HR: 14:30h
AN: S12D-03    [PDF]
TI: What Mantle Processes Determine Isotopic
AU: * Tackley, P J
EM: ptackley@ucla.edu
AF: Department of Earth and Space Sciences, UCLA, 595 Charles Young Drive E, Los Angeles, CA 90095-1567 United States
AU: * Tackley, P J
EM: ptackley@ucla.edu
AF: Institute of Geophysics and Planetary Physics, UCLA, 595 Charles Young Drive E, Los Angeles, CA 90095-1567 United States
AU: Xie, S
EM: sxie@ess.ucla.edu
AF: Department of Earth and Space Sciences, UCLA, 595 Charles Young Drive E, Los Angeles, CA 90095-1567 United States
AB: Isotopic measurements on Mid Ocean Ridge Basalts and Ocean Island Basalts indicate effective `ages' (from e.g., U-Pb or Sm-Nd systems) in the range 1-2 billion years- much less than the age of the Earth, even though melting should have been much more vigorous early on and skewed the mean time since melting to older values. This relatively young `age' has generally been explained in terms of stretching of heterogeneities by mantle convection, which might reduce them to dimensions too small to be individually distinguishable in short timescales of less than 1 Gyr. On the other hand, published numerical models that use tracers to track differentiated material (Christensen and Hofmann, 1994, Davies, 2002) suggest that Earth-like `ages' can be obtained without taking stretching-induced erasure of tracer signatures into account, although this might effectively happen if the lengthscale for sampling the isotope systems was large enough. In those models, the only explicit mechanism for resetting isotope systems was re-melting, but for this to explain the isotopic ages observed for basalts, the global rate of melting in the recent past would have had to be very much higher than present-day values. To investigate stretching vs. re-melting we have conducted numerical experiments of a cooling mantle with plate tectonics, differentiation and evolution of important isotopic systems. The time of last melting and the total strain is tracked on each tracer (in addition to isotopic information). The results confirm that a model matching today's crustal production rate and with a reasonable secular cooling history generates `ages' that are substantially larger than those observed, with the extent of crustal settling above the CMB making some difference but not enough. The effect of sampling lengthscale on observed `age' is also tested and found to be insufficient to explain the data. Thus, these results reaffirm the importance of stretching as a key mechanism for effectively deleting older heterogeneities. From analysis of strain vs. age and matching of the observed ages, it is estimated that erasure of heterogeneities occurs at strains of 10$^3$-10$^4$, somewhat larger than has often been assumed.
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 3670 Minor and trace element composition
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8125 Evolution of the Earth
SC: Seismology [S]
MN: 2003 Fall Meeting