major elements in continental crust like “calc-alkaline” arc andesite & dacite

2. major elements in continental crust like “calc-alkaline” arc andesite & dacite Kelemen CMP 1995

3. compared to igneous protolith, Sr removed by chemical weathering, Zr, Ti, … enriched in detrital zircon & rutile Kelemen et al., AGU Mon 2003

4. some island arcs have lavas ~ continental crust Kelemen et al., AGU Mon 2003

6. igneous 3 recipes for continental crust 1. crystal fractionation of primitive basalt at high fO2 and/or H2O plus “delamination” of dense stuff* 2. mixing of primitive and evolved igneous rocks plus “delamination” of dense stuff* 3. crystal fractionation of primitive andesite plus “delamination” of dense stuff* *: or cumulates lie below Moho, or “relamination” of buoyant stuff ^ in this talk: delamination? use trace elements to choose 1-3

7. Region map

8. redrawn from DeBari & Coleman, 1989

9. Aleut I-B Chugach Mtns S Sand SE Talkeetna Mtns NW Talkeetna Mtns Tonga Mar Talkeetna arc did not intrude pre-existing oceanic or continental crust instead, like an ophiolite or oceanic crust, the arc was emplaced “into itself” Rioux et al, GSA Bull 2007

10. numbers of samples, not necessarily representative of proportions of rock types magma mixing, AFC, … crystal fractionation & melting

11. crystal fractionation model of Greene et al., J. Petrol., 2006

12. Bernard map & crustal sectionfrom DeBari & Coleman, 1989 first 30% crystallization only represented by a few hundred meters of cumulate pyroxenite and gabbro near Moho 30% xl’n Cpx Mg# 92 to 85

13. Delta rho cpx’te & arc lc Jull & Kelemen, JGR 2001

14. Behn & Kelemen JGR 2006

16. D C B A if you “like” A or B, then lots more magma mixing or strongly calc-alkaline crystal fractionation than we initially expected or modeled, or …

17. D C B A A or B … substantially more SiO2-rich primitive magmas than we initially expected or modeled … maybe early in arc history? volcanics mostly eroded, plutonics largely preserved?

18. Region map approx length of SNB

19. approx length of Alaska Peninsula batholith

20. A B C D Behn & Kelemen JGR 2006 Takahashi et al Geology 2007

27. 1974 two stage distillation: melt at ridge melt again in subduction enrich low melting pt components

31. 2% fluid A M 2% melt arc/MORB enrichments via melt, not aqueous fluid boninites removed here boninites removed here MORB cont crust Kelemen et al Treatise on Geochemistry 2003

32. Peacock et al. PEPI 2005 pre-2000 style post-2000 models

33. VERY simple geodynamic models Why so sure? Geochemists say it’s right VERY simple geochemical models Why so sure? Geophysicists say it’s right

35. Kelemen et al., Treatise on Geochem 2003 Hacker et al., 2004

36. Rho granulite & arc lc Jull & Kelemen, JGR 2001

38. most models don’t satisfy constraints which basically require T’ at about 45 km

39. drips sorting products of calc-alkaline differentiation: 1. throw dense rocks off the bottom Jull & Kelemen, JGR 2001

40. sorting products of calc-alkaline differentiation: 2. hide mafic rocks below the Moho Santos et al J Pet 2002

41. sorting products of calc-alkaline differentiation: 3. “relamination” of buoyant products Brad Hacker and his fan club, in prep. Hannes Broeckner & colleagues

42. very rare Kelemen et al., Treatise on Geochem, 2003

43. making lavas ~ continental crust primitive andesite ~ 20-30% “cumulates” wet and oxidized 50-60% “cumulates” mixing 60-80% “cumulates”

44. numbers of samples, not necessarily representative of proportions of rock types

45. crystal fractionation & melting crystal fractionation & melting

47. oceanic arc lavas all samples, 0.5 < Mg# < 0.7 arc plutons wt% SiO2 continental arc lavas wt% SiO2 Kelemen et al., Treatise on Geochem 2003 wt% SiO2

49. mid-ocean ridge lavas don’t “go there”

50. Behn & Kelemen JGR 2006