of rocks of intermediate density, mostly compounds of O, Mg, Fe, Si New continental crust may be produced during partial melting of mantle material. Evidence for mantle composition: Sampled by xenoliths, occasionally exposed by crustal deformation (ophiolites) Peridotite Eclogite What is eclogite? Seismic velocities match both rocks Must melt to form basaltic magma Peridotite melting max 40% Eclogite melting nearly 100% 6

7 Mantle compositional estimates Models on the right are still reasonable today: Pyrolite: a mix of mantle samples Anderson s model adds eclogite, to undo melt depletion Recycled crust 7

8 Earth s Crust Lighter rocks floated to the surface of the magma ocean. The crust is formed of light materials with low melting temperature and is 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: => Earth cooled enough for solid crust + liquid water within 100 Ma after the Giant impact (Moon > 4.47Ga) 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: 8

9 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 Continents are complex, oceanic crust systematic Forms at mid-ocean ridges, cools away from ridge until ~180Ma Made ~entirely of basalt - expected from (partial) melting of the mantle All other solar system crusts are basaltic 9

10 Hot spots are also largely basaltic (e.g. Hawaii). Hotspot melting probably deeper How to make continental crust Mantle melting makes basalt (45-55% SiO 2 ), so how to make rocks with SiO 2 > 60% 10

11 Continental crust age distribution Low density continental crust does not subduct, it just folds. Continents up to 4 Ga, only continental mass recycled is small amounts of sediment on oceanic plates (small flux) Land keeps being added 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) 11

12 11/11/15 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 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) 12

13 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