The influence of melt on the LPO, seismic properties and dynamics of mantle plumes
Abstract
Patterns of seismic anisotropy are indicators of both deformation mechanisms and melt distribution in the mantle and, as such, are primary sources of information on mantle flow dynamics. Recent experimental constraints on the influence of melt distribution on lattice preferred orientation (LPO) of deformed partially molten rocks impact interpretations of seismic anisotropy data, based on 3 observations: i) In melt-free samples deformed at low differential stresses, olivine a-axes (seismic fast direction) align in the shear direction; ii) when melt is present and pockets align due to stress, olivine a-axes form girdles; iii) when melt segregates into melt-rich networks, the resulting partitioning of strain and modification of stress fields cause alignments of a-axes 90o to the shear direction. Here, we propose tests for the presence of melt-modified fabrics based on 1) field observations in the Oman ophiolite and 2) published seismic observations from the Iceland hotspot. Plume-like structures provide good test sites because of strong 3-D gradients in flow and melt production, but to what extent can small-scale diapiric flow provide insight into the dynamics of large-scale plumes? 1) In the Semail Massif, frozen mantle flow structures suggest the presence of small scale (< 10 km diameter) diapiric flows. Although direct evidence of melt (frozen pockets) are rare, abundant evidence for large fluxes of melt in the area exist, including significant weakening of mantle rocks that enabled small-scale diapiric flow. In olivine LPOs from this region, increasingly strong maxima develop with increasing distance from the center of the diapir, though in preliminary results, the effects of melt as observed in our experiments are not obvious. 2) In large-scale plumes, the third mechanism (shear-normal a-axes) may develop. In SKS splitting observations of upper mantle beneath Iceland (e.g., Li and Detrick, EPSL, 2003), fast directions tangential, as opposed to radial, to the plume center are often observed. In deforming partially-molten mantle, effects of melt geometry on seismic anisotropy must be considered and compared to the effects of LPO. We propose a model involving melt segregation, strain partitioning, and olivine a-axis rotation to explain these tangential fast-directions in plumes.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.U53A..01H
- Keywords:
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- 3000 MARINE GEOLOGY AND GEOPHYSICS;
- 3902 Creep and deformation;
- 7203 Body wave propagation;
- 8121 Dynamics;
- convection currents and mantle plumes;
- 8162 Rheology: mantle