Upper Mantle Melt Distribution From Petrologically Constrained Magnetotellurics

Abstract

Three parameters: temperature, hydrogen content, and the presence of partial melt, are the dominant controls on the rheology of the convecting upper mantle. As such, they determine the dynamics that control plate tectonics and continental evolution. Since hydrogen depresses the peridotite solidus temperature, these parameters are strongly linked petrologically. We have developed a genetic algorithm code to statistically assess the likelihood that a section of upper mantle contains partial melt. This code uses magnetotelluric observations and petrological constraints on composition and solidus temperatures and allows for uncertainties in the geotherm and the electrical conductivity structure. We have applied this code to the convecting upper mantle beneath (1) a stable continent (the Superior Craton); (2) a hot spot (Tristan da Cunha); (3) stable, old oceanic lithosphere (the northwest Pacific Ocean); and (4) young oceanic lithosphere (adjacent to the East Pacific Rise). Results show that the volume of melt in the convecting upper mantle is heterogeneous. The highest melt proportions are beneath the hot spot while little to no melt is required in the other regions. All regions show low water contents (generally <50 wt ppm in olivine) in the shallow convecting upper mantle, making it unlikely that water causes a large or sharp viscosity contrast between the lithosphere and the convecting mantle. Results differ significantly for different experimental olivine hydrogen conductivity models, highlighting the importance of reconciling these experimental constraints.

Magnetotelluric Constraints on the Temperature, Composition, Partial Melt Content, and Viscosity of the Upper Mantle Beneath Svalbard

“…With the geotherm thus constrained, the MT data can be used to estimate composition and partial melt content. We used the genetic algorithm code developed by Selway et al (2019) to calculate the hydrogen and partial melt contents of the upper mantle from the MT model. This code calculates the likelihood that partial melt is present in a given region and calculates the range of melt fractions and hydrogen contents that can fit an input resistivity and temperature range with associated uncertainties.…”

“…Where melt was present, its proportion was capped at a volume fraction of 1%, since larger amounts of melt would be expected to migrate out of the region (Holtzman, 2016). All other parameters are as described in Selway et al (2019).…”

A reduced order approach for probabilistic inversions of 3-D magnetotelluric data I: general formulation

“…Heise et al, 2008;Comeau et al, 2016), detect partial melting in the deep mantle (e.g. Evans et al, 2005;Selway et al, 2019), and study the thermochemical structure of the lithosphere in general (e.g. Meqbel et al, 2014;Jones et al, 2017;Fullea et al, 2011).…”

Some Remarks on the Electrical Conductivity of Hydrous Silicate Minerals in the Earth Crust, Upper Mantle and Subduction Zone at High Temperatures and High Pressures

“…As usual, it is well known that the hydrous minerals and rocks are the major carrier of water in the subducted geotectonic environment and NAMs can carry trace amounts of water into the deep uppermost mantle. Since the first theoretical calculations of electrical conductivity on hydrous olivine from Karato (1990) based on the proton diffusion of Nernst-Einstein equation, there are a large quantity of electrical conductivity results to be reported from the laboratory-based electrical conductivity experiments, theoretical calculations of molecular dynamics, and filed geophysical observation of magnetotellurics (or electromagnetic depth soundings) [2][3][4][5][6][7][8]. Furthermore, by virtue of these acquired high-pressure laboratory-based conductivity results, we can extrapolate the water content, distribution state, storage form, and migration mechanism of water in the deep Earth interior.…”

“…Ti  , M Ti  , etc. Here, M Ti  denotes the trivalent titanium ion of metallic site in the olivine crystalline lattice, M Ti  denotes the tetravalent titanium ion of metallic site in the olivine crystalline lattice) and hydrogen-related defects (such as (3 ) Si H  , (4 ) Si H  , etc.Here,(3 ) Si H  denotes three hydrogen vacancies of silicon site in the olivine crystalline lattice,(4 ) …”