Efficient Element Recycling in Subducting Sediments

3. efficient recycling of elements from subducting sediments, including Th (and Be) with low solubility in aqueous fluids probably requires partial melting of subducting sediment Plank et al, Geol. 2003 80% recycling of subduced sediment Th Plank & Langmuir, Nature 1993

4. Class et al., G-cubed 2000

5. Aleutians Nicaragua Morris & Ryan, ToG 2003, 2007

6. Be is immobile in metamorphic fluids into eclogite facies Be (in phengite) is incompatible during melting of eclogite 10Be from sediments is efficiently recycled into arc magmas via melting constant Be (1 ppm) eclogite Bebout et al. GCA 1993

7. correlated enrichment of Ba & Th, enrichment of Be, requires melt, not fluid

9. Kessel et al. Nature 2005

10. Kessel et al. Nature 2005

11. Kessel et al. Nature 2005

12. 1974 two stage distillation: melt at ridge melt again in subduction enrich low melting pt components

13. Kay, JVGR 1978 Yogodzinski et al., 1995 primitive andesites with >100 ppm Ni and Mg# >70% form via reaction between melts of subducting eclogite and residual peridotite in the mantle wedge

14. most arc lavas are not HREE depleted: no garnet in residue? 20% eclogite melting 40% eclogite melting Fijian lavas Gill CMP 1974

15. also Ringwood, 1974

16. constraints on subduction zone & arc geotherms arc • PT of mantle-melt equilibration • PT of arc lower crust • Vp and Vs structure • topography • arc heat flow, heat flow gradient subduction zone • fore-arc heat flow • PT of HP & UHP rocks • partial melting of subducting stuff

19. metamorphic PT, Talkeetna arc section temperature, °C

27. arc • PT of mantle-melt equilibration ~1300°C, 45 km • PT of arc lower crust ~1000°C, 30 km • Vp and Vs structure ~ 6% slow just below Moho melt (NOT H2O fluid) stable in shallow mantle • topography weak wedge in upper 100-200 km • arc heat flow ≥ 700°C, 20 km

28. constraints on subduction zone & arc geotherms arc • PT of mantle-melt equilibration • PT of arc lower crust • Vp and Vs structure • topography • arc heat flow, heat flow gradient subduction zone • fore-arc heat flow • PT of HP & UHP rocks • partial melting of subducting stuff

29. Simon Peacock, AGU Monograph 2003 Table 1: Recent estimates of subduction zone shear stresses Subduction zone Shear stress Reference Match of thermal models with surface heat flow Continental 14 - 27 Tichelaar & Ruff 1993 Cascadia ~ 0 Hyndman & Wang 1995 Nankai ~ 0 Peacock & Wang 1999 NE Japan 10 Peacock & Wang 1999 Kermadec 40 ± 17 von Herzen et al. 2001 Blueschists (high P – low T conditions) Franciscan < 20-30 Peacock 1992 Mariana 18 ± 8 Peacock 1996 Dynamical modeling of trench topography Oceanic 15 - 30 Zhong & Gurnis 1994 Upper plate stress field Cascadia < 10 Wang et al. 1995 These data limit heat production via frictional shear heating beneath the fore-arc to ~ 30 mW/m2 from 0 to 50 km depth, DT ~ 40°C

30. constant viscosity models needed a rigid upper layer (but no way to get T’ at 45 km depth) Peacock et al EPSL 1993

31. constant viscosity models generally predicted sub-solidus subduction zone temperatures

32. most arc lavas are not HREE depleted: no garnet in residue? 20% eclogite melting 40% eclogite melting Fijian lavas Gill CMP 1974

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

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

37. constant viscosity models needed a rigid upper layer (but no way to get T’ at 45 km depth) Peacock et al EPSL 1993

38. most T- and stress-dependent models still have a rigid upper layer (no way to get T’ at 45 km depth) Van Keken et al., G3, 2002

39. England & Wilkins, GJI 2004

41. T-dependent viscosity, no rigid layer (like Rowland & Davies, 1999) Kelemen, Rilling, Parmentier, Mehl & Hacker, 2003

44. 1021 Pa s no T dependence 20 Myr 1021 Pa s no T dependence 200 Myr 1021 Pa s Q/(RT) = 20 20 Myr 1021 Pa s Q/(RT) = 20 200 Myr Kelemen, Rilling, Parmentier, Mehl & Hacker, 2003

45. 1021 Pa s Q/(RT) = 20 20 Myr 1021 Pa s Q/(RT) = 20 200 Myr 1018 Pa s Q/(RT) = 20 20 Myr 1018 Pa s Q/(RT) = 20 200 Myr Kelemen, Rilling, Parmentier, Mehl & Hacker, 2003

46. Kelemen, Rilling, Parmentier, Mehl & Hacker, 2003

47. Kelemen, Rilling, Parmentier, Mehl & Hacker, 2003

48. 1021 Pa s Q/(RT) = 20 20 Myr 1021 Pa s Q/(RT) = 20 200 Myr 1018 Pa s Q/(RT) = 20 20 Myr 1018 Pa s Q/(RT) = 20 200 Myr Kelemen, Rilling, Parmentier, Mehl & Hacker, 2003

50. T- and stress-dependent viscosity models yield higher subduction zone temperatures Kelemen, Rilling, Parmentier, Mehl & Hacker, 2003