1 SUMMARY OF MANTLE TEMPERATURES DON L. ANDERSON 2006

2 Bottom lines Geophysical global estimates of mantle temperature are slightly higher and have a larger range than petrological estimates from mature spreading ridges (away from ‘plume influence’) The conduction geotherm may extend deeper than the average depth of MORB extraction (~280 vs ~100 km) The deeper geotherm is subadiabatic Middles of long-lived plates can be 30-50˚C hotter than at mature ridges; “new” ridges can give hotter MORB

3 POTENTIAL TEMPERATURES (Tp) Global Geophysical Inversion [heatflow, spreading rates] 1410± 180°C [Kaula, 1983] (entire range) Ridges –[melt petrology] 1370±70°C [Asimow, 2006] (2 sigma) –[peridotites] ±100°C [Bonatti et al., 1993] –[subsidence rates] ±100°C [Perrot et al., 1998] Kolbeinsey Ridge – °C [Korenaga, Kodaira] Lower mantle – °C [Zhao, Anderson, Stacey, Stixrude] Compare with McKenzie and Bickle [1988] –Tp= 1280°C ± 20°C for normal mantle (thereby implying plumes for Tp>1300 ° C)

4 TEMPERATURE BUMP IN UPPER MANTLE Internal heating and secular cooling are expected to decrease the radial geothermal gradient away from an adiabat. Modeling shows that the average thermal gradient is expected to be significantly subadiabatic through much of the interior of the mantle There may be a maximum in T near km below the plate and below the depth of MORB extraction The geotherm is unlikely to be a TBL joining with an adiabat at the ‘lithosphere’-asthenosphere boundary (ala McKenzie, who uses the term ‘lithosphere’ for the TBL )

5

6 * *more realistically, C

7 McKenzie and colleagues assume the upper mantle to be homogeneous and isothermal. They adopt a cold subsolidus potential temperature of 1280°C ± 20°C for normal mantle. Sleep adopted  T’s of >200 C to represent plumes Most other ‘hotspots’ and LIPs have no thermal or heat flow anomaly (see

8 Temperature: Iceland Foulger et al. (2005)

9 The total range sampled by ‘normal’ ridges inferred from petrology is ˚C (Asimow, 2006) or 1475˚C if Iceland is a ridge and is not built on a continental fragment (this includes crustal thicknesses of 3-11 km and includes 'ridge-like' ridges, away from complexities that are likely to confound simple relationships between potential temperature, crustal thickness, and melt composition such as active flow, fertile sources, non-steady flow, focusing, EDGE effects).

10 These petrological estimates are now consistent with long-standing geophysical estimates. Kaula (1983) estimated the minimal upper mantle temperature variations that are consistent with observed heat flow and plate velocities. At the fully convective level, about 280 km depth, temperature variations are at least ± 180˚C, averaged over 500 km spatial dimensions. Tp under ridges was estimated at 1410˚C. There is some indication that MORB at the onset of spreading are ~50˚ hotter

11 Melting Temperatures (solidi extrapolated to P=0) Eclogite 1100˚C (extrapolated from 1 MPa) Peridotite 1300˚C (…from 1 MPa) Melting anomalies may be due to fertility streaks

12 The potential temperature of the present upper mantle is 1400±200˚C based on bathymetry, subsidence, heat flow, tomography, plate motions, discontinuity depths (Anderson, 2000). Temporal variations of ~200˚C over ~200 Myr are expected. Secular cooling of 100˚C in 1 Ga is plausible. Temperatures at onset of spreading may be ~50˚C warmer McKenzie and Bickle (1988) assumed the upper mantle to be homogeneous and isothermal. They adopted a cold subsolidus potential temperature of 1280 ± 20˚C for normal mantle and thus require hot plumes elsewhere. If normal mantle temperatures are 1400 ± 200°C, or even 1350 ± 150°C, there is no thermal requirement for hot mantle plumes. Convection simulations without plumes give the above ranges in temperature

13 RIDGES Eclogite solidus Peridotite liquidus Eclogite liquidus HISTOGRAM FROM ASIMOW (2006)

14 Cold eclogite can be negatively buoyant but it can have low shear wave velocities & low melting point. Fertile eclogite blobs can be brought into shallow mantle by entrainment or displacement or by melting

15 Dry peridotite can only melt in shallow mantle

16 COLD ECLOGITE CAN MIMIC HOT UPWELLING Dense cold eclogite can have low seismic velocities I ridges Presnell, Gudfinnsson, Herzberg

17 Extreme case of subadiabatic gradient ARCS; the hot mantle wedge paradox Kelemen et al, 2002

18 Figure 1. Predicted geotherms beneath arcs from thermal modeling (small symbols and fine lines), compared to petrological estimates of PT conditions in the uppermost mantle and lowermost crust in arcs (large symbols and thick lines). Most petrological estimates are several 100˚ hotter than the highest temperature thermal models at a given depth. Wide grey lines illustrate a plausible thermal structure consistent with the petrological estimates. Such a thermal structure requires adiabatic mantle convection beneath the arc to a depth of ~ 50 km, instead of minimum depths of ~ 80 km or more in most thermal models. Deeper mantle may be hotter than usually modeled.

19 Mantle Temperature Variations Beneath Back-arc Spreading Centers Inferred from Seismology, Petrology, and Bathymetry Douglas A. Wiens*, Katherine Kelley. Terry Plank Earth and Planetary Science Letters Compare max T with Hawaii

20 The currently high flux at Hawaii is unusual Van Ark & Lin, 2004 The quasi-periodic variations in the flux along the Hawaiian ridge may be due to fertile streaks or stress variations rather than pulsation of a plume. The highest flux is on the young lithosphere between the Murray and Molokai FZ

21 A fertility streak can be due to subduction of an aseismic ridge or seamount chain (about 20 are currently entering subduction zones) Hawaiian swell can be due to a buried buoyant load at 120 km depth (Van Ark & Lin, 2004).

22 In an internally heated mantle or in a mantle that is cooled by cold slabs, the geotherm becomes subadiabatic. This means that shallow mantle temperatures can be hotter than at ~600 km. Actual mantle temperatures and their variations are greater, and the melting temperatures can be less, than assumed in plume modelling.