Self-consistent interpretation of magnetotelluric observations for understanding the nature of oceanic lithosphere and asthenosphere
Abstract
Marine magnetotelluric (MT) observations give estimates on electrical conductivity structure of the oceanic upper mantle. The obtained electrical conductivity distribution can be interpreted in terms of the parameters controlling that of mantle mineral and melt phases. The parameters are temperature, small amount of H2O and CO2 dissolved in nominally anhydrous minerals and melt and the fraction of melt if the mantle is partially molten. The relations between the conductivity of each phase and these parameters have widely explored by laboratories. The electrical conductivity obtained from the MT observation can be converted into one of the parameters using the relations. It is impossible to constrain all the parameters at once from the electrical conductivity alone and use of other independent information and/or reasonable assumptions are indispensable to distinguish the impacts of each parameter. The consideration of the reduction of peridotite solidus due to the H2O and CO2 and partition of them in peridotite and melt can couple the parameters mutually and they can be constrained more by self-consistent manner. Assuming a one-dimensional thermal structure based on the plate cooling model, we investigated five parameters, mantle potential temperature, the thickness of thermally conductive plate, contents of H2O and CO2 in the bulk mantle, and the crustal conductivity that explains the MT responses in the two areas, the northwest (Area A) and southeast (Area B) of the Shatsky Rise in the northwestern Pacific, obtained through the Normal Oceanic Mantle (NOMan) project. A grid search inversion was applied to the MT data to find an acceptable range of the five parameters. Advantages of this approach are; 1) we can obtain the thermal structure and volatile contents that explain the MT data as well as the conductivity profile under a statistical criterion, 2) we can investigate the trade-off relation between the model parameters, and 3) we can investigate various conditions of interest, for example, necessity of partial melt. The results indicate that thickness of the thermally conductive plate is significantly thinner for Area A than for Area B but the potential temperature can be comparable between the two area, although the lithospheric ages are old and similar (130 Ma for Area and 140 Ma for Area B). We also found that small amount (< 0.1 %) of carbonated-silicate melt must exist if the mantle contains CO2.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMDI13C0022B
- Keywords:
-
- 3006 Marine electromagnetics;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3025 Marine seismics;
- MARINE GEOLOGY AND GEOPHYSICS;
- 7208 Mantle;
- SEISMOLOGY;
- 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS