Melt segregation and strain partitioning: Implications for seismic anisotropy and mantle flow

Science

Volumn 301, Issue 5637, 2003, Pages 1227-1230

  Holtzman, B K ^a   Kohlstedt, D L ^a   Zimmerman, M E ^a   Heidelbach, F ^b   Hiraga, T ^a   Hustoft, J ^a  

^a University of Minnesota   (United States)

^b UNIVERSITÄT BAYREUTH   (Germany)

Author keywords

[No Author keywords available]

Indexed keywords

ANISOTROPY; DEFORMATION; EARTH (PLANET); STRAIN;

SEISMIC ANISOTROPY;

SEISMIC PROSPECTING;

DEFORMATION; MANTLE CONVECTION; MELTING; SEISMIC ANISOTROPY; STRAIN PARTITIONING;

ANISOTROPY; ARTICLE; ASTRONOMY; DYNAMICS; PRIORITY JOURNAL; SHEAR FLOW;

EID: 0042178292     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.1087132     Document Type: Article

References (27)

1. D. Mainprice, Tectonophysics 279, 161 (1997).
2. A. Nicolas, N. I. Christensen, Formation of Anisotropy in Upper Mantle Peridotites - A Review, Geodynamic Series (American Geophysical Union, Washington, DC, 1987), vol. 16.
3. S. Zhang, S. Karato, Nature 375, 774 (1995).
4. M. Bystricky, K. Kunze, L Burlini, J.-P. Burg Science 290, 1564 (2000).
5. H. Jung, S. Karato, Science 293, 1460 (2001).
6. G. Hirth, D. L. Kohlstedt, Earth Planet, Sci. Lett. 144, 93 (1996).
7. B. K. Holtzman, N. J. Groebner, M. E. Zimmerman, S. B. Ginsberg, D. L. Kohlstedt, Geochem. Geophys. Geosyst. 4, 8607 (2003).
8. Material and Methods are available as supporting material on Science Online.
9. M. E. Zimmerman, S. Zhang, D. L Kohlstedt, S. Karato, Geophys. Res. Lett. 26, 1505 (1999).
10. D. McKenzie, J. Petrol. 25, 713 (1984).
11. B. L. Adams, S. I. Wright, K. Kunze, Metall. Trans. 24A, 819 (1993).
12. W. B. Durham, C. Goetze, J. Geophys, Res. 82, 5737 (1977).
13. M. S. Paterson, in Physics of Strengh and Plasticity, A. S. Argon, Ed. (MIT Press, Cambridge, MA, 1969), pp. 377-392.
14. M. J. Daines, D. L Kohlstedt, J. Geophys. Res. B102, 10257 (1997).
15. The fabric in Fig. 2B would normally be interpreted as slip on the (010)[001] system. Such a change in dominant slip system could result from locally high stresses due to the strong chromite grains activating the (010)[001] slip system in olivine, which is much stronger than (010)[100] (12). However, this possibility is negated by the fact that the same CPO exists in the olivine and MORB with FeS sample, which forms bands with a weak melt phase (i.e., FeS) in place of strong chromite inclusions. Furthermore, TEM images of the dislocation structures revealed no evidence for a preponderance of dislocations with [001] Burgers vectors or with high local stresses (i.e., increased dislocation density). Because no evidence exists for a change in the relative strength of the slip systems, another explanation for the a-c switch must be invoked.
16. The absence of variation in the CPO around and within a band supports the conclusion that deformation in the bands does not contribute to or strongly modify the CPO (SOM Text, section 3). At least two reasons argue against formation of the observed CPO in the melt-rich bands: (i) To maintain a stable orientation during simple shear, the melt within the bands must move relative to the solid (7), Thus, the strain associated with a band ata glven Iocation wiU at a given location witt probably not be high enough to modify the CPO. (ii) The deformation mechanisms may be different in the two regions because of increased melt fraction in the melt-rich bands. Granular flow or grain boundary sliding accommodated by diffusion with rigid rotation of the grains in the melt-rich bands would reduce the strength of the CPO produced in the lenses, but would not produce a different CPO.
17. The whole sample undergoes general shear, with a larger amount of simple shear (∼300%) and a smaller component of axial shortening (∼20%) (i.e., "transpression"). Models incorporating the influence of transpressional geometry of deformation on olivine CPOs produce girdles in a and c axes because the flattening component efficiently orients a and c axes radially (24). Because the simple shear component of deformation is partitioned between the bands and lenses, in the lenses the shear plane has a high angle to the principal compressive stress (lenses β = 45° + 20°, bands α = 45° - 20°) and, thus, the flattening component of strain is greater. The effect of the highly asymmetric strain partitioning is superimposed on the effect of transpression on the CPO and possibly on the MPO-CPO effect; the bands accommodate much of the strain in the shear direction,@ whereas the lenses accommodate most of the strain normal to the shear direction. Furthermore, the bands may accommodate some differential movement between the lenses normal to the shear direction, but cannot accommodate internal deformation of the lenses. The internal deformation of the lenses creates the observed alignments of a axes.
18. "Complex" often implies that seismic fast directions are not parallel to plate motions. However, for this proposed relation between flow and anisotropy to be useful, we must also explain why and when "normal" relations between flow and anisotropy exist. In the MELT experiment study area (25), seismic fast directions are parallel to plate motions. In purely passive upwelling, as in the East Pacific Rise, the flow may be sufficiently two-dimensional that the extrusion required to re-align a axes may not exist.
19. J. B. Gaherty, Science 293, 1645 (2001).
20. X. Yang, K. M. Fischer, G. A. Abers, J. Geophys. Res. B100, 18165 (1995).
21. I. Bjarnason, P. G. Silver, G. Rumpker, S. C. Solomon, J. Geophys. Res. 107, 2382 (2002).
22. In all three examples, researchers have discussed the potential role of high water fugacity and high stress in modifying the relation between seismic anisotropy and flow direction, as discussed in (5). The water-stress mechanism and the one proposed here are likely to be mutually exclusive. When melting begins, olivine quickly dehydrates due to the strong partitioning of water into the melt (26), which is a likely scenario in a sub-ridge melting region and a plume head. Thus, a water-dependent mechanism may not apply to partially molten regions of the mantle.
23. In natural rock samples, new criteria must be used to identify the CPO reported here. Without knowledge of the shear direction and evidence of strain partitioning in a network of melt-rich bands, it would not be possible to distinguish this CPO from any other produced by a slip on b planes. To identify the effects of strain partitioning, one would have to establish the orientation of the a axes in relation to either the network geometry or some regional or macroscopic indicator of shear sense.
24. A. Tommasi, B. Tikoff, A. Vauchez, Earth Planet. Sci. Lett. 168, 173 (1999).
25. C. J. Wolfe, S. C. Solomon, Science 280, 1230 (1998).
26. K. Koga, E. Hauri, M. Hirschmann, D. Bell, Geochem. Geophys. Geosyst, 4, 1019 (2003).
27. Support for this research was provided by NSF EAR-0126277, NSF OCE-0002463, NSF OCE-0327143, a Japanese Society for the Promotion of Science Fellowship (to T.H.), a Fulbright Fellowship to France (to B.K.H.), and DAAD grant 315-ab and NSF grant INT-0123224 for collaborations between members of the University of Minnesota and the Bayerisches Geoinstitut.