Making the Earth’s crust - The generation and destruction of crustal masses through time

1. Making the Earth’s crust - The generation and destruction of crustal masses through time Mihai Ducea University of Arizona, Department of Geosciences Tucson, AZ, 85721

2. Outline • The paradox of continental formation • Making basalts, the oceanic crust • Plate tectonics, now and in the geologic past; • Continental arcs- the possible solution • The need for a recycling mechanism • Future challenges

3. Continental crustal paradox • Continental crust is being generated by several melting stages of the Earth;s mantle; it is essentially “distilled” from the mantle • All mantle melts are basaltic • Continents are granitic on average • WE DON’T QUITE UNDERSTAND HOW CONTINENTS FORM(ED)

4. Granites, granodiorites, tonalites= continental crust

5. Basalt and gabbro density-3 g/cm3 Granite-2.7 g/cm3

6. Oceanic lithosphere- different from continental: Basaltic crust, relatively thin (5-7 km), leads to topographic lows

7. Heat engine- very efficient Earth differentiation- primarily by magmatism Mantle convection- Mostly solid state Melting shallow by adiabatic decompression Lithosphere- the cold lid at the top

8. Chondrites- bulk Earth Iron meteorites=core-like

9. Mantle xenoliths- PERIDOTITES

10. CONVECTION T - scale ~ plate motions Length scales - appear much more complicated than the ridge-trench systems

11. Convection cells penetrate to the core-mantle boundary

12. Making the oceanic crust

14. Young------- Old

17. The oceanic crust • Young (< 200 Ma) • Forms at mid-ocean ridges, the oceanic crust and lithosphere cools away from ridge • Made entirely of basalt - which is what we expect to form by partial melting of the mantle • All other solar system “crusts” are basaltic

18. In addition- experiments carried out under any possible conditions of mantle melting have demonstrated unambiguously that basaltic melts should be what we get out of the mantle. Basaltic melts are ~ 50 SiO2, granites are 65% SiO2. If anything (e.g. higher degrees of melting) we’d expect lower than 50% SiO2 melts.

19. It is instructive to check if this applies to another product of mantle melting - “mantle plumes” or “hot spots” like Hawaii. The answer: YES, they’re also made of basalts.

24. What happens at subduction zones? • Oceanic plates are young; older oceanic crust has been subducted • Obviously it had to be recycled- otherwise the Earth should be increasing its volume • At subduction zones, magmatism tends to follow the subduction margin - the resulting product is a magmatic “arc”

28. Island arcs - classic example- the Aleutians arc On average - the composition is also basaltic

29. But then there are the continents • When did they form? • How did they form? • Answers: we think they form gradually over the 4.5 Ga history of the Earth • We also think they form by remelting basalts • That requires an additional “residual” reservoir that we don’t see anywhere

30. Evidence for PT goes back to the Archean. Faster motions, more melt, smaller continents (the continental nuclei known as cratons or “croutons”)

32. Continents-succession of orogenic events

33. Ages of continental rocks • Young ages are determined biostratigraphically • Older rocks are being determined geochronologically

34. Zircons - as old as 40- 4.2 Ga; evidence for continental crust

35. Adding mass to a continent Materials get accreted to the continental margin and become terranes, “exotic terranes”; I.e. blocks of unrelated origin that were once far apart but got assembled by accretion onto a continental margin

36. E.g. the Pacific

37. Classic example: the North American Cordillera

38. But a factory that transforms basalts into grantioids has to exist • The only reasonable place- the continental arcs • If not- we simply don’t have a good enough environment today (or in the recent geologic past) that makes continents • The key place where all may can be explained: the Sierra Nevada of California

40. North American batholiths Coast batholith Idaho batholith Sierra Nevada batholith Peninsular Ranges batholith

41. Igneous Intrusions 5-10 km 100-1000000 km3 ????

42. Typically shallow exposures and no deep “windows” into the arc

43. 1 part melt requires 1 to 2 parts residue

44. Sierra granitic thickness is 30-35 km, much larger than previously thought (8-10 km) three deep crustal exposures (30-35 km)

45. BIG SUR (Santa Lucia Mts) Granulites (feldspar +garnet +pyroxenes)

46. M i o c e n e P l i o c e n e Q u a t e r n a r y C S N B - C e n t r a l S i e r r a N e v a d a b a t h o l i t h Xenoliths

47. Minerals: clinopyroxene garnet orthopyroxene amphibole rutile, apatite Textures 1. Large grain sizes 2. Cumulate textures =IGNEOUS textures Gar Cpx

48. Key new observations • The arc is much thicker than thought • Requires about 1:1 or 2;1 residue to melt ratio • Much of the residue is not granulitic as all textbooks argue, but it is “ECLOGITIC”

49. ECLOGITE = garnet + pyroxene Garnet- much denser than any other crustal mineral

50. Arc root density vs. temperature Mafic lower crust