Dehydration Melting At The Top Of The Lower Mantle: Summary

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One of the article “Dehydration Melting At The Top Of The Lower Mantle.” Suggests evidence of large water reservoirs trapped inside the transition layer of the earth sub surface. The study by Brandon Schmandt argues that there are the huge water reservoirs that could be the cause of dehydration melting seen on the mantle layer of the earth sub surface (Schmandt et al 1266). According to his paper, water content found in the upper mantle from the mid-ocean ridge basalts weigh about 0.02 to 0.05 percent weight. However, significant potential can be seen following the discovery of between 3 and 1 percent weight, some 410 km to 660 km deep into the mantle layer. The water is stored in large mineral deposits of wadsleyitte and ringwoodite. The movement of this minerals from the upper layers of the transition layer down through the mantle layer to the lower transition layer cause some filtration of elements and in that case if the water content exceeds the water levels in the transition layer then possibility of dehydration melting is very real. This would be caused by the downward flow towards the mantle.

According to “Seismic Evidence of Effects of Water on Melt Transport in The Lau Back-Arc Mantle” the study presents some very high resolution 3d seismic data of the Lau Basin. The study places constrain on melting variations on the mantle layer in a region made of extremely extended gradients of water content on the mantle layer. In the study, use of Rayleigh wave tomography is employed together with data from a land and an ocean bottom surface based network seismic imaging of the basin. The two are used to help understand the phase velocity periods and the shear velocity of the waves seen in the upper mantle layer (Wei et al. 395-398).

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Due to external and internal variation of factors like temperature and the composition of the mantle plus the melt which have significant influence on the seismic velocity. Use of SV wave velocity (Seismic Velocity) estimates is adopted so that corrections of the water effects are made. For radial anisotropy a rather unexpected result is observed where low shear velocity is observed beneath the spreading centers of the basin places where melt generation is expected. This however explains the analogy of partial melt as a dominant factor in the region with high water content. From the study, though, it became rather difficult to determine full extent of the effects of melt on the seismic velocity. This is occasioned by limited and insufficient method to conduct a quantitative interpretation (Lawrence, Jesse, & Michael 255).

According to “Broad Plumes Rooted At The Base Of The Earth’s Mantle Beneath Major Hotspots” the effects of underground activities can be associated with development of plumes and other related volcanic structures. In order to detect the plumes the study suggests use of whole mantle imaging techniques that combine computations of wavefield and the seismic waveforms. With this technique, broad and quasi vertical structures that resemble conduits can be observed on or near hotspot regions. The premise that necessitated this study is the proposal by Morgan some 40 years ago, that hotspot volcanoes are evidence of existence of plumes that originates from deep mantle. The existence of the plumes and their actual depth has always been debatable. However, according to the study, hotspots can be anchored from the core mantle boundary. This can be illustrated by the distribution of hotspots in respect to observed mantle circulation (French, Scott & Barbara 96).

Finally, to control velocity and expected changes in correlation with the lower mantle and surface mantle, numerical circulation models are used (Schmandt, et al 1265). There is also the use of radial variants to compute viscosity and to make appropriate prescriptions about surface motions. Attenuation temperature has been employed as a control factor to determine the melt. This has shown that an increase in temperature which leads to a significant decrease in velocity (Lawrence, Jesse, & Michael 255).

  • French, Scott W. and Barbara Romanowicz. “Broad Plumes Rooted At The Base Of The Earth’s Mantle Beneath Major Hotspots”. Nature, vol 525, no. 7567, 2015, pp. 95-99. Nature Publishing Group
  • Lawrence, Jesse F., and Michael E. Wysession. “Seismic evidence for subduction‐transported water in the lower mantle.” Earth’s Deep Water Cycle (2006): 251-261.
  • Schmandt, B. et al. “Dehydration Melting At The Top Of The Lower Mantle”. Science, vol 344, no. 6189, 2014, pp. 1265-1268. American Association For The Advancement Of Science (AAAS)
  • Wei, S. Shawn et al. “Seismic Evidence Of Effects Of Water On Melt Transport In The Lau Back-Arc Mantle”. Nature, vol 518, no. 7539, 2015, pp. 395-398. Nature Publishing Group

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